Photoeffects at semiconductor-electrolyte interfaces (ACS symposium series)

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Author: Arthur J. Nozik (Ed.)

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Photoeffects at SemiconductorElectrolyte Interfaces

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

Photoeffects at SemiconductorElectrolyte Interfaces Arthur J. Nozik, EDITOR

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Solar Energy Research Institute

Based on a symposium sponsored by the Division of Colloid and Surface Chemistry at the 179th Meeting of the American Chemical Society, Houston, Texas, March 25-26, 1980.

146

ACS SYMPOSIUM SERIES

AMERICAN

CHEMICAL

SOCIETY

WASHINGTON, D. C. 1981

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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Library of Congress CIP Data Photoeffects at semiconductor-electrolyte interfaces. (ACS symposium series; 146 ISSN 0097-6156) Includes bibliographies and index. 1. Photochemistry—Congresses. 2. Photoelectricity— Congresses. 3. Semiconductors—Congresses. I. Nozik, Arthur J., 1936. II. American Chemical Society. Division of Colloid and Surface Chemistry. III. Series: American Chemical Society. ACS symposium series; 146. QD701.P47 ISBN 0-8412-0604-X

541.3'5 ASCMC8

80-27773 146 1-416 1981

Copyright © 1981 American Chemical Society A l l Rights Reserved. The appearance of the code at the bottom of the first page of each article in this volume indicates the copyright owner's consent that reprographic copies of the article may be made for personal or internal use or for the personal or internal use of specific clients. This consent is given on the condition, however, that the copier pay the stated per copy fee through the Copyright Clearance Center, Inc. for copying beyond that permitted by Sections 107 or 108 of the U.S. Copyright Law. This consent does not extend to copying or transmission by any means—graphic or electronic—for any other purpose, such as for general distribution, for advertising or promotional purposes, for creating new collective works, for resale, or for information storage and retrieval systems. The citation of trade names and/or names of manufacturers in this publication is not to be construed as an endorsement or as approval by ACS of the commercial products or services referenced herein; nor should the mere reference herein to any drawing, specification, chemical process, or other data be regarded as a license or as a conveyance of any right or permission, to the holder, reader, or any other person or corporation, to manufacture, reproduce, use, or sell any patented invention or copyrighted work that may in any way be related thereto. PRINTED IN T H E UNITED STATES O F AMERICA

American Chemical Society Library 1155 16th St., N.W. Washington, D.C. 20036 Interfaces; Nozik, A.; In Photoeffects at Semiconductor-Electrolyte ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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A C S Symposium Series M . Joan Comstock, Series Editor

Advisory Board David L. Allara

James P. Lodge

Kenneth B. Bischoff

Marvin Margoshes

Donald D. Dollberg

Leon Petrakis

Robert E. Feeney

Theodore Provder

Jack Halpern

F. Sherwood Rowland

Brian M . Harney

Dennis Schuetzle

W. Jeffrey Howe

Davis L. Temple, Jr.

James D. Idol, Jr.

Gunter Zweig

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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FOREWORD The ACS SYMPOSIUM SERIES was founded in 1974 to provide

a medium for publishing symposia quickly in book form. The format of the Series parallels that of the continuing ADVANCES IN CHEMISTRY SERIES except that in order to save time the papers are not typeset but are reproduced as they are sub mitted by the authors in camera-ready form. Papers are re viewed under the supervision of the Editors with the assistance of the Series Advisory Board and are selected to maintain the integrity of the symposia; however, verbatim reproductions of previously published papers are not accepted. Both reviews and reports of research are acceptable since symposia may embrace both types of presentation.

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

PREFACE

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T

his volume, based on the symposium "Photoeffects at SemiconductorElectrolyte Interfaces," consists of 25 invited and contributed papers. Although the emphasis of the symposium was on the more basic aspects of research in photoelectrochemistry, the covered topics included applied research on photoelectrochemical cells. This is natural since it is clear that the driving force for the intense current interest and activity in photoelectrochemistry is the potential development of photoelectrochemical cells for solar energy conversion. These versatile cells can be designed either to produce electricity (electrochemical photovoltaic cells) or to produce fuels and chemicals (photoelectrosynthetic cells). Thefirst12 chapters of this volume are concerned with the vital sub ject of the effects of surface properties on photoelectrochemical behavior. This includes work on the effects of the chemical modification of semi conductor electrode surfaces either through molecular derivatization or ionic adsorption; the effects of surface structural defects and surface elec tronic states on photo-induced charge transfer across semiconductor-elec trolyte interfaces; the kinetics of competing chemical reactions on semicon ductor electrode surfaces; catalytic effects on semiconductor surfaces; and the problems of photocorrosion of semiconductor electrodes. Chapters 13-15 deal with new electrode materials (oxide semiconductors) and structures (protective layers and interfacial chlorophyll layers). Chapters 16 and 17 relate to the basic energetics of the semiconductor-electrolyte interface (potential distribution and the effects of charge inversion leading to band-edge unpinning), while Chapters 18-22 present results on new chemical systems and phenomena associated with photoelectrochemistry. These latter include luminescence studies, surfactant assemblies, a new model based on the effects of ionic desorption, studies of carbanion oxi dation on semiconductor surfaces, and the behavior of molten-salt electro lytes. Finally, the volume concludes with three chapters on electrochemical photovoltaic cell devices dealing with models for current-voltage charac teristics, stability performance, and solid electrolytes. The exceptional interest and ferment in photoelectrochemistry has been manifested in 1980 by the appearance of at least five major confer ences, symposia, or workshops in thefieldwith international participation, including the present symposium. It is apparent from these meetings, as well as from the burgeoning amount of published literature, that photo– ix In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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electrochemistry is a vital, productive, and stimulating field of science in which much significant and exciting scientific progress can be expected for a long time. Although very rapid progress has also been made in the applications of photoelectrochemistry, it is still too early to predict what impact photoelectrochemistry will have on practical solar energy conver sion systems. We can be confident, however, that basic research in photo electrochemistry will continue to produce important new knowledge, and that attractive potential applications to our energy problems will continue to provide the impetus for the work. Solar Energy Research Institute Golden, Colorado 80401 October 3, 1980

ARTHUR J. NOZIK

x

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

1 The Influence of Surface Orientation and Crystal Imperfections on Photoelectrochemical Reactions at Semiconductor Electrodes

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HEINZ GERISCHER Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-1000 Berlin 33, West Germany

It is well known that the surface orientation of crystals and imperfections in the surface, l i k e grain boundaries or dislo cations, affect largely the reaction rates at electrodes made of metals or semiconductors. Such effects are most pronounced in those reactions where atoms leave their position in a crystal lat tice or have to be incorporated into such one. These processes are connected with activation barriers which are particularly high for semiconductors where the chemical bonds between the components of the crystal lattice are highly directed and localized. If we con sider photoelectrochemical reactions at semiconductors we have ad ditionally to discuss the influence of these factors on light ab sorption and i t s consequences. Factors which control the y i e l d of photocurrents Photocurrents and photovoltages are induced by the generation of excessive mobile charge carriers. In a semiconductor these are electron hole pairs generated by light absorption. The size of the effects is largely dependent on how far the recombination between electrons and holes can be prevented. An efficient separation of electron hole pairs occurs only in the space charge layer beneath the semiconductor/electrolyte contact. Large efficiencies can be reached if this space charge layer forms a high enough energy bar rier for the two charge carriers to encounter each other. Such a situation is found in a depletion layer of n-type or p-type semi conductors or in an inversion layer (1,2). Here we shall not con sider insulating materials where one can use high external elec tric fields to obtain charge separation (3). Assuming that we have such a situation favorable for charge separation, we have to consider what factors influencing the effi ciency of charge separation in an illuminated semiconductor elec trode are affected by crystal orientation or crystal imperfec tions. Five such factors are l i s t e d in the following table:

0097-6156/81/0146-0001$05.00/0 © 1981 American Chemical Society In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

2

PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE

(1) (2) (3) (k) (5)

INTERFACES

Schottky b a r r i e r height : Δψ Schottky b a r r i e r e x t e n s i o n : tP r a t e o f surface recombination: r mean d i f f u s i o n l e n g t h o f m i n o r i t i e s : L p e n e t r a t i o n depth o f l i g h t : — with α = absorptivity The quantum y i e l d o f the photocurrent f o r an e l e c t r o d e i l l u minated from the f r o n t s i d e can be c a l c u l a t e d from a simple model d e s c r i b e d by Gartner (h) p r o v i d e d some s i m p l i f y i n g assumptions are a p p l i c a b l e . T h i s model i s shown i n F i g u r e 1. I f surface recombina t i o n can be n e g l e c t e d , the quantum y i e l d φ i s obtained as g

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Φ = 1 -

1

I

aL

exp (- aW)

0)

The equation above can be approximated i f the width o f the space charge l a y e r i s s m a l l compared w i t h the p e n e t r a t i o n depth o f the l i g h t 1/a by

**wfï

i f « . V « 1

(2)

The width o f the space charge l a y e r depends on the height o f the Schottky b a r r i e r according t o TT

W =

J/ 2ε

I 1

ε e

ο

Δφ /se · Ν

11 ^

(3)

ο where Ν i s the donor or acceptor c o n c e n t r a t i o n , ε, ε and e have t h e i r u s u a l meaning o f p e r m i t t i v i t y and elementary e2ectric°charge. These equations c o n t a i n f o u r parameters o f Table 1 and i n d i c a t e how the quantum y i e l d i s a f f e c t e d by these f a c t o r s . The surface recombination r a t e i s only important i f the b a r r i e r height i s low and can otherwise be neglected. This was assumed i n the d e r i v a t i o n o f Equation (1) which r e q u i r e s a h i g h enough b a r r i e r h e i g h t . The main e f f e c t o f c r y s t a l o r i e n t a t i o n i s caused by d i f f e r e n t b a r r i e r h e i g h t s on d i f f e r e n t c r y s t a l f a c e s . I t i s w e l l known t h a t V o l t a - p o t e n t i a l d i f f e r e n c e s are dependent on c r y s t a l o r i e n t a t i o n because the surface d i p o l e d i f f e r s f o r d i f f e r e n t f a c e s . I n the case o f a semiconductor e l e c t r o d e t h i s means t h a t the f l a t band p o t e n t i a l which can be determined e x p e r i m e n t a l l y (5_,6.,χ) depends on surface o r i e n t a t i o n . Consequently, the band bending at the same p o s i t i o n o f the Fermi l e v e l i n the b u l k o f the semiconductor, i . e . at the same e l e c t r o d e p o t e n t i a l , d i f f e r s f o r d i f f e r e n t f a c e s . Figure 2 gives a schematic p i c t u r e f o r such d i f f e r e n c e s . Besides the e f f e c t s o f d i f f e r e n t surface d i p o l e s , the con c e n t r a t i o n and energy p o s i t i o n o f surface s t a t e s depend a l s o l a r g e l y on surface o r i e n t a t i o n w i t h the r e s u l t t h a t the e l e c t r i c excess charge i n surface s t a t e s can be very d i f f e r e n t on d i f f e r e n t s u r f a c e s . This i s i n d i c a t e d i n F i g u r e 3 by a comparison between the f l a t band s i t u a t i o n and t h e s i t u a t i o n at equal e l e c t r o d e p o t e n t i a l f o r d i f f e r e n t s u r f a c e s . Case (a) i s a surface f r e e o f surface

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

GERiscHER

Surface Orientation and Crystal Imperfections

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-1/a

Helmholtz double layer Figure 1.

Geometric parameters characterizing the semiconductor-electrolyte contact

E c

'"~Η= redox

E

~ ffredoxE

surface

Figure 2.

I

F

surface I

s u r f a c e ΙΠ

Energy scheme for different surfaces of a semiconductor electrode at equilibrium with the same redox system

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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4

PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE INTERFACES

s t a t e s , case ("b) a surface w i t h acceptor s t a t e s and case (c) a surface w i t h donor s t a t e s both having energy l e v e l s c l o s e t o the conduction band. Since the excess charge i n these surface s t a t e s changes, w i t h v a r y i n g p o l a r i z a t i o n , t h e band bending depends i n a d i f f e r e n t way on t h e v o l t a g e a p p l i e d . Besides such e l e c t r o n i c surface s t a t e s which can i n t e r a c t e i t h e r w i t h e l e c t r o n s i n t h e b u l k o f the semiconductor or w i t h a redox system i n t h e e l e c t r o l y t e , we have t o consider another type o f excess charge a t the s u r f a c e . This stems from adsorbed ions or from i o n i c groups attached t o t h e surface o f the semiconductor. This i s w e l l known from t h e pH dependence of the f l a t band potent i a l o f semiconducting oxides {8) o r the dependence o f the f l a t band p o t e n t i a l o f s u l f i d e s on t h e s u l f i d e concentration i n s o l u t i o n (£). Since surfaces o f d i f f e r e n t o r i e n t a t i o n w i l l i n t e r a c t d i f f e r e n t l y w i t h such i o n i c charge, t h i s again w i l l a f f e c t the p h o t o e l e c t r o c h e m i c a l processes v i a the d i f f e r e n t b a r r i e r heights at d i f f e r e n t surface o r i e n t a t i o n . E l e c t r i c charge i n surface s t a t e s or i o n i c groups on the s u r face w i l l p a r t i c u l a r l y depend on imperfections found i n the s u r f a c e . Imperfections i n t h e b u l k w i l l a f f e c t the extension o f a Schottky b a r r i e r s i n c e such imperfections can form t r a p s f o r e l e c t r i c charge. Such a case has been s t u d i e d f o r example at z i n c oxide ( 1 0 ) . The r a t e o f surface recombination i s only important at low b a r r i e r h e i g h t . S i n c e , however, the b a r r i e r height depends on both f a c t o r s , o r i e n t a t i o n and i m p e r f e c t i o n s , there are d i f f e r e n t potent i a l ranges, where t h i s e f f e c t has t o be taken i n t o account. The mean d i f f u s i o n l e n g t h o f the m i n o r i t i e s i s d r a s t i c a l l y dependent on c r y s t a l i m p e r f e c t i o n s . I t may a l s o be dependent on surface o r i e n t a t i o n , i f they have an a n i s o t r o p i c m o b i l i t y . O p t i c a l a n i s o t r o p y i s necessary f o r seeing an i n f l u e n c e o f c r y s t a l o r i e n t a t i o n on the p e n e t r a t i o n depth o f the l i g h t w h i l e c r y s t a l imperf e c t i o n s w i l l only a f f e c t t h e p e n e t r a t i o n depth i n a range o f wave lengths where l i g h t a b s o r p t i o n i n t h e c r y s t a l i s very weak. The photocurrents are very s m a l l i n t h i s case which w i l l not be d i s cussed here. We conclude from t h i s d i s c u s s i o n t h a t a very complex c o r r e l a t i o n between s t r u c t u r e and photoelectrochemical behavior i s t o be expected and i t w i l l o f t e n be d i f f i c u l t t o decide what may be the main i n f l u e n c e . The f o l l o w i n g examples are s e l e c t e d under the aspect t o demonstrate some e f f e c t s o f surface o r i e n t a t i o n and c r y s t a l imperfections i n systems where they are very pronounced. M a t e r i a l s w i t h a l a r g e a n i s o t r o p y o f the c r y s t a l p r o p e r t i e s are the best candidates f o r t h i s purpose. Therefore semiconductors w i t h l a y e r s t r u c t u r e which have been introduced i n t o p h o t o e l e c t r o chemical s t u d i e s by T r i b u t s c h ( 1 1 , 1 2 , 1 3 ) are predominantly used as examples.

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

1.

GERiscHER

Surface Orientation and Crystal Imperfections

5

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Photoredox r e a c t i o n s at d i f f e r e n t surfaces of l a y e r e d semiconductors Molybdenum s e l e n i d e and molybdenum s u l f i d e e l e c t r o d e s s h a l l demonstrate the i n f l u e n c e o f d i f f e r e n t surface o r i e n t a t i o n on photoredox r e a c t i o n s . Figure k represents the c r y s t a l s t r u c t u r e o f MoS^ -where one can immediately see t h a t on the van der Waals-surface e x c l u s i v e l y s u l f u r atoms are exposed t o the contact -with the e l e c t r o l y t e . They should behave very d i f f e r e n t l y from any other surface where the molybdenum atoms are not so -well s h i e l d e d from i n t e r a c t i o n w i t h the e l e c t r o l y t e s o l u t i o n . Photocurrent v o l t a g e curves have been s t u d i e d w i t h molybdenum s e l e n i d e c r y s t a l s of d i f f e r e n t o r i e n t a t i o n and d i f f e r e n t p r e t r e a t ment. F i g u r e 5 represents r e s u l t s f o r three t y p i c a l surfaces of ntype MoSe ( j J O . An e l e c t r o d e w i t h a very smooth surface cleaved p a r a l l e l t o the van der Waals-plane shows a very low dark current i n contact w i t h the KI c o n t a i n i n g e l e c t r o l y t e since i o d i d e cannot d i r e c t l y i n j e c t e l e c t r o n s i n t o the conduction band and can only be o x i d i z e d by h o l e s . At a b i a s p o s i t i v e from the f l a t band p o t e n t i a l ^fb ^ d e p l e t i o n l a y e r i s formed a photocurrent can be observed as shown i n t h i s F i g u r e . This photocurrent reaches a s a t u r a t i o n at a p o t e n t i a l about 300 mV more p o s i t i v e than when surface recombination becomes n e g l i g i b l e . An e l e c t r o d e w i t h another surface s t r u c t u r e which was not cleaved but used as grown appeared t o c o n t a i n faces of other o r i e n t a t i o n . I t s current v o l t a g e behavior i s seen i n curve 2 of F i g ure 5 . This e l e c t r o d e had a s i m i l a r b l o c k i n g character i n the dark as the f i r s t one. The most prominent d i f f e r e n c e i s t h a t the onset of the photocurrent i s s h i f t e d t o more anodic p o t e n t i a l s and i s somewhat l e s s steep than at the f i r s t e l e c t r o d e . The surface s t r u c t u r e s o f these two e l e c t r o d e s are shown i n the microscopic p i c t u r e s of F i g u r e 6 (15). I t appears t h a t the second e l e c t r o d e s t i l l contains a l a r g e p o r t i o n of van der Waals-planes w i t h some s u r faces of other o r i e n t a t i o n i n between. Although the l a t t e r ones are s t i l l i n a c t i v e i n the dark, they have a d i f f e r e n t d i p o l e o r d i f f e r i n a d s o r p t i o n of charged ions from s o l u t i o n . Therefore the f l a t band p o t e n t i a l i s s h i f t e d i n t o anodic d i r e c t i o n . A d r a s t i c a l l y d i f f e r e n t behavior was found i f the e l e c t r o d e was prepared by a mechanical cut normal t o the van der Waals-surf a c e . The dark current increases s t e e p l y above a c r i t i c a l anodic p o t e n t i a l as i s seen i n curve 3 of F i g u r e 5 . Only a s m a l l photocurrent can be observed which q u i c k l y becomes i n d i s t i n g u i s h a b l e from the dark c u r r e n t , i f the e l e c t r o d e p o t e n t i a l i s f u r t h e r increased a n o d i c a l l y . This shows t h a t the recombination r a t e i s much higher at t h i s k i n d of surface and t h a t e l e c t r o n i n j e c t i o n i n t o the cond u c t i o n band i s now c a t a l y z e d by surface s t a t e s generated by the formation of steps and other surface imperfections (j_6 ) . Figure 7 shows another set of examples f o r a molybdenum s u l fide electrode ) . Current voltage curve No. 1 gives the r e s u l t for a smooth e l e c t r o d e surface cleaved along the van der Waalsw

e r e

a

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

6

PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE INTERFACES

(c)

(a) Ecflat c: band E potential E

^4

negative surface charge._ 5

F

. positive ^surface

C

F

ψ charge

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fc

, vacant acceptor states

reverse bias

plane, free of surface states

plane with donor type surface states

plane with acceptor type surface states

Figure 3. Energy correlations for three different surfaces of an η-type semicon ductor at the flat band potential (upper row) and at equal anodic bias flower row): (a) plane free of surface states; (b) plane with surface states of acceptor character; (c) plane with surface states of donor character

M0S2

layer

van der Waals distance

M0S2

layer

1 •=Mo 2 Η - MoS

Figure 4.

2

0=S

lattice

W

V

J

unit

cell

Model of a 2 Η-crystal lattice for MoS

2

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

GERiscHER

Surface Orientation and Crystal Imperfections

n-MoSe

ι

in 1M \ (i I:80mW/cm AN 11 2

I

2

ε

1

I

.

/

/ / / /

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Ί

/

0

y -0,5

2

h

/- ^ J 2_____-_-

y/

da

0

r

i

1

I

0,5

1.0

electrode

potential vs.SCE [V]

Figure 5. Current-voltage curves in the dark and under illumination (solar-like light with 80 mW/cm ) of MoSe electrodes with different surface structure (\A): (1) 1-c face, cleaved (van der Waals face); (2) ||-c face, as grown; (3) ||-c face, mechanically cut; (electrolyte: 1M KCl + 0.05M KI) 2

2

Figure 6. Pictures of the surfaces used in Experiments 1 and 2 of Figure 5

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

8

PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE INTERFACES

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Τ

J

I

0

^05

I

I

UÔ

^V5

ELECTRODE POTENTIAL

[Vvs.SCE]

Figure 7. Current-voltage curves in the dark and under illumination (680-nm light, 1 mV/cm ) of a MoS electrode of ±-c orientation at different states of surface perfection (15): (1) freshly cleaved; (2) after 25-min photocorrosion (solar light of AM 1) at 1 V in 1M KCl solution; (3) after 115-min photocorrosion, same conditions as (2); electrolyte: 1M KCl + 0.05M KI 2

2

SCE

e l e c t r o n - ι ho le pair separation ι

Figure 8. Model for charge distribution and course of the electric potential at a semiconductor with two different adjacent surface areas in contact with an electro lyte

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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1.

GERiscHER

Surface Orientation and Crystal Imperfections

9

plane. The dark current i s again very s m a l l and the photocurrent reaches s a t u r a t i o n very q u i c k l y . With continuous exposure t o photoc o r r o s i o n i n the absence of the redox system the e l e c t r o d e surface becomes more and more l e s s p e r f e c t . An i n c r e a s i n g number of steps i s formed on the s u r f a c e and the dark current increases as shown i n curves No. 2 and 3. The onset of the photocurrent i s simultane o u s l y h i f t e d t o more anodic p o t e n t i a l s . However, i n t h i s example, the s a t u r a t i o n photocurrent seems p r a c t i c a l l y not t o decrease. In order t o e x p l a i n these e f f e c t s one has t o assume t h a t the e l e c t r i c double l a y e r s t r u c t u r e i s d i f f e r e n t at d i f f e r e n t faces ( 1 6 ) . While the van der Waals-face should not c o n t a i n surface s t a t e s which can accumulate excess e l e c t r i c charge, other faces o r surfaces w i t h s t r u c t u r a l defects form surface s t a t e s w i t h such a p r o p e r t y . Therefore, depending on the s i g n of the charge i n these surface s t a t e s , the space charge underneath the surface w i l l , a t a given p o s i t i o n of the Fermi l e v e l i n the b u l k , be l a r g e r o r s m a l l e r i n these regions than underneath the van der Waals-planes. Figure 8 shows a model where the excess charge on the surface i s p o s i t i v e and t h e r e f o r e the extension of the d e p l e t i o n l a y e r i n the η-type semiconductor i s s m a l l e r i n t h i s r e g i o n beneath the surface than i n the other r e g i o n s . Recombination i s enhanced at the defec t i v e p a r t s of the surface w h i l e the photocurrents are u n a f f e c t e d on the other p a r t s as i s i n d i c a t e d i n t h i s p i c t u r e . I f the surface s t a t e s would p i c k up negative charge, we would have the opposite s i t u a t i o n w i t h a more extended space charge l a y e r beneath the surface areas c o n t a i n i n g such d e f e c t s . This would s h i f t the onset of the photocurrent t o more negative p o t e n t i a l s and could improve the photocurrent y i e l d . I t appears t h a t such a s i t u a t i o n can a l s o be found i n some cases. Photocurrent

and a n i s o t r o p y of l i g h t

absorption

A n i s o t r o p i c c r y s t a l s have a l i g h t absorption c o e f f i c i e n t de pending on the d i r e c t i o n of the l i g h t wave and i t s p o l a r i z a t i o n ( 1 7 ) . This again can be demonstrated i n e l e c t r o c h e m i c a l e x p e r i ments w i t h l a y e r e d c r y s t a l s ( 1 8 ) . Some r e s u l t s obtained w i t h g a l l i u m s e l e n i d e c r y s t a l s are shown i n the f o l l o w i n g F i g u r e s . G a l l i u m s e l e n i d e , Ga Se^, i s a l a y e r e d m a t e r i a l where the Ga^-molecules are enclosed between two selenium l a y e r s . The s t r u c t u r e i s shown i n F i g u r e 9· The a n i s o t r o p y o f the o p t i c a l ab s o r p t i o n has been s t u d i e d w i t h very t h i n c r y s t a l s (}9). I t was found t h a t the absorption c o e f f i c i e n t f o r l i g h t p o l a r i z e d normal t o the l a y e r s i s higher than f o r l i g h t p o l a r i z e d p a r a l l e l t o the l a y e r s . Because we were not able t o prepare smooth surfaces o r i e n t e d p a r a l l e l t o the c - a x i s , we s t u d i e d the photocurrents obtained under i l l u m i n a t i o n w i t h p o l a r i z e d l i g h t at v a r i o u s angles of i n cidence. Since s - p o l a r i z e d l i g h t has the e l e c t r i c a l v e c t o r p a r a l l e l t o the van der Waals-planes independent of the angle of i n cidence, the photoresponse o f t h i s l i g h t could be used f o r a nor m a l i z a t i o n of the photocurrent s p e c t r a obtained. This was neces-

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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Figure 9.

PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE INTERFACES

Structure model of a Ga Se crystal 2

2

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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1.

GERISCHER

Surface Orientation and Crystal Imperfections

11

sary "because w i t h v a r y i n g angle o f incidence the d i f f r a c t e d l i g h t beam e n t e r i n g the c r y s t a l v a r i e s d i r e c t i o n and the p e n e t r a t i o n depth o f l i g h t must be c o r r e c t e d f o r t h i s . What i s more important f o r a comparison, the i l l u m i n a t e d area o f the e l e c t r o d e v a r i e s w i t h v a r y i n g angle o f incidence i n a not f u l l y c o n t r o l l a b l e manner. These d i f f e r e n c e s can, however, be excluded i f one measures the r e l a t i v e s i z e o f the photocurrents f o r s- and p - p o l a r i z e d l i g h t at every wave l e n g t h and r e l a t e s these values t o the photocurrent spectrum obtained at normal incidence where no d i f f e r e n c e between s- and p - p o l a r i z e d l i g h t was found i n accordance w i t h the t h e o r e t i c a l e x p e c t a t i o n . A f u r t h e r c o r r e c t i o n has t o be made f o r the d i f f e r e n c e o f the r e f l e c t i v i t i e s f o r s- and p - p o l a r i z e d l i g h t . A s e t o f r e s u l t s obtained i n such experiments i s shown i n Figure 10. One sees t h a t t h e l i g h t absorption w i t h p - p o l a r i z e d l i g h t leads t o a d r a s t i c i n c r e a s e of the photocurrents i n the whole s p e c t r a l range. This corresponds t o the l a r g e r absorption c o e f f i c i e n t f o r l i g h t w i t h p - p o l a r i z a t i o n . Since such d i f f e r e n c e s can only be seen i n the photocurrents i f the quantum y i e l d i s s m a l l - c f Equation ( 1 ) and ( 2 ) - t h e d i f f u s i o n l e n g t h of e l e c t r o n s , L , must be s m a l l compared t o the p e n e t r a t i o n depth o f the l i g h t 1/a. Figure 11 gives another example performed i n the same way w i t h a g a l l i u m selenide c r y s t a l o f another (lower) doping concentrat i o n . The pronounced d i f f e r e n c e s from the previous Figure i n d i c a t e c l e a r l y t h a t i t i s not only the d i f f e r e n t absorption c o e f f i c i e n t which c o n t r o l s the height o f the photocurrents. The d i f f u s i o n l e n g t h o f the m i n o r i t y c a r r i e r s normal t o the surface i s of equal importance f o r the quantum y i e l d as Equation ( 1 ) and ( 2 ) have shown. Therefore the r e l a t i o n between the y i e l d s f o r s- and p-pol a r i z e d l i g h t w i l l s t r o n g l y depend on the l i f e time of the m i n o r i t y c a r r i e r s . Both s p e c t r a should c o i n c i d e i f L » 1/a and . This can p a r t l y e x p l a i n the d i f f e r e n c e s found f o r the two e l e c trodes . The somewhat p r e l i m i n a r y r e s u l t s shown here demonstrate t h a t l a r g e e f f e c t s o f t h i s k i n d can be found and one has t o take i n t o account the anisotropy o f l i g h t absorption i n the study of p h o t o e f f e c t s at semiconductor e l e c t r o d e s . J(

The r o l e o f c r y s t a l imperfections C r y s t a l imperfections p l a y an enormous r o l e i n semiconductor e l e c t r o c h e m i s t r y . Examples f o r imperfections i n the surface have already been given i n the previous s e c t i o n . Imperfections i n the b u l k mainly i n f l u e n c e recombination. This i s always seen i n the decreased y i e l d s a f t e r mechanical surface p o l i s h i n g where a great number of c r y s t a l defects has been generated. Such c r y s t a l s show l a r g e photocurrents only a f t e r c a r e f u l e t c h i n g u n t i l a l l mechanical d e f e c t s have been removed from the boundary l a y e r i n which the phot o c u r r e n t s are generated. An example i s given i n F i g u r e 12 f o r the photocurrents observed at a z i n c oxide e l e c t r o d e . I t has been shown t h a t mechanically formed defects i n z i n c oxide act as t r a p s f o r holes and as recombination centers (10). T h e i r presence i n the po-

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE INTERFACES

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12

Figure 10. Photocurrent spectra for ppolarized light at different angles of inci dence, normalized to s-polarized light, represented by the spectrum at 0° (GaSe electrode with N = 7 Χ 10 cm' at V = -0.7 V; electrolyte: 1M H SO ) 16

3

4

8CE

2

k

LOO

500

600 wavelength

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

700 InmJ

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1.

GERiscHER

Surface Orientation and Crystal Imperfections

13

s i t i v e l y charged s t a t e decreases the extension of the space charge l a y e r . Therefore the photocurrent i s reduced i n two ways, by enhanced recombination and by a decrease of the e l e c t r o n hole p a i r s e p a r a t i o n e f f i c i e n c y of the space charge l a y e r . S i m i l a r r e s u l t s have been observed w i t h CdS (20) and GaAs(21)• One more example f o r the r o l e of imperfections i n the bulk and on the surface s h a l l be demonstrated here. This i s t h e i r i n f l u e n c e on recombination luminescence which can be q u i t e i n d i c a t i v e f o r the type of defect which i s i n v o l v e d . Luminescence can be s t u d i e d by i l l u m i n a t i o n of the c r y s t a l w i t h l i g h t of s h o r t e r wave l e n g t h as w i d e l y done i n s o l i d s t a t e i n v e s t i g a t i o n s . In the case of an e l e c t r o c h e m i c a l system one can observe luminescence a l s o due t o the i n j e c t i o n of m i n o r i t i e s i n t o the s u r f a c e . This has been done w i t h semiconductors o f s m a l l band gap very e a r l y (22_,23). Semiconductors w i t h a wider band gap have f i r s t been s t u d i e d by Beckmann and Memming (2h) who c o u l d observe luminescence at GaP from recombination v i a surface s t a t e s . We have a p p l i e d t h i s technique t o z i n c oxide where hole i n j e c t i o n can be reached w i t h very o x i d i z i n g redox species (25). F i g u r e 13 shows the r e s u l t of such experiments. The luminescence observed covers two s p e c t r a l ranges. One can be a t t r i b u t e d t o i n terband recombination and corresponds t o the l i g h t absorption edge of z i n c oxide. There i s a t a i l at longer wave lengths which can come e i t h e r from recombination v i a surface s t a t e s or v i a bulk s t a t e s having energy l e v e l s w i t h i n the gap. Mechanical p o l i s h i n g of the surface changes the s p e c t r a l d i s t r i b u t i o n of the luminescence l i g h t d r a s t i c a l l y as shown i n t h i s F i g u r e . The interband luminescence i s l a r g e l y reduced but not the t a i l at longer wave l e n g t h s . This can be taken as an i n d i c a t i o n t h a t the longer wave l e n g t h range of luninescence stems from surface s t a t e recombinat i o n s i n c e the mechanical p o l i s h i n g w i l l c e r t a i n l y decrease the l i f e time of m i n o r i t y c a r r i e r s i n the b u l k and enhance r a d i a t i o n l e s s recombination. On the other hand, the same treatment might i n c r e a s e the number of surface s t a t e s due t o the formation o f d i f f e r e n t l y o r i e n t e d s u r f a c e areas o r of surface d e f e c t s . I t appears reasonable t h a t the luminescence v i a surface s t a t e s cannot be quenched by the i n c r e a s e d c o n c e n t r a t i o n of defects i n the b u l k and may even i n c r e a s e due t o a higher c o n c e n t r a t i o n of such s t a t e s . More systematic i n v e s t i g a t i o n s are necessary t o analyze the d e t a i l s of such processes. However, t h i s i s a good example showing how s e n s i t i v e p h o t o e f f e c t s i n semiconductors respond t o s t r u c t u r a l changes of the surface and i n the boundary l a y e r underneath the i n t e r f a c e . Conclusions Surface o r i e n t a t i o n and i m p e r f e c t i o n s o f the surface or the b u l k expose very d r a s t i c a l i n f l u e n c e s on the photoelectroehemical behavior of semiconductor e l e c t r o d e s . This i s very important f o r a l l a p p l i c a t i o n s of such systems, f o r example f o r the conversion

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE INTERFACES

electrode

Figure 12.

potential

vs. S C E

[V]

Photocurrent-voltage curves for a ZnO electrode after two different surface pretreatments (electrolyte: 7 M KCl)

λ Cnm] Figure 13. Luminescence from a ZnO electrode into which holes are injected by SOj,' radicals (25) (spectra for two different surface pretreatments)

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

1.

GERISCHER

Surface Orientation and Crystal Imperfections

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of s o l a r energy ( 1 U , 2 6 , 2 7 ) . I t appears t h a t h i g h e f f i c i e n c i e s can o n l y be reached -with m a t e r i a l s and surfaces where a l l the e f f e c t s which favor recombination between e l e c t r o n s and holes can be minimized. No surface s t a t e s should be present which can p i c k up charge o f the same s i g n as t h a t o f the m i n o r i t y c a r r i e r s since t h i s would reduce t h e band bending under i l l u m i n a t i o n by an i n creased voltage drop i n the Helmholtz double l a y e r and would d i m i n i s h t h e photovoltage output a c c e s s i b l e from such a contact. Besides t h i s , these f a c t o r s have a l s o a v e r y l a r g e i n f l u e n c e on the c o r r o s i o n r e a c t i o n s which c o u l d not be discussed here although they are extremely dependent on surface imperfections ( j j S ). The o p t i c a l a n i s o t r o p y o f most semiconductor m a t e r i a l s , which has been neglected n e a r l y completely i n e l e c t r o c h e m i c a l experiments u n t i l now, must a l s o be taken i n t o account. Acknowledgement : I want t o thank Mrs. M. Lubke f o r performing t h e experiments w i t h GaSe. Literature Cited 1.

e.g. Milnes, A. G . ; Feucht, D. L. "Heterojunction and Metal Semiconductor Junctions", Academic Press: New York - London, 1972.

2.

Gerischer, Η . , in "Light-Induced Charge Separation in Biology and Chemistry", Eds. H. Gerischer and J. J. Katz; Dahlem Konferenzen: Berlin, 1979; p. 61.

3.

cf Gerischer, H . ; W i l l i g , F. "Topics in Current Chemistry", Vol. 61; Springer-Verlag: Berlin, 1976; p. 31-84.

4.

Gärtner, W. W. Phys. Rev., 1959, 116, 84.

5.

Myamlin, V. Α . ; Pleskov, Yu. V. "Electrochemistry of Semi conductors"; Plenum Press: New York, 1967·

6.

Gerischer, Η . , in "Physical Chemistry", Vol. IX A: "Electro chemistry"; Eds. H. Eyring, D. Henderson, W. Jost; Academic Press: New York, 1970; p. 463-542.

7.

Van den Berghe, R. A. L.; Cordon, F.; Gomes, W. P. Surf. S c i . , 1973, 39, 368.

8.

Lohmann, F . Ber. Bunsenges. Phys. Chem., 1966, 70, 87, 428.

9.

Inoue, T . ; Watanabe, T . ; Fujishima, Α . ; Honda, K. B u l l . Chem. Soc. Japan, 1979, 52, 1243.

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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Gerischer, H . ; Hein, F.; Lübke, M . ; Meyer, E.; Pettinger, B . ; Schöppel, H. Ber. Bunsenges. Phys. Chem., 1973, 77, 284.

11.

Tributsch, Η . , Z. Naturforsch., 1977, 32a, 972.

12.

Tributsch, Η . , Ber. Bunsenges. Phys. Chem., 1978, 82,

13.

Tributsch, Η . , J. Electrochem.

14.

Gobrecht, J., Thesis, Technical University B e r l i n , 1979.

15.

Kautek, W., Thesis, Technical University Berlin, 1980.

16.

Kautek, W.; Gerischer, H . ; Tributsch, H. Ber. Bunsenges. Phys. Chem., 1979, 83, 1000.

17.

e.g. Greenaway, D. L.; Harbeke, G. "Optical Properties and Band Structure of Semiconductors", Pergamon Press: Oxford 1968.

18.

Gerischer, H . ; Gobrecht, J.; Turner, J. Ber. Bunsenges. Phys. Chem., 1980, 84, 596-601.

19.

Akhundov, G. Α . ; Musaev, S. Α . ; Bakhyshev, A. E.; Musaev, L . G. Sov. Phys. Semicond., 1975, 9, 95.

20.

Van den Berghe, R. A. L.; Gomes, W. P . ; Cordon, F. Ber. Bunsenges. Phys. Chem., 1973, 77, 291.

21.

Van Meirhaage, R. L.; Cordon, F.; Gomes, W. P. Ber. Bunsen ges. Phys. Chem., 1979, 83, 236.

22.

Gobrecht, H . ; Bender, J.; Blaser, R.; Hein, F.; Schaldach, M . ; Wagemann, H. G. Phys. L e t t . , 1965, 16, 132.

23.

Bernhard, J.; Handler, P. Surf. S c i . , 1973, 40, 141.

24.

Beckmann, Κ. H . ; Memming, R. J. Electrochem. Soc., 116, 368.

25.

Pettinger, B; Schöppel, H. R.; Gerischer, H. Ber. Bunsenges. Phys. Chem., 1976, 80, 849.

26.

Heller, Α . ; Chang, K. C . ; Miller B. J. Electrochem. 1977, 124, 697.

27.

Lewerenz, H. J.; Heller, Α . ; DiSalvo, F. J. J. Am. Chem. Soc., 1980, 102, 1877.

Soc.,

1978, 125,

169.

1086.

1969,

Received October 23, 1980.

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

Soc.,

2 Carrier Recombination at Steps in Surfaces of Layered Compound Photoelectrodes H. J. LEWERENZ, A. HELLER, H . J. L E A M Y , and S. D. FERRIS

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Bell Laboratories, Murray Hill, NJ 07974

The performance of n-WSe and n-MoSe photoanodes differing in surface morphology has been investigated. A correlation between the smoothness of the surface as determined by scanning electron micros copy and solar conversion efficiency was found. With a smooth WSe photoanode, a solar-to-electrical conversion efficiency of ~5% is reached in the n-WSe /2MKI-0.05MI /C cell. Charge collection electron (EBIC) microscopy on p-WSe shows that steps on the surface of layered sem iconductors are recombination sites. The deleterious effect of steps is explained by deflection of minority carriers towards recombination sites at the edges of steps by an electric field component which parallels the layers. 2

2

2

2

2

2

Layered compound transition metal dichalcogenides gained recent interest as electrode material in semiconductor liquid junction solar cells (1-5). These materials show improved stability when used as pho toanodes even in oxidizing solutions such as relatively non-toxic air stable cells, which do not require hermetic seals. The improvement in stability to photocorrosion has been attributed to the fact that excitation of the layered dichalcogenides involves transitions between hybridized metal d-bands, leaving the bonding of the illuminated semiconductors relatively unaffected (2,3). Review of the metal-metal distances which determine the d-d-interaction and hence also the d-d-splitting, suggests that WSe , MoSe and WS are likely to be particularly stable when used as photoanodes. Because of uncertainties even in the most basic data for these materials none can be ruled out or preferred in photoelectrochemi– 2

2

2

0097-6156/81/0146-0017$05.00/0 © 1981 American Chemical Society In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE INTERFACES

cal solar cells. Thus, for example, band gaps ranging from 1.16eV (6) to 1.56eV (7) have been reported for WSe . The differences in the data may be due to the methods of preparations of the different crystals (8J£). With respect to semiconductor liquid junction solar cells, the unknown parameters include flat band positions, photoelectrode kinetics and com patibility with various redox couples (10). Substantial differences in the performance of the various layered dichalcogenides in solar cells have been reported. Molybdenum dichalcogenides appeared to have higher short circuit currents, open circuit voltages and fill factors (Γ1). The first investigation of p-WSe revealed losses, of unknown origin, due to car rier recombination (12). The best solar conversion efficiency reached with this material was of 2% (under laboratory conditions). This paper analyzes the causes of the losses in n-WSe and n-MoSe photoanodes and proposes ways to reduce these. Tungsten and molybdenum diselenide crystallize in the molybdenite structure with the space group symmetry D£ -P /mmc (13, 14). The arrangement of the metal atoms with respect to each other is that of a close packed hexago nal layer. Each of these layers is surrounded by two adjacent closepacked planes containing the anions, so that the cations occupy trigonal prismatic sites. Strong covalent bonding is assumed within the layers (15, 16, 17) whereas the interlayer attractive forces are predominantly characterized by van der Waals interactions. This particular bonding results in the remarkable anisotropic properties of layered semiconduc tors. The peculiar features of the band structures of the transition lay ered metal compounds include the following (16): (i) Substantially split bonding and antibonding orbitals result from the strong covalent bond between metal and chalcogen s and ρ orbi tals. The resulting energy separation of the respective bands is rela tively large (16). (ii) The interaction between the metal d- and chalcogen sp-orbitals is weaker and causes the d-bands to be located within the metalchalcogen sp-gap. (iii) The d and the d _ , type subbands are strongly hybridized. The hybridization produces an energy gap within the d-bands. Theoretical calculations as well as experimental data show that the highest occupied band has ^-character (15. 16. 18) whereas the four other d-subbands are unoccupied. The n—WSe band structure is presented schematically in Figure 1. A band gap of 1.35eV is assumed (8). 2

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h

z2

xl

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yl

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Carrier Recombination

Experimental

Single crystals of n-WSe and n-MoSe were grown using chlorine transport; for p-WSe , TeCl was used as transport agent Q£). The dop ing of these samples was approximately the same (~8 χ 10 cm~ ). The area of the crystals varied between 0.2cm and 0.5cm and their thickness from 0.1mm to 0.5mm. To avoid differences in doping or composition all samples studied were selected from the same batch of crystals. The mounting procedure of the WSe and MoSe photoanodes has been described earlier (20). The electrochemical experiments were performed in the normal three-electrode potentiostatic configuration. All potentials are referred to that of the r/lj redox couple. Except in solar efficiency measurements, which were performed in sunlight, a 150W tungsten iodine lamp (Oriel Corp.) was used. All solutions were prepared from deionized water with analytical grade chemicals. 2

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2

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The Correlation Between Surface Morphology and Solar Cell Performance of WSe and MoSe Photoanodes 2

2

Initial inspection of the as-grown crystals, revealed substantial vari ations in shape, thickness and crystal morphology. Examination with a light microscope showed large differences in surface topography, which were particularly pronounced in WSe samples. The main features, found on the surfaces were: (a) crack-like steps (b) deep ruptures (c) heaps of small, thin crystallites lying on the surface (d) very small terraces associated with growth spirals (e) smooth but curved (concave or convex) surfaces (f) smooth planar single crystalline surfaces of hexagonal shape 2

In order to study the influence of surface morphology on the pho toanode performance, samples differing in surface structure have been selected. Electron micrographs of representative areas of three different n-WSe electrodes are shown in Fig. 2. Substantial differences in surface structure ranging from nearly smooth (Fig. 2a) through moderately structured (Fig. 2b) to extremely high structured (Fig. 2c) are noted. 2

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PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE INTERFACES

Figure 2.

Electron micrographs of representative areas of three n-WSe samples

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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Carrier Recombination

In order to obtain a semiquantitative estimate of the respective amount of the non-horizontal surface area on each sample, we counted the height and length of the dislocations, which were mostly step-like. This was done by (a) taking electron micrographs at two different angles, thus obtaining a stereoscopic view of the surface and (b) by estimating step-heights with "defocusing" under a light microscope. Since the sur face structure is to a large extent semi-macroscopic, we only considered step heights larger than Ιμίη. The non-horizontal surface area was estimated to be 5% of the horizontal area in case of sample A (Fig. 2a) -20% for sample Β (Fig. 2b) and -80% in case of sample C (Fig. 2c) which had particularly high steps. The current voltage curves corresponding to these samples, and measured in the n-WSe /2MKl-50mMl /C cell are shown in Fig. 3. Drastic differences in short circuit current, open circuit voltage and fill factors are found. The smoothest electrode, sample A, shows the highest values whereas the most structured sample C shows the lowest ones. Sample A exhibits an open circuit voltage of 0.63V and a fill factor of 0.37 with the maximum power point at 0.35V. The fill factors of the approximately four times more structured sample Β is 0.25, the open circuit voltage is 0.5V and the short circuit current is down by a factor of 2.5. The fill factor of sample C is only 0.23, the open circuit voltage is 0.39V and the short cir cuit current is one third of that of sample A. Practically the same correlation is found on n-MoSe electrodes. The electron micrographs of representative areas of two samples, differing in surface morphology, are shown in Fig. 4. Sample A, although not completely smooth, reveals considerably less surface struc ture than sample B. The resulting solar cell performance of these elec trodes is shown in Fig. 5. The smoother photoanode (Fig. 4a) shows substantially higher values of fill factor, open circuit voltage and short circuit current than the more structured one (Fig. 4b). After the results established a correlation between surface structure and cell performance in n-WSe and n-MoSe electrodes, crystals with smooth surfaces were selected to build semiconductor liquid junction solar cells of increased efficiency. The electron micrographs of these n-WSe samples, labeled D, Ε and Η are shown in Fig. 6. Their surfaces reveal almost no structure. Current voltage curves of these and addi tional samples are displayed in Fig. 7. With the best these anodes, labelled D-H, have open circuit voltages approaching 0.6V and exhibit fill factors as high as 0.55. The short circuit current of sample F, with some surface structure, is slightly lower than that of the other electrodes. 2

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PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE INTERFACES

PHOTO VOLTAGE / V

Figure 3.

Current-voltage characteristics of the samples shown in Figure 2 in 2M KI-0.05M I solution (the solution was exposed to air) 2

Figure 4.

Electron micrographs of representative areas of two n- MoSe samples

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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1

PHOTOVOLTAGE/V

Figure 5.

Figure 6.

Current-voltage characteristics of the samples shown in Figure 4 in 2M KI-0.5M h solution (air)

Electron micrographs of representative areas of three smooth n-WSe samples

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE INTERFACES

.2

.4

.6

PHOTO VOLTAGE /V

Figure 7. Current-voltage characteristics of the samples shown in Figure 6 (D, E, H). Also shown are the current-voltage characteristics of additional smooth samples F and G; solution: 2M KI-0.05M I (air). 2

14 r

PHOTO VOLTAGE / V

Figure 8. Current-voltage characteristic of photoanode D (see also Figures 6 and 7) under 92.5 mW/cm insolation in the n-WSe /2M KI-0.05M I /C cell 2

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Carrier Recombination

LEWERENZ ET AL.

In order to get an estimate of the solar-to-electrical conversion efficiency on layered compounds, sample D has been measured in sun light. The result, obtained at 92.5mW/cm insolation is shown in Fig. 8. The maximum power point is at 0.33V and 10.7mA/cm , with a resulting solar conversion efficiency of 3.7%. As is evident from Fig. 7, some samples show better overall performance than sample D. The best of these, sample G, the surface of which was accidentally damaged before being measured in the sun, had a maximum power output which exceeded that of sample D by a factor of 1.4 bringing the estimated solar conversion efficiency to 5.2%. The existence of recombination sites at or near the surface of a semiconductor is known to affect the short circuit current spectra in three respects: first, the current efficiency decreases at all wavelengths; second, the loss being greatest at short wavelengths; third, under intense illumination (with a laser beam) the quantum efficiency declines further (21,22). In Fig. 9, the photocurrent spectra of two parts of the same n—WSe electrode are shown. Spectra measured on a relatively smooth part of the sample are displayed in Fig. 9a. The spectra of a highly struc tured part are shown in Fig. 9b. It is seen that the current from the structured surface is lower for all wavelengths and a drastic drop of current efficiency under intense illumination ("laser on") is noted. No such behavior is observed in Fig. 9a on the smoother part of the sample. The spectra with the "laser on" in Fig. 9 were obtained at an irradiance of 2mW/cm at 6328À with a He-Ne laser. Comparison of the current efficiencies of the smoother and the highly structured parts of the crystal shows a decline of the current by a factor of 4 under low level illumina tion through a monochromator. Additional illumination with laser light causes a further decrease by a factor of 5, thus reducing the quantum efficiency to 5% of that of the smooth part of the sample. 2

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2

Identification of Recombination Sites by EBIC

A powerful method for the characterization of the origin of losses on structured surfaces is provided by the EBIC (Electron Beam Induced Current) technique also known as charge collection microscopy (23, 24). A typical EBIC experiment is shown schematically in Fig. 10. A Schottky barrier is formed between a semiconductor and a metal. An electron beam impinges through the metal and creates electron hole pairs in the depletion region of the semiconductor. The charge carriers are separated by the electric field in the space charge region and are detected as collected current l in the external circuit. In the presence of recomc

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE INTERFACES

Figure 9. Photocurrent spectra of two parts of the same n-WSe electrode: (a) relatively smooth part and (b) heavily structured part. The arrows indicate the assumed band edge; ( ) the relative quantum yields at very low levels of illumination; ( ) the relative quantum yields when the sample is illuminated also by a superimposed laser beam of 2 mW/cm at 6328 A. The spectra are corrected for the response of the system. 2

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Carrier Recombination

bination sites, I is decreased, and by scanning the electron beam across the surface, one obtains information on the location and effectiveness of recombination sites. We utilized this technique on a structured p—WSe sample (25) (good quality Schottky barriers with η-type materials were difficult to form). The Schottky junction was made by evaporation of 500Â Al onto the p-WSe surface (26). An ohmic back contact was obtained by eva porating 300Â of Zn followed by 1000Â of Au onto the back of the sam ple. Figs. 2 and 4 reveal that the surfaces of layered compounds are often terraced. We have therefore investigated the charge collection efficiency on a part of the WSe sample which showed very fine steps under the scanning electron microscope. Fig. 11a is an electron micro graph of a typical stepped surface and Fig. lib shows the corresponding EBIC-picture. In the latter, high charge collection efficiency is indicated by dark areas and poor efficiency by light areas. Fig. 11 shows very poor collection efficiency at and near the edges of steps and provides direct proof that steps act as recombination sites. The effect of steps on the collection efficiency can be more quantitatively displayed if I is recorded during a single lateral scan of the electron beam. Fig. 12 shows a typical result, obtained on another part of the sample. Clearly, the collection current l drops whenever the electron beam scans across a step, demon strating further the recombination at these. c

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c

c

Discussion

The results in Section 3 and 4 demonstrate that steps on the surface of layered semiconductors are recombination sites, and hence predom inantly responsible for the poor cell performance of structured electrodes. We therefore proceed in examining the role of steps in lay ered compounds. First, we consider the ideal case of an atomically smooth sample as shown in Fig. 13 for WSe . Here, we have assumed an average absorp tion coefficient of M0 cm in the energy range above 1.5 eV (7) hence the light intensity l drops to / /e within 1000À. With a static dielectric function e (perpendicular to the layer structure) of 9(27), a barrier height of 0.6V and a carrier concentration of 810 cm~ , a space charge layer thickness of 860 Â is obtained in the depletion approximation. Thus within the accuracy of the above approximation, the absorption length L and the depletion layer width W have very similar values (as shown in Fig. 13) indicating that most of the incoming light is absorbed in the space charge region. The absorption of light results in creation of electron hole pairs, predominantly in the depletion region, and is then followed, as usual, by transport of minority carriers to the surface. In layered compounds, however, mobility within the layers is high, while mobility perpendicular 2

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PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE INTERFACES

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ELECTRON BEAM

SEMICONDUCTOR (p-TYPE)

BACK CONTACT

Figure 10.

Schematic of an EBIC experiment with a p-type semiconductor as photoactive part (l denotes the charge collection current) c

Figure 11. (a) Electron micrograph of a stepped part of a p-WSe photocathode; (b) EBIC picture of the same part of the p-WSe sample; the charge collection efficiency is lowest for light and highest for dark areas. 2

2

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LEWERENZ ET AL.

Carrier Recombination

Zr< ο

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Lu or

x/ARB. UNITS

Figure 12. Collection current of an EBIC experiment obtained by scanning the electron beam across a stepped surface of a p-WSe sample (x-direction); beam voltage 20 kV, beam current 6.5 χ 10~ A. The arrows indicate the position of the steps. 2

10

c-AXIS ELECTROLYTE

f

Figure 13. Absorption length L« and depletion layer width W of an ideally smooth WSe electrode in contact with an electrolyte, assuming ej_ == 9 and n = 8 Χ 10 cm' 6

2

c

3

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PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE INTERFACES

to the layers is low. The charge transport in the perpendicular direction is related to randomly distributed stacking faults, which interconnect the layers, introducing an extrinsic conduction mechanism (28). The pres ence of stacking faults leaves the translational invariance parallel to the layers unaffected whereas it destroys translational invariance perpendicu lar to them. Thus, layered semiconductors may be viewed as one dimensional disordered systems with extrinsic conduction paths parallel to the c-axis. The pronounced anisotropy of the electrical conductivity in layered compounds (8,15) suggests that the charge carriers move, on their way to the surface, predominantly within the layers, i.e. parallel to the main surface as shown in Fig. 14 (in which the relative path of the charge car riers within the layers is compressed). The random character of interlayer charge transport due to extrinsic conduction leads to a variety of possible paths, two of which are represented in Fig. 14. The presence of a step introduces two significant changes. First, in contrast to a smooth surface which does not exhibit unsaturated bonds in the direction perpendicular to the surface, strong covalent bonds are exposed to the electrolyte at the edge of a step. These are likely to chemisorb species from the environment and introduce surface states within the band gap (29). These are recognized recombination sites and shunts in solar cells, known to reduce fill factors, current collection efficiencies and open circuit voltages (30, 31). Second, the exposure of a step to the conductive electrolyte results in a space charge region parallel to the layers. The depth of this region is given by the differences in the static dielectric constant parallel and per pendicular to the layers. For WSe , e,, = 16 and e = 9(27), hence in case of homogenous doping, the width of the depletion region parallel to the layered structure, W is increased approximately by a factor of compared with W = 8 6 0 Â . In case of MoSe , this anisotropy in W is somewhat larger, since e = 4.9 and e = 20(32), but also not particu larly substantial. We will consider two physically different situations: (a) the height d of the step is comparable with the extension of the depletion layer (d ~W) and (b) the step is much higher than the depletion width (d > > W). In the first case, charge carriers are generated in a region of the sample in which two electric fields exist: one parallel, the other perpen dicular to the layers. Minority carriers, created within the depletion region w will drift towards the edge of the step due to the acting electric field and the highly anisotropic conductivity. A step can thus be regarded as a collector of minority carriers which otherwise would have reached the surface that parallels the layers in the semiconductor. Such deflection of carriers is shown schematically in Fig. 15. Once the minor2

±

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±

2

±

M

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In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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31

Carrier Recombination ELECTROLYTE

* Λ

I

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4

2

Figure 14. Schematic of trajectories of minority carriers for an ideally smooth surface in contact with an electrolyte. The random character of the interlayer charge transport is also indicated (small arrows). The average minority carrier motion is given by the large arrows.

ELECTROLYTE DEPLETION REGION W,II

c-AXIS

Figure 15. Schematic of trajectories of minority carriers for a surface with a step exposed to the electrolyte (the step height d is assumed to equalize approximately the depletion layer width)

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PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE INTERFACES

c-AXIS •

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ELECTROLYTE

Figure 16. Schematic of the influence of steps on diffusion processes in case d > > W. The lined areas indicate the extension of the depletion layer parallel or perpendicular to the layered structure, W and W ± , respectively, and \± denotes the minority carrier diffusion length perpedicular to the layered structure (the horizontal radius of the ellipses is compressed somewhat). N

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33

ity carriers reach the edge of a step, most of them will undergo recombi nation due to the high density of surface states. In case d > > w, an electric field and a space charge layer parallel to the layer structure is present in a region, in which light absorption is negligible and no field perpendicular to the layers exists. However, the efficiency of solar cells is a function of the minority carrier diffusion length (33, 34). In WSe , evidence has been obtained, that this length /_,_ is in the order of 10" cm.(6, 25) The presence of a step associated with an electric field parallel to the layers leads to a deflection of diffusing minority carriers towards the edge where recombination occurs as schematically indicated in Fig. 16. The diffusion coefficient D is propor tional to the carrier mobility μ, which is highly anisotropic in layered semiconductors thus the diffusion profile is elliptic as shown in the figure. Hence, a step will also reduce the effective minority carrier diffusion length and lifetime, causing further deterioration in solar cell performance. In summary, it has been demonstrated that surface morphology is critically important in determining the performance of solar cells with layered compound semiconductors. Steps on structured surfaces of tran sition metal dichalcogenides have been identified as carrier recombina tion sites. The region defined by the depth of the space charge layer parallel to the van der Waals planes can be considered as essentially "dead" in the sense that its photoresponse is negligible. As the "step model" predicts, marked improvement in solar cell performance is found on samples with smooth surfaces. Two conclusions concerning future application of layered com pounds in solar energy converting devices can be derived from the present investigation: (i) the growth of large area smooth samples is highly desirable (ii) a method to reduce the disadvantageous effects of steps on sur faces of layered semiconductors is needed 2

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4

If neither of these goals can be realized, layered semiconductors may not become useful electrode material in either semiconductor liquid junction or Schottky junction devices. Fortunately, evidence is already being obtained that the negative effects due to steps can be at least tem porarily and partially alleviated (35, 36). Future development of chemi cal methods to inhibit deflection of minority carriers to the edges of steps and to reduce the high recombination rates at steps may open the way for the use of polycrystalline layered chalcogenide semiconductors in solar cell devices.

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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PHOTO-EFFECTS AT SEMICONDUCTOR-ELECTROLYTE INTERFACES

REFERENCES (1) Gerischer, H . Electroanal. Chem. and Interfac. Electrochem. (1975), 58, 263. (2) Tributsch, H . Z . Naturforsch. (1977), 32a, 972. (3) Tributsch, H . Ber. Buns. Ges. Phys.Chem. (1977), 81, 361. (4) Tributsch, H . Ber. Buns. Ges. Phys. Chem. (1978), 82, 169. (5) Tributsch, H . J. Electrochem. Soc. (1978), 125, 1086. (6) Kautek, W., Gerischer, H . , Tributsch, H . J. Electrochem. Soc. in press. (7) Frindt, R. F., J. Phys. Chem. Solids, (1963), 24, 1107. (8) Upadhyayula, L. C., Loferski, J. J., Wold., Α . , Giriat, W., Kershaw, R., J . Appl. Phys. (1968), 39, (10), 4736. (9) Levy, F., Schmid, Ph. Berger, H . Phil. Mag. (1976), 34, 1129. (10) Menezes, S., DiSalvo, F. J. Miller, B., J. Electrochem. Soc. (1978), 125, 2085. (11) Gobrecht, J., Tributsch, H . , Gerischer, H . , J. Electrochem. Soc. (1978), 125, 2085. (12) Gobrecht, J . , Gerischer, H . , Tributsch, H . Ber. Buns. Ges. Phys. Chem., (1978), 82, 1331. (13) Kershaw, R., Vlasse, M . , Wold, Α . , Inorganic Chem. (1967), 6, (8), 1599. (14) Monzack, J . , Richamn, M . H . , Metallography (1972), 5, 279. (15) Wilson, J. Α . , Yoffe, A . D . , Adv. Phys. (1969), 18, 193. (16) Mattheis, L . F., Phys. Rev. Β (1973), 8, (8), 3729. (17) White, R. M., Lucovsky, G . Sol. St. Comm., (1972), 11, 1369. (18) Title, R. S., Shafer, M . W., Phys. Rev. Lett. (1972), 28, (13), 808. (19) Schaefer, H . in "Chemical Treansport Reactions", Academic Press, (1964). (20) Lewerenz, H . J . , Heller, Α . , DiSalvo, F. J . , J. Am. Chem. Soc., (1980), 102, 1877. (21) Heller, Α . , Chang, K. C., Miller, B., J . Am. Chem. Soc. (1978), 100, 684. (22) Chang, K. C., Heller, Α . , Miller, B., J. Electrochem. Soc. (1977), 124, (5), 697. (23) Leamy, H . J . , Kimerling, L. C., Ferris, S. D., in "Scanning Electron Microscopy" (1976), Part IV, Vol. 1, ed. Johari, Ο., (IIT Res. Inst., Chicago Ill), p. 529. (24) Wu, C. J., Wittry, D. B. J . Appl. Phys. (1978), 49, 2827. (25) Lewerenz, H . J., Ferris, S. D., Leamy, H . J., Doherty, C . J., to be published. (26) Clemen, C., Saldana, X . I., Munz, P., Bucher, E . , Phys. Stat. Sol. (1978), A47, 437.

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(27) Zeppenfeld, Κ. Opt. Comm. (1970), 1, 377. (28) Fivaz, R., Schmid, Ph. E., in "Physics and Chemistry of Materi als with Layered Structures", Vol. 4, ed. P. A . Lee, D. Reidel, Holland (1976). (29) Ahmed, S. M . , Gerischer, H . , Electrochimica Acta, (1979), 24, 705. (30) Parkinson, Β. Α . , Heller, Α . , Miller, B., J. Appl. Phys. (1978), 33, 521. (31) Parkinson, Β. Α . , Heller, Α . , Miller, B., J. Electrochem. Soc., (1979), 126, 954. (32) Neville, R. Α . , Evans, B. L . Phys. Stat. Sol. (1976), B73, 597. (33) Gosh, A . K . , Maruska, H . P. J. Electrochem. Soc. (1977), 124, 1516. (34) Graff, K . , Fischer, H . Top. Appl. Phys. (1979), 31, 173. (35) Parkinson, Β. Α . , Furtak, T. E . , Canfield, D . , Kam, K . , Kline, G., to be published. (36) Lewerenz, H . J., Heller, Α . , Gallagher, P. K . , Menezes, S., Miller, B., to be published. Received October 23, 1980.

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

3 Rate of Reduction of Photogenerated, SurfaceConfined Ferricenium by Solution Reductants Derivatized n-Type Silicon Photoanode-Based Photoelectrochemical Cells N A T H A N S. LEWIS and MARK S. WRIGHTON

1

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Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139

N-type semiconductors can be used as photoanodes in electrochemical cells (1, 2, 3), but photoanodic decomposition of the photoelectrode often competes with the desired anodic process (1, 4, 5). When photoanodic decomposition of the electrode does compete, the u t i l i t y of the photoelectrochemical device is limited by the photoelectrode decomposition. In a number of instances redox additives, A, have proven to be photooxidized at n-type semiconductors with essentially 100% current efficiency (1, 2, 3, 6, 7, 8, 9). Research in this laboratory has shown that immobilization of A onto the photo anode surface may be an approach to stabilization of the photo anode when the desired chemistry i s photooxidation of a solution species B, where oxidation of Β is not able to directly compete with the anodic decomposition of the "naked" (non-derivatized) photoanode (10, 11, 12). Photoanodes derivatized with a redox reagent A can effect oxidation of solution species Β according to the sequence represented by equations (1) - (3) (10-15).

+

Thus, A is oxidized by the photogenerated h , and the photogenerated A+surf in turn oxidizes B. By such a mechanism, the photooxidation of Β is possible for wavelengths of light that w i l l create e - h pairs in the n-type semiconductor (≥ E ) and for processes where the chemistry represented by equation (3) is spontaneous in a thermodynamic sense. The (A /A) reagent system must also result in a suppression of the anodic decomposition, equation (4), in order to achieve surf

-

+

g

+

surf

1

Address correspondence to this author. 0097-6156/81/0146-0037$05.00/0 © 1981 American Chemical Society In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

38

PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE INTERFACES

a durable i n t e r f a c e f o r p h o t o o x i d a t i o n when the naked e l e c t r o d e undergoes photoanodic decomposition i n the presence of B . Large values of k are needed to achieve photoanode-based c e l l s having h i g h e f f i c i e n c y . Measurement of k can be made d i r e c t l y , s i n c e d e r i v a t i z e d photoanodes are two s t i m u l i response systems (10-16). O x i d a t i o n of A ^ r e q u i r e s 2l g l i g h t and some e l e c t r o d e p o t e n t i a l , E f . îîowever, r e d u c t i o n of ^surf. ^ y P semiconductor only r e q u i r e s a s u f f i c i e n t l y negative p o t e n t i a l . This two s t i m u l i response ( l i g h t and p o t e n t i a l ) allows e v a l u a t i o n of the r a t e constant k by measuring the time dependence of the A * f c o n c e n t r a t i o n i n the dark i n the presence o f Β (16) . The A+ ^. c o n c e n t r a t i o n can be monitored by e i t h e r a negative sweep or step"from the p o s i t i v e p o t e n t i a l needed i n the photogeneration of A ^ ^ f , equation (5) , and e t

e t

E

o

n

β

n _ t

e

e t

u r

Downloaded by 148.204.30.87 on October 21, 2009 | http://pubs.acs.org Publication Date: March 2, 1981 | doi: 10.1021/bk-1981-0146.ch003

U

e"

+ A

f

+

. — A . (5) surf. surf. i n t e g r a t i n g the charge passed i n the r e d u c t i o n o f A g . Separation o f the charge passed corresponding t o r e d u c t i o n o f A g f from that a s s o c i a t e d w i t h r e d u c t i o n of B+ can be accomplished by moving B away from the photoanode by s t i r r i n g . The key f a c t i s t h a t even though the p o s i t i v e l i m i t may be more p o s i t i v e than E°(A+ /A) . , regeneration o f A g ~ f after r e a c t i o n according to equation ( 3 ) , w i l l not occur i n the dark. Thus, the consumption o f Ag" f by Β i s d e t e c t a b l e by the d e c l i n e i n Ag" f c o n c e n t r a t i o n (16) measured by cathodic current a s s o c i a t e d w i t h i t s r e d u c t i o n a f t e r a r e a c t i o n time t-j_ a t s p e c i f i e d c o n d i t i o n s . D i r e c t monitoring of A g " f c o n c e n t r a t i o n i n t h i s sense i s not p o s s i b l e on a r e v e r s i b l e e l e c t r o d e , such as Pt o r Au, s i n c e the ( A / A ) f r a t i o depends only on Ef, I f ^ s u r f . does e f f e c t chemistry according t o equation ( 3 ) , the A|urf. i s regenerated t o an extent that depends only on Ef (16). However, i n d i r e c t procedures f o r e v a l u a t i n g k do e x i s t f o r r e v e r s i b l e e l e c t r o d e s , p a r t i c u l a r l y when the o x i d a t i o n o f Β a t the naked e l e c t r o d e occurs a t a n e g l i g i b l e r a t e compared to the r a t e a t an e l e c t r o d e d e r i v a t i z e d w i t h the ( A / A ) f system (17). I n photoelectrochemical c e l l s h i g h e f f i c i e n c y depends on having a h i g h quantum y i e l d f o r e l e c t r o n flow, Φ , a t a l l l i g h t i n t e n s i t i e s t o be used. I f k i s too small, Φ may be l e s s than u n i t y because back e l e c t r o n t r a n s f e r , equation ( 5 ) , can compete when E f i s s u f f i c i e n t l y negative. Negative values o f E f are d e s i r a b l e , s i n c e the extent to which A f -»• A+ p can be d r i v e n u p h i l l w i t h l i g h t , and here Β -> B , depends on E {!) . F u r t h e r , if k i s too s m a l l , d i r e c t e" - h recombination, equation ( 6 ) , + 6 e - h y heat and/or l i g h t (6) may occur when the A f Ag f conversion i s complete. When k i s l a r g e , back e l e c t r o n t r a n s f e r and recombination can s t i l l be competitive i f the c o n c e n t r a t i o n o f Β i s too low. I f B i s present back r e a c t i o n , equation ( 7 ) , can c o n t r i b u t e t o u r f

u r

β

+

Q11T

ur

f

β

ur

ur

β

ur

e

+

s u r

β

e t

+

s u r

e t

e

θ

s u r

#

ur;

+

f

+

e t

k

g u r

u r

e t

+

+

A

, + Β surf.

k

7

+

> A * + Β surf, A

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

(7)

3.

LEWIS AND WRIGHTON

39

Reduction of Ferricenium

a low v a l u e of Φ . L i g h t i n t e n s i t y i s a c o n s i d e r a t i o n f o r two reasons. F i r s t , the r a t e o f e" - h recombination i s a " b i m o l e c u l a r " process whereas the other e" o r h processes are "unimolecular"; Φ might be lower at h i g h e r l i g h t i n t e n s i t y . Second, when Β i s being consumed according to equation (3) i t can only be r e p l e n i s h e d at the i n t e r f a c e at a mass t r a n s p o r t c o n t r o l l e d r a t e ; the e x c i t a t i o n r a t e can exceed the mass t r a n s p o r t r a t e r e s u l t i n g i n a low s t e a d y - s t a t e value of Φ . In t h i s a r t i c l e we wish to a m p l i f y on our previous s t u d i e s (10-16) of d e r i v a t i z e d photoanode s u r f a c e s by r e p o r t i n g new r e s u l t s r e l a t e d to the measurement of k f o r η-type S i photoanodes d e r i v a t i z e d w i t h ( l , l - f e r r o c e n e d i y l ) d i c h l o r o s i l a n e , I . We r e p o r t that a number of s o l u t i o n reductants Β can be o x i d i z e d β

+

+

β

e t

Downloaded by 148.204.30.87 on October 21, 2009 | http://pubs.acs.org Publication Date: March 2, 1981 | doi: 10.1021/bk-1981-0146.ch003

f

Fe

SiCl

2

I by the photogenerated, s u r f a c e - c o n f i n e d f e r r i c e n i u m w i t h a value of k that exceeds 6 χ 10& cm^ mol"-*- s ~ l a t 298 K. Larger values of k cannot be measured owing to d i f f i c u l t i e s a s s o c i a t e d w i t h mass t r a n s p o r t c o n t r o l l e d r a t e s . This would correspond t o a homogeneous b i m o l e c u l a r r a t e constant o f >6 χ 10^ M""-*- s ~ l . In p r a c t i c a l terms t h i s means t h a t k i s l a r g e enough to y i e l d a good value of Φ at s o l a r i r r a d i a t i o n i n t e n s i t i e s and a t g e n e r a l l y a c c e s s i b l e concentrations of B. However, the extent to which the o x i d a t i o n of Β can be d r i v e n u p h i l l , Ey, i s g e n e r a l l y modest (0.4 - 0.5 V a t o p e n - c i r c u i t ) compared t o Eg = 1.1 eV f o r S i . Small values of Ey g i v e low o v e r a l l o p t i c a l energy conversion e f f i c i e n c y . e t

e t

e t

θ

Background and Working Hypothesis N-type S i d e r i v a t i z e d w i t h I i s b e l i e v e d to have the i n t e r f a c e s t r u c t u r e and e n e r g e t i c s represented by Scheme I (11). Taking E ° ( F e C p / 0 ) to be +0.43 V v s . SCE i n EtOH from measurements f o r Au or Pt e l e c t r o d e s d e r i v a t i z e d w i t h I , (18, 19) Ey f o r the ( F e C p ° ) . + ( F e C p ) s u r f . i d a t i o n can be up to -0.60 V (10). That i s , ' t h e value o f E of the photoanode where the ( ^ C P 2 ^ ) f r a t i o i s one i s ~0.60 V more negative than f o r Au or Pt i n the best cases. More t y p i c a l l y , the value of Ey i s 0.3-0,4 V as shown i n F i g u r e 1 f o r e l e c t r o d e s (Pt vs. i l l u m i n a t e d η-Si) c h a r a c t e r i z e d by c y c l i c voltammetry. +

2

surf#

+

2

surf

2

o x

f

e

+

s u r

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE

INTERFACES

(-) C o n d u c t i o n Band Ε

=

-0.3V

CB

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E°(FeCp^

/ 0

)

s u r f !

=

+0.43V

t (+) Scheme I

I n t e r f a c e e n e r g e t i c s f o r an η-type S i photoanode a t the f l a t - b a n d c o n d i t i o n showing the formal p o t e n t i a l f o r a s u r f a c e - c o n f i n e d f e r r i c e n i u m / f e r r o c e n e reagent r e l a t i v e to the p o s i t i o n of the top of the valence band,E^^,and the bottom o f t h e conduction band,E_ , at the i n t e r f a c e between the S i s u b s t r a t e and the r e d o x / e l e c t r o l y t e system. I n t e r f a c e e n e r g e t i c s apply to an EtOH/0.1 M [n-Bu^N^ClO^ e l e c t r o l y t e system. B

0.0

+0.4

P O T E N T I A L , V vs

SCE

Figure 1. Typical comparison of cyclic voltammetry (100 mV/s) for Pt vs. n-type Si in CH CN/0.1M [n-Bu^ClOj, derivatized with (1 J -ferrocenediyl)dichlorosilane. f

3

The Pt exhibits a reversible wave in the dark whereas the η-type Si exhibits no oxidation current unless illuminated with > E light (632.8 nm, ~ 50 mW I cm ). The photoanodic peak is more negative than the anodic peak on Pt, reflecting the extent to which ferrocene -> ferricenium can be driven uphill (380 mV in this case). For η-Si, ( ) represents the reduction when the light is not switched off at the +0.6 V positive limit; ( ) on the cathodic sweep corresponds to the dark reduction. 2

g

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

3.

LEWIS AND WRIGHTON

41

Reduction of Ferricenium

The main p o i n t i s that a t a photoanode p o t e n t i a l o f Ï~0,4 V v s . SCE the ( F e C p 2 / ° ) f . r a t i o i s t y p i c a l l y >10 and the a v a i l a b l e oxidant i s (FeCp2 )surf.· Thus, any m a t e r i a l Β t h a t i s o x i d i z a b l e w i t h (FeCp2: ) s o l u t i o n should be o x i d i z a b l e w i t h the η-type S i photoanode d e r i v a t i z e d w i t h I (10-16). N-type S i d e r i v a t i z e d w i t h I can be used i n ^ O / e l e c t r o l y t e s o l u t i o n , u n l i k e the naked η-type S i that i s r a p i d l y p a s s i v a t e d by photoanodic growth of an oxide l a y e r on the surface (10, 11, 12, 16). As a guide t o understanding the heterogeneous o x i d a t i o n o f Β by ( F e C p 2 ) u r f . > adopt the theory of Marcus (20, 21). However, we underscore the f a c t that redox r e a c t i o n o f the s u r f a c e - c o n f i n e d redox system, l i k e the s o l u t i o n system, i s accompanied by s o l v a t i o n changes (16). Heterogeneous e l e c t r o n t r a n s f e r a t a naked e l e c t r o d e need not i n v o l v e an e l e c t r o d e s o l v a t i o n term. Q u a l i t a t i v e l y , Marcus p r e d i c t s that k w i l l be l a r g e when the (B+/B) and FeCp2 /0) self-exchange are f a s t and when the d r i v i n g f o r c e i s l a r g e . +

sur

+

i

+

w

n

e

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S

e t

+

Results Solvent Dependence of I O x i d a t i o n In an e a r l i e r study, (16), we found the r a t e law represented by equation (8) t o be a p p r o p r i a t e f o r B=I~ i n EtOH o r H2O s o l v e n t . Rate

=

k

e t

t(

F e C

2

P

+

B

>

2 1

U n i t s A n a l y s i s : Rate, molcm" s~ ; k r

[(FeCp

+ 2

)

s u r f e

L

molcm

( 8 )

rf.^ ^

S U

—2

1

, cm^ol^s" ;

^

; [ B ] , molcni

We assume the same r a t e law t o g e n e r a l l y apply when Β i s a one-electron reductant. The a b i l i t y to prove the r a t e law f o r B=I"~ stems from a r a t h e r low value o f k , Table I . I n t e r e s t i n the photochemical o x i d a t i o n o f I " t o I ^ ~ f o r energy storage purposes and the ~ 1 0 - f o l d d i f f e r e n c e i n k i n EtOH v s . H2O prompted us t o determine k f o r B=I"~ f o r s e v e r a l other s o l v e n t s . Values o f k , E ^ ( I ~ ) , and E ° ( F e C p 2 / ° ) f . i n the v a r i o u s s o l v e n t s used are given i n Table I . The E ^ ( I " " ) values are data from the l i t e r a t u r e (22) and our own measurements, and the E ° ( F e C p 2 ^ ) r f . from the c y c l i c voltammetry peak p o s i t i o n s f o r P t e l e c t r o d e s f u n c t i o n a l i z e d w i t h I i n the v a r i o u s s o l v e n t s . Together, these data provide i n f o r m a t i o n concerning the d r i v i n g f o r c e f o r the o x i d a t i o n o f I by (FeCp2 )surf. various solvents. Values o f k were determined as p r e v i o u s l y reported f o r H 0 o r EtOH s o l v e n t (16). The d e r i v a t i z e d e l e c t r o d e i s f i r s t c h a r a c t e r i z e d by c y c l i c voltammetry i n s o l v e n t / e l e c t r o l y t e s o l u t i o n without added I " . The value of k i s then determined from the time dependence of the s u r f a c e - c o n c e n t r a t i o n o f (FeCp2 )surf. i n the presence o f v a r i a b l e I " c o n c e n t r a t i o n and as a f u n c t i o n of s o l v e n t . The ( F e C p 2 ) u r f . generated i n a l i n e a r p o t e n t i a l sweep from -0.6 to +0.5 V vs. SCE w h i l e the e t

e t

e t

+

e t

o x d n

sur

O X ( i r i e

+

a

r

e

s u

+

i

n

t

n

e

e t

2

e t

+

+

i

s

S

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE INTERFACES

42

Table I . Reduction of Surface-Confined Ferricenium by Iodide i n Various Solvents a t 298 K.

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P o t e n t i a l , V vs. SCE Solvent/Electrolyte EtOH/[n-Bu N]ci0

E°(FeCp

+/0,a ) 1

2

, -,b E^(I ) w

T

k

e t >

3^-1-1° cm mol s 4

0.43

0.42

3 χ 10 '

Η 0 (pH=1.3)/NaC10

0.35

0.30

Glacial Acetic Acid/[n-Bu N]C10

0,42

0.42

1 χ 10 4 3 χ 10

EtOH/Toltiene (1/1)/-

0.44

0.44

.4 3 χ 10

0.48

0.40

0.50

0,32

4

4

4

4

[n-Bu N]C10 4

4

CH Cl /£n-Bu N]C10 2

2

3

4

4

CH.CN/In-Bu.NJC10/

6 m 1Q 8 >6 χ 10 4

e

Formal p o t e n t i a l f o r surface-confined (FeCp ) as determined by slow sweep c y c l i c voltammetry f o r P t e l e c t r o d e s d e r i v a t i z e d w i t h ( 1,1 -ferrocenediyl)dichlorosilane. T

^Data are quarter wave p o t e n t i a l s f o r I*~ o x i d a t i o n reported i n r e f . 22 and measured a t P t , see Experimental. °Heterogeneous e l e c t r o n t r a n s f e r r a t e constant, see equation (8) i n t e x t , f o r I o x i d a t i o n determined as i n r e f . 16. ^Data from r e f , 16. e

+

Assuming [ ( F e C p ) 2

s

u

r

f ] = 10

mol/cm^.

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

3.

43

Reduction of Ferricenium

LEWIS AND WRIGHTON

e l e c t r o d e i s i l l u m i n a t e d a t a s u f f i c i e n t l y high 632.8 nm l i g h t i n t e n s i t y (-50 mW/cm ) t o i n s u r e that the ( F e C p 2 / 0 ) f ratio at +0.5 V vs. SCE i s >10/1. The l i g h t i s then switched off at the p o s i t i v e l i m i t . Concentration of (FeCp2 )surf. * » t±> i n the dark i s then determined by h o l d i n g the p o t e n t i a l at +0.5 V vs. SCE f o r a time, t and then sweeping the p o t e n t i a l a t >300 mV/s t o -0.6 V vs. SCE w h i l e monitoring cathodic current corresponding t o (FeCp2 )surf. r e d u c t i o n . Another way t o vary r e a c t i o n time, t-^, i s t o simply use no delay a t the p o s i t i v e l i m i t and to vary the sweep r a t e on the negative scan. This method has been used as our r o u t i n e method of determining k (16). E q u i v a l e n t data were a l s o obtained by measuring [ ( F e C p 2 ) f . ] by doing a p o t e n t i a l step from +0.5 V t o -0.6 V v s . SCE and i n t e g r a t i n g c u r r e n t . S o l u t i o n s are s t i r r e d t o remove o x i d i z e d product, Ιβ", from the v i c i n i t y of the e l e c t r o d e . +

s u r

+

i

v s

t i m e

5

+

e t

+

Downloaded by 148.204.30.87 on October 21, 2009 | http://pubs.acs.org Publication Date: March 2, 1981 | doi: 10.1021/bk-1981-0146.ch003

s u r

P l o t s of l n { [ F e C p 2 ] o / t P 2 ] t = t * ± with a slope = k b l [ l ~ ] . C o n s i s t e n t l y , the slopes are d i r e c t l y proportional to Since k = k [(FeCp f . L the value o f k i s given by d i v i d i n g k ^ ^ by s u r f a c e coverage. By h o l d i n g the e l e c t r o d e at +0.5 V vs. SCE and i r r a d i a t i n g w i t h a s u f f i c i e n t l y high l i g h t i n t e n s i t y t o i n s u r e that e x c i t a t i o n r a t e i s not r a t e l i m i t i n g , the s t e a d y - s t a t e photocurrent should be p r e d i c t a b l e u s i n g equation ( 8 ) . Indeed, using the surface coverage, t y p i c a l l y ~10~^ mol/cm^, and the k s given i n Table I c a l c u l a t e d and observed s t e a d y - s t a t e photocurrents are i n good agreement. For CH3CN solvent we can only p l a c e a lower l i m i t on the value of k . Using a d e r i v a t i z e d r o t a t i n g d i s k photoelectrode we f i n d that the s t e a d y - s t a t e photocurrent at +0.5 V vs. SCE i s d i r e c t l y p r o p o r t i o n a l t o ω^, the square-root of the r o t a t i o n v e l o c i t y . Such a dependence i s expected when the l i m i t i n g current i s c o n t r o l l e d by mass t r a n s p o r t (23, 24, 25) and not by the electron transfer rate ( i . e . k i s very l a r g e ) . For CH3CN s o l v e n t , the s t e a d y - s t a t e photocurrent i s independent of coverage of [ ( F e C p ) s u r f . ] 5 expected f o r a mass t r a n s p o r t c o n t r o l l e d r a t e . But f o r every other solvent system i n v e s t i g a t e d we f i n d values of k that are s m a l l compared to what would be expected f o r mass t r a n s p o r t l i m i t e d c u r r e n t s ; steady-state c u r r e n t s do not depend on ω or whether the s o l u t i o n i s s t i r r e d when the current i s not mass t r a n s p o r t l i m i t e d . U n f o r t u n a t e l y , the l a r g e value of k associated with I o x i d a t i o n i n CH3CN i s accompanied by an unstable i n t e r f a c e . For reasons that we do not p r e s e n t l y understand, the η-type S i e l e c t r o d e s d e r i v a t i z e d w i t h I are not durable enough i n CH^CN s o l u t i o n t o s u s t a i n I ~ o x i d a t i o n f o r prolonged periods o f time. However, the e l e c t r o d e s do s u r v i v e long enough t o e s t a b l i s h that the r a t e of I"" o x i d a t i o n i s l i m i t e d by mass t r a n s p o r t . At the highest ω from our 200p r.p.m. motor our s t r i c t l y l i n e a r p l o t s of l i m i t i n g current vs. ω e s t a b l i s h that the heterogeneous r a t e constant, k [(FeCp2 )surf.]> _>0.06 cm/sec (16). Thus, i f [(FeCp2 ) s u r f . ] ^ ^Vcm2, which approximates a 'monolayef' of reagent exposed to the s o l u t i o n , k i s ^ 6 χ 10 cm^mol~~^s~l. F e C

+

+

v s

t

a

r

e

l

i

n

e

a

r

t =

0

s v c

+

o b s v d

t

e t

0

2

s

u

r

s v

T

e t

e t

e t

+

a

s

2

e t

e t

2

+

i

s

e t

+

s

e t

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

44

PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE INTERFACES

Reduction of S u r f a c e - F e r r i c e n i u m by One-Electron Reductants I n our e a r l i e r work, we showed that a number o f reagents could be o x i d i z e d according to equations (1) - (3) where k is so l a r g e that the r a t e i s l i m i t e d by mass t r a n s p o r t (16), Thus, as f o r Β = IT i n CH CN,k i s >6 χ 1 0 cm^mol^is-l f o r an e f f e c t i v e s u r f a c e coverage of - 1 0 " mol/cm . The reagents p r e v i o u s l y s t u d i e d (16), Β = F e ( n - C H c ) , F e C n ^ ^ H ^ M e ) , F e ( n - C H ) ( n 5 - C H P h ) , Fe(r| -indenyl) , and [Fe(CO) ( n - C H ) ] , are a l l one-electron reductants that have f a s t self-exchange r a t e s and should, t h e r e f o r e , reduce ( F e C p 2 ) f . r a p i d l y . Table I I l i s t s some of our e a r l i e r data along w i t h i n f o r m a t i o n for s e v e r a l other systems t h a t we have now determined to have l a r g e values of k . F i g u r e 2 shows the s o r t of d i r e c t evidence that shows t h a t ( F e C p 2 ) f . can o x i d i z e s o l u t i o n reductants. The f i g u r e shows c y c l i c voltammograms f o r the d e r i v a t i z e d e l e c t r o d e i n the s t i r r e d EtOH/0.1 M n-Bu^N CIO4 e l e c t r o l y t e s o l u t i o n f i r s t i n the absence of Β and then i n the presence of Β = Co(bipy)3^ at 3.0 mM, The photocurrent a t tjie p o s i t i v e l i m i t of +0.3 V vs. SCE i s d i r e c t l y p r o p o r t i o n a l to i n the presence of 3.0 mM Co(bipy)3 . The c a t h o d i c current a s s o c i a t e d w i t h ( F e C p 2 ) f . ( P2°) urf. on the negative sweep i n the dark from +0.3 V v s . SCE i s completely absent i n the presence of 3.0 mM C o ( b i p y ) < ^ a t the scan r a t e used. The l a c k of a r e t u r n wave f o r the ( F e C p 2 ) f "** (FeCp °) s u r f , r e d u c t i o n d i r e c t l y evidences complete r e d u c t i o n of the (FeCp2^) urf. by C o ( b i p y ) 3 ^ . L i n e a r p l o t s of l i m i t i n g current vs. establish k to be >6 χ 10 cm m o l ' ^ s ' l . Representative data f o r e q u i l i b r a t i o n of (FeC]>2 )surf. s o l u t i o n ferrocene are shown i n F i g u r e 3 where the l i m i t i n g c u r r e n t v a r i e s l i n e a r l y w i t h s o l u t i o n ferrocene c o n c e n t r a t i o n a t a f i x e d ω and v a r i e s l i n e a r l y w i t h ur2 f o r a f i x e d s o l u t i o n ferrocene c o n c e n t r a t i o n . Such data have been obtained f o r a l l of the couples l i s t e d i n Table I I and f o r I " i n CH3CN. The measurement of the mass t r a n s p o r t r a t e constant by monitoring E ( E e C p 2 ) f . ] vs. t ^ i n the presence of the v a r i o u s f a s t reductants g e n e r a l l y gives a value that does not accord w e l l w i t h the s t e a d y - s t a t e photocurrents. This s i t u a t i o n r e s u l t s even though the s t e a d y - s t a t e photocurrent d e n s i t y f o r r o t a t i n g , d e r i v a t i z e d d i s k e l e c t r o d e s and the current d e n s i t y at a r o t a t i n g r e v e r s i b l e d i s k e l e c t r o d e (e.g. Pt) are n e a r l y the same f o r a l l reagents when c o r r e c t e d f o r minor d i f f e r e n c e s i n d i f f u s i o n constants. A r e p r e s e n t a t i v e s i t u a t i o n i s shown i n F i g u r e 4 where c y c l i c voltammetry of an η-type S i photoanode, d e r i v a t i z e d w i t h I , i s shown f o r s e v e r a l s i t u a t i o n s . Included are data f o r Β = ferrocene and 1 , l - d i m e t h y l f e r r o c e n e under i d e n t i c a l c o n d i t i o n s . These two s o l u t i o n reductants r e s u l t i n i d e n t i c a l s t e a d y - s t a t e photocurrents at +0.5 V v s . SCE, but as shown i n F i g u r e 4b vs. 4c the 0.5 mM 1,1'-dimethyIferrocene consumes more of the C(FeCp2 ) s u r f ] than does the 0.5 mM ferrocene under i d e n t i c a l c o n d i t i o n s . e t

s

3

et

1 0

2

5

2

5

5

2

5

5

5

5

5

4

2

5

5

+

s u r

et

Downloaded by 148.204.30.87 on October 21, 2009 | http://pubs.acs.org Publication Date: March 2, 1981 | doi: 10.1021/bk-1981-0146.ch003

+

s u r

+

+

FeC

s u r

s

+

+

s u r

2

+

t n e

S

e t

+

w

i

+

s u r

T

+

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

t

n

4

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

6

6

J

5

-C H )

2

5

5

5

4

2

5

5

4

+0.34

9

CH C1

2

CH C1

EtOH

EtOH

9

2

sulfolane

2

k

1

X

>6 >6

X

X

X

X

X

X

>6

>6

>6

>6

>6

>6

1

;

;

j

1

ίο

1&

io

{

io

ίο

1

ίο

io

io

10 X

>6 >6

10

X X

>6

, cn^mol^s" *

e t

See equation (8) o f t e x t . A l l k * s here a r e lower l i m i t s , s i n c e observed r a t e i s mass t r a n s p o r t l i m i t e d (e.g. F i g u r e 3 ) , see t e x t .

5

[Fe(CO)(n -C H )]

5

+0.17

5

+0.36

Fe(ri^-indenyl)

2

+0.22

CH C1

+0.46 2

EtOH

EtOH

2

2

H0

H0

EtOH

Solvent

+0.45

I r r e v . Oxdn @+0.2 @ P t

-0.20

+0.20

+0.37

+0.46

4

+

E°(B /B) , V v s . SCE

Fe(n -c H )(n -c H Ph)

5

Fe(n -C H Me)

Fe(n

2

5

2+

2+

(Me dtc)"

3

Ru(NH )

Fe(CN)

3

4

Co(bpy)

Reductant, Β

Table I I . Rate Constants f o r Reduction of S u r f a c e - F e r r i c e n i u m by V a r i o u s Reductants a t 298 K.

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PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE INTERFACES

Figure 2.

Cyclic voltammetric (100 mV/s) characterization of an η-type Si elec trode. 9

2

(a) Derivatized with (1 ,V-ferrocenediyl)dichlorosilane (5 X 10~ molI cm ) in stirred EtOH/O.lM [n-Bu^ClO^. ( ) the dark current; ( ) the effect of illumination (632.8 nm, ~ 50 mW I cm ) from -0.5 V to +0.3 V. The light is switched off at +0.3 V showing that photogenerated ferricenium can be reduced in the dark on the negative sweep, (b) Same as in (a) except 3.0 m M Co(bipy) Cl is in the stirred electrolyte solu tion. Note enhanced photoanodic current indicating Co(bipy) -> Co(bipy) and the lack of a return wave for ferricenium -» ferrocene showing that Co(bipy) + formation occurs via surface ferricenium + Co(bipy) -» surface ferrocene + Co(bipy) *. 2

3

2

2+

3+

3

3

3

3

2+

3

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

3

3

LEWIS AND WRIGHTON

Reduction of Ferricenium

47

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3.

ν

Figure 3. Photocurrent vs. ω * at +0.5 V vs. SCE from an η-type Si disk deriva tized with 5 X 10~ mol/cm of ferricenium/ferrocene from (1 ,r~ferrocenediyl)dichlorosilane. 9

2

The solution is EtOH/O.lM [n-Bu, N]ClO^/ferrocene and illumination is at 632.8 nm, ~50 W/cm . The strict linearity of the plots shows that oxidation of solution ferrocene is mass transport-limited for all ω used and for each ferrocene concentration used. t

2

American Chemical Society Library 1155 16th St., N.W. Washington, O.C. 20036 In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

PHOTOEFFECTS

A T SEMICONDUCTOR-ELECTROLYTE INTERFACES

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48

Figure 4. Cyclic voltammetry (100 mV/s) in stirred EtOH/O.lM [n-Bu,,N]ClO solutions for η-type Si derivatized with (1J -ferrocenediyl)dichlorosilane (5 X 10 mol/cm ) with illumination at 632.8 nm, ~ 50 mW/cm from negative initial poten tial to the positive limit at -\-0.5 V. h

f

2

9

2

Light is switched off at +0.5 V vs. SCE for the cathodic sweep. In (a) there is no added reductant; (b), (c), and (d) contain 0.5mM ferrocene, 1 ,Γ-dimethylferrocene, and acetylferrocene, respectively. Acetylferrocene does not attenuate the surface ferricenium -> surface ferrocene wave since it is not a sufficiently powerful reductant. Ferrocene and 1 ,Γ-dimethylferrocene both attenuate the surface ferricenium -» surface ferrocene wave. But 1 ,Γ-dimethylferrocene is more effective under identical conditions despite the fact that the same, mass transport-limited, steady-state photocurrent is found for these two reductants. These data suggest that after the light is switched off the reduction of surface ferricenium is controlled partially by mass transport and partly by the electron transfer rate (see text).

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

LEWIS AND WRIGHTON

3.

49

Reduction of Ferricenium

T

Since the 1,1 -dimethylferrocene consumes more of the (FeCp ) f we conclude that i t i s a f a s t e r reductant than ferrocene. C o n s i s t e n t l y , the r a t i o of cathodic peak areas f o r the reductants examined i s s t r i c t l y maintained f o r a v a r i e t y of scan r a t e s , pulse times t c o n c e n t r a t i o n s of reductant, and s t i r r i n g r a t e s . Using a s i m i l a r technique we order the r a t e s f o r s e v e r a l other reagents: +

2

s u r

β

i

?

F e ( - i n d e n y l ) > [ Fe(CO) (η - C H ) ] ~ Fe(n -C H Me) > 5

5

n

2

5

5

Fe(r|~*-C,_Hj-) £ ~ phenylferrocene

5

4

5

4

2

>> a c e t y l f errocene

The a c e t y l f e r r o c e n e does not consume the (FeCp2 ) urf.» F i g u r e 4d, because the r e a c t i o n i s not ther mo dynamically spontaneous. The c o n f l i c t i n our data i s that s t e a d y - s t a t e photocurrents f o r the f a s t reductants i s the same, but the ( F e C p 2 ) f . ^ consumed at d i f f e r e n t r a t e s i n the dark f o r the v a r i o u s f a s t reductants. The data demand the c o n c l u s i o n t h a t r e d u c t i o n of F e ( C p 2 ) f . can become l i m i t e d p a r t i a l l y by mass t r a n s p o r t and p a r t i a l l y by k a t some p o i n t i n the r e a c t i o n ( l a r g e f r a c t i o n a l consumption of ( F e C p 2 ) f . ) dark. This seems reasonable i n view of the expected r a t e law, equation ( 8 ) , the d e c l i n i n g ; ( F e C p ) s u r f . ] * a constant mass t r a n s p o r t r a t e of f r e s h s o l u t i o n reductant. +

Downloaded by 148.204.30.87 on October 21, 2009 | http://pubs.acs.org Publication Date: March 2, 1981 | doi: 10.1021/bk-1981-0146.ch003

S

+

c

a

n

e

s u r

+

s u r

e t

+

i

n

t

n

e

s u r

+

a n c

2

Discussion Data given i n Table I f o r mediated I ~ o x i d a t i o n r a t e , k , a n d f o r e n e r g e t i c s o f the process E°(FeCp2 /°) urf. - E ^ " ) seem to suggest that both s o l v e n t and d r i v i n g f o r c e f o r r e a c t i o n can i n f l u e n c e k . I n CH^CN where the d r i v i n g f o r c e i s greatest, we f i n d l a r g e values f o r k . However, i t does not seem that 0.18 V (CH CN) v s . 0.08 V (CH C1 ) would account f o r a f a c t o r o f 1 0 i n r a t e , and we conclude t h a t medium e f f e c t s can c o n t r i b u t e t o r a t e v a r i a t i o n as w e l l . I n support o f t h i s c o n c l u s i o n we note that H 0 y i e l d s a r a t e constant about a f a c t o r o f 10 lower than the other s o l v e n t s (EtOH, g l a c i a l a c e t i c a c i d , EtOH/toluene) where the d r i v i n g f o r c e i s even s l i g h t l y s m a l l e r . A c o m p l i c a t i o n i s that self-exchange r a t e s are not known f o r a l l of the media used. This precludes a d e t a i l e d i n t e r p r e t a t i o n o f the data w i t h i n the framework o f Marcus theory (20, 21). With respect t o s o l a r energy conversion and p h o t o e l e c t r o chemical s y n t h e s i s i n v o l v i n g I -> Iβ" o x i d a t i o n , i t i s noteworthy that s i g n i f i c a n t v a r i a t i o n i n k can be brought about by v a r i a t i o n o f the e l e c t r o l y t e s o l u t i o n . The data suggest that CH3CN c o u l d be a good s o l v e n t i n terms o f the measured v a l u e o f k , but the d u r a b i l i t y of the i n t e r f a c e i n CH3CN i s too poor. Perhaps a mixture o f CH3CN/H2O would prove workable. Two p o i n t s o f i n t e r e s t r e g a r d i n g d e r i v a t i z e d e l e c t r o d e s emerge from the data f o r mediated I " o x i d a t i o n . F i r s t , i t i s noteworthy that f e r r i c e n i u m i n H2O w i l l not o x i d i z e I ; the e t

+

v s

1

S

e t

e t

4

3

2

2

2

et

-

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

50

PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE INTERFACES

thermodynamics are such that the r e a c t i o n i s not spontaneous (26). However, we do f i n d that (FeCp2 )surf. f d e r i v a t i z a t i o n w i t h I does spontaneously o x i d i z e , a l b e i t s l o w l y , I " i n H2O. Apparently, the s u r f a c e - c o n f i n e d system has a s u f f i c i e n t l y more p o s i t i v e p o t e n t i a l that I"" can be o x i d i z e d . Since monosubstituted ferrocenes do not have a p o t e n t i a l i d e n t i c a l to that f o r ferrocene i t s e l f , the s u r f a c e reagent may be enough d i f f e r e n t that i t s o x i d i z i n g power can be greater. I t may a l s o be t h a t , d e s p i t e the general s i m i l a r i t y i n E° s f o r surface-bound and s o l u t i o n species (27), the d i f f e r e n c e i n E° brought about by b i n d i n g the reagent to the s u r f a c e i s s u f f i c i e n t to change the d i r e c t i o n of spontaneity f o r a given redox process. Second, the d i r e c t p r o p o r t i o n a l i t y of steady-state photocurrent to £ ( F e C p 2 ) f ] f o r k < (mass t r a n s p o r t l i m i t e d ) i n d i c a t e s that a l l surface-bound m a t e r i a l i s I " a c c e s s i b l e . This i s c o n s i s t e n t w i t h the a b i l i t y of s m a l l anions to penetrate throughout the redox polymer. Large anions have been e l e c t r o s t a t i c a l l y bound to p o l y c a t i o n i c m a t e r i a l confined to an electrode s u r f a c e (28), and such bound anions s l o w l y exchange w i t h smaller counterions. We do not f i n d evidence f o r slow I " or I^"" i o n d i f f u s i o n i n and out of polymer on e l e c t r o d e s d e r i v a t i z e d w i t h J . Larger c o u n t e r i o n s , however, do e f f e c t the e l e c t r o c h e m i c a l behavior as p r e v i o u s l y noted (19). As shown i n Table I I , there are a number of reagents that w i l l r a p i d l y reduce ( F e C p 2 ) f . · The value of k >_ 6 χ 1 0 cm^mol" s""l comes from the o b s e r v a t i o n f o r I ~ i n CH^CN and a l l reductants i n Table I I that photocurrent d e n s i t y i s d i r e c t l y p r o p o r t i o n a l to for r o t a t i n g disk electrodes. The h i g h e s t ω allows the c o n c l u s i o n that l ^ v g ^ >^ 0.06 cm/s. D i v i d i n g by a coverage of ~10~10 mol/cm gives k > 6 χ 10 cnr m o l " s ~ . We are not c e r t a i n that 1 0 ~ mol/cm i s the proper coverage to use; we take t h i s v a l u e because the data i n f a c t show l i m i t i n g photocurrent to be independent of coverage i n the range ~6 χ 10^° to 1 χ 10" mol/cm . The ~ 1 0 ~ mol/cm i s the l i k e l y coverage that would correspond to a monolayer of reagent derived from a molecule as l a r g e as I . But some c a u t i o n should be e x e r c i s e d i n i n t e r p r e t i n g k that i s assigned from k b p I n our e a r l i e r work (16) we used a coverage of -10"^ mol/cm to represent the a p p r o p r i a t e monolayer coverage but we b e l i e v e t h i s to be too h i g h , s i n c e <10~9 mol/cm w i l l , e x p e r i m e n t a l l y , r e s u l t i n the same s t e a d y - s t a t e photocurrent» The value of k >_ 6 χ 1 0 cuAnol^s*" i n the u s u a l u n i t s f o r a bimolecular r a t e constant f o r homogeneous s o l u t i o n i s > 6 χ 105 M ~ l s ~ l , The ferrocene self-exchange constant i s 5 χ 1 0 M~T -1 (29). Various cross r e a c t i o n s of s u b s t i t u t e d ferrocenes and f e r r i c e n i u m d e r i v a t i v e s have b i m o l e c u l a r r a t e constants t h a t exceed 10^ M""^s""l where the e q u i l i b r i u m constant exceeds u n i t y (30). F u r t h e r , i n the cross r e a c t i o n s , the r a t e constants v a r i e d by almost two orders of magnitude f o r a change i n d r i v i n g f o r c e of -0.25 V (30). Thus, the data i n Table I I r e l a t i n g to the f e r r o c e n e - l i k e molecules i s reasonable. +

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In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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The cross r e a c t i o n r a t e s and self-exchange r a t e s p r e d i c t k > 6 χ 10^ M " ~ l s ~ l , provided that the s u r f a c e - c o n f i n e d reagent has the same a c t i v i t y as the s o l u t i o n s p e c i e s . While we have not ordered k s q u a n t i t a t i v e l y , the order of k ^ ' s f o r the f e r r o c e n e - l i k e reagents does appear to depend i n a systematic way on the d r i v i n g f o r c e f o r r e a c t i o n . C o ( b i p y ) self-exchange i s f a i r l y slow (31) , and we f e l t that t h i s species might reduce ( F e C p 2 ) f . slowly enough that k could a c t u a l l y be measured. However, i t too has too l a r g e a k t o measure. The dimethyldithiocarbamate, Me2dtc~, i s i r r e v e r s i b l y o x i d i z e d by (FeCp2 )surf. * again the r a t e constant i s too l a r g e t o measure. The product(s) from t h i s r e a c t i o n have not yet been i d e n t i f i e d . Ferrocene i n s u l f o l a n e i s reported t o have a low r a t e constant f o r heterogeneous exchange (32) ; we examined t h i s system w i t h the hope of being able t o measure k , but found that ferrocene again has too l a r g e a k t o measure. The aqueous R u i N ^ ) ^ and Fe(CN) ^"" show that water s o l u b l e redox reagents can be found that g i v e l a r g e values o f k . Indeed, the s i m i l a r i t y o f E° from I " / I " and F e ( C N ) ~ / ~ and t h e >10 change i n r a t e constant shows that the I ~ o x i d a t i o n has a k i n e t i c b a r r i e r i n ^ 0 . We could a t t r i b u t e t h i s k i n e t i c d i f f i c u l t y s o l e l y t o the two-electron process t o form I3"" v s . the one-e]ectron o x i d a t i o n of Fe(CN)^ . However, t h i s e x p l a n a t i o n i s not e n t i r e l y s a t i s f a c t o r y i n view of the r e s u l t s f o r I " i n CH3CN. The F e ( C N ) ^ ' ^ " system would seemingly be a good couple f o r a p h o t o e l e c t r o c h e m i c a l c e l l f o r t h e conversion o f l i g h t t o e l e c t r i c i t y but here long term d u r a b i l i t y i s again a s e r i o u s problem: F e ( C N ) " / ~ i s phot o s e n s i t i v e (33) and the long term d u r a b i l i t y of e l e c t r o d e s d e r i v a t i z e d w i t h I i n the presence of high concentrations of Fe(CN)^^~/^~ has not been demonstrated. S e v e r a l of the r e v e r s i b l e redox couples c o u l d , i n f a c t be used i n a c e l l f o r e l e c t r i c i t y generation. However, the output photovoltage, Ey, a s s o c i a t e d w i t h these η-type S i / f e r r i c e n i u m / ferrocene photoanodes i s only i n the 0.3-0.4 V range a t openc i r c u i t . Such i s l i k e l y too low t o be u s e f u l i n p r a c t i c a l schemes f o r s o l a r energy conversion. The approach o f dérivâtization, though, may be a p p l i e d t o other photoanode m a t e r i a l s t o r e a l i z e improved e f f i c i e n c y and d u r a b i l i t y . e t

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Summary Photogenerated, s u r f a c e - c o n f i n e d f e r r i c e n i u m can be reduced by a number o f reductants i n c l u d i n g I , C o ( b i p y ) ^ , ferrocene, l , l - d i m e t h y l f e r r o c e n e , phenylferrocene„ Fe(n^-indenyl)2? [ F e ( C O ) ( n - C H ) ] , R u ( N H ) ^ , F e ( C N ) ~ and d i m e t h y l d i t h i o carbamate. With the exception o f I"", a l l g e n e r a l l y reduce the surface-confined f e r r i c e n i u m w i t h an observed heterogeneous r a t e constant o f >0.06 cm/s which corresponds t o a b i m o l e c u l a r r a t e constant > 6 χ 10^ M ~ l s ~ l , assuming that a monolayer (~10-10 mol/cm ) o f the s u r f a c e - f e r r i c e n i u m p a r t i c i p a t e s i n the -

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In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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r e a c t i o n . While good k i n e t i c s f o r I " o x i d a t i o n can be obtained i n CH3CN, d u r a b i l i t y of the photoelectrode i s not good enough to promise long term o p e r a t i o n . For e l e c t r o d e / s o l v e n t / e l e c t r o l y t e / r e d o x couple combinations that are durable and where good k i n e t i c s o b t a i n , low output photovoltage remains a problem f o r η-type S i e l e c t r o d e s d e r i v a t i z e d w i t h ferrocene reagents. Acknowledgements We thank the United States Department of Energy, O f f i c e of B a s i c Energy Sciences, D i v i s i o n of Chemical Science f o r support of t h i s r e s e a r c h . N.S.L. acknowledges support from the John and Fannie Hertz Foundation 1977-present, and M.S.W. as a Dreyfus Teacher-Scholar Grant r e c i p i e n t , 1975-1980. We acknowledge the a s s i s t a n c e of Dr. A.B. B o c a r s l y i n some aspects of t h i s work. Experimental Chemicals Ferrocene ( A l d r i c h Chemical Co.) was p u r i f i e d by s u b l i m a t i o n . B i s i n d e n y l i r o n (34), phenylferrocene (35)> C ( n - C H ) F e ( C O ) ] 4 (36) Co(bipy)3Cl2 * 71^0 ( 3 7 ) , and sodium dimethyldithiocarbamate (_3δ) were prepared by l i t e r a t u r e methods. 1,1 -Dimethylferrocene ( P o l y s c i e n c e s ) , a c e t y l f e r r o c e n e , and l , l - d i a c e t y l f e r r o c e n e ( A l d r i c h ) were p u r i f i e d by chromatography on alumina w i t h hexane as e l u a n t . K^Fe(CN)^ was used as r e c e i v e d ( M a l l i n c k r o d t ) , as was anhydrous NaCK>4 (G. F r e d e r i c k Smith). R u ( N H 3 ) 6 was c o n v e n i e n t l y prepared by e l e c t r o c h e m i c a l r e d u c t i o n at -0.3 V vs. SCE at a Hg p o o l e l e c t r o d e of R u ( N H ) C l ( A l f a Ventron) i n pH = 4.0 HCIO4/O.I M NaClO^. The s o l u t i o n was then a c i d i f i e d to pH = 2.0 w i t h HCIO4 j u s t before use, and manipulated under Ar. P o l a r o g r a p h i c grade [n-Bu^NJClO^ (Southwestern A n a l y t i c a l Chemicals) was d r i e d at 353 Κ f o r 24 hours and s t o r e d i n a d e s s i c a t o r u n t i l use. [n-Bu^Njl (Eastman) was r e c r y s t a l l i z e d twice from a b s o l u t e EtOH. Absolute EtOH, s p e c t r o q u a l i t y i s o o c t a n e , CH2CI2, CH3CN, and toluene were used as r e c e i v e d , as was reagent grade a c e t i c a c i d and s u l f o l a n e . A l l aqueous s o l u t i o n s were prepared from doubly d i s t i l l e d d e i o n i z e d water. 5

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Electrodes S i n g l e - c r y s t a l Sb-doped, p o l i s h e d η-type S i wafers ((111) face exposed) were obtained from General Diode Co., Framingham, MA. The wafers were 0.25 mm t h i c k and had r e s i s t i v i t i e s of 4-5 ohm-cm. E l e c t r o d e s were fashioned as p r e v i o u s l y reported (12). Ohmic contacts were achieved by rubbing Ga-In e u t e c t i c onto the back s i d e of the e l e c t r o d e a f t e r a 48% HF etch and H2O r i n s e . The e l e c t r o d e was attached to a Cu w i r e w i t h Ag epoxy, and the Cu w i r e was passed through a 4 mm Pyrex tube. The e l e c t r o d e s u r f a c e was defined by i n s u l a t i n g a l l other surfaces w i t h o r d i n a r y epoxy, y i e l d i n g e l e c t r o d e s of areas 3-15 mm . 2

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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C i r c l e s o f 5 mm were cut u l t r a s o n i c a l l y from the o r i g i n a l S i wafers f o r use as r o t a t i n g d i s k e l e c t r o d e s . Mounting was c a r r i e d out as above i n a 6 mm o.d. c a p i l l a r y tube, except t h a t the Cu wire was r e p l a c e d by a Hg contact through the c a p i l l a r y . Ordinary epoxy i n s u l a t i o n was used s p a r i n g l y and care was taken t o maintain as f l a t a S i e l e c t r o d e surface as p o s s i b l e . For c a l i b r a t i o n purposes, r o t a t i n g P t d i s k s from c i r c l e s o f Pt f o i l were mounted i n e x a c t l y the same manner. R o t a t i n g d i s k e l e c t r o d e s were mounted v e r t i c a l l y and s t i r r e d by a v a r i a b l e speed motor from P o l y s c i e n c e s , Inc. R o t a t i o n v e l o c i t i e s were c a l i b r a t e d by two methods: (1) a s l i t t e d p i e c e o f cardboard was mounted on the d i s k s h a f t and the time response o f a photodiode was recorded on an o s c i l l o s c o p e , and (2) p l o t s o f the l i m i t i n g current as a f u n c t i o n o f K^Fe(CN)^ c o n c e n t r a t i o n i n 2 M KC1 y i e l d e d s t r a i g h t l i n e s whose slope i s uh (D f o r F e ( C N ) - i s 6.3 χ 1 0 ~ cm /sec) ( 3 9 ) . Agreement between the two methods was b e t t e r than 10%, and the motor was found t o be extremely s t a b l e over long p e r i o d s o f time. Befure use, a l l S i s u r f a c e s were etched i n concentrated HF and r i n s e d w i t h d i s t i l l e d H2O. E l e c t r o d e s to be d e r i v a t i z e d were then immersed i n 10 Έ NaOH f o r 60 seconds, washed w i t h H 0 followed by acetone, and then a i r d r i e d . D e r i v a t i z a t i o n was accomplished by exposing the p r e t r e a t e d e l e c t r o d e s t o d r y , degassed isooctane s o l u t i o n s o f ( 1 > 1 - f e r r o c e n e d i y l ) d i c h l o r o s i l a n e f o r 2-4 hours ( 1 9 ) . The e l e c t r o d e s were then r i n s e d w i t h isooctane f o l l o w e d by EtOH. 4

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Electrochemistry A l l experiments were performed i n s i n g l e compartment Pyrex c e l l s equipped w i t h a s a t u r a t e d calomel reference e l e c t r o d e (SCE), P t wire c o u n t e r e l e c t r o d e , and the a p p r o p r i a t e working e l e c t r o d e . I r r a d i a t i o n was s u p p l i e d by a beam expanded He-Ne l a s e r o f -50 mW/cm (5 mW t o t a l ) a t 632.8 nm, Laser i n t e n s i t i e s were measured u s i n g a T e k t r o n i x J16 d i g i t a l radiometer equipped w i t h a J6502 probe, and were adjusted t o d e s i r e d i l l u m i n a t i o n l e v e l s by use o f Corning t r a n s m i s s i o n f i l t e r s . C y c l i c v o l t a mmetry measurements were obtained w i t h a P r i n c e t o n A p p l i e d Research Model 173 p o t e n t i o s t a t equipped w i t h a Model 179 d i g i t a l coulometer and d r i v e n by a Model 175 v o l t a g e programmer. P o t e n t i a l step experiments were performed u s i n g the same apparatus, and the coulometer readings were v e r i f i e d by o s c i l l o s c o p i c current-time t r a c e s on a T e k t r o n i x 564B storage o s c i l l o s c o p e w i t h Type 2B67 time base p l u g - i n . C y c l i c voltammetry t r a c e s were recorded on a Houston Instrument Model 2000 X-Y recorder and current-time p l o t s were obtained u s i n g a Hewlett-Packard s t r i p chart recorder. The supporting e l e c t r o l y t e s were 0.1 M [n-Bu N]C10 f o r CH3CN, EtOH, C H C 1 , EtOH/toluene (1:1 v / v ) . s u l f o l a n e , and g l a c i a l a c e t i c a c i d s o l v e n t s , and 1.0 M NaC10 /0.01-0.1 M HC10 f o r H 0. Ei^ values f o r I " o x i d a t i o n were obtained a t P t 2

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r o t a t i n g d i s k e l e c t r o d e s a t 600 r.p.m.; e x c e l l e n t agreement w i t h l i t e r a t u r e values (22), where a v a i l a b l e , was obtained. P o t e n t i a l values f o r surface-attached m a t e r i a l were obtained by d e r i v a t i z i n g P t and Au surfaces as p r e v i o u s l y described (18, 1 9 ) , and t a k i n g E°(FeCp )surf. to be the a r i t h m e t i c mean of the anodic and cathodic peak p o s i t i o n s ; g e n e r a l l y peak-to-peak separations were l e s s than 10 mV a t a 20 mV/s scan r a t e . K i n e t i c Measurements For measurement of k , d e r i v a t i z e d e l e c t r o d e s were c y c l e d i n a s o l u t i o n o f appropriate solvent and e l e c t r o l y t e u n t i l s t a b l e c y c l i c voltammetric parameters were obtained a t 100 mV/sec scan r a t e . C y c l i c voltammograms a t s e v e r a l scan r a t e s were recorded w i t h i l l u m i n a t i o n f o r the anodic p o r t i o n of the scan, but the l i g h t was blocked a t the anodic l i m i t by a s o l e n o i d d r i v e n by the t r i g g e r output of the PAR 175 v o l t a g e programmer, Stock s o l u t i o n s of reductant were prepared as needed and a l i q u o t s i n j e c t e d i n t o the Pyrex e l e c t r o c h e m i c a l c e l l immediately p r i o r t o use. C y c l i c voltammetry data was then c o l l e c t e d f o r the same s e t o f scan r a t e s and i l l u m i n a t i o n c o n d i t i o n s i n the presence of s o l u t i o n reductant t o o b t a i n k i n e t i c data. The e l e c t r o d e s were r i n s e d w i t h solvent and checked f o r decay i n the absence o f reductant between every k i n e t i c measurement. A t l e a s t four d i f f e r e n t concentrations o f reductant were used f o r each s e t of data p o i n t s . From t h i s data, cathodic currents a s s o c i a t e d w i t h t h e r e d u c t i o n of surface-attached f e r r i c e n i u m were i n t e g r a t e d manually t o determine the time and c o n c e n t r a t i o n dependence o f the extent of consumption o f s u r f a c e - c o n f i n e d f e r r i c e n i u m . The r e a c t i o n time a s s o c i a t e d w i t h a c y c l i c voltam m e t r i c sweep was chosen as the p e r i o d from the anodic l i m i t t o the peak cathodic current i n the c y c l i c voltammogram. P o t e n t i a l step experiments were performed by scanning a n o d i c a l l y a t 500 mV/sec to the anodic l i m i t (+0.5 V v s . SCE), h o l d i n g at t h i s l i m i t i n the dark f o r a time t ^ , and then p u l s i n g c a t h o d i c a l l y back t o -0.6 V vs. SCE where the r e d u c t i o n was observed. +/0

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Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9.

Wrighton, M.S., Acc. Chem. Res., 1979, 12, 303. Nozik, A.J., Ann. Rev. Phys. Chem., 1978, 29, 189. Bard, A.J., Science, 1980, 207, 139. Gerischer, H. J. Electroanal. Chem., 1977, 82, 133. Bard, A.J.; Wrighton, M . S . , J. Electrochem. Soc., 1977, 124, 1706. E l l i s , A . B . ; Kaiser, S.W.; Wrighton, M.S., J. Am. Chem. Soc., 1976, 98, 1635, 6418, and 6855. Hodes, G . , Nature (London), 1980, 285, 29. Parkinson, B . A . ; Heller, Α.; M i l l e r , B . , Appl. Phys. L e t t . , 1978, 33, 521. Nakatani, K . ; Matsudaira, S.; Tsubomura, H . , J. Electrochem. Soc., 1978, 125, 406.

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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11. 12.

13. 14.

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15. 16. 17. 18.

19. 20. 21. 22. 23.

24. 25. 26. 27.

28. 29. 30. 31. 32. 33. 34. 35.

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Wrighton, M.S.; Bocarsly, A . B . ; Bolts, J . M . ; Bradley, M.G.; Fischer, A . B . ; Lewis, N.S.; Palazzotto, M.C.; Walton, E.G., Adv. Chem. Ser., 1980, 184, 269. Bocarsly, A . B . ; Walton, E . G . ; Wrighton, M.S., J. Am. Chem. Soc., 1980, 102, 3390. Bolts, J . M . ; Bocarsly, A . B . ; Palazzotto, M.C.; Walton, E.G.; Lewis, N.S.; Wrighton, M . S . , J. Am. Chem. Soc., 1979, 101, 1378. Bolts, J . M . ; Wrighton, M.S., J. Am. Chem. Soc., 1979, 101, 6179. Bocarsly, A . B . ; Walton, E . G . ; Bradley, M.G.; Wrighton, M.S., J . Electroanal. Chem., 1979, 100, 283. Bolts, J . M . ; Wrighton, M.S., J. Am. Chem. Soc., 1978, 100, 5257. Lewis, N.S.; Bocarsly, A . B . ; Wrighton, M.S., J. Phys. Chem., 1980, 84, 2033. Murray, R.W., Acc. Chem. Res., 1980, 13, 135 and references therein. Wrighton, M.S.; Austin, R . G . ; Bocarsly, A . B . ; Bolts, J . M . ; Haas, O.; Legg, K . D . ; Nadjo, J.; Palazzotto, M. C., J . Electroanal. Chem., 1978, 87, 429. Wrighton, M.S.; Palazzotto, M.C.; Bocarsly, A . B . ; Bolts, J.M. Fischer, A . B . ; Nadjo, L., J. Am. Chem. Soc., 1978, 100, 7264. Marcus, R . A . , Ann. Rev. Phys. Chem., 1964, 15, 155. Marcus, R . A . , J . Chem. Phys., 1965, 43, 679. Janz, D. J.; Tomkins, R . P . T . , "Nonaqueous Electrolytes Handbook", Vol. 2, Academic Press: New York, 1972. Piebarski, S.; Adams, R . N . , in "Physical Methods of Chemistry", Part IIA, Weissberger, A. and Rossiter, B . , eds., Wiley-Intersciences: New York, 1971, Chapter 7. Galus, Ζ . , Adams, R . N . , J. Phys. Chem., 1963, 67, 866. Levich, V . G . , "Physicochemical Hydrodynamics", Prentice-Hall: Englewood C l i f f s , New Jersey, 1962. Yeh, P . ; Kuwana, T., J. Electrochem. Soc., 1976, 123, 1334. Lenhard, J . R . ; Rocklin, R.; Abruna, H . ; William, K . ; Kuo, K . ; Nowak, R.; Murray, R.W., J. Am. Chem. Soc., 1978, 100, 5213. Oyama, N . ; Anson, F.C., J. Electrochem. Soc., 1980, 127, 247. Yang, E . S . ; Chan, M . ; Wahl, A.C., J. Phys. Chem., 1975, 79, 2049. Pladziewicz, J . R . ; Espenson, J.H., J. Am. Chem. Soc., 1973, 95, 56. Farina, R.; Wilkins, R . G . , Inorg. Chem., 1968, 7, 514. Armstrong, N.R.; Quinn, R . K . ; Vanderborgh, N . E . , J. Electro chem. Soc., 1976, 123, 646. Balzani, V . ; Carassiti, V . , "Photochemistry of Coordination Compounds", Academic Press: New York, 1970. King, R . B . , "Organometallic Synthesis", Academic Press: New York, 1965, p. 73. Broadhead, G . ; Pauson, P . L . , J. Chem. Soc., 1955, 367.

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56 36. 37. 38. 39.

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King, R . B . , Inorg. Chem., 1966, 5, 2227. Burstall, F . H . ; Nyholm, R . S . , J. Chem. Soc., 1952, 3570. Delepine, M . ; Haller, Α . , Comptes Rendus de L'Academie des Sciences, 1907, 144, 1126. Adams, R . N . , "Electrochemistry at Solid Electrodes", Marcel Dekker: New York, 1969.

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Received October 3, 1980.

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

4 Chemical Control of Surface and Grain Boundary Recombination in Semiconductors ADAM HELLER

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Bell Laboratories, Murray Hill, NJ 07974

The density and distribution of surface and grain boundary states between the edges of valence and conduction bands of a semiconductor, which affect the electron hole recombination velo cities, can be controlled by chemisorption reactions. Strongly exoergic chemisorption reactions reduce the density of surface states in the band gap, leading to substantial decrease in the recombina tion velocity at grain boundaries and surfaces and thus to an im provement in the performance of solar cells. Chemisorption of oxygen reduces the surface recombination velocities in Si, Ge and InP. With single crystals of n-GaAs, reduction of the surface recombination velocity from 10 cm/sec to 3x10 cm/sec, doubling of the band gap luminescence intensity, and increase in the solar -to-electrical conversion efficiency of the cell n-GaAs|0.8M K Se-0.1M K Se -1M KOH|C from 8.8% to 12% are ob served upon the chemisorption of a submonolayer quantity of Ru . With thin, chemically vapor deposited films of polycrystal line (9µm average grain size) n-GaAs diffusion of Ru ions into the grain boundaries increases the current collection efficiency in electron beam induced current measurements by 50% and the solar energy conversion efficiency of the n-GaAs|0.8M K Se-0.1M K Se -1M KOH|C cell fourfold to 7.3%. Co -absorbing Ru and Pb raises the efficiency of the thin film cell to 7.8%. 6

2

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Recombination of electrons and holes at grain boundaries in thin, polycrystalline films of semiconductors is a key problem that requires solution if efficient and inexpensive solar cells are to be developed. This problem is quite similar to the extensively stu died phenomenon of recombination at semiconductor surfaces. Both involve the electronic states associated with the abrupt discontinuity in the chemical bonding at an interface. 0097-6156/81/0146-0057$05.25/0 © 1981 American Chemical Society In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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Semiconductor surfaces have been the subject of numerous studies during the past three decades. The extent and number of past studies can be appreciated by a review of the references of the books of Many, Goldstein, Grover and of Morrison. Chapters 3, 5, 7 and 9 of the first book and chapter 9 of the second show that workers in the field have been quite aware of chemical effects on surface recombination velocities. Indeed, in their 1952 paper on "Surface Recombination of Germanium" Brattain and Bardeen pointed out that there is relatively little change in the surface recombination velocity (vj when η or ρ crystals are exposed to 0 /0 , humid N or 0 and dry 0 . However, follow ing a suggestion of C. S. Fuller, they did observe, "that v could be changed from the order of 10 cm/sec. to greater than 10 cm/sec and back again by exposure . . . to NH OH fumes and then to HC1 fumes respectively." During the 30 years that have elapsed since the pioneering work of Brattain and Bardeen, infor mation has been gathered on the effect of gases, etchants, ions, adsorbed organic molecules and physical damage at or near the surface. We have analyzed a small part of the accumulated infor mation and will show that a simple chemical model can account for many of these effects. This model allows the application of chemical concepts and techniques to the reduction of the recombi nation velocity at surfaces and grain boundaries. We have applied these concepts to the case of n-GaAs, and have reduced the sur face and grain boundary recombination velocity by chemisorbing Ru and other ions. Chemical Model Electron-hole recombination velocities at semiconductor inter faces vary from 10 cm/sec for Ge to 10 cm/sec for GaAs. Our first purpose is to explain this variation in chemical terms. In phy sical terms, the velocities are determined by the surface (or grain boundary) density of trapped electrons and holes and by the cross section of their recombination reaction. The surface density of the carriers depends on the density of surface donor and acceptor states and the (potential dependent) population of these. If the states are outside the band gap of the semiconductor, or are not populated because of their location or because they are inaccessi ble by either thermal or tunneling processes, they do not contri bute to the recombination process. Thus, chemical processes that substantially reduce the number of states within the band gap, or shift these, so that they are less populated or make these inacces sible, reduce recombination velocities. Processes which increase the surface state density or their population or make these states accessible, increase the recombination velocity. 1

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In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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4.

HELLER

Surface and Grain Boundary Recombination

A hypothetical semiconductor with dangling or highly strained bonds at its surface, such as would be formed if the crystal were cleaved under perfect vacuum at a temperature low enough to prevent surface relaxation and reconstruction will have a high density of surface states and a high recombination velocity. This can be derived from elementary chemical bonding theory. When a chemical bond is formed between two atoms the states occupied by the bonding electrons are split to form a lower energy, occu pied or bonding state and a higher unoccupied or antibonding state. States due to weakly or partially bound atoms or to atoms with strained bonds (for example, double carbon carbon bonds on diamond) are therefore always between the product states of strongly bound atoms. As valence and conduction bands evolve from the initial diatomic states, upon the extension of the bonding in the lattice, unbound and partially bound surface atoms, or sur face atoms with strained bonds, introduce states within the band gap. (Figure 1) If such a hypothetical surface with strained bonds or with weakly or partially bound surface atoms is allowed to relax and reconstruct, the bonding at the surface will still be weaker than in the bulk and the splitting usually less than the band gap. Thus, clean, relaxed semiconductor surfaces have high densities of sur face states and, unless the cross sections for recombination are unusually low, also exhibit high recombination velocities. This need not be the case, however, if another element, ion or molecule is chemisorbed on the surface. In this case, the atoms with strained bonds and the partially or weakly bound atoms responsible for recombination may react with the chemisorbed species. If the amount of free energy released is of the order of magnitude (or larger than) the band gap, i.e., the chemisorption is strong, there will be adequate displacement of the surface or grain boundary states (Fig. 2) to sweep clean part or all of the region between the edges of the valence and conduction bands. If the splitting is now adequate to displace the surface states to positions either outside the band gap or to positions where they are less populated or are inaccessible to the majority carrier by both thermal scaling of the surface barrier or by tunneling through the barrier (Fig. 2c), the recombination velocity will be reduced. If the chemisorption is weak, the splitting of the surface states will not be adequate to displace the surface states from within the band gap, or to shift these to positions where they cannot be po pulated by thermal or tunneling processes. (Fig. 2b) Thus, weak chemisorption merely redistributes or even increases the density of surface states within the band gap, and with it the surface recombination velocity. The concepts of strong and weak splitting

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

PHOTOEFFECTS

AT

SEMICONDUCTOR-ELECTROLYTE

INTERFACES

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cb

Figure 1. Simple chemical bonding model showing that unbound or partially bound atoms on a semiconductor surface contribute states within the band gap. The states of unbound atoms (a) are split upon partial bonding (b), then further split when the fully bound species (c) is formed. Evolution of the periodic lattice broadens the bonding states to form the valence band (vb) and the antibonding orbitals to form the conduction band (cb). In the process of band formation, the unbound and partially bound states (a and b) remain between vb and cb.

—

SR*

cb

SR*

/

Γ l-AF

vb

-i-SR

Figure 2.

A surface state (S) located near the conduction band (cb) (Mt) upon the chemisorption of a reagent (R).

is split

If the release of energy (AF) is small, the splitting is small (center) and the number of product states (SR and SR*) within the band gap, near the conduction band, increases. If the chemisorption is strong, a substantial amount of free energy is released fright) and the number of surface states within the band gap (or near the conduction band) decreases.

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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4.

HELLER

Surface and Grain Boundary Recombination

are presented in Fig. 2. Whether states between the band edges are added or substantially removed upon "contamination" of the surface by a chemisorbed reagent depends on the initial position of these states and on -AF, the change in free energy which pro duces the splitting. When -AF is large, states are likely to be re moved. When -AF is small, states are usually added. In summary, a simple chemical picture of surface recombina tion is presented. The surface or grain boundary recombination velocity decreases when the appropriate surface species is reacted with a strongly chemisorbed species. It increases when a species is weakly chemisorbed. We shall now illustrate this concept for six extensively studied semiconductors, Ge, Si, GaP, GaAs, InP and InSb. Ge Brattain and Bardeen measured the surface recombination velocity (v ) of oxidized (ozone and peroxide exposed) germani um and found it to be slow, about 170 cm/sec for p-type and 50 cm/sec for n-type/Ge. Following complete oxidation, these values change only slightly in a variety of ambients. The recombi nation velocity increases by three orders of magnitude, to >10 cm/sec, when the oxidized surface is exposed to ammonia, and reverts back to ~10 cm/sec when the surface is exposed to HC1. We interpret these observations as follows. Chemisorption of reactive oxygen on water-free Ge produces a Ge0 film. The standard-free energy of formation for bulk Ge0 is -115 Kcal/mole. For the reaction of oxygen at a surface with unsa turated Ge bonds, AF is likely to be more negative. Thus, the splitting of the surface states upon oxidation displaces these states to domains above the edge of the conduction band and below the edge of the valence band. As the density of states within the band gap is reduced, the surface recombination velocity is also re duced. Humid ammonia vapor attacks the Ge0 film. Reoxidation by air in the presence of an acid (humid HC1) restores the film. Si The existence of today's silicon based microelectronics tech nology is evidence for the low surface recombination velocity of oxidized Si. The velocity is less than 10 cm/sec. ' We explain the low surface recombination velocity by the sweeping of surface states from the region between the edges of the conduction and valence bands upon oxidation of the surface. The standard free energies of formation of crystalline and fused quartz from bulk Si are -192 and -191 Kcal/mole respectively. The standard free energy change for a Si surface is likely to be 3

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PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE INTERFACES

more negative. Thus the edges of the conduction and valence bands of Si are well within those of Si0 . We note in this context that in Si based MIS (metalinsulator-semiconductor) solar cells one of the roles of the 20-60Â thick Si0 layer may well be reduction of the recombination velo city at the Si surface. Chambouleyron and Soucedo noted a de crease in the recombination velocity at the conductive Sn0 /Si in terface relative to that at the Si surface and Michel and Lasnier find a recombination velocity of less than 2xl0 cm/sec at the con ductive indium tin oxide/Si interface. In both cases heating the metal oxides present on the elemental Si produces an intermediate Si0 layer. The grain boundary recombination velocity in polycrystalline Si, which reaches 10 cm/sec , is well above v at the air (oxygen)/Si interface. We attribute the difference to the fact that Si grain boundaries are either not oxidized or are oxidized only to SiO, a "black", small band gap material. Were it possible to grow 20-50Â thick dioxide layers at the boundaries, the density of grain boundary states could be decreased yet carriers could tunnel through. This would increase the efficiency of small grained Si based solar cells. Presently available oxidation methods lead to excessively thick oxide near the 0 exposed surface and to little or no oxidation deeper in the polycrystalline material. Exposure of silicon to atomic hydrogen increases the surface recombination velocity. The free energy of formation of SiH , the most stable of the hydrides of silicon, is only — lOKcal/mole. Since four electron pairs are shared in the forma tion of the molecule, the free energy of formations per Si-Η bond is only —2.5 Kcal or about 0.1 eV. Because of the weak chem isorption, heating of the silicon to temperatures above 500°C is adequate to release the hydrogen. Our model explains the in crease in surface recombination velocity by the weak chemisorp tion of hydrogen, which may increase the density of surface states within the band gap (see Fig. 2b). The observation of Seager and Ginley that grain boundaries of polycrystalline silicon are passivated by atomic hydrogen is ex plained by the filling of empty or part empty states upon reaction with hydrogen and the consequent reduction in their population. In p-type (but not η-type) silicon, the effect may also be due to penetration of hydrogen into the Si surface, which leads to heavy doping of the material near the boundary. Such doping makes the grain boundary states less accessible under circumstances dis cussed at the end of the paper. We note that penetration of hy drogen into Si and doping were suggested bv Law , and that 2

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In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

4.

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Surface and Grain Boundary Recombination

15

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lies , Soclof and lies and Sah and Lindholm have shown that heavy doping near boundaries is a means to reduce the grain boundary recombination velocity.

InP and GaP Casey and Buehler have shown that the surface recombination velocity of n-InP (~5xl0 carriers/cm ) is low, ~10 cm/sec. Suzuki and Ogawa have recently reported a sequence of surface treatments that cause substantial changes in the surface recombi nation velocity of InP. They found that in freshly vacuum cleaved (110) faces v is much greater than at air exposed faces and that the quantum efficiency of band gap luminescence in creases by an order of magnitude when the freshly cleaved face is exposed to air. This suggests that the surface recombination velo city is reduced when 0 is chemisorbed. The changes are explained as follows: The density of surface states within the band gap on freshly cleaved InP is high. As a result, the surface recombination velocity is high and the lumines cence efficiency is low. Chemisorption of oxygen splits the sur face states, as large band gap, colorless InP0 is formed. Reduction in the surface recombination velocity of GaP, from 1.7 χ 10 cm/sec to 5xl0 cm/sec, is observed upon exposure to a CF plasma in whichfluorineis known to be present. Again, the product of chemisorption offluorineon the surface is likely to be a large band gap material such as GaF , which straddles the edges of the conduction and valence bands of GaP. GaAs and InSb For both GaAs and InSb, even when exposed to air or oxy gen, y is high, typically 10 cm/sec. ' * ' ' Thurmond et al have shown that no arsenic containing oxide (i.e. As 0 , As 0 GaAs0 or GaAs0 ) will co-exist in thermodynamic equilibrium with GaAs. For example, the standard free energy change for the reaction 17

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In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE INTERFACES

is -62Kcal/mole. Consequently, elemental As is a constituent of the GaAs/oxide interface in thermal, plasma and anodically pro duced oxides. Since surface GaAs is more reactive than bulk GaAs, elemental As must always be a constituent of the first monolayer on GaAs in air. The presence of arsenic at the interface implies that surface states within the band gap will be introduced (see Fig. 1). We as sociate the high surface recombination velocity with the presence

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2627 2 8

of arsenic. The formation of elemental As on the GaAs surface explains the difference in behavior of InP and of GaAs. In InP the thermodynamically stable phase that results from oxidation of the surface is colorless InP0 which straddles the band gap. In GaAs it is Ga 0 and small band gap As. The standard free energy change in the reaction 4

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llnSb + Sb 0 — ln 0 + 4Sb 2

3

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3

is -33 Kcal/mole for bulk InSb. For an InSb surface, it is prob ably more negative. Consequently, Sb will be present at the inter face, and will increase v,. Indeed, Skountzos and Euthymious re port v, ~10 cm/sec for InSb in air. 6

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Effect of Chemisorbed Ru on n-GaAs Surfaces and Grain Boundaries It is evident that in order to reduce the high surface recombi nation velocity of GaAs, it is necessary to remove the elemental arsenic at the air interface and to stabilize the surface with a strongly chemisorbed species. This can be accomplished by a se quence of surface treatments consisting of oxidation, dipping into either a base or into a basic selenide-diselenide solution and dip ping into an acidic R u solution. ' The function of the base or the basic selenide diselenide solution is to clean the GaAs surface. The base dissolves the amphoteric gallium oxide and the acidic ar senic oxides. The selenide-diselenide solution dissolves residual arsenic, when present, by the reaction 3+

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In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

4.

HELLER

Surface and Grain Boundary Recombination

2As + 3Set -» 2AsSef

which will also leave a monolayer of selenide on the surface. The acid Ru solution either removes this layer or the selenide layer allows the Ru to diffuse through it. In either case, the Ru is strongly chemisorbed on the n-GaAs surface. Bulk doping by Ru is ruled out by Rutherford backscattering studies. The latter show that the Ru remains at the surface and does not diffuse into the bulk even at 300°C. The number of Ru atoms on the surface corresponds to substantially less than a monolayer. Measurement of the decay time of the band gap luminescence fol lowing Ru treatment reveals a dramatic increase in the carrier lifetime. Figure 3 shows the luminescence decay following excita tion by light pulses of several nanosecond duration of a GaAs cry stal that had received three surface treatments. The slowest de cay is observed for the crystal with an epitaxial GaAlAs layer grown on it. The carrier recombination velocity at the GaAlAsGaAs interface is less than 500 cm/sec. Here, the observed de cay rate represents the sum of the nonradiative bulk recombina tion and radiative bulk recombination processes. When the GaA lAs layer is removed by HC1 to expose the GaAs surface, v in creases to 10 cm/sec. The luminescence decay is now fast and is dominated by the surface recombination process. Chemisorp tion of Ru increases the carrier lifetime by reducing the surface recombination velocity. Analysis of the luminescence decay time as a function of the thickness of the GaAs sample shows a de crease in recombination velocity from 10 cm/sec to 3xl0 cm/sec following Ru treatment. It is notable that the effect of Ru persists in spite of oxidation of the surface in air, suggesting that the ruthenium stays at the interface between the oxide layer and n-GaAs. Recent results of Rowe, who measured the depth profile of the Ru concentration at the air-GaAs interface by Auger spectroscopy, prove this point. 3+

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Woodall et al. have analyzed the relationship between sur face recombination velocity and the steady state band gap luminescence in GaAs. They calculate for 534nm excitation that a decrease in v from 10 cm/sec to 10 cm/sec will triple the quan tum efficiency at a 2.5μπι deep p-n junction if the hole diffusion length, L , is 3μΐη, and the electron diffusion length, L„ is 4μΐη. 6

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In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE INTERFACES

10*

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TIME (nsec)

Figure 3. Decay of the band gap luminescence in the same n-GaAs crystal (a) with GaAlAs windows on both sides; (b) with a GaAlAs window on one side and an airexposed GaAs surface on the other; (c) same as (b) after chemisorption of Ru on the GaAs surface. For details, see Ref. 33. 3+

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

4.

HELLER

Surface and Grain Boundary Recombination

They also show that as L and L„ decrease or as carrier concentra tion increases there is less improvement in the quantum yield. In agreement with these calculations, we find for samples with L ~ 2μΐη a doubling of the luminescence intensity. Chemisorption of a fraction of a monolayer of Ru also im proves the performance of n-GaAs based solar cells. The solarto-electrical conversion efficiency of a n-GaAs|0.8M K Se-0.1M K S e - l M KOH|C semiconductor-liquid junc tion solar cell increases from 8.8% to 12%. ' Current voltage characteristics for the cell, before and after Ru chemisorption, are shown in Fig. 4. The improvement is in the fill factor. At po tentials approaching open circuit, fewer electrons recombine at the semiconductor liquid interface with holes after R u is chemisorbed. The exceptionally strong bond between R u and the nGaAs surface is evidenced by the fact that the improvement in cell performance persists for weeks. Ru is not the only ion capable of improving the efficiency of n-GaAs based solar cells. Lead chemisorbed from a basic plumbite solution, as well as Ir and R h chemisorbed from acids are also effective. These ions are, however, not as strongly chem isorbed as Ru . Consequently, they are more readily desorbed and produce a lesser improvement. Experiments with combined plumbite and ruthenium chemisorption show a further small im provement. Ions that are not chemisorbed do not affect the performance of semiconductor liquid junction solar cells. Weakly chemisorbed ions produce inadequate splitting of surface states between the edges of the conduction and valence band and increase rather than decrease the density of the surface states in the band gap and thus the recombination velocity. Bi is an example of such an ion. As seen in Figure 5, it decreases the efficiency of the p

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n-GaAs|o.8M K S e - 0 . l M K S e - l M KOH|C cell. Since the chemisorp 2

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tion of Bi is weak, the deterioration in performance is tem porary. The ion is desorbed in ~10 min. and the cell recovers. It is notable that once strongly chemisorbed Ru has been ad sorbed on an n-GaAs surface, Bi causes little deterioration in cell performance. This suggests that the two ions react with the same chemical species or "site" on the GaAs surface. The basic thesis of this paper, the displacement of interface states by strongly chemisorbed species, is also applicable to grain boundaries. 3+

3+

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PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE INTERFACES

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1.0

CELL VOLTAGE 3+

Figure 4. Effect of chemisorbed Ru on the current-voltage characteristics of the n-GaAs/0.8M K Se-0.1M K Se -lM KOH/C solar cell (( ; freshly etched; ( ) Ru * chemisorbed) 2

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3

CELL VOLTAGE 3+

Figure 5. Effect of chemisorbed Bi on the current-voltage characteristics of the n-GaAs/0.8M K Se-0JM K Se -lM KOH/C solar cell (( ; freshly etched; ( ) Bi chemisorbed) 2

2

2

3+

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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Surface and Grain Boundary Recombination

Our model predicts that a strong chemisorption reaction which is effective in reducing the recombination velocity on the surface of a crystal, will do the same at its grain boundary. Like a sur face, a grain boundary is a discontinuity which introduces states in the gap between the edges of the conduction and valence bands. These can be shifted by chemisorption. To accomplish the re quired grain boundary reaction, advantage is taken of the fact that the rate of diffusion of a reactant in a grain boundary is much fas ter than the rate of its diffusion in the bulk. Thus, chemisorption reactions can be carried out without substantial doping of the bulk of the semiconductor. It is easier to improve the efficiency of thin film cells of direct gap semiconductors (which have light adsorbtion coefficients of ( a « 1 0 c m ) , than of indirect band gap materi als ( a«10 -10 cm )- The reason is that grain boundary diffusion to depths of 10"cm is adequate in the first, while the second re quires diffusion to depths of 10~ - 10~ cm. GaAs is a direct band gap semiconductor. Films of Ιμπι thickness absorb nearly all the photons with energies exceeding the 1.4 eV bandgap. Upon diffusion of R u into boundaries of 34μπι grains of n-GaAs (produced by chemical vapor deposition onto a graphite substate coated with a 500Â film of germanium) we observed a fourfold increase in the solar conversion efficiency (from 1.2% to 4.8%) in the n-GaAs|0.8M K S e - 0 . l M K2Se -lM KOHlC cell. Since the relevant area over which electron-hole recombination may take place is much larger than in a single crystal, the improvement is far more dramatic. Fig. 6 shows the current voltage characteristics obtained with a polycrystalline n-GaAs photoanode before and after chem isorption of Ru at the grain boundaries. The deep diffusion of Ru into the grain boundaries is evidenced by the fact that etchants, which attack the surface of n-GaAs and completely re verse the improvement in solar cells following Ru chemisorption in single crystals, reverse the improvement in polycrystalline films only in part, unless substantial (>1000Â) film thickness is re moved. Using an improved chemically vapor deposited film of n-GaAs on graphite, with a 9μΐη average grain size, a solar-to-electrical conversion efficiency of 7.3% is reached after Ru is chemisorbed at the grain boundaries. By chemisorbingfirstRu from an acid, then Pb from a basic aqueous solution, the efficiency of the po lycrystalline thin film n-GaAs|0.8M K Se-0.1M K S e - l M KOHlC cell 5

2

5

_1

-1

4

3

2

3+

39

2

2

3+

3+

3+

3+

40

3+

2+

2

2

2

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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PHOTOEFFECTS

CELL

VOLTAGE

3+

Figure 6. Effect of chemisorption of Ru on the grain boundaries of a chemically vapor-deposited, thin-film n-GaAs photoanode. The grains are of 3-5 ^m in size. The current-voltage characteristics shown are for the n-GaAs/0.8M K Se-0.1M K Se -lM KOH/C cell. (( ; freshly etched; ( ; Ru * chemisorbed) 2

3

2

2

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

4.

HELLER

Surface and Grain Boundary Recombination

is increased to 7.8%, the highest value for any GaAs cell of such small grain size. The current voltage characteristics of the 7.8% efficient cell following Ru and Pb treatments are shown in Fig. 7. The improvement in performance following chemisorption of Ru at n-GaAs grain boundaries is not limited to semiconductor liquid junction solar cells. Charge collection scanning electron mi croscopy at a gold-n-GaAs Schottky junction shows a drastic reduction in grain boundary recombination following the Ru and Ru plus Pb treatments. Figure 8 shows charge collection scan ning electron micrographs for the same polycrystalline film before and after R u treatment. While chemisorbed Ru reduces the surface and grain boun dary recombination on n-GaAs, it has no effect on p-GaAs. This can be accounted for if the chemisorption reaction decreases only the density of electron trapping states near the conduction band edge but not of hole trapping states near the valence band edge. It appears that in GaAs chemisorption of Ru + splits electron trap ping surface states near the edge of the conduction band to form states above the edge of this band and states near the valence band (Fig. 9). While such a reaction prevents electrons from reaching the surface in η-type materials (Fig. 9, left) it does not prevent holes from doing the same in p-type materials (Fig. 9, right). Since holes and electrons are, respectively, abundant at surfaces of illuminated η and ρ type GaAs, only recombination at surfaces of η-type materials is reduced. One may speculate about the causes of strong chemisorption of some ions and the weak chemisorption of others. The four metals, with strongly chemisorbed ions on GaAs, (Ru, Pb, Ir, Rh) have several stable oxidation states and thus radii, some of which approach those of the lattice components. The orbitals of these metals and ions also have substantial mixed sp (—60%) and d(~40%) character, which makes varying orbital hybridizations and thus a range of orbitals of different directionality possible. In some cases, orbitals binding at two or more surface sites can be envisaged. The recombination velocity at a grain boundary can be re duced not only by reducing the density of grain boundary states, but also by diffusing a dopant into the boundary and heavily dop ing the nearby region of the grain. If n -n or p -p junctions are formed, the space charge region associated with the boundary is shrunk and minority carriers are no longer pulled to the grain boundary (by the field associated with the space charge region) 38

3+

2+

3+

3+

3+

2+

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3+

3+

3

41

+

+

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE INTERFACES

CELL VOLTAGE

Figure 7. Current-voltage characteristics of an n-GaAs/0.8M K Se-0.1M K Se 1M KOH/C cell made with a chemically vapor-deposited, thinfilmof n-GaAs on W-coated graphite with Ru ^—) and with Ru * and Pb ( ) chemisorbed on the grain boundaries. The grains are of 9-μπι average size. The GaAs layer is 20 μτη thick. 2

3

3

2+

Figure 8. Charge collection scanning electron micrographs for a gold-polycrystalline n-GaAs junction (a) before and (b) after Ru * treatment. The darker the area the less the recombination. 3

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

2

2

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4.

HELLER

73

Surface and Grain Boundary Recombination

CB

CB

VB

VB

"N

-A-Ru

-A-Ru

CB

CB

VB

VB

-A-Ru -B

n-GaAs

Figure 9.

ρ - GaAs

3+

Model for splitting of surface states upon chemisorption of Ru and p-GaAs. 3+

on vi

Surface states before Ru chemisorption (top) and after (bottom) are shown for n-GaAs (left) and p-GaAs fright). Note that the splitting of the electron trapping surface states decreases the density of State A, potentially accessible to the majority carrier in n-GaAs, but not in p-GaAs, where the recombination controlling State Β is a hole trap.

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

υ

υ

-

CB η

η

+

η

π

η

Figure 10, Heavy (η ) doping of η-type semiconductors or heavy (p ) doping of p-type semicon ductors near grain boundaries introduce barriers that reduce transport of minority carriers to the boundaries

+

r~u—"JJ—υ

£

ce

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4.

HELLER

Surface and Grain Boundary Recombination

from the bulk of the grain. The high-low doping also produces a reflecting barrier to minority carrier diffusion (Fig. 10). Heavy doping has been proposed earlier ' ' as a means to reduce the recombination velocity at grain boundaries and Di Stefano and Cuomo , who phosphorus doped p-Si grain boundaries, were suc cessful in reducing recombination. In retrospect, we also explain as being due to the formation of n-n junctions the improvement in the conversion efficiency of solar cells made with hot-pressed, polycrystalline n-CdSe upon diffusion of cadmium metal, and η-type dopant, into the boun daries. The effectiveness of the doping approach is limited by the residual thermal diffusion of minority carriers to grain boundaries if n -n or p -p junctions are formed. Such a loss does not exist if the grain boundary recombination velocity is reduced by the strong chemisorption process proposed in this paper. With this method single crystal efficiency is approached in polycrystalline films made of small grained semiconductors. Indeed, we reach two thirds of the efficiency of single crystal n-GaAs cells with 20μπι thick chemically deposited films of n-GaAs on graphite. We would have reached a still higher fraction of the single crystal efficiency had we been able to achieve a non-reflective surface by etching wavelength sized hillocks into the film surface, as we did in single crystal GaAs. Based on our observations, we have reason to believe that sin gle crystal performance will be approached in future thin film, po lycrystalline semiconductor based solar cells with grain boundary recombination velocities reduced by strongly chemisorbed species. ACKNOWLEDGMENTS Extensive and enjoyable discussions with Harry J. Leamy and Barry Miller are acknowledged. The author also thanks H. D. Hagstrum and J. C. Philips for reviewing the manuscript. 10 15 16

42

+

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43

+

+

3840

30

31

REFERENCES (1) Many, Α . , Goldstein, Y . and Grover, Ν . B., "Semiconduc tor Surfaces", 1965, North Holland Publishing Co., Amsterdam. (2) S. R. Morrison, "The Chemical Physics of Surfaces", 1977, Plenum Press, Ν . Y . (3) Brattain, W. H . , and Bardeen, J., Bell System Journal, 1952, 32, 1.

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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(4) Pastrzebski, L . , Gatos, H . C., and Lagowski, J . , J. Appl. Phys. 1977, 48, 1730. (5) Moore, A . R., and Nelson, H . , R C A Rev., 1956, 17, 5. (6) Hogarth, C. Α . , Proc. Phys. Soc. (London), 1956, B69, 791. (7) Petrusevich, V. Α . , Fiz. tver. Tela, 1959, 1, 1695. (8) Chambouleyron, I. and Saucedo, E . , Sol. Energy Mater. 1979, 1, 299. (9) Michel, J. and Lasnier, M . F., Proc. Intnl. Conf. on Pho tovoltaic Solar Energy, Luxembourg, 1977, p. 125. (10) Sah, C. T., and Lindholm, F. Α . , IEEE 12th Photovoltaic Specialists Conf., Baton Rouge, L A , 1976, p. 93. (11) Law, J. T., J. Appl. Phys., 1961, 32, 600. (12) Law, J. T., J. Appl. Phys., 1961, 32, 848. (13) Law, J. T., "Semiconductor Surfaces", in Zemel, J. N . , Editor, Pergamon Press, Oxford, 1960, p. 9. (14) Seager, C. H . , and Ginley, D. S., Appl. Phys. Lett. 1979, 34, 337. (15) P. A . Iles, Workshop Proceedings on Photovoltaic Conversion of Solar Energy and Terrestrial Application, Cherry Hill, October 1973, p. 71. (16) S. I. Soclof and P. A . Iles, Proc. 11th IEEE Photovoltaic Specialists Conference, May 6-8, 1975, p. 56. (17) Casey, H . C., Jr. and Buehler, E . , Appl. Phys. Lett. 1977, 30, 247. (18) Suzuki, T., and Ogawa, M . , Appl. Phys. Lett. 1979, 34, 447. (19) L . L . Vereikina, Khim. Svoistva Metody. Anal. Tugo plavkikh Sodein, Edited by G . V. Samsonov, Naukova Dumka, Kiev, 1969, p. 37. (20) Stringfellow, G . B., J. Vac. Sci. and Technol. 1976, 13, 908. (21) Jastrzebski, L . , Lagowski, J., and Gatos, H . C., Appl. Phys. Lett. 1975, 27, 537. (22) Jastrzebski, L . , Gatos, H . C. and Lagowski, J. J. Appl. Phys., 1977, 48, 1730. (23) Hoffman, C. Α . , Jarasiunas, K . , Gerritsen, H . J., and Nurmikko, Α. V., Appl. Phys. Lett., 1978, 33, (24) Skountzos, P. Α . , and Euthyimiou, 1977, 48, 430. (25) Thurmond, C. D., Schwartz, G . P., Schwartz, B., J. Electrochem. Soc., 1980, 127,

536. P. C., J. Appl. Phys., Kamlott, G . W., and 1366.

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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Surface and Grain Boundary Recombination

(26) Schwartz, G . P., Griffiths, J. E . , DiStefano, D., Gualteri, G. J., and Schwartz, B., Appl. Phys. Lett., 1979, 34, 742. (27) Watanabe, K . , Hashiba, M . , Hirohata, Y . , Nishino, M . , and Yamashina, T., Thin Solid Films, 1979, 56, 63. (28) Schwartz, G . P., Griffiths, J. E . , and Schwartz, B., J. Vac. Sci. Technol., 1979, 16, 1383. (29) Skountzos, P. Α . , and Euthymiou, P. C., J. Appl. Phys., 1977, 48, 430. (30) Parkinson, Β. Α., Heller, A . and Miller, B., Appl. Phys. Lett., 1978, 33, 521. (31) Heller, Α . , Parkinson, Β. Α . , and Miller, B., Proc. 12th IEEE Photovoltaic Specialists Conf., Washington, June 5-8, 1978, p. 1253. (32) Parkinson, Β. Α . , Heller, Α . , and Miller, B., J. Electro chem. Soc., 979, 126, 954. (33) Nelson, R. J., Williams, J. S., Leamy, H . J., Miller, B., Parkinson, Β. Α . , and Heller, Α . , Appl. Phys. Lett., 1980, 36, 76. (34) Thrush, E. J., Selway, P. R., and Henshall, G . D., Elec tron. Lett. (GB), 1979, 15, 156. (35) Rowe, J. E . , private communication. (36) Woodall, J., Pettit, G . D., Chappell, T., and Hovel, H . J., J. Vac. Sci. Tech., 1979, 16, 1389. (37) Chang, K. C., Heller, A . and Miller, B., Science, 1977, 196, 1097. (38) Heller, Α . , Lewerenz, H . J., and Miller, B., Ber. Bunsengesellschaft, Phys. Chem., 1980, 00, 0000. (39) Johnston, W. D., Jr., Leamy, H . J., Parkinson, Β. Α . , Heller, Α . , and Miller, B., J. Electrochem. Soc., 1980, 127, 90. (40) Heller, Α . , Miller, B., Chu, S. S., and Lee, Y. T., J. Am. Chem. Soc., 1979, 101, 7633. (41) L . Pauling, "The Nature of the Chemical Bond", 3rd Edi tion, Cornell University Press, Ithaca, 1960, pp. 417-9. (42) DiStefano, T. H . , and Cuomo, J. J., Appl. Phys. Lett., 1977, 30, 351. (43) Miller, B., Heller, Α . , Robbins, M . , Menezes, S., Chang, K. C., and Thomson, J. Jr., J. Electrochem. Soc., 1977, 124, 1019. Received October 3, 1980.

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

5 Charge-Transfer Processes in Photoelectrochemical Cells D. S. GINLEY and M . A. BUTLER

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Sandia National Laboratories, Albuquerque, NM 87185

The promise of photoelectrochemical devices of both the photovoltaic and chemical producing variety has been discussed and reviewed extensively.(1,2,3,4) The c r i t e r i a that these cells must meet with respect to s t a b i l i t y , band gap and flatband poten tial have been modeled effectively and i n a systematic fashion. However, it is becoming clear that though such models accurately describe the general features of the device, as i n the case of solid state Schottky barrier solar c e l l s , the detailed nature of the interfacial properties can play an overriding role i n deter mining the device properties. Some of these interface proper ties and processes and their potential deleterious or beneficial effects on electrode performance w i l l be discussed. Due to the chemical potential difference for species i n the electrolyte and the photoelectrode, and by virtue of the fact that the electrode can be run i n forward and reverse bias con figurations, a number of important processes at the interface can be discerned. In each case, we w i l l be concerned with the energy required for the process under consideration to occur and i t s resulting effects on photoelectrode performance. We can think of these processes as being of four basic types: chemi sorption, the desired electron or hole charge transfer, surface decomposition and electrochemical ion injection. In the rest of the paper we w i l l briefly summarize our present understanding of each. Chemisorption Chemisorption is the process by which various ions are ad sorbed on the semiconductor surface with the formation of a chemical bond and can affect a number of important c e l l param eters. As has been previously discussed,(5,6) the amount of band bending i n the depletion region determines the amount of external bias (V ) needed for chemically producing PECs (photoelectro chemical cells) or the open-circuit potential (V ) for wet bais

oc

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p h o t o v o l t a i c c e l l s . Figure 1 i l l u s t r a t e s the energy l e v e l d i a gram f o r a t y p i c a l PEC. Under e q u i l i b r i u m c o n d i t i o n s and no i l l u m i n a t i o n , the Fermi l e v e l i n the semiconductor should e q u i l i brate a t the redox p o t e n t i a l i n the e l e c t r o l y t e . Therefore, the amount of band bending i s determined by the d i f f e r e n c e between the redox l e v e l i n the e l e c t r o l y t e , j > <* flatband p o t e n t i a l of the semiconductor, V ^ . The f l a t b a n d p o t e n t i a l i s a measure of the semiconductor e l e c t r o n a f f i n i t y r e l a t i v e to a reference e l e c t r o d e i n the e l e c t r o l y t e . I t i n c l u d e s not only the i n t r i n s i c e l e c t r o n a f f i n i t y which can be c a l c u l a t e d using e l e c t r o n e g a t i v i t y arguments(_3) but a l s o the p o t e n t i a l drop across any d i p o l e l a y e r t h a t may e x i s t at the i n t e r f a c e . Since a d i p o l e l a y e r can a r i s e due to cheraisorption of i o n s , the f l a t b a n d p o t e n t i a l and t h e r e f o r e V\ or V*. ^ w i l l be determined to some oc bias extent by the chemisorption process. A cut and p o l i s h e d semiconductor surface g e n e r a l l y contains a l a r g e amount of disorder and a s i g n i f i c a n t d e n s i t y of defect s t a t e s . The e l e c t r o c h e m i c a l p o t e n t i a l f o r an i o n adsorbed on t h i s surface i s g e n e r a l l y q u i t e d i f f e r e n t from that of the same i o n i n the e l e c t r o l y t e . I f the f r e e energy of the adsorbed i o n i s s i g n i f i c a n t l y more negative than the h y d r a t i o n energy of the i o n s , they w i l l be adsorbed. Consequently, there i s a strong i n t e r a c t i o n of those ions with the surface. I f the chemisorpt i o n energy i s d i f f e r e n t f o r the anion and the c a t i o n , one w i l l be p r e f e r e n t i a l l y adsorbed g i v i n g r i s e to a net surface charge. A compensating charge l a y e r w i l l e x i s t i n the e l e c t r o l y t e r e s u l t ing i n a c a p a c i t o r l i k e s t r u c t u r e . The p o t e n t i a l drop across t h i s d i p o l e l a y e r i s given by AV = Q/C where Q i s the net adsorbed charge and C the capacitance of the d i p o l e l a y e r . Since at e q u i l i b r i u m the e l e c t r o c h e m i c a l p o t e n t i a l of the adsorbed ions must equal the e l e c t r o c h e m i c a l p o t e n t i a l of the same ions i n s o l u t i o n and s i n c e the e l e c t r o c h e m i c a l p o t e n t i a l of a species i n a given phase depends on i t s a c t i v i t y , we can change the number of adsorbed anions and c a t i o n s by a l t e r i n g t h e i r concent r a t i o n i n s o l u t i o n . This w i l l r e s u l t i n a change i n the net adsorbed charge and give r i s e to the Nernstian dependence of on i o n c o n c e n t r a t i o n : E

an

r e <

t n e

o x

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r

V

fb

= V

(PZZP) - * i I n — - — fb zF a

K L

p z z p

where (PZZP) i s the i n t r i n s i c f l a t b a n d p o t e n t i a l determined by the semiconductor e l e c t r o n a f f i n i t y , " z " i s the i o n charge F i s Faraday's constant and "a" the a c t i v i t y . C l e a r l y , a t some p o i n t the number of adsorbed p o s i t i v e and negative charges w i l l be equal and the p o t e n t i a l drop across the Helmholtz l a y e r w i l l be e f f e c t i v e l y zero. Only the small d i p o l a r c o n t r i b u t i o n of adsorbed water or other n e u t r a l molecules w i l l provide a potent i a l gradient. At t h i s p o i n t , the Point of Zero Zeta P o t e n t i a l

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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GINLEY AND BUTLER

Charge-Transfer Processes

Figure 1. Energy level diagram for a photoelectrochemical cell illustrating the relationship between the electron affinity (EA) and the flatband potential (V/&). Energy levels are shown for zero external bias (E = 4.75 eV). 0

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(PZZP), the f l a t b a n d p o t e n t i a l i s a d i r e c t measure of the semi conductor e l e c t r o n a f f i n i t y (EA). This p o i n t may be l o c a t e d a n a l y t i c a l l y by v i r t u e of the f a c t that a semiconductor powder i n a s o l u t i o n of adsorbing ions a c t s as a b u f f e r f o r those ions everywhere but at the PZZP. Thus, p o t e n t i a l d r i f t or d i f f e r e n t i a l potentiometric titrations(7^,8^) can be employed to determine the PZZP as i l l u s t r a t e d i n Figure 2 f o r CdS. Once the PZZP i s determined i n t h i s f a s h i o n , a d i r e c t comparison of EA and i s p o s s i b l e and has been done f o r a v a r i e t y of semiconductors. (9^) Figure 3 i l l u s t r a t e s the · pH data f o r p-GaP and shows good agreement between the p r e d i c t e d fb e l e c t r o n e g a t i v i t y c a l c u l a t i o n s and the observed value.(9) The p o t e n t i a l drop across the Helmholtz l a y e r i s thus impor tant to the b i a s i n g requirements of a PEC and i s d i r e c t l y a f f e c t e d by the chemisorption of ions at the semiconductor/ electrolyte interface. The chemisorption of ions plays another r o l e i n determining the p r o p e r t i e s of PEC d e v i c e s . The adsorbed ions may create the chemical intermediates or s p e c i f i c r e a c t i o n s i t e s necessary f o r charge t r a n s f e r and chemical product formation. The r a p i d k i n e t i c s i n p h o t o e l e c t r o l y s i s and wet p h o t o v o l t a i c c e l l s are i n no small part due to the f a c t that i n most of these systems the redox species are s t r o n g l y adsorbed. K n o t e k ( 1 0 , l l ) and others (12,13) have shown that the nature of the T i 0 surface and the species adsorbed on i t g r e a t l y a f f e c t i t s c a t a l y t i c p r o p e r t i e s . We have observed that p-GaP seems to be an e f f e c t i v e hydrogen e l e c t r o d e because of the adsorbed aqueous species on the surface as r e f l e c t e d by the N e r n s t i a n dependence of on pH i n Figure 3. Thus, i t appears i n many cases the chemisorption of ions a t the i n t e r f a c e i s a necessary step f o r r a p i d charge t r a n s f e r to occur. This c l e a r l y has a strong bearing on recent observations of Fermi l e v e l pinning i n v a r i o u s PEC devices where strong chemi s o r p t i o n does not occur.(14,15) The i n f l u e n c e of chemisorbed ions i s thus seen i n the poten t i a l drop across the Helmholtz l a y e r and i n the c a t a l y t i c a b i l i t y of the surface. Considerable work remains to be done on the chemical pretreatment of surfaces to maximize the c a t a l y t i c nature of the surface and enhance the adsorption of appropriate ionic species. ν δ

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V

a t

t t i e

P Z Z P

f r o m

2

Surface Decomposition While i t i s r e l a t e d to and sometimes dependent upon chemi s o r p t i o n e f f e c t s , one pathway f o r e l e c t r o d e decomposition can be looked upon as a separate type of charge t r a n s f e r phenomenon. Here, charge t r a n s f e r g e n e r a l l y occurs between species i n the semiconductor near the surface and not across the i n t e r f a c e to species i n the e l e c t r o l y t e . This charge t r a n s f e r can be h i g h l y

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

Charge-Transfer Processes

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GiNLEY AND BUTLER

Figure 2.

=

Determination of the S concentration at the PZZP of the CdS anode.

(top) Data taken by the pX drift technique. In particular, we plot grams of CdS added to a 600 mL-sulfide solution of known initial pS and 0.1 M KOH vs. measured pS . Points on continuous lines represent successive additions to the same solution, (bottom) Three differential potentiometric titrations of a suspended amount of CdS, 1, 5, 10 gm in 600 mL 0.1 M KOH with Na _S solution. The range of change of pS with added titrant, dpS /dS, is plotted vs. pS=. It is of note that the same peak for the PZZP is observed in both curves and that dpS /dx scales inversely with the amount of added CdS. =

=

=

2

=

=

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE INTERFACES

+ 1.0

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+ 0. 5 h

0. o h

Figure 3. Experimentalflatbandpotential for p-GaP vs. pH ( ) has a slope of 59 mV/pH unit; (O) calculated V at the measured PZZP; (A) this work; (%) G. Horowitz (26)) / 6

ρ 1R

t

decomp ox red , R

, R

ox red

, R

1

p decomp

b

Figure 4. Relative positions of hole decomposition potentials for the semicon ductor and desired redox potentials for a photoelectrochemical cell using xv-type semiconducting electrodes. All examples are thermodynamically unstable, but (b) is kinetically more stable than (a).

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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a c t i v a t e d by the a d s o r p t i o n of ions on the surface since hydra t i o n energies play a n important r o l e i n e l e c t r o d e d i s s o l u t i o n . However, the fundamental decomposition pathway i s bond breaking i n the semiconductor s u r f a c e . G e r i s c h e r ( 1 6 ) , Bard and Wrighton(17) have r e c e n t l y discussed a simple model f o r the thermodynamic s t a b i l i t y of a range of photoelectrodes. As has been discussed p r e v i o u s l y , except f o r the r a r e case where the anodic and cathodic decomposition poten t i a l s l i e outside the band gap, the e l e c t r o d e w i l l be i n t r i n s i c a l l y unstable a n o d i c a l l y , c a t h o d i c a l l y , o r both.(16) I t i s the r e l a t i v e o v e r p o t e n t i a l of the redox r e a c t i o n of i n t e r e s t compared to that of the appropriate decomposition p o t e n t i a l which determines the r e l a t i v e k i n e t i c s and thus s t a b i l i t y of the e l e c t r o d e as i l l u s t r a t e d i n Figure 4. The cathodic and anodic decomposition p o t e n t i a l s may be roughly estimated by thermody namic free-energy c a l c u l a t i o n s but these numbers may not be t r u l y r e p r e s e n t a t i v e due to the m e d i a t i o n of surface e f f e c t s . Once again the p r e c i s e nature of the i n t e r f a c e plays a c r u c i a l r o l e i n determining the decomposition products observed. This i s c l e a r l y i l l u s t r a t e d i n the case of p-GaP as shown i n Figure 5. I n a 0.1 M NaOH s o l u t i o n decomposition i s by Ga l o s s from the surface as the hydroxide, and photocurrent decay i s r a p i d as a phosphate l a y e r b u i l d s o n the s u r f a c e . I n 0.1 M ΗβΡΟ^ s o l u t i o n decay i s a g a i n r a p i d but i n t h i s case the Ga i s not s o l u b i l i z e d but deposits on the surface as the metal. I n the more o x i d i z i n g acids such as H2SO4 both Ga and Ρ are removed from the surface and the photocurrent remains high as the surface i s e s s e n t i a l l y photoetched. While desorption i s o p e r a t i v e to some extent at most semi c o n d u c t o r / e l e c t r o l y t e i n t e r f a c e s , i n some cases surface p a s s i v a t i o n can occur. Here charge t r a n s f e r across the i n t e r f a c e i s used to e s t a b l i s h covalent bonds with e l e c t r o l y t e s p e c i e s , which r e s u l t s i n changes i n surface composition. This i s t y p i f i e d by the well-known oxide f i l m growth on n-GaP and n-GaAs surfaces i n aqueous s o l u t i o n s . I n these cases, however, the p a s s i v a t i o n process can be competed with e f f e c t i v e l y by the use of high con c e n t r a t i o n s of other redox species such as the polychalcogenides. Charge Transfer We have thus f a r t a l k e d about the chemisorption of ions a t the s e m i c o n d u c t o r / e l e c t r o l y t e i n t e r f a c e and charge t r a n s f e r i n the semiconductor surface l a y e r . The main charge t r a n s f e r pro cess of i n t e r e s t i s the t r a n s f e r of e l e c t r o n s and holes across the s e m i c o n d u c t o r / e l e c t r o l y t e i n t e r f a c e to the d e s i r e d e l e c t r o l y t e species r e s u l t i n g i n t h e i r o x i d a t i o n or r e d u c t i o n . For any semiconductor, e l e c t r o d e charge t r a n s f e r can occur w i t h o r without i l l u m i n a t i o n and with the j u n c t i o n biased i n the forward or reverse d i r e c t i o n .

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE INTERFACES

1000

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τ—ι—ι—ι—ι

2000

3000

1

1

4000 1—

Coulombs

Figure 5. Photocurrent for p-GaP illuminated with white light as a function of total charge passed through the interface. Note the different scales for different electrolytes.

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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One of the major questions that remains to be answered i s the d e t a i l e d mechanism of charge t r a n s f e r . For redox couples which l i e i n the gap of the semiconductor, i s o e n e r g e t i c e l e c t r o n t r a n s f e r would r e q u i r e the existence of a n appropriate surface s t a t e . While such s t a t e s have been p o s t u l a t e d , l i t t l e d i r e c t evidence of t h e i r e x i s t e n c e i s a v a i l a b l e . An a l t e r n a t e p o s s i b i l i t y i s an i n e l a s t i c (non-isoenergetic) e l e c t r o n t r a n s f e r process such as i s commonly observed i n s o l i d s t a t e devices.(18) As i n d i c a t e d i n Figure 1, i f a semiconductor i s biased to d e p l e t i o n i n contact w i t h a n e l e c t r o l y t e , a photocurrent can be generated upon i l l u m i n a t i o n . This occurs because the photoe x c i t e d m a j o r i t y c a r r i e r s are d r i v e n by the e l e c t r i c f i e l d i n the d e p l e t i o n l a y e r to the counter e l e c t r o d e and m i n o r i t y c a r r i ers migrate to the i n t e r f a c e where they are trapped at the band edge. Nozik has r e c e n t l y speculated that hot m i n o r i t y c a r r i e r i n j e c t i o n may play a r o l e i n supra-band edge r e a c t i o n s . ( 1 9 ) Here the t r a n s f e r time of the hot c a r r i e r s i s balanced against t h e i r t h e r m a l i z a t i o n time constant. I n many PEC systems the chemical k i n e t i c s f o r the primary charge t r a n s f e r process at the i n t e r f a c e are not observed at the l i g h t i n t e n s i t i e s of i n t e r e s t f o r p r a c t i c a l devices and the i n t e r f a c e can be modeled as a Schottky b a r r i e r . This i s true because the inherent o v e r p o t e n t i a l , the energy d i f f e r e n c e between where m i n o r i t y c a r r i e r s are trapped at the band edge and the l o c a t i o n of the a p p r o p r i a t e redox p o t e n t i a l i n the e l e c t r o l y t e , d r i v e s the r e a c t i o n of i n t e r e s t . The Schottky b a r r i e r assumption breaks down near zero b i a s where the e f f e c t s of i n t e r face s t a t e s o r surface recombination become more important.(13) The p r e c i s e nature of the i n t e r a c t i o n of the p o t e n t i a l determining species w i t h the semiconductor surface w i l l determine the apparent redox p o t e n t i a l s i n the e l e c t r o l y t e . K i n e t i c s would be a n t i c i p a t e d to be of importance i n those cases where there i s not strong chemisorption of the p o t e n t i a l determining s p e c i e s . This i s the case f o r CdS i n a c i d i c or b a s i c aqueous s o l u t i o n where photocurrents are nonlinear at l o w - l i g h t i n t e n s i t i e s and the dependence of V o n pH i s non-Nernstian.(20) Recent observat i o n s by Bard and Wrighton(14,15) i n d i c a t e that Fermi l e v e l pinning and t h e r e f o r e supra-band edge charge t r a n s f e r can occur i n S i and GaAs i n those systems ( i . e . , O^CN/t n-Bu^N]C10^) w i t h v a r i o u s redox couples where l i t t l e e l e c t r o l y t e i n t e r a c t i o n i s anticipated. That surface i n t e r a c t i o n s play such a r o l e c l e a r l y demands that some s o r t o f surface s t a t e concept be invoked. However, no simple techniques have y i e l d e d d i r e c t i n f o r m a t i o n about the nature of such s t a t e s . To e x p l a i n charge t r a n s f e r , i s o e n e r g e t i c e l e c t r o n or hole processes are normally invoked w i t h a subsequent t h e r m a l i z a t i o n of the e l e c t r o n or hole i n the semiconductor. This u n f o r t u n a t e l y n e c e s s i t a t e s the e x i s t e n c e of a surface s t a t e at the l e v e l of the redox p o t e n t i a l . This may of n e c e s s i t y occur when strong chemisorption i s present. However, i n those cases, f b

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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where t h i s i s not the case, i n e l a s t i c e l e c t r o n t r a n s f e r may occur. Such charge t r a n s f e r processes at s o l i d s t a t e i n t e r f a c e s are w e l l known( 18) and i n v o l v e the e x c i t a t i o n of v i b r a t i o n a l modes, d i s c r e t e e l e c t r o n i c t r a n s i t i o n s and c o l l e c t i v e e x c i t a t i o n s such as surface plasraons. I t seems v e r y p l a u s i b l e that s i m i l a r processes should occur at the s e m i c o n d u c t o r / e l e c t r o l y t e i n t e r face. One way to probe the e x i s t e n c e of such surface s t a t e s i s to u t i l i z e another charge t r a n s f e r phenomenon, t h a t of photoemission. (21) Here e l e c t r o n s or holes may be i n j e c t e d d i r e c t l y i n t o the e l e c t r o l y t e from an i l l u m i n a t e d , b i a s e d e l e c t r o d e as i l l u s t r a t e d f o r e l e c t r o n photoemission i n Figure 6. Though photoemission from a semiconductor d i r e c t l y i n t o an aqueous e l e c t r o l y t e has yet to be observed conclusively(22^), the tech nique shows considerable promise as a s u r f c e s e n s i t i v e probe f o r PECs. Figure 7 i l l u s t r a t e s the y i e l d curves f o r a WOg e l e c t r o d e i n 1 Ν - H2S0/ .(23) A p o s i t i v e photocurrent i s observed u n t i l a s u f f i c i e n t l y negative p o t e n t i a l i s achieved so that hydrogen bronze formation occurs at the e l e c t r o d e surface. At t h i s point e l e c t r o n photoemission occurs. The nature of the p o s i t i v e photo current i s not known. Threshold data, as shown i n the i n s e r t i o n Figure 7, i n d i c a t e that i t i s true photoemission being observed since the thresholds s h i f t l i n e a r l y with p o t e n t i a l and agree w i t h those observed by others f o r photoemission from metal e l e c t r o d e s . ( 2 3 ) The peak i n the y i e l d curve at 3.5 eV i s i n t e r e s t i n g and unexpected and may be due to r e f l e c t i v i t y changes or m o d i f i c a t i o n s i n the f i n a l d e n s i t y of s t a t e s . Hole photoemission may a l s o occur i n a p p r o p r i a t e l y b i a s e d PECs, although t h i s process has not yet been observed. The major problem w i t h the o b s e r v a t i o n of photoemission from semi conducting e l e c t r o d e s i s i n t e r f e r e n c e from the much l a r g e r photoc u r r e n t s produced by the e x i s t e n c e of the Schottky b a r r i e r at the i n t e r f a c e . Thus hole or e l e c t r o n t r a n s f e r can f o l l o w a number of path ways across the s e m i c o n d u c t o r / e l e c t r o l y t e i n t e r f a c e . F i r s t , one can have d i r e c t o x i d a t i v e or r e d u c t i v e charge t r a n s f e r to s o l u t i o n species r e s u l t i n g i n d e s i r e d product formation. Second, one can have d i r e c t charge t r a n s f e r r e s u l t i n g i n surface m o d i f i c a t i o n , such as oxide f i l m growth on GaP or CdS i n aqueous PECs. F i n a l l y , one can have photoemission of e l e c t r o n s or holes d i r e c t l y i n t o the e l e c t r o l y t e . A l l of these processes provide some i n f o r m a t i o n about the e l e c t r o n i c s t r u c t u r e of the i n t e r f a c e . t

E l e c t r o m i g a t i o n of Ions We have discussed the e f f e c t s of chemisorption and of e l e c t r o n and hole t r a n s p o r t across the s e m i c o n d u c t o r / e l e c t r o l y t e i n t e r f a c e . These have been shown to play a l a r g e r o l e i n deter mining e l e c t r o d e p r o p e r t i e s . Recently another mechanism of

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

GiNLEY AND BUTLER

Charge-Transfer Processes

• Photoemission

threshold

+ -3

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+ -2

+ -1 H /H +

0

0H-/0

2

+ +1

+-2

++3

++4 n-Semlconductor

Figure 6.

1 Ν - H S0 2

4

The energy level structure for an n-semiconductor-electrolyte interface as is appropriate for electron photoemission.

The vertical axis is in volts relative to the saturated calomel reference electrode (SCE). Photoemission in aqueous media is best performed in the electrochemical window, be tween the hydrogen and oxygen evolution redox potentials, where small dc currents allow easier detection of the photoemission current. Protons are used for scavenging the photoexcited electrons.

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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5001

Il Ζ 5

1

I

3.0

1

I

3.5

1

1

I

I

4.0 4.5 Photon Energy (eV)

r

ι

I

5.0

5.5

Figure 7. Normalized photorespouses yield spectra for W0 in IN H SOi,. The data are taken in order of decreasing negative potential at the potentials indicated. A11 of the currents are cathodic. The insert shows Fowler plots used to determine the photoemission threshold at each potential. 3

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

2

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91

charge t r a n s p o r t across the i n t e r f a c e has been recognized and demonstrated to play a r o l e i n determining PEC c h a r a c t e r i s t i c s . (11,24) The f i e l d i n the d e p l e t i o n r e g i o n i s only ~1 v o l t but t h i s p o t e n t i a l i s normally dropped across a d e p l e t i o n l a y e r of <1 ym depth. This g i v e s r i s e to e l e c t r i c f i e l d s of 10 volts/cm and l a r g e r . These f i e l d s , which are l a r g e enough t o promote the movement of s m a l l charged i o n s , can e a s i l y be manipulated by the e x t e r n a l b i a s . This movement of ions can e f f e c t the s t a b i l i t y , f l a t b a n d and s p e c t r a l response of a semi conducting e l e c t r o d e . Figure 8 i l l u s t r a t e s the c r y s t a l s t r u c t u r e s f o r TiC^ and GaP, which are t y p i c a l examples of a η-type semiconducting oxide and a p-type main group semiconductor which show promise as photoe l e c t r o d e s . Both m a t e r i a l s have channels through the l a t t i c e perpendicular to the exposed f a c e . From a simple geometric argument a c r i t i c a l r a d i u s f o r ions that can f i t i n t o the chan n e l s has been determined f o r each m a t e r i a l . For r u t i l e t h i s i s R = 0.94 Â and f o r GaP i t i s R = 0.8 5 Â. These numbers t e l l us immediately that we w i l l only have to worry about the m i g r a t i o n of p o s i t i v e ions i n these and most other semiconductors since negative ions have too l a r g e a r a d i u s , the smallest being F~~ at 1.33 Â and 0~ a t 1.32 Â. There do e x i s t , however, a large number of p o s i t i v e l y charged ions w i t h an appropriate r a d i u s to f i t i n t o the channels. The i n f l u e n c e on anions a t the semiconductor surface can be l a r g e however and t h i s w i l l be i l l u s t r a t e d l a t e r f o r F~ on SrTiC^. Under normal operating c o n d i t i o n s , the f i e l d s i n the d e p l e t i o n r e g i o n would be expected to cause the m i g r a t i o n of p o s i t i v e ions f o r n- and p-type semiconductors as shown i n Figure 9. This c l e a r l y has l a r g e consequences f o r the defect doped metal oxides where i n t e r s t i t i a l metal atoms would be expected to be leached slowly from the surface changing the doping p r o f i l e . For TiOo and SrTiO^ t h i s i s i n f a c t e x a c t l y what i s observed. Figure 10 shows the r e s u l t s of the anodic aging of a l i g h t l y d o p e d SrTiO^ wafer on i t s s p e c t r a l response. As expected, the removal of i n t e r s t i t i a l T i ( R = .76 Â) a l t e r s the doping p r o f i l e and t h i s widens the d e p l e t i o n r e g i o n width a l l o w i n g f o r the c o l l e c t i o n of e l e c t r o n - h o l e p a i r s from photons with a longer wavelength and a deeper p e n e t r a t i o n depth. T h i s , i n e f f e c t , red s h i f t s the photoresponse. The doping p r o f i l e a f t e r anodic aging has been modeled by Butler(_5) and a t y p i c a l example i s shown i n Figure 11. A f t e r aging the TiC^ the r e s u l t a n t c o n c e n t r a t i o n gradient causes the room temperature m i g r a t i o n of T i i n t e r s t i a l s and over a period of one week the doping p r o f i l e i s r e s t o r e d to near that of the v i r g i n sample. For SrTiOg the a c t i v a t i o n energy f o r movement of the i n t e r s t i t i a l s i s considerably l a r g e r and the a n o d i c a l l y aged p r o f i l e i s r e l a t i v e l y s t a b l e at room temperature. At ~300°C and above the i n t e r s t i t i a l s can move more f r e e l y . Since only small changes i n sample s t o i c h i o m e t r y <1 ppm occur no changes i n c

Q

3 +

C

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE INTERFACES

Θ Θ

φ

(ft

Θ

Θ

0

Θ Θ

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F/gwre 5.

Θ

Θ Θ

C001D

Θ

Θ

ClI0D

RUTILE

ZINC

Θ BLENDE

Crystal structures for Ti0 (rutile) (leftj and GaP (zinc blende) (right). The critical radii for these two are 0.94 À and 0.85 A, respectively. 2

Anodic bias

η-Semi conductor depletion

Cathodic b i a s

n-Seniconductor accumulation

Figure 9. The electric fields and directions of positive ion electromigration in the near-surface region of both n- and p-type semiconductors under anodic and cathodic bias

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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GiNLEY AND BUTLER

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Charge-Transfer Processes

Figure 10. Electrochemical anodic aging experiments on a lightly doped SrTiO wafer. All experiments were performed in 1M NaOH. ((1) virgin SrTiO ; (2) anodically aged 21 h at 5 V vs. SCE; (3) annealed in air 16 hat 300° C) s

s

350 WAVELENGTH (nm)

0.2

0.3 Θ.4 DEPTH Cpm)

0.5

0.6

Figure 11. The measured defect density profile in 3 Ω-cm Ti0 sample aged under constant voltage (-{-5 V) for 30 h. The data is normalized to the bulk density. 2

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

94

PHOTOEFFECTS

AT SEMICONDUCTOR-ELECTROLYTE

INTERFACES

an

^fb P d d none are observed as shown i n the top h a l f of Table I. While enhanced r e d response i s c l e a r l y u s e f u l i n these p h o t o e l e c t r o d e s , the consequences i n terras of o v e r a l l e l e c t r o d e s t a b i l i t y may outweigh any advantage gained. C l e a r l y s u b s t i t u t i o n a l doping would avert these problems and may be a p r e f e r a b l e technique. a

r

e

e x

e c t e

Table 1 DEPENDENCE OF V'fb ON AGING CONDITIONS FOR SrTiOo fU

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Bias

Illumination

vs SCE

Conditions Virgin

-1.11

Ano die

On

+5 V v s SCE PC* 21 h r s .

-1.15

Anodic

On

+1 V v s SCE PC 4 days

-1.15

Virgin

-1.17

Cat ho die

Off

.2 mA-CC** 25 h r s .

- .95

Cathodic

Off

.2 mA-CC 21 h r s .

- .96

*PC - P o t e n t i a l C o n t r o l **CC - Current C o n t r o l The anodic and cathodic aging experiments were done consecutively.

For a p-type semiconductor operated i n the normal f a s h i o n o r for an η-type m a t e r i a l under c a t h o d i c b i a s (accumulation) i t i s p o s s i b l e to get p o s i t i v e i o n i n j e c t i o n i n t o the semiconductor surface. As we have p r e v i o u s l y discussed, the e f f e c t s of such i n j e c t i o n can be q u i t e dramatic i n c l u d i n g f r a c t u r e of the e l e c trode surface as i l l u s t r a t e d i n Figure 12. I n aqueous media proton i n j e c t i o n i s most l i k e l y to occur and can have pronounced e f f e c t s on e l e c t r o d e performance. Figure 13 i l l u s t r a t e s the changes i n the s p e c t r a l response of a SrTiO^ e l e c t r o d e that has had H i n j e c t e d i n t o the near-surface r e g i o n by cathodic aging. The shape o f the s p e c t r a l response curve remains b a s i c a l l y unchanged as a consequence of the f a c t that the d e p l e t i o n l a y e r w i d t h i s r e l a t i v e l y unchanged. The o v e r a l l quantum e f f i c i e n c y i s s i g n i f i c a n t l y reduced, which suggests that the i n j e c t e d H s i g n i f i c a n t l y i n c r e a s e s recombination center d e n s i t i e s . Since +

+

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

GiNLEY AND BUTLER

95

Charge-Transfer Processes

Figure 12. Photograph (4χ magnifica tion) of an electrochemically deuteriumdoped rutile wafer. Doping was accom plished by current-controlled cathodic aging an undoped TiO wafer in LiOD, D 0 for 3 days at 10 mA. This resulted in a shattering of the sample in the region exposed to the electrolyte.

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5.

z

2

Figure 13. Electrochemical cathodic ag ing experiments under current-controlled conditions (ce) on a lightly doped SrTi0 wafer. All experiments were performed in 1M NaOH. ((1) virgin SrTiO ; (2) cathodically aged 0.2 mA-cc for 25 h; (3) 0.0Vvs.SCEfor27h) 3

300

350 WAVELENGTH (nm)

400

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

s

96

PHOTOEFFECTS AT

SEMICONDUCTOR-ELECTROLYTE INTERFACES

the amount of hydrogen i n j e c t e d i s l a r g e , the e l e c t r o n a f f i n i t y of the surface region should change and t h e r e f o r e changes would be expected i n V ^ . The lower p o r t i o n of Table I i l l u s t r a t e s as a f u n c t i o n of c a t h o d i c aging. The l a r g e change i n i t i a l l y represents a s a t u r a t i o n of a v a i l a b l e s i t e s . The subsequent l a c k of change i s an i n d i c a t i o n that a d d i t i o n a l hydrogen penetrates f u r t h e r and that the c o n c e n t r a t i o n of hydrogen cannot go beyond the o r i g i n a l s a t u r a t e d l e v e l . We can use the e l e c t r o n e g a t i v i t y model(7^j8) to q u a n t i f y the amount of hydogen i n c o r p o r a t e d . The bulk e l e c t r o n e g a t i v i t y f o r hydrogenated SrTiO^ w i l l be: x

3

x

1 /

X(SrTi0 H ) = [X(Sr)X(Ti)X (0)X (H)] (

5 + x

3

>

(2)

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From the measured f l a t b a n d p o t e n t i a l and the relationship(_24) EA = V rb •+· Ε ο + Δ,re + à px

x

f K

(3) '

where E i s the d i f f e r e n c e between the vacuum l e v e l and the SCE, i s a s m a l l c o r r e c t i o n term f o r the d i f f e r e n c e between the Fermi l e v e l and the conduction band and Δ i s the p o t e n t i a l drop across the Helmholtz l a y e r due to adsorbed charge, we can c a l c u l a t e the m a t e r i a l s apparent e l e c t r o n a f f i n i t y . We know as well that Q

X

EA(mat) = ( n i a t

b u l k

) - 1/2 Ε (mat)

(4)

thus EA(observed) =

3

x

[X(Sr)X(Ti)X (0)X (H)]

1 / ( 5 + x )

- 1/2 E ( S r T i 0 ) g

3

(5)

Since Eg remains unchanged from the v i r g i n v a l u e , ( F i g . 13) sub s t i t u t i n g we f i n d X equals 0.2 or there i s one hydrogen f o r every f i v e t i t a n i u m s . This r a t i o i s a l s o found f o r T1O2 where i t i s s u b s t a n t i a t e d by s t i m u l a t e d d e s o r p t i o n experiments.(11) The hydrogen occupies a number of d i f f e r e n t s i t e s both hydroxyl and hydride as observed by i n f r a r e d spectroscopy.(11) As curve three i n Figure 13 shows, anodic aging of the sample removes a l a r g e p o r t i o n of the hydrogen a s s o c i a t e d with recombination c e n t e r s . But as the f l a t b a n d i n d i c a t e s and surface measurements show, a s i g n i f i c a n t p o r t i o n of the hydrogen cannot be removed i n t h i s fashion. An i n t e r e s t i n g e f f e c t has been observed i n p-GaP.(25) I f the cathodic breakdown regime i s a t t a i n e d (10 V or greater vs SCE) i n the dark at cathodic c u r r e n t s of 10 mA or g r e a t e r , luminescence i s observed from the e l e c t r o d e . This luminescence i s a s s o c i a t e d w i t h the i n j e c t i o n of ions i n t o the semiconductor. The lumines cence i s broad band and occurs both above and below the band gap as i l l u s t r a t e d i n Figure 14a. Table I I i l l u s t r a t e s the depen-

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

5.

GINLEY AND BUTLER

97

Charge-Transfer Processes

586

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(a)

3.5

Ι.Θ

1.5

2.6

2.5

3.8

P h o t o n E n e r g y CeNO

lb)

redox

Ε Iectr©I y t e

p-GoP

Figure 14. (a) Spectral distribution of the luminescence at a reverse-biased pGaP/electrolyte interface. The electrolyte is 0.15M HNO and the current density flowing through the interface is ~ 20 mA/cm . The low-energy limit of the spec trum is determined by the photomultiplier sensitivity, (b) Strongly cathodically biased p-GaP/electrolyte interface. Hot electrons are created by tunneling from valence to conduction bands. These may decay radioactively to fill empty states created by cation infection or drive other redox reactions. s

2

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

98

PHOTOEFFECTS

AT

SEMICONDUCTOR-ELECTROLYTE

INTERFACES

dence of the luminescence on the i o n s i z e . C l e a r l y ions of r a d i u s greater than R cannot be i n j e c t e d and thus do not cause luminescence. Ion i n j e c t i o n has been confirmed w i t h forward s c a t t e r i n g experiments which have detected L i i n j e c t i o n to depths of g r e a t e r than 0.5 microns.(25) A p o s s i b l e luminescence mechanism i s i l l u s t r a t e d i n Figure 14b. I n j e c t e d c a t i o n s create vacant e l e c t r o n i c s t a t e s below the conduction band of the semiconductor which may be f i l l e d by r a d i a t i v e decay of hot e l e c t r o n s i n j e c t e d i n t o the conduction band. c

Table 2

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EFFECTS OF VARIOUS IONS ON THE LUMINESCENCE IN p-GaP

Ion

Crystal Ionic R a d i i (Â)

Steady State Luminescence Yes**

—

4

Mg "*

0.66

Yes

Li+

0.68

Yes

0.74

Yes

4

Zn "*

R (GaP) = 0.85 c

Na

+

4

4

Cd " " K

+

NH

+ 4

Â

0.97

No

0.97

No

1.33

No

1.43

Yes

*Handbook of Chemistry and P h y s i c s , ed. by C. D. Hodgman (Chem. Rubber. Co., Cleveland, 1962) pg. 3507. **0nly at concentrations l e s s than 0.5 M.

Anions are nominally too l a r g e to be i n j e c t e d down channels i n the semiconductor. However, under strong anodic bias there may be e l e c t r o c h e m i c a l i n t e r a c t i o n s of the anions with the semiconductor surface ( p o s s i b l y by an i o n exchange mechanism). Figure 15 shows the s p e c t r a l response and I-V c h a r a c t e r i s t i c s f o r a SrTiO^ e l e c t r o d e i n 1 Μ KOH a f t e r being aged f o r two days a t +5 V vs SCE i n 1 M KF. These e l e c t r o d e p r o p e r t i e s appear constant with respect to operation i n a PEC. The quantum e f f i ciency a t 0 V vs SCE i s s u b s t a n t i a l l y improved as i s the c o l l e c -

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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GiNLEY AND BUTLER

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Charge-Transfer Processes

wavelength (nm)

Figure 15. (top) Spectral response curves for a reduced SrTi0 electrode before and after aging in F" solution, (bottom,) I-V data for the same aging conditions. The I-V and spectral response curves were obtained in 0.7M NaOH, and the electrode was aged in 1M KF for 2 days at 3

potential vs SCE

+5

V vs.

SCE.

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE

INTERFACES

t i o n e f f i c i e n c y near the f l a t b a n d p o t e n t i a l s This increase i n the f i l l f a c t o r and enhancement of surface charge t r a n s f e r may be i n d i c a t i v e of improved e l e c t r o d e k i n e t i c s or the e l i m i n a t i o n of surface recombination. Work i s c u r r e n t l y i n progress to determine which process i s important.

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Summary Four main charge t r a n s f e r phenomena appear important i n PECs. Cheraisorption of ions creates a p o t e n t i a l drop across the Helmholtz double l a y e r and provides chemical intermediates which can s i g n i f i c a n t l y a l t e r the k i n e t i c s of the r e a c t i o n . Surface bond breaking i s a potent means of e l e c t r o d e degradation. E l e c t r o n and hole t r a n s p o r t across the s e m i c o n d u c t o r / e l e c t r o l y t e i n t e r f a c e may be e l a s t i c or i n e l a s t i c and give r i s e to c l a s s i c a l e l e c t r o c h e m i s t r y . Photoemission may a l s o occur g i v i n g r i s e t o n o n c l a s s i c a l products and can be used as a u s e f u l probe of the i n t e r f a c i a l e l e c t r o n i c s t r u c t u r e . E l e c t r o m i g r a t i o n of ions can change doping p r o f i l e s and be used by way of e l e c t r o c h e m i c a l i o n i n j e c t i o n to modify e l e c t r o d e surface and near-surface r e g i o n s . Much remains to be understood concerning the d e t a i l s of these processes. Such an understanding i s v i t a l t o the s u c c e s s f u l production of an optimum photoelectrode. *This work was supported by the M a t e r i a l s Sciences D i v i s i o n , O f f i c e of Basic Energy Sciences, U. S. Department of Energy under Contract DE-AC04-76-DP00789.

Literature Cited 1.

2. 3. 4. 5. 6.

7. 8. 9.

Gerischer, H. i n "Physical Chemistry: An Advanced Treatise," Eds. Eyring, Η . , Henderson, D . , Jost, W. (Academic Press, New York, 1970) Vol. 9A. Nozik, A. J.; Ann. Rev. Phys. Chem., 1978, 29, 189. Butler, M. A. and Ginley, D. S.; J. Mat. S c i . , 1980, 15, 1. Gerrard, W. A. and Rouse, L . M . ; J. Vac. Sci. Technol., 1978, 15, 1155. Butler, Μ. Α . ; J. Electrochem. Soc., 1979, 126, 338. Schwerzel, R. E.; Brooman, E. W.; Craig, R. Α.; V. Ε. Wood in "Semiconductor Liquid-Junction Sola Cells," edited by Heller, A. (Electrochemical Society, Princeton, 1977) p. 293. Butler, M. A. and Ginley, D. S.; J. Electrochem. Soc., 1978, 125, 228. Ginley, D. S. and Butler, Μ. Α . ; J. Electrochem. Soc., 1978, 125, 1968. Butler, M. A. and Ginley, D. S.; J. Electrochem. Soc., (June 1980).

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

101

5. GINLEY AND BUTLER Charge-Transfer Processes 10. 11. 12. 13.

14.

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15. 16. 17. 18. 19.

20. 21. 22. 23. 24. 25. 26.

Knotek, M. L.; Abstract 339, p. 869, The Electrochemical Society Extended Astracts, Philadelphia, PA, May 8-13, 1977. Ginley, D. S. and Knotek, M. L.; J. Electrochem. Soc., 1979, 126, 2163. Wilson, R. H . ; Abstract 415, p. 1038, The Electrochemical Society Extended Abstracts, Seattle, WA, May 21-26, 1978. Somorjai, G. Α.; Abstract, Solar Energy and Photoelectronic Processes Symposium, p. 420, 109th AIME Annual meeting, Las Vegas, NV, February 24-28, 1980. Bocarsly, A. B . ; Bookbinder, D. C . ; Dorainey, R. N . ; Lewis, N. S.; Wrighton, M. S.; J. Amer. Chem. Soc., 1980, 102, 0000. Bard, A. J.; Bocarsly, A. B.; Fan, F. F . ; Walton, E. G . ; Wrighton, M. S.; J. Amer. Chem. Soc., 1980, 102, 0000. Gerischer, H . ; J. Electroanal. Chem., 1977, 82, 133. A. J. Bard and M. S. Wrighton; J. Electrochem. Soc., 1977, 124, 1706. Duke, C. B.; "Tunneling in Solids," Academic Press, New York 1969. Turner, J. and Nozik, A. J.; Abstract 84, C o l l . and Surface Science Division, American Chemical Society Abstracts, Houston, TX, March 23-28, 1980. Watanabe, T . ; Fujishima, Α.; Honda, Α.; Chem. L e t t . , 1975, 897. Gerischer, H . ; Kolb, D. M.; Sass, J. K . ; Adv. i n Phys., 1978, 27, 437. Gurevich, Y. Y . ; Krotova, M. D . ; Pleskov, Y. V . ; J. Electroanal. Chem., 1977, 75, 339. Butler, Μ. Α . ; Surface Science, i n press. Butler, M. A. and Ginley, D. S.; Chem. Phys. L e t t . , 1977, 47, 319. Butler, M. A. and Ginley, D. S.; Appl. Phys. L e t t . , 1980, 36, 845. Horowitz, G.; J. Appl. Phys., 1978,

49,

3571.

Received October 15, 1980.

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

6 The Role of Interface States in Electron-Transfer Processes at Photoexcited Semiconductor Electrodes R. H . WILSON

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Corporate Research and Development, General Electric Company, P.O. Box 8, Schenectady, NY 12301

Electronic energy levels localized at the surface of a semiconductor have frequently been used to explain, experimentally observed currents at semiconductor-electrolyte junctions. These surface or interface states are invoked when the observations are inconsistent with direct electron transfer between the conduction or valence band of the semiconductor and electronic states of an electrolyte in contact with the semiconductor. Charge carriers in the semiconductor bands are transferred to surface states that have energies within the bandgap of the semiconductor. From these states electrons can move isoenergetically across the interface to or from electrolyte states in accordance with the widely accepted view of electron transfer. The process by which the semiconductor carriers reach the surface to react with surface states must be considered. The case of greatest importance under photoexcitation is with the semiconductor biased to depletion as shown in Figure 1. While it is possible for semiconductor carriers to reach the surface of the semiconductor through tunneling, or impurity conduction processes, these processes have not been shown to be important in most examples of photoexcited semiconductor electrodes. Consequently, these processes will be ignored here in favor of the normal transport of carriers in the semiconductor bands. Furthermore, only carriers within a few kT of the band edges will be considered, i.e., "hot" carriers will be ignored. Figure 1 illustrates different modes of electron transfer between electrolyte states and carriers in the bands at the semiconductor surface. If the overlap between the electrolyte levels and the semiconductor bands is insufficient to allow direct, isoenergetic electron transfer, then an inelastic, energy-dissipating process must be used to explain experi mentally observed electron transfer. Duke has argued that a complete theory for electron transfer includes terms that allow direct, inelastic processes. The probability of such processes, however, has not been treated quantitatively. On the other hand, inelastic transfer of carriers in the bands to surface states is well known and reaction rates sufficient to 1-9

10

11,12,13

0097-6156/81/0146-0103$05.00/0 © 1981 American Chemical Society In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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AT

SEMICONDUCTOR-ELECTROLYTE

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Figure 1. Schematic of various electron transfer processes between semiconductor carriers at the surface and electrolyte and surface states

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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explain many of the experimental observations can be readily estimated. The objective of this paper is to focus on this reaction between carriers and surface states and to emphasize the reaction cross section as an important parameter in charge transfer as well as in surface recombin ation. The role of surface states in charge transfer at the semiconductorelectrolyte interface is contrasted with effects at semiconductor-vacuum, semiconductor-insulator and semiconductor-metal interfaces. Factors affecting the magnitude of the reaction cross section are discussed and the theoretical understanding of the capture process is briefly reviewed. Finally, some experimental observations are discussed in which charge transfer to surface states is important. The emphasis is on methods to be quantitative in describing the role of surface states by determining their density and reaction cross sections. Some previously published observations as well as preliminary new results are used to illustrate the role of surface bound species as charge transfer surface states. Quantitative Reaction Rates Localized states in the bulk of a semiconductor that have energies within the bandgap are known to capture mobile carriers from the conduction and valence bands.— The bulk reaction rate is determined by the product of the carrier density, density of empty states, the thermal velocity of the carriers and the cross-section for carrier capture. These same concepts are applied to reactions at semiconductor surfaces that have localized energy levels within the bandgap.— — In that case the electron flux to the surface, F , reacting with a surface state is given by 1

n

F

n

N

v

n= s eO"n n

<*>

where Ν is the density of electrons in the conduction band at the surface, Ν isthe area density of empty surface states, CT is the electron capture cross section of the surface states and ν is the thermal velocity of electrons. This is clearly an oversimplified description of the capture process which hides many of the complexities in the phenomenological parameter, CT. Nevertheless, this approach has been usefully applied in describing the kinetics of semiconductor surface states in a variety of circumstances. For holes near the edge of the valence band with density, ρ , at the surface the analogous expression for the hole flux to the surface, F , reacting with a filled surface state of area density, N , is s

p

r

F

r^ = P c f N

<^ n V

(

2

)

ρ *s 1 Ρ ρ By including terms to describe the rate of emission of electrons and holes from filled and empty surface states respectively, constraining the total number of surface states to by N + N = and assuming that the states do not interact, the electron-hole recombination at. the.surface has been analyzed— — in analogy with H a l l - S h o c k l e y - R e a d ^ * ^ recombin ation. These methods are important in the study of semiconductorx

s

e

2

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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insulator interfaces.— Similar rgethods are applied to the kinetics at semiconductor-gas interfaces.^— When the semiconductor surface is coupled through a capacitance small compared with the semiconductor space charge capacitance (semiconductor-gas, semiconductor-vacuum, metal-thick insulator-semiconductor) the surface potential is very sensi tive to the net change in surface states. (In the standardized terminology for metal-insulator-semicor^uctor structures this charge is referred to as interface trapped charge).— In the case of a metal-semiconductor junction the semiconductor surface is closely coupled to the metal. As a result electrons in the conduction band at the surface see a high density of empty metal states into which they can cross the interface isoenergetically. Similarly, valence band holes see a high density of filled electron states in the metal. As a result, electron transfer is usually treated as direct transfer in a thermionic process and surface states are2^5^ated to a minor role in surface generation and recombination effects.— — By contrast, electrolyte states are much more limited in their distribution than metal conduction band states so that in many cases electron transfer through surface states may be the dominant process in semiconductor-electrolyte junctions. On the other hand, in contrast to vacuum and insulators, liquid electrolytes allow substantial interaction at the interface. Ionic currents flow, adsorption and desorption take place, solvent molecules fluctuate around ions and reactants and products diffuse to and from the surface. The reactions and kinetics of these processes must be considered in analyzing the behavior of surface states at the semiconductor-electrolyte junction. Thus, at the semiconductorelectrolyte junction, surface states can interact strongly with the electro lyte but from the point of view of the semiconductor the reaction of surface states with the semiconductor carriers should still be describable by equations 1 and 2. In the next section factors that affect the reaction cross section are discussed. It is argued that electrolyte species on the surface of the semiconductor can qualify as surface states. In the subsequent section several examples of such surface states will be discussed. 1

Factors Affecting Cross Section As a hole or electron moves through a semiconductor its cross section for encountering neutral impurity atom should be an atomic dimension, about 10" cm . The probability that the carrier be captured during that encounter, however, may be much less than one, depending on the energy of the available impurity level and the quantum state of the carrier and impurity levels. When the capture occurs an amount of energy equal to the difference between the band edge and the impurity level is lost by the electron. This energy must be carried away by photon and/or phonon emission. The maximum phonon energy in the available phonon spectrum of the semiconductor crystal is generally less than one-tenth of an electron-volt. Consequently, for radiationless capture (no photon emission) of a carrier by a deep level more than a tenth of an electronvolt from the band, emission of many phonons is required. In that case

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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107

the capture probability per enççunte^ might be quite low so that cross sections on the order of 10" cm are expected^ Observed cross sections, however, are frequently on the order of 10" cm . (For a more complete discussion see Ref. 14, p. 91.) Theory to explain such large cross-j sections is not well established. The cascade capture model of Lax has had some success but a multiphonon emission process is more appropriate for deep levels.— A similar situation exists for carrier capture by surface states. Relatively large capture cross sections are observed but no adequate theoretical treatment exists. Theory to describe the capture process is greatly complicated by presence of the surface. The carrier motion as well as the vibrational behavior of the crystal is perturbed by the surface. What does seem clear, however, is that the surface state should be tightly enough bound to the crystal lattice so that phonon emission is possible. In addition, the state should be close enough to the semiconductor to overlap the wave function of the semiconductor carrier. As a working definition, a surface state can be any electron energy level within the bandgap of the semiconductor located at its surface that is coupled to the semiconductor lattice strongly enough to allow inelastic capture of carriers from the semiconductor bands. Several examples of possible surface states are illustrated in Figure 2. In the next section experimental manifestations of some of these are described. 1

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?

Experimental Examples Sulfide ions on CdS. When Na2S is added to an aqueous electrolyte the flatband voltage of a CdS electrodes shifts to more negative-potentials by an amount linear in the log of the Na^S concentration.— The change in flatband voltage is due to a change in voltage across the Helmholtz layer. The Nernstian dependence on concentration suggests that some form of negative sulfide ion is bound to the semiconductor surface by a chemical reaction. An estimate of the sulfide ion concentra tion on the surface has been made using a simple capacitive model of the Helmholtz layer.— The result is shown in Figure 3 based on the flatband voltage measurements of Inoue et al.— In addition, when their currentvoltage data are replotted relative to the flatband voltage as shown in Figure 4, significant effects in addition to the flatband voltage changes are evident. The photocurrent onset is shifted to more negative poten tials than observed in the absence of sulfide. The role of the sulfide ions can be interpreted in several ways. One interpretation presumes that the photocurrent onset in the absence of sulfide is determined by electron-hole recombination. The sulfide ions on the surface are then supposed to be bound to these surface recombination levels rendering them unavilable for recombination reac tions. The charge transfer reactions could then proceed at lower voltages. In this case the corrosion suppression role of the sulfide ions would be to reduce the oxidized corrosion site before a cadmium ion could go into solution. A variation on this theme is to consider the corrosion site to be the recombination state, i.e., the site on the surface that normally leads to corrosion when oxidized by a photoexcited hole can be

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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PHOTOEFFECTS

CB.

AT

/ / / / J ^ \

SEMICONDUCTOR-ELECTROLYTE

INTERFACES

C.B. r9

V.B.

C.B.^^t1 V.B.

Figure 2.

V.B.

Illustration of various kinds of surface states that can react with carriers at the surface of the semiconductors.

(a) an intrinsic surface state of the semiconductor that may be due to lattice termination or a dangling bond. Subsequent reactions of such a state may lead to corrosion, (b) A specfically adsorbed electrolyte species. After reacting with a carrier it may desorb. (c) An intermediate in a multielectron reaction. After first hole capture, for example, the intermediate product may form a surface state capable of a second hole capture with subsequent release of the final product to solution, (d) A molcule attached to the surface by chemical treatment prior to immersing in the electrolyte. Such a surface state could, for example, be oxidized by a hole and subsequently be reduced from the electrolyte.

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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NooS ADDED

109

(MOLES/LITER) Journal of the Electrochemical Society

Figure 3.

Surface density Ν of adsorbed sulfide ions calculated from the change in flatband potential V reported in Ref. 26 (21). / b

Journal of the Electrochemical Society

Figure 4. Current-voltage curves of a CdS electrode in 0.2M Na^O^ from Ref. 26. Figure 10 replotted relative to flatband voltage. (1) No Na S added; (2) 5 X 10' M Na S; (3) 10' M Na S; (4) 10' M Na S; (5) lO'M Na S (21). 4

2

3

2

2

2

2

2

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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reduced by an electron from the conduction band before the next step in the corrosion process takes place. In the absence of sulfide this recombination would be the dominant process at low voltages when the surface concentration of electrons is high. When sulfide is present in the electrolyte the sulfide ions could deposit on the corrosion sites and prevent their oxidation thereby simultaneously suppressing recombination and allowing current onset at lower voltages. Another interpretation would be to suppose that the adsorbed sulfide ion forms a surface state that can be directly oxidized by a hole in the valence band. In this case the shift in current onset to lower voltages would be due to an increase in the charge transfer rate rather than the decrease in the recombination rate discussed in the preceeding paragraph. The corrosion suppression associated with the sulfide could then be partially attributed to the rapid kinetics of hole capture by these surface sulfide ions and partially due to reduction of oxidized corrosion sites by sulfide ions in solution. It is not the objective here to choose between these alternatives. Additional experimental work is needed to do this. The objective of this work is to show how both the corrosion sites and adsorbed sulfide ions can be treated as surface states and to further suggest that quantitative treatment of these surface states can be useful in considering experi ments to elucidate the role of the sulfide ions. First, by referring to Figure 4· it can be seen that change of more than two orders of magnitude in sulfide concentration in solution causes only a small change in the photocurrent onset voltage. This is readily understood from Figure 3 if the adsorbed sulfide ions rather than the solutio^ ions are responsible for the change in current onset. Thus, about 3x10 sulfide ions per square centimeter are responsible for the shift in photocurrent onset. Since this number is only a few percent of the available atomic sites on the surface it would be surprising if that few sulfide ions could passivate all the corrosion sites. On the other hand, if the adsorbed sulfide ions act as surface states for direct, inelastic capture of valence band holes their surface reaction constant for holes is S =N ( Γ ν >(3 χ 10 cm" ) (10"° cm ) (1θ' cm/sec) = 3x10^ cm/sec on tne assvPmiftion that the coulomb attraction of the negatively charge sulfide ions fot the positive holes would make the cross section greater than 10 cm . If the corrosion sites are treated as surface states on the CdS (as seems logical since the cadmium atoms that eventually receive the holes are part of the semiconductor surface) the first step corrosion rate constant for holes, S , may be analyzed in a similar fashion S =N C7^ v,. To explain the shift in current onset.£«Sgis required. Assuming TSJ =iÎ0^ cm"", S <10 cm/sec if

9 Z

Z

c

c

<

c

c

c

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

WILSON

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Interface States in Electron-Transfer Processes

Some additional experiments relevant to the data in Figure 4 are suggested by the preceding discussion. In particular, if the corrosion sites also act as the recombination centers that control current onset in the absence of sulfide ions (as discussed earlier) then there are no oxidized recombination centers before exposure to light. In that case a CdS electrode biased at a voltage below the saturated portion of curve 1 in Figure k would show a higher initial current than indicated in curve 1 and then decay in time to the curve 1 value. This situation can be analyzed by: dN -j^ce

=pN fj'vρ - ce Ν k ^"cf C cpT vρ- n sN ce <7~ en νη - sρ Ν ce cp v

H

v

lN

(3) v

/

where N is the surface density of oxidized corrosion sites that have captured one hole, Ν , is the density of corrosion sites that are still filled (N x+N =N , the total density of corrosion sites) and k is a rate constant for the reaction of the oxidized corrosion site with the electrolyte. The net photo current, j , associated with these processes is

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c e

c

ce

c

r

j c = p Ν ^ ν +p Ν <7"' ν - η Ν < Γ ~ *s cf cp ρ s ce cp p s ce cn n r

ν

F

1N

v

n

iN

v

(4) v

^

7

When an n-CdS electrode is suddenly illuminated with light capable of producing holes in the CdS, j , would almost immediately reach some large value (equal to or less than the saturation current of curve 1) and then decay to the steady state value of curve 1 as the steady state value of Ν is approached according to equation 3. If such a transient does not occuf^the oxidized corrosion site acting as a recombination state is not the controlling factor in the photocurrent onset. Another quantitative consideration involves the rate of replacement of the sulfide ions on the surface during illumination assuming that they are the states with which the holers react. The rate at which sulfide ions reach the CdS surface for £ χ 1 0 " M concentration can be estimated to be greater than 10 i% \ *^f * 2 of Figure 4 the first plateau occurs at j / q - 10 cm" sec" suggesting that the value of Ν is not limited by the replacement of the sulfide ions. The plateau in curve 2 has been quantitatively attributed to a change in su^ide ion concentration at the surface due to a diffusion-limited process.— If that is the case capacitance measurements made under the operating conditions of the plateau in curve 2 would indicate a reversion of the flatband voltage to that of curve 1 with no sulfide in solution. If such a shift is observed then operation in the plateau area does not help elucidate the mechanism of charge transfer since the same diffusion limitation occurs for surface ion or solution ion transfer. The objective of the preceding discussion of CdS electrodes has been to point out the possible role of two types of identifiable surface states: one a state intrinsic to the CdS surface, presumably associated with Cd, which is eventually ionized by reaction with a hole and dissolved into solution; the other is a sulfide ion deposited on the CdS surface from solution. By considering these as surface states capable of inelastic c m

s e c

wn

o rc u r v e

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capture of carriers from the semiconductor bands quantitative estimates of their role in charge transfer were made to illustrate the usefulness of this approach. 28 Oxygen evolution on TiO . In a previous report— the dark reduction peak (curve 3 in Figure 5) produced by photoexciting a TiO^ electrode was discussed. The extra dark current was analyzed using equation & and ?

n = n exp- q(V-V )/kT s

b

fb

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where n, is the bulk electron density and V.^ is the flatband voltage. The resulting current during a negative voltage sweep at a rate dV/dt is

where N. is the area density of oxidized surface states before the sweep was initiated and V is the voltage at which the current, j, is maximum. The cross section, ^P, for electron capture by these states is given by

All of the parameters except N. and cr are determined by other observations. N. is determined by tne area under the curve and cr by the position of the cfurrent peak. Comparison of experimental and calculated curves are shown in Figure 6. A better fit could be achieved by assuming a distribution of cross sections, but that did not seem justified. More experimental work is needed but the surface state analysis fits the available experimental observations in all essential details with reason able values for N. and <Γ. As discussed previously, the surface states responsible for the reduction peak could be intrinsic surface states or states associated with a surface-attached intermediate in the series of reactions leading to evolution. The latter possibility was deemed to be more likely since no change in voltage across the Helmholtz layer (no change in capacitance) was observed when these states are in the oxidized form. Regardless of the nature of the surface state it is clear that it can capture an electron from the conduction band producing cathodic current. This cathodic current balances the anodic current produced when the photoexcited holes produced the oxidized surface state. The net result of these two processes is electron-hole recombination leading to no net current. This recombination process is what controls the voltage of photocurrent onset as can be seen in curve 2 of Figure 5. The results summarized here illustrate the important role surface states play in O 2 evolution from photoexcited T1O2 and provide an example of a quantitative determination of the density and electron capture cross section of these states. Attached molecules. A molecule attached to a semiconductor surface could behave as a surface state if it has an energy level within the

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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40

I (fio)

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20

-0.6 V

S C E

-0.4 -0.2 (VOLTS)

Journal of the Electrochemical Society

Figure 5. Current-voltage curves of a Ti0 electrode in 1 .OM KOH with 200 mV/s scan from 0 V (SCE). Curve 1, in dark; Curve 2, with 350-nm light on; Curve 3, in dark after 30 s illumination with 350-nm light atOV (SCE) (28). 2

ν ^

(VOLTS) (b ) +0.2

+0.4

V

Journal of the Electrochemical Society

-); ((analogous Figure 6. Comparison of extra experimental reduction current to the difference in Curves 1 and 3 in Figure 5) with calculated (( ) using Equation 5 in (a) 1.0M KOH and (b) 0.5M H SO using N i = 6.7 Χ 10 cm' and 1.9 Χ 10 cm' , respectively with peak positions matched to experimental curves 13

2

13

h

2

(2S).

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bandgap. The cross section for carrier capture would depend on the nature of the attachment process, the strength of the bond and the distance from the surface. Figure 7 shows some preliminary results obtained from ^ T i O electrode to which Ru(4-OHL 3 - C O O H o - p h e n X b i p y y P F ^ ^ was attached using a silane process.— The curves are analogous to Tigure 5 except the photoexcitation prior to curve 3 in Figure 7 was with 470nm light. This wavelength light does not produce electron hole pairs in T i C ^ but is at the maximum of absorption of the ruthenium compound. The photocurrent is presumed to be due to transfer of an electron from the excited state of the attached material to the conduction band of the TiC>2 leaving behind the oxidized compound. The current decreases in time indicating that the reduced form is not being regenerated. The reduction peaks in curve 3 of Figure 7 are presumed to be due to the transfer of an electron from the conduction band to the oxidized compound. After the voltage sweep of curve 3 the current reverts to curve 1 and the 470 nm photocurrent reverts to its original value. The process is repeatable and delays of several minutes between photoexcita tion and voltage sweep show little change in curve 3. Peaks at about -0.21V. and -0.5^V can be distinguished. Using -0.2 V/sec scan rate, 10 cm" for n^, 10 cm/senior v^ and -0.7V i§ equation 6 gives cross sections of 5 χ 10" cm and «2 χ 10^ cm respectively for those peaks. Each yields about 3 χ 10 cm" for its surface density. It should be emphasized that these results are preliminary and subject to modification as further checks and measurements are made. They are included here to illustrate how a surface attached molecule can be treated quantitatively as a surface state.

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Q

Conclusions There is a growing tendency to invoke surface states to explain electron transfer at semiconductor-electrolyte interfaces. Too frequently the discussion of surface states is qualitative with no attempt to make quantitative estimates of the rate of surface state reactions or to measure any of the properties of these surface states. This article summarizes earlier work in which charge transfer at the semiconductorelectrolyte interface is analyzed as inelastic capture by surface states of charge carriers in the semiconductor bands at the surface. This approach is shown to be capable of explaining the experimental results within the context of established semiconductor behavior without tunneling or i m purity conduction in the bandgap. Methods for measuring the density and cross section of surface states in different circumstances are discussed. A working definition of a surface state as any energy level within the bandgap that is bound to the surface sufficiently to allow inelastic electron transfer to or from the semiconductor bonds was introduced. This allows adsorbed electrolyte species, reaction intermediates and attached layers to be considered as surface states. The experimental observations discussed illustrate such states.

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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WILSON

1

Interface States in Electron-Transfer Processes

Figure 7. Current-voltage curves for a Ru(4-OH,3-COOH,o-phen)(bipy) (PF ) coated Ti0 electrode in pH 7 Na SO . Current curves analogous to Curves 1 and 3 in Figure 5 except the illumination was at -\-0.5 V and with 470 nm light. 2

2

2

k

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While the ability to treat capture cross sections theoretically is very primitive and the experimental data on capture cross sections are very limited this phenomenological parameter seems to be an appropriate meeting place for experiment and theory. More work in both of these areas is needed to characterize and understand the important role of surface states in electron transfer at semiconductor-electrolyte inter faces. Acknowledgements

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The author gratefully acknowledges many helpful discussions with LA Harris and the TiO- coating work of PA Piacente. This work was supported in part by the Department of Energy Office of Basic Energy Sciences. LITERATURE CITED 1.

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Vandermolen, J., Gomes, WP, Cardon, F . , Investigation of the Kinetics of Electroreduction Processes at Dark ΤiO and SrTiO Single Crystal Semiconductor Electrodes, J. Electrochem. Soc., 127, 324, 1980. 2

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Skockley, W., and Read, WT, Jr., Statistics of the Recombinations of Holes and Electrons, Phys. Rev., 87, 835, 1952.

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Many, H., Relation Between Physical and Chemical Processes on Semiconductor Surfaces, CRC Critical Reviews in Solid State Scie nces, 4, 515, 1974.

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Balestra, C L , Lagowski, J . , and Gatos, HC, Determination of Surface State Parameters from Surface Photovoltage Transients, Surface Science, 64, 457, 1977.

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Lax, M., Cascade Capture of Electrons in Solids, Phys. Rev., 119, 1502, 1960.

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Henry, CH and Lang, DV, Nonradiative Capture and Recombination by Multiphonon Emission in GaAs and GaP, Phys. Rev. B, 15, 989, 1977.

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Inoue, T., Watanabe, T., Fujishima, Α., and Honda, K., Competitive Oxidation at Semiconductor Photoanodes, in Semiconductor Liquid-Junction Solar Cells, Heller, Α., Ed., The Electrochemical Society, Princeton, NJ, 1977, 210.

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Wilson, RH, Analysis of Charge Transfer at Photoexcited CdS Electrodes with Na S in the Electrolyte, J . Electrochem. Soc., 126, 1187, 1979.

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2

28.

Wilson, RH, Observation and Analysis of Surface States on TiO Electrodes in Aqueous Electrolytes, J . Electrochem. Soc., 127, 228, 1980.

29.

Piacente, PA, Determination of Ground and Excited State pK values of Several New Ru(II) Complexes, MS Thesis, Rensselaer Polytechnic Inst., Troy, NY, August 1979.

30.

Abruna, HD, Meyer, TJ, Murray, RW, Chemical and Electrochemical Properties of 2,2'-bipyridyl Complexes of Ruthenium Covalently Bound to Platinum Oxide Electrodes.

2

a

Received October 3, 1980.

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

7 Competing Photoelectrochemical Reactions on Semiconductor Electrodes W. P. GOMES, F. V A N OVERMEIRE, D. V A N M A E K E L B E R G H , and F. V A N D E N KERCHOVE Rijksuniversiteit Gent, Laboratorium voor Fysische Scheikunde, Krijgslaan 271, B-9000 Gent, Belgium F. CARDON

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Rijksuniversiteit Gent, Laboratorium voor Kristallograti en Studie van de Vaste Stof, Krijgslaan 271, B-9000 Gent, Belgium

1. Introduction As part of the research on solar energy conversion by photoelectrochemical methods, a considerable amount of investigation has been carried out during the last half decade on the stabili zation of illuminated η-type semiconductor electrodes by means of competing hole reactions with reducing agents added to the solution. However, very l i t t l e attention has hitherto been paid to the influence of the light intensity upon the relative rates of the competing reactions. Recently, it has been reported that the ratio of the rate of anodic photodissolution of the ZnO elec trode versus that of electrooxidation of dissolved halide ions is light-intensity dependent, and an interpretation has been given to this effect based upon the concept of the quasi-Fermi level (_1). Independently, we have also found that the stabiliza tion of n-GaP by Fe(CN)g" ion in aqueous medium is light-intensi ty dependent, and here, the effect has been interpreted in kine tic terms (2, 3). In the present work, we have extended the ex perimental cTata on light-intensity dependent stabilization of η-type III-V semiconductor electrodes, as well as the kinetic analysis of this phenomenon. 2. Experimental The measurements were made by means of the rotating ringdisk technique. The ring consisted of Au (for the determination of Fe(CN)3") or of amalgamated Au (for the determination of Fe(III)-EDTA). The disk consisted of either n-InP (N = 5 χ 10 cnr ) or n-GaP (N = 1.6 χ 10 cm" ). The characteristic di mensions of the RRDE were rj = 2.50 mm, = 2.65 mm and r^ = 2.95 mm (for InP) or r = 3.65 mm (for GaP). For both semicon ductors, the (III) face was exposed to the electrolyte. Before each experiment, the electrode surface was etched, the etchant being 0.3 % Br in CH0H for InP and 0.5 Μ KOH + 1 M K Fe(CN)

l f i iD

n

3

17

3

D

3

2

3

0097-6156/81/0146-0119$05.00/0 © 1981 American Chemical Society In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

3

6

120

PHOTOEFFECTS

INTERFACES

in H2O for GaP, respectively. The indifferent electrolyte con sisted of 0.25 M KoS0 plus buffer in H 0, the buffer being either phtalate (pH - 3.8) or borax (pH - 9 . 2 ) . Reducing agents added to the solution were either Fe(II)-EDTA (at pH = 3.8) or Fe(CN)$" (at pH = 9 . 2 ) . Three types of interfaces were studied, i.e. n-InP/Fe(II)-EDTA, n-GaP/Fe(II)-EDTA and n-GaP/Fe(CN)g". For the disk potential, a value within the saturation photocur rent region was chosen (0.0 V vs. the sulphate electrode for InP and 0.0 or + 1.0 V vs. the sulphate electrode for GaP). The elec trode was illuminated with a Hg lamp, and the Fe(III) complexes formed by photoelectrochemical oxidation were reduced at the ring (at - 1.4 V and - 0.80 V vs. the sulphate electrode for Fe(III)-EDTA and Fe(CN)3", respectively). By changing the rota tion velocity of the RRDE between 500 and 2000 rpm, it was certi fied that the effects further described were not due to ratelimiting transport of the reactants from the solution to the disk. The results were expressed in terms of the stabilization ra tio s, given by 4

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AT SEMICONDUCTOR-ELECTROLYTE

2

1

s

= v^

(i)

where Ιγ represents the current corresponding to the anodic oxi dation of the reducing agent Y (the Fe(II) complex), and Ip the total photocurrent measured at the disk. The value of Ιγ was de termined from the measured ring current increase Δΐ^ by means of the relationship n

Ι

γ

= - Δίβ/Ν

(2)

Ν being the collection efficiency of the RRDE. For the latter, the theoretical value was used, as it follows from the characte ristic dimensions of the RRDE. In one case (nGaP/Fe(CN)g~) where from independent measurements, the conditions are known under which s = 1, we were able to check that the experimental Ν value is in good agreement with the calculated one. All results which will be reported pertain to steady-state values of both I and s. At relatively high values of the ratio of photocurrent density vs. concentration at pH = 3 . 8 , an initial decrease of the photocurrent was observed, indicating the forma tion of some surface layer. Such cases have not been taken into consideration here. All results were obtained at room temperature and with the exclusion of oxygen. Rh

3. Results For the three systems considered, the stabilization ratio s was determined as a function of the total photocurrent density jp = I / A (A = surface area) and the concentration of reducing agent y. In all cases, s was found to increase with increasing n

pn

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

7.

Competing Photoelectrochemical Reactions

GOMES ET AL.

y and to decrease with increasing j . . shown in Figures 1 and 2.

121

Typical examples are

p

4. Discussion 4.1. Kinetic analysis. The observed light-intensity effect should permit a deeper insight into the mechanism of photoelec trochemical reactions. In this framework, a kinetic analysis of the stabilization process will now be made for an η-type compound semiconductor AB, which is being oxidized photoelectrochemically according to the over-all reaction equation

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AB + η h

+

—>

products

(3)

with η = 6 for GaP (4, b) and 6 < η < 8 for InP (6). The oxida tion of the dissolveïï reducing agent Y will be taken to be oneequivalent, i.e. γ +

+ h

—>

Ζ

(4)

and, for simplicity, the reverse step of reaction (4) will be assumed to be negligible. Reaction (3) must obviously be complex, and we will assume that in a first step, a hole arriving from the interior reacts at the semiconductor surface (AB) to form some surface interme diate according to

(AB)

k

+ +h

l

+

X. ,

(5)

-1 ki and k_^ being the forward and reverse rate constants respecti vely. It will be further assumed that the competing reactions with the reducing agent Y either occurs with holes coming from the bulk directly, according to . k n

Y+ h

—^>

Ζ

case (Η)

(6)

or with the intermediate, according to k" Y + Xj — ^ Ζ + (AB) case (X) (7) Reactions (6) and (7) are denoted as the cases (H) and (X) res pectively. We will use the symbol j to indicate the number of holes consumed in electrochemical reactions per unit of surface area and per second. Hence, i f e represents the elementary charge,

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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122

PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE INTERFACES

jph/μΑ cm-2

Figure 1. Stabilization ratio s as a function of the photocurrent density } at dif ferent concentrations of solved reducing agent y, n-InP/Fe(II)-EDTA (rotation speed of the electrode 1000 rpm) ph

0

50

100

150

200

j p h / μ A.cm-2 Faraday Discussions

Figure 2.

Plot of s vs. ]

ph

for different y, n-GaP/Fe(II)-EDTA (1) (rotation speed 1000 rpm)

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

7.

GOMES ET AL.

Competing Photoelectrochemical Reactions

123

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Assuming that Y is not adsorbed, the following expressions can then be written for s j , the fraction of j associated with the anodic oxidation of Y: - case (H) :

sj = k y ρ

- case (X) :

sj =

(9)

Q

y χ

χ

(10)

with ρ the free hole concentration at the electrode surface and x
Κι = k_i

+

k"i y

(11)

As we have assumed that the reactions (6) and (7) do not occur simultaneously, it follows that k^, Ξ k in the case (H). As for the case ( X ) , we will assume tnat the consumption of X^ pre dominantly occurs through reaction (7), so that here, k!!]y » k_^ and hence k^ - k"^y. We will now consider two possibilities for the nature of the intermediate Xj. The first possibility will be that Xj is localized, i.e. represents an electron missing in a well-deter mined surface bond; this case will be denoted by (L). The al ternative is that Xi is mobile within the surface, in a manner analogous to that of a free hole in the bulk. Energetically speaking, X^ might then belong to a surface energy band. This case will be denoted in what follows by the symbol (M). If one further takes into consideration the possibilities that reaction (5) may be either irreversible (I) or reversible (R), a series of different possible reaction mechanisms can now be discussed, constituting combinations of the alternatives described above. A survey of these mechanisms is given in Table I.

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

124

PHOTOEFFECTS

AT SEMICONDUCTOR-ELECTROLYTE

(AB) + h

Table I.

+

S

INTERFACES

^

Xj

Expressions of the stabilization (L) Xj localized:

ratio for different reaction me chanisms.

J

(H) Y + h —> Ζ

(I) (AB) + h -> Xj

+

+

s

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1

(R) +

Χ

S

(M)(H)(I)

s Φ f(j)

s /

(L)(H)(R)

(M)(H)(R)

f(j)

2

2

2

χ

(L)(X)

(X) Y + X ->Z + (AB) x

2

2

(L)(H)(I)

2

! (AB) + h

—» X

2Xj

+

Xj + h -> X

(M) X^ mobile:

s = f (},

S

(M)(X)

s

2

2

y

Case (L). It is logical to assume here that the further oxidation of an AB unit occurs through consecutive hole-capture steps, so that reaction (5) is followed by the steps x

+

1

h

X _ + h (1

X

+

x

+

1

2

products

1

(13)

which for simplicity will be taken to be irreversible. As a con sequence of the steady-state which is established, the formation rates of all η oxidation products of AB will be equal and hence each given by (l-s)j/n, since (l-s)j is the fraction of j corres ponding to the photodecomposition of AB. For the formation rate of Xj, one has: (l-s)j/n = k

- k^Xj

l P

;

(14)

for that of the higher intermediates: (l-s)jVn = k x p = 2

x

= ^χ _!Ρ η

(15)

Elimination of Xj from eqns. (14) and (15) leads to: k', k,p

η

I

k~p

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

(16)

7.

GOMES ET AL.

125

Competing Photoelectrochemical Reactions

In the case (L), the two alternatives (H) or (X) for the oxida tion of Y can be considered. For the alternative (H), distinc tion can further be made between the case where reaction (5) would be either irreversible (I) or reversible (R). This leads to the following possible mechanisms. - Mechanism Ο-^(Η)^Ι) : since here, Ξ = 0> combina tion of eqns. (?) and (16) leads to an expression of the type s/(l-s) * y

(17)

which is independent of j and hence of the light intensity. - Mechanism iL^(H)jR| assuming that the reverse reaction (5) is fast with respect to reaction (12), so that k'_j Ξ k_ » k^p, it follows from eqns. (9) and (16) that

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:

1

2

2

(li 1-s J - Mechanism jL]^(X) : we recall that we have postulated in this case that the reverse reaction (5) can be neglected with respect to reaction (7), so that kl^ - ku^y. By combination of eqns. (10), (15) and (16), one gets an expression of the type v

(19) Case (M). Let us now consider the alternative in which Xi stands for a mobile surface intermediate. In this case, we will postulate that the two-equivalent oxidation product X is formed by the encounter of two Xj species, that the subsequent oxidation reactions occur with X^ species and that all these reaction steps are irreversible. 2

2X,

— ^ k

X +X 2

X~

(20)

3 >

x

X

3

J

(21)

k +

X

r o d u c t s

22

V l l P ( ) (for simplicity, the same symbols for the rate constants have been used as for case (H)). Under steady-state conditions, the following rate equations can be written here:

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

126

PHOTOEFFECTS

j

= kjp - k

(l-s)j/n The

- 1

x

1

= k2xx2

AT SEMICONDUCTOR-ELECTROLYTE

INTERFACES

+ kQyp = k3x2x1

l a s t term i n e q n . (23) d i f f e r s

(23) =

= knxn_1x1

f r o m z e r o i n t h e c a s e (H)

(24) only.

Here a g a i n , t h e a l t e r n a t i v e s (H) and (X) w i l l be d i s c u s s e d , and i n t h e f o r m e r c a s e , d i s t i n c t i o n w i l l be made between t h e p o s s i b i l i t i e s of the f i r s t step i n the decomposition r e a c t i o n being either i r r e v e r s i b l e or r e v e r s i b l e . Hence, the f o l l o w i n g cases c a n be c o n s i d e r e d .

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- Mechanism | M J | ( H ) X I | : p u t t i n g k l i a k _ i = 0 i n e q n . ( 2 3 ) and c o m b i n i n g t h i s w i t h e q n . ( 9 ) , a r e l a t i o n s h i p o f t h e t y p e (17) is obtained. - Mechanism (M)(H)XR^ : h e r e we w i l l assume f o r s i m p l i c i t y t h a t a q u a s T - e q u i T i b r i u m e s t a b l i s h e s i n r e a c t i o n ( 5 ) , so t h a t k

l

p

=

k

- l x l

(25)

From e q n s . ( 9 ) , ( 2 4 ) and ( 2 5 ) t h e n f o l l o w s an e x p r e s s i o n f o r s o f the type o f r e l a t i o n s h i p ( 1 8 ) . - Mechanism X M ^ ( X ) : a r e l a t i o n s h i p o f t h e t y p e (18) i s o b t a i n e d f r o m t h e a p p r o p r i a t e e x p r e s s i o n s , i . e . e q n s . (10) and ( 2 4 ) . The r e s u l t s o f t h i s k i n e t i c a n a l y s i s have been i n c l u d e d i n Table I. I t c a n be seen t h a t , i f b o t h t h e a n o d i c d e c o m p o s i t i o n o f t h e s e m i c o n d u c t o r and t h e a n o d i c o x i d a t i o n o f t h e c o m p e t i n g r e a c t a n t w o u l d o c c u r by i r r e v e r s i b l e h o l e - c a p t u r e s t e p s ( ( L ) ( H ) ( I ) o r ( M ) ( H ) ( I ) ) , as was h i t h e r t o g e n e r a l l y a c c e p t e d , t h e s t a b i l i z a t i o n s h o u l d be i n d e p e n d e n t o f l i g h t i n t e n s i t y , i n c o n t r a d i c t i o n w i t h the r e s u l t s d e s c r i b e d above. The mechanism i n w h i c h t h e r e d u c i n g a g e n t r e a c t s by d o n a t i n g an e l e c t r o n t o a l o c a l i z e d s u r f a c e h o l e ( ( L ) ( X ) ) l e a d s t o an e x p r e s s i o n i n w h i c h s i s a f u n c t i o n of the v a r i a b l e ( y / j ) o n l y . The t h r e e o t h e r mechanisms c o n s i d e r e d l e a d t o t h e r e l a t i o n s h i p o f t h e type ( 1 8 ) , i n which s i s a f u n c t i o n of ( y 2 / j ) . 4 . 2 . Interpretation of experimental r e s u l t s . In o r d e r t o compare t h e f o r e g o i n g c o n c l u s i o n s w i t h e x p e r i m e n t a l f i n d i n g s , we have r e p l o t t e d t h e d a t a f o r I n P / F e ( I I ) - E D T A o f F i g u r e 1 a s a f u n c t i o n o f t h e v a r i a b l e ( y / j p h ) i n F i g u r e 3 and o f ( y 2 / j p h ) i n Figure 4. I t c a n be seen t h a t , whereas s c a n n o t be c o n s i d e r e d as a f u n c t i o n o f ( y / j p n ) o n l y , t h e data a r e r e a s o n a b l y w e l l grouped on one s i n g l e c u r v e when p l o t t e d as s v s . ( y V j p n ) . The same c o n c l u s i o n i s o b t a i n e d f o r G a P / F e ( I I ) - E D T A (compare F i g u r e s 5 and 6) and f o r G a P / F e ( C N ) £ " ( 3 ) . In F i g u r e 7 f i n a l l y , t h e r e s u l t s o f F i g u r e 2 have been r e p l o t t e d as l o g ( s 2 / ( l - s ) ] v s . l o g ( y 2 / j p n ) > d e m o n s t r a t i n g t h a t r e l a t i o n s h i p (18) g i v e s an a c c e p t a b l e d e s -

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

7.

GOMES ET AL.

1

1

"

127

Competing Photoelectrochemical Reactions

η - InP • Fe(n) - EDTA

^

v

m -

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-ι

-

-15

-5

1

1

-4.5

-4

1

-3.5

-3

-2.5 log(y/jph)

Figure 3.

Plot of log s vs. log (y/j ); same data as in Figure 1; (y/'] n) in arbitrary units ph

p

ι

1

Γ

log ( y / j 2

Figure 4.

2

p

h

ϊ

2

Plot of log s vs. log (y /] ); same data as in Figure 1; (y /] h) in arbi trary units p1}

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

P

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128

PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE INTERFACES

log(y/jp > h

Faraday Discussions

Figure 5.

Plot of log s vs. log (y/] h); same data as in Figure 2; (y/) h) in arbitrary units (7) P

ι

" -

—τ

P

-

ι

τ

η - GaP • Fe(ir) - EDTA

-

X

Sx

_ lι

-8

Iι

Iι

I

-7.5

-7

-6.5

I

I

-6

-5.5

log(y /j ) 2

p h

Faraday Discussions

Figure 6.

2

2

Plot of log s vs. log (y /) ); same data as in Figure 2; (y /] ) in arbi trary units (1) ph

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

ph

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7.

GOMES ET AL.

Competing Phoîoelectrochemical

2

129

Reactions

2

2

Figure 7. Plot of log [s /(l — s)] vs. log (y /] h); same data as in Figure 2; (y /] h) in arbitrary units (1) P

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

P

130

PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE INTERFACES

cription of the stabilization kinetics. (One has to consider that, with s /(l-s) as the variable, experimental errors become relatively important for s either close to zero or to one). According to the foregoing discussion, it must hence be con cluded that either the first step in the decomposition process is reversible, or that the mechanism is that in which the decomposi tion involves a bimolecular step between two mobile surface holes Xl and in which the reducing agent reacts by donating an electron to such a partially broken surface bond. It must be finally noted that for all light-intensity depen dent cases discussed here, the stabilization ratio was found to decrease with increasing light intensity. All reaction schemes proposed here to explain this effect are based upon the fact that the decomposition reaction is two- or multi-equivalent (see Table I). It might hence well be that this multi-equivalence is at the origin of the observed light-intensity effect on stabilization. This effect may constitute a problem for photoelectrochemical solar cells which for economic reasons may be used under concen trated sunlight, since this would lead to greater deterioration of the electrode under concentrated light.

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2

Abstract. The stabilization of illuminated n-type III-V semiconductor electrodes through competing hole capture by reducing agents ad ded to the aqueous solution has been studied as a function of concentration and of the light intensity. The main result con cerns the observed light-intensity dependence. From a kinetic analysis of the stabilization process, i t follows that two types of reaction mechanisms can be held responsible for the observed kinetics. Literature Cited. 1. Kobayashi, T . ; Yoneyama, H . ; Tamura, H . : Chem. Letters, 1979, 457. 2. Gomes, W.P.: Electrochem. Soc. Meetings Extended Abstracts, 1979, vol. 79-2, 1565. 3. Van Overmeire, F.; Vanden Kerchove, F.; Gomes, W.P.; Cardon, F.: Bull. Soc. Chim. Belg., 1980, 89, 181. 4. Madou, M . J . ; Cardon, F . ; Gomes, W.P.: Ber. Bunsenges. Phys. Chem., 1977, 81, 1186. 5. Kohayakawa, K . ; Fujishima, Α . ; Honda, K.: Nippon Kagaku Kaishi, 1977, 6, 780. 6. Ellis, A . B . ; Bolts, J.M.; Wrighton, M.S.: J. Electrochem. Soc., 1977, 124, 1603. 7. Cardon, F . ; Gomes, W.P.; Vanden Kerchove, F.; Vanmaekelbergh, D.; Van Overmeire, F.: Faraday Discussions, 1980, nr. 70, in print. RECEIVED October 3,

1980.

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

8 Factors Governing the Competition in Electrochemical Reactions at Illuminated Semiconductors H I D E O T A M U R A , H I R O S H I Y O N E Y A M A , and T E T S U H I K O K O B A Y A S H I

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Department of Applied Chemistry, Faculty of Engineering, Osaka University, Yamadakami, Suita, Osaka 565, Japan

Electrochemical reactions at metal electrodes can occur at their redox potential i f the reaction system is reversible. In cases of semiconductor electrodes, however, different situations are often observed. For example, oxidation reactions at an illuminated η-type semiconductor electrode commence to occur at around the flat-band potential Ef^ irrespective of the redox potential of the reaction E j i f Ef^ is negative of E j (1»1»2). Therefore, it is difficult to control the selectivity of the electrochemical reaction by controlling the electrode potential, and more than one kind of electrochemical reactions often occur competitively. The present study was conducted to investigate factors which affect the competition of the anodic oxidation of halide ions X" on illuminated ZnO electrodes and the anodic decomposition of the electrode itself. These reactions are given by Eqs 1 and 2, respectively: r e c

+

ZnO + 2p 2X~

+

+ 2p

o x

r e (

+ Zn + X

2+

+ l/20

2

o x

(1) (2)

2

Until now, it has been pointed out that at least two factors play important roles in determining the degree of competition, neglecting kinetic factors. One is concerned with thermodynamics, and predicts that the most negative E j for a reaction will be the most reactive. The theoretical background for this concept was given by Gerischer (4,5j and by Bard and Wrighton (6), and experimental work to verify this concept has been done by Honda's group (Z>8»2) which included ZnO electrodes. Another important factor, which was proposed by Gerischer (5,10), is illumination intensity, and we have already reported preliminary results to support this view (Y\) for the present electrode/electrolyte systems. r e (

o x

Experimental The competitive reactions were studied by using the rotating 0097-6156/81/0146-0131$05.00/0 © 1981 American Chemical Society In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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r i n g - d i s k electrode technique ( 7 , 1 2 ) . A schematic i l l u s t r a t i o n of the experimental setup i s given i n F i g . 1. A single crystal of 2 mm t h i c k n e s s was m a c h i n e d t o 5 . 8 mm d i a m e t e r a n d mounted i n a T e f l o n e l e c t r o d e h o l d e r as t h e d i s k e l e c t r o d e . A platinum ring e l e c t r o d e h a v i n g 7 . 0 mm i n n e r d i a m e t e r a n d 9 . 0 mm o u t e r d i a m e t e r was a l s o mounted i n t h e T e f l o n h o l d e r c o a x i a l t o t h e ZnO d i s k electrode. P r e t r e a t m e n t s o f t h e e l e c t r o d e were c a r r i e d o u t by s o a k i n g f i r s t i n HC1 f o r 10 s and t h e n i n H-PO* f o r 5 m i n , f o l l o w e d by washing w i t h d e - i o n i z e d water. The RRDE was t h e n s e t i n a RRDE m e a s u r i n g s y s t e m ( N i k k o K e i s o k u , model D P G S - 1 ) . The r o t a t i o n r a t e o f t h e e l e c t r o d e was u s u a l l y 1000 rpm e x c e p t where s p e c i a l l y n o t e d . E l e c t r o l y t e s were o f r e a g e n t g r a d e c h e m i c a l s and t w i c e d i s t i l l e d water. A 500 W s u p e r h i g h p r e s s u r e m e r c u r y a r c lamp ( U s h i o E l e c t r i c , moàet USH-500D) was used a s a l i g h t s o u r c e . A h o r i z o n t a l l i g h t p a t h f r o m t h e lamp was b e n t v e r t i c a l l y by a m i r r o r t o i l l u m i n a t e t h e e l e c t r o d e from t h e bottom o f t h e c e l l . A l l the p o t e n t i a l s c i t e d i n t h i s o a p e r a r e r e f e r r e d t o a SCE w h i c h was u s e d as a r e f e r e n c e e l e c t r o d e i n t h i s s t u d y . A c c o r d i n g t o Eqs 1 and 2 , h a l o g e n m o l e c u l e s , z i n c i o n s and oxygen m o l e c u l e s a r e p r o d u c e d by t h e c o m p e t i t i v e r e a c t i o n s . I f t h e P t r i n g e l e c t r o d e i s s e t a t a p o t e n t i a l where o n l y h a l o g e n m o l e c u l e s a r e s e l e c t i v e l y r e d u c e d , t h e d i s k c u r r e n t due t o t h e o x i d a t i o n o f h a l i d e i o n s i D c a n t h e n be e s t i m a t e d by d i v i d i n g t h e measured r i n g c u r r e n t Î R by t h e c o l l e c t i o n e f f i c i e n c y o f t h e r i n g e l e c t r o d e N: iD(X")

= i R ^ ) " ^

( 3 )

With estimated i D ( X " ) , the percentage o f o x i d a t i o n o f h a l i d e Φ ( Χ ~ ) c a n be d e t e r m i n e d a s a r a t i o o f IQ t o t h e t o t a l d i s k photocurrent measured:

Φ(ΧΊ = [i (X')/i D

tota1

]-100

ions

(4)

Therefore, i t i s important to f i n d out the p o t e n t i a l o f the Pt r i n g e l e c t r o d e where o n l y h a l o g e n m o l e c u l e s a r e s e l e c t i v e l y r e d u c e d . The p o t e n t i a l o f t h e P t r i n g e l e c t r o d e most s u i t a b l e f o r d e t e c t i o n o f Xp was d e t e r m i n e d a s t h e p o t e n t i a l where r e d u c t i o n o f X 2 o c c u r s u n d e r d i f f u s i o n - l i m i t e d c o n d i t i o n s b u t oxygen and Zn^ were n o t r e d u c e d a t a l l . The p o t e n t i a l s t h u s d e t e r m i n e d were 0 V f o r I 2 , 0 . 0 5 V f o r B r 2 , and 0 . 2 V f o r C l 2 when 0 . 5 M I C S O , was used a s t h e base e l e c t r o l y t e . I n c a s e s where t h e base e l e c t r o l y t e was more a c i d i c t h a n 0 . 5 M K 2 S 0 * , t h e s e p o t e n t i a l s were a l i t t l e changed d e p e n d i n g on t h e pH v a l u e o f e l e c t r o l y t e s . Results F i g u r e 2 g i v e s i Q and i R as a f u n c t i o n o f t h e d i s k e l e c t r o d e p o t e n t i a l i n 0 . 5 M K 2 S 0 4 c o n t a i n i n g 10"2M K I . I t i s seen t h a t i R

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

8.

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1

Figure 1. Schematic of experimental setup for measurements of the rotating ring-disk electrode: (1) dual potentiogalvanostat; (2) ZnO disk electrode; (3) Pt ring electrode; (4) Teflon electrode holder; (5) electrolytic cell; (6) N gas inlet; (7) Pt counter electrode; (8) SCE; (9) mirror; (10) Hg arc lamp 2

c ^04

'

0

'

OA

'

08

Potential of disk electrode/V

' vsSCE

Figure 2. Ring current and disk current as a function of potential of the ZnO disk electrode. Potential of Pt ring: 0 V vs. SCE; rotation rate: 1000 rpm; solution: 10- MKIin 0.5M K SO . 2

2

k

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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A T SEMICONDUCTOR-ELECTROLYTE

INTERFACES

was proportional to in, and both ip and TR became saturated at disk potentials positive of 0.5 V. The ratio of I R to in, which was 0.35 in this case, did not vary with increasing concentration of KI and/or with decreasing pH values of electrolytes. Furthermore, under experimental conditions giving the results shown in this figure, no signal for reduction of O2 could be seen in I R - T D curves at ring electrode potentials where oxygen can be reduced in addition to the reduction of I 2 . Therefore, this ratio, 35%, was judged to show the collection efficiency of the ring electrode N. The value obtained was almost the same as that given theoretically (13). Figure 3 shows Φ(Χ ) (X = I, Br) as a function of the concen tration of electrolytes for two different disk currents. According to this figure, a large Φ(Χ~) was obtained for a high concentration of KI, as easily expected from general electrode kinetics. However, it is noticed that the concentration dependence of Φ(Χ") is different even for the same kind of halide ions i f the ip chosen^ is different. In order to investigate this point in detail, Φ(Χ") was obtained as a function of ip for different concentration of halide ions. Figure 4 shows results for iodide ions. It is seen in this figure that in a region of relatively small ip, Φ(Ι") is relatively high and constant with concentration. However, it decreases with an increase in ip i f the magnitude of the disk photocurrent exceeds a certain critical value which depends on the concentration of iodide ions. The decrease in the percentage was judged to reflect a simple mass transport problem, because Φ ( Γ ) increased when the rotation rate of_the electrode was increased. In the region where the constant Φ(Ι~) was observed, it was not influenced by the rotation rate of the electrode but was varied by changing the pH values of electrolytes, as shown in Figure 5. According to this figure, Φ ( Γ ) increased with increasing acid concentration of the electrolytes. Figure 6 shows Φ{ΒΓ") as a function of ip for a variety of electrolyte concentrations. By comparing this figure with Figure 4, it is noticed that the dependency of Φ ( Β θ on ip is quite different from the case of iodide ions. Φ(Β^) increases with an increase in ip in a region of relatively small disk currents, and complete suppression of the anodic dissolution could be achieved under certain critical ip in solutions having a concentration about thousand times higher than that required for iodide solutions. In a region of photocurrents where an ascending trend of Φ ( Β θ was observed, the percentage was not changed by the rotation rate of the electrode, suggesting that Φ ( Β θ in this region is not controlled by any factor on the solution side. As in the case of iodide ions, Φ(Β^) was distinctly affected by solution pH, and was high for solutions of low pH. The results to show this are given in Figure 7. Figure 8 shows Φ(01") as a function of ip. The ability of chloride ions to suppress the anodic decomposition of the electrode

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

TAMURA ET AL.

Competition at Illuminated Semiconductors

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8.

135

Figure 3. Percentage of oxidation of Γ and Br as a function of the concentration of halide ions for two different disk cur rents: (-Ο-0-) 10 A; (-·-, 4.5 X W Α; (-Ο- · - ; KI in 0.5M K SO,,; (-0-, KBr in 0.5M ^SO^; rotation rate—1000 rpm 5

5

2

_l

10"*

I

I I I

III

10"°

• '

Disk

10

1

'

I

I I ι I III

10

current/A Chemistry Letters

Figure 4. Percentage of oxidation of /", Φ(Γ), as a function of disk current for a variety of concentrations of KI in 0.5M Κ ΞΟ/,. Concentration (Μ): (-Ο-) 1 X ΙΟ*; (-V-; 3.16 χ ΙΟ ; (-0-) 1 χ 10~ ; (-A-) 3.16 X 10 ; 1 X 10 (U). 2

3

3

A

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

A

136

PHOTOEFFECTS

AT

SEMICONDUCTOR-ELECTROLYTE

INTERFACES

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100h

Figure 5. As in Figure 4, but for a variety of solution pH (concentration of KI: 10~ M\ rotation rate: 1000 rpm; pH value: (-Ο-) 4.2; (-0-) 5.5; (-A-) 6.2; (-D-)8.0) 4

100h

Chemistry Letters

Figure 6. Plot of Φ(ΒΓ) as a function of disk photocurrent for a variety of concen trations of KBr in 0.5M K SO (concentration (M)* (-Ο-) 1 Χ ΙΟ ; (-0-) 1 χ 70-2; (-Δ-j 3.16 Χ ΙΟ ; (~Π~) 1 X 10~ ; rotation rate: 1000 rpm (11) 1

2

h

4

4

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

Competition at Illuminated Semiconductors

137

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8. TAMURA ET AL.

Figure 8. Plot of &(CÏ) as a function of disk current for different concentrations of KCl in 0.5M K SOj, (rotation rate: 1000 rpm; concentration (M): (-0-) 1.0; (-0-) 0.562; (-A-) 0.316) 2

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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138

PHOTOEFFECTS

AT

SEMICONDUCTOR-ELECTROLYTE INTERFACES

LU

χ ζ

^decomp "

_

Îr /Br" 2

•*

cn -6 ο α ο Η3

Electrode

~

Electrolyte Chemistry Letters

Figure 9. Energetic correlation between ZnO electrodes and halide solutions at pH = 6.0 E , E , and E denote the energy level of the conduction band edge, valence band edge, and the decomposition reaction of ZnO, respectively f l l j . c

r

dccomp

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

8.

TAMURA ET AL.

139

Competition at Illuminated Semiconductors

was much poorer than that of bromide ions, as judged from a comparison of Figures 6 with 8. Although ascending trends of Φ(01~) with iQ were observed also in this case, the complete suppression of the electrode decomposition was not achieved even in solutions as high as 1.0 M.

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Discussion In Figures 4 to 8, Φ(Χ~) is given as a function of i n . Since in. is the saturated photocurrent and is proportional to tne illumination intensity, the results given in these figures can be read in such a way that Φ(Χ~) is given as a function of the illumination intensity. From the results given above, therefore, the following are important factors affecting the degree of competition: the concentration of halide ions, the illumination intensity, the acidity of the electrolytes, the redox potential of the electrolytes, and kinetic factors. The importance of the first factor, the concentration, is clear because the anodic photocurrent due to oxidation of halide ions should be proportional to the product of the concentration of positive holes at the electrode surface and that of halide ions in solution. When ip becomes large, the supply of halide ions to the electrode surface by diffusion_becomes unable to follow i , resulting in_a decrease of Φ(Χ~), which depends on the concen tration of X". Up to date, the importance of illumination intensity has been paid l i t t l e attention. According to the results obtained, Φ ( Β ^ ) and Φ(Π~) were noticeably affected by the illumination intensity, although this was not the case for iodide ions. The ascending trend of Φ(Χ") observed with increasing ip, which was invariant with changing the rotation rate of the electrode, is believed to reflect an illumination intensity effect. Figure 9 shows the energetic correlation of the ZnO electrode and the electrolyte at pH = 6. According to this figure, the energy level of the I"/l2 couple is higher than that of the anodic decomposition reaction of ZnO, while a reverse relation is seen for the cases of Br"/Br2 and C I " / C I 2 - By changing the pH value from 6 to 2, these energetic correlations are not upset, because the redox potentials of the competitive reactions are not changed by chanqina the pH values of electrolytes as long as the solution conditions are acidic. It is noticed from such energetic correlations that the illumination intensity effect is dependent on the relative positions of the energy levels of the competing reactions. The energy level of the quasi-Fermi level of positive holes, pEp, which gives a measure of the average energy of positive holes, is given by Eq 5. D

p

E

F

=E v

kT[Zn(p + p*)/N ] 0

v

(5)

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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In t h i s e q u a t i o n , Ev i s t h e e n e r g y l e v e l o f t h e v a l e n c e band e d g e , Ny t h e e f f e c t i v e d e n s i t y o f s t a t e s o f t h e v a l e n c e b a n d , p * t h e c o n c e n t r a t i o n o f p h o t o - g e n e r a t e d p o s i t i v e h o l e s , and pp t h a t o f p o s i t i v e h o l e s i n t h e r m a l e q u i l i b r i u m i n t h e d a r k w h i c h can be e v e n t u a l l y i g n o r e d i n a s e m i c o n d u c t o r o f l a r g e band gap s u c h as ZnO. A c c o r d i n g t o t h i s e q u a t i o n , pEp a p p r o a c h e s t h e v a l e n c e band w i t h an i n c r e a s e i n t h e i l l u m i n a t i o n i n t e n s i t y . Under dynamic c o n d i t i o n s i n which electrochemical r e a c t i o n s are o c c r i n g , photog e n e r a t e d p o s i t i v e h o l e s a r e consumed. In a d d i t i o n , some f r a c t i o n o f t h e s e h o l e s r e c o m b i n e w i t h e l e c t r o n s and a r e a n n i h i l a t e d . T h e r e f o r e , the determination o f the c o n c e n t r a t i o n of p o s i t i v e holes a t the i l l u m i n a t e d s u r f a c e o f the e l e c t r o d e i s q u i t e d i f f i c u l t , making i t a l s o d i f f i c u l t t o e s t i m a t e the e x a c t p o s i t i o n o f p E p . Even s o , t h e i n c r e a s e o f Φ ( Χ " ) w i t h an i n c r e a s e i n i p can be q u a l i t a t i v e l y u n d e r s t o o d f r o m t h e s h i f t o f pEp w i t h i l l u m i n a t i o n intensity. The e n e r g e t i c p o s i t i o n o f t h e a n o d i c d e c o m p o s i t i o n r e a c t i o n o f ZnO i s h i g h e r t h a n t h o s e o f B r 2 / B r " and C I 2 / C T . W i t h an i n c r e a s e i n the i l l u m i n a t i o n i n t e n s i t y , p E ? at the s u r f a c e s h i f t s down f i r s t t o r e a c h t h e e n e r g y l e v e l o f t h e a n o d i c d e c o m p o s i t i o n r e a c t i o n of the e l e c t r o d e . The a n o d i c d i s s o l u t i o n o f t h e e l e c t r o d e i s t h e r e f o r e p r e d o m i n a n t u n d e r i l l u m i n a t i o n o f r e l a t i v e l y low intensity. I f the r a t e constant of the anodic decomposition r e a c t i o n i s so l a r g e t h a t t h e r e a c t i o n p r o c e e d s r e v e r s i b l y , t h e r a t e o f p o s i t i v e h o l e c o n s u m p t i o n w i l l be h i g h enough n o t t o b r i n g about a f u r t h e r s h i f t o f pEp towards the lower l e v e l w i t h f u r t h e r i n c r e a s e i n the i l l u m i n a t i o n i n t e n s i t y . However, i f t h i s i s not t h e c a s e , t h e p E p w i l l t h e n s h i f t down t o r e a c h E B r . / B r " a n c ' C l 2 / C l " d e p e n d i n g on t h e n a t u r e o f e l e c t r o l y t e s . In t h i s c a s e , t h e a n o d i c o x i d a t i o n o f X" becomes f e a s i b l e . I f the rate constant of the a n o d i c o x i d a t i o n r e a c t i o n o f h a l i d e i o n s i s l a r g e r than t h a t o f the anodic decomposition r e a c t i o n of the e l e c t r o d e i t s e l f , t h e f o r m e r r e a c t i o n must o c c u r p r e f e r e n t i a l l y . The i n c r e a s e i n Φ ( Β ^ ) and Φ ( 0 1 " ) w i t h i n c r e a s i n g i l l u m i n a t i o n i n t e n s i t y i s b e l i e v e d to r e f l e c t such e f f e c t s o f the i l l u m i n a t i o n i n t e n s i t y . I f t h e r a t e c o n s t a n t o f t h e a n o d i c o x i d a t i o n o f X" i s s m a l l e r t h a n t h a t of the decomposition r e a c t i o n , the percentage o f the o x i d a t i o n o f h a l i d e i o n s w i l l n o t be a p p r e c i a b l y i n c r e a s e d w i t h i n c r e a s i n g illumination intensity. In t h e c a s e o f i o d i d e s o l u t i o n s , t h e energy l e v e l of the redox e l e c t r o l y t e i s h i g h e r than t h a t o f the a n o d i c d e c o m p o s i t i o n r e a c t i o n o f ZnO, so t h a t p r e f e r e n t i a l oxidation of iodide ions occurs. The o b s e r v e d i n v a r i a n c e o f Φ ( Ι " ) w i t h i l l u m i n a t i o n i n t e n s i t y seems t o r e f l e c t t h a t t h e r a t e c o n s t a n t o f t h e a n o d i c o x i d a t i o n o f i o d i d e i o n s i s l a r g e enough f o r p E p t o be p i n n e d a t a f i x e d e n e r g e t i c p o s i t i o n g f E I ? / j - . Under s u c h c i r c u m s t a n c e s , a r e l a t i v e p o s i t i o n o f pEp to tne e n e r g e t i c position o f t h e d e c o m p o s i t i o n r e a c t i o n i s a l s o f i x e d , so t h a t t h e r a t i o o f the anodic o x i d a t i o n o f i o d i d e ions to the decomposition r e a c t i o n is invariable. E

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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As Figures 5 and 7 show, Φ(Χ~) was large in solutions of low pH. It is known from the energetic correlation between ZnO and electrolytes that the energy level of the redox electrolyte relative to that of the anodic decomposition reaction is invariant with changing pH values of the electrolytes. Therefore, the pH dependency observed cannot be explained from a point of thermo dynamics. The energy separation of Εχ /χ- from the valence band edge increases with an increase in acidity of electrolytes, making i t difficult to explain the observed pH dependence from the point of view of overlaps of the density of states of electrolytes and the valence band. It may be plausible to assume that the energy levels of surface states match well with those of halide ions with decreasing pH values of the electrolytes (8). However, similar pH dependencies of Φ(Χ~) are also observed on other n-type semiconducting oxides such as Ti02, WO3, and a-Fe2Û3 (14). Furthermore, increasing trends of the activity for the anodic oxidation of iodide and bromide ions with an increase in acidity of electrolytes has been reported for SnÛ2 (15). Similar pH effects are also observed for the anodic oxidation of chloride ions at metallic conductive Ru02 electrodes (16). Considering such a broad pH dependence of the reactivity of halide ions, arguments based on surface states do not seem valid. Based on surface chemistry arguments, the double layer structure of metal oxide surfaces is effected by the solution pH (17,18), and hydroxide groups of the surface becomes less abundant with a decrease in solution pH. The process may be represented by the following equation (18);

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2

m+

M (0H) (H 0) ^J^ - Η m

2

n

^(OHJm-i

("2θ)

η+1

]

+1

(6)

In this equation, M represents a surface metal cation and the sum of m and η fulfills the coordination of M. Such a change in surface conditions is reflected in the pH dependence of the flatband potential of ZnO electrodes (19). Judging from the point of zero charge (pzc) of ZnO, which is at about pH=8.7 (20), the surface of the electrode must be charged positively in the soltuion chosen in the experiments. Then, there may arise specific adsorption of halide ions onto the cationic sites, and a mechanism is postulated that the observed pH effects of Φ(Χ") is due to contribution from the specifically adsorbed halide ions. Measurements of the flat-band potential of ZnO electrodes as a function of the concentration of iodide ions, however, gave no indication of the specific adsorption. Then, this model is ruled out. Another model for giving an explanation of the pH dependence of the reactivity of halide ions may be that surface cations serve as effective sites for adsorption of reaction intermediates which are produced in the course of the anodic oxidation of halide ions. Usually, the anodic oxidation of halide ions is believed to

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

142

PHOTOEFFECTS

AT SEMICONDUCTOR-ELECTROLYTE

INTERFACES

proceed according to the following equation: X" + P followed by

+

+ Xad

(7)

either Xad

+

Xad -

X

(8)

2

or

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Xad

+ Χ" + Ρ

·>

0)

X2

If the adsorption of neutral halogen atoms occurs on the surface cation sites with replacing surface-bound water, then the amount of the adsorbed intermediates will be high in acidic solutions, as implied by Eq 6 , resulting in an increase in apparent reactivity of halide ions with decreasing pH values. The importance of the thermodynamic correlation for competing reactions has already been discussed from theoretical and experimental points of view as described in the Introduction. The difference in reactivity created by difference in the redox potential of X2/X" systems is clearly reflected in the results obtained in the present study in that the concentration required to stabilize the electrode is different among the kind of halide ions ( lowest for iodide but highest for chloride ions). Conclusion We have discussed the important factors which determine the degree of competition. These factors are usually intermixed and hence make the observed phenomena complicated. However, the present study has revealed that there are cases where the illumination intensity plays an important role in determining the degree of competition. Furthermore, it is suggested from the present study that stabilization of electrodes will be achieved by appropriate choices of electrolytes, solution pH, and illumination intensity even for electrode/electrolyte systems of thermodynamic instability. Abstract The anodic oxidation of iodide, bromide and chloride ions at illuminated ZnO electrodes, which occurs in competition with the anodic decomposition of the electrode i t s e l f , was studied as functions of halide ion concentration, illumination intensity and solution pH in order to investigate factors which affect the degree of competition. The reactivity of halide ions, obtained under fixed conditions, was in the order of I > Br > Cl ,reflecting the importance of the redox potential in determining the reactivity. The illumination intensity was found to influence on the degree of competition, and the effects were different among the kind of -

-

-

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

8. TAMURA ET AL.

Competition at Illuminated Semiconductors

143

halide ions. The observed phenomena are interpreted from the point of shift of the quasi-Fermi level of positive holes with illumination. The pH values of electrolytes was also found to have a marked effect on determining the degree of competition in such a way that the apparent reactivity of halide ions increased with decreasing pH values. The importance of the hydration structure of the electrode surface is suggested.

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Literature Cited

A.

1. Gerischer, H . , in "Physical Chemistry," Vol. IX A, Eyring, H . ; Henderson, D.; Jost, W., Ed. Academic Press, New York, Ν. Y . , 1976; p. 463. 2. Memming, R., in "Electroanalytical Chemistry," Vol. 12., Bard, J., E d . , Dekker, New York, Ν. Y . , 1979; p. 1. 3. Bard, A. J. J. Photochem.,1979, 10, 59. 4. Gerischer, H. J. Electroanal. Chem., 1977, 82, 133. 5. Gerischer, H. J. Vac. S c i . Technol., 1978, 15, 1422. 6. Bard, A. J.; Wrighton, M. S. J. Electrochem. Soc., 1977, 124, 1706. 7. Inoue, T . ; Watanabe, T . ; Fujishima, Α . ; Honda, K . ; Kohayakawa, H. J. Electrochem. Soc., 1977, 124, 719. 8. Inoue,T.; Fujishima, Α . ; Honda, K. B u l l . Chem. Soc. J p n . , 1979, 52, 3217. 9. Fujishima, Α . ; Inoue, T . ; Honda, K. Chem. L e t t . , 1978, 377. 10. Gerischer, H . , in "Solar Power and Fuels," Bolton, J . R., Ed. Academic Press, New York, Ν. Y . , 1977; p. 77. 11. Kobayashi, T . ; Yoneyama, H . ; Tamura, H. Chem. L e t t . , 1979, 457. 12. Memming, R. Ber. Bunsenges. Phys. Chem., 1977, 81, 732. 13. Albery, W. J.; Bruckenstein, S. Trans. Farady Soc., 1966, 62, 1920. 14. Kobayashi, T . ; Yoneyama, H . ; Tamura, H. to be published. 15. Yoneyama, H . ; Laitinen, H. A. J. Electroanal. Chem., 1977, 79, 129. 16. Arikado, T.; Iwakura, C.; Tamura, H. Electrochim. Acta, 1978, 23, 9. 17. Ahmed, S. M. J. Phys. Chem., 1969, 73, 3546. 18. Boehm, H. Discuss. Farady Soc., 1972, 52, 264. 19. Lohmann, F. Ber. Bunsenges. Phys. Chem., 1966, 70, 87, 428. 20. Block L.; DeBruyn, P. L. J. Colloid Interf. S c i . , 1972, 32, 518. Received October 3, 1980.

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

9 Electrochemical Behavior and Surface Structure of Gallium Phosphide Electrodes Y. N A K A T O , A . TSUMURA, and H . TSUBOMURA

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Department of Chemistry, Faculty of Engineering Science, Osaka University, Toyonaka, Osaka, 560 Japan

The p r a c t i c a l success of semiconductor e l e c t r o c h e m i c a l photo c e l l s depends on how to prevent the photo-corrosion of the e l e c trode m a t e r i a l s . The v a r i o u s e l e c t r o c h e m i c a l processes a t the surface of semiconductor photo-electrodes e l e c t r o n t r a n s f e r as w e l l as decomposition r e a c t i o n s have been discussed so f a r mainly by taking account of the s t a t i c e l e c t r o n i c energy l e v e l s of the semiconductors and the s o l u t i o n ; that i s to say, the conduction band edge, E , the valence band edge, E , the redox p o t e n t i a l s of the redox couple i n the s o l u t i o n , E°(Ox/R), the decomposition p o t e n t i a l , E^; e t c . ( j L - 6 ) . However, the competition between the e l e c t r o n t r a n s f e r process and the decomposition r e a c t i o n paths should b e t t e r be understood from a k i n e t i c point of view (4). Namely, i f the r a t e of the former i s f a s t e r than the l a t t e r , the photoanode i s maintained s t a b l e . A l s o , i t seems v i t a l l y important to take i n t o account the presence of the surface s t a t e whose energy and s t r u c t u r e may be dynamically changed by the e l e c t r o d e reactions. In t h i s paper, we w i l l r e p o r t our experimental f i n d i n g s on the photo-anodic behavior of η-type g a l l i u m phosphide (GaP) i n aqueous e l e c t r o l y t e and d i s c u s s them based on a p i c t u r e of the r e a c t i o n intermediates, which are to play an important r o l e on the r e a c t i o n pathway as w e l l as on the c r e a t i o n of photo-voltages and photoc u r r e n t s . The main point i s that the surface band energies (de p i c t e d f o r an η-type semiconductor i n F i g . 1 ) , which play the most important r o l e i n the e l e c t r o d e processes, by no means remain con stant, although t h i s has been t a c i t l y assumed to be the case i n many previous papers, but change during the photoelectrode pro cesses by the accumulation of surface intermediates and of surface charge ( 7 , 8 ) . For l a t e r d i s c u s s i o n s , we a l s o d e f i n e a p o t e n t i a l U , which i s a p o t e n t i a l a t which the i n v e r s e square of the d i f f e r e n t i a l capacitance 1/C^ tends to zero as determined from the 1/C^ v s p o t e n t i a l p l o t (Mott-Schottky p l o t ) . I t i s r e l a t e d to E§ i n the f o l l o w i n g way: c

v

s

0097-6156/81/0146-0145$05.00/0 © 1981 American Chemical Society In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

PHOTOEFFECTS

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146

AT SEMICONDUCTOR-ELECTROLYTE

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Journal of the Electrochemical Society

Figure 1. Schematic of the band structure and energy terms of a semiconductor Π)

Journal of the Electrochemical Society

Figure 2.

Current-potential curve for the illuminated (lll)-face of an n-GaP electrode in aO.lM NaOH solution (1 )

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

9.

NAKATO E T A L .

E

c

-

"

e

U

GaP Electrode

s+

147

Δ

where e i s the elementary charge and Δ a small energy d i f f e r e n c e between the conduction band edge i n the bulk and the Fermi l e v e l . Experimental The η-type GaP used was a s i n g l e c r y s t a l i n the form of wafers, 99.999%pure and doped with s u l f u r to the c o n c e n t r a t i o n of 2 to 3 χ 1 0 cm~3 (Yamanaka Chemical I n d u s t r i e s L t d . ) . The p-type GaP used was doped with z i n c to 3.7 χ 10 cm~3 (Sanyo E l e c t r i c Co., L t d . ) . Both were cut perpendicular to the [ l l l ] - a x i s . The ohmic contact was made by vacuum d e p o s i t i o n of indium on one face of the c r y s t a l , followed by heating a t ca. 500 °C f o r 10 min. The s i d e connected with a wire was covered with epoxy r e s i n . Before the experiment, the c r y s t a l s were p o l i s h e d and_etched with warm aqua r e g i a . The (111)-face (Ga face) and the ( l l l ) - f a c e (P face) were d i s t i n g u i s h a b l e by microscopic i n s p e c t i o n of the etched s u r f a c e s , the former very rough and the l a t t e r smooth. The c u r r e n t - p o t e n t i a l curve was obtained with a Hokutodenko HA-101 p o t e n t i o s t a t . The d i f f e r e n t i a l capacitance was measured mostly with a Yokogawa-Hewlett-Packard U n i v e r s a l Bridge 4265B, having a modulation frequency of 1 kHz and a modulation amplitude of 20 mV (peak to peak). The frequency dependence of Mott-Schottky p l o t s were checked by connecting a f u n c t i o n generator t o the bridge. The e l e c t r o d e was i l l u m i n a t e d through a quartz window with a 250 watt high pressure mercury lamp. The l i g h t i n t e n s i t y was attenuated by use of n e u t r a l density f i l t e r s made of metal nets. Solutions were prepared from deionized water by using reagent grade chemicals, i n most cases, without f u r t h e r p u r i f i c a t i o n . They were deaerated by bubbling n i t r o g e n and s t i r r e d with a magnetic s t i r r e r . The pH of the s o l u t i o n s i n the range of 3 to 11 was con t r o l l e d by using b u f f e r mixtures; acetate, phosphate, borate, or carbonate, each a t about 0.01 M, where M means mol/drn^. T h i s a b b r e v i a t i o n i s used throughout the present paper. 1 7

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7

Results and Discussions 1. The E f f e c t of I l l u m i n a t i o n . In an a l k a l i n e s o l u t i o n , an n-GaP e l e c t r o d e , (111) surface, under i l l u m i n a t i o n shows an anodic photocurrent, accompanied by q u a n t i t a t i v e d i s s o l u t i o n of the e l e c trode. The c u r r e n t - p o t e n t i a l curve shows considerable h y s t e r i s i s as seen i n F i g . 2; the anodic c u r r e n t , scanned backward, (toward l e s s p o s i t i v e p o t e n t i a l ) begins to decrease at a p o t e n t i a l much more p o s i t i v e than the onset p o t e n t i a l of the anodic current f o r the forward scanning, the l a t t e r being s l i g h t l y more p o s i t i v e than the U value i n the dark, U ( d a r k ) . These r e s u l t s suggest that the GaP surface with backward scanning develops an o x i d i z e d s t r u c t u r e , which i s a c t i n g as a pre cursor, or precursors, to the anodic d i s s o l u t i o n r e a c t i o n s . s

s

American Chemical Society Library 1155 16th St., N.W.

In Photoeffects at Washington, Semiconductor-Electrolyte Interfaces; Nozik, A.; D.C. 20036 ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

148

PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE

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We have found that a good l i n e a r Mott-Schottky p l o t can be obtained f o r the e l e c t r o d e not only i n the dark but a l s o under weak i l l u m i n a t i o n ( F i g . 3). This means that, even under anodic p o l a r i z a t i o n and i l l u m i n a t i o n , the s t a t e of the surface i s main tained at a constant c o n d i t i o n . The U value determined from the i n t e r c e p t of the p l o t with the a b s c i s s a i s somewhat l e s s negative than the U value determined s i m i l a r l y i n the dark. The U values i n the dark and under i l l u m i n a t i o n , r e s p e c t i v e l y , at v a r i o u s pH are given i n F i g . 4 f o r the (111) face of GaP, together with the value, the l a t t e r defined as the p o t e n t i a l at zero current f o r backward scanning (see F i g . 2). The l i n e a r dependence of U ( d a r k ) on pH i s understood, as i s g e n e r a l l y the case f o r some semiconduc tors (9,10), to be the r e s u l t of the acid-base e q u i l i b r i u m on the s u r f a c e . The d e v i a t i o n of U ( i l l . ) (the U value under i l l u m i n a t i o n ) from U ( d a r k ) increases s l i g h t l y with the i l l u m i n a t i o n i n t e n s i t y , and i s explained by assuming the accumulation of s u r f a c e intermediates. That i s , when the e l e c t r o d e i s under i l l u m i n a t i o n , the holes generated by i l l u m i n a t i o n are drawn to the s u r f a c e by the b u i l t - i n p o t e n t i a l gradient, and cause anodic decomposition of the e l e c t r o d e . As Gerischer suggested f o r Ge or GaAs e l e c t r o d e s (5,9,11), intermediates are formed as the precursors i n the decom p o s i t i o n or d i s s o l u t i o n path: g

s

s

g

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s

g

s

-P

Ρ-

v

w

-P -Ga -OH

P - Ga"

Ρ

Ga-OH

P - Ga— OH

Ga- P—

OH

The extent of the accumulation of such intermediates depends on t h e i r r a t e s of the formation and those of the ensuing decomposi t i o n (or d i s s o l u t i o n ) r e a c t i o n s . I f the l a t t e r are not high, the t o t a l d e n s i t y of such surface intermediates becomes so h i g h that an a p p r e c i a b l e s u r f a c e p o t e n t i a l Δψ i s created by the e l e c t r i c double l a y e r formed by the charge unbalance of these intermediates, as w e l l as by the approach of counter ions and the h y d r a t i o n around these intermediates. The experimentally obtained d i f f e r e n c e between U ( d a r k ) and U ( i l l . ) can be a t t r i b u t e d to t h i s Δψ. The surface p o t e n t i a l p o s t u l a t e d above should a f f e c t the U and the U Q mentioned p r e v i o u s l y by an equal magnitude. However, the s h i f t of from U ( d a r k ) as seen i n F i g . 4 i s much l a r g e r than that of U ( i l l . ) . T h i s l a r g e s h i f t of the former may be ex p l a i n e d by assuming that the surface intermediates capture e l e c trons i n the conduction band and act e f f e c t i v e l y as recombination centers. G

g

S

g

s

2. The Surface P o t e n t i a l a r i s i n g from the I n t e r a c t i o n between the Surface " S t a t e s " and the Redox Couples i n the S o l u t i o n . When the f e r r i c y a n i d e / f e r r o c y a n i d e redox couple i s present i n a 0.1 Ν NaOH s o l u t i o n , the dark cathodic current of the n-GaP (111)-face sets out at — 1 . 1 V (SCE) , showing t h a t an e l e c t r o n t r a n s f e r occurs

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

ΝAKATO E T A L .

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GaP Electrode

Figure 4. The U (dark), U (illuminated), and U for the (lll)-face of n-GaP in solutions of 0.05M Να 30^ and buffers. The anodic photocurrentsflowingduring measurements are shown in units of liAcm' . 8

s

0

a

2

2

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

150

PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE

INTERFACES

from the conduction band of n-GaP to the f e r r i c y a n i d e . T h i s onsetp o t e n t i a l i s ca. 0.6 V more anodic than the onset p o t e n t i a l f o r the cathodic current which corresponds to H 2 e v o l u t i o n observed i n a s o l u t i o n l a c k i n g the redox couple ( F i g . 5), the l a t t e r being c l o s e to U ( d a r k ) . T h i s q u i t e l a r g e anodic d e v i a t i o n i s analogous to the anodic s h i f t of U Q from U ( d a r k ) mentioned above, and i n d i c a t e s the formation of surface intermediates which a c t as an e f f e c t i v e e l e c t r o n t r a n s f e r mediator as w e l l as being r e s p o n s i b l e f o r s h i f t i n g the s u r f a c e band energy E^. As F i g . 6 shows, the dark cathodic current f o r a p-GaP e l e c t r o d e i n the presence of the f e r r i c y a n i d e / f e r r o c y a n i d e couple sets out at ca. 0 V (SCE), i n d i c a t i n g that hole i n j e c t i o n to the valence band of GaP occurs from the f e r r i c y a n i d e . T h i s suggests that holes are i n j e c t e d by f e r r i cyanide i n t o GaP and supports the above idea that holes are chem i c a l l y relaxed at the surface to form surface intermediates. s

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s

A good l i n e a r Mott-Schottky p l o t was obtained f o r the (111)face of n-GaP i n a s o l u t i o n of f e r r i c y a n i d e / f e r r o c y a n i d e , as shown i n F i g . 7. The U values determined from such p l o t s , i n e l e c t r o l y t e s o l u t i o n s e i t h e r with the absence or with the presence of the redox couple ( U and U ( r e d o x ) , r e s p e c t i v e l y ) , are p l o t t e d at v a r i o u s pH ( F i g . 8). Although the U changes l i n e a r l y with pH, the U ( r e d o x ) stays constant from pH 5 to pH 10, y i e l d i n g a constant d i f f e r e n c e of 1.7 V with the observed redox p o t e n t i a l E(0x/R) of the f e r r i c y a n i d e / f e r r o c y a n i d e couple. This r e s u l t can be under stood by again assuming that a surface p o t e n t i a l , a r i s i n g from the accumulated surface intermediates, ( i n c l u d i n g s u r f a c e trapped h o l e s ) , brings down the e l e c t r o n i c energy of the surface-trapped hole (as w e l l as the surface band energies) to the l e v e l of the redox p o t e n t i a l of the redox couple i n the s o l u t i o n and achieves e l e c t r o n exchange e q u i l i b r i u m . The surface trapped hole may be v i s u a l i z e d as the e l e c t r o n d e f i c i e n t Ga-P bond, s t a b i l i z e d by the d i s t o r t i o n of the surface Ga-P framework and the h y d r a t i o n or the approach of OH" ions i n the s o l u t i o n . There a l s o might p o s s i b l y be c o n t r i b u t i o n s from the intermediates formed by the chemical r e l a x a t i o n processes thereof. However, i n the present d i s c u s s i o n , we t e n t a t i v e l y assume that only one species a surface trapped hole i s r e s p o n s i b l e f o r the e l e c t r o c h e m i c a l behavior. This s p e c i e s , and the unreacted Ga-P bonds, would c o n s t i t u t e a redox system which i s c h a r a c t e r i z e d by a redox p o t e n t i a l Eft r e l a t e d to the s u r f a c e p o t e n t i a l ψ: G

S

S

G

G

kT

E

h

=

E£

+ -£±-lnC

h

+

ψ

+

const.

where C^ i s the d e n s i t y of the surface trapped hole which i s under r e v e r s i b l e e l e c t r o n i c e q u i l i b r i u m with the s o l u t e . In most cases, the e f f e c t of ψ on E^ i s much stronger than the second term of the above equation. With t h i s view, the energy i n t e r v a l s between the E^, E^ and -eEj^ becomes n e a r l y constant, because these l e v e l s a l l change e q u a l l y with ψ. We can determine t h e i r r e l a t i v e p o s i t i o n s by the

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

9.

NAKATO E T A L .

151

GaP Electrode

U / V vs SCE

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-2.0

-1.5

-1.0

0

Journal of the Electrochemical Society

Figure 5. Current-potential curves for the (lll)-face of n-GaP in aO.lM NaOH solution, in the absence (a) and the presence (b) of 0.05M potassium ferrocyanide and 0.05M potassium ferricyanide (1 ).

Figure 6. Dark cathodic currents at a p-GaP electrode in solutions of 0.05M potassium ferricyanide and 0.05M potassium ferrocyanide

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

PHOTOEFFECTS

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U / V vs SCE

Figure 7. Mott-Schottky plots for the (lll)-face of an n-GaP electrode in solu tions containing 0.05M potassium ferricyanide and 0.05M potassium ferrocyanide

Figure 8. U values for the (lll)-face of n-GaP (dark) at various concentrations of the ferricyanide/ferrocyanide couple (equal concentrations): (Φ) OM; (O) 0.005M; (A) 0.05M; (A) 0.4M; E(Ox/R) redox potential of the redox couple determined by the cyclic voltammetry; (U ) the U for a p-GaP in the absence of the redox couple. s

p

s

s

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

9.

NAKATO E T A L .

153

GaP Electrode

f o l l o w i n g argument. As mentioned before, at the e q u i l i b r i u m with the f e r r i c y a n i d e / f e r r o c y a n i d e couple the d i f f e r e n c e between U and E(Ox/R) i s 1 . 7 V. Then, by taking Δ to be 0 . 1 V, the d i f f e r e n c e between and E ( 0 x / R ) becomes 1 . 8 V. I f we set E = Ε(ferricy anide/f errocyanide) , which i s observed to be 0 . 2 V (SCE), then from the band gap, Ε , of GaP ( 2 . 3 V ) , E^ i s c a l c u l a t e d to be 0 . 5 V below Ε^. As an example, the U of n-GaP at pH 6 with no redox couple present i n the s o l u t i o n i s — 1 . 6 V (SCE). T h i s shows that Ε^ without a redox couple i s ca. + 0 . 1 V (SCE). I t i s then under stood that the i n t e r a c t i o n of a f e r r i c y a n i d e / f e r r o c y a n i d e couple s h i f t s Eft, as w e l l as U , by 0 . 1 V to the anodic d i r e c t i o n at pH 6 ( F i g . 1 1 ) . At pH 1 0 , the s h i f t i s 0 . 4 V. The breakdown at a pH higher than 1 0 of the p a r a l l e l i s m be tween the U (redox) and the redox p o t e n t i a l of the redox couple i n s o l u t i o n can be explained by assuming that the r a t e of the d i s s o l u t i o n r e a c t i o n , caused by the attack of H 0 or OH" on the sur face trapped hole, i s so high i n t h i s pH range that the e l e c t r o n exchange e q u i l i b r i u m at the i n t e r f a c e i s no longer achieved. For the ( Ϊ 1 Ϊ ) face of n-GaP, the measured U (redox) changed almost l i n e a r l y with the pH, showing that the surface-trapped hole i s l e s s s t a b l e than that f o r the case of the ( 1 1 1 ) f a c e above men tioned. In the presence of a f e r r i c / f e r r o u s ( F e ^ / F e ) couple, and that of tetraammine copper (II) Cu(NH3)^^ i o n , the measured U values a l s o s h i f t e d . The redox p o t e n t i a l f o r the f e r r i c / f e r r o u s couple, both i n equal concentrations, i s + 0 . 5 V (vs SCE) i n a low pH r e g i o n , and that f o r the tetraammine copper (II)/(IŒ) couple i s 0 to — 0 . 2 V, depending on the c o n c e n t r a t i o n of ammonia. In the presence of a vanadate/vanadite ( V ^ / V ) couple at pH < 3, whose formal redox p o t e n t i a l i s very h i g h l y negative ( — 0 . 5 V (SCE)), the U s h i f t e d very l i t t l e and was the onset p o t e n t i a l f o r the cathodic c u r r e n t . I t was pointed out p r e v i o u s l y ( 1 2 , 1 3 ) that n-GaP emitted luminescence under cathodic p o l a r i z a t i o n i n contact with a f e r r i c y a n i d e or other oxidant s o l u t i o n . We have a l s o measured the electrochemiluminescence spectrum i n the presence of the f e r r i c y a n i d e / f e r r o c y anide couple ( F i g . 9). The spectrum i s s i m i l a r to that observed by P e t t i n g e r , Schoppel and G e r i s c h e r , while that measured by Beckman and Memming showed two peaks. The luminescence peak measured by us i s at 1 . 6 eV, which i s i n rough agreement with the energy d i f f e r ence between E | and E^ ( 1 . 8 eV). The luminescence can be observed only when the cathodic current i s f a i r l y strong and only when the redox couple i s present. Hence the luminescence can be probably assigned to an e l e c t r o n i c t r a n s i t i o n from the conduction band to the s u r f a c e trapped hole. s

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3. S t a b i l i t y of I l l u m i n a t e d n-GaP i n Redox S o l u t i o n s . F i g u r e 1 0 shows the current-potential curves f o r the n-GaP e l e c t r o d e under i l l u m i n a t i o n , i n the presence of f e r r o u s oxalate F e ^ 2 0 4 ) 2 ^ i f e r r o c y a n i d e Fe(CN) ^~, together with that f o r the s o l u t i o n without a

6

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

n

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AT SEMICONDUCTOR-ELECTROLYTE

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154

Figure 10. Current-potential curves for the (lll)-face of n-GaP under illumination at pH 6.0 in the presence of 0.05M ferrous oxalate (a) and 0.05M ferrocyanide (b), together with that of a solution containing only 0.05M Na SO as a supporting electrolyte 2

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

k

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9.

NAKATO E T A L .

GaP Electrode

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a reducing agent. For the case of f e r r o u s oxalate, (pH 6.0), the onset p o t e n t i a l f o r the photoanodic current i s l a r g e l y moved to the negative, and the i - U curve has l o s t i t s strong h y s t e r e t i c be havior, i n d i c a t i n g that the surface trapped holes are e f f e c t i v e l y quenched by f e r r o u s oxalate and that the e l e c t r o d e i s kept from c o r r o s i o n . The sharp increase of the cathodic current a t — 1 . 3 V i s undoubtedly due to hydrogen e v o l u t i o n . For the case of f e r r o cyanide, the onset p o t e n t i a l of photoanodic current i s somewhat more cathodic but the h y s t e r e s i s s t i l l remains, i n d i c a t i n g that there are s t i l l some surface trapped h o l e s . The e l e c t r o d e i s , however, kept i n t a c t from c o r r o s i o n , the photocurrent showing no decay even at the magnitude of 100 μΑ/cm , i n c o n t r a s t to the case of the s o l u t i o n c o n t a i n i n g no reducing agent, where the photocur rent decays q u i c k l y by the oxide f i l m formation i f the i n i t i a l value of the photocurrent i s higher than 30 μΑ/cm . A l l these r e s u l t s can be explained i n terms of the model pro posed above ( c f . F i g . 11). Namely, with f e r r o u s oxalate having a standard redox p o t e n t i a l E° (Ox/R) of — 0 . 2 V (SCE), which i s a l i t t l e more negative than the E^ of the surface trapped hole l o cated ca. 0.5 V above E^, the surface trapped hole i s e f f e c t i v e l y quenched by the r a p i d r e d u c t i o n , and the photoanodic current flows without decomposition. With f e r r o c y a n i d e , having an Ε(Ox/R) of 0.2 V (SCE), which i s more p o s i t i v e than the E^ of the surface trapped hole, the surface trapped holes are accumulated to the extent that the surface p o t e n t i a l created w i l l l e v e l i t down to the Ε(Ox/R) of the redox couple. At t h i s p o i n t , the r a t e s of nuc l e o p h i l l i c a t t a c k of E^O and OH" to the surface trapped holes are s t i l l low and the e l e c t r o d e decomposition i s prevented. Concluding Remarks In the d i s c u s s i o n s by many authors of the energy conversion e f f i c i e n c y of semiconductor photoelectrochemical systems, i t has been t a c i t l y assumed that the maximum t h e o r e t i c a l photovoltages produced i s the d i f f e r e n c e between E | ( i n u n i t s of eV) and Ε(Ox/R). The best conversion e f f i c i e n c y should then be obtained with a redox couple whose standard redox p o t e n t i a l i s as low as p o s s i b l e , with a reasonable margin x, say 0.3 V, above E ^ ( F i g . 11). From t h i s i t f o l l o w s that the maximum photovoltage obtainable i s equal to the band gap, E , i n an eV u n i t , minus a small margin χ plus Δ. It has been pointed out by some authors (1,2) that f o r a semi conductor having a thermodynamic decomposition p o t e n t i a l , E^, i n between E and E^, a redox couple with a standard redox p o t e n t i a l , E°, more negative than E^ i s needed i n order to operate the photoanode without decomposition. Then, the maximum photovoltage a t t a i n a b l e i s U - E , which i s o f t e n much lower than Ε - Δ - χ . For GaP, t h i s i s only 0.8 V (_4) ( F i g . 11). In t h i s regard, the main c o n c l u s i o n of the present paper i s as f o l l o w s : 1) The n-GaP photoanode can be operated i n a s t a b l e c o n d i t i o n with g

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In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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AT SEMICONDUCTOR-ELECTROLYTE

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Electrolyte (pH = 6.0)

Figure 11. Energy diagram of the interface between n-GaP and an electrolyte solution of pH 6.0

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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a reducing agent (e.g., f e r r o c y a n i d e ) having E° more p o s i t i v e than the decomposition p o t e n t i a l E^. This means that the t h e o r e t i c a l l i m i t of the output photovoltage can be higher than U - E^. 2) The best photovoltage can be obtained with a redox couple having E° s l i g h t l y higher (say — 0 . 2 V) than the E ^ of surface trapped hole (0.1 V), e.g., f e r r o u s oxalate. The use of a redox couple having E° much more p o s i t i v e than that has no merit because i n that case the surface trapped holes a r e accumulated and the sur face p o t e n t i a l moves up so that not only E^ but a l s o U are shifted more p o s i t i v e , thus reducing the t h e o r e t i c a l l i m i t of the photovoltage. F i n a l l y i t i s pointed out that these conclusions f o r n-GaP can be extended to other v a r i o u s η-type semiconductors f o r general c r i t e r i a of the performance of photoelectrochemical systems. s

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Abstract The current-potential curves of the n-GaP electrode were studied in aqueous solutions in the dark and under illumination, in connection with the surface conduction band energy Esc of the GaP, which is equivalent to the electrode potential U determined from the Mott-Schottky plots. From the hysteresis in the currentpotential curves, and the change of U under anodic polarization caused by the action of light or by an oxidizing agent in the solu tion, it has been concluded that the surface trapped holes or sur face intermediates of the anodic decomposition reactions are rather stable in the region of pH between 5 and 10, and, by the accumula tion of these species, a surface potential is built up, causing the shift of U to the positive. In such a pH region, the U values are fixed by the presence of a redox couple, e.g., ferricyanide/ ferrocyanide. It is deduced that the redox couple is in an elec tron transfer equilibrium with the surface trapped holes having a 'standard'redox potential E°(trapped hole) of 0.5 eV above the valence band edge. Electrochemiluminescence spectrum was observed and attributed to the electron transition from the conduction band to the surface trapped hole. Based on these results, it has been theoretically concluded that a photoelectrochemical cell can be operated as a stable system when a redox couple is present in the aqueous phase which has E°(redox) slightly more negative than E° (trapped hole). This conclusion has been experimentally varified. The photovoltage of such a photocell can have the theoretical limit defined by the difference between U and E°(trapped hole), which is much higher than those previously quoted rather pessimistically, on the basis of the thermodynamic decomposition potentials. s

s

s

s

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Literature Cited 1. Gerischer, H. J. Electroanal. Chem., 1977, 82, 133. 2. Bard, A. J.; Wrighton, M. S. J. Electrochem. Soc., 1977, 124, 1706.

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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3. Fujishima, Α.; Inoue, T.; Watanabe, T.; Honda, K. Chem. Lett. 1978, 375; Inoue, T.; Watanabe, K.; Fujishima, Α.; Honda, K. Bull. Chem. Soc. Jpn., 1979, 52, 1243. 4. Memming, R. J. Electrochem. Soc., 1978, 125, 117. 5. Gerischer, H.; Ed. "Topics in Applied Physics, Solar Energy Conversion"; vol. 31, Springer: Berlin, Heidelberg, New York, 1979. 6. Yoneyama, H.; Tamura, H. Chem. Lett., 1979, 457. 7. Nakato, Y.; Tsumura, Α.; Tsubomura, H. J. Electrochem. Soc., 1980, 127, 1502. 8. Nakato, T.; Tsumura, Α.; Tsubomura, H. ibid., submitted for publication. 9. Gerischer, H.; Hoffmann-Perez, M.; Mindt, W. Ber. Bunsenges. phys. Chem., 1965, 69, 130. 10. Lohmann, F., ibid., 1966, 70, 428. 11. Gerischer, H.; Mindt, W. Electrochim. Acta, 1968, 13, 1329; Gerischer, H. Surf. Sci., 1969, 13, 265. 12. Beckmann, Κ. H.; Memming, R. J. Electrochem. Soc., 1969, 116, 368. 13. Pettinger, B.; Schöppel, H. -R.; Gerischer, H. Ber. Bunsenges. phvs. Chem., 1976, 80, 849.

RECEIVED October

3, 1980.

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

10 Surface Aspects of Hydrogen Photogeneration on Titanium Oxides F. T. WAGNER, S. FERRER, and G. A. SOMORJAI

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Materials and Molecular Research Division, Lawrence Berkeley Laboratory, and Department of Chemistry, University of California, Berkeley, CA 94720

Strontium titanate and titanium dioxide have received considerable attention as materials for photoanodes and photocatalysts in the dissociation of water (1,2,3), and in other photoassisted reactions. Knowledge of how the surface composition and electronic structure of these materials change under illumination when in contact with gases or liquid electrolytes is essential i f detailed understanding of the mechanisms of semiconductor photochemistry is to be achieved. Although these wide bandgap oxides do not exhibit gross photocorrosion under most reaction conditions (2) and would appear less susceptable to possible Fermi-level pinning than many semiconductors with smaller bandgaps (4), more subtle surface chemical effects have been documented. Evidence for photocorrosion (5,6,7), surface-state mediation of electron and hole transfer to electrolyte species (8,9), and the dependence of quantum efficiencies on surface preparation techniques (10), indicate important roles for surface species on wide bandgap materials. Most detailed studies of water photodissociation on SrTiO 3

and TiO have concentrated on photoelectrochemical cells (PEC cells) operating under conditions of optimum efficiency, that is with an external potential applied between the photoanode and 2

counterelectrode. We have become i n t e r e s t e d i n understanding and improving r e a c t i o n k i n e t i c s under c o n d i t i o n s of zero a p p l i e d p o t e n t i a l . Operation at zero a p p l i e d p o t e n t i a l permits simpler e l e c t r o d e c o n f i g u r a t i o n s (11) and i s e s s e n t i a l to the development of photochemistry at the gas-semiconductor i n t e r f a c e . Reactions at the gas-sold, rather than l i q u i d - s o l i d , i n t e r f a c e might permit the use o f m a t e r i a l s which photocorrode i n aqueous e l e c t r o l y t e . The g a s - s o l i d i n t e r f a c e i s a l s o more amenable to the a p p l i c a t i o n of u l t r a h i g h vacuum surface a n a l y t i c a l techniques. In t h i s paper the hydroxide concentration dependence of the r a t e of hydrogen production i n S r T i 0 systems (JL2) i s discussed i n l i g h t of s u r f a c e a n a l y t i c a l r e s u l t s . The s u r f a c e elemental composition before and a f t e r i l l u m i n a t i o n i n various aqueous e l e c t r o l y t e s has been monitored with Auger e l e c t r o n spectroscopy 3

0097-6156/81/0146-0159$05.00/0 © 1981 American Chemical Society In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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and i s compared with the composition obtained by u l t r a h i g h vacuum s u r f a c e p r e p a r a t i o n techniques. Auger spectroscopy, while l e s s s e n s i t i v e than photoelectron spectroscopies to s u b t l e changes i n the o x i d a t i o n s t a t e s o f s u r f a c e s p e c i e s , i s more e a s i l y a p p l i e d to the i m p e r f e c t l y c l e a n surfaces obtained i n b a s i c aqueous e l e c t r o l y t e s using present technology. Carbon and s i l i c o n i m p u r i t i e s are found on surfaces exposed to e l e c t r o l y t e s ; the carbonaceous species have some f i l l e d s t a t e s which may make them e f f e c t i v e f o r the mediation o f charge t r a n s f e r across the i n t e r f a c e . The e f f e c t s o f s u r f a c e p l a t i n i z a t i o n on p h o t o a c t i v i t y a r e discussed and evidence f o r a thermal r e a c t i o n between Pd and T i 0 surfaces i s given. (13) A hydrogen-producing s t o i c h i o m e t r i c photoreaction occurs between pre-reduced S r T i 0 and ^.O" T o r r water vapor.(14) At these low pressures surface T I and hydroxyl species can be observed by photoelectron s p e c t r o s c o p i e s . Comparison of the r e a c t i o n c o n d i t i o n s r e q u i r e d f o r hydrogen photogeneration from low pressure water vapor and from aqueous e l e c t r o l y t e allows some s p e c u l a t i o n as to the r o l e s of hydroxyl s p e c i e s . 2

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II.

Experimental

I I . 1 . L i q u i d - s o l i d I n t e r f a c e Experiments. S i n g l e c r y s t a l wafers f o r experiments i n l i q u i d e l e c t r o l y t e were cut to w i t h i n 2% o f the (111) face and etched 3-5 minutes i n molten NaOH h e l d i n a gold l i n e d c r u c i b l e . Wafers were then r i n s e d i n water, soaked 5 minutes i n aqua r e g i a , r i n s e d , soaked 5 minutes i n high p u r i t y 35% aqueous NaOH (Apache SP 7329), r i n s e d i n 7M-ft t r i p l y d i s t i l l e d water, and a i r d r i e d . L i q u i d phase hydrogen photogeneration experiments were c a r r i e d out with a gas chromatographic d e t e c t i o n system described i n more d e t a i l elsewhere.(12) C r y s t a l s rested i n a 2-10 ml pool of e l e c t r o l y t e w i t h i n a b o r o s i l i c a t e glass vacuum f l a s k . The det e c t i o n system was s e n s i t i v e t o r a t e s of a t l e a s t 5 x 1 0 molecules H /hr-cm S r T i 0 (=5 monolayers/hr), but slow r a t e s of oxygen production could not be followed. A 500W high pressure mercury lamp provided a f l u x of bandgap photons o f 1 0 c m s" . The e l e c t r o l y t e f o r most experiments was compounded from r e agent-grade m a t e r i a l s and low c o n d u c t i v i t y water. However, i n some experiments high p u r i t y (Apache SP7329 35% NaOH) or u l t r a p u r i t y ( A l f a 87864 30% NaOH) s o l u t i o n s were employed. Glassware f o r these experiments was prepared by soaking i n 1:1 H S0<,;HN0 , thoroughly r i n s i n g i n 7M-ft water, and r i n s i n g i n the e l e c t r o l y t e to be used. A f t e r being i l l u m i n a t e d , the c r y s t a l s were r i n s e d i n low c o n d u c t i v i t y water, allowed to dry i n a i r , and t r a n s f e r r e d i n t o e i t h e r a P h y s i c a l E l e c t r o n i c s 590 scanning Auger microprobe o r i n t o a UHV chamber equipped with a sample enetry l o c k , a V a r i a n c y l i n d r i c a l m i r r o r Auger analyzer, and a glancing incidence e l e c t r o n gun. Unless otherwise noted, c r y s t a l s r e c e i v e d no argon s p u t t e r i n g , heating, or other c l e a n i n g treatments in vacuo. 15

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II-2 G a s - s o l i d I n t e r f a c e Experiments, Low pressure photor e a c t i v i t y experiments were c a r r i e d out i n a UHV chamber previousl y described (15) equipped with an e l e c t r o n analyzer f o r Auger, photoelectron, and low r e s o l u t i o n energy l o s s s p e c t r o s c o p i e s . D 0 vapor was admitted i n t o the chamber through a v a r i a b l e leak v a l v e . Photogenerated p r o j e c t s were detected with a UTI quadrupole mass spectrometer. The system was s e n s i t i v e to H generation r a t e s as low a 0.3 monolayers/hour and had s t i l l higher s e n s i t i v i t y to oxygen. A P h y s i c a l E l e c t r o n i c s s i n g l e - p a s s CMA and a P h i 4-grid LEED Optics u n i t were used i n the s t u d i e s of metal f i l m s on T i 0 . Pd and Au f i l m s were evaporated from high-density alumina e f f u s i o n sources. S r T i 0 and T i 0 c r y s t a l s f o r gas-phase s t u d i e s were p o l i s h e d with l y diamond paste. P r e - r e d u c e d " S r T i 0 c r y s t a l s were baked four hours i n flowing hydrogen at 1270 K. T i 0 c r y s t a l s were r e duced in vacuo. Clean surfaces were produced by A r bombardment and thermal annealing. 2

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Results

III-l. Hydrogen production and surface stoichiometry. Hydrogen production was observed upon i l l u m i n a t i o n of both s t o i c h i o m e t r i c and pre-reduced S r T i 0 c r y s t a l s i n concentrated NaOH s o l u t i o n s . On metal-free c r y s t a l s , r a t e s o f hydrogen production of 20 and up to 100 monolayers per hour (1 monolayer 5 1 0 molecules/cm S r T i 0 ) were commonly observed, corresponding to a quantum e f f i c i e n c y f o r photons with hv>3.2 eV of 0.03-0.15%. Hydrogen production r e q u i r e d bandgap r a d i a t i o n and could be maintained f o r hours (Figure 1 ) . Rates of hydrogen e v o l u t i o n from platinum-free c r y s t a l s i n v a r i o u s e l e c t r o l y t e s are l i s t e d i n Table I . The r a t e of hydrogen production increases at high 0H~ concentrations.(12) S i m i l a r r a t e s were observed i n 10M NaOH prepared from reagent-grade NaOH (heavy metal i m p u r i t i e s 1 ppm s e n s i t i v i t y , Pt not r e p o r t e d ) . No hydrogen production was observed i n 10M NaClO^ compounded from a reagent c o n t a i n i n g 5 ppm heavy metal i m p u r i t i e s . F i g u r e 2 shows Auger s p e c t r a of water-rinsed S r T i 0 c r y s t a l s (A) before i l l u m i n a t i o n , (B) a f t e r i l l u m i n a t i o n i n 30% NaOH, and (C) a f t e r i l l u m i n a t i o n i n NaC10 . (Spectrum D w i l l be discussed l a t e r . ) These s p e c t r a were taken with high beam current d e n s i t i e s to allow more ready d e t e c t i o n of i m p u r i t i e s . F i g u r e 3 shows T i - 0 Auger s p e c t r a p r e v i o u s l y taken a t low beam currents (<0.1]iA) to minimize beam damage to the s u r f a c e . The 0(507 eV/Ti(380 eV) peak r a t i o of 2.6 i s the same before and a f t e r hydrogen-producing i l l u m i n a t i o n i n 30% NaOH, but i s higher (3.8) a f t e r i l l u m i n a t i o n i n 10M NaCIO*. C r y s t a l s i l l u m i n a t e d i n other e l e c t r o l y t e s (pure water, 1 M H S0*J gave s p e c t r a s i m i l a r to those f o r i l l u m i n a t i o n 3

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A

2

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

162

PHOTOEFFECTS

Table I.

AT SEMICONDUCTOR-ELECTROLYTE

INTERFACES

Hydrogen Photogeneration from Stoichiometric, Initially Metal-Free, SrTiO, in Various Electrolytes (prereduced crystal; 30% NaOH = 10M)

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3

Average y i e l d (monolayers/hr)

Run duration/ Hr

Normalized H yield (monolayers)

30% reagent NaOH

12

1300

110

30% u l t r a - p u r e NaOH

17

940

55

30% high p u r i t y NaOH, acetate added to 8xlO" M

18

1250

70

30% reagent NaOH Pt (IV) added to 4X10~4M

12

19,200

1600

10M reagent NaCIOz,

24

7M-ft water

13

1M H S 0

15

0

0

10

600

60

Electrolyte

2

2

2

0

0 3

36

4

40% reagent NaOH

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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WAGNER E T A L .

Hydrogen Photo generation on Titanium Oxides

163

Nature

Figure 1. Hydrogen accumulation as a junction of time on a stoichiometric, metalfree SrTiOs crystal in 20M NaOH (41)

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE

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- B

- C

1

-c

1 100

200

300

400

ELECTRON ENERGY

500

600

700

800

900

(EV)

Figure 2. Auger spectra of water-rinsed stoichiometric crystals (A) after pretreatment described in Experimental section; (B) after illumination in 30% (10M) NaOH (H produced); (C) after illumination in 10M NaClO (no H produced); (D) after illumination in 30% (10M) NaOH with Pt(IV) added to4X 10' M 2

k

2

4

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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10.

WAGNER E T A L .

Hydrogen Photogeneration on Titanium Oxides

165

i n NaOH. although no hydrogen was produced. The Sr (66 eV)/Ti (380 eV) peak r a t i o s before and a f t e r i l l u m i n a t i o n i n NaOH were a l s o i d e n t i c a l a t 0.11. Oxygen mapping with the scanning Auger microprobe showed no regions of c r y s t a l i l l u m i n a t e d i n NaOH to be s i g n i f i c a n t l y depleted i n oxygen, with a s p a t i a l r e s o l u t i o n o f ^3y. Thus no obvious development o f macroscopic anodic and c a t h odic regions of the c r y s t a l occurs. A f t e r c r y s t a l i l l u m i n a t i o n , spectrophotometry examination of the e l e c t r o l y t e by the p e r t i t a n a t e method (16) showed no d i s s o l v e d t i t a n i u m with a s e n s i t i v i t y on the order of 10 monolayers. I t appears that no i r r e v e r s i b l e change i n the surface stoichiometry of c o n s t i t u e n t elements accompanies the slow photogeneration o f hydrogen on metal-free c r y s t a l s i n aqueous NaOH. Although a change i n stoichiometry occurred i n NaClO^, no such change occurs i n other e l e c t r o l y t e s which a r e a l s o i n e f f e c t i v e f o r hydrogen production. I I I - 2 . Surface Contaminants. Most d i s c u s s i o n s of the r o l e o f surface s t a t e s i n the photochemistry of metal oxides have cons i d e r e d p r i m a r i l y s t a t e s i n the metal-oxygen system (8), induced by adsorption of major species from the e l e c t r o l y t e (17), or generated by i n t e n t i o n a l d e r i v i t i z a t i o n of the surface.(18) Surface i m p u r i t i e s may a l s o play a r o l e i n mediating charge t r a n s f e r across the i n t e r f a c e . A major carbon impurity (272 eV) was observed on a l l c r y s t a l s contacted with l i q u i d water. T y p i c a l carbon coverage was on the order of one monolayer, as estimated from Auger s e n s i t i v i t y f a c t o r s based on T i 0 and g r a p h i t e . The carbon peak shape was more c h a r a c t e r i s t i c of graphite or a part i a l l y hydrogenated carbon l a y e r than of a carbide (19) and the s i g n a l was too intense to be due to a carbonate.(20,21) F i g u r e 9 shows Auger s p e c t r a o f S r T i 0 a f t e r A r s p u t t e r i n g (A), exposure to room a i r (B), and exposure to t r i p l y d i s t i l l e d water. Even on t h i s h i g h l y r e a c t i v e (22) sputtered surface the majority o f carbon contamination a r i s e s from exposure to l i q u i d water rather than to air. UPS spectra of clean A r sputtered and in vacuo carbon-contaminated surfaces a r e shown i n F i g u r e 4. On the c l e a n , sputtered surface a f i l l e d s t a t e due to T i l i e s 0.6 eV below the conduct i o n band.(22) Carbon-induced f i l l e d s t a t e s l i e i n a broad peak with considerable i n t e n s i t y between the valence band edge and the T i ^ peak. Frank et reported evidence that a s t a t e l y i n g about 1.2 eV above the valence band mediates e l e c t r o n t r a n s f e r from T i 0 e l e c t r o d e s . Although these carbon s t a t e s a r e as of now poorly defined and have not been d i r e c t l y i m p l i c a t e d i n any aqueous photochemistry, t h e i r n e a r l y ubiquitous presence should be considered i n d i s c u s s i o n s of charge t r a n s f e r a t r e a l oxide surfaces. The presence of carbon contamination on S r T i 0 surfaces r a i s e s the question of whether a carbonaceous s p e c i e s , rather than water, i s o x i d i z e d during hydrogen photogeneration on P t - f r e e 2

+

3

+

3 +

+

2

3

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

166

PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE

INTERFACES

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N'(E)

350

390 430 E L E C T R O N ENERGY

470 (EV)

510

Figure 3. Low-beam current (< 0.1 fiA) Auger spectra: (A) before illumination; (B) after illumination in 30% (10M) NaOH; (C) after illumination in 10M CaClO k

Figure 4. UPS spectra of clean, Ar*-sputtered SrTi0 before (A) and after (B) contamination with about 1 monolayer C in vacuo 3

BINDING ENERGY (EV)

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

10.

167

Hydrogen Photogeneration on Titanium Oxides

WAGNER E T A L .

c r y s t a l s , where d i f f i c u l t i e s with d e t e c t i o n of slow oxygen product i o n and the low p h o t o a c t i v i t y leave the i s s u e unresolved. Several authors (23,24) have reported photoassisted r e a c t i o n s between carbon and water y i e l d i n g some hydrogen and C0 • PhotoKolbe r e a c t i o n s of carboxylates have a l s o been demonstrated (25). However, n e i t h e r a d d i t i o n of acetate to 0.08M nor an u n i n t e n t i o n a l gross contamination o f the 10M NaOH e l e c t r o l y t e with charred epoxy residue caused s i g n i f i c a n t a c c e l e r a t i o n o f hydrogen product i o n i n our experiments. The presence of carbon monolayers on S r T i 0 shows the need f o r c a u t i o n i n e v a l u a t i n g photoreactions where the t o t a l product y i e l d i s on the order of one monolayer or less. S i l i c o n (Auger peaks a t 92 and 1613 eV) appeared on S r T i 0 surfaces a f t e r i l l u m i n a t i o n i n both NaOH (where hydrogen was produced) and i n NaCIOz, (where no hydrogen e v o l u t i o n occurred) . Less S i deposited from NaOH than from NaCIOz,, but s t i l l l e s s S i appeared a f t e r i l l u m i n a t i o n i n other e l e c t r o l y t e s from which no hydrogen was evolved. I t appears that high a l k a l i c a t i o n concentrations a c c e l e r a t e the etching o f the b o r o s i l i c a t e r e a c t i o n v e s s e l . Surfaces i l l u m i n a t e d i n NaCIO*, e x h i b i t e d an unusual oxygen peak shape, most c l e a r l y v i s i b l e i n the h i g h e r - r e s o l u t i o n s p e c t r a of F i g u r e 5 as the f e a t u r e with an i n f l e c t i o n point a t 484.5 eV. Knotek (26) has reported a s i m i l a r peak shape on a T i 0 surface exposed to water vapor and aged in vacuo. The f e a t u r e may be due to a p e c u l i a r form of h y d r o x y l a t i o n or p e r o x i d a t i o n of the s u r f a c e . However, Legare e t a l . ( 2 7 ) reported a s t r i k i n g l y s i m i l a r spectrum f o r a i r o x i d i z e d s i l i c o n and a s c r i b e d t h i s peak to a bulk plasmon i n S i 0 . The ambiguity i n t h i s data produced by s i l i c o n contamina t i o n i s i n d i c a t i v e of the problems encountered i n the use of glassware with h i g h l y concentrated e l e c t r o l y t e s . Some advantages may be found i n the use o f adherent t h i n f i l m s of e l e c t r o l y t e s . ( 1 2 ) 2

3

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3

2

2

I I I - 3 . M e t a l l i c Impurities and Surface M e t a l l i z a t i o n . K r a e u t l e r and Bard (28) have demonstrated that metal i o n impurit i e s i n aqueous e l e c t r o l y t e s r e a d i l y p l a t e out on i l l u m i n a t e d T i 0 . No heavy metal contaminants were observed on S r T i 0 surfaces i l l u m i n a t e d i n the e l e c t r o l y t e s used here, though surface coverages l e s s than 1% of a monolayer would have remained undet e c t e d . When H P t C l was added to a concentration of 4xlO »M (2000 monolayers Pt i n s o l u t i o n ) i n 30% NaOH and a s t o i c h i o m e t r i c c r y s t a l was i l l u m i n a t e d f o r 12 hours t h e r e i n , platinum deposited unevenly onto the i l l u m i n a t e d s u r f a c e . As the Pt p l a t e d out, the r a t e of hydrogen e v o l u t i o n increased from an i n i t i a l 50 monolayers /hr. to 2000 monolayers/hr. F i g u r e ID shows the spectrum o f the t h i c k e s t part of the Pt d e p o s i t , which formed around a hydrogenc o n t a i n i n g bubble trapped under the c r y s t a l . The Auger equivalent of 3 or 4 monolayers of platinum i s present. The carbon/titanium peak r a t i o appears s i g n i f i c a n t l y smaller than observed on metalf r e e s u r f a c e s , p o s s i b l y due to carbon-consuming photoreactions (23, 24) or s e l e c t i v e d e p o s i t i o n of Pt on carbon. 2

3

_z

2

6

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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168

PHOTOEFFECTS

AT SEMICONDUCTOR-ELECTROLYTE

Figure 5. Details of oxygen Auger spectra of SrTiOs crystals: (A) Af -sputtered; (B) illuminated in 7M Q H 0; (C) illuminated in 30% NaOH; (D) illuminated in 10M NaClOj,; (E) illuminated in 1M H SO . Inflection point of main oxygen peak at 501 eV.

INTERFACES

2

2

460

k

Xl/4

XI dN(E) dE

\

i Ti LMM

1 300

0 KLL

f Pd

E

(eV)

1 400

490 (EV)

dN(E) dE

PdMNN 200

480

Xl/4

XI X 1

|

470

E L E C T R O N ENERGY

1

200

1

Ti LMM

MNN 1 300

0KLL

0(506)/Ti(382)=Q33

E

400 (eV)

500

Figure 6. Auger spectra of Pd film evaporated onto Ti0 before and after annealing ((left) ~ 12 monolayers Pd deposited on Ti0 (110) at room temperature, E = 3.1 KeV; (right; after 5 min anneal at 700°C) 2

2

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

{

10.

WAGNER E T A L .

Hydrogen Photogeneration on Titanium Oxides

169

Reducible oxides such as T i 0 and S r T i 0 have been shown to e x h i b i t strong metal-support i n t e r a c t i o n e f f e c t s i n a number of c a t a l y t i c reactions(29). I t i s p o s s i b l e that d i r e c t m e t a l l i z a t i o n of oxide semiconductors may lead to somewhat d i f f e r e n t chemi s t r y from that obtainable with d i s c r e t e oxide and metal e l e c trodes. Bahl et al.(30) have undertaken photoemission s t u d i e s of p l a t i n i z e d S r T i 0 surfaces and have found in vacuo evidence of p a r t i a l negative charge t r a n s f e r from S r T i 0 to each surface Pt atom. Under more severe c o n d i t i o n s i n t e r m e t a l l i c compounds may form.(13) F i g u r e 6 shows the Auger spectrum of about 12 monolayers of Pd evaporated onto vacuum-reduced T i 0 . Both the t i t a n i u m and oxygen Auger s i g n a l s are almost completely masked by the o v e r l y i n g Pd. Upon annealing above 700°C the T i , but not the 0, s i g n a l grows i n d i c a t i n g a d i f f u s i o n of T i to the s u r f a c e . The T i peak shape i s more m e t a l l i c i n character than that seen on c l e a n T i 0 . The T i d i f f u s i o n i s more pronounced on Ar-sputtered surfaces with a high Ti3+ c o n c e n t r a t i o n and i s not observed f o r Au on T i 0 or Pd on a-Al 0 . 2

3

3

3

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2

2

2

2

3

F i g u r e 7 shows a schematic r e p r e s e n t a t i o n of low energy e l e c t r o n d i f f r a c t i o n patterns on the T i 0 ( 1 1 0 ) and the v i c i n a l (320) " stepped" s u r f a c e s . The p a t t e r n i n F i g u r e 7A was obtained a f t e r annealing a Pd f i l m on e i t h e r s u r f a c e at V500°C; spots due to the s u b s t r a t e and to (111)-faced Pd c r y s t a l l i t e s were observed.The p a t t e r n i n 7B developed on the stepped surface a f t e r annealing a t higher temperature; superimposed on the patterns due to the subs t r a t e and P d ( l l l ) i s another s e t of spots whose l a t t i c e parameter i s c o n s i s t e n t with the hexagonal b a s a l plane of a known i n t e r m e t a l l i c compound, P d T i . ( 3 1 ) I t i s not yet c l e a r whether such metal-metal oxide r e a c t i o n s can be stimulated photochemically. Such r e a c t i o n s could modify the photoelectrochemical p r o p e r t i e s of the system, as platinum-niobium i n t e r m e t a l l i c s have proven s u p e r i o r to pure platinum f o r the e l e c t r o c a t a l y s i s of oxygen r e duction at elevated temperatures.(32) 2

3

I I I - 5 . Comparison of Hydrogen Production with Current Measurements. To allow d i r e c t comparison of hydrogen e v o l u t i o n r e s u l t s from s t o i c h i o m e t r i c metal-free c r y s t a l s i n aqueous e l e c t r o l y t e with r a t e s obtained from a photoelectrochemical c e l l , d i s c r e t e S r T i 0 and p l a t i n i z e d platinum e l e c t r o d e s were mounted i n the vacuum r e a c t i o n f l a s k . The S r T i 0 e l e c t r o d e was prepared from a c r y s t a l p o l i s h e d with l y diamond paste, etched i n molten NaOH, and reduced 4 hours i n hydrogen at 1270 K. Contact was made through Ga-In e u t e c t i c and s i l v e r epoxy, and the contact was i n s u l a t e d with UHV-grade epoxy. Current measurements were made v i a the v o l t a g e drop across an 110, r e s i s t o r placed across the e l e c t r i c a l vacuum feedthroughs to which the e l e c t r o d e s were attached. F i g u r e 8 shows the r e s u l t s of simultaneous measurements of i n t e grated photocurrent and hydrogen accumulation, as measured with the gas chromatograph, i n NaOH e l e c t r o l y t e s of v a r y i n g c o n c e n t r a t i o n . 3

3

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170

PHOTOEFFECTS

AT SEMICONDUCTOR-ELECTROLYTE

INTERFACES

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Structure A (110 and stepped)

Structure B (stepped surface)

a"* II T i 0

2

[lloj

Figure 7. Schematics of LEED patterns and their real-space interpretations for Pd on Ti0 (110) and (320). Key to spots: (%) substrate; (A) Pd( 111 )-faced crystallites; ([J) intermetallic crystallites. 2

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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WAGNER E T A L .

0

Hydrogen Photogeneration on Titanium Oxides

20 40 Time (min)

60

0

20 40 Time (min)

171

60

Figure 8. Simultaneous measurements of H yield and integrated current passed as a function of NaOH electrolyte concentration for a n-SrTiO /Pt PEC cell ((left) gas chromatography; (right) coulometry). Each vertical division corresponds to the equivalent of 5000 monolayers H produced. ((A) 0.01 F; (O) 1 F; (V) 10 F; (D)20F) 2

s

2

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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172

P H O T O E F F E C T S AT

SEMICONDUCTOR-ELECTROLYTE

INTERFACES

The same trend of i n c r e a s i n g photoresponse with i n c r e a s i n g hydroxide concentration i s seen i n both measurements, showing that the hydroxide dependence of hydrogen production i s not simply an a r t i f a c t of the greater s o l u b i l i t y of hydrogen i n l e s s concentrated s o l u t i o n s preventing escape of photogenerated hydrogen to the gas phase. However, some of the d i s c r e p a n c i e s between current and hydrogen measurements i n d i l u t e s o l u t i o n s may be due to t h i s s o l u b i l i t y f a c t o r . The r o l e of d i s s o l v e d oxygen and hydrogen i n a l t e r i n g the e f f e c t i v e "Fermi l e v e l " of the s o l u t i o n (33) has not yet been thoroughly i n v e s t i g a t e d . Although oxygen i s much l e s s s o l u b l e i n concentrated than i n d i l u t e NaOH s o l u t i o n s , i t a l s o d i f f u s e s much l e s s r a p i d l y at higher OH" concentrations.(34) The r e l a t i v e oxygen concentrations i n s o l u t i o n s of v a r i a b l e OH" c o n c e n t r a t i o n deaerated by vacuum pumping (as used here) or by argon or n i t r o g e n purging f o r f i n i t e times remain undetermined. I I I - 6 . Stoichiometry of Vacuum Prepared vs. Water-dipped SrTi0 . A l l of the Auger s p e c t r a i n Figures 2, 3, and 5 were taken a f t e r r i n s i n g the c r y s t a l i n t r i p l e d i s t i l l e d water to r e move e l e c t r o l y t e r e s i d u e which would i n t e r f e r e with Auger a n a l y s i s . F i g u r e 9 shows the changes wrought upon an A r sputtered S r T i 0 surface prepared in vacuo (A), upon exposure to room a i r f o r two minutes (B), or a f t e r a one minute r i n s e i n t r i p l e d i s t i l l e d water (C). The vacuum-prepared s u r f a c e , with or without a i r exposure, shows a higher Sr (68 eV/Ti 383 eV) r a t i o (0.5) than i s seen i n the s u r f a c e exposed i n l i q u i d water (0.2). Tench and Raleigh (35) showed that immersion i n water a l s o removed Sr from S r T i 0 cleaved i n a i r . The e f f e c t i s too l a r g e to be accounted f o r as d i f f e r e n t i a l a t t e n u a t i o n by the carbon monolayer. The higher Sr concentration of the vacuum-prepared surface should lead to a lower surface e l e c t r o n a f f i n i t y , g i v i n g e l e c t r o n s i n the conduction band under f l a t - b a n d c o n d i t i o n s (which are l i k e l y on s t r o n g l y i l l u m i n a t e d , P t - f r e e c r y s t a l s ) greater r e d u c t i v e power than would be a t t a i n a b l e at the water-dipped s u r f a c e . Since r i n s i n g i n water can change the stoichiometry of vacuumprepared c r y s t a l s , i t i s p o s s i b l e that t h i s r i n s i n g step may obscure r e v e r s i b l e changes i n surface stoichiometry which occur during immersion and/or i l l u m i n a t i o n i n d i v e r s e e l e c t r o l y t e s . The use of v o l a t i l e e l e c t r o l y t e s or gas-phase reactants i s thus desirable. 3

+

3

3

I I I - 7 . Hydrogen Photogeneration from Low-Pressure Water Vapor. Water vapor can r e a c t with oxygen vacancies of i l l u m i n ated pre-reduced S r T i 0 surfaces to y i e l d hydrogen and l a t t i c e oxide.(14) Vacuum-prepared (111) surfaces of pre-reduced and s t o i c h i o m e t r i c S r T i 0 were heated to 400°C i n 10" Torr D 0 i n a UHV system equipped with a quadrupole mass spectrometer. Illumi n a t i o n of the pre-reduced c r y s t a l caused an i n c r e a s e i n the D pressure of the system equivalent to D production of 3 monol a y e r s / h r . No such e f f e c t was seen on the s t o i c h i o m e t r i c c r y s t a l . 3

7

3

2

2

2

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

10.

WAGNER E T A L .

Hydrogen Photogeneration on Titanium Oxides

173

E l e c t r o n energy l o s s s p e c t r a showed that the surface T±3+ conc e n t r a t i o n was smaller during i l l u m i n a t i o n i n water vapor than during dark exposure to water vapor. In 10 Torr D 0 photoproduction of HD, but not D was observed. At c r y s t a l temperatures below 200°C photoproduction of n e i t h e r D or HD could be measured. No molecular or atomic oxygen photogeneration was observed under any c o n d i t i o n s . In the absence of D 0, heating the pre-reduced c r y s t a l to 400°C caused the slow e v o l u t i o n of hydrogen from the pre-reduced bulk, but no p h o t o e f f e c t s were seen i n the absence of water vapor. Both oxygen vacancies and hydrogen appear to d i f f u s e towards the surface at 400°C. An oxygen d i f f u s i o n c o e f f i c i e n t of 7 x l 0 " c m S (obtained by e x t r a p o l a t i n g the data of Paladino (26) to 400°C) would allow 3 x l 0 oxygen vacanc i e s - c m S ~ l to reach the surface i n i t i a l l y from a c r y s t a l with a bulk vacancy concentration of 10 cm~^. T h i s r a t e seems adequate to account f o r the l a c k of gas-phase oxygen production i n t h i s experiment. No hydrogen photoproduction was observed on prereduced c r y s t a l s near room temperature i n water vapor at up to 20 Torr pressure.(12) 2

2

2

2

12

2

1 3

2

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IV.

Discussion

IV.1 Hydrogen Production on M e t a l - f r e e C r y s t a l s . Hydrogen i s photogenerated on metal-free, as w e l l as ' p l a t i n i z e d S r T i 0 c r y s t a l s i n aqueous a l k a l i e l e c t r o l y t e s . The use of e l e c t r o l y t e s of higher p u r i t y d?.d not s i g n i f i c a n t l y decrease the r a t e of hydrogen production. S i m i l a r r e s u l t s were obtained i n the adherent e l e c t r o l y t e f i l m s i n which the t o t a l amount of m e t a l l i c i m p u r i t i e s was s e v e r a l orders of magnitude l e s s than i n the bulk l i q u i d e l e c t r o l y t e . ( 1 2 ) Hydrogen photoproduction was observed from the more r i g o r o u s l y P t - f r e e c r y s t a l s exposed to water vapor. While a c a t a l y t i c r o l e f o r very low l e v e l s of m e t a l l i c i m p u r i t i e s can not be r u l e d out, i t appears that the c l e a n oxide s u r f a c e does have some r e s i d u a l a c t i v i t y . Weber (37) has r e ported T a f e l p l o t s f o r hydrogen e v o l u t i o n on anodized sodium tungsten bronze, another p e r o v s k i t e semiconductor, under c o n d i t i o n s c a r e f u l l y designed to prevent platinum contamination. On t h i s m a t e r i a l currents equivalent to hydrogen photoproduction on metal-free S r T i 0 (^2yA) r e q u i r e d an o v e r p o t e n t i a l of 150 mV. As t h i s i s l e s s than the d i f f e r e n c e between the f l a t b a n d potent i a l of S r T i 0 and the hydrogen redox l e v e l (2) i t i s q u i t e p o s s i b l e that the SrTi03 i t s e l f has s u f f i c i e n t c a t a l y t i c a c t i v i t y to account f o r the observed production. 1

3

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During hydrogen e v o l u t i o n on metal-free S r T i 0 photogenerated e l e c t r o n s must d i f f u s e to the surface against an e l e c t r i c f i e l d w i t h i n the d e p l e t i o n l a y e r which tends to d r i v e e l e c t r o n s i n t o the c r y s t a l bulk. K r a u e t l e r and Bard (25) have proposed the existence of shallow surface e l e c t r o n traps to account f o r reduct i v e chemistry on n-type oxides. The only e l e c t r o n - t r a p p i n g s u r face species as yet i d e n t i f i e d by UPS on S r T i 0 or T i 0 i s the 3

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174

P H O T O E F F E C T S AT S E M I C O N D U C T O R - E L E C T R O L Y T E

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J

Ti s t a t e . Though i t has been impossible to monitor t h i s s t a t e at the l i q u i d - s o l i d i n t e r f a c e , the T i ^ c o n c e n t r a t i o n decreases during hydrogen producing i l l u m i n a t i o n i n water vapor. One would expect an i n c r e a s e i n T i ^ upon i l l u m i n a t i o n i f photopopul a t i o n of t h i s e l e c t r o n trap s t a t e c o n t r o l l e d the r e a c t i o n r a t e . It thus appears that even on metal-free S r T i 0 conduction-band e l e c t r o n s are the primary reductants. Since s i m i l a r r e a c t i o n r a t e s occur on pre-reduced and s t o i c h i o m e t r i c c r y s t a l s with d i s parate d e p l e t i o n l a y e r widths, the e l e c t r o n s do not tunnel through the d e p l e t i o n l a y e r . With no Pt to provide an o u t l e t f o r e l e c trons at p o t e n t i a l s f a r p o s i t i v e of the f l a t b a n d p o t e n t i a l , strong i l l u m i n a t i o n would f l a t t e n the bands almost completely and allow e l e c t r o n s to reach the semiconductor s u r f a c e . The presence of both e l e c t r o n s and holes at the surface could lead to unique chemistry as w e l l as high surface recombination r a t e s . On s t o i c h i o m e t r i c c r y s t a l s d i f f u s e p l a t i n i z a t i o n of the i l l u m i n a t e d surface a c c e l e r a t e d hydrogen production, while p l a t i n i z a t i o n of dark areas had l i t t l e e f f e c t . Short d i f f u s i o n d i s tances f o r e l e c t r o n s (and/or p o s s i b l y hydrogen atoms) between S r T i 0 and Pt centers are b e n e f i c i a l to hydrogen production. Since a d d i t i o n of platinum, a good hydrogen e v o l u t i o n c a t a l y s t , does not a l t e r the hydroxide concentration e f f e c t , the e f f e c t bears on oxygen, r a t h e r than hydrogen production. +

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V-2. The Hydroxide Concentration E f f e c t . The higher rates of hydrogen e v o l u t i o n obtained i n h i g h l y concentrated a l k a l i n e e l e c t r o l y t e s are not p r e d i c t e d by simple p h o t o e l e c t r o chemical theory s i n c e the pH dependence of the f l a t b a n d p o t e n t i a l i s expected to be the same as that of the hydrogen and oxygen r e dox p o t e n t i a l s . ( 2 ) Although the photoresponse of S r T i 0 PEC c e l l s have been shown to be independent of e l e c t r o l y t e pH 02), these measurements were taken at a high a p p l i e d p o t e n t i a l . Kawai and Sakata (38) have found that pH-related d i f f e r e n c e s i n the dynamic response~To pulsed l i g h t of T i 0 e l e c t r o d e s disappear upon a p p l i c a t i o n of an e x t e r n a l anodic p o t e n t i a l . At zero a p p l i e d potent i a l surface recombination i s more r a p i d , and the speed of t r a n s fer of charge c a r r i e r s to the s o l u t i o n may become c r i t i c a l to the conversion e f f i c i e n c y of the system. Small changes i n band bending due to s p e c i f i c chemical e f f e c t s would a l s o be more important at zero a p p l i e d p o t e n t i a l . pH-Dependent changes i n surface stoichiometry could increase band bending by decreasing the e l e c t r o n a f f i n i t y of the s u r f a c e , as probably occurs during A r s p u t t e r i n g . However, no such stoichiometry changes have been observed by Auger, and more p u r e l y k i n e t i c explanations of the hydroxide e f f e c t should be considered. That r a t e - l i m i t i n g step i n oxygen production may be (1) abs o r p t i o n of a f a c i l e hole-acceptor s p e c i e s , (2) hole t r a n s f e r to an absorbed or e l e c t r o l y t e s p e c i e s , or (3) desorption of o x i d i z e d products. These steps must compete with bulk and surface recomb i n a t i o n processes. Williams and Nozik (39) have shown that 3

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WAGNER E T A L .

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step (2) i s l i k e l y to be h i g h l y i r r e v e r s i b l e and the s o l i d - l i q u i d i n t e r f a c e , c a s t i n g doubt onto whether step (3) could be r a t e controlling. Comparison of the c o n d i t i o n s r e q u i r e d f o r hydrogen photogeneration from aqueous e l e c t r o l y t e s and from low pressure water vapor may shed some l i g h t on steps (1) and (2). Hydrogen photogeneration from 10" Torr water vapor occurs only on pre-reduced c r y s t a l s at elevated temperatures where a s t o i c h i o m e t r i c r e a c t i o n with reduced centers ( T i ^ - V o ~ ) from the c r y s t a l bulk i s p o s s i b l e . These oxygen vacancies could r e a c t with zero v a l e n t oxygen produced at the surface i n a r e a c t i o n analogous to that y i e l d i n g 0 ( g ) i n aqueous s o l u t i o n . A l t e r n a t e l y , the oxygen from water may never be o x i d i z e d above the - I I or - I s t a t e , and holes may be d i r e c t l y accepted by the reduced center. Figure 10 shows the energies of known surface species (from UPS s p e c t r a (22)) and estimates of the f i l l e d s t a t e d i s t r i b u t i o n of s e v e r a l aqueous species (17) r e l a t i v e to the S r T i 0 band edges. A l l of the UPS-detectable hydroxide s t a t e s formed upon adsorption of low pressure water vapor l i e w e l l below the band edge, where n e i t h e r thermalized nor somewhat hot holes could be accepted. On the c l e a n , water vapor exposed surface only the T i ^ species could accept h o l e s , though considerable r e l a x a t i o n of the hole must occur. Oxidation of T i ^ i s , i n f a c t , observed upon i l l u m i n a t i o n . The f i l l e d s t a t e d i s t r i b u t i o n of aqueous OH" i s b e l i e v e d to overlap the S r T i 0 valence band edge, and r a p i d i s o e n e r g e t i c hold t r a n s f e r i s p o s s i b l e , l e a d i n g to the c a t a l y t i c o x i d a t i o n of water. At high e l e c t r o l y t e concentrations hydroxide-hydroxide r e a c t i o n s may i n c r e a s e the overlap, thereby f a c i l i t a t i n g hole t r a n s f e r . I f the aqueous energy l e v e l s are as shown, no adsorbed s u r f a c e species would be needed to mediate charge t r a n s f e r , though carbon s t a t e s are a v a i l a b l e at the proper energy. 7

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I t should be noted that while UPS could not detect an adsorbed hydroxide species with surface coverage l e s s than 10% of a monolayer, such a species could be chemically a c t i v e . The same hole-acceptor could be present at both the g a s - s o l i d and l i q u i d s o l i d i n t e r f a c e , but the r a t e of i t s formation may be inadequate to compete with oxygen d i f f u s i o n i n t o the bulk from the g a s - s o l i d i n t e r f a c e . Mu uera (40) has found that measurable r a t e s of r e s t o r a t i o n of a hydroxyl species l i n k e d to the p h o t o a c t i v i t y of T i 0 powders f o r oxygen desorption r e q u i r e treatments harsher than immersion i n l i q u i d water. n

2

IV-3. Summary. No Auger detectable changes i n surfaces composition c o r r e l a t e with the higher r a t e s of hydrogen photogeneration on metal-free S r T i 0 observed i n h i g h l y concentrated aqueous a l k a l i n e e l e c t r o l y t e . P l a t i n i z e d c r y s t a l s and PEC c e l l s show s i m i l a r enhancement of p h o t o a c t i v i t y at high hydroxide conc e n t r a t i o n s . Although no hydrogen photogeneration i s seen from water vapor at pressures up to 20 Torr on c r y s t a l s near room temperatures, hydrogen photogeneration does occur on pre-reduced c r y s t a l s at temperatures where oxygen d i f f u s i o n i n t o the bulk i s 3

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

PHOTOEFFECTS

AT SEMICONDUCTOR-ELECTROLYTE

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Figure 10. Filled levels of surface and aqueous species referenced to SrTiO band edges and H , O redox couples s

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SrTiO, 3

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ELECTROLYTE

PH = I3

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r a p i d . K i n e t i c s of h o l e - t r a n s f e r or h y d r o x y l a t i o n may determine the o v e r a l l r e a c t i o n r a t e a t zero a p p l i e d p o t e n t i a l . Carbon s t a t e s l y i n g w i t h i n the S r T i 0 forbidden gap may mediate charge t r a n s f e r i n some photochemical r e a c t i o n systems, and intermeta l l i c compound formation may a l t e r the c a t a l y t i c p r o p e r t i e s of d i r e c t l y m e t a l l i z e d t i t a n i f e r o u s semiconductors. 3

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Acknowledgement This work was supported by the D i v i s i o n of Chemical Sciences, O f f i c e of Basic Energy Sciences, United States Department of Energy under Contract No. W-7405-ENG-48. The authors would l i k e to thank Dr. John Wang f o r a s s i s t a n c e with the scanning Auger microprobe and Dr. P h i l l i p Ross f o r the use of h i s Auger apparatus.

Literature Cited 1. Fujishima, A., Honda, K., Bull. Chem. Soc. Jpn., 1971, 44, 1148 2. Wrighton, M.S., Ellis, A.B., Wolczanski, P.T., Morse, D.L., Abrahamson, H.B., Ginley, D.S., J. Am. Chem. Soc. 1976, 98, 2774. 3. Mavroides, J.G., Kafalas, J.A., Kolesar, D.F., Appl. Phys. Lett. 1976, 28, 241. 4. Bard, A.J., Bocarsly, A.B., Fan, F.F., Walton, E.G., Wrighton, M.S., J. Am. Chem. Soc., 1980, 102, 3671. 5. Schrager, M., and Collins, F.C., J. Appl. Phys., 1975, 46, 1934. 6. Hurlen, T., Acta Chem. Scan., 1959, 13, 365. 7. Harris, L.A., Wilson, R.H. J. Electrochem. Soc.,1976,123,1010. 8. Frank, S.M., Bard, A.J. J. Am. Chem. Soc.1975, 97, 7427. 9. Frank, S.M., Bard, A.J. J. Am. Chem. Soc. 1977, 99, 303. 10. Wilson, R.H., Harris, L.A., Gerstner, M.E., J. Electrochem. Soc. 1979, 126, 844. 11. Nozik, J.A., Appl. Phys. Lett. 1977, 30, 567. 12. Wagner, F.T., Somorjai, G.A. J.Am. Chem. Soc. 1980, 102, 5494 13. Wanger, F.T., Somorjai, G.A. to be published. 14. Ferrer, S., Somorjai, G. A. to be published. 15. Lo,W.J.,and Somorjai, G.A., Phys. Rev. B, 1978, 17, 4842. 16. Mühlebach, J., Müller, K., Schwarzenbach, G., Inorg. Chem. 1970, 9, 2381. 17. Sakata, T., Kawai, T., Ber. Bunsenges. Phys. Chem., 1979, 83 486.

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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18. Tomkiewicz, M., J. Electrochem. Soc., 1980, 127, 1518. 19. Haas, T.W., Grant, J.T., Dooley, G.J.III, J. Appl. Phys. 1972, 43, 1853. 20. Weber, R.E., R-D Magazine, Oct. 1972. 21. Ross, P.N., unpublished data. 22. Ferrer, S., and Somorjai, G.A., Surface Sci. 1980, 94, 41. 23. Sato, S., White, J.M., Chem. Phys. Lett., 1980, 70, 131. 24. Kawai, T., Sakata, T., Nature, 1979, 282, 283. 25. Kraeutler, B., Bard., A.J., J. Am. Chem. Soc., 1978, 100, 5985. 26. Knotek, M.L., in "Proc. Symp. on Electrode Materials and Processes for Energy Conversion and Storage," eds. McIntyre, J.D.E., Srinivasan, S., Will, G. (Proceedings, Vol. 77-6, The Electrochem. Soc., Princeton, NJ, 1977). 27. Legaré, P., Maire, G., Carriére, B., DeVille, J.P., Surface Science, 1977, 68, 348. 28. Kraeutler, B., Bard, A.J., J. Am. Chem. Soc., 1978, 100, 431F. 29. Tauster, S.J., Fung, S.C., Garten, R.L., J. Am. Chem. Soc. 1978, 100, 170. 30. Bahl, M.K., Tsai, S.C., Chung, Y.W., Phys. Rev. B., 1980, 21, 1344. 31. Nishimura, J., Hiramatsu, T., J. Japan Inst. Metal, 1958, 22, 38. 32. Ross, P.N., National Fuel Cell Seminar Abstracts, 1980, 42. 33. Meraming, R., Electrochimica Acta, 1980, 25, 77 34. Gubbins, K.E., Walker, R.D., J. Electrochem, Soc. 1965, 112, 469. 35. Tench, D.M., Raleigh , D.O., in "Electrocatalysis on Non-Metallic Surfaces," NBS Spec. Pub. 455, 1976, p.229. 36. Paladino, A.E., Rubin, L.G., Waugh, J.S., J. Phys. Chem. Solids, 1965, 26, 391. 37. Weber, M.F., "Electrocatalytic Activity and Surface Properties of Tungsten Bronzes," Ph.D. Thesis, Iowa State University, 1977, p.76. 38. Kawai, T., Sakata, T., Chem. Phys. Lett. in press. 39. Williams, F., Nozik, A.M., Nature, 1978, 271, 137. 40. Mumuera, G., Rives-Arnau, V., Saucedo, A., J. Chem. Soc. Faraday Trans. I, 1979, 75, 736. 41. Wagner, F.T., Somorjai, G.A., Nature, 1980, 285, 559. RECEIVED October

9, 1980.

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

11 Photocorrosion in Solar Cells The Enhanced Effectiveness of Stabilization Agents Due to Oxide Films 1

S. ROY MORRISON, M A R C J. MADOU, and K A R L W. FRESE, JR.

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SRI International, Menlo Park, CA 94025

Studies of the corrosion of the n-type silicon electrode show that Fe(II) EDTA, Fe(II) cyano and ferrocene (the last in ethanol) stabilize silicon temporarily against photocorrosion, but only do so after a thin oxide layer is formed. Analysis of voltammetric C/V and I/V curves suggests the reason for this behavior: a voltage develops across the oxide that raises the energy levels of the reducing agent in solution, relative to the energy of the valence band edge, and increases their hole capture efficiency. This effect may also be present with other systems, e.g. GaAs and GaP, where for some stabilizing agents the greatest stability is found at intermediate pH where the Ga2O is relatively insoluble. An important side benefit of the observation is the possibility of analyzing the voltammetry data for the reorganization energy, λ. Preliminary values of λ are given. Finally the disadvantages of requiring a thin oxide on a photoelectrochemical solar cell are briefly discussed. 3

In the development of photoelectrochemical (PEC) s o l a r c e l l s , one of the most d i f f i c u l t problems i s the c o r r o s i o n problem. In any s o l v e n t , but p a r t i c u l a r l y i n solvents with water present, anodic currents flowing from the s o l i d to the s o l u t i o n w i l l u s u a l l y lead t o c o r r o s i o n . S p e c i f i c a l l y the c o r r o s i o n w i l l take the form of anodic o x i d a t i o n of the semiconductor, with the products remaining as a f i l m , d i s s o l v i n g i n t o the s o l u t i o n , or evolving as a gas. Any such a c t i o n w i l l degrade the s o l a r c e l l . We have been studying s i l i c o n as a PEC c e l l , p a r t l y because s i l i c o n would be an e x c e p t i o n a l l y v a l u a b l e m a t e r i a l f o r s o l a r c e l l s i f the c o r r o s i o n could be c o n t r o l l e d , and p a r t l y because the c o r r o s i o n product, Si02, has p a r t i c u l a r l y simple characteristics: i t i s g e n e r a l l y i n s o l u b l e i n aqueous s o l u t i o n s , and i t

1

Supported by the Solar Energy Research Institute

0097-6156/81/0146-0179$05.00/0 © 1981 American Chemical Society In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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i s a good i n s u l a t o r . Thus s i l i c o n not only has p o t e n t i a l f o r a s o l a r c e l l m a t e r i a l , but i t i s of great i n t e r e s t f o r s t u d i e s to generate new b a s i c information regarding photocorrosion. In t h i s report we w i l l t r y to summarize our observation (1) that the presence of a t h i n oxide l a y e r on the surface of s i l i c o n has a dominating e f f e c t on the e f f e c t i v e n e s s of s t a b i l i z i n g agents i n the s o l u t i o n . The mechanism i s simple. As photoproduced holes are captured by i n t e r f a c e s t a t e s , a v o l t a g e develops across the oxide layer and the energy l e v e l s of the ions i n s o l u t i o n become higher r e l a t i v e to the edge of the valence band. Then the photoproduced holes are more e a s i l y captured by the energy l e v e l s of the s t a b i l i z i n g agent. Now the increased a b i l i t y of the reducing agent to capture holes a r i s e s because the valence band edge becomes i s o e n e r g e t i c with the higher density p a r t of the Gaussian d i s t r i b u t i o n of energy l e v e l s (2,3). Thus i f we p l o t the current to the s t a b i l i z i n g agent as a f u n c t i o n of the v o l t a g e across the oxide l a y e r , the most important parameter i n the r e l a t i o n s h i p i s the r e o r g a n i z a t i o n energy, \. Then another f e a t u r e of i n t e r e s t i n t h i s oxide l a y e r model i s that from such measurements we can evaluate the parameter \. In the d i s c u s s i o n to f o l l o w , f i r s t we w i l l consider the experimental observations, secondly, we w i l l go i n t o the theory of the measurement of X and of how the presence of the oxide improves the e f f e c t i v e n e s s of the s t a b i l i z i n g agent. As mentioned above, s i l i c o n has been found to be a very v a l u a b l e m a t e r i a l with which to study the i n f l u e n c e of the oxide l a y e r . T h i s i s because the oxide l a y e r on s i l i c o n i s an e x c e l l e n t i n s u l a t o r , i . e . there i s no problem w i t h c a r r i e r t r a n s p o r t through impurity bands. T h i s makes the i n t e r p r e t a t i o n much simpler with s i l i c o n than i t would be, f o r example, with the oxide on g a l l i u m arsenide (4) where the oxide seems to have s u f f i c i e n t conductance to complicate the i n t e r p r e t a t i o n . Results and D i s c u s s i o n A l a c k of s t a b i l i z a t i o n of the s i l i c o n means that when holes reach the surface the oxide c o n t i n u a l l y grows. In our measurements on s i l i c o n , the l o s s of s t a b i l i t y i s thus observed by the decrease i n photocurrent at a given v o l t a g e as the oxide grows. In a t y p i c a l run we c y c l e the e l e c t r o d e p o t e n t i a l of the s i l i c o n from s t r o n g l y cathodic to s t r o n g l y anodic and back. I f the holes o x i d i z e the s i l i c o n during the anodic p a r t i n the c y c l e , then the current at a given v o l t a g e becomes lower during the next c y c l e because of the growth of the i n s u l a t i n g s i l i c o n oxide. The l o s s of current i s assumed due to lower tunneling p r o b a b i l i t y . I f on the other hand, the s t a b i l i z i n g agent i s e f f e c t i v e , the current at a given v o l t a g e i s maintained at a constant l e v e l through several cycles. In the bottom of F i g u r e 1 we show a s e r i e s of sweeps from cathodic to anodic, with a saturated s o l u t i o n of ferrocene i n a c e t o n i t r i l e present i n the s o l u t i o n . The e f f e c t of the growing

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MORRISON

Figure 1. Voltammetry under illumination with ferrocene in acetonitrile for stability. The system does not stabilize the silicon, as shown by the decrease in current in consecutive Curves 1-4.

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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182

PHOTOEFFECTS

AT

SEMICONDUCTOR-ELECTROLYTE

INTERFACES

oxide on the current v o l t a g e c h a r a c t e r i s t i c s i s c l e a r . The photoc o r r o s i o n at a given v o l t a g e i s lower i n each successive c y c l e . We show r e s u l t s where the s t a b i l i z i n g agent i s i n e f f e c t i v e r a t h e r than r e s u l t s with no s t a b i l i z i n g agent present because the i n t e n t i s to show the slow change i n s u c c e s s i v e c y c l e s due to a slowly i n c r e a s i n g oxide t h i c k n e s s . With no s t a b i l i z i n g agent present the current decreases a f t e r one anodic sweep. In the top h a l f of F i g u r e 1 the behavior of the c a p a c i t y i s shown, as we sweep from the accumulation l a y e r with i t s very high c a p a c i t y toward a d e p l e t i o n l a y e r with i t s r a t h e r low c a p a c i t y . On the f i r s t sweep (#1) a s u b s t a n t i a l band bending can be induced by the p o s i t i v e e l e c t r o d e p o t e n t i a l as i n d i c a t e d by the low capac i t y a t t a i n e d . In s u c c e s s i v e sweeps however we are unable to develop as much band bending at the same e l e c t r o d e p o t e n t i a l . The c a p a c i t y found to be frequency-independent, i s assumed to r e f l e c t the v o l t a g e drop i n the space charge r e g i o n , and the i n a b i l i t y to reach a low c a p a c i t y simply means that the v o l t a g e drop appears across the oxide. In F i g u r e 2 we show the equivalent r e s u l t s with a reasonably e f f e c t i v e s t a b i l i z i n g agent, f e r r o c y a n i d e , present. In t h i s f i g u r e we show r e s u l t s with HF present (no oxide) f o r comparison as curve (d). The f i r s t c y c l e of a f r e s h l y etched e l e c t r o d e i n the 0.1 M potassium ferrocyanide/water s o l u t i o n i s i n d i c a t e d i n the current/voltage c h a r a c t e r i s t i c s as curve ( a ) . Now i n curve (a) the current begins at the same v o l t a g e as i t begins with HF present. In both cases there i s no oxide present on the surface so the current i s c h a r a c t e r i s t i c of an o x i d e - f r e e s i l i c o n sample. However, as anodic current begins to flow, the curve (a) deviates from the HF curve. Curve (a) resembles curve (1) i n F i g u r e 1, and the d e v i a t i o n from the HF curve can be a t t r i b u t e d to c o r r o s i o n (oxide growth). So curve (a) i n d i c a t e s the development of an oxide at the surface of the s i l i c o n . As we sweep back cathodi c , we sweep along curve (b). Following t h i s f i r s t c y c l e , the onset of current i s s h i f t e d as shown i n the anodic sweep to curve (b) . Now the s i g n i f i c a n t d i f f e r e n c e between F i g u r e 1 and Figure 2 i s that curve (b) of F i g u r e 2 i s s i m i l a r f o r the anodic and cathodic sweep, and i s r e p r o d u c i b l e i n successive c y c l e s . There i s apparently no r a p i d oxide growth; there i s no f u r t h e r change i n the c h a r a c t e r i s t i c s of the sample as was observed i n F i g u r e 1. A f t e r the oxide i s grown i n the f i r s t c y c l e , the r e s u l t s of F i g u r e 2 thus i n d i c a t e s t a b i l i t y against photocorrosion. A l though not shown i n the f i g u r e , i t i s found that the photocurrent r i s e s to a s a t u r a t i o n value c l o s e to the s a t u r a t i o n current observed with the same l i g h t i n t e n s i t y with HF present i n the s o l u t i o n . As f u r t h e r i n d i c a t i o n that the changes from curve (d) to curve (b) i s due to an oxide l a y e r , we have i n t e n t i o n a l l y formed an oxide. A f t e r a t h i n oxide i s grown on s i l i c o n ( i n a 0.1 M KC1 e l e c t r o l y t e ) , and then the sample i s t r a n s f e r r e d to a s o l u t i o n containing a s t a b i l i z i n g agent, curve (d) i s observed d i r e c t l y , bypassing the "oxide growing" step of curve ( a ) .

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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11.

MORRISON

ET

AL.

Photocorrosion in Solar Cells

183

The mechanism by which the oxide promotes s t a b i l i t y can be determined by observing the c a p a c i t y / v o l t a g e r e l a t i o n s h i p i n the top h a l f of F i g u r e 2. The curve again shows the case with HF present i n the s o l u t i o n . Curve (b) i n the C/V p l o t corresponds to curve (b) i n the current/voltage p l o t . The i n i t i a l p a r t of the C/V c h a r a c t e r i s t i c s under i l l u m i n a t i o n , more cathodic than about -0.2 v o l t s (SCE), looks very much the same as the c h a r a c t e r i s t i c with the HF present. However, as the e l e c t r o d e p o t e n t i a l i s made more p o s i t i v e , a p l a t e a u i n the C/V curve (b) appears. The p l a teau i n the c a p a c i t y extends from an a p p l i e d v o l t a g e of about -0.3 to +0.1 v o l t s (SCE). Because the capacity r e f l e c t s the v o l tage i n the space charge r e g i o n , i t i s c l e a r that a constant c a p a c i t y means that the v o l t a g e i n the space charge r e g i o n i s constant. Thus the change i n a p p l i e d voltage must appear e l s e where. S p e c i f i c a l l y , the changes i n a p p l i e d voltage must appear across the oxide l a y e r . Thus as we sweep the a p p l i e d v o l t a g e from -0.3 to +0.1 v o l t s , the v o l t a g e appears across the oxide l a y e r , r a i s i n g the energy l e v e l of ions i n s o l u t i o n r e l a t i v e to the energy l e v e l of the valence band i n the s i l i c o n . E v e n t u a l l y the energy l e v e l s of the ferrocyanide come c l o s e enough to the energy l e v e l of the valence band so that holes can tunnel through the oxide, reaching the reducing agent energy l e v e l s i n s o l u t i o n . In Figure 3 we show a case to i n d i c a t e that the sample i s indeed changed i n a way c o n s i s t e n t with such a development of an oxide l a y e r . Curve ( i ) i n d i c a t e s that the c a p a c i t y / v o l t a g e r e l a t i o n s h i p before any oxide i s grown. Curve ( i i ) i s r e p r e s e n t a t i v e of a c a p a c i t y / v o l t a g e curve with the l i g h t on, which can be r e peated s e v e r a l times. Curve ( i i i ) i s again i n the dark with no holes present, a f t e r these repeated c y c l e s under i l l u m i n a t i o n . I t i s observed that the c y c l e s under i l l u m i n a t i o n caused a s h i f t i n the f l a t band p o t e n t i a l of the sample that i s c o n s i s t e n t with the presence of another phase or an adsorbed species at the s u r face. I t i s noted that i n F i g u r e 3 s t a b i l i z a t i o n i s found with ferrous (EDTA) ions i n the s o l u t i o n . Ferrocene i n ethanol i s a t h i r d e f f e c t i v e s t a b i l i z i n g agent (5). The e f f e c t i v e n e s s of various s t a b i l i z i n g agents can be approximately compared by comparing the number of c y c l e s that can be a p p l i e d to the sample before degradation due to excessive oxide growth i s observed. With equivalent v o l t a g e and photocurrent, such as the values shown i n Figure 2, and with a sweep speed of 0.01 V/s, f e r r o cyanide ions s t a b i l i z e the s i l i c o n f o r about h a l f a dozen c y c l e s . T y p i c a l l y , with f e r r o u s (EDTA) present, the curves are unchanged f o r about 10 to 20 c y c l e s and with ferrocene i n ethanol present the curves are unchanged f o r s i g n i f i c a n t l y more. For any of these systems, however, exceeding some photocurrent l i m i t (the l i m i t depending on the s t a b i l i z i n g agent) or some voltage l i m i t (close to + 0.16 SCE) w i l l r e s u l t i n r a p i d degradation of the c h a r a c t e r i s t i c s as t y p i f i e d by F i g u r e 1.

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

184

PHOTOEFFECTS

4.10"

8

AT

SEMICONDUCTOR-ELECTROLYTE

I

I

I

I

I

INTERFACES

I

y

—

/

\//7

(c)

(b)

I

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0

-

f/(b)

J? A

^l^^ II

fa /

0.3

0.2

0.1

0

M -0.2

i -0.4

l

-0.6

l

-0.8

-1.0

VOLTS (V versus SCE)

Figure 2. Volt ammo grams of capacity and current for n-silicon under illumination under various conditions: (a) Fe(CN)f , no HF, first cycle; (b) same solution, subsequent cycles; (c) higher illumination intensity; (d) HF added 4

VOLTS (V versus SCE)

Figure 3.

Shift in dark capacity due to oxide growth in ferrous-EDTA: (i) first curve in dark; (ii) one of several curves in light; (Hi) dark again

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

Downloaded by 148.204.30.87 on October 21, 2009 | http://pubs.acs.org Publication Date: March 2, 1981 | doi: 10.1021/bk-1981-0146.ch011

11.

MORRISON

185

Photocorrosion in Solar Cells

ET AL.

In Figure 4 we suggest a band model f o r the s i l i c o n under three b i a s c o n d i t i o n s : (a) s t r o n g l y cathodic, (b) f l a t band, and (c) s t r o n g l y anodic. The energy l e v e l s i n s o l u t i o n are considered a constant reference system and are shown together with the s i l i con band diagram i n Figure 4(b). Now with a s t r o n g l y cathodic b i a s , as i n Figure 4(a), a s i g n i f i c a n t negative charge may accumulate i n the i n t e r f a c e s t a t e s between the s i l i c o n and the s i l i c o n oxide. Such charging i s suggested by the f l a t band s h i f t i n Figure 2 between curve (d) and curve (b) or (c) as i s observed i n the cathodic region (< -0.2 V SCE) of the C/V curves. With the negative charge i n i n t e r f a c e s t a t e s a voltage appears across the oxide as i s i n d i c a t e d i n F i g u r e 4(a) and the valence band of the semiconductor w i l l be s u b s t a n t i a l l y r a i s e d from the energy l e v e l s of the reducing agent i n s o l u t i o n . We have i n d i c a t e d i n curve (b) (the f l a t band case), that here a l s o the energy l e v e l of the valence band i s too high f o r e f f e c t i v e hole t r a n s f e r to the r e ducing agent i n s o l u t i o n . Now i n the s t r o n g l y anodic case, under i l l u m i n a t i o n , F i g u r e 4 ( c ) , hole trapping w i l l occur at the i n t e r face s t a t e s . The switch from negative to p o s i t i v e charge on the i n t e r f a c e s t a t e s w i l l be observed as a p l a t e a u on the C/V characteristics. The p o s i t i v e charge w i l l lead to a s u b s t a n t i a l lowering of the valence band r e l a t i v e to the ions i n s o l u t i o n , as i n d i c a t e d i n Figure 4 ( c ) , and now holes not only can come to the s u r f a c e , but when they reach the surface they are at an energy h i g h l y favorable f o r tunneling through t o the energy l e v e l s of the reducing agent. We can e a s i l y develop the theory i n more mathematical d e t a i l , e s p e c i a l l y the equations f o r low surface b a r r i e r V , and use the r e s u l t s to estimate X the r e o r g a n i z a t i o n energy of the reducing agent. The r e o r g a n i z a t i o n energy i s a parameter i n the expression d e s c r i b i n g the p r o b a b i l i t y that the energy l e v e l of the i o n i n s o l u t i o n has the energy E. Because of f l u c t u a t i o n s i n the p o l a r i z a t i o n of the medium and i n the bond lengths between the i o n and i t s l i g a n d s , the energy of the i o n f l u c t u a t e s widely i n energy with a normal d i s t r i b u t i o n f u n c t i o n , as i n d i c a t e d i n Figure 4, dominated by X. Figure 5 i n d i c a t e s the symbolism used. We assume that the current to the s u r f a c e , J , i s p r o p o r t i o n a l both to the density of holes at the surface and to the density of ions i n s o l u t i o n with a donor l e v e l i s o e n e r g e t i c with the valence band edge. The p r o p o r t i o n a l i t y constant i n c l u d e s a tunneling p r o b a b i l i t y exp (-yx ), with y constant. s

9

a

Q

J

a

=

B

p

s

e x p

( E

£-

E

) 2 / 4 m

vs~ red

}

e x

P("Yx )

(D

Q

From F i g u r e 5, with E-p (SCE) defined as zero, and (as common with the band model) the p o t e n t i a l s more p o s i t i v e toward the bottom of the f i g u r e , we f i n d : -

( E

E

vs - red

)

qV

" m

V

" <* s "

( E

^>

+

E

« °

+

X

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

(2)

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186

PHOTOEFFECTS

Figure 4.

AT

SEMICONDUCTOR-ELECTROLYTE

INTERFACES

Band model of oxidized silicon.

The energy levels in the solution are kept constant, and the applied voltage shifts the bands in the oxide and the silicon. The Gaussian curves in Figure 4b represent the ferrocyanide/ ferricyanide redox couple with an excess of ferrocyanide. E° is the standard redox potential of iron cyanide. With this, one can construct (a) to represent conditions with an accumulation layers, (b) with flatbands, where for illustration, we assume no charge in interface states, and (c) with an inversion or deep depletion layer (high anodic potential).

J

a

= Bp exp|-(E s

where (E

v$

v$

- E

- E ) /4XkT}exp(- x ) red

red

2

7

) =qV

- q.V - (E

m

s

g

0

- n) + q E ° + X

Assume P = P * exp{qV /kT} s

b

s

then CnJ - qV /kT = const - ( ^ ) a

s

|(qV

m

- q V - (E s

g

- M) + q E ° + X) /\] 2

and curve fitting to give slope = 1/4kT provides best X.

Figure 5.

Determination of X

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

11.

MORRISON

Now

at low V

ET

s

AL.

we

Photocorwsion in Solar Cells

can assume: p

where p

2

= p* exp{qV /kT}

(3)

s

i s the s u r f a c e , p* the bulk hole d e n s i t y .

g

Then 2

- qV /kT = const - (4kT)"" {qV - qV -(Eg-p,) + qE° +X} /X s m s (4) The p h y s i c a l p i c t u r e i s as f o l l o w s . As the valence band edge i s lowered r e l a t i v e to the energy l e v e l s of the reducing agent, that i s , E - E j becomes s m a l l e r , the anodic current w i l l change. The parameter that dominates t h i s change i s the parameter X. A l l the slope-determining parameters i n Equation 4 except X are measurable. The anodic current i s measured d i r e c t l y , of course; the s u r f a c e b a r r i e r V i s measured by measuring the c a p a c i t y of the s i l i c o n s u r f a c e ; the energy gap of the s i l i c o n and i t s Fermi energy [L are known from r e s i s t i v i t y measurements, and Eo i s the standard e l e c t r o d e p o t e n t i a l of the couple. From a p l o t according to Equation (4) we f i n d the value of X that gives a slope of (4 k T ) " . By curves such as shown i n F i g u r e 2 we have been able to analyze X i n p r e l i m i n a r y measurements, f i n d i n g the order of 0.45 eV f o r ferrocene and the order of 0.9 eV f o r f e r r o cyanide. C a r e f u l experiments intended to determine X more accur a t e l y using t h i s method are i n progress. XnJ

1

a

v s

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187

r e (

s

1

Concluding Remarks The a b i l i t y to measure X f o r v a r i o u s reducing agents i n s o l u t i o n i s of course c r i t i c a l i n photocorrosion. The understanding of the r o l e of t h i n p a s s i v a t i n g ( p o s s i b l y oxide) l a y e r s i s a l s o important. I t i s observed i n the present study that the presence of the oxide may be d e s i r a b l e and i n some cases i s probably necessary to make the s t a b i l i z i n g agents e f f e c t i v e i n preventing photoc o r r o s i o n . T h i s observation i s not l i m i t e d to s i l i c o n . It i s w e l l known that g a l l i u m arsenide (5) and g a l l i u m phosphide (6) show most s t a b i l i t y under the c o n d i t i o n s of pH where the g a l l i u m oxide i s i n s o l u b l e i n an aqueous s o l u t i o n . By analogy to the present work we suggest that i n these cases one a l s o apparently needs a t h i n oxide to promote the greatest s t a b i l i t y against photocorrosion. To i l l u s t r a t e i n s l i g h t l y more d e t a i l how the oxide can be e f f e c t i v e i n photocorrosion, Equation 1, g i v i n g the anodic current to the s t a b i l i z i n g agent as a f u n c t i o n of the thickness of the oxide, i s p l o t t e d i n F i g u r e 6. A c t u a l l y we p l o t the maximum current that the s t a b i l i z i n g agent can accept. The value of AE = E E d> zero voltage across the oxide, and X are i n d i c a t e d as the parameters f o r the curves of F i g u r e 6. We have used 0.5 f o r the t u n n e l i n g c o e f f i c i e n t y and 1 0 l 3 - 2 f the i n t e r f a c e s t a t e d e n s i t y (which determines the maximum v o l t a g e obtainable across the oxide) and 3 f o r the d i e l e c t r i c constant of the oxide. v s

w

l

t

n

re

cm

o

r

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

188

AT

SEMICONDUCTOR-ELECTROLYTE

INTERFACES

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PHOTOEFFECTS

Figure 6. Maximum hole current to a reducing agent as a function of oxide thickness. Equation 1 is plotted, with p assumed constant and incorporated into J . AE = E — E with no oxide, A. is the reorganization energy, and other constants have been chosen as described in the text. s

v s

red

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

0

11.

MORRISON

ET AL.

Photocorrosion in Solar Cells

189

In F i g u r e 6, where we p l o t the maximum current that the s t a b i l i zing agent can accept, i f the current r i s e s above the value i n d i cated, the surface i s by d e f i n i t i o n unstable. Consider f o r examp l e the curve where AE = 1 and X = 0.7. I f we consider a current of, say 10 4 j A/cm2, i t i s observed that with no oxide there i s no s t a b i l i t y , so the oxide w i l l grow. The oxide w i l l grow, according to t h i s curve, u n t i l i t i s 5 X i n t h i c k n e s s . Then i t w i l l provide s t a b i l i t y . I f , however, the oxide grows beyond 18 X i n thickness then the s t a b i l i z i n g agent can no longer capture the holes, p r i m a r i l y because with the value of y chosen, the t u n n e l ing current i s too low. Then again the s i l i c o n w i l l no longer be s t a b l e . Of course i n p r a c t i c e 100% s t a b i l i t y cannot be expected f o r any value of oxide t h i c k n e s s — e v e n i f 99.99% s t a b i l i t y i s achieved, the 0.01% of the holes causes i r r e v e r s i b l e oxide growth i n t h i s system where there i s no mechanism f o r oxide removal. Thus i n the experiments reported above, a l a s t i n g s t a b i l i t y was never reached. In conclusion i t should be pointed out that the s t a b i l i t y of the s i l i c o n or other semiconductor against photocorrosion i s not the only important parameter i n PEC s o l a r c e l l s . Free current flow from the s i l i c o n valence band t o the reducing agent i n s o l u t i o n must be p o s s i b l e i n order to have an e f f i c i e n t s o l a r c e l l . Thus with s t a b i l i z i n g agents such as we have been d i s c u s s i n g i n the present experiments, where an oxide i s needed, a compromise must be reached between the improved c o r r o s i o n r e s i s t a n c e of the m a t e r i a l s and the poor current flow c h a r a c t e r i s t i c s . An oxide may or may not a f f e c t the open c i r c u i t voltage or the short c i r c u i t current adversely, but the f i l l f a c t o r of the s o l a r c e l l w i l l s u f f e r most due t o the increased voltage across the oxide as the photocurrent i n c r e a s e s . -

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Q

Literature Cited 1.

Madou, M. J., Frese, Jr., K. W., and Morrison, S. R., J. Phys. Chem. (submitted for publication). 2. Gerischer, H., Z. Phys. Chem., 1961, NF 27, p. 48 3. Morrison, B. R., "Electrochemistry at Semiconductor and Oxidized Metal Electrodes," Plenum: New York, 1980 (in press). 4. Frese, Jr., K. W. and Morrison, S. R., J. Vac. Sci. Tech., 1980, 17, 609. 5. Wrighton, M. S., Bolts, J. M., Bocarsley, A. B., Palazzotto, M. C., and Walton, E. G., J. Vac. Sci. Tech., 1978, 15, 1429. 6. Madou, M. J., Frese, Jr., K. W., and Morrison, S. R., J. Electrochem. Soc., 1980, 127, 987. 7. Madou, M. J., Cardon, F., and Gomes, W. P., Ber. Bunsenges, Phys. Chem., 1977, 81, 1186. RECEIVED

October 3, 1980.

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

12 Conditions for Rapid Photocorrosion at Strontium Titanate Photoanodes R. E. SCHWERZEL, E. W. BROOMAN, H . J. BYKER, E. J. DRAUGLIS, D. D. L E V Y , L. E. V A A L E R , and V. E. WOOD

Downloaded by 148.204.30.87 on October 21, 2009 | http://pubs.acs.org Publication Date: March 2, 1981 | doi: 10.1021/bk-1981-0146.ch012

Battelle Columbus Laboratories, 505 King Avenue, Columbus, OH 43201

In 1912, the great Italian photochemist, Giacomo Ciamician, published a remarkable paper entitled "The Photochemistry of the Future" (1) in which he considered the wealth of benefits which might be gained by the photochemical utilization of solar energy for the production of useful chemical materials. In discussing the role of plant crops (or biomass, as we would say now) as solar energy transducers, he suggested that: "The harvest, dried by the sun ought to be converted, in the most economical way, entirely into gaseous fuel, taking care during this operation to fix the ammonia (by the Mond process, for instance) which should be returned to the soil as nitrogen fertilizer together with all the mineral substances contained in the ashes". This elusive goal of efficient, economical fuel production from renewable biomass resources has stimulated research efforts around the world since Ciamician's time. While much progress has been made, the problems involved are far from solved, and the production of gaseous fuels from biomass is s t i l l too expensive to be economically feasible on a large scale. Nonetheless, recent developments i n s e v e r a l aspects of photoelectrochemistry have o f f e r e d renewed promise that the production of u s e f u l f u e l s with s o l a r energy might indeed become a v i a b l e process. Of p a r t i c u l a r i n t e r e s t i n t h i s context has been the f i n d i n g that the Kolbe r e a c t i o n , the anodic o x i d a t i o n of c a r b o x y l i c acids (Equation 1) ( 2 ) , can be made to occur at n-type oxide semiconductor photoanodes to the v i r t u a l exc l u s i o n of oxygen formation ( 3 , 4 , 5 ) .

2 C H 3 C O 2 H AG

0

C H6 +2 C 0 +H 2

2

2

= - 2 2 . 3 kJ/mole ( - 5 . 3 kcal/mole)

0097-6156/81/0146-0191$05.00/0 © 1981 American Chemical Society In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

(1)

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192

PHOTOEFFECTS

AT

SEMICONDUCTOR-ELECTROLYTE

INTERFACES

While t h i s r e a c t i o n i s s u b s t a n t i a l l y exothermic (6), i t provides an i n t r i g u i n g approach to the production of f u e l s from renewable resources, as the required acids ( i n c l u d i n g a c e t i c a c i d , b u t y r i c a c i d , and a v a r i e t y of other simple a l i p h a t i c c a r b o x y l i c a c i d s ) can be produced i n abundant y i e l d s by the enzymatic fermentation of simple sugars which are, i n t u r n , a v a i l a b l e from the m i c r o b i o l o g i c a l h y d r o l y s i s of c e l l u l o s i c biomass materials (7). These considerations have l e d us to suggest the concept of a "tandem" p h o t o e l e c t r o l y s i s system, i n which a s o l a r p h o t o e l e c t r o l y s i s device f o r the production of f u e l s v i a the photo-Kolbe r e a c t i o n might derive i t s a c i d - r i c h aqueous feedstock from a biomass conversion plant for the h y d r o l y s i s and fermentation of crop wastes or other c e l l u l o s i c materials (4.). As one aspect of our recent studies i n t h i s f i e l d , we have sought to extend the range of c o n d i t i o n s under which the photo-Kolbe r e a c t i o n could be conducted, so as to explore the s e n s i t i v i t y of the process to such v a r i a b l e s as l i g h t i n t e n s i t y , pH, concentration of c a r b o x y l i c a c i d , and so on. It has gradually become apparent that under at l e a s t some of our experimental c o n d i t i o n s , the strontium t i t a n a t e photoanodes can undergo severe photodegradation. When t h i s occurs, v i s i b l e p i t s or c r a t e r s are formed i n the i l l u m i n a t e d p o r t i o n of the e l e c t r o d e a f t e r s e v e r a l hours or days of exposure to focused l i g h t from an Eimac 150W xenon arc lamp. This obs e r v a t i o n i s t o t a l l y unprecedented; a f t e r a l l , strontium t i t a n a t e i s one of the few m a t e r i a l s that has been unanimously reported i n the l i t e r a t u r e to be a robust, s t a b l e photoanode (8-16). We have conducted numerous r e p l i c a t e c o n t r o l experiments to determine whether a procedural e r r o r or an equipment malf u n c t i o n (such as, f o r example, the leakage of a l t e r n a t i n g current from the l i n e c i r c u i t s i n t o the dc c i r c u i t s of the p h o t o e l e c t r o l y s i s apparatus) could have been responsible f o r the observed e f f e c t s . U l t i m a t e l y , these c o n t r o l experiments led us to add an o s c i l l o s c o p e to the d i a g n o s t i c equipment (to check f o r ac leakage), to replace each of the major e l e c t r o n i c components ( i n c l u d i n g the p o t e n t i o s t a t , voltage scan u n i t , and electrometer) with a l t e r n a t i v e components of comparable q u a l i t y , and to replace the simple, one-compartment c e l l we had been using with a newly designed two-compartment c e l l , so as to minimize the p o s s i b i l i t y that cathodic products could somehow a f f e c t the s t a b i l i t y of the strontium t i t a n a t e photoanodes. In a d d i t i o n , the electrodes used i n the new c e l l were mounted to t h e i r Pyrex support tubes using heat-shrinkable T e f l o n tubing rather than epoxy cement. As before, the new c e l l had a Pyrex window through which the semiconductor electrode could be i r r a d i a t e d , a Luggins c a p i l l a r y p o s i t i o n e d c l o s e to the surface of the i l l u m i n a t e d e l e c t r o d e , and i n t e g r a l gas burets above each e l e c t r o d e f o r the c o l l e c t i o n of gas samples.

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

12.

SCHWERZEL

ET

SrTiOs Photoanodes

AL.

Despite these precautions, marked c o r r o s i o n was s t i l l observed on some, but not a l l , of the n - S r T i 0 3 photoanodes obtained from four d i f f e r e n t sources. The c o r r o s i o n appeared to be most severe a f t e r s e v e r a l experiments ( t o t a l l i n g t y p i c a l l y 20 hours or more of use as an e l e c t r o d e ) had been conducted under photo-Kolbe r e a c t i o n c o n d i t i o n s . A f i n e white f i l m was a l s o observed to form g r a d u a l l y on the i r r a d i a t e d areas of n-SrTi03 when the a c i d e l e c t r o l y t e ( t y p i c a l l y 2N H 2 S O 4 ) was used i n the absence of added a c e t i c a c i d .

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C h a r a c t e r i s t i c s of the Photocorrosion

Process

The magnitude of the problem can be appreciated by comparing the c r y s t a l s marked A and B i n Figure 1. C r y s t a l A i s t y p i c a l of the c o n d i t i o n of our n - S r T i 0 3 e l e c t r o d e s j u s t p r i o r to etching and mounting; i t i s smooth, shiny ( a f t e r p o l i s h i n g with a 1.5y diamond paste c l o t h ) , and somewhat transparent. C r y s t a l B i l l u s t r a t e s the degree of photocorr o s i o n which occurred i n a s i m i l a r n-SrTi03 e l e c t r o d e a f t e r approximately 50 hours of i r r a d i a t i o n (during s e v e r a l experiments) under t y p i c a l photo-Kolbe c o n d i t i o n s , i n t h i s case aqueous s u l f u r i c a c i d c o n t a i n i n g a c e t i c a c i d . The s e v e r e l y eroded area i s l o c a t e d where the l i g h t beam was focused on the e l e c t r o d e ; the r e s i d u a l p i t t i n g on the surface i s probably due to s c a t t e r e d l i g h t s t r i k i n g the e l e c t r o d e . V i r t u a l l y no photo-Kolbe products have been observed by mass spectrometry i n the gas evolved from the decomposing e l e c trodes; the primary gaseous product i s oxygen, although v a r i a b l e amounts of C 0 have been observed at times. Thus, there appears to be a competition between e l e c t r o d e decomp o s i t i o n and the photo-Kolbe r e a c t i o n under these c o n d i t i o n s . While we have not yet c a r r i e d out d e t a i l e d k i n e t i c measurements on the r a t e of photocorrosion, our impression i s that the process i s r e l a t i v e l y i n s e n s i t i v e to the s p e c i f i c composition of the strontium t i t a n a t e . Trace element comp o s i t i o n s , obtained by spark-source mass spectrometry, are presented i n Table I f o r the four boules of n - S r T i 0 3 from which electrodes have been cut. Photocorrosion has been observed i n samples from a l l four boules. In a l l cases, the electrodes were cut to a thickness of 1-2 mm using a diamond saw, reduced under H at 800-1000 C f o r up to 16 hours, p o l i s h e d with a diamond paste c l o t h , and etched with e i t h e r hot concentrated n i t r i c a c i d or hot aqua r e g i a . Ohmic cont a c t s were then made with gallium-indium e u t e c t i c a l l o y , and a wire was attached using e l e c t r i c a l l y conductive s i l v e r epoxy p r i o r to mounting the e l e c t r o d e on a Pyrex support tube with e i t h e r epoxy cement or heat-shrinkable T e f l o n tubing. Some information about the nature of the photocorrosion process i s provided by a comparison of the U V / v i s i b l e ab2

2

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

194

PHOTOEFFECTS

TABLE I .

AT SEMICONDUCTOR-ELECTROLYTE

TRACE ELEMENT COMPOSITION OF n-SrTiO

Q

INTERFACES

BOULES

Sample Element

x

(b)

2

3(d)

(c)

4

(e)

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j-e.

0.3

1.0

0.3

<1,0

<0.05

<0.05

<0.3

20.0

10.0

50.0

Not reported

^3

^3

^3

^5

w

2.0

Ta

<0.03

30.0

0.2

Nb

<0.05

Ba Fe

(a)

Analyses are reported as p a r t s per m i l l i o n (by weight), and were obtained by spark-source mass spectrometry. The i d e n t i t y of the bulk m a t e r i a l as SrTiO^ was confirmed by x-ray powder d i f f r a c t i o n measurements.

(b)

Obtained from N a t i o n a l Lead Company some 5 or 6 years ago f o r another p r o j e c t at Battelle-Columbus, and made a v a i l a b l e to us i n 1977 when our research i n p h o t o e l e c t r o l y s i s began. Our i n i t i a l photoKolbe experiments were c a r r i e d out using e l e c t r o d e s cut from t h i s boule.

(c)

This sample was k i n d l y provided by Mr. Fred Wagner, of the M a t e r i a l s Science Laboratory, The U n i v e r s i t y of C a l i f o r n i a at Berkeley (1978).

(d)

Purchased

from Commercial C r y s t a l L a b o r a t o r i e s

(1978). (e)

Purchased

from Atomergic

L a b o r a t o r i e s (1979),

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

12.

SCHWERZEL

ET

AL.

SrTiOs Photoanodes

s o r p t i o n spectra of the e l e c t r o d e s before and a f t e r extended irradition. As Figure 2 i l l u s t r a t e s , the photocorrosion process i s accompanied by a dimunition of the band edge below 400 nm and a pronounced broadening of the v i s i b l e absorbance around the T i ^ band at 505 nm. The magnitude of the v i s i b l e absorbance does not change a p p r e c i a b l y , remaining about 0.7 to 0.8 at 505 nm before and a f t e r c o r r o s i o n f o r these e l e c t r o d e s . To o b t a i n these s p e c t r a , a t h i n (<1 mm) s i n g l e c r y s t a l wafer was used as an e l e c t r o d e a f t e r i t had been reduced to a b r i g h t blue c o l o r (as viewed by transmitted l i g h t ) , p o l i s h e d , and etched as described above. A f t e r the e l e c t r o d e had been corroded by i r r a d i a t i o n under photo-Kolbe c o n d i t i o n s f o r some 20 hours, i t was demounted from i t s holder and p o l i s h e d again. These spectra therefore r e f l e c t changes i n the bulk semiconductor, which are manifested by a change i n c o l o r from the o r i g i n a l blue to g r e y i s h - t a n .

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+

Further i n s i g h t i s provided by the F o u r i e r - t r a n s f o r m i n f r a r e d absorption spectrum of the c o r r o s i o n residue which could be scraped from the surface of a n - S r T i 0 3 e l e c t r o d e a f t e r p a r t i a l photocorrosion i n the mixed H2S04:H0Ac e l e c t r o l y t e . As Figure 3 shows, the spectrum contains strong a b s o r p t i o n bands at 460 cm""l and 620 cm"*, which are c h a r a c t e r i s t i c of S r T i 0 3 # However, i t a l s o contains strong bands at 620 cm"- , 650 cm" , 1000 cm"" , 1100 cm , 1140 cm"" , and 1205 cm"" , which are c h a r a c t e r i s t i c of S r S 0 4 , and two weaker bands at 410 cm"" and 425 cm" , which appear to be d i a g n o s t i c f o r S r ( 0 A c ) . There i s no s t r i k i n g evidence f o r the presence of T i 0 2 > although i t s i n f r a r e d spectrum i s r e l a t i v e l y f e a t u r e l e s s . The assignments reported here have been confirmed by recording the i n f r a r e d spectra of authentic samples of SrSO^ and S r ( 0 A c ) . Since S r ( 0 A c ) i s s i g n i f i c a n t l y more s o l u b l e i n water than i s SrS04 (which p r e c i p i t a t e s immediately when a d i l u t e s o l u t i o n of S r ( 0 A c ) i s added to d i l u t e s u l f u r i c a c i d ) i t i s p l a u s i b l e that the photocorrosion process may be a c c e l e r a t e d under these experimental c o n d i t i o n s by the formation of S r ( 0 A c ) and the subsequent p r e c i p i t a t i o n of S r S 0 4 at the e l e c t r o d e s u r f a c e . In s u l f u r i c a c i d s o l u t i o n s alone, the formation of an i n s o l uble l a y e r of the s u l f a t e on the e l e c t r o d e surface may serve to i n h i b i t the r a p i d bulk c o r r o s i o n which has been observed i n the presence of a c e t i c a c i d . F i n a l l y , we note that the photocorrosion process i s s t r o n g l y pH-dependent, o c c u r r i n g most r e a d i l y i n s t r o n g l y a c i d s o l u t i o n s , and that the presence of a c a r b o x y l i c a c i d i s required f o r the occurrence of severe photocorrosion. In Table I I we present a n a l y t i c a l r e s u l t s , based on i n d u c t i v e l y coupled argon plasma (ICP) emission spectroscopy, f o r r e p r e s e n t a t i v e e l e c t r o l y t e s o l u t i o n s a f t e r 6-8 hr. of photo-Kolbe e l e c t r o l y s i s with n - S r T i 0 3 anodes. I t can be seen that the formation of s o l u b l e strontium and t i t a n i u m species i s 1

1

1

1

-1

1

1

1

2

2

2

2

2

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

196

PHOTOEFFECTS

AT

SEMICONDUCTOR-ELECTROLYTE

INTERFACES

Figure 1. Comparison of n-SrTiO photoelectrodes (A) before and (B) after corrosion under photo-Kolbe reaction conditions Downloaded by 148.204.30.87 on October 21, 2009 | http://pubs.acs.org Publication Date: March 2, 1981 | doi: 10.1021/bk-1981-0146.ch012

s

Figure 2. UV/visible absorption spectra of n-SrTiO (A) before and (B) after partial photocorrosion in 2N H SO containing 0.5N HO Ac (about 20 h irradiation) s

2

If

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

12.

SCHWERZEL

ET AL.

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TABLE I I .

SrTiOs Photoanodes

197

ANALYSIS OF ELECTROLYTES FOR PHOTOCORROSION PRODUCTS FROM n - S r T i 0 o

Sample

pH

Electrolyte

1

0.2M NaOH + 0.5M N a O A c

(b)

2

0.5M HOAc + 0.5M N a O A c

(c)

3

1.0M H S0. + 0.5M H O A c

(c)

o

[Sr]

( a )

[Ti]

( a )

0.10

<0.05

5

0.25

<0.05

0

4.10

2.20

13

(a)

Concentration of strontium and titanium, r e s p e c t i v e l y , i n yg/ml, as determined by i n d u c t i v e l y coupled argon plasma (ICP) emission spectroscopy ( J a r r e l l - A s h ICP spectrometer).

(b)

Bias p o t e n t i a l , 0.0 V vs. SCE.

(c)

Bias p o t e n t i a l , +1.0 V vs. SCE.

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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198

PHOTOEFFECTS

3600

3200

2800

A T SEMICONDUCTOR-ELECTROLYTE

2400

2000

Frequency, cm'

1200

1600

INTERFACES

800

400

1

Figure 3. Fourier transform IR absorption spectra of corrosion residue from n-SrTi0 electrode formed after about 20 h irradiation in 2N H SO containing 0.5N HO Ac 3

2

k

Conduction Bond EFB -+0.04V -+0.49V

0.35V S r ( 0 A c ) 2 + T1O2 •H 2 O ^ S r T i 0 3 + 2 H 0 A Sr2+ + T i 0 2

+

C

+ C O 2 + C H 3 O H + 1/202 + 4 e " — S r T i 0 3 + H O A c

-+0.68V

S r S 0 4 ( c ) + T . 0 2 + + 1/202

-+0.92V

Sr2+ + T i 0 2 + + 0 2 ( g ) + 4 e " - + S r T i 0 3

-+0.99V

O2 + 4 H + 4e~

- + 1.16V

S r ( C H 0 ) 2 (aq) + TiC>2

- + 1.75V

Range of Potentials Reported for Photo-Kolbe Reaction

+

2

+

H 2 O + 2e - S r T i 0 3 + H 2 S 0 4 ( a q )

2H20

3

( 2 C H 3 C O 2 H + 2e~ -

1/202 + H 2 O + 2e~ ^ S r T i 0 3 + 2 H 0 A c

+

C2H

6

+2C0

2

+ H ) at 2

P l a t i n u m Anodes

Valence Band Volts vs SCE

Figure 4.

Estimated redox potentials of possible photocorrosion processes in strontium titanate at pH = 0 (V vs. SCE)

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

12.

SCHWERZEL

ET

AL.

SrTiOs Photoanodes

g r e a t l y a c c e l e r a t e d i n the s t r o n g l y a c i d ( p H = 0 ) s o l u t i o n , and that only trace amounts of S r ^ can be detected i n the other s o l u t i o n s . I t i s not yet c l e a r whether the presence of s u l f u r i c a c i d i s required f o r r a p i d c o r r o s i o n , or whether the r a p i d c o r r o s i o n observed i n the s o l u t i o n s containing s u l f u r i c a c i d simply r e f l e c t s the e f f e c t of low pH. The same e l e c trode was used a l l f o r these experiments, and a l l i r r a d i ations were c a r r i e d out with the sample apparatus f o r approximately equal periods of time; thus, the r e l a t i v e amounts of strontium found i n the s o l u t i o n s provide an i n d i c a t i o n of the r e l a t i v e rates of photocorrosion i n the d i f f e r e n t e l e c t r o l y t e s . As noted before, l i t t l e or no photocorrosion could be detected v i s u a l l y i n e i t h e r a c i d i c or b a s i c e l e c t r o l y t e s under comparable c o n d i t i o n s i n the absence of added a c e t i c a c i d (or a c e t a t e ) .

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+

Proposed Mechanism of

Photocorrosion

The electrochemical redox p o t e n t i a l of s e v e r a l p o s s i b l e decomposition r e a c t i o n s at pH = 0 ( r e l a t i v e to the p o t e n t i a l of the saturated calomel e l e c t r o d e ) , which have been estimated from thermodynamic parameters (6,17-21), are shown schematically i n Figure 4 . The band l e v e l s are shown f o r o p e n - c i r c u i t c o n d i t i o n s . The standard electrode p o t e n t i a l s were c a l c u l a t e d from the f r e e energies of formation, which are summarized below i n Table I I I . A l l but one of the r e a c t i o n s i n Figure 4 lead to the formation of the s o l u b l e T i O ^ i o n ; t h i s seems c o n s i s t e n t with the observed changes i n the v i s i b l e absorption spectrum of the s o l i d e l e c t r o d e . I t may a l s o be that other titanium species are formed i n s o l u t i o n , such as peroxytitanium complexes l i k e IfyTiC^. We have no d i r e c t evidence as to the i d e n t i t y of the s o l u t i o n species at t h i s time, and have l i m i t e d the candidate c o r r o s i o n r e a c t i o n s shown i n Figure 4 to those f o r which thermodynamic data are r e a d i l y a v a i l a b l e . Nonetheless, the f a c t that t i t a n i u m i s observed i n the e l e c t r o l y t e only upon extensive photocorrosion (and then i n smaller amounts than strontium) suggests that the i n i t i a l photocorrosion process involves the l o s s of strontium from the S r T i 0 3 , with the formation of S r ( 0 A c ) 2 or SrSC>4. However, we cannot r e a l l y choose among the p o s s i b l e photoc o r r o s i o n r e a c t i o n s on the b a s i s of the information p r e s e n t l y available. Some a d d i t i o n a l i n s i g h t as to p o s s i b l e mechanisms of the photocorrosion process can be gained from a more d e t a i l e d c o n s i d e r a t i o n of the e f f e c t s of pH on the band l e v e l s i n S r T i 0 3 and on the redox p o t e n t i a l s of oxygen formation and the photo-Kolbe r e a c t i o n . These data, along with the band l e v e l s f o r T i 0 2 , are shown i n Figure 5 . I t i s important to remember that the photocorrosion process occurs i n com+

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

PHOTOEFFECTS

200

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TABLE I I I .

Compound

SrTi0 SrS0 Ti0

4

(c)

3

(c) (c)

2

Sr(OAc) Sr

2 +

2

(aq)

(aq)

AT SEMICONDUCTOR-ELECTROLYTE

STANDARD FREE ENERGIES OF FORMATION

AG

f

(kcal/mole)

Reference

-378.8

17

-318.9

6

-203.8

6

-311.8

6

-133.2

6 18, 19

(aq)

-138.2

(aq)

-177.34

6

(g)

-94.26

6

H 0 (1)

-56.69

6

CH 0H (aq)

-42.12

20

HOAc (aq)

-96.19

21

Ti0

2 +

H S0 2

co

2

4

2

3

INTERFACES

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

SCHWERZEL

ET AL.

SrTiOs Photoanodes

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12.

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

201

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202

PHOTOEFFECTS

AT

SEMICONDUCTOR-ELECTROLYTE

INTERFACES

p e t i t i o n with both the photo-Kolbe r e a c t i o n and normal p h o t o e l e c t r o l y s i s (although oxygen i s g e n e r a l l y evolved i n p a r a l l e l with photocorrosion), and that t h i s competition i s dominated not by thermodynamics (as a l l three processes are e n e r g e t i c a l l y allowed) but by surface k i n e t i c e f f e c t s which can only be speculated on at present. The c r u c i a l observa t i o n , however, i s that the photo-Kolbe r e a c t i o n occurs c l e a n l y and i n f a l l i b l y on n-Ti02> p a r t i c u l a r l y i n a c i d s o l u t i o n (J5), with l i t t l e i f any degradation of the t i t a n i u m d i o x i d e , while on n-SrTi03 the photo-Kolbe r e a c t i o n sometimes occurs (3,4) and sometimes i s overwhelmed by photocorr o s i o n and/or oxygen e v o l u t i o n . As was i n d i c a t e d by the r e s u l t s i n Table I, the success or f a i l u r e of the photo-Kolbe r e a c t i o n to occur on SrTi03 appears to be unrelated to the c o n c e n t r a t i o n of doping imp u r i t i e s present i n the SrTi03. S i m i l a r l y , there seems to be no obvious c o r r e l a t i o n with the extent of r e d u c t i o n of the SrTi03, as s i m i l a r behavior has been observed on c r y s t a l s which ranged i n appearance from b r i g h t blue to n e a r l y black. We speculate, then, that the behavior we observe may r e l y on the presence of defect surface s t a t e s which can provide optimal overlap with the SrTi03 c o r r o s i o n p o t e n t i a l s , and most l i k e l y with the oxygen e v o l u t i o n p o t e n t i a l as w e l l , f o r those e l e c t r o d e s which undergo photocorrosion. I f t h i s i s t r u e , then the o x i d a t i o n of c a r b o x y l i c a c i d s should be the p r e f e r r e d process on SrTi03 photoanodes, i n the absence of such defect surface s t a t e s . We see from F i g ure 5 that the range of p o t e n t i a l s reported f o r the normal Kolbe r e a c t i o n (at platinum) a c t u a l l y crosses the valence band l e v e l s of both SrTi03 and Ti02 i n the n e u t r a l pH r e gion. I t may w e l l be that at high pH, the photo-Kolbe p o t e n t i a l l i e s at or below the valence band edge f o r these semiconductors, c o n s i s t e n t with the observation that photoKolbe products are not observed under these c o n d i t i o n s . Where there i s d i r e c t overlap with the valence band edge, the e l e c t r o n t r a n s f e r process may be so f a c i l e as to give r i s e to the Hofer-Moest r e a c t i o n (£), i n which the intermediate a l k y l r a d i c a l i s i t s e l f o x i d i z e d (while i t i s s t i l l adsorbed to the e l e c t r o d e surface) to give a carbonium i o n . The r e a c t i o n of t h i s carbonium i o n with the aqueous e l e c t r o l y t e would then y i e l d water-soluble products such as methanol, i n keeping with our observation that anodic gas e v o l u t i o n i s suppressed under these c o n d i t i o n s . In a c i d i c s o l u t i o n s , where the Kolbe r e a c t i o n i s e n e r g e t i c a l l y allowed, i t s k i n e t i c competition with the other r e a c t i o n s on SrTi03 thus depends on the absence of defect surface s t a t e s which are present i n some electrode c r y s t a l s and not i n others.

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

12.

SCHWERZEL

ET AL.

SrTiOs Photoanodes

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Summary and Conclusions According to t h i s p i c t u r e , then, the photo-Kolbe react i o n should be the p r e f e r r e d r e a c t i o n f o r d e f e c t - f r e e SrTi03 photoanodes i n a c i d i c s o l u t i o n s , while the Hofer-Moest react i o n should be p r e f e r r e d i n a l k a l i n e s o l u t i o n s . The presence of defect surface s t a t e s on some electrodes would thus def l e c t the anodic chemistry of these electrodes toward photoc o r r o s i o n and oxygen e v o l u t i o n , to the detriment of the des i r e d fuel-forming r e a c t i o n s . We do not y e t know what the o r i g i n or nature of such surface states might be, nor indeed can we o f f e r d i r e c t evidence that they e x i s t . However, they provide a p l a u s i b l e r a t i o n a l e f o r the behavior we observe, as w e l l as a l o g i c a l target f o r f u t u r e e x p l o r a t i o n . They a l s o c a l l a t t e n t i o n to a c r u c i a l problem which i s common to a l l of photoelectrochemistry a t present: the understanding and k i n e t i c c o n t r o l of e n e r g e t i c a l l y allowed r e a c t i o n s at the semiconductor-electrolyte i n t e r f a c e . I t i s toward t h i s end that f u t u r e experiments must be d i r e c t e d i f p h o t o e l e c t r o chemistry i s to become a v i a b l e means of s o l a r energy u t i l i zation. Acknowledgement This research was supported by the Solar Energy Research I n s t i t u t e , U.S. Department of Energy. We thank Dr. Fred Wagner ( U n i v e r s i t y of C a l i f o r n i a , Berkeley) f o r a s s i s t a n c e with the preparation of s e v e r a l samples of strontium t i t a n ate.

Abstract We report here that strontium titanate photoanodes, which previously have been thought to be impervious to electrolytic attack, can undergo rapid photocorrosion under photo-Kolbe reaction conditions in acidic electrolyte containing both sulfuric acid and acetic acid. Little, if any, corrosion is observed in aqueous sulfuric acid alone, and virtually no corrosion occurs in alkaline solutions, with or without added acetate. Furthermore, the corrosion process is independent of the trace element composition of the SrTiO , and occurs only on some, but not a l l , photoelectrodes cut from any given boule of SrTiO under the same experimental conditions. We suggest that the observed behavior is consistent with the presence of defect surface states which provide good overlap with the potentials of several plausible SrTiO corrosion reactions. The photoKolbe reaction is thus considered to be the preferred reaction on defect-free SrTiO electrodes in acid solution, with photocorrosion dominating for kinetic reasons on those elec3

3

3

3

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

203

204

PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE INTERFACES

trodes which have been etched insufficiently or otherwise treated so as to provide appropriately located surface states.

Literature Cited 1. G. Ciamician, Science, 36, 385 (1912).

Downloaded by 148.204.30.87 on October 21, 2009 | http://pubs.acs.org Publication Date: March 2, 1981 | doi: 10.1021/bk-1981-0146.ch012

2. A. K. Vijh and B. E. Conway, Chemical Reviews, 67, 623 (1967). 3. R. E. Schwerzel, "Methods for the Photochemical Utilization of Solar Energy", paper presented at the 29th Southeast Regional Meeting of the American Chemical Society, Tampa, Florida (November 9-11, 1977). 4. R. E. Schwerzel, E. W. Brooman, R. A. Craig, D. D. Levy, F. R. Moore, L. E. Vaaler, and V. E. Wood, in "Solar Energy: Chemical Conversion and Storage", R. R. Hautala, R. B. King, and C. Kutal, Eds., The Humana Press, Inc., Clifton, N.J. (1979), pp. 83-115. 5. B. Kraeutler and A. J. Bard, J. Amer. Chem. Soc., 100, 5985 (1978), and references cited therein. 6. "CRC Handbook of Chemistry and Physics", 53rd Ed., Chemical Rubber Publishing Company, Cleveland, Ohio (1972). 7. See, for instance J. E. Sanderson, D. L. Wise and D. G. Augenstein, "Liquid Hydrocarbon Fuels from Aquatic Biomass", Paper No. 27 presented at the Second Annual Fuels from Biomass Symposium, Rensselaer Polytechnic Institute, Troy, New York (June 20-22, 1978). 8. M. S. Wrighton, A. G. Ellis, P. T. Wolczanski, D. Morse, H. H. Abraharason, and D. Ginley, J. Amer. Chem. Soc., 98, 2774 (1976). 9. J. Mavroides, J. Kafalas, and D. Kolesar, Appl. Phys. Lett., 28, 241 (1976). 10. T. Watanabe, A. Fujishima, K. Honda, Bull. Chem. Soc. Japan, 49, 355 (1976). 11. H. H. Kung, H. S. Jarret, A. W. Sleight, and A. G. Ferretti, J. Appl. Phys., 48, 2463 (1977). 12. A. B. Bocarsly, J. M. Bolts, P. C. Cummins, and M. S. Wrighton, Appl. Phys. Lett., 31, 568 (1977).

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

12. SCHWERZEL ET AL.

SrTiOs Photoanodes

13. M. S. Wrighton, P. J. Wolczanski, and A. B. Ellis, J. Solid State Chem., 22, 17 (1977). 14. K. Honda, and T. Watanabe, Elektrokhimiua, 13, 924 (1977). 15. H. P. Maruska and A. K. Ghosh, Solar Energy,20,443 (1978). 16. F. Vanden Kerchove, J. Vandermolen, and W. P. Gomes, Ber. Bunsenges, Phys. Chem., 83, 230 (1979).

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17. L. A. Zharkova, Zh. Fiz. Khim., 36, 985 (1962).

ΔGf°

18. A. J. Bard and M. S. Wrighton, J. Electrochem. Soc., 124, 1706 (1977). 19. Estimated from the cell potential for: TiO + 1/2 O + 2e --> TiO , whereE°=+1.422VvsH /H as given by A. J. Bard and M. S. Wrighton, J. Electrochem. Soc., 124, 1706 (1977), and for TiO =-203.8kcal/mole. 2+

-

2

2

+

2

2

20. From AGf° forCH OH(1)and ideal solution correction to 1 molal aqueous solution. 3

21. Same as Reference 19. At pH = 0 essentially all of the acetic acid should be present as HOAc(aq). RECEIVED October 21, 1980.

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

205

13 Photoelectronic Properties of Ternary Niobium Oxides K. DWIGHT and A. WOLD

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Department of Chemistry, Brown University, Providence, RI 02912

Ferric oxide is known to have an optical band gap of about 2 eV, and the literature contains many reports of photoelectrochemical measurements on "conducting", n-type Fe O (1, 2, 3, 4, 5). Unfortunately, any attempt to reduce ferric oxide results in the formation of magnetite as a distinct separate phase, and there is no solubility of this spinel in the corundum structure (5, 6). Thus, all the properties reported above for Fe O were measured either on multiphase samples or on samples which con tained impurities. The ternary iron oxides, as exemplified by the iron-niobium system, offer an opportunity to obtain single-phase, conducting n-type iron oxides; in which the conductivity can be controlled by means of chemical substitution. At first glance, FeNbO and FeNb O might appear to be very different materials. Yet as MM'O and MM' O they merely represent superstructures of the basic α-PbO structure obtained under the conditions of prepara tion (7). Consequently, they form a solid solution in which the two valence states of iron are uniformly distributed throughout a single homogeneous phase (8). However, many ternary systems incorporate a second photoac tive center in addition to the [FeO ] octahedra: in the present case, [NbO ] octahedra. The interaction between such multiple centers has not previously been investigated. In the present work, interband transitions are observed which appear characteris tic of niobium centers, together with other transitions character istic of the iron centers. Since these are homogeneous, single -phasematerials, this result suggests that caution should be exer cised when applying the conventional band model to such oxide semiconductors. For materials with a single photoactive center, it is gener ally observed that the optical band gap and flat-band potential are interrelated, so that lower band gaps appear to be accompanied by more positive flat-band potentials (4,9). Nevertheless, the non-active Α-site ions in such ternary compounds as BaTiO , SrTiO , Ba Sr Nb O and Sr Nb O do have a perturbing effect 2

3

2

3

4

2

6

4

2

6

2

6

6

3

3

0.5

0.5

2

6

2

2

7

0097-6156/81/0146-0207$05.00/0 © 1981 American Chemical Society In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

208

PHOTOEFFECTS AT

SEMICONDUCTOR-ELECTROLYTE

INTERFACES

(A, J_9 9_, 10) . Consequently, i f m u l t i p l e photoactive centers can maintain s u f f i c i e n t l y independent existence i n a s i n g l e compound, i t would be conceivable that s i g n i f i c a n t d e v i a t i o n s from the u s u a l c o r r e l a t i o n of high f l a t - b a n d p o t e n t i a l s with low band-gap ener g i e s might occur. E f f e c t s of Composition

and S t r u c t u r e

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Before proceeding to ternary oxides with m u l t i p l e photoactive centers, the e f f e c t s of composition and s t r u c t u r e upon such photoe l e c t r o n i c p r o p e r t i e s as o p t i c a l band gap and f l a t - b a n d p o t e n t i a l f o r a g i v e n a c t i v e center should be considered. I t w i l l be seen that composition appears to p r i m a r i l y a f f e c t the f l a t - b a n d poten t i a l , whereas the band gap i s more s e n s i t i v e to s t r u c t u r e . Sr

N b

i s

a

d

e

f

e

c

t

e r o v

S r N b 0 i s a pyrochlore; Ba~ 5 Q s 2 ° 6 P " skite. In both m a t e r i a l s , the [NbO^] octahedra are the only pho t o a c t i v e c e n t e r s . As shown i n F i g u r e 1, the f l a t - b a n d p o t e n t i a l of the pyrochlore i s more negative by 0.4 v o l t s , and i t s band gap i s correspondingly l a r g e r , as would be expected. But the respec t i v e r o l e s of s t r u c t u r e and composition cannot be deduced from t h i s comparison alone. BaTiO^ and SrTiO^ are both p e r o v s k i t e s and have n e a r l y the same o p t i c a l band gaps. Yet the f l a t - b a n d p o t e n t i a l of SrTiO^ i s 0.6 v o l t s more negative than f o r the barium analog, a d i f f e r e n c e comparable i n magnitude to that noted above f o r the n i o b a t e s . Furthermore, i t can be seen from F i g u r e 1 that the band gap i n the r u t i l e TiO^ i s s i g n i f i c a n t l y lower than i n these p e r o v s k i t e t i tanates. Thus, the behavior i n both the t i t a n i u m and niobium systems i s c o n s i s t e n t w i t h the hypothesis that the Α-site c a t i o n i s p r i m a r i l y r e s p o n s i b l e f o r v a r i a t i o n i n f l a t - b a n d p o t e n t i a l w h i l e the s t r u c t u r e i s p r i m a r i l y r e s p o n s i b l e f o r v a r i a t i o n i n o p t i c a l band gap. Of course, i t has been noted elsewhere that other p r o p e r t i e s such as the magnitude of the quantum e f f i c i e n c y a l s o depend upon s t r u c t u r e (10). From F i g u r e 1 i t i s evident that ^ ° 3 °4» Ti0 a l l have r e l a t i v e l y p o s i t i v e f l a t - b a n d p o t e n t i a l s , which i s pre sumably a c h a r a c t e r i s t i c of the i r o n . The band gap i n the t i t a n ate appears to be a s s o c i a t e d with the [TiO^] octahedra; that i n the niobate appears to match f e r r i c oxide w i t h i n s t r u c t u r a l v a r i ability. From such a cursory a n a l y s i s , there would appear to be no e f f e c t from the presence of a second photoactive center i n these two m a t e r i a l s . However, the existence of such an e f f e c t can be demonstrated by the a p p l i c a t i o n of a r e c e n t l y proposed technique f o r the study of interband t r a n s i t i o n s having energies greater than the " o p t i c a l " band gap (11) . Standard procedures e x i s t f o r the e x t r a c t i o n of band-gap i n f o r m a t i o n from measurement of the o p t i c a l absorp t i o n c o e f f i c i e n t , which has been shown to be p r o p o r t i o n a l to the quantum e f f i c i e n c y (photocurrent d e n s i t y d i v i d e d by the i n c i d e n t 2

2

7

9

F e N b

a

n

d

F e

2

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

3

13.

DWIGHT A N D WOLD

209

Ternary Niobium Oxides

PHOTOELECTRONIC

PROPERTIES

4.0

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Ο Sr^bgOy

3,5 o

>

SnTi0

3

ο ο

Β

α

ο .

5

^ Α

Ο ΒαΤί0

3

3.0 Ο TiO,

Ο FeTiO,

ο wo

< m

a

2. 5 Η

< υ 0. Ο

Ο FeNbO,

2.0

1.5

-1.5

-1.0

-0.5

-0.0

0.5

FLAT-BAND POTENTIAL vs SCE ( H=13) p

Figure 1.

Optical band gaps and fiat-band potentials (adjusted to pH = 13) for some photoanode materials (A, 7, 9, 10)

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

210

PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE

INTERFACES

l i g h t f l u x ) under c o n d i t i o n s a p p l i c a b l e to the m a t e r i a l s c o n s i d ered here (11, 12). The p h o t o e l e c t r o l y s i s experiment provides an e f f e c t i v e sampling r e g i o n much thinner than can be obtained by p o l i s h i n g c r y s t a l s , thereby extending the range of measurement to much higher energies. Since t h i s technique i s not y e t widely known, an o u t l i n e of i t s p r i n c i p a l f e a t u r e s i s presented i n the following section. Band-Gap A n a l y s i s

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Under moderate i r r a d i a t i o n , the r e a c t i o n r a t e i n a photoelec t r o l y s i s c e l l i s l i m i t e d by the a r r i v a l r a t e of holes a t the anode surface (12), i n which case the quantum e f f i c i e n c y η i s given by: η = 1 - [exp (-aW)]/(l

+ aLp)

where a i s the o p t i c a l absorption c o e f f i c i e n t , L i s the hole d i f f u s i o n length, and W i s the width of the d e p l e t i B n l a y e r (12). Also, W - [2εε ο ο

(V-V„)/eN] lb ο

and L

ρ

i

;

* [εε (kT/ )/eN ] ο ο

/

1 / 2

Δ

e

s i n c e the hole d i f f u s i o n length i s determined by bulk recombin a t i o n i n h i g h l y d e f e c t i v e oxides (11, 12). The d i e l e c t r i c constant ε can be estimated to be of the order of 100, and the donor c o n c e n t r a t i o n Ν can be estimated from the measured c o n d u c t i v i t y , a c t i v a t i o n energy,and H a l l m o b i l i t y to be of the order of 10 cm" . Then W ζ 10 cm and L i s even smaller, so that expansion of the exponential y i e l S s a quantum e f f i c i e n c y p o r p o r t i o n a l to the o p t i c a l absorption c o e f f i c i e n t even f o r l a r g e values of o. The o p t i c a l absorption c o e f f i c i e n t f o r a s i n g l e in|erband ^ t r a n s i t i o n i s r e l a t e d to the photon energy by α ^ (hv) (hv - Eg) where Eg i s the band gap and n depends upon the character of the t r a n s i t i o n (n = 0.5 f o r allowed d i r e c t t r a n s i t i o n s ; n = 2 f o r allowed i n d i r e c t ones). Thus, i f experimental values f o r α are m u l t i p l i e d by hv and are p l o t t e d as (ahv) against hv, then a s t r a i g h t l i n e i n t e r s e c t i n g the energy a x i s a t Eg w i l l be obtained when n c o r r e c t l y c h a r a c t e r i z e s the t r a n s i t i o n . Since the t o t a l o p t i c a l absorption c o e f f i c i e n t α f o r a compound comprises the sum of such c o n t r i b u t i o n s from successive interband t r a n s i t i o n s , i t s complete a n a l y s i s must proceed i n stages. Each t r a n s i t i o n i s c h a r a c t e r i z e d i n t u r n , s t a r t i n g from the lowest energy, whereupon i t s c o n t r i b u t i o n to the absorption i s extrapolated to higher ener gies and subtracted from the t o t a l a. However, the simple d e t e r mination of the interband t r a n s i t i o n energies does not r e q u i r e t h i s elaborate process, the onset of each a d d i t i o n a l c o n t r i b u t i o n

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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13.

DWiGHT A N D WOLD

Ternary Niobium Oxides

211

to the t o t a l α being c l e a r l y v i s i b l e as an abrupt i n c r e a s e i n the slope of the graph of (ahv) v s hv. Furthermore, higher-ener gy d i r e c t t r a n s i t i o n s can o f t e n be i d e n t i f i e d unambiguously w i t h out s u b t r a c t i n g the c o n t r i b u t i o n s from lower-energy i n d i r e c t ones. The absorption c o e f f i c i e n t increases w i t h i n c r e a s i n g photon energy, and each s u c c e s s i v e t r a n s i t i o n adds to the r a t e of i n crease. Consequently, the a n a l y s i s of higher-energy t r a n s i t i o n s i s l i m i t e d by the maximum v a l u e of α which can be measured, which i s i n v e r s e l y p r o p o r t i o n a l to the thickness of the sample. In the p h o t o e l e c t r o l y s i s experiment, the d e p l e t i o n l a y e r forms a very narrow sampling r e g i o n , so that the maximum value of α measurable by η i s l a r g e . T h i s permits the determination of interband t r a n s i t i o n s w e l l above the energy of the lowest band gap (11). In order to i l l u s t r a t e the power and r e l i a b i l i t y of t h i s ana l y t i c a l procedure, the quantum e f f i c i e n c y η measured f o r S r T i 0 ~ , being p r o p o r t i o n a l to a, has been mult^p^ied by the photon energy hv and i s p l o t t e d i n F i g u r e 2 as (nhv) * v s hv. The l i n e a r i t y of the lowest-energy s e c t i o n of t h i s graph (with n = 2) c h a r a c t e r i z e s the t r a n s i t i o n as i n d i r e c t . The energy i n t e r c e p t y i e l d s the v a l u e of 3.2 eV f o r the lowest band gap, which i s i n good agreement with previous a b s o r p t i o n measurements (13) and with the c a l c u l a t i o n of Kahn and Leyendecker (14). The abrupt i n c r e a s e i n slope a t 3.37 eV s i g n a l s the presence of a higher-energy t r a n s i t i o n , i n accord with the increased ab s o r p t i o n found by e l e c t r o m o d u l a t i o n measurement (15). The de crease i n slope at 3.5 eV corresponds to a s a t u r a t i o n of t h i s con t r i b u t i o n to the t o t a l a b s o r p t i o n and i s not understood. Never t h e l e s s , s e v e r a l other m a t e r i a l s g i v e evidence of s i m i l a r behavior. F i n a l l y , the t r a n s i t i o n at 3.74 eV agrees both w i t h the e l e c tromodulation s p e c t r a (15) and w i t h the band s t r u c t u r e c a l c u l a t i o n (14). This higher-energy s e c t i o n does not appear g r e a t l y d i f f e r ent from the r e s t of F i g u r e 2, although there i s some curvature of the data. Hgwever, t h i s r e g i o n becomes t r u l y l i n e a r when r e p l o t ted as (nhv) versus hv, which e s t a b l i s h e s the d i r e c t character of the high-energy t r a n s i t i o n . Results and D i s c u s s i o n The quantum e f f i c i e n c y data f o r the d e f e c t pyrochlore Ba^ 5 Q 5 2 ° 6 P F i g u r e 3 (10) . I t shows an i n d i r e c t band gap a t 3.4 eV with a " t a i l " extending to n e a r l y 2.6 eV. The higher-energy t r a n s i t i o n a t 4.4 eV shows some curvature of the data, and indeed, corresponds to a d i r e c t t r a n s i t i o n when r e p l o t ted as (nhv) versus energy (10). S i m i l a r data f o r the pyrochlore 8 Γ ^ ^ 2 0 ^ i s p l o t t e d i n F i g u r e 4. Here the p r i n c i p l e i n d i r e c t band gap occurs a t 3.9 eV w i t h a "tail to n e a r l y 3.4 eV. The data beyond 4.3 eV cannot be i n t e r preted q u a n t i t a t i v e l y because of a breakdown i n the c o n d i t i o n s required f o r the band-gap a n a l y s i s , but there i s an i n d i c a t i o n of Sr

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In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

13.

DWiGHT AND WOLD

Ternary Niobium Oxides

213

a t r a n s i t i o n i n the v i c i n i t y of 4.7 eV. Thus the behavior i s q u a l i t a t i v e l y s i m i l a r to that observed i n the p e r o v s k i t e niobate, only s h i f t e d to higher energies. The analagous r e s u l t s f o r the corundum F e 0 are given i n Figure 5. This shows an i n d i r e c t band gap a t 1.85 eV together with a d i r e c t band gap a t 2.5 eV ( n ) . Such simple behavior i s i n sharp contrast with the complex succession of t r a n s i t i o n s shown i n Figure 6 f o r FeNbO^ (7). Here the lowest-energy t r a n s i t i o n a t 2.05 eV i s c l e a r l y i n d i r e c t . I t i s followed by s e v e r a l higherenergy t r a n s i t i o n s a t 2.68, 2.93, 3.24, and 4.38 eV, each g i v i n g r i s e t o a sudden increase i n the slope of the curve, but so c l o s e together as to preclude r e l i a b l e determination of d i r e c t or i n d i r e c t character. The l o c a t i o n s of these a d d i t i o n a l interband t r a n s i t i o n s a r e h i g h l y suggestive. That a t 2.68 eV appears to c o r r e l a t e with the 2.58 eV t r a n s i t i o n f o r F ^ O ^ F i g u r e 5; those a t 3.24, 2.9, and 4.38 eV a r e reminiscent of the i n d i r e c t t r a n s i t i o n a t 3.4 eV, i t s " t a i l , " and the d i r e c t t r a n s i t i o n a t 4.4 eV shown i n Figure 3 f o r Ba^ S r Nb 0,. Thus the data f o r FeNbO^ show a l l the c h a r a c t e r i s t i c s of the LNbO/] octahedra i n a d d i t i o n to a l l the c h a r a c t e r i s t i c s of the [FeO.] centers. The greater s i m i l a r i t y to the p e r o v s k i t e niobate can be a t t r i b u t e d to c l o s e r agreement be tween t h e i r Nb-0 bond strengths as compared with those i n the pyrochlore s t r u c t u r e .

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Summary and Conclusions When only a s i n g l e species of photoactive center i s present i n a compound, the presence of a non-active, Α-site c a t i o n pro duces a c h a r a c t e r i s t i c s h i f t i n the f l a t - b a n d p o t e n t i a l . A change i n s t r u c t u r e , however, w i l l i n general produce a s h i f t i n the op t i c a l band-gap energy. This i s accompanied by corresponding s h i f t s i n any other, higher-energy interband t r a n s i t i o n , but the q u a l i t a t i v e f e a t u r e s remain the same, and hence appear t o be c h a r a c t e r i s t i c of the p a r t i c u l a r photoactive center. When two species of photoactive centers a r e simultaneously present, the higher f l a t - b a n d p o t e n t i a l appears to dominate. But i t i s evident that both species c o n t r i b u t e t h e i r c h a r a c t e r i s t i c sets of interband t r a n s i t i o n s to the ensemble. In t h i s respect, these oxide semiconductors behave d i f f e r e n t l y than the conven t i o n a l , broad-band semiconductors. I t would appear that d i f f e r e n t photoactive centers remain a t l e a s t p a r t i a l l y independent. However, f u r t h e r experimentation embracing a v a r i e t y of t e r nary systems w i l l be required to determine the degree of i n t e r a c t i o n between such m u l t i p l e centers. P r e l i m i n a r y r e s u l t s f o r Fe^WO, confirm the s u p e r p o s i t i o n of two c h a r a c t e r i s t i c sets of interband t r a n s i t i o n s . The o p t i c a l band gap and f l a t - b a n d poten t i a l are e s s e n t i a l l y the same as i n FeNbO,, but the quantum e f f i ciency i s considerably g r e a t e r . This suggests that there may be some enhancement of the photoresponse due to i n t e r a c t i o n between the i r o n and tungsten centers.

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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DWiGHT AND WOLD

215

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In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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Acknowledgements The authors would l i k e to acknowledge the support of the O f f i c e of Naval Research, A r l i n g t o n , V i r g i n i a f o r the support of Kirby Dwight. The authors would a l s o l i k e to acknowledge the Solar Energy Research I n s t i t u t e , Golden, Colorado as w e l l as the M a t e r i a l s Research Laboratory Program at Brown U n i v e r s i t y f o r t h e i r support.

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Literature

Cited

1. Hardee, K. L.; Bard, A. J. J. Electrochem. Soc., 1976, 123, 1024 and J. Electrochem. Soc., 1977, 124, 215. 2. Quinn, R. K.; Nasby, R. D.; Baughman, R. J. Mat. Res. Bull., 1976, 11, 1011. 3. Yeh, L. R.; Hackerman, N. J. Electrochem. Soc., 1977, 124, 833. 4. Kung, H. H.; Jarrett, H. S.; Sleight, A. W.; Ferretti, A. J. Appl. Phys., 1977, 48, 2463. 5. Merchant, P.; Collins, R.; Kershaw, R.; Dwight, K.; Wold, A. J. Solid State Chem., 1979, 27, 307. 6. Salmon, O. N. J. Phys. Chem., 1961, 65, 550. 7. Koenitzer, J.; Khazai, B.; Hormadaly, J.; Dwight, K.; Wold, A. J. Solid State Chem., to be published. 8. Turnock, A. C. J. of the Am. Ceramic Soc., 1966, 49, 177. 9. Nozik, A. J. Ann. Rev. Phys. Chem., 1978, 29, 189. 10. Hormadaly, J.; Subbarao, S. N.; Kershaw, R.; Dwight, K.; Wold, A. J. Solid State Chem., to be published. 11. Koffyberg, F. P.; Dwight, K.; Wold, A. Solid State Communi cations, 1979, 30, 433. 12. Butler, M. A. J. Appl. Phys., 1977, 48, 1914. 13. Weakliem, Η. Α.; Burk, W. J.; Redfield, D.; Korsum, V. R.C.A. Review, 1975, 36, 149. 14. Kahn, A. H.; Leyendecker, A. J. Phys. Rev., 1964, 135, A1321. 15. Mack, S. Α.; Handler, P. Phys. Rev., 1974, B9, 3415. R E C E I V E D October 3,

1980.

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

14 Electrode Band Structure and Interface States in Photoelectrochemical Cells JOHN G. MAVROIDES, JOHN C. F A N , and HERBERT J. ZEIGER

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Lincoln Laboratory, Massachusetts Institute of Technology, Lexington, M A 02173

E f f i c i e n t u t i l i z a t i o n of s o l a r energy by means of photoe l e c t r o l y s i s r e q u i r e s an e l e c t r o d e material that i s not only s t a b l e but also has an e l e c t r o n a f f i n i t y that i s small enough to give s u f f i c i e n t band bending. In a d d i t i o n , s a t i s f a c t o r y material must have an energy gap that i s well matched to the s o l a r spectrum. The energy gaps of both TiO and SrTiO , which have values of 3.0 and 3.2 eV, r e s p e c t i v e l y , are considerably too large to s a t i s f y t h i s l a t t e r requirement. Furthermore no b e t t e r s i n g l e chemical compound has been found. This suggests t r y i n g a combination of compounds f o r electrodes. We w i l l d e s c r i b e our program to develop e l e c t r o d e s , with the d e s i r e d electrochemical p r o p e r t i e s , by such an approach. In p a r t i c u l a r , we w i l l discuss composite s t r u c t u r e s and s o l i d s o l u t i o n s . This i s an ongoing program and what w i l l be presented are mainly the concepts with only a few p r e l i m i n a r y results. 2

Composite

3

Electrodes

B e g i n n i n g w i t h c o m p o s i t e e l e c t r o d e s t h e s i m p l e s t scheme h e r e i s t o c o a t t h e s u r f a c e o f a s m a l l gap s e m i c o n d u c t o r t h a t i s w e l l matched t o t h e s o l a r s p e c t r u m b u t w h i c h i s e l e c t r o c h e m i c a l l y u n s t a b l e w i t h a t h i n f i l m o f a wide gap, e l e c t r o c h e m i c a l l y s t a b l e semiconductor. To d e m o n s t r a t e t h e f e a s i b i l i t y o f u s i n g s u c h a c o m p o s i t e e l e c t r o d e , t h e f i l m must be t h i n enough - o f t h e o r d e r o f 5 0 - 1 0 0 Â o r l e s s - so t h a t a t l e a s t some o f t h e p h o t o g e n e r a t e d c a r r i e r s i n t h e s m a l l bandgap m a t e r i a l can tunnel through t o t h e e l e c t r o l y t e . Furthermore t h e f i l m must n o t have any c r a c k s o r p i n h o l e s s i n c e t h e s e w o u l d

0097-6156/81/0146-0217$05.00/0 © 1981 American Chemical Society In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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a l l o w t h e e l e c t r o l y t e t o p e n e t r a t e t h r o u g h t o t h e s m a l l bandgap m a t e r i a l and c a u s e d i s s o l u t i o n . The f o r m a t i o n o f such a t h i n d e f e c t - f r e e f i l m by c o n v e n t i o n a l t e c h n i q u e s i s v e r y d i f f i c u l t and a l t h o u g h many a t t e m p t s have been r e p o r t e d , s u c c e s s has not been a c h i e v e d {1_ 2J. We a r e i n v e s t i g a t i n g a s l i g h t l y more complex t y p e o f c o m p o s i t e e l e c t r o d e , i n w h i c h t h e s m a l l bandgap s e m i c o n d u c t o r i s p r o t e c t e d by a much t h i c k e r , t w o - p h a s e f i l m s u c h as shown i n F i g u r e 1. Here t h e low bandgap s u b s t r a t e m a t e r i a l i s CdSe, w h i c h has an ες = 1.7 eV, and t h e l a r g e bandgap m a t e r i a l i s SrTi03. In t h i s a r r a n g e m e n t t h e p r o t e c t i v e c o a t i n g , w h i c h i s o f t h e o r d e r o f 50QÂ t h i c k , c o n s i s t s o f s m a l l g r a i n s o f CdSe (~ 5C& in size) i n a matrix of SrTi03. During p h o t o e l e c t r o l y s i s , the h o l e - e l e c t r o n p a i r s a r e g e n e r a t e d i n t h e CdSe and s e p a r a t e d by the depletion f i e l d . The h o l e s move t o t h e s u b s t r a t e - f i l m i n t e r f a c e and t h e n t u n n e l t h r o u g h S r T i 0 3 t o t h e n e a r e s t g r a i n o f CdSe. A f t e r a few jumps between g r a i n s , t h e h o l e s r e a c h t h e electrolyte. T h i s f i l m s h o u l d be e f f e c t i v e i n p r o t e c t i n g t h i s s u b s t r a t e p r o v i d e d t h e g r a i n s are i s o l a t e d from each o t h e r . In t h i s case the g r a i n s t h a t are i n i t i a l l y i n c o n t a c t w i t h the e l e c t r o l y t e w i l l be d i s s o l v e d away u n t i l a c o n t i n u o u s p r o t e c t i v e surface of SrTi03 i s l e f t in contact with the e l e c t r o l y t e . F i g u r e 2 i s a s c h e m a t i c e n e r g y band d i a g r a m o f t h i s c o m p o s i t e e l e c t r o d e i n d i c a t i n g how t h e m i n o r i t y c a r r i e r s f r o m t h e b u l k CdSe v a l e n c e band t u n n e l t h r o u g h t h e S r T i 0 3 t o CdSe g r a i n s v i a s u r f a c e s t a t e s . The e a s y p a s s a g e o f m i n o r i t y c a r r i e r s through the i n t e r f a c e t o the e l e c t r o l y t e r e q u i r e s t h a t t h e v a l e n c e band edge o f CdSe o v e r l a p t h e s u r f a c e s t a t e s i n t h e bandgap o f t h e S r T i 0 3 . For f r e e passage of t h e m a j o r i t y c a r r i e r s between t h e two s e m i c o n d u c t o r s , t h e i r e l e c t r o n a f f i n i t i e s s h o u l d be e q u a l so t h a t t h e p o t e n t i a l b a r r i e r a t t h e i n t e r f a c e s between them i s n e g l i g i b l e . These c o n d i t i o n s a r e f a i r l y w e l l met i n t h e C d S e - S r T i Û 3 s y s t e m . In c o n n e c t i o n w i t h t h i s s y s t e m a number o f q u e s t i o n s a r i s e . F i r s t o f a l l , b e c a u s e o f i n t e r a c t i o n and s c a t t e r i n g e f f e c t s , t h e o p t i c a l a b s o r p t i o n o f t h e t w o - p h a s e f i l m c o u l d be s i g n i f i c a n t l y g r e a t e r than t h a t o f t h e i n d i v i d u a l components. In t h i s c a s e i t w o u l d be n e c e s s a r y t o s h i n e t h e l i g h t o n t o t h e back s u r f a c e o f t h e s u b s t r a t e and t h e s u b s t r a t e w o u l d have t o be v e r y t h i n , o f t h e o r d e r o f t h e d i f f u s i o n l e n g t h ; s e c o n d l y , even i f t h e c a r r i e r s a r e e x c i t e d t h r o u g h t h e p r o t e c t i v e l a y e r and w i t h i n t h e d e p l e t i o n r e g i o n o f t h e s u b s t r a t e , how e f f i c i e n t l y would t h e y t r a n s f e r t o t h e e l e c t r o l y t e ? What w o u l d t h e n a t u r e o f t h e s t a t e s a t t h e i n t e r f a c e s be and how e f f i c i e n t l y w o u l d c h a r g e t r a n s f e r between t h e g r a i n s t o t h e e l e c t o l y t e . What w o u l d t h e t u n n e l i n g e f f i c i e n c y be and w o u l d c a r r i e r s c a t t e r i n g be a p r o b l e m ? T h i r d l y , would the f i l m f a b r i c a t i o n process cause work damage t o t h e s u b s t r a t e ?

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9

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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PHOTOEFFECTS

AT S E M I C O N D U C T O R - E L E C T R O L Y T E

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F i n a l l y how w o u l d one make s u c h a f i l m ? One way i s shown i n F i g u r e 3. What i s shown h e r e s c h e m a t i c a l l y i s an r f s p u t t e r i n g system. The s u b s t r a t e i s mounted on a r o t a t i n g d i s c and a m i c r o p r o c e s s o r - c o n t r o l l e d u n i t i s used t o p r o d u c e s e q u e n t i a l s p u t t e r i n g o f two d i f f e r e n t t a r g e t s , one f o r e a c h component, w i t h p r o v i s i o n s f o r a d j u s t i n g t h e p r o p o r t i o n s o f t h e two com p o n e n t s i n t h e f i l m t o any d e s i r e d v a l u e . A n n e a l i n g o r some o t h e r t r e a t m e n t a f t e r t h e s p u t t e r i n g may be n e c e s s a r y t o i n s u r e t h a t t h e S r T i 0 3 f o r m s a c o n t i n u o u s m a t r i x w i t h t h e CdSe embedded i n i t .

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A l 1 oys Another approach t o t h e e l e c t r o d e problem i s t h a t of a l l o y i n g e i t h e r T i 0 2 o r S r T i 0 3 w i t h a n a l o g o u s compounds t o f o r m s o l i d s o l u t i o n s w h i c h c o u l d have l o w e r bandgaps and s t i l l r e t a i n t h e other desirable properties. A l l o y i n g i s w i d e l y used f o r a d j u s t i n g t h e bandgaps i n s o l i d s o l u t i o n s o f s u c h s e m i c o n d u c t o r s as Ge and S i , and GaAs and GaP, where t h e o u t e r e l e c t r o n s o c c u p y s and ρ o r b i t a l s . In t h e s e b r o a d b a n d m a t e r i a l s an a d e q u a t e d e s c r i p t i o n o f t h e e l e c t r o n i c s t a t e s i s p r o v i d e d by t h e band t h e o r y a p p r o x i m a t i o n ; as a r e s u l t a l l o y i n g s h i f t s t h e e n e r g y gap a p p r o x i m a t e l y l i n e a r l y w i t h com p o s i t i o n a l t h o u g h t h e r e a r e some c a s e s i n w h i c h an i n t e r m e d i a t e c o m p o s i t i o n can have e i t h e r a maximum o r a minimum bandgap. T r a n s i t i o n m e t a l s s u c h as T i , however, have o u t e r d e l e c t r o n s w h i c h e x h i b i t a s t r o n g t e n d e n c y t o be l o c a l i z e d and t h e s i m p l e o n e - e l e c t r o n band model i s f r e q u e n t l y not a p p l i c a b l e t o t r a n s i t i o n - m e t a l compounds. C o n s e q u e n t l y t h e s e compounds and t h e i r s o l i d s o l u t i o n s a r e g e n e r a l l y not so w e l l u n d e r s t o o d , a l t h o u g h some e x p e r i m e n t a l d a t a and t h e o r e t i c a l work on them exists. F o r t h e s e a l l o y s not o n l y can you g e t bandgap c h a n g e s , b u t you can a l s o i n t r o d u c e s e p a r a t e bands o f l e v e l s due t o t h e i n d i v i d u a l c o n s t i t u e n t s o f t h e a l l o y , and t h e s e can be p a r t i a l l y r e s p o n s i b l e f o r lower energy t r a n s i t i o n s . For example, i n the c a s e o f m e t a l l i c a l l o y s o f Ni w i t h C u , one f i n d s n o t j u s t a s i n g l e s e t o f d band s t a t e s , but two s e t s o f s t a t e s , one c o r r e s p o n d i n g t o Ni d s t a t e s and t h e o t h e r t o Cu s t a t e s (_3). I t i s w e l l known t h a t t h e v a l e n c e and c o n d u c t i o n bands o f T i O 2 a r e d e r i v e d m a i n l y f r o m t h e oxygen 2 ρ s t a t e s and t i t a n i u m 3 d s t a t e s , r e s p e c t i v e l y , (4J as shown i n F i g u r e 4 ( a ) . Less i s known o f t h e compounds w h i c h a r e c o m p l e t e l y m i s c i b l e w i t h T i U 2 » namely TaO?, WO2, Nb02, VO2 and Μ0Ο2. The f i r s t t h r e e o f t h e s e compounds a r e s e m i c o n d u c t o r s w i t h a band s t r u c t u r e s i m i l a r t o T i O 2 and e n e r g y gaps c l o s e t o 3 eV. VO2 on t h e o t h e r h a n d , i s a s e m i c o n d u c t o r b e l o w 65°C where i t undergoes a semiconductor-to-metal t r a n s i t i o n . I t s bandgap o f O.7 eV s e p a r a t e s t w o - s u b b a n d s b o t h d e r i v e d f r o m vanadium d - s t a t e s as shown i n F i g u r e 4 ( b ) . X-ray photoemission data i n d i c a t e that

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

MAVROIDES E T

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HEATER

Figure 3. Simultaneous RF sputtering module

Ti0

2

(a)

V0

2

Mo0

(b)

(c)

2

Figure 4. Energy band diagrams (after Réf. 4) for Ti0 , V0 , and MoO 2

2

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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t h e gap i n VO2 a n a l o g o u s t o t h e p - t o - d gap i n T i O 2 i s l e s s t h a n 2 eV. T h e r e f o r e one m i g h t e x p e c t t h a t s o l i d s o l u t i o n s c o n t a i n i n g a l i m i t e d c o n c e n t r a t i o n o f VO2 w i l l have t h e same b a s i c band s t r u c t u r e as T i 0 2 but w i t h a s m a l l e r bandgap. Even l e s s i s known about Μ0Ο2, e x c e p t t h a t i t i s a s e m i m e t a l ; i t s t h e o r e t i c a l e n e r g y band d i a g r a m , as p r o p o s e d by Goodenough, i s shown i n Fig. 4(c). A n o t h e r i n t e r e s t i n g s y s t e m i s based on a l l o y s o f S r T i Û 3 w i t h o t h e r ΑΒΟ3 compounds o f t h e p e r o v s k i t e s t r u c t u r e and i s s u g g e s t e d by t h e work o f T r i b u t s c h and c o - w o r k e r s , who have f o u n d s t a b l e p h o t o g a l v a n i c a c t i o n by u s i n g o p t i c a l e x c i t a t i o n between n o n b o n d i n g d o r b i t a l s o f t r a n s i t i o n m e t a l d i c h a l c o g e n i d e s such as MoSe2 ( 5 ^ , 7 ^ ) · The same mechanism e x p l a i n s t h e p h o t o e l e c t r o c h e m i c a l s t a b i l i t y o f Fe2Û3 ( 9 , 1 0 , 1 1 , 1 2 ) and YFe03 (.13). However, a b i a s v o l t a g e must be a p p l i e d to o b t a i n p h o t o e l e c t r o l y s i s with e l e c t r o d e s of the l a t t e r compounds, s i n c e t h e i r e l e c t r o n a f f i n i t i e s a r e t o o h i g h . S r T i 0 3 - L a F e 0 3 s o l i d s o l u t i o n s a p p e a r i n t e r e s t i n g because we f e e l t h a t such a l l o y s c o u l d e x h i b i t s u b s t a n t i a l s o l a r a b s o r p t i o n due t o t r a n s i t i o n s between t h e F e ^ d - l e v e l s o f LaFe03 w h i l e r e t a i n i n g a l a r g e p r o p o r t i o n o f t h e f a v o r a b l e band b e n d i n g o f SrTiÛ3. In t e r m s o f t h e e n e r g y d i a g r a m s o f F i g u r e 4 S r T i 0 3 w o u l d be r e p r e s e n t e d by ( a ) a n d LaFeÛ3 by (b) where t h e d - t o - d gap i s now ~ 2 eV. The p r e p a r a t i o n and c h a r a c t e r i z a t i o n o f p e r o v s k i t e b a s e d s o l i d s o l u t i o n s , i n c l u d i n g a 50/50 L a F e 0 3 - S r T i Û 3 a l l o y , has been r e p o r t e d v e r y r e c e n t l y by Rauh e t al (14). F o r t h e l a t t e r compound, t h e s e r e s e a r c h e r s o b s e r v e d p h o t o r e s p o n s e a t photon e n e r g i e s i n t h e 2 eV r a n g e , but w i t h v e r y low p h o t o c u r r e n t s . +

Experiments P r e l i m i n a r y experiments with the composite e l e c t r o d e s i n d i c a t e t h a t o p t i c a l s c a t t e r i n g does n o t a p p e a r t o be a problem; the a c t i o n s p e c t r a of the composite f i l m s are s i m i l a r t o t h o s e o f t h e b a r e s u b s t r a t e e l e c t r o d e s . The c u r r e n t - v o l t a g e c u r v e s , such as shown a t t h e b o t t o m o f F i g u r e 5 f o r a 50/50 c o m p o s i t i o n o f CdSe and S r T i 0 3 , i n d i c a t e t h a t c a r r i e r s c a t t e r i n g i s a p r o b l e m . Under i l l u m i n a t i o n , t h e i n i t i a l c o m p o s i t e e l e c t r o d e s d i s p l a y v e r y low p h o t o c u r r e n t s (~ 1 0 " ^ o f t h e s u b s t r a t e m a t e r i a l ) and a l i n e a r dependence o f c u r r e n t on e l e c t r o d e p o t e n t i a l under b o t h c a t h o d i c and a n o d i c c o n d i t i o n s , w i t h no s a t u r a t i o n e f f e c t i n t h e a n o d i c d i r e c t i o n . Such b e h a v i o r i s s u g g e s t i v e of a b a r r i e r i n which the c a r r i e r f l o w i s d i f f u s i o n l i m i t e d , w h i c h w o u l d be t h e c a s e i f c o l l i s o n s a t t h e C d S e - S r T i Û 3 g r a i n b o u n d a r i e s were s i g n i f i c a n t ( 1 5 ) . Annealing t h e e l e c t r o d e i n an A r atmosphere t o ~ 80(PC and t h e n s l o w l y c o o l i n g i n c r e a s e s t h e p h o t o r e s p o n s e by one o r d e r o f m a g n i t u d e

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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Electrode Band Structure and Interface States 223

but how much improvement w i l l u l t i m a t e l y be o b t a i n e d i s n o t y e t known. Most o f t h e T i Û 2 and S r T i Û 3 a l l o y s were made by f i r i n g m i x t u r e s o f powders o f t h e a p p r o p r i a t e o x i d e s . Stoichiometric m i x t u r e s o f t h e s t a r t i n g m a t e r i a l s , w h i c h had a p u r i t y o f a t l e a s t 99.995% a s o b t a i n e d f r o m J o h n s o n M a t t h e y C h e m i c a l s L i m i t e d , a r e f i n e l y ground and s i n t e r e d i n a i r a t ~ 1100°C f o r a b o u t 12 h o u r s . The m a t e r i a l s a r e t h e n p r e s s e d i n t o d i s c s , a b o u t 1 cm i n d i a m e t e r and 1 mm t h i c k , and t h e n r e d u c e d , d e p e n d i n g on t h e a l l o y and c o m p o s i t i o n , a t t e m p e r a t u r e s between 100(PC and 140(PC and t i m e s r a n g i n g f r o m 3 t o 12 h o u r s i n an a t m o s p h e r e o f e i t h e r f o r m i n g gas o r A r i n o r d e r t o o b t a i n c o n d u c t i v i t i e s w h i c h r a n g e between 1 0 " and 1 0 " 3 mho/cm. A l t h o u g h we have f a b r i c a t e d s o l i d s o l u t i o n s o f T i Û 2 w i t h T a 0 2 , NbÛ2, VO2 and M0O2, t h e d i s c u s s i o n h e r e w i l l be l i m i t e d t o t h e Nb a l l o y s . In t h e c a s e o f S r T i Û 3 , o n l y r e s u l t s o f a l l o y s between t h i s s e m i c o n d u c t o r and LaFeÛ3 w i l l be g i v e n . F i g u r e 6 shows t h e v a r i a t i o n o f i n t e g r a t e d p h o t o c u r r e n t v e r s u s photon e n e r g y f o r s e v e r a l e l e c t r o d e s o f Nb c o m p o s i t i o n , namely f o r 5 , 10 and 20% Nb02 i n T i 0 2 « A l s o shown f o r c o m p a r i s o n i s t h e r e s u l t f o r a TiO2 e l e c t r o d e w h i c h was r e d u c e d so as t o be c o n d u c t i n g . In t h e s e measurements t h e e l e c t r o d e s , w h i c h a r e a l l η - t y p e , were h e l d a t a p o t e n t i a l o f + 1.0 V r e l t o SCE. A 150 W Xe lamp i n c o m b i n a t i o n w i t h l o w - e n e r g y - p a s s c o l o r f i l t e r s was used t o o b t a i n t h e s e a c t i o n s p e c t r a . Such a t e c h n i q u e , w h i l e only y i e l d i n g approximate a c t i o n s p e c t r a , i s u s e f u l i n c a s e s where t h e p h o t o r e s p o n s e i s l o w . I t w i l l be n o t e d t h a t a l l t h e e l e c t r o d e s have maximum r e s p o n s e i n t h e UV b u t t h a t t h e T i - N b e l e c t r o d e s have a r e s p o n s e w h i c h e x t e n d s b e l o w 3 eV i n t o t h e v i s i b l e r e g i o n . U n a l l o y e d T i Û 2 a l s o showed some r e s p o n s e i n t o t h e v i s i b l e , b u t i t was two o r d e r s o f magnitude lower than f o r t h e a l l o y s . S t a r t i n g w i t h t h e dashed c u r v e s , an i n c r e a s e i n t h e l o w e n e r g y p h o t o r e s p o n s e i s o b t a i n e d as t h e c o m p o s i t i o n i s i n c r e a s e d f r o m 5 t o 10% Nb and a s m a l l d e c r e a s e as t h e c o m p o s i t i o n i s f u r t h e r i n c r e a s e d t o 20% The d e c r e a s e i n t h e l a t t e r a l l o y i s no doubt due t o t h e f a c t t h a t i t i s a t w o - p h a s e m a t e r i a l as i d e n t i f i e d by x - r a y d i f f r a c t i o n patterns. A t t h e h i g h e n e r g y end o f t h e s p e c t r u m , on t h e o t h e r h a n d , a d e c r e a s e i n p h o t o r e s p o n s e i s f o u n d i n g o i n g f r o m Τ1Ό2 t o i n c r e a s i n g amounts o f Nb. T h i s t e n d s t o s u g g e s t t h a t t h e same mechanism w h i c h p r o d u c e s t h e l o w e n e r g y s e n s i t i z a t i o n a l s o causes t h e lowered high energy response i n agreement w i t h t h e o b s e r v a t i o n s o f Maruska and Ghosh f o r doped T i 0 2 e l e c t r o d e s ( 1 6 ) . T h i s i s n o t c o m p l e t e l y t r u e , h o w e v e r , as t h e s o l i d c u r v e l a b e l l e d (B) on t h i s p l o t i n d i c a t e s . This curve i s t h e a c t i o n s p e c t r u m o f t h e o r i g i n a l 5% Nb sample l a b e l l e d (A) t a k e n a f t e r i t was r e o x i d i z e d i n a i r a t 1000PC f o r 10 m i n u t e s and t h e n , r e d u c e d a g a i n . T h i s h e a t t r e a t m e n t has i m p r o v e d n o t o n l y t h e l o w e n e r g y end o f t h e s p e c t r u m b u t a l s o t h e h i g h e n e r g y

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1

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

224

PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE 1 1 1 -WITH WHITE LIGHT FROM Hg SOURCE

1

1

1

1

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1

CdSe

0 O.2

I„

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^ ^ Γ ^ ^ ι

-O.5-O.4

-O.3

-O.2

1

-O.1

1 O.1

0

1 O.2

VOLTAGE RELATIVE TO SCE

1 O.3

1 O.4

O.5

(V)

Figure 5. Comparison between l-V curves of a single-crystal, bare CdSe elctrode (top) and a composite electrode (bottom) with light ( ) and in the dark ( ) (pH—4.7)

1.8

2.2

2.6

3.0

3.4

3.8

4.2

hv (eV)

Figure 6. Integrated photocurrent vs. photon energy obtained with a 150-W Xe lamp in combination with low-energy-pass color filters. Electrodes are biased at +1.0 V relative to SCE (pH 10). See text for difference between (A) and (B).

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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end. The r e s u l t s u g g e s t s t h e need f o r more d e t a i l e d and c a r e f u l c h a r a c t e r i z a t i o n s ( b o t h p h y s i c a l and c h e m i c a l ) o f t h e b a s i c m a t e r i a l p r o p e r t i e s i n order t o o p t i m i z e t h e photoresponse. F i g u r e 7 i s a comparison o f t h e r e s p o n s i v i t y ( t h a t i s , t h e v a r i a t i o n o f t h e n o r m a l i z e d p h o t o c u r r e n t w i t h photon e n e r g y ) , o f t h e 5% Nb a l l o y e d sample ( l a b e l l e d (B) i n F i g u r e 6 ) , o f T i 0 2 s i n g l e c r y s t a l and o f a NbÛ2 powder d i s c e l e c t r o d e . The peak r e s p o n s e o f t h e a l l o y i n t h e u l t r a v i o l e t i s one o r d e r o f m a g n i t u d e down f r o m t h a t o f T i Û 2 ; t h e a l l o y a l s o has a r e s p o n s e i n t h e v i s i b l e w h i c h , however, i s s e v e r a l o r d e r s down f r o m t h a t i n t h e UV. The s i n t e r e d NbU2 e l e c t r o d e r e s p o n s e peaks i n t h e UV b u t has a t a i l i n r e s p o n s e w h i c h e x t e n d s i n t o t h e v i s i b l e . The p h o t o r e s p o n s e i n t h e UV i s f a s t w h i c h s u g g e s t s t h a t t h i s r e s p o n s e a r i s e s f r o m e l e c t r o n i c t r a n s i t i o n s between t h e v a l e n c e and c o n d u c t i o n b a n d s , and g i v e s a v a l u e f o r t h e e n e r g y gap o f NbÛ2 o f about 3 . 3 e V . The v i s i b l e p h o t o r e s p o n s e on t h e o t h e r hand i s s l o w e r and p r o b a b l y a r i s e s f r o m s t a t e s w i t h i n t h e bandgap. The r e s p o n s e o f t h e T i o . 9 5 N b o . 0 5 O 2 a l l o y i s a l m o s t a r e p l i c a o f t h e NbÛ2 r e s p o n s e b u t w i t h an a p p a r e n t s h i f t o f t h e e n e r g y gap t o l o w e r e n e r g i e s (~ 2.7 eV) and a l s o a h i g h e r responsivity. T h i s r e p r e s e n t s a s h i f t o f t h e e n e r g y bands r a t h e r than a t a i l i n g o f t h e Ή Ο 2 response which takes place w i t h doping ( 1 6 ) . I n t e r e s t i n g l y , t h i s e x t r a p o l a t i o n suggests t h a t t h e a l l o y m i g h t have a bandgap w h i c h i s l e s s t h a n t h a t o f t h e two s t a r t i n g m a t e r i a l s . Such e f f e c t s have been o b s e r v e d i n w i d e band s e m i c o n d u c t o r s , e . g . , t h e ZnSe-ZnTe s y s t e m ( 1 7 ) . The f a c t t h a t t h e s t r u c t u r e i n t h e a l l o y a c t i o n spectrum i s s i m i l a r t o t h a t o f p u r e r e d u c e d NbÛ2 i m p l i e s t h a t s i m i l a r s t a t e s e x i s t i n t h e a l l o y as i n t h e r e d u c e d NbÛ2 and t h a t t h e l o w e n e r g y r e s p o n s e i n t h e a l l o y i s n o t due m a i n l y t o s t r a i n s a r i s i n g f r o m inhomogeniety o r a l l o y i n g . C o n d u c t i v i t y versus temperature p l o t s i n d i c a t e t h a t r e l a t i v e l y s h a l l o w e n e r g y l e v e l s below t h e c o n d u c t i o n band a r e i n t r o d u c e d by t h e r e d u c t i o n p r o c e s s . Thus t h e s e s t a t e s a r e b u l k and n o t s u r f a c e s t a t e s . F i g u r e 8 compares t h e r e s p o n s i v i t i e s o f a p u r e LaFeÛ3 powder d i s c , s i n g l e - c r y s t a l S r T i Û 3 and 20% LaFeÛ3 i n S r T i Û 3 . Curve A, t h e r e s p o n s i v i t y o f t h e a l l o y , i n d i c a t e s response both i n t h e v i s i b l e as w e l l as i n t h e UV. R e c e n t l y o t h e r methods o f e l e c t r o d e f a b r i c a t i o n have been r e p o r t e d w i t h r e l a t i v e l y h i g h e f f i c i e n c i e s ; f o r e x a m p l e , A u g u s t y n s k i e t a l ( 1 8 ) have p r e p a r e d t h i n f i l m s o f T i 0 2 doped w i t h m e t a l s s u c h as A l and Be on T i s u b s t r a t e s by a s p r a y i n g t e c h n i q u e . A n o t h e r f a b r i c a t i o n t e c h n i q u e which i s q u i t e simple i n p r i n c i p l e i s t o f i r e oxide m i x t u r e s on T i m e t a l i n t h e method r e c e n t l y d e s c r i b e d by Guruswamy and B o c k r i s ( 1 9 ) who s t u d i e d L a C r 0 3 - T i 0 2 e l e c t r o d e s . Here a g a i n a v e r y t h i n f i l m o f m a t e r i a l i s formed and a s i g n i f i c a n t photoresponse i s o b t a i n e d . Fabrication technique i s

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1.8

2.2

2.6

4.2

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Figure 7. Comparison between responsivities of single-crystal n-Ti0 , pressed-disc n-Nb0 , and pressed-disc n-Ti Nb sO . Electrodes are biased at +1.0 V rela tive to SCE (pH = 10). 2

2

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MAVROIDES E T A L .

Figure 8. Comparison between responsivities of single-crystal n-SrTiO , presseddisc p-LaFeOs, and n-O.8 SrTiO -O.2 LaFeO . SrTiO and the alloys are biased at +1.0 V relative to SCE while p-LaFeO is biased at—O.7 V relative to SCE (pH = 10). See text for difference between (A) and (B). s

3

s

s

s

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Figure 9.

Comparison between I-V curves of O.8 SrTiO -O.2 LaFeO film and single-crystal SrTiO (pH = 10) s

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Electrode Band Structure and Interface States 229

thus very important i n determining photoresponse, e s p e c i a l l y f o r alloys. Curve (Β) o f F i g u r e 8 g i v e s t h e photoresponse f o r a t h i n e l e c t r o d e p r e p a r e d e s s e n t i a l l y by t h e method o f Guruswamy and Bockris. I t w i l l be n o t e d t h a t a l t h o u g h b o t h e l e c t r o d e s (A) and (B) have t h e same v i s i b l e p h o t o r e s p o n s e , w h i c h i s i n t e r m e d i a t e between t h a t o f t h e two end m a t e r i a l s b u t v e r y l o w , t h e f i l m , e l e c t r o d e ( B ) , has a much h i g h e r r e s p o n s e f r o m t h e UV i n t o t h e visible. I f these a l l o y s a r e t o prove u s e f u l , t h e i r low energy r e s p o n s e needs t o be r a i s e d , p e r h a p s by a c o m b i n a t i o n o f i m p r o v e d f a b r i c a t i o n t e c h n i q u e s and i d e n t i f i c a t i o n o f t h e optimum a l l o y c o n c e n t r a t i o n . F i g u r e 9 compares t h e I-V c u r v e s o f e l e c t r o d e s formed f r o m t h e S r T i 0 3 - L a F e 0 3 a l l o y f i l m and t h e s i n g l e - c r y s t a l S r T i 0 3 a t pH = 1 0 . I t w i l l be o b s e r v e d t h a t ( w i t h a 150 W Xe lamp) t h e onset o f photocurrent f o r t h e a l l o y i s a t ~ -O.95 V r e l a t i v e t o t h e SCE j u s t as n e g a t i v e as f o r S r T i 0 3 . However, w i t h a l i t t l e b i a s t h e S r T i Û 3 e l e c t r o d e g i v e s a much l a r g e r p h o t o c u r r e n t , i n d i c a t i n g much l e s s h o l e - e l e c t r o n r e c o m b i n a t i o n . In summary, we have d e s c r i b e d t h e main c o n c e p t s o f two components o f o u r p r o g r a m , one on c o m p o s i t e m a t e r i a l s and t h e o t h e r on a l l o y s . The e x p e r i m e n t s on c o m p o s i t e m a t e r i a l s a r e s t i l l n o t advanced enough t o t e l l how e f f e c t i v e such m a t e r i a l s w i l l be as e l e c t r o d e s . The r e s u l t s on t h e a l l o y s show some p r o m i s e , and p o i n t t o t h e need f o r a more b a s i c m a t e r i a l s a p p r o a c h i n o r d e r t o o b t a i n i m p r o v e d e l e c t r o d e s . T h i s work was s u p p o r t e d by t h e S o l a r E n e r g y R e s e a r c h I n s t i t u t e .

Abstract Presently available semiconducting electrodes suitable for the photoelectrolysis of water are inefficient because their action spectra are not well matched to the solar spectrum. In terms of band structure, the energy gaps of the usual stable semiconducting electodes are too large. Semiconductors with smaller energy gaps, on the other hand, are usually not photoelectrochemically stable; furthermore, they have electron affinities which are too large. One solution to these contradictory requirements, the use of layer type compounds with d-energy bands, has been recently investigated Tributsch and co-workers. Another possible solution is the use of composite electrodes. In such electrodes, interface states become particularly significant. Not only are the states at the electrode-electrolyte interface important, but also those at interfaces between elements of the composite electrode. In this talk we will discuss the electronic properties of several kinds of composite electrodes in terms of energy band structure and interface states, as well as their fabrication and operation in photoelectrochemical cells. This work was supported by the Solar Energy Research Institute.

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Literature

AT SEMICONDUCTOR-ELECTROLYTE

INTERFACES

Cited

1. Kohl, P. Α.; Frank, S. N.; Bard, A. J., J. Electrochem. Soc., 1977, 124, 225. 2. Tomkiewicz, M. and Woodall, J. M., J. Electrochem. Soc., 1977, 124, 1436. 3. Yu, Κ. Y.; Helms, C. R.; Spicer, W. Ε.; Chye, P. W., Phys. Rev., 1977, 15, 1629. 4. Goodenough, J. Β.,"Metallic Oxides", in Progress in Solid State Chemistry, Editor H. Reiss, (Pergam mon Press, New York) 1972, p. 145. 5. Tributsch, Η., Ber. Bunsenges. Phys. Chem., 1977, 81, 362. 6. Tributsch, H., Ber. Bunsenges. Phys. Chem., 1978, 82, 169. 7. Tributsch, Η., Ber. Bunsenges. J. Electrochem. Soc., 1978, 125, 1086. 8. Gobrecht, J.; Tributsch, H.; Gerischer, H., J. Elec trochem. Soc., 1978, 125, 2085. 9. Hardee, K. L. and Bard, A. J., J. Electrochem. Soc., 1976, 123, 1024. 10. Quinn, R. K.; Nasby, R. D.; Baughman, R. J., Mater. Res. Bull., 1976, 11, 1011. 11. Shu, L.; Yen, R.; Hackerman, N., J. Electrochem. Soc., 1977, 124, 833. 12. Kennedy, J. H. and Frese, Jr., K. W., J. Electro chem. Soc., 1978, 125, 709. 13. Butler, Μ. Α.; Ginely, D. S.; Eibschutz, M., J. Appl. Phys., 1977, 48, 3070. 14. Rauh, R. D.; Buzby, J. M.; Reise, T. F.; Alkartis, S. Α., J. Phys. Chem., 1977, 83, 2221. 15. Crowell, C. R. and Sze, S. Μ., Solid-State Electron. 1966, 9, 1035. 16. Maruska, H. P. and Ghosh, Α. Κ., Solar Energy Mater. 1979, 1, 237. 17. Larach, S.; Shrader, R. E.; Stocher, C. F., Phys. Rev., 1957, 108, 587. 18. Augustynski, J.; Hinden, J.; Stalder, Chs., Electro chem. Soc., 1977, 124, 1063. 19. Guruswamy, V. and Bockris, J. O. Μ., Solar Energy Mater., 1979, 1, 441. R E C E I V E D October 17,

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In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

15 Photoelectrochemical Systems Involving SolidLiquid Interfacial Layers of Chlorophylls TSUTOMU MIYASAKA and KENICHI HONDA

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Department of Synthetic Chemistry, Faculty of Engineering, University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113, Japan

Photoelectrochemical conversion from visible light to elec tric and/or chemical energy using dye-sensitized semiconductor or metal electrodes is a promising system for the in vitro simula tion of the plant photosynthetic conversion process, which is con sidered one of the fundamental subjects of modern and future photoelectrochemistry. Use of chlorophylls(Chls) and related compounds such as porphyrins in photoelectric and photoelectro chemical devices also has been of growing interest because of its close relevance to the photoacts of reaction center Chls in photo synthesis. Although Chl, as well as most of biological pigments isolated from living organisms, is unstable under ambient conditions, the usefulness of Chl as a photoreceptor in in vitro studies is im portant for the following reasons: (a) its capability of utilizing red incident light, (b) strong redox reactivities in its excited states, (c) the occurrence of highly efficient energy migration among excited molecules, (d) the ability to form a wide variety of photoactive derivatives absorbing in far red region, such as Chl-nucleophile aggrega tes, and (e) the availability of its surfactant structure which enables the ideal incorporation of the molecule into membrane struc tures. Based on these characteristics, a number of investigations of the photoelectrochemical as well as the photovoltaic effects of in vitro Chl have appeared during the last decade. Besides developing an efficient model system, the goal of light conversion studies using in vitro Chl is to obtain as much useful information about the photobehavior of Chl as possible in order to identify the in vivo reaction mechanisms involved. To this end, various photoelectrochemical approaches have emphasized the molecular configuration and geometry of Chl. Although studies in this field are currently progressing rapidly, this paper will review the photoelectrochemical behavior of Chl from the physical and photochemical perspectives. 0097-6156/81/0146-0231$05.25/0 © 1981 American Chemical Society In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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1.

Morphological

Aspects of C h l o r o p h y l l I n t e r f a c i a l Layers

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We are p r i m a r i l y concerned with s e v e r a l t y p i c a l modes of the Chl i n t e r f a c i a l l a y e r r e l e v a n t to p h o t o e l e c t r i c and p h o t o e l e c t r o chemical systems. Since the Chl molecule, a s u r f a c t a n t pigment c o n s i s t i n g of a p a r t i a l l y h y d r o p h i l i c porphyrin r i n g and a t o t a l l y hydrophobic p h y t o l chain, i s i n s o l u b l e i n water, i t i s general l y employed f o r p h o t o e l e c t r i c and photoelectrochemical measure ments i n the form of a s o l i d (or q u a s i - s o l i d ) f i l m deposited on an e l e c t r o d e s u r f a c e . Such Chl f i l m s are d i v i d e d i n t o three t y p i c a l modes according to molecular c o n f i g u r a t i o n . 1) Amorphous Films. An amorphous f i l m i s g e n e r a l l y pre pared by solvent evaporation of a dry organic s o l u t i o n of Chl on a s o l i d substrate s u r f a c e . The vacuum sublimation technique, which i s widely employed f o r most s y n t h e t i c dyes, i s not a p p l i c a b l e to Chl due to p o s s i b l e thermal degradation of the pigment. The red absorption peak of a dry amorphous Chl a f i l m i s around 675-680 nm. This i s r e d - s h i f t e d from that of monomeric Chl a (660 nm) i n organic s o l u t i o n (1,2), i n d i c a t i n g that aggregated forms of Chl a such as dimers and oligomers (absorbing i n the red at 670-680 nm (2 3^) ) are i n v o l v e d . 9

2) M a c r o c r y s t a l l i n e C h l o r o p h y l l and I t s F i l m s . The c r y s t a l l i z a t i o n e f f e c t i s a c h a r a c t e r i s t i c of Chl. Jacob et at.(5) e s t a b l i s h e d evidence f o r the s i g n i f i c a n t r o l e of water i n the f o r mation of m a c r o c r y s t a l l i n e Chl. L a t e r , B a l l s c h m i t e r and Katz (6) explained t h i s m i c r o c r y s t a l l i n e form of Chl a, absorbing i n the red around 740 nm, i n terms of (Chl α - Η 2 θ ) adduct. A great deal of study has focused on the s t r u c t u r a l and photochemical charac t e r i z a t i o n of v a r i o u s Chl-I^O aggregate species (2,3,4,6-13). The relevance of these f a r - r e d absorbing aggregates as s t r u c t u r a l models to the photosynthetic r e a c t i o n center Ρ700 (14), which i s b e l i e v e d to i n v o l v e a s p e c i a l p a i r of Chl a, has a l s o been a sub j e c t of intense study (3,4,7,9,10,11,13); f o r example, s t r u c t u a l proposals f o r Chl α - Η 0 species have suggested that the water molecule i s bound to Chl a with i t s oxygen coordinated to a cen t r a l magnesium and with i t s hydrogen to the C-9 keto carbonyl group (_7>13) or the C-10 keto carbonyl (9_,10) i n the porphyrin r i n g . I t i s known that other n u c l e o p h i l e s can a l s o a s s o c i a t e to form aggregates s i m i l a r to Chl-H 0 species (8). I t should be noted that the formation of these hydrated Chl species as w e l l as t h e i r l i g h t r e a c t i o n s may be involved more or l e s s i n the other types of Chl f i l m s prepared on e l e c t r o d e s , with the extent depending s i g n i f i c a n t l y on the method of preparing the f i l m as w e l l as the e x p e r i mental c o n d i t i o n s . Chl α - Η 0 m i c r o c r y s t a l s (745 nm) (5,6,10,11) and other hy drated Chl species are r e a d i l y obtained by allowing a small η

2

2

2

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amount of water to remain i n s o l u t i o n s of Chl a d i s s o l v e d i n s u i t able nonpolar organic solvents. Films can be formed on the s o l i d ( e l e c t r o d e ) s u r f a c e by solvent evaporation. A Chl a-H2Û macrocryst a l l i n e f i l m can be prepared conveniently by the e l e c t r o d e p o s i t i o n technique e s t a b l i s h e d by Tang and Albrecht (15); the m i c r o c r y s t a l s ( p o s i t i v e l y charged) i n 3-methylpentane suspension are deposited on the e l e c t r o d e substrate (cathode) under a p p l i e d f i e l d s (about 1 kV/cm ), allowing f o r uniform f i l m s as t h i c k as s e v e r a l hundred monolayers. This m i c r o c r y s t a l l i n e f i l m (740-5 nm) can e a s i l y be converted to an anhydrous and amorphous form (675 nm) by heating the f i l m below 70°C. 3) Monolayer Assemblies o f C h l o r o p h y l l Monolayer and m u l t i l a y e r f i l m s of Chls have been studied i n d e t a i l since 1937 (16,17). From the b i o l o g i c a l point of view, Chl monolayer arrays are of i n t e r e s t because they may be important i n the s t r u c t u r e of the c h l o r o p l a s t - l a m e l l a e or s o - c a l l e d t h y l a k o i d membrane (18,19) Due to t h e i r s u r f a c t a n t s t r u c t u r e s , Chls are among the i d e a l and s t a b l e monolayer-forming pigments, and surface pressure-molecular area (Π-Α) isotherms of t h e i r monolayers formed a t air-water i n t e r f a c e s have been w e l l c h a r a c t e r i z e d (20). I t i s b e l i e v e d that at the water surface Chl molecules are o r i e n t e d with the porphyrin r i n g s and p h y t o l chains d i r e c t e d upward and the h y d r o p h i l i c e s t e r linkages downward. Monolayers and mixed monolayers of Chls a r e prepared on a n e u t r a l agueous b u f f e r surface and can be deposited at a c o n t r o l l e d surface concentration onto a s o l i d substrate by the Langmuir-Blodgett technique (21). Such preparation and deposi t i o n techniques are s p e c i f i e d i n l i t e r a t u r e s (20,22,23). As a t y p i c a l f e a t u r e , a monolayer f i l m of pure Chl a possesses absorp t i o n peaks a t 675-680 nm and 435-440 nm with corresponding o p t i c a l d e n s i t i e s of O.008-O.01 and O.01-O.013 i n the red and blue bands, r e s p e c t i v e l y (24,25). The r e d - s h i f t s observed i n the absorption peaks with respect to Chl a i n s o l u t i o n may r e s u l t from d i p o l e d i p o l e i n t e r a c t i o n between Chl molecules (20).

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9

Various i n e r t compounds such as f a t t y a c i d s , f a t t y a l c o h o l s , and l i p i d s behave as two-dimensional d i l u e n t s f o r Chl monolayers and lead to the formation of homogeneously mixed monolayers (20). These d i l u e n t s have f a c i l i t a t e d the study o f Chl-Chl energy t r a n s f e r w i t h i n a two-dimensional plane as a f u n c t i o n of the intermol e c u l a r Chl separation (26,27). In s u f f i c i e n t l y d i l u t e mixed monolayers, a majority of the Chl molecules are thought to e x i s t i n the monomeric s t a t e , with t h e i r mutual aggregations e f f e c t i v e l y suppressed w i t h i n the geometrically c o n t r o l l e d , ordered c o n f i g u r a t i o n . M u l t i l a y e r s ( b u i l t - u p monolayers) of Chl a have a l s o been studied (23) and u t i l i z e d f o r p h o t o v o l t a i c studies (see the next s e c t i o n ) . The molecular o r i e n t a t i o n i n such Chl a m u l t i l a y e r s has been ascertained from the observed dichroism i n s p e c t r o p o l a r i z a t i o n measurements with respect to absorption (23) and emission (28). I t has been reported that Chl a i n both m i c r o c r y s t a l l i n e f i l m s (5,10) and monolayer assemblies (23) are f a i r l y s t a b l e over long periods of time.

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE INTERFACES

was 2%. S i m i l a r d i f f e r e n c e s between c r y s t a l l i n e and amorphous f i l m s were reported e a r l i e r by Corker and Lundstrom (42). However, they mentioned that the photοconductive and r e c t i f y i n g nature of the s o l i d Chl a l a y e r i s not a t t r i b u t a b l e to the postulated organic semiconductivity of Chl a, but more l i k e l y to an oxide l a y e r e x i s t ing on the metal surface which functions as a r e c t i f y i n g b a r r i e r and to a l a r g e number of trapped e l e c t r o n s which act l i k e dopants w i t h i n the l a y e r . Such a concept i s compatible with McCree's p o s t u l a t i o n (34), although there are d i f f e r e n c e s i n the f i l m c o n f i g u r a t i o n s i n t h e i r systems. Whether or not Chl i s regarded i n t r i n s i c a l l y as an organic semiconductor, the s o l i d Chl l a y e r i n contact with a metal does d i s p l a y a p-type p h o t o v o l t a i c e f f e c t , and i t s e f f i c i e n c y depends s i g n i f i c a n t l y on the morphology of the Chl l a y e r as w e l l as the nature of the metal. The e f f e c t corresponding t o a p-type photoconductor can a l s o be expected at the j u n c t i o n of a metal /Chl/ l i q u i d i n a photoelectrochemical system. Such a presumption i s i n f a c t compatible with the photoelectrochemical behavior observed f o r most of Chl-coated metal e l e c t r o d e s , as w i l l be shown l a t e r . 3.

E l e c t r o c h e m i c a l Energetics of C h l o r o p h y l l

The ground s t a t e o x i d a t i o n and reduction p o t e n t i a l s of Chl a and b i n v a r i o u s organic s o l u t i o n s have been measured by a number of i n v e s t i g a t o r s . These experimental data as w e l l as other impor tant energy parameters f o r Chls have been covered i n a recent review by Seely (43). I t i s noted that the p o t e n t i a l values r e ported by various i n v e s t i g a t o r s are not i n good agreement. This might r e f l e c t the d i f f i c u l t y (due to j u n c t i o n p o t e n t i a l s ) i n cor r e l a t i n g the observed redox p o t e n t i a l s (versus, e.g., Ag) i n organic media to those i n aqueous media r e f e r r e d to the SCE (saturated calomel electrode) or the NHE, r a t h e r than r e s u l t s from solvent e f f e c t s . Taking the various sources of data i n t o account, onee l e c t r o n o x i d a t i o n and r e d u c t i o n p o t e n t i a l s f o r Chl a are +O.52 to +O.62 V v s . SCE and -1.08 to -1.25 V v s . SCE, r e s p e c t i v e l y ; those f o r Chl b are +O.56 to +O.72 V v s . SCE and -1.03 t o - 1 . 2 9 V v s . SCE, r e s p e c t i v e l y . We r e c e n t l y measured the redox p o t e n t i a l s f o r Chls by c y c l i c voltammetry (mainly i n DMF medium), employing ferrocene as a standard redox sample to c o r r e c t f o r the reference e l e c t r o d e p o t e n t i a l . The r e s u l t s obtained were +O.59 V and -1.17 V v s . SCE f o r Chl a o x i d a t i o n and reduction (one-electron), and +O.63 V and'v-1.1 V v s . SCE f o r Chl b o x i d a t i o n and r e d u c t i o n (44). A l l are w i t h i n the range of p r e v i o u s l y reported values. Using s i n g l e t and t r i p l e t e x c i t a t i o n energies reported f o r Chl a, 1.85 eV (45) and 1.33 eV (46), and f o r Chl b, 1.91 eV (45) and 1.39 eV (46), r e s p e c t i v e l y , o x i d a t i o n p o t e n t i a l s of Chls i n the ground and e x c i t e d s t a t e s are presented i n Figure 1 and compared with redox p o t e n t i a l s of other b i o l o g i c a l l y important reagents. Another t o p i c of i n t e r e s t i s the r e a c t i o n of e x c i t e d s t a t e Chl a with water. This occurs as a r e s u l t o f the primary photo-

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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15.

MiYASAKA AND HONDA

2.

I n t r i n s i c P h o t o e l e c t r i c a l Behavior of S o l i d C h l o r o p h y l l Films

Interfacial Layers of Chlorophylls

235

Since photoelectrochemical measurements deal mostly with s o l i d i n t e r f a c i a l l a y e r s of Chl i n contact with e l e c t r o d e s , i t i s of prime importance to understand the p h o t o e l e c t r i c p r o p e r t i e s of Chl i n the s o l i d s t a t e . P r i o r to any photoelectrochemical s t u d i e s , a number of i n v e s t i g a t i o n s of the photoconductive behav i o r of Chl were performed based on the hypothesis (29,30) that charge separation i n photosynthetic primary r e a c t i o n s may r e s u l t from the photoconduction i n Chl systems. Among the e a r l y s t u d i e s of Chl photophysics were those by Nelson (31) and Arnold and Macley (32) who measured p h o t o c o n d u c t i v i t i e s of solvent-evaporated Chl f i l m s on platinum g r i d s and of monolayer assemblies on c o l l o i dal g r a p h i t e g r i d s , r e s p e c t i v e l y . Therein and co-workers (33) studied the p h o t o c o n d u c t i v i t y of m i c r o c r y s t a l l i n e Chl l a y e r s sand wiched between two e l e c t r o d e s and c h a r a c t e r i z e d the s o l i d Chl l a y e r as a p-type organic semiconductor. On the other hand, a l a t e r study by McCree (34) pointed out that monolayer assemblies of Chl are undoubtedly i n e f f i c i e n t p h o t o c o n d u c t o r s . A similar conclusion was made by Reucroft and Simpson (35) u s i n g Chl a m u l t i l a y e r s coated on Sn02 or Sn e l e c t r o d e s . However, the photo v o l t a i c e f f e c t of Chl a was enhanced s i g n i f i c a n t l y by superimposed monolayers of an e l e c t r o n acceptor or doner. This e f f e c t o f the added acceptor l a y e r s has a l s o been e s t a b l i s h e d by Shkuropatov et al. (36) and Janzen and Bolton (37) u s i n g amorphous f i l m s and monolayers of Chl a , r e s p e c t i v e l y , placed on A l e l e c t r o d e s . High power e f f i c i e n c i e s of p h o t o v o l t a i c e f f e c t s due to metal / Chl l a y e r contacts have r e c e n t l y been reported using metal / Chl a I metal sandwich-type dry c e l l s . Meilanov et al. (38) reported quantum e f f i c i e n c i e s as high as 10% f o r the charge gen e r a t i o n i n amorphous Chl a f i l m s between A l e l e c t r o d e s . Tang and Albrecht (39,40), employing the e l e c t r o d e p o s i t i o n technique, have e x t e n s i v e l y studied the metal / m i c r o c r y s t a l l i n e Chl a I metal sand wich c e l l s with respect to v a r i o u s combinations of metals with d i f f e r e n t work f u n c t i o n s . In t h e i r systems, m i c r o c r y s t a l l i n e Chl a (the hydrated form absorbing at 745 nm) behaved as an e f f i c i e n t p-type semiconductor, and the highest power conversion e f f i c i e n c y , 5 χ 1 0 ~ % at 745 nm (O.7% photocurrent quantum e f f i c i e n c y ) , was obtained i n the c e l l mode, Cr / Chl a I Hg (40). They a t t r i b u t e d the p h o t o v o l t a i c e f f e c t to a p o t e n t i a l b a r r i e r (Schottky b a r r i e r ) formed at the contact of the low work-function metal (e.g.,Cr) and the p-type organic semiconductor ( c r y s t a l l i n e Chl a). More r e c e n t l y Dodelet et al. (41) studied A l / Chl a I Ag p h o t o v o l t a i c c e l l s (the A l / Chl a contact i s photoactive) where the p h o t o c o n d u c t i v i t i e s of m i c r o c r y s t a l l i n e l a y e r s and amorphous (anhydrous) l a y e r s were compared. The power conversion e f f i c i e n c y i n the former U O.2%) was much higher than i n the l a t t e r U O.04%), which supports the contention that the c r y s t a l l i n e form i s a b e t t e r photoconductor than the amorphous form. In t h i s study, the maximum photocurrent quantum e f f i c i e n c y f o r the c r y s t a l l i n e Chl a 2

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE

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236

Si 1

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t

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In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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MIYASAKA AND

HONDA

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s y n t h e t i c processes i n terms of the enzyme-assisted o x i d a t i o n of water by the e x c i t a t i o n of photosystem I I r e a c t i o n center Chl a (P680) and reduction of NADP (or protons i n the presence of hydrogenase) by the e x c i t a t i o n of photosystem I r e a c t i o n center Chl a (P700). From Figure 1, one can see that the e x c i t e d s i n g l e t s t a t e of in vitro Chl a, as w e l l as Chl b, can reduce water on an energ e t i c b a s i s since an o v e r p o t e n t i a l of about O.7 V i s a v a i l a b l e . However, i t i s known (47,48) that e x c i t e d Chl a i n s o l u t i o n undergoes e l e c t r o n t r a n s f e r r e a c t i o n s with redox species e x c l u s i v e l y v i a a l o n g - l i v e d t r i p l e t s t a t e ( l i f e t i m e , ca. 2 ms (49)). Since the o x i d a t i o n p o t e n t i a l s of ground and t r i p l e t e x c i t e d s t a t e Chls are s i t u a t e d c l o s e to the o x i d a t i o n (+O.57 V v s . SCE) and reduction (-O.66 V v s . SCE) p o t e n t i a l s of water, r e s p e c t i v e l y , i n n e u t r a l s o l u t i o n , the photodecomposition of water by t r i p l e t e x c i t e d Chls, namely, Chl*

+

Chl"*" +

H

+

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^

Chl"*" +

1/2

Chl

H

+

+

H +

(1)

2

1/2

0

2

(2)

may not be e f f i c i e n t at pH 7. On the other hand, e i t h e r r e a c t i o n might be promoted when the pH of s o l u t i o n (i.e.,redox p o t e n t i a l of water) i s v a r i e d so as to favour the r e a c t i o n . This conjecture i s c o n s i s t e n t with the demonstration of water s p l i t t i n g e i t h e r i n an a c i d i c e l e c t r o l y t e ( f o r the r e a c t i o n 1) or i n an a l k a l i n e e l e c t r o l y t e ( f o r the r e a c t i o n 2) as w i l l be described i n the next s e c t i o n . In connect i o n with r e a c t i o n 2, Watanabe and Honda (50) r e c e n t l y found that the r a t e constant f o r the r e a c t i o n of the Chl a c a t i o n r a d i c a l with water i n a c e t o n i t r i l e medium i s extremely low. However, the redox p o t e n t i a l as w e l l as the r e a c t i o n behavior i n v e s t i g a t e d i n organic s o l u t i o n s may not be d i r e c t l y a p p l i c a b l e to the energetics of a s o l i d Chl layer-aqueous s o l u t i o n i n t e r f a c e , the general c o n d i t i o n i n a photoelectrochemical system. 4.

Photoelectrochemical Systems Involving Semiconductor and Metal Electrodes

Chlorophyll-Coated

1) General Technique f o r C h l o r o p h y l l U t i l i z a t i o n . In photoe l e c t r o c h e m i c a l measurements, the methods commonly employed f o r preparing Chl i n t e r f a c i a l l a y e r s on e l e c t r o d e substrates are solvent evaporation, e l e c t r o d e p o s i t i o n ( f o r c r y s t a l l i n e Chl), and monolayer d e p o s i t i o n techniques, as o u t l i n e d p r e v i o u s l y . Chl i s p u r i f i e d from c h l o r o p l a s t e x t r a c t s , u s u a l l y obtained from spinach leaves, by dioxane p r e c i p i t a t i o n method (51) and conventional sugar column chromatography (52). For r a p i d and easy preparation, the method r e c e n t l y developed by Omata and Murata (53) i s s a t i s f a c t o r y ; s y n t h e t i c (DEAE-) Sepharose i s s u b s t i t u t e d f o r sugar on the column. Besides using spectroscopic c r i t e r i a (52), the p u r i t y of Chl samples can be checked r e a d i l y by means of s i l i c a - g e l t h i n l a y e r chromatography (54). C o l o r l e s s contaminations i n Chl

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE INTERFACES

238

preparations can be determined by measuring Chl monolayer IT - A isotherms (20). S p e c t r a l parameters f o r Chl a , such as e x t i n c t i o n c o e f f i c i e n t , i n various p o l a r and nonpolar organic solvents have been i n v e s t i g a t e d by Seely (1). 2) Chlorophyll-Coated Semiconductor E l e c t r o d e s . Chl has f i r s t been employed by T r i b u t s c h and C a l v i n (55^56) i n dye s e n s i t i z a t i o n studies of semiconductor e l e c t r o d e s . Solvent-evapo rated f i l m s of Chl a Chl b and b a c t e r i o c h l o r o p h y l l on n-type semiconductor ZnO electrodes ( s i n g l e c r y s t a l ) gave anodic s e n s i t i z e d photocurrents under p o t e n t i o s t a t i c conditions i n aqueous e l e c t r o l y t e s . The photocurrent a c t i o n spectrum obtained f o r Chl a showed the red band peak at 673 nm corresponding c l o s e l y to the amorphous and monomeric s t a t e of Chl a. The a d d i t i o n of supers e n s i t i z e r s (reducing agents) increased the anodic photocurrents, and a maximum quantum e f f i c i e n c y of 12.5% was obtained f o r the photocurrent i n the presence of phenylhydrazine. Based on the observed strong s u p e r s e n s i t i z a t i o n e f f e c t , generation of the anodic photocurrent was explained i n terms of an e l e c t r o n t r a n s f e r mechanism i n v o l v i n g s e v e r a l p o s s i b l e pro cesses. For example, e l e c t r o n i n j e c t i o n from e x c i t e d Chl a ( s i n g l e t and/or t r i p l e t ) i n t o the conduction band of ZnO could take place followed by a r a p i d reduction of the Chl a r a d i c a l c a t i o n by a reducing agent, and/or Chl a could undergo photoreduction by the reducing agent to produce a r a d i c a l anion which subsequently i n j e c t s an e l e c t r o n to the conduction band. P a r t i c i p a t i o n of both s i n g l e t and t r i p l e t e x c i t e d s t a t e s of Chl a i n the above e l e c t r o n t r a n s f e r processes was proposed. Knowing that the s i n g l e t s t a t e donor l e v e l of Chl a (around -1.3 V vs. SCE) i s located consider ably above the conduction band edge of ZnO (ca. -O.75 V vs. SCE at pH 7 (57) ) while the t r i p l e t donor l e v e l (around -O.75 V v s . SCE) i s s i t u a t e d very c l o s e to the ZnO l e v e l , i t seems reasonable to assume that the d i r e c t e l e c t r o n i n j e c t i o n from the e x c i t e d Chl a should occur predominantly v i a the s i n g l e t s t a t e . Since only a short time (ca. 10" s (58) ) i s necessary f o r e l e c t r o n i n j e c t i o n from a s e n s i t i z i n g dye to a semiconductor, e l e c t r o n t r a n s f e r from an e x c i t e d s i n g l e t s t a t e of Chl a ( l i f e t i m e ca. 6 χ 10~ s (59) ) must be efficient. In c o n t r a s t , another p o s s i b l e process i n i t i a t e d by the reduction of an e x c i t e d Chl α by a reducing agent, i f i n v o l v e d , i s considered favourable to the t r i p l e t s t a t e of Chl a, i n view of the f a c t that the p h o t o i o n i z a t i o n of Chl a by the r e a c t i o n with redox species has so f a r been found only v i a t r i p l e t e x c i t a t i o n (43,47,48). Besides ZnO e l e c t r o d e , i t was a l s o found that n-type CdS can be s e n s i t i z e d by Chls. We have studied the photoelectrochemical behavior of Chl a and Chl b on an n-type Sn0 (60) o p t i c a l l y transparent e l e c t r o d e (OTE; thickness of the Sn0 l a y e r on the g l a s s substrate, ca. 2000 A; donor d e n s i t y , 1 0 c m " ) . Chl monolayer assemblies, deposited by means of the Langmuir-Blodgett technique (20,21), were employed. The use of such monolayer assemblies as i n t e r f a c i a l dye l a y e r s

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3

9

1

9

2

2

2 0 - 2 1

3

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

o

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i n photoelectrochemical s t u d i e s has the f o l l o w i n g advantages : (a) Strong attachment of dye l a y e r s to the e l e c t r o d e surface can be e s t a b l i s h e d v i a h y d r o p h i l i c and/or hydrophobic i n t e r a c t i o n s , without accompanying chemical r e a c t i o n s of the dye, (b) two-dimensional homogeneous dye l a y e r s can be formed on the electrode surface, (c) the surface concentration as w e l l as the mean i n t e r m o l e c u l a r distance of the dye can be c o n t r o l l e d , and (d) the t o t a l thickness of the dye l a y e r s can be p r e c i s e l y con t r o l l e d at the molecular l e v e l . Besides these p r a c t i c a l advantages, i t i s of b i o l o g i c a l importance to study the photoeffects of Chl i n an ordered s t r u c t u r e , because such a s t r u c t u r e i s a c r u c i a l f a c t o r i n r e g u l a t i n g energy migra t i o n among Chls as w e l l as promoting e l e c t r o n t r a n s f e r processes i n photosynthetic organisms. Photoredox r e a c t i o n s i n v o l v i n g Chl incorporated i n t o a r t i f i c i a l membranes have long been studied by the use of b i l a y e r l i p i d membranes (BLM) (61). BLM studies are based on photogalvanic e f f e c t s caused by e x c i t e d Chl and mediated v i a the s o l u t i o n , while our membrane-electrode system deals with the d i r e c t capture of an e l e c t r o n (or hole) from photoexcited Chl at the e l e c t r o d e - s o l u t i o n i n t e r f a c e . Monolayer and mixed monolayers of Chl a and/or Chl b were deposited on a Sn0 OTE at a constant surface pressure of 10-20 dyn/cm. This r e s u l t e d i n a c l o s e l y packed, ordered f i l m on the electrode. The high transmittance of the OTE allowed f o r the d i r e c t measurement of the absorbance of a monolayer at the electrode-electrolyte interface. Quantum e f f i c i e n c i e s f o r the generation of photocurrents were c a l c u l a t e d using t h i s information. Monolayers of Chl a as w e l l as Chl b gave anodic s e n s i t i z e d photo currents under p o t e n t i o s t a t i c c o n d i t i o n s i n aqueous e l e c t r o l y t e containing a reducing agent (hydroquinone, H Q) (25). The s p e c t r a l dependence of the photocurrent matched the absorption spectrum of Chl at a S n 0 2 - e l e c t r o l y t e i n t e r f a c e . The spectra are shown i n Figure 2; the photoresponse of Sn0 i t s e l f i s n e g l i g i b l e over the v i s i b l e region due to i t s l a r g e band gap (3.5-3.8 eV). Under open c i r c u i t c o n d i t i o n , Chl α-coated Sn0 developed negative photovoltages
2

2

2

2

2

2

9

2

2

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

240

PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE

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INTERFACES

ο

\

\

CL

\

\

J\

\\ \ \

"

400

'

50Ô ' WAVELENGTH

600 (nm)

'

700

Figure 2. Photocurrent spectra for a Chl sl monolayer ( ) and a Chl b monolayer ( ) on a Sn0 electrode. Electrolyte, aqueous phosphate buffer (O.025M, pH 6.9) containingO.05-O.1Mhydroquinone; electrode potential,O.05V vs. SCE. The red band maxima of both spectra are normalized. 2

sf

V

τ,*

Λ Cond. Band

Figure 3. Scheme for the electron transfer in dye sensitization process at the interfacial layer of Chl on Sn0 electrode 2

Sn02

Chl

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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15.

MiYASAKA A N D HONDA

Interfacial Layers of Chlorophylls

241

Attempts have been made to i n t e r s p e r s e a s u i t a b l e i n e r t s u r f a c t a n t d i l u e n t i n the monolayer i n order to c o n t r o l the sur face concentration as w e l l as the mean i n t e r m o l e c u l a r separation of Chl. Since Chl has h i g h l y o v e r l a p i n g absorption and emission (fluorescence) s p e c t r a , e f f i c i e n t m i g r a t i o n of e x c i t a t i o n energy among molecules i s allowed. I t has been w e l l e s t a b l i s h e d both i n s o l u t i o n s (62,63) and i n membranes (26,27,59) that Chl undergoes strong self-quenching of s i n g l e t e x c i t a t i o n energy at high concen t r a t i o n . This concentration quenching phenomenon may a r i s e from the formation of non-fluorescent p a i r traps of Chl possessing a c r i t i c a l separation (64) and can be suppressed e f f e c t i v e l y by the homogeneous d i l u t i o n of the Chl. Among the s u r f a c t a n t s which can act as i d e a l two-dimensional d i l u e n t s f o r a Chl a monolayer, we chose the p h o s p h o l i p i d , d i p a l m i t o y l l e c i t h i n (DPL) (65), which i s one of the main components of b i o l o g i c a l membranes. Two-dimensional homogeneity i n the Chl α-DPL mixed monolayer system was confirmed by the Π - Α isotherms as w e l l as by the observed enhancement of the fluorescence y i e l d of the mixed mono l a y e r upon decreasing the Chl a surface c o n c e n t r a t i o n (66). I t was found that the quantum e f f i c i e n c y of the anodic photo current i n the Chl α-DPL system was increased from 3-4% i n the pure Chl a monolayer up to about 25% (65) i n h i g h l y d i l u t e d mixed monolayers. This increase i n e f f i c i e n c y was accompanied by red s h i f t s of the photocurrent peak p o s i t i o n s i n the red and blue bands (e.g., i n the red, from 675 nm (pure Chl a) to 665-670 nm (Chl α/DPL ^ 1/49)) and probably r e f l e c t s the change i n chromophore-chromophore i n t e r a c t i o n between Chl a molecules. The considerable e f f i c i e n c i e s obtained apparently r e s u l t from the suppression of the self-quenching of Chl a by separating the Chl-Chl i n t e r m o l e c u l a r d i s t a n c e . Based on photoelectrochemical measurements, an e x t e n t i o n of t h i s study may e l u c i d a t e the energy i n t e r a c t i o n s between Chl and other photosynthetic pigments i n the mixed monolayer systems. The e f f e c t of the f i l m thickness on the photocurrent e f f i ciency was i n v e s t i g a t e d by the use of b u i l t - u p m u l t i l a y e r s of Chl a (67). Since a s i n g l e Chl a monolayer i s about 14 Â t h i c k (16), the p r e c i s e c o n t r o l of the f i l m thickness i s p o s s i b l e by s t a c k i n g monolayers. The magnitude of the anodic photocurrent increased with the number of monolayers u n t i l i t was saturated at around 10-15 l a y e r s , while the quantum e f f i c i e n c y of the photocurrent monotonously decreased. The l a t t e r phenomenon may r e f l e c t enhanced e x c i t a t i o n energy quenching among neighbouring monolayers i n the three-dimensional arrey and/or an increase i n t o t a l e l e c t r i c r e s i s t a n c e of the m u l t i l a y e r with i n c r e a s i n g number of l a y e r s . Lowering of the anodic p h o t o e f f e c t might a l s o be a t t r i b u t e d to p-type photoconductivity developing i n t h i c k e r stacks of monolayers which leads to the reinforcement of the cathodic p h o t o e f f e c t of Chl a. From the above r e s u l t s we conclude that a monolayer-thick f i l m having w e l l i n t e r s p e r s e d Chl i s most e f f i c i e n t f o r the

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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INTERFACES

conversion of l i g h t energy using C h l - s e n s i t i z e d semiconductor electrodes. The photocurrent generated by a Chl a m u l t i l a y e r i n contact with an aqueous e l e c t r o l y t e without any added redox agents has been measured as a f u n c t i o n of pH of the s o l u t i o n (67). S i g n i f i c a n t i n c r e a s e i n the anodic photocurrent was observed at pHs higher than 7. Since the photooxidized Chl a ( c a t i o n r a d i c a l ) i s considered to o x i d i z e water b e t t e r at higher pH above pH7 (see S e c t i o n 3), water probably behaves as a reducing agent ( s u p e r s e n s i t i z e r ) f o r Chl a i n a l k a l i n e s o l u t i o n s . In connection with the study using Chl-coated Sn02 e l e c t r o d e s , an attempt has been reported to coat a c h l o r o p l a s t - or algae-immob i l i z e d polymer f i l m onto Sn(>2 OTE (68), and water o x i d a t i o n at the i l l u m i n a t e d e l e c t r o d e has been performed. Chl-coated semiconductor (n-type) e l e c t r o d e s have thus f a r been s t u d i e d using ZnO, CdS, and S n 0 2 , a l l of which act as e f f i c i e n t photoanodes f o r converting v i s i b l e l i g h t . Such C h l - s e n s i t i z e d photoanodes could be regarded as in vitro models f o r the photosystem I I (oxygen e v o l u t i o n ) f u n c t i o n i n photosynthesis, p-type semiconductor e l e c t r o d e s have not been u t i l i z e d s u c c e s s f u l l y to produce cathodic C h l - s e n s i t i z e d photocurrents with s a t i s f a c t o r y efficiencies. On the other hand, Chl-coated metal e l e c t r o d e systems seem to overcome t h i s problem. 3) Chlorophyll-Coated Metal E l e c t r o d e s . Photoelectrochemical r e a c t i o n s at Chl-coated metal e l e c t r o d e s have been i n v e s t i g a t e d respecting the v a r i o u s configurâtional modes of Chl. Platinum e l e c t r o d e s have widely been employed as substrates of Chl f i l m s . Takahashi and co-workers (69,70,71) reported both cathodic and anodic photocurrents i n a d d i t i o n to corresponding p o s i t i v e and negative photovoltages at solvent-evaporated f i l m s of a Chloxidant mixture and a Chl-reductant mixture, r e s p e c t i v e l y , on platinum e l e c t r o d e s . Various redox species were examined, respect i v e l y , as a donor or acceptor added i n an aqueous e l e c t r o l y t e (69). In a t y p i c a l experiment (71), NAD and F e ( C N ) g , each d i s s o l v e d i n a n e u t r a l e l e c t r o l y t e s o l u t i o n , were employed as an acceptor f o r a photocathode and a donor for a photoanode, r e s p e c t i v e l y , and the photoreduction of NAD at a Chl-naphthoquinone-coated cathode and the photooxidation of Fe(CN)£ at a Chl-anthrahydroquinone-coated anode were performed under e i t h e r short c i r c u i t c o n d i t i o n s or p o t e n t i o s t a t i c c o n d i t i o n s . The r e d u c t i o n of NAD at the photocathode was demonstrated as a model f o r the photosynthetic system I . In t h e i r s t u d i e s , the photoactive species was a t t r i b u t e d to the composite of Chl-oxidant or -reductant (70). A p-type semiconductor model was proposed as the mechanism f o r photocurrent generation at the Chl photocathode (71). Photoelectrochemical r e a c t i o n s of hydrated m i c r o c r y s t a l l i n e Chl a e l e c t r o d e p o s i t e d on platinum e l e c t r o d e s have been s t u d i e d by Fong and co-workers (72.,73'Ζ^ΖΙ). The experiments were p e r f omed i n short c i r c u i t e l e c t r o c h e m i c a l c e l l s . In aqueous e l e c t r o l y t e s

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

15.

MiYASAKA A N D HONDA

Interfacial Layers of Chlorophylls

without added redox agents, the m i c r o c r y s t a l l i n e (hydrated aggre gates of) Chl α-coated, p l a t i n i z e d - p l a t i n u m e l e c t r o d e s gave cathodic photocurrents upon i l l u m i n a t i o n , and the photocurrent peaked i n the red a t 745 nm. The quantum e f f i c i e n c y of the photo current was increased i n a c i d i c s o l u t i o n s and was i n the order of 10"" e l e c t r o n s / i n c i d e n t photon (73). T h e i r s t u d i e s i n p a r t i c u l a r focused on the in vitro s i m u l a t i o n of the photosynthetic water s p l i t t i n g r e a c t i o n by using f a r - r e d absorbing Chl a-H2Û s p e c i e s . These species have long been proposed as the in vitro structural models f o r photosynthetic r e a c t i o n centers ( r e f e r to Section 1 ) . Using v a r i o u s photoactive Chl a-B^O s p e c i e s , such as Chl a-H^O, (Chl a-H 0)2, (Chl a - 2 H 0 ) , Fong et al. p o s t u l a t e d , on the b a s i s of redox t i t r a t i o n experiments (76) and ESR observations of the r e a c t i o n of e x c i t e d Chl α hydrate with water (11), that the dihydrated Chl a oligomers, (Chl a - 2 H 0 ) , are the most powerful redox species and are capable of both reducing and o x i d i z i n g water under f a r red e x c i t a t i o n . Evidence f o r the water s p l i t t i n g r e a c t i o n a t an i l l u m i n a t e d Chl α dihydrate-platinum ( p l a t i n i z e d ) e l e c t r o d e was claimed (74) based on the mass spectrometric analy s i s of the p h o t o l y t i c products from i s o t o p i c a l l y l a b e l e d water. More r e c e n t l y , photoreduction of C 0 i n t h i s system has a l s o been undertaken (75). In view of the f a c t that the Fong s experiments i n v o l v e d repeated p l a t i n i z a t i o n of the surfaces of the platinum and Chl α l a y e r s (74,75), the observed p h o t o r e a c t i v i t y may a r i s e from a p h o t o c a t a l y t i c e f f e c t of the composite of Chl α hydrates and p l a tinum r a t h e r than Chl α hydrates themselves. In t h i s r e s p e c t , i t has been reported that Pt d i s p e r s i o n s c a t a l i z e d y e - s e n s i t i z e d photoreduction of water (77,78). Another type of Chl i n t e r f a c i a l l a y e r employed on a metal e l e c t r o d e was a f i l m c o n s i s t i n g of ordered molecules. V i l l a r (79) studied short c i r c u i t cathodic photocurrents a t m u l t i l a y e r s of Chl α and b b u i l t up on semi-transparent platinum e l e c t r o d e s i n an e l e c t r o l y t e c o n s i s t i n g of 96% g l y c e r o l and 4% KCl-saturated aqueous s o l u t i o n . Photocurrent quantum e f f i c i e n c i e s of m u l t i l a y e r s and of amorphous f i l m s prepared by solvent evaporation were compared. The highest e f f i c i e n c y (about 10 ^ e l e c t r o n s / absorbed photon, c a l c u l a t e d from the paper) was obtained with Chl α m u l t i l a y e r s , and the amorphous f i l m s of Chl α proved to be l e s s e f f i c i e n t than Chl b m u l t i l a y e r s . We have i n v e s t i g a t e d the photocurrent behavior of m u l t i l a y e r s of a Chl α-DPL (molar r a t i o 1/1) mixture on platinum i n an aqueous e l e c t r o l y t e without added redox agents (80). Cathodic photo currents with quantum e f f i c i e n c i e s i n the order of 10~^ were ob tained with f i l m s c o n s i s t i n g of a s u f f i c i e n t number of monolayers. The photocurrent was increased i n a c i d i c s o l u t i o n s . However, no appreciable photocurrent was observed with a s i n g l e monolayer coated on platinum. The l a t t e r f a c t most probably r e s u l t s from minimal r e c t i f y i n g property of the metal surface and/or an e f f i c i e n t energy quenching of dye e x c i t e d s t a t e s by f r e e e l e c t r o n s i n 3

2

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243

2

n

2

n

2

f

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

244

PHOTOEFFECTS

AT SEMICONDUCTOR-ELECTROLYTE

INTERFACES

the metal (81,82). This i n d i c a t e s that a reasonably t h i c k f i l m of Chl, which may behave l i k e a photoconductor, i s r e q u i r e d to produce measurable photocurrents on metal e l e c t r o d e s . Aizawa and Suzuki (83,84,85,86) u t i l i z e d , as an ordered system, l i q u i d c r y s t a l s i n which Chl was immobilized. E l e c t r o d e s were prepared by solvent-evaporating a s o l u t i o n c o n s i s t i n g of Chl and a t y p i c a l nematic l i q u i d c r y s t a l , such as n-(p-methoxybenzyli d e n e ) - p - b u t y l a n i l i n e , onto a platinum s u r f a c e . C h l - l i q u i d c r y s t a l e l e c t r o d e s i n a c i d i c b u f f e r s o l u t i o n s gave cathodic photocurrents accompanied by the e v o l u t i o n of hydrogen gas (83). This was the f i r s t demonstration of photoelectrochemical s p l i t t i n g of water using in vitro Chl. Of p a r t i c u l a r i n t e r e s t i n these s t u d i e s i s the e f f e c t of s u b s t i t u t i n g the c e n t r a l metal i n the Chl molecule. In c o n t r a s t to Chl (Mg-pheophytin) which behaves as a photocathode, Mn-pheophytin immobilized i n a l i q u i d c r y s t a l functioned as a photoanode, producing oxygen ( o x i d i z i n g water) i n a l k a l i n e b u f f e r s o l u t i o n s (84,85). More r e c e n t l y , Ru-pheophytin was a l s o examined and found to generate cathodic photocurrent with a quantum e f f i c i e n c y of around O.5% (86). The e f f e c t of the c e n t r a l metal on the s i g n of photoresponse may r e f l e c t v a r i a t i o n s of redox pot e n t i a l among these metal pheophytin analogues. In these s t u d i e s , the authors suggested that a Chl-water adduct formed i n the l i q u i d c r y s t a l system was the photoactive species (83,85). Taking a general view of the above s t u d i e s , we note that Chl-coated metal (platinum) e l e c t r o d e s commonly f u n c t i o n as photocathodes i n a c i d i c s o l u t i o n s , although the photocurrent e f f c i e n c i e s tend to be lower compared to systems employing semiconductors. This cathodic photoresponse may a r i s e from a p-type photoconduct i v e nature of a s o l i d Chl l a y e r and/or formation of a contact b a r r i e r at the metal-Chl i n t e r f a c e which c o n t r i b u t e s to l i g h t - i n duced c a r r i e r s e p a r a t i o n and leads to photocurrent generation.

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T

5.

Other Related Systems I n v o l v i n g Model Compounds

Among what have been widely employed as model compounds f o r Chl, are p o r p h y r i n s , phthalocyanines, and some photoactive t r a n s i t i o n metal complexes, which are more s t a b l e and e a s i e r to o b t a i n than Chl. I n t e r f a c i a l l a y e r s of these i n s o l u b l e compounds are g e n e r a l l y prepared by means of vacuum sublimation or solvent evaporation. Tetraphenylporphine (TPP) and other metal porphyrine d e r i v a t i v e s coated on platinum (87^,88^,89) or gold (89,90) e l e c t r o d e s have been i n v e s t i g a t e d i n photoelectrochemical modes. Photocurrents reported are cathodic or anodic, depending on the pH as w e l l as the composition of the e l e c t r o l y t e employed. Photocurrent quantum e f f i c i e n c i e s of 2% (89) to 7% (87) were reported i n systems u s i n g water i t s e l f or methylviologen as the redox species i n aqueous e l e c t r o l y t e . Photocurrent generation at Zn-TPP-coated metal cathodes (89) was i n t e r p r e t e d i n terms of a r e c t i f y i n g e f f e c t of the Schottky b a r r i e r formed at a metal-p-type

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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15.

MiYASAKA A N D HONDA

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245

semiconductor contact, and i t s e f f i c i e n c y was found to i n c r e a s e with i n c r e a s i n g work f u n c t i o n of the metal. Wang (91) obtained considerable negative photovoltages (1.1 V under f i l t e r e d i n c i d e n t l i g h t from 150 W tungsten lamp) and anodic photocurrents with Zn-TPP m u l t i l a y e r s on A l e l e c t r o d e s i n contact with a f e r r o / f e r r i redox e l e c t r o l y t e . Based on the Wang's system, Kampas et al. (92) r e c e n t l y compared the quantum e f f i c i e n c i e s of a wide v a r i e t y of amorphous metal porphyrin d e r i v a t i v e s . The highest quantum e f f i c i e n c y , around 10%, was obtained with Mg-porphyrin analogues having more negative o x i d a t i o n p o t e n t i a l s . The p h o t o e f f e c t was a t t r i b u t e d to the formation of Schottky type contact i n terms of metal (Al) / i n s u l a t o r ( A I 2 O 3 ) / p-type semiconductor (porphyrins) system. Photoelectrochemical behavior of metal phthalocyanine s o l i d f i l m s (p-type photoconductors) have been s t u d i e d a t both metal (93,94,95,96) and semiconductor (97^98) e l e c t r o d e s . Copper phthalocyanine vacuum-deposited on a Sn02 OTE (97) d i s p l a y e d photocurrents with signs depending on the thickness of f i l m as w e l l as the e l e c t r o d e p o t e n t i a l . Besides anodic photocurrents due to normal dye s e n s i t i z a t i o n phenomenon on an n-type semiconductor, enhanced cathodic photocurrents were observed with t h i c k e r f i l m s due to a bulk e f f e c t (p-type photoconductivity) of the dye l a y e r . Meier et al. (95) studied the cathodic photocurrent behavior of various metal phthalocyanines on platinum e l e c t r o d e s where the dye l a y e r acted as a t y p i c a l p-type organic semiconductor. Another model compound, the t r i s ( 2 , 2 - b i p y r i d i n e ) r u t h e n i u m ( I I ) complex, has prompted c o n s i d e r a b l e i n t e r e s t because i t s w a t e r - s p l i t t i n g p h o t o r e a c t i v i t y has been demonstrated i n v a r i o u s types of photochemical systems (77,99,100,101). Memming and Schroppel (102) have attempted to deposit a monolayer of a surf actant Ru(II) complex on a SnÛ2 OTE. In aqueous s o l u t i o n , an anodic photocurrent a t t r i b u t a b l e to water o x i d a t i o n by the e x c i t e d t r i p l e t Ru complex was observed. A maximum quantum e f f i c i e n c y of 15% was obtained i n a l k a l i n e s o l u t i o n . Other dye-layer-coated photoelectrode systems have been studied but are not covered i n t h i s review due to space r e s t r i c t i o n s . T h i s area of study, i n c l u d i n g the photoelectrochemistry of Chl, i s s t i l l i n an inchoate but r a p i d l y developing stage. !

6. Concluding Remarks: Relevance

to Photosynthetic Model Systems.

Chl-coated semiconductor (n-type) e l e c t r o d e s and metal e l e c t rodes can a c t as e f f i c i e n t photoanodes and photocathoes, respect i v e l y , f o r v i s i b l e l i g h t conversion. The former system f u n c t i o n s as a d y e - s e n s i t i z e d semiconductor e l e c t r o d e , while the l a t t e r i s presumably d r i v e n by the photoconductive p r o p e r t i e s of a Chl s o l i d l a y e r and/or charge s e p a r a t i o n i n v o l v i n g the Chl-metal contact barrier. Morphological d i f f e r e n c e s i n the s o l i d l a y e r of Chl are c r u c i a l f o r determining the p h o t o e l e c t r i c p r o p e r t i e s of Chl.

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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H

INTERFACES

2

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hydrogenase

I!

II H 0 2

P700

u

Mn-enzyme ) P 6 8 0 02^

^rAcceptor Acceptor

Λ

• Donor

red>

+ -. I ext. circuit

m

Chli

Donor semiconductor photo anode

metal photocathode

Figure 4. Schematic for the photoelectrochemical simulation of the photosynthetic electron-pumping processes ("upper sketch) by means of a Chl-semiconductor photoanode and a Chl-metal photocathode

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

Downloaded by 148.204.30.87 on October 21, 2009 | http://pubs.acs.org Publication Date: March 2, 1981 | doi: 10.1021/bk-1981-0146.ch015

15.

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247

Accordingly, such f a c t o r s as molecular c o n f i g u r a t i o n or surface concentration of Chl and thickness o f the l a y e r s i g n i f i c a n t l y a f f e c t the e f f i c i e n c y of photocurrent generation i n electrochemi c a l systems. As pointed out p r e v i o u s l y , C h l - s e n s i t i z e d photoelectrodes are important t o studies of in vitro s i m u l a t i o n of the photosynthetic l i g h t conversion process. In photosynthetic primary processes, the pumping of e l e c t r o n s which enables the s p l i t t i n g of water i s achieved v i a two photosystems i n which l i g h t - i n d u c e d charge sepa r a t i o n s occur e f f i c i e n t l y coupled with o n e - d i r e c t i o n a l e l e c t r o n t r a n s f e r s i n redox systems. Though t o t a l process f o r the photo s y n t h e t i c water s p l i t t i n g r e a c t i o n n e c e s s a r i l y i n v o l v e s enzyme-assisted dark r e a c t i o n s as secondary processes, the primary processes f o r electron-pumping could be simulated e l e c t r o c h e m i c a l l y using a couple of C h l - s e n s i t i z e d photoelectrodes which have inverse r e c t i f y i n g p r o p e r t i e s and d i f f e r e n t redox l e v e l s ( i . e . , work f u n c t i o n s ) . An example i s sketched i n Figure 4; i n t h i s model, Chl-semiconductor photoanode and Chl-metal photocathode, corresponding r e s p e c t i v e l y to photosystem I I and photosystem I , are combined i n s e r i e s with an e x t e r n a l c i r c u i t which takes the place of in vivo e l e c t r o n t r a n s f e r chain across the t h y l a k o i d membrane. In order that the photocathode (system I) possesses e n e r g e t i c a l l y higher p o t e n t i a l s than photoanode (system I I ) , d i f f e r e n t molecular c o n f i g u r a t i o n s of Chl, e.g., hydrated aggregates or monolayer assemblies, may be a p p l i c a b l e to each photoelectrode. Based on such an e l e c t r o c h e m i c a l l y simulated system, f u r t h e r m o d i f i c a t i o n and c h a r a c t e r i z a t i o n of the C h l - c o n t a i n i n g i n t e r f a c i a l l a y e r on electrodes are expected t o c o n t r i b u t e to the d i s covery of u s e f u l information on the e l e c t r o n and energy t r a n s f e r r e a c t i o n s i n v o l v i n g Chl and other compounds of photosynthetic importance. Acknowledgements The authors wish to thank Drs. A. Fujishima for their useful discussions.

Literature 1. 2. 3. 4. 5.

and T. Watanabe

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16 Supra-Band-Edge Reactions at SemiconductorElectrolyte Interfaces Band-Edge Unpinning Produced by the Effects of Inversion 1

J. A . TURNER, J. MANASSEN , and A . J. NOZIK

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Solar Energy Research Institute, Golden, CO 80401

I.

Introduction

A recent modification (1-3) of the conventional model (4) for photoelectrochemical reactions suggests that photo-generated minority carriers may, under certain conditions, be injected into the electrolyte before they reach thermal equilibrium within the semiconductor space charge layer. This process is called "hot carrier injection. More efficient conversion of optical energy into chemical energy may be possible with hot carrier injection because a greater fraction of the incident photon energy can be deposited in the electrolyte to do chemical work. Experiments designed to test for hot electron injection from p-type semiconductors were conducted by probing reduction reactions having redox potentials more negative than (i.e., above) the conduction band edge. The oxidized components of such redox couples should not be reduced by thermalized photo-generated electrons coming from the conducting band edge. However, hot electrons, having more negative potential, could reduce the higher lying redox couples. The energy level diagram for this process is shown in Figure 1. Analogous results should be obtained for oxidation reactions having redox potentials more positive than the valence band edge of n-type photoanodes. We call such oxidation or reduction reactions lying outside the semiconductor band gap "supra band-edge reactions". Experiments using p-Si in non-aqueous electrolytes (5,6) indicate that the reduction of redox species with redox potentials more negative than the conduction band edge could apparently be achieved. However, further analysis showed that these results are not caused by hot electron injection, but by the unpinning of the semiconductor band edges at the semiconductor-electrolyte interface; this unpinning effect is caused by the creation of an inversion layer at the p-Si surface. This is an important effect, especially for small band gap semiconductors, that has received little attention On Sabbatical leave from the Weizmann Institute of Science, Rehovot, Israel. 1

0097-6156/81/0146-0253$05.00/0 © 1981 American Chemical Society In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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INTERFACES

Fermi Level

Figure 1. Hot electron injection from a p-type semiconductor electrode. The photogenerated electron is injected into the electrolyte to reduce A to A' before it is thermalized in the space-charge layer. A thermalized electron emerging at the conduction band edge (E ) would have insufficient energy to drive the redox couple A/A: c

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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16.

TURNER E T A L .

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255

in photoelectrochemistry. Unpinned band edges i n p h o t o e l e c t r o c h e m i c a l s y s t e m s have a l s o r e c e n t l y been p r o p o s e d by o t h e r w o r k e r s . B a r d , e t a l . (7J e x p l a i n t h e e f f e c t as t h e r e s u l t o f a l a r g e d e n s i t y o f s u r f a c e s t a t e s , w h i l e M o r r i s o n (8.) i n v o k e s i n t e r f a c e s t a t e s a r i s i n g from o x i d e l a y e r s . The u n p i n n e d band edges can move w i t h a p p l i e d p o t e n t i a l , maki n g t h e s e m i c o n d u c t o r e l e c t r o d e behave s i m i l a r t o a m e t a l e l e c t r o d e i n t h a t changes i n a p p l i e d p o t e n t i a l o c c u r a c r o s s t h e B e l m h o l t z l a y e r r a t h e r than a c r o s s t h e semiconductor space charge l a y e r . This s i t u a t i o n , o f c o u r s e , d r a s t i c a l l y a l t e r s the energy r e l a t i o n s h i p s between t h e s e m i c o n d u c t o r bands and t h e e l e c t r o l y t e r e d o x p o t e n t i a l s compared t o t h e c o n v e n t i o n a l model ( 4 ) ; i n t h e l a t t e r , t h e s e m i c o n d u c t o r bands a r e p i n n e d a t t h e s u r f a c e , t h e p o t e n t i a l d r o p a c r o s s t h e H e l m h o l t z l a y e r i s c o n s t a n t , and p o t e n t i a l changes i n t h e e l e c t r o d e p r o d u c e c o r r e s p o n d i n g changes i n t h e band b e n d i n g w i t h i n t h e semiconductor space charge l a y e r . The d i f f e r e n c e b e tween p i n n e d and u n p i n n e d band edges i s i l l u s t r a t e d i n F i g u r e 2. In t h e f o r m e r c a s e , t h e p o s i t i o n s o f t h e c o n d u c t i o n and v a l e n c e band edges a t t h e i n t e r f a c e (E£ and Ey) a r e f i x e d w i t h r e s p e c t t o the e l e c t r o l y t e redox l e v e l s , w h i l e i n the l a t t e r case the p o s i t i o n s o f E and E§ w i t h r e s p e c t t o t h e e l e c t r o l y t e r e d o x l e v e l s v a r y w i t h e l e c t r o d e p o t e n t i a l . T h u s , w i t h u n p i n n e d band e d g e s , r e d o x c o u p l e s l y i n g o u t s i d e t h e band gap under f l a t band c o n d i t i o n s can l i e w i t h i n t h e band gap under band b e n d i n g c o n d i t i o n s o r under i l l u m i n a t i o n . II.

Experimental

Results

The e x p e r i m e n t s were p e r f o r m e d w i t h s i n g l e c r y s t a l (111) p - S i e l e c t r o d e s w i t h a r e s i s t i v i t y o f a b o u t 5.5 ohm cm; non-aqueous e l e c t r o l y t e s were used c o n s i s t i n g o f a b s o l u t e m e t h a n o l c o n t a i n i n g t e t r a m e t h y l a m m o n i u m c h l o r i d e (TMAC) o r a c e t o n i t r i l e c o n t a i n i n g t e t r a e t h y l ammonium p e r c h l o r a t e ( T E A P ) . The f l a t - b a n d p o t e n t i a l s o r p - S i i n t h e two e l e c t r o l y t e s were d e t e r m i n e d f r o m M o t t - S c h o t t k y p l o t s ( i n the dark) i n the d e p l e t i o n range o f the p-Si e l e c t r o d e , from o p e n - c i r c u i t p h o t o p o t e n t i a l m e a s u r e m e n t s , and f r o m t h e v a l u e s o f e l e c t r o d e p o t e n t i a l a t which anodic photocurrent i s f i r s t obs e r v e d i n n - t y p e S i e l e c t r o d e s . These t h r e e methods a l l y i e l d e d c o n s i s t e n t f l a t - b a n d p o t e n t i a l v a l u e s f o r p - S i o f + O.05V (vs SCE) ± . 0 5 V i n b o t h m e t h a n o l and a c e t o n i t r i l e . These v a l u e s , combined w i t h t h e d o p i n g d e n s i t y and t h e band gap o f 1.12 eV f o r p - S i p l a c e s t h e c o n d u c t i o n band edge i n m e t h a n o l and a c e t o n i t r i l e a t -O.85V (vs S C E ) . The s u p r a b a n d edqe r e d o x c o u p l e s c h o s e n f o r t h e two e l e c t r o l y t e s were 1,3 d i m e t h o x y - 4 - n i t r o b e n z e n e (&>=-!.0V vs SCE)for m e t h a n o l , and 1 n i t r o n a p h t h a l e n e ( E = - l . 0 8 ) , 1,2 d i c h l o r o 4 m i t r o b e n z e n e ( E = - O . 9 5 ) , and a n t h r a q u i n o n e ( E = - O . 9 5 ) f o r a c e t o n i t r i l e . These r e d o x couples l i e from O.1 V to O.24V above t h e c o n d u c t i o n band edge o f p-Si, and h e n c e , i n t h e c o n v e n t i o n a l model, c o u l d n o t be photoreduced by p - S i . P h o t o c u r r e n t - v o l t a g e d a t a showed t h a t a l l t h e s u p r a - b a n d - e d g e r e d o x s p e c i e s l i s t e d above a r e r e d u c e d upon i l l u m i n a t i o n o f t h e p - S i e l e c t r o d e ; no a p p r e c i a b l e r e d u c t i o n was a c h i e v e d i n t h e d a r k . T y p i c a l r e s u l t s f o r p - S i w i t h methanol and a c e t o n i t r i l e a r e shown 0

o

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0

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Pinned Band Edges v = u -u B

E

Unpinned Band Edges v * u - u

U'e-

B

E

( b

) b

Ε;

s

Figure 2. Pinned vs. unpinned band edges. In the former case the position of E and Έ remain fixed with respect to the electrolyte redox couples as the electrode potential is varied. In the latter case, E and E change with applied potential. c

8

ν

s

c

s

v

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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TURNER E T A L .

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i n F i g u r e 3. The l i g h t i n t e n s i t y dependence o f t h e p h o t o c u r r e n t v o l t a g e b e h a v i o r f o r a n t h r a q u i n o n e i n a c e t o n i t r i l e i s shown i n F i g u r e 4 . A t h i g h l i g h t i n t e n s i t y (442 mW/cm , w h i t e l i g h t f r o m xenon lamp) o n l y a c a t h o d i c r e d u c t i o n p h o t o c u r r e n t i s o b s e r v e d . As t h e l i g h t i n t e n s i t y i s r e d u c e d , an o x i d a t i o n wave grows i n t h a t d e v e l o p s on t h e a n o d i c sweep. T h i s o x i d a t i o n wave i s b e l i e v e d t o r e p r e s e n t t h e r e - o x i d a t i o n o f reduced anthraquinone a t t h e p-Si electrode. I n F i g u r e 5, i t i s seen t h a t t h e o x i d a t i o n wave c a n be generated i f t h e high l i g h t i n t e n s i t y i s suddenly reduced to zero on t h e a n o d i c r e t u r n sweep a f t e r r e d u c t i o n has o c c u r r e d a t t h e high l i g h t i n t e n s i t y . A l t h o u g h t h e r e s u l t s i n F i g u r e s 3 a r e t h o s e t h a t w o u l d be e x pected f o r hot e l e c t r o n i n j e c t i o n e f f e c t s , a d d i t i o n a l experiments and a n a l y s e s i n d i c a t e t h a t h o t e l e c t r o n p r o c e s s e s a r e n o t o c c u r r i n g here. The f i r s t t y p e o f e x p e r i m e n t a l r e s u l t t h a t was i n c o n s i s t e n t w i t h hot e l e c t r o n i n j e c t i o n i n v o l v e d the wavelength depen dence o f t h e r e d u c t i o n c u r r e n t f o r t h e s u p r a - b a n d - e d g e r e d o x s p e c ies. T h i s i s shown i n F i g u r e 6 f o r 1,3 d i m e t h o x y - 4 - n i t r o b e n z e n e i n m e t h a n o l . The r e l a t i v e quantum e f f i c i e n c y o f t h e p h o t o r e d u c t i o n c u r r e n t was f o u n d t o be i n d e p e n d e n t o f w a v e l e n g t h o v e r t h e r a n g e 400 nm t o 800 nm. Above 800 n , t h e quantum e f f i c i e n c y d r o p s and r e a c h e s z e r o a t a b o u t 1100 nm; t h i s d r o p - o f f i s c o n s i s t e n t w i t h t h e 1,12 eV band gap o f S i . A s e c o n d s e t o f e x p e r i m e n t s i n v o l v e d c a p a c i t a n c e measurements as a f u n c t i o n o f e l e c t r o d e p o t e n t i a l , a c s i g n a l f r e q u e n c y , and light intensity. T y p i c a l r e s u l t s f o r p - S i i n a c e t o n i t r i l e and m e t h a n o l a r e shown i n F i g u r e s 7 and 8 , In a c e t o n i t r i l e ( F i g u r e 5), t h e d a r k c a p a c i t a n c e f i r s t d e c r e a s e s w i t h i n c r e a s e d band b e n d i n g and t h e n a t a b o u t - O . 3 V ( v s SCE) r e m a i n s a t a f l a t minimum a t a c s i g n a l f r e q u e n c i e s above 100 H z . Under i l l u m i n a t i o n , t h e c a p a c i tance increases sharply with i n c r e a s i n g l i g h t i n t e n s i t y a t a l l po t e n t i a l s ; t h e c a p a c i t a n c e - v o l t a g e c u r v e shows a minimum a t a b o u t -O.1 V ( v s SCE) and i n c r e a s e s r a p i d l y a t h i g h e r n e g a t i v e p o t e n tials. A t f r e q u e n c i e s b e l o w 100 H z , t h e c a p a c i t a n c e a l s o i n c r e a s e s i n t h e d a r k w i t h i n c r e a s i n g n e g a t i v e p o t e n t i a l beyond - 0 , 3 V ( v s SCE). S i m i l a r r e s u l t s a r e o b t a i n e d w i t h m e t h a n o l ( F i g u r e 6) e x cept t h a t a sharp decrease i n capacitance i s a l s o g e n e r a l l y o b s e r v e d a t a l l f r e q u e n c i e s a t p o t e n t i a l s more n e g a t i v e t h a n - O . 3 t o -O.5V ( v s S C E ) . T h i s d r o p i s a s s o c i a t e d w i t h r e d u c t i o n o f t r a c e amounts o f w a t e r i n t h e e l e c t r o l y t e w h i c h l e a d s t o l a r g e dark c u r r e n t f l o w .

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2

Ill.

Discussion

F o r t h e p a r t i e l u a r p - S i e l e c t r o d e used i n t h e s e e x p e r i m e n t s ( d o p i n g d e n s i t y o f 2 χ 10 cm~3) t h e p r e d i c t e d w a v e l e n g t h d e p e n dence o f t h e p h o t o - r e d u c t i o n c u r r e n t f o r h o t c a r r i e r p r o c e s s e s s h o u l d show no p h o t o c u r r e n t f o r w a v e l e n g t h s l o n g e r t h a n 440 nm. T h i s i s because t h e d e p l e t i o n w i d t h o f t h e p-Si e l e c t r o d e w i t h 1 v o l t band b e n d i n g i s 870 nm, and p h o t o g e n e r a t e d e l e c t r o n s a r r i v i n g a t t h e s u r f a c e from t h e f i e l d - f r e e r e g i o n ( a f t e r c r o s s i n g t h e s p a c e - c h a r g e l a y e r ) w o u l d be t h e r m a l i z e d . T h a t i s , t h e d e p l e t i o n

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Electrode Potential (vs SCE), Volts

H1.0

Applied Physics Letters Figure 3. Photoreduction on p-Si or redox couples with redox potentials lying above the conduction band edge as determined by dark fiat-band potential measurements, (a) Photoreduction of 1,3 dimethoxy-4-nitrobenzene in methanol (E = —1.0 V vs. SCE); (b) photoreduction of anthraquinone in acetonitrile (E = -O.95 V vs. SCE); E for p-Si in both cases in the dark is -O.85 V vs. SCE; ( ; dark; ( ; light (5). 0

0

c

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50

-4 -300

-O.3

O.0

Ε vs. SSCE, Volts

Figure 4. Light intensity dependence of the photoreduction current of anthraquinone in acetonitrile. An anodic wave grows in at about O.0 V (vs. SSCE) with decreasing light intensity. (( ) O.9 mw/cm ; ( ) 50 mw/cm ; ('---) 442 mw/cm ; anthraquinone (ImM); O.5M TEAP, Acetonitrile, 50 mV/s scan) 2

2

2

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INTERFACES

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ρ+O.5

-2.0

-1.5

-1.0

-O.5

+O.5

Ε vs. SSCE, Volts 2

Figure 5. Current-voltage curve for anthraquinone (400 mw/cm ) in acetonitrile at high light intensities: ( ) the current response after the light is shut off during the anodic sweep. An oxidation wave at about O.0 V (vs. SSCE) develops if the light is shut off.

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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Figure 6. Wavelength dependence of the photoreduction current of 1,3 dimethoxy4-nitrobenzene (20mM) in methanol (1M TMAC)

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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+O.2

O.0 -O.2 -O.4 Electrode Potential (vs.SCE), Volts

-O.6

Applied Physics Letters Figure 7. Capacitance vs. p-Si electrode potential in acetonitrile as a function of frequency and light intensity (5) (( ) 10 Hz; ( ; 10 Hz; ( ) 5 χ 10 Hz) 2

3

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TURNER E T A L .

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2.001

ol +O.2

1 O.0

1 -O.2

1 -O.4

1 -O.6

1— -O.8

Electrode Potential (vs S C E ) , Volts

Applied Physics Letters Figure 8. Dark capacitance vs. p-S/ electrode potential in methanol as a function of frequency (5) (( ; 10 , 5 χ 10 Hz; ( ) 5 X 10 Hz; ( ) 10 Hz) 2

1

3

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PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE INTERFACES

w i d t h i s w i d e enough t o a l l o w s u f f i c i e n t e l e c t r o n - p h o n o n c o l l i s i o n s t o d i s s i p a t e t h e band b e n d i n g p o t e n t i a l e n e r g y as phonon excitation (heat). In o r d e r t o c r e a t e h o t e l e c t r o n s a t t h e s u r f a c e o f t h e s a m p l e , i t w o u l d be n e c e s s a r y t o i l l u m i n a t e t h e p - S i e l e c t r o d e w i t h p h o t o n s o f e n e r g y g r e a t e r t h a n a b o u t 2.8 eV ( w a v e l e n g t h s l e s s t h a n 440 nm). These p h o t o n s have a b s o r p t i o n c o e f f i c i e n t s g r e a t e r t h a n 2 χ 1 0 c m " and w o u l d be a b s o r b e d w e l l w i t h i n t h e s p a c e c h a r g e l a y e r ; t h e y w o u l d a l s o have e n e r g i e s s u f f i c i e n t l y g r e a t e r t h a n t h e band gap s u c h t h a t p h o t o e l e c t r o n s w o u l d be c r e a t e d a t t h e S i s u r f a c e w i t h e n e r g i e s a t l e a s t O.3 eV above t h e c o n d u c t i o n band e d g e . T h i s e x c e s s e n e r g y w o u l d be s u f f i c i e n t t o d r i v e t h e s u p r a - b a n d - e d g e r e d u c t i o n s u n d e r i n v e s t i g a t i o n ; photons w i t h w a v e l e n g t h s l o n g e r t h a n 440 nm w o u l d be a b s o r b e d d e e p e r i n t o t h e c r y s t a l and c o u l d n o t produce e l e c t r o n s t h a t would a r r i v e a t the s u r f a c e w i t h s u f f i c i e n t energy to d r i v e the r e d u c t i o n r e a c t i o n . Thus, the f a c t t h a t the w a v e l e n g t h dependence o f t h e r e d u c t i o n c u r r e n t i n F i g u r e 6 f o l l o w s t h e a b s o r p t i o n edge o f S i means t h a t h o t c a r r i e r e f f e c t s c a n n o t be occurring here. I t i s i n s t r u c t i v e t o compare t h e d a t a i n F i g u r e s 7 and 8 w i t h t h e i d e a l c u r v e s o b t a i n e d f o r a m e t a l - i n s u l a t o r - s e m i c o n d u c t o r (MIS) device (9J. For MIS d e v i c e s , t h e s p a c e - c h a r g e d e n s i t y and t h e c a p a c i t a n c e d e c r e a s e i n t h e d e p l e t i o n r e g i o n and r e a c h a minimum value at the onset of the i n v e r s i o n r e g i o n . The i n v e r s i o n r e g i o n o c c u r s when t h e band b e n d i n g i s s u f f i c i e n t l y l a r g e s u c h t h a t t h e Fermi l e v e l a t t h e s u r f a c e l i e s c l o s e r t o t h e m i n o r i t y c a r r i e r band r a t h e r t h a n t o t h e m a j o r i t y c a r r i e r band. As t h e i n v e r s i o n region develops, the space-charge d e n s i t y i n the semiconductor surface increases r a p i d l y . The e f f e c t o f t h i s i n c r e a s e d c h a r g e d e n s i t y on t h e measured c a p a c i t a n c e depends upon t h e f r e q u e n c y o f t h e a p p l i e d ac s i g n a l . A t h i g h f r e q u e n c y (>100 Hz f o r S i / S i 0 2 ) t h e e l e c t r o n c o n c e n t r a t i o n c a n n o t f o l l o w t h e ac s i g n a l , and t h e measured c a p a c i t a n c e i s f l a t and m i n i m i z e d i n t h e i n v e r s i o n r e g i o n . However, a t l o w f r e q u e n c y (<100 Hz) t h e e l e c t r o n s i n t h e s p a c e c h a r g e l a y e r can f o l l o w t h e ac s i g n a l and t h e c a p a c i t a n c e i n c r e a s e s w i t h the i n c r e a s i n g degree o f i n v e r s i o n . The e f f e c t o f i l l u m i n a t i o n i n t h e s e e x p e r i m e n t s i s t o i n c r e a s e t h e measured c a p a c i t a n c e i n t h e i n v e r s i o n r e g i o n a t t h e h i g h f r e q u e n c i e s s u c h t h a t t h e low frequency behavior i s produced. The c a p a c i t a n c e d a t a p r e s e n t e d i n F i g u r e s 7 and 8 g e n e r a l l y e x h i b i t t h e b e h a v i o r d e s c r i b e d above f o r MIS ( m e t a l / S i 0 2 / p - S i ) d e v i c e s . A l t h o u g h t h e p - S i sample i s e t c h e d i n HF s o l u t i o n b e f o r e each r u n , i t i s a l s o e x p o s e d t o a i r and a t h i n o x i d e l a y e r ( M 0 - 2 0 A) e x i s t s on t h e s u r f a c e . The e f f e c t o f t h e o x i d e l a y e r i s t o i n h i b i t c u r r e n t f l o w and f a c i l i t a t e t h e m a i n tenance o f a l a r g e charge d e n s i t y i n the semiconductor space charge layer. The c r e a t i o n o f a l a r g e s p a c e - c h a r g e d e n s i t y i n t h e s e m i c o n d u c t o r (and a c o r r e s p o n d i n g i n c r e a s e i n c a p a c i t a n c e ) can c a u s e t h e bands t o become u n p i n n e d s i n c e changes i n a p p l i e d p o t e n t i a l can now o c c u r a c r o s s t h e H e l m h o l t z l a y e r . 4

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1

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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16.

TURNER ET A L .

265

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As seen i n F i g u r e 7, t h e e f f e c t o f l i g h t on t h e s y s t e m i s t o further increase the capacitance in the inversion l a y e r . This, o f c o u r s e , enhances t h e u n p i n n i n g e f f e c t as t h e c a p a c i t a n c e o f t h e s p a c e - c h a r g e - l a y e r approaches o r exceeds t h a t o f t h e H e l m h o l t z layer. To c h e c k t h e s e e f f e c t s f u r t h e r , d e t a i l e d s t u d i e s were made (10) o f t h e c a p a c i t a n c e - v o l t a g e b e h a v i o r o f p - S i e l e c t r o d e s i n a c e t o n i t r i l e as a f u n c t i o n o f o x i d e t h i c k n e s s and l i g h t i n t e n s i t y . O x i d e l a y e r s w i t h t h i c k n e s s e s v a r y i n g from 25 A t o 667 A were grown under c o n t r o l l e d c o n d i t i o n s ( H J 2 ) and measured w i t h an ellipsometer. The c a p a c i t a n c e - v o l t a g e d a t a f o r t h e s e samples were measured as a f u n c t i o n o f l i g h t i n t e n s i t y , and t h e d a t a u n e q u i v o c a l l y showed t h e c h a r a c t e r i s t i c b e h a v i o r e x p e c t e d f o r MIS d e v i c e s (9.). These s t u d i e s c o n f i r m e d t h a t i n v e r s i o n c o u l d be r e a d i l y i n duced i n p - S i e l e c t r o d e s , r e s u l t i n g i n band edge u n p i n n i n g e f f e c t s . F i n a l l y , t h e r e s u l t s i n F i g u r e s 4 and 5 a r e c o n s i s t e n t w i t h band edge u n p i n n i n g due t o i l l u m i n a t i o n i n t h e i n v e r s i o n r e g i o n . The e n e r g y l e v e l d i a g r a m p r e s e n t e d i n F i g u r e 9 e x p l a i n s t h e b e h a v i o r i n F i g u r e s 4 and 5. In t h e d a r k , t h e s u p r a b a n d - e d g e c o u p l e A/A" l i e s above t h e c o n d u c t i o n band edge o f t h e p - t y p e e l e c t r o d e . In i n v e r s i o n and under m o d e r a t e i l l u m i n a t i o n , t h e bands move up such t h a t t h e r e d o x c o u p l e l i e s n e a r t h e c o n d u c t i o n band e d g e . The p h o t o c u r r e n t - v o l t a g e c h a r a c t e r i s t i c h e r e w o u l d be s e m i - r e v e r s i b l e i n t h a t o x i d a t i o n c o u l d f o l l o w r e d u c t i o n , b u t t h e peaks w o u l d be s p l i t w i d e a p a r t . W i t h s t r o n g e r i l l u m i n a t i o n , t h e bands move f u r t h e r upward s u c h t h a t t h e r e d o x c o u p l e l i e s w e l l w i t h i n t h e band g a p . T h i s p r o d u c e s i r r e v e r s i b l e p h o t o r e d u c t i o n b e h a v i o r . A sudden s h u t - o f f o f t h e i l l u m i n a t i o n under t h e s e c o n d i t i o n s w o u l d move t h e bands downward, such t h a t o x i d a t i o n o f t h e r e d u c e d A " s p e c i e s c o u l d o c c u r on t h e a n o d i c sweep. T h i s b e h a v i o r i s what i s o b s e r v e d i n F i g u r e s 4 and 5.

A/A-

A/A-

Dark

Moderate Illumination (Semi-Reversible Photoreduction)

A/A-

Strong Illumination (irreversible photoreduction)

Figure 9. Energy level diagrams showing movement of semiconductor band edges with respect to the redox potential of the electrolyte as a function of illumination intensity

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Acknowledgement The work r e p o r t e d h e r e was s u p p o r t e d by t h e U.S. D e p a r t m e n t o f Energy, O f f i c e o f B a s i c Energy S c i e n c e s , D i v i s i o n o f Chemical Sciences. Literature

Cited

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1.

Boudreaux, D.S.; Williams, F.; Nozik, A.J. J. Appl. Phys., 1980, 51, 2158. 2. Nozik, A.J.; Boudreaux, D.S.; Chance, R.R.; Williams, F. Advances in Chemistry Series, 1980, 184, 155 (Amer. Chem. Soc. Washington). 3. Williams, F.; Nozik, A.J. Nature, 1978, 271, 137. 4. Gerischer, H. Topics in Applied Physics, Vol. 31: Solar Energy Conversion, B.O. Seraphin, ed., Springer-Verlag, Berlin, 1979. 5. Turner, J.; Manassen, J.; Nozik, A.J. Applied Physics Letts., 1980, 37, 488. 6. Bocarsly, A.B.; Bookbinder, D.C.; Dominey, R.N.; Lewis, N.S.; Wrighton, M.S. J. Amer. Chem. Soc., 1980, 102, 3683. 7. Bard, A.J.; Bocarsly, A.B.; Fen, F.; Walton, E.G.; Wrighton, M.S. J. Amer. Chem. Soc., 1980, 102, 3671. 8. Morrison, S.R., private communication. 9. Sze, S.M. Physics of Semiconductor Devices, Wiley-Interscience, N.Y., Chap. 9, 1969. 10. Turner, J.; Klausner, M.; Nozik, A.J., to be published. 11. Deal, B.E. J. Electrochem. Soc., 1963, 110, 527. 12. Archer, R.J. J. Electrochem. Soc., 1957, 104, 444. R E C E I V E D October 3, 1980.

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

17 Study of the Potential Distribution at the Semiconductor-Electrolyte Interface in Regenerative Photoelectrochemical Solar Cells MICHA TOMKIEWICZ, JOSEPH K. LYDEN, R. P. SILBERSTEIN, and FRED H. POLLAK Downloaded by 148.204.30.87 on October 21, 2009 | http://pubs.acs.org Publication Date: March 2, 1981 | doi: 10.1021/bk-1981-0146.ch017

Department of Physics, Brooklyn College of CUNY, Brooklyn, NY 11210

L i q u i d j u n c t i o n s o l a r c e l l s (1) o f f e r the p o s s i b i l i t y of high e f f i c i e n c y energy conversion using low cost e l e c t r o d e m a t e r i a l s . In a d d i t i o n , an expensive j u n c t i o n f a b r i c a t i o n process i s not needed since the j u n c t i o n i s formed at the semic o n d u c t o r - e l e c t r o l y t e i n t e r f a c e simply by immersing the semiconductor i n the appropriate e l e c t r o l y t e . In t h i s paper we d i s cuss some of the problems and techniques i n e v a l u a t i n g the s t r u c ture of the space charge r e g i o n i n such c e l l s under operating c o n d i t i o n s . S p e c i f i c a l l y we d i s c u s s s e v e r a l recent r e s u l t s on regenerative s o l a r c e l l s with a s i n g l e c r y s t a l CdSe photoanode, a metal counter-electrode, and a Na0H/S /S 1:1:1M e l e c t r o l y t e s o l u t i o n . (2) The combined energy diagram of such a c e l l i s shown i n F i g . 1 with a few of the charge accumulation modes that can determine the p o t e n t i a l d i s t r i b u t i o n at the j u n c t i o n . This i s a h i g h l y s i m p l i f i e d energy diagram. We are assuming that the only p o t e n t i a l drop i s at the space charge l a y e r of the semiconductor, that there i s a homogeneous d i s t r i b u t i o n of dopants, and that the p o t e n t i a l d i s t r i b u t i o n i s one-dimensional. E f f e c t s of surface roughness and l a t e r a l inhomogeneity i n dopant d i s t r i b u t i o n s are neglected. A l l these e f f e c t s w i l l have to be included f o r a more complete c h a r a c t e r i z a t i o n of the j u n c t i o n . Within the one-dimensional model which i s presented i n F i g . 1, i t i s important to c h a r a c t e r i z e the dark charge c a r r i e r d i s t r i b u t i o n before i t s r o l e i n the l i g h t - i n d u c e d charge t r a n s f e r process i s considered. Therefore, our emphasis i s on dark " e q u i l i b r i u m " measurements. We w i l l i l l u s t r a t e the d i f f i c u l t i e s and the o p p o r t u n i t i e s which are associated with two complementary measuring techniques: Relaxation Spectrum A n a l y s i s and E l e c t r o l y t e E l e c t r o r e f l e c t a n c e . Both techniques provide information on the p o t e n t i a l d i s t r i b u t i o n at the j u n c t i o n of a " r e a l " semiconductor. Due to the i n d i v i d u a l c h a r a c t e r i s t i c s of each system, care must be taken before d i r e c t l y applying the r e s u l t s which were obtained on our samples to other, s i m i l a r l y prepared c r y s t a l s . =

0097-6156/81 /0146-0267$05.00/0 © 1981 American Chemical Society In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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Figure 1. Combined energy diagram for a regenerative photoelectrochemical cell with n-CdSe as the anode, metallic cathode and polysulfide as the electrolyte. The diagram indicates some of the charge accumulation modes that might contribute to the potential distribution at the interface. ((QD) ionized donors; (QDT) deep traps; (Qss) surface state; (Q ) compensating charge in the electrolyte) E

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

17.

TOMKiEWicz E T A L .

Potential Distribution at Interface

269

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Relaxation Spectrum A n a l y s i s The d e t a i l s of t h i s method f o r a n a l y z i n g the impedance of photoelectrochemical c e l l s were published elsewhere. (3) We assume that the v a r i o u s modes of charge accumulation are a d d i t i v e and that they are compensated by the charge accumulation i n the Helmholtz l a y e r . T h i s a d d i t i v i t y of charges w i l l make the d i f f e r e n t i a l capacitance of the v a r i o u s modes appear i n p a r a l l e l with each other, each with i t s own r e l a x a t i o n time. The equival e n t c i r c u i t i s presented i n F i g . 2. The b a s i c premise of t h i s technique i s to use the frequency d i s p e r s i o n to c o n s t r u c t the equivalent c i r c u i t i n terms of a set of completely p a s s i v e e l e ments. Once t h i s i s accomplished, one t r i e s to determine the p h y s i c a l o r i g i n of these elements by monitoring t h e i r values as a f u n c t i o n of such parameters as p o t e n t i a l , pH, doping l e v e l , temperature, e t c . T h i s technique was used to study a non-regene r a t i v e s o l a r c e l l with T1O2 as the photoanode, (4) and parameters such as the doping d i s t r i b u t i o n , and the pH dependence of the f l a t b a n d p o t e n t i a l and capacitance of the Helmholtz l a y e r were determined. We present here p r e l i m i n a r y r e s u l t s on regenerative c e l l s with CdSe as the photoanode and a p o l y s u l f i d e s o l u t i o n as the e l e c t r o l y t e . An a l t e r n a t i v e approach that was used i n the past was to t r e a t the photoelectrochemical c e l l as a s i n g l e RC element and to i n t e r p r e t the frequency d i s p e r s i o n of the "capacitance" as i n d i c a t i v e of a frequency d i s p e r s i o n of the d i e l e c t r i c constant. (5) In i t s simplest form the frequency d i s p e r s i o n obeys the Debye equation. (6) I t can be shown that i n t h i s simple form the two approaches are f o r m a l l y e q u i v a l e n t (7) and the d i f f e r e n c e r e s i d e s i n the p h y s i c a l i n t e r p r e t a t i o n of modes of charge accumul a t i o n , t h e i r r e l a x a t i o n time, and the mechanism f o r d i e l e c t r i c r e l a x a t i o n s . T h i s ambiguity i s not unique to l i q u i d j u n c t i o n c e l l s but extends to s o l i d j u n c t i o n s where m i c r o s c o p i c mechanisms f o r the d i e l e c t r i c r e l a x a t i o n such as the presence of deep traps were assumed. In our experience with CdSe regenerative c e l l s we have found that u n l i k e T1O2 (4) the complete frequency range of the impeda n c e measurements extends to the l i m i t s of our experimental c a p a b i l i t y , which i s 10 MHz. At these high f r e q u e n c i e s , the i n ductance of v a r i o u s components i n the measuring apparatus seems to p l a y an important r o l e . While attempts were made to reduce i t s c o n t r i b u t i o n s to the impédance, we were unable to e l i m i n a t e i t and thus we included i t i n the equivalent c i r c u i t i n F i g . 2. The high frequency values of the r e a l and imaginary p a r t s are used to c a l c u l a t e L,Cg£ and Rg, and these values are used to c a l c u l a t e Bgg as p r e v i o u s l y d e s c r i b e d . (3) F i g . 3 shows the r e l a x a t i o n spectrum of s i n g l e c r y s t a l CdSe i n 1M NaOH/S"/S e l e c t r o l y t e , with a p o t e n t i a l U=+O.8 V vs NaOH/S /S 1:1: 1M e l e c t r o d e which was s e p a r a t e l y measured to be at -O.72 V vs SCE. The sharp d i p i n the imaginary part i s f u l l y c o n s i s t e n t with the presence of the i n d u c t i v e element i n s e r i e s as shown i n F i g . 2 =

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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EQUIVALENT CIRCUIT

HIGH FREQUENCY' Z

HF

• R +J S

= R

s

(»L--SgL-)

+ JX

ω

ω

J Csc

Csc

LOW FREQUENCY' B

s

s

,

£

Ci

Figure 2. Assumed generalized equivalent circuit of the semiconductor—electrolyte interface. Reduced equivalent circuit at high frequencies and the expression for the impedance at low and high frequencies.

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

Potential Distribution at Interface

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TOMKIEWICZ E T A L .

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

271

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PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE INTERFACES

while the two smaller dips i n the r e a l part are not c o n s i s t e n t with t h i s p r e s e n t a t i o n . They could r e s u l t from the presence of i n d u c t i v e elements p a r a l l e l to Rg. The p o s s i b l e r o l e of s t r a y capacitance i n the system i s a l s o a matter f o r f u r t h e r i n v e s t i gation. Noting these u n c e r t a i n t i e s , we have evaluated the equival e n t c i r c u i t and present the r e s u l t s i n F i g . 3. The p o t e n t i a l dependence of the three c a p a c i t i v e elements i s shown i n F i g s . 4 and 5. As shown i n F i g . 4 the agreement of the Csc data with the Mott-Schottky r e l a t i o n i s good and i n d i c a t e s that C$ç o r i g i n a t e s from the majority c a r r i e r s i n the space charge region. The normalized slope i n d i c a t e s a doping l e v e l of 8 χ 10*°/cm3, and the i n t e r c e p t i s at -O.96 V vs NaOH/S /S 1:1:1M e l e c t r o d e . The p o t e n t i a l dependence of the other two c a p a c i t i v e elements are given i n F i g . 5. These elements do not obey the MottSchottky r e l a t i o n and t h e i r o r i g i n i s s t i l l being i n v e s t i g a t e d . P r e l i m i n a r y measurements have been made on Au Schottky b a r r i e r s on CdSe s i n g l e c r y s t a l s . N e i t h e r Relaxation Spectrum A n a l y s i s nor Deep L e v e l Transient Spectroscopy (10) has provided e v i dence f o r the slower r e l a x i n g s t a t e s . These n u l l r e s u l t s and the p o t e n t i a l dependence of C and C2 i n d i c a t e that these slower s t a t e s might be associated with the c r y s t a l - l i q u i d i n t e r f a c e . However, a word of caution i s i n d i c a t e d as the c r y s t a l s used f o r the Schottky b a r r i e r work had a much lower doping l e v e l (5 χ 10 / cm as determined from t h e i r MottSchottky data) than the c r y s t a l s used i n the p h o t o e l e c t r o chemical c e l l s .

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=

Electrolyte

Electroreflectance

E l e c t r o l y t e E l e c t r o r e f l e c t a n c e (EER) i s a s e n s i t i v e o p t i c a l technique i n which an a p p l i e d e l e c t r i c f i e l d at the surface of a semiconductor modulates the r e f l e c t i v i t y , and the detected s i g n a l s are analyzed using a l o c k - i n a m p l i f i e r . EER i s a powerful method f o r studying the o p t i c a l p r o p e r t i e s of semi conductors, and considerable experimental d e t a i l i s a v a i l a b l e i n the l i t e r a t u r e . ( Π , JL2, 13, 14, JL5) The EER spectrum i s a u t o m a t i c a l l y normalized with respect to field-independent o p t i c a l p r o p e r t i e s of surface f i l m s ( f o r example, s u l f i d e s ) , e l e c t r o l y t e s , and other experimental p a r t i c u l a r s . S i g n i f i c a n t l y , the EER spectrum may contain features which are s e n s i t i v e to both the AC and the DC a p p l i e d e l e c t r i c f i e l d s , and can be used to monitor i n s i t u the p o t e n t i a l d i s t r i b u t i o n at the l i q u i d j u n c t i o n i n t e r f a c e . (Γ4, JL5, 16, 17, 18) We have measured the EER spectra of s i n g l e c r y s t a l CdSe i n a p o l y s u l f i d e e l e c t r o l y t e using the same c o n f i g u r a t i o n f o r which the r e l a x a t i o n spectrum a n a l y s i s was a p p l i e d . Prelimi nary r e s u l t s are shown i n F i g . 6. Two p r i n c i p a l s p e c t r a l features can be seen i n F i g . 6 near 1.75 and 2.16 eV, corresponding to the E ( d i r e c t gap at k=0) and E ( s p i n - o r b i t s p l i t component) peaks p r e v i o u s l y A B

Q

C

G

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

TOMKIEWICZ ET AL.

Potential Distribution at Interface

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17.

POTENTIAL (Vvs NaOH/S /S) =

Figure 4. Mott-Schottky plot of C from Figure 3 sc

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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AT SEMICONDUCTOR-ELECTROLYTE

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274

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TOMKiEWicz ET AL.

Potential Distribution at Interface

observed. (11) As the p o t e n t i a l i s reduced from +O.5 V to -1.0 V the s p e c t r a l lineshape changes s i g n i f i c a n t l y . Additiona l measurements c a r r i e d out with zero DC a p p l i e d v o l t a g e have shown that the i n t e n s i t y of the spectrum s c a l e s l i n e a r l y with the amplitude of the modulation v o l t a g e from V = O.05 V to V = 1.0 V peak to peak. T h i s i n d i c a t e s that the EER spectra are " l o w - f i e l d spectra. ( 1 , J5, 16, 17) However, EER spect r a i n the l o w - f i e l d regime are expected to be independent of the DC e l e c t r i c f i e l d a t the surface, hence, independent of p o t e n t i a l . The v a r i a t i o n i n the EER lineshape as a f u n c t i o n of p o t e n t i a l may be a r e s u l t of the mismatch between the e l e c t r i c f i e l d p r o f i l e near the surface of the semiconductor and the p e n e t r a t i o n depth of the l i g h t . (13, 14, 15) In t h i s case, the EER spectra would be independent of p o t e n t i a l f o r values corresponding to deep d e p l e t i o n , where the f i e l d over the penet r a t i o n depth of the l i g h t i s r e l a t i v e l y constant. A d d i t i o n a l data show that t h i s r e l a t i o n i s approximately maintained f o r p o t e n t i a l s i n the range +O.5 V to +1.0 V. F o r l o w - f i e l d e l e c t r o r e f l e c t a n c e , the s i g n a l observed a t the fundamental frequency i s expected to pass through a minimum as we approach the f l a t b a n d p o t e n t i a l . (J3, 18) Despite the complications i n the lineshape due to f i e l d nonuniformity w i t h i n the penetrat i o n depth of the l i g h t , the EER spectra of F i g . 6 do show that the s i g n a l i n t e n s i t y goes through a minimum a t U = -O.8 V vs NaOH/S /S e l e c t r o d e . T h i s value f o r the f l a t b a n d p o t e n t i a l i s i n approximate agreement with that from the capacitance data. As the f l a t b a n d p o t e n t i a l i s crossed, the energy bands at the surface go from d e p l e t i o n to accumulation and the EER s i g n a l i s expected to reverse i n s i g n , (13, 18) as can be observed i n F i g . 6 f o r U = -1.0 V. By measuring the amplitude and phase of the EER s i g n a l a t a s p e c t r a l f e a t u r e f o r which the photon energy i s independent of the e l e c t r o d e p o t e n t i a l , a very accur a t e value of the f l a t b a n d p o t e n t i a l can be obtained simply by scanning through the p o t e n t i a l range. The appropriate c o n d i t i o n s are approximately s a t i s f i e d f o r hv = 1.74 eV, the energy gap of CdSe, which i s near the average p o s i t i o n of the main e l e c t r o r e f l e c t a n c e peak. F i g . 7 shows the EER s i g n a l f o r h v = 1.74 eV as a f u n c t i o n of p o t e n t i a l under c o n d i t i o n s i d e n t i c a l with those f o r the data i n F i g . 6. F o r comparison, the v a r i a t i o n i n the photocurrent due to chopped white l i g h t , as a f u n c t i o n of the e l e c t r o d e p o t e n t i a l , i s a l s o shown. We can see that the turn-on p o t e n t i a l and the i n v e r s i o n o f the EER s i g n a l agree, i n d i c a t i n g that the l i q u i d j u n c t i o n c e l l r e s e m b l e s a "well behaved" diode. For the uniform f i e l d case the EER method should be s u p e r i o r to both the capacitance technique which r e l i e s upon the separate e v a l u a t i o n of the v a r i o u s charge accumulation modes, and the turn-on p o t e n t i a l technique which i s dependent on k i n e t i c f a c t o r s . a c

a c

11

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275

=

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WAVELENGTH (A)

PHOTON ENERGY

(eV)

Figure 6. Variation with potential of the electrolyte electroreflectance spectra of single-crystal n-CdSe in polysulfide solution (CdSe Τ = 300 Κ in NaOH/S=/S 1:1:1; modulation = O.2 V)

Figure 7. Potential sweep of the electroreflectance signal at 1.74 eV superimposed on potential sweep of chopped light photoresponse

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

17.

TOMKIEWICZ E T A L .

Potential Distribution at Interface

211

Conclusions

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We have extended the technique of Relaxation Spectrum A n a l y s i s to cover the seven orders of magnitude of the e x p e r i mentally a v a i l a b l e frequency range. This frequency range i s required f o r a c o m p l e t e d e s c r i p t i o n of the equivalent c i r c u i t f o r our CdSe-polysulfide e l e c t r o l y t e c e l l s . The f a s t e s t r e l a x i n g c a p a c i t i v e element i s due to the f u l l y i o n i z e d donor s t a t e s . On the b a s i s of t h e i r p o t e n t i a l dependence e x h i b i t e d i n the c e l l data and t h e i r i n d i c a t e d absence i n the p r e l i m i nary measurements of the Au Schottky b a r r i e r s on CdSe s i n g l e c r y s t a l s , the slower r e l a x i n g c a p a c i t i v e elements are t e n t a t i v e l y associated with charge accumulation a t the s o l i d - l i q u i d interface. The c l o s e agreement among the f l a t b a n d p o t e n t i a l s determined by the capacitance measurements, e l e c t r o l y t e e l e c t r o r e f l e c t a n c e , threshold f o r l i g h t - i n d u c e d charge separation, i n d i c a t e s that these c e l l s are " w e l l behaved." This c l o s e agreement a l s o suggests that the open c i r c u i t voltage should approach the t h e o r e t i c a l value — the d i f f e r e n c e between the p o l y s u l f i d e redox p o t e n t i a l and the flatband p o t e n t i a l of the p h o t o e l e c t r o d e — o n c e the p o l a r i z a t i o n a t the counter e l e c t r o d e and the dark current are minimized. The dependence of the e l e c t r o r e f l e c t a n c e lineshape on the e l e c t r o d e p o t e n t i a l o f f e r s the promise of d i r e c t measurement of the e l e c t r i c f i e l d at the j u n c t i o n and of the p e n e t r a t i o n of the l i g h t r e l a t i v e to the electric field. To f u l l y r e a l i z e t h i s promise a f u l l a n a l y s i s of the lineshape i s r e q u i r e d .

ABSTRACT The Relaxation Spectrum Analysis was carried out for a cell consisting of n-CdSe in a liquid junction configuration with NaOH/S /S 1:1:1M as the electrolyte. Three parallel RC elements were identified for the equivalent circuit of this cell, and the fastest relaxing capacitive element obeys the Mott-Schottky relation. Electroreflectance spectra were measured for n-CdSe in the liquid junction configuration, and variations of the lineshape as a function of potential were observed. As the potential was reduced below the flatband potential, the electroreflectance signals changed sign. The potential at which this change occurs correlates well with the turn-on potential for light-induced photocurrent and with the inter cept of the Mott-Schottky plot. =

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Research supported in part by the Solar Energy Research Institute under contract XS-9-832-1 and in part by the Office of Naval Research under contract N00014-78-C-0718.

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Literature

Cited

1. Heller, Α., ed.; "Semiconductor Liquid Junction Solar Cells" The Electrochemical Society, Inc. Princeton, N.J., 1977. 2. a Hodes, G., Manassen, J., and Cahen, D., Nature 261, 406 (1976). b Ellis, A.B., Kaiser, S.W. and Wrighton, M.S.; J. Am. Chem. Soc. 98, 1635 (1976). 3. Tomkiewicz, M.; J. Electrochem. Soc. 126, 2220 (1979). 4. Tomkiewicz, M.; J. Electrochem. Soc. 126, 1505 (1979). 5. Dutoit, E.C., Van Meirhaeghe, R.L., Cardon, F. and Gomes, W.P.; Berichte der Bunsen-Gesellshaft 79, 1206 (1975). 6. Debye, P.; Phys. Ζ 35, 101 (1934). 7. Cole, K.S. and Cole, R.H.; J. Chem. Phys. 9, 341 (1941). 8. Losee, D.L.; Appl. Phys. Lett. 21, 54 (1972). 9. Crowell, C.R. and Nakano, K.; Solid State Electron. 15, 605 (1972). 10. Lang, D.V.; J. Appl. Phys. 45, 3014, 3023 (1974). 11. Cardona, Μ., Shaklee, K.L. and Pollak, F.H.; Phys. Rev. 154, 696 (1967). 12. Cardona, M.; "Modulation Spectroscopy" (Academic Press, New York 1969). 13. Seraphin, B.O.; "Semiconductors and Semimetals," Vol. 9, eds. Willardson, R.L. and Beer, A.C. (Academic Press, New York 1972) P. 1. 14. Hamakawa, Y. and Nishino, T.; "Optical Properties of Solids: New Developments," ed. Seraphin, B.O. (NorthHolland, Amsterdam 1976) P. 225. 15. Aspnes, D.E.; "Handbook on Semiconductors," Vol. 2, ed. Balkanski, M. (North-Holland, New York 1980) P. 109. 16. Aspnes, D.E.; Surf. Sci. 37, 418 (1973). 17. Aspnes, D.E.; Phys. Rev. B10, 4228 (1974). 18. Pond, S.F. and Handler, P.; Phys. Rev. B6, 2248 (1972); B8, 2869 (1973); Pond, S.F.; Surf. Sci. 37, 596 (1973). R E C E I V E D October 3, 1980.

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

18 Luminescence and Photoelectrochemistry of Surfactant Metalloporphyrin Assemblies on Solid Supports JOHN E . BULKOWSKI, RANDY A . BULL, and STEVEN R. SAUERBRUNN

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Department of Chemistry, University of Delaware, Newark, D E 19711 Photoelectrochemical cells based on charge separation at semiconductor-liquid junctions offer promise as a practical means for converting solar energy into electricity or storable chemical fuels (1). One approach to achieving such devices involves the sensitization of wide-gap semiconductors by modifi cation of their surfaces with visible-light absorbing dyes. In this well-established spectral sensitization process (2,3,4), the light-excited dye molecules transfer majority carriers into the semiconductor, and the interfacial energetics are advantageously employed to inhibit back reactions. For instance, if the energy levels are suitably matched, dyes bound to n-type semiconductor electrodes inject electrons into the conduction band upon illumination. The electrons are swept away from the surface into the bulk of the semiconductor as a result of the space -charge layer developed beneath the semiconductor surface. In the electrochemical cell, the photooxidized dyes trapped at the surface are reduced by appropriate redox species in the electro lyte. Coupling of the photoelectrode via an external circuit to a counter electrode also immersed in the electrolyte completes the cell. For dyes absorbing in the visible region, wide-gap semi conductors are chosen to maximize electron transfer from the excited dye molecules on the surface to the solid (5,6). However, a major difficulty associated with this approach is the restriction of overall quantum conversion efficiencies due to low light absorption by thin dye layers. Thick dye layers, although they may absorb all of the light, do not result in significantly greater conversion efficiencies, since they suffer from increased quenching probabilities and large resistances. These properties of thick dye films are discouraging with regard to fabricating practical photoelectrochemical cells. Consequently, we are employing a molecular design approach to systematically examine photoinduced charge and energy transfer in highly ordered dye assemblies of a controllable architecture. The aim is to understand the fundamental sensitization processes, both within

0097-6156/81/0146-0279$05.00/0 © 1981 American Chemical Society In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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t h e a s s e m b l i e s and a t t h e i n t e r f a c e s , i n o r d e r t o u l t i m a t e l y realize useful sensitization materials. The o b j e c t i v e o f t h i s m o l e c u l a r d e s i g n a p p r o a c h i s t h e unique three-dimensional a r r a y s c h e m a t i c a l l y represented i n F i g u r e 1. I n t h i s i d e a l i z e d s t r u c t u r e , c o o r d i n a t i o n compounds a r e o r g a n i z e d i n d i s t i n c t monomolecular l a y e r s . Each l a y e r i s c o n n e c t e d t o a d j a c e n t ones by b i f u n c t i o n a l m o l e c u l a r l i n k a g e s c o o r d i n a t e d to the a x i a l s i t e s of the m e t a l c e n t e r s . The dye m o l e c u l e s we a r e u s i n g i n o u r e f f o r t s t o s y n t h e s i z e s u c h a s t r u c t u r e a r e m e t a l l o p o r p h y r i n s , whose c o o r d i n a t i o n c h e m i s t r y and s t r u c t u r a l c h a r a c t e r i s t i c s a p p e a r t o be w e l l s u i t e d t o accomplishing the s y n t h e t i c goals. The i n t e r l a y e r l i n k a g e s t o be u s e d a r e b i f u n c t i o n a l b a s e s s u c h as p y r a z i n e , w h i c h i s shown i n t h e i l l u s t r a t i o n . Their functions are two-fold: to p r o v i d e b i n d i n g s i t e s f o r p o s i t i o n i n g s u c c e s s i v e l a y e r s i n the a r r a y , and t o p r o v i d e an a d j u s t a b l e i n n e r - s p h e r e pathway f o r p h o t o i n d u c e d charge t r a n s f e r between a d j a c e n t l a y e r s . Although i t i s not our i n t e n t to develop the d e t a i l e d r a t i o n a l e behind o u r d e s i g n h e r e , i t s h o u l d be n o t e d t h a t c o n s t r u c t i o n o f s u c h an a r r a y i n a s t e p - w i s e manner w o u l d p r o v i d e a c o n v e n i e n t means f o r t u n i n g t h e o p t i c a l and e l e c t r i c a l p r o p e r t i e s o f t h e f i l m s architecture. For example, f o r m a t i o n of a m u l t i - l a y e r e d assembly w i t h s u c c e s s i v e l a y e r s c o m p r i s e d o f m e t a l l o p o r p h y r i n s w i t h dec r e a s i n g r e d o x p o t e n t i a l s c o u l d r e s u l t i n t h e g e n e r a t i o n o f an asymmetric f i l m . The s y n t h e t i c s t r a t e g y we a r e f o l l o w i n g t o d e v e l o p t h e s e ordered f i l m s i n v o l v e s a combination of monolayer techniques and c o o r d i n a t i o n c h e m i s t r y s u b s t i t u t i o n r e a c t i o n s . T h i s s y n t h e t i c a p p r o a c h i s d e p i c t e d i n F i g u r e 2 f o r a two l a y e r e d s y s t e m i n w h i c h t h e l a y e r s a r e i n t e r c o n n e c t e d by p y r a z i n e . The f i r s t s t e p i s f o r m a t i o n o f a t e m p l a t e l a y e r on a s o l i d s u p p o r t . This i s a c c o m p l i s h e d by t r a n s f e r r i n g an o r d e r e d l a y e r o f s u r f a c t a n t m e t a l l o p o r p h y r i n s f r o m an a i r - w a t e r i n t e r f a c e o n t o a h y d r o p h o b i c a l l y t r e a t e d s u p p o r t s u r f a c e by p a s s i n g t h e s u p p o r t down through the o r i e n t e d f i l m (Step 1 ) . I t i s imperative that t h i s m o n o l a y e r i s homogeneous and t h a t t h e m o l e c u l a r p l a n e s o f e a c h p o r p h y r i n molecule are o r i e n t e d p a r a l l e l to the support s u r f a c e , l e a v i n g the m e t a l c e n t e r s exposed at the h y d r o p h i l i c i n t e r f a c e . The s e c o n d and t h i r d s t e p s i n v o l v e s u c c e s s i v e a d d i t i o n s o f t h e p y r a z i n e and n o n - s u r f a c t a n t w a t e r s o l u b l e m e t a l l o p o r p h y r i n comp l e x e s t o t h e t e m p l a t e submerged i n t h e aqueous p h a s e ( S t e p s 2 and 3, r e s p e c t i v e l y ) . The t w o - l a y e r e d a s s e m b l y i s t h e n removed from the water through a p r o t e c t i v e monolayer of s t e a r i c a c i d to prevent d i s r u p t i o n of the f i l m s t r u c t u r e (Step 4 ) . I f instead o f r e m o v i n g t h e a s s e m b l y i n S t e p 4, S t e p s 2 and 3 w e r e s u c c e s s i v e l y repeated, m u l t i - l a y e r e d assemblies having the e s s e n t i a l f e a t u r e s d e s c r i b e d i n F i g u r e 1 m i g h t be r e a l i z e d . Since achievement of template formation i s c r u c i a l to the s u c c e s s o f t h i s a p p r o a c h , much o f o u r e f f o r t t o d a t e has b e e n d i r e c t e d at e l u c i d a t i n g the monolayer p r o p e r t i e s of v a r i o u s por1

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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Figure 1. Schematic of the proposed dye assembly where M is coordinated in the plane of a macrocyclic ligand (e.g., a por phyrin) and pyrazine is axially coordi nated between two metals in adjacent layers

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Figure 2. Representation of the dye assembly construction procedure: (O-) stearic acid; (-%-) surfactant metalloporphyrin (MTOAPP); and (-%-) water soluble porphyrin

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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p h y r i n compounds. T h i s h a s l e d t o o u r work w i t h s u r f a c t a n t p o r p h y r i n d e r i v a t i v e s o f the type s c h e m a t i c a l l y represented i n F i g u r e 3. The r e s u l t s o f o u r m o n o l a y e r a n d l u m i n e s c e n c e s t u d i e s w i t h t h e s e m o d i f i e d p o r p h y r i n s do i n f a c t i n d i c a t e t h a t t h e y f o r m w e l l - o r d e r e d monolayers a t the a i r - w a t e r i n t e r f a c e w i t h c h a r a c t e r i s t i c s s u i t a b l e f o r template formation. T h i s i s i n agreement with reported results f o r s i m i l a r surfactant porphyrin derivatives Ç7,8). The p u r p o s e o f t h i s a r t i c l e i s t o p r e s e n t t h e r e s u l t s of our l u m i n e s c e n c e and p h o t o e l e c t r i c s t u d i e s w i t h these p o r p h y r i n d e r i v a t i v e s , s i n c e t h e y d e m o n s t r a t e t h a t t h e s e compounds p r o v i d e a good o p p o r t u n i t y f o r e x a m i n i n g e l e c t r o n a n d e n e r g y t r a n s f e r phenomena a t a u n i q u e l y d e f i n e d d y e s e n s i t i z e d s e m i c o n d u c t o r electrolyte interface. The r e s u l t s a r e a l s o d i s c u s s e d w i t h r e g a r d to the molecular design o f the novel p h o t o s e n s i t i v e f i l m s . Experimental P r e p a r a t i o n and P u r i f i c a t i o n o f M a t e r i a l s . The t e t r a o c t a d e c y l a m i d e d e r i v a t i v e ( t T O A P P ) o f m e s o - t e t r a ( a , a , a , a - o - a m i n o p h e n y l ) p o r p h y r i n was p r e p a r e d b y m o d i f i c a t i o n o f t h e " p i c k e t - f e n c e " p o r p h y r i n p r o c e d u r e o f C o l l m a n and c o w o r k e r s (90. P u r i f i c a t i o n was b y c o l u m n c h r o m a t o g r a p h y o n s i l i c a g e l u s i n g g r a d i e n t e l u t i o n w i t h e t h e r - p e t r o l e u m e t h e r s o l v e n t combinations. The c h r o m a t o g r a p h i c p r o c e d u r e was r e p e a t e d u n t i l t h e p o r p h y r i n s were determined t o be g r e a t e r than 99% i s o m e r i c a l l y pure. P u r i t y was c h e c k e d u s i n g h i g h - p e r f o r m a n c e l i q u i d c h r o m a t o graphy by comparing t h e pure product t o a standard m i x t u r e o f t h e four p o s s i b l e atropisomers. Refluxing the free-base surfactant p o r p h y r i n w i t h z i n c a c e t a t e i n dimethylformamide and n i c k e l c h l o r i d e i n c h l o r o f o r m - e t h a n o l gave t h e z i n c a n d n i c k e l m e t a l l o porphyrin derivatives,respectively. O c t a d e c a n e (Eastman) a n d s t e a r i c a c i d ( A l d r i c h ) were r e c r y s t a l l i z e d from acetone and a c e t o n i t r i l e , r e s p e c t i v e l y . A l l o t h e r o r g a n i c s o l v e n t s a n d i n o r g a n i c compounds w e r e r e a g e n t grade o r b e t t e r and used a s r e c e i v e d . T r i p l y d i s t i l l e d water (from an a l l g l a s s system h a v i n g permanganate and s u l f u r i c a c i d s t a g e s ) was u s e d f o r t h e m o n o l a y e r s u b p h a s e s a n d t h e e l e c t r o l y t e solutions. S o l i d s u p p o r t s w e r e e i t h e r 25 χ 75 mm g l a s s m i c r o s c o p e s l i d e s ( F i s h e r b r a n d ) o r 25 χ 75 mm Sb doped S n 0 c o a t e d g l a s s s l i d e s ( P r a c t i c a l P r o d u c t s C o . ) . The S n 0 c o a t i n g was 3000A t h i c k a n d had a r e s i s t a n c e o f 100 Ω/Ο. O p t i c a l t r a n s m i t t a n c e was 80-85% i n t h e v i s i b l e r e g i o n above 400 nm. The g l a s s m i c r o s c o p e s l i d e s w e r e t r e a t e d w i t h h o t o r g a n i c s o l v e n t s , h o t HNOo, d i l u t e NHOH, and t h e n w e r e r i n s e d s e v e r a l t i m e s w i t h t r i p l y d i s t i l l e d w a t e r . They w e r e a i r d r i e d i n a n i n v e r t e d p o s i t i o n . The S n 0 o p t i c a l l y t r a n s p a r e n t e l e c t r o d e s (OTE's) were t r e a t e d w i t h h o t CHCI3 a n d CHOH, e t c h e d w i t h l S O , a n d t h e n washed s e v e r a l t i m e s w i t h t r i p l y d i s t i l l e d water. A conductive s i l v e r coating (Practical ?

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P r o d u c t s Co.) was p a i n t e d a s a c o n t a c t o n one e n d o f t h e OTE, w h i c h was t h e n c u r e d i n a n o v e n a t 160°C. A f t e r c l e a n i n g a n d d r y i n g , t h e g l a s s s l i d e s a n d O T E s were s t o r e d i n s e a l e d i n d i v i d u a l g l a s s c o n t a i n e r s u n t i l use. 1

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Monolayer Techniques. The g e n e r a l methods u s e d f o r f o r m i n g and m a n i p u l a t i n g mono l a y e r s a r e d e s c r i b e d b y Kuhn and c o w o r k e r s ( 1 0 ) . Pressure-area c u r v e s w e r e d e t e r m i n e d o n a p a r a f f i n c o a t e d Cenco L a n g m u i r b a l a n c e c a l i b r a t e d b y s u s p e n d i n g w e i g h t s f r o m a n a t t a c h e d s i d e arm o f known l e n g t h . The c a l i b r a t i o n was c h e c k e d p e r i o d i c a l l y b y mea s u r i n g the p r e s s u r e - a r e a curve f o r s t e a r i c a c i d (SA). The f i l m s a t t h e a i r - w a t e r i n t e r f a c e w e r e f o r m e d b y d e l i v e r i n g 100-200 μΐ o f a p p r o x i m a t e l y 10" M s o l u t i o n s o f the r e q u i r e d s u r f a c t a n t m i x t u r e s i n c h l o r o f o r m o n t o t h e aqueous s u r f a c e u s i n g a c a l i b r a t e d 200 y l s y r i n g e . The s u r f a c e - p r e s s u r e a r e a i s o t h e r m s w e r e measured by c o m p r e s s i n g t h e f i l m i n s u c c e s s i v e s t e p s a t one m i n u t e i n t e r vals. The aqueous s u b p h a s e , u n l e s s n o t e d o t h e r w i s e , was b u f f e r e d a t pH 6.5 w i t h 5 χ 10-5 M N a H C 0 a n d c o n t a i n e d 3 χ 1 0 " M C d C l . The p r e c l e a n e d g l a s s s l i d e s f o r t h e s p e c t r o s c o p i c s t u d i e s w e r e p r e c o a t e d o n one s i d e w i t h t h r e e l a y e r s o f cadmium stéarate ( f o r m e d b y a d d i t i o n o f SA t o t h e c a d m i u m - c o n t a i n i n g s u b p h a s e ) by p a s s i n g two s l i d e s p o s i t i o n e d b a c k - t o - b a c k t h r o u g h t h e monol a y e r t h r e e t i m e s a t a r a t e o f 1.0 cm/min. The s u r f a c e p r e s s u r e was h e l d c o n s t a n t a t 30 dyne/cm b y a w e i g h t and p u l l e y t r a n s f e r apparatus w i t h a motor d r i v e n l i f t (11). D e p o s i t i o n o f the s u r f a c t a n t p o r p h y r i n m i x t u r e s was b y p a s s i n g t h e stéarate c o a t e d s l i d e s down t h r o u g h t h e a p p r o p r i a t e f i l m a t a c o n s t a n t p r e s s u r e o f 20 dyne/cm. I f a s i n g l e p o r p h y r i n l a y e r was r e q u i r e d on t h e s u p p o r t , t h e p o r p h y r i n m o n o l a y e r was t h e n removed f r o m t h e s u b p h a s e s u r f a c e and a s t e a r i c a c i d f i l m was f o r m e d t h r o u g h w h i c h t h e s l i d e c o n t a i n i n g t h e p o r p h y r i n m o n o l a y e r was removed. F o r f o r m a t i o n o f two p o r p h y r i n dye l a y e r s f a c e - t o - f a c e o n t h e s u p p o r t , t h e s l i d e was s i m p l y d i p p e d a n d t h e n removed t h r o u g h the p o r p h y r i n f i l m on the subphase. D e p o s i t i o n o f s u r f a c t a n t p o r p h y r i n s o n t h e S n 0 O T E s f o r t h e p h o t o e l e c t r i c measurements was b y v e r t i c a l r e m o v a l o f t h e OTE (two s l i d e s a t a t i m e w i t h the g l a s s s u r f a c e s f a c i n g each o t h e r ) through the p o r p h y r i n m i x t u r e o n t h e aqueous s u b p h a s e . S u r f a c t a n t p o r p h y r i n (MTOAPP): s t e a r i c a c i d : o c t a d e c a n e m o n o l a y e r s i n t h e r a t i o o f 1:4:3 w e r e h e l d a t a c o n s t a n t p r e s s u r e o f 25 dyne/cm f o r t h e s e d e p o s i t i o n s . I n t h i s s u p p o r t - f i l m arrangement, the p o r p h y r i n r i n g s were p o s i t i o n e d next t o the Sn0 s u r f a c e . Coated s l i d e s were s t o r e d i n s e p a r a t e g l a s s c o n t a i n e r s i n the dark b e f o r e use. Deposition r a t i o s w e r e r o u t i n e l y measured and u s e d a s a c r i t e r i o n t o a s s e s s the q u a l i t y o f the c o a t i n g s . 4

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PHOTOEFFECTS AT

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Luminescence Measurements. C o r r e c t e d l u m i n e s c e n c e s p e c t r a were determined w i t h a P e r k i n E l m e r MPF-44B s p e c t r o p h o t o f l u o r i m e t e r e q u i p p e d w i t h a R-928 p h o t o m u l t i p l i e r and a DCSU-2 m i c r o p r o c e s s o r . C a l i b r a t i o n was w i t h a P e r k i n - E l m e r s t a n d a r d t u n g s t e n lamp u n i t . E x c i t a t i o n and e m i s s i o n s p e c t r a o f m o n o l a y e r s on t h e g l a s s s l i d e s were f r o m t h e f r o n t s i d e w i t h t h e s l i d e p o s i t i o n e d i n a sample h o l d e r w i t h a n o n - f l u o r e s c i n g b l a c k b a c k g r o u n d . The e x c i t i n g l i g h t was a t an a n g l e o f 30° to the normal o f the sample s l i d e w i t h d e t e c t i o n of l u m i n e s c e n c e a t r i g h t a n g l e s t o t h e i n c i d e n t l i g h t t h r o u g h a 565 nm c u t - o f f f i l t e r ( T u r n e r , S h a r p Cut #22).

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Photoelectrochemical

Measurements.

The e l e c t r o c h e m i c a l c e l l c o n s i s t e d o f t h e p o r p h y r i n t r e a t e d SnO OTE mounted as a window on a m a c h i n e d T e f l o n b l o c k w h i c h c o n t a i n e d a compartment f o r t h e e l e c t r o l y t e s o l u t i o n . The s e m i c o n d u c t o r - p o r p h y r i n f a c e c o n t a c t e d t h e e l e c t r o l y t e w i t h an a r e a o f 1.0 c m . E l e c t r i c a l c o n t a c t t o t h e S n 0 e l e c t r o d e was made v i a a c o n d u c t i v e s i l v e r c o a t i n g on one end. A p l a t i n u m w i r e and a s a t u r a t e d c a l o m e l e l e c t r o d e (SCE) s e r v e d as t h e c o u n t e r and r e f e r e n c e e l e c t r o d e s , r e s p e c t i v e l y . The SCE c o n t a c t e d t h e e l e c t r o l y t e v i a a salt bridge. The c e l l compartment was p r o v i d e d w i t h an i n l e t and o u t l e t f o r i n t r o d u c t i o n o f t h e e l e c t r o l y t e . The s u p p o r t i n g e l e c t r o l y t e was O.1 M KC1 and t h e pH was maintained a t 7.0 u s i n g a p h o s p h a t e b u f f e r . The e l e c t r o l y t e was r i g o r o u s l y deaerated w i t h a N purge. C u r r e n t v e r s u s p o t e n t i a l measurements w e r e made u s i n g c i r c u i t r y s i m i l a r t o t h a t d e s c r i b e d by Honda and coworkers (12). The p o t e n t i o s t a t and s i g n a l programmer w e r e o f c o n v e n t i o n a l d e s i g n and c o n s t r u c t e d by us f r o m c o m m e r c i a l l y a v a i l a b l e components. P h o t o c u r r e n t s were measured by a K e i t h l e y e l e c t r o m e t e r ( M o d e l 610C) and r e c o r d e d on an X-Y r e c o r d e r . The c e l l mounts and a l l e l e c t r i c a l components were c o n n e c t e d t o a common g r o u n d . T y p i c a l c e l l i n t e r n a l r e s i s t a n c e s were 20-30 k f i . The l i g h t s o u r c e f o r c u r r e n t v e r s u s p o t e n t i a l and c u r r e n t v e r s u s t i m e measurements was a 300-watt ELH lamp ( G e n e r a l E l e c t r i c ) o p e r a t e d a t an i n t e g r a t e d i r r a d i a n c e o f c a . 75 mw/cm above 350 nm. A 350 nm c u t - o f f f i l t e r was p o s i t i o n e d between t h e e l e c t r o c h e m i c a l c e l l and t h e l i g h t s o u r c e . The p h o t o c u r r e n t b a c k g r o u n d s o f S n 0 e l e c t r o d e s h a v i n g o n l y a cadmium stéarate l a y e r w e r e 10" l e s s t h a n t h o s e h a v i n g t h e p o r p h y r i n dye m o n o l a y e r s . D e t e r m i n a t i o n o f a c t i o n s p e c t r a was a c c o m p l i s h e d by p l a c i n g t h e e l e c t r o c h e m i c a l c e l l i n t h e sample chamber o f t h e P e r k i n - E l m e r MPF-44B s p e c t r o m e t e r and u s i n g t h e 1 5 0 - w a t t Xe l i g h t s o u r c e and e x c i t a t i o n o p t i c s f o r i r r a d i a t i n g the e l e c t r o d e . 2

2

2

R e s u l t s and

Discussion

Monolayer P r o p e r t i e s . P r e v i o u s s t u d i e s o f dye s e n s i t i z a t i o n w i t h p o r p h y r i n m a t e r i a l s

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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18.

BULKOWSKi E T A L .

Surfactant Metalloporphyrin Assemblies

285

( 1 3 - 1 9 ) have u s u a l l y i n v o l v e d f o r m a t i o n o f s o l i d f i l m s by t e c h niques such as s o l u t i o n e v a p o r a t i o n o r vapor d e p o s i t i o n . L i t t l e i s known a b o u t t h e s t r u c t u r e s o f s u c h f i l m s . A s a r e s u l t , we have b e e n s t u d y i n g methods f o r g e n e r a t i n g w e l l - d e f i n e d p o r p h y r i n assemb l i e s u s i n g m o n o l a y e r methods a t t h e a i r - w a t e r i n t e r f a c e ( 1 0 ) . Our a t t e m p t s t o g e n e r a t e m o n o l a y e r s o f m e t a l l o t e t r a p h e n y l p o r p h y r i n s b y s p r e a d i n g s o l u t i o n s o f them o n a n aqueous s u b p h a s e d i d not r e s u l t i n the g e n e r a t i o n o f monomolecular l a y e r s . As expected, a h i g h d e g r e e o f a g g r e g a t i o n and m i c r o c r y s t a l l i n i t y was r e a d i l y apparent. Compression o f these f i l m s r e s u l t s i n curves a t y p i c a l of monolayers w i t h e x t r a p o l a t e d area/molecule values l e s s than 15 A / m o l e c u l e a t o b s e r v a b l e s u r f a c e p r e s s u r e s . Additionof s u r f a c t a n t compounds known t o enhance m o n o l a y e r - f o r m i n g propert i e s (e.g., s t e a r i c a c i d ) i n the spreading process d i d not a l l e v i a t e the aggregation problems. Codeposition o f mixtures of v a r i o u s s u r f a c t a n t p y r i d i n e bases (e.g., an octadecylamide d e r i v a t i v e o f 4 - a m i n o p y r i d i n e ) and m e t a l l o t e t r a p h e n y l p o r p h y r i n s r e s u l t e d i n no v i s i b l e a g g r e g a t i o n . Pressure-area isotherms c h a r a c t e r i s t i c o f monomolecular l a y e r s were o b s e r v e d ; however, the p o r p h y r i n area/molecule v a l u e s ranged from 8 0 - 1 2 0 Â / m o l e c u l e d e p e n d i n g on t h e n a t u r e o f t h e m e t a l l o t e t r a p h e n y l p o r p h y r i n . T h i s suggests that the porphyrins are e i t h e r aggregated, o r i e n t e d p e r p e n d i c u l a r l y t o t h e aqueous s u r f a c e , o r t u c k e d up i n s i d e t h e surfactant layer. Although these systems are u n s u i t a b l e f o r t e m p l a t e f o r m a t i o n i n o u r a s s e m b l y p r o c e d u r e , t h e y may b e u s e f u l f o r examining dye s e n s i t i z a t i o n o r i e n t a t i o n a l e f f e c t s a t e l e c t r o d e surfaces. To a c h i e v e p o r p h y r i n m o n o l a y e r s s u i t a b l e f o r use i n o u r p r o p o s e d a s s e m b l y scheme, we examined t h e p r o p e r t i e s o f s u r f a c t a n t p o r p h y r i n s o f t h e g e n e r a l t y p e shown i n F i g u r e 3 . A t y p i c a l s u r f a c e p r e s s u r e - a r e a i s o t h e r m f o r I T O A P P o n a pH 6 . 5 b u f f e r e d NaHCO aqueous s u b p h a s e i s g i v e n i n F i g u r e 4 . T h i s c u r v e i s c h a r a c t e r i s t i c o f f o r m a t i o n o f monomolecular l a y e r s a t the a i r - w a t e r i n t e r f a c e (20). E x t r a p o l a t i o n o f the condensed phase r e g i o n to zero pressure gives a value o f ca. 160 Â / m o l e c u l e f o r the p o r p h y r i n , w h i c h i s i n agreement w i t h p r e v i o u s o b s e r v a t i o n s f o r t h i s t y p e o f compound ( 8 ) . S i m i l a r v a l u e s a r e f o u n d f o r b o t h t h e Zn and NiTOAPP d e r i v a t i v e s . This area/molecule value i s c o n s i s t e n t w i t h that expected f o r the p o r p h y r i n r i n g o r i e n t e d n e a r l y p a r a l l e l t o t h e aqueous s u r f a c e ( v a l u e s o f c a . 1 6 5 Â / molecule are c a l c u l a t e d from c r y s t a l s t r u c t u r e s o f t e t r a p h e n y l porphyrin). The d e c r e a s e i n a r e a t o c a . 1 3 0 A / m o l e c u l e upon c o m p r e s s i o n t o 2 0 dyne/cm p r o b a b l y r e s u l t s f r o m t i l t i n g o f t h e p o r p h y r i n heads a s t h e l o o s e l y p a c k e d c h a i n s ( t o t a l a r e a o f c a . 80 A / m o l e c u l e ) a r e s q u e e z e d i n t o a t i g h t e r a r r a n g e m e n t . This c o u l d r e s u l t i n some o v e r l a p o f t h e p o r p h y r i n π o r b i t a l s y s t e m . The p o r p h y r i n o r i e n t a t i o n i n d i c a t e d b y t h i s r e s u l t i s e n c o u r a g i n g w i t h r e g a r d t o u s i n g t h e s e compounds f o r t e m p l a t e f o r m a t i o n t o b u i l d up t h e p r o p o s e d p o r p h y r i n - c o n t a i n i n g a r r a y s d e s c r i b e d earlier.

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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286

PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE

INTERFACES

Figure 3. Schematic of a surfactant metalloporphyrin, MTOAPP

Figure 4. Surface pressure-area isotherm of H TOAPP (aqueous phase, 5 X 10~ M NaHCO ; pH 6.5; air atmosphere at 25°C) 5

2

s

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

18.

BULKOWSKi E T A L .

Surfactant Metalloporphyrin Assemblies

287

M i x e d m o n o l a y e r p r o p e r t i e s o f t h e MTOAPP d e r i v a t i v e s w i t h v a r i o u s c o m b i n a t i o n s o f SA and o c t a d e c a n e w e r e d e t e r m i n e d t o i n v e s t i g a t e ways f o r c o n t r o l l i n g t h e s u r f a c e c o n c e n t r a t i o n u s i n g photoinert spacers. The o b j e c t i v e was t o c h a r a c t e r i z e t h e i r s t a b i l i t y and h o m o g e n e i t y upon d i l u t i o n . Surface pressure-area i s o t h e r m s f o r d i f f e r i n g m i x i n g r a t i o s o f ZnTOAPP a n d SA a r e g i v e n i n F i g u r e 5. Two f e a t u r e s o f t h i s f a m i l y o f c u r v e s i n d i c a t e h o m o g e n e i t y o f m i x i n g ( 2 0 ) . They a r e : 1) i n c r e a s i n g breakdown p r e s s u r e a s a f u n c t i o n o f m i x i n g r a t i o , a n d 2) t h e c a l c u l a t e d curve, F i g u r e 5 c , which i s based on a d d i t i v i t y o f t h e areas o f p u r e ZnTOAPP a n d SA ( a 1:4, ZnTOAPP:SA m i x t u r e ) does n o t a g r e e w i t h t h e e x p e r i m e n t a l l y d e t e r m i n e d c u r v e , F i g u r e 5 c . Re s u l t s o b t a i n e d f o r a ZnTOAPP-octadecane m i x t u r e i n d i c a t e d t h a t a t l o w m i x i n g r a t i o s ( l e s s t h a n 1:10, ZnTOAPP:octadecane) l a y e r s a r e s t a b i l i z e d r e l a t i v e t o p u r e ZnTOAPP. Maximum f i l m s t a b i l i t y was o b s e r v e d f o r t h e 1:3 m i x t u r e o f ZnTOAPP:octadecane. I t a p p e a r s t h a t t h e o c t a d e c a n e m o l e c u l e s c a n b e accommodated b y v a c a n c i e s i n t h e l o o s e l y packed hydrophobic r e g i o n . Studies o f mixtures o f p o r p h y r i n - s t e a r i c acid-octadecane i n d i c a t e d that a 1:4:3 r a t i o o f ZnTOAPP:SA:octadecane g i v e s good, s t a b l e m o n o l a y e r s so t h i s c o m b i n a t i o n was c h o s e n f o r t h e p h o t o e l e c t r o c h e m i c a l measurements. U s i n g e s t a b l i s h e d methods ( 1 0 ) , t h e ZnTOAPP m o n o l a y e r s w e r e r e a d i l y t r a n s f e r r e d t o s o l i d supports, such as s u i t a b l y t r e a t e d g l a s s s l i d e s o r SnO o p t i c a l l y t r a n s p a r e n t e l e c t r o d e s .

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1

Luminescence S t u d i e s . L u m i n e s c e n c e s p e c t r o s c o p y was u s e d a s a s e n s i t i v e t e c h n i q u e t o c h a r a c t e r i z e t h e ZnTOAPP l a y e r s a t t h e a i r - s u p p o r t i n t e r f a c e . F i g u r e 6 compares t h e e m i s s i o n ( λ > 435 nm) a n d e x c i t a t i o n (X 660 nm) s p e c t r a o f a m o n o l a y e r o f ZnTOAPP a n d SA ( 1 : 4 ) on a h y d r o p h o b i c g l a s s support (arrangement o f l a y e r s i s glass/SA/ ZnTOAPP/SA) w i t h t h o s e o f a Ι Ο " M ZnTOAPP c h l o r o f o r m s o l u t i o n . T h e r e i s a s l i g h t b a t h o c h r o m i c s h i f t ( c a . 10 nm) o f t h e m o n o l a y e r p e a k s a n d a b r o a d e n i n g o f t h e S o r e t band w h i c h q u a l i t a t i v e l y agrees w i t h r e s u l t s reported f o r vapor deposited s o l i d f i l m s o f ZnTPP on q u a r t z ( 2 1 , 2 2 ) . These s p e c t r a show t h a t o u r compounds a r e q u i t e p u r e ( e . g . , no a d d i t i o n a l p e a k s a t 630 a n d 690 nm due t o c h l o r i n i m p u r i t i e s (22)) and t h a t they a r e n o t aggregated. Even 10 M p o r p h y r i n s o l u t i o n e x c i t a t i o n s p e c t r a o f t h e s e com pounds show s e v e r a l a d d i t i o n a l bands i n t h e S o r e t r e g i o n due t o i n t e r m o l e c u l a r i n t e r a c t i o n s . A l s o , t h e b r o a d e n i n g and peak s h i f t s a r e r a t h e r i n s e n s i t i v e t o d i l u t i o n o f t h e m o n o l a y e r s w i t h SA, a l t h o u g h f l u o r e s c e n c e i n t e n s i t i e s do s i g n i f i c a n t l y d e c r e a s e . F i g u r e 7 shows t h e s e l f - q u e n c h i n g o f t h e 660 nm f l u o r e s c e n c e e m i s s i o n ( n o r m a l i z e d t o r e l a t i v e i n t e n s i t y p e r chromophore u s i n g c o n c e n t r a t i o n v a l u e s b a s e d o n o u r aqueous s u b p h a s e m o n o l a y e r d a t a ) as a f u n c t i o n o f d i l u t i o n o f t h e ZnTOAPP b y SA. A s h a r p d e c r e a s e i n t h e q u e n c h i n g o c c u r s a t ZnTOAPP:SA r a t i o s o f 1:4, a n d a c o n βχ

e m >

6

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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288

PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE

A R E A PER M O L E C U L E

INTERFACES

( A )

Figure 5. Surface pressure-area isotherms for ZnTOAPP.SA mixing ratios— (a) pure ZnTOAPP; (b) 1:2; (c) 1:4; (c ) 1:4 calculated; (d) 1:10; (e) 1:500; and (f) pure SA. Aqueous phase contains 5 X 10' M NaHCO buffer, pH 6.5, and 3 X 10~ M CdCl . f

5

s

4

2

100 >-

400

450

500

550

WAVELENGTH

600

650

700

(nm)

6

Figure 6. Excitation and emission spectra of a 10~ M solution of ZnTOAPP in chloroform ( ) and a monolayer of ZnTOAPP on a hydrophobically treated, Cd-SA glass support ( ). Relative intensities are arbitrarily adjusted.

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

Downloaded by 148.204.30.87 on October 21, 2009 | http://pubs.acs.org Publication Date: March 2, 1981 | doi: 10.1021/bk-1981-0146.ch018

18.

BULKOWSKI ET A L .

Surfactant Metalloporphyrin Assemblies

289

s t a n t l i m i t i n g v a l u e i s r e a c h e d a t a r a t i o o f c a . 1:10. This i s c o n s i s t e n t w i t h t h e f o r m u l a t i o n o f a homogeneous, t i g h t l y p a c k e d monolayer on the support s u r f a c e . A heterogeneous s u r f a c e o f i s l a n d s o f p o r p h y r i n and s p a c e r s w o u l d b e e x p e c t e d t o show l i t t l e e f f e c t upon SA d i l u t i o n . A l s o , t h i s luminescence data i n d i c a t e s t h a t energy t r a n s f e r a s s o c i a t e d w i t h s e l f - q u e n c h i n g takes p l a c e o v e r v e r y s h o r t ranges ( i . e . l e s s than 1 Â s p a c i n g s between p o r p h y r i n r i n g p e r i p h e r i e s ) . T h i s may e v e n r e q u i r e c o n t a c t o f o r b i t a l s on a d j a c e n t c h r o m o p h o r e s a s m i g h t b e e x p e c t e d f o r p o r p h y r i n s a r r a n g e d i n a t i l t e d c o n f i g u r a t i o n o n t h e s u r f a c e . By s l i g h t s e p a r a t i o n u s i n g SA, t h e q u e n c h i n g e f f e c t p e r m o l e c u l e i s d e c r e a s e d b y a f a c t o r o f 3. I n c l u s i o n o f v a r i o u s r a t i o s o f n o n - f l u o r e s c i n g NiTOAPP t r a p s i n t o t h e ZnTOAPP m o n o l a y e r (MT0APP:SA, 1:4) g i v e s t h e dependency i n d i c a t e d i n F i g u r e 8. T h i s r e s u l t i n d i c a t e s s i g n i f i c a n t e n e r g y t r a n s f e r f r o m t h e Zn c e n t e r s t o t h e t r a p s , even a t c o n c e n t r a t i o n s o f one t r a p p e r one h u n d r e d d y e c e n t e r s . The c u r v e shape a g r e e s w e l l w i t h one c a l c u l a t e d f o r a d i p o l a r - d i p o l a r c o u p l i n g mechanism (23) b e t w e e n p o r p h y r i n s . P r e l i m i n a r y evidence w i t h a two-layer s y s t e m i n w h i c h t h e ZnTOAPP m o n o l a y e r (ZnTOAPP:SA, 1:4) c o n c e n t r a t i o n was h e l d c o n s t a n t , b u t w h i c h was i n f a c e - t o - f a c e c o n t a c t w i t h a s e c o n d NiTOAPP-SA m o n o l a y e r o f v a r y i n g N i c o n c e n t r a t i o n , i n d i c a t e s a s i m i l a r i n t e n s i t y dependence o n m i x i n g r a t i o a s was f o u n d f o r t h e s i n g l e l a y e r s y s t e m . T h i s r e s u l t s u g g e s t s t h a t i n t e r l a y e r q u e n c h i n g a l s o o c c u r s v i a a d i p o l a r - d i p o l a r mechanism. However, t h e c r i t i c a l d i s t a n c e f o r q u e n c h i n g ( I / I o = O.5) was f o u n d t o b e l e s s t h a n t h a t measured f o r t h e s i n g l e l a y e r a s s e m b l y . T h i s i n d i c a t e s t h a t t h e r e i s l e s s e f f e c t i v e c o u p l i n g between w e a k l y i n t e r a c t i n g chromophores i n a d j a c e n t l a y e r s t h a n b e t w e e n m o l e c u l e s t i g h t l y p a c k e d i n i n d i v i d u a l l a y e r s . We a r e c u r r e n t l y studying these e f f e c t s i n greater d e t a i l . Photoelectrochemical

Studies.

Our i n i t i a l p h o t o e l e c t r o c h e m i c a l s t u d i e s have b e e n c o n d u c t e d w i t h m o n o l a y e r s o f ZnTOAPP:SA:octadecane m i x t u r e s i n t h e r a t i o o f 1:4:3. They a r e d e p o s i t e d d i r e c t l y on t h e SnO OTE's w i t h t h e s u r f a c t a n t p o r p h y r i n head g r o u p s i n c o n t a c t w i t h t h e e l e c t r o d e surface. The e l e c t r o l y t e c o n t a i n e d O.1 M KC1 and was m a i n t a i n e d a t pH 7.0 w i t h a p h o s p h a t e b u f f e r . The e l e c t r o l y t e was d e o x y g e n a t e d b y a N2 gas p u r g e . An a n o d i c p h o t o c u r r e n t was g e n e r a t e d under s h o r t - c i r c u i t c o n d i t i o n s which i n c r e a s e d w i t h a p p l i e d potential. The a n o d i c p h o t o c u r r e n t i s c o n s i s t e n t w i t h e l e c t r o n i n j e c t i o n t o w a r d t h e Sn02« A d d i t i o n a l l y , a n open c i r c u i t v o l t a g e was m e a s u r e d upon i r r a d i a t i o n b y t h e ELH lamp s o u r c e . The o b s e r v e d p h o t o v o l t a g e ( c a . 10 mV) was n e g a t i v e a n d c o n s i s t e n t w i t h the g e n e r a t i o n o f an anodic photocurrent. N e g l i g i b l e photoe f f e c t s were o b s e r v e d f o r a Sn02 OTE c o a t e d w i t h j u s t a cadmium stéarate l a y e r u n d e r i d e n t i c a l c o n d i t i o n s . T h i s b e h a v i o r i s c o n s i s t e n t w i t h a n e n e r g y l e v e l scheme f o r

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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15

5 1 α PURE ZnTOAPP MIXING

ι

ι

1

10

1—

100

RATIO (SA/ZnTOAPP)

Figure 7. Dependence offluorescenceintensity (\ , 660 nm) of ZnTOAPP on SA.ZnTOAPP mixing ratios. Mixtures are deposited on a hydrophobically treated, Cd-SA glass slide and have a Cd-SA outer layer as represented in the insert. em

ι

MIXING

RATIO

(ΖηΤΟΑΡΡ/ΝιTOAPP)

Figure 8. Dependence offluorescencequenching on ZnTOAPP.NiTOAPP mixing ratios (MTOAPPiSA, 1:4) in a monolayer on Cd-SA-treated glass with a Cd-SA outer layer as represented in the insert

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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18.

BULKOWSKI ET A L .

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291

e l e c t r o n t r a n s f e r i n w h i c h the e l e c t r o n donor i s s i t u a t e d a t a more n e g a t i v e p o t e n t i a l t h a n t h e c o n d u c t i o n band edge o f t h e semiconductor. The f l a t b a n d p o t e n t i a l f o r SnC a t pH 7.0 i s -O.45 V v s . SCE ( 2 4 ) . The e n e r g y l e v e l o f t h e e x c i t e d d o n o r i s approximated from i t s o x i d a t i o n redox p o t e n t i a l and e x c i t a t i o n e n e r g y ( 4 ) . The o x i d a t i o n p o t e n t i a l f o r ZnTOAPP i s t a k e n a s +O.70 V, b a s e d o n r e p o r t e d v a l u e s f o r s u b s t i t u t e d ZnTPP's ( 2 5 ) ; t h e s i n g l e t e x c i t a t i o n e n e r g y i s 1.88 eV, b a s e d o n t h e 660 nm emission. This r e s u l t s i n a c a l c u l a t e d o x i d a t i o n redox p o t e n t i a l f o r t h e e x c i t e d s i n g l e t o f -1.18 V v s . SCE. The e x c i t e d t r i p l e t p o t e n t i a l w o u l d b e e x p e c t e d t o b e no g r e a t e r t h a n O.4-O.6 V l e s s t h a n t h e s i n g l e t , b a s e d o n known Zn p o r p h y r i n t r i p l e t e n e r g i e s (26). Since these excited state l e v e l s are e n e r g e t i c a l l y higher t h a n t h e SnO c o n d u c t i o n b a n d , e l e c t r o n t r a n s f e r t o t h e s e m i c o n d u c t o r s h o u l d b e p o s s i b l e v i a e i t h e r pathway t o p r o d u c e a n o d i c photocurrents w i t h these m e t a l l o p o r p h y r i n s e n s i t i z e r s . A t y p i c a l time response f o r a s h o r t - c i r c u i t e d photocurrent i n t h e p r e s e n c e o f h y d r o q u i n o n e ( H Q ) a s a n added s o l u t i o n r e d o x s p e c i e s i s shown i n F i g u r e 9. These p h o t o c u r r e n t s w e r e s t a b l e f o r s e v e r a l hours. I n the absence o f H Q i n the e l e c t r o l y t e , t h e p h o t o c u r r e n t a l s o i n c r e a s e d r a p i d l y upon t h e o n s e t o f i l l u m i n a t i o n , b u t s u b s e q u e n t l y d e c a y e d e x p o n e n t i a l l y t o 70% o f i t s i n i t i a l v a l u e i n a h a l f - d e c a y t i m e o f c a . 25 s. T h i s b e h a v i o r i s s i m i l a r t o that observed f o r c h l o r o p h y l l monolayers d e p o s i t e d on Sn02 ( 1 2 ) . P h o t o c u r r e n t s u n d e r p o t e n t i a l l y - c o n t r o l l e d c o n d i t i o n s were a l s o s t a b l e upon i l l u m i n a t i o n , b u t e x h i b i t e d s l o w e r decay c h a r a c t e r i s t i c s when t h e l i g h t was t u r n e d o f f . T h i s e f f e c t i s unusual and i s c u r r e n t l y under f u r t h e r i n v e s t i g a t i o n . A t y p i c a l photocurrent a c t i o n spectrum i s i l l u s t r a t e d i n F i g u r e 10 t o g e t h e r w i t h t h e e x c i t a t i o n s p e c t r u m o f t h e ZnTOAPP monolayer. The good c o r r e s p o n d e n c e b e t w e e n t h e two c u r v e s i n d i c a t e s t h a t t h e dye i s , i n f a c t , t h e s o u r c e o f t h e p h o t o c u r r e n t s observed. We a r e c u r r e n t l y e x p l o i t i n g t h e u n i q u e a b i l i t y t o c o n t r o l the a r c h i t e c t u r e i n these monolayers t o f u r t h e r i n v e s t i g a t e t h e i r p h o t o e l e c t r o c h e m i c a l p r o p e r t i e s w i t h r e s p e c t t o such f a c t o r s a s l i g h t i n t e n s i t y , s o l u t i o n and f i l m r e d o x components, a s s e m b l y s t r u c t u r e , dye o r i e n t a t i o n , e t c . We a r e p a r t i c u l a r l y i n t e r e s t e d i n u s i n g t h e m o n o l a y e r s t r u c t u r a l i n f o r m a t i o n we now have t o c o r r e l a t e f i l m e l e c t r o n i c p r o p e r t i e s w i t h c h a r g e t r a n s f e r e f f e c t s a t both the d y e - s o l i d and d y e - l i q u i d i n t e r f a c e s . Acknowledgement T h i s w o r k was s u p p o r t e d b y t h e U n i v e r s i t y o f D e l a w a r e R e s e a r c h F o u n d a t i o n and b y t h e U.S. D e p a r t m e n t o f E n e r g y ( G r a n t No. DE-FG02-79ER-10533).

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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eg

1

,

I

ι

I

Ο

1

2

3

4

TIME

.

I

I

5

6

(sec)

Figure 9. Short-circuited photocurrent vs. time for a monolayer of ZnTOAPP (ZnTOAPP.SA, 1:4) directly on a Sn0 OTE (O.1M KCl, pH 7.0,O.05MH Q, N purged) 2

2

2

ι ~~

Γ

WAVELENGTH

(nrrO

Figure 10. Action spectrum of a monolayer of ZnTOAPP (ZnTOAPP.SA :octadecane, 1:4:3) directly on a Sn0 OTE: O.1M KCl, pH 7.0, electrode potential is +O.3 V vs. SCE, Ν purged; ( ) the excitation spectrum (\ m, 660 nm) of ZnTOAPP monolayer at SnO -electrolyte interface 2

2

e

2

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

18.

BULKOWSKI

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Literature

ET AL.

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Cited

1. Wrighton, M.S. Acc. Chem. Res., 1979,9,303. 2. Gerischer, H. in "Physical Chemistry: An Advanced Treatise," Eyring, H.; Henderson, D.; Jost, W., Eds.; Academic Press, New York, 1970; Vol. 9A, Chapter 5. 3. Memming, R. in "Electroanalytical Chemistry," A.J. Bard, Ed.; Marcel Dekker, New York, Vol. 11; 1978. 4. Gerischer, H. Topics in Current Chemistry, 1976, 61, 31. 5. Memming, R. Photochem. Photobiol., 1972, 16, 325. 6. Spitler, M.T.; Calvin, M. J. Chem. Phys., 1977, 66, 4294. 7. Whitten, D.G.; Eaker, D.W.; Horsey, B.E.; Schmehl, R.H.; Worsham, P.R. Ber. Bunsenges. Phys. Chem., 1978, 82, 858. 8. Mercer-Smith, J.A.; Whitten, D.G. J. Am. Chem.Soc.,1979, 101, 6620. 9. Collman, J.P.; Gagne, R.R.; Reed, C.A.; Halbert, T.R.; Lang, G.; Robinson, W.T. J. Am. Chem. Soc., 1975, 97, 1427. 10. Kuhn, H.; Mobius, D.; Bucher, H. "Physical Methods of Chemistry," Vol. 1; Part 3B; Weissburger, Α.; Rossiter, B., Eds.; Wiley, New York, 1972; p. 588. 11. Bucher, H.; Eisner, O.; Mobius, D.; Tillmann, P.; Wiegand, J. Z. Phys. Chem., 1969, 65, 152. 12. Miyasaka, T.; Watanabe, T.; Fujishima, Α.; Honda, K. J. Am. Chem. Soc., 1978, 100, 6657. 13. Meier, H. Topics in Current Chemistry, 1976, 61, 85. 14. Wang, J.H. P r o c . Natl. Acad. Sci., U.S.A., 1969, 62, 653. 15. Adler, A.D. J. Polymer Sci. Part C, 1970, 29, 73. 16. Adler, A.D.; Varadi, V.; Wilson, N. Ann. N.Y. Acad. Sci., 1975, 244, 685. 17. Umezawa, Y.; Yamamura, T. J. Chem. Soc. Chem. Comm., 1978, 1106. 18. Tang, C.W. U.S. Patent No. 4,164,431, August, 1979. 19. Umezawa, Y.; Yamamura, T. J. Electroanal. Chem., 1979, 95, 113. 20. Gaines, G.L., Jr. "Insoluble Monolayers at Liquid-Gas Interfaces"; Wiley Interscience, New York, 1966. 21. Tanimura, K.; Kawai, T.; Sakata, T. J. Phys. Chem., 1979, 83, 2639. 22. Tanimura, K.; Kawai, T.; Sakata, T. J. Phys. Chem., 1980, 84, 751. 23. Kuhn, H. J. Photochem., 1979, 10, 111. 24. Mollers, F.; Memming, R. Ber. Bunsenges. Phys. Chem., 1975, 76, 469. 25. Wolberg, A. Isr. J. Chem., 1975, 12, 1031. 26. Hopf, F.R.; Whitten, D.G. in "Porphyrins and Metalloporphyrins," Smith, K.M., Ed.; Elsevier, New York, 1975; Chapter 16. RECEIVED October 3, 1980.

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

19 Effects of Temperature on Excited-State Descriptions of Luminescent Photoelectrochemical Cells Employing Tellurium-Doped Cadmium Sulfide Electrodes ARTHUR B. ELLIS and BRADLEY R. KARAS

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Department of Chemistry, University of Wisconsin—Madison, Madison, WI 53706 The need for alternate energy sources has led to the rapid development of photoelectrochemical cells (PECs). A PEC consisting of an η-type semiconductor, a counterelectrode, and a suitably chosen electrolyte can convert optical energy directly into chemical fuels and/or electricity Q»2.»3,4). We recently reported that tellurium-doped CdS (CdS:Te) mimics undoped CdS in its ability to sustain the conversion of monochromatic ultraband gap light (>2.4 eV; λ <500 nm (5)) into electricity at 7% efficiency in PECs employing aqueous polychalcogenide electrolytes (6,7,90. A novel feature of the CdS:Te photoanodes is that they emit (A 600 nm for 100 ppm CdS:Te) with O.1% efficiency while effecting the oxidation of polychalcogenide species. Luminescence results from the introduction of intraband gap states by the substitution of Te for S in the CdS lattice. Because of its lower electron affinity, Te sites trap holes which can then coulombically bind an electron in or near the conduction band to form an exciton. Subsequent radiative collapse of this exciton leads to emission (10,11,12,13). In the context of the PEC, emission thus serves as a probe of electron-hole (e~- h ) pair recombination which competes with e~~- h pair separation leading to photocurrent. Except for intensity, the emitted spectral distribution is found to be independent of the presence and/or composition of polychalcogenide electrolyte, excitation wavelength (Ar ion laser lines, 457.9-514.5 nm) and intensity (<30 mW/cm), and applied potential (-O.3V vs. SCE to open circuit) (6 ] 8,9). Optical penetration depth plays a significant role in the PEC properties we observe. The absorptivity of 100 ppm CdS:Te for 514.5 nm light is 10 cm"" 0,3,11,12,13). Since the depletion region in which ê"- h pairs are efficiently separated by band bending to yield photocurrent is 10~4-io~5 cm thick (14), a significant fraction of 514.5 nm light is absorbed beyond this region. We therefore expect and observe greater emission intensity and smaller photocurrent with 514.5 nm excitation than with ultraband gap wavelengths (8) for which the CdS:Te max

+

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0097-6156/81/0146-0295$05.00/0 © 1981 American Chemical Society In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

PHOTOEFFECTS AT

296

3

SEMICONDUCTOR-ELECTROLYTE INTERFACES

1

a b s o r p t i v i t y i s l C r - l O - cm (5,10,11,12,13). A d d i t i o n a l l y , whereas t h e r e i s l i t t l e p o t e n t i a l dependence of e m i s s i o n i n t e n s i t y w i t h 514.5 nm e x c i t a t i o n , a s t r o n g dependence i s o b s e r v e d w i t h u l t r a b a n d gap i r r a d i a t i o n : i n p a s s i n g f r o m -O.3V v s . SCE t o open c i r c u i t i n aqueous p o l y c h a l c o g e n i d e e l e c t r o l y t e s , i n c r e a s e s i n emission i n t e n s i t y of 15-1400% o b t a i n (6,7,8.,9) . T h i s i s i n accord w i t h the premise t h a t v a r i a t i o n s of p o t e n t i a l correspond t o a l t e r a t i o n s i n t h e d e g r e e o f band b e n d i n g ( 1 4 ) . T e m p e r a t u r e i s a n o t h e r PEC p a r a m e t e r w h i c h can p o t e n t i a l l y m o d i f y t h e e f f i c i e n c i e s o f p h o t o c u r r e n t and l u m i n e s c e n c e . Among t h e m a t e r i a l s whose t e m p e r a t u r e d e p e n d e n t PEC p r o p e r t i e s h a v e b e e n s t u d i e d a r e Sn02 ( 1 5 ) , T i 0 (16) and C u I n S ( 1 7 ) . Undoped CdS has a known o p t i c a l band gap t e m p e r a t u r e d e p e n d e n c e o f -5.2x10-4 eV/°K b e t w e e n 90 and 400 Κ ( 1 8 ) . Owing t o t h e g e n e r a l s i m i l a r i t y o f CdS:Te t o CdS, we a n t i c i p a t e d a c o m p a r a b l e r e d s h i f t i n the onset of a b s o r p t i o n . I n t h i s p a p e r we summarize t h e r e s u l t s o f t e m p e r a t u r e s t u d i e s on CdS:Te-based PECs e m p l o y i n g aqueous p o l y s e l e n i d e (9) and s u l f i d e e l e c t r o l y t e s .

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2

2

Experimental S i n g l e - c r y s t a l p l a t e s o f v a p o r - g r o w n , 100 ppm CdS:Te w e r e purchased from Cleveland C r y s t a l s , I n c . , C l e v e l a n d , Ohio. E m i s s i v e s p e c t r a l f e a t u r e s were c o n s i s t e n t w i t h those p r e v i o u s l y r e p o r t e d f o r CdS:Te (6-13) and c o n f i r m e d ( R o e s s l e r ' s c o r r e l a t i o n , ( 1 2 ) ) t h a t t h e Te c o n c e n t r a t i o n was <100 ppm. The 5 x 5 x 1 mm s a m p l e s had r e s i s t i v i t i e s o f 2 ohm-cm ( f o u r p o i n t p r o b e method) and w e r e o r i e n t e d w i t h t h e 5x5 mm f a c e p e r p e n d i c u l a r t o t h e c-axis. Samples w e r e f i r s t e t c h e d w i t h 1:10 ( v / v ) Br2/MeOH and t h e n p l a c e d i n an u l t r a s o n i c c l e a n e r t o remove r e s i d u a l B r . The e l e c t r o l y t e was e i t h e r s u l f i d e , 1M 0H"/1M S , or p o l y s e l e n i d e , t y p i c a l l y 5M OH""/O.1M Se ~/O.001M S e " ; s h o r t o p t i c a l p a t h l e n g t h s ( 500 nm. E l e c t r o d e and e l e c t r o l y t e p r e p a r a t i o n a s w e l l as e l e c t r o c h e m i c a l and o p t i c a l i n s t r u m e n t a t i o n e m p l o y e d h a v e b e e n d e s c r i b e d p r e v i o u s l y ( 8 ) . E l e c t r o l y t e s were m a g n e t i c a l l y s t i r r e d and b l a n k e t e d u n d e r N d u r i n g u s e . A s s e m b l i n g t h e PEC i n s i d e t h e s a m p l e compartment o f a s p e c t r o p h o t o f l u o r o m e t e r p e r m i t t e d measurement o f e m i s s i o n s p e c t r a l d a t a (200-800 nm; V> nm b a n d w i d t h ) . F r o n t - s u r f a c e e m i s s i v e p r o p e r t i e s w e r e r e c o r d e d by i n c l i n i n g t h e p h o t o e l e c t r o d e a t 45° t o b o t h t h e i n c i d e n t C o h e r e n t R a d i a t i o n CR-12 A r i o n l a s e r beam (501.7 o r 514.5 nm) and t h e e m i s s i o n d e t e c t i o n o p t i c s . I n a l l experiments the 3 mm d i a . beam was 10X expanded and masked t o f i l l the e l e c t r o d e s u r f a c e ; i n c i d e n t i n t e n s i t i e s were g e n e r a l l y <10 mW/cm . T e m p e r a t u r e o f t h e PEC was a d j u s t e d a s p r e v i o u s l y described (9). 2

2

2

2

2

2

2

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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R e s u l t s and D i s c u s s i o n E m i s s i v e and P h o t o c u r r e n t P r o p e r t i e s . Three c h a r a c t e r i s t i c s o f e m i s s i v e PECs w h i c h m i g h t d i s p l a y t h e r m a l e f f e c t s a r e t h e e m i s s i v e s p e c t r a l d i s t r i b u t i o n , e m i s s i o n i n t e n s i t y , and p h o t o c u r r e n t . We o b s e r v e d t h e e m i s s i o n s p e c t r u m o f 100 ppm CdS:Te i n b o t h p o l y s e l e n i d e and s u l f i d e e l e c t r o l y t e s f r o m 20-100°C. Lowr e s o l u t i o n s p e c t r a o f t h e v a r i o u s samples r e v e a l e d r e d - s h i f t s o f X w i t h i n c r e a s i n g t e m p e r a t u r e o f a t most 5-10 nm; t h e d i s p l a c e ment o f X f o r many s a m p l e s , h o w e v e r , was w i t h i n t h e b a n d w i d t h o f t h e s p e c t r o m e t e r ( 5 nm). T h e s e r e s u l t s a r e i n t h e r a n g e o f an e x t r a p o l a t i o n o f R o e s s l e r ' s d a t a (12) f r o m w h i c h we w o u l d predict a red-shift i n V o f ' W - l l nm b e t w e e n 20 a n d 100°C. P a r a l l e l s h i f t s h a v e a l s o b e e n r e p o r t e d f o r CdS:Te a b s o r p t i o n and e x c i t a t i o n s p e c t r a o v e r s u b a m b i e n t t e m p e r a t u r e r a n g e s ( 1 3 ) . These s p e c t r a i n c o r p o r a t e a l o w - e n e r g y band w h i c h d i s t i n g u i s h e s CdS:Te f r o m CdS a n d , i n f a c t , masks t h e e x a c t p o s i t i o n o f t h e CdS:Te band gap ( 7 , 8 , 1 1 , 1 2 , 1 3 ) . S e v e r a l o t h e r f e a t u r e s o f t h e CdS:Te e m i s s i o n s p e c t r u m a r e p a r t i c u l a r l y r e l e v a n t i n t h e c o n t e x t o f PEC experiments. For a g i v e n temperature the s p e c t r a l d i s t r i b u t i o n i s i n d e p e n d e n t o f w h e t h e r 501.7 o r 514.5 nm e x c i t a t i o n i s u s e d a n d o f e l e c t r o d e p o t e n t i a l b e t w e e n +O.7 V v s . A g ( p s e u d o r e f e r e n c e e l e c t r o d e , PRE) a n d t h e o n s e t o f c a t h o d i c c u r r e n t . Such a n i n s e n s i t i v i t y to p o t e n t i a l indicates that the energies of the i n t r a b a n d gap Te s t a t e s a r e a f f e c t e d b y p o t e n t i a l i n t h e same manner a s t h e c o n d u c t i o n and v a l e n c e band e n e r g i e s . m a x

m a x

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ax

A v e r y p r o f o u n d t e m p e r a t u r e e f f e c t was o b s e r v e d f o r t h e emission i n t e n s i t y . Figure 1 presents an emission-temperature p r o f i l e a t open c i r c u i t i n s u l f i d e e l e c t r o l y t e ; t h e r e l a t i v e i n v a r i a n c e o f t h e sample's s p e c t r a l d i s t r i b u t i o n w i t h temperature a l l o w e d u s t o m o n i t o r e m i s s i o n i n t e n s i t y a t t h e band maximum. E m i s s i o n i n t e n s i t y was matched f o r 501.7 a n d 514.5 nm e x c i t a t i o n a t 20°C u s i n g 17 t i m e s a s much 501.7 nm i n t e n s i t y . Over t h e 20-100°C e x c u r s i o n e m i s s i o n i n t e n s i t y i s s e e n t o d r o p b y f a c t o r s o f 8 and 30 f o r 501.7 a n d 514.5 nm e x c i t a t i o n , r e s p e c t i v e l y . These f a c t o r s a r e c o n s i s t e n t w i t h t h e 10- t o 2 0 - f o l d d e c l i n e observed i n p o l y s e l e n i d e e l e c t r o l y t e ; i n those experiments there a l s o appeared t o be l i t t l e p o t e n t i a l dependence o f t h e r e s u l t s (9). S i m i l a r t h e r m a l quenching d a t a has been r e p o r t e d f o r d r y CdS:Te s a m p l e s i r r a d i a t e d w i t h UV l i g h t ( 1 0 , 1 2 , 1 3 ) , e l e c t r o n beams ( 1 1 ) , and α p a r t i c l e s ( 1 9 ) . The t e m p e r a t u r e dependence o f the d e c l i n e i n e m i s s i o n i n t e n s i t y has been l i n k e d t o t h e i o n i z a t i o n e n e r g y o f t h e Te-bound h o l e , O.2 eV ( 1 0 , 1 1 , 1 2 , 1 3 , 1 9 ) . G i v e n t h e i n h e r e n t l y c o m p e t i t i v e n a t u r e o f e m i s s i o n and p h o t o c u r r e n t , i t s h o u l d n o t b e s u r p r i s i n g t h a t p h o t o c u r r e n t was g e n e r a l l y observed t o i n c r e a s e w i t h temperature. F i g u r e 2 i s a p h o t o c u r r e n t - t e m p e r a t u r e p r o f i l e o b t a i n e d a t +O.7 V v s . A g (PRE) i n s u l f i d e e l e c t r o l y t e ; t h e l i g h t i n t e n s i t y and s u l f i d e c o n c e n t r a t i o n employed e n s u r e d t h a t p h o t o c u r r e n t s w e r e l i m i t e d b y e x c i t a t i o n r a t e and n o t b y mass t r a n s p o r t , i . e . , p h o t o c u r r e n t s r

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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30

40 50 60 70 TEMPERATURE, °C

80

90

INTERFACES

100

Figure 1. Relative emission intensity monitored at 600 nm vs. temperature in 1M OH/1M S ' electrolyte of CdS:Te (100 ppm) excited at open circuit with 514.5 (\Z\) and 501.7 nm (O) light in identical geometries. The excitation intensity at 501.7 nm is ~ 17X that at 514.5 nm in order to match approximately room temperature emission intensities. 2

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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40

50

60

70

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90

299

100

TEMPERATURE, °C

Figure 2. Relative photocurrent vs. temperature for the CdS.Te (100 ppm) photoelectrode of Figure 1 in 1M OH/1M S ~ electrolyte excited in identical geometries with equivalent intensities (ein/s) of 514.5 nm (Φ) and 501.7 nm (O) light at +O.7 V vs. Ag (PRE). The scale is such that the 25°C., 501.7 nm photocurrent has been arbitrarily set to 100 and corresponds to a current density of ~ O.38 mA/ cm and a photocurrent quantum yield of ~ O.66. 2

2

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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were not saturated with r e s p e c t to l i g h t i n t e n s i t y and were not a f f e c t e d by s t i r r i n g . At 20°C f o r matching i n t e n s i t i e s (ein/sec) 15 times the photocurrent i s observed with ultraband gap 501.7 nm e x c i t a t i o n as with band gap edge 514.5 nm l i g h t . As the temperature i s i n c r e a s e d to 100°C., a modest i n c r e a s e i n 501.7 nm photocurrent i s observed, while about an order of magnitude growth i s obtained f o r 514.5 nm e x c i t a t i o n . In f a c t , at 100°C the 514.5 nm photocurrent has reached 60% of the ultraband gap photocurrent. S i m i l a r e f f e c t s were observed i n p o l y s e l e n i d e e l e c t r o l y t e f o r both CdS:Te and undoped CdS ( 9 ) . The photocurrent enhancement f o r 514.5 nm e x c i t a t i o n i s a p r e d i c t a b l e consequence of an a b s o r p t i o n edge which red s h i f t s with temperature: As the a b s o r p t i v i t y f o r 514.5 nm l i g h t i n c r e a s e s with temperature, p r o g r e s s i v e l y l a r g e r f r a c t i o n s of l i g h t w i l l be absorbed i n the d e p l e t i o n r e g i o n . In t h i s sense the a c c e l e r a t e d d e c l i n e i n emission i n t e n s i t y observed f o r 514.5 nm r e l a t i v e to 501.7 nm e x c i t a t i o n (Figure 1) may i n c l u d e a c o n t r i b u t i o n r e f l e c t i n g decreasing o p t i c a l p e n e t r a t i o n depth; weaker emission would be expected as 514.5 nm l i g h t a c q u i r e s s t a t u s as an ultraband gap wavelength, s i n c e near-surface n o n r a d i a t i v e recombination s i t e s could play a more s i g n i f i c a n t role i n excited-state deactivation Ç 7 , 8 ) . Although the r a t e at which 514.5 nm photocurrent i n c r e a s e s with temperature i s i n reasonable accord with the CdS o p t i c a l band gap temperature c o e f f i c i e n t , the e f f e c t of potential-dependent a b s o r p t i v i t y must be considered. E l e c t r o a b s o r p t i o n measurements have been made on undoped CdS at room temperature. They r e v e a l that f o r e l e c t r i c f i e l d s of 105 V/cm, the change i n a b s o r p t i v i t y , Δα, i s + l x l 0 and - 4 x l 0 cm at 515 and 500 nm, r e s p e c t i v e l y (20). While t h i s e f f e c t helps to b l u r the discrepancy i n 514.5 and 501.7 nm o p t i c a l p e n e t r a t i o n depths, the r e s u l t a n t a b s o r p t i v i t i e s are s t i l l s u f f i c i e n t l y d i s p a r a t e r e l a t i v e to the width of the d e p l e t i o n r e g i o n to y i e l d very d i f f e r e n t photocurrents at 295 K. To our knowledge e l e c t r o a b s o r p t i o n data are not p r e s e n t l y a v a i l a b l e f o r CdS:Te. A crude attempt to gauge the magnitude of t h i s e f f e c t at 295 Κ f o r 514.5 nm l i g h t suggests i t i s small (8). In the absence of e l e c t r o a b s o r p t i o n data over the 20-100°C range, however, the e l e c t r i c f i e l d and thermal e f f e c t s are not completely decoupled and the r e s u l t s presented here should be so t r e a t e d . We should p o i n t out that the temperature e f f e c t s on emission i n t e n s i t y and photocurrent are completely r e v e r s i b l e . Although t h i s r e s u l t suggests that e l e c t r o d e s t a b i l i t y obtains over the d u r a t i o n of the experiments, the p r o p e r t i e s measured may not be very s e n s i t i v e to v a r i a t i o n s i n s u r f a c e or near-surface composition. There i s now c o n s i d e r a b l e evidence, i n f a c t , that s u r f a c e r e o r g a n i z a t i o n processes do occur i n CdS- and CdSe- based PECs i n polychalcogenide e l e c t r o l y t e s (Γ7, 21-26). In p a r t i c u l a r , the occurrence of such an exchange r e a c t i o n f o r CdS:Te i n p o l y s e l e n i d e e l e c t r o l y t e would y i e l d CdSe to whose lower band gap 3

-1

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3

19.

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(1.7 eV (27)) some of the observed p r o p e r t i e s could be a t t r i b u t e d . The r e v e r s i b i l i t y of the temperature e f f e c t s as w e l l as the s i m i l a r behavior seen i n s u l f i d e e l e c t r o l y t e argue against such an e x p l a n a t i o n . A d d i t i o n a l l y , we have employed low l i g h t i n t e n s i t i e s (<10 mW/cm ) and current d e n s i t i e s (<2 mA/cm with a t o t a l charge of g e n e r a l l y <1 C/cm2) to f u r t h e r minimize exchange processes. But we do recognize that our techniques by no means r u l e out the p o s s i b i l i t y of s u r f a c e r e o r g a n i z a t i o n processes a t some l e v e l . Simultaneous measurement of c u r r e n t , luminescence and v o l t a g e can be presented i n i L V curves which summarize much of our data. We f i n d that the r a t i o of o p e n - c i r c u i t to i n - c i r c u i t luminescence i n t e n s i t y ( φ / Φ ) i s a u s e f u l expression of the emission's p o t e n t i a l dependence, with the i n - c i r c u i t value taken at a p o t e n t i a l where the photocurrent i s s a t u r a t e d . Low values of photocurrent quantum y i e l d , φ times smaller (curve A v s . curve B ) than f o r 514.5 nm l i g h t . The r a t i o of Φ / φ i s 1.0 f o r 514.5 nm (curve B ) and 3.5 (curve A ) for501.7nm i l l u m i n a t i o n . I n c r e a s i n g the temperature to 49°C i n c r e a s e s the photo current from 501.7 nm l i g h t by M 5 % (curve C) and diminishes the luminescence i n t e n s i t y by a f a c t o r of 2. However, a s i m i l a r φ / φ r a t i o of 3.4 obtains (curve C ) . At 86°C the 514.5 nm photocurrent i n c r e a s e s by a f a c t o r of almost 8 (curve D). Despite i t s approximately 1 0 - f o l d drop i n i n t e n s i t y , emission from 514.5 nm e x c i t a t i o n now e x h i b i t s a p o t e n t i a l dependence with a Φ / Φ v a l u e of 1.27 (curve D - note 10X s c a l e expansion). S i m i l a r non-unity Φ Γ / Φ Γ l were observed i n s u l f i d e e l e c t r o l y t e s a t temperatures exceeding 8 0 ° C . 2

2

Γ

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Γ

χ

Γ Ο

Γ

χ

Γ Ο

Γ ο

Γ

Γ

1

1

Γ ο

f

Γ

1

1

Γ ο

Γ

1

Γ Ο

Γ

v

a

u

e

s

0

%

I n t e r r e l a t i o n s h i p s of E x c i t e d - S t a t e Decay Routes. The i L V curves conveniently d i s p l a y the competitive nature of photo current and luminescence i n t e n s i t y as e x c i t e d - s t a t e d e a c t i v a t i o n pathways. Our a n a l y s i s i s l i m i t e d i n the sense that we have obtained absolute numbers f o r φ but have had to content ourselves with r e l a t i v e φ measurements. We l a c k measures of n o n r a d i a t i v e recombination e f f i c i e n c y ( φ ) > although they now appear to be χ

r

ηΓ

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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POTENTIAL, VOLTS vs Ag (PRE)

Journal of the Electrochemical Society Figure 3.

Current-luminescence-voltage (iLV) curves for a 100-ppm CdS.Te electrode in polyselenide electrolyte.

Unprimed, solid-line curves are photocurrent (left-hand scale) and primed, dotted-line curves are emission intensity (right-hand scale) monitored at X ~ 600 nm. Curves A and A' result from excitation at 501.7-nm, 23°C; Curves Β and B' from 514.5-nm, 23°C; Curves C and C., 49°C and 501.7-nm excitation; Curves D and D\ 86°C., 514.5-nm irra diation. Note that the ordinate of Curve D' has been expanded by a factor of 10. Equivalent numbers of 501.7- and 514.5-nm photons were used to excite the photoelectrode in identical geometric configurations. The exposed electrode area is ~ O.41 cm , corresponding to an estimated φ for 501.7-nm excitation at 23°C and +O.7 V vs. Ag (PRE) of ~ O.50, uncorrected for solution absorbance and reflectance losses (9). max

2

χ

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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o b t a i n a b l e by the technique o f photothermal spectroscopy (28,29). T r e a t i n g t h e s e a s t h e o n l y p o s s i b l e d e c a y r o u t e s y i e l d s eq. 1 φ + φ + φ x r nr Y

Y

T

= 1

(1) +

A t open c i r c u i t φ = 0 and p h o t o g e n e r a t e d e~- h p a i r s a r e f o r c e d to recombine. The r a t i o o f φ t o φ i s shown t o b e w a v e l e n g t h dependent by comparing the o p e n - c i r c u i t e m i s s i o n i n t e n s i t i e s o f c u r v e A and c u r v e B i n F i g u r e 3. T h i s r a t i o i s a l s o t e m p e r a t u r e d e p e n d e n t , a s shown i n F i g u r e 1. C o r r e l a t i o n o f φ , φ , and φ i n v o l v e s knowing the r e l a t i v e e x t e n t t o w h i c h n o n r a d i a t i v e l y and r a d i a t i v e l y r e c o m b i n i n g e h " " p a i r s are prevented from recombining, i . e . , t h e i r r e l a t i v e c o n t r i b u t i o n t o p h o t o c u r r e n t . T h r e e p o s s i b l e schemes a r e : p h o t o c u r r e n t , φ , i n t e r c o n v e r t s (1) e x c l u s i v e l y w i t h φ ; ( 2 ) e x c l u s i v e l y w i t h φ ; (3) w i t h b o t h φ and φ such t h a t φ = k φ f o r any φ . Scheme 1 i s u n l i k e l y b e c a u s e o f t h e r e l a t i v e m a g n i t u d e s o f φ and φ . M e a s u r e d v a l u e s o f φ and φ a r e 1 0 " and 1 0 " , r e s p e c t i v e l y , f o r band gap edge e x c i t a t i o n ( 8 ) . I n p a s s i n g f r o m +O.7 V v s . A g (PRE) t o open c i r c u i t , a 1 0 0 - f o l d i n c r e a s e i n emission i n t e n s i t y i s predicted. T h i s i s i n c o n s i s t e n t w i t h the i n s e n s i t i v i t y d i s p l a y e d i n c u r v e B , F i g u r e 3. Scheme 2 a r g u e s t h a t φ w i l l be independent o f p o t e n t i a l . While i t appears t h a t c u r v e B ( F i g u r e 3) a g r e e s w i t h t h i s , i t i s c o n t r a d i c t e d b y c u r v e s A , C , a n d D\ A l t h o u g h n o t p e r f e c t , Scheme 3 i s most c o m p a t i b l e w i t h o u r d a t a . T h i s a s s u m p t i o n when combined w i t h eq. 1 l e a d s t o a s i m p l e r e l a t i o n s h i p f o r m o n o c h r o m a t i c e x c i t a t i o n b e t w e e n φ and φ /φ : Γ

f

η

Γ

1

χ

Γ

η

Γ

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1

χ

Γ

η Γ

η Γ

r

Γ

η

Γ

χ

3

Γ

χ

Γ

1

χ

1

Γ

f

r

T

χ

A

Γ

=

Τ-Γ-

(2)

Table I consists o f a compilation o f φ / Φ r a t i o s as a f u n c t i o n o f φ . Our r e s u l t s a n d t h o s e p r e s e n t e d f o r p-GaP and n-ZnO a r e i n r o u g h agreement w i t h t h i s s i m p l e m o d e l ( 8 , 9 , 3 0 , 3 1 , 3 2 ) . C o n s t r u c t i o n o f a more r e f i n e d m o d e l a w a i t s i n c o r p o r a t i o n o f other data (nonexponential l i f e t i m e s , electroabsorption, carrier properties, intensity effects, quantitative evaluation of Φ b y p h o t o t h e r m a l s p e c t r o s c o p y , e.g.) and e x a m i n a t i o n o f other systems. Γ

Γ

χ

Η

Γ

Acknowledgment We a r e g r a t e f u l t o t h e O f f i c e o f N a v a l R e s e a r c h f o r s u p p o r t o f t h i s work. BRK a c k n o w l e d g e s t h e s u p p o r t o f t h e E l e c t r o c h e m i c a l S o c i e t y t h r o u g h a J o s e p h W. R i c h a r d Summer F e l l o w s h i p . D a v i d J . M o r a n o , D a n i e l K. B i l i c h , and H o l g e r H. S t r e c k e r t a r e t h a n k e d f o r t h e i r a s s i s t a n c e w i t h some o f t h e measurements.

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

304

PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE

Table I .

R e l a t i o n s h i p Between φ Φ

Φ

χ

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and φ

/φ

a

INTERFACES

(9)

/Φ

ο 1.00 1.01 1.05 1.11 1.25 1.43 1.67 2.00 2.50 3.33 5.00 10.00

O.001 O.01 O.05 O.10 O.20 O.30 O.40 O.50 O.60 O.70 O.80 O.90 1.00

oo

C a l c u l a t e d from eq.2 where φ i s the photocurrent quantum y i e l d , and Φ Γ / Φ Γ * i ° °f emission quantum y i e l d s between open c i r c u i t and the p o t e n t i a l where φ i s measured. Journal of the Electrochemical Society Abstract χ

i

s

t

i e

r a t

0

χ

The effect of temperature on excited-state deactivation processes in a single-crystal, n-type, 100 ppm CdS:Te-based photo electrochemical cell (PEC) employing aqueous polychalcogenide electrolytes is discussed. While serving as electrodes these materials emit (λ ~600 nm). Photocurrent (quantum yield Φ) from ultraband gap (≥2.4 eV;λ ≤500 nm) 501.7nm excitation increases modestly by ≤20% between 20° and 100°C; photocurrent from band gap edge 514.5 nm excitation increases by about an order of magnitude, reaching ~50-100% of the room temperature 501.7 nm value. High lighting the competitive nature of emission and photocurrent as excited-state decay processes, luminescence (quantum yield Φ) declines over the same temperature regime by factors of between 10 and 30. At most, modest red shifts of λ (<10 nm) are observed in the spectral distribution of emission with temperature. These effects are discussed in terms of optical penetration depth, band bending, and the known red shift of the CdS absorption edge with temperature. Correlations involving Φ and Φ suggested by the data are discussed. max

x

r

max

x

Literature 1. 2. 3. 4. 5.

r

Cited

Bard, A.J. Science, 1980, 207, 139. Memming, R. Electrochim. Acta, 1980, 25, 77. Wrighton, M.S. Acc. Chem. Res., 1979, 12, 303. Nozik, A.J. Ann. Rev. Phys. Chem., 1978, 29, 189. Dutton, D. Phys. Rev., 1958, 112, 785.

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

19.

6. 7. 8. 9. 10. 11. 12. 13.

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14. 15. 16. 17. 18. 19. 20.

21. 22. 23. 24. 25. 26. 27. 28.

29. 30. 31. 32.

ELLIS AND KARAS

Luminescent Photoelectrochemical Cells

305

Ellis, A.B.; Karas, B.R. J. Am. Chem. Soc., 1979, 101, 236. Ellis, A.B.; Karas, B.R. Adv. Chem. Ser., 1980, 184, 185. Karas, B.R.; Ellis, A.B. J. Am. Chem. Soc., 1980, 102, 968. Karas, B.R.; Morano, D.J.; Bilich, D.K.; Ellis, A.B. J. Electrochem. Soc., 1980, 127, 1144. Aten, A.C.; Haanstra, J.H.; deVries, H. Philips Res. Rep., 1965, 20, 395. Cuthbert, J.D.; Thomas, D.G. J. Appl. Phys., 1968, 39, 1573. Roessler, D.M. J. Appl. Phys. 1970, 41, 4589. Moulton, P.F., Ph.D. Dissertation, Massachusetts Institute of Technology, 1975. Gerischer, H. J. Electroanal. Chem., 1975, 58, 263. Wrighton, M.S.; Morse, D.L.; Ellis, A.B.; Ginley, D.S.; Abrahamson, H.B. J. Am. Chem. Soc., 1976, 98, 44. Butler, M.A.; Ginley, D.S. Nature, 1978, 273, 524. Heller, Α.; Miller, B. Adv. Chem. Ser., 1980, 184, 215. Bube, R.H. Phys. Rev., 1955, 98, 431. Bateman, J.E.; Ozsan, F.E.; Woods, J.; Cutter, J.R. J. Phys. D. Appl. Phys., 1974, 7, 1316. Blossey, D.F.; Handler, P. in "Semiconductors and Semimetals"; Willardson, R.K., Beer, A.C.,Eds.; Academic Press, New York, 1972; Vol. 9, Chapter 3 and references therein. Noufi, R.N.; Kohl, P.Α.; Rodgers, J.W. Jr.; White, J.M.; Bard, A.J. J. Electrochem. Soc., 1979, 126, 949. Heller, Α.; Schwartz, G.P.; Vadimsky, R.G.; Menezes, S.; Miller, B. ibid., 1978, 125, 1156. Cahen, D.; Hodes, G.; Manassen, J. ibid., 1978, 125, 1623. Gerischer, H.; Gobrecht, J. Ber. Bunsenges Phys. Chem., 1978, 82, 520. Hodes, G.; Manassen, J.; Cahen, D. Nature (London), 1976, 261, 403. DeSilva, K.T.L.; Haneman, D. J. Electrochem. Soc., 1980, 127, 1554. Wheeler, R.G.; Dimmock, J.O. Phys. Rev., 1962, 125, 1805. Fujishima, Α.; Brilmyer, G.H.; Bard, A.J. in "Semiconductor Liquid Junction Solar Cells", Heller, Α., Ed.; Proc. Vol. 77-3: Electrochemical Society, Inc.: Princeton, N.J. 1977, p. 172. Fujishima, Α.; Maeda, Y.; Honda, K.; Brilmyer, G.H.; Bard, A.J. J. Electrochem. Soc., 1980, 127, 840. Ellis, A.B.; Karas, B.R.; Streckert, H.H. Faraday Discuss. Chem. Soc., in press. Beckmann, K.H.; Memming, R. J. Electrochem. Soc., 1969, 116, 368. Petermann, G.; Tributsch, H.; Bogomolni, R. J. Chem. Phys., 1972, 57, 1026.

R E C E I V E D October 3, 1 9 8 0 .

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

20 The Role of Ionic Product Desorption Rates in Photoassisted Electrochemical Reactions HAROLD E. H A G E R

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Department of Chemical Engineering, University of Washington, Seattle, WA

98195

A rapidly growing interest in the photoelectrolysis of water has been evident in recent published research. The goal of this work is the development of a system which can directly convert solar energy to a storable chemical fuel (H2) by driving an endoergic reaction (H2O -*. H2 + hP2). Practical utilization of photo-assisted electrolysis systems is hampered by poor overall conversion efficiencies. Essentially, this problem results from poor quantum efficiencies at low bias. The transient current response of photo-electrodes to steppedillumination changes suggests itself as a method of mechanisti cally interpreting this quantum efficiency problem. Though such transients have been studied for p-type GaP (1) and a number of η-type transition metal compounds (2, 3, 4,.5, 60, published explanations of the observed behavior are not well developed. Below stepped-illumination experiments are presented for the photo-assisted electrolysis of water using η-type TiC or SnC photoanode/dark Pt cathode systems. An analysis of these results will be performed, focusing on the influence of the anodic halfcell reaction products upon the electronic state of the semi conductor/electrolyte interface. Experiment The p h o t o - a s s i s t e d e l e c t r o l y s i s current v s . time scans were obtained with the f o l l o w i n g experimental set-up: three e l e c t r o d e c e l l , i n c l u d i n g : a. Pt b l a c k reference e l e c t r o d e b. smooth Pt counter e l e c t r o d e c. η-type TiC or SnC working e l e c t r o d e A 2h i n c h diameter by 1/8" t h i c k quartz p l a t e was attached to the bottom of the c e l l , forming a UV window. A pen-ray quartz UV lamp and power supply were employed f o r the i l l u m i n a t i o n source. The lamp was housed i n a minibox f i x e d with a s h u t t e r (ILEX No. 3 Universal). The p o t e n t i a l d i f f e r e n c e between working and reference 0097-6156/81/0146-0307$07.50/0 © 1981 American Chemical Society In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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308

PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE INTERFACES

e l e c t r o d e s was c o n t r o l l e d v i a a p o n t i o s t a t (PAR Model 173) employed i n conjunction with a u n i v e r s a l programmer (PAR Model 175). E l e c trochemical current was determined by a m p l i f i c a t i o n ( K e i t h l y Model 160B d i g i t a l multimeter) of the instantaneous voltage drop across a reference r e s i s t o r . The multimeter output went to a d i f f e r e n t i a l a m p l i f i e r module (Tektronix 5A20N), employed as the y-input for the current vs. time o s c i l l o s c o p e scans (Tektronix 5103N). The h o r i z o n t a l r a t e of sweep was c o n t r o l l e d by a time base/amp module (Tektronic 5B10N). The b i a s between the working and reference e l e c t r o d e was monitored by a second multimeter, or by d i r e c t scope measurement using a d i f f e r e n t i a l a m p l i f i e r module. The e l e c t r o l y t e s o l u t i o n s were made up from de-ionized water which was obtained by passing d i s t i l l e d water through an ion ex changer (Barnstead p u r i f i e r with #DO809 c a r t r i d g e ) . A l l a c i d s used were reagent grade. Two methods of preparing T1O2 were employed. Technique I u t i l i z e d the flame d e p o s i t i o n of small T1O2 p a r t i c l e s , producing a uniform T1O2 f i l m on a T i substrate. The Ti02 was formed by passing T1CI4 * 2 stream through the flame of a hydrogen/ oxygen t o r c h . A l t e r n a t i v e l y , T1O2 coated p l a t e s were produced by b r i n g i n g a sheet of T i f o i l i n t o d i r e c t contact with a bunsen flame (7). T h i s i s designated technique I I . Following d e p o s i t i o n , the Ti02 coated p l a t e s were subjected to a r e d u c t i o n treatment under H2 at 600°C f o r 2 hours. A f t e r v e r i f i c a t i o n of a good e l e c t r i c a l contact at the back surface of the T i sheet, a copper p l a t e with an e l e c t r o d e lead was attached to the e l e c t r o d e back with s i l v e r p a i n t . The e l e c t r o d e back and edges were covered with s i l i c o n e rubber adhesive (GE RTV 108). A chemical vapor d e p o s i t i o n method was employed f o r the Sn02 electrode preparation. A s o l u t i o n of 5 weight % SnCl i n ethanol was sprayed on a g l a s s p l a t e held at 450°C. The e l e c t r o d e contact was obtained by coating with s i l v e r paint along one edge of the Sn02 f i l m and then p u t t i n g down a s t r i p of copper f o i l of equal width. The contact was i n s u l a t e d by applying a coating of epoxy (Devcon 5-minute). n

a n

Experimental Results Two types of experimental current scans were performed. After i n t r o d u c i n g the photoanode i n t o s o l u t i o n , successive current v s . p o t e n t i a l ( i vs. φ) scans were performed at slow sweep r a t e s (2mV/S). A l l of the photoanode m a t e r i a l s studied e x h i b i t e d an i n i t i a l d r i f t of the i vs. φ scans to more anodic b i a s . This occurred f o r both uninterrupted scans as w e l l as i n t e r m i t t e n t scanning. The o r i g i n of t h i s aging phenomena i s not understood. However, i t was observed that a f t e r approximately t h i r t y minutes the i vs. φ scans achieved a p r o f i l e which was reproduced during f u r t h e r c y c l i c sweeps. This r e s u l t was taken as a c r i t e r i o n f o r proper e l e c t r o d e p r e - c o n d i t i o n i n g .

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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20.

HAGER

309

Ionic Product Desorption Rates

Our emphasis i n t h i s paper w i l l be to d e s c r i b e and analyze current vs. time scans determined a f t e r making stepped-changes i n the i l l u m i n a t i o n i n t e n s i t y . These scans were performed a f t e r proper photoanode p r e - c o n d i t i o n i n g was i n d i c a t e d . T y p i c a l experimental r e s u l t s a r e shown i n Figure 1 f o r Ti02 e l e c t r o d e s made by technique I . Figure 2 presents the r e s u l t s for a s i m i l a r study with an Sn02 photoanode. These scans were made i n 1.0N H2SO4. Current vs. time p r o f i l e s obtained with technique I T1O2 photoanodes i n e l e c t r o l y t e made up by i n t r o d u c i n g v a r i o u s a d d i t i o n s of HC1 t o 120 ml of 1.0N H2SO4 e x h i b i t e d g e n e r a l l y s i m i l a r behavior (Figure 3 ) . Overshoot of the steady s t a t e l e v e l i s an i n t r i g u i n g f e a t u r e of many o f the t r a n s i e n t current responses. This behavior occurs f o l l o w i n g both opening and c l o s i n g the l i g h t b o x s h u t t e r . The f o l l o w i n g observations c h a r a c t e r i z e t h i s "overshoot phenomena: 1. The magnitude of the overshoot current, r e l a t i v e to the d i f f e r e n c e between the photo-assisted and dark current steady s t a t e l e v e l s , decreases with i n c r e a s i n g b i a s . 2. The timescale f o r decay of the overshoot current i s de pendent upon the i l l u m i n a t i o n i n t e n s i t y , ψ, the e l e c t r o l y t e , and bias: (a) the decay time decreases with i n c r e a s i n g φ. (b) For s o l u t i o n s c o n t a i n i n g H2SO4 and no HC1 the decay time decreases with i n c r e a s i n g b i a s . (The decay time f o r s o l u t i o n s c o n t a i n i n g both H2SO4 and HC1 pass through a minimum with i n c r e a s i n g b i a s . ) (c) At constant φ and b i a s , the decay time increases with i n c r e a s i n g HC1 a d d i t i o n . 11

Photoanode Response under Stepped-Illumination. In the f o l lowing paragraphs, arguments are developed which a s c r i b e the ex perimental observations to i o n i c adsorption at the photoanode/ e l e c t r o l y t e i n t e r f a c e . The form of a n a l y s i s was chosen to c l e a r l y demonstrate the r o l e of i o n i c products upon the a s s o c i a t e d h a l f c e l l r e a c t i o n charge t r a n s f e r processes. The anodic h a l f - c e l l r e a c t i o n o c c u r r i n g at the photoanode/ e l e c t r o l y t e i n t e r f a c e may be w r i t t e n : 2H 0 + 4 h * 4 H + 0 +

[1]

+

2

2

Occurrence of the photo-assisted e l e c t r o l y s i s r e a c t i o n , from l e f t to r i g h t , produces an increased H concentration at the photoanode surface. We have p r e v i o u s l y analyzed the photo-assisted c o r r o s i o n of η-type m a t e r i a l s during photoanode a p p l i c a t i o n s (8). This work suggested that the H ions produced during the e l e c t r o l y s i s r e a c t i o n underwent desorption from the photoanode surface much l e s s r e a d i l y than would be suspected f o r f u l l y hydrated H i o n s , and i s summarized i n Appendix I. The f o l l o w i n g Y& i o n adsorption balance i s proposed: +

+

+

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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Figure 1. Oscilloscope traces of current vs. time response following opening and closing of the lightbox shutter, Type I Ti0 photoanode, l.ON H SO ((a)O.4V; (b)O.5V; (c)O.6V; (d)O.7V) 2

2

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

h

31

Ionic Product Desorption Rates

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HAGER

Figure 2. Oscilloscope traces of current vs. time response following opening and closing of the tlightbox shutter, Sn0 photoanode, l.ON H SO ((top)O.0V; (middle,) O.2 V; fbottomj O.4 V) 2

2

Jf

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

312

PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE

INTERFACES

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ΔΦ=O.4ν

Figure 3. Oscilloscope traces of current vs. time response following opening and closing of the lightbox shutter, Type I Ti0 photoanode, 120 mL 7.0N H SO, + 75 mL 1.0N HCl (top) O.4 V; (middle; O.5 V; (bottom; O.6 V) 2

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

2

t

20.

H

313

Ionic Product Desorption Rates

HAGER

+

accumulation

= H

+

1

generation - IT" desorption

S

[2]

s

where Vjjf = H generation r a t e = photo-assisted e l e c t r o l y s i s r e a c t i o n r a t e and C g i s the surface concentration a t t = 0 f o r a photoelectrode i n i t i a l l y under dark steady s t a t e c o n d i t i o n s . Solving eqn. [ 3 ] with the i n i t i a l c o n d i t i o n +

+

C+ Downloaded by 148.204.30.87 on October 21, 2009 | http://pubs.acs.org Publication Date: March 2, 1981 | doi: 10.1021/bk-1981-0146.ch020

R

=0°

[4]

at t = 0

we o b t a i n

C

=

C

%

+

i

(

1

~

e

x

p

~

[

k

d

e

s

t

]

)

[

5

]

or

des 2 where Γ i s the number o f s i t e s / c m . Consider the e l e c t r o n i c energy diagram f o r an η-type semicon ductor, as shown i n Figure 4. The t o t a l p o t e n t i a l energy i n the semiconductor conduction band a t x, r e l a t i v e t o the corresponding band energy deep i n the bulk may be expressed a s :

Tot where the f i r s t term on the r i g h t represents the conduction band energy a t x, r e l a t i v e to the bulk l e v e l , as given from the s o l u t i o n of P o i s s o n s equation. The second term on the r i g h t accounts for the image p o t e n t i a l energy lowering. T h i s c o n t r i b u t i o n a r i s e s from the i n t e r a c t i o n between the e l e c t r o n , at p o s i t i o n x, and i t s a s s o c i a t e d image charge, induced at -x, and i s a p p l i c a b l e only f o r very small r a t i o s of semiconductor dopant concentrations to e l e c t r o l y t e concentrations, such that H i s very much l a r g e r than the e l e c t r i c a l double l a y e r thickness i n s o l u t i o n , d . In p r a c t i c e , t h i s l i m i t a t i o n i s not s e r i o u s l y r e s t r i c t i v e . Moderate e l e c t r o l y t e concentrations (t O.5N) are r e q u i r e d by s o l u t i o n IR drop concerns, y i e l d i n g d on the order o f a few ang stroms. Optimal semiconductor doping l e v e l s are d i c t a t e d by de v e l o p i n g H with the approximate value of a " l , where a i s the semi c o n d u c t o r a b s o r p t i o n c o e f f i c i e n t , and H i s t h e d e p l e t i o n width. As a c o n s e q u e n c e , a t y p i c a l o r d e r o f mag n i t u d e f o r H i s 1 m i c r o n (10 Â ) . Thus t h e r a t i o H / d 1

g

s

g

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE

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314

Figure 4.

INTERFACES

Energy diagram of a Schottky barrier formed at an η-type semiconduc tor-electrolyte interface

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

20.

Ionic Product Desorption Rates

HAGER

315

has the order of magnitude 1 0 , and the charge d e n s i t y d i s t r i b u t i o n a t the semiconductor/electrolyte i n t e r f a c e c l o s e l y resembles that f o r the semiconductor/metal i n t e r f a c e of a Schottky b a r r i e r . F i n a l l y , the term Φβ(χ) represents the p o t e n t i a l energy of i n t e r a c t i o n between the e l e c t r o n at χ and the net charge adsorbed at the e l e c t r o d e / e l e c t r o l y t e i n t e r f a c e . In the f o l l o w i n g t r e a t ment we w i l l assume that under the i l l u m i n a t i o n c o n d i t i o n s employed i n the experiments reviewed above, only the i n t e r f a c i a l H concentration i s a p p r e c i a b l y a l t e r e d by changes i n the i l l u m i n a t i o n l e v e l , ψ. Thus we n e g l e c t the photo-adsorption/desorption of other i o n i c s p e c i e s . Appendix I I d e l i n e a t e s our approach f o r determining the ad s o r p t i o n p o t e n t i a l energy, Φ . In essence, t h i s was accomplished by breaking Φ i n t o two terms, with one accounting f o r d i r e c t i n t e r a c t i o n with the c e n t r a l H i o n . The second term accounts f o r d i r e c t i n t e r a c t i o n between the e l e c t r o n and successive nearest neighbor H i o n s . The f i n a l expression f o r the adsorption poten t i a l energy a t the conduction band energy maximum, χ = ϋ i s given by:

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β

+

9

Φ (« Β

where

4

-=2L

= = = = =

2 θ

Η+

y -1 L n=l n

(w jf + £)ε ε ε' w τ\-

«

£ r

H

+

-1 1.57 - tan

£ + w —, ε

[8]

nr.

semiconductor d i e l e c t r i c constant s o l u t i o n d i e l e c t r i c constant l o c a t i o n of adsorbed H i o n radius f r a c t i o n a l H i o n coverage +

+

The d i s c r e t e charge model o f equation [8] i s suggested by combined capacitance-photocurrent v s . b i a s s t u d i e s (9, 10). These r e s u l t s i n d i c a t e that the adsorbed charge i s l o c a l i z e d , y i e l d i n g a l o c a l p o t e n t i a l d i s t r i b u t i o n which can not be simply described by an averaged charge density approach, such as i s obtained from s o l u t i o n of P o i s s o n s equation. The value of I i s determined from the c o n d i t i o n : 1

άΦ. ΡΕ, Tot = 0 dx Using r e l a t i o n s

[7] and [8] i n eqn. [9] we o b t a i n :

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

[9]

PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE INTERFACES

316

2

2 % 4q 6 +

£ + w

» l {τΓ n=l

R

1

ετ. H+

(nr/eT)

1

1+

— ε

-1 2qN

r

-(w - A) - 0

JJ

[10]

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E v a l u a t i o n of Photocurrent E l e c t r o c h e m i c a l processes which pass through a surface i n t e r mediate species w i l l e x h i b i t a pseudo-capacitive current response (11). T h i s form of behavior i s observable i n Figure 1 ( d ) . Indeed, at high b i a s (Figure 1(d)) the t r a n s i e n t current response appears to be s o l e l y determined by t h i s pseudo-capacitance, f o r the capacitance values determined from these scans (M.0"" F) are two orders of magnitude l a r g e r than t y p i c a l semiconductor space charge capacitances (12). These observations, and a p p l i c a t i o n of K i r c h o f f ' s law, sug gest that the time dependent e l e c t r o c h e m i c a l current d e n s i t y , J e ( t ) , may be w r i t t e n as the product of the f r a c t i o n a l interme d i a t e surface coverage, Θ. ( t ) , and the d i f f e r e n c e between the photo-generation and recombination current d e n s i t i e s , J ( t ) and J ( t ) , respectively. Thus: t

r

je(t) = e .

n t

(t)[J (t)

J (t)]

L

[11]

r

where J" (t) i s the t o t a l e l e c t r o n / h o l e p a i r generation current f l u x minus the bulk recombination c u r r e n t , and J ( t ) i s the i n t e r f a c i a l recombination current f l u x , normalized per u n i t area of intermediate coverage. The recombination current d e n s i t y , J , can be t r e a t e d e f f e c t i v e l y as a Schottky b a r r i e r diode current d e n s i t y . I n c l u d i n g both thermionic emission and d i f f u s i o n charge t r a n s p o r t mechanisms (13) J can be w r i t t e n as L

r

r

r

(E -E ) C

qNv Dr J

r

exp

"

kT

qv

F

exp

app kT

[12]

1+-

where i s the recombination v e l o c i t y . The maximum p o t e n t i a l d i f f e r e n c e between the conduction band i n the space-charge r e g i o n and the bulk conduction band i s expressed as V 2 , and ( E Q - E ) i s the energy d i f f e r e n c e between the conduction and Fermi l e v e l s i n the bulk (see Figure 4). The a p p l i e d v o l t a g e i s designated by V p. F i n a l l y , v i s the e f f e c t i v e d i f f u s i o n v e l o c i t y a s s o c i a t e d witn e l e c t r o n t r a n s p o r t from the edge of the d e p l e t i o n r e g i o n to the top of the conduction band b a r r i e r , as d e f i n e d (13) by f

a

D

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

20.

317

Ionic Product Desorption Rates

HAGER

-1 Φ P E

(χ) /kT dx \ Tot

where μ i s the e l e c t r o n m o b i l i t y . In summary, we have developed eqns. [8], [10], and [13] f o r the e v a l u a t i o n o f Φ ( & ) , and v , r e s p e c t i v e l y . Two computer programs f o r determination o f £, v , and Φ β ( ) are l i s t e d i n Appendices I I and I I I o f reference [ 9 ] . The former accounts f o r adsorption p o t e n t i a l energy e f f e c t s . The l a t t e r n e g l e c t s these c o n t r i b u t i o n s . Table I presents the r e s u l t s of such computations f o r a broad range of semiconductor m a t e r i a l parameters. The system parameters v f and k g g are taken from Figures 1(a), (b), and (c) correspon ding to a p p l i e d p o t e n t i a l s of O.4, O.5, and O.6V, r e s p e c t i v e l y . The p o t e n t i a l energy b a r r i e r i s lowered by accounting f o r the adsorption p o t e n t i a l energy c o n t r i b u t i o n . Note that though the absolute magnitude of the b a r r i e r lowering i n c r e a s e s with i n c r e a s i n g a p p l i e d p o t e n t i a l , the r e l a t i v e b a r r i e r lowering, (ΔΡΕ)/ΡΕ , decreases with higher b i a s (Table I I ) . The l a t t e r r e s u l t i s i n q u a l i t a t i v e agreement with our experimental observa t i o n 1. Transient Response. The current a n a l y s i s performed above demonstrates that H" i o n adsorption at the photoanode/electrolyte i n t e r f a c e decreases the e l e c t r o n i c energy b a r r i e r f o r e l e c t r o n t r a n s f e r from the bulk conduction band t o the e l e c t r o d e s u r f a c e . We explore below the hypothesis that the observed overshoot c u r rent vs. time behavior a r i s e s from t h i s e f f e c t . In essence, the time dependent change i n produces an a s s o c i a t e d change i n the b a r r i e r height f o r e l e c t r o n t r a n s f e r t o the semiconductor s u r f a c e , a l t e r i n g the surface e l e c t r o n / h o l e recombination r a t e . Determination of 6 i ( t ) i s made with the f o l l o w i n g assump tions: β

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[13]

D

0

1-

n t

e

r a t e of intermediate formation = K [ 1 - i f

r a t e of intermediate d e s t r u c t i o n = A t r a n s i e n t balance

(Appendix

i n t

n t

(t)]

(t)

[14i] [14ii]

I I I ) on the intermediate y i e l d s : form d i s

1 - e" =

Γ

[

1 + Κ,. /K dis form

1

5

]

f

The surface recombination current d e n s i t y , J , has the form: r

J

-Φ/kT - e B

r

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

[16]

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

50

1 7

1 8

1 6

1 7

1 8

io

io

io

io

io

X

X

X

X

X

5.0

5.0

5.0

5.0

5.0

10

10

10

1 6

1 7

1 8

5.0 χ I O

5.0 χ 1 0

DC

100

100

100

50

5.0 χ I O

RN(l/cm )

3

50

1 6

io

X

5.0

10

io

10

1 8

io

X

X

5.0

1 7

10

DC

5.0

1 6

io

X

5.0

3

RN(l/cm )

TABLE I .

4.4

Q

X

X

X

X

X

X

X

X

io"

io"

io-

1θ"

7

io"

2.5 χ I O "

4.1 χ I O "

7.1 χ 1 0 ~

7

7

7

7

7

7

7

io"

io-

IO"

7

io"

£ (cm)

1.3

2.3

4.1

1.6

2.8

4.9

2.7

X

(cm)

7.6

Q

7

7

7

Q

o

X

X

X

X

X

X

X

X

X

io

io

io

3

3

1 0

6.8 χ 1 0

1.9 χ I O

3.8 χ

Q

.331

.361

.378

.285

.335

.363

.036

.187

.278

PE (EV)

4

. 1 0 9

.274

.371

Q

PE (EV)

Δφ = O.5V

3

3

2

io

3

2

4

3

3

4

io

io

io

io

10

VT> (cm/s)

6.9

2.1

6.5

1.0

3.1

9.4

3.6

9.6

2.4

VD (cm/s)

Δφ = O.4V

X

X

X

X

X

X

X

io"

7

6

6

1.2 χ 1 θ "

1.9 χ 1 θ "

7

7

7

io~

io-

HT

IO"

7

io"

io"

£(cm)

3.1

5.5

9.8

4.0

6.8

1.2

1.9

£(cm)

6

6

X

X

X

X

X

X

X

io

io

io

io

io

io

io

1.0 χ

2.4 χ

3

3

2

4

3

2

3

1 0

I O

VD(cm/s)

7.7

2.2

6.7

1.3

3.4

9.8

2.9

VD(cm/s)

4

3

(EV)

. 0 1 3

. 2 2 1

P E (EV)

.250

.315

.352

.151

.257

.319

.139

PE

The P o s i t i o n o f Maximum P o t e n t i a l Energy, £, the Maximum P o t e n t i a l Energy B a r r i e r , PE, and the E l e c t r o n D i f f u s i o n V e l o c i t y , VD. Subscript ο i n d i c a t e s Adsorption has been Neglected.

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In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

10

10

50

50

50

1 7

1 8

1 6

1 7

1 8

10

10

10

10

io

io

10

10

X

X

X

X

X

X

X

X

5.0

5.0

5.0

5.0

5.0

5.0

5.0

5.0

100

100

1 7

1 8

100

10

1 6

10

X

5.0

1 6

DC

RN(l/cm )

3

5.0 χ 10

5.0 χ 1 0

ΙΟ" ΙΟ" ioΙΟ"

X

X

X

X

io" 10"

X

X

1.2

ίο"

ΙΟ"

X

X

ίο"

X

2.1

3.7

1.4

2.5

4.4

2.3

3.9

6.7

o

* (cm)

7

7

7

7

7

7

7

7

7

7

4.7

1.4

4.5

7.0

2.1

6.5

2.3

6.5

1.6

X

X

X

X

X

X

X

X

X

10

10

io

10

10

io

10

10

10

3

3

2

3

3

2

4

3

3

VD (cm/s) Q

3

3

2

Q

.523

.557

.576

.472

.528

.559

.188

.362

.465

PE

(EV)

.427

.459

.477

.378

.431

3

3

(EV)

.461

0

2

PE

Δφ = O.6V

5.3 χ 1 0

1.7 χ 1 0

3.9 χ 1 0 "

2.2 χ 1 0 "

100

100

1 7

5.0 χ 1 0

1.2 χ 1 0 ~

7

1.5 χ 1 0 "

50

1 6

5.0 χ 1 0

100

7

2.6 χ 1 0 ~

50

1 8

5.0 χ 1 0

Q

5.4 χ 1 0

7

1 7

"I

8.8 χ 1 0

7

7

2.5 χ 1 0

4.6 χ ÎO"* 7.7 χ 1 0

VD (cm/s) ο

50

0

* (cm)

1 6

DC

Δφ = O.5V

5.0 χ 1 0

RN(l/cm )

3

TABLE I (Cont.)

5.5 χ 1 0 1.8 χ 1 0

7

9.6 χ 1 0 ~

3.0

5.4

9.6

3.7

6.6

1.2

1.1

1.9

X

X

X

X

X

X

X

X

io"

10~

io-

ΙΟ"

ΙΟ"

ίο"

ίο"

io-

£(cm)

7

7

7

7

7

6

6

6

3.0 χ 1 0 ~

5.4 χ 1 0 "

5.3

1.5

4.6

8.7

2.3

6.8

9.2

1.9

X

X

X

X

X

X

X

X

io

io

io

io

io

io

io

io

VD(cm/s)

3

2

4

3

2

3

3

3

2

3

3

2

3

3

6.2 χ 1 0

1.0 χ 1 0

w ?

7

6.6 χ 1 0 "

7

2.8 χ 1 0

7

3.8 χ 1 0

8.2 χ 1 0

6

VD(cm/s)

1.2 χ 1 0 "

l (cm)

Downloaded by 148.204.30.87 on October 21, 2009 | http://pubs.acs.org Publication Date: March 2, 1981 | doi: 10.1021/bk-1981-0146.ch020

.432

.504

.546

.318

.440

.510

.077

.304

PE (EV)

.341

.410

.449

.234

.348

.414

PE (EV)

Downloaded by 148.204.30.87 on October 21, 2009 | http://pubs.acs.org Publication Date: March 2, 1981 | doi: 10.1021/bk-1981-0146.ch020

320

PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE

TABLE I I .

INTERFACES

R e l a t i v e B a r r i e r Lowering f o r the Conditions of Table I , with RN = 5.0 χ 1 0 / c m . 16

3

ε

bias

Δ PE

Δ PE bias

Δ PE PE ο

10

O.4

.139

.348

.50

0,5

.15

.300

.40

O.6

.161

.268

.35

O.4

.044

.11

.121

O.5

.047

.094

.102

O.6

.049

.082

.088

O.4

.026

.065

.069

O.5

.028

.056

.059

O.6

.030

.050

.052

50

100

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

20.

321

Ionic Product Desorption Rates

HAGER

Using eqn. [ 8 J , and the approximation t a n χ = D/.zy χ t o r χ « we can w r i t e : be +/kT J « e [17 H

r

f o r 6jjf << 1. Here b i s a constant. Under the c o n d i t i o n s o f the experiments reviewed above, b i of order leV. At room temperature kT = 0.0256. Thus f o r 6fl+ « 0.0256 be^/kT

be

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8

and eqn.

1

R

+

" k T -

+

[17] may be r e w r i t t e n a s : (J

- J

r

) -

e„+

ο

[18]

"

where J i s the value o f J i n the l i m i t 6^+ 0. L e t us assume that Θ + (t=0) = 0. Noting that J i s independent o f t , a p p l i c a t i o n of eqn. [6] g i v e s : r

o

r

Η

r

V >

J

" r

s

s

-

A

J

r

s

s

Q

e

_

k

d

e

s

t

[19]

where A J i s the d i f f e r e n c e between the steady s t a t e recombina tion c u r r l S t density, J , and the recombination r a t e a t zero coverage, J . S u b s t i t u t i n g r e l a t i o n s [15] and [19] i n eqn. [11], r

r

s

r

J (t

.>

= l^lH

where Κ = K K» = K

and

s

Q

J

+ K

f o r m

d i s

LL

/K

J

" < r

.

( t

>

j

- w J v

)J

" AJ e r

[20]

d l s

f o r f f i

C o l l e c t i n g terms i n eqn.

J

s s

r

+ s

s

e

~ ^ r

[20] we o b t a i n :

s

s

- J

L

^

W

or

ν

Je(t)

J

where δ = àJ /J r

r

L

-

-Kt/- ^ δ

" des

and J -J-r L ss

= 3.

T

ss

L

k

t

>i

,

δ

^desM

L

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

r991

322

PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE INTERFACES

Figure 5 shows computer generated p l o t s of — β

y

/(1+K )

V S #

t f o r the b i a s c o n d i t i o n s of Figures 1 ( a ) , ( b ) , and (c) respectively. The parameter k was taken from the appropriate f i g u r e , l e a v i n g Κ and the r a t i o 6/3 as two unknown v a r i a b l e s . Values f o r these parameters were chosen to give the best agreement between the form of the computed scan and the a s s o c i a t e d experimental p l o t . Figure 5 demonstrates that the shape of the t r a n s i e n t current response f o l l o w i n g opening of the shutter can be reasonably r e p r o duced by the model developed above, except f o r the very short time l i m i t . T h i s discrepancy may a r i s e from f a i l u r e to account f o r capacitance e f f e c t s i n the current monitoring c i r c u i t . Further e f f o r t s to analyze the experimental steady s t a t e J l e v e l s confront a s e r i e s of problems. E s s e n t i a l l y , these a r i s e from the need to a s s i g n values to a number of semiconductor/ s o l u t i o n p r o p e r t i e s (e.g., μ, ε, ε , and 0+) with i n s u f f i c i e n t a v a i l a b l e information to make these assignments. Methods of ex p e r i m e n t a l l y determining these unknown parameters are being ex plored. d e s

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e

1

R e l a t i o n to Other Work Bokris and Uosaki ÇL) have studied t r a n s i e n t photo-assisted e l e c t r o l y s i s current f o r systems i n c l u d i n g a p-type semiconductor photocathode and dark Pt anode. A set of current v s . time scans taken with a ZnTe photocathode system i s shown i n Figure 6. The r e s u l t s of Figure 6 e x h i b i t the same general behavior as our experimental scans reported above: the magnitude of the overshoot current r e l a t i v e to the d i f f e r e n c e between the steady s t a t e dark and i l l u m i n a t e d current l e v e l s decreases with i n c r e a s i n g b i a s (or decreasing photocathode p o t e n t i a l ) . We suggest that these r e s u l t s a r i s e from adsorption e f f e c t s which are the cathodic complements to the anodic phenomena outl i n e d above. The e l e c t r o l y s i s h a l f - c e l l r e a c t i o n at the photocathode : 2H 0 + 2e~ * H 2

2

+ 20H~

[23]

increases the l o c a l OH"" concentration at the photocathode/electrol y t e i n t e r f a c e , j u s t as the anodic h a l f - c e l l r e a c t i o n produces an i n c r e a s e i n the l o c a l H concentration at the photoanode/electrolyte interface. As recombination current i n a photocathode was t r e a t e d as an e l e c t r o n current to the photoanode s u r f a c e , the photocathode r e combination current may be viewed as a hole c u r r e n t . Correspond i n g l y , 0H~ ions at the photocathode/electrolyte i n t e r f a c e lower the energy b a r r i e r f o r hole transport to the photocathode s u r f a c e . Figure 7 d i s p l a y s computer generated p l o t s of J ( t ) / $ ( 1 + K ) vs. t f o r comparison with the t r a n s i e n t current responses reported +

f

e

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

20.

HAGER

Ionic Product Desorption Rates

323

by Bockris and Uosaki (1). The normalized current expression employed i s the cathodic equivalent to the anodic case o u t l i n e d above. Summary

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We have performed an experimental study of photo-assisted e l e c t r o l y s i s f o r i l l u m i n a t e d η-type Ti02 photοanode/dark Pt cathode systems. A n a l y s i s o f these r e s u l t s i n d i c a t e s that the e l e c t r o n i c s t a t e o f the semiconductor/electrolyte i n t e r f a c e i s i n f l u e n c e d by the e l e c t r o l y s i s r e a c t i o n products, i n a manner not p r e v i o u s l y accounted f o r . S p e c i f i c a l l y , the o x i d a t i o n h a l f - c e l l r e a c t i o n : +

2H 0 + 4 h * 0 2

2

+ 4H

+

a l t e r s the l o c a l H* i o n concentration a t the photoanode/electrol y t e i n t e r f a c e . A g e n e r a l i z e d model has been presented demonstrat ing that the i n t e r f a c i a l i o n concentration s t r o n g l y a f f e c t s t r a n s port processes a s s o c i a t e d with the space charge recombination c u r r e n t . Our r e s u l t s show that the IT*" i o n coverage decreases the energy b a r r i e r f o r e l e c t r o n transport to the photoanode s u r f a c e , i n c r e a s i n g the recombination c u r r e n t . A review o f photo-assisted e l e c t r o l y s i s s t u d i e s performed with ρ-type semiconductor photocathode/dark Pt anode systems sug gests that a complementary phenomena a r i s i n g from the presence of 0H~ ions produced during the r e d u c t i o n h a l f - c e l l r e a c t i o n , 2H 0 + 2e~ £ H 2

2

+ 20H"

augments the hole current to the e l e c t r o d e / e l e c t r o l y t e i n t e r f a c e , again i n c r e a s i n g the surface recombination c u r r e n t . These conclusions a r e s i g n i f i c a n t and of p r a c t i c a l concern. Ionic products formed during both the anodic and cathodic e l e c t r o l y s i s h a l f - c e l l r e a c t i o n s a l t e r the l o c a l net charge at the e l e c t r o d e / e l e c t r o l y t e i n t e r f a c e so as to increase the recombina t i o n c u r r e n t s . Hence f o r both photo-assisted h a l f - c e l l r e a c t i o n s the e l e c t r o c h e m i c a l current i s reduced by t h i s phenomena. The r e l a t i v e s i z e o f t h i s e f f e c t i s l a r g e r a t lower b i a s , i n d i c a t i n g that t h i s phenomena presents a serious dilemma to workers designing photo-assisted conversion systems with high o v e r a l l energy con version e f f i c i e n c i e s . Acknowledgement s The support provided by the Department of Energy and the N a t i o n a l Science Foundation i s g r a t e f u l l y acknowledged.

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

324

PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE

INTERFACES

Δ Φ =O.40 V

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a

0

TIME (sec)

(xO.I) 18

LJ

or

0

1

2

3 TIME (sec)

4

5 (xl)

Figure 5. Comparison between the experimental photocurrent vs. time profile and Equation 22: (a) O.4 V; (b—facing page,) O.5 V; (c—facing pagej O.6 V

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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HAGER

Ionic Product Desorption Rates

Δ φ =O.60V C

TIME (sec)

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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326

PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE

INTERFACES

TIME (sec)

TIME (sec)

Journal of the Electrochemical Society Figure 6. Traces of the current vs. time response following opening and closing of the lightbox shutter, ZnTe photocathode, 1.0N NaOH (1): (a) -O.95 V (NHE); (b) -1.15 V (NHE); (c) -1.36 V (NHE)

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

20.

HAGER

327

Ionic Product Desorption Rates

Appendix I Consider a photo-assisted water e l e c t r o l y s i s c e l l , i n c o r p o r a t i n g a photoanode and dark metal cathode. I l l u m i n a t i o n of the η-type semiconductor photoanode with a d e p l e t i o n space charge r e g i o n r e s u l t s i n a net flow o f p o s i t i v e vacancies, o r h o l e s , t o the s e m i c o n d u c t o r / e l e c t r o l y t e i n t e r f a c e . Here the hole ( h ) may be accepted by the reduced form o f the oxygen redox couple. A l t e r n a t i v e l y , redox l e v e l s of the semiconductor m a t e r i a l may l i e a t energies such that the hole, d i r e c t l y o r through a m u l t i step process described below, accepts an e l e c t r o n from a semicon ductor surface atom, producing concomittant surface o x i d a t i o n . A f u l l d e s c r i p t i o n o f photo-assisted semiconductor c o r r o s i o n i s not a v a i l a b l e . We have undertaken an a n a l y s i s of t h i s pheno mena, r e l y i n g on aqueous s o l u t i o n e l e c t r o c h e m i c a l e q u i l i b r i a con s i d e r a t i o n s (16). It has been experimentally observed that GaP i s s t a b l e i n the dark i n weakly a c i d i c s o l u t i o n s (15). T h i s s t a b i l i t y has been a s c r i b e d (15) to the formation o f Ga2Û3 a t the semiconductor s u r f a c e , with t h i s oxide f i l m e f f e c t i v e l y p a s s i v a t i n g the s u r f a c e . E l e c t r o c h e m i c a l e q u i l i b r i a a n a l y s i s (16) i n d i c a t e s that Ga2Û3 i s s t a b l e i n aqueous e l e c t r o l y t e s with 3.0 < pH < 11.2 up t o p o t e n t i a l s a p p r e c i a b l y more anodic than the p o t e n t i a l a s s o c i a t e d with the o x i d a t i o n o f H2O t o O2 (Figure 8 ) . Yet upon i l l u m i n a t i o n , η-type GaP employed as a photoanode i n an aqueous s o l u t i o n at pH = 4.7 i s unstable (17). Thus, under i l l u m i n a t i o n , semiconductor photoelectrodes may corrode even though the bulk pH i s i n s u f f i c i e n t l y a c i d i c / b a s i c t o d r i v e such a l a t t i c e d i s s o l u t i o n process. We propose that t h i s observed photo-assisted c o r r o s i o n occurs v i a the f o l l o w i n g mechanism: [1] Photo-produced h o l e s , h , a r r i v e a t the e l e c t r o d e surface, o x i d i z i n g H2O to O2:

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+

+

4

2H 0 + 4h ' + 0 2

2

+ 4H

+

+

[2] The production of two H i o n s , f o r every molecule of water o x i d i z e d , promotes a surface H"" coverage which i s substan t i a l l y higher than the dark value. [3] For H+ surface coverages l a r g e r than that obtained i n the dark a t a pH o f 3.0, Ga2Û3 i s no longer s t a b l e , undergoing d i s s o l u t i o n v i a the f o l l o w i n g r e a c t i o n : 1

Ga 0 2

3

+ 6H+ + 2 G a

H +

l

( s o l n ) + 3H 0 2

Indeed, our a n a l y s i s suggests that the ions produced during the e l e c t r o l y s i s r e a c t i o n undergo removal from the photoanode surface region much l e s s r e a d i l y than would be expected f o r simple d i f f u s i o n . A p o s s i b l e , more complete, explanation may be that the H*" i o n i s produced on the surface with an incomplete h y d r a t i o n sheath, f a c i l i t a t i n g a d i r e c t H+ i o n / e l e c t r o d e

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

328

PHOTOEFFECTS

AT SEMICONDUCTOR-ELECTROLYTE

INTERFACES

a ~

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< Ε

O.8 1.2

TIME (sec)

1.6

-O.95 V (NHE)

Figure 7. Comparison between the experimental photocurrent vs. time profile and the cathodic equivalent of Equation 22: (a) —O.95 V (NHE); (b—facing pagej -1.15 V (NHE); fc—facing pagej -1.36 V (NHE)

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

HAGER

Ionic Product Desorption Rates

TIME (sec)

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-1.16 V(NHE)

-31 UJ

0

O.2

O.4

O.6 03 TIME (sec)

1.0

Ό

O.2

O.4

O.6 O.8 TIME (sec)

1.0

c

1.2

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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330

PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE

Figure 8.

INTERFACES

Theoretical conditions of corrosion, immunity, and passivation of gallium, at 25°C., assuming passivation by afilmof a-Ga 0 (16) 2

3

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

20.

331

Ionic Product Desorption Rates

HAGER

i n t e r a c t i o n . Desorption of the Έ& i o n can then occur only by s u f f i c i e n t l y overcoming the electrode/ΗΓ " i o n i n t e r a c t i o n energy to draw the Έ& i o n away from the e l e c t r o d e s u r f a c e . T h i s permits i n s e r t i o n of (a) water molecule(s) between the i o n and e l e c trode, completing the h y d r a t i o n sheath, p r i o r to desorption of the f u l l y hydrated i o n . The oxide redox energy l e v e l s f o r a l l elements except gold are cathodic to the redox l e v e l of the H2O/O2 couple. Gold, however, i s an i m p r a c t i c a l component f o r compound semiconductors. A l l other compound semiconductors employed as e l e c t r o l y s i s photoanodes w i l l undergo s u r f a c e o x i d a t i o n i n aqueous e l e c t r o l y t e s to produce a s u r f a c e oxide f i l m which normally c o n s t i t u t e s the stable surface of the photoelectrode. Our proposed mechanism i n d i c a t e s that the proton induced oxide d i s s o l u t i o n r e a c t i o n a r i s e s from product IT" i o n i n t e r a c t i o n s with the oxide anion ( 0 ) . The arguments j u s t presented suggest that a l l η-type semi conductor photoanodes which r e s i s t c o r r o s i o n under c o n d i t i o n s of Η 0 to O2 o x i d a t i o n must have (a) s t a b l e elemental component(s) at low pH. Moreover, s i n c e the oxide d i s s o l u t i o n r e a c t i o n i s a s s o c i a t e d with e s s e n t i a l l y the same K*~/0 i n t e r a c t i o n s , the low pH s t a b i l i t y l i m i t w i l l be at approximately the same value a s t a b l e * >Pourbaix (16) has prepared t h e o r e t i c a l s t a b i l i t y diagrams of p o t e n t i a l v s . pH f o r many common metals and nonmetalloids. A review of these r e s u l t s i n d i c a t e s that semiconductor compounds of Au, I r , P t , Rd, Ru, Z , S i , Pd, Fe, Sn, W, Ta, Nb, or T i should serve as r e l a t i v e l y a c i d - s t a b l e photoanodes f o r the e l e c t r o l y s i s of water. Indeed, a l l of the s t a b l e p h o t o - a s s i s t e d anode m a t e r i a l s reported i n the l i t e r a t u r e , as of March, 1980 (see Table I I I ) contain at l e a s t one element from t h i s s t a b i l i t y l i s t , with the exception of CdO. Kung and co-workers (18) observed that the CdO photoanode was s t a b l e at a bulk pH of 13.3. The Pourbaix diagram f o r Cd (16) shows that an oxide f i l m p a s s i v a t e s Cd over the con c e n t r a t i o n range 10.0 < pH < 13.5. Hence the d e s o r p t i o n of the product H i o n f o r the p a r t i c u l a r case of CdO must be exception a l l y f a c i l e ; without producing an e f f e c t i v e s u r f a c e pH lower than 10.O. T h i s anamolous behavior f o r CdO i s not w e l l understood.

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1

=

2

=

3

r

+

Appendix I I Determination of Adsorption P o t e n t i a l /direct interaction ι with c e n t r a l H+ ion)

ΦΒ

Φ

Β

-

-Aid

[(w

φ B

Energy

d i r e c t i n t e r a c t i o n with l s u c c e s s i v e nearest neighbors)

7

2

2

2

1

{(w |'- x ) + n R ) } ]

dx

2

χ) ε

2

n=/j +m

2

m=0

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

(1)

332

PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE INTERFACES

TABLE I I I .

Stable Water E l e c t r o l y s i s Photoanodes

Material

τιο

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F e

Band gap energy (eV)

References

* 2

-3.0 2.2

2°3

wo

-2.6**

3

18-23 20,23. 18,20

SrTi0

3

3.2

18,24

BaTi0

3

3.3

18,25

3.4

26.

2.2

27

3.5

28,29

CaTiOj Fe Ti0 2

KTa0

5

3

Hg Ta 0

?

1.8

18

Hg Nb O

y

1.8

18

3.2

29

2.4

18

3.4

30

2

2

2

P b T i

2

W

1.5 0.5°6.5

N b

2°5 YFe0

3

PbFe 0 1 2

9

2.6

31

2.3

18

CdFe 0. 2 4 Zr0

2.3

18

5.0

18

CdO

2.1

18

o

2

Sn0

-3.6***

2

reported Ε

2.9 - 3.2

reported Ε

2.4 - 2.3

reported E

3.5 - 3.3

(

29,32

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

20.

Ionic Product Desorption Rates

HAGER

where ε ε' w R -A

= = = = =

semiconductor d i e l e c t r i c constant s o l u t i o n d i e l e c t r i c constant l o c a t i o n o f adsorbed average separation between adsorbed ions d i s t a n c e t o conduction band energy maximum

I n t e g r a t i n g (1)

Φ

4q

=

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(w|+Jl)

2

00

ε(2 2 2 [Σ {(w —) + n R

r v

r

/

ε' , 2,,- 2w — χ + χ }]

0

e

From r e f . 14, equation 109 / λ. j 2.-1, 2 -l.çx+bx (c+bx+ax) dx = r~ tan ( )

where g = 4ac - b . Thus (2) may be reduced t o :

=

Φ

Β

, ε' (wf+*)ε

2 4£l ε

n=~

2x-2w ( — — )

9

Σ

tan"

/g

1

where 2

2

2

g = 4 [ ( w ) + n R ] - 4(w | )

2

2

= 4n R

2

or / (w Now

-, ι

+£)ε

—

-, -2£-2w —

-

n=l

- tan~l(-°°) » tan

(°°) =

90° 360

c

* 2Π =

1.57

Thus we can w r i t e 2 Φ

= Z S L

Β

, ε' (w+il)e

4q

-1 Σ η " < 1.57+tan ( E R n=l χ

- λ- w { — )

Assuming a square a d s o r p t i o n p a t t e r n R

=

v-v"

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE

334

where r

INTERFACES

= H+ i o n r a d i u s = fractional H

+

i o n coverage

employing these r e l a t i o n s i n (5) we o b t a i n 2

Φ

=

β

4q

φ (w .| +£)ε

ε

γ

ej*f

2

— Η+

-I-

.

Σ

η

=

η"

1

1.57+tan

1

1

w

-

( — ) " f f f f "

(6)

1

Appendix I I I

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Time Dependence o f F r a c t i o n a l Intermediate Surface Coverage J

e

(t)

v

=

J

Θ. intermediate

r a t e o f adsorption = K

(t) * ( J ( t ) - J ( t ) ) L r

v

J

f

o

W

V

W T

v

(1 - e

m

±

n

t

e

m

w

y

)

r a t e o f desorption = Κ,, Θ. d i s interm Balance : Accumulation -

d

=

°interm dt

adsorption - desorption

=

(

form =

*n [Κ, form

I n i t i a l Condition

S

C

=

Κ-

- (K* form

1v

4

u

Θ = 0

at

form -

r

+ K

form ciis

1 - (1 + Κ)Θ. interm.

interm

1- t + c

t = 0

dis

interm,

]

-

t

form K

or

)

7J 4

form*

form d i s

n

+K dis

v

form £n [ K

dis intern.

£

Γ

h

}

interm.

K - (K + K , . ) θ. „. form form d i s interm.

" K+K, form d i s

t

1

=

, form e""

+

K

dis. Γ ;

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

20.

HAGER

Ionic Product Desorption Rates

335

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Abstract A growing awareness that photo-assisted electrochemical systems may provide the best device technology for some direct solar energy conversion applications is manifested by the rapidly expanding literature in this area. Nevertheless, poor device per formance remains a major problem in this research. This is parti cularly true for many of the photogenerative systems which have been studied. The most widely investigated photogenerative reac tion has been the photo-electrolysis of water. Growing evidence suggests that unfavorable photo-assisted water electrolysis cell performances arise, at least in part, from product ion adsorbate effects. The latter appears to contribute both to materials stability and photon utilization efficiency vs. bias problems. This product ion phenomena should also be of importance, in general, for any system incorporating photoelectrodes which utilize depletion space charge fields. "Literature Cited" 1. Bockris, J. O'M.; Uosaki, K. J. Electrochem. Soc., 1977, 124, 1348. 2. Hardee, K. S.; Bard, A. F. J. Electrochem. Soc., 1977, 123, 1025. 3. Mavroides, J. G. "The Electrochemical Society Softbound Sym posium Series", PV 77-3; Princeton, N.J., 1977; p. 84. 4. Pavlov, D.; Zanova, S.; Papazov, G. J. Electrochem. Soc., 1977, 124, 1523. 5. Jeh, L.-C. R.; Hackerman, N. J. Electrochem. Soc., 1977, 124, 833. 6. Curran, J. S.; Gissler, W.; J. Electrochem. Soc., 1979, 126, 56. 7. Fujishima, Α.; Kohayakawa, K.; Honda, K. J. Electrochem. Soc., 1975, 122, 1488. 8. Hager, Η. E.; Ollis, D. F. Abstract 247, The Electrochemical Society Extended Abstracts, Fall Meeting, Pittsburgh, Pennsyl vania, Oct. 15-20, 1978. 9. Hager, H.E.,Ph.D. Thesis, Princeton University, 1979. 10. Wilson, R. H. Personal communication. 11. Bockris, J. O'M. "Modern Electrochemistry" V2; Plenum Press: New York, 1970.

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12. Myamlin, V.; Pleskov, Y. "Electrochemistry of Semiconductors"; Plenum Press: New York, 1967. 13. Crowell, C. R.; Sze, S. M. Solid State Electronics, 1966, 9, 695. 14. Weast, R. C., Ed. "Handbook of Chemistry and Physics"; 54th Edition, CRC Press: Cleveland, Ohio, 1973. 15. Memming, R.; Schwandt, G. Electrochim. Acta, 1968, 13, 1299. 16. Pourbaix, M. "Atlas of Electrochemical Equilibria in Aqueous Solutions"; Pergamon Press: New York, 1966. 17. Nakato, Y.; Abe, K.; Tsubomura, H. Ber. Bunsen-Gesellschaft, 1976, 80, 1002. 18. Kung, H.; Jarrett, H.; Sleight, Α.; Ferretti, A. J. Appl. Phys. 1977, 48, 2463. 19. Mavroides, J.; Tcherner, D.; Kafalos, J.; Kolesar, D. Mat. Res. Bull., 1975, 10, 1023. 20. Hardee, K.; Bard, A. J. Electrochem. Soc., 1977, 124, 215. 21. Tiyishima, Α.; Honda, K. Nature, 1972, 238, 37. 22. Wrighton, M.; Ginley, D.; Wolczanski, P.; Ellis, Α.; Morse, D.; Linz, A. Proc. Nat. Acad. Sci. USA, 1975, 72, 1518. 23. Hardee, K.; Bard, A. J. Electrochem. Soc., 1976, 123, 1024. 24. Wrighton, M.; Ellis, Α.; Wolczanski, P.; Morse, D.; Abrahamson, H.; Ginley, D. J. Am. Chem. Soc., 1976, 98, 2774. 25. Kennedy, J.; Ftese, K. J. Electrochem. Soc., 1976, 123, 1683. 26. Tchernov, D. I. Abstract C3,"Int. Conf. Photochem. Conv. Storage Solar Energy," London, Ontario, 1976. 27. Ginley, D.; Butler, M. J. Appl. Phys., 1977, 48, 2019. 28. Ellis, Α.; Kaiser, S.; Wrighton, M. J. Phys. Chem., 1976, 80, 1325. 29. Bolta, J.; Wrighton, M. J. Phys. Chem., 1976, 80, 2641. 30. Clechet, P.; Martin, J.; Oliver, R.; Vallouy, O. C. R. Acad. Sci., 1976, C282, 887. 31. Butler, M.; Ginley, D.; Eibschutz, M. J. Appl. Phys., 1977, 48, 3070. 32. Kim, H.; Laitenen, H. J. Electrochem. Soc., 1975, 122, 53. RECEIVED

October 3,

1980.

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

21 Carbanion Photooxidation at Semiconductor Surfaces M A R Y E ANNE FOX and ROBERT C. OWEN

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Department of Chemistry, University of Texas at Austin, Austin, T X 78712

Within the last decade, many inventive procedures for the quantum utilization of visible light by organic or inorganic ab sorbers have been developed. The most efficient of these often involve electron exchange reactions. Here, a vexing problem persists: rapid thermal recombination of the electron-hole pairs can regenerate the ground state and effectively waste the energy of the absorbed photon. We have reasoned that this back reaction could be inhibited, or at least dramatically slowed, if an ex cited anion M (eqn 1) were used as the donor rather than a neu tral molecule M (eqn 2). The reversal of equation (1), governed only by the typically low electron affinity of the photoproduced -

radical, should be less favored than the back reaction in equa tion (2), where an electrostatic attraction within the photoproduct pair favors reversion to the ground state. Indeed, in this analysis, recapture of a photo-ejected electron by the oxidized primary photoproduct formed from a dianion should be even less favorable. This approach to the inhibition of electron recap ture is therefore complementary to the use of acceptors which form metastable one electron reduction products upon photoexcita tion of an electron donor. In fact, the most effective acceptors (e.g., methyl viologen) are usually those which operate on this same electrostatic principle, i.e., where electrostatic attraction of the oxidized and reduced primary photoproducts is minimized. In the cases considered here, one might reasonably expect compar able stabilization of the primary photoproducts in equations (1) or (2) by the presence of neutral electron acceptors. Consequent ly, the relative inhibition of back electron transfer in simple processes like equations (1) and (2) should find direct parallel in the presence of neutral electron acceptors.

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Further i n h i b i t i o n would a l s o be a n t i c i p a t e d i f the environment of the e l e c t r o n - h o l e p a i r i n t e r f e r e s with recombination. Separation of such p a i r s has been e f f e c t i v e l y achieved i n the e l e c t r i c f i e l d formed at the i n t e r f a c e between a semiconductor and an e l e c t r o l y t e s o l u t i o n (1). Accordingly, we have examined a s e r i e s of e x c i t e d organic anions, both i n homogeneous s o l u t i o n and at a semiconductor e l e c t r o d e i n a photoelectrochemical c e l l . As expected, redox photochemistry o c c u r r i n g at the semiconductor surface d i f f e r s s i g n i f i c a n t l y from that found under homogeneous cond i t i o n s . We have used t h i s a l t e r e d r e a c t i v i t y as a s y n t h e t i c technique f o r c o n t r o l l e d o x i d a t i v e coupling r e a c t i o n s and as an i n v e s t i g a t i v e t o o l f o r e s t a b l i s h i n g mechanism i n v i s i b l e l i g h t p h o t o l y s i s of h i g h l y a b s o r p t i v e anions. Our experimental procedure p a r a l l e l s that described e a r l i e r i n the c o n s t r u c t i o n of an a n i o n i c photogalvanic c e l l (22. A very t h i n l a y e r (-1 mm) of anhydrous s o l u t i o n containing the absorpt i v e anion i s sandwiched between an o p t i c a l f l a t and a poised semiconductor e l e c t r o d e . Upon i r r a d i a t i o n of t h i s s t i r r e d s o l u t i o n , photocurrents between the i l l u m i n a t e d e l e c t r o d e and a dark platinum counterelectrode can be monitored. (See reference 2 f o r experimental d e t a i l . ) By choosing an appropriate wavelength r e gion from the i r r a d i a t i o n source, we may e x c i t e p r e f e r e n t i a l l y e i t h e r the d i s s o l v e d (or adsorbed) anion or the semiconductor i t self. Redox r e a c t i o n s o c c u r r i n g i n the s t i r r e d s o l u t i o n may be followed i n s i t u by c y c l i c voltammetry or by withdrawing an a l i quot f o r chemical a n a l y s i s by standard spectroscopic and/or chromatographic techniques. P r e p a r a t i v e e l e c t r o l y s e s were conducted i n the same c e l l employing a PAR (Princeton A p p l i e d Research) p o t e n t i o s t a t and d i g i t a l coulometer. S o l u t i o n phase photolyses were conducted by i r r a d i a t i n g sealed, degassed, pyrex ampoules c o n t a i n i n g the r e a c t i v e anions and were analyzed as above. By e x c i t i n g the red-orange c y c l o o c t a t e t r a e n e d i a n i o n 1 i n the presence of c y c l o o c t a t e t r a e n e i n our photoelectrochemical c e l l (n-Ti02/NH3/Pt), we were able to observe photocurrents without d e t e c t a b l e decomposition of the a n i o n i c absorber Ç 2 ) . Presumably, a r a p i d dismutation of the photooxidized product i n h i b i t e d e l e c tron recombination, producing a s t a b l e hydrocarbon whose cathodic reduction at the counter e l e c t r o d e regenerates the o r i g i n a l mixture e s s e n t i a l l y q u a n t i t a t i v e l y (eqn 3 ) .

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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We hope to confirm the p r i n c i p l e o f t h i s mechanism by demonstra t i n g that c h a r a c t e r i z a b l e chemical r e a c t i o n s might ensue i f the i n i t i a l photooxidation ( u n l i k e eqn 3) were i r r e v e r s i b l e . Previous work i n our l a b o r a t o r y (3) and i n others (A) has e s t a b l i s h e d that the primary photoprocess i n a v a r i e t y of e x c i t e d carbanions i n v o l v e s e l e c t r o n e j e c t i o n . T h i s photooxidation w i l l generate a r e a c t i v e free r a d i c a l i f recapture o f the e l e c t r o n i s i n h i b i t e d . P a r a l l e l generation of these same carbon r a d i c a l s by e l e c t r o c h e m i c a l o x i d a t i o n r e v e a l s an i r r e v e r s i b l e anodic wave, c o n s i s t e n t with r a p i d chemical r e a c t i o n by the o x i d i z e d organic species (5). L i t t l e chemical c h a r a c t e r i z a t i o n o f the products has been attempted, however (6). A t y p i c a l c y c l i c voltammetric t r a c e f o r the anodic o x i d a t i o n of the f l u o r e n y l anion 2 at platinum i s shown i n F i g u r e 1. The o x i d a t i o n p o t e n t i a l f o r t h i s and s e v e r a l other resonance s t a b i l i z e d carbanions l i e s conveniently w i t h i n the band gap of n-type T 1 O 2 i n the non-aqueous s o l v e n t s , and hence i n a range s u s c e p t i b l e to photoinduced charge t r a n s f e r . Furthermore, dimeric products (e.g., b i f l u o r e n y l ) can be i s o l a t e d i n good y i e l d (55-80%) a f t e r a one Faraday/mole c o n t r o l l e d p o t e n t i a l (+1.0 eV vs Ag q u a s i reference) o x i d a t i o n at platinum. I f an e l e c t r o n acceptor i s a v a i l a b l e i n homogeneous s o l u t i o n , photochemical r e a c t i o n can be observed. For example, when 2 i s excited (λ 350 nm) i n anhydrous d i m e t h y l s u l f o x i d e (DMSO), methylation occurs, u l t i m a t e l y g i v i n g r i s e to 9,9-dimethylf l u o r e n e i n >80% y i e l d . By analogy with T o l b e r t ' s mechanism f o r photomethylation i n DMSO (4), such a process may be i n i t i a t e d by e l e c t r o n t r a n s f e r to DMSO to form a caged r a d i c a l - r a d i c a l anion pair from which subsequent C-S cleavage occurs (eqn 4 ) .

I f no acceptor i s present, recapture o f the photoejected e l e c t r o n w i l l be r a p i d and the photon energy w i l l be l o s t . In accord with t h i s p r e d i c t i o n , a f t e r overnight p h o t o l y s i s of a tetrahydrofuran s o l u t i o n o f 2, under c o n d i t i o n s i d e n t i c a l to those described above, 2 can be recovered e s s e n t i a l l y unchanged. E x a c t l y p a r a l l e l r e s u l t s have been obtained with t e t r a p h e n y l cyclopentadienide (3). That a semiconductor e l e c t r o d e can prevent b a c k - e l e c t r o n t r a n s f e r f o l l o w s from d e t e c t i o n of dimer (dihydrooctaphenyl-

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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25

eV (vs.

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I Ag)

Figure 1.

1.5

1.0

-O.5

O.5

-1.0

Electrochemical oxidation offluorenyllithium (DMSO, LiClO (O.1M), room temperature, scan rate: 50 mV s' ) k

1

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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FOXAND OWEN

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341

f u l v a l e n e ) (7) by HPLC and NMR, when l i t h i u m t e t r a p h e n y l c y c l o pentadienide i s excited at the surface of doped η-type TiU2 c r y s t a l held a t O.0 eV (vs Ag) and connected with a platinum e l e c trode i n DMSO containing LiC10i+ (8) as i n e r t e l e c t r o l y t e (eqn 5 ) .

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3

Phi+

Phi*

Photooxidation occurs at p o t e n t i a l s w e l l negative o f the anodic wave. As dimer formation occurs photo-currents (O.1-3.9 μΑ) can be detected, implying photoinduced change t r a n s f e r . It i s a l s o s i g n i f i c a n t to note that dimeric products a r e formed whether the anion or the semiconductor i s e x c i t e d . I t i s p o s s i b l e to s e l e c t i v e l y e x c i t e d the anion by f i l t e r i n g out l i g h t of wavelengths shorter than the onset of band gap i r r a d i a t i o n i n n-Ti02, t.e.yby cut o f f f i l t e r s . For very t h i n l a y e r s o f i r r a d i a t e d s o l u t i o n i n our c e l l , incomplete l i g h t absorption by the solution/adsorbate occurs when u n f i l t e r e d l i g h t i s used as the e x c i t a t i o n source. Under these c o n d i t i o n s , where s i g n i f i c a n t e x c i t a t i o n of the semiconductor occurs simultaneously with anion e x c i t a t i o n , g r e a t l y enhanced photocurrents a t t r i b u t a b l e to band gap i r r a d i a t i o n i n the presence of donors are observed. Thus o x i d a t i o n i s achieved r e s p e c t i v e l y e i t h e r i f an e l e c t r o n i s i n j e c t e d i n t o the conduction band from the a n i o n i c e x c i t e d s t a t e or i f an e l e c t r o n i s t r a n s f e r r e d to the photogenerated hole i n the valence band. As we have shown p r e v i o u s l y , s e n s i t i z a t i o n by adsorbed o r chemically attached dyes (9) should t h e r e f o r e ex tend the wavelength response of l a r g e band gap semiconductors i n a process p a r a l l e l to that observed here. Analogous r e s u l t s are being obtained f o r the f l u o r e n y l anion and other resonance s t a b i l i z e d anions. Although the mechanistic d e t a i l s are s t i l l under i n v e s t i g a t i o n and w i l l be discussed e l s e where, these experiments demonstrate that the semiconductor i n t e r face i s e f f e c t i v e i n i n h i b i t i n g e l e c t r o n recapture i n anion p h o t o l y s i s , i n e s t a b l i s h i n g mechanism i n p o s s i b l e charge t r a n s f e r photoreactions, and i n a c t i n g as a c a t a l y t i c s u r f a c e i n u s e f u l o x i d a t i v e photocoupling r e a c t i o n s . Acknowledgement. We are g r a t e f u l to the U.S. Energy f o r F i n a n c i a l support.

Department o f

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References and Notes 1.

(a) Bard, A.J., J. Photochem. 1980, 10, 59; (b) Nozik, A.J., Ann. Rev. Phys. Chem. 1978, 29, 189; (c) Memming, R. in "Electroanalytical Chemistry", A.J. Bard, ed., M. Dekker, N.Y. Vol. II, 1978; (d) Archer, M.D., J. Appl. Electrochem. 1975, 5, 17; (e) Gerischer, H. in "Physical Chemistry - An Advanced Treatise", H. Eyring, D. Henderson, and W. Jost, eds., Academic Press, N.Y., 463 (1970).

2.

Fox, M.A.; Kabir-ud-Din,

3.

Fox, M.A.; Singletary,

4.

For examples, see Tolbert, L.M., J. Am. Chem. Soc. 1978, 100, 3952; Dvorak, V.; Michl, J., ibid. 1976, 98, 1080; and Fox, M.A., Chem. Rev. 1979, 79, 259.

5.

Lochert, P.; Federlin,

6.

For exceptions to this generalization, see Schafer, H.; Azrak, A.A., Chem. Ber. 1972, 105, 2398; Borhani, K.J.; Hawley, M.D., J. Electroanal. Chem., 1979, 101, 407.

7.

Pauson, P.L.; Williams, B.J., J. Chem. Soc., 1961, 4158.

8.

A control experiment established that LiClO has no effect on the homogeneous photolysis of the lithium salt of 2. At conversions of anion greater that ~20%, the efficiency of dimer formation decreases. Whether this can be attributed to further oxidation of the anion derived from dimer or to cathodic cleavage of dimer is under investigation.

9.

Fox, M.A.; Nobs, F.J.; Voynick, T.A., J. Amer. Chem. Soc. 1980, 102, 4029, 4036.

J. Phys. Chem. 1979, 83, 1800. N.J.,

Solar Energy, 1980, in press.

P., Tetrahedron Lett. 1973, 1109.

4

R E C E I V E D October 15, 1980.

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

22 Fundamental Aspects of Photoeffects at the n-Gallium Arsenide-Molten-Salt Interphase R. J. G A L E , P. SMITH, P. SINGH, K. RAJESHWAR, and J. DUBOW

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Department of Electrical Engineering, Colorado State University, Fort Collins, CO 80523

Detailed studies of the semiconductor/electrolyte interphase boundary are necessary in order to understand fully the operation of photoelectrochemical cells. It is important to be able to establish the potential distribution throughout the interphasial regions because the surface potential barrier governs the separation of photogenerated electrons and holes and thus the energy conversion efficiency. Additionally, it is important to identify those factors that control or modify both the faradaic chargetransfer reactions occurring across the solid/liquid interface and the recombination processes within the semiconductor. For any particular system, a complete analysis in terms of the chemical identification of all the interposed molecules and their role would be an arduous, if not impossible task. Present knowledge of the catalytic and dielectric properties of the species existing in surface atomic layers and in the inner double layer region adjacent to semiconductor electrodes is particularly limited. We are restricted therefore to trying to identify the key factors affecting the output performance of cells and to devise experimental approaches that might ultimately provide a complete description of the photoelectrochemical processes in molecular detail. This work attempts to model a semiconductor/molten salt electrolyte interphase, in the absence of illumination, in terms of its basic circuit elements. Measurement of the equivalent electrical properties has been achieved using a newly developed technique of automated admittance measurements and some progress has been made toward identification of the frequency dependent device components (1). The system chosen for studying the semiconductor/ molten salt interphase has the configuration n-GaAs/AlCl : 1butylpyridinium chloride (BPC) melt/vitreous C., with the ferrocene/ ferricenium ion redox couple as the liquid phase charge carrier. Photoelectrochemical cell electrolytes can be divided broadly into two classes, aqueous and nonaqueous. If aqueous mixtures are included within the former classification, the non3

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aqueous t y p e s can be s u b d i v i d e d f u r t h e r i n t o t h o s e based upon o r g a n i c o r i n o r g a n i c s o l v e n t s and m o l t e n s a l t e l e c t r o l y t e s , e a c h o f t h e s e r e q u i r i n g some means o f i o n i c c o n d u c t i o n a n d / o r a s u i t a b l e redox system. The r a t i o n a l e f o r s t u d y i n g t h e s e m i c o n d u c t o r / molten s a l t i n t e r p h a s e i s t h a t i n g e n e r a l , from c o r r o s i o n c o n s i d e r a t i o n s , nonaqueous e l e c t r o l y t e s a p p e a r t o be more a t t r a c t i v e t h a n t h e aqueous f o r use w i t h t h e t r a d i t i o n a l s e m i c o n d u c t o r m a t erials. Inherent advantages o f d e v e l o p i n g molten s a l t e l e c t r o l y t e s y s t e m s f o r c e l l s t o p r o d u c e e l e c t r i c i t y f r o m l i g h t have been d i s cussed elsewhere (2).

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Experimental Materials. S i n g l e c r y s t a l s o f S n - d o p e d n-GaAs o f o r i e n t a t i o n (100) were o b t a i n e d f r o m L a s e r Diode L a b , I n c . w i t h t h e f o l l o w i n g q u o t e d p r o p e r t i e s , N = 2.3 χ 1 0 c m " , ρ = O.0071 Ωαη, μ = 3900 cm V " s " . Ohmic c o n t a c t was a c h i e v e d w i t h t h e r m a l l y e v a p o r a t e d Ge-Au a l l o y a n n e a l e d i n N a t 400 C f o r 15 m i n u t e s . A Teflon c o a t e d Cu w i r e was a t t a c h e d w i t h s i l v e r epoxy cement and t h e s i n g l e c r y s t a l and t h e Ohmic c o n t a c t masked w i t h a n o n c o n d u c t i n g epoxy c e m e n t . S u r f a c e p r e p a r a t i o n o f t h e c r y s t a l s was s e q u e n t i a l l y a 30 sec e t c h i n 6M HC1, a d i s t i l l e d w a t e r r i n s e , a 1 m i n u t e immersion i n H S 0 : H 0 : H 0 (3:1:1 v o l . ) , a d i s t i l l e d water r i n s e , and a f i n a l r i n s e w i t h a b s o l u t e e t h a n o l . The wet a s s e m b l y was t h e n p l a c e d i m m e d i a t e l y i n t o t h e a n t e - c h a m b e r o f t h e d r y b o x t o be vacuum d r i e d . The c o u n t e r e l e c t r o d e c o m p r i s e d a p l a t e o f v i t r e o u s C ( A t o m e r g i c C h e m e t a l s , I n c . ) and t h e r e f e r e n c e e l e c t r o d e f o r a l l e x p e r i m e n t s c o n s i s t e d o f O.5 mm A l w i r e ( A l f a Y e n t r o n P u r a t r o n i c g r a d e , m4N8) immersed i n 2:1 m o l a r r a t i o A l C l : BPC m e l t , t h e p r e p a r a t i o n o f 1 - b u t y l p y r i d i n i u m c h l o r i d e and m e l t s have been d e s c r i b e d in e a r l i e r p u b l i c a t i o n s , e.g (3). 1 7

3

D

2

1

1

2

2

4

2

2

2

3

Apparatus

and

Technique

C i r c u i t r y used t o o b t a i n c a p a c i t a n c e - v o l t a g e d a t a f o r M o t t S c h o t t k y p l o t s was s i m i l a r t o t h a t u s e d p r e v i o u s l y ( 2 J . Linear v o l t a g e ramps ( 5 - 1 0 m V s " ) and 10 mV peak/peak ac s i g n a l s were added t o a PAR 173 P o t e n t i o s t a t / G a l v a n o s t a t f r o m a PAR 179 U n i v e r s i t y Programmer and I t h a c a D y n a t r a c 3 l o c k - i n a m p l i f i e r , r e spectively. The v o l t a g e o u t p u t f r o m t h e c u r r e n t f o l l o w e r o f a PAR 179 D i g i t a l C o u l o m e t e r was c o n n e c t e d t o t h e s i g n a l i n p u t o f t h e L I A and t h e c a p a c i t a n c e - v o l t a g e c u r v e s were d i s p l a y e d on a PAR RE 0074 X-Y r e c o r d e r . These measurements were made on c e l l s t h e r m o s t a t e d t o 40 + 1 C w i t h i n a d r y b o x . Most measurements were made in absolute darkness or i n c e l l s c a r e f u l l y screened w i t h Al f o i l to exclude l i g h t . A schematic o f the a u t o m a t i c network a n a l y s i s system used f o r p h o t o e l e c t r o c h e m i c a l c e l l measurements i s shown i n F i g u r e 1. The ac s i g n a l o f 20 mV p/p a m p l i t u d e i s a p p l i e d w i t h a H e w l e t t - P a c k a r d 3320B f r e q u e n c y s y n t h e s i z e r t o t h e c e l l and t o t h e r e f e r e n c e s i g 1

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

22.

GALE

345

n-GaAs-Molten-Salt Interphase

ET AL.

n a l i n p u t o f a HP 3570A a u t o m a t i c n e t w o r k a n a l y z e r . A h i g h speed c u r r e n t - v o l t a g e c o n v e r t e r ( T e l e d y n e P h i l b r i c k 1435) and c u r r e n t b o o s t e r w i t h a f l a t f r e q u e n c y r e s p o n s e f r o m d c * t o 1 MHz i s used t o buffer the analyzer. C e l l a d m i t t a n c e s c a n be c a l c u l a t e d f r o m t h e measured G ( s ) = (G.j + s C j ) / ( G + s C ) i n w h i c h G a n d G a r e t h e f

f

i

f

c o n d u c t a n c e s , C. and C t h e c a p a c i t a n c e s o f t h e c e l l and t h e f e e d back c o m p o n e n t s , r e s p e c t i v e l y , and s e q u a l s jo>. The d e s k t o p comp u t e r (HP 9825A) c o m m u n i c a t e s v i a t h e I E E E - 4 8 8 s t a n d a r d i n t e r f a c e bus w i t h t h e s y n t h e s i z e r a n d n e t w o r k a n a l y z e r t o sweep f r e q u e n c y , t o c o l l e c t g a i n and phase d a t a , t o c a l c u l a t e c a p a c i t a n c e and c o n d u c t a n c e v a l u e s and t h e n t o s t o r e t h i s i n f o r m a t i o n on f l e x i b l e discs. In t h e s e e x p e r i m e n t s , t h e b i a s was a d j u s t e d m a n u a l l y u s i n g a l o w n o i s e power s u p p l y u n t i l t h e r e q u i r e d p o t e n t i a l d r o p was o b t a i n e d between t h e r e f e r e n c e and w o r k i n g e l e c t r o d e s . Measurements were made i n two f r e q u e n c y r a n g e s f r o m 5-50 Hz and 5 0 - 1 0 0 KHz. D e t a i l s c o n c e r n i n g t h e c a l i b r a t i o n o f t h e s y s t e m and c o m p a r i s o n o f r e s u l t s o b t a i n a b l e t o t h e d a t a a c c e s s i b l e f r o m impedance b r i d g e t e c h n i q u e s have been o u t l i n e d a l r e a d y f o r m e t a l - i n s u l a t o r - s e m i conductor (MIS)/semiconductor-insulator-semiconductor (SIS) s o l a r c e l l s ( ] ) . Dummy c e l l c a l i b r a t i o n c u r v e s gave e r r o r s n o m i n a l l y < 3 p e r c e n t f o r g a i n and m o d e r a t e phase a n g l e s .

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f

Results

and D i s c u s s i o n

Voltammetric Investigations. L i n e a r sweep v o l t a m m e t r y p r o v i d e s a s i m p l e , s e n s i t i v e a n d r a p i d means o f o b t a i n i n g a c u r r e n t voltage p r o f i l e of the semiconductor/electrolyte interphase. Figu r e 2 i l l u s t r a t e s t h e d a r k I-V c u r v e s a t a n-GaAs e l e c t r o d e i n b a s i c O.8:1 ( A 1 C 1 : BPC r e s p e c t i v e l y ) and a c i d i c 1.5:1 m o l a r ratio melts. In b a s i c m e l t s , a peak a t - O . 6 t o - O . 7 5 V i n c r e a s e s w i t h i l l u m i n a t i o n and by s c a n n i n g f u r t h e r i n t h e p o s i t i v e d i r e c i o n past t h e r e s t p o t e n t i a l a t -O.48 V ( F i g u r e 3 ) . T e n t a t i v e l y , t h i s r e d u c t i o n peak may be a s s i g n e d t o a homogeneous a r s e n i c c h l o r i d e s p e c i e s a t t h e e l e c t r o d e s u r f a c e b e c a u s e a s i m i l a r peak i s o b t a i n e d a t a C e l e c t r o d e by t h e a d d i t i o n o f A s C l t o t h e m e l t ( F i g u r e 4 ) . An a n o d i c peak i n F i g u r e 4 a t +O.47 V a p p e a r s o n l y a f t e r t h e c a t h o d i c r e a c t i o n a t -O.4 V b u t , p r o b a b l y due t o f i l m i n g , r e p e a t e d s c a n s c a u s e t h e waves t o d i s t o r t and d i m i n i s h . The e l e c t r o c h e m i s t r y o f g a l l i u m chloroaluminate species i n a high temperature Al C l : NaCl: KC1 e u t e c t i c has been s t u d i e d ( 4 ) . The s t a n d a r d e l e c t r o d e p o t e n t i a l s a r e r e p o r t e d t o be G a ( I I I ) / G a ( I ) = O.766 V , G a ( I I I ) / G a ( 0 ) = O.577 V and G a ( I ) / G a ( 0 ) = O.199 V v e r sus A l ( A l C I : NaCl: KC1 e u t . ) r e f e r e n c e . Extremely low e l e c t r o a c t i v i t y was i n d i c a t e d a t a C e l e c t r o d e a f t e r t h e a d d i t i o n o f G a C l t o t h e b a s i c room t e m p e r a t u r e m e l t b u t t h e p e a k s a t c i r c a +O.2 V and +O.8 V i n t h e voltammograms o f n-GaAs may be t e n t a t i v e l y a s s i g n e d t o compounds o f G a ( I ) and G a ( I I I ) , r e s p e c t i v e l y , e i t h e r homogeneous o r s u r f a c e bonded. 3

3

3

3

3

B o t h g a l l i u m and a r s e n i c compounds

exist

i n several

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

oxidation

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346

PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE

Figure 1.

INTERFACES

Computer-controlled network analysis system for photoelectrochemical cell admittance measurements

Figure 2. Cyclic voltammograms of n-GaAs in O.8:1 (upper) and 1.5:1 (lower) melts at 50 mVs' and 20 mVs' , respectively, nonilluminated, 40°C 1

1

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

347

n-GaAs-Molten-Salt Interphase

Downloaded by 148.204.30.87 on October 21, 2009 | http://pubs.acs.org Publication Date: March 2, 1981 | doi: 10.1021/bk-1981-0146.ch022

GALE E T A L .

Figure 4.

Cyclic voltammogram (initial scan) of AsCl (O.136 mL) added to O.95:1 melt (12.12 g), ν = 100 mVs , C electrode 3

1

American Chemical Society Library 1155 16th St., N.W. Washington, D.C. 20036 In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

348

PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE

INTERFACES

s t a t e s and t h e p r o b a b i l i t y o f c o m p l e x e q u i l i b r i a w i t h h a l o - s p e c i e s and t h e d e g r e e o f i r r e v e r s i b i l i t y o f t h e e l e c t r o n - t r a n s f e r p r o c e s s e s makes a b s o l u t e c o r r e l a t i o n s o f p o t e n t i a l peaks w i t h d e f i n i t i v e species d i f f i c u l t . An e s s e n t i a l p r o b l e m t o o , i s t o d i s t i n g u i s h between t h e t r a c e amounts o f d i s s o l v e d p r o d u c t s f r o m c o r r o s i o n o f t h e s e m i c o n d u c t o r and c h e m i c a l s t a t e s t h a t may be p r e s e n t i n t h e surface atomic l a y e r . E l e c t r o l y t e Composition

Effects

The m a j o r e q u i l i b r i u m p r o c e s s i s g i v e n by e q u a t i o n ( 1 } ,

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2 A i d /

c o n t r o l l i n g the solvent

A1 C1 " 2

7

acidity

+ Cl"

(1)

From a p o t e n t i o m e t r i c model f o r t h e c h l o r a l u m i n a t e s p e c i e s p r e s e n t i n t h e s e i o n i c m i x t u r e s , an e q u i l i b r i u m c o n s t a n t has been d e r i v e d u s i n g t h e mole f r a c t i o n s c a l e , Κ * 7.0 χ 1 0 " at 40° (3J. A pCl" can be d e f i n e d a n a l o g o u s t o pOH" i n aqueous m e d i a , pCl~= - l o g i o C l T M i x t u r e s w i t h an e x c e s s o f A l C l w i l l be L e w i s a c i d s c o n t a i n i n g t h e e l e c t r o n d e f i c i e n t A 1 C 1 " i o n w i t h some A 1 C 1 " i o n , w h e r e a s when t h e m o l a r r a t i o becomes < 1:1 A l C l : BPC, r e s p e c t i v e l y , t h e m e l t w i l l be t e r m e d b a s i c and c o n t a i n d o n o r C l " i o n s t o g e t h e r w i t h A l Cl 1+" i o n . S u r f a c e i n t e r a c t i o n s between t h e s u b s t r a t e m a t e r i a l and t h e e l e c t r o l y t e can a f f e c t s i g n i f i c a n t l y t h e p e r f o r m a n c e o f c e l l s ( 5 ) and t h e s e a r e o f two main t y p e s ( i ) s o l v e n t a c i d - b a s e i n d u c e d i o n i z a t i o n o f s u r f a c e compounds, and ( i i ) t h e a c c u m u l a t i o n o f s o l u t e species i n the inner double l a y e r region at the semi conductor surface ( s p e c i f i c or superequivalent ion adsorption, organic adsorbates, e t c . ) . Such a c l a s s i f i c a t i o n i s n o t c l e a r c u t because the phenomenological d e f i n i t i o n o f s p e c i f i c i o n a d s o r p t i o n a t m e t a l e l e c t r o d e s (6) does n o t d i s c u s s t h e c h e m i c a l s t a t e o f t h e s p e c i e s , t h e e s s e n t i a l c r i t e r i o n b e i n g t h a t an e x c e s s o f t h e s p e c i e s must be p r e s e n t a t t h e p o t e n t i a l o f z e r o c h a r g e . Different i a t i o n o f t h e s e p r o c e s s e s r e q u i r e s a knowledge o f t h e c h e m i c a l forces involved in the e q u i l i b r i a or the adsorption ( e l e c t r o s t a t i c plus chemical f o r i o n s ) , the extent of i o n i z a t i o n or the adsorp t i o n i s o t h e r m , and t h e s u r f a c e p o t e n t i a l p a r a m e t e r s a s s o c i a t e d with e i t h e r process. Q u a n t i t a t i v e expressions of these e f f e c t s a r e r e s t r i c t e d i n t h e l i t e r a t u r e . An e x p l a n a t i o n o f t h e f l a t - b a n d s h i f t f o u n d i n aqueous s o l u t i o n s a r i s e s f r o m a c o n s i d e r a t i o n o f surface e q u i l i b r i a of the t y p e , 1 3

3

2

7

4

3

-M0H + OH"

-MO" + H 0

An a d d i t i o n a l p o t e n t i a l d r o p o c c u r s a c r o s s ( 7 ) , given by, Δφ

η |

= const

(2)

2

the Helmholtz

- (2.3RT/F){[pH] + 1 o g ( f

M n

-

. X

layer,

M n

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

-)}

(3)

22.

GALE

ET

i n w h i c h A<j>

DL

349

n-G aAs-Molten-Salt Interphase

AL.

i s t h e p o t e n t i a l drop a c r o s s

t h e d o u b l e l a y e r and

f j Q - and X Q - r e s p e c t i v e l y a r e t h e a c t i v i t y c o e f f i c i e n t and mole f r a c t i o n o f i o n i z e d groups i n the s u r f a c e . A l i n e a r r e l a t i o n be tween t h e f l a t b a n d p o t e n t i a l , Y . , and t h e pH s h o u l d be e x p e c t e d as l o n g as t h e c o n t r i b u t i o n f r o m t h e s e c o n d t e r m i s p a r e n t h e s i s i s m i n i m a l . G r y s e , Gomes, Cardon and V e n n i k (8) have d i s c u s s e d the e f f e c t o f the double l a y e r p o t e n t i a l drop. F i g u r e 5 c o n t a i n s a summary o f t h e M o t t - S c h o t t k y p l o t s o b t a i n e d a t 1 KHz and d i f f e r e n t e l e c t r o l y t e c o m p o s i t i o n s r a n g i n g f r o m O.8:1 t o 1.75:1 m o l a r r a t i o s . The i n t e r c e p t s were c a l c u l a t e d from the l e a s t squares g r a d i e n t s taken i n the v o l t a g e r e g i o n s where f a r a d a i c p r o c e s s e s a r e l e a s t s i g n i f i c a n t , namely 2 . 0 t o O.6 V f o r t h e a c i d i c and 1.0 t o -O.5 V f o r t h e b a s i c m e l t s . Faradaic p r o c e s s e s become a p p a r e n t as h y s t e r e s i s i n t h e C-V p l o t s and t h e s e e f f e c t s give r i s e to considerable s c a t t e r in data, p a r t i c u l a r l y n e a r t h e 1:1 m o l a r r a t i o c o m p o s i t i o n . Least squares analyses o f t h e s e d a t a g i v e a s h i f t o f t h e f l a t - b a n d w i t h p C l " o f O.066 V ( s t a n d a r d d e v i a t i o n , σ = O.009 V ) , o r a p p r o x i m a t e l y ( 2 . 3 RT/F) V a t 4 0 ° . T h i s A V . / p C l ~ v a l u e f o r t h e (100) o r i e n t a t i o n n-GaAs i s a p p r o x i m a t e l y o n e - h a l f t h a t o b t a i n e d w i t h (111) n-GaAs c r y s t a l s ( 2 ) , i n d i c a t i n g t h a t t h e c r y s t a l s u r f a c e atom d e n s i t y and t y p e can be a s i g n i f i c a n t f a c t o r i n t h e i n t e r a c t i o n s between s u b s t r a t e and electrolyte. F l a t - b a n d p o t e n t i a l v a l u e s f o r (100) and (111) n - G a A s / m o l t e n s a l t i n t e r p h a s e s and f o r t h e (111) n-GaAs/aqueous e l e c t r o l y t e i n t e r p h a s e a r e compared i n T a b l e I.

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f

f

TABLE Xtal

I

Orientation

n-GaAs

aqueous

pH

II

AlC1 :BPC 3

II

n-GaAs

(100)face

Ref.

-O.95V

(£)

f b

(TTT)face

n-GaAs ( l l l ) f a c e

V (NHE)

A1C1 :BPC 3

II

2.1

pH 12

-1.50V

pCl "2

-1.25V

pCl "12

+O.1OV

pCl"2

-O.80V

This

work

pCl'l 2

-O.20V

This

work

(2)

T y p i c a l l y i n aqueous e l e c t r o l y t e s t h e f l a t - b a n d p o t e n t i a l v a r i a t i o n p e r pH u n i t i s ( 2 . 3 RT/F) V ( 7 , S M 5 ) b u t i n t h e h i g h t e m p e r a t u r e A l C l : N a C l m o l t e n e l e c t r o l y t e s , U c h i d a and c o w o r k e r s Q 6 > Π3 have r e p o r t e d f l a t - b a n d s h i f t s o f 2 ( 2 . 3 R T / F ) V p e r p C l " u n i t f o r Sb-doped S n 0 and n - T i 0 semiconductors. 3

2

2

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

350

INTERFACES

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PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE

Figure 6.

Power curves for n-GaAsO.124Mferrocene cell, electrolyte unstirred, illumination ~ 100 mW cm' 2

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

22.

GALE

ET AL.

n-GaAs-Molten-Salt Interphase

351

The c o n s e q u e n c e s o f q u i t e s m a l l c h a n g e s i n t h e s u r f a c e c h a r g e can be s i g n i f i c a n t t o d e v i c e p e r f o r m a n c e . Figure 6 i l l u s t r a t e s t h e i l l u m i n a t e d c u r r e n t - v o l t a g e b e h a v i o r o f a ( 1 0 0 ) n-GaAs e l e c trode i n a melt c o n t a i n i n g t h e f e r r o c e n e / f e r r i c e n i u m redox couple. At t h e exact n e u t r a l p o i n t , V . -O.91 V , o r 1.04 V n e g a t i v e o f t h e redox p o t e n t i a l o f t h e f e r r o c e n e c o u p l e . As t h e band gap o f n-GaAs i s a p p r o x i m a t e l y 1.44 V , a n e g a t i v e s h i f t o f t h e band edges o f O.4 Y w o u l d i m p r o v e t h e o v e r l a p between t h e r e d o x c o u p l e e n e r g y l e v e l s and t h e v a l e n c e band e d g e . T h u s , i t may be p o s s i b l e t o " t u n e " p h o t o e l e c t r o c h e m i c a l c e l l s f o r o p t i m a l p e r f o r m a n c e by d e l i b e r a t e l y v a r y i n g t h e extent o f t h e surface charge w i t h s u i t able adsorbates. These p h o t o e f f e c t s have been d i s c u s s e d i n g r e a t er d e t a i l elsewhere (18). The m a g n i t u d e o f t h e e r r o r s i n d e t e r m i n i n g t h e f l a t - b a n d p o t e n t i a l by c a p a c i t a n c e - v o l t a g e t e c h n i q u e s c a n be s i z a b l e b e c a u s e ( a ) t r a c e amounts o f c o r r o s i o n p r o d u c t s may be a d s o r b e d on t h e s u r f a c e , ( b ) i d e a l p o l a r i z a b i l i t y may n o t be a c h i e v e d w i t h r e g a r d to e l e c t r o l y t e decomposition processes, (c) surface states a r i s i n g f r o m c h e m i c a l i n t e r a c t i o n s between t h e e l e c t r o l y t e and s e m i c o n d u c t o r c a n d i s t o r t t h e C-V d a t a , and ( d ) c r y s t a l l i n e i n h o m o g e n e i t y , d e f e c t s , o r b u l k s u b s t r a t e e f f e c t s may be m a n i f e s t e d a t the s o l i d e l e c t r o d e causing frequency d i s p e r s i o n e f f e c t s . In t h e n e x t s e c t i o n , i t w i l l be shown t h a t t h e e q u i v a l e n t p a r a l l e l c o n d u c t a n c e t e c h n i q u e e n a b l e s more d i s c r i m i n a t o r y and p r e c i s e a n a l y ses o f t h e i n t e r p h a s i a l e l e c t r i c a l p r o p e r t i e s .

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f

Automated A d m i t t a n c e Measurements Some o f t h e e x p e r i m e n t a l t e c h n i q u e s t h a t have been d e v e l o p e d f o r s t u d i e s o f MIS s o l a r c e l l s , f o r e x a m p l e , m i g h t be a p p l i e d s u c c e s s f u l l y to photoelectrochemical c e l l s . F e a t u r e s common t o e l e c t r o c h e m i c a l s o l a r c e l l s and m e t a l - o x i d e - s e m i c o n d u c t o r ( M 0 S ) , M I S , o r S I S d e v i c e s have been r e v i e w e d i n a t h e o r e t i c a l c o m p a r i s o n o f t h e i r u n d e r l y i n g modes o f o p e r a t i o n (19). An e s s e n t i a l s t a r t i n g p o i n t f o r d e v i c e a n a l y s i s i s t o be a b l e t o model e q u i v a l e n t c i r c u i t t h a t a c c u r a t e l y r e p r e s e n t and r e f l e c t t h e m a t e r i a l p r o p e r t i e s and c e l l b e h a v i o r i n b o t h t h e s t a t i c a n d , h o p e f u l l y , t h e o p e r a t i o n a l modes. T o m k i e w i c z ( 2 0 ) has a p p l i e d a r e l a x a t i o n s p e c t r u m analysis technique t o the n-Ti0 /aqueous e l e c t r o l y t e interphase and he c o u l d a s s i g n p a s s i v e e l e m e n t s t o two s p a c e c h a r g e l a y e r s w i t h d i f f e r e n t doping l e v e l s . In t h e l i g h t o f p r e v i o u s work w h i c h has d e m o n s t r a t e d ( 2 T j t h a t t h e e q u i v a l e n t p a r a l l e l c o n d u c t a n c e t e c h n i q u e i s more s e n s i t i v e t h a n c a p a c i t a n c e measurements f o r d e r i v i n g s u f a c e s t a t e i n f o r m a t i o n i n MIS d e v i c e s , we have made a s e r i e s o f t h e s e measurements on a p h o t o e l e c t r o c h e m i c a l c e l l . For o u r i n i t i a l e v a l u a t i o n s , t h e i n t e r f a c e was n o t i l l u m i n a t e d and no r e d o x c o u p l e was added t o t h e e l e c t r o l y t e . B o t h t h e c a p a c i t a n c e and c o n d u c t a n c e methods d e r i v e e q u i v a l e n t i n f o r m a t i o n as f u n c t i o n s o f t h e a p p l i e d v o l t a g e and f r e q u e n c y , however, t h e i n a c c u r a c i e s that a r i s e i n the treatment o f data g e n e r a l l y are l a r g e r using 2

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

352

PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE INTERFACES

c a p a c i t a n c e measurements ( 2 1 ) . F i g u r e 6 i l l u s t r a t e s an e q u i v a l e n t c i r c u i t f o r the semiconductor/molten s a l t interphase. The c a p a c i t a n c e and r e s i s t i v e e l e m e n t s a s s o c i a t e d w i t h t h e l a r g e a r e a c o u n t e r e l e c t r o d e and t h e c e l l g e o m e t r y have been i g n o r e d i n t h i s analysis. Mathematical treatments f o r equivalent c i r c u i t o f t h i s type f o r a s i n g l e l e v e l and a c o n t i n u u m o f s u r f a c e s t a t e l e v e l s have been p r e s e n t e d e l s e w h e r e ( e . g . , 1 0 , 2Ό, 2 1 , 2 2 J . The a d m i t t a n c e o f the reduced c i r c u i t a f t e r removing the i n f l u e n c e o f R L , h

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Y = 6 + jB

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sc

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1

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The unknown p a r a l l e l RC e l e m e n t s t h a t c o m p r i s e t h e impedance ζ i n F i g u r e 7 c a u s e a s e r i e s o f maxima i n t h e G/ω v s . ω d a t a a t ωτ = 1 , from which the m a j o r i t y c a r r i e r time c o n s t a n t τ o f a s i n g l e l e v e l i n t e r f a c e s t a t e , R C., o r any g e n e r a l RC t i m e c o n s t a n t may be o b tainable. Superposition o f curves occurs i f the i n d i v i d u a l time constants are c l o s e l y spaced. F i g u r e 8 i l l u s t r a t e s t h e d a t a p o i n t s and t h e o r e t i c a l c u r v e f i t s f o r t h e (100) n - G a A s / O . 8 : l m e l t i n t e r p h a s e . To i d e n t i f y t h e s p a c e c h a r g e c a p a c i t a n c e and t h e b u l k r e s i s t a n c e e l e m e n t s , t h e f r e q u e n c y i n d e p e n d e n t c a p a c i t a n c e s were d e r i v e d f r o m t h e maxima i n l o g (G/ω) v s . log(aj) p l o t s , e . g . , F i g u r e 8 , a s (G/œ)max s/ and c o n d u c t a n c e s f r o m t h e maxima i n log(a)C) v s . l o g ω p l o t s e . g . , F i g u r e 9 . The p r o c e d u r a l s t e p s f o r m o d e l i n g were ( a ) f i r s t l y , e s t a b l i s h t h e l i m i t i n g l o w f r e q u e n c y c o n d u c t a n c e G<j = 1 / R f a r *> as d e s c r i b e d a b o v e , t h e R b u l k C c p a r a m e t e r s , ( b ) model a c u r v e f o r the c i r c u i t o b t a i n e d from the b a s i c elements, ( c ) s u b s t r a c t t h e o r e t i c a l curve from experimental data t o determine d i f f e r e n c e maxima a n d , f i n a l l y , ( d ) s u c e s s i v e t r i a l and e r r o r i n s e r t i o n o f RC v a l u e s c o n s t i t u t i n g ζ t o a t t a i n b e s t c u r v e f i t t o e x p e r i m e n t a l data. I n F i g u r e 8 , t h e c i r c u i t e l e m e n t R f a r O.83 ΜΩ d e t e r m i n e s t h e d e g r e e t o w h i c h i d e a l p o l a r i z a b i l i t y has been a c h i e v e d i . e . , t h e a b s e n c e o f c o r r o s i o n p r o c e s s e s and f a r a d a i c l o s s e s due t o e l e c t r o a c t i v e i m p u r i t i e s o r e l e c t r o l y t e decomposition (cf. n-Ti02/ aqueous i n t e r p h a s e ( 2 2 ) , R f = 10 ΚΩ). A d d i t i o n o f a l a r g e p s e u d o - c a p a c i t y o f Rfar d i d not a l t e r the s l o p e o f the low f r e quency s i m u l a t e d p o r t i o n o f t h e c u r v e . A v a l u e o f C f r - O.1 yF c a u s e s a peak a t l o w f r e q u e n c i e s (<100 H z ) . No s u c h p e a k s were observed i n the present d a t a . The i n f l u e n c e o f d e c r e a s i n g t h e e l e c t r o l y t e conductance o r i n c r e a s i n g o v e r p o t e n t i a l s a t the = c

2

anc

c

S

=

a r

a

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

GALE

ET AL.

353

n-Ga As-Molten-Salt Interphase

Figure 7. Equivalent circuit for inter phase: (Rdiei) resistance of semiconductor; (Reiec) electrolyte resistance, (Rf ) fara daic resistance; (C ) space charge capaci tance; (C ) double-layer capacitance; and (z) parallel impedances associated with surface states, faradaic reactions, etc. ar

sc

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DL

2

Figure 8. n-GaAs in O.8:1 melt at O.0 V bias, area O.06 cm . Experimental (G/ω) values are crosses: ( ) the theoretical curve for R and single Kuik C circuit; (----) curve insertion of high frequency parallel RC; and ( ) final curve fit. Lower points represent the difference between theoretical and experimental curves. far

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

sc

354

PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE

-3r—τ

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INTERFACES

ΓΤΤ

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ο» ο

6

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I

ι ι ι I

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2

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Figure 9.

ι

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11 6

FREQUENCY

n-GaAs in O.8:1 melt, O.0 V bias. Experimental coC values (Φ) and theoretical curve ( )

15

12 h

Figure 10. data; (

Cole-Cole plot for n-GaAs in O.8:1 melt, biasO.0V: (%) experimental ) theory for circuit insert of Figure 8; ( ) single R C hemisphere

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

22.

GALE E T A L .

355

n-GaAs-Molten-Salt Interphase

s e c o n d a r y e l e c t r o d e i s t o l o w e r t h e f r e q u e n c y o f t h e maximum a s s o c i a t e d w i t h t h e space charge c a p a c i t a n c e and i t s s e r i e s r e s i s t a n c e , RbulkA n o t h e r method f o r c o m p a r i s o n o f e x p e r i m e n t a l d a t a a n d d e r i v e d c i r c u i t components i s t o examine t h e C o l e - C o l e p l o t s ( 2 3 ) . F i g u r e 10 p o r t r a y s t h e i m a g i n a r y and r e a l a d m i t t a n c e s f o r t h e e x p e r i m e n t a l d a t a and i t i s a p p a r e n t t h a t t h e t o t a l r e duced c i r c u i t f i t s more c l o s e l y t h a n t h e t h e o r e t i c a l model f o r a s i n g l e RC c element. Dispersive d i s t o r t i o n o f t h ecurves i s e v i d e n t a b o u t 10 KHz i n e x p e r i m e n t a l d a t a p o s s i b l y c a u s e d by t h e p r e s e n c e o f f a s t s u r f a c e s t a t e s w h e r e a s i t a p p e a r s above 100 KHz i n a dummy c e l l o f s i m i l a r f o r m a t . I t i s n e c e s s a r y , h o w e v e r , t o i n c l u d e two RC e l e m e n t s f o r ζ i n o r d e r t o o b t a i n a good f i t t o t h e e x p e r i m e n t a l d a t a . The i n s e r t i n F i g u r e 8 d e s c r i b e s t h e complete c i r c u i t i n terms o f i t s p a s s i v e elements. A h i g h e r f r e q u e n c y RC e l e m e n t i n ζ o f a s y e t u n d e t e r mined p h y s i c a l o r i g i n , appeared both a t b a s i c (O.8:1) and a c i d i c ( 1 . 5 : 1 ) i n t e r p h a s e s w i t h (100) n-GaAs. I f i t a r i s e s from a s u r f a c e s t a t e , an o r d e r o f m a g n i t u d e e s t i m a t e o f i t s d e n s i t y f r o m the expression N = C / e i s 4 χ 1 0 c m " . The l o w e r f r e q u e n c y RC e l e m e n t i n t h e b a s i c m e l t s c a n be a s s o c i a t e d w i t h t h e f a r a d a i c p r o c e s s seen i n c y c l i c v o l t a m m e t r y a t c i r c a - O . 7 V. T h i s i s shown i n F i g u r e 1 1 , where t h e m a g n i t u d e o f i t s e f f e c t has i n c r e a s ed. The i n f l u e n c e o f t r a c e f a r a d a i c p r o c e s s e s i n t h e a c i d i c m e l t s a l s o i s m a n i f e s t a t f r e q u e n c i e s 5 0 - 5 0 0 H z . A more d e t a i l e d d i s c u s s i o n o f t h e s e e x p e r i m e n t a l p r o c e d u r e s and t h e o r e t i c a l m o d e l i n g i s a v a i l a b l e ( 2 4 ) . F i n a l l y , t h e frequency independent c a p a c i t a n c e values C o b t a i n e d a t d i f f e r e n t b i a s v a l u e s have been u s e d t o o b t a i n v a l u e s by M o t t - S c h o t t k y a n a l y s e s f o r t h e f l a t - b a n d p o t e n t i a l s i n t h e O.8:1 a n d 1.5:1 m e l t s . These v a l u e s a r e shown i n F i g u r e 5 and c o r r e s p o n d w e l l t o t h e d a t a o b t a i n e d from slow v o l t age sweep c a p a c i t a n c e m e a s u r e m e n t s . Obviously, the equivalent p a r a l l e l c o n d u c t a n c e method c a n p r o v i d e a d e t a i l e d p i c t u r e o f t h e f r e q u e n c y e f f e c t s a n d b e t t e r i n d i c a t e w h i c h f r e q u e n c i e s a r e most r e l i a b l e f o r extracting information f o r specific interphasial phenomena.

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s

1 1

s

s

s

2

s

c

Abstract Linear sweep voltammetry, capacitance-voltage and automated admittance measurements have been applied to characterize the n-GaAs/room temperature molten salt interphase. Semiconductor crystal orientation is shown to be an important factor in the manner in which chemical interactions with the electrolyte can influence the surface potentials. For example, the flat-band shift for (100) orientation was (2.3RT/F)V per pCl unit com pared to 2(2.3RT/F)V per pCl for (111) orientation. The manner in which these interactions may be used to optimize cell perform ance is discussed. The equivalent parallel conductance method has been used to identify the circuit elements for the non-illum inated semiconductor/electrolyte interphase. The utility of this -

-

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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356

PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE

INTERFACES

Figure 11. Illustration of influence of irreversible faradaic process on (G/ω) data, O.8:1 melt, bias:O.65V ((X) experimental data; ( ) theoretical curve for equiva lent circuit insert)

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

22.

GALE ET AL.

357

n-GaAs-Molten-Salt Interphase

technique in providing information on surface states, trace faradaic processes and reliability of Mott-Schottky analyses, is demonstrated using the n-GaAs/AlCl -Butylpyridinium Chloride interphase as a representative example. 3

Acknowledgement Support funds were provided by The Solar Energy Research Institute under Contract No. XP-9-8002-9.

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2.

Singh, P.; Rajeshwar, K.; DuBow, J.B.; Job, R. Soc., 1980, 102, 4676.

3.

Gale, R.J.; Gilbert, 18, 2723.

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Anders, U.; Plambeck, J.A.

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Rajeshwar, K.; Singh, R.; Singh, P.; Gale, R.J.; DuBow, J.; J. App. Physics (In Press).

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Cardon, F.; Gomes, W.P. J. Phys. D: Appl. Phys. 1978, 11, L63.

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Inorg. Chem, 1979,

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10. Dutoit, E.C.; Van Meirhaeghe, R.L.; Cardon, F.,; Gomes, W.P. Ber. Bunsenges Phys. Chem., 1975, 79, 1206. 11. Dutoit, E.C.; Cardon, F., Gomes, W.P. ibid. 1976, 80, 475. 12.

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J. Appl. Physics, 1978, 49, 3571

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Tomkiewicz, M. J. Electrochem. Soc., 1979, 126, 1505. Uchida, I.; Urushibata, H.; Toshima, S. J. Electoanal. Chem., 1979,

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Cole, R.H.; Cole, K.S. J. Chem. Phys., 1941, 9, 341.

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Smith, P. M.Sc. thesis,

Colorado State University

Electrochem.

Soc.,

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(1980).

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(1980).

RECEIVED November 19, 1 9 8 0 .

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

23 Analysis of Current-Voltage Characteristics of Illuminated Cadmium Selenide-Polysulfide Junctions JOSEPH REICHMAN and MICHAEL A. RUSSAK

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Research Department A01/26, Grumman Aerospace Corporation, Bethpage, N Y 11714 Comparisons of experimental I-V characteristics with those predicted by theoretical models ( J J are commonly made to analyze the effects of illumination on semiconductor junction devices. These comparisons have not normally been made for semiconductorelectrolyte (S-E) junctions, most likely due to the lack of suitable theoretical models. In this paper the calculated I-V characteristics using a model that includes depletion region recombination (2) are com pared with experimental results for single-crystal CdSe electrode/polysulfide electrolyte junctions. In particular the predicted effects of redox ion concentration and light intensity on the I-V characteristics are analyzed and compared with exper imental data. Theoretical

Discussion

Models. The most commonly used model t o a n a l y z e t h e I-V p e r f o r m a n c e o f S-E j u n c t i o n s i s t h a t o f G a r t n e r (3) a s g i v e n by the f o l l o w i n g Ig= q F [ l - exp(-oW)/(l

+ oL)]

where F i s t h e a b s o r b e d m o n o c h r o m a t i c s o l a r f l u x , α i s t h e a b s o r p t i o n c o e f f i c i e n t , W i s t h e d e p l e t i o n w i d t h , and L i s t h e minority carrier diffusion length. T h i s e q u a t i o n shows a weak dependence o f c u r r e n t on v o l t a g e t h r o u g h t h e f u n c t i o n a l d e p e n dence o f d e p l e t i o n w i d t h on t h e s q u a r e r o o t o f p o t e n t i a l drop a c r o s s i t . T h i s e q u a t i o n o f t e n g i v e s a good f i t t o e x p e r i m e n t a l d a t a a t s h o r t c i r c u i t c o n d i t i o n s ( t h e r e d o x p o t e n t i a l ) and a t r e v e r s e b i a s ( a n o d i c p o l a r i z a t i o n f o r an η - t y p e s e m i c o n d u c t o r electrode). However, i n t h e f o r w a r d b i a s r a n g e ( c a t h o d i c p o l a r i z a t i o n ) , t h e r e g i o n of i n t e r e s t f o r p h o t o v o l t a i c performance a n a l y s i s , t h e G a r t n e r model p r e d i c t s a d e c r e a s e i n c u r r e n t c o n s i d e r a b l y smaller than i s observed e x p e r i m e n t a l l y . T h i s i s due t o t h e n e g l e c t o f t h e v a r i o u s r e c o m b i n a t i o n mechanisms t h a t c o n tribute to t h e decrease i n photocurrent with increasing

0097-6156/81 /0146-0359$05.00/0 © 1981 American Chemical Society In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE INTERFACES

p h o t o v o l t a g e ( o r d e c r e a s i n g band b e n d i n g ) . The G a r t n e r m o d e l , o r i g i n a l l y derived f o r a Schottky b a r r i e r i n reverse b i a s , only accounts f o r recombination o f c a r r i e r s generated i n the bulk o f the semiconductor. O t h e r mechanisms t h a t can c o n t r i b u t e t o photocurrent loss a r e : 1. D i f f u s i o n of d e p l e t i o n region generated m i n o r i t y c a r r i e r s t o t h e b u l k and s u b s e q u e n t r e c o m b i n a t i o n 2. Recombination i n the depletion region 3. Recombination a t the i n t e r f a c e 4. O p p o s i n g dark c u r r e n t . R e c o m b i n a t i o n i n t h e d e p l e t i o n l a y e r c a n become i m p o r t a n t when t h e c o n c e n t r a t i o n o f m i n o r i t y c a r r i e r s a t t h e i n t e r f a c e exceeds t h e m a j o r i t y c a r r i e r c o n c e n t r a t i o n . Under i l l u m i n a t i o n minority c a r r i e r buildup a t the semiconductor-electrolyte i n t e r f a c e c a n o c c u r due t o s l o w c h a r g e t r a n s f e r . Thus s u r f a c e i n v e r s i o n may o c c u r and r e c o m b i n a t i o n i n the depletion region can become t h e d o m i n a n t mechanism a c c o u n t i n g f o r l o s s i n photocurrent. Mechanisms 1 a n d 2 a r e i n c l u d e d i n t h e model t h a t i s used here f o r comparison w i t h experimental d a t a . Interface recombin ation and dark c u r r e n t e f f e c t s a r e n o t i n c l u d e d ; h o w e v e r , t h e e x p e r i m e n t a l d a t a have been a d j u s t e d t o e x c l u d e t h e e f f e c t s o f dark c u r r e n t . To i n c l u d e t h e a d d i t i o n a l b u l k and d e p l e t i o n l a y e r recombination l o s s e s , the d i f f u s i o n equation f o r minority c a r r i e r s i s s o l v e d u s i n g boundary c o n d i t i o n s r e l e v a n t t o t h e S-E j u n c t i o n ( i . e . , the photocurrent i s l i n e a r l y r e l a t e d to t h e con c e n t r a t i o n of minority c a r r i e r s a t the i n t e r f a c e ) . Using this boundary condition and a s s u m i n g q u a s i - e q u i l i b r i u m conditions ( f l a t q u a s i - F e r m i l e v e l s ) (4) i n t h e d e p l e t i o n r e g i o n , t h e f o l lowing current-voltage r e l a t i o n s h i p i s obtained. I = qvp

w

exp(q

Φι,/kT)

v/here ν i s t h e r a t e c o n s t a n t f o r c h a r g e t r a n s f e r , i s the p o t e n t i a l d r o p a c r o s s t h e d e p l e t i o n r e g i o n and p i s t h e h o l e c o n c e n t r a t i o n a t t h e d e p l e t i o n edge. To d e t e r m i n e p t h e f o l l o w i n g e q u a t i o n i s used w

w

K

w

L

κ + (K2 + 2A

ν 4Α0

2

2 Ί J

where A = q v e x p / k j ) + q L / τ , C = I + qvp exp /kT) and Κ = - n k l ( 2 e e / q / / T . In t h e a b o v e , L i s t h e h o l e d i f f u s i o n l e n g t h , τ i s t h e l i f e t i m e , p i s t h e e q u i l i b r i u m hole d e n s i t y , and Φ β i s t h e e q u i l i b r i u m band b e n d i n g v o l t a g e . These e q u a t i o n s a r e good a p p r o x i m a t i o n s when Φ i s n o t t o o s m a l l and a r e e q u i v a l e n t t o t h a t g i v e n i n [2) where t h e exchange c u r r e n t p a r a m e t e r i s u s e d i n s t e a d o f t h e charge t r a n s f e r r a t e c o n s t a n t . More a c c u r a t e b

a

0

b o

y

0

0

0

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

23.

REiCHMAN

A N D RUSSAK

Illuminated CdSe-Polysulfide Junctions

361

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e q u a t i o n s t h a t a p p l y t o t h e e x t i r e range o f p o t e n t i a l have been used i n t h e a n a l y s i s g i v e n h e r e ( 5 ) . The e q u a t i o n s however a r e somewhat complex and t h e r e f o r e a r e n o t p r e s e n t e d i n t h i s p a p e r . Predicted Results. The e f f e c t s o f c h a r g e t r a n s f e r r a t e c o n s t a n t , v , and i l l u m i n a t i o n i n t e n s i t y were s e l e c t e d f o r a n a l y s i s and c o m p a r i s o n w i t h e x p e r i m e n t . P r e d i c t e d r e s u l t s b a s e d on t h e model d e s c r i b e d above showing t h e e f f e c t s o f ν on t h e I-V c h a r a c t e r i s t i c s a r e g i v e n i n F i g u r e 1. Using the values o f the p a r a m e t e r s g i v e n i n t h e c a p t i o n , t h i s f i g u r e shows t h a t a s ν g e t s s m a l l e r , l o s s e s i n c u r r e n t due t o d e p l e t i o n r e g i o n r e c o m bination o c c u r a t h i g h e r v a l u e s o f band b e n d i n g . T h i s i s due to the higher c o n c e n t r a t i o n of m i n o r i t y c a r r i e r s a t the i n t e r face t h a t i s r q u i r e d to s u s t a i n a given p h o t o c u r r e n t as ν d e creases. This leads to higher recombination rates i n the d e p l e t i o n r e g i o n as t h e band b e n d i n g i s r e d u c e d . The e f f e c t o f v a r y i n g l i g h t i n t e n i t y on t h e I-V c h a r a c t e r i s t i c s a r e shown i n F i g u r e 2. Because t h e c u r v e s a r e n o r m a l i z e d t o t h e i n c i d e n t i n t e n s i t y , t h e y s h o u l d s u p e r p o s e each o t h e r i f t h e r e c o m b i n a t i o n l o s s e s were l i n e a r l y d e p e n d e n t on i n t e n s i t y . However, o u r model p r e d i c t s t h a t r e c o m b i n a t i o n l o s s e s i n t h e d e p l e t i o n region i s p r o p o r t i o n a l to t h e square r o o t of i n t e n sity. Thus l o s s e s due t o d e p l e t i o n r e g i o n r e c o m b i n a t i o n become r e l a t i v e l y l e s s s i g n i f i c a n t a t h i g h e r i n t e n s i t i e s and c o n s e q u e n t l y , f o r a g i v e n band b e n d i n g , h i g h e r c o l l e c t i o n e f f i c i e n cies result. F o r t h e p a r a m e t e r s used h e r e , t h e a d d i t i o n a l r e c o m b i n a t i o n due t o d i f f u s i o n from t h e d e p l e t i o n r e g i o n t o t h e b u l k do n o t become s i g n i f i c a n t u n t i l t h e band b e n d i n g becomes s m a l l . Discussion

o f Experiment

Experimental Procedure. E x p e r i m e n t a l I-V d a t a f o r a CdSe s i n g l e c r y s t a l e l e c t r o d e i n p o l y s u l f i d e e l e c t r o l y t e was used t o compare w i t h p r e d i c t i o n s o f t h e model t h a t i n c l u d e s d e p l e t i o n region recombinations. The c r y s t a l was o b t a i n e d f r o m C l e v e l a n d C r y s t a l C o r p . , C l e v e l a n d , O h i o , and p r e p a r e d f o r measurements a c c o r d i n g t o p r o c e d u r e s f o u n d i n t h e l i t e r a t u r e (6). Electrodes composed o f Na2S and s u l f u r i n 1M NaOH was used w i t h Argon gas bubbling through t h e c e l l d u r i n g measurements. Voltammetric measurements were made w i t h a PAR model 173 p o t e n t i o s t a t and Model 175 u n i v e r s a l programmer u s i n g a scan r a t e o f 10 MV/sec. S p e c t r a l measurements were made u s i n g t h e o u t p u t o f a t u n g s t e n h a l o g e n l a m p . M o n o c h r o m a t i c i n t e n s i t i e s were o b t a i n e d by u s i n g n a r r o w bandpass filters. I n t e n s i t i e s were measured u s i n g a Elppley thermopile detector. Electrode Character!zation. To m i n i m i z e t h e number o f a d j u s t a b l e p a r a m e t e r s o f t h e m o d e l , t h e o p t i c a l and e l e c t r o n i c p r o p e r t i e s o f t h e CdSe c r y s t a l e l e c t r o d e were meaured. Diffuse

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE

INTERFACES

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1.0 r -

BAND BENDING, V O L T S

Figure 1. Calculated effect of rate constant on I-V characteristics (parameters: lifetime τ = 10' s; Ν = 1.5 Χ 10 cm' ; L = 1 μτη; a = 3 Χ 10 cm- ) ((A) 1000; (B)100; (C) 10; (D) 1 cm/s) 9

17

3

4

1

1.0 r -

0

O.2

O.4

O.6

O.8

BAND BENDING, V O L T S

Figure 2. Calculated effects of intensity on I-V characteristics (parameters as in Figure 1 with ν = 10 cm/s) ((A) 10 mA/cm ; (B) 1.0 mA/cm ; (C) O.1 mA/cm ) 2

2

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

2

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23.

REiCHMAN A N D R U S S A K

Illuminated CdSe-Polysulfide Junctions

363

r e f l e c t a n c e measurements a s a f u n c t i o n o f w a v e l e n g t h from t h e e l e c t r o d e s u r f a c e immersed i n t h e e l e c t r o l y t e compartment were obtained. The a b s o r p t i o n c o e f f i c i e n t as a f u n c t i o n o f wavel e n g t h was o b t a i n e d from r e f l e c t a n c e and t r a n s m i t t a n c e m e a s u r e Tients o f CdSe t h i n f i l m s u s i n g conventional methods. The c a r r i e r d e n s i t y was o b t a i n e d from measurements o f r e s i s t i v i t y and H a l l v o l t a g e u s i n g t h e Van de Pauw method ( 7 J . The f l a t b a n d p o t e n t i a l was o b t a i n e d by d e t e r m i n i n g t h e p o t e n t i a l a t which a p h o t o r e s p o n s e i s o b s e r v e d u s i n g a h i g h i n t e n s i t y chopped l i g h t source. A d d i t i o n a l l y , M o t t - S c h o t t k y p l o t s were a l s o made f o r additional v e r i f i c a t i o n o f f l a t band p o t e n t i a l and c a r r i e r density. The r e s u l t s o f t h e s e measurements a r e summarized i n T a b l e I. Measurements o f p h o t o c u r r e n t a s a f u n c t i o n o f w a v e l e n g t h a t s h o r t c i r c u i t c o n d i t i o n s ( - O . 8 V r e l SCE) were made. The d a t a tfas r e d u c e d t o i n t e r n a l quantum e f f i c i e n c y by u s i n g t h e measured values of r e f l e c t a n c e , c o r r e c t i n g f o r e l e c t r o l y t e absorption, and m e a s u r i n g t h e i n c i d e n t i n t e n s i t y . The r e s u l t s a r e shown i n Figure 3 together with a best f i t to the data using the Gartner model ( E q u a t i o n 1 ) . The d e p l e t i o n w i d t h was c a l c u l a t e d u s i n g t h e measured v a l u e s o f c a r r i e r d e n s i t y and f l a t band p o t e n t i a l . F o r a d i f f u s i o n l e n g t h o f 1 ym and t h e measured v a l u e s o f t h e a b s o r p t i o n c o e f f i c i e n t s , a good f i t t o t h e d a t a was o b t a i n e d , a s seen f r o m F i g u r e 3 . The I-V c h a r a c t e r i s t i c s were o b t a i n e d a t v a r y i n g m o n o c h r o m a t i c i n t e n s i t i e s u s i n g a chopped beam a t a w a v e l e n g t h o f 6 5 0 nm. The i n t e r n a l quantum e f f i c i e n c y ( o r c o l l e c t i o n e f f i c i e n c y ) was t h e n d e t e r m i n e d , as a f u n c t i o n o f band b e n d i n g voltage, u s i n g t h e r e f l e c t a n c e and t r a n s m i t t a n c e d a t a . T h i s was done by f i r s t s u b t r a c t i n g t h e dark c u r r e n t from t h e p h o t o c u r r e n t as o b t a i n e d from t h e chopped l i g h t r e s p o n s e . Comparison o f Theory & Experiment To compare t h e e f f e c t s o f i n t e n s i t y , I-V c h a r a c t e r i s t i c s were o b t a i n e d f o r t h e C d S e / p o l y s u l f i d e j u n c t i o n a t i n t e n s i t i e s o f 3 . 7 and O.37 ma/cm a t a w a v e l e n g t h o f 6 5 0 nm. The r e s u l t s u s i n g t h e above d a t a r e d u c t i o n method, a r e shown i n F i g u r e 4 t o g e t h e r w i t h c a l c u l a t e d f i t s t o t h e d a t a u s i n g t h e model t h a t includes depletion layer recombination. The a d j u s t a b l e c o n s t a n t s were c a r r i e r l i f e t i m e and c h a r g e t r a n s f e r r a t e c o n s t a n t . O t h e r p a r a m e t e r s were o b t a i n e d from t h e p r e v i o u s meaurements. As c a n be seen from F i g u r e 4 r e a s o n a b l y good f i t s t o t h e d a t a a r e o b t a i n e d f o r v a l u e s o f band b e n d i n g >O.2 V. The p r e d i c t e d decreasing importance of d e p l e t i o n region recombination with i n c r e a s i n g i n t e n s i t y i s observed e x p e r i m e n t a l l y . At low values o f band b e n d i n g , e f f e c t s n o t i n c l u d e d i n t h e m o d e l , such a s interface recombination and changes i n p o t e n t i a l d r o p across t h e H e l m h o l t z l a y e r may become i m p o r t a n t and e x p l a i n t h e d i s crepancy. 2

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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364

PHOTOEFFECTS

Table I.

PARAMETER

AT SEMICONDUCTOR-ELECTROLYTE

INTERFACES

CdSe Single Crystal Electrode Characterization

MEASUREMENT

VALUES

ABSORPTION

TRANSMITTANCE

5x 1 0 c m "

COEFFICIENT (a)

& REFLECTANCE

TO 7 χ 1 0 c m "

REFLECTANCE

INTEGRATING SPHERE

O.10-O.12

CARRIER DENSITY

H A L L METHOD

1.5 χ 1 0

1 7

MOTT-SCHOTTKY

1χ 10

cm"

CHOPPED LIGHT MOTT-SCHOTTKY

-1.60 vs SCE -1.47 vs SCE

D

FLATBAND POTENTIAL ( V ) fb

3

1

4

1 7

cm" 3

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

1

3

REiCHMAN

AND

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RUSSAK

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23.

Figure 4. Comparison of theory with experiment: effects of intensity (parameters as in A of Figure 1 with ν = 225 cm/s) (( ) experimental; ( ) calculated; (A) 3.7 mA/cm intensity; (B)O.37mA/cm intensity) 2

2

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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ο

0

O.2

O.4 BAND BENDING, VOLTS

O.6

O.8

Figure 5. Comparison of theory with experiment: effect of redox ion concentration (parameters to fit data were as in Figure 1 with ν = 225 cm/s for A and ν = 22.5 cm/s for Β) (( ) experimental; ( ; calculated; (A) 2.5M Na S/lM S/1M KOH; (Β)O.25MNa S/O.1M S/1M KOH) 2

2

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

23.

REICHMAN A N D RUSSAK

367

The e f f e c t s o f c h a r g e t r a n s f e r r a t e c o n s t a n t were d e t e r mined by v a r y i n g t h e c o n c e n t r a t i o n o f t h e r e d o x i o n s . The r e s u l t s a r e shown i n F i g u r e 5 f o r e l e c t r o l y t e o f c o m p o s i t i o n 2 . 5 M N a S / l M S/1 M kOH a n d O.25 M N a S / O . 1 M S/1 M KOH. The c a l c u l a t e d c u r v e s were o b t a i n e d by a s s u m i n g t h e r a t e c o n s t a n t s d i f f e r e d from one a n o t h e r by an o r d e r o f m a g n i t u d e t o c o r r e s p o n d to t h e redox i o n c o n c e n t r a t i o n change. F o r t h e higher concen t r a t i o n e l e c t r o l y t e , a good f i t was o b t a i n e d . At the lower con c e n t r a t i o n , h o w e v e r , t h e f i t was p o o r w i t h t h e p r e d i c t e d r e s u l t showing a much more r a p i d c u r r e n t v a r i a t i o n . T h i s may be due t o an i n s t a b i l i t y i n t h e s u r f a c e l a y e r due t o b u i l d u p o f s u l f u r (J$) which w o u l d t h e n make ν a f u n c t i o n o f c u r r e n t d e n s i t y . As t h e c u r r e n t d e c r e a s e s due t o r e c o m b i n a t i o n , t h e s u l f u r l a y e r g e t s thinner. T h i s i n c r e a s e s ν and t h e r e b y r e d u c e s t h e r e c o m b i n a t i o n l o s s e s making f o r a more g r a d u a l d e c l i n e i n p h o t o c u r r e n t . This e f f e c t i s e x p e c t e d t o be more s i g n i f i c a n t a t t h e l o w e r r e d o x i o n concentration. 2

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Illuminated CdSe-Polysulfide Junctions

2

Summary The o b s e r v e d d e c r e a s e i n p h o t o c u r r e n t w i t h d e c r e a s i n g band bending ( i n c r e a s i n g photovoltage) f o r C d S e / p o l y s u l f i d e j u n c t i o n s have been a c c o u n t e d f o r u s i n g a model t h a t h a s d e p l e t i o n r e g i o n r e c o m b i n a t i o n a s t h e m a j o r l o s s mechanism. T h i s i s based o n r e a s o n a b l y good f i t s o f p r e d i c t e d b e h a v i o r t o e x p e r i m e n t a l I-V d a t a f o r w e l l c h a r a c t e r i z e d e l e c t r o d e s where l i g h t i n t e n s i t y a n d r e d o x i o n c o n c e n t r a t i o n v a r i a t i o n s were s t u d i e d . The s i g n i f i c a n t d e p a r t u r e from p r e d i c t i o n s o f t h e l o w r e d o x i o n c o n c e n t r a t i o n I-V d a t a c a n be a s c r i b e d t o n o n l i n e a r boundary c o n d i t i o n s . T h i s a p p e a r s due t o t h e dependence o f t h e c h a r g e t r a n s f e r r a t e on c u r r e n t d e n s i t y c a u s e d by s u l f u r b u i l d u p on t h e s u r f a c e . Acknowledgements The a u t h o r s thank C. C r e t e r and J . D e C a r l o f o r t h e i r a s s i s t a n c e i n o b t a i n i n g much o f t h e e x p e r i m e n t a l d a t a . P a r t i a l sup p o r t f o r t h i s s t u d y was p r o v i d e d by SERI u n d e r C o n t r a c t X P - 9 8002-8. Abstract The current-voltage (I-V) characteristics of semiconductor/ electrolyte junctions under illumination are analyzed using a model that has depletion region recombination as the dominant loss mechanism. The effects of intensity and charge transfer rate constant on the I-V characteristics are studied theoreti cally and experimentally using single-crystal CdSe electrodes in polysulfide electrolyte. Reasonably good fits to the data were obtained using a minimum number of adjustable model parameters except at small band bending and for the low redox ion

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

368

PHOTOEFFECTS

AT SEMICONDUCTOR-ELECTROLYTE

concentration. These deviations included in the model. Literature 1. 2.

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3. 4. 5. 6. 7. 8.

are

ascribed

to

INTERFACES

effects

Cited

Hovel, H.; "Semiconductors and Semimetals, Vol 11, Solar Cells;" Academic Press; NY, 1975. Reichman, J.; "The Current-Voltage Characteristics of Semiconnductor-Electrolyte Junction Photovoltaic Cells;" Appl Phys Ltr, 36, p 574, 1980. Gartner, W.W.; Phys Rev; 116, p 84, 1959. Many, Α., Goldstein, Y. and Grover, N.B.; "Semiconductor Surfaces;" J. Wiley and Sons, NY, p 156, 1965. Reichman, J.; to be published. Miller, B. and Heller,, Α.; J. Electrochem Soc; 124, p 694, 1978. Van der Pauw, L.J.; Philips Research Reports; 13, p 1, 1958. Cahan, D., Manassen, J. and Hodes, G.; Solar Cell Matls; 1, p 343, 1979.

R E C E I V E D October 3, 1980.

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

not

24 Stability of Cadmium-Chalcogenide-Based Photoelectrochemical Cells DAVID CAHEN, G A R Y HODES, JOOST MANASSEN, and RESHEF T E N N E

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The Weizmann Institute of Science, Rehovot, Israel

Cd-chalcogenides (CdS, CdSe, CdTe) a r e among the most s t u d i e d m a t e r i a l s as photoelectrodes i n a photoelectrochemical c e l l (PEC) (jL>.2>3>40 . I n t e r e s t i n such PEC's stems from the f a c t that, i n aqueous p o l y s u l f i d e or p o l y s e l e n i d e s o l u t i o n s , a d r a s t i c decrease i n photocorrosion i s observed, as compared to other aqueous s o l u t i o n s , while reasonable conversion e f f i c i e n c i e s can be a t t a i n e d . An important c o n s i d e r a t i o n , from the p r a c t i c a l p o i n t of view, i s that t h i n f i l m p o l y c r y s t a l l i n e photoelectrodes can be prepared, by various methods, with conversion e f f i c i e n c i e s of more than h a l f of those obtained with s i n g l e c r y s t a l e l e c t r o d e s and with b e t t e r s t a b i l i t y c h a r a c t e r i s t i c s than those obtained with s i n g l e c r y s t a l based PEC s U,4,5). Recently we showed that PEC's using a l l o y s of Cd(Se,Te) as photoelectrodes can be prepared with s t a b i l i t y c h a r a c t e r i s t i c s much b e t t e r than those obtained f o r CdTe based c e l l s and with s i m i l a r conversion e f f i c i e n c e s (6>7) . Those e f f i c i e n c i e s could be improved c o n s i d e r a b l y by s e v e r a l chemical and p h o t o e l e c t r o chemical s u r f a c e treatments. Here we w i l l consider mainly the s h o r t - c i r c u i t current (SCC) output s t a b i l i t y c h a r a c t e r i s t i c s of both small g r a i n , t h i n f i l m CdSe and Cd(Se,Te)-based PEC s as w e l l as those of s i n g l e c r y s t a l CdSe-based c e l l s , where d i f f e r e n t c r y s t a l faces a r e exposed to the s o l u t i o n , a f t e r they have undergone any of a s e r i e s of s u r f a c e treatments. These s t u d i e s show a strong dependence o f the output s t a b i l i t y on s o l u t i o n composition, on r e a l e l e c t r o d e s u r f a c e area, on s u r f a c e treatment, on c r y s t a l face and on c r y s t a l s t r u c t u r e (for the Cd(Se,Te) a l l o y s ) (1,2,7). We suggest a predominantly k i n e t i c explanation f o r these phenomena, i n v o l v i n g the f i n e balance between the r a t e of the photocorrosion r e a c t i o n on the one hand and that of the regenerat i v e redox r e a c t i o n s on the other hand. Because we a r e i n t e r e s t e d , u l t i m a t e l y , i n p r a c t i c a l d e v i c e s , our aim i s to t r y to a t t a i n long term s t a b i l i t y (months-years). 1

?

0097-6156/81/0146-0369$05.00/0 © 1981 American Chemical Society In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE INTERFACES

Therefore u n s a t i s f a c t o r y s t a b i l i t y by our d e f i n i t i o n does not n e c e s s a r i l y r e f l e c t short term behaviour (minutes-hours). Experimental S i n g l e c r y s t a l s of CdSe were obtained from Cleveland C r y s t a l s Inc., as w e l l as from W. G i r i a t (iVIC-Caracas), and were n-type with t y p i c a l r e s i s t i v i t i e s of 3-15 Ω-cm (VLOm~3 donor concen t r a t i o n ) . C r y s t a l o r i e n t a t i o n was checked by X-ray d i f f r a c t i o n . E l e c t r o d e s were prepared from them as described elsewhere ( 1 ) . P o l y c r y s t a l l i n e CdSe and Cd(Se,Te) e l e c t r o d e s were prepared by p a i n t i n g a s l u r r y of the powder on a T i s u b s t r a t e and subsequent annealing ( 5 ) . The Cd(Se,Te) powders were prepared from CdSe and CdTe by c o s i n t e r i n g (7). S o l u t i o n p r e p a r a t i o n has been described before ( 1 ) . Except f o r the outdoors t e s t s , which used 2-electrode c e l l s with CoS counterelectrode (8), a l l s t a b i l i t y t e s t s were done under p o t e n t i o s t a t i c c o n t r o l at SCC, using Pt counter and r e f e rence e l e c t r o d e s . A s t a b i l i z e d q u a r t z - l 2 lamp provided i l l u m i n a t i o n . Constant temperature was maintained using a thermostatted water bath surrounding the PEC.

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C

E t c h i n g . The p o l y c r y s t a l l i n e e l e c t r o d e s were etched, before use i n p o l y s u l f i d e s o l u t i o n , i n 3% HNO3 i n cone. HC1 f o r sees. Single c r y s t a l CdSe e l e c t r o d e s were p o l i s h e d , before use, down to O. 3., with alumina p o l i s h . T h e i r v a r i o u s etching treatments were as follows : 1. Chromic a c i d etch only; 10 sees, i n Cr03:HCl:H 0 (6:10:4 w/w). 2. Aqua r e g i a etch; 20 sees, i n aqua r e g i a . 3. Aqua regia/chromic a c i d etch; VL0 sees, i n s o l u t i o n of "1", a f t e r normal aqua r e g i a e t c h . 4. Photoetch; 5 sees, i n HN0 :HC1:H 0 (O.3:9.7:90 ν / ν ) , under AML i l l u m i n a t i o n , as photoelectrode i n PEC with carbon countere l e c t r o d e , a f t e r aqua r e g i a etch (Tenne and Hodes, Appl. Phys. Lett., i n press). 2

3

2

Results During the l a s t few years we have c a r r i e d out s e v e r a l out doors t e s t of PEC's c o n t a i n i n g t h i n f i l m p o l y c r y s t a l l i n e CdSe pho t o e l e c t r o d e s and CoS c o u n t e r e l e c t r o d e s . Figure 1 shows the r e s u l t s of one such t e s t c a r r i e d out during 1979, with an initially high e f f i c i e n c y c e l l . I t i s c l e a r from the f i g u r e that the drop i n e f f i c i e n c y a f t e r 2 months i s due mainly to parameters a f f e c t i n g the SCC. More recent t e s t s , using s e v e r a l of the improvements that can be obtained from the r e s u l t s discussed below, showed CdSe c e l l s of a s i m i l a r type to be s t a b l e w i t h i n 10% f o r 6 months. The s o l u t i o n used i n the c e l l of f i g u r e 1 was chosen i n Dec. 1978 and i t s most important c h a r a c t e r i s t i c i s the [ S ] / [ S ] (added q u a n t i t i e s ) of O.5. Figure 2 shows the considerable importance of t h i s r a t i o on the output s t a b i l i t y of CdSe-based P E C s . We see =

f

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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24.

CAHEN

ET

Cd-Chalcogenide Photoelectrochemical Cells

AL.

371

2

Figure 1. Results of an 8-month outdoors test of PEC containing aO.8-cm thin film, painted CdSe photoelectrode (not photoetched), CoS counter electrode, and 1M KOH, 2M S-, 1M S, ImM Se solution. (OCV) open-circuit voltage; (SCC) short-circuit current; (EFF) solar conversion efficiency (~ A M 1.5). Between mea surements the cell operated on maximum power (68 Ω load). No appreciable change infill-factoroccurred during the test.

Figure 2. Time dependence of short-circuit current of polycrystalline painted CdSe photoelectrodes as a function of solution composition. All solutions contained O.8M KOH and O.8M S . S/S ratio indicated next to plots. Dashed line shows behavior in 3.2M S solution at 50°C All other experiments done at 35°C., potentiostatically. Light intensities adjusted to obtain identical initial current densities. =

=

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

372

PHOTOEFFECTS

AT

SEMICONDUCTOR-ELECTROLYTE

INTERFACES

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=

that the [ S ] / [ S ] of f i g u r e 1 i s one of the l e a s t favourable ones and that a r a t i o of 2 would y i e l d much b e t t e r r e s u l t s . I f we compare the output s t a b i l i t y of p o l y c r y s t a l l i n e and s i n g l e c r y s t a l CdSe c e l l s we f i n d the p o l y c r y s t a l l i n e CdSe c e l l s to be much more s t a b l e ( f i g u r e 3 ) . Figure 3B furthermore s t r e s s e s the importance of the i n i t i a l current d e n s i t y i n determining the output s t a b i l i t y of such c e l l s . (Note that the i n i t i a l current d e n s i t i e s of f i g u r e s 2 and 3 correspond to an i l l u m i n a t i o n i n t e n s i t y equivalent to 3.5 -5xAKL s o l a r i n t e n s i t y , and as such these experiments are a c c e l e rated l i f e - t i m e t e s t s . Such t e s t s may be much more severe than the f a c t o r s 3.5-5 would i m p l y ) . For comparison, f i g u r e 3B shows r e s u l t s not only f o r the r e l a t i v e l y most s t a b l e face of a s i n g l e c r y s t a l (2) and a p o l y c r y s t a l l i n e t h i n l a y e r prepared by p a s t i n g (5), but a l s o f o r a pressed p e l l e t of CdSe (2). This l a s t e l e c t r o d e shows s t a b i l i t y behaviour intermediate between that of the two other e l e c t r o d e s . This behav i o u r i s evident a l s o i n the decrease i n output s t a b i l i t y , which i s l e s s steep than that of the s i n g l e c r y s t a l e l e c t r o d e . In f i g u r e 4 we show how the output s t a b i l i t y of a c e l l using a s i n g l e c r y s t a l e l e c t r o d e with a s p e c i f i c c r y s t a l face exposed to the s o l u t i o n can vary depending on the surface treatment given to the e l e c t r o d e , before use. For comparison we i n c l u d e the s t a b i l i t y behaviour of the, l e a s t unstable, (1120) face, which was given the most favourable surface treatment (photoetch). Figure 5 provides a c l o s e look at the faces a f t e r the v a r i o u s s u r f a c e treatments. (The completely f e a t u r e l e s s chromic a c i d etched-face i s not shown). The p r o g r e s s i v e l y i n c r e a s i n g s u r f a c e area (from A-C) i s e v i d e n t . The nature of the small C O . l y ) holes obtained a f t e r purposely photocorroding the e l e c t r o d e (termed "photoetching") i s p r e s e n t l y under i n v e s t i g a t i o n . Although f i g u r e s 2-5 i n d i c a t e how we may improve the output s t a b i l i t y of CdSe-based photoelectrochemical c e l l s (and, i n some cases t h e i r conversion e f f i c i e n c y , though never much beyond 4% f o r t h i n - f i l m p o l y c r y s t a l l i n e based ones), a p r a c t i c a l device should have a considerably higher s o l a r conversion e f f i c i e n c y together with an output s t a b i l i t y at l e a s t equal to that obtained f o r the best CdSe-based c e l l s . CdTe with a bandgap of 1.45eV (at RT) as compared to 1.7eV f o r CdSe would be a l o g i c a l choice f o r a t t a i n i n g t h i s goal but, u n f o r t u n a t e l y , c e l l s using n-CdTe are not s t a b l e i n p o l y s u l f i d e s o l u t i o n s under " s o l a r illuminâtion" c o n d i t i o n s . (Such c e l l s are claimed to be s t a b l e i n p o l y s e l e n i d e and p o l y t e l l u r i d e s o l u t i o n s ( 9 ) ) . Also a d d i t i o n of tens of m i l l i m o l a r q u a n t i t i e s of Se to p o l y s u l f i d e s o l u t i o n s was shown to improve the output s t a b i l i t y of CdSe c e l l s (2). Here we are, f o r p r a c t i c a l reasons, i n t e r e s t e d i n s o l u t i o n s c o n t a i n i n g no, or only minimal q u a n t i t i e s of Se and Te because of the much lower t o x i c i t y and lower a i r s e n s i t i v i t y of pure p o l y s u l f i d e s o l u t i o n s ) . Figure 6 shows, however, that a l l o y s of CdSe T e can have o p t i c a l bandgaps comparable to that of CdTe, even i f x>O.5 (10). CdSe and CdTe form homogeneous a l l o y s over the whole composition x

x

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

CAHEN ET AL.

Cd-Chalcogenide Photoelectrochemical Cells

373

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24.

Figure 3. (A) Dependence of short-circuit current on total charge passed for painted polycrystalline ( ) and single crystal ( , (1120) face) CdSe photoelectrodes. Solution contained 1M each KOH, S , and S. (B) Same as 3A but at higher initial current density. Behavior of pressed pellet CdSe electrode (2) included as well. Other conditions as for 3A (but 5mM Se added to the solution) and for Figure 2. The behavior of the single crystal from 3A is shown, too, for comparison. (Data obtained at Bell Labs.). =

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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374

PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE INTERFACES

Figure 4. Logarithmic plot of time dependence of short-circuit current for singlecrystal CdSe photoelectrode, (1010) face exposed to solution: (. . -) after chromic acid etch; (-.--) after aqua regia/chromic acid etch; ( ) after aqua regia etch; ( ) after aqua regia/photoetch; (A) (1010); (B) (1120). Further conditions as in Figure 3A.

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

CAHEN

ET

AL.

Cd-Chalcogenide Photoelectrochemical Cells

375

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24.

Figure 5. Scanning electron microscope pictures of single-crystal CdSe after several surface treatments. (0001) face, Cd-side, shown. (1120) and (1010) faces behave similarly. (A) After aqua regia/chromic acid etch; (B) after aqua regia etch; (C) after aqua regia/photoetch.

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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376

PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE

1 0 - 3 0 At. %

50

INTERFACES

70

Se (χ in C d S e T e , ) x

x

Figure 6. Direct optical bandgap of Cd(Se,Te) as a function of Se content and crystal structure. Data for well-annealed samples, at room temperature. (After Ref. 10) (1).

Ο X

>

(nm)

500

(eV) WAVELENGTH (λ) Nature Figure 7. Spectral response of polycrystalline, thin-film, painted CdSe .esTe .35 photoelectrode in 1M KOH, S ,S solution. Corrected for photon density wavelength dependence (6). 0

=

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

0

24.

CAHEN

ET AL.

377

Cd-Chalcogenide Photoelectrochemical Cells

range (as do CdS and CdSe (10,11) and CdS and CdTe (10)). The a l l o y can have e i t h e r the hexagonal, w u r t z i t e , s t r u c t u r e or the cubic, s p h a l e r i t e , one, depending on i t s composition and the con d i t i o n s o f preparation (10,12). Table I shows that the l a t t i c e parameters of Cd(Se,Te) a l l o y s depend on the a l l o y stoichiometry i n a rather l i n e a r fashion, i . e . Vegard s law i s obeyed (although d e v i a t i o n s of up to one percent are observed). f

Table I Cd S e

x

Te . 1-

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ο

ο

ο

X

Hex(%)

a (A)

c(A)

O.00

100

4.31

7.02

0

.70

.30

100

4.38

7.165

0

.65

.35

>95

4.40

7.18

<5

6.23

>90

4.42

7.20

<10

6.24

1.00

Cubic (%)

a (A)

1-x

Q

(6.05)

-

.63

.37

.60

.40

70 (±10)

4.43

7.23

30(±10)

6.26

.57

.43

30(±10)

4.44

7.24

70(±10)

6.27

.55

.45

<5

4.44

7.25

>95

6.28

.50

.50

0

-

-

100

6.30

1.00

0

100

6.48

O.00

(4.60)

(7.50)

C r y s t a l l o g r a p h i c data f o r C d S e T e , from X-ray powder d i f f r a c t i o n . Stoichiometries from r e l a t i v e amounts of s t a r t i n g m a t e r i a l s , and from Vegard's law. Percentages o f cubic and hexagonal s t r u c tures estimated from powder d i f f r a c t i o n i n t e n s i t i e s . L a t t i c e dimensions i n parentheses are l i t e r a t u r e data. x

- x

Figure 6 shows moreover that the two s t r u c t u r e s can c o e x i s t over a range o f s t o i c h i o m e t r i e s . The s p e c t r a l response of a PEC con t a i n i n g n-CdSe £5 Te 35 as photoanode i s shown i n f i g u r e 7. An e f f e c t i v e o p t i c a l bandgap of VL.45eV, i n agreement with f i g u r e 6, i s observed. Such c e l l s can, under optimal c o n d i t i o n s , a t t a i n s o l a r conversion e f f i c i e n c i e s o f 8% ( 6 ) . Here we w i l l consider t h e i r output s t a b i l i t y as a f u n c t i o n o f stoichiometry of the photoelectrode or, more p r e c i s e l y , as a f u n c t i o n of the c r y s t a l s t r u c t u r e of the photoelectrode. Figure 8 i l l u s t r a t e s the pro nounced d i f f e r e n c e s i n s t a b i l i t y between c e l l s w i t h electrodes having d i f f e r e n t s t o i c h i o m e t r i e s . The strong decrease i n s t a b i l i t y which occurs when the stoichiometry i s a l t e r e d only s l i g h t l y , suggests that another f a c t o r besides composition, plays a r o l e . As i n d i c a t e d on the f i g u r e t h i s f a c t o r i s the c r y s t a l s t r u c t u r e . Electrodes that have a predominantly hexagonal s t r u c t u r e are much more s t a b l e than those having mainly the cubic s t r u c t u r e . This i s e

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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20

30

INTERFACES

40

TIME (hrs.)

Figure 8. Time dependence of short-circuit current for several Cd(Se,Te) elec trodes in 2 M KOH, S , and S. Further details as in Figure 2. See Table I for deter mination of stoichiometry (upper numbers, Se/Te) and crystal structure flower num bers; (1). =

Journal of the Electrochemical Society Figure 9. Scanning electron microscope pictures of painted Cd chalcogenide films: (A) CdfSeo.65Teo.35); (B) CdSe (3χ larger magnification than A). Both surfaces were etched before the experiment (5).

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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true a l s o at a f i x e d s t o i c h i o m e t r y . C e l l s with C d S e Q Q TeQ photoelectrodes could be prepared with a higher hexagonal content than that shown i n the f i g u r e , and then showed a s t a b i l i t y c l o s e r to that found f o r the CdSeQ TeQ 35 - c o n t a i n i n g c e l l . The surface morphology of such a l l o y electrodes i s shown i n f i g u r e 9A and compared to that of CdSe e l e c t r o d e s ( f i g u r e 9B), prepared by the same " p a s t i n g " method (5). The increased g r a i n s i z e o f the a l l o y electrodes can be a t t r i b u t e d to the double annealing process needed f o r t h e i r p r e p a r a t i o n . A s i n g l e annealing step (of the s l u r r y on the T i - s u b s t r a t e ) s u f f i c e s f o r p r e p a r a t i o n of the CdSe e l e c t r o d e s . As we s h a l l see below, t h i s increased g r a i n s i z e can have a detrimental e f f e c t on output s t a b i l i t y . e

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Discussion The r e s u l t s presented above f o r CdSe-based p o l y s u l f i d e c e l l s ( f i g u r e s 1-4) can be understood i n terms o f the competition b e t ween regenerative redox r e a c t i o n s ( S oxidation) and photocorros i o n o f the semiconductor as i l l u s t r a t e d s c h e m a t i c a l l y i n f i g u r e 10. T h i s scheme i s based on the model f o r d e a c t i v a t i o n of CdSe and CdS p o l y c r y s t a l l i n e - and s i n g l e c r y s t a l e l e c t r o d e s , proposed p r e v i o u s l y (1,13). This model i n v o l v e s the above mentioned two p o s s i b l e r e a c t i o n paths open to a photocorrosion r e a c t i o n produces zerovalent Se or S, and C d i o n s , whose r a p i d r e p r e c i p i t a t i o n leads to the formation of a f i n e l y p o l y c r y s t a l l i n e top l a y e r of CdS (1,14, Cahen and Vandenberg, submitted) and, i n the case of CdSe, to S/Se s u b s t i t u t i o n ( , 2 1 4 ) . Although one can argue that the d e a c t i v a t i o n o f CdSe i s due to the formation of a chemically d i f f e r e n t (CdS) l a y e r on i t , by i n t r o d u c i n g a b a r r i e r f o r holes on t h e i r way to the surface, f i g u r e 11 shows that CdS behaves r a t h e r s i m i l a r to CdSe. T h i s suggests that the surface r e s t r u c t u r i n g alone, e.g. d i f f e r e n c e s i n g r a i n s i z e , and, p o s s i b l y , a d i f f e r e n t doping l e v e l of the top l a y e r o f CdS, as compared to the bulk, may s u f f i c e to d e s t a b i l i z e the e l e c t r o d e , by i n t r o d u c i n g a new r e s i s t a n c e to current flow. The r e s u l t s shown i n f i g u r e 1 can be understood i n the l i g h t o f these f i n d i n g s , as we would expect the SCC to be a f f e c t e d mostly by t h i s increased r e s i s t a n c e . The explanation f o r the r e s u l t s o f f i g u r e 2 can be found i n the extreme l e f t - h a n d process of f i g u r e 10, namely the d i s s o l u t i o n of S from the s u r f a c e . P r e v i o u s l y we showed how the s o l u t i o n compos i t i o n can i n f l u e n c e the t r a n s i e n t photocurrents i n CdSe/Sj PEC's, i n a manner which can be c o r r e l a t e d with the S - d i s s o l v i n g a b i l i t y of the s o l u t i o n (15). An optimum s o l u t i o n composition was found for a [ S ] / [ S ] r a t i o of 1.5-2. Figure 2 shows that the same r a t i o exerts an optimal s t a b i l i z i n g i n f l u e n c e on the c e l l . This can be r a t i o n a l i z e d by c o n s i d e r i n g the e f f e c t of elemental S on the e l e c t r o d e s u r f a c e . Such s u l f u r w i l l i n h i b i t the flow of holes to S from the s o l u t i o n (or, l i m i t the a v a i l a b i l i t y of adsorbed S a t the surface) and thus increases the p r o b a b i l i t y o f photocorrosion. (A more e l e c t r o c h e m i c a l way of l o o k i n g a t t h i s i s to consider the =

2 +

=

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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AT

REGENERATIVE

SEMICONDUCTOR-ELECTROLYTE

Cd

INTERFACES

PHOTOCORROSION

Se

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REDOX REACTIONS

SeS

=

(solution) counterelectrode

fast

CdS (surface) polycrystalline

Figure 10. Scheme of competing reactions at illuminated CdSe surface. The process on the extreme left is the main rate-limiting step, while the one on the right can lead to photoelectrode corrosion.

10

20

30

TIME (hrs.)

Figure 11. Time dependence of SCC for thin-film polycrystalline CdS and CdSe photoelectrodes (1M KOH, 2M S , and O.2M S solution). Note unfavorable [ 5 ] / [ 5 ] (see Figure 2), chosen here to minimize solution absorption. Further details as in Figure 2. =

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

=

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s h i f t i n p o t e n t i a l s caused by the presence of a, r a t h e r non-conducting, s u l f u r l a y e r on the s u r f a c e . T h i s reasoning leads to the same c o n c l u s i o n s ) . I t i s q u i t e l i k e l y t h a t , i n i t i a l l y , the photoc o r r o s i o n r e a c t i o n w i l l a f f e c t e l e c t r o d e performance only m i n i m a l l y . However, because photocorrosion leads to r e s t r u c t u r i n g of the e l e c t r o d e top l a y e r , which causes a decrease i n h o l e flow to the surface as w e l l , i t i s , i n a sense, an a u t o c a t a l y t i c r e a c t i o n , at l e a s t once i t has proceeded beyond a c r i t i c a l point (a c r i t i c a l e f f e c t i v e thickness of the r e s t r u c t u r e d top l a y e r ) . T h i s autoc a t a l y t i c nature of the d e s t a b i l i z a t i o n process can be seen i n f i g u r e 3 f o r the s i n g l e c r y s t a l e l e c t r o d e . It i s c l e a r then from the above that -photoelectrode s t a b i l i t y w i l l depend on the r e l a t i v e r a t e s of the two competing processes (13). Increasing the i l l u m i n a t i o n i n t e n s i t y may favour the photocorrosion r e a c t i o n at constant maximal r a t e of s u l f i d e o x i d a t i o n and s u l f u r removal (constant s o l u t i o n composition). On the other hand an i n c r e a s e i n s o l u t i o n temperature increases the r e g e n e r a t i v e redox r e a c t i o n r a t e as w e l l as the r a t e of s u l f u r removal, and, at constant i l l u m i n a t i o n i n t e n s i t y w i l l l e a d to increased s t a b i l i t y ( f i g u r e 2). (In the Result's s e c t i o n we sounded a cautionary note regarding the use of a c c e l e r a t e d l i f e time t e s t s . In view of the autocatal y t i c nature of the degradation process, and the f a c t that at low current d e n s i t i e s an e l e c t r o d e may f u n c t i o n w e l l , even though some surface r e s t r u c t u r i n g takes place (1), t h i s c a u t i o n can be understood. In simple terms: 5,000 C/cm? at 4xAMl t e l l s us only that at AMI the e l e c t r o d e w i l l be s t a b l e during passage of at l e a s t t h i s amount of charge. I t may very w e l l be s t a b l e f o r 50,000 C/cm , however I ) . 2

Also the r e s u l t s of f i g u r e s 3 and 4 can be understood i n terms of t h i s competition. The regenerative redox r e a c t i o n ( S oxidation) w i l l , as any e l e c t r o c h e m i c a l process, depend on the r e a l surface area, because t h i s w i l l determine the true current d e n s i t y . The higher the r e a l surface area the lower the true current density and the more the regenerative redox r e a c t i o n w i l l be favoured over the photocorrosion process, which depends mainly on the h o l e f l u x to the s u r f a c e . Figure 9B shows the r a t h e r small g r a i n s i z e of our t h i n f i l m CdSe e l e c t r o d e s . Therefore, at equal geometric current d e n s i t i e s , the true current d e n s i t y passing through the p o l y c r y s t a l l i n e t h i n f i l m e l e c t r o d e . Figure 3B furthermore shows that an e l e c t r o d e with true s u r f a c e area i n t e r mediate between these, does indeed show a s t a b i l i t y behaviour b e t ween those of the s i n g l e c r y s t a l and t h i n f i l m e l e c t r o d e . The r e s u l t s of f i g u r e 4 are now c l e a r as w e l l , e s p e c i a l l y i n the l i g h t of the SEM p i c t u r e s of f i g u r e 5. The v a r i o u s s u r f a c e t r e a t ments i n f l u e n c e , i n t e r a l i a , the true s u r f a c e area of the e l e c t rodes and thus t h e i r s t a b i l i t y behaviour. P r e s e n t l y we are studying the e f f e c t s of these surface treatments f u r t h e r , as, f o r example, the improved conversion e f f i c i e n c y of photoetched t h i n f i l m p o l y c r y s t a l l i n e e l e c t r o d e s (6) cannot be explained by t h i s surface area e f f e c t . C l e a r l y a l s o the p r e v i o u s l y observed (2), =

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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and here confirmed d i f f e r e n c e s i n s t a b i l i t y between e l e c t r o d e s with d i f f e r e n t c r y s t a l faces exposed to the s o l u t i o n , cannot be a t t r i b u t e d to surface area e f f e c t s , as the various surface t r e a t ments a l l had s i m i l a r e f f e c t s on the three c r y s t a l faces s t u d i e d : (1010), (0001) and (1120). A p o s s i b l e reason f o r the d i f f e r e n c e s i n s t a b i l i t y between these faces (whichwe found to have decreas i n g s t a b i l i t y i n the order (1120), (1010) (0001) f o r a l l surface treatments) may be found i n the number of bonds that has to be broken to d i s l o d g e a Cd i o n from the s u r f a c e . In the (1120) plane t h e r e a r e two in-plane bonds and one i n t o the bulk phase; f o r the (1010) plane there i s only one in-plane bond and one or two i n t o the bulk phase; the (0001) plane has no in-plane bonds but 1 or 3 i n t o t h e bulk phase. In t h i s , admittedly, s i m p l i f i e d p i c t u r e the (1120) plane i s the s t a b l e one because 3 bonds need to be broken to d i s r u p t i t . A l t e r n a t i v e l y one can look at the p o s s i b i l i t y f o r strong s u r f a c e - s o l u t e i n t e r a c t i o n , which can i n fluence the r e l a t i v e e n e r g e t i c s of the two competing processes and which, i n i t s turn, w i l l be dependent on the number of nonbonded e l e c t r o n s a v a i l a b l e at the s u r f a c e . Future experiments w i l l t r y to c l a r i f y these hypotheses through the use of other crystal faces. We now turn to the strong c r y s t a l s t r u c t u r e dependence of the output s t a b i l i t y of Cd (Se,Te)-based PEC's. In f i g u r e 12 we compare the two c r y s t a l s t r u c t u r e s that can be adopted by the a l l o y s . Both s p a c e - f i l l i n g and s t i c k - a n d - b a l l models are shown, because the c l o s e r e l a t i o n s h i p between the two s t r u c t u r e s can be best appreciated from the s p a c e - f i l l i n g models, while the Cd and chalcogen c o o r d i n a t i o n i s seen c l e a r e s t i n the s t i c k - a n d - b a l l s model. (The hexagonal phase i s e s s e n t i a l l y a close-packed ABAB hexagonal s t a c k i n g of t e t r a h e d r a l l y coordinated Cd atoms, and the cubic phase has ABCABC s t a c k i n g . The f i r s t c o o r d i n a t i o n s h e l l i s i d e n t i c a l i n both forms). Several p o s s i b l e explanations can be forwarded to e x p l a i n t h i s dependence. For example, because the output s t a b i l i t y of hexagonal CdSe depends on the c r y s t a l face exposed to the S~ s o l u t i o n , p r e f e r r e d o r i e n t a t i o n of the, more s t a b l e , (1120) plane, which i s known to be a p r e f e r r e d cleavage plane of the w u r t z i t e phase could e x p l a i n the improved s t a b i l i t y of the hexagonal phase over that of the cubic one. (The s p h a l e r i t e (110) plane i s c r y s t a l l o g r a p h i c a l l y equivalent to the w u r t z i t e (1120) one). Comparison between observed X-ray powder i n t e n s i t i e s and c a l c u l a t e d ones (on the b a s i s of the known c r y s t a l s t r u c t u r e s ) shows indeed some p r e f e r r e d o r i e n t a t i o n i n these pasted electrodes, but r a t h e r of the w u r t z i t e (1010) plane, and of the c r y s t a l l o g r a p h i c a l l y s i m i l a r s p h a l e r i t e (111) one. Probably we are d e a l i n g here with an i n t r i n s i c d i f f e r e n c e between these two s t r u c t u r e types. T h e i r s o l i d s t a t e chemistry shows the w u r t z i t e s t r u c t u r e to be the more r i g i d one ( i n terms of s u b s t i t u t i o n , f o r example). This could imply a d i f f e r e n c e i n Cd-X (X=S,Se,Te) bond s t r e n g t h s . These bond strengths are, according to Pauling's i o n i c i t y concept, i n f l u e n c e d by the

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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Figure 12. Spacefilling(topj and stick-and-ball (bottom,) models of wurtzite (leftj and sphalerite (right,) structures

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE INTERFACES

e f f e c t i v e i o n i c charges on the atoms and these charges can be c a l c u l a t e d from p i e z o e l e c t r i c c o e f f i c i e n t s . For ZnS (the only case where these constants are well-known f o r both s t r u c t u r e types) Zn i s found to have a higher e f f e c t i v e i o n i c charge i n the hexagonal s t r u c t u r e , implying stronger bonding, than i n the cubic one. A l s o , from f i g u r e 6 we see that the cubic form has a c o n s i s t e n t l y lower o p t i c a l bandgap than the hexagonal one (over the range where they can c o e x i s t ) . This we can a s s o c i a t e , be i t somewhat l o o s e l y , with a weaker i n t e r a c t i o n between Cd and X ( i n a l l II-VI compounds that adopt these s t r u c t u r e s , t h i s d i f f e r e n c e i n bandgaps i s found). Since the s t a b i l i t y of Cd chalcogenide photoelectrodes i n pol y s u l f i d e s o l u t i o n depends on the r e l a t i v e p r o b a b i l i t i e s of s e l f o x i d a t i o n , which leads to breaking a Cd-X bond,and i n t e r f a c i a l charge t r a n s f e r , a small change i n bond strength (only 1-1.5 Kcal/mole, from the d i f f e r e n c e s i n bandgaps,i.e. 2-3% of the t o t a l bond d i s s o c i a t i o n energy) may i n f l u e n c e the semiconductor s t a b i l i t y profoundly i f i t i s assumed that the d i f f e r e n c e i n a c t i v a t i o n energies f o r the cleavage of the Cd-X bond,in the two s t r u c t u r e s , i s of the order of 1-1.5 Kcal/mole too. T h i s can be p i c t u r e d s c h e m a t i c a l l y as follows:

Here f- and f2 represent the bond s t r e n g t h of the Cd-X bond, d i c t a t e d by bulk c r y s t a l s t r u c t u r e , and that between the s u r f a c e Cd and the probably adsorbed, p o l y s u l f i d e species from s o l u t i o n , resp. The l a r g e r f , the more l i k e l y i t i s that the photogenerated h o l e , h , w i l l r e a c t with the p o l y s u l f i d e s p e c i e s , i . e . the lower the p r o b a b i l i t y of s e l f - o x i d a t i o n . From t h i s e x p l a n a t i o n f o r the observed d i f f e r e n c e s i n output s t a b i l i t y , we may i n f e r that the decomposition p o t e n t i a l of any CdX depends on i t s c r y s t a l s t r u c t u r e . In view of the r e s u l t s presented here and i n r e f . 2, i t i s l i k e l y that t h i s p o t e n t i a l i s a f u n c t i o n of exposed c r y s t a l face as w e l l . Further i n s i g h t i n those dependences, development of optimal s u r f a c e treatments, and use of e l e c t r o d e s with optimal g r a i n s i z e , may a l l be of help i n d e v i s i n g s t r a t e g i e s to o b t a i n even more s t a b l e CdX/S photoelectrochemical systems. +

=

n

Acknowledg ement s P a r t i a l support of the U.S. I s r a e l B i n a t i o n a l Science Foundat i o n , Jerusalem, I s r a e l , and Ormat Turbines L t d . , Yavneh, I s r a e l , i s g r a t e f u l l y acknolwedged. DC thanks B e l l Labs, f o r h o s p i t a l i t y and A. H e l l e r , B. M i l l e r and M. Robbins f o r p r o v i d i n g some of the e l e c t r o d e s used f o r Figure 3B. Abstract The output stability of CdS, CdSe and to some extent, Cd(Se,Te)/polysulfide photoelectrochemical cells can be explained

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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in terms of a competition between the semiconductor self-oxidation and the desired polysulfide oxidation. This stability is shown to be dependent on solution composition, real electrode surface area, crystal structure and crystal face. Optimal solutions have maximal sulfur dissolution ability. For CdSe increase in the electrode surface area causes an improvement in output stability, thus exp laining the observed differences between polycrystalline and single crystal electrodes, and between single crystal electrodes which are given different surface treatments. Among these, a deliberate photocorrosion process is found to lead to greatest stability. Variations in output stability between single crystal faces may be connected with differences in the number and nature of bonds to surface atoms. Cd(Se,Te) electrodes have reasonable stabilities only if they have a predominantly wurtzite structure. The inferior behaviour of such electrodes with the cubic structure is explained in terms of small differences in bond strengths, which affect the probabilities of the above-mentioned reactions. Literature 1. 2.

3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

Cited

Cahen, D.; Hodes, G.; Manassen, J. J. Electrochem. Soc., 1978, 125, 1623. Heller, Α.; Miller, Β. Adv. Chem., 1980, 184, 215; Miller, B.; Heller, Α.; Robbins, M.; Menezes, S.; Chang, K.C.; Thomson Jr., J. J. Electrochem. Soc., 1977, 124, 1019. Ellis, A.B.; Kaiser, S.W.; Wrighton, M.S. J. Amer. Chem. Soc. 1976, 98, 6855. Russak, M.A.; Reichman, J.; Witzke, H.; Deb, S.K.; Chen, S.N. J. Electrochem. Soc., 1980, 127, 725. Hodes, G.; Cahen, D.; Manassen, J.; David, M. J. Electrochem. Soc., 1980, in press. Hodes, G. Nature (London), 1980, 285, 29. Hodes, G.; Manassen, J.; Cahen, D. J. Amer. Chem. Soc., 1980, in press. Hodes, G.; Manassen, J.; Cahen, D. J. Electrochem. Soc., 1980, 127, 544. Ellis, A.B.; Bolts, J.M.; Kaiser, S.W.; Wrighton, M.S. J. Amer. Chem. Soc., 1977, 99, 2839;2848. Tai, H.; Nakashima, S.; Hori, S. Phys. Stat. Sol. Α., 1975, 30(a), K115. Noufi, R.N.; Kohl, P.Α.; Bard, A.J. J. Electrochem. Soc., 1978, 125, 376. Stuckes, A.D.; Farrell, G. J. Phys. Chem. Solids, 1964, 25, 477. Cahen, D.; Manassen, J.; Hodes, G. Sol. En. Mater., 1979, 1, 343. Gerischer, H.; Gobrecht, J. Ber. Bunsenges. Phys. Chem., 1978, 82, 520. Lando, D.; Manassen, J.; Hodes, G.; Cahen, D. J. Amer. Chem. Soc., 1979, 101, 3969.

RECEIVED October 3, 1980.

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

25 Photoeffects on Solid-State Photoelectrochemical Cells PETER G. P. A N G and ANTHONY F. SAMMELLS

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Institute of Gas Technology, IIT Center, 3424 South State Street, Chicago, IL 60616

Photoeffects in solid-state photovoltaic devices have been demonstrated for a variety of interfaces, e.g., at p-n junctions and Schottky barriers. For such devices a series of highly-controlled fabrication steps are necessary since high-purity semiconductor materials are required to reduce electron-electron hole recombination reactions at trap sites. Greater simplicity in solar cell fabrication can, however, be achieved with liquid-junction photoelectrochemical cells (PEC). For such devices a junction is formed merely by introducing a semiconductor electrode into a redox electrolyte. Illumination of, for example an n-type semiconductor, can result in oxidation of a redox species via its reaction with electron holes photogenerated in the semiconductor space charge region. Solar energy conversion efficiencies achieved with such liquid-junction devices have recently been approaching those of conventional solid-state photovoltaic devices. Photoelectrochemical devices have the inherent advantage that either electricity or useful chemical species, potentially available for later electrochemical discharge, can be produced at the interface, whereas only direct electricity generation occurs with solid-state photovoltaic cells. High solar energy conversion efficiencies have been accomplished by narrow band-gap materials which can utilize a large fraction of the solar spectrum. However, in many cases such materials can manifest photoanodic corrosion effects with time, particularly when the redox species equilibrium potential is close to the semiconductor decomposition potential. Approaches to stabilize those semiconductors have included the evaluation of both organic and inorganic nonaqueous electrolytes. Most work is, however, at an early stage. Extending the thought of identifying electrolytes having higher stability and lower nucleophilicity has led us to consider the utility of solid electrolytes for photoelectrochemical devices. No work has been previously reported on the evaluation of solid-state photoelectrochemical systems. It is difficult to conceive a regenerative solid-state PEC because of the ionic specificity of most solid electrolytes. This has led us to consider semiconductor/solid electrolyte interfaces, where an immobile redox species might be present, either in a solid electrolyte lattice 0097-6156/81/0146-0387$05.00/0 © 1981 American Chemical Society In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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p o s i t i o n , o r as a s e p a r a t e p h a s e , i n i n t i m a t e c o n t a c t w i t h t h e s o l i d electrolyte. I l l u m i n a t i o n of a semiconductor/solid e l e c t r o l y t e i n t e r f a c e c o n t a i n i n g such a redox s p e c i e s might r e s u l t i n e i t h e r o x i d a t i o n o r r e d u c t i o n o f t h i s s p e c i e s , d e p e n d i n g upon w h e t h e r t h e s e m i c o n d u c t o r i s a photoanode or a photocathode. Because the redox s p e c i e s i s i m m o b i l e , s u c h a d e v i c e w o u l d s t o r e t h e e n e r g y p r o d u c e d . To some e x t e n t t h e v i a b i l i t y o f t h i s a p p r o a c h w i l l , i n p a r t , be d e p e n d e n t upon t h e a c c o m m o d a t i o n o f a c o n c e n t r a t i o n o f m o b i l e c a t i o n s somewhat g r e a t e r than t h a t r e q u i r e d f o r acceptable i o n i c c o n d u c t i v i t y across such a s o l i d - s t a t e c e l l . This i s necessary to preserve electroneut r a l i t y i n t h e p r o x i m i t y o f t h e i m m o b i l e r e d o x s p e c i e s . From c r y s t a l l o g r a p h i c c o n s i d e r a t i o n s , b e t a - a l u m i n a has a p a r t i c u l a r l y s u i t a b l e l a t t i c e s t r u c t u r e f o r t h i s c o n c e p t . H e r e , t h e a and b a x e s a r e p e r i o d i c a l l y s e p a r a t e d by A1-0-A1 b o n d s , between w h i c h t h e m o b i l e sodium i o n s m i g r a t e . A s i g n i f i c a n t e x c e s s o f s o d i u m i o n s c a n be a c commodated w i t h i n t h i s m a t e r i a l . I n t h e c o u r s e o f t h i s w o r k s o l i d e l e c t r o l y t e s e v a l u a t e d have i n c l u d e d b e t a - a l u m i n a , l i t h i u m i o d i d e , and r u b i d i u m s i l v e r i o d i d e . U n f o r t u n a t e l y , t h e s o d i u m i o n c o n d u c t i v i t y o f b e t a - a l u m i n a a t room t e m p e r a t u r e i s t o o low t o be o f u t i l i t y i n s o l i d - s t a t e p h o t o e l e c t r o c h e m i c a l d e v i c e s . Any p h o t o p o t e n t i a l gene r a t e d w o u l d p r o b a b l y be i n s u f f i c i e n t t o overcome r e s i s t a n c e l o s s e s w i t h i n the s o l i d e l e c t r o l y t e . Because of the h i g h i o n i c c o n d u c t i v i t y of the rubidium s i l v e r i o d i d e a t ambient temperature, e x p e r i m e n t a l e m p h a s i s was p l a c e d on t h i s m a t e r i a l . -1

R u b i d i u m s i l v e r i o d i d e , R b A g I ç , has a c o n d u c t i v i t y o f O.27 ohm cm a t a m b i e n t t e m p e r a t u r e s (1) and has f o u n d a p p l i c a t i o n i n g a l v a n i c c e l l s ( 2 ) . S o l i d e l e c t r o l y t e s have a l s o been s y n t h e s i z e d t h r o u g h compound f o r m a t i o n b e t w e e n A g i and v a r i o u s s u b s t i t u t e d ammonium h a l i d e s (3-50 and a l s o b e t w e e n A g i and s u l f o n i u m i o d i d e s (6). The s i n g l e c r y s t a l s t r u c t u r e s o f c e r t a i n u n i q u e p h a s e s have b e e n i n v e s t i g a t e d (7-11) t o d e t e r m i n e t h e s t r u c t u r a l f e a t u r e s t h a t p e r m i t f a s t ion d i f f u s i o n i n s o l i d s . I t was o b s e r v e d t h a t t h e i o d i d e i o n s f o r m f a c e - s h a r i n g polyhedra ( g e n e r a l l y tetrahedra) w i t h the s i l v e r ions s i t u a t e d w i t h i n these polyhedra. On t h e a v e r a g e t h e r e a r e two o r three vacant s i t e s per s i l v e r i o n , thus p e r m i t t i n g ready d i f f u s i o n through the faces of the polyhedra. P h o t o e f f e c t s a t the s e m i c o n d u c t o r / R b A g I s i n t e r f a c e were i n v e s t i g a t e d w i t h the o b j e c t i v e of i d e n t i f y i n g the u t i l i t y of t h i s s o l i d e l e c t r o l y t e material i n solid-state photoelectrochemical devices with storage. -1

Experimental S o l i d - s t a t e c e l l s w e r e f a b r i c a t e d u s i n g R b A g I s ( R e s e a r c h Org a n i c I n o r g a n i c , Sun V a l l e y , C a l i f o r n i a ) , s i l v e r powder ( C e r a c , I n c . ) and P b l 2 a n d / o r i o d i n e powder. L a y e r s o f t h e a p p r o p r i a t e m i x t u r e s w e r e p r e p a r e d i n s i d e a c o m m e r c i a l KBr d i e and p r e s s e d a t 2000 k g / c m u s i n g a C a r v e r l a b o r a t o r y p r e s s . The r e s u l t i n g c e l l ( a b o u t 2mm t h i c k ) was s a n d w i c h e d b e t w e e n a c o u n t e r e l e c t r o d e and a t r a n s p a r e n t s e m i conductor working e l e c t r o d e . Current c o l l e c t i o n to the working e l e c t r o d e was made by a t r a n s p a r e n t c o n d u c t i n g g l a s s . The conducting 2

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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g l a s s e l e c t r o d e was t i n o x i d e - c o a t e d C o r n i n g 7059 g l a s s w i t h a t h i c k n e s s o f O.032" and a s t a n d a r d r e s i s t i v i t y o f 100 ± 30 ohms p e r s q u a r e . A s m a l l s i l v e r w i r e r e f e r e n c e e l e c t r o d e was i n s e r t e d i n s i d e the c e l l v i a a s m a l l h o l e . The p h o t o e l e c t r o c h e m i c a l p r o p e r t i e s o f t h e s e m i c o n d u c t o r / s o l i d e l e c t r o l y t e i n t e r f a c e was i n v e s t i g a t e d u s i n g a W e n k i n g ST-72 p o t e n t i o s t a t and a n O r i e l 150-Watt x e n o n l i g h t s o u r c e . Monochromatic l i g h t was o b t a i n e d b y u s i n g an O r i e l M o d e l 7240 monochromator. The s l i t o p e n i n g o f t h e monochromator was s e t a t 1.5mm, t h u s g i v i n g a s p e c t r a l b a n d p a s s o f 10 nm. The e l e c t r o d e p o t e n t i a l was measured u s i n g a Wenking PPT-70 e l e c t r o m e t e r . A T a c u s s e l t y p e GSTP2B p u l s e sweep g e n e r a t o r was u s e d t o c o n t r o l t h e p o t e n t i o s t a t . The c u r r e n t v o l t a g e r e l a t i o n s h i p s o f t h e e l e c t r o d e s w e r e r e c o r d e d on a H e w l e t t P a c k a r d 7046 X-Y r e c o r d e r . An η-type p o l y c r y s t a l l i n e CdSe was made by t h e r m a l vacuum e v a p o r a t i o n o f CdSe powder ( 9 9 . 9 9 9 % , C e r a c , I n c . ) on t h e c o n d u c t i n g g l a s s . T h i s was a c c o m p l i s h e d by u s i n g a n Edwards 306 t h i n - f i l m e v a p o r a t o r a t a p r e s s u r e o f 10*" T o r r . The e l e c t r o d e was a n n e a l e d i n a i r a t 400°C f o r 30 m i n u t e s and c o o l e d down s l o w l y . 6

R e s u l t s and

Discussion

Photoactivity ofRbAg I . Before e v a l u a t i n g photoeffects a t the semiconductor-solid e l e c t r o l y t e i n t e r f a c e , i t i s f i r s t necessary t o c h a r a c t e r i z e any p h o t o a c t i v i t y p r e s e n t i n t h e s o l i d e l e c t r o l y t e itself. R u b i d i u m s i l v e r i o d i d e can be e x p e c t e d t o p o s s e s s some pho t o s e n s i t i v i t y b y analogy t o the p h o t o v o l t a i c e f f e c t demonstrated by t h e b i n a r y s i l v e r h a l i d e s (12» 13.). T h i s was p e r f o r m e d by e v a l u a t i n g a c e l l o f c o n f i g u r a t i o n Cond. g l a s s / P b I + R b A g I s / R b A g i / A g + R b A g i I / C o n d . g l a s s . S o l i d - s t a t e c e l l s b a s e d upon t h i s s o l i d e l e c t r o l y t e u s u a l l y c o n s i s t o f a s i l v e r anode and a c o m p l e x e d i o d i n e c a t h o d e ( 1 4 ) . T h i s c e l l was mounted b e t w e e n two t r a n s p a r e n t c o n d u c t i n g g l a s s e l e c t r o d e s . Upon i l l u m i n a t i o n o f t h e Cond. g l a s s / P b l 2 + RbAgi I i n t e r f a c e , t h i s s i d e developed p o s i t i v e p h o t o p o t e n t i a l s o f b e t w e e n 3 and 50 mV. However, a f t e r i n i t i a l l y p a s s i n g b o t h a n o d i c as w e l l a s c a t h o d i c c u r r e n t s t h r o u g h t h i s i n t e r f a c e , l a r g e r p h o t o p o t e n t i a l s c o u l d be a c h i e v e d . D u r i n g passage o f anodic c u r r e n t a t t h i s i n t e r f a c e , d a r k a r e a s a p p e a r e d on t h e e l e c t r o l y t e s u r f a c e . This might be e x p l a i n e d by the f o r m a t i o n of f r e e i o d i n e at t h i s i n t e r f a c e . L a r g e r p h o t o p o t e n t i a l s were r e a l i z e d a t these d a r k e r a r e a s . F o r e x a m p l e , a p o t e n t i a l o f 450 mV i n t h e d a r k and 850 mV u n d e r i l l u m i n a t i o n was f o u n d v e r s u s t h e s i l v e r r e f e r e n c e e l e c t r o d e . T h i s i s shown i n F i g u r e 1. However, when t h e l i g h t beam was n o t f o c u s e d o n t h e d a r k e n e d s p o t , t h e p h o t o p o t e n t i a l was o n l y a b o u t 5 mV. L a r g e p h o t o p o t e n t i a l s were s t i l l d e t e c t e d a t w a v e l e n g t h s l o n g e r t h a n 4750 A, when a s h a r p - c u t f i l t e r was u s e d i n t h e l i g h t p a t h . The r e s p o n s e o f t h e c e l l u n d e r t h i s c o n d i t i o n i s shown b y t h e s e c o n d d e f l e c t i o n shown i n F i g u r e 1. The a c t i o n s p e c t r a o f t h e c e l l w i l l be d e s c r i b e d l a t e r i n more d e t a i l . u

s

2

t

f

5

5

P h o t o e x c i t a t i o n o f t h e R b A g I s can be e x p e c t e d t o r e s u l t i n t h e formation o f electron-hole p a i r s . The p h o t o e l e c t r o n s w i l l d i f f u s e f r o m t h e s u r f a c e i n t o t h e b u l k b e c a u s e o f t h e i r much g r e a t e r d r i f t m o b i l i t y compared t o t h e p h o t o h o l e s . A net p o s i t i v e space charge

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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INTERFACES

Figure 1. Photopotential in the cell: cond. glass/Pblt + RbAgJs/RbAgJg/Ag + RbAg l /cond. glass. The cond. glass/PbI + RbAg I interface was illu minated with 200 mW /cm xenon light. k 5

2

!f

5

2

Figure 2. Current-voltage characteristics of the cell: cond. glass/Pbl + RbAg l / RbAg,JrJΆ g + Rb A g,J /cond. glass under chopped illumination (200 mW/cm ) at 25°C (voltage scan rate: 25 mV/s) 2

k 5

2

5

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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w i l l p r o b a b l y b e f o r m e d n e a r t h e i l l u m i n a t e d e l e c t r o d e due t o t h i s h i g h e r c o n c e n t r a t i o n o f p h o t o h o l e s . T h e s e p h o t o h o l e s c a n be e x p e c t e d t o o x i d i z e any f r e e s i l v e r p r e s e n t a t t h i s i n t e r f a c e . T h e s e p r o cesses can l e a d t o a p o s i t i v e photovoltage a t the i l l u m i n a t e d e l e c t r o d e , w h i c h c a n b e c o r r e l a t e d t o Dember-type p h o t o v o l t a g e ( 1 3 , 1 5 ) . The c u r r e n t - v o l t a g e c h a r a c t e r i s t i c s o f t h e c e l l w e r e a n a l y z e d by s w e e p i n g t h e c e l l v o l t a g e u s i n g a p o t e n t i o s t a t . The c e l l v o l t a g e was m e a s u r e d a s t h e v o l t a g e b e t w e e n t h e i l l u m i n a t e d s i d e v e r s u s t h e n o n - i l l u m i n a t e d s i l v e r s i d e . U n d e r chopped x e n o n l i g h t i l l u m i n a t i o n , c a t h o d i c a s w e l l a s a n o d i c p h o t o c u r r e n t s w e r e r e c o r d e d , as shown i n F i g u r e 2. T h e s e p h o t o c u r r e n t s , a s w e l l a s t h e p h o t o p o t e n t i a l i n F i g u r e 1, i n d i c a t e t h a t p h o t o e x c i t e d c h a r g e c a r r i e r s a r e p r o d u c e d i n a d d i t i o n t o the s i l v e r ions present i n the dark. This c o r r e l a t e s w e l l w i t h t h e H a l l e f f e c t o f R b A g I s i n t h e d a r k and u n d e r i l l u m i n a t i o n t h a t has been p r e v i o u s l y r e p o r t e d ( 1 6 ) . H a l l s i g n a l s i n t h e d a r k a r e c a u s e d b y t h e Ag+ i o n c a r r i e r s . Under i l l u m i n a t i o n t h e s i g n o f the H a l l v o l t a g e , however, i s r e v e r s e d . From t h i s , and b y v i r t u e o f t h e m a g n i t u d e and t e m p e r a t u r e dependence o f H a l l m o b i l i t y , i t was c o n c l u d e d t h a t t h e H a l l s i g n a l s u n d e r i l l u m i n a t i o n was p r i m a r i l y c o n t r i b u t e d by p h o t o e l e c t r o n s . I f we assume t h a t t h e d a r k a r e a s a r e f o r m e d by p h o t o l y t i c i o d i n e , then the p o s i t i v e p h o t o p o t e n t i a l s i n F i g u r e 1 can be expected t o l e a d t o l a r g e r c a t h o d i c c u r r e n t s . H e r e t h e i o d i n e w i l l be r e d u c e d t o i o d i d e . Under c o n t i n u o u s i l l u m i n a t i o n , i t h a s b e e n shown t h a t c a t h o d i c p h o t o c u r r e n t s c a n b e drawn f o r many h o u r s . When t h e p o t e n t i a l o f t h e i l l u m i n a t e d i n t e r f a c e was made a n o d i c , i o d i d e w i l l b e o x i d i z e d t o i o d i n e on t h e s u r f a c e . Photogenerated charge c a r r i e r s may r e d u c e t h e i n t e r n a l r e s i s t a n c e o f t h e c e l l , t h e r e b y i n c r e a s i n g t h e e f f e c t i v e c u r r e n t d e n s i t y upon i l l u m i n a t i o n , a s shown i n F i g u r e 2. However, a t l a r g e c e l l v o l t a g e s , t h e e l e c t r o l y t e w i l l decompose, a c c o r d i n g t o (14) : 2 RbAgi+Is + 4 A g + 2 R b l + 4 A g i , w i t h t h e c o n c u r r e n t e q u i l i b r a t i o n R b l 3 £ R b l + l£. I o d i n e g e n e r a t e d b y s u c h mechanisms may b e r e s p o n s i b l e f o r t h e d a r k a r e a s on t h e e l e c t r o l y t e s u r f a c e . An i n c r e a s e i n the anodic dark current at v o l t a g e s higher t h a n 900 mV i n F i g u r e 2 c a n b e c o r r e l a t e d w i t h t h i s p r o c e s s . The e x p e r i m e n t , h o w e v e r , was p e r f o r m e d w i t h n o c o m p e n s a t i o n f o r r e s i s t a n c e and p o l a r i z a t i o n l o s s e s w i t h i n t h e s o l i d e l e c t r o l y t e c e l l . Hence, t h e v o l t a g e s f o r t h e i n c e p t i o n o f e l e c t r o c h e m i c a l r e a c t i o n s may d i f f e r f r o m v a l u e s e x p e c t e d f r o m a r e v e r s i b l e e l e c t r o d e . I t i s of i n t e r e s t to i n v e s t i g a t e the p h o t o s e n s i t i v i t y of RbAgIs s o l i d e l e c t r o l y t e doped w i t h f r e e i o d i n e . H e r e t h e s u r f a c e o f t h e p r e s s e d e l e c t r o l y t e was c o a t e d w i t h s o l i d i o d i n e f o r a few s e c o n d s u n t i l i t became l i g h t brown. The c e l l was a s s e m b l e d b e t w e e n t r a n s parent conducting g l a s s current c o l l e c t o r s . The p o t e n t i a l o f t h e i o d i n e - t r e a t e d s u r f a c e was m e a s u r e d a g a i n s t a s i l v e r r e f e r e n c e e l e c t r o d e . Due t o t h e h i g h a c t i v i t y o f t h e i o d i n e , t h e p o t e n t i a l was 661 mV i n t h e d a r k . T h i s s u r f a c e i m m e d i a t e l y d e v e l o p e d a l a r g e p o s i t i v e p h o t o p o t e n t i a l o f a b o u t 100 mV upon i l l u m i n a t i o n w i t h t h e x e n o n light. The c u r r e n t - p o t e n t i a l c h a r a c t e r i s t i c s o f t h i s s u r f a c e was e s s e n t i a l l y t h e same a s t h e one shown i n F i g u r e 2. However, an i n c r e a s e i n t h e d a r k c u r r e n t i n t h e a n o d i c d i r e c t i o n was n o t i c e d . The d e p e n d e n c y o f t h e p h o t o p o t e n t i a l o f t h i s c e l l upon t h e w a v e l e n g t h 3

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PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE INTERFACES

of the i l l u m i n a t i n g l i g h t i s shown i n Figure 3. Below 300 nm, essen t i a l l y no p h o t o p o t e n t i a l was detected. This was not s u r p r i s i n g s i n c e the g l a s s and the t i n oxide l a y e r on the conducting g l a s s e l e c t r o d e would absorb l i g h t with wavelengths below t h i s value. The band-gap of Sn0 i s w i t h i n t h i s range, i . e . , 3.8 eV. T h i s would correspond to a wavelength of about 326 nm. The conducting g l a s s had no photo a c t i v i t y when i t was i l l u m i n a t e d i n l i q u i d e l e c t r o l y t e s . In Figure 3 the photoresponse peak of t h i s s o l i d e l e c t r o l y t e system occurred a t a wavelength of about 425 nm. The p h o t o p o t e n t i a l decreased at longer wavelength. The peak at 425 nm can be c o r r e l a t e d t o the 425 nm band i n the o p t i c a l absorption spectrum of RbAgi+Is. The spectrum a c t u a l l y showed four bands at 485, 425, 375, and 335 nm (17). The 425 nm band was a t t r i b u t e d t o a forbidden i n t e r n a l t r a n s i t i o n i n the A g which becomes permitted because o f the t e t r a h e d r a l c o o r d i n a t i o n of the Ag i o n i n R b A g I s . The p h o t o p o t e n t i a l , as w e l l as the anodic and cathodic photocurrents were seen up t o wavelengths approaching 700 nm. The p h o t o p o t e n t i a l was a l s o measured as a f u n c t i o n of l i g h t i n t e n s i t y . At 200 mW/cm xenon l i g h t i l l u m i n a t i o n , a p h o t o p o t e n t i a l of about 100 mV was measured. The p h o t o p o t e n t i a l decreased upon a t t e n u a t i o n of the l i g h t i n t e n s i t y with n e u t r a l d e n s i t y f i l t e r s . A p l o t of the p h o t o p o t e n t i a l versus the r e l a t i v e l i g h t i n t e n s i t y i s shown i n F i g u r e 4. The p h o t o p o t e n t i a l i s p r o p o r t i o n a l to the loga rithm of the l i g h t i n t e n s i t y , as observed i n the Dember photovoltage of AgBr (13). 2

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+

+

2

CdSe/Solid E l e c t r o l y t e J u n c t i o n . I t has been demonstrated i n the previous s e c t i o n that RbAg l5 i s p h o t o s e n s i t i v e . Our primary goal i n t h i s study was a c t u a l l y t o see p h o t o a c t i v i t y at a semiconductor/ s o l i d e l e c t r o l y t e i n t e r f a c e . An η-type CdSe was chosen f o r t h i s study since a t h i n - f i l m , p a r t i a l l y transparent e l e c t r o d e could be f a b r i c a t e d by thermal vacuum evaporation on a transparent conducting g l a s s . A c e l l of the c o n f i g u r a t i o n CdSe on Cond. glass/12 + R b A g I s / RbAgIs/Cond. g l a s s was f a b r i c a t e d . A small quantity of i o d i n e was introduced onto the s o l i d e l e c t r o l y t e surf ace as redox s p e c i e s . The CdSe e l e c t r o d e was then placed i n contact with t h i s s u r f a c e . The p o t e n t i a l of the CdSe was measured versus a s i l v e r wire reference e l e c t r o d e . The p o t e n t i a l i n the dark was 650 mV and i t moved to about 300 mV under 200 mW/cm xenon l i g h t i l l u m i n a t i o n . However, the ac t u a l l i g h t i n t e n s i t y reaching the CdSe/solid e l e c t r o l y t e i n t e r f a c e was s i g n i f i c a n t l y lower. T h i s r a p i d negative p h o t o p o t e n t i a l i s shown i n Figure 5. T h i s phenomenon i s analogous to the behavior of n-CdSe i n l i q u i d - j u n c t i o n photoelectrochemical c e l l s . The dependency of the CdSe e l e c t r o d e p o t e n t i a l upon the wave length of the l i g h t i s shown i n Figure 6. The spectrum was not cor rected f o r v a r i a t i o n s i n l i g h t i n t e n s i t y by the monochromator. The mean i n t e n s i t y at 625 nm was 70 yW/cm with v a r i a t i o n s at other wave lengths estimated as ±25 uW/cm . No p h o t o p o t e n t i a l was seen below 300 nm. Between 300 and 375 nm, a p o s i t i v e p h o t o p o t e n t i a l was de t e c t e d . This could have the same o r i g i n as the p h o t o p o t e n t i a l of the Cond. glass/RbAg I i n t e r f a c e shown i n Figure 3. A l a r g e negalf

2

2

2

i+

5

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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ANG

AND

SAMMELLS

Solid-State Photoelectrochemical Cells

393

Figure 3.

Potential of cond. glass/1 + RbAgJs interface vs. Ag as a function of wavelength

Figure 4.

Photoresponse of cond. glass/1 + intensity

2

2

RbAgJJcond.

glass vs. light

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

394

PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE

INTERFACES

DARK

r

< CO

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>

LIGHT 1

1

200 TIME, s

Figure 5.

Photopotential of CdSe/I + RbAgJrJRbAgJJcond. mination with 200 mW/cm xenon light 2

glass, back-illu-

2

Figure 6.

Potential of CdSe in contact with I, + RbAgJs vs. Ag as a function of the wavelength of the light

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

25.

A N G AND SAMMELLS

Solid-State Photoelectrochemical Cells

395

t i v e p h o t o p o t e n t i a l can be seen i n Figure 6 a t wavelengths longer than 375 nm, up to about 800 nm, which corresponds c l o s e l y to the 1.7 eV band-gap of CdSe. Thus, the negative p h o t o p o t e n t i a l can be assigned to the CdSe/solid e l e c t r o l y t e i n t e r f a c e . The photoresponse between 300 and 500 nm i n F i g u r e 6 might very w e l l be a superposit i o n of the p o s i t i v e p h o t o p o t e n t i a l of the Cond. g l a s s / R b A g I s i n t e r f a c e and the negative p h o t o p o t e n t i a l of the CdSe/RbAgIs i n t e r face. The magnitude of the negative p h o t o p o t e n t i a l was l a r g e s t a t 625 nm. The dependency of t h i s p h o t o p o t e n t i a l upon the i n t e n s i t y of the i n c i d e n t l i g h t was a l s o analyzed. Figure 7 shows the negat i v e p h o t o p o t e n t i a l as a f u n c t i o n of i n c i d e n t o p t i c a l power. The r e l a t i v e s c a l e a t 10° corresponds to 70 yW/cm , where the photopot e n t i a l was —85 mV. With approximately 100 mW/cm white xenon l i g h t , photopotentials up to —300 mV have been detected. At the very low l i g h t i n t e n s i t y of O.7 yW/cm a t 625 nm, photopotentials of —2 mV were s t i l l observed. At moderate l i g h t i n t e n s i t i e s the photopotent i a l i s p r o p o r t i o n a l to the logarithm of the l i g h t i n t e n s i t y . The c u r r e n t - p o t e n t i a l c h a r a c t e r i s t i c s of the CdSe e l e c t r o d e were studied under chopped i l l u m i n a t i o n . The r e s u l t i s shown i n Figure 8. The o p e n - c i r c u i t p o t e n t i a l versus Ag was 350 mV under xenon l i g h t . When the p o t e n t i a l of the CdSe e l e c t r o d e was scanned i n the p o s i t i v e d i r e c t i o n , anodic photocurrents flowed. At high p o t e n t i a l s the photocurrent increased, but the dark current increased as w e l l . When scanned i n the negative d i r e c t i o n , a small cathodic current flowed due t o reduction of i o d i n e . I n i t i a l l y , small anodic photocurrent d e f l e c t i o n s were seen under chopped i l l u m i n a t i o n near the r e s t i n g p o t e n t i a l . However, at higher cathodic p o l a r i z a t i o n the photocurrent became cathodic. A cathodic photocurrent has already been demonstrated by RbAgi+Is as shown i n F i g u r e 2. B r i n g i n g the p o t e n t i a l more cathodic than the e q u i l i b r i u m p o t e n t i a l of s i l v e r r e s u l t e d i n a l a r g e cathodic current corresponding to reduction of s i l v e r ions to s i l v e r . When the p o t e n t i a l was brought back to the anodic d i r e c t i o n , an anodic current flowed due t o r e o x i d a t i o n of the s i l v e r . U s u a l l y , the p h o t o p o t e n t i a l decreased when the e l e c t r o d e i s brought to near 0 mV versus s i l v e r . T h i s could be due t o p l a t i n g of s i l v e r on the e l e c t r o d e surface. However, upon r e o x i d a t i o n of the s i l v e r the p h o t o p o t e n t i a l was r e s t o r e d . The decomposition of RbAgi to e i t h e r s i l v e r or i o d i n e species may create l i m i t a t i o n s to the use of t h i s s o l i d e l e c t r o l y t e f o r photoelectrochemical c e l l s . 2

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2

2

Conclusion We have demonstrated p h o t o a c t i v i t y at s o l i d e l e c t r o l y t e / C o n d . g l a s s i n t e r f a c e s . The p o s i t i v e photopotentials could, however, be a Dember-type photovoltage. Cathodic photocurrents passed when the c e l l was discharged. I l l u m i n a t i o n may y i e l d i o d i n e on the i l l u m i nated s i d e with some reduced species (probably s i l v e r ) i n the bulk of the e l e c t r o l y t e . When CdSe was i n contact with the s o l i d e l e c t r o l y t e c o n t a i n i n g i o d i n e redox s p e c i e s , negative photopotentials and anodic photocurrents were detected. This i s analogous to the behavior of n-type semiconductor i n l i q u i d - j u n c t i o n s o l a r c e l l s . In the case of s o l i d

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

396

INTERFACES

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PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE

Figure 8.

Current-potential characteristics of CdSe/I + RbAg,I interface under chopped illumination (voltage scan rate: 40 mV/s) 2

5

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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ANG

AND

SAMMELLS

P-TYPE PHOTOCATHODE

Solid-State Photoelectrochemical Cells

397

ΓΛΝ-ΤΥΡΕ PHOTOANODE X

Figure 9. Photoelectrochemical cell with a solid electrolyte containing reductant and oxidant on each side (the charging circuit contains rectifiers to prevent selfdischarge in the dark)

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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398

PHOTOEFFECTS AT

SEMICONDUCTOR-ELECTROLYTE

INTERFACES

e l e c t r o l y t e s , the redox species are immobile. T h i s opens up the pos s i b i l i t y of c o n s t r u c t i n g a s o l i d - e l e c t r o l y t e photoelectrochemical c e l l with a storage c a p a b i l i t y as shown i n Figure 9. The s o l i d s t a t e e l e c t r o l y t e device would c o n s i s t of three phases i n intimate contact where i n the i d e a l case the c r y s t a l l a t t i c e s t r u c t u r e of the reductant, oxidant, and s o l i d e l e c t r o l y t e phases would be s i m i l a r to f a c i l i t a t e r a p i d movement of ions between each phase. The reductant and oxidant p o r t i o n s of the device would contain s e l e c t e d redox species e i t h e r introduced i n t o f i x e d immobile l a t t i c e s i t e s i n the s o l i d e l e c t r o l y t e ( i n the i d e a l case) or dispersed and i n intimate contact with the s o l i d e l e c t r o l y t e i n each of the r e s p e c t i v e compartments. The reductant and oxidant e l e c t r o d e s would possess both i o n i c and e l e c t r o n i c c o n d u c t i v i t y , whereas the separating s o l i d e l e c t r o l y t e would possess only i o n i c c o n d u c t i v i t y . A f t e r s e p a r a t i o n of e l e c t r o n s and holes i n the η-type semiconductor, the f o l l o w i n g general r e a c t i o n s may occur during charge: At oxidant e l e c t r o d e — nC

, +h

+

+ M . l(lattice)

+

-

MÎ!fHÏ l(lattice)

+

(n-DC

+

At reductant e l e c t r o d e — (n-l)C

+ M 2Î ( l a t t„.i c e,) + e-

+

Un'll2 ( l a t t i c e ï) +

+

n

C

+

where: Μ. represents a redox species which i s o x i d i z e d by an e l e c tron hole h ; C represents a mobile i o n conductor; and M2 repre sents a redox species which i s reduced by an e l e c t r o n e". During discharge, the f o l l o w i n g r e a c t i o n may occur: +

+

+ (n+l) 1(lattice)

M+fa- ) 2(lattice) 1

W

M" + 1(lattice) 11

10

M* 2(lattice)

The b a s i c requirement of the redox oxidant i n contact with n-type semiconductors i s that i t has an e q u i l i b r i u m p o t e n t i a l more negative than the decomposition p o t e n t i a l of the semiconductor and more p o s i t i v e than the lower edge of the semiconductor conduction band. The b a s i c requirement of the reductant e l e c t r o l y t e i s that i t s redox e q u i l i b r i u m p o t e n t i a l be negative of the oxidant e l e c t r o l y t e and more p o s i t i v e than the lower edge of the semiconductor conduction band. More work w i l l be necessary at c h a r a c t e r i z i n g s o l i d e l e c t r o lyte/semiconductor i n t e r f a c e s with those s o l i d e l e c t r o l y t e s a v a i l able before s a t i s f a c t o r y s o l i d - s t a t e devices capable of photocharge can be r e a l i z e d . Acknowledgment

for

T h i s work was sponsored by the Solar Energy Research I n s t i t u t e the United States Department of Energy.

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

25.

A N G AND S A M M E L L S

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Owens, B.B.; Oxley, J.E.; Sammells, A.F., "Applications of Halogenide Solid Electrolytes," in Topics in Applied Physics, 1977, Vol. 21, Solid Electrolytes, Geller, S., Ed., Berlin: Springer-Verlag.

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Radhakrishna, S.; Hariharan, K.; Jagadeesh, M.S. J. Appl. Phys., 1979, 50, 4883.

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F. J. Appl. Phys., 1969, 40, 66.

1972, 29, 937.

R E C E I V E D October 3, 1980.

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

INDEX Β

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A

Absorption, light 1 Backscattering studies, Rutherford.... 65 photocurrent and anisotropy of 9-11 Band Acetonitrile 255 bending 80, 296, 360, 363 anthraquinone in 257 edge(s), pinned vs. unpinned 256/ current-voltage curve for 260/ unpinning produced by the light intensity dependence of the effects of inversion 253-265 photoreduction current of.. 259/ unpinning of the semiconductor.. 253 capacitance vs. p-Si electrode gap 255 potential 262/ analysis 210-211 Acids, fatty 233 for Ba r,Sr N b 0 212/ Aging for FeNb0 215/ cathodic 96 for F e 0 214/ experiments, electrochemical 95/ for Sr Nb 0 214/ conditions for SrTi0 dependence for SrTi0 212/ of semiconductor flatband optical 208,209/ potential on 94/ of Sn0 392 doping profile after anodic 91 of «-type T i 0 339 experiments, electrochemical anodic 93/ levels, effects of pH on 199 Alcohols, fatty 233 model of oxidized silicon 186/ Alloy(s) 220-222 potential, flat208, 209/, 355, 363 stoichiometry 377 structure and energy terms of a β-Alumina 388 semiconductor 146/ mes0-tetra(a,a,a,a-0-Aminophenyl)structure and interface states in porphyrin, tetraoctadecylamide photoelectrochemical cells, derivative (H TOAPP) of 282 electrode 217-230 Anisotropic properties of layered theory approximation 220 semiconductors 18 Bifluorenyl 339 Anisotropy of light absorption, Bilayer lipid membranes 239 photocurrent and 9-11 hydrolysis of cellulosic 192 Anisotropy, optical 4 Biomass materials, microbiological.... 192 Anode surface, rate of holes at 210 /ns(2,2'-Bipyridine)ruthenium(II) Anodic complex 245 aging, doping profile after 91 Bismuth 67 aging experiments, electrochemical 93/ 59 dissolution reactions 147 Bonding theory, chemical 0

05

2

6

4

2

2

3

3

2

7

3

2

2

2

oxidation of carboxylic acids of halide ions of the semiconductor photocurrents Anthraquinone in acetonitrile current-voltage curve for light intensity dependence of the photoreduction current of Arsenic compounds oxides trichloride (AsCl ), cyclic voltammogram of Auger spectroscopy

191 141 179 395 257 260/

C C d S e T e i . , crystallographic data for 377 Co(bipy) * 44 CoS counterelectrode(s) 370, 371/ CuInS 296 Cadmium -chalcogenide -based photoelectrochemical cells, stability of 369-385 films, scanning electron micro scope pictures of painted.... 378/ photoelectrodes, stability of 384 x

2

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2

259/ 345 64

3

347/ 160

403

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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404

PHOTOEFFECTS

AT SEMICONDUCTOR-ELECTROLYTE

Cadmium (continued) selenide (CdSe) 218,369, 370 and CdS polycrystalline, deactivation of 379 cells, output stability of polycrystalline 372 cells, output stability of singlecrystal 372 electrode, single-crystal bare 224/ photoelectrode(s) 370,371/ short-circuit current of polycrystalline painted 371/ time dependence of shortcircuit current for singlecrystal 374/ time dependence of shortcircuit current for thinfilm polycrystalline and CdS 380/ polysulfide junction 363 analysis of current-voltage characteristics of illuminated 359-368 potential of 394/ scanning electron microscope pictures of single-crystal... 375/ single-crystal 272 electrode characterization 364t solid electrolyte junction 392-395 -SrTiOs composite electrode, representation of 219/ surface, competing reactions at illuminated 380/ (Se,Te) direct optical band-gap of 376/ (Se,Te) electrodes, time dependence of short-circuit current for several 378/ sulfide (CdS) 369 and CdSe photoelectrodes, time dependence of short-circuit current for thin-film polycrystalline 380/ electrode, current-voltage curves of 109/ electrodes, effects of temperature on excited-state descriptions of luminescent photoelectrochemical cells employing telluriumdoped 295-304 polycrystalline, deactivation of CdSe and 379 sulfide ions on 107-112 : tellurium 298/ -doped 295 electrode 302/ exchange reaction for 300 telluride (CdTe) 369,370

INTERFACES

rt-Cadmium selenide (/i-CdSe) as the anode, energy diagram for a regenerative photoelectrochemical cell with 268/ in polysulfide solution, electrolyte electroreflectance spectra of single-crystal 276/ single-crystal in polysulfide, impedance response curves for 271/ Capacitance-voltage techniques 351 Carbanion photooxidation of semiconductor surfaces 337-341 Carbon impurity(ies) 160, 165 Carboxylic acid, anodic oxidation o f . 191 Carrier recombination at steps in surfaces of layered compound photoelectrodes 17-33 Carriers, minority 30, 33 Cathodic aging 96 experiments, electrochemical 95/ Cathodic photocurrents, short-circuit. 243 Cation concentrations, high-alkali 167 Cellulosic biomass materials, microbiological hydrolysis of 192 Chalcogenide photoelectrodes, stability of 384 Charge -collection microscopy 25-27 -transfer processes in photoelectrochemical cells 79-100 -transfer rate constant 361, 367 transport, interlayer 30 Chemisorption 59,79-82 of fluorine 63 plumbite 67 reactions 69 of a reagent 60/ ruthenium 67 splitting of surface states upon 73/ Chlorophyll(s) -coated electrodes metal 242-244 semiconductor 238-242 and metal, photoelectrochemical systems involving 237-244 electrochemical energetics of 235-237 energy migration among 239 and its films, microcrystalline 232 interfacial layers, morphological aspects of 232-233 monolayer assemblies of 233 monolayers deposited on Sn0 291 one-electron oxidation potentials for 236/ photocurrent spectra for 240/ photoelectrochemical systems involving solid-liquid interfacial layers of 231-247 2

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

405

INDEX

Chlorophyll(s) (continued) photophysics 234 utilization, general technique for 237-244 Chloroplast lamellae 233 Cole-Cole plot for rc-GaAs 354/ Composite electrodes 217-220 Contaminants, surface 165-167 Conversion efficiency, improved 381 Conversion efficiency of semicon ductor-liquid junction solar cell, solar-to-electrical 67 Corrosion 331 of gallium 330/ product(s) 351 Si0 179 suppression role of the sulfide ions 107 Counterelectrode, CoS 371/ Crystal(s) Auger spectra of water-rinsed stoichiometric 164/ GaSe 9, 11 structure of 10/ hydrogen evolution from platinumfree 161 hydrogen production on metalfree 173-174 imperfections 4 on photoelectrochemical reac tions at semiconductor elec trodes, the influence of sur face orientation and 1-15 the role of 11-13 MoSe 5 Crystalline inhomogeneity 351 Current disk 133/, 135/, 137/ experimental reduction 113/ output stability characteristics, short-circuit 369 response, pseudo-capacitive 316 ring 133/ -voltage characteristics of the cell 390/ of illuminated CdSe/polysulfide junctions, analysis of 359-368 of /i-WSe 22/, 24/

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2

2

2

D Decay routes, interrelationships of excited-state Decomposition electrolyte processes reactions surface Dember-type photovoltage of AgBr

301-303 352 351 145 82-85 391 392

Depletion layer, recombination in 360 Deposition techniques, monolayer 237 Desorption 85 rates, in photo-assisted electrochem ical reactions, role of ionic product 307-335 Destabilization process, autocatalytic nature of 381 Dichalcogenides, molybdenum 18 Dichalcogenides, transition metal 222 layered compound 17 Diffusion, recombination due to 361 Dihydrooctaphenylfulvalene 339 1,3-Dimethoxy-4-nitrobenzene in methanol 257 photoreduction current of 261/ Dimethyldithiocarbamate 51 1, Γ-Dimethylferrocene 49 Dipalmitoyllecithin 241 Dissolution reactions, anodic 147 Doping boundaries 71, 75 density 255 distribution 269 levels, optimal semiconductor 313 profile after anodic aging 91 Dye(s) 341 assembly construction procedure.... 281/ sensitization process, electron transfer in 240/ visible-light absorbing 279 Ε EBIC (see Electron Beam Induced Current) Electric charge in surface states 4 Electrical conductivity in layered compounds 30 Electricity generation, direct 387 Electroactive impurities 352 Electrochemical behavior and surface structure of GaP electrodes 145-157 equilibria analysis 327 reactions at illuminated semicon ductors, factors governing the competition in 131-143 reactions, role of ionic product de sorption rates in photoassisted 307-335 solar cells 351 Electrochemistry 53 semiconductor 11 Electrode(s) band structure and interface states in photoelectrochemical cells 217-230

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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406

PHOTOEFFECTS

AT SEMICONDUCTOR-ELECTROLYTE

Electrode(s) (continued) CdS:Te 302/ characterization 361-363 CdSe single crystal 364/ chlorophyll-coated metal 242-244 composite 217-220, 224/ cyclic voltammetric characterization of an «-type 46/, 48/ dark cathodic currents at a p-GaP 151/ disk potential 132 rotating 53 ring 133/ ZnO 133/ effects of temperature on excitedstate descriptions of luminescent photoelectrochemical cells employing telluriumdoped CdS 295-304 electrochemical behavior and surface structure of GaP 145-157 >z-GaP current-potential curve for 146/ electroluminescence spectra from 154/ Mott-Schottky plots of 152/ and halide solutions, energetic correlation between ZnO 138/ ion adsorption at metal 348 MoSe 5 current-voltage curves of If MoSo 5 current-voltage curves in 8/ photoanodic decomposition of 37 photoelectrochemical systems involving chlorophyll-coated semiconductor and metal 237-244 platinized platinum 169 potential of silicon 180 potentials, ring 134 semiconductor chlorophyll-coated 23 8-242 competing photoelectrochemical reactions on 119-130 energy scheme for different surfaces of 3/ the influence of surface orientation and crystal imperfections on photoelectrochemical reactions 1-15 the role of interface states in electron-transfer processes at photoexcited 103-116 single-crystal bare CdSe 224/ time dependence of short-circuit current for several Cd(Se,Te) 378/ «-type Sn0 optically transparent. 238 Electrodeposition 237 Electroluminescence spectra from an «-GaP electrode 154/ 2

2

INTERFACES

Electrolysis half-cell reaction at the photocathode 322 Electrolysis photoanodes, stable water 332? Electrolyte(s) absorption 363 acidity of 139 composition effects 348-351 contact geometric parameters characterizing the semiconductor/ 3/ containing reductant and oxidant on each side, photoelectrochemical cell with a solid 397/ decomposition 352 doped with free iodine, photosensitivity of RbAg I solid 391 electroreflectance 267, 272-276 interface energy level structure for an -semiconductor/ 89/ luminescence at a reversebiased p-GaP/ 97/ strongly cathodically biased p-GaP/ 97/ junction, CdSe/solid 392-395 non-aqueous 255 for photocorrosion products from «-SrTiO , analysis of 1911 photoelectrochemical cell 343 polyselenide 297 polysulfide 361 redox potential of 139 sulfide 297 Electromigration, electricfieldsand directions of positive ion 92/ Electromigration of ions 88-100 Electromodulation measurements 211 Electron affinity, surface 172 Beam Induced Current (EBIC) 25 experiment, schematic of 28/, 29/ identification of recombination sites by 25-27 diffusion velocity 318/—319? flux 105 hole paris 1 -hole recombination 112 velocities at semiconductor interfaces 58-61 injection, hot 254/ -pumping processes, photoelectrochemical simulation of the photosynthetic 246/ recapture, inhibition of 337 transfer 145 in dye sensitization process 240/ and hole 159 mediator 150 processes 239 4

5

a

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

407

INDEX

Electron (continued) transfer (continued) processes (continued) at photoexcited semiconductor electrodes, the role of interface states in 103-116 schematic of various 104/ Electronegativity model 96 Electroneutrality 388 Electroreflectance electrolyte 267 Electroreflectance signal 276/ Electrostatic attraction 337 Energy(ies) band diagram of the CdSe-SrTi0 composite electrodes 219/ diagram for T i 0 221/ surface 123 barrier, electronic 317 conversion efficiency 343 solar 387 conversion, solar 41,49 determination of adsorption potential 331 free 80 hydration 80 levels, Gaussian distribution of 180 levels, oxide redox 331 maximum potential 318i—319? migration among chlorophylls 239 phonon 106 photon 253 integrated photocurrent vs 224/ reorganization 185 separation 141 terms of a semiconductor, band structure and 146/ zinc porphyrin triplet 291 Etching 370 Evaporation, solvent 237 3

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2

F

FeNb0 , band-gap analysis for 215 Fe 0 , band-gap analysis for 214/ Faradaic process, influence of irreversible 356/ Fatty acids 233 Fatty alcohols 233 Ferricenium by iodide, reduction of surfaceconfined 42/ by one-electron reductants, reduction of surface 44-49 rate constants for reduction of surface 45? redox couple, ferrocene/ 351 by solution reductants, rate of reduction of photogenerated, surface-confined 37-54 4

2

3

Ferricyanide/ferrocyanide couple 152 redox 148 Ferrocene(s) 50 in ethanol 183 /ferricenium redox couple 351 voltammetry under illumination with 181/ (l,r-Ferrocenediyl)dichlorosilane 39 Ferrocyanide 154/, 155, 182 couple, ferricyanide/ 152 redox 148 Ferrous -EDTA, oxide growth in 194/ ions 183 oxalate 154/, 155 Films, amorphous 232 Flat-band potential(s) 355, 363 Fluorenyl lithium, electrochemical oxidation of 340/ Fluorescence intensity 290/ Fluorescence quenching 290/ Fluorine, chemisorption of 63 Free energies of formation, standard. 200/ G Gallium (Ga) arsenide (GaAs) photoanode 70/ arsenide (GaAs), surface recombination velocity of 63-64 compounds 345 corrosion of 330/ immunity of 330/ oxide (Ga 0 ) 64 at the semiconductor surface, formation of 327 passivation of 330/ phosphide (GaP) electrodes, electrochemical behavior and surface structure of 145-157 phosphide (GaP), surface recombination velocity of 63 selenide (Ga Se ) crystal(s) 9, 11 structure of 10/ «-Gallium arsenide («-GaAs) 353/, 354/ Cole-Cole plot for 354/ cyclic voltammograms of 346/ junction, charge collection scanning electron micrographs, gold polycrystalline 72/ molten-salt interphase, photoeffects at 343-357 power curves for 350/ surfaces and grain boundaries, effect of chemisorbed ruthenium on 64-75 voltammograms of 347/ «-Gallium phosphide («-GaP) current-potential curves of 149/, 151/, 154/ 2

3

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2

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

408

PHOTOEFFECTS

AT SEMICONDUCTOR-ELECTROLYTE

n-G allium phosphide (rc-GaP) (continued) current-potential curves of (continued) electrode 146/ electrode, electroluminescence spectra from 154/ electrode, Mott-Schottky plots of 152/ + Fe(II)-EDTA...120, 122/, 128/, 129/ /Fe(CN) 120 in redox solutions, stability of illuminated 153-155 p-Gallium phosphide (p-GaP) effects of various ions on the luminescence in 98/ electrode, dark cathodic currents at 151/ electrolyte interface, luminescence at a reverse-biased 97/ electrolyte interface, strongly cathodically biased 97/ flatband potential for 84/ photocurrent for 86/ Gartner model 360 Germanium, surface recombination velocity of 61 Grain boundary(ies) chemisorption of ruthenium on 70/ doping semiconductors near 74/ recombination in semiconductors, chemical control of surface and 57-75 Graphite 165 4

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c

H HoTOAPP, surface pressure-area isotherm of Halide ions, anodic oxidation of ions, concentration of solutions, energetic correlation between ZnO electrodes and Hexagonal, Wurtzite structure Hole(s) at the anode surface, rate of current to a reducing agent as a function of oxide thickness, maximum pairs, electron photo-produced transfer, electron and Hydrogen accumulation on metal-free SrTi0 crystal bronze formation evolution from platinum-free crystals and lattice oxide

286/ 141 139 138/ 377 210 188/ 1 327 159

3

163/ 88 161 172

INTERFACES

Hydrogen (continued) oxygen redox couples, SrTi0 band edges and photogeneration from low-pressure water vapor on titanium oxides, surface aspects of production with current measurements, comparison of on metal-free crystals on SrTi0 systems and surface stoichiometry yield and integrated current, measurements of Hydrolysis of cellulosic biomass materials, microbiological Hydroxide concentration effect 3

3

176/ 162/, 173 172-173 159-172 169-172 173-174 159 161-165 171/ 192 174-175

I ICP (inductively coupled argon plasma) emission spectroscopy... 195 Illumination 148 chopped 390/ the effect of 147-148 with ferrocene, voltammetry under 181/ intensity 131, 139, 309, 361 photoanode response under stepped 309-316 voltammograms of capacity and current for η-Si under 184/ Immunity of Gallium 330/ Imperfections, crystal 4 the role of 11-13 Impurity(ies) carbon 165 and silicon 160 conduction processes 103 electroactive 352 metal ion 167 and surface metallization, metallic 167-169 Indium antimonide (InSb), surface recombination velocity of 63 Indium phosphide (InP), surface recombination velocity of 63 Inductively coupled argon plasma (ICP) emission spectroscopy 195 Injection, hot carrier 253 Intensity effects of 365/ fluorescence 290/ incident 363 and photocurrent, temperature effects on emission 300 relative emission 298/

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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INDEX

409

Interface(s) experiments, liquid-solid 160-161 experiments, gas-solid 161 in regenerative photoelectrochem ical solar cells, potential distri bution at the semiconductorelectrolyte 267-277 semiconductor-gas 106 semiconductor-insulator 105-106 states in electron-transfer processes at photoexcited semiconductor electrodes, the role of 103-116 states in photoelectrochemical cells, electrode band structure and 217-230 Intermetallics, platinum-niobium 169 Interphase, equivalent circuit for 353/ Inversion, band-edge unpinning pro duced by the effects of 253-265 Inversion layer, capacitance in 265 Iodide oxidation 49 solvent dependence of 41-43 Iodide, reduction of surface-confined ferricenium by 42/ Iodine 388 cathode, complexed 389 photolytic 391 photosensitivity of RbAg I-„ solid electrolyte doped with free 391 reduction of 395 Ton adsorption at metal electrodes 348 Ions, electromigration of 88-100 Ionic product desorption rates in photo-assisted electrochemical reactions, role of 307-335 Iron-niobium system 207 4

Κ

Kinetic measurements

54

L /?-LaFeOa, pressed-disk 227/ Layer, depletion 1 Layer, inversion 1 Layered compound photoelectrodes, carrier recombination at steps in sur faces of 17-33 compounds, electrical conductivity in 30 semiconductors 33 Lead iodide (Pbl ) 388 LEED patterns and their real-space interpretations for lead on TiOo 170/ Ligand, macrocyclic 281/ Light absorption, photocurrent and aniso tropy of 9-11 2

Light (continued) conversion studies 231 intensity 257, 392 dependence of the photoreduc tion current of anthraquinone in acetonitrile 259/ effect 121 photoresponse, chopped 276/ p-polarized 9, 11 photocurrent spectra for 12/ ç-polarized 9, 11 xenon 390/ illumination, chopped 391 white 395 Lipid(s) 233 membranes, bilayer 239 Liquid-junction solar cells 267 Lithium iodide 388 Lithium tetraphenylcyclopentadienide 341 Luminescence 296, 301 in p-GaP, effects of various ions on 98/ measurements 284 and photoelectrochemistry of surfactant metalloporphyrin assemblies on solid supports 279-291 recombination 13 at a reverse-biased p-GaP-electrolyte interface 97/ studies 287-289 M Marcus theory 41,49 Melt interphase 352 Metal electrode(s) chlorophyll-coated 242-244 ion adsorption at 348 photoelectrochemical systems involving chlorophyll-coated semiconductor and 237-244 -free crystals, hydrogen production on 173-174 ion impurities 167 oxide semiconductor 351 transition complexes, photoactive 244 compounds, «-type 307 dichalcogenides 222 layered compound 17 Metallic impurities and surface metallization 167-169 Metalloporphyrin, surfactant 286/ assemblies on solid supports, luminescence and photoelectrochemistry of 279-291 ordered layer of 280

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

410

PHOTOEFFECTS

AT SEMICONDUCTOR-ELECTROLYTE

Metallotetraphenylporphyrins, mono layers of 285 Methanol 255 dark capacitance vs. p-Si electron potential in 263/ 1,3-dimethoxy-4-nitrobenzene in 257 photoreduction current of 261/ Methyl viologen 337 Microelectronics technology, siliconbased 61 Microscopy, charge collection 25-27 Mixing, homogeneity of 287 Molybdenum dioxide (Mo0 ) energy band diagram for 221/ dichalcogenides 18 diselenide (MoSe ) 18 electrodes 5 current-voltage curves of If photoanodes 17-33 surface morphology and solar cell performance of 19-25 disulfide (MoS ) electrode(s) 5 current-voltage curves in 8/ disulfide, model of 2 H-crystal lattice for 6/ -Molybdenum diselenide 0z-MoSe ) current-voltage characteristics of 23/ -Molybdenum diselenide samples, electron micrographs of 22/ Monolayer(s) action spectrum of 292/ properties 284-287 techniques 283 of ZnTOAPP:SA:octadecane mixtures 289 Mott-Schottky intercept, variation of 350/ Mott-Schottky plot(s) 145, 148, 149/, 150 of rt-GaP electrode 152/ 2

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2

2

Oxidation (continued) iodide 49 solvent dependence of 41-43 offluorenyllithium, electrochemical 340/ half-cell reaction 323 hydrogen and lattice 172 potentials for chlorophyll, oneelectron 236/ Oxide(s) films, the enhanced effectiveness of stabilization agents due to 179-189 growth in ferrous-EDTA 184/ photoelectronic properties of ternary niobium 207-216 -semiconductor, metal351 thickness, maximum hole current to a reducing agent as a function of 188/ Oxygen evolution on TiOo 112 mapping 165 production, rate-limiting step in 174 redox couples, SrTi0 band edges and hydrogen 176/ 3

Ρ

2

Ν Niobate, perovskite 213 Niobium intermetallics, platinum169 oxides, photoelectronic properties of ternary 207-216 system, iron207 /t-Niobium dioxide, pressed-disk 226/ Nucleophilicity 387 Ο

Overshoot current vs. time behavior Overshoot phenomena Oxidation anodic of carboxylic acids of halide ions of the semiconductor

317 309

INTERFACES

Palladium film evaporated onto TiOo, Auger spectra of 168/ Palladium on TiOo, L E E D patterns and their real-space interpreta tions for 170/ Passivation of gallium 330/ Pasting method 379 Perovskite(s) 208 niobate 213 structure 222 Peroxytitanium complexes 199 Phenylferrocene 49 Phonon energy 106,253 Phonon excitation 265 Photoanode(s) 265 conditions for rapid photocorrosion at SrTiOa 191-204 FTIR spectra of corrosion residue from SrTiO* 198/ interface energetics for an «-type Si 40 MoSe 17-33 surface morphology and solar cell performance of 19-25 response under steppedillumination 309-316 surfaces, derivatized 39 WSe 17-33 surface morphology and solar cell performance of 19-25 Photoanodic decomposition of electrode 37 2

2

191 141 179

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

INDEX

411

Photocathode, electrolysis half-cell reaction at 322 Photocathode system, (ZnTe) 322 Photocorrosion 159 process, characteristics of 193-199 products from «-SrTi0 , analysis of electrolytes for 197/ proposed mechanism of 199-202 in solar cells 179-189 at SrTi0 photoanodes, conditions for rapid 191-204 Photocurrent(s) anodic 395 and anisotropy of light absorption 9-11 decay 85 density, stabilization ratio as a function of 122/ disk 136/ evaluation of 316-317 factors that control the yield of 1-4 as a function of wavelength 363 limit 183 onset, voltage of 112 vs. photon energy, integrated 224/ properties, emissive and 297-301 quantum yield of 2 saturated 139 short-circuit cathodic 243 temperature effects on emission intensity and 300 vs. time, short-circuited 292/ Photodegradation 192 Photodissociation, water 159 Photoeffects at n-GaAs-molten salt interphase 343-357 Photoeffects on solid-state photoelectrochemical cells 387-398 Photoelectrochemical cell(s) admittance measurements 346/ with Az-CdSe as the anode, energy diagram for regenerative.... 268/ charge-transfer processes i n . 7 9 - 1 0 0 derivatized «-type Si photoanodebased 37-54 electrode band structure and interface states in 217-230 electrolyte 343 energy level diagram for 81/ photoeffects on solid-state 387-398 solar 17-18 potential distribution at semiconductor-electrolyte interface in regenerative 267-277 with solid electrolyte containing reductant and oxidant on each side 397/ 3

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3

Photoelectrochemical (continued) cell(s) (continued) stability of cadmium-chalcogenide based 369-385 employing tellurium-doped CdS electrodes, effects of temperature on excited-state description of luminescent 295-304 measurements 284 reactions on semiconductor electrodes 119-130 the influence of surface orientation and crystal imperfections on 1-15 splitting of water 244 studies 289-291 synthesis 49 systems involving chlorophyllcoated semiconductor and metal electrodes 237-244 systems involving solid-liquid interfacial layers of chlorophylls 231-247 Photoelectrode(s) CdSe 371/ short-circuit current of polycrystalline-painted 371/ time dependence of short-circuit current for single crystal.... 374/ time dependence of short-circuit current for thin-film polycrystalline CdS and 380/ CdS:Te 399/ carrier recombination at steps in surfaces of layered-compound 17-33 derivatized rotating-disk 43 spectral response of polycrystalline, thin-film, painted CdSe r>Te , , 376/ stability of cadmium chalcogenide 384 step model of layered-compound 17-33 thermodynamic stability of 85 thin-film polycrystalline 369 Photoemission 88 Photoetching 372 Photogalvanic cell, anionic 338 Photogeneration, hydrogen 162/ from low-pressure water vapor 172-173 on titanium oxides, surface aspects of 159-172 Photoholes 391 Photo-Kolbe reaction 195, 199 Photolytic iodine 391 Photooxication at semiconductor surfaces, carbanion 337-341 Photopotential of CdSe/L + RbAgJ-, 396/ Photopotential in the cell 390/ ()6

( 3r

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

Downloaded by 148.204.30.87 on October 21, 2009 | http://pubs.acs.org Publication Date: March 2, 1981 | doi: 10.1021/bk-1981-0146.ix001

412

PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE INTERFACES

Photoproduction, hydrogen 173 Photoredox reactions at different surfaces of layered semiconductors. 5-9 Photoreduction of water, dyesensitized 243 Photosynthetic model systems 245-247 Photothermal spectroscopy 303 Photovoltage(s) 4 Dember-type 391 Phthalocyanines 244 Phytol chain(s) 233 totally hydrophobic 232 Piezoelectric coefficients 384 Platinum 167 electrodes, platinized 169 -niobium intermetallics 169 vs. «-type Si, cyclic voltammetry for 40/ Plumbite chemisorption 67 Polarization 148 Polarized light, p9, 11 photocurrent spectra for 12/ Polarized light, s9,11 Polycrystalline-painted CdSe photoelectrodes, short-circuit current of 371/ Polycrystalline photoelectrodes, thinfilm 369 Polyselenide electrolyte(s) 297, 361 Polysulfide junction, CdSe/ 363 analysis of current-voltage characteristics of illuminated 359-368 Porphyrin(s) 231,244 derivatives, surfactant 282 molecule 280 rings 233 hydrophilic 232 Potassium bromide (KBr) 136/ Potassium chloride (KC1) 137/ Pyrazine 280, 281/ Pyrochlore structure 213 Q Quantum efficiency(ies) 159, 161, 208 internal 363 Quenching, fluorescence 290/ R

Recombination 11 in the depletion layer 360 due to diffusion 361 electron-hole 112 velocities at semiconductor interfaces 58-61 luminescence 13 in semiconductors, chemical control of surface and grain boundary 57-75

Recombination (continued) sites by Electron Beam Induced Current (EBIC), identification of 25-27 at steps in surfaces of layered- compound photoelectrodes, carrier 17-33 surface(s), velocity of GaAs 63-64 GaP, surface 63 germanium 61 InSb 63 InP 63 silicon 61-62 Redox couple(s) ferricyanide/ferrocyanide 148 in solution, surface potential from the interaction between surface and 148-153 SrTi0 band edges and hydrogen, oxygen 176/ energy levels, oxide 331 ion concentration, effect of 366/ potential(s) 131,199 of the electrolytes 139 of possible photocorrosion processes in SrTiOa 198/ reaction, regenerative 381 solutions, stability of illuminated «-GaP in 153-155 Reducing agent as a function of oxide thickness, maximum hole current to 188/ Reductants, rate of reduction of photogenerated, surface-confined ferricenium by solution 37-54 Reductants, reduction of surfaceferricenium by one-electron 44-49 Reflectance 363 Relaxation spectrum analysis 269-272 Reorganization energy 185 Resistance elements, bulk 352 Rubidium silver iodide (RbAgJ-,) 388, 389 optical absorption spectrum of 392 photoactivity of 389-392 solid electrolyte doped with free iodine, photosensitivity of 391 Ruthenium chemisorption 67 on grain boundaries 70/ on «-GaAs surfaces and grain boundaries, effect of chemisorbed 64-75 -pheophytin 244 Rutherford backscattering studies 65 Rutile wafer, electrochemically deuterium-doped 95/ 3

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

413

INDEX

s Sn0 296 band-gap of 392 chlorophyll monolayers deposited on 291 dark platinum cathode systems, Ti0 307 optically transparent electrode, «-type 238 Sr Nb 0 , band-gap analysis for 214/ Salt interphase, photoeffects at the «-GaAs-molten 343-357 Scattering carrier 222 effects, interaction and 218 optical 222 Schottky barrier 2, 27, 234 energy diagram of 314/ junction solar cells 33 Selenide 65 Semiconductor(s) anodic oxidation of 179 band structure and energy terms of 146/ chemical control of surface and grain boundary recombination in 57-75 direct band-gap 69 doping levels, optimal 313 electrochemistry 11 electrode(s) chlorophyll-coated 238-242 competing photoelectrochemical reactions on 119-130 energy scheme for different surfaces of 3/ the influence of surface orientation and crystal imperfections on photoelectrochemical reactions at 1-15 the role of interface states in electron-transfer processes at photoexcited 103-116 -electrolyte contact, geometric parameters characterizing 3/ -electrolyte interface(s) generalized equivalent circuit of. 270/ in regenerative photoelectrochemical solar cells, potential distribution at 267-277 supra-band-edge reactions at 253-265 factors governing the competition in electrochemical reactions at illuminated 131-143 flatband potential on aging conditions for SrTi0 , dependence of 94/ near grain boundaries, doping of 74/ 2

2

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2

2

7

3

Semiconductor(s) (continued) interfaces electron-hole recombination velocities at 58-61 -gas 106 -insulator 105-106 layered 33 anisotropic properties of 18 photoredox reactions at different surfaces of 5-9 liquid-junction solar cell(s) 17-33 solar-to-electrical conversion efficiency of 67 and metal electrodes, photoelectrochemical systems involving chlorophyll-coated 237-244 metal oxide 351 metallization of oxide 169 molecules attached to 112-114 solar cells, metal-insulator62 surface(s) 58 atoms on 60/ carbanion photooxidation of.337-341 cut and polished 80 formation of G a 0 at 327 «-type 37-54 energy correlations for surfaces of 6/ «-Semiconductor-electrolyte interface, energy level structure for 89/ Sensitization process, spectral 279 Silicon 179 band model of oxidized 186/ -based microelectronics technology 61 dioxide (Si0 ), corrosion product 179 electrode potential of 180 grain boundaries of polycrystalline 62 impurities 160 surface recombination velocity o f . 6 1 - 6 2 «-type cyclic voltammetry for platinum vs 40/ disk 47/ electrode, cyclic voltammetric characterization of 46/, 48/ photoanode-based photoelectrochemical cells, derivatized.37-54 photoanode, interface energetics for 40 wafers 52 «-Silicon under illumination, voltammograms of capacity and current for 184/ p-Silicon electrode potential in acetonitrile, capacitance vs 262/ electrode potential in methanol, dark capacitance vs 263/ photoreduction on 258/ 2

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In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

414

PHOTOEFFECTS

Silver 388 anode 389 bromide (AgBr), Dember photovoltage of 392 halides, binary 389 iodide (Agi) 388 reoxidation of 395 Sodium ions 388 Solar cell(s) electrochemical 351 liquid-junction 267 semiconductor 17-33 solar-to-electrical conversion efficiency of 67 metal-insulator-semiconductor. 62 performance of MoSe photoanodes, surface morphology and 19-25 performance of WSe photoanodes, surface morphology and 19-25 photocorrosion in 179-189 photoelectrochemical 17-18 potential distribution at the semiconductor-electrolyte interface in regenerative 267-277 Schottky junction 33 conversion efficiency 18, 372 -to-electrical conversion efficiency of semiconductor-liquidjunction solar cell 67 energy conversion 49 efficiencies 387 illumination conditions 372 Solute interaction, surface382 Solution reductants, rate of reduction of photogenerated, surfaceconfined ferricenium by 37-54 Solvent dependence of iodide oxidation 41-43 Spectroscopy Auger 160 inductively coupled argon plasma (ICP) emission 195 photothermal 303 Spectrum analysis, relaxation 269-272 technique 351 Sphalerite structure(s) 382, 383/ cubic 377 Spinach leaves 237 Splitting, water 237 photoelectrochemical 244 Sputtering module, simultaneous R F . . 221/ Stability characteristics, short-circuit current output 369 2

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Stability (continued) of polycrystalline CdSe cells, output 372 of single-crystal CdSe cells, output 372 Stabilization agents due to oxide films, the enhanced effectiveness of.179-189 ratio for different reaction mechanisms 124/ ratio as a function of photocurrent density 122/ Stearic acid, protective monolayer of.. 280 Step model of layered-compound photoelectrodes 17-33 Strontium titanate (SrTi0 ) 159, 169, 218 alloys of 222 band-edges and hydrogen, oxygen redox couples 176/ band-gap analysis for 212/ composite electrode, representation of CdSe219/ crystal(s) 161 hydrogen accumulation on metal-free 163/ oxygen Auger spectra of 168/ dependence of semiconductor flatband potential on aging conditions for 94/ photoanode, FTIR spectra of corrosion residue from 198/ photoanodes, conditions for rapid photocorrosion at 191-204 redox potentials of possible photocorrosion processes in.... 198/ single-crystal 228/ stoichiometry of vacuum prepared vs. water-dipped 172 systems, hydrogen production in.... 159 «-Strontium titanate («-SrTi0 ) analysis of electrolytes for photocorrosion products from 1 97/ boules, trace element composition of 194/ photoelectrodes 196/ single-crystal 227/ and «-Ti0 , effects of pH on the energy levels of 201/ UV/visible absorption spectra of.... 196/ Sulfide electrolytes 297 Sulfide ions on CdS 107-112 corrosion suppression role of 107 surface density of absorbed 109/ Sulfonium iodides 388 Supra-band edge reactions 87 Surface contaminants 165 decomposition 82-85 3

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In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

INDEX

415

Surface (continued) metallization, metallic impurities and 167-169. morphology and solar cell perform ance of MoSeo photoanodes .19-25 morphology and solar cell perform ance of WSe photoanodes 19-25 orientation and crystal imperfec tions on photoelectrochemical reactions at semiconductor electrodes, the influence of 1-15 passivation 85 plasmons 88 potential from the interaction be tween surface states and redox couples in solution 148-153 -solute interaction 382 states, electric charge in 4 states, illustration of 108/ stoichiometry, hydrogen production and 161-165

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2

Τ

«-TiO 9-,Nb 0 ϋ Ο , pressed-disk 226/ Tellurium-doped CdS 295 Tetraethylammonium perchlorate (TEAP) 255 Tetrahydrofuran solution, photolysis of 339 Tetramethylammonium chloride (TMAC) 255 Tetroactadecylamide derivative (HoTOAPP) of meso-tetra. o-aminophenyl)porphyrin 282 Tetraphenylcyclopentadienide 339 Tetraphenylporphine 244 Thermodynamic(s) 131 stability of photoelectrodes 85 Thylokoid membrane 233 Titanium dioxide (TiOo) 159, 165, 296 band-gap of «-type 339 crystal structure for 92/ electrode, current-voltage curves of 113/, 115/ L E E D patterns and their real-space interpretations for Pd on 170/ oxygen evolution on 112 as the photoanode 269 type I 310/, 312/ powders, photoactivity of 175 or SnOo photoanode/dark Pt cathode systems 307 valence and conduction bands of 220 «-Titanium dioxide («-TiO->) /aqueous electrolyte interphase 351 effects of pH on the energy levels of «-SrTiO. and 201/ single-crystal 226/ 0

0

2

{

Titanium oxides, surface aspects of hydrogen photogeneration on 159-172 TMAS (tetramethylammonium chloride) 255 Trace element composition of Az-SrTiOa boules 194/ Transfer processes in photoelectro chemical cells, charge79-100 Transition metal complexes, photoactive 244 compounds, «-type 307 dichalcogenides 222 layered-compound 17 Tungsten diselenide (WSe ) 18 electrode 29/ photoanodes 17—33 surface morphology and solar cell performance 19-25 Tungsten disulfide (WS ) photoanodes 17 «-Tungsten diselenide («-WSe ) band structure of 20/ current-voltage characteristics of 22/, 24/ electrode, photocurrent spectra of 26/ electron micrographs of 20/, 23/ photocathode, electron micrograph of 28/ Tunneling 103 2

2

2

U Unpinning of the semiconductor band edges

253

V

Vacuum sublimation technique 232 van der Waals surface 5 Vanadium oxide, energy band diagram for 221/ Voltage 301 current-, characteristics of the cell 390/ illuminated CdSe/polysulfide junctions, analysis of 359-368 «-WSe., 22/, 24/ limit 183 techniques, capacitance351 Voltammetric characterization of an «-type Si electrode, cyclic 46/, 48/ Voltammetric investigations 345-348 Voltammetry, cyclic 54 for platinum vs. «-type Si 40/ Voltammetry under illumination with ferrocene 181/ W Wafers, «-type Si Water dissociation of

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

52 159

416

PHOTOEFFECTS

AT SEMICONDUCTOR-ELECTROLYTE

Water (continued) dye-sensitized photoreduction of photo-assisted electrolysis of photodissociation photoelectrochemical splitting of... splitting Wurtzite structure 382, hexagonal

Ζ

243 307 159 244 237 383/ 377

X 390/ 391 395

ZnTOAPP, surface pressure-area isotherms for ZnTOAPP:SA:octadecane mixtures, monolayers of Zinc oxide (ZnO) disk electrode electrodes and halide solutions, energetic correlation between prophyrin triplet energies tellurite (ZnTe) photocathode system

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Xenon light illumination, chopped white

INTERFACES

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

288/ 289 4 133/ 138/ 291 326/ 322