MIRAGE EFFECT STUDIES OF ELECTROCHEMICAL PROCESSES

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MIRAGE EFFECT STUDIES OF ELECTROCHEMICAL PROCESSES

B. Royce, D. Voss, A. Bocarsly

To cite this version:

B. Royce, D. Voss, A. Bocarsly. MIRAGE EFFECT STUDIES OF ELECTROCHEM- ICAL PROCESSES. Journal de Physique Colloques, 1983, 44 (C6), pp.C6-325-C6-329.

�10.1051/jphyscol:1983653�. �jpa-00223212�

JOURNAL DE PHYSIQUE

Colloque C6, supplement au nolO, Tome 44, o c t o b r e 1983 page C 6 - 3 2 5

MIRAGE EFFECT STUDIES OF ELECTROCHEMICAL PROCESSES

B.S.H. Royce, D. Voss and A. B o c a r s l y *

Applied Physics and Materials Laboratory

*Department of Chemistry

Princeton University, Princeton, N e w Jersey 08544, Y.S.A.

Rdsurn6 - On a 6 t u d i i l a dicompos i t i o n c h i m i q u e d 1 6 1 e c t rodes semi - c o n d u c t r i c e s e t l e s e f f e t s de 116change de l a charge s u r l e s couples d 1 0 x y d o r 6 d u c t i o n en s o l u t i o n a u moyen de I ' e f f e t de mi rage. L ' u s a g e s p e c t r o s c o p i q u e de c e t t e

& t h o d e p e r m t l a d i t e c t i o n des f i l m s de s u r f a c e e t on a mesur6 l e s c o e f f i - c i e n t s de d i f f u s i o n des i o n s d 1 0 x y d o r 6 d u c t i o n p r o v e n a n t de I ' i c h a n g e de l a charge a u n i v e a u des i l e c t r o d e s de m6tal en se s e r v a n t des c h a n g e w n t s de I

'

i n d i c e de r 6 f r a c t i o n a s s o c i i s

>

ces r 6 a c t i o n s .

A b s t r a c t - The p h o t o c h e m i c a l decomposi t i o n o f semi c o n d u c t i n g e l e c t r o d e s and the e f f e c t s o f charge exchange t o redox s p e c i e s i n s o l u t i o n have been s t u d i e d by means o f t h e M i r a g e E f f e c t . S p e c t r o s c o p i c use o f t h i s method p e r m i t s t h e d e t e c t i o n o f s u r f a c e f i l m s and d i f f u s i o n c o e f f i c i e n t s o f r e d o x i o n s g e n e r a t e d by c h a r g e exchange a t metal e l e c t r o d e s have been measured u s i n g t h e r e f r a c t i v e i n d e x changes a s s o c i a t e d wi t h these r e a c t i o n s .

I n t e r f a c i a l c h a r g e t r a n s f e r processes i n e l e c t r o c h e m i c a l s y s tens a r e i m p o r t a n t i n a number o f a r e a s o f t e c h n o l o g i c a l and s c i e n t i f i c i n t e r e s t . I n systems i n v o l v i n g more than one e l e c t r o a c t i v e s p e c i e s c o n v e n t i o n a l e l e c t r o c h e m i c a l methods [I ] may n o t be a b l e t o i d e n t i f y t h e r e l a t i v e c o n t r i b u t i o n s o f t h e i n d i v i d u a l s p e c i e s t o t h e o v e r a l l r e a c t i o n and s p e c i e s s p e c i f i c complementary methods a r e r e q u i r e d . O p t i c a l t e c h - n i q u e s a r e c a p a b l e o f p r o v i d i n g t h e d e s i r e d i n f o r m a t i o n and t h e m i r a g e e f f e c t [2,3,

41

p r o v i d e s a method o f o p t i c a l l y i n t e r r o g a t i n g r e a l i s t i c e l e c t r o d e s w h i c h may be opaque, rough, p o l y c r y s t a l l i n e and hence l i g h t s c a t t e r i n g . Due t o the l a r g e temper- a t u r e c o e f f i c i e n t o f the r e f r a c t i v e i n d e x o f l i q u i d s t h e method i s w e l l s u i t e d t o a s p e c t r o s c o p i c s t u d y o f t h e 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 . Charge t r a n s f e r p r o - cesses f r o m an e l e c t r o d e t o a r e d o x c o u p l e i n s o l u t i o n a l t e r t h e c o n c e n t r a t i o n p r o - f i l e s i n t h e e l e c t r o l y t e . The c o n c e n t r a t i o n dependence o f the r e f r a c t i v e i n d e x i s c i r c a p e r mole and i o n i c c o n c e n t r a t i o n changes due t o charge t r a n s f e r r e a c t i o n s may, t h e r e f o r e , be e a s i l y d e t e c t e d u s i n g t h e c o n v e n t i o n a l mi rage e f f e c t geometry b u t w i t h o u t i l l u m i n a t i o n o f t h e e l e c t r o d e .

T h i s paper i s concerned w i t h mi rage e f f e c t s t u d i e s o f e l e c t r o c h e m i c a l systems i n - v o l v i n g p h o t o c o r r o s i o n processes t a k i n g p l a c e a t s e m i c o n d u c t i n g e l e c t r o d e s and t r a n s p o r t p r o c e s s e s f o l l o w i n g charge exchange t o s o l u t i o n s p e c i e s a t metal e l e c t r o d e s .

PHOTOCORROSION OF SEMI CONDUCT1 NG ELECTRODES

The e x p e r i m e n t a l c o n f i g u r a t i o n employed f o r p h o t o c o r r o s i o n s t u d i e s o f ZnSe and CdTe s e m i c o n d u c t i n g e l e c t r o d e s i s shown i n F i g u r e 1 . The s e m i c o n d u c t o r i s mounted on an i n s u l a t i n g h o l d e r and ohmic c o n t a c t i s made t o i t s r e a r s u r f a c e . A p l a t i n i z e d p l a t - inum gauze and a calomel r e f e r e n c e e l e c t r o d e a r e a l s o p r o v i d e d t o form a t h r e e e l e c - t r o d e e l e c t r o c h e m i c a l c e l l . The c e l l windows p e r m i t t u n a b l e monochromatic r a d i a t i o n t o be focused on t h e sample s u r f a c e . The probe beam d e f l e c t i o n i s d e t e c t e d u s i n g a

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983653

C6-326 JOURNAL DE PHYSIQUE

UDT p o s i t i o n d e t e c t o r and a PAR 5204 l o c k - i n a m p l i f i e r . S i g n a l c o l l e c t i o n and p r o - c e s s i n p i s performed by an HPgE25A d a t a c o l l e c t i o n system.

OROER FILTER - B E W

SFUTTER

F i g u r e 1 . Schematic Diagram o f t h e P h o t o e l e c t rochemi c a l M i rage E f f e c t A p p a r a t u s . For b o t h ZnSe and CdTe one o f t h e d e c o m p o s i t i o n p r o d u c t s o f t h e p h o t o e l e c t r o c h e m i c a l r e a c t i o n remains on t h e s u r f a c e as a t h i n f i l m and t h e m i r a g e e f f e c t s i g n a l c o r r e s - ponds t o t h e t h e r m a l l y t h i n l i m i t d i s c u s s e d by Jackson e t a l 151, f o r w h i c h the s i g - n a l , S, i s r e l a t e d t o t h e f i l m t h i c k n e s s , k , and t h e o p t i c a l a b s o r p t i o n c o e f f i c i e n t , a, by: S = A [ l - e x p i - a E ) ] where A i s a c o n s t a n t t h a t can be e x p e r i m e n t a l l y e v a l -

u a t e d i n t h e s i g n a l s a t u r a t i o n l i m i t .

The p h o f o c o r r o s i o n o f ZnSe [ 6 ] s i n g l e c r y s t a l s was f o l l o w e d i n e l e c t r o l y t e s made f r o m s t a n d a r d F i s h e r b u f f e r s o l u t i o n s . The samples were { I l l } p l a t e l e t s o f a b o u t 0 . 5 x 0 . 5 x 0 . 2 cm 3

,

t h e s u r f a c e r e c e i v i n g a chemical p o l i s h w i t h a 1 % Bromine-Methanol s o l u t i o n . P h o t o c o r r o s i o n was caused b y i l l u m i n a t i n g t h e sample w i t h 460 nm l i g h t w i t h c i

d u r i n g a t pH4

r c a 1 mw/cm2 i n c i d e n t f l u x ; no d e t e c t a b l e chanoe i n t h e s u r f a c e f i l m o c c u r r e d t h e s p e c t r a l measurement. F i g u r e 2 shows PDS p h o t o c o r r o s i o n s p e c t r a o b t a i n e d a f t e r 20 mins and 18 h r s o f i l l u m i n a t i o n i n open c i r c u i t . The p h o t o c o r r o s i o n

W l U N ENERGY ( * V )

2.0 22 2 4 26

( a )

l b )

WVELENGTH ( nm I

F i g u r e 2. ZnSe P h o t o c o r r o s i o n a t pH4. a) 20 mins, b ) 18 h r s .

r e a c t i o n : ZnSe+Zh = 2n2++ SeO ( s u r f a c e ) has been p r e v i o u s l y d i s c u s s e d b y W i l l iams [ 7 ] and by Gautron e t a l [ 8 ] . The d a t a o f F i g u r e 2 were f i t t o t h e known s p e c t r a l l y dependent a b s o r p t i o n c o e f f i c i e n t o f amorphous s e l e n i u m and t h e c o r r o s i o n l a y e r was

found t o have a t h i c k n e s s o f (0.85

+

0.2) x cm a f t e r t h e 18 h r p h o t o c o r r o s i o n . Auger measurements c o n f i r m e d t h e presence o f t h e s e l e n i urn s u r f a c e f i l m .

The CdTe 191 s t u d i e s p r o v i d e d o p t i c a l a b s o r p t i o n d a t a t h a t c o u l d be used t o i d e n t i f y d e p a r t u r e s from s t o i c h i o m e t r y i n t h i n p o l y c r y s t a l l i n e f i l m s f o r s o l a r c e l l a p p l i c a - t i o n s and reproduced such d e p a r t u r e s f r o m s t o i c h i o m e t r y i n s i n g l e c r y s t a l s u s i n g e l e c t r o c h e m i c a l methods. The p o l y c r y s t a l l i n e f i l m s were arown on e i t h e r i n s u l a t i n g o r c o n d u c t i n g g l a s s s u b s t r a t e s u s i n g t h e G r a d i e n t R e c r y s t a l l i z a t i o n Growth method

[ l o ]

and had t h i c k n e s s e s o f about 100 m i c r o n s w i t h a columnar s t r u c t u r e normal t o t h e f i l m s u r f a c e . The p h o t o t h e r m a l d e f l e c t i o n s p e c t r a o b t a i n e d w i t h t h e s e samples a r e shown i n F i g u r e 3A. The f i l m s grown on t h e i n s u l a t i n g s u b s t r a t e had t h e absorp- t i o n a s s o c i a t e d w i t h t h e normal band edge o f CdTe whereas t h e f i l m s produced on t h e c o n d u c t i n g s u b s t r a t e showed a d d i t i o n a l a b s o r p t i o n f e a t u r e s i n t h e 850-1160 nm r e g i o n . P r e v i o u s p h o t o c o r r o s i o n s t u d i e s by Vazquez-Lopez e t a1 [I 1 ] i n d i c a t e d t h a t exposure o f CdTe samples t o band gap r a d i a t i o n i n a SnCI2 e l e c t r o l y t e g i v ? r i s e t o t h e f o r - m a t i o n o f a T e l l u r i u m s u r f a c e l a y e r . A s i n g l e c r y s t a l was p h o t o c o r r o d e d under open c i r c u i t c o n d i t i o n s i n an a c i d i f i e d SnCI2 e l e c t r o l y t e by i l l u m i n a t i n g i t f o r 20 mins w i t h the defocused r a d i a t i o n from a 1 mW He/Ne l a s e r p r o d u c i n g t h e s p e c t r a shown i n

F i g u r e 38. The p u r e c r y s t a l spectrum i s i n good agreement w i t h t h a t o b t a i n e d from

F i g u r e 3A. T h i n F i l m s . a) I n s u l a t i n g s u b s t r a t e , b) Conducting s u b s t r a t e . 38. S i n g l e c r y s t a l . a) l i n t r e a t e d , b ) Photocorroded i n SnCI2.

-

f i l m s d e p o s i t e d on t h e i n s u l a t i n g s u b s t r a t e whereas t h e f i lms d e p o s i t e d on t h e con- d u c t i n g s u b s t r a t e have a spectrum c h a r a c t e r i s t i c o f a t e l 1 u r i u m excess. T h i s behav- i o r i s n o t y e t understood a1 though s i m i l a r Te-excesses have been r e p o r t e d f o r CdTe s i n g l e c r y s t a l s on w h i c h g o l d c o n t a c t s have been e l e c t rochemical l y deposi ted [12]

and on samples t h a t were chemical l y e t c h e d [ I

31.

Dl FFUSl ON PROCESSES I N THE ELECTROLYTE

The t r a n s p o r t parameters o f e l e c t r o l y t e s p e c i e s a r e o f c o n s i d e r a b l e i n t e r e s t f o r e v a l u a t i n g e l e c t r o ~ h e m i c a l r e a c t i o n s . The mi rage e f f e c t p r o v i d e s a s e n s i t i v e probe f o r changes i n t h e l o c a l r e f r a c t i v e i n d e x t h a t accompanies t h e c r e a t i o n o f new s o l u - t i o n s p e c i e s and has been a p p l i e d t o measure c o n c e n t r a t i o n p r o f i l e s and the d i f f u - s i o n c o e f f i c i e n t o f t h e one e q u i v a l e n t f e r r i - f e r r o c y a n i d e r e d o x s p e c i e s . The appa- r a t u s used f o r these measurements had e s s e n t i a l l y t h e same c o n f i g u r a t i o n as shown i n F i g u r e 1 , however, t h e s i g n a l was g e n e r a t e d by c o n c e n t r a t i o n a s s o c i a t e d r e f r a c t i v e i n d e x g r a d i e n t s r a t h e r than a thermal g r a d i e n t . The e l e c t r o c h e m i c a l c e l l was mount- ed on a p r e c i s i o n t r a n s l a t i o n s t a g e p r o v i d i n g a method f o r a d j u s t i n g t h e p o s i t i o n o f t h e probe l a s e r beam w i t h r e s p e c t t o t h e p l a t i n u m e l e c t r o d e . The e l e c t r o d e poten- t i a l was c o n t r o l l e d by a p o t e n t i o s t a t and a s t e p v o l t a g e was used t o i n i t i a t e t h e charge t r a n s f e r r e a c t i o n . W i t h t h e p r o b e l a s e r s i t u a t e d a d i s t a n c e , x, f r o m t h e 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 a t i m e d e l a y was observed between t h e a p p l i c a t i o n o f

JOURNAL DE PHYSIQUE

t h e v o l t a g e s t e p and t h e l a s e r beam d e f l e c t i o n . Measurement o f t h i s d e l a y t i m e as a f u n c t i o n o f the d i s t a n c e x i n d i c a t e d t h a t : A t = x /4D 2 where D i s t h e d i f f u s i o n co- e f f i c i e n t o f t h e e l e c t r o c h e m i c a l l y produced s p e c i e s . F i g u r e 4 shows t h e p l o t o f A t vs. x2 o b t a i n e d w i t h an e l e c t r o l y t e o f 50 mM F ~ ( C N ) ~ - / ~ O mkt F ~ ( c N ) : - i n 0.5 M K2S04

6

a t room temperature. The s l o p e o f t h i s l i n e y i e l d s a d i f f u s i o n c o e f f i c i e n t o f 2 - 1

( 9 . 4 k 0 . 2 ) x cm sec w h i c h agrees w i t h p u b l i s h e d v a l u e s [14-171 f r o m r o t a t i n g d i s c and s p e c t r o e l e c t r o c h e m i c a i measurements. Since t h i s method i s a d i r e c t measure o f t h e d i f f u s i v e t r a n s p o r t i t i s independent o f processes l i m i t i n g t h e charge t r a n s - f e r a t t h e e l e c t r o d e .

= ' -

F i g u r e 4 . D i f f u s i o n Data f o r F ~ ( C N ) ~ - / ~ - i n 0.5 M K2S04.

Examination o f t h e t i m e dependence o f t h e l a s e r beam d e f l e c t i o n showed t h a t a s m a l l o p p o s i t e s i g n a l o c c u r r e d p r i o r t o t h e c o n c e n t r a t i o n g r a d i e n t a s s o c i a t e d d e f l e c t i o n . T h i s r e v e r s e d e f l e c t i o n i s t h o u g h t t o b e a s s o c i a t e d w i t h a r a p i d thermal wave r e - s u l t i n g f r o m t h e e l e c t r o c h e m i c a l Pel t i e r e f f e c t [ 1 8 ] . l ndependent measurements o f the c o n c e n t r a t i o n dependence o f t h e e l e c t r o l y t e r e f r a c t i v e i n d e x were made and i t was found t h a t (dn/dc) had t h e v a l ues (7.2 2 0.1) x p e r m l e and (5.2 2 0.1) x p e r m l e f o r F ~ ( c N ) : - and F~(cN):- r e s p e c t i v e l y . These values show t h a t (dn/dc)Ac f o r t h e charge t r a n s f e r r e a c t i o n i s f r o m t h r e e t o f o u r o r d e r s o f magnitude l a r g e r than t h e v a l ue o f ( d n / d ~ ) A T a s s o c i a t e d w i t h P e l t i e r h e a t . Independent measurements o f t h e t h e r m a l d i f f u s i o n process a r e now i n p r o g r e s s u s i n g b o t h s t e p f u n c t i o n and p e r i o d i c h e a t i n g methods.

CONCLUS l ONS

The measurements r e p o r t e d above have i n d i c a t e d t h e u t i l i t y o f the m i r a g e e f f e c t f o r t h e s t u d y o f e l e c t r o c h e m i c a l processes. The charge t r a n s f e r r e a c t i o n s wi t h s o l u- t i o n s p e c i e s a r e b e i n g combined w i t h s t a n d a r d e l e c t r o c h e m i c a l methods t o o b t a i n s p e c i e s s p e c i f i c i n f o r m a t i o n . I t i s o f i n t e r e s t t o p o i n t o u t a f e a t u r e o f t h e m i r a g e e f f e c t t e c h n i q u e w h i c h has n o t y e t been e x p l o i t e d . The measurements d i s c u s s - ed above used a probe l a s e r o f f i x e d wavelength, however, w i t h a t u n a b l e l a s e r probe t h e l a s e r beam d e f l e c t i o n a s s o c i a t e d w i t h a g i v e n c o n c e n t r a t i o n p r o f i l e w i l l have a w a v e l e n g t h dependence due t o t h e r e f r a c t i v e i n d e x anomaly t h a t accompanies 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 e l e c t r o l y t e s p e c i e s and t h e Kramers-Kronig e q u a t i o n s can be used t o o b t a i n t h e i r c h a r a c t e r i s t i c a b s o r p t i o n s p e c t r a . T h i s method i s s i m i l a r t o t h e s p e c t r o e l e c t r o c h e m i c a l method b e i n g e x p l o r e d by McCreery's group [ 1 9 ] . Both methods o f f e r t h e r a p i d response and h i g h s p a t i a l r e s o l u t i o n so d e s i r a b l e i n t h e s t u d y o f charge t r a n s f e r processes.

ACKNOWLEDGMENTS

T h i s work was s u p p o r t e d i n p a r t by AROD-DAAG29-80C-0053. The a u t h o r s a l s o w i s h t o acknowledge t h e c o n t r i b u t i o n s o f F. Sanchez-Sinencio, A. Kahn, R. W i l l i a m s , W.M.

Yim, J .G. ttendoza-Alvarez, 0 . i e l a y a - A n g e l

,

C. Menezes, R. T r i b o u l e t , R. G o l d s t e i n and R. M u r a t o r e t o t h e e x p e r i m e n t s d i s c u s s e d .

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2. D. F o u r n i e r , A.C. Boccara and J. Bardoz, App. Phys. L e t t .

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(1980) 130 3. J . C . Murphy and L.C. Aamodt, J. App. Phys.

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(1981) 4903, and J . App. Phys.

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