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Submitted on 1 Jan 1983
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DIFFERENCE PHOTOACOUSTIC AND
DIFFERENCE REFLECTANCE SPECTRA OF THE ELECTRODE-ELECTROLYTE INTERFACE, THIN
OXIDE FILMS ON GOLD AND PLATINUM
J. Dohrmann, U. Sander
To cite this version:
J. Dohrmann, U. Sander. DIFFERENCE PHOTOACOUSTIC AND DIFFERENCE RE-
FLECTANCE SPECTRA OF THE ELECTRODE-ELECTROLYTE INTERFACE, THIN OXIDE
FILMS ON GOLD AND PLATINUM. Journal de Physique Colloques, 1983, 44 (C6), pp.C6-281-C6-
284. �10.1051/jphyscol:1983644�. �jpa-00223203�
DIFFERENCE PHOTOACOUSTIC AND DIFFERENCE REFLECTANCE SPECTRA OF THE ELECTRODE-ELECTROLYTE INTERFACE, T H I N OXIDE F I L M S ON GOLD AND PLATINUM
J . K . Dohrmann a n d U . S a n d e r
I n s t i t u t ~?2r PhysikaZische fiernie d e r Freien ~ n i v e r s i t a t BerZZr,, Il'akust~asse 3, D-1000 B e r l i n 39, Ep.R.G.
Resume - Des s p e c t r e s PA o n t e t e t r a c e s " i n s i t u " pour des e l e c t r o d e s -tine e t d ' o r , nues ou c o u v e r t e s d'oxyde. Les r e s u l t a t s de PA c a l i b r e e s o n t compares 3 ceux de r e f l e x i o n .
A b s t r a c t - PA s p e c t r a have been measured i n s i t u o f oxide-covered and o x i d e - f r e e Au and P t e l e c t r o d e s . The c o r r e s p o n d i n g d i f f e r e n c e PA s p e c t r a a r e g i v e n . I n t e n s i t y - c a l i b r a t e d PA d a t a a r e compared w i t h r e f l e c t i v i t y d a t a o f these systems.
PAS has p r o v e n t o be a u s e f u l t o o l f o r t h e i n s i t u i n v e s t i g a t i o n o f t h e 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 /1,2/. Changes i n t h e o p t i c a l p r o p e r t i e s o f t h e i n t e r f a c e caused by e l e c t r o c h e m i c a l processes can be observed w i t h o u t t h e need o f e l a b o r a t e e l e c t r o d e p r e p a r a t i o n . F o r m i r r o r - e l e c t r o d e s made by vacuum-deposition o f m e t a l f i l m s on po- l i s h e d s u b s t r a t e s , s p e c u l a r r e f l e c t i o n s p e c t r o s c o p y i s u s u a l l y employed f o r i n s i t u s t u d i e s o f t h e 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 / 3 / . The e x a m i n a t i o n o f t h e r e l a t i o n s between PA and r e f l e c t i o n s p e c t r o s c o p i e s i s d e s i r a b l e i n o r d e r t o assess t h e scope and l i m i t a t i o n o f i n s i t u PAS. We r e p o r t a comparison between PA and r e f l e c t i v i t y d a t a f o r g o l d and p l a t i n u m e l e c t r o d e s i n aqueous s o l u t i o n a t p o t e n t i a l s where t h e e l e c t r o d e s u r f a c e i s e i t h e r o x i d e - f r e e o r covered w i t h a t h i n o x i d e f i l m . Such a comparison r e q u i r e s i n t e n s i t y - c a l i b r a t i o n o f t h e PA s p e c t r a . The r e f l e c t i v i t y d a t a have been c a l c u l a t e d f r o m p u b l i s h e d v a l u e s o f t h e o p t i c a l c o n s t a n t s o f t h e m a t e r i a l s o r have been t a k e n d i r e c t l y f r o m t h e l i t e r a t u r e .
EXPERIMENTAL
The w o r k i n g e l e c t r o d e s were c i r c u l a r sheets o f u n p o l i s h e d g o l d (99.96 %, 0.1 mm t h i c k ) o r p l a t i n u m (99.5 %, 0.05 mm t h i c k ) . The i l l u m i n a t e d f r o n t s u r f a c e was i n c o n t a c t w i t h t h e e l e c t r o l y t e s o l u t i o n . An e l e c t r e t microphone (Sennheiser, KE 13-227) was p l a c e d a t t h e back s u r f a c e ( F i g . 1 ) . A t t h e m o d u l a t i o n f r e q u e n c y used ( 2 0 Hz) t h e e l e c t r o d e s were t h e r m a l l y t h i n . The e l e c t r o d e p o t e n t i a l was c o n t r o l l e d by means o f a p o t e n t i o s t a t .
The s p e c t r o m e t e r was a double-beam i n s t r u m e n t equipped w i t h a 450 W Xe lamp and an f / 2 monochromator (Jobin-Yvon, HL). F o r d e t a i l s , see Ref. / I / . The s p e c t r a were t a k e n a t normal i n c i d e n c e . The s p e c t r a l bandwidth was 10 nm.
F o r i n t e n s i t y - c a l i b r a t i o n of t h e s p e c t r a , t h e PA s i g n a l o f t h e e l e c t r o d e was mea- s u r e d a f t e r anodic d e p o s i t i o n o f ca. l pm t h i c k l a y e r o f b l a c k PbO T h i s l a y e r con- v e r t s v i r t u a l l y a l l t h e i n c i d e n t l i g h t i n t o h e a t , w h i l e t h e c h a n g e Z b i n t h e thermal p r o p e r t i e s and i n t h e t h i c k n e s s o f t h e e l e c t r o d e a r e n e g l i g i b l y s m a l l .
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983644
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F i g . 1
-
E l e c t r o c h e m i c a l c e l l f o r i n s i t u ?AS. WE, RE, CE: working, r e f e r e n c e , and c o u n t e r e l e c t r o d e s , r e s p e c t i v e l y ; Q: q u a r t z window; ?4: microphone.RESULT AN0 DISCUSSION
I t can be shown /4/ t h a t t h e i n t e n s i t y - c a l i b r a t e d PA spectrum, P/PoI o f an o p t i c a l l y opaque and t h e r m a l l y t h i n e l e c t r o d e i s connected w i t h t h e r e f l e c t i v i t y , R, of t h e 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 by
P denotes t h e PA s i g n a l o f t h e PbO covered e l e c t r o d e . E q . ( 1 ) assumes t h a t t h e P ~ O l a y e r i s i d e a l l y b l a c k and t h a e luminescence o r p h o t o e l e c t r o c h e m i c a l processes do g o t o c c u r . The systems s t u d i e d h e r e f u l f i l l these c o n d i t i o n s s u f f i c i e n t l y w e l l . The comparison between P/P and ( 1 - R ) f o r o x i d e - f r e e Au and P t e l e c t r o d e s i n 0.2
M
Pb(N0 ) 2 ( F i g . 2, a and b ) O i n d i c a t e s f a i r agreement f o r Au and good agreement f o r P t . ~ a e r e f l e c t i v i t y has been c a l c u l a t e d by means o f F r e s n e l ' s E q u a t i o n w i t h t h e
F i g . 2 - C a l i b r a t e d PA s p e c t r a and c a l c u l a t e d r e f l e c t i v i t y o f ( a ) g o l d (99.96 %, u n p o l i s h e d ) and ( b ) p l a t i n u m (99.5 %, u n p o l i s h e d ) a t 0 V vs NHE i n 0.2 M aqueous Pb(N03)2.
o p t i c a l c o n s t a n t s , n and k, o f g o l d /5,6/ and p l a t i n u m (D.M. Kolb, u n p u b l i s h e d va- l u e s ) and t a k i n g n = 1.333 f o r t h e t r a n s p a r e n t aqueous s o l u t i o n . The d e v i a t i o n between P / P and (1-R) f o r g o l d cannot be e x p l a i n e d b y a c a l i b r a t i o n e r r o r , s i n c e t h e r e s i d u a ? r e f l e c t i v i t y o f PbO i s l e s s t h a n 6 ?;. The d e v i a t i o n i s a t t r i b u t e d p r i - m a r i l y t o s u r f a c e roughness o f t 6 e e l e c t r o d e / 4 / .
Anodic p o l a r i z a t i o n o f g o l d and ~ l a t i n u m i n 1 !4 HC104 produces t h i n o x i d e f i l m s on the e l e c t r o d e s u r f a c e . F i g . 3 shows t h e PA s i g n a l a t 400 nm o f t h e p l a t i n u m e l e c -
F i g . 3 - PA s i g n a l a t 400 nm ( t o p ) and c y c l i c voltammogram ( b o t t o m ) o f t h e P t e l e c t r o d e i n 1 N HC1D4.
The u n c a l i b r a t e d PA s p e c t r a o f t h e oxide-covered and o x i d e - f r e e g o l d and p l a t i n u m e l e c t r o d e s a r e shown i n F i g . 4. Assuming t h e c o m p o s i t i o n Au 0 and PtO f o r t h e o x i d e f i l m s , t h e t h i c k n e s s was 1.3 nm f o r Au 0 a t 1.73 V an$ 8 . 4 nm f o r PtO a t
1.5 V vs NHE as o b t a i n e d by c o u l o m e t r y . A t 0223 V, b o t h m e t a l s a r e o x i d e - f r e e .
F i g . 4
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Bottom: PA s p e c t r a o f t h e ( a ) Au, ( b ) P t e l e c t r o d e i n 1 M HC104 a t 0 . 2 9 V vs NHE ( b a r e s u r f a c e ) and a t ( a ) 1.73 V, ( b ) 1.5 V ( o x i d e - f i l m c o v e r e d ) . Top: D i f - ference PA s p e c t r a o f t h e o x i d e - f i l m s , ( a ) 1.3 nm Au 0 on Au, ( b ) 0.4 nm PtO on P t . Open c i r c l e s i n F i g . 4b a r e f o r t h e d i f f e r e n c e r % f ? e c t i v i t y spectrum, -AR/R, o f PtO on P t f r o m Ref. / 9 / . The f e a t u r e s near 400 and 625 nrn i n t h e PA s p e c t r a a r e f r o m a base l i n e i r r e g u l a r i t y o f t h e PA s p e c t r o m e t e r .I n s t u d i e s o f t h i n s u r f a c e f i l m s b y r e f l e c t a n c e techniques, t h e d i f f e r e n c e r e f l e c t i - v i t y (DR) spectrum, AR/R, i s u s u a l l y measured / 3 / . ( A R = R'-R, where R' and R a r e t h e r e f l e c t i v i t i e s o f t h e f i l m - c o v e r e d and t h e b a r e s u r f a c e , r e s p e c t i v e l y . ) By a;a- l o g y , we d e f i n e t h e d i f f e r e n c e PA (DPA) spectrum by t h e q u a n t i t y AP/(Po-P). P, P
,
and Po denote t h e PA s i g n a l s o f t h e bare, t h e f i l m - c o v e r e d , and t h e Pb02-covered e l e c t r o d e , r e s p e c t i v e l y , and AP = PI-P. From Eq. (1) i t f o l l o w s t h a t
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The DPA s p e c t r a o f t h e o x i d e f i l m s on g o l d and p l a t i n u m a r e a l s o shown i n F i g . 4 The o p t i c a l p r o p e r t i e s of o x i d e - f i l m s on g o l d /8/ and p l a t i n u m /9/ have been s t u - d i e d i n d e t a i l by M c I n t y r e and K o l b who a p p l i e d s p e c u l a r r e f l e c t i o n spectroscopy t o m i r r o r - e l e c t r o d e s . The peak a t 500 nm i n t h e DPA spectrum o f t h e 1.3 nm o x i d e - f i l m on g o l d ( F i g . 4 a ) i s a l s o p r e s e n t i n t h e DR spectrum r e p o r t e d f o r t h e f i l m t h i c k - ness o f 0.6 nm / 8 / . The peak i n t e n s i t y o f 0.15 i 0.02 i n t h e DPA spectrum agrees s u f f i c i e n t l y w e l l w i t h t h e v a l u e o f -AR/R = 0.12 c a l c u l a t e d by means o f t h e M c I n t y r e - Aspnes-approximation
/ l o /
f o r a f i l m t h i c k n e s s o f 1.3 nm and normal i n c i d e n c e ( o p t i - c a l c o n s t a n t s o f Au 0 and Au f r o m Ref. / 8 / ) . , S a t i s f a c t o r y agreement i s a l s o ob- t a i n e d between t h e6 ~ a
spectrum o f t h e 0.4 nm o x i d e - f i l m on p l a t i n u m ( F i g . 4b) and t h e n o r m a l - i n c i d e n c e DR spectrum, -AR/R, r e p o r t e d f o r t h e same e l e c t r o c h e m i c a l con- d i t i o n s /9/ (open c i r c l e s i n F i g . 4 b ) .The r e s u l t suggests t h a t i n t e n s i t v - c a l i b r a t e d PA and r e f l e c t a n c e measurements a r e e q u i v a l e n t f o r t h e e l e c t r o d e s s t u d i e d h e r e . There i s no doubt, however, t h a t r e - f l e c t i o n spectroscopy a t m i r r o r - e l e c t r o d e s i s s u p e r i o r t o PAS w i t h r e s p e c t t o b o t h s e n s i t i v i t y and accuracy. R e f l e c t i o n spectroscopy i s , t h e r e f o r e , t h e method o f c h o i c e f o r t h e d e t e r m i n a t i o n o f t h e o p t i c a l c o n s t a n t s o f e l e c t r o d e m a t e r i a l s and s u r f a c e f i l m s , i f m i r r o r - e l e c t r o d e s can be ~ r e o a r e d . W i t h u n o o l i s h e d e l e c t r o d e s , PAS i n c o n j u n c t i o n w i t h i n t e n s i t y - c a l i b r a t i o n o f f e r s a c o n v e n i e n t means t o c h a r a c t e r i z e t h e o p t i c a l p r o p e r t i e s o f t h e 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 .
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