APPLICATION OF THE VISIBLE ELECTROREFLECTANCE SPECTROSCOPY TO THE CHARACTERIZATION OF PROCESSES OCCURRING ON GOLD ELECTRODES

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APPLICATION OF THE VISIBLE

ELECTROREFLECTANCE SPECTROSCOPY TO THE CHARACTERIZATION OF PROCESSES

OCCURRING ON GOLD ELECTRODES

C. Nguyen van Huong, C. Hinnen

To cite this version:

C. Nguyen van Huong, C. Hinnen. APPLICATION OF THE VISIBLE ELECTRORE-

FLECTANCE SPECTROSCOPY TO THE CHARACTERIZATION OF PROCESSES OCCUR-

RING ON GOLD ELECTRODES. Journal de Physique Colloques, 1983, 44 (C10), pp.C10-513-C10-

516. �10.1051/jphyscol:198310105�. �jpa-00223465�

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Colloque CIO, supplément au n°12, Tome M, décembre 1983 page C10-513

APPLICATION OF THE VISIBLE ELECTROREFLECTANCE SPECTROSCOPY TO THE CHARACTERIZATION OF PROCESSES OCCURRING ON GOLD ELECTRODES

C. Nguyen Van Huong.and C. Hinnen

Laboratoire d'Electroohimie Interfaciale du CN.R.S,, 1, Vlace A. Briand, 92195 Meudon Principal Cedex, France

Résumé - Les phénomènes d'adsorption sur des surfaces d'Au (110) et (311) sont étudiés en couplant une analyse électro- chimique et des mesures d'électroréflectance. Ces expériences permettent également de caractériser une réaction rédox entre molécules de cytochrome adsorbées.

Abstract - Adsorption phenomena at Au (110) and (311) surfa- ces are investigated by coupling electrochemical analysis and electroreflectance measurements. These experiments also allow to characterize a redox reaction in adsorbed cytochrome.

INTRODUCTION

The coupling of optical method and electrochemical analysis allows in situ investigations of adsorption. The presence of a polarization anisotropy of the normal incidence electroreflectance (er) spectra on the Au(llO) and (311) surfaces is used here to study some adsorp- tive behaviour of gold electrodes [l] . We firstly investigate the ad- sorption of ether molecules and bromide ions on these faces and secondly give evidence for the presence of a redox reaction between adsorbed cytochrome C, molecules.

EXPERIMENTAL

The optical arrangement for measuring relative reflectance changes has been described [2,5"]. The optical measurements were made with linearly polarized light at normal incidence. The electrochemical behaviour of the electrodes was checked by capacity measurements and voltammograms.

RESULTS

1. Characterization of the adsorptive behaviour Au (110) and (311) surfaces

1.1. Adsorption of ethyl ether and bromide on Au(llO) surface Ai ionic substance (Br- anions) and a neutral one ( ethyl ether) are used and allow us to compare the er spectra obtained in two complete- ly different kinds of adsorption [ 4 ] . In the first case, the adsorbed amount varies approximately linearly with the electric field and in the second case, quadratically. This explains the main characteristic features of the capacity curves given in fig. 1 relative to the two

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:198310105

CIO-514 JOURNAL DE PHYSIQUE

cases. The maximum adsorption of t h e s u r f a c e i s obtained i n t h e f i r s t case a t t h e most p o s i t i v e p o t e n t i a l preceding t h e e l e c t r o d e d i s s o l u - t i o n and i n t h e second one a t t h e p o t e n t i a l of t h e lowest capacity v a l u e . The c o r r e s p o n d i n g e l e c t r o r e f l e c t i o n s p e c t r a a r e r e p o r t e d i n

f i g . 2, t h e s p e c t r a r e l a t i v e to t h e c a s e when t h e l a y e r a d j a c e n t t o t h e m e t a l i s composed e n t i r e l y of w a t e r m o l e c u l e s a r e r e p o r t e d f o r comparison ( f i g . 2 a )

.

One must n o t e t h e invariance of t h e s p e c t r a when t h e adsorbed l a y e r i s composed of e t h e r ( f i g . 2 c ) , a n d t h e complete change i n t h e case of a bromide l a y e r ( f i g . 2 b ) . I n p a r t i c u l a r t h e p o l a r i z a t i o n anisotropy i s unchanged i n t h e f i r s t case and reversed w i t h a

n o t i c e a b l e enhancement i n t h e second c a s e . I t maybe deduced f r o m t h e s e r e s u l t s t h a t t h e e t h e r l a y e r a c t s a s a t r a n s p a r e n t one without any chemical i n t e r a c t i o n with gold, a t l e a s t n o t s t r o n g e r than i n t h e case of a water l a y e r . On t h e o t h e r hand t h e bromide l a y e r i n t e r a c t s s t r o n g l y with t h e gold s u r f a c e s , so s t r o n g l y t h a t t h i s i n t e r a c t i o n completely p e r t u r b s t h e arrangement of t h e outermost m e t a l l i c l a y e r . 1 . 2 . S t r u c t u r a l evolution of t h e surface during adsorption

This kind of adsorption with chemical i n t e r a c t i o n can be more p r e c i s e - l y s t u d i e d a s a f u n c t i o n of coverage with bromide ions. We present i n f i g . 3 t h e e l e c t r o r e f l e c t i o n s p e c t r a d i s p l a y e d on A u ( l l 0 ) and Au(311) f a c e s , r e c o r d e d a t ' p o t e n t i a l s c o r r e s p o n d i n g t o t h e same c h a r a c t e r i s t i c f e a t u r e s i n t h e c a p a c i t y c u r v e s f o r t h e two p l a n e s :

a ) t o t h e f i r s t maximum

,

^b) t o t h e f i r s t minimum and c ) t o t h e p l a t e a u p r e c e d i n g t h e e l e c t r o d e d i s s o l u t i o n where a f u l l coverage i s assumed t o b e r e a c h e d ( s e e f i g . l a ) .

It i s remarkaSle t h a t t h e completely covered s u r f a c e s e x h i b i t comple- t e l y d i f f e r e n t o p t i c a l p r o p e r t i e s . The s a t u r a t e d (311) f a c e i s i s o - t r o p i c whereas t h e f u l l y covered (110) f a c e d i s p l a y s t h e g r e a t e s t a n i s o t r o p i c c h a r a c t e r with a marked divergence from t h e gold spectrum, i n p a r t i c u l a r around 3 eV f o r t h e $

//

^[110] response.

A t low coverage, f o r both f a c e s t h e enhanced o p t i c a l s i g n a l s concern t h e [110] d i r e c t i o n and can be i n t e r p r e t e d a s a f i l l i n g with bromile i o n s along t h e r a i l s . Then a t higher coverage near 0.4 , t h e inver- s i o n of t h e a n i s o t r o p i c s i g n would i n d i c a t e a r e c o n s t r u c t i o n of t h e s u r f a c e due t o t h e r a t h e r s t r o n g i n t e r a c t i o n between gold atoms and bromide ions. The r e c o n s t r u c t i o n mechanism i s d i f f e r e n t f o r t h e two f a c e s a t s a t u r a t i o n and t h i s can be c o r r e l a t e d t o t h e s u p e r f i c i a l atomic roughness of t h e two bare planes. The most open (311) one i s favourable t o make an arrangement i n a c l o s e r packed atomic s t r u c t u r e than t h e more marked grooved (110) s t r u c t u r e .

2 . C h a r a c t e r i z a t i o n of a redox r e a c t i o n between adsorbed s p e c i e s A r a t h e r d i f f e r e n t kind of information c a n be gained i n t h e case of an adsorbed l a y e r whose components a r e absorbing i n t h e v i s i b l e region. This i s t h e case of a heme p r o t e i n (cytochrome

c3)

which changes i t s absorption spectrum from oxidation of t h e f e r r o u s i r o n t o f e r r i c i r o n . When a gold e l e c t r o d e is covered with an i r r e v e r s i b l y adsorbed l a y e r of cytochrome C, t h e f e a t u r e s of t h e e r s p e c t r a d r a s -

2

t i c a l l y vary a s a f u n c t i o n of t h e applied p o t e n t i a l i n t h e reglon where t h e cytochrome remains f i r m l y adsorbed without chemical degra- dation. The appearance of a b i s i g n a t e e r s i g n a l ( f u l l curve i n f i g . 4) a t 410 nm ( v ) and of two well-defined s t r u c t u r e s a t 520 nm ( 8 ) and 550 nm ( n ) i n t h e region of t h e capacity peak E ( f i g . 4) can be explained by t h e e x i s t e n c e of a redox r e a c t i o n o c c u r r i n g between

ferrous and ferric iron of the adsorbed cytochrome. This is con- firmed by the disappearance of these structures (dotted curves) when the mean potential is far from E where the species are completely reduced or oxidized. P

These results show that er can be used as a direct and positive probe of the electron transfer process for adsorbed cytochrome. This cons- titutes a unique way of investigation of this process which cannot be demonstrated unambiguously by means o f electrochemical investiga- tion alone.

REFERENCES

1. NGUYEN VAN HUONG C

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^{HINNEN C}

.

and PARSONS R., J. Electroanal. Chem. 105 (1979)

397.

2. HINNEN C., NGUYEN VAN HUONG C., ROUSSEAU A. and DALEERA J.P., J. Electroanal. Chem.

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HINNEN C., NGUYEN VAN HUONG C. et DALBERA J.P., J. Chim. Phys.

79

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PARSONS R., Surf. Sci.

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316.

Fig 1.- Capacity potential curve of Au (110) (---) in supporting electrolyte, (a) in pre- sence of ~ r - , (b) in presence of ethylether

f = 1 5 H z 6E = 5 mV rms

Fig 2.- Electroreflection spectra obtained on Au(ll0) in presence of a fully covered surface ; a) with a water layer ; b) w i t h a bromide layer ; c) with an ether layer.

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Fig 3.- Comparative electrorcflcction spectra o b t a ~ n c d on Au(ll0)

a n d Au (311) as a iunction of coverage with BK-

.

Fig 4 . - a) voltammogram and b) capacity curve of Au (110) in presence of an adsorbed layer of cytochrome C3.

c) (-) er spectrum obtained at the potential E of the capacity peak.

(--- ,

. ..

.) er spectra at