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IN SITU IR-ATR-SPECTROSCOPY OF ELECTRODE SURFACES
H. Neugebauer, A. Neckel, G. Nauer, N. Brinda-Konopik, F. Garnier, G.
Tourillon
To cite this version:
H. Neugebauer, A. Neckel, G. Nauer, N. Brinda-Konopik, F. Garnier, et al.. IN SITU IR-ATR-
SPECTROSCOPY OF ELECTRODE SURFACES. Journal de Physique Colloques, 1983, 44 (C10),
pp.C10-517-C10-520. �10.1051/jphyscol:198310106�. �jpa-00223466�
JOURNAL DE PHYSIQUE
Colloque CIO, supplément au n°12, Tome HH, décembre 1983 page C1Û-517
IN SITU IR-ATR-SPECTROSCOPY OF ELECTRODE SURFACES
H. Neugebauer, A. Neckel, G. Nauer, N. Brinda-Konopik, F. G a m i e r * and G. Tourillon*
Institut für Physikalische Chemie der Universität Wien, A-1090 Wien, Währingerstrasse 42, Austria
*Laboratoire de Photoehimie Solaire du C.N.R.S., 2, rue Henri Dunant, 94320 Thiais, France
Laboratoire de Photochimie Solaire du C.N.B.S., 2, vue Henri Vunant, 94320 Thiais, France
Résume - Dans ce papier on présente une méthode rapide pour l'étude in situ des surfaces d'électrodes en contact avec un électrolyte, utili- sant la spectroscopie infra-rouge à transformée de Fourier. On étudie les réactions électrochimiques de substances organiques polymérisées conduc- trices, par spectroscopie en réflexion totale atténuée utilisant des élé- ments réflecteurs semiconducteurs recouverts de couches minces métalliques évaporées. On discute quelques résultats sur le processus d'oxydation de films de polymères hétéroatomiques sur des couches minces de platine.
A b s t r a c t - In t h i s paper a f a s t method f o r i n s i t u i n v e s t i g a t i o n s of electrode surfaces i n contact with an e l e c t r o l y t e using Fourier transform infrared spectroscopy i s demonstrated. With attenuated t o t a l r e f l e c t i o n spectroscopy (ATRS) using semiconducting r e f l e c t i o n elements with evapo- r a t e d t h i n m e t a l l a y e r s the e l e c t r o c h e m i c a l r e a c t i o n s of c o n d u c t i n g p o l y m e r i z e d o r g a n i c s u b s t a n c e s a r e i n v e s t i g a t e d . Some r e s u l t s on t h e oxidation process of heteroaromatic polymer films on t h i n platinum l a y e r s are d i s c u s s e d .
In t h e l a s t y e a r s . e x t e n s i v e r e s e a r c h work has been done i n t h e f i e l d of i n s i t u i n v e s t i g a t i o n s of e l e c t r o d e s i n contact with an e l e c t r o l y t e / 1 / . A c h a r a c t e r i s t i c point of a l l proposed and applied techniques i s the p o s s i b i l i t y to c h a r a c t e r i z e an electrode i n contact with an e l e c t r o l y t e a t d e f i n i t e p o t e n t i a l s or during c o n t r o l - led electrochemical experiments, e.g. c y c l i c voltammetry.
Most of these methods use electromagnetic waves as s t i m u l a t i o n and detection s i g - n a l s , e.g. l i g h t i n the uv or v i s i b l e s p e c t r a l range ( e l l i p s o m e t r y , r e f l e c t i o n spectroscopy) or infrared r a d i a t i o n . Spectroscopic i n v e s t i g a t i o n s in the infrared s p e c t r a l range can yield information about the molecular s t r u c t u r e of the electrode surface and the strength of the chemical bonds of adsorbed s p e c i e s .
Some methods have been d e v e l o p e d , such as EMIBS ( e l e c t r o c h e m i c a l l y modulated infrared r e f l e c t a n c e spectroscopy) / 2 / and IRRAS (infrared r e f l e c t a n c e absorption spectroscopy) / 3 / . Another approach to obtain the infrared spectra of an electrode s u r f a c e i s t h e a t t e n u a t e d t o t a l r e f l e c t i o n s p e c t r o s c o p y (ATKS) / 4 / . In t h i s c a s e s e m i c o n d u c t i n g r e f l e c t i o n e l e m e n t s , e.g. germanium, s i l i c o n or cadmium t e l l u r i d e , are used. The working electrode in electrochemical c e l l s c o n s i s t s of one of these r e f l e c t i o n elements covered with a t h i n m e t a l l i c layer / 5 / . This m e t a l l i c layer can be used as a s u b s t r a t e m a t e r i a l for other r e a c t i o n s , e.g. polymerization of organic substances. For our i n v e s t i g a t i o n s of the oxidation r e a c t i o n s of polymerized h e t e - r o a r o m a t i c s u b s t a n c e s ( 3 - m e t h y l t h i o p h e n e and t h i o p h e n e ) we use t h i n p l a t i n u m l a y e r s . By means of Fourier transform infrared spectroscopy rapid changes of the surface during dynamic electrochemical experiments can be detected.
In figure 1 the t y p i c a l r a d i a t i o n path in the spectroelectrochemical c e l l i s demon- s t r a t e d . The c e l l i s made from t e f l o n and i s c o n s t r u c t e d a s a flow c e l l , a s d e -
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:198310106
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Fig. 1: Path of r a d i a t i o n i n t h e s p e c t r o e l e c t r o c h e ~ n i c a l equipment.
s c r i b e d e a r l i e r /6/. The c o u n t e r e l e c t r o d e i s a platinum f o i l , t h e r e f e r e n c e e l e c - t r o d e a s a t u r a t e d calolnel e l e c t r o d e (SCE) o r a s i l v e r / s i l v e r n i t r a t e e l e c t r o d e i n a c e t o n i t r i l e . A f t e r improving t h e s e a l i n g of t h e c e l l i t was p o s s i b l e t o work w i t h an evacuated s p e c t r o m e t e r (FTIR s p e c t r o m e t e r Bruker IPS 11 3V, Karlsruhe, FRG) t o e l i m i n a t e i n s t a b i l i t i e s caused by f l u s h i n g t h e i n s t r u m e n t w i t h i n e r t gas. The e l e c - t r o c h e m i c a l equipment c o n s i s t e d of a p o t e n t i o s t a t ( ~ O T B , J a i s s l e , FRG), a sweep g e n e r a t o r (PPRI, HiTekInstruments, UK) and a X-Y-plotter (2900083, Bryans, UK).
The p o l y m e r i z a t i o n r e a c t i o n s were performed i n a 0.4 molar s o l u t i o n of 3-methyl- t h i o p h e n e o r t h i o p h e n e i n a c e t o n i t r i l e w i t h tetrabutylammoniumperchlorate (0.1 molar) as s u p p o r t i n g e l e c t r o l y t e . The p o l y m e r i z a t i o n s t a r t s a t a n o d i c p o t e n t i a l s ( + I 3 V a g a i n s t SCE, r e s p e c t i v e l y +1.3 '1 a g a i n s t Ag/AgN03 i n t h e c a s e of thiophene) and t h e p o l y m e r s a p e p r o d u c e d i n t h e c o n d u c t i n g o x i d i z e d f o r m on t h e p l a t i n u m l a y e r . The o x i d i z e d f o r m s w e r e r e d u c e d t o t h e n o n - c o n d u c t i n g n e u t r a l f o r m s by applying a p o t e n t i a l of 0.0 V a g a i n s t SCE (-0.5 V a g a i n s t A ~ / A ~ N o ? ) on t h e working e l e c t r o d e . The o b t a i n e d polymer f i l m can be o x i d i z e d and reduced r e p e a t e d l y d u r i n g c y c l i c v o l t a m m e t r i c e x p e r i m e n t s . The p o t e n t i a l s w e e p a p p l i e d t o t h e c e l l was 2 mV/sec, t h e p o t e n t i a l l i m i t s w e r e 0.0 V and +1.0 V a g a i n s t SCE f o r 3 - m e t h y l t h i o - phene, i n t h e c a s e of thiophene -0.5 V and +1.0 V a g a i n s t Ag/AgrJ03. Znterferograms were measured d u r i n g t h e o x i d a t i o n and r e d u c t i o n h a l f cycle. I n a p o t e n t i a l range o f a b o u t 3 0 t o 4 0 mV a number o f i n t e r f e r o g r a m s w e r e a d d e d and s t o r e d on a d i s k . T h i s addizig of i n t e r f e r o g r a m s b e f o r e p e r f o r m i n g t h e F o u r i e r t r a n s f o r m a t i o n i s n e c e s s a r y t o improve t h e s i g n a l l n o i s e r a t i o of t h e s p e c t r a .
I n f i g u r e 2 a t h e c y c l i c voltamm6gram o f a p o l y t h i o p h e n e l a y e ? on t h e p l a t i n u m e l e c t r o d e i s displayed. F i g u r e 2b shows t h e i n f r a r e d s p e c t r a o b t a i n e d d u r i n g t h e o x i d a t i o n . The r e f e r e n c e s p e c t r u m f o r t h i s p l o t , w h i c h i s s u b t r a c t e d from e a c h c u r v e , i s t h e s p e c t r u m o f t h e e l e c t r o l y t e and was r e c o r d e d b e f o r e s t a r t i n g t h e p o l y m e r i z a t i o n , The a b s o r p t i o n of t h e polymer on t h e platinum e l e c t r o d e i n c r e a s e s v e r y s t r o n g l y d u r i n g t h e o x i d a t i o n p r o c e s s and i s r e l a t e d t o t h e change i n t h e e l e c t r i c a l behaviour. Furthermore some t y p i c a l v i b r a t i o n bands occur a d d i t i o n a l l y t o t h e bands of t h e n e u t r a l form. The s t r o n g i n t e n s i t y of t h e bands growing d u r i n g t h e o x i d a t i o n may be explained by a v i b r o n i c i n t e n s i t y enhancement e f f e c t s i m i l a r t o t h e e f f e c t obtained during t h e doping p r o c e s s of p o l y a c e t y l e n e /7/.
I n f i g u r e 3 t h e i n f r a r e d s p e c t r a measured d u r i n g t h e o x i d a t i o n p r o c e s s o f a poly-3- methylthiophene l a y e r on a platinum e l e c t r o d e i s displayed. The i n s e r t shows t h e c y c l i c voltammogram o f t h i s l a y e r . I n t h i s c a s e t h e r e f e r e n c e s p e c t r u m i s t h e s p e c t r u m of t h e n e u t r a l f o r m o f t h e p o l y m e r 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 . C o r r e s p o n d i n g t o t h e i n c r e a s e o f t h e e l e c t r i c a l c o n d u c t i v i t y t h e i n f r a r e d ab- s o r p t i o n of t h e f i l m i n c r e a s e s and some c h a r a c t e r i s t i c v i b r a t i o n bands o c c u r d u r i n g t h e o x i d a t i o n p r o c e s s . T h e s e b a n d s h a v e no c o u n t e r p a r t s i n t h e n e u t r a l f o r m and i n c r e a s e w i t h t h e e l e c t r o c h e m i c a l o x i d a t i o n process. A d d i t i o n a l l y t h e i n t e n s i t i e s o f some b a n d s grow w i t h d i f f e r e n t r a t e s d u r i n g t h e o x i d a t i o n p r o c e s s . A b a n d a t 1100 cm-l o c c u r s i n t h e h i g h o x i d a t i o n s t a t e . T h i s band i s a s s o c i a t e d w i t h t h e C104- a n i o n / 8 / , and r e s u l t s from t h e i n t e r c a l a t i o n of p e r c h l o r a t e a n i o n s i n t o t h e p o l y m e r f i l m d u r i n g t h e o x i d a t i o n p r o c e s s /9/. An e x a c t i n t e r p r e t a t i o n o f a l l
F i g . 2a: C u r r e n t d e n s i t y / p o - t e n t i a l curves:
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e l e c t r o c h e m i c a l polyme- r i z a t i o n o f t h i o p h e n e (sweep r a t e 2 m ~ / s e c )-
o x i d a t i o n and r e d u c t i o n of t h e p o l y t h i o p h e n e l a y e r (sweep r a t e 2 m ~ / s e c )ig. 2b: ATR-IR-spectra acorded d u r i n g t h e oxi- i t i o n o f p o l y t h i o p h e n e . t c h s p e c t r u m o r i g i n a t e s rom 20 added i n t e r f e r o - rams. R e f e r e n c e spec- rum: e l e c t r o l y t e s o l u - ion. The i n s e r t e d poten- i a l v a l u e s r e f e r t o t h e y c l i c voltammogram i n ig. 2a.
Fig. 3: ATR-IR-spectra r e c o r d e d d u r i n g t h e o x i d a t i o n o f p o l y - 3 - m e t h y l t h i o p h e n e . Each s p e c t r u m o r i g i n a t e s from 128 added i n t e r f e - r o g r a m s , r e f e r e n c e s p e c t r u m i s t h e spec- t r u m o f t h e n e u t r a l form of poly-+methyl- thiophene.
The i n s e r t shows t h e c y c l i c voltammogram o f t h e o x i d a t i o n and re- d u c t i o n o f t h e poly-3- methylthiophene layer.
CIO-520 JOURNAL
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PHYSIQUEv i b r a t i o n bands o f t h e polymer and of t h e i r behaviour d u r i n g t h e o x i d a t i o n process ( s h i f t of t h e maxima e.g.) cannot be described a t t h i s moment, but work i s c a r r i e d out t o g e t more i n f o r m a t i o n about t h e s t r u c t u r e and t h e normal v i b r a t i o n modes of t h e s e organic conducting polymers.
The ATR spectroscopy i n combination w i t h a f a s t F o u r i e r transform equipment a l l o w s t o g e t i n f o r m a t i o n of a n e l e c t r o d e s u r f a c e i n t h e i n f r a r e d s p e c t r a l range during d y n a m i c e l e c t r o c h e m i c a l e x p e r i m e n t s and i s s u i t a b l e f o r t h e c h a r a c t e r i z a t i o n of r e a c t i o n products and t h e e l u c i d a t i o n of r e a c t i o n mechanisms.
Acknowledgement
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We g r a t e f u l l y acknowledge t h e f i n a n c i a l support given by t h e"Fonds s u r Fijrderung d e r w i s s e n s c h a f t l i c h e n Forschung" of Austria.
We a l s o t h a n k Prof. R.Kellner, I n s t i t u t f i i r A n a l y t i s c h e Chemie d e r T e c h n i s c h e n U n i v e r s i t l t Wien, f o r h i s kind permission t o use t h e FTIR-equipment.
References
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