j. electrochem. soc. 1988 stilwell 2254 62

7/28/2019 J. Electrochem. Soc. 1988 Stilwell 2254 62

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Electrochem istry of C on du ctive Polym ersI I . E l e c t r o c h e m i c a l S t u d i e s o n G r o w t h P r o p e r t ie s o f P o l y a n i l i n e

D a v i d E . S t i l w e l l * '1 a n d S u - M o o n P a r k *Departm ent o f Chemistry, University of New Mexico, Albuquerque, New Mexico 87131

A B S T R A C TD e t a i ls o f t h e g r o w t h p r o p e r t i e s o f p o l y a n i li n e ( P A ) h a v e b e e n s t u d i e d e m p l o y i n g e l e c t r o c h e m i c a l t e c h n i q u e s ( p o te n -t i a l c y c l i n g m e t h o d ) a n d t h e r e s u l t s a r e r e p o r t e d . T h e e l e c t r o c h e m i c a l g r o w t h m a y b e c h a r a c t e r i z e d a s a u t o c a t a ly t i c , a n dt h e g r o w t h r a t e i s f i rs t o r d e r i n a n i l i n e c o n c e n t r a t i o n . T h e g r o w t h r a t e is a l s o d e p e n d e n t o n t h e a m o u n t o f P A f i lm o n t h ee l e c t r o d e a s w e l l a s t h e n u m b e r o f p o t e n t i a l c y c l e s . T h e h i g h e s t o x i d i z e d P A , a t t a c k e d b y a n a n i l i n e m o l e c u l e , g r o w s t o al o n g e r c h a in , a n d a n e t r e d u c t i o n o f t h e P A f il m u p o n a n i li n e a d d i t i o n w o u l d r e n d e r t h e f i l m c o n d u c ti n g . T h i s m e c h a n i s me x p l a i n s w h y t h e f i l m c o n t i n u e s t o g r o w a t p o t e n ti a l s w h e r e o n l y p a s s i v a t in g b e h a v i o r w o u l d b e p r e d i ct e d . I n a d d i t i o n, t h eg r o w t h w a s d i v i d e d i n to r e g i o n s o f w e l l - d e fi n e d a n d p o o r l y d e f i n e d ( a m o r p h o u s ) p h a s e s ; a t l o n g e r o x i d a t i o n t i m e s d u r i n gg r o w t h , s i d e r e a c t i o n s s e e m t o p l a y m o r e i m p o r t a n t r o le s c o m p a r e d t o th e e a r l i e r p h a s e o f t h e g r o w t h p r o c e s s.

T h e g r o w i n g i n t e re s t i n c o n d u c t i v e p o l y m e r s h a sp r o m p t e d r e n e w e d s t u d i e s o n p o l y an i l in e . U n l i k e m o s tc o n d u c t i n g p o l y m e r s , p o l y a n i l i n e ( P A ) i s r e a d i ly p r e p a r e df r o m a q u e o u s s o l ut i o ns , a n d t h e c o n d u c t i n g f o r m i s s t a b lei n a i r a n d w a t e r . P A ' s h a v e p r o t o n e x c h a n g e a n d r e d o xp r o p e r t i e s , a r e i n s o l u b l e i n t h e c o m m o n o r g a n i c s o l v e n t s ,a r e h y g r o s c o p i c , a n d h a v e a n i l l -d e f i n e d m e l t i n g p o i n t(1, 2). A n u m b e r o f u s e s h a v e b e e n d e m o n s t r a t e d f o r p o l y -a n i l i n e ; i t is u s e d i n b a t t e r i e s ( 2- 5) , i n i o n e x c h a n g e ( 6, 7) , t op r o t e c t m e t a l s a n d s e m i c o n d u c t o r s f r o m c o r r o s i o n (8 -1 0),i n e l e c t r o c h r o m i c d i s p l a y s ( 11 , 1 2), a n d i n a m i c r o e l e c t r o n -i c d e v i c e ( 13 ).P A c a n b e p r e p a r e d e i t h e r c h e m i c a l l y o r e l e c t r o c h e m i -c a ll y . C h e m i c a l s y n t h e s i s m e t h o d s r e q u i r e a s t r o n g o x i -d a n t s u c h a s p e r s u l f a t e s ( 2 , 4 , 5 ) o r p e r c h l o r a t e s a n d h y d r o -g e n p e r o x i d e ( 14 -1 6). E l e c t r o c h e m i c a l t e c h n i q u e s i n c l u d ep o t e n t i o s t a t i c , g a l v a n o s t a t i c , a n d p o t e n t i a l c y c l i n g m e t h -o d s (1 , 3 , 8 -1 3 , 17 -2 5). I n g e n e r a l , c h e m i c a l m e t h o d s o f p r e p -a r a t i o n l a c k t h e c o n t r o l o f re a c t i o n c o n d i t i o n s . T h e p o t e n -t ia l c y cl i n g m e t h o d h a s b e e n c l a i m e d t o p r o d u c e f i lm sw i t h s u p e r i o r a d h e s i o n , s m o o t h n e s s , a n d o p t i c a l p r o p e r -t i e s ( 1 8 , 2 2 , 2 5 ) , a l t h o u g h K i t a n i et al. ( 2 0 ) c o u l d n o t d e t e c ta n y d i f f e r e n c e i n t h e e l e c t r i c a l o r c h e m i c a l p r o p e r t i e s b e -t w e e n f i lm s g r o w n b y u s i n g t h e p o t e n ti o s t a t ic a n d t h e p o -t e n t ia l c y c l i n g m e t h o d s o n p l a t i n u m s u rf a c es . T h e p o t e n -t i o st a t ic m e t h o d h a s b e e n c i t e d a s p r o d u c i n g s u p e r i o rf i l m s o n p - S i (1 0). I n t h e m a j o r i t y o f i n s t a n c e s , h o w e v e r , n op a r t i cu l a r r e a s o n h a s b e e n g i v e n f or u s in g t h e e x p e r i m e n -t a l g r o w t h c o n d i t i o n s r e p o r t e d , a l t h o u g h , i n a l l c a s e s , t h ef i lm s g r o w n i n a c i d i c s o l u t io n s r e s u l t e d i n e l e c t r o a c t i v ep o l y a n i l i n e f i lm s .T h e r e h a v e b e e n a f e w s tu d i e s w h e r e t h e g r o w t h m e c h -a n i s m s f o r P A f i l m s a r e m e n t i o n e d (1 , 8 , 1 7, 2 2). T h eb u i l d i n g u p o n t h e b a si s p - a m i n o d i p h e n y l a m i n e a n i l in ed i m e r , a s f ir s t s u g g e s t e d b y M o h i l n e r et al. ( 1) , a n d l a t e r b yV o l k o v et al. (1 7) a s w e l l a s M e n g o l i et al. ( 8) h a s b e e n f o r -w a r d e d i n t h e l i t e ra t u r e . C a r l i n et al. (2 2) e x a m i n e d t h e o x i -d a t i o n o f a n i l in e a t t h e t e r m i n i o f t h e p o l y m e r i c c h a i nu s i n g o p t i c a l m i c r o s c o p y . T h e f i lm g r o w t h c h a r a c t e r i s t i c sa r e n o t w e l l u n d e r s to o d . F o r f i l m s g r o w n b y t h e p o t e n t i a lc y c l i n g m e t h o d , t h e f i lm g r o w t h h a s b e e n r e p o r t e d t o b el i n e a r ( 20 ) a s w e l l as q u a d r a t i c w i t h t h e c y c l e n u m b e r (2 5).A d e c r e a s e i n th e g r o w t h r a t e w i t h t h i c k n e s s w a s r e p o r t e db y P a u l et al. ( 13 ) f o r p o t e n t i o s t a t i c a l l y g r o w n f i lm s .I n o u r c u r r e n t c o m m u n i c a t i o n , w e d e s c r i b e o u r d e t a i le ds t u d i e s o n t h e g r o w t h p r o p e r t i e s o f P A f il m s e m p l o y i n ge l e c t r o c h e m i c a l t e c h n i q u e s . T h e p o l y a n i l i n e f i l m s u s e d i no u r w o r k w e r e a l l p r e p a r e d b y t h e p o t e n t i a l c y c l in gm e t h o d ( 3, 9 , 17 , 1 8, 2 0 , 2 2 , 2 5) . D u r i n g t h e c o u r s e o f o u rw o r k , a v a r i e t y o f a n i l i n e c o n c e n t r a t i o n s , e l e c t r o l y t e c o m -p o s i t i o n s , a n d p o t e n t i a l c y c l i n g l im i t s w e r e e m p l o y e d . As y s t e m a t i c s t u d y o f th e g r o w t h r a t e a s a f u n c t io n o f c y c l en u m b e r a n d a n i l i n e c o n c e n t r a t i o n w a s , h o w e v e r , l i m i t e dt o g r o w t h c o n d i t i o n s i n 1 M H ~ SO 4 , w i t h t h e p o t e n t i a l c y -

* E l e c t r o c h e m i c a l S o c i e t y A c t i v e M e m b e r .1 P r e s e n t a d d r e s s : N a v a l W e a p o n s C e n t e r , C h i n a L a k e , C a l i f o r n i a93555.

c l e d b e t w e e n t h e l i m i t s o f - 0 . 1 9 t o 1 .2 V vs. A g / A g C 1 ( s a t u -r a t e d K C 1 ) .E x p e r i m e n t a l

T h e e l e c t r o c h e m i c a l m e a s u r e m e n t s w e r e c ar r ie d o u tw i t h a P A R M o d e l 1 73 p o t e n t i o s t a t / g a l v a n o s t a t w i t h t h e1 79 d i g i t a l c o u l o m e t e r a n d t h e 1 75 u n i v e r s a l p r o g r a m m e ra c c e s s o r i e s . F o r t h e e l e c t r o c h e m i c a l c e ll , w e e m p l o y e d as t a n d a r d t h r e e - e le c t r o d e s y s t e m c o n t a i n e d i n a d u a l c o m -p a r t m e n t c e ll . A f i n e p o r o s i t y g l a s s f r i t w a s u s e d t o s e p a -r a te t h e w o r k i n g e l e c t r o d e f r o m t h e c o u n t e r e l e c t r o d e . Ap l a t i n u m d i s k ( a r e a = 0 . 03 7 -+ 0 .0 0 2 c m 2) o r a p l a t i n u m c y -l i n d r i c a l f o i l ( a re a = 9 . 5 - + 0 .2 c m ~) w a s u s e d a s t h e w o r k i n ge l e c t r o d e . P r i o r t o u s e , t h e p l a t i n u m d i s k e l e c t r o d e w a sp o l i s h e d t o a f i n al s m o o t h n e s s o f a r o u n d 0 .1 ~ m w i t h s l u r -r i e s o f a l u m i n a p o l i s h i n g p o w d e r ( F i s h e r) . F o r th e g r o w t hs t u d i e s , t h e d i s k e l e c t r o d e w a s u s e d ; a n d f o r f i l m w e i g h td e t e r m i n a t i o n s , t h e l a rg e a r e a e le c t r o d e w a s e m p l o y e d .T h e r e f e r e n c e e l e c t r o d es w e r e e n c a s e d w i t h a L u g g i np r o b e . A n A g / A g C 1, s a t u r a t e d K C 1 e l e c t r o d e w a s u s e d a s ar e f e r e n c e . I n s o m e c a s e s , f i l m s w e r e g r o w n o n a r o t a t i n gd i s k e l e c t r o d e ( R D E ) . T h e g e o m e t r i c a r e a o f t h i s e l e c t r o d ew a s 0 .4 61 c m 2. T h e e x p e r i m e n t a l d e t a i ls f o r t h e r o t a t i n gd i s k a n d f o r t h e r o t a t i n g r i n g - d i s k e l e c t r o d e ( R R D E ) a r ed e s c r i b e d e l s e w h e r e ( 26~ .T h e p l a t i n u m e l e c t ro d e s w e r e p r e c l e a n e d a n d c o n d i -t i o n e d b y p o t e n t i a l p u l s i n g ( 0 . 5 s p u l s e w i d t h ) b e t w e e n t h eh y d r o g e n a n d o x y g e n e v o l u t i o n re g i o ns . T h is p r o c e d u r er e s u lt s i n f o r m i n g a p l a t i n u m s u r f a ce w i t h r e p r o d u c i b l ee l e c t r o c h e m i c a l b e h a v i o r s ( 27 -2 9) . I n a d d i t i o n , a c l e a n p l a t -i n u m s u r f a c e e x h i b it s c h a r a c t e r is t i c c y c l ic v o l t a m m o -g r a m s i n 1 M H 2 S O 4 (3 0) , w h i c h w e r e u s e d f o r q u a l i t y c o n -t r ol . T h r o u g h o u t t h e s e s t u d i e s , a n a e r o b i c c o n d i t i o n s w e r em a i n t a i n e d w i t h a n it r o g e n o r a r g o n a t m o s p h e r e . T h e g a sw a s p a s s e d t h r o u g h a g as p u r i f i e r ( M a t h e s o n N o . 6 40 6) a n dt h e n t h r o u g h a g a s b u b b l e r f i ll e d w i t h d o u b l y d i s t i l l e dw a t e r t o m i n i m i z e e v a p o r a t i o n i n t h e c e ll .T h e c h e m i c a l s w e r e a l l r e a g e n t g r a d e o r b e tt e r . T h e s u l -f u r i c a c i d (U l t r e x , B a k e r ) w a s u s e d a s r e c e i v e d . A n i l i n e( E a s t m a n ) w a s u s e d a f t e r d is t i l l in g o v e r z in c d u s t t o e l i m i -n a t e t h e o x i d i z e d i m p u r i t i e s . T h e c l e a r a n i l i n e s o l u t io nw a s s t o r e d i n t h e d a r k u n d e r a n a r g o n o r n i t ro g e n a t m o -s p h e r e. A l l o f t h e a q u e o u s s o l u ti o n s w e r e p r e p a r e d w i t hd o u b l y d i s t i l l ed d e i o n i z e d w a t e r .

Resu l tsCyclic voltammetry.--Shown i n F i g . 1 a r e r e p r e s e n t a t i v ec y c l ic v o l t a m m o g r a m s ( C V 's ) t a k e n d u r i n g f i l m gr o w t h .T h e p e a k d e s i g n a t i o n s s h o w n i n F ig . l c w i l l b e f o l l o w e d i nt h i s d i s c u s s i o n . F r o m t h e C V ' s , a n u m b e r o f p o i n t s c a n b em a d e r e g a r d i n g t h e g r o w t h m e c h a n i s m . F i rs t , t h e a n i li n e

o x i d a t i o n w a v e , p e a k E ( s e e F i g . l c ), d i s a p p e a r s a f t e r ac o u p l e o f c y c l e s . T h i s a t t r i b u t e s t r o n g l y s u g g e s t s t h a t t h eg r o w t h d o e s n o t c o n s i s t o f t h e s i m p l e b u i l d u p o f l a y er s o nt h e s u r f a c e f r o m t h e a n i l in e o x i d a t i o n p r o d u c t s d u r i n g t h ei n i t ia l p h a s e o f t h e g r o w t h p r o c e s s . N o t e a l s o t h a t t h e m o s ta n o d i c o x i d a t i o n w a v e o f P A f o r m e d , p e a k D , a p p e a r s2 2 5 4

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Fig. 2 . Growth C V curves for P A . Grow n i n 1M H 2S O4 + 0 . 082Ma n i l i n e b e t w e e n - 0 . 1 9 a n d 1 . 2V v s . A g/AgCI at a scan rate ot~ S Om V / s . E l e c t r o d e a r e a = 0 . 0 3 7 c m 2.

Fi g. 1 . Th e e f f e c t o f s c a n r a t e o n P A g r o w t h c y c l i c v o l t a m m o g r a m sg r o w n i n 1 M s u l f u r i c a c i d w i t h [ a n i l i n e ] = 0 . 0 3 3 M . P t e l e c t r o d e a r e a =0 . 037 (cm ) 2. (a) S ca n rate = 20, (b) S O, (c) 1 00 m V/s.a r o u n d 2 0 0- 40 0 m V n e g a t i v e w i t h r e s p e c t t o t h e d i s a p -p e a r i n g a n i l i n e w a v e . T h e c o r r e s p o n d i n g r e d u c t i o n p e a k( D ' ) i s, h o w e v e r , q u e n c h e d a s t h e s c a n r a t e d e c r e a s e s , s u g -g e s t i n g t h e o x i d a t i o n p r o d u c t , g e n e r a t e d a t C V p e a k D a n da c c u m u l a t e d o n t h e e l e c t r o d e s u r fa c e , re a c t s w i t h t h ea n i l i n e , a n d t h a t t h i s r e a c t i o n i s t h e r a t e - l i m i t i n g s t e p . W eb e l i e v e t h a t t h i s s p e c i e s i s a d i c a t i o n o r a d i r a d i c a l d i c a t i o ni n p o l y a n i l i n e w h i c h , i n t u r n , a c c e p t s a n e l e c t r o n f r o ma n i l i n e . F o l l o w i n g t h i s a d d i t i o n r e a c t i o n , th e s u r f a c eb o u n d p r o d u c t i s r e o x i d i z e d o n t h e n e x t a n o d i c s c a n, ap r o c e s s t h a t r e p e a t s i t s e l f t o s o m e d e g r e e . A t s l o w e r s c a nr a t e s t h e e f f e c t i v e o x i d a t i o n t i m e i s a p p a r e n t l y l o n g s u c ht h a t u p o n s w e e p r e v er s a l, t h e s t ea d y - s ta t e c o n c e n t r a t i o n o ft h e i n t e r m e d i a t e b e c o m e s l o w . T h e q u e n c h i n g o f t h e r e-d u c t i o n w a v e ( D ' ) i s a l so d e p e n d e n t o n t h e a n i l in e c o n c e n -t r a t i o n . T h i s p o i n t i s i l l u s t r a t e d b y c o m p a r i n g t h e g r o w t hb e h a v i o r i n 0 . 08 2 M a n i l i n e , s h o w n i n F i g . 2 , w i t h t h eg r o w t h a t 0. 03 3 M a n i li n e , s h o w n i n F i g . l b , u n d e r o t h e r -w i s e i d e n t i c a l c o n d i t i o n s .I t w a s s h o w n t h a t t h e o x i d a t i o n o f t h i s p a r t i c u l a r s u r f a c es p e c i e s i n t o t h e d i r a d i c a l o r d i c a t i o n w a s c r u c i a l f o rg r o w t h t o o c c u r. T h is w a s d e m o n s t r a t e d b y i n c r e m e n t i n gt h e s w i t c h i n g p o t e n t i a l i n a p o s i t i v e d i r e c t i o n f o r a p a r -t i a ll y g r o w n f i l m a n d n o t i n g w h e n g r o w t h s t ar t s. C y c l i n gt o v a l u e s l e s s p o s i t i v e t h a n t h e f o o t o f t h i s o x i d a t i o n w a v e( p e a k D ) r e s u l t e d i n n o g r o w t h . F i l m g r o w t h w a s o n l yn o t e d w h e n t h e p o t e n t i al a s s u m e d a f i n al v a l u e o f g r e a t e rt h a n 0 . 7 - 0 . 8 V vs . A g / A g C 1 i n 1 M H 2S O 4 .E v i d e n c e f o r a c a t a l y t ic c u r r e n t i s a l so s h o w n i n F ig . 3. I nt h i s f i g u r e t h e C V f o r t h e l a s t g r o w t h c y c l e ( A ) i s c o m p a r e dt o t h e s e c o n d C V ( B ) f o r t h e s a m e f i l m t a k e n i n a s o l u t i o nc o n t a i n i n g o n l y t h e 1 M H 2 SO 4 e l e c t r o l y t e . T h e o x i d a t i o nc u r r e n t f o r p o t e n t i a l s p o s i t i v e t o t h e l a s t o x i d a t i o n w a v e

( p e a k D ) i s l a r g e r in s o l u t i o n c o n t a i n i n g a n i l i n e s u g g e s t i n gt h e r e o x i d a t i o n o f t h i s s p e c i e s, a l t h o u g h t h e r e i s s o m e b a s el i n e sh i ft . T h e f a c t t h a t f i l m s d o i n d e e d g r o w u n d e r p o t e n -t i o s t a t i c c o n d i t i o n s ( 8- 10 , 13 , 19 , 2 0 -2 5 ), c o u p l e d w i t h o u rf i n d i n g s t h a t t h e a n i li n e o x i d a t i o n w a v e d i s a p p e ar s ,i m p l i e s t h a t s o m e o t h e r p r o c es s , i:e., a u t o c a t a l y s i s , m u s to c c u r . A l s o n o t e t h a t t h e q u e n c h i n g e f f e c t f o r t h e r e d u c -t i o n o f p e a k D ' i s e v i d e n t o n l y i n t h e s o l u t i o n c o n t a i n i n ga n i l i n e .F u r t h e r e v i d e n c e w h i c h i n d i c at e s t h a t th e o x i d a t i o np r o d u c t r e s p o n s i b l e f o r g r o w t h i s i n d e e d a su r f a c e b o u n ds p e c i es i s p r o v i d e d b y o u r r e s u l t s o n t h e C V b e h a v i o r o fP A o n a r o t a t i n g d i s k e l e c t r o d e . I n F i g . 4 a t h e e f f e c t o f r o -t a t i n g t h e e l e c t r o d e i n s o l u t i o n s c o n t a i n i n g o n l y t h e e le c -t r o l y t e is s h o w n . N o t e t h a t t h e r e d u c t i o n w a v e ( D ' ) h a s ap e a k s h a p e a n d d o e s n o t d i s a p p e a r u p o n r o t a t i o n . I f t h eo x i d a t i o n p r o d u c t w h i c h b e g i n s t o a p p e a r a t - 0 . 7 V w a s

A A

I I I I I I 11 2 0.8 0.4 0

P o t e n t i a l , V v s . A g /A g C IF ig. 3 . C o m p a r i s o n o f c y cl ic v o l t a m m o g r a m s f o r P A. S c a n r a t e = 5 0m V / s . E l e c t r o d e a r e a = 9 . S c m ~. (A ) Last grow th curv e i n I M H zS O4 +0 . 0 5 S M a n i l i n e . ( B) S e c o n d s c a n i n 1 M H2SO4.

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2256 J. Electrochem. Sac.: E L E C T R O C H E M I C A L S C I E N C E A N D T E C H N O L O G Y Septemb er 1988

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P o t e n t i a l , V v s . A g A g C /Fig. 5 . The growth behavior for PA und er hydrodyn am ic condi t ions in] M H ~S 0 4 + 0 . 0 8 8 M a n i l i n e . P o t e n t i a l l i m i t s ; -0 . 1 to 1~2V vs. Ag/

A gC I . S c a n ra t e = 5 0 m V / s . R o t a t i o n ra t e = 1 0 0 0 rp m . T h e c u rv e ss h o w n a re f o r t h e g ro w t h c y c l e n o . ' s w h i c h a re i n d i c a t e d o n t h e f i g u re .

b

.= R~j' 0 ! 8 o ' 4 ' l ,

P o t e n t i a l , V v s . A g /A g C IF ig. 4 . ( a ) C y c l i c v o l t a m m e t r i c b e h a v i o r f o r a P A f i l m o n a R D E i n I MH2S04. Scan rate = 50 mV/s. Rotat ion ra te = 1000 rpm. E lectrode ar ea0 . 4 6 c m 2 . ( b ) T h e e f f e c t o f ro t a t i n g t h e e l e c t ro d e d u r i n g g ro w t h i n1 M H =S 0 4 + 0 . 0 8 3 M a n i l i n e . S c a n ra t e = 5 0 m V / s . U n m a rk e d c u rv e i st h e g ro w t h b e h a v i o r f o r t h e | S t h c y c l e i n a q u i e t s o l u ti o n . T h e c u r v em a r k e d " R " i s t h e b e h a v i o r f o r t h e 1 6 t h c y c l e w i t h t h e r o t a t i o n r a t e =1 0 0 0 rp m .

soluble, then it would have been swept away and the re-duction current profile would not assume the peak shapeshown. I n Fig. 4b the effects of rotati ng the electrode dur-in g growth is shown. The rotation was turned on at the endof a growth cycle and the rotational effects exhibi ted werefor the next higher cycle number. Note that there is no cur-rent plateau for aniline oxidation and that the area underthe CV does not drastically change. Aniline, therefore, iszmt oxidized at the mass-transfer limited rate and the ma-jority of the features seen in the CV reflect surface reac-tions. The peak (B,B') seen at ~0.5V tends to, however, dis-appear when rotated; thus, this product should be solutionsoluble. The peak potential is in the same potential regionas for the be nzo /hyd roqu inon e (BQ/HQ) couple in 1MH2SO4. We believe that this product is benzoquinone (BQ)produced via hydrolysis reactions. The species corres-ponding to the redox peak (C,C') seen adjacent to the BQpeak is unknown. This species could be benzidine (31-35),or i t could represent some intermediate PA oxidationstate.

Alternatively, if it is accepted that peaks B and B' repre-sent the HQ/BQ couple then it is reasonable to assign thepeaks at C and C' to the p-aminophenol/benzoquinone-imine (PAP/QI) redox pair. The redox potential for thePAP/QI couple lies about 100 mV positive to the HQ/BQcouple. The QI would be produced as an intermediate hy-

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4/9

Vol . 135, No. 9 E L E C T R O C H E M I S T R Y O F C O N D U C T I V E P O L Y M E R S 2257

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drolysis product between quinonediimine and p-benzo-quinone. The oxidation of PAP followed by the hydrolysisof the qui noneimine has been well s tudied and is aclassic example of an EC (electrochemical followed bychemical) reaction mechanism. Indeed, the mea surementsof the relative peak hei ghts for QI and BQ reduc tion ha vebeen use d in studies aime d at determini ng the rate of hy-drolysis of QI into BQ. This topic has be en discusse d in de-tail by Adams (27). It should be point ed out also that themajor degradation product detected, when PA was oxi-dized at potentials more positive than CV peak D in anelectrolyte solution without aniline present was BQ(36, 37).PA films were also grown under conditions where theelectrod e was conti nuousl y rotated, shown in Fig. 5. Thealmost total loss of activity for aniline oxidation betweenthe first and second cycle is clearly and drama tically dem-onstrated in this figure. The general features of growingPA films by this rotation method are similar to the growthbehavior in unstirred solutions. The actual growth,though, is around five times slower than the growth meas-ured for films prepared on the same electrode under iden-tical conditions in quiet solutions; for example, the c hargeunde r the cathodic port ion of the CV for the 10th and 20thcycl e was 10.73 and 26.05 mC whe n gro wn und er contin u-ous rotat ion, w hile it was 54.41 and 151.2 mC for gro wth inthe quiet solution. This lower rate may be due to thethrusti ng of the g rowing fibers away from the electrode bythe centrifugal force. When grown under continuous rota-tion conditions, a green product with a radial pattern wasdeposited On the Teflon surface surrounding the platinumdisk electrode. The lower rate may also be due to smallersize PA products being swept away, which would havebeen deposited otherwise and grown further.We have since grown PA under a wide range of condi-tions and in all cases the aniline wave is inhibited; concur-rent with this, is the appea rance of the surface bound spe-cies at potentials less than t he initial aniline oxidation.

G r o w t h a n d g r o w t h r a t e . - - T h e growth of the PA filmswas monitored as a function of potential cycle number bymeasur ement of the charge under the cathodic portion ofthe CV (20), obtained for each growth cycle. The growthrate was taken as the average difference in charge betweenthe prec eding and following cycle number. The aniline

concentration in the growth solution ranged from -0.01 to-0.33M.Typical examples of the growth curves that were ob-tained are shown in Fig. 6. The growth rates deter minedfrom the growth cu rves are shown in Fig. 7. Note that, ex-cept for growth conditions with very low aniline concen-trations, there is a growth region that is characterized by agrowth rate which increases linearly with cycle number.After some cycling time, there are negative deviations inthe growth rates which finally reach some ma xim umvalue. For cycles beyond the maximum, the rate de-creases.This change in the growth behavior is related to filmthickness as well as the total oxidation time at potentialsmore posit ive than CV peak D. The cycle number, wherenegative deviat ions in the growth rate are first noted, fallsfro m 30 - 35 cycl es for gro wth in 0.02 - 0.03M anilin e to15 - 20 for growt h in 0.16 - 0.32M anil ine. Char ge, an dhence thickness, was not the only parameter exerting thiseffect; for example, the charge measu red for the 20th cyclewas 3 mC in 0.0328M aniline, but it was 100 mC in 0.164Manilin e (Fig. 6). This poin t is furth er il lustrat ed by co mpari-son of the g rowth CV's at high cycle numbers, shown inFig. 8. Note that the c hanges in growt h rate occur concur-rently with significant differences in the shapes of theCV's. Equally important, though, is the fact that despite anorder of magnitude difference in the current scale, theCV's are similar in appearance, suggesting that the totaloxidation time may be an import ant factor in the processwhich results in the obse rved changes in growth rate.The morphological changes seen in the CV's at longeroxidation times, which cause the lower observed growthrates, could be brought about by a buildup of cross-linkedand other degrada tion by-products in the film (2). Thisbuildup of presumably undesirable reaction productswould result in films with relatively poor charge transferkinetics, i.e., poor conductance. Associated with the slowcharge transfer would be capacitance and IR drop distor-tions. Oyama's group has found that the effect ive diffusioncoefficient of PA changes by about two orders of magni-tude, d epend ing on growth conditions (23). In addition,cross-linking which would form bonds on the phenylgroups (2), may i nhibit forma tion of the radical cationform. Additions on the phenyl group inhibit radical cationformation for violene com pound s (38, 39).



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2258 J. EIectrochem. Soc.: E L E C T R O C H E M I C A L S C I E N C E A N D T E C H N O L O G Y Sep tem ber I988

Fig. 8. Compar ison o f growth CV's . (a ) Growth cyc les 1 -25 in 1Ms u l fu r ic a c i d + 0 . 1 6 M a n i l i n e . (b ) Gr o w th c y c l e s 2 2 -4 7 , a n d 5 5 in 1 MH2504 + 0 .033M ani l ine . (See Fig. lb for cyc les 1 -20) .

Further evidence that the charge transfer becomesslower at longer oxidation times is shown from the in-crease in the pe ak widt h at half height for the first anodicpeak (take, peak A, for example). This width changes from100-120 mV, for the linear growth rate region, to well over200 mV for the films at higher cycle number. This increasein the peak widt h at half the height impli es slower electrontransfer kinetics (40).The same general features as mentioned above were ob-served for films that were grow n to less positive switchingpotentials . The cycle number where morphologicalchanges in the CV are first noted is comparative ly higher,but this was at the expense of a lower growth rate due to

the decrease in net oxidation time. For example, a filmgrow n in 0.1M H2SO4 cont aini ng 0.1M aniline, c ycled at ascan rate of 100 mV/s between -0.156 to +0.844V vs. Ag/AgC1, did not exhibit amorphous growth until about the150th cycle, yet, the charge for that cycle was only -3.5mC. Note in Fig. 6 that this charge is reached after only fiveto ten cycles under normal growth conditions, when com-paring films grown in solutions at similar aniline concen-tra tio ns (0.082M).Increasing the scan rate is another way to decrease thenet oxi dation time per cycle, and was found to play an im-portant role in the growth rate. In Fig. 9, the pea k height ofthe first anodic wave (peak A) has been plotted vs. scanrate at a constant growth cycle number. If the films wereall of the sam e thickness, then the peak height woul d in-crease linearly with scan rate (40). It was d eter mined fromthe slope of the log/log plot that the peak he ight decrea sedto the -0.55 power with scan rate. Since the growth ismuc h less at higher scan rates, the oxi dation time is one ofthe limi ting factors in the growth mechanism. This findingis consistent with the observations given earlier on thequenc hing effect of peak D' with lower scan rates. In otherwords, aniline does not react rapidly with the oxidized spe-cies. If this were so, then t he scan rate de pend ency wouldbe seen to assume some limiting value.

At comparatively low aniline concentrations, below0.02M, the results were ambiguous. This behavior may re-flect the compe ting nature b etween growth and degrada-tion. The degradation of PA when subje cted to high oxida-tion potentials in electrolyte solutions has been no ted (11,13, 25, 37, 41, 45). Grow th in low a niline conc entr ati onscould be more strongly influenced by these degradationreactions.To briefly summarize, the growth conditions which re-sult in a change in the growth rate, and in morphologicalchanges in the volt ammograms have been identified; theseconditions were avoided in all of our subsequent work,and only those growth conditions which produ ce filmswith sharp and well-defined volta mmetr ic peaks were em-ployed in the results describe d below.Over the region where the growth rate increases linearlywith cycle number, the film growth expr essed in total ca-thodic charge (mC) may be characterized byGro wth (mC) = k[An] x (cycle no.) 2 + C [1]

Plots of this growth function for various aniline concentra-tions are shown in Fig. 10. The dev iati on from linearity, forcycle numbers beyond around 20-25, is shown in the fig-1, 4

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F i g . 1 1 . ( a , t o p ) G ro w t h ra t e v s . a n i l i n e c o n c e n t r a t i o n f o r c y c le s # 5a n d # 2 0 . ( b, b o t t o m ) L o g/l o g p l o t o f t h e d a t a i n (a ) .u r e; h o w e v e r , o n l y t h e d a t a p o i n t s b e t w e e n 5 a n d 2 0 - 2 5c y c l e s w e r e u s e d t o d e t e r m i n e t h e b e s t f it l i ne s . T h e l in e a rc o r r e l a t i o n c o e f f i c i e n t r a n g e d f r o m 0 . 9 9 9 6 t o 0 .9 9 9 7 . T h ev a l u e s f o r k a n d x w e r e c o m p u t e d f r o m g e o m e t r i c p l o t s o ft h e s l o p e ( s l o p e = k [ A n ] =) vs. t h e a n i l i n e c o n c e n t r a t i o n ;t h u s , b y f i x i n g t h e r e a c t i o n o r d e r i n c y c l e n u m b e r a t 2 , t h ea n i l i n e r e a c t i o n o r d e r , x , w a s 1 . 97 _+ 0 . 15 (9 5 % c o n f i d e n c el i m i t ) a n d k = 7 . 9 + - 2 (9 5 % c o n f i d e n c e l i m i t ) , r e s p e c t i v e l y .T h e y - i n t e r c e p t , C , i n E q . [ 1] w a s n e a r l y z e r o a n d a l w a y sp o s i t i v e , r e f le c t i n g t h a t a n i n d u c t i o n t i m e i s r eq u i r e d f o rg r o w t h . N o t e i n F ig . 1 a n d 2 t h e l a r ge s h i f t s i n t h e p e a ks h a p e a n d p o s i t i o n d u r i n g t h e f ir s t f e w g r o w t h c y c l e s . T h et r a n s i t i o n s o c c u r i n g d u r i n g t h e f i rs t f e w c y c l e s a r e u n-k n o w n . T h e b e h a v i o r f o r t h e f ir st t w o c y c l e s a r e t h e s a m ea s t h a t w e h a v e n o t e d f o r P A i n a l k a l i n e s o l u t io n s . I n a l k a -l i n e s o l u t i o n s , h o w e v e r , t h e c h a r g e b u i l d u p c o n t i n u e s t od e c r e a s e w i t h c y c l e n u m b e r a n d t h e r e s u l t i n g f i lm i s e le c -t r o i n a c t i v e . O n t h e o t h e r h a n d , i n a c i d s o l u t i o n s , s o m e s u r-f a c e s p e c i e s i s p r e s e n t w h i c h a l l o w s t h e f i lm g r o w t h t oc o n t i n u e , a s a lr e a d y d i s c u s s e d . T h e c o n c e n t r a t i o n o f t h i ss p e c i e s d u r i n g t h e e a r ly g r o w t h s t a g es i s p r o b a b l y q u i t el o w w h i c h w o u l d a c c o u n t f o r t h e o b s e r v e d i n d u c t i o np e r i o d .

T h e r e a c t i o n o r d e r f o r a n i l in e , i n a d d i t i o n t o t h e a b o v es l o p e m e t h o d , m a y a l s o b e d e t e r m i n e d f r o m p l o t s o f t h eg r o w t h r a t e vs. a n i l i n e c o n c e n t r a t i o n a t s o m e c o n s t a n tc y c l e n u m b e r . T h e g r o w t h r a t es ( m C / c y c l e ) t a k e n a t a p a r-t i c u l a r c y c l e n u m b e r ( F i g . 7 ) w e r e p l o t t e d vs. t h e a n i l i n ec o n c e n t r a t i o n . T h e r es u l t s f o r c y c l e s n o . 5 a n d c y c l e n o . 20a r e s h o w n i n F i g . l l a . T h e r e a c t i o n o r d e r i n a n i l i n e d e t e r -m i n e d f r o m t h e l o g / l o g p l o t s ( F i g . l l b ) w a s 1 .9 5 _+ 0 .0 8 (9 5 %c .1.) f o r x a n d 7 .6 + 1 .5 (9 5 % c o n f i d e n c e l i m i t ) fo r k , w h i c ha r e i n c l o s e a g r e e m e n t w i t h t h e p r e v i o u s v a l u e s t h a t w e r e

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o b t a i n e d . T h u s , a t c o n s t a n t s c a n r a te , f i l m g r o w t h w i t ht i m e i s a p p r o x i m a t e d b y

G r o w t h ( m C ) ~ - k [ A n ] 2 ( c y c l e n o . ) 2 [ 2]T h e d e p e n d e n c e o f t h e g r o w t h r a te w i t h o x i d a t i o n t i m ei s c o n s i s t e n t w i t h a n a u t o c a t a l y t i c g r o w t h m e c h a n i s m ;

h o w e v e r , s i n c e t h e P A f i l m is a s o l i d o n t h e e l e c t r o d e s u r-f a c e , a n a u t o c a t a l y t i c e f f ec t i s m o r e l i k e l y to b e a s s o c i a t e dw i t h t h e e f fe c t i v e s u r f a c e a r e a ra t h e r t h a n t h e f i lm v o l u m e .A s t h e f i l m g r o w s , t h e f ib e r s m a y b r a n c h o u t ( 22 ), i n c r e a s -i n g t h e :a re a a n d t h u s , t h e g r o w t h r a t e. A n e f f e c t i v e a r e ac a n b e C o n s i d e r ed a s a m e a s u r e o f t h e n u m b e r o f r e a c t i v es i te s . I f t h e i n c r e a s i n g r a t e w i t h c y c l e n u m b e r i s d u e t o t h i sa r ea e ff e c t , t h e n f i l m s w i t h c o n s t a n t a r e a s h o u l d e x h i b i t ag r o w t h r a te w h i c h w o u l d d e p e n d o n l y o n a n i l i n e c o n c e n -t r a t i o n .T o t e s t t h i s p o s t u l a t e, t h e g r o w t h r a t e a t c o n s t a n t a r eaw a s e s t i m a t e d b y a s s u m i n g t h a t f i lm s w i t h t h e s a m ec h a r g e w o u l d h a v e t h e s a m e e ff e c t i v e a r ea , e v e n t h o u g ht h e a c t u a l v a l u e f o r t h e a r e a i s n o t k n o w n . T h e r e su l t s w eo b t a i n e d f o r p l o t s o f t h e g r o w t h r a te a t c o n s t a n t c h a r g e vs.a n i l i n e c o n c e n t r a t i o n a r e s h o w n i n F i g . 12 . T h e l i n ea r r el a -t i o n s h i p i n d i c a t e s t h a t t h e g r o w t h r a t e i s f ir s t o r d e r ina n i l i n e c o n c e n t r a t i o n w h e n a r e a e f fe c t s a r e t a k e n i n t o a c -c o u n t .

A s s u g g e s t e d f r o m t h e s e f i nd i n g s , a n d a s s h o w n b e l o w , ap l o t o f t h e g r o w t h r a te vs. [ A n ] ( c h a r g e ) 1j2 s h o u l d b e l i n e a r .T o v e r i f y t l 4 is r e l a t i o n , w e o b t a i n b y r e c a s t i n g E q . [2 ]( c h a r g e ) lj2 = k l ~ [ A n ] ( c y c l e n o . ) [ 3 ]

T a k i n g t h e d e r i v a t i v e o f E q . [3 ] y i e l d sd ( c ha r ge ) / d ( c y c l e ) = 2 k l / 2 [ A n ] ( c h a r g e ) lj 2 [ 4 ]

a n d , t h e r e f o r eG r o w t h R a t e ( m C / c y c l e ) = k ' [ A n ] ( c h a r g e ) i n [ 5]

w h e r e k ' = 2ki / 2.N o t e t h a t i n t e g r a t i o n o f E q . [5 ] y i e l d s E q . [ 1] b a c k . T h e r e -

l a t i o n s h i p d e s c r i b e d b y E q . [5 ] i s p l o t t e d i n F i g . 1 3 a f o rc o n s t a n t c h a r g e . T h e d a t a p o i n t s a r e t h e s a m e a s t h o s es h o w n i n F ig . 12 . T h e d a t a p o i n t s s h o w n i n F ig . 1 3b a r ef r o m a s et o f g r o w t h r a t es t a k e n d u r i n g g r o w t h ( c o n s t a n ta n i l i n e c o n c e n t ra t i o n ) p l o t t e d vs. ( c h a r g e ) lj2 . N o t e h e r e t h a tt h e d e v i a t i o n a t h i g h e r r a te s r ef l ec t s t h e c h a n g e i n r e a c t i o no r d e r d i s c u s s e d p r e v i o u s l y .

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2 2 6 0 J . E l e c t r o c h e m . S o c . : E L E C T R O C H E M I C A L S C I E N C E A N D T E C H N O L O G Y S e p t e m b e r 1 9 8 8F o r a d i s k e l e c t r o d e a n d f o r f il m t h i c k n e s s e s m u c h l e s st h a n t h e d i s k r a d i u s , t h e f il m v o l u m e w i l l b e a p p r o x i -m a t e l y e q u a l t o a c y l i n d e r w i t h r a d i u s e q u a l t o t h e d i s ka n d w i t h a l e n g t h e q u a l t o t h i c k n e s s . F r o m t h e r e s u l ts ,s h o w n i n F i g . 4 b , t h e c h a r g e w a s s h o w n t o b e d u e , f o r t h em o s t p a rt , to s u r f a ce p h e n o m e n a a n d n o t t o s o l ut i o n s p e -c i es . O v e r t h e r a n g e w h e r e t h e g r o w t h r a t e d o e s n o tc h a n g e , w e h a v e s h o w n t h a t a l i n e a r t h i c k n e s s t o c h a r g er e l a t i o n s h i p h o l d s (4 2) . A c c e p t i n g t h i s , t h e n E q . [5 ] b e -

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G r o w t h R a t e ( P / c y c l e ) = k " ( A n ) ( P ) u2 [ 6]w h e r e , P m a y b e t h e fi l m t h i c k n e s s , v o l u m e , m a s s , o rm o l e s o f a n i l i n e u n i t s / c m 3.

A n i d e a l a u t o c a t a l y t i c r e a c t i o n i n s o l u t i o n i s f i rs t o r d e r i nP ( 43). T h i s d i s c r e p a n c y c a n b e e x p l a i n e d i n t e r m s o f t h es o l i d f i l m . F o r t h i c k e r f i l m s , a l e s s e r f r a c t i o n o f t h e t o t a lf i lm i s e x p o s e d t o t h e s o l u t i o n ; f o r e x a m p l e , a f il m o f a rb i -t r a r y t h i c k n e s s o f 1 0 w o u l d h a v e a r e a c t i o n t h i c k n e s s o f3 - 3 .5 , b u t o n e w i t h a n a r b i t r a r y t h i c k n e s s o f 10 0 w o u l do n l y i n c r e a s e t o 1 0. C o n t a i n e d w i t h i n t h i s r e a c t i o n v o l u m ei s s o m e n u m b e r o f r e a c t io n s i te s w h i c h i s a s s o c ia t e d w i t hs o m e e f f e c t i v e a r e a. O u r r e s u l t s i m p l y t h a t t h e i n c r e a s e i ng r o w t h r a t e r e f l e c ts a n i n c r e a s e i n t h e e f f e c t i v e ar e a. T h ea v e r a g e v a l u e f o r k ' w a s 5 .3 1 .2 ( 9 5% c o n f i d e n c e l i m i t ),w h i c h g i v e s k = ( k ' /2 ) 2 = 7 . 02 . T h i s v a l u e i s i n g o o d a g r e e -m e n t w i t h k - v a l u e s g i v e n e a rl i er .

D i s c u s s i o nI n o r d e r to d i s cu s s t h e g r o w t h m e c h a n i s m , w e b e g i n o u rd i s c u s s i o n w i t h C V p e a k a s s i g n m e n t s w i t h i n t h e f ra m e o fo u r w o r k a n d s o m e P A p r o p e r t ie s r e p o r t e d i n t h e l i t er a -t u r e . W e h a v e a l r e a d y s p e c u l a t e d o n t h e p o s s i b l e s o u r c e o fC V c u r r e n t s o b s e r v e d f o r p e a k s B a n d C (F i g. l c) . W e w i l ld i s c u s s t h i s f u r t h e r b e l o w , b u t f o r t h e a s s i g n m e n t o f o t h e rp e a k s , i.e., A a n d D , a b r i e f r e v i e w o f e l e c t r o n s p i n r e s o -n a n c e ( e s r) a n d o t h e r p r o p e r t i e s o b s e r v e d c o n c u r r e n t l yw i t h C V r e c o r d i n g i s i n or d e r .R e c e n t l y G l a r u m a n d M a r s h a l l ( 44 ) r e p o r t e d t h a t in s i tue s r s ig n a l s w e r e o b s e r v e d a t b o t h e l e c t r o d e 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 C V p e a k s A a n d D f o r t h i n ( - 1 0 0 n m ) f i l m s .R a d i c a l p r o p e r ti e s w e r e d e t e c t e d b e t w e e n t h e t w o p o t e n -t i al b o u n d a r i e s . T h e p e a k i n t h e e s r s i g n a l w a s n o t o b -s e r v e d , h o w e v e r , f o r t h e t h i c k P A f i lm a t t h e p o t e n t i a l c o r -r e s p o n d i n g t o t h e C V p e a k D . A l s o, w e h a v e r e c e n t l yr e p o r t e d t h a t t h e r e w e r e w e l l - d e f i n e d t r a n s i t i o n s i n s p e c -t r a l p r o p e r t i e s a t t h e s e t w o p o t e n t i a l s ( 45). A t C V p e a k A ,t h e l e u c o fo r m o f P A , e x h i b i t i n g a n a b s o r p t i o n m a x i m u m(em a~) a t 3 2 8 n m , w a s o x i d i z e d t o a n o t h e r f o r m h a v i n g a em ~xa t 4 4 0 n m . A t p o t e n t i a l s m o r e p o s i t i v e t h a n C V p e a k D , i tw a s o b s e r v e d t h a t a s p e c i e s w i t h it s Emax a t 65 0 n m w a s g e n -e r a t e d . T h i s p a r t i c u l a r s p e c i e s , w h e n g e n e r a t e d , w a s f o u n d

t o b e r e s p o n s i b l e f o r l e a d i n g P A t o d e g r a d a t i o n p r o d u c t s(37, 45).T h e s e o b s e r v a t i o n s a r e c o n s i s t e n t w i t h t h e f a c t t h a tm a j o r t r a n s i t i o n s o c c u r i n P A f i l m s w i t h r e s p e c t t o t h e i rp h y s i c a l a s w e l l a s c h e m i c a l p r o p e r t i e s . A t p e a k A , w e b e -l i e v e t h a t r a d i c a l c a t io n s a r e g e n e r a t e d u p o n o x i d a t i o n ,w h i c h p e r s i s t i n t h e f i lm s u n t i l t h e p o t e n t i a l r e a c h e s av a l u e d e s i g n a t e d a s D . A t p e a k D , d i r a d i c a l d i c a t i o n s m a yb e g e n e r a t e d , w h i c h e s t a b li s h a n e q u i l i b r i u m w i t h t h e i ro t h e r r e s o n a n c e f o r mH H H- e -

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T h u s , w e c o n c l u d e h e r e t h a t o n l y C V p e a k s A a n d D a r er e l e v a n t t o th e r e d o x c h e m i s t r y o f P A . W h e n t h e p o t e n t i a lb e c o m e s m o r e p o s i t i v e t h a n D , d i r a d i c a l d i c a t io n s a r e g e n -e r a t e d , w h i c h e i t h e r a l lo w s a f u r t h e r P A g r o w t h , i f a n i l i n ei s p r e s e n t i n t h e s o l u t i o n , o r a d e g r a d a t i o n r e a c t i o n ( v ia h y -d r o l y s is ) t o o c c u r w i t h o u t a n i l i n e p r e s e n t .A s a l r e a d y d i s c u s s e d , w e b e l i e v e t h a t C V p e a k B o r ig i -n a t e s f r o m t h e H Q / B Q r e d o x p a ir ; b o t h s t a n d a r d a d d i t i o nm e t h o d s a n d v o l t a m m e t r y a t th e r o t at i n g d i s k s u p p o r t t h isc o n c l u s i o n . I t a p p e a r s t h a t t h e r e a re B Q / H Q c o m p o n e n t st h a t a r e a n d a r e n o t a f f e c t e d b y r o t a ti o n . I n o t h e r w o r d s ,t h e a m o u n t o f t h e B Q / H Q p a i r t h a t c a n b e r e m o v e d b y r o-t a t i o n a p p e a r s t o b e l i m i t e d . W e b e l i e v e t h i s to b e d u e t oa d s o r b e d B Q , a n d i n d e e d B Q s h o w s a s t ro n g t e n d e n c y t ob e a d s o r b e d , w h e n e x p e r i m e n t s a r e r u n i n a n e l e c tr o l y t es o l u t io n c o n t a i n i n g B Q . B o t h p e a k s B a n d C b e c o m eh i g h l y v i s ib l e at l o n g e r g r o w t h t i m e s o f P A o r w h e n P A i so x i d i z e d e x t e n s i v e l y . T h i s i n d i c a t e s t h a t b o t h p e a k s a r e r e-l a t e d to c e r ta i n d e g r a d a t i o n p r o d u c t s . P e a k C , h o w e v e r ,s h o w s c h a r a c t e r i s t i c s o f t h o s e t h a t a r e a d s o r b e d . W e b e -l i e v e , t h e r e f o r e , t h a t i t is n o t u n r e a s o n a b l e t o a s s i g n t h i s t ot h e r e d o x p r o c e s s o f P A P / Q I .F o r t h e g r o w t h o f P A f i lm s , t h e d i r a d i c a l d i c a t i o n p r o -d u c e d a t a p o t e n t i a l m o r e p o s i t i v e th a n p e a k D p l a y s t h em o s t i m p o r t a n t r o l e . T h i s s p e c i e s , b e i n g a n e n e r g e t i c e l e c -t r o p h i l e , e x t r a c t s a n e l e c t r o n f r o m a n i l i n e p r o d u c i n g i t sr a d i c a l c a t io n , w h i c h f o r m s a b o n d w i t h a n o t h e r r a d i c a l.A s a r e s u lt , t h e d i r a d ic a l d i c a t i o n j u s t p r o d u c e d b e c o m e sr e d u c e d . T h i s w h o l e p r o c e s s i s p r o p e r l y c a l l e d a c a t a l y t i cm e c h a n i s m (40 ). T h u s t h e o x i d a t i o n c u r r e n t a t p e a k Dw o u l d b e e x p e c t e d t o s h o w e n h a n c e m e n t i n t h e p r e s e n c eo f a n i l i n e ( F ig . 3 ). A g r o w t h m e c h a n i s m c o n s i s t e n t w i t h

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G C h e m i c s l+ N H i% 2

P r o l o n a t e d F o r m s

S c h e m e I I. E C gro w t h m e c h a n i s m

e l e c t r o c h e m i c a l

Gc h e m i c a l

R - - ~ - - R + H H

H + H +R--N N--R+ R - - N N - - RH H HN H9H

S c h e m e I II . C h e m i c a l o r e l e c tr o c h e m i c a l a d d i ti o n s w i t h i n t h e c h a i nthese findings is

(PA ) ~m --~ (PA) ~ + ne- [7](PA)n~ + An --~ --* ~ (PA),~ l~ R [8]

where An = aniline, (PA)sere is partially oxidized PA withradical cations present, and (PA) ~ is a diradical dicationform of PA. When this process is cycled once, a total ofthree (3) electrons are removed, n becomes n + 1, and thehighest oxidation state is reattained. An autocatalysismechanism indicates that the PA growth is not due to asimple b uildup of layers on the surface.Possible growth mechanisms that are consistent withour findings and whi ch follow the overall growth mecha-nis m (reactions [7] and [8]) are s hown in Fig. 14. The homo -lytic addition, Scheme I, is similar to the initial dimeri-zation reactio n for the oxidation of aniline (31). Thisschem e invol ving radicals is reasonable as the nitrog enradicals are electrophilic, and have bee n exte nsively citedas intermediatesin aniline oxidation mechanisms (1, 27,31, 34, 35, 39). Altern ative ly, it may be that the anilin e addi-tions and the subseq uent reactions are sufficiently rapidsuch that the highest oxidation state, the alternating quin-oid/aromatic form, does not predominate. Instead, someintermediate PA SEM-OX form could be the predominatespecies in the aniline containing solutions. This postulateis supported by our observations on the quenching effectof peak D' as a function of scan rate and aniline concent ra-tion, as was sho wn in Fig. 1 thr ough 3.The reaction in Scheme II is considered unlikely as itcan be expected that the sites for nucleophilic attack bythe aromatic amine would predominate on the carbons ofthe quinonediimine functional group (27, 46-49).The homolytic addition reaction in Scheme III is thesame as that sho wn in Sch eme I exce pt that the site for ad-dition is within the chain rather than at the end of a grow-ing chain. This results in a branched product that couldcross-link with anot her chain. The contr ibution o f thismechanism, although minor, may explain the differencesin conductivities observed for PA's prepared by differentmethods. The PA with a cross-linked network would havelower conductivities.Regardless of path, an importan t point that we wishto make is that t he aniline addition has the net effect of re-ducing the oxi dation level of the PA. The partially reducedfilm, SEM form, may regain conductivity; this would ex-plain why the film continues to grow at potentials wherePA has bee n shown to be an insulator in solutions contain-ing only the supporting electrolyte (13, 25). The conductiv-ity of PA during growth has not b een reported. A net re-duction, upon dimerization, for aniline oxidation wasshown to occur (35). A mediated charge transfer reaction,which would have the same effect of reducing the film,

does not occur with other species such as Br- or[Ru(CN)~] 4- (23).Concurrent with the desired growth reaction could beundesired side reactions, such as benzidine-type linkages,carbazole formation, cross-linking structures and by-prod-ucts of the hydrolysis reactions (2, 32). The observedchanges in the growth rate and the CV shape suggests thatthese side reactions occur to a significant extent at laterstages of the growth.A favorable side reaction is quite possible+ =N + H20

This reacti on is just the rever se of the hydrolysis reaction.Imine, or Schi ff base formation from carbon yl groups iswell-k nown and has bee n studi ed exten sively (46-49). Thecarbonyl and imine forms exist in equilibrium. The equi-librium is pH dependent and has been studied usinganili ne in sim pler syste ms (48, 49). A logical ext ensi on o fthe reaction type sho wn above would involve the additionof aniline to the quinonediimine itself

The ext ent of the above favorable reactions are probablyminor. We have provided evidence that, in the regionwhere the growth rate is linear with cycle number, theaniline does not react rapidly with the oxidized speciesand t hen stop (see Fig. 1 thro ugh 3 and 9). Whet her the sefavorable side reactions occur during the later growthstages is unknown.C o n c l u s i o n s

Over a limited range of aniline concentratio ns and po-tential cycle number, we determined the following filmgrowth beh avior for PA:1. The film growth is autocatalytic.2. The growth rate is proportional to the 1/2 power withrespect to film volume, and it is first order with respec t toaniline concentration.3. The growth rate is proportional to the cycle numberand to the square of the aniline concentrati on when PAvolu me effects are not taken into account.4. A surface bound species is involve d in growth mech-anism.A growth mechanism consistent with these findings hasbeen presented.

Manuscript submitted Oct. 9, 1987; revised manuscriptreceived Dec. 30, 1987.REF EREN CES

1. D. M. Mohilner, R. N. Adams, and W. J. Argersinger,Jr., J. Am. Chem. Soc., 84, 3618 (1962).2. R. DeSurville, M. Jozefowicz, L.-T. Yu, J. Perichon,and R. Buvet, Electrochim. Acta, 13, 1451 (1968).3. A. Kitani, M. Kaya, and K. Sasaki, This Journal, 133,1069 (1986).4. A. G. MacDiarmid, J.-C. Chiang, M. Halperm, W.S.Huang, J. R. Krawczyk, R .J. Mammone , S. L. Mu,N.L. Somasiri, and W. Wu, Polymer Preprints, 25 ,248 (1984).5. A. G. MacDiarmid, S. L. Mu, N.L. Somasiri, and W.Wu, Mol. Cryst. Liq. Cryst., 121, 187 (1985).6. R. Messin a, C. Sarazi n, L. T. Yu, and R. Buve t, J. Chim.Phys., 73, 919 (1976).7. N. Oyama, T. Ohsaka, and T. Shimizu , Anal. Chem., 57 ,1526 (1985).8. G. Mengo li, M. T. Munari, P. Bia cco, an d M. M. Mus-iani, J. Appl. Poly mer Sci., 26, 4247 (1981).9. R. Noufi, A. J. Nozik, J. White, an d L. F. Warren, ThisJournal, 129, 2261 (1982).10. B. Aurian-Bla jeni, I. Taniguc hi, and J. O'M. Bockris, J.Electroanal. Chem. Interracial Electrochem., 149, 291(1983).11. T. Kobayashi , H. Yoneya ma, and H. Tamur a, ibid., 161,419 (1984).12. S. Gottesfeld, A. Redondo, and S. W. Feldberg, ThisJournal, 134, 271 (1987).

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2262 J . E l e c t r o c h e m . S o c . : E L E C T R O C H E M I C A L S C I E N C E A N D T E C H N O L O G Y S e p t e m b e r 1 9 8 813. E. W. Paul, A. J. Ricco, and M. S. Wrighton, J . Phys .C h e m . , 89, 1441 (1985).14. A. G. Gre en and A. E. Woodhead, J. C h e m . S o c .(Trans . ) , 97, 2388 (1910).15. A. G. Gree n and A. E. Woodhead, ib id . , 101, 1117 (1912).16. a, J. Langer, S o l i d S t a t e C o m m u n . , 26, 839 (1978); b, J.Langer, Ma ter . S c i ., 10, 174 (1984).17. A. Volkov, G. Tourill on, P.-C. Lacaze, an d J.-E. Du-bois, J. E lec t roana l . Chem. I n t e r f ac ia l E l e c t rochem. ,115, 279 (1980).18. A. F. Diaz and J. A. Logan, ib id . , 115, 111 (1980).19. T. Ohsaka, Y. Ohnuki, N. Oyama, G. Katagiri, and K.Kamisako, ib id . , 161, 399 (1984).20. A. Kitani, J. Izumi , J. Yano, Y. Hiromoto, a nd K. Sas-aki, Bu l l . Chem. Soc . J pn . , 57, 2254 (1984).21. M. Breitenbach and K.-H. Heckner, J . E l e c t roana l .C h e m . I n t e r f a c i a l E l ec t r oc h e m . , 43, 267 (1973).22. C. Carlin, L. J. Ke pley, an d A. J. Bar d, T h i s J o u r n a l ,132, 353 (1985).23. N. Oyama, Y. Ohnu ki, K. Chiba, and T. Ohsaka, C h e m .Le t t . ( J pn . ) , 1759 (1983).24. T. Ohsaka, K. Chiba, and N. Oyama, N i p p o n K a g a k uK a i s h i , 457 (1986).25. S. H. Glar um and J. H. Marshall, T h i s J o u r n a l , 134, 142(1987).26. J. R. Wilso n an d S.-M. Par k, T h i s J o u r n a l , 129, 149(1982).27. R. N. Adams, "El ectroc hemist ry at Solid Electrodes,"Marcel Dekker, Inc., New York (1969).28. H. Angerste in-Kozlows ka, B. E. Conway, an d W.A.Sharp, J . E l e c t roana l . Chem. I n t e r f ac ia l E l e c t ro -chem. , 43, 9 (1973).29. A. C. Chialvo, W. E. Triaca, an d A. J. Arv ia, ib id . , 146,93 (1983).30. A. Hu bba rd, Acc . Ch em. Res . , 13, 183 (1980).31. W. E. Britton, in "The C hemist ry of Func tio nalGroups; Supplement F, The Chemistry of Amine,Nitroso, and Nitro Compounds, and Their Deriva-tives," Part 1, S. Patai, Editor, Wiley Interscience,New York (1982).32. R. F. Nelson, in "Tec hniq ue of Electroorg anic Synthe-sis, Part 1, Tech niqu es of Ch emistry," Vol. 5, N. L.Weinberg, Editor, Wiley Interscience, New York(1970).33. F. Fichter, "Organische Electrochemie," Steinkoph,

Dresden, Ger man y (1942): Unive rsit y Microfilms,An n Arbor, MI orde r no. op 23625.34. R. L. Ha nd an d R. F. Nelson, J . A m . C h e m . S oc . , 96, 850(1974).35. J. Bac on and R. N. Adam s, J . A m . C h e m . S oc ., 90, 6596(1968).36. T. Kobayashi, H. Yoneyama, an d H. Tamura, J. Electro-a n a t . C h e m . I n t e r f a c i a l E l e c tr o c h em . , 177,293 (1984).37. D. E. Stilwell and S.-M. Park, T h i s J o u r n a l , In press.38. S. F. Nelson, in "Fre e Radic als," Vol. 2, J. Kochi, Edi -tor, Jo hn Wiley & Sons, In c., New York (1973).39. A. J. Bard, A. Ledwith, and H. J. Shine, in "Adva ncesin Physical Organic Chemistry," Vol. 15, V. Gold,Editor, Acade mic Press, Lon don (1976).40. A.J . Bard and L. R. Faul kner, "Electro chemica l Meth-ods," J oh n Wi ley & Sons, Inc., NY (1980).41. V. P. Par ini, Z. S. Kakako va, a nd A. A. Berlin, Vyso-k o mo l e k u l . S o e d i n . , 3, 1870 (1961).42. D. E. Stilw ell and S.-M. Park, T h i s J o u r n a l , In press.43. P. W. Atkins, " Physic al Chemis try," p. 860, Freema n,San Francisco (1978).44. S. H. Glarum and J. H. Marshall, J. P h y s . C h e m . , 90,6O76 (1986).45. D. E. Stilwell and S.-M. Park, in "Electrode Materialsand Processes for Energy Convers ion and Storage,"S. Srinivas an, S. Wagner, and H. Wrolblowa, Editors,p. 83, The Electrochemical Society Softbound Pro-ceedi ng Series, Penni ngt on, NJ (1987).46. a, A. Bruylan ts and E. Feyma nts -de Medicis, in "TheChemistry of Functiona l Groups: The Chemistry ofthe Carbon-Nitrogen Double Bond," S. Patai, Editor,Chap. 10, Wiley Inte rsc ienc e, New York (1970). b,K. T. Finle y and L. K. J. Tong, in "The Chemi stry ofFunct ional Groups: The Chemistry of the Carbon-Nitrogen Double Bond," S. Patai, Editor, Chap. 10,Wiley Interscienc e, New York (1970).47. W. P. Jencks , in "Progress in Physical Organic Chem-istry," Vol. 2, Cohe n, et al., Editors, Wiley Inter-scienc e, New York (1964).48. St. Berger and A. Rieker, in "The Chemist ry of Func -tional Groups: The Chemistry of the QuinoinoidCom pou nds ," Par t 1, S. Patai, Edito r, Chap. 4, p. 216,Wiley Interscien ce, New York (1974).49. E. H. Cordes a nd W. P. Jen cks , J. A m. C h e m . S o c . , 84,

832 (1962).

M igrat ion C ons iderations in Ch ronop otent iom etr ic Analys isC e c i li a Y . M a k * '1 a n d H u k Y . C h e h *

D e p a r t m e n t o f C h e m i c a l E n g i n e e r i n g a n d A p p l i e d C h e m i s tr y , C o l u m b i a U n i v er s it y , N e w Y o r k , N e w Y o r k 10 02 7

ABSTRACTA theoretical analysis has been conducte d to evaluate the effect of migration in galvanostatic processes a nd to providea fundam enta l basis for assessing the limits of applicability of Sand's equati on in chronopotenti ometric measurements.The results of the comput ation show that the diffusion potential is a significant driving force for migration transportwithin the diffusion layer. The square root of the trans ition time is inversely proportional to the applied curre nt density,irrespective of the degree of migrational interaction in the system. Three case s tudies are p resented to illustrate the effectof migra tion in different electrochemica l processes.

Chronopot entiometry is a techniq ue that has com monlybeen used to study mass transport behavior in electro-chemical systems. In this method, the transient electrodepotential is monitored under controlled current condi-tions. The meth od yields quantitative measur ement of thecharacteristic time when a distinct shift in the electrodepotential takes place. The potential transition is indicativeof the attainment of a critical condition at the electrodesurface where the prevailing mass transport mech anis mhas become limiting. Often, Sand's equati on (1) is used torelate the applied current density and the tran sition time*Electrochemical Society Active Member.1Present address: IBM Corporation, Thomas J. Watson ResearchCenter, Yorktown Heights, New York.

to the surface concentration of the species involv ed in theelectrochemical reaction.Sand's equation is derived from the consideration of un-steady diffusion transport in a semi-infinite medium.Therefore, its application is limited to galvanostatic pro-cesses in which there is no bulk convective motion andnegligib le migrat ional effects. The last co nditi on is usual lyassumed to be satisfied in systems which contain an ex-cess amount of supporting electrolyte. The supportingelectrolyte increases the conducti vity of the solution,thereby reduces the electric field in the system. Since inthose cases, the ionic current within the diffusion layer isprincipally supported by diffusion, it is valid to use Sand'sequati on to predict the surface concentration of the mi norspecies. However, one may ask: How mu ch is a "mini mal"ecsdl org/site/terms useaddress Redistribution subject to ECS license or copyright; see160 36 202 135Downloaded on 2013-06-27 to IP

j. electrochem. soc. 1988 stilwell 2254 62

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