the influence of the aqueous growth medium on the growth

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The Influence of the Aqueous Growth Medium on the

Growth Rate, Composition, and Structure of Hydrous

Iridium Oxide Films

Birss, Viola I.; Pickup, Peter G.

The Electrochemical Society

Pickup, Peter G. and Birss, Viola I. (1988). "The Influence of the Aqueous Growth Medium on the

Growth Rate, Composition, and Structure of Hydrous Iridium Oxide Films". Journal of the

Electrochemical Society, Vol. 135(1): 126-133.

http://hdl.handle.net/1880/44747

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126 J. Electrochem. Soc.: E L E C T R O C H E M I C A L

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14B, 451 (1983). 5. "Statpak Users Guide, Computing Centre," 3rd ed.,

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(1969). 8. I. Karakaya and W. T. Thompson, J. Chem. Thermo., 18,

859 (1986). 9. A. D. Pelton and S. N. Flengas, Can. J. Chem., 47, 2283

(1969). 10. I. Karakaya and W. T. Thompson, ibid., 63, 2808 (1985);

ibid., 63, 3634 (1985). 11. T. Ostvold, Ph.D. Thesis, Universi ty of Trondheim,

Trondheim, Norway (1971). 12. T. Ostvold, High Temp. Sci., 4, 51 (1972). 13. W. T. Thompson, C. W. Bale, and A. D. Pelton,

"F*A*C*T Facility for the Analysis of Chemical Thermodynamics ," McGill University/Ecole Poly-

S C I E N C E A N D T E C H N O L O G Y January 1988

technique, Montreal, Que., Canada (1979). 14. I. Barin, O. Knacke, and O. Kubaschewski, "Thermo-

chemical Properties of Inorganic Substances," Springer-Verlag, Berlin, Germany (1977).

15. K. Grjotheim, J. L. Holm, and M. Rotnes, Acta Chem. Scand., 26, 3802 (1972).

16. V. E. Plyushchev, L. N. Komissarova, L. V. Meshchani- nova, and L. M. Akukina, Russ. J. Inorg. Chem., 1, 220 (1956).

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18. P. L. Lin, A. D. Pelton, and C. W. Bale, J. Am. Ceram. Soc., 62, 414 (1979).

19. H. Flood and S. Urnes, Z. Electrochem., 59, 834 (1955). 20. D. E. Neil, H. M. Clark, and R. H. Wiswall, J. Chem.

Eng. Data, 10, 21 (1965). 21. A. D. Pelton and W. T. Thompson, Can. J. Chem., 48,

1585 (1970). 22. S.N. Flengas and A. S. Kucharski, ibid., 49, 3971 (1971). 23. Z.-C. Wang, Trans. Faraday Soc., 86, 375 (1986).

The Influence of the Aqueous Growth Medium on the Growth Rate, Composition, and Structure of Hydrous Iridium Oxide Films

Peter G. Pickup 1 and Viola I. Birss

Department of Chemistry, University of Calgary, Calgary, Alberta, Canada T2N 1N4

ABSTRACT

The electrochemical growth of Ir oxide in acidic, neutral, and basic LiC104 solutions has been investigated. Dissolu- tion of the oxide during growth is much less in these LiC104 solutions than in H2SO4 solutions, which have generally been used for Ir oxide growth, and this allows higher growth rates to be achieved. The oxide filrns grown in LiC104 solutions have a greater mechanical stability and appear to be less hydrated than those grown in H~SO4 solutions. The empirical formula weight of the oxidized form of Ir oxide, grown in neutral LiC104 and dried in air, was found to be 260 -+ 10, indicative of an empirical formula of IrO2 �9 2H20. The potential at which the oxide is oxidized to an extent of one electron per Ir atom (in the oxide) has also been determined for various media. This allows the future determination of the Ir content of Ir oxide films by cyclic voltammetry.

I t has been known for some years that thick hydrous i r id ium oxide films can be grown at i r id ium metal elec- t rodes unde r par t icular potent ia l cycling condi t ions (1-6). This oxide exhibi ts a reversible Ir(IV)/(III) oxida- t ion state change which t ransforms it from a colorless insulat ing material (Ir(III)) to a black metallic conductor (Ir(IV)) (7, 8). It is, therefore, of interest, for applications in electrochromic devices. It also appears to have applications in the area of electrocatalysis, for reactions such as chlorine evolution (9) and oxygen evolution (1).

Hydrous Ir oxide has general ly been grown and stud- ied in dilute H2SO4, but it can be grown in a wide variety of aqueous media. Mozota and Conway (5) have shown that it can be grown in aqueous Na2CO3 or HC104. Burke and Scannel l (10) have reported a comprehens ive s tudy of Ir oxide in basic solutions. These authors found signif icant differences be tween films grown in NaOH and those grown in H2SO4. The base-grown films were less s table and were reported t o h a v e a less open structure than those grown in acidic solutions.

We have been interes ted in opt imizing the condi t ions for Ir oxide preparat ion. In a previous s tudy (11), we demonstrated that, in 0.5M H2SO4, opt imum growth rates were obtained using a potential pulsing, rather than potential scanning, method. We also explained the reported (12, 2) dependence of the growth rate upon the potential limits of the growth cycles (11). In the present work, the op t imum aqueous growth medium was sought.

The growth of Ir oxide films in a variety of aqueous media was invest igated here. We a t tempted to de te rmine how the oxide growth rate, composit ion, and s t ructure can be influenced by changing the growth medium. The

1 Present address: Department of Chemistry, Memorial Univer- sity of Newfoundland, St. John's, Newfoundland, Canada A1B 3X7.

inf luence of the electrolyte pH was of p r imary interes t and, therefore, in most of this work, the electrolyte contained 1M LiC104 so that the pH could be changed without any other significant changes occurr ing in the solu- t ion composition.

The composit ion and structure of Ir oxide films grown elect rochemical ly in acidic solut ions has been investigated previously by several groups (3, 9, 13-17). Using weight measurement s of oxide-coated Ir electrodes; McIntyre and co-workers (3) es tabl ished that the main redox react ion of e lectrochemical ly grown Ir oxide is due to the Ir(IV/III) couple, a l though the potent ia l at which the react ion was complete was not de termined. These workers also demonstrated that the density of this oxide (ca. 2g cm -3) is much less than that of anhydrous IrO~ (11.68g cm-3). However, the composi t ion of the oxide films was not determined.

Because of the r emain ing uncer ta in ty concern ing the composi t ion of e lectrochemical ly grown Ir oxide films, we have repeated the weighing exper iments descr ibed by McIntyre and co-workers (3) and also have analyzed the growth medium and the oxide films for their Ir content. The combinat ion of these two techniques allows the empirical formula weight (EFW) of the oxide to be determined without ambiguity. We have carried out these exper iments for oxide films grown in different media in order to determine whether the growth medium influences the oxide composition. We have also used scanning elec- t ron microscopy to invest igate s t ructural differences in the oxide as a function of method and medium of oxide preparation.

Experimental Electrochemistry.--All electrochemical exper iments

were carried out in convent iona l th ree -compar tment

Vol. 135, No. 1 S T R U C T U R E O F H Y D R O U S I rO2 F I L M S 127

g l a s s ce l l s , u n d e r a r g o n a t r o o m t e m p e r a t u r e . W o r k i n g e l e c t r o d e s w e r e e i t h e r I r w i r e (99.9%; J o h n s o n M a t t h e y ) or fo i l (99.9%; M e t a l C r y s t a l s a n d O x i d e s L i m i t e d , En - g l and ) . D e t a i l s o f - e l e c t r o d e c o n s t r u c t i o n a n d s u r f a c e t r e a t m e n t h a v e b e e n g i v e n e l s e w h e r e (11, 18). T h e w i r e was e l e c t r o c h e m i c a l l y p o l i s h e d a n d h a d a r o u g h n e s s fac- t o r of 2.4 -+ 0.2 (11). T h e foi l w a s p o l i s h e d w i t h s u c c e s - s ive ly f iner g r a d e s of d i a m o n d p a s t e (Micro M e t a l l u r g i c a l L i m i t e d ) , d o w n to 25 ~m. B e f o r e e a c h e x p e r i m e n t , b o t h t y p e s of e l e c t r o d e s w e r e e l e c t r o c h e m i c a l l y c l e a n e d at ca. 2V vs. S S C E in 10% HzSO4 for ca. 5 m i n . O c c a s i o n a l po- t e n t i a l p u l s e s i n t o t h e h y d r o g e n e v o l u t i o n r e g i o n (e.g., - 0 . 7 V ) h e l p e d to r e m o v e o x i d e r e m a i n i n g o n t h e e lec- t r o d e f r o m t h e p r e v i o u s e x p e r i m e n t . All e l e c t r o d e a reas r e f e r r e d to in t h i s p a p e r a re g e o m e t r i c areas .

C o u n t e r e l e c t r o d e s w e r e e i t h e r P t w i r e or gauze , a n d R H E , SCE, or S S C E r e f e r e n c e e l e c t r o d e s w e r e used . All p o t e n t i a l s a r e q u o t e d r e l a t i v e to t h e s a t u r a t e d s o d i u m c a l o m e l e l e c t r o d e , S S C E (+236 m V vs. t h e NHE), u s e d in- s t e a d of t h e S C E to a v o i d f r i t c l o g g i n g b y p r e c i p i t a t e d KC104.

A n E G & G P A R C 173 p o t e n t i o s t a t w as u s e d w i t h a P A R C 175 U n i v e r s a l P r o g r a m m e r a n d a P A R C 179 digi- t a l c o u l o m e t e r . V o l t a m m o g r a m s w e r e r e c o r d e d on Hew- l e t t - P a c k a r d 7044A X-Y or 7090A r e c o r d e r s .

S c a n n i n g electron mic roscopy . - -Scann ing e l e c t r o n mi- c r o s c o p y w a s p e r f o r m e d u s i n g a C a m b r i d g e S t e r e o s c a n 2500. T h e S E M s a m p l e s w e r e n o t c o a t e d w i t h go ld or g r a p h i t e . G e n e r a l l y , t h e o x i d e c o a t e d e l e c t r o d e s w e r e f irst r i n s e d w i t h w a t e r a n d ace tone , a n d t h e n a i r d r i e d be- fo re t h e S E M s t u d i e s . H o w e v e r , a c r i t i c a l p o i n t d r y i n g m e t h o d was u s e d in s o m e cases .

Cr i t i ca l p o i n t d r y i n g (19) is a m e t h o d u s e d to d r y S E M s a m p l e s w i t h a m i n i m u m of s t r u c t u r a l d a m a g e . T h e sam- p le is t a k e n f r o m w a t e r a n d s o a k e d s u c c e s s i v e l y in wa te r / a c e t o n e m i x t u r e s o f i n c r e a s i n g a c e t o n e c o n t e n t ( t0 m i n in 50% a n d 90% a c e t o n e w a s e m p l o y e d he re ) . F i n a l l y , a f t e r s o a k i n g in p u r e a c e t o n e , t h e s a m p l e is t r a n s f e r r e d to l i q u i d COs, w h i c h is t h e n h e a t e d to a b o v e i ts c r i t i c a l p o i n t a n d r e m o v e d .

A n a l y s i s . - - A C a h n 25 a u t o m a t i c e l e c t r o b a l a n c e was u s e d for w e i g h i n g Ir foil e l e c t r o d e s ( typ ica l ly 0.2 c m • 1 • 0.005 cm; ca. 10 mg). F o r e l e c t r o d e s w e i g h i n g less t h a n 20 mg , m e a s u r e m e n t s c o u l d b e m a d e to a n a c c u r a c y of 1 ~g.

I n o r d e r to d e t e r m i n e t h e c o m p o s i t i o n of I r o x i d e f i lms , t h e f o l l o w i n g p r o c e d u r e w a s u s e d (3). T h e o x i d e was g r o w n on a w e i g h e d I r foil in ca. 3 m l of t h e g r o w t h m e d i u m a n d a cyc l ic v o l t a m m o g r a m was r e c o r d e d at 10 mV/s . T h e o x i d e e l e c t r o d e was t h e n o x i d i z e d so t h a t ap- p r o x i m a t e l y o n e e l e c t r o n p e r I r a t o m in t h e o x i d e h a d b e e n p a s s e d (see be low) , r e m o v e d f r o m t h e cell, w a s h e d w i t h w a t e r a n d ace tone , a n d air d r i e d for ca. 5 to 10 rain. T h e o x i d e - c o a t e d foi l w a s t h e n r e w e i g h e d , a n d in s o m e cases , d r i e d f u r t h e r a n d r e w e i g h e d . F i n a l l y , t h e o x i d e w a s d i s s o l v e d in 12M of h o t HC1 b y t h e a d d i t i o n of a few d r o p s of 30% HzO2. T h e foi l w a s w a s h e d w i t h w a t e r a n d a c e t o n e , a i r d r i e d , a n d r e w e i g h e d . T h e g r o w t h m e d i u m a n d t h e s o l u t i o n c o n t a i n i n g t h e d i s s o l v e d o x i d e w e r e t h e n a n a l y z e d for Ir.

I r i d i u m a n a l y s i s w a s a c c o m p l i s h e d b y t h e S n C 1 J H B r m e t h o d (20). (NH4)2IrC16 ( A l d r i c h ) w a s u s e d to p r e p a r e s t a n d a r d s in t h e 0.4-2.0 p p m Ir r ange . T h e r e p o r t e d ana ly- s is m e t h o d was m o d i f i e d s o m e w h a t for t h i s work . To en- su r e t h a t t h e I r was in t h e s a m e f o r m in all s a m p l e s , t h e u n k n o w n s a n d t h e s t a n d a r d s w e r e b o i l e d for a t l e a s t l h w i t h NaC1 a n d HC1.

G e n e r a l l y , HC1 (12M, 2g), NaC1 (0.2g), a n d w a t e r w e r e a d d e d to a l l s a m p l e s to g ive a f ina l v o l u m e of ca. 10 ml . T h e s o l u t i o n s w e r e t h e n c o v e r e d a n d s i m m e r e d on a h o t p l a t e fo r ca. l h . W a t e r w as a d d e d as r e q u i r e d to g ive a f ina l v o l u m e of ca. 5 ml . 5 m l of 48% H B r w e r e a d d e d to e a c h s a m p l e a n d h e a t i n g was c o n t i n u e d . Af t e r 30 ra in , 5 m l o f a 25% s o l u t i o n o f SnClz - 2H20 in 48% H B r w e r e a d d e d to e a c h s a m p l e . A f t e r a f u r t h e r 2 min , t h e s a m p l e s w e r e r a p i d l y c o o l e d a n d d i l u t e d to 25 m l w i t h wate r . Ab-

s o r b a n c e s a t 396 n m w e r e m e a s u r e d w i t h a S h i m a d z u UV-240 s p e c t r o p h o t o m e t e r . T h e a v e r a g e m o l a r a b s o r p - t i v i t y for t h e s t a n d a r d s was d e t e r m i n e d to b e 3.0 + 0.3 • 104 M -1 c m - ' . I t was f o u n d t h a t a d d i t i o n of 0.5M H2SOt or 1M LiC1OJ0 .1M L i O H to t h e s t a n d a r d s d i d n o t s ignif i - c a n t l y c h a n g e t h i s va lue .

Al l c h e m i c a l s w e r e r e a g e n t g r a d e or b e t t e r (F i sher ) a n d w e r e u s e d as r ece ived .

Results and Discussion Ir ox ide g r o w t h in acidic , neu tra l , a n d basic l i t h i u m

perch lora te solut ions . - - -Acidic l i t h ium perchlorate (aq ) . - -F igure 1 s h o w s cyc l i c v o l t a m m o g r a m s of a n I r e l e c t r o d e in 1M LiCIOJOAM HC104 b e f o r e (- - -) a n d a f t e r ( ) o x i d e g r o w t h in t h i s so lu t ion . T h e o x i d e was g r o w n u s i n g 600 p o t e n t i a l p u l s e s b e t w e e n - 0 . 3 a n d + 1.2V a t 0.5 Hz. T h e a p p e a r a n c e of t h e Ir o x i d e v o l t a m m o g r a m in t h e ac id ic L i C 1 Q s o l u t i o n is a l m o s t i d e n t i c a l to t h a t of Ir oxide in 0.5M H2SO4 (2).

T h e p r i n c i p a l r e v e r s i b l e w a v e a t E ~ = 0.56V is d u e to t h e I r ( IV) / ( I I I ) c o u p l e a n d is t h o u g h t to i n v o l v e H + a n d a n i o n (X) i n s e r t i o n d u r i n g o x i d e r e d u c t i o n (18). E x p e r i - m e n t s c a r r i ed o u t in H F s o l u t i o n s (18) y i e l d e d cyc l ic vol- t a m m o g r a m s w h i c h w e r e v e r y s i m i l a r to t h a t s h o w n in Fig. 1. A c h e m i c a l ana ly s i s of t h e c o n t e n t s of t h e film, in b o t h t h e o x i d i z e d a n d r e d u c e d f o r m s , s h o w e d (18) t h a t t h e o x i d a t i o n a n d r e d u c t i o n of I r o x i d e f i lms o c c u r s ac- c o r d i n g to r e a c t i o n [1] in H X so lu t ions .

(IrO~ - nH~O).~ + 5e- + 7H + + 2X-

= [Ir503(OH)~- 5nH20]2+(X )2 [1]

R e a c t i o n [1] is a lso c o n s i s t e n t w i t h t h e 80 to 90 m V / p H d e p e n d e n c e of I r o x i d e r e d u c t i o n a n d o x i d a t i o n , f o u n d b y u s a n d o t h e r s (10, 18). I n t h e 1M LiC1OJ0.1M HC104 so- l u t i o n of Fig. 1, X - is e x p e c t e d to b e C104-. A l t h o u g h th i s is a r a t h e r l a rge ion, i t is f ea s ib l e t h a t i t c a n m o v e in t h e p o r e s of t h e o x i d e f i lm, a n a l o g o u s to t h e c a s e o f SO4 z-, f o u n d b y E S C A a n a l y s i s to b e p r e s e n t in I r o x i d e f i lms g r o w n in su l fu r i c ac id s o l u t i o n s (16).

T h e o t h e r f e a t u r e s of t h e v o l t a m m o g r a m of I r o x i d e in ac id (Fig. 1) h a v e no t b e e n c lea r ly a s s igned . I t is t h o u g h t t h a t h i g h e r o x i d a t i o n s t a t e s of I r (Ir(V) or I r (VI)) a re f o r m e d in t h e p o t e n t i a l r e g i o n a f t e r t h e m a i n p e a k a n d b e f o r e t h e o n s e t o f o x y g e n e v o l u t i o n a t ca. + 1V (6). F o r e x a m p l e , b y E S C A ana lys i s , I r (VI) h a s b e e n f o u n d to b e p r e s e n t in t h i c k I r o x i d e f i lms f o r m e d e l e c t r o c h e m i c a l l y in su l fu r i c ac id s o l u t i o n s (16).

N e u t r a l l ( t h ium perchlorate (aq ) . - -F igure 2 i l l u s t r a t e s t h e e l e c t r o c h e m i c a l g r o w t h a n d cycl ic v o l t a m m e t r i c be- h a v i o r of I r o x i d e in n e u t r a l a q u e o u s LiC104 (pH - 6). T h e v o l t a m m o g r a m of t h e o x i d e in 1M LiC104 is r a t h e r c o m p l e x a n d u n u s u a l , d u e to t h e c o m p e t i t i o n b e t w e e n

ia ~ 2 I 2 m A/c m

. . . . . . . . t ~ -j-En=l

1.0

E,VvsSSCE Fig. 1. Cyclic voltammograms (100 mV/s) of Ir in O.IM HCIO4/IM

LiCIO4, before (- - -) and after (- ) 600 potential pulses between -0.3 and +1.2V at 0.S Hz.

128 J. Electrochem. 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 January 1988

a

l m A / c m

-1 .0 / 0 1. :)

/ ..... E,V w SSCE ~

Fig. 2. Cyclic voltammograms (100 mV/s) of Ir in 1M LiCI04 (pH "6), before (- - -) and after ( ) 180 potential pulses between - i .2

and + 1.0V at 0.5 Hz~

H § and Li § insert ion (18) and to variations of the electrolyte pH wi th in the oxide dur ing potent ia l cycl ing (2, 10, 18). This will be d iscussed in greater detai l later in this paper. The main electrochemical reactions which are t hough t to occur in neutral LiC1Q are shown as reactions [2] and [3] (18), where the M ion probably has a tran- sient exis tence within the reduced form of Ir oxide (18), being replaced by H + with t ime

IrO2 �9 2H20 + e- + M s = M+[IrO(OH)2 �9 H20]- [2]

I r Q . 2H20 + e- + H § = IrO(OH) �9 2H20 [3]

Ir ox ide growth in neutra l solut ions causes the pH of the solution to gradually change because the amounts of oxygen and hydrogen evolved at the potent ia l l imits used for oxide growth are generally not equal. In order to min imize these pH changes, 0.01M Na~B407 was some- t imes added to buffer the neutra l LiC104 solution. This did not appear to alter the growth or e lectrochemistry of the result ing oxide.

Basic lithium perchlorate (aq).--Figure 3 i l lustrates the growth of Ir oxide in aqueous 1M LiC104/0.1M LiOH. The oxide e lect rochemist ry in this medium is similar to that obta ined in 0.1M NaOH by Burke and Scannel l (10). However , the growth rate is approx imate ly three t imes higher than that reported by Burke and Scannell. This is p robab ly due to the fact that potent ia l puls ing (11), ra ther than the less efficient potent ia l scanning technique, was used for oxide growth in the present work.

The main e lec t rochemica l react ion (E ~ = -0.39V) in basic media is best described by reaction [4] (18). The mi- nor wave at E ~ = -0.08V has not been assigned

(M+)3[Ir~O~o(OH)3 - 7H20] 3- + 5e- + 7H~O

= (M+)[Ir506(OH)4 �9 10H20]- + 2M + + 7 OH- [4]

The Composition and Structure of Ir Oxide F i lms . - Chemical analysis.--Table I contains the resul ts of the we igh ing and Ir analysis exper iments . The weight mea- surements yield the weight of the dry oxide film in addi- t ion to the sum of the weight of Ir in the oxide, plus the Ir that dissolved during oxide growth. The chemical analysis of the solutions give the weight of Ir in the oxide and the weight of dissolved It. The cyclic vo l tammetry gives charge (or number of electrons) as a function of potential in each solution. Thus, the empir ica l formula weight (EFW) of the oxide can be calculated and using Faraday's law, the potent ia l at which the oxide is oxidized to the extent of one electron per Ir atom in that solution (E,~1) can then be es t imated. These resul ts are also g iven in Table I.

a

5mA/cm 2

En_-I

1 t .~176 *~176 ."

~176176 ~

. . s . ~ ~.,,. / /

I " " "'% "~

E, Vvs SSCE

Fig. 3. Cyclic voltarnmograms (100 mV/s) of Ir in 0.1M LiOH/1M LiCIO4, after 0 ( . . . . . ), 450 (- - -), 900 ( . . . . . ), and 1350 ( ) potential pulses between - 1 . 2 and +0 .6V at 0.5 Hz.

The sum of the two Ir analyses (Ir dissolved + Ir in oxide) should be equal to the weight loss of the Ir foil during the experiments , so that a good method of checking the accuracy of the results is available. Inspect ion of the data in Table I reveals that the sum of the Ir analyses genera l ly agrees wi th the weight loss of the foil wi th in expe r imen ta l error (-+ 1 ~g). This confirms the accuracy of the weighings and analyses.

In exper iment 2, however, the sum of the Ir analyses is significantly less (by 4 ~g) than the weight loss of the foil. This d i sc repancy may be due to loss of oxide f rom the foil during the rinsing, drying, and weighing procedures. SEM studies (see below) indicate that this is particularly l ikely for Ir oxide films grown in 0.5M H~SO4. E x t r e m e care should, therefore, be taken to avoid loss of oxide in this way. The error caused by oxide loss was minimized by report ing the average of the weight of Ir in the oxide from the Ir analysis of the oxide and from the weight loss of the foil minus the dissolved Ir (see footnote d in Table I).

The empirical formula weights (EFW) of Ir oxide films, g iven in Table I, indicate that Ir oxide films grown in neutral or basic LiC104 can be represented as IrO2.2H20 (EFW = 260.2). However, analysis of the Li + content of an I r oxide film that had been oxidized in basic LiC104 showed that it contained ca. 0.5 Li § ion per Ir atom (18). Thus the oxide grown in this m e d i u m could be represented as Li[Ir204(OH)(H~O)3] (EFW = 263.2 per Ir). [r oxide films that had been oxidized in neutra l LiC104 were found to contain very little Li + (18), al though significant amounts of Li + are found in the r educed form of the oxide.

The films grown in 0.5M H2SO4 have a s ignif icant ly h igher empir ica l formula weigh t than those grown in

Vol. 135, No. 1 S T R U C T U R E O F H Y D R O U S I r O 2 F I L M S

Table I. Weight and Ir anolysis data for Ir oxide films (air dried)

129

Weight (~g)~ Ir analysis (~g) Expe r imen t

n u m b e r Growth m e d i u m Oxide Ir h Dissolved ~ Oxide EFW d E,=~(V) ~

1 0.5M H2SO4 f 50 43 8 34 280 2 0.5M H~SO4 f 49 41 6 31 290 3 0.5M H2SO4 f 43 37 8 - - 280 4 1.0M LiC1OJ 63 47 1 - - 260 5 1.0M LiC104 ~ 61 46 1 44 260 6 1.0M LiC104 ~ - - - - - - 38 - - 7 0.1M LiOH/ 60 45 1 44 260

1.0M LiC104 g 8 0.1M LiOH/ - - - - - - 38 - -

1.0M LiC104 ~

0.96 0.93 0.88 0.91 0.79 0.83 0.11

0.17

o r

~-+ 1 ~g. b Total weight loss of foil. Should be equal to weight of Ir in oxide plus weight of Ir dissolved dur ing oxide growth. r Ir dissolved dur ing oxide growth. d Empirical formula weight of oxide. Calculated from

EFW = AWIr • weight of oxide/analyzed weight of Ir in oxide

EFW = AW~r • weight of oxide/(weight loss of foil - weight Ir dissolved).

Where both calculations were possible, the average is given. Es t imated error is -+ 10. AW,r is the atomic weight of Ir = 192. Potential (vs. SSCE) at which the oxide is oxidized to ex ten t of one electron per Ir a tom in oxide film.

f Oxide grown by potential pu ls ing (0.5 Hz) be tween -0.26 and + 1.24V for 30 rain. Oxide grown by potential pu ls ing (0.5 Hz) between - 1.20 and + 1.20V for 30 min.

L i C 1 0 4 s o l u t i o n s a n d a r e , t h e r e f o r e , p r o b a b l y m o r e h y - d r a t e d , e.g., IrO2 - 3 H 2 0 ( E F W = 278.2). I t s h o u l d b e n o t e d t h a t l o o s e l y a t t a c h e d w a t e r p r e s e n t i n i t i a l l y i n s m a l l p o r e s o r v o i d s w i t h i n t h e o x i d e m a y h a v e b e e n r e m o v e d b y t h e a c e t o n e r i n s i n g p r o c e d u r e , p r i o r to t h e h e a t i n g s t e p s . T h e e x t r a w e i g h t i n H 2 8 0 4 s o l u t i o n s m a y a l s o b e d u e to t h e p r e s e n c e o f SO42-, w h i c h h a s b e e n d e t e c t e d i n I r o x i d e f i l m s , g r o w n i n H2SO4 s o l u t i o n s , b y s e v e r a l re - s e a r c h g r o u p s (2, 16).

T h e d a t a i n T a b l e I w a s o b t a i n e d u s i n g o x i d e f i l m s w h i c h w e r e d r i e d i n a i r fo r 5 to 10 m i n . C l e a r l y , m o s t o f t h e a d h e r i n g w a t e r a n d a n y w a t e r i n t h e p o r e s o f t h e o x - i d e f i l m i s l o s t r a p i d l y f r o m t h e o x i d e u n d e r t h e s e m i l d d r y i n g c o n d i t i o n s . S o m e f u r t h e r w e i g h t m e a s u r e m e n t e x p e r i m e n t s w e r e c a r r i e d o u t i n o r d e r t o d e t e r m i n e w h e t h e r t h e r e m a i n i n g w a t e r c o u l d b e r e m o v e d . T h e re - s u l t s o f t h e s e e x p e r i m e n t s a r e s h o w n i n T a b l e II.

D r y i n g i n a i r f o r l o n g e r p e r i o d s p r o d u c e s s o m e f u r t h e r w e i g h t l o s s a n d d r y i n g i n a v a c u u m a t a n e l e v a t e d t e rn - p e r a t u r e c a u s e s e v e n f u r t h e r w e i g h t l o s s . T h e v a c u u m - d r i e d f i l m s w e r e f o u n d t o b e v e r y d i f f i c u l t to r e m o v e f r o m t h e u n d e r l y i n g I r foi l . T h e y c o u l d b e r e m o v e d e l ec - t r o c h e m i c a l l y a t + 2 V i n 10% H2S O 4 b u t t h e d i s l o d g e d o x - i d e d i d n o t d i s s o l v e a n d t h e r e f o r e I r a n a l y s i s o f t h e o x i d e c o u l d n o t b e p e r f o r m e d . H o w e v e r , t h e d a t a i n T a b l e I in - d i c a t e s t h a t t h e w e i g h t o f I r i n a n o x i d e f i l m c o r r e s p o n d s t o 78 + 2% a n d 97 + 1% o f t h e w e i g h t l o s s o f t h e fo i l , i n 0 . 5 M H2SO4 a n d 1M L i C 104 , r e s p e c t i v e l y . T h e e m p i r i c a l f o r m u l a w e i g h t s i n T a b l e I I w e r e c a l c u l a t e d o n t h i s b a s i s ( s ee f o o t n o t e a i n T a b l e II a n d d i n T a b l e I).

I t i s c l e a r f r o m T a b l e I I t h a t I r o x i d e f i l m s g r o w n i n e i t h e r 0 . 5M H2SO4 o r 1M L i C 1 0 4 c a n b e d e h y d r a t e d to g i v e IrO~ ( F W = 224). T h e o x i d e g r o w n i n 0 .5M H2SO4 c a n b e d r i e d to I r O , �9 2 H 2 0 ( F W = 260) a t r o o m t e m p e r a t u r e i n a i r a n d to IrO2 i n a v a c u u m a t >70~ T h e s e r e s u l t s i n d i - c a t e t h a t t h e SO42- c o n t e n t o f t h i s o x i d e m u s t b e l e s s t h a n o n e i o n p e r s i x IrO2 u n i t s . A n a l y s i s o f t h e F - c o n t e n t o f I r o x i d e f i l m s w h i c h h a d b e e n o x i d i z e d i n 0 .2M H F a l s o in- d i c a t e d t h a t a n i o n i n c o r p o r a t i o n i n t o t h e o x i d i z e d f o r m o f t h e o x i d e w a s m i n i m a l (18).

B a s e d u p o n t h e r e s u l t s i n T a b l e s I a n d II , i t c a n b e s t a t e d t h a t t h e r e i s l i t t l e d i f f e r e n c e i n t h e c o m p o s i t i o n o f I r o x i d e f i l m s g r o w n i n t h e d i f f e r e n t m e d i a u s e d i n t h i s w o r k , a l t h o u g h t h e f i l m s g r o w n i n 0 .5M H2SO4 m a y b e s l i g h t l y m o r e h y d r a t e d .

T h e En=l r e s u l t s i n T a b l e I a r e e x t r e m e l y u s e f u l b e - c a u s e t h e a m o u n t o f I r i n a n I r o x i d e f i l m c a n n o w b e d e - t e r m i n e d b y c y c l i c v o l t a m m e t r y . I n t h e p a s t , t h e r e l a t i v e q u a n t i t y o f I r p r e s e n t i n a n I r o x i d e f i l m h a s b e e n e s t i - m a t e d f r o m t h e c h a r g e d e n s i t y p a s s e d d u r i n g a v o l t a m - m e t r i c s w e e p (3, 5, 6, 11), b u t a n a b s o l u t e d e t e r m i n a t i o n

h a s n o t b e e n p o s s i b l e . I n 0 .5M H2SO4, t h e c h a r g e d e n s i t y i n t e g r a t e d u p to + 1 .30V vs. R H E (qox.,.3) w a s u s e d a s a re l - a t i v e m e a s u r e o f t h e o x i d e q u a n t i t y (11). T h i s u p p e r p o - t e n t i a l f o r t h e i n t e g r a t i o n w a s c h o s e n s o m e w h a t a r b i t r a - r i l y b e c a u s e E,=, h a d n o t b e e n d e t e r m i n e d a t t h a t t i m e .

I r o x i d e f i l m q u a n t i t i e s c a n n o w b e e x p r e s s e d i n m o l e s o f I r p e r u n i t e l e c t r o d e a r e a (F) b y u s i n g Eq . [1]. I t i s a s - s u m e d h e r e t h a t a l l o f t h e I r s i t e s i n t h e f i lm a r e e l e c t r o a c - t i ve , p a r t i c u l a r l y i n s l o w p o t e n t i a l s w e e p e x p e r i m e n t s

F = q . . . . . , /F [1]

q,,x.n=, i s t h e c h a r g e d e n s i t y p a s s e d d u r i n g a s l o w p o t e n - t i a l s w e e p f r o m t h e o n s e t o f o x i d a t i o n o f t h e o x i d e (e.g., - 0 . 1 V in F ig . 1) to En=,. F is i n d e p e n d e n t o f t h e m e d i u m i n w h i c h t h e v o l t a m m o g r a m is r e c o r d e d a n d so a c o m - p a r i s o n o f En=, i n d i f f e r e n t m e d i a i s n o w p o s s i b l e . T h e av- e r a g e E.=I v a l u e s f r o m T a b l e I a r e 0.92 -+ 0.04, 0.84 -+ 0.06, a n d 0.14 -+ 0.04V, fo r I r o x i d e i n 0 .5M H2SO4, 1M LiC104, a n d 0 .1M L i O H / 1 M L i C 1 Q , r e s p e c t i v e l y . En- , f o r I r o x i d e i n 0 .1M H C 1 O J 1 M LiC104 w a s e s t i m a t e d to b e 0 .95V f r o m a n e x p e r i m e n t i n w h i c h a n I r o x i d e c o a t e d e l e c t r o d e w a s g r o w n in t h e s o l u t i o n a n d t h e n t r a n s f e r r e d to a s o l u t i o n fo r w h i c h t h e En=, is k n o w n (0 .1M L i O H / 1 M LiC104).

Table II. Weight data for Ir oxide films as a function of drying method

Empirical Weight of oxide Weight formula

(~g) loss of weight ~ foil

Growth m e d i u m A B C (~g) A B C

0.5M H2804 b 55 - - 51 (60) 46 280 - - 260 0.5MH2SO4 b 43 - - 40 (70) 37 280 - - 260 0-5MH~SO4 b'c 222 215 185(90) 194 270 260 230 1.0MLiC104/ 123 I l l 104(90) 88 270 250 230

0.01M Na2B407 ~

A. Dried for 5-10 min in air. B. Dried for > 16h in air. C. Dried in v a c u u m at 60~176 for lh.

M a x i m u m tempera ture (~ given in parentheses .

a Calculated by a s s u m i n g that the weight of Ir in the oxide is 78% of t he foil we igh t loss in 0.5M H2SO4 and 97% in 1M LiC104 (i.e., f rom data in Table I).

b Oxide g rown by potent ia l pu l s i ng (0.5 Hz) be tween -0 .26 and +1.24V for 30 rain.

~ Larger electrode. Weighings on Mettler Micro Gramatic balance to -+5 ~g.

a Oxide g rown by potent ia l pu l s ing (0.5 Hz) be tween - 1.20 and +I.10V for 60 min.

130 J. Electrochem. Soc.: ELECTROCHEMICAL SCIENCE AND TECHNOLOGY January 1988

The pos i t ions of these E.~I po ten t ia l s on the v o l t a m m o - g r a m s s h o w n in Fig. 1-3 have been m a r k e d on the figures. I t is c lear tha t at all pH's , Ir ox ide can be r eve r s ib ly oxi- d i zed to an e x t e n t of g r ea t e r t h a n one e l e c t r o n pe r Ir. I t can also be seen tha t the ox ida t ion of Ir ox ide f rom Ir(III) to Ir(IV) does no t occur as a s ingle peak and tha t the gen- e r a t i o n of h i g h e r o x i d a t i o n s ta tes of Ir (Ir(V) or Ir(VI)) m a y ac tua l ly c o m m e n c e before the ox ide has b e e n com- p le te ly t r a n s f o r m e d to the Ir(IV) form. Final ly , it shou ld be n o t e d tha t , i f t h e a s s u m p t i o n tha t e a c h I r s i te in t he o x i d e p r o v i d e s one e l e c t r o n (Eq. [1]) had b e e n un jus - t i f ied, t h e En=l v a l u e s w o u l d e i t h e r h a v e b e e n v e r y low, i.e., only a smal l f rac t ion of the total ox ide cha rge w o u l d h a v e p a s s e d at E,=I, or e x c e s s i v e l y h igh , i.e., p o s i t i v e of t he po ten t i a l for o x y g e n evolu t ion . The fact tha t En=, ap- pea r s at a r e a s onab l e po in t in t he cycl ic v o l t a m m o g r a m s suppor t s the ini t ia l hypo the s i s of the i n v o l v e m e n t of one e l ec t ron per Ir si te in the oxide.

Scanning electron microscopy.--Figure 4 s h o w s sev- e ra l s c a n n i n g e l e c t r o n m i c r o g r a p h s of s o m e e lec t ro - c h e m i c a l l y g r o w n Ir o x i d e f i lms f o r m e d on p o l i s h e d Ir su r faces . T h e fi lm s h o w n in Fig. 4A was g r o w n in 0.5M H2SO4 and was d r i ed in air a f te r s o a k i n g in w a t e r for 10 rain. This f i lm is e x t e n s i v e l y c r acked and shows poor me- chan i ca l s tabi l i ty, as seen by the fact tha t f lakes of ox ide h a v e fa l l en f r o m the e l e c t r o d e (top r i g h t - h a n d a rea of

Fig. 4A). This t ype of s t ruc tu re has been r epo r t ed previous ly by Mozota and C o n w a y (9).

T h e fi lm s h o w n in Fig. 4B was g r o w n u n d e r t he s a m e c o n d i t i o n s as t he film in Fig. 4A. Howeve r , this film was s u b s e q u e n t l y dr ied by the cri t ical po in t m e t h o d (see Ex- p e r i m e n t a l sec t ion) . I t can be seen tha t t h e use of th is d r y i n g m e t h o d l eads to a s imi l a r c r a c k e d s t r uc tu r e , al- t h o u g h t h e c r acks (Fig. 4B) are s ign i f i can t ly n a r r o w e r t h a n t hose in Fig. 4A, ind ica t ive of less sh r inkage of the o x i d e film. Also , no loss of f lakes o f o x i d e was s e e n by t h e S E M e x a m i n a t i o n (Fig. 4B), and , h e n c e , t he fi lm is p r o b a b l y m o r e a d h e r e n t than tha t s h o w n in Fig. 4A. The fac t t ha t th i s c r i t i c a l - p o i n t d r i ed fi lm is s t i l l c r a c k e d p r o b a b l y ind ica tes tha t the c r ack ing occurs d u r i n g ox ide g rowth , as r epo r t ed by Mozota and C o n w a y (9). Howeve r , t h e o x i d e can o b v i o u s l y suf fe r f u r t h e r d a m a g e d u r i n g s u d d e n drying.

T h e f i lm s h o w n in Fig. 4C was g r o w n in a q u e o u s 1M LiC1OJ0.01M Na2B407 and d r i ed in air a f t e r s o a k i n g in w a t e r for 10 min . T h e d e g r e e of c r a c k i n g for th is f i lm is s i gn i f i c an t l y less t h a n tha t of t he fi lm g r o w n in 0.5M H2SO4 (Fig. 4A). This ind ica tes tha t s t ruc tu ra l d i f fe rences m u s t ex is t b e t w e e n the films g rown in the two d i f fe ren t media . The S E M p h o t o g r a p h s canno t revea l t hese differences , b u t t h e r e su l t s of t h e c h e m i c a l a n a l y s e s g i v e n in t he p r ev ious sec t ion (Tables I and II) m a y p rov ide s o m e c lues . T h e o x i d e g r o w n in 0.5M H2SO4 a p p e a r s to r e t a in

Fig. 4. Scanning electron micrographs of Ir oxide films. Films A, B, and D were grown in 0 .5M H~SO4 (F - 7 • 10 -7 mol/cm2). Film C was grown in 1M L iCIOJ0,01M Na.2B407 (1" = 6.4 • 10 v m01/cm2). Film B was dried by the critical point method, while the rest were dried in air.

Vol . 135, No . 1 STRUCTURE OF HYDROUS Ir02 FILMS 131

m o r e w a t e r t h a n t h e o x i d e g r o w n in 1M LiC104. P e r h a p s t h e a c i d - g r o w n o x i d e h a s a m o r e o p e n a n d h y d r a t e d s t r u c t u r e t h a t is m o r e s u s c e p t i b l e to s h r i n k a g e a n d c r a c k i n g d u r i n g d ry ing . I t s h o u l d b e r e c a l l e d t h a t w a t e r w h i c h c o u l d h a v e b e e n p r e s e n t w i t h i n m i c r o p o r e s of t h e o x i d e m a y h a v e b e e n r e m o v e d b y a c e t o n e r in s ing . I f t h i s w a t e r h a d a l so b e e n m o n i t o r e d , i t is p o s s i b l e t h a t t h e a c i d - g r o w n o x i d e s w o u l d h a v e y i e l d e d a s i g n i f i c a n t l y g r e a t e r w a t e r c o n t e n t t h a n t h o s e g r o w n in n e u t r a l so lu- t ions . I t s h o u l d also b e n o t e d t h a t t h e g r o w t h of f i lms in ac id s o l u t i o n s r e s u l t e d in s i gn i f i c an t l y h i g h e r d i s s o l u t i o n r a t e s . O x i d e d i s s o l u t i o n c o u l d a l so b e l i n k e d to loss of m e c h a n i c a l s t r e n g t h of t h e oxide .

O n e a i m of t h i s S E M s t u d y was to m e a s u r e t h e p o r o s i t y o f a n o d i c I r o x i d e f i lms. H o w e v e r , no h i g h e r m a g n i f i c a - t i o n m i c r o s t r u c t u r e t h a n t h a t s h o w n in Fig. 4 c o u l d b e re- so lved . I t m u s t b e c o n c l u d e d tha t , in all cases , t h e p o r e s in t h e o x i d e are less t h a n 50 n m in d i a m e t e r .

S E M is a u s e f u l t e c h n i q u e for m e a s u r i n g t h e t h i c k n e s s o f f i lms. F i g u r e 4D s h o w s a c r o s s - s e c t i o n a l v i e w of a n o x i d e - c o a t e d Ir su r face , f o r m e d in 0.5M H~SO4. T h e f i lm c o n t a i n e d 6.6 • 10 -7 m o l c m -2 of I r a n d w as a b o u t 0.8 to 0.9 ktm t h i c k . T h i s y i e l d s a u s e f u l r e l a t i o n s h i p b e t w e e n o x i d e q u a n t i t y a n d f i lm t h i c k n e s s of ca. 7.8 • 10 -7 too l c m 2 of I r o x i d e p e r m i c r o n of fi lm, or a f i lm d e n s i t y o f ca. 2.0g c m -3 ( a s s u m i n g a n E F W of 260). T h i s d e n s i t y is in ex- c e l l e n t a g r e e m e n t w i t h t h e v a l u e of ca. 2.0g c m -3 re- p o r t e d b y M c I n t y r e a n d c o - w o r k e r s (3).

Ef fec t o f p o t e n t i a l l i m i t s on Ir o x i d e g r o w t h r a t e s . - - I n o r d e r to b u i l d u p a t h i c k h y d r o u s o x i d e f i lm a t Ir , re- p e a t e d p o t e n t i a l c y c l i n g is r e q u i r e d a n d in e a c h cyc le , o n l y h a l f a m o n o l a y e r of I r c an b e c o n v e r t e d to h y d r o u s o x i d e (11). F i g u r e 5 s h o w s t h e e f fec t on t h e o x i d e g r o w t h r a t e of u s i n g d i f f e r e n t p o t e n t i a l c y c l i n g l i m i t s in ac id i c , n e u t r a l , a n d b a s i c LiC104 s o l u t i o n s . T h e r e s u l t s a re s imi- la r to t h o s e o b t a i n e d in o t h e r m e d i a (2, 5, 10, 12) a n d ind i - c a t e t h a t o u r m o d e l for I r o x i d e g r o w t h in 0.5M H 2 S Q (11) m a y b e a p p l i c a b l e at all pH ' s .

I n t h i s m o d e l , h y d r o u s o x i d e g r o w t h c o m m e n c e s w h e n a t h i n c o m p a c t o x i d e in i t i a l ly g rows (at p o t e n t i a l s a b o v e ca. +0.3V vs. S S C E in 0.5M H2SO4) on t h e Ir m e t a l s u r f a c e a n d t h e o u t e r l aye r of t h i s o x i d e b e c o m e s h y d r a t e d (hy- d r a t e d su r f ace layer) a t p o t e n t i a l s g r e a t e r t h a n ca. + 1V in 0.5M H2SO4. T h e i n n e r c o m p a c t o x i d e c a n b e e l e c t r o - c h e m i c a l l y r e d u c e d to Ir m e t a l b u t t h e h y d r a t e d s u r f a c e l aye r c a n n o t b e e l e c t r o c h e m i c a l l y r e d u c e d in t h e access i - b l e r a n g e o f p o t e n t i a l a n d , t h e r e f o r e , as t h e p o t e n t i a l is c o n t i n u o u s l y cyc led , i t a c c u m u l a t e s o n t h e e l e c t r o d e sur- f ace as a f i lm of h y d r o u s ox ide . I f t h e l o w e r p o t e n t i a l l i m i t is i n s u f f i c i e n t to r e d u c e t h e i n n e r c o m p a c t o x i d e , t h e n t h e h y d r a t e d s u r f a c e l aye r r e m a i n s b o u n d to i t a n d is n o t r e l e a s e d i n to t h e h y d r o u s o x i d e layer .

T h e g e n e r a l t r e n d s s h o w n in Fig. 5 c an b e eas i ly u n d e r - s t o o d in t e r m s of t h i s m o d e l . T h u s , t h e i n c r e a s e in g r o w t h r a t e as t h e l o w e r p o t e n t i a l l i m i t (E_) is d e c r e a s e d is d u e to t h e i n c r e a s e d e x t e n t of r e d u c t i o n of t h e i n n e r c o m p a c t ox ide . T h i s l e a d s to a n i n c r e a s e in t h e a m o u n t o f n e w h y d r a t e d s u r f a c e l a y e r w h i c h c a n b e g e n e r a t e d a n d r e l e a s e d i n to t h e b u l k h y d r o u s o x i d e d u r i n g e a c h cycle of t h e p o t e n t i a l . T h e i n c r e a s e in g r o w t h r a t e as t h e up- p e r p o t e n t i a l l i m i t (E+) is r a i s e d is d u e to a n i n c r e a s e in t h e q u a n t i t y of t h e h y d r a t e d s u r f ace l aye r t h a t is f o r m e d i n e a c h cyc le . T h e g r o w t h of o x i d e p e r cyc le n e v e r ex- c e e d s t h e l im i t of h a l f a m o n o l a y e r of I r m e t a l (ca. 2.8 • 10 -9 m o l cm-2), as p r e d i c t e d b y t h e m o d e l (11).

A n u n d e r s t a n d i n g of t h e de t a i l s of Fig. 5 r e q u i r e s con- s i d e r a t i o n of two f u r t h e r a spec t s . T h e s e are (i) t h e e f fec t o f p H o n t h e g r o w t h of t h e h y d r a t e d s u r f ace l aye r a n d t h e r e d u c t i o n of t h e i n n e r c o m p a c t o x i d e a n d (ii) t h e p H c h a n g e s in t h e s o l u t i o n c lose to t h e e l e c t r o d e c a u s e d b y t h e I r o x i d e e l e c t r o c h e m i s t r y a n d b y H2 a n d 02 e v o l u t i o n .

O x i d e g r o w t h a n d r e d u c t i o n wil l o c c u r at l o w e r p o t e n - t i a l s (vs. t h e p H i n d e p e n d e n t S S C E ) as t h e p H is in- c r e a s e d (21). O x i d a t i o n of t h e o x i d e a n d O2 e v o l u t i o n wi l l b o t h c a u s e t h e p H c lo se to t h e e l e c t r o d e to d e c r e a s e , w h i l e o x i d e r e d u c t i o n a n d H~ e v o l u t i o n wi l l c a u s e i t to i n c r e a s e (22). T h e f ixed p o t e n t i a l l i m i t s of E_ = - 1.2V a n d E§ = + 1.5V vs. S S C E u s e d to o b t a i n t h e da t a for Fig. 5 a re

, o

1.5 Growth

Rate ~1

+2.0 -I:0 0 +1.0 E+ !A,=.e) or E_(•,m.O), Vvs SSCE

Fig. 5. Average Ir oxide growth rates over 150 potential pulses at 0.5 Hz in 0.1M HCIO4/1M LiCIO4 (A, A), neutral IM LiCIO4 (D, I ) , and 0.1M I . iOH/IM LiCIO4 (Q, O) as a function of the potential limits. The open points represent experiments where E+ was constant at + 1.5V, while E_ was varied. For the solid points, E_ was - 1.2V and E+ was varied.

su f f i c i en t to c a u s e e x t e n s i v e H2 a n d 02 e v o l u t i o n , r e spec - t ive ly , a t all p H ' s used .

T h e d a t a for ac id ic (pH = 1) LiC104 in Fig. 5 is t h e m o s t s t r a i g h t f o r w a r d . T h e E+ d a t a (A) r e p r e s e n t s t h e r a t e of g r o w t h of t h e o x i d e a t a c o n s t a n t E v a l u e of - 1 . 2 V b u t w i t h a v a r y i n g E+. T h i s is e q u i v a l e n t to a l t e r i n g t h e ex- t e n t of f o r m a t i o n of t h e h y d r a t e d s u r f a c e l aye r as a func - t i o n of t h e a n o d i c p o t e n t i a l l imit . T h e E_ d a t a (A) r ep re - s e n t s t h e r a t e of o x i d e g r o w t h w h e n E+ w a s c o n s t a n t a t 1.5V a n d E_ was va r i ed . T h i s c a n b e c o n s i d e r e d as a s t u d y o f t h e r a t e of r e d u c t i o n of t h e i n n e r c o m p a c t o x i d e as a f u n c t i o n of E .

T h e E for b a s i c LiC104 (�9 r e p r e s e n t s t h e r a t e of i n n e r c o m p a c t o x i d e r e d u c t i o n at p H - 13, e x c e p t for E_ v a l u e s m o r e p o s i t i v e t h a n - 0 . S V , w h e r e t h e o x i d e g r o w t h r a t e ( - i n n e r o x i d e r e d u c t i o n ra te ) is h i g h e r t h a n w o u l d b e e x p e c t e d a t p H - 13 (21). T h e r e a s o n for t h i s is t h a t t h e H § g e n e r a t e d at E§ (+ 1.5V) in t h e p r e v i o u s c a t h o d i c p u l s e c a u s e s t h e p H c lo se to t h e e l e c t r o d e to b e s i g n i f i c a n t l y l o w e r t h a n 13 a t t h e b e g i n n i n g of t h e c a t h o d i c pu l se . T h u s s o m e i n n e r c o m p a c t o x i d e r e d u c t i o n c a n o c c u r a t E_ > - 0 . 8 V , b e f o r e t h e s o l u t i o n p H e q u i l i b r a t e s .

A s i m i l a r a r g u m e n t c a n b e a p p l i e d to t h e E_ d a t a for n e u t r a l LiC104 (D). F o r E_ > - 0 . 5 V , t h e H § g e n e r a t e d a t E+ m a i n t a i n s t h e s o l u t i o n p H c lo se to t h e e l e c t r o d e a t a low v a l u e (i.e., - 1 ) a n d so t h e h y d r o u s o x i d e g r o w t h ra t e (-= i n n e r c o m p a c t o x i d e r e d u c t i o n ra te) is t h e s a m e as for a c i d i c LiC104. H o w e v e r , for E_ < - 0 . 5 V , t h e O H - g e n e r - a t e d b y o x i d e r e d u c t i o n a n d H~ e v o l u t i o n a t E_ i n h i b i t s i n n e r c o m p a c t o x i d e r e d u c t i o n and , there fore '~ t h e hy- d r o u s o x i d e g r o w t h r a t e vs. E_ p lo t goes t h r o u g h a min i - m u m as i t s w i t c h e s f r o m ac id ic to bas i c b e h a v i o r .

T h e E§ d a t a for n e u t r a l LiC104 (11) r e f l ec t s t h e r a t e of h y d r a t e d s u r f a c e l aye r g r o w t h in w e a k l y bas i c s o l u t i o n s b e c a u s e t h e s o l u t i o n c lose to t h e e l e c t r o d e is m a d e bas i c at E . H y d r o u s o x i d e g r o w t h , t h e r e f o r e , o c c u r s a t s ignif i- c a n t l y l o w e r v a l u e s of E+ t h a n in acid.

T h e E+ d a t a for ba s i c LiC104 (0 ) is a n o m a l o u s as i t fal ls b e t w e e n t h e a c i d i c a n d n e u t r a l da ta . H y d r a t e d s u r f a c e l a y e r f o r m a t i o n is e x p e c t e d to o c c u r a t l o w e r p o t e n t i a l s as t h e p H is i n c r e a s e d a n d so h y d r o u s o x i d e g r o w t h s h o u l d o c c u r at l o w e r v a l u e s of E+ as t h e pH is i n c r e a s e d . A c lue to t h e o r ig in of t h i s a n o m a l y l ies in t h e " s t e p " in t h e E+ d a t a for ba s i c LiC104 at p o t e n t i a l s b e t w e e n +0.75 a n d + 1.0V. Th i s s t ep c o r r e s p o n d s to t h e p o t e n t i a l r e g i o n in t h e v o l t a m m o g r a m b e t w e e n o x y g e n e v o l u t i o n f r o m a b a s i c s o l u t i o n ( - 0 . 8 V ) a n d o x y g e n e v o l u t i o n f r o m n e u - t r a l H20 (E > 1.2V). T h i s i n d i c a t e s t h a t o x y g e n e v o l u t i o n , i.e., t h e p r o t o n s r e l e a s e d b y o x y g e n e v o l u t i o n , e n h a n c e s h y d r o u s o x i d e g r o w t h in s o m e way in bas i c so lu t i ons .

132 J. Electrochem. Soc.: E L E C T R O C H E M I C A L

Burke and Scannell (10) have reported that the release of protons from the oxide at E+ enhances hydrous oxide growth and that acidification of the solution close to the e lec t rode is probably necessary for hydrous oxide growth to occur in base. The present results confirm the enhancement of oxide growth by proton release at E+ in basic media. However, the E§ data for neutral LiC104 solut ions clearly show that the solution near the electrode does not need to become acidic. With E_ = - 1.2V and E+ = +0.5V, the OH- produced at E_ would be sufficient to maintain a high pH close to the electrode. However, hydrous oxide growth still occurs under these condi t ions (Fig. 5). Thus, it appears that the low hydrous oxide growth rates in basic media are due to suppression of oxide growth by high OH- concentra t ions . It may be that the ox ide begins to dissolve when the pH is very high (10).

For E_ < -1 .0V and E§ > +1.25V, the data in Fig. 5 is the same, wi thin exper imenta l error, for acidic, neutral , and basic solut ions. These potent ia ls are sufficient in magni tude to rapidly reduce the inner compact oxide (at nega t ive potent ials) and p roduce the hydra ted surface layer (at posit ive potentials) at any pH. The pH changes close to the e lec t rode caused by H~ and O2 evolu t ion at these potent ia ls p robably swamp any effects due to the pH of the bulk solution.

An important feature of the data in Fig. 5 is that the oxide growth rate cont inues to increase as E+ is raised above IV. In 0.5M H2SO4 or 0.1M NaOH (i.e., no Li + present), the growth rate rises to a m a x i m u m at ca. 1.25 (5, 12) or 0.55V (10), respectively, and then decreases sharply as E+ reaches values at which significant oxygen evolut ion occurs. This is probably due to the increased rate of dissolution of the oxide once O~ evolut ion commences (3, 5, 12). Similar maxima have also been reported for Ir oxide growth in 0.5M Na2CO3 and 1M HCtO, (5).

The resul ts of the chemica l analyses given in Table I show tha t Ir ox ide dissolut ion dur ing growth in neutra l or basic LiCIO4 solutions is very low and is much less than in 0.5M H2SO4. This then explains the above difference and supports the hypothesis that the apparent decrease in oxide growth rate at high potentials in some media is due to oxide dissolution. The reason for the low solubility of Ir oxide in the LiCIO4 solutions is not clear.

Comparison of growth rates in different aqueous solu- t ions.--Oxide growth rates in the var ious media are shown in Fig. 6 as a function of the number of Cycles of potent ial . In all solutions, the growth rate decl ines s ignif icant ly as the oxide th ickens (after more cycles). This is presumably due to the uncompensa ted resistance of the hydrous oxide film, which increases as the film th ickens . The effect is greater in the LiC104 solut ions than in 0.5M H2SO4. This is p robably due to the lower conduct ivi ty of the LiC104 solutions and possibly a lower porosi ty of Ir oxide films grown in LiC104 so lu t ions .The app rox ima te theore t ica l l imit shown in Fig. 6 corre- sponds to one monolayer of IrO:, which appears to be the m a x i m u m amount that can become hydrated in a single cycle of potential (11).

The resul ts in Fig. 6 clearly show that h igher oxide growth rates can be achieved in LiC104 solutions than in 0.5M H2SO4. It should be noted that the growth rates in Fig. 6 for 0.5M H2SO4 (O) are the op t imum values (i.e., op- t imized potent ia l l imits) for cycl ing at 0.5 Hz, whereas the growth rates at 0.5 Hz for the LiCIO4 solutions (Q, A, R) could p resumab ly be increased even fur ther by rais- ing E+ (see Fig. 5).

The p r imary reason for the h igher oxide growth rates in the LiC104 solutions is that higher values of E+ can be used. In 0.5M H2SO4, the oxide growth rate begins to decrease at E+ greater than + 1.25V because of d issolu t ion of the oxide at these E+ values (see above). If the opti- m u m oxide growth conditions at 0.5 Hz in 0.5M H~SO4 are used for acidic LiC104, then very similar growth rates to those in 0.5M H2SO4 (0) are obta ined [Fig. 6 (D)]. The only difference is a more rapid decrease in growth rates with increasing film thickness for the LiC104 solution vs. 0.5M HzSO4, as discussed above.

S C I E N C E A N D T E C H N O L O G Y J a n u a r y 1988

3-t approx, theoretical limit

Average

l Growth Rate

(109m~ cn12cyc~1) 2-1_

0 I I t 0 1 2 3 4 # of cycles (xlO -3)

Fig. 6. Average growth rates as a function of the number of growth cycles for Ir oxide growth in various media using potential pulsing at 0 . 5 Hz. (O) IM LiCIO4, E_ = -1 .4V , E+ = + ] .6V. (A) 0.1M LiOH/IM LiCIO4, E_ = -1 .4V , E+ = +1.6V. (m) 0.1M HCIOJ IM LiCIO4, E_ = - 1.4V, E+ = + 1.6V. (O) 0.5M H2SO~, E_ = -0 .26V , E+ = + 1.24V. (D) 0.1M HCIOJ1M LICIO4, E_ = -0 .3V , E+ = +1.2V.

The results in Fig. 6 also show that pH does not have a great influence upon the rate of hydrous oxide growth in LiC104 solutions when E_ = -1.4V and E+ = +l.6V. The data for acidic, neutral, and basic LiC104 solutions fit the same curve.

Burke and Scannel l (10) have repor ted that Ir oxide growth is slower in base than in acid. The present results contradict this and show that the overall solution composition, ra ther than jus t the pH, de te rmines the Ir ox ide growth rate.

Ir idium oxide electrochemistry in neutral lithium perchlorate solutions.--The e lec t rochemis t ry of Ir oxide in neut ra l LiC104 is ra ther unusua l and deserves fur ther discussion. F igure 7 shows some v o l t a m m o g r a m s of Ir oxide (grown in 1M LiC1OJ0.1M HC104) in neutra l 1M LiC104. It should be noted that this oxide film is thinner than the one used in Fig. 2. Figure 7A shows the effect on the v o l t a m m o g r a m of changing the potent ia l l imi ts of the scan. It can clearly be seen that the reduct ion peak at -0.295V is associated with the oxidation of the film at potentials of -0 .7V ( ). When the upper potential l imit is ma in ta ined below the potent ia l of this ox ida t ion process ( . . . . . . ), the reduc t ion wave at -0.295V is absent and a broad revers ib le wave is ob ta ined at -0.100V. When the lower potent ia l l imit is above -0.295V (- - -), then the oxidation peak at +0.595V is absent.

Thus, there appear to be two major redox processes occurr ing in this solution, a reversible process occurring at -0.100V and an i r revers ib le process cen tered at ca. + 0.15V (oxidation above 0.5V, reduct ion below - -0.2V). This in te rpre ta t ion is suppor ted by the effect of sweep rate (s) on the vo l tammogram (Fig. 7B). Note that the cur- rent sensitivities (S) were chosen so that S/s is the same for each vol tammogram. The posit ion of the wave for the revers ib le process, seen in the anodic scan, is indepen- dent of the sweep rate and the peak currents increase linearly with the sweep rate, as expected for a surface process (23). The oxidat ion and reduct ion peaks of the other major process shift to h igher and lower potent ia ls , respectively, as the scan rate is increased and the peak currents increase less than linearly with sweep rate. This indicates that a new process having di f ferent kinet ics is now observed.

The cyclic v o l t a m m e t r y of Ir oxide in neutra l LiC104 can be exp la ined if it is a ssumed that Li § H+; and OH- are all involved in the e lectrochemical reactions and that the ox ida t ion and reduc t ion processes occur r ing at >0.5V and < -0.2V are related to pH changes which o c -

VoL 135, No. 1 S T R U C T U R E O F H Y D R O U S IrO2 F I L M S 133

A I 0 . 1 m A / c m 2

r ~ ~ , ' " ' " ' " - i '::"+" + I i , J I

. . . . .

B Is ..

-o.s ~ 0~ .:' r.0 �9 . \ . .-. / /

E, V vs SSCE

Fig. 7. Cyclic voltammograms of Ir oxide in 1M LiCIO+ (pH ~ 6). A. Voltamrnograms at 100 mV/s with various potential limits. B. Voltam- mograms at 0.02 ( ), 0.2 (- - -), and 2 ( . . . . . ) V/s. S/scan rate = 1 (mA/cm2)/(V/s).

cur within the oxide film with potential cycling in these neut ra l solut ions (2, 10). As the oxide is oxidized, H + is genera ted (18, 22) and so the inter ior of the oxide be- comes acidic. Hence; the oxidat ion of the oxide is seen at more posit ive potentials (ca. 0.4 to 1.0V), characterist ic of Ir oxide oxidation in acidic media (Fig. 1). During reduc- t ion of the oxide, H § is consumed, and the main reduc- t ion wave is now seen at a potent ia l (-0.295V) that is charac ter i s t ic of Ir oxide reduc t ion in basic media (Fig. 3).

The reversible process occurr ing at ca. -0.1V does not appear to be affected by these pH changes and it is therefore likely that Li + is involved in the redox process in the 1M LiC104 solution. Thus, it is proposed here that the oxidation peak at ca. -0 .1V involves expuls ion of Li + from the film and the reduc t ion peak at ca. -0 .1V [Fig. 7A ( . . . . . )] involves Li + inser t ion into the Ir oxide. Fur the r ev idence for the Li + inser t ion/expulsion mechanism has been obta ined from analysis of the ion conten t of the Ir ox ide films (18), and has also been repor ted for Ir oxide films in Li+-containing 2-methyl - te t rahydrofuran solutions (14).

Conclusions Neutra l aqueous LiC104 solut ions have been found to

be super ior to aqueous H2SO4 as a m e d i u m for the growth of hydrous Ir ox ide films. Higher oxide growth rates can be ach ieved and less d issolu t ion of the oxide occurs during growth in neutral vs. acidic growth media. Also, the resultant Ir oxide films have a greater mechanical stability and are probably less hydrated when LiC10~ is used, as compared to sulfuric acid grown films.

The empi r ica l formula weight of the oxidized form of Ir oxide, grown in neut ra l LiC10+ and dr ied in air, was found to be 260 -+ 10, indicat ive of an empirical formula of IrO~ �9 2H20. Vacuum drying reduced the formula weight to 230 +- 10, indicating dehydrat ion to IrO2.

A useful pa ramete r (En=l), which indicates the potential at which one electron has been passed per Ir atom in an oxide film, has been determined in a number of the solut ions in this work. This permits the accurate measurement of the quant i ty of Ir oxide film at an electrode surface from the cyclic voi tammetr ic response.

Acknowledgment The authors gratefully acknowledge financial support

f rom the Natural Sc iences and Engineer ing Research Council of Canada and from Allied Canada Incorporated.

Manuscr ip t submi t ted Jan. 15, 1987; revised manu- script received Ju ly 2, 1987.

Universi ty of Calgary assisted in meeting the publica- t ion Costs of this article.

REFERENCES 1. L. D. Burke, in "Electrodes of Conductive Metallic Ox-

ides," Part A, S. Trasatti, Editor, Elsevier Pub- lishing, New York (1980).

2. S. Gottesfeld and J. D. E. McIntyre, This Journal, 126, 742 (1979).

3. J. D. E. McIntyre, W. F. Peck, and S. Nakahara, ibid., 127, 1264 (1980).

4. G. Beni, C. E. Rice, and J. L. Shay, ibid., 127, 1342 (1980).

5. J. Mozota and B. E. Conway, Electrochim. Acta., 28, 9 (1983).

6. L. D. Burke and D. P. Whelan, J. Electroanal. Chem., 1fi2, 121 (1984).

7. S. H. Glarum and J. H. Marshall, This Journal, 127, 1467 (1980).

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the influence of the aqueous growth medium on the growth

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