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886 J . E l e c t r o c h e m . S o c . : ELECTROCHEMICAL SCIENCE AND TECHNOLOGY J u n e I 9 7 82 5 0

> 2 0 0

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I ] I I I IO I,O 2.0 5.O 4 . 0 5 . 0 6 . 0pHF i g . 2 . The depend ence of the photogalvanie response for thesolution as a funct ion of pH.

As to why the max im um photopotential should occuraround pH 3.4, we have no ready answer, for thisphenome non has not been studied. However, i t appearsthat, since the species -TH2 + is generat ed by light, it isevident from Eq. [1] that the observed photopote ntialshould depend on the s tabili ty of the entity at theelectrode surface. Conceivab ly the acetate ion ac tingas a ligand can somehow either alter the redox po-tential of the system or increase the rate of inter-facial electron transfer. Additional experiments usinga variety of ligands are of obvious interest.

Summarizing, we have shown that the photoresponseof the conventional iron thioni ne system can be e n-hanced wi th a slight modification of the solution. Inaddition, an interes ting pH dependence was observed.The finding presented here merits fur ther exploration.A c k n o w l e d g m e n t sThis work was supported, in part, by a postdoctoralenergy- relate d fellowship to J. M. Mountz from t h eNational Science Foundati on and the College of Natu-

ral Sciences, Michigan State University.Manuscript submitted Oct. 14, 1977; revised manu-scrip t rec eive d Feb. 13, 1978.Any discussion of this paper will appear in a Discus-sion Section to be published in the December 1978JOURNAL. All disc ussi ons for th e D ec em be r 1978 Dis -cussion Section should be submitted by Aug. 1, 1978.P u b l i c a t i o n c o s t s o~ t h i s a r t i c l e w e r e a s s i s t e d b yM i c h i g a n S t a t e U n i v e r s it y .

REFERENCES1. G. Porter and M. D. Archer, l n t e rd~ sc i p . Sc i , . R ev . ,1, 119 (1976).2. E. Rabinowitch, J . C h e m . P h y s . , 8, 551, 560 (1940).3. A. E. Potter, Jr. and L. H. Thaller, S o l . E n e r g y , 3 ,1 (1959).4. L. J. Miller, Tech. Ser. Rep., AD 282, 878, U.S. Dept.of Comme rce Washi ngto n, D.C. (1962).5. W. D. K. Clark and J. A. Eckert, S o l . E n e r g y , 1 7 ,147 (1975).6. D. E. Hall, J. A. Eckert, N. N. Lichtin, and P. D.Wildes, T h i s J o u r n a l , 123, 1705 (1976).7. N. N. Lichtin, in "Photochemi cal Conversio n andStorage of Solar Energy," J. R. Bolton, Editor,Academic Press, New York (1977).8. J. M. Mountz and H. T. Tien, S o l . E n e r g y , To bepublished.9. R. Gomer, E l e c t r o c h i m . A c t a , ~(}, 13 (1975).

Elec trochem ica l O x ida t ion and Red uct ion o fTh in F i lms o f Pruss ian B lue

V e r n o n D . N e f fD e p a r t m e n t o ~ C h e m i s t r y , K e n t S t a t e U n i v e r s i ty , K e n t , O h i o 44 24 2

We would like to communi cate some interesting pre-limi nary results concerning the electrochemical be-havior of thin films of Prussian blue. There are actu-ally two well-characterized forms of Prussian blue(PB) which have the formulas KFenIF e I(CN)6 andFenI4[Fen(CN)6]~ (1). The latter compound is pre-pared fr om a solution contain ing a large excess of fer-ric ion. In addition to PB itself, two additional, closelyrelated compounds are of interest. These are Everitt 'ssalt K2FenFeII(CN)6, which is colorless, and Berli nGreen FemFem(CN)~ (2). All three compo~mds havethe same basic cubic crystal st ruct ure consisting ofiron ions bridged by the cyanide groups, althoughsubtle s tructur al modifications may occur, depend ingon the stoichiometry (3, 4). The physical and chemicalproperties of these compounds have been reviewed byRobin and Day (5).We have discovered a method for preparing thinadherent films of PB on various metal and semicon-ductor substrates . The particular experiment we wishto discuss is concerned with a thin PB film on aplatinum foil electrode. The film is obtained from asolution containing equimolar amo unts of ferric chlo-ride and potassium ferricyanide. A one-to-one com-plex between ferric ion and ferri cyanide ions forms

K e y w o r d s : e l e c t r o c h e m i c a l o x i d a t io n , e l e c t r o c h e m i c a l r e d u c -t i o n , ] P r u s s i a n b l u e f i l m s .

in such solutions (6). The solutio n must be freshlyprepared because the complex will slowly polymerizeand eventually a colloidal precipitate of insolubleBerlin green is formed (7).A fres h ~oluti on 0.01M in re age nt grade FeC18 9 6H20and 0.01M in reagent grade KsFe(CN)6 was pre pared in

tripl y distil led water. A 1 cm~ pl ati num foil electrode,with platinum lead encased in Teflon, was cathodizedfor 1 hr at 1 mA in a 1M HC1 solution. The elect rodewas placed in the ferric ferricyanide solution and,after several minutes a blue f ilm was formed on thesurface. The electrode was removed from the solutionand rinsed thoroughly with dis ti l led water.A cyclic voltammogram of the PB electrode in quiet1M KC1 solution is shown in Fig. 1. The voltammo-gram was obtained with a PAR Model 173 potentio-stat equipped with a Model 175 voltage programmer.The interesting feature of this voltamm ogram is thaton the anodic side of the sweep (0.6V v s . SCE) theelectrode is bright blue, whereas on the cathodic side(0.0V v s . SCE) it is colorless. Furthermore it is pos-

sible to switch rapidly between the blue and colorlessstates with the selecting switch on the potentiostat.The electrode reaction evid entl y occurs in the filmitself and corresponds to the ultimat e reduction of PBto the colorless "Everit t 's salt."

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P R U S S I A N B L U E T H I N F I L M S 88 7

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VoL 125, No. 6

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Fig. 1. Single sweep voltammogram of a thin film of Prussianblue on platinum foil in 0.1M KCI solutio n. The scan rate is 10mV/sec.

A s e c o n d o b s e r v a t i o n i s c o n c e r n e d w i t h t h e o x i d a -t i o n o f t h e P B f i l m a t m o r e a n o d i c p o t e n t i a l s . A t a p o -t e n t i a l o f a p p r o x i m a t e l y 1 . 0 V vs. S C E t h e f i l m b e c o m e sg r e e n . I n t h i s ca s e t h e fi l m h a s a p p a r e n t l y b e e n o x i -d i z e d t o B e r l i n g r e e n . A g a i n t h e c o l o r c a n b e s w i t c h e dr a p i d l y b e t w e e n b l u e a n d g r e e n b u t t h e f i l m b e g i n s tod e t e r i o r a t e a f t e r s e v e r a l c y c l e s p r e s u m a b l y b e c a u s e o f

t h e s i m u l t a n e o u s o x i d a t i o n o f c h l o r i d e i o n a n d t h ef o r m a t i o n o f s u r f a c e o x i d e o n t h e p l a t i n u m .T h e d e t a i l s o f t h e s e e l e c t r o c h e m i c a l r e a c t i o n s a rec u r r e n t l y u n d e r i n v e s t i g a t i o n .M a n u s c r i p t s u b m i t t e d S e p t . 2 9 1 97 7; r e v i s e d m a n u -sc r ip t r e c e iv e d D e c . 1 , 1977.A n y d i s c u s s i o n o f th i s p a p e r w i l l a p p e a r i n a D i s c u s -s i o n S e c t i o n t o b e p u b l i s h e d i n t h e D e c e m b e r 1 9 7 8JOU RN AL. A l l d i sc uss ions fo r t he D e c e m be r 1978 D i s -

c uss ion Se c t ion shou ld b e sub mi t t e d b y A ug . 1 , 1978.Publication costs o~ this article were assisted byKent State University.R E F E R E N C E S1 . D . D a v ids on a nd L . A . We lo , J. Phys. Chem., 3~,1191 (1928).2 . J . F . D unc a n a nd P . W. R . Wig le y , J. Chem. Soc.,1120 (1963).3 . J . F . K e g in a nd F . D . Mi l e s , Nature (London), 137,577 (1~36).4 . A . Lud i a nd H . U . G ude l , Struct. Bonding (Berlin)14, 1 (1973).5 . M. B . Rob in a nd P . D a y , Adv. Inorg. Chem. Radio-chim., 10, 247 (1967).6 . J . A_. Ib ers ~nd N. D avid son , J. Am. Chem. Soc.,73, 476 (19 61).

7. D . E . K n a p p , P h . D. d i s s e r t a t i o n , K e n t S t a t e U n i -ve r s i t y , K e n t , O h io (1966 ) .

D I S C U S S I O NS E C T I O N I~ ~ ......

T h i s D i s c u s s i on S e c t i o n i n c l u d e s d i s c u s s io n o f p a p e r s a p p e a r i n gin t h e Journal of The ELectrochemical Society , Vol . 124 , No . 7 , 9 ,a n d 1 2, J u l y , S e p t e m b e r , a n d D e c e m b e r 1 97 7.

Fabricating Patterns in Tin Oxide Filmby Electrochemical Reaction

B. J. Baliga and S. K. Ghandhi(pp. 1059-1060, Vol. 124, No. 7)L F . C h a n g : l T h e t e c h n i q u e o f f ab r i c a t i n g p a t t e r n so f t in o x i d e f i l m s b y e l e c t r o c h e m i c e a l r e a c t i o n h a s a l sob e e n d e v e l o p e d i n o u r l a b o r a t o r y . 2 W e h a v e f o u n d t h a tt h e o v e r - a l l e t c h i n g r a t e o f t i n o x i d e f i l m i s n o t s i g -n i f i c a n t l y d e p e n d e n t o n t h e c o n c e n t r a t i o n o f H C1 inw a t e r a s s h o w n b y B a l i g a e~ al.3 H o w e v e r , w e w o u l d

l i k e to p o i n t o u t a n i m p o r t a n t f a c t c o n c e r n i n g t h ese l e c t ion o f a c r i t i c a l H C1 c onc e n t ra t i on .S i n c e t h e o v e r - a l l e t c h i n g p r o c e s s c o n s i s t s o f t w oc o n s e c u t i v e s t e p s : ( i ) e l e c t r o c h e m i c a l r e d u c t i o n o f t i no x i d e t o t i n ( t h e d a r k c o l o r a t i o n o f m e t a l t i n c a n b e d i -r e c t l y o b s e rv e d d u r i n g t h e e t c h i n g a n d a n a l y z e d b y i n -t e r r u p t i n g t h e e t c h i n g p r o c e s s ) ; a n d (ii) e t c h i n g o f t i nb y H C 1, i t i s n e c e s a r y t o s e l e c t t h e e l e c t r o l y t e c o m p o s i -t i o n s u c h t h a t t h e r e d u c t i o n r a t e o f t i n o x i d e ( s t e p i ) i sh i g h e r t h a n t h e e t c h i n g r a t e o f t i n m e t a l ( s t e p ii).T h i s s e l e c t io n g u a r a n t e e s t h a t a l l t h e d e t a i l p a t t e r n s t ob e e t c h e d a r e e t c h e d a w a y w i t h n o e l e c t r i c a l l y i s o -l a t e d o x i d e r e g i o n s f o r m e d o n ~ h e g l a s s s u b s t r a t e . W eh a v e f o u n d t h a t , f o r a c h i e v i n g b e s t e t c h i n g r e s u l t s ,t h e c o n c e n t r a t i o n o f H C I in w a t e r s h o u l d b e a p p r o x i -m a t e l y 7 % . A n e t c h e d p a t t e r n w i t h r e s o l u t i o n o n t h eo rde r o f 20 ~m i s show n in F ig . i .

I I B M T h o m a s J . W a t s o n R e s e a r c h C e n t e r , Y o r k t o w n H e i g h t s ,N e w Y o r k 1 0 5 9 8 .I . F . C h a n g , W . P. H o r n b e r g e r , a n d H . H . T a u b , IBM Tech. Dis-closure Bull., 16, 3463 (1974).~ B . J . B a l i g a a n d S . K . G h a n d i , This Journal , 124, 1059 (1977).

Fig. I. Etched pattern in tin oxide film on glass substrate byelectrochemical react ion technique (the ruler indicates 0.05 mmper division).

D u r i n g t h e c o u r s e o f o u r w o r k , w e h a v e a l s o d i s -c o v e r e d s o m e p r i o r a r t i n t h e p a t e n t l i t e r a t u r e 4,5 de-s c r i b i n g g e n e r a l m e t a l o x i d e e t c h i n g b y e l e c t r o c h e m i -c a l r e a c t ion .B . J . B a l i g a a n d S . K . G h a n d h i : 6 W e a g r e e w i t hC h a n g t h a t b e t t e r e t c h i n g c a n b e a c h i e v e d b y e n s u r i n gR. F . Shaw, US Pat . 3 ,507 ,759 (1970) .5 R. E . Szu p i l l o , US Pa t . 3 ,616,349 (1971) .G e n e r a l E l e c t r ic C o r p o r a t e R e s e a r c h & D e v e l o p m e n t , S c h e n e c -t a d y , N e w Y o r k 1 23 09 .

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