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JOU RN AL OF APPLIED ELECTRO CHEM ISTRY 25 (1995) 841-845

C a t h o d i c r e d u c t i o n a n d i n f r a r e d r e f l e c t a n c e s p e c t r o s c o p yo f b a s i c c o p p e r ( I I ) s a l t s o n c o p p e r s u b s t r a t eJ . Y . M A L V A U L T , J. L O P I T A U X , D . D E L A H A Y E , M . L E N G L E TUniver si t~ de Rouen, Laborator ie d 'Ana lyse , de Spec t roscopie e t de T rai t ement de Sur face des Matdr iau x , IU T ,76821 Mont -Sain t -Aignan Cedex , FranceRece ived 24 Oc tober 1994; revised 26 Janu ary 1995

I n f r a r e d r e f l e c t a n c e s p e c t r o s c o p y a n d c a t h o d i c r e d u c t i o n h a v e b e e n u s e d f o r t h e c h a r a c t e r i z a t i o n o fc o p p e r c o r r o s i o n p r o d u c t s f o r m e d b y a t m o s p h e r i c c o r r o s io n . T h e f o l l o w i n g b a si c c o p p e r ( II ) s a lt sw e r e s y n t h e s iz e d : a t a c a m i t e a n d p a r a t a c a m i t e ( C u 2 C I (O H ) 3 ) , b r o c h a n t i t e ( C u 4 S O 4 (O H ) 6 ) , p o s n j a k i t e( C u 4 S O 4 ( O H ) 6 . H 2 0 ) , m a l a c h i t e ( C u 2 C O 3 (O H ) 2 ) , g e r h a r d i t e (C u 2 N O 3 ( O H ) 3 ) . T h e s a m p l e s f o ra n a ly s i s w e r e o b t a i n e d b y i n c r u s t a ti o n o f s u b m i c r o n p a r t ic l es o f t h e c o m p o u n d o n t h e s u r f a c e o fO F H C c o p p e r c o u p o n s . I n f r a r e d r e fl e c t a n c e s p e c t r o s c o p y s h o w s t h a t , u n l i k e c o p p e r o x i d e s, th e r ef l ec -t i o n s p e c t r a o f th i n l a y e r s o f b a s i c c o p p e r ( I I ) s a lt s o n c o p p e r a r e s i m i l a r to t h o s e o b t a i n e d b y t h e u s u a lK B r - p e l l e t m e t h o d f o r t h e l o c a t i o n o f t h e b a n d s . H o w e v e r , s o m e d i f fe r e n c es o c c u r i n t h e in t e n s it i e s o ft h e b a n d s b e t w e e n t h e t w o m o d e s . I n f r a r e d r e f l e c ta n c e s p e c t r o s c o p y a ll o w s a n e a s y i d e n t if i c a t io n o fb a s i c c o p p e r ( I I ) s al ts : c h l o r i d e s c a n b e i d e n t if i e d b y t h e i r C u - O - H b e n d i n g m o d e s ; s u l f a te s , su l fi te ,n i t r at e a n d c a r b o n a t e b y t h e i n t e r n al m o d e o f t h e c o r re s p o n d i n g a n i o n (u 3 r e gi o n ). T h e c a t h o d i cr e d u c t i o n a n a l y s e s o f t h e s a lt s o n c o p p e r d i s p l a y o n l y a w i d e p e a k i n t h e ra n g e f r o m - 0 . 6 t o- 0 . 5 5 V v s S C E , i n d i c a t i n g t h a t o n e s t e p is i n v o l v e d . T h e p o t e n t i a l o f t h i s c a t h o d i c r e d u c t i o n p e a ki s d i s t i n g u i s h a b l e f r o m t h e o n e o f c o p p e r ( I ) o x id e s ( - 0 . 9 V v s S C E ) . T h e r e b y t h e r e l at i v e p r o p o r t i o n so f c o p p e r ( I I ) ) s al ts a n d c o p p e r ( I ) o x i d e s p r e s e n t i n t h e c o r r o s i o n l a y e r c a n b e d e t e r m i n e d .

1. IntroductionThe in t e r a c t ion o f m e ta l s w i th t he a tm osp he r e a nd theg r ow th o f su r f a c e f i lm s o f c o r r o s ion p r oduc t s ha veim por t a n t c onse que nc e s f o r t e c hno logy . The f o r m a -t ion of th in f i lms on meta ls an d a l loys used in e lec -t ronics and e lec t r ica l contac ts may inf luence thee l ec t ri c al a nd m a gne t i c p r ope r t ie s o f t he c om pone n t sand lead to fa i lure .The ana lys is of cor ros ion f i lms requires h ighlysens i t ive sur face techniques . The charac te r iza t ion ofa tm osphe r i c c o r r o s ion p r oduc t s de ve lope d on c oppe rc a n be c a r r i e d ou t by no nde s t r uc t ive m e tho ds l i ke lowa ng le X - r a y d i f f r a c t ion f o r t he de t e r m ina t ion o f c r y s-t a l l i ne pha se s a nd op t i c a l m e thods . P ho ton ba se dtechniques ( i . r . and Raman spec t roscopy) possessim por t a n t a dva n ta ge s ove r phys i c a l m e thods , l i keX P S , S I M S a nd e l e c t r on m ic r op r obe , ba se d on e le c-t rons or ions as probing par t ic les . Opt ica l methodsdo no t r e qu i r e h igh va c uum c ond i t i ons du r ing thea na lys i s a nd the r e f o r e c a n be u se d a t a m b ie n t a tm os -pher ic condi t ions . These techniques a ! low in situstudies [1, 2].C a thod ic r e duc t ion o f ox ide s f o r m e d on c op pe r ha sbe e n de sc r ibe d by Eva ns a nd M i l e y [ 3] . The se a u tho r ssugge s t t ha t t he i n i t i a l pa r t o f t he po te n t i a l - t im ec u r ve i s r e l a t e d to t he r e duc t ion o f c up r ous ox idea nd the s e c ond pa r t t o t he r e du c t ion o f t he c up r icoxide . Pops and Hen nessy [4] have prop osed a ga lva-nos t a t i c m e thod , ba se d on the s a m e c onc lu s ions , f o r

de t e r m in ing ox ide s f o r m e d on c oppe r i n t he w i r eindus t r y . The c a thod ic r e duc t ion t e c hn ique o f ox ideth i ckne s s de t e r m ina t ion i s c u r r e n t ly u se d in i ndu s t r ya s pa r t o f t he qua l i t y c on t r o l t e s t i ng p r og r a m a ndthe a f o r e m e n t ione d in t e rp r e t a t i on ha s be e n inc lude din a s tand ard spec i f ica t ion [5]. I t i s now wel l e s tab-l ishe d tha t t he spe c if ic a ti ons o f t he A S T M s t a nda r dB49-50 resul t in subs tant ia l e r rors due to an inco r rec tide n t if i c at i on o f t he tw o ox ide s C uO a nd C u20 ,[6-8] . Coulometr ic reduc t ion i s a technique used forqua n t i t a t i ve m e a su r e m e n t s o f t he t o t a l a m o un t o f c o r -r o s ion p r oduc t s . H ow e ve r , t he po te n t i a l a s soc i a t e dwith reduc ing each cor ros ion spec ies inc ludes con-s iderable e lec t rochem ica l over lap . The pu rpos e o fth is s tudy is to charac te r ize the copper ( I I ) sa l t sf o r m e d on c oppe r du r ing a tm osphe r i c c o r r o s ion bye l e c tr oc he m ic a l a nd op t i c a l m e thods .2. Experimental detailsThe bas ic coppe r ( I I ) sal t s were prepared accord ing toproced ures previou s ly publ ished: ch lor ides [9] , ca rbo -na te [10] , n i t r a te [11] and su l fa tes [12] . Th e samplesob ta ine d w e r e c ha r a c t e r i z e d by X - r a y d i f f r a c t ion .C oppe r ox ide s w e r e f o r m e d by the r m a l ox ida t ion o fc oppe r c oupons . P ow de r c oppe r ox ide s w e r e c om m e r -c i a l p r oduc t s w h ic h w e r e g r ound , t he subm ic r on icpar t ic les be ing separa ted by sedimenta t ion .The samples , ana lysed by inf ra red ( i . r . ) r e f lec -t a nc e spe c t ro sc opy , w e r e ob ta ine d by inc r us t a t i on o f

0021-891X 1995 Chap man & Hall 841

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8 4 2 J . Y . M A L V A U L T ET AL.

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2 0 0 0 1 7 0 0 1 4 0 0 1 1 O 0 8 0 0 5 0 0 2 0 0+ - W a v e n u m b e r / c m 1

F i g . 1 . I n f r a r e d s p e c t r a o f C u 4 ( O H ) 6 N O 4 : s p e c u l a r r e fl e c t a n c e o f at h i n l a y e r o f b r o n c h a n t i t e a t 8 0 ( a ) a n d t r a n s m i s s i o n i n K B r p e l l e t(b ) .s u b m i c r o n i c p a r t i cl e s o f t h e c o m p o u n d i n th e s u r f a c eo f O F H C c o p p e r c o u p o n s . T h e b u l k m e t a l s p e ci m e n sw e r e m e c h a n i c a l l y p o l i s h e d ( 1 # m g r a d e d i a m o n dp a s t e ) . T h e l a y e r f o r m e d w a s c h e c k e d w i t h a l o wa n g l e X - r a y d i f f r a c t o m e t e r u s i n g C u K ~ r a d i a t i o n( Ine l equip me nt , inc idence angle : 5) . The v ibra t iona ls p e c t r a w e r e r e c o r d e d o n a N i c o l e t F T I R 7 1 0 s p e c tr o -p h o t o m e t e r ( a n a l y z e d r a n g e 5 0 0 0 - 2 2 5 c m - 1 ) . T w ot e c h n i q u e s w e r e u s e d : t h e s t a n d a r d K B r p e l l e t t e c h -n i q u e a n d t h e s p e c u l a r r e f l e c t i o n t e c h n i q u e f o r s u r -face ana lys i s ( inc idence angle : 80) .

For e l ec t rochemica l ana lys i s , c i rcu la r copper e l ec -t r o d e s , 0 . 5 m m t h i c k w i t h 1 . 1 3 c m 2 s u r f a c e a r e a,w e r e u s e d . T h e e l e c t r o c h e m i c a l r e d u c t i o n c u r v e sw e r e d e t e r m i n e d u s i n g a s a m p l e . h o l d e r , a n e l e c tr o c h e -m i c a l c e ll fi t te d w i t h a p l a t i n u m c o u n t e r e l e c t r o d e a n da s a t u r a t e d c a l o m e l e l e c t r o d e ( S C E ) a s r e f e r e n c e .B e f o r e e a c h r u n t h e e l e c t r o l y t e ( 0 . 1 M N a z B 4 0 7 ;a t p H i 9 , t h e s o lu b i li ty o f t h e d if f er e n t c o p p e r

c o m p o u n d s , o x i d e s a n d h y d r o x i d e s i n t h i s s o l u t i o ni s m i n i m a l ) w a s c a r e f u l l y p u r g e d w i t h n i t r o g e n . T h er e d u c t i o n s p e e d w a s 0 . 5 m V s - 1 .3 . I R re f lec tance s pec t ros copy o f bas i c copper ( H ) s a l t s

T h e b e h a v i o u r o f t h e s p e c u l a r r e fl e c ta n c e o f t h in o x i -des f i lms on meta l s has been s tudied theore t i ca l ly ande x p e r i m e n t a l ly i n t h e i n f r a r e d r e g io n . T h e o p t i c a l c o n -s t a n ts a n d a b s o r p t i o n b a n d s a r e c a l c u l a t e d b y u si n gt h e m o d e l o f a s y s te m o f i n d e p e n d e n t a d s o r b e d h a r -m o n i c o s c i l l a t o r s . T h i s m o d e l i s d e d u c e d f r o m t h ec l as s ic a l t h e o r y o f b o u n d e l e c t ro n s a p p l i e d t o d i e l ec -t r i c mate r i a l s . An exce l l en t agreement be tween ca lcu-l a t e d a n d e x p e r i m e n t a l s p e c t r a i s o b s e r v e d f o r t h eC u 2 0 / C u s y s t e m [ 1 3] . T h e s p e c t r a a r e c l o s e ly d e p e n -d e n t o n t h e a n g l e o f in c i d e n c e o f th e r a d i a t i o n o nt h e s u r f a c e a n d o n t h e t h i c k n e ss o f o x i d e o n t h e m e t a l -l ic s u b s t r a t e b u t n o t o n t h e n o n s t o i c h i o m e t r y o f t h ec u p r o u s o x i d e ( th e r e f le c t an c e s p e c t r u m o f C u 3 0 2 i ss imi la r to tha t o f Cu2 0 [14, 15] ) . Never the les s , thein te rpre ta t ion of re f lec tance spec t ra is m ore d i f f i cu l tf o r s u c h s y s t e m s t h a n t h a t o f tr a n s m i s s i o n s p e c tr a ,s ince the e f fec t s of rad ia t ion inc idence angle , th i ck-nes s of oxide f i lms , op t i ca l cons tan t s of sur facep r o d u c t s a n d p h a s e c h a n g e s d u r i n g t h e r e f l e c t i o nproces s a re c r i t i ca l cons idera t ions in re f l ec tance spec-t roscopy. The re f l ec tance spec t ra d i f fe r in band loca-t i o n s a n d b a n d i n t e n s i t i e s f r o m t h e t r a n s m i s s i o ns p e c t r a f o r t h e o x i d e - m e t a l s y s t e m s .

The feas ib i l i ty of employing re f l ec tance inf ra reds p e c t ro s c o p y t o s t u d y t h e c o m p o s i t io n o f c o m p l e xf il m s o n m e t a l s u r f a c e s l ik e c h r o m a t e c o n v e r s i o n c o a t -i n g s h a s b e e n d e m o n s t r a t e d [ 16 ]. A p r e v i o u s s t u d y o ft h e Z n C r O 4 / F e a n d C r P O 4 / F e s y s te m s h as s h o w n t h a tthe C rO 2- a nd PO 3- ion s a re eas i ly ident i f ied by inf ra -red re f l ec tance spec t rosc opy unde r a near graz ing inc i-d e n c e b y t h e lo n g i t u d i n a l o p t i c a l c o m p o n e n t s o f th eu3 v ibra t ion , i .e ., a t 960 and 108 0cm -~ , respec t ive ly[ 17]. I t appe ars tha t the in te rpre ta t ionof the re f l ec tance spec t ra for these sys tems i s no t

T a b l e 1 . T r a n s m i s s i o n a n d s p e c u l a r r e fl e c ta n c e i .r . s p e c t r a o f b r o c h a n t i t e C u 4 ( O H ) 6 S O 4

O H s t r e t e h i n g f r e q u e n c i e s/ c m - 1 C u - O H b e n d i n g e n e r g w s /c m - 1 R e f3 5 9 6 3 5 7 2 3 7 0 4 3 3 8 8 3 2 7 2 9 9 0 9 4 5 8 7 8 780 734 [181

T 3 5 9 0 3 5 6 9 3 4 0 4 3 3 8 9 3 2 7 8 9 8 6 9 4 5 8 7 3 776 734 [19]3 5 8 4 3 5 6 3 3 3 8 5 3 2 7 4 9 8 5 9 4 4 8 7 6 7 8 0 7 3 4 *

R 3 5 5 6 3 3 8 2 3 2 7 2 9 8 7 9 4 7 8 7 3 7 8 1 7 4 3 *I n t e r n a l v i b r a t i o n s o f t h e S O ~ - i o n s /c m - 1 C u - O s t r et c h in g R e f

[171u 3 u 1 u 4 u 2 o r C u - O [18]1 1 2 9 1 1 2 3 1 1 1 7 1 0 9 0 9 9 5 6 3 0 6 0 4 513 485 420 338 318 [18]

T 1 1 1 9 1 0 8 8 9 8 6 6 2 9 6 0 2 510 485 420 338 315 [19]1 1 1 9 1 0 8 9 9 8 5 6 2 8 6 0 2 5 1 0 4 8 4 4 2 5 3 3 5 3 1 8

R 1 1 3 0 1 0 8 2 9 8 7 6 3 2 6 0 0 5 1 3 4 8 7 4 2 1 3 3 7 T , R t r a n s m i s s i o n o r r e f l e c ta n c e s p e c t r u m .3 5 9 6 , 1 1 2 9 m a i n b a n d s .* T h i s s t u d y .

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C A T H O D I C R E D U C T I O N A N D I R S O F B A S IC C u (I I) S A L T S O N C O P P E R 8 4 3Table 2. Specular reflectance i.r. spectra of basic copper(I1) salts in the 1500-300 cm I regionCu2(OH)3CI Cu4(OH)6S04 , H2 0 Cu2(OH)3NO 3 Cu2(OH)2CO 3Paratacamite Atacamite Posnjakite Gerhardite Malachite

985 987 (m) ]/948 (m) }921 917 |897 J

86 3830 848741

585 600(m)520 519

482 (m)455 448405 415

C u - O - H

1501 }1419 1425 u31356 1391

1121 ] 10981072 1 u2 1048 (m) 1049 (m)98 0

875 877 878817 (m)75 3

691 (m) 697629 (m)601 (m) 587509 (m) 510 (m) 523

50 542 736 433 030 3

c o m p l i c a t e d b y d i s t o r t e d b a n d s h a p e s a n d f r e q u e n c ys h i f t s f o r t h i n f i l m s , w h e n c o m p a r e d t o a b s o r p t i o ns p e c t r a o f t h e p u r e b u l k m a t e ri a ls .C o n s e q u e n t l y , o n e p u r p o s e o f t hi s p a p e r i s a s ys -t e m a t i c s t u d y , b y t h is t e c h n i q u e , o f t h e m a i n p r o d u c t sf o r m e d d u r i n g t h e a t m o s p h e r i c c o r r o s i o n o f c o p p e r .T h e t r a n s m i t t a n c e a n d r e f le c t an c e s p e c t r a o f t h eC u 4 ( O H ) 6 S O 4 c o m p o u n d ( b r o c h a n t i t e ) a r e p r e s e n t e din F ig . 1 . An exce l l en t agreement i s obse rved con-c e r n in g t h e v i b r a t i o n a l f r e q u e n c ie s o f O H s t re t c h -i n g m o d e s , in t e r n a l m o d e s o f th e S O ] - i o n s a n dC u - O - H bending mo des (Table 1). Neverthe less ,t h e i n te n s it y o f t h e C u - O - H b e n d i n g m o d e s a n dC u - O s t re t c h in g m o d e s d e c r e as e o n t h e re f le c ta n c espec t ra .

T h e t r a n s m i s s i o n s p e c t r u m p r e s e n t e d i n t h i s s t u d yi s i n a g r e e m e n t w i t h t h o s e p r e v i o u s l y p u b l i s h e d[18 , 19] . Acc ordin g to Lutz [19] , the in te rna l v ibra -t io n s o f th e S O l - ( u l , u2 a n d /"4) i o n s i n b r o c h a n t i t ea re over l apped, and even over l a id , a s we l l a sp a r t l y c o u p l e d w i t h t h e v i b r a t i o n s o f t h e s i x cr y s -t a l lographica l ly d i f fe rent OH a l so present in tha treg ion .

T h i s s p e c t r o s c o p i c s t u d y o f b r o c h a n t i t e a l l o w s u s t oconc lude tha t , un l ike oxide sys tems , the spec t ra oft h i n f il m s o f b a s ic c o p p e r ( I I ) s a lt s o n c o p p e r o b t a i n e di n t h e r e f le c t io n m o d e a r e s i m i la r t o t h o s e r e c o r d e d b yt h e u s u a l K B r p e l l e t m e t h o d .In th i s s tudy a s imi la r ana lys i s has been extendedt o o t h e r c o p p e r ( I I ) c o m p o u n d s ( T a b l e 2 ) . F r o mthe v ibra t ion a l f requenc ies of the copp er su l f it eC u 2 ( I ) S O 3 C u ( I I ) S O 3 , H 2 0 i t i s e v i d e n t t h a t t h e

f o r m a t i o n o f s u lf it es m a y b e c h a r a c t e r i z ed b y i . r.spec t roscopy [21] : V ibra t iona l f requenc ies for su l f i t eions in co pp er su lf i te [21] 1025, 1008, 977, 912, 636,5 6 4 a n d 5 1 9 c m - 1 .

In conc lus ion , the d i f fe rent bas ic copper ( I I ) s a l t sd e v e l o p e d o n c o p p e r d u r i n g a t m o s p h e r i c c o r r o s i o nmay be ident i f i ed by i . r . r e f l ec tance spec t roscopy.Specif ic frequencies are ass igned as fol lows: ( i ) Chlor-ides : ' C u - O - H ben ding mod es ; (i i) su lf it e, su l fa t es ,n i tr a t e, c a r b o n a t e : i n t e r n a l m o d e s o f t h e c o r r e s p o n d -ing an ion (u3 r e g io n ) . T h e O H s t re t c h i n g m o d e r e g i o nc a n n o t b e u s e d b e c a u s e o f a l a rg e o v e r l a p o b s e r v e df o r d i f f e re n t c o m p o n e n t s i n t h e s p e c t r a o f d i f fe r e n tc o m p o u n d s .

4. Electrochemical study4.1. Analysis of cuprous oxide films fo rme d by thermaloxidation or by incrustation o f submicronic particlesA br ie f ana lys i s of the exper ime nta l da ta , p revious lypubl i shed [7] shows tha t the e l ec t rochemica l reduc-t i o n o f c o p p e r o x i d e s in c o m p l e x l a y e r s g i v e s r is e t othe fo l lowing reac t ions :F o r C u OC u 2 + + e - > C u + i n t he r a ng e f r o m ]

- 0.60 to - 0.75 V / (1)(CuO)C u + + e - ~ C u n e a r - 0 . 8 0 V (2)(CuO)

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844 J.Y. MALVAULT E T A L .0 . 60 . 5

0 . 3 (~ 0 . 2

O l

- 0 . - 0 . 4 - 0 . 5 - 0 . 6 -0 . 7 - 0 . 8 -0 . 9 - 1 . 0 -1 . 1 - 1 . 2 - 1 . 3 - 1 . 4E / V vs S CE

F ig . 2 . E le c t roc he m ic a l r e duc t ion c u rve s o f c uprous ox ide s fo rm e db y t h e r m a l o x i d a t i o n o f O F H C c o p p e r s a m p l e s (a ) o r b y i n c r u s t a -t i o n o f C u 2 0 o n b u l k c o p p e r ( b ).and for copper(I) oxidesCu + + e- ~ Cu in the range from - 0.85to - 0.95 V (3)( C u 2 0 o r C u 3 0 2 )(An overlap of Reactions 2 and 3 may be observed:increasing thickness shifts the peak positions tomore negative potentials).

These results, in excellent agreement with those ofother authors [6], contradict the conclusions of Popsand Hennessy [4] who mistook CuO for Cu20 in thestudy of the reduction of oxide layers formed on cop-per wires (the same error is observed in the specifica-tion of the ASTM standard B49-50 [5]).

Figure 2 presents the electrochemical reductioncurves of cuprous oxide films formed by thermal oxi-dation of OFHC copper samples (a) or by incrusta-tion of Cu20 submicronic particles in the surface ofa circular mechanically polished sheet (b). In thetwo voltammograms the first minor peak near-0.5 V vs SCE is attributable to the precursor CuxO[22] and the major peak at -0.85V vs SCE to thereduction reaction Cu ++ e- ~ Cu of the nonstoichiometric cuprous oxides. The precursor ofcuprite already present on copper substrates hasbeen developed by thermal oxidation. From atomprobe, XPS and Auger studies, it can be concludedthat the precursor CuxO has a mixed valency charac-ter containing interstitial Cu scattered in the Cu(I)oxide phase [22]. At 473 K, three steps preceedingthe formation of CuO are always observed: forma-tion of the precursor CuxO and growth of thecuprous oxides according to reactions:

2 Cu + 02 ~ Cu203 C u 2 0 + 0 2 ~ 2 C u 3 0 2

> 0 . 1 00 . 0 90 . 0 8

, , 0 . 0 7E 0 . 0 6< 0 , 0 5E 0 . 0 4~ " 0 . 0 30 . 0 2

0 . 0 10- 0 . 4 - 0 , 5 - 0 . 6 - 0 . 7 - 0 . 8 - 0 , 9 - 1 . 0 - 1. 1 - 1 . 2

E / V vs S CEF i g . 3 . 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 a t h i n l a y e r o f b r o c h a n t i t ei n c r u s t e d o n b u l k c o p p e r .The study of the grazing X-ray diffraction diagramsand the electron probe microanalysis has revealedthe successive steps of copper oxidation [23], in agree-ment with dat a of reference [24]. Czanderna hasobtained a composition CUO0.67 (or Cu302) by lowtemperature oxidation (120-160C) of polycrystal-line [25] or ( 1 0 0) single crysta l films o f copper [26].This composition has been considered as a grossdefect structure of Cu20 corresponding to one cop-per vacancy per unit cell on the average. A deviationfrom stoichiometry of this magnitude has not beenreported previously and, in fact, Cu302 is probablya unique metastable phase. Cu302 and Cu20 maybe identified only by grazing X-ray diffraction andby optical methods (mainly photoluminescencemeasurements) [23] . I.r. reflectance spectra ofCu 20 /C u and Cu3 O2/Cu systems are similar and con-sist of thin oxide layers (0.1-0.2 #m) of one band inthe range 660-645 cm -1 (LO mode) [14, 15].

The formation of Cu302 by thermal oxidation at473 K is very rapid (the EPMA analysis of an ETPcopper sample oxidized 30 min at 473 K reveals tha tthe main const ituent o f the oxide layer is Cu302).Consequently, it may be concluded from this studythat the differentiation of the two cuprous oxidesCu20 and Cu302 by cathodic reduction is impossible.4.2. Catho d ic r educ t ion o f bas ic copper ( I I ) s a l t s on

me ta l l i c s ubs t r a te s .A volt ammogram observed for the reduction of a thinlayer of brochantite incrusted in the surface of coppersheet is shown on Fig. 3. The potential scans at0.5 mVs -1 in 0.1 M Na2B407 for the reduction of allbasic copper(II) salts incrusted in copper displayonly one broad peak in the range -0.60 to -0.55Vvs SCE indicating that one step is involved; seeTable 3.

Table 3.

Compounds Cu2(OH)3CI Cu4( On)6 so 4 Cu2(OIt)2CO3(on OF HC c oppe r )A t ac ami t e P ar a t ac ami t e B r oc han t i t e Mal ac h i t e

P e a k p o s i t i o n / V v s S C E - 0 . 5 5 - 0 . 6 1 - 0 . 5 6 - 0 . 5 8

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