The preparation and properties of metallic optically transparent electrodes

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The preparation and properties of metallic optically

transparent electrodes

Citation for published version (APA):

Janssen, L. J. J., & Kuhn, A. T. (1983). The preparation and properties of metallic optically transparent electrodes. Surface Technology, 20(1), 41-49. https://doi.org/10.1016/0376-4583(83)90075-4

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10.1016/0376-4583(83)90075-4 Document status and date: Published: 01/01/1983

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Surface Technology, 20 (1983) 41 - 49 41

T H E P R E P A R A T I O N A N D P R O P E R T I E S O F M E T A L L I C O P T I C A L L Y T R A N S P A R E N T E L E C T R O D E S

L. J. J. JANSSEN

Department o f Chemical Technology, Laboratory o f Electrochemistry, Eindhoven Uni- versity o f Technology, P.O. B o x 513, 5600 MB Eindhoven (The Netherlands)

A. T. KUHN

Department o f Biomaterials Science, Institute o f Dental Surgery, Eastman Dental Hospital, 256 Gray's Inn Road, London W C I X 8LD (Gt. Brilain)

(Received May 25, 1983)

S u m m a r y

T h e uses o f o p t i c a l l y t r a n s p a r e n t e l e c t r o d e s ( O T E s ) are b r i e f l y discussed t o g e t h e r with t h e d i f f e r e n t t y p e s o f O T E . T h e review is f o c u s e d on metallic O T E s and a s u r v e y o f the various t y p e s , t h e i r m e a n s o f p r e p a r a t i o n a n d t h e i r o p t i c a l a n d electrical p r o p e r t i e s is included.

1. I n t r o d u c t i o n

All t o o o f t e n , e l e c t r o c h e m i s t s or c o r r o s i o n scientists find t h e m s e l v e s , like t h e blind, c u r i o u s t o " s e e " a c t u a l l y w h a t is t a k i n g place at t h e i n t e r f a c e w h i c h is usually t h e f o c u s o f interest. A l t h o u g h " s e e i n g " m i g h t m e a n ac- t u a l l y viewing an e l e c t r o d e or an e l e c t r o d e p r o c e s s with t h e n a k e d e y e or w i t h a m i c r o s c o p e , it will m o r e p r o b a b l y involve t h e m o r e s o p h i s t i c a t e d m e t h o d s o f s p e c t r o s c o p y t o d e t e c t t h e p r e s e n c e o f a given species or to m o n i t o r its c o n c e n t r a t i o n with t i m e . It is n o w a c c e p t e d t h a t a v e r y l i m i t e d n u m b e r o f c o n c e p t u a l a p p r o a c h e s exist. T h e y i n c l u d e (a) specular reflec- t a n c e , (b) t r a n s m i s s i o n and (c) a t t e n u a t e d t o t a l r e f l e c t a n c e ( A T R ) . T h e c o n f i g u r a t i o n s f o r t h e s e are s h o w n in Fig. 1. S p e c u l a r r e f l e c t a n c e will n o t be c o n s i d e r e d f u r t h e r b e y o n d the o b v i o u s c o m m e n t t h a t , a l t h o u g h it m a k e s n o d e m a n d s on t h e p r o p e r t i e s o f the e l e c t r o d e itself, t h e i n c i d e n t and t h e r e f l e c t e d b e a m s o f light m u s t pass t h r o u g h a s o l u t i o n w h i c h will c e r t a i n l y a t t e n u a t e t h e m t o a g r e a t e r or t o a lesser e x t e n t . Nevertheless, various w o r k e r s [1] have m a d e r e m a r k a b l e strides in bringing this m e t h o d (over a wide w a v e l e n g t h b a n d ) t o t h e p o i n t w h e r e it is r o u t i n e l y used. In t h e o t h e r t w o a p p r o a c h e s , h o w e v e r , t h e m e t h o d is based o n t h e t r a n s m i s s i o n o f inci- d e n t a n d r e f l e c t e d light t h r o u g h an o p t i c a l l y t r a n s p a r e n t e l e c t r o d e ( O T E ) . T o achieve this, a m a t e r i a l or c o m b i n a t i o n o f materials is r e q u i r e d , a n d it is t h e a c h i e v e m e n t o f such a c o m b i n a t i o n t h a t will be c o n s i d e r e d h e r e .

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42 ( a ) C

iO aO

I-Z (b) (c)

Fig. 1. Three alternative configurations for speetroelectrochemistry: (a) specular reflec- tanee through solution (C, working electrode; a, counterelectrodes); (b) transmission (plan view) (C, platinum OTE; a, eounterelectrodes; b, reference electrode); (c) ATR using a germanium hemicylinder g with a platinum film OTE C (a, countereleetrode).

T w o p r e l i m i n a r y c o m m e n t s are r e q u i r e d . Firstly, even if a single material was f o u n d to possess t h e criteria o f high t r a n s m i s s i o n o f light and g o o d electrical p r o p e r t i e s (subjects w h i c h will be c o n s i d e r e d b e l o w ) it m i g h t be o f l i m i t e d i n t e r e s t in t h a t t h e m a j o r i t y o f e l e c t r o c h e m i c a l r e a c t i o n s r e l a t e t o a small n u m b e r o f e l e c t r o d e materials, n o t a b l y lead, m e r c u r y and plati- n u m , a n d (unless t h e m a t e r i a l in q u e s t i o n h a d r e m a r k a b l e e l e c t r o e a t a l y t i c p r o p e r t i e s ) m o s t i n t e r e s t , r i g h t l y or w r o n g l y , is in t h e s e materials. T h e s e c o n d c o m m e n t is t h a t , a l t h o u g h t h e t r a n s m i s s i o n c o n f i g u r a t i o n (Fig. l ( b ) ) is h i s t o r i c a l l y p r o b a b l y t h e o l d e s t and m o s t well established, it suffers t h e disadvantages o f b o t h t h e r e f l e c t a n c e m e t h o d

(i.e.

light passing t h r o u g h t h e s o l u t i o n ) and t h e O T E s y s t e m s (in w h i c h t h e e l e c t r o d e s are d i f f i c u l t t o p r e p a r e a n d n o t as r o b u s t as massive materials). O n l y t h e s i m p l i c i t y o f use, in t h a t h o p e f u l l y such cells can be i n s e r t e d i n t o c o m m e r c i a l U V - v i s i b l e s p e c t r o m e t e r s with m i n i m a l a d a p t a t i o n , can be set against t h e s e disadvantages. Given all t h e a b o v e f a c t o r s , an e n t i r e l y h e a l t h y " c o m p e t i t i o n " is e x p e c t e d b e t w e e n t h e users o f t h e various a p p r o a c h e s and, in this c o m p e t i - t i o n , d e v e l o p m e n t s in e l e c t r o n i c s

(e.g.

phase-sensitive d e t e c t i o n ) have over- w h e l m i n g l y f a v o u r e d t h e r e f l e c t a n c e a p p r o a c h . In a d d i t i o n , n o v e l materials ( p e r h a p s as " s p i n - o f f s " f r o m a i r c r a f t a p p l i c a t i o n s , solar e n e r g y d e v e l o p m e n t s o r e l e c t r o c h r o m i c displays) m i g h t be e x p e c t e d t o m a k e an i m p a c t on O T E d e v e l o p m e n t s , and o p t i c a l fibre t e c h n o l o g y c o u l d be e q u a l l y i m p o r t a n t . T o

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43

some e x t e n t , the choice o f approach is governed by the i n f o r m a t i o n desired in the e x p e r i m e n t . ATR is, by its nature, useful only to m o n i t o r the layer o f solution very close to the electrode surface. True transmission will e m b r a c e changes w h e t h e r at the surface or in bulk solution, as will specular reflec- tance. However, the use o f twin-beam s p e c t r o s c o p y t o offset background or solution absorbance is one means by which surface p h e n o m e n a can be em- phasized,

e.g.

as shown by Goelz and Heineman [2] who e x a m i n e d specific adsorption o f anions. Modulation of electrode potential, an approach highly developed by Bewick and coworkers, gives even m o r e accurate results.

2. Applications

Although OTEs have been used to view processes such as bubble evolution, their main i m p o r t a n c e must lie in spectroscopic analysis o f reaction p r o d u c t s or intermediates in solution or adsorbed species on the electrode surface. In either case, steady state or non-steady-state conditions m a y be o f interest. A review o f the literature shows t h a t n o t all workers have been aware o f the c o r r e c t use o f OTEs

(e.g.

their useful potential range) and in the present review their preparation, cells for their use and related m e t h o d o l o g y are considered.

3. T y p e s o f optically transparent electrode

Existing OTEs can be divided into three categories as follows.

(a) Thin c o n d u c t i n g films can be s u p p o r t e d on glass, quartz or special IR-transparent materials. Such films can be c o n d u c t i n g oxides {such as SnO2) or metals (platinum or gold). T h e y may be " d u p l e x " films where one metal film is deposited o n t o a n o t h e r metal film or o n t o an oxide layer.

(b) "Massive" OTEs are made of d o p e d silicon, germanium or o t h e r materials.

(c) "Minigrid" electrodes are those where a material such as gold is fabri- cated as a fine grid, exposing a high surface area to the t r a n s m i t t e d light.

We shall focus on the first o f these categories in the present review, and the criteria o f i m p o r t a n c e will be (a) the optical properties (transmission as a f u n c t i o n o f wavelength), (b) the electrical resistance of the film (in ohms per square) and (c) the chemical and electrochemical properties including the potential range over which the OTE can be e m p l o y e d and the electro- catalytic behaviour.

4. General characteristics o f thin film optically transparent electrodes

A most valuable review by Haacke [3] entitled " T r a n s p a r e n t con- ducting coatings" is mainly directed towards applications such as solar

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4 4

e n e r g y , liquid c r y s t a l d i s p l a y s a n d o t h e r t e c h n o l o g i e s using similar m a t e r i a l s . H a a c k e b e g a n w i t h a c o n s i d e r a t i o n o f t h e " f i g u r e o f m e r i t " a p p r o a c h t o t h e c l a s s i f i c a t i o n o f O T E s : T10 ~ T C = - - - Rs w h e r e q~TC is t h e figure o f m e r i t , T is t h e o p t i c a l t r a n s m i s s i o n a n d R s is t h e s h e e t r e s i s t a n c e o f t h e m a t e r i a l ( c o a t i n g ) . As he s h o w e d , t h e figure o f m e r i t c a n also b e e x p r e s s e d b y t h e e q u a t i o n ( ~ T C = o t exp(--10c~t) w h e r e o (~2 1 c m 1) is t h e electrical c o n d u c t i v i t y a n d c~ ( c m 1) is t h e o p t i c a l a b s o r p t i o n c o e f f i c i e n t . H a a c k e p r e s e n t e d a p l o t o f figure o f m e r i t v e r s u s film t h i c k n e s s f o r t h i n films o f c o p p e r , gold a n d C d z S n O 4 a n d f o r a " h y p o t h e t i c a l s e m i c o n d u c t o r " . T h e p e r f o r m a n c e o f t h e C d z S n O 4 is seen to a p p r o a c h t h a t o f a h y p o t h e t i c a l s e m i c o n d u c t o r . With s u c h an a p p r o a c h , t h e individual m a t e r i a l s c o n s i d e r e d as O T E s c a n be discussed. T h e s e m a t e r i a l s c a n be d i v i d e d i n t o m e t a l l i c ( a n d q u a s i - m e t a l l i c ) films o n t h e o n e h a n d a n d s e m i c o n d u c t o r s o n t h e o t h e r h a n d . 5. Metallic f i h n o p t i c a l l y t r a n s p a r e n t e l e c t r o d e s As H a a c k e p o i n t e d o u t , t h e m a n n e r in w h i c h t h i n films o f m e t a l are f o r m e d r e s u l t s in m u c h h i g h e r s h e e t r e s i s t a n c e values f o r such films t h a n m i g h t be e x p e c t e d f r o m d a t a o n b u l k c o n d u c t i v i t y . A n u m b e r o f t r i c k s e x i s t t o m i n i m i z e t h e island f o r m a t i o n o f c l u s t e r s o f m e t a l a t o m s w i t h i n t e r v e n i n g gaps; H a a c k e d e s c r i b e d a f e w o f t h e s e t r i c k s b r i e f l y , a l t h o u g h h e e m p h a s i z e d t h a t t h e p r o d u c t i o n o f such m e t a l l i c film O T E s is m a i n l y a m a t t e r o f undis- c l o s e d " k n o w - h o w " . It is g e n e r a l l y a c c e p t e d t h a t , f o r t h e v e r y small t h i c k - nesses in q u e s t i o n , t h e s e m e t a l films do n o t o b e y t h e B e e r - L a m b e r t law. H a e m a n [4] has s h o w n t h a t t h e l o g a r i t h m o f t r a n s m i s s i o n T (%) d e c r e a s e s l i n e a r l y w i t h t h e t h i c k n e s s ( f o r t h i c k n e s s e s g r e a t e r t h a n 0.1 - 0.2 p m ) w i t h a p r o p o r t i o n a l i t y f a c t o r w h i c h d e p e n d s on t h e m e t a l used. S o m e values o f t r a n s m i s s i o n t a k e n f r o m t h e l i t e r a t u r e f o r p l a t i n u m , gold, nickel a n d lead films 0.6 p m t h i c k are 0 . 0 8 % (at 550 n m ) , 5% (at 550 n m ) , 0 . 0 3 % (at 503 n m ) a n d 0.5% (at 503 n m ) r e s p e c t i v e l y . A m o n g t h e m a t e r i a l s w h i c h h a v e b e e n i n v e s t i g a t e d b y e l e c t r o c h e m i s t s are gold, p l a t i n u m a n d c a r b o n a n d v a r i o u s alloys. T h e p r o b l e m is to d e p o s i t a t h i n a n d c o h e r e n t film o f a m e t a l w i t h s u f f i c i e n t c o r r o s i o n r e s i s t a n c e t o allow its use o v e r a r e a s o n a b l y w i d e r a n g e o f p o t e n t i a l s . T h e t r a d e - o f f b e t w e e n t h i c k e r films ( i m p r o v e d electrical c o n d u c t a n c e ) a n d t h i n n e r films ( i m p r o v e d o p t i c a l t r a n s m i s s i o n ) is n o w evi- d e n t . A f u r t h e r d a n g e r is t h a t , a l t h o u g h t h e film r e m a i n s u n a t t a c k e d , it m a y be lifted a w a y f r o m t h e s u b s t r a t e b y h y d r o g e n gas e v o l u t i o n or b y similar p h e n o m e n a .

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The means by which metal film OTEs have been prepared have passed t h r o u g h several phases. Initially, a metal such as platinum was painted o n t o a substrate using a preparation in which the metal was dissolved in organic oils. A f t e r the metal had been painted on, it was fired. Workers such as Ports

e t al. [5] gave directions as t o the best means o f application o f the paint, the n u m b e r of.coats etc. Later [6] the t e c h n i q u e o f " s p i n n i n g " was e m p l o y e d to ensure the m o s t even application o f the paint. However, most recently, the p r e f e r r e d m e t h o d o f application has been some f o r m o f vacuum evaporation. Simultaneous with these ideas has been the recognition that annealing after application improves the electrical characteristics o f the film and also that, rather than deposition o f the metal directly o n t o glass or quartz, an im- proved result is obtained when the metal is deposited o n t o an oxide [7]. More r e c e n t l y [8], the use o f tin oxide has been advocated so t h a t n o t only do the general benefits o f oxides result but also the additional c o n d u c t i v i t y o f the oxide layer can be usefully e m p l o y e d .

Thus Ports e t al. [5] have described h o w a platinum OTE may be form- ed by brushing Liquid Platinum 1 (Engelhard Industries Ltd.) o n t o glass with a fine brush and by firing the film so f o r m e d at 650 - 680 °C, just below the fusion t e m p e r a t u r e o f the glass (borosilicate in their case) o n t o which it was applied. Either a single coat or successive applications and firings m a y be e m p l o y e d . Pons e t al. r e p o r t e d a n u m b e r o f tricks such as the i m p o r t a n c e o f the direction o f the brush stroke. The platinum paint m a y be t h i n n e d with a t h i n n e r such as d i c h l o r o m e t h a n e before application. Inspection u n d e r the m i c r o s c o p e shows t h a t these films are n o t quite u n i f o r m and t h a t t h e y contain holes. Annealing is thus beneficial as might be e x p e c t e d and results in an optical density o f a b o u t 0.4%, with a sheet resistance value o f 25 ~2/D. Even f u r t h e r details are contained in the paper by Van Benken and Kuwana [9] who m e n t i o n e d t h a t such OTEs are stable to anodization, boiling in HNO 3 or immersion in HC1 solution. However, even a brief period o f hydrogen evolution will suffice to lift o f f the film. Mercuric ions (although n o t m e r c u r y metal) are also damaging. Gold films have a lower electrical resistance than platinum films for the same optical density and a plot showing typical optical densities is given in Fig. 2. Pons e t al. [5] also suggested t h a t palladium films, especially when deposited o n t o KRS5 may have special use with a wider IR window (5 - 16 p m ) than those obtainable on germanium

1.0 b 0-5 0 I I I I I I 3 0 0 5 0 0 7 0 0 Wavelength (nm)

Fig. 2. Absorbance vs. wavelength: curve a, p l a t i n u m on quartz (electrical resistance, ~ / ~ ) ; curve b, gold on Bi203 on quartz (electrical resistance, 2.5 ~ / D ) . (After ref. 10.)

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46 ( 2 - 1 0 p m ) . T h e s a m e w o r k e r s s u g g e s t e d t h a t p a l l a d i u m m i g h t h a v e I R t r a n s p a r e n c y o v e r a w i d e r r a n g e (5 - 16 p m ) o n H R S 5 t h a n it has o n ger- m a n i u m (2 - 10 p m ) . V a n B e n k e n a n d K u w a n a [9] d e s c r i b e d t h e d e p o s i t i o n o f p l a t i n u m films d i r e c t l y o n t o b o r o s i l i c a t e or o t h e r glasses or q u a r t z b y e v a p o r a t i o n in a v a c u u m o f (1 - 7) X 10 3 T o r r . T h e glass was first c l e a n e d b y using a g l o w d i s c h a r g e . F o r gold t h e film was a p p l i e d o v e r a Bi203 or P b O 2 c o a t i n g . All films w e r e a n n e a l e d a f t e r use. T h e p l a t i n u m s u r f a c e s w e r e b r i g h t , u n i f o r m a n d silvery. T h e y were m e c h a n i c a l l y s t a b l e , e x c e p t w h e n o x i d i z e d a f t e r h y d r o g e n e v o l u t i o n f o r a f e w s e c o n d s o r d u r i n g t h e d e p o s i t i o n o f m e r c u r y . N o r m a l w a s h i n g a n d cleaning, b o i l i n g in c o n c e n t r a t e d H N O 3 or p r o l o n g e d i m m e r s i o n in HCI s o l u t i o n d i d n o t d i s t u r b t h e film, n o r did c o n t a c t w i t h m e t a l l i c m e r c u r y .

On t h e basis o f e q u a l r e s i s t a n c e , gold films w e r e m o r e t r a n s p a r e n t t h a n p l a t i n u m films. T h e u n d e r c o a t i n g u s e d w i t h gold films i m p r o v e d t h e i r m e c h a n i c a l a n d electric'a] c h a r a c t e r i s t i c s and, in this, t h e p r e s e n t a u t h o r s c a n be said t o h a v e a n t i c i p a t e d t h e P t - S n O 2 O T E o f L a i t i n e n [ 8 ] .

T h e c h a r a c t e r i s t i c s o f t h e s e films are seen in Figs. 2 a n d 3. T h e r e a p p e a r to be f e w e q u a l l y d e t a i l e d a c c o u n t s o f p r e p a r a t i o n a n d p r o p e r t i e s o f such e l e c t r o d e s . Nasielski e t al. [ 1 1 ] m e n t i o n e d gold e l e c t r o d e s p r e p a r e d b y " s u b l i m i n g gold o n t o glass". T h e a v e r a g e t h i c k n e s s was 50 n m a n d t h e t r a n s m i t t a n c e a t 5 5 0 n m was 25% -+ 5%. O n c e t h e p l a t i n u m a n d t h e gold O T E s h a d b e e n d e v e l o p e d , o t h e r t y p e s o f O T E f o l l o w e d . T h e H g - P t O T E is f o r m e d b y t a k i n g a p l a t i n u m O T E ( f o r m e d b y v a p o u r d e p o s i t i o n ) a n d e l e c t r o c h e m i c a l l y d e p o s i t i n g m e t a l l i c m e r c u r y o n t o it f r o m a d i l u t e a q u e o u s s o l u t i o n . A r e l a t i v e l y t h i n l a y e r o f m e r c u r y ( 5 - 50 n m ) i m p a r t s t o t h e p l a t i n u m a " m e r c u r y - l i k e c h a r a c t e r " ,

i,e. t h e high h y d r o g e n e v o l u t i o n o v e r v o l t a g e w h i c h allows c a t h o d i c p r o - cesses t o be s t u d i e d at this e l e c t r o d e is u n h i n d e r e d b y h y d r o g e n f o r m a t i o n . H e i n e m a n a n d K u w a n a [ 1 2 ] d e s c r i b e d t h e p r e p a r a t i o n o f p l a t i n u m O T E s ( r e s i s t a n c e , a b o u t 10 gZ c m - 2 ) , w h i c h w e r e w a s h e d a n d dried a n d t h e n c l e a n e d in a p l a s m a d i s c h a r g e f o r 5 m i n . A f t e r this t h e p l a t i n u m O T E s w e r e i n s e r t e d i n t o a cell w h i c h was filled w i t h a 0.5 m M s o l u t i o n o f Hg2(NO3) 2 in an a c e t a t e b u f f e r o f p H 4. This was d e o x y g e n a t e d f o r 30 m i n a n d t h e O T E was s w i t c h e d f r o m o p e n c i r c u i t t o a p o t e n t i a l o f 0 . 0 0 V m e a s u r e d w i t h r e s p e c t t o a 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 . T h e o p t i c a l r e s p o n s e was 1 I 0 1 2 3 4 ['ime (h) (a) (b) ~ 2 o 1 0 I ~ l 0 1 2 3 4 'l'i m ~ ( h )

Fig. 3. The effect of annealing on platinum films: (a) optical absorbance vs. time; (b) electrical resistance vs. time. (After ref. 10.)

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m o n i t o r e d at 609 nm and the c u r r e n t passing at the same time was inte- grated to allow an estimation of the charge o f the m e r c u r y deposition and hence its thickness. A c o r r e c t i o n for the background c u r r e n t was made in this case. 10 mC o f m e r c u r y per square c e n t i m e t r e (corresponding to a layer a b o u t 15 nm thick) yielded a mercury-like OTE. Heineman and Kuwana [13] also described a cell design which allows admission o f a d e o x y g e n a t e d solution directly into the cell, and a f u r t h e r m o d i f i c a t i o n is shown [14] in which the electrode area is r e d u c e d t o 3 mm 2 and the hold-up volume o f e l e c t r o l y t e is r e d u c e d to a b o u t 1 ml. Heineman and Goelz [15] have described the application of an H g - P t OTE in the transmission m o d e to moni- t o r the specific absorption of anions. A more detailed a c c o u n t of the same work was s u b s e q u e n t l y r e p o r t e d [2]. Goelz and Heineman [16] have re- p o r t e d a s t u d y in which these electrodes were characterized in terms o f their electrochemical behaviour on v o l t a m m e t r i c stripping and their appearance u n d e r the optical microscope. F o r a useful stable electrode, Goelz and H e i n e m a n advocated a m e r c u r y layer 5 nm thick. Transmission values o f 85% are shown. Optical m i c r o s c o p y taken in c o n j u n c t i o n with the vol- t a m m e t r i c data shows h o w excess m e r c u r y deposition leads to globules o f the metal on the surface. These globules can be r e m o v e d by anodic oxidation, leaving behind the P t - H g intermetallic phase which is the desired structure. Discussing the longevity o f these OTEs, Goelz and Heineman suggested t h a t t o o m u c h m e r c u r y leads to amalgamation o f the platinum all the way down to the glass substrate, resulting in peeling. If just sufficient m e r c u r y is deposited to f o r m the amalgam, but w i t h o u t the f o r m a t i o n o f droplets, a lifetime o f 8 h or more can result. In addition to their use in the transmission approach, the use o f P t - H g OTEs in internal reflection s p e c t r o s c o p y has also been described [17] using a commercially available cell unit. Data are shown for m e t h y l e n e blue, using a single reflection and time-averaging methods.

The greater o p a c i t y o f metals to light o f longer wavelength led to problems with IR-transparent OTEs. Mattson and Smith [18] described a t t e m p t s by earlier workers t o o v e r c o m e such problems. Metal films o f less than a p p r o x i m a t e l y 3 nm thickness are IR transparent. However, t h e y do n o t have sufficient electrical conductivity. For this reason, Mattson and Smith devised the carbon film OTE using a germanium internal reflection element. The carbon was coated by a commercial organization and, when deposited, had a resistance o f 2000 - 5000 ~ / D for a film a b o u t 30 nm thick, corresponding to a value greater than t h a t of bulk graphite by a f a c t o r o f 10. In their paper [18] a comparison is made between background scans f o r platinum, carbon and germanium prism OTEs and data for p r o t e i n a c e o u s films and o t h e r organic species are shown. The carbon films afford t r a n s p a r e n c y over the entire range o f use with the substrate (germanium). Large ger- m a n i u m prisms with 13 reflections at the G e - C interface gave transmissions from 4% to 8%. Germanium microprisms with t h r e e reflections gave 15% transmission. U n c o a t e d prisms gave 25% - 30% transmission.

A nickel OTE has been used by Janssen and coworkers [19, 20]. These are also commercially prepared by the deposition o f nickel (0.35 × 10 -4 ram)

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o n t o an SnO2 (7 × 10 4 m m ) l a y e r on a glass s u b s t r a t e . T h e t r a n s m i s s i o n was a b o u t 12% a n d t y p i c a l electrical r e s i s t a n c e s w e r e 10 - 20 Yt/[:]. E l e c t r o l y t i c b u b b l e f o r m a t i o n ( b o t h h y d r o g e n a n d o x y g e n ) o n t h e s e e l e c t r o d e s was v i e w e d t h r o u g h a m i c r o s c o p e . K O H s o l u t i o n s o f u p t o 7 M w e r e u s e d at t e m p e r a t u r e s f r o m a m b i e n t t o 80 °C. G e n e r a l l y , t h e s e e l e c t r o d e s lasted f o r at least 1 d a y a n d b e c a u s e o f its c h e m i c a l s t a b i l i t y an N i - S n O 2 e l e c t r o d e was m o s t u s e f u l f o r o x y g e n e v o l u t i o n [ 2 1 ] .

T h e A u - C r O T E has also b e e n used b y o n e o f us ( L . J . J . J . ) to s t u d y h y d r o g e n b u b b l e f o r m a t i o n in K O H . This e l e c t r o d e was b a s e d on an u n d e r - l a y e r o f c h r o m i u m 10 n m t h i c k w i t h 40 n m o f gold a b o v e . T h e t r a n s p a r e n c y was 9% a n d t h e e l e c t r i c a l r e s i s t a n c e was 2.7 YZ/[N. H o w e v e r , o x y g e n evolu- t i o n led to lifting o f t h e gold film f r o m t h e surfaces.

De Angelis e t al. [ 1 4 ] d e s c r i b e d a c a r b o n O T E d e p o s i t e d o n t o glass or q u a r t z a n d a m e r c u r y - c o a t e d v e r s i o n o f a similar e l e c t r o d e . As in t h e p r e v i o u s w o r k o f M a t t s o n a n d S m i t h [ 1 8 ] t h e e l e c t r o d e s w e r e p r e p a r e d b y a c o m m e r - cial o r g a n i z a t i o n w h i c h u s e d e l e c t r o n b e a m e v a p o r a t i o n o f c a r b o n . F i l m r e s i s t a n c e s o f 1 0 0 0 - 1 7 0 0 ~ / [ ] w e r e o b t a i n e d , a n d t h e H g - C O T E b e h a v e d e l e c t r o c h e m i c a l l y in a s i m i l a r w a y to m e r c u r y itself. F i n a l l y , t h e use o f O T E s in n o n - a q u e o u s m e d i a s h o u l d be m e n t i o n e d , t h e p a p e r b y Osa a n d K u w a n a [ 2 2 ] b e i n g an e x c e l l e n t o v e r v i e w o f this a r e a o f t h e s u b j e c t . O t h e r i n f o r m a t i o n p e r t a i n i n g t o n o n - a q u e o u s s o l v e n t s a n d O T E s c a n be f o u n d in ref. 10. In this s h o r t p a p e r t h e " h a r d w a r e " a s p e c t s o f m e t a l l i c O T E s h a v e b e e n r e v i e w e d ( s e m i c o n d u c t i n g t y p e s i n c l u d i n g t h e i m p o r t a n t S n O 2 t y p e s h a v e n o t b e e n c o v e r e d here). In a d d i t i o n , t h e electrical a n d e l e c t r o c h e m i c a l p r o p - erties o f O T E s , w h i c h basically r e s e m b l e t h o s e o f t h e p a r e n t m e t a l s , w i t h t h e c a v e a t t h a t h y d r o g e n e v o l u t i o n ( a n d p r e s u m a b l y also o x y g e n e v o l u t i o n ) m a r k s t h e d a n g e r z o n e s in w h i c h t h e y c a n n o t be used, h a v e also b e e n discussed. F o r a d e s c r i p t i o n o f cells i n c o r p o r a t i n g t h e s e O T E s a n d t h e m e t h o d - o l o g y u n d e r l y i n g t h e i r use, e s p e c i a l l y f o r t h e d e r i v a t i o n of t r a n s i e n t d a t a , o n e o f t h e fullest, b e s t a n d m o s t d e t a i l e d a c c o u n t s is given in ref. 10, to w h i c h t h e r e a d e r is r e f e r r e d a n d f r o m w h i c h s o m e i n f o r m a t i o n has b e e n o b t a i n e d f o r this s h o r t review.

Acknowledgment

T h e a u t h o r s are g r a t e f u l to Dr. Alan B e w i c k f o r his v a l u a b l e c o m m e n t s on this p a p e r .

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