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Thick oxide growth on gold in base
J. Electroanal. Chem., 125 (1981) 387--399 Elsevier Sequoia S.A., Lausanne -- Printed in The Netherlands
387
T H I C K O X I D E G R O W T H O N G O L D IN B A S E
LAURENCE D. BURKE and MARY McRANN Chemistry Department, University College, Cork (Ireland) (Received 15th September 1980; in revised form 20th February 1981)
ABSTRACT The formation of thick oxide films (ca. 100 monolayer equivalents at 2.3 V) on a gold anode was observed under steady-state polarization conditions in base. As reported by other authors for acid, the layer produced was of duplex character with a compact, largely anhydrous inner film at the metal surface, and a much thicker, porous, highly hydrated outer film at the oxide--solution interface. These deposits were readily distinguished, and quantified coulometrically, due to differences in reduction potential under cathodic sweep conditions. The onset of thick film growth occurred in a potential region where linear Tafel behaviour is observed for oxygen gas evolution on gold, and changes in oxygen coordination of cations in the outer region of the compact layer under these conditions is assumed to enhance the conversion of material in the outer region of the latter to the more hydrated form. As compared with acid, oxide growth was much slower (and reached a limiting value) in base, the outer film was reduced less readily and inhibition of oxygen gas evolution in the thick film region (at ca. 2.35 V) was observed. The results were accounted for by assuming increasing hydroxide ion coordination by cations in the hydrous layer with increasing pH -- the more highly coordinated species being thermodynamically more stable but kinetically less capable of rearranging to form a crystalline product. The relationship between the hydrous film on gold and those produced on a wide range of other transition metals is briefly outlined.
INTRODUCTION I n t e r e s t in the e l e c t r o c h e m i c a l o x i d a t i o n o f n o b l e metals has increased in r e c e n t years with the a d v e n t o f p o t e n t i a l c y c l i n g t e c h n i q u e s [ 1 - - 3 ] for p r o d u c ing t h i c k o x i d e films w h i c h display interesting charge storage, e l e c t r o c a t a l y t i c [4,5] and e l e c t r o c h r o m i c [6] b e h a v i o u r . T h e l o w a f f i n i t y o f gold for o x y g e n is d e m o n s t r a t e d b y the fact t h a t o x i d e f o r m a t i o n on this m e t a l [7] in acid o n l y c o m m e n c e s above 1.30 V (HE), a p p r e c i a b l y higher t h a n the reversible o x y g e n p o t e n t i a l in a q u e o u s media. F u r t h e r m o r e , o x y g e n gas e v o l u t i o n on this m e t a l is s t r o n g l y inhibited; it o n l y b e c o m e s appreciable [8] above a b o u t 2.0 V (HE) where, again in acid, it is a c c o m p a n i e d b y t h e f o r m a t i o n o f a t h i c k o x i d e film [9--11]. C o m p a r e d with t h e w o r k in acid, little a t t e n t i o n has b e e n d e v o t e d as y e t t o o x i d e f o r m a t i o n o n gold in base. A c c o r d i n g t o H o a r e [ 1 2 ] , o n l y a m o n o l a y e r o f A u : O 3 is p r o d u c e d o n a n o d i z i n g in alkaline solution, the latter b e c o m i n g y e l l o w on p r o l o n g e d a n o d i z a t i o n due t o the p r o d u c t i o n o f soluble a u r a t e species. R e c e n t w o r k b y Arvia and c o - w o r k e r s [ 1 3 , 1 4 ] has s h o w n t h a t at p o t e n tials l o w e r t h a n t h a t associated with m o n o l a y e r o x i d e f o r m a t i o n ( w h i c h is 0022-0728/81/0000---0000/$02.50 © 1981 Elsevier Sequoia S.A.
388 slightly lower for gold in base as compared with acid [15] ), a reversible reaction, involving adsorbed OH species, takes place. According to Kirk and coworkers [ 16], the three peaks observed in this region when using fast (> 1 V s -1) potential sweep techniques are related to hydroxyl formation at the three main crystal faces, (110), (100) and (111), at the gold surface. The present work is concerned largely with oxide formation on gold in base at higher potentials. Multilayer oxide growth can occur in this region to give a duplex film; however, unlike the behaviour in acid [ 11 ], the rate of thick oxide growth under potentiostatic conditions in base decreases with time, or with increasing film thickness, apparently due to a gradual change in the nature of the outer film. EXPERIMENTAL Both working and counter electrodes consisted of gold wire (1 mm diam., ca. 1 cm 2 exposed geometric area, 99.999% pure, Koch--Light Laboratories) sealed directly into soda glass. A three-compartment cell, with a sintered glass disc separating the working and counter compartments, was used. The potential of the working electrode was measured (and is quoted) with respect to a hydrogen electrode in the same solution. A Luggin capillary, with a glass tip placed a b o u t 1 mm away from the surface of the working electrode, was used to minimize the effect of potential drop across the electrolyte. Solutions were made up using AnalaR grade chemicals and triply distilled water; the liquid in the working electrode c o m p a r t m e n t was stirred almost continuously with a flow of purified nitrogen gas. The solutions used included 1.0 mol dm -3 H2SO4, pH = 0.5; 1.0 mol dm -3 NaOH, pH = 14.0; 1.0 mol dm -3 H3PO4 (+ NaOH), pH = 1--10. Potential control was achieved with the aid of a potentiostat (Wenking, Model 68 FR 0.5) controlled either by a linear sweep generator (Hewlett Packard, Model 3310A) or, for work involving intermediate oxide growth at constant potential, a stepping potentiometer (Wenking, Model SMP 69). Currents were recorded as a potential drop across a standard resistor and the extent of oxide growth was measured in terms of the charge associated with the reduction peak. Oxygen evolution currents in base at high potentials were in the region of 10 mA (surface passivation for this reaction occurs in the thick film region [8]); since the resistance between the working electrode and the tip of the Luggin capillary was a b o u t 1 ~2, the maximum iR error in the recorded potential values is about 0.01 V. RESULTS A typical cyclic voltammogram for gold in base is shown in Fig. 1. Surface oxidation c o m m e n c e d at a lower potential in base as compared with acid, and the three well-established peaks for monolayer oxide growth on gold in the latter [17] were not observed with the solution of high pH. A further broad peak above 1.5 V (peak maximum at ca. 1.8 V) in base, did not appear to be due to oxide film growth. For example, plots of the charge for oxide reduction vs. oxide formation potential (see Figs. 6 and 8) showed no sudden iricrease in coverage in this region. Since peak currents at 1.8 V varied considerably from
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Fig. 1. Cyclic v o l t a m m o g r a m (40 m V s -1 ) for gold in n i t r o g e n - s t i r r e d 1.0 m o l d m -3 N a O H at 25°C. Fig. 2. P e a k s p l i t t i n g d u r i n g t h e course o f o x i d e r e d u c t i o n ( c a t h o d i c sweep s p e e d = 50 m V s -1 ) in: (a) 1 m o l d m -3 H2SO4 w h e r e t h e film was g r o w n at 2.2 V for 10 s; (b) 1.0 m o l d m -3 N a O H w h e r e t h e film was g r o w n at 2.4 V for 10 s. T = 25Oc.
one run to another it appeared likely that some other surface process, e.g. oxygen gas evolution or metal dissolution, was involved -- the surface becoming deactivated for this process above 1.8 V. Marked increases in oxide film thickness, and a linear Tafel slope for oxygen gas evolution on gold in both acid and base, only occurred above 2.0 V. Investigation of the growth of these thicker films was complicated by splitting of the reduction peak. As illustrated in Fig. 2, two, and occasionally three, reduction peaks were observed. The second peak appeared most readily in acid where the minimum oxide formation potential required to observe this type of behaviour was in the region of 2.1 V; it appeared somewhat less readily with base (pH ~ 14), and least readily of all in a phosphate buffer of intermediate pH. Peak splitting for the reduction process was observed more clearly in all cases on raising the oxide formation potential and extending the duration of the oxide growth reaction. In general, reduction of oxide films grown at high potentials in acid gave only two cathodic peaks. In the case of phosphate buffer solution a further peak appeared as a narrow shoulder on the anodic side of the second oxide reduction peak for films grown above 2.3 V. Similar results were obtained for
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Fig. 4. T y p i c a l Tafel p l o t s for o x y g e n gas e v o l u t i o n o n gold in 1.0 m o l d m -3 N a O H , T = 25°C. T h e p o t e n t i a l was a l t e r e d in 10 m V steps at a rate of 1 step m i n -1 a n d t h e c u r r e n t d e n s i t y values given are t h o s e at t h e e n d o f t h e 1 m i n period. T h e r e a c t i v a t i o n o f t h e surface w h i c h c o m m e n c e d at a b o u t 2.48 V o n t h e s u b s e q u e n t c a t h o d i c sweep was a c c o m p a n i e d b y d e t a c h m e n t o f a t h i c k o x i d e layer.
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Fig. 5. Variation of the potential of the oxide reduction m a x i m a with solution pH at 25°C: (o) first oxide reduction peak; (A) second oxide reduction peak. Oxide grown at 2.2 V for 10s. Fig. 6. Effect of oxide formation potential on oxide growth (as measured by the charge associated with subsequent reduction) on gold in 1.0 m o l d m -3 NaOH at 25°C: (o) total oxide; (A) oxide reduced at the first peak; (D) oxide reduced at the second peak. The oxide formation potential was maintained in each case for a period of 1 rain.
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Fig. 7. Effect of oxide growth time on the extent of oxidation of a gold electrode in 1.0 m o l d m -3 NaOH at 25°C. Oxide formation potentials: (D) 2.4 V; (o) 2.3 V; (~) 2.2 V.
392
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Fig. 8. Effect of oxide formation potential on the total charge associated with reduction of oxide films of limiting thickness (oxide growth period = 10 min) in the ease of a gold electrode in 1.0 mol dm-3 NaOH at 25°(?.
1.0 mol dm -3 NaOH on extending the oxide growth period to longer times (5=--10 min). While the height of the shoulder altered little with increasing oxide formation time, the charge associated with the final oxide reduction peak increased considerably under these conditions. The variation of these reduction peak potentials with oxide formation conditions for gold in base is summarized in Fig. 3. Some results for oxygen gas evolution on gold in base are illustrated in Fig. 4. With increasing potential a linear Tafel region was observed at about 2.2 V; it is worth noting that the onset of inhibition at more anodic values almost coincided with the appearance of the shoulder, or intermediate peak, in the cathodic stripping experiments. The potential difference between the two main r e d u c t i o n peaks increased with increasing pH. As illustrated in Fig. 5, this increased separation arises largely from the rapid cathodic drift of the second reduction peak with increasing pH. The slope of the upper line in this diagram is --6.5 mV per unit pH, while that of the lower is --26.5 mV per unit pH. Oxide coverage, as measured by the charge associated with the reduction peaks, is given as a function of oxide formation potential in Fig. 6. The charge associated with the first reduction peak increased almost linearly with potential and within a range of 1--10 min was virtually independent of the time for oxide growth. The sharp increase in oxide growth above 2.0 V (HE) is associated mainly with material which is reduced in the course of the second cathodic peak. The increase in total oxide reduction charge with time for oxide growth is illustrated in Fig. 7 for three different oxide formation potentials. Similar plots for lower values of the latter showed lower charge values which reached constant levels at shorter times. At higher oxide formation potentials [ 2.3 and 2.4 V (HE)], the charge associated with the second reduction peak increased rapidly with oxide formation time over the first 5 min -- however, the values in
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Fig. 9. E f f e c t o f p H o n t h e e x t e n t o f o x i d e g r o w t h a t a gold e l e c t r o d e in a q u e o u s s o l u t i o n at 25QC. P r i o r t o r e d u c t i o n t h e gold was a n o d i z e d for 10 s at t h e f o l l o w i n g p o t e n t i a l values:
(-) 2.5 v; (A) 2.4 v; (o) 2.3 v; (o) 2.2 v; (A) 2.1 V; (o) 2.0 V.
each case approached a limit after a growth period of a b o u t 10 min. Figure 8 is a plot of these limiting oxide coverages in base as a function of the oxide formation potential. A plot of the charge associated with oxide reduction, taken after a relatively short period of oxide growth, as a function of electrolyte pH for a range of oxide formation potentials (Fig. 9), clearly demonstrated that thick anodic film growth on gold occurred most readily in highly acidic solution. DISCUSSION
The processes involved in the initial stages of metal oxidation have been discussed recently by Conway [18]. Radical species such as OH and O are produced as a chemisorbed layer.on the metal surface -- this has been confirmed in the case of gold in acid by Gol'dshtein and co-workers [19]. The chemisorption is sensitive to the crystallographic orientation of the electrode surface [ 17] -hence the three peaks in the monolayer oxide formation region of gold at low pH. With increasing coverage, oxidation becomes progressively more irreversible in character owing to rearrangement of the adsorbed layer, i.e. place-exchange reactions occur in which O and OH species replace gold ions in the outer layers of the metal surface. Cathodic reduction of monolayer oxide films on gold occurs by an island mechanism [ 7], i.e. oxide removal proceeds by o u t w a r d two-dimensional reaction from reduction centres which probably correspond to flaws or weaknesses in the original oxide film.
394 The absence of the three peaks in the oxide formation region of gold in base (Fig. 1) suggests that the mechanism of surface oxidation in this medium differs from that in acid. Electrochemical measurements [13,16,20] indicate that oxygen species chemisorb on gold in solutions of high pH at potentials well below the main anodic peak. According to Kirk and co-workers [16] a monolayer of hydroxyl species is produced in this region. They observed three peaks (at about --0.2 to +0.3 V, +0.3 to 0.7 V, and +0.7 to +1.3 V respectively) which were attributed to the influence of crystallographic orientation at the metal surface on the activity of the chemisorbed radicals. The main anodic peak above 1 V (Fig. 1) in this case probably corresponds to the conversion of surface hydroxide (AuOH) to the higher oxide (Au203) or hydroxide (Au(OH)3). Before considering the mechanism of thick oxide growth on gold it may be useful to estimate values for the thickness of such layers. This is frequently carried o u t using the charge involved in the formation or reduction of the film and assuming a value in the region [7,17] of 0.38 mC cm -2 (real area) for the charge associated with an oxygen monolayer on polycrystalline gold. Complications arising in base which limit the accuracy of this technique include the previously mentioned incomplete oxide to metal interconversion, which could result in a maximum error factor of 2 at low coverage, and changing surface roughness -- especially in the thick film region. Ideally a further correction should be applied for double-layer charging; however, since the value involved is uncertain, and usually rather small, it was ignored in the present case. Assuming monolayer oxygen coverage at 1.5 V - - t h i s corresponds to the minimum just above the main anodic peak (Fig. 1) -- a charge of 1.2 mC cm -2 at this point (Fig. 6) gives a roughness factor of 3--6 (depending u p o n whether the film in base is regarded as being totally or only half reduced). Similarly, small roughness factors for polycrystalline gold have been reported by Schultze [21]. Assuming that this factor remains constant even over the thick film region, the compact inner layer increased almost linearly with potential (Fig. 6) from about 1 to 4.3 oxygen monolayers on going from 1.5 to 2.4 V. The charge for total oxide reduction-in the limiting case (Fig. 8) at 2.4 V was a b o u t 128 mC cm -2, giving a net oxide coverage of a b o u t 107 monolayers. The equation for Au203 film thickness (d) quoted b y Dickertmann and co-workers [17] is d = QcVm/r6F
(1)
where Vm is the molar volume taken as 40 cm 3 mo1-1. If the mean roughness factor r in the present case is taken as 4.5, then at 2.4 V the thickness of the compact film is estimated, to be a b o u t 1.3 nm, while the maximum value for the outer layer (in terms of c o m p a c t Au203) is a b o u t 18.4 nm. In considering the limitations of this approach it should be borne in mind that the outer layer is probably not only highly hydrated but also microporous, i.e. at best the present approach gives only rough relative, rather than absolute, film thickness values. According to Schultze [21] the inner compact film on gold is practically anhydrous while the outer film is extensively hydrated. Oxide growth involves outward migration of Au 3÷ ions through the c o m p a c t layer [11,21], the thickness of the latter increasing almost linearly (Fig. 6) with increasing electrode potential, i.e. with increasing drop in chemical potential across the inner film.
395 Above 2.0 V the cations in the outer region of the film undergo a change in coordination state and rearrange to give a porous film. It is n o t certain whether this rearrangement involves total or only partial detachment of cations in the outer region. Total detachment, i.e. a dissolution--precipitation mechanism [ 11,21 ], should favour gold dissolution, e.g. aurate formation in base. Since the latter reaction, as examined by testing the cell solution by atomic absorption spectroscopy after electrolysis in the thick film region, was found to be almost negligible, the alternative -- oxide hydration without dissolution -- is worth considering. The hydration of the anhydrous film is similar in many ways to a phase transformation process [22] where the reaction is usually slow due to the high activation energy associated with breaking of primary coordination bonds. It should, however, be borne in mind that above 2.0 V cations in the outer region of the oxide film are involved in oxygen gas evolution and the mechanism of the latter reaction involves a surface redox process. According to Krasil'shchikov [23] this reaction includes (among others) the two steps: Au203 + O- = Au204 + e-
(I)
and Au204 -+ Au203 + O
(II)
In step (I) the primary coordination state of the Au(111) cations in the initially anhydrous film is altered, and in the subsequent decomposition of the unstable oxide (step (II)) loss of oxygen may involve its replacement by H20 or OHspecies to give hydrated or hydroxylated oxyeations. The latter may reoxidize and evolve further oxygen via steps (I) and (II), or rearrange to form the nucleus of an outer hydrated film. A further factor influencing this rearrangement may be physical changes induced at the anode surface by vigorous evolution of gas bubbles. It is well known [24] that freshly prepared h y d r o u s oxides are highly reactive and alter with time from an amorphous to a more regular structure. Among the possible reactions involved in this latter process are aggregation phenomena (combination of tiny nuclei to form larger aggregates), recrystallization (resulting not only in more stable structures within the micropartieles but also increasing the bonding between the latter and decreasing the internal surface area of the agglomerates), Ostwald ripening (growth of larger particles at the expense of smaller ones) and chemical interaetion (e.g. polymerization of oxycation species due to oxygen-bridging reactions). The net result is a porous outer film of hydrated oxide microparticles and an inner c o m p a e t layer, the latter continuously maintained during the growth of the outer layer b y the presence of a gradient in ehemical potential which is confined largely to the anhydrous film. It m a y be noted here that dissolution--precipitation reactions may occur during the rearrangement of the hydrated film, i.e. as a side-reaction. Formation of dissolved species cannot, therefore, be regarded as conclusive evidence that the sole mechanism for hydrated film formation in this t y p e of reaction involves a dissolution--precipitation (or dissolution--hydrolysis) mechanism. The influence of pH both on the kinetics of thick film growth (Fig. 9) and the ease of reduction of the resulting film (Fig. 5) is quite interesting. With hydrous films in general [25], as with many hydrated ions in aqueous solution [26], hydrolysis can occur due to loss of protons (or gain of hydroxide ions) on
396
raising the pH of the aqueous phase. It is well established in analytical chemistry [ 24] that the rearrangement velocity of freshly prepared h y d r o u s oxides decreases with increasing number of cation-coordinated hydroxide groups. A more rapid rearrangement at low pH (and, hence, a more crystalline product) would explain the faster rate of hydrated film formation under these conditions (Fig. 9). The rate of formation of the hydrated layer in acid is known [11] to be largely independent of time or film thickness -- evidently under these conditions the rearrangement of the outer layer is so fast that the rate-determining step is the formation of the hydrous layer from the c o m p a c t material. In base, on the other hand, inhibition of hydrous oxide rearrangement, due to the increased hydroxide coordination, apparently results in a more amorphous film. A decreasing abiIity of water or hydroxide ions to penetrate through the latter would explain both the lower rate of hydrous oxide formation (Fig. 9) and the inhibition of the oxygen gas evolution process (Fig. 4) on gold in base at high pH. A less hydrated variety of the outer oxide (which may well be the main inhibiting species) is apparently produced -- the evidence for this at present is the appearance of the intermediate peak in the reduction process for layers grown above 2.2 V (Fig. 3). Such a peak is not observed in acid. As outlined in the next paragraph, increased hydroxide incorporation (or, perhaps more accurately, increased stabilization of the hydrated o x y h y d r o x i d e gold complex aggregates in the outer layer) can also account for the unusual potential--pH behaviour of these hydrous films. The shift in peak potential with pH, especially that for the hydrous film (lower curve in Fig. 5), is unusual, as reversible oxide potentials are normally independent of pH when measured with respect to a reversible hydrogen electrode in the same solution. We have discussed this problem recently [25] in the case of hydrous films produced in iridium, rhodium and iron, where shifts similar to those observed with gold were noted. With these other metals the shift is clearly of thermodynamic origin as, among other features, peak potentials (e.g. in the case of iron) are virtually independent of sweep rate. Such results were attributed to the effect of hydrolysis on the activity of the transition metal species in the hydrous layer. In the case of gold the situation is somewhat more complex as the process involved is an oxide--metal rather than an oxide--oxide transition. Current results in the case of gold show that all oxide reduction peaks in both acid and base tend to more anodic values with decreasing sweep rate (500--5 mV 's-t). While there may be ohmic and kinetic contributions to this peak shift, the main factor (as in the case of the charge storage films on iridium, etc.) appears to be of thermodynamic origin. For instance, the rate of increase in peak potential On decreasing the rate was found to be greater for the reduction of the compact than for the hydrous layer (especially in base). Furthermore, the large separation between the peak values for the reduction of the hydrous film in acid and base did n o t decrease on lowering the sweep speed. The increased stability of hydrous transition metal species in base, including n o t only iridium, rhodium and iron, but also lead species in homogeneous solution [27], appears to be due to the ability of coordinated species such as water molecules to hydrolyse according to the reaction: Pb(OH)2H2OI ~ Pb(OH)~ + H ÷
(III)
397
This change in coordination (which could also occur by direct hydroxide ion incorporation) apparently only occurs with highly hydrated films. The stability of the resulting complex is obviously enhanced on increasing pH, and, in principle, the resultant lowering of the Pb(II) or Au(III) activity would explain the unusual lowering of the reduction potential for the hydrous species -- irrespective as to whether the latter is present in solution or in the porous film. The variation in activity in the present case extends over the entire pH range -possibly there is interaction between the cation sites in the film (which must be electronically interconnected) resulting, as in related materials [ 28], in the loss of individuality in the hydrolysis processes. To explain a shift of approximately --88.5 mV, i.e. 3/2 (2.303 R T / F ) , per pH unit (with respect to a pH-independent reference electrode), the electrode reaction must involve t h e transfer of a ratio of three protons (or h y d r o x i d e ions) to t w o electrons. Structural determination of these hydrous films is likely to be quite difficult owing to their amorphous nature [ 29] and their tendency to desiccate under high vacuum conditions [ 30]. One possibility, which can at least explain the electrochemical data and is n o t unreasonable in terms of the bonding in hydrous oxide films, is that the material is present as a non-linear polymer or aggregate of structure: I
.2°
o ( 1OH \o/1\o
.2°
t-120
H J|
n
While stressing the tentative nature of this proposed structure, its analogy with those proposed for isopolymolybdates and isopolytungstates [31 ] (where linking of octahedral hydrated oxyhydroxides to yield a range of aggregation products is well known) is worth noting. Reduction of such a species according to the reaction [--Au20~(OH)3 3 H20--]n + 9n H ÷ + 6 n e - = 2 n Au + 9n H20 (acid)
(V)
or
[--Au20~(OH)3 3 H20--]n + 6 h e - = 2 n Au + 9n O H - (base)
(VI)
would account for the observed potential--pH shift. Furthermore, the extensive hydration would be expected to inhibit extensive cross-linking between the chains and, hence, explain the open structure and the absence of e x t e n d e d electronic band structure in this t y p e of film [32]. With regard to the peak shift for the inner layer (lower curve in Fig. 5), it is interesting that a similar small potential--pH shift was n o t e d recently for a minor peak at low potentials in the case of rhodium [33]. Although these inner compact films are far less hydrated, the small shift could be due to hydrolysis effects associated with a limited a m o u n t of hydrous material in this region. We believe that the increased stability of the hydrous as compared with the compact film is an important factor in the growth of charge storage films on transition metals under potential cycling conditions, especially in base. In many instances the lower limit used in such experiments results in major reduction of
398 the compact layer -- thus resulting in activation of the metal surface. The significantly lower reduction potential for the hydrous film ensures that the latter can accumulate at the surface unless the lower limit is pushed to significantly more cathodic values. Although it was suggested earlier [ 11] that the anodically formed oxide layers on gold were unique, it now appears in fact that they have m a n y similarities with thick oxide films grown on other transition metals. For example, thick film growth occurs on platinum in acid [34] at potentials above 2.0 V; the layer is again of duplex character as shown by the splitting of the reduction process. Furthermore, m o l y b d e n u m at low potential in base [35], like gold in acid [ 11], can be oxidized with formation of a thick film at a rate which is independent of time or film thickness. On the other hand, thick oxide growth on various other metals (e.g. iridium [1] and tungsten [36] in acid, or rhodium [3], nickel [37] and iron [38] in base) occurs under potential cycling conditions at a rate which decreases with increasing film thickness -- behaviour similar to that observed in the present case with gold at constant potential in base (Fig. 7). In general, it appears that when these metals are oxidized in aqueous media under conditions where dissolution is negligible, a largely anhydrous oxide, of limited thickness, is produced initially. Conversion to a somewhat more stable hydrous oxide can in some cases (Pt and Au) occur under vigorous oxygen evolution conditions, but in most cases extensive reduction of the compact film seems necessary. Possibly in the latter case a more dispersed layer of metal atoms is produced on the electrode surface which is then oxidized more extensively on the subsequent anodic sweep. On further cycling the latter material is converted to the hydrous layer while the compact oxide is renewed by further oxidation of the underlying metal. Limiting hydrous oxide thickness in all cases is probably due to limited access of H20 and OH- species to the region close to the metal surface.
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D.N. B u c k l e y anti L.D. B u r k e , J. C h e m . Soc. F a r a d a y 1, 71 ( 1 9 7 5 ) 1 4 4 7 . D.N. B u c k l e y , L.D. Burke a n d J . K . M u l c a h y , J. C h e m . Soc. F a r a d a y 1, 72 ( 1 9 7 6 ) 1 8 9 6 . L.D. B u r k e a n d E.J.M. O'Sullivan, J. E l e c t r o a n a l . C h e m . , 93 ( 1 9 7 8 ) 11. S. G o t t e s f e l d a n d S. Srinivasan, J. E l e c t r o a n a l . C h e m . , 86 ( 1 9 7 8 ) 89. L.D. B u r k e a n d E.J.M. O'Sullivan, J. E l e c t r o a n a l . C h e m . , 97 ( 1 9 7 9 ) 123. J.D.E. M e I n t y r ¢ , W.F. P e c k a n d S. N a k a h a r a , J. E l e c t r o c h e m . Soc., 127 ( 1 9 8 0 ) 1 2 6 4 . J.W. S c h u l t z e a n d K.J. V e t t e r , Bet. Bunsenges. Phys. C h e m . , 75 ( 1 9 7 1 ) 4 7 0 . J . J . M a c D o n a l d a n d B.E. G o n w a y , Proe. R o y . Soc., A 2 6 9 ( 1 9 6 2 ) 4 1 9 . G. G r u n e b e r g , E l e e t r o c h i m . A c t a , 10 ( 1 9 6 5 ) 3 3 9 . D.A.J. R a n d a n d R. Woods, J : E l e c t r o a n a l . C h e m . , 31 ( 1 9 7 1 ) 29. M.M. L o h r e n g e l a n d J.W. S c h u l t z e , E l e c t r o c h i m . A c t a , 21 ( 1 9 7 6 ) 9 5 7 . J.P. H o a r e , The E l e c t r o c h e m i s t r y o f O x y g e n , Interseience, New Y o r k , 1 9 6 8 , p. 48. R. C o r d o v a O., M.E. Martins a n d A.J. A r v f a , E l e c t r o c h i m . A c t a , 25 ( 1 9 8 0 ) 4 5 3 . R. C o r d o v a O., M.E. Martins a n d A.J. Arvfa, J. E l e c t r o c h e m . Soc., 1 2 6 ( 1 9 7 9 ) 1 1 7 2 . D.E. I e e n h o w e r , H.B. U r b a c h a n d J . H . H a r r i s o n , J. E l e c t r o c h e m . Soc., 1 1 7 ( 1 9 7 0 ) 1 5 0 0 . D.W. Kirk, F . R . F o u l k e s a n d W.F. G r a y d o n , J. E l e c t r o c h e m . Soc., 1 2 7 ( 1 9 8 0 ) 1 0 6 9 . D. D i c k e r t m a n n , J.W. S e h u l t z e a n d K.J. V e t t e r , J. E l e c t r o a n a l . C h e m . , 55 ( 1 9 7 4 ) 4 2 9 . B.E. C o n w a y a n d H. A n g e r s t e i n - K o z l o w s k a , in A.D. F r a n k l i n (Ed.), E l e c t r o c a t a l y s i s at Non-Metallic S u r f a c e s , N.B.S. Special P u b l i c a t i o n 4 5 5 , U.S. G o v e r n m e n t Printing Office, W a s h i n g t o n , 1 9 7 6 , pp. 107--124. 19 M.D. G o l ' d s h t e i n , Ts.I. Z a l k i n d a n d V.I. Veselovskii, E l e c t r o k h i m i y a , 8 ( 1 9 7 2 ) 6 0 6 a n d 9 ( 1 9 7 3 ) 699. 20 J.P. C a r t a n d N.A. H a m p s o n ° J. E l e c t r o c h e m . Soc., 119 ( 1 9 7 2 ) 3 2 5 .
399 21 J.W. S c h u l t z e , E l e c t r o c h i m . A c t a , 17 ( 1 9 7 2 ) 4 5 1 . 2 2 C . N . R . R a o a n d K . J . R a o i n H. Reiss ( E d . ) , P r o g r e s s in S o l i d - S t a t e C h e m i s t r y , Vol. 4, P e r g a m o n Press, O x f o r d , 1 9 6 7 , C h . 4. 2 3 A.I. K r a s i l ' s h c h i k o v , R u s s . J. P h y s . C h e m . , 3 7 ( 1 9 6 3 ) 2 7 3 . 2 4 I.M. K o l t h o f f a n d E . B . S a n d e l l , T e x t b o o k o f Q u a n t i t a t i v e I n o r g a n i c A n a l y s i s , C o l l i e r - M a c M i l l a n , L o n d o n 3 r d e d n . , 1 9 5 2 , C h . 7. 2 5 L . D . B u r k e , M.E. L y o n s , E . J . M . O ' S u l l i v a n a n d D.P. W h e l a n , J . E l e c t r o a n a l . C h e m . , 1 2 2 ( 1 9 8 1 ) 4 0 3 . 2 6 J. B u r g e s s , M e t a l I o n s i n S o l u t i o n , Ellis H o r w o o d , C h i c h e s t e r , E n g l a n d , 1 9 7 8 , p p . 2 5 9 - - 3 0 9 . 27 J. L i n g a n e , C h e m . R e v . , 2 9 ( 1 9 4 1 ) 1. 2 8 S. H a d z i - J o r d a n o v , H . A n g e r s t e i n - K o z l o w s k a , M. V u k o v i c a n d B . E . C o n w a y , J. E l e c t r o c h e m . S o c . , 125 (1978) 1471. 2 9 D. M i t c h e l l , D . A . J . R a n d a n d R . W o o d s , J. E l e c t r o a n a l . C h e m . , 6 4 ( 1 9 7 7 ) 1 1 7 . 3 0 S . H . G l a r u m a n d J . H . M a r s h a l l , J. E l e c t r o c h e m . S o c . , 1 2 7 ( 1 9 8 0 ) 1 4 6 7 . 31 K . - H . T y t k o a n d O. G l e m s e r , A d v . I n o r g . C h e m . R a d i o c h e m . , 19 ( 1 9 7 6 ) 2 3 9 . 32 L.D. Burke and E.J.M. O'Sullivan, J. Electroanal. Chem., 117 (1981) 155. 3 3 L . D . B u r k e a n d E . J . M . O ' S u l l i v a n , J. E l e c t r o a n a l . C h e m . , in p r e s s . 3 4 J. Balej a n d O. S p a l e k , C o l l e c t . C z e c h . C h e m . C o m m u n . , 3 7 ( 1 9 7 2 ) 4 9 9 . 3 5 T h . H e n m a n n a n d G. H a n c k , Ber. B u n s e n g e s . P h y s . C h e m . , 71 ( 1 9 6 7 ) 4 0 4 . 36 L.D. Burke, T.A.M. Twomey and D.P. Whelan, J. Electroanal. Chem., 107 (1980) 201. 3 7 L . D . B u r k e a n d D.P. W h e l a n , J . E l e c t r o a n a l . C h e m . , 1 0 9 ( 1 9 8 0 ) 3 8 5 . 3 8 L . D . B u r k e a n d O . J . M u r p h y , J. E l e c t r o a n a l . C h e m . , 1 0 9 ( 1 9 8 0 ) 3 7 9 .
387
T H I C K O X I D E G R O W T H O N G O L D IN B A S E
LAURENCE D. BURKE and MARY McRANN Chemistry Department, University College, Cork (Ireland) (Received 15th September 1980; in revised form 20th February 1981)
ABSTRACT The formation of thick oxide films (ca. 100 monolayer equivalents at 2.3 V) on a gold anode was observed under steady-state polarization conditions in base. As reported by other authors for acid, the layer produced was of duplex character with a compact, largely anhydrous inner film at the metal surface, and a much thicker, porous, highly hydrated outer film at the oxide--solution interface. These deposits were readily distinguished, and quantified coulometrically, due to differences in reduction potential under cathodic sweep conditions. The onset of thick film growth occurred in a potential region where linear Tafel behaviour is observed for oxygen gas evolution on gold, and changes in oxygen coordination of cations in the outer region of the compact layer under these conditions is assumed to enhance the conversion of material in the outer region of the latter to the more hydrated form. As compared with acid, oxide growth was much slower (and reached a limiting value) in base, the outer film was reduced less readily and inhibition of oxygen gas evolution in the thick film region (at ca. 2.35 V) was observed. The results were accounted for by assuming increasing hydroxide ion coordination by cations in the hydrous layer with increasing pH -- the more highly coordinated species being thermodynamically more stable but kinetically less capable of rearranging to form a crystalline product. The relationship between the hydrous film on gold and those produced on a wide range of other transition metals is briefly outlined.
INTRODUCTION I n t e r e s t in the e l e c t r o c h e m i c a l o x i d a t i o n o f n o b l e metals has increased in r e c e n t years with the a d v e n t o f p o t e n t i a l c y c l i n g t e c h n i q u e s [ 1 - - 3 ] for p r o d u c ing t h i c k o x i d e films w h i c h display interesting charge storage, e l e c t r o c a t a l y t i c [4,5] and e l e c t r o c h r o m i c [6] b e h a v i o u r . T h e l o w a f f i n i t y o f gold for o x y g e n is d e m o n s t r a t e d b y the fact t h a t o x i d e f o r m a t i o n on this m e t a l [7] in acid o n l y c o m m e n c e s above 1.30 V (HE), a p p r e c i a b l y higher t h a n the reversible o x y g e n p o t e n t i a l in a q u e o u s media. F u r t h e r m o r e , o x y g e n gas e v o l u t i o n on this m e t a l is s t r o n g l y inhibited; it o n l y b e c o m e s appreciable [8] above a b o u t 2.0 V (HE) where, again in acid, it is a c c o m p a n i e d b y t h e f o r m a t i o n o f a t h i c k o x i d e film [9--11]. C o m p a r e d with t h e w o r k in acid, little a t t e n t i o n has b e e n d e v o t e d as y e t t o o x i d e f o r m a t i o n o n gold in base. A c c o r d i n g t o H o a r e [ 1 2 ] , o n l y a m o n o l a y e r o f A u : O 3 is p r o d u c e d o n a n o d i z i n g in alkaline solution, the latter b e c o m i n g y e l l o w on p r o l o n g e d a n o d i z a t i o n due t o the p r o d u c t i o n o f soluble a u r a t e species. R e c e n t w o r k b y Arvia and c o - w o r k e r s [ 1 3 , 1 4 ] has s h o w n t h a t at p o t e n tials l o w e r t h a n t h a t associated with m o n o l a y e r o x i d e f o r m a t i o n ( w h i c h is 0022-0728/81/0000---0000/$02.50 © 1981 Elsevier Sequoia S.A.
388 slightly lower for gold in base as compared with acid [15] ), a reversible reaction, involving adsorbed OH species, takes place. According to Kirk and coworkers [ 16], the three peaks observed in this region when using fast (> 1 V s -1) potential sweep techniques are related to hydroxyl formation at the three main crystal faces, (110), (100) and (111), at the gold surface. The present work is concerned largely with oxide formation on gold in base at higher potentials. Multilayer oxide growth can occur in this region to give a duplex film; however, unlike the behaviour in acid [ 11 ], the rate of thick oxide growth under potentiostatic conditions in base decreases with time, or with increasing film thickness, apparently due to a gradual change in the nature of the outer film. EXPERIMENTAL Both working and counter electrodes consisted of gold wire (1 mm diam., ca. 1 cm 2 exposed geometric area, 99.999% pure, Koch--Light Laboratories) sealed directly into soda glass. A three-compartment cell, with a sintered glass disc separating the working and counter compartments, was used. The potential of the working electrode was measured (and is quoted) with respect to a hydrogen electrode in the same solution. A Luggin capillary, with a glass tip placed a b o u t 1 mm away from the surface of the working electrode, was used to minimize the effect of potential drop across the electrolyte. Solutions were made up using AnalaR grade chemicals and triply distilled water; the liquid in the working electrode c o m p a r t m e n t was stirred almost continuously with a flow of purified nitrogen gas. The solutions used included 1.0 mol dm -3 H2SO4, pH = 0.5; 1.0 mol dm -3 NaOH, pH = 14.0; 1.0 mol dm -3 H3PO4 (+ NaOH), pH = 1--10. Potential control was achieved with the aid of a potentiostat (Wenking, Model 68 FR 0.5) controlled either by a linear sweep generator (Hewlett Packard, Model 3310A) or, for work involving intermediate oxide growth at constant potential, a stepping potentiometer (Wenking, Model SMP 69). Currents were recorded as a potential drop across a standard resistor and the extent of oxide growth was measured in terms of the charge associated with the reduction peak. Oxygen evolution currents in base at high potentials were in the region of 10 mA (surface passivation for this reaction occurs in the thick film region [8]); since the resistance between the working electrode and the tip of the Luggin capillary was a b o u t 1 ~2, the maximum iR error in the recorded potential values is about 0.01 V. RESULTS A typical cyclic voltammogram for gold in base is shown in Fig. 1. Surface oxidation c o m m e n c e d at a lower potential in base as compared with acid, and the three well-established peaks for monolayer oxide growth on gold in the latter [17] were not observed with the solution of high pH. A further broad peak above 1.5 V (peak maximum at ca. 1.8 V) in base, did not appear to be due to oxide film growth. For example, plots of the charge for oxide reduction vs. oxide formation potential (see Figs. 6 and 8) showed no sudden iricrease in coverage in this region. Since peak currents at 1.8 V varied considerably from
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one run to another it appeared likely that some other surface process, e.g. oxygen gas evolution or metal dissolution, was involved -- the surface becoming deactivated for this process above 1.8 V. Marked increases in oxide film thickness, and a linear Tafel slope for oxygen gas evolution on gold in both acid and base, only occurred above 2.0 V. Investigation of the growth of these thicker films was complicated by splitting of the reduction peak. As illustrated in Fig. 2, two, and occasionally three, reduction peaks were observed. The second peak appeared most readily in acid where the minimum oxide formation potential required to observe this type of behaviour was in the region of 2.1 V; it appeared somewhat less readily with base (pH ~ 14), and least readily of all in a phosphate buffer of intermediate pH. Peak splitting for the reduction process was observed more clearly in all cases on raising the oxide formation potential and extending the duration of the oxide growth reaction. In general, reduction of oxide films grown at high potentials in acid gave only two cathodic peaks. In the case of phosphate buffer solution a further peak appeared as a narrow shoulder on the anodic side of the second oxide reduction peak for films grown above 2.3 V. Similar results were obtained for
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1.0 mol dm -3 NaOH on extending the oxide growth period to longer times (5=--10 min). While the height of the shoulder altered little with increasing oxide formation time, the charge associated with the final oxide reduction peak increased considerably under these conditions. The variation of these reduction peak potentials with oxide formation conditions for gold in base is summarized in Fig. 3. Some results for oxygen gas evolution on gold in base are illustrated in Fig. 4. With increasing potential a linear Tafel region was observed at about 2.2 V; it is worth noting that the onset of inhibition at more anodic values almost coincided with the appearance of the shoulder, or intermediate peak, in the cathodic stripping experiments. The potential difference between the two main r e d u c t i o n peaks increased with increasing pH. As illustrated in Fig. 5, this increased separation arises largely from the rapid cathodic drift of the second reduction peak with increasing pH. The slope of the upper line in this diagram is --6.5 mV per unit pH, while that of the lower is --26.5 mV per unit pH. Oxide coverage, as measured by the charge associated with the reduction peaks, is given as a function of oxide formation potential in Fig. 6. The charge associated with the first reduction peak increased almost linearly with potential and within a range of 1--10 min was virtually independent of the time for oxide growth. The sharp increase in oxide growth above 2.0 V (HE) is associated mainly with material which is reduced in the course of the second cathodic peak. The increase in total oxide reduction charge with time for oxide growth is illustrated in Fig. 7 for three different oxide formation potentials. Similar plots for lower values of the latter showed lower charge values which reached constant levels at shorter times. At higher oxide formation potentials [ 2.3 and 2.4 V (HE)], the charge associated with the second reduction peak increased rapidly with oxide formation time over the first 5 min -- however, the values in
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each case approached a limit after a growth period of a b o u t 10 min. Figure 8 is a plot of these limiting oxide coverages in base as a function of the oxide formation potential. A plot of the charge associated with oxide reduction, taken after a relatively short period of oxide growth, as a function of electrolyte pH for a range of oxide formation potentials (Fig. 9), clearly demonstrated that thick anodic film growth on gold occurred most readily in highly acidic solution. DISCUSSION
The processes involved in the initial stages of metal oxidation have been discussed recently by Conway [18]. Radical species such as OH and O are produced as a chemisorbed layer.on the metal surface -- this has been confirmed in the case of gold in acid by Gol'dshtein and co-workers [19]. The chemisorption is sensitive to the crystallographic orientation of the electrode surface [ 17] -hence the three peaks in the monolayer oxide formation region of gold at low pH. With increasing coverage, oxidation becomes progressively more irreversible in character owing to rearrangement of the adsorbed layer, i.e. place-exchange reactions occur in which O and OH species replace gold ions in the outer layers of the metal surface. Cathodic reduction of monolayer oxide films on gold occurs by an island mechanism [ 7], i.e. oxide removal proceeds by o u t w a r d two-dimensional reaction from reduction centres which probably correspond to flaws or weaknesses in the original oxide film.
394 The absence of the three peaks in the oxide formation region of gold in base (Fig. 1) suggests that the mechanism of surface oxidation in this medium differs from that in acid. Electrochemical measurements [13,16,20] indicate that oxygen species chemisorb on gold in solutions of high pH at potentials well below the main anodic peak. According to Kirk and co-workers [16] a monolayer of hydroxyl species is produced in this region. They observed three peaks (at about --0.2 to +0.3 V, +0.3 to 0.7 V, and +0.7 to +1.3 V respectively) which were attributed to the influence of crystallographic orientation at the metal surface on the activity of the chemisorbed radicals. The main anodic peak above 1 V (Fig. 1) in this case probably corresponds to the conversion of surface hydroxide (AuOH) to the higher oxide (Au203) or hydroxide (Au(OH)3). Before considering the mechanism of thick oxide growth on gold it may be useful to estimate values for the thickness of such layers. This is frequently carried o u t using the charge involved in the formation or reduction of the film and assuming a value in the region [7,17] of 0.38 mC cm -2 (real area) for the charge associated with an oxygen monolayer on polycrystalline gold. Complications arising in base which limit the accuracy of this technique include the previously mentioned incomplete oxide to metal interconversion, which could result in a maximum error factor of 2 at low coverage, and changing surface roughness -- especially in the thick film region. Ideally a further correction should be applied for double-layer charging; however, since the value involved is uncertain, and usually rather small, it was ignored in the present case. Assuming monolayer oxygen coverage at 1.5 V - - t h i s corresponds to the minimum just above the main anodic peak (Fig. 1) -- a charge of 1.2 mC cm -2 at this point (Fig. 6) gives a roughness factor of 3--6 (depending u p o n whether the film in base is regarded as being totally or only half reduced). Similarly, small roughness factors for polycrystalline gold have been reported by Schultze [21]. Assuming that this factor remains constant even over the thick film region, the compact inner layer increased almost linearly with potential (Fig. 6) from about 1 to 4.3 oxygen monolayers on going from 1.5 to 2.4 V. The charge for total oxide reduction-in the limiting case (Fig. 8) at 2.4 V was a b o u t 128 mC cm -2, giving a net oxide coverage of a b o u t 107 monolayers. The equation for Au203 film thickness (d) quoted b y Dickertmann and co-workers [17] is d = QcVm/r6F
(1)
where Vm is the molar volume taken as 40 cm 3 mo1-1. If the mean roughness factor r in the present case is taken as 4.5, then at 2.4 V the thickness of the compact film is estimated, to be a b o u t 1.3 nm, while the maximum value for the outer layer (in terms of c o m p a c t Au203) is a b o u t 18.4 nm. In considering the limitations of this approach it should be borne in mind that the outer layer is probably not only highly hydrated but also microporous, i.e. at best the present approach gives only rough relative, rather than absolute, film thickness values. According to Schultze [21] the inner compact film on gold is practically anhydrous while the outer film is extensively hydrated. Oxide growth involves outward migration of Au 3÷ ions through the c o m p a c t layer [11,21], the thickness of the latter increasing almost linearly (Fig. 6) with increasing electrode potential, i.e. with increasing drop in chemical potential across the inner film.
395 Above 2.0 V the cations in the outer region of the film undergo a change in coordination state and rearrange to give a porous film. It is n o t certain whether this rearrangement involves total or only partial detachment of cations in the outer region. Total detachment, i.e. a dissolution--precipitation mechanism [ 11,21 ], should favour gold dissolution, e.g. aurate formation in base. Since the latter reaction, as examined by testing the cell solution by atomic absorption spectroscopy after electrolysis in the thick film region, was found to be almost negligible, the alternative -- oxide hydration without dissolution -- is worth considering. The hydration of the anhydrous film is similar in many ways to a phase transformation process [22] where the reaction is usually slow due to the high activation energy associated with breaking of primary coordination bonds. It should, however, be borne in mind that above 2.0 V cations in the outer region of the oxide film are involved in oxygen gas evolution and the mechanism of the latter reaction involves a surface redox process. According to Krasil'shchikov [23] this reaction includes (among others) the two steps: Au203 + O- = Au204 + e-
(I)
and Au204 -+ Au203 + O
(II)
In step (I) the primary coordination state of the Au(111) cations in the initially anhydrous film is altered, and in the subsequent decomposition of the unstable oxide (step (II)) loss of oxygen may involve its replacement by H20 or OHspecies to give hydrated or hydroxylated oxyeations. The latter may reoxidize and evolve further oxygen via steps (I) and (II), or rearrange to form the nucleus of an outer hydrated film. A further factor influencing this rearrangement may be physical changes induced at the anode surface by vigorous evolution of gas bubbles. It is well known [24] that freshly prepared h y d r o u s oxides are highly reactive and alter with time from an amorphous to a more regular structure. Among the possible reactions involved in this latter process are aggregation phenomena (combination of tiny nuclei to form larger aggregates), recrystallization (resulting not only in more stable structures within the micropartieles but also increasing the bonding between the latter and decreasing the internal surface area of the agglomerates), Ostwald ripening (growth of larger particles at the expense of smaller ones) and chemical interaetion (e.g. polymerization of oxycation species due to oxygen-bridging reactions). The net result is a porous outer film of hydrated oxide microparticles and an inner c o m p a e t layer, the latter continuously maintained during the growth of the outer layer b y the presence of a gradient in ehemical potential which is confined largely to the anhydrous film. It m a y be noted here that dissolution--precipitation reactions may occur during the rearrangement of the hydrated film, i.e. as a side-reaction. Formation of dissolved species cannot, therefore, be regarded as conclusive evidence that the sole mechanism for hydrated film formation in this t y p e of reaction involves a dissolution--precipitation (or dissolution--hydrolysis) mechanism. The influence of pH both on the kinetics of thick film growth (Fig. 9) and the ease of reduction of the resulting film (Fig. 5) is quite interesting. With hydrous films in general [25], as with many hydrated ions in aqueous solution [26], hydrolysis can occur due to loss of protons (or gain of hydroxide ions) on
396
raising the pH of the aqueous phase. It is well established in analytical chemistry [ 24] that the rearrangement velocity of freshly prepared h y d r o u s oxides decreases with increasing number of cation-coordinated hydroxide groups. A more rapid rearrangement at low pH (and, hence, a more crystalline product) would explain the faster rate of hydrated film formation under these conditions (Fig. 9). The rate of formation of the hydrated layer in acid is known [11] to be largely independent of time or film thickness -- evidently under these conditions the rearrangement of the outer layer is so fast that the rate-determining step is the formation of the hydrous layer from the c o m p a c t material. In base, on the other hand, inhibition of hydrous oxide rearrangement, due to the increased hydroxide coordination, apparently results in a more amorphous film. A decreasing abiIity of water or hydroxide ions to penetrate through the latter would explain both the lower rate of hydrous oxide formation (Fig. 9) and the inhibition of the oxygen gas evolution process (Fig. 4) on gold in base at high pH. A less hydrated variety of the outer oxide (which may well be the main inhibiting species) is apparently produced -- the evidence for this at present is the appearance of the intermediate peak in the reduction process for layers grown above 2.2 V (Fig. 3). Such a peak is not observed in acid. As outlined in the next paragraph, increased hydroxide incorporation (or, perhaps more accurately, increased stabilization of the hydrated o x y h y d r o x i d e gold complex aggregates in the outer layer) can also account for the unusual potential--pH behaviour of these hydrous films. The shift in peak potential with pH, especially that for the hydrous film (lower curve in Fig. 5), is unusual, as reversible oxide potentials are normally independent of pH when measured with respect to a reversible hydrogen electrode in the same solution. We have discussed this problem recently [25] in the case of hydrous films produced in iridium, rhodium and iron, where shifts similar to those observed with gold were noted. With these other metals the shift is clearly of thermodynamic origin as, among other features, peak potentials (e.g. in the case of iron) are virtually independent of sweep rate. Such results were attributed to the effect of hydrolysis on the activity of the transition metal species in the hydrous layer. In the case of gold the situation is somewhat more complex as the process involved is an oxide--metal rather than an oxide--oxide transition. Current results in the case of gold show that all oxide reduction peaks in both acid and base tend to more anodic values with decreasing sweep rate (500--5 mV 's-t). While there may be ohmic and kinetic contributions to this peak shift, the main factor (as in the case of the charge storage films on iridium, etc.) appears to be of thermodynamic origin. For instance, the rate of increase in peak potential On decreasing the rate was found to be greater for the reduction of the compact than for the hydrous layer (especially in base). Furthermore, the large separation between the peak values for the reduction of the hydrous film in acid and base did n o t decrease on lowering the sweep speed. The increased stability of hydrous transition metal species in base, including n o t only iridium, rhodium and iron, but also lead species in homogeneous solution [27], appears to be due to the ability of coordinated species such as water molecules to hydrolyse according to the reaction: Pb(OH)2H2OI ~ Pb(OH)~ + H ÷
(III)
397
This change in coordination (which could also occur by direct hydroxide ion incorporation) apparently only occurs with highly hydrated films. The stability of the resulting complex is obviously enhanced on increasing pH, and, in principle, the resultant lowering of the Pb(II) or Au(III) activity would explain the unusual lowering of the reduction potential for the hydrous species -- irrespective as to whether the latter is present in solution or in the porous film. The variation in activity in the present case extends over the entire pH range -possibly there is interaction between the cation sites in the film (which must be electronically interconnected) resulting, as in related materials [ 28], in the loss of individuality in the hydrolysis processes. To explain a shift of approximately --88.5 mV, i.e. 3/2 (2.303 R T / F ) , per pH unit (with respect to a pH-independent reference electrode), the electrode reaction must involve t h e transfer of a ratio of three protons (or h y d r o x i d e ions) to t w o electrons. Structural determination of these hydrous films is likely to be quite difficult owing to their amorphous nature [ 29] and their tendency to desiccate under high vacuum conditions [ 30]. One possibility, which can at least explain the electrochemical data and is n o t unreasonable in terms of the bonding in hydrous oxide films, is that the material is present as a non-linear polymer or aggregate of structure: I
.2°
o ( 1OH \o/1\o
.2°
t-120
H J|
n
While stressing the tentative nature of this proposed structure, its analogy with those proposed for isopolymolybdates and isopolytungstates [31 ] (where linking of octahedral hydrated oxyhydroxides to yield a range of aggregation products is well known) is worth noting. Reduction of such a species according to the reaction [--Au20~(OH)3 3 H20--]n + 9n H ÷ + 6 n e - = 2 n Au + 9n H20 (acid)
(V)
or
[--Au20~(OH)3 3 H20--]n + 6 h e - = 2 n Au + 9n O H - (base)
(VI)
would account for the observed potential--pH shift. Furthermore, the extensive hydration would be expected to inhibit extensive cross-linking between the chains and, hence, explain the open structure and the absence of e x t e n d e d electronic band structure in this t y p e of film [32]. With regard to the peak shift for the inner layer (lower curve in Fig. 5), it is interesting that a similar small potential--pH shift was n o t e d recently for a minor peak at low potentials in the case of rhodium [33]. Although these inner compact films are far less hydrated, the small shift could be due to hydrolysis effects associated with a limited a m o u n t of hydrous material in this region. We believe that the increased stability of the hydrous as compared with the compact film is an important factor in the growth of charge storage films on transition metals under potential cycling conditions, especially in base. In many instances the lower limit used in such experiments results in major reduction of
398 the compact layer -- thus resulting in activation of the metal surface. The significantly lower reduction potential for the hydrous film ensures that the latter can accumulate at the surface unless the lower limit is pushed to significantly more cathodic values. Although it was suggested earlier [ 11] that the anodically formed oxide layers on gold were unique, it now appears in fact that they have m a n y similarities with thick oxide films grown on other transition metals. For example, thick film growth occurs on platinum in acid [34] at potentials above 2.0 V; the layer is again of duplex character as shown by the splitting of the reduction process. Furthermore, m o l y b d e n u m at low potential in base [35], like gold in acid [ 11], can be oxidized with formation of a thick film at a rate which is independent of time or film thickness. On the other hand, thick oxide growth on various other metals (e.g. iridium [1] and tungsten [36] in acid, or rhodium [3], nickel [37] and iron [38] in base) occurs under potential cycling conditions at a rate which decreases with increasing film thickness -- behaviour similar to that observed in the present case with gold at constant potential in base (Fig. 7). In general, it appears that when these metals are oxidized in aqueous media under conditions where dissolution is negligible, a largely anhydrous oxide, of limited thickness, is produced initially. Conversion to a somewhat more stable hydrous oxide can in some cases (Pt and Au) occur under vigorous oxygen evolution conditions, but in most cases extensive reduction of the compact film seems necessary. Possibly in the latter case a more dispersed layer of metal atoms is produced on the electrode surface which is then oxidized more extensively on the subsequent anodic sweep. On further cycling the latter material is converted to the hydrous layer while the compact oxide is renewed by further oxidation of the underlying metal. Limiting hydrous oxide thickness in all cases is probably due to limited access of H20 and OH- species to the region close to the metal surface.
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