Applied Surface Science 55 (1992) 297-301 North-Holland

s u r f a c e s c h ~ n c e

Surface segregation induced by chemisorption at the alloy/solution interface

V. L~z~rescu 2, O. Radoviei b and M. Vass a a Institute o f Physical Chemistry, Spl. Independent.el 202, Bncharest 77 208, Romania

G¢,neral Chemistry Department, Polytechnic Institote o f Bucharest, Spl. Independent, ei 313, Bncharest, Romania

Received 24 June t991; accepted for publication 25 November 1991

Au-Ag vacuum-deposited alloy films with a small content of gold (5-10 at%) were found to exhibit unusual potentiodynamic behaviuur in alkaline media. The changes observed in the I - E profiles during the first 10-12 potential scans preceding the attainment of the stationary, shape were assigned to a phenomenon of surface segregation of silver induced by the chemisorption of oxygen atoms, resulting in a HO- discharge step.

I. Introduction

The chemical compositions of alloy surface and alloy bulk are frequently different as a result of either the thermodynamic equilibrium condi- tions or non-equilibrium phenomena. Usually, the surface becomes enriched in the component with the lower heat of sublimation after an annealing treatment, respectively the lower rate of sputter- ing after an ion bombardment, or in that one forming the stronger bonds with the adsorbate after a gas-phase chemisorption process.

As the phase diagramme reveals, silver and gold form continuous series of solid solutions over the entire compositional range [1]. Since the sublimation heat of gold exceeds that of silver by more than 100 k J / g [2], a pronounced surface enrichment in silver is expected to occur for their thermally equilibrated alloys, according to the regular solution model developed by van Santen and Boersma [3] for disordered monophasic bi- nary alloys,

However, the experimental data do not sup- port these predictions. Whatever the investigated alloy sample was, epitaxially grown films [4], bulk ingots [5] or foils [6], no appreciable surface seg- regation has been observed. Even the results re-

ported by Overbury and Somorjai [7], apparently discordant, turned out to be in good agreement with these conclusions [6].

In clear-cut contrast with all these findings stands the observation of significant (although much less than the theoretical predictions) sur- face segregation of silver for alumina-supported alloy catalysts reported by Toreis and Verykios [8]. Their results could be explained, in our opin- ion, provided that the specific conditions of the catalyst preparation are taken into account. Un- like the alloys discussed before, all of them ob- tained in high vacuum or inert atmosphere, the catalysts were exposed to oxygen during their preparation [8]. Therefore, it seems plausible to suppose that the surface excess of silver is, in this case, only a consequence of the preferential in- teraction of oxygen with one of the alloy compo- nents.

This assumption found an unexpected support in the results of cyclic voltammetry investigations on the behaviour of A u - A g alloys in alkaline solutions that revealed a definite phenomenon of surface segregation of silver induced by oxygen ehemisorption at the alloy/solution interface.

Studying the effects of the oxygen interaction with A u - A g alloys at the solid/gas and the

0169-4332/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved

V. Ldzdrescu a aL / Segregation indaced by chemisorption at the alloy/sohaion interface

solid/solution interfaces [9] it has been found that during repeated potential scans, the alloys with low percentages of gold (5-10 at%) exhib- ited an unusual potentiodynamie behaviour point- ing out important changes in their surface com- position. It is the aim of this paper to report on this interesting phenomenon.

2. Experimental

The measurements were carried out on poly- crystalline alloy films vacuum-deposited on opti- cally polished glass substrates. Every precaution had been taken in order to get a homogeneous material. Yhe two components were simultane- ously evaporated from closely positioned but sep- arate sources containing the pure metals, at indi- vidual constant evaporation rates. A heated sub- strate as weii as a subsequent annealing for 30 min have been used to enhance the atom mobility after the condensation. The heating temperature (350°C) was chosen to be equal to the highest of the Tammann temperatures of the two compo- nents (Au), where surface diffusion is known [10,11] to become appreciable. The residual gas pressure did not exceed 10 -8 Torr during the evaporation and the annealing steps.

The thickness of the deposit was greater than 3000 .~ in order to avoid both the influence of the substrate [12] and the effect of thickness fluctua- tions. The latter ones, usually less than 100 ,~ under such circumstances [13] could not bring about changes in the electrochemical behaviour of the film, observed to be identical with that of the massive metal starting with 1200 ,~ [!4-16].

The exgeriments were performed at room tem- perature in 1M KOH solution in doubly distilled water, in a three-compartment cell of conven- tional design. Two platinum wires embedded in the glass surface ensured the electrical connec- tion to the measuring circuit. The working elec- trode potential measured relative to a saturated calomel electrode (SCE) was controlled by a PAR-EG&G potentiostat (Model 173) fed by a PAR-EG & G Universal Programmer (Model 179). The cyclic voltammograms taken with a constant

scan rate of 10 mV s-~ were recorded with a Hewlett-Packard recorder.

3. Results and discussion

As seen in figs. I and 2, the I - E profiles exhibited under these circumstances by the alloys containing 5 to 10 at% Au-Ag underwent contin- uous changes during the repe~ ted potential scans. The second anodic peak (B), greatly enhanced in the course of the first 5-6 scans was noticed to be slowly diminished in the following ones. Ade- quately, the corresponding cathodic peaks suf- fered similar changes. Such a peculiar behaviour has never been observed for either simple silver films [17] or alloys with a higher content of gold [9].

That the detailed shape of the voltammogram is very sensitive to trace impurities as well as to surface structure [18-20], providing, as it has been claimed [18], a genuine "finger-print" for a clean and definite surface is well-known. Thereby, there is no doubt that the first voltammogram recorded for each alloy separately gives direct information on every change happened in the surface composition and structure. As a matter of fact, a conclusive image of the alloying effects operating in this case has been recently reported [91.

On the other hand, the complete miscibility of the components [1], the moderate exothermic en- thalpy of formation [21] indicating that these al- loys equilibrate fairly readily and the clear evi- dence on the absence of the surface segregation phenomenon [4-6] when they are thermally equi- librated under vacuum conditions are good rea- sons to suppose that a film of uniform composi- tion was obtained. Besides, unlike most of the modern techniques developed for the met- al/vacuum interfaces, which furnish only local information, the voltammograms give integrated information fl'om the whole investigated surface [221.

Consequently, neither the alloying effects, nor the alloy non-homogeneity could be responsible for the changes in the I - E curves reported here. Therefore, the origin of the continuous modifica-

V. L~z~rescu et aL / Segregation induced by chemisorption at the alloy ~solution interface

tion of the voltammogram under consideration should lie in the nature of the electrode processes involved.

A careful examination of some of the charac- teristic features of the anodic oxidation processes taking place on Ag electrodes in alkaline solu- tions [23,24] led to the conclusion that the well- known ability of silver to incorporate oxygen from the gas phase has a correspondent in one of its

first steps. It has been shown that after a minor dissolution process (A), a non-stoichiometric sub- surface oxide is formed by diffusion into the silver lattice of the chemisorbed oxygen, resulting in a H O - discharge step (B). Development of a thick layer of stoichiometric AgzO (C) followed by the formation of a mixed Ag(i)Ag(IIl)O 2 (D) ends the anodic oxidatioa of silver in alkaline media, as generally agreed [25-27].

l/mA Io.i mA D

• . ~ ~ , ~ ~ f 0.i~0.~ o .~ E/~v~ sc~

(0l

l/mA [ 0.1 mA B

A

' " ' o

I (c}

Fig. 1. Cyclic voltammograms of 5 at% Au-Ag alloy in IM KOH solution recorded during the repeated potential scans: (a) Ist; (b) 6th; (c) 10th.

300 V. Ldz~rescu et al. / Segregation induced by chemisorption at the alloy/sohttion interface

Thus, it is the electrode process (B) involved in the subsurface oxide layer growth that is mostly influenced in the above-mentioned experiments. This process was shown to be strongly dependent on the more or less porous structure of the vac- uum-evaporated film structure [17] suggesting the oxygen diffusion into the bulk as the rate-de- termining step. Therefore, its variations brought about by the repeated potential scans as well as the fluctuations of its cathodic correspondent re-

flect changes in the alloy surface and subsurface layer structure. Tile considerable increases ob- served during the first 5-6 runs and the subse- quent decreases noticed in the course of the last ones regarding the peak B height and width sug- gest that we witness a metal distension phe- nomenon soon followed by its opposite.

Since it is known [28,29] that the presence of oxygen on the surface causes an increase in the self-diffusion coefficient of the silver atoms by a

I/rnA [ 0.2 mA C D

- 0.2 0 A ~ . ~

(aJ

I/mA T J. 0.2 mA BC

~__...--~ • ~ .2~...c.~ . . . . ~ [~" O. 0.4 0.6 E/VvsSCE (hi

I 0.1 mA D

Fig. 2. Cyclic voltammugrams of 10 at% Au-Ag al!oy in IM KOH solution recorded during the repeated potential scans: (a) lst; (b) 5th; (e) 12th.

v. LSzdresctt et aL / Segregation induced by chemisorption at the alloy ~sob((ion #(terrace

f ac to r o f 100, it may be s u p p o s e d tha t o n e o f the c o n s e q u e n c e s w o u l d be the " p u s h i n g d o w n " 1o the b o t t o m layers of the gold a t o m s f o u n d in small p e r c e n t a g e s , This p rocess resul ts firstly in subsu r f ace layers with a m o r e p o r o u s s t r uc tu r e which t u r n e d in to m o r e c o m p a c t ones , finally.

T h e s t a t i ona ry s h a p e o f the cyclic v o l t a m m o - g r a m s r e c o r d e d for bo th 5 a n d 10 a t % A u - A g alloys r e sembl ing closely t h a t o f the c o m p a c t e l e c t rode ins tead o f t ha t c o r r e s p o n d i n g to the silver film o b t a i n e d in the s ame cond i t i ons [17] is conclus ive p r o o f in this respect . T h e fact t ha t such a b e h a v i o u r has no t b e e n obse rved lor p u r e si lver f i lms gives f u r t h e r suppor t to these a s sump- tions.

Th i s is, in o u r view, c l e a r ev idence o f a phe- n o m e n o n of su r f ace s e g r e g a t i o n i n d u c e d by c h e m i s o r p t i o n a t the a l l o y / s o l u t i o n in te r face . As f a r as we know, it is the first r e p o t o f this k ind, a d d i n g a va luab le a r g u m e n t in f avour o f the idea t ha t a close s imilar i ty exists indee¢! b e t w e e n the p h e n o m e n a t ak ing p lace a t the so l i c : /gas a n d the s o l i d / s o l u t i o n in te r faces .

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