Surface segregation induced by chemisorption at the alloy/solution interface

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<ul><li><p>Applied Surface Science 55 (1992) 297-301 North-Holland </p><p>sur face sch~nce </p><p>Surface segregation induced by chemisorption at the alloy/solution interface </p><p>V. L~z~rescu 2, O. Radoviei b and M. Vass a a Institute of Physical Chemistry, Spl. Independent.el 202, Bncharest 77 208, Romania </p><p>G,neral Chemistry Department, Polytechnic Institote of Bucharest, Spl. Independent, ei 313, Bncharest, Romania </p><p>Received 24 June t991; accepted for publication 25 November 1991 </p><p>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. </p><p>I. Introduction </p><p>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. </p><p>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, </p><p>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- </p><p>ported by Overbury and Somorjai [7], apparently discordant, turned out to be in good agreement with these conclusions [6]. </p><p>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. </p><p>This assumption found an unexpected support in the results of cyclic voltammetry investigations on the behaviour of Au-Ag alloys in alkaline solutions that revealed a definite phenomenon of surface segregation of silver induced by oxygen ehemisorption at the alloy/solution interface. </p><p>Studying the effects of the oxygen interaction with Au-Ag alloys at the solid/gas and the </p><p>0169-4332/92/$05.00 1992 - Elsevier Science Publishers B.V. All rights reserved </p></li><li><p>V. Ldzdrescu a aL / Segregation indaced by chemisorption at the alloy/sohaion interface </p><p>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. </p><p>2. Experimental </p><p>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 (350C) 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. </p><p>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]. </p><p>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&amp;G potentiostat (Model 173) fed by a PAR-EG &amp; G Universal Programmer (Model 179). The cyclic voltammograms taken with a constant </p><p>scan rate of 10 mV s-~ were recorded with a Hewlett-Packard recorder. </p><p>3. Results and discussion </p><p>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]. </p><p>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. </p><p>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. </p><p>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- </p></li><li><p>V. L~z~rescu et aL / Segregation induced by chemisorption at the alloy ~solution interface </p><p>tion of the voltammogram under consideration should lie in the nature of the electrode processes involved. </p><p>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 </p><p>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 HO- 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]. </p><p>l/mA Io.i mA D </p><p> .~~,~ ~ f 0.i~0.~ o.~ E/~v~ sc~ (0l </p><p>l/mA [ 0.1 mA B </p><p>A </p><p>' " ' o </p><p>I (c} </p><p>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. </p></li><li><p>300 V. Ldz~rescu et al. / Segregation induced by chemisorption at the alloy/sohttion interface </p><p>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- </p><p>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. </p><p>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 </p><p>I/rnA [ 0.2 mA C D </p><p>- 0.2 0 A~. ~ </p><p>(aJ </p><p>I/mA T J. 0.2 mA BC </p><p>~__...--~ ~ .2~...c.~ . . . . ~ [~" O. 0.4 0.6 E/VvsSCE (hi </p><p>I 0.1 mA D </p><p>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. </p></li><li><p>v. LSzdresctt et aL / Segregation induced by chemisorption at the alloy ~sob((ion #(terrace </p><p>factor of 100, it may be supposed that one of the consequences would be the "push ing down" 1o the bot tom layers of the gold atoms found in small percentages, This process results firstly in subsurface layers with a more porous structure which turned into more compact ones, finally. </p><p>The stat ionary shape of the cyclic vo l tammo- grams recorded for both 5 and 10 a t% Au-Ag alloys resembl ing closely that of the compact e lectrode instead of that cor respond ing to the silver film obta ined in the same condit ions [17] is conclusive proof in this respect. The fact that such a behaviour has not been observed lor pure silver fi lms gives fur ther support to these assump- tions. </p><p>This is, in our view, c lear evidence of a phe- nomenon of surface segregat ion induced by chemisorpt ion at the a l loy /so lut ion interface. As far as we know, it is the first repot of this kind, add ing a valuable a rgument in favour of the idea that a close similarity exists indee! between the phenomena taking place at the sol ic : /gas and the so l id /so lut ion interfaces. </p><p>]References </p><p>[I] M. Hansen, Constitution of Binary Alloys (McGraw-Hill. New York, 1958). </p><p>[2] C.J. Smithells, Metals Reference Book II (Bulterworths, London, 1962). </p><p>[3] R,A. van Santen and M.A.M. Boersma, J. Catal. 34 (1974) 13. </p><p>[4] S.C. Fain, Jr, and J.M. McDavid, Phys, Rev. B 9 (1974) 5089, </p><p>[5] R. Bouwman, L.H. Toneman, M.A.M. Boersma and R.A. van Santen. Surf. Sci. 59 (1076) 72. </p><p>16] M. Yabumoto, K. Watanabe and T. Yamashina, Surf. Sci. 77 (1978) 615. </p><p>[7] S.H. Overbury and G.A. Somorjai, Surf, Sci. 55 (1976) 209. </p><p>[8] N. Toreis and X.E. Vc, rykios, J. Catal. 108 (1987) 161. [9] V..L~z~rescu and M. Vass, Rev. Roum. Chim. 36 (1991), </p><p>in press. [10] S.L Gregg, The Surface Chemistry of Solids (Chapman &amp; </p><p>Hall, London, 1961). [11] R, Bouwman and W.M,H. Sachtler. Surf. Sci. 24 (1971) </p><p>350. [12] D. Henning and K.G. Wed, Z, Phys. Chem. (NF) 98 </p><p>(1975) 149. [13] F. Holland, Vacuum Deposition of Thin Films (Chapman </p><p>&amp; Hall, London, 1961). [14] I. Vartires. Stud. Cercet. Chim. 18 (1970) 683. [15] O. Radovici and M.E. Macovschi. Rev. Room. Chim. 16 </p><p>(197t) 16. [16] M.E. Macovschi, Stud. Cereet. Chim, 20 (1972)849. [17] V. I~zarescu, O. Radovici and M. Vass, Rev. Roum. </p><p>Chim. 32 (1987) 895. [18] R. Parsons, Surf. Sci. 101 (1980)316. [19] B.E. Conway, H. Angerstein-Kozlowska and F.C, Ho, J. </p><p>Vac. Sci. Technol. 14 (1977) 351. [20] B.E. Conway. Prog. Surf. Sei. 16 (1984) 1. [21] R.L. Moss and L. Whally, Adv. Catal. 22 (1972) 115. [22] J. Clavilier and J.P. Chauvineau, J. Electroanal. Chem. </p><p>100 (1979) 461. [23] V. l~z-~rescu. O. Radovici and M. Vass, Electrochem. </p><p>Acta 30 (1985) 1407. [24] V. Lfiz~rescu, O. Radovici and M. Vass, Rev. Roum. </p><p>Chim. "31 (1986) 461. [25] P. Stonehart. Electroehim. Acta 13 (1968) 1789. [2,5] B.V. Tilak, R.S. Perkins, H. Angerstein-Kozlowska and </p><p>B.E. Conway. Electrochim. Acta 17 (1972) 1447. [27] B.G. Pound, D.D. Macdonald and J.V. Tomlinson, Elec- </p><p>trochim. Acta ~ (1980) 563. [28] G.E. Rhead, Acta Met. 13 (1965) 233. [29] S.K. Sharma and J. Spitz, Thin Solid Films 56 (1979) LI7. </p></li></ul>


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