Underpotential deposition of Zn2+ ions on Au(111), Au(100) and Au(110) electrodes

Journal of Electroanalytical

Chemistry, 376 (1994) 203-206

203

Short communication

Underpotential deposition of Zn2+ ions on Au( ill), and Au( 110) electrodes *

Au( 100)

Shinobu Moniwa and Akiko Aramata ** Catalysis Research Center, Hokkaido

University Sapporo 060 (Japan)

(Received 13 April 1994; in revised form 9 May 1994)

1. Introduction

Underpotential deposition (UPD) on single crystal electrodes has been extensively studied because of the interest in submonolayer modified well-defined surfaces. The structure of UPD, especially UPD of Cu2+ ions on Au and Pt single crystals has been observed by various techniques such as STM [l], EXAFS [2,3], XANES [3,4], FTIR [5], radiotracer [61 and QCM [7]. Recently, we have studied UPD of Zn2+ ions on polycrystal Au, Pt, and Pd at various pH [8,9], where we discussed the number of electrons transferred at the UPD process, which values involve uncertainty because of unspecified surface structure. In this note, we report UPD of Zn2+ on Au(lll), Au(lOO), and Au(l10) in phosphate buffer, NaClO,, and NaCl solutions where special attention is paid to the number of electrons transferred in the UPD processes in phosphate solutions, since UPD of Cu2+ on Au(ll1) is reported to take place by transfer of one and two electrons at the positively and negatively positioned UPD peaks, respectively [ 101. Experimental details are given elsewhere [ll]. Crystals of Au(lll), Au(lOO), and A&10) were prepared by the Clavilier method 1121, where special caution is needed in cooling the Au crystal which must proceed more slowly than in the case of Pt. Phosphate salts (Kanto), Zn(C10,),6H,O (Kishida), NaCl (Merck) in reagent grade were used without further purification. After flame annealing and Ar saturated pure-water quenching treatments of the crystal, it was put in contact with a deaerated solution prepared with purified water (Millipore pure-water supply), while the electrode potential was imposed in the double layer In honour of Dr. A. Hamelin for her scientific accomplishment. * To whom correspondence should be addressed.

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0022-0728/94/$7.00 SSDZ 0022-0728(94)03547-G

region. Electrode potentials in the following are expressed with respect to saturated calomel electrode (SCE). 2. Results and discussion Figure 1 shows cyclic voltammograms (CVs> at the sweep rate of 20 mV/s on the three low-index Au electrodes in phosphate solutions pH 4.6 and 5.4. These surfaces were confirmed to be well-defined surfaces by the current profiles for adsorbed oxygen species formation [13,14]. In Fig. 1, different W’s feature at negative and positive sweeps does not seem to show a different process since it appears together when negative potential limits were changed with the same amount of charge densities at the both sweeps. The Au(ll1) exhibits irreversible Zn2+ UPD main peaks at -0.45 and -0.39 V for negative and positive sweeps, but on Au(100) and Au(ll0) these peaks are nearly reversible within 0.03 V. This irreversibility observed on Au(lll1, being independent of the sweep rates studied, was not observed on polycrystal. This is likely to be taken to be associated with a relatively long terrace of Au(ll1). The reconstruction of gold surfaces in neutral solutions was suggested by double layer capacity hysterisis with the direction of potential sweeps [13], which was later confirmed in acidic solutions by the STM measurements [15-171. Although there is no such previous observation in phosphate solution to our knowledge, the double layer capacity, observed on Au faces in KPFs solution [18], showed surface reconstruction when the negative limit potential Elim = -0.6 V. Detailed examination of reconstruction was carried out for Au(100) in acidic solution [16,17] and showed that Au0001 is reconstructed to (5 X 27) by holding potential at a negative charge density and the reconstruction from (1 X 1) to (5 X 27) takes place at more negative 0 1994 - Elsevier Science S.A. All rights reserved

S. Moniwa, A. Aramata / Underpotential deposition of Zn2 + ions

204

1 (4

potentials in H,SO, than in HCIO,. This suggests that reconstruction during negative-going potential sweep in phosphate buffer solution does not occur when Elim= -0.6V. In the case of Au(lOO), the surface reconstruction was examined by the change of the CV as Elimis altered, as shown in Fig. 2. When Eli,,, > -0.8V in a 0.1 M NaCl solution, the CV nearly remains identical, but in a 0.1 M NaClO, the CV shows an increase of current at - 0.5 V < E < - 0.35 V; the former suggests

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Fig. 2. Cyclic voltammograms at 20 mV s-l on AuWO). (a) 1 x 10e4 M Zn(ClO,), + O.lM NaClO, and (b) 2 X 10m4 M Zn(ClO,), + 0.1 M NaCl. _J

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-500 E / mV vs. &X Fig. 1. Cyclic voltammograms at 20 mV s-l on Au(lll), ( -); Au(lOO), (------I; Au(ll0) (- - --); for (a) pH 4.6 phosphate solution with 1 x 10W3 M Zn(ClO& and (- - - - -) for Au (poly) (b) pH 5.4 phosphate solution with 2X low4 M Zn(ClO,),.

no change of surface structure consequent on change of Elimand the latter possibly shows a change of Au(lOO)-(1 x 1) to (5 x 27) which has Au(ll1) facets, since the peak at E = -0.44V is characteristic of Au(lll), and is absent on polycrystal Au. These results suggest that curves of Fig. 1 are taken at surfaces free from reconstruction over a major proportion of their surface. The UPD shift potentials, AE,, of the main UPD peak were 0.71 V for A&11), and 0.64 V for Au(100) and Au(ll0) at [Zn2’] = 1 X lop3 M for pH 4.6 and 2 x lop4 M for pH 5.4 from Fig. 1. AE, was independent of pH, at pH 4.6 and 5.4, and of the nature of the solution. The inhibition of the hydrogen evolution reaction by Zn2+ UPD observed on polycrystal Au was observed to a greater or lesser extent on the single crystals studied (Fig. la>. Figure 2(b) shows two UPD peaks of -0.54 and - 0.77 V in NaCl solution, the former peak is quite reversible at negative and positive sweeps, and the latter has not been found in

S. ~on~wa, A. Aramata / Unde~t~ni~~ de~~ition of Zn2 f ions

phosphate and perchlorate solutions. These suggest that the IJPD process is strongly correlated with anion specific adsorption. In Ref. [193 for polycrystal Pt, Pd and Au electrodes, the number of electrons present during WPD of Zn2+

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Log ([Zn2+j/M) Fig. 4. Electrode potential shift of the main peaks with the change of Zn2+ ion concentrations in pH 4.2 phosphate solution. 0, Au(lll); A, Au(100); 0, Au(llO).

ions, y, on Au was found to be 1.5 at pH 5.0, differing from 2 on Pt and Pd. In the case of UPD of Cu2+ ion on Au(lll), the shift of electrode potentials with the change of Cn2+ ion concentration showed that y was 1 for the positive UPD peak and 2 for the negative UPD peak. Therefore, we examined y by changing concentration of Zn2+ ions, where y is obtained by the following relation, provided that the coverage of Zn2+ UPD, 8, is constant, y=

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Fig. 3. Cyclic voltammograms at 20 mV s-l on (a) A&111), (b) Au(100), and (c) Au(ll0) in pH 4.2 phosphate solution with different Zn*+ ion concen~ations. ( -----I, 10-3 M; ( . . . . . *), 4x m-4 M; (------f, 1iY4 M.

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where Epeak is the potential at the UPD peak, aznz+ is the activity of Zr? ions which is replaced by concentration of Zn2* ions since the concentration is low enough with respect to concentration of phosphate buffer and the activity coefficient remains constant, Figure 3 shows CV at di~erent Zn2+ concentrations on Au(lll), Au(lOO), and Au(ll0) in pH 4.6 phosphate solution. At Epesk, we have the condition of constant charge density associated with UPD, QUPD, since Q,,ro was 20.5 & 0.3, 30.4 rf: 0.8, and 20.1 f 0.7 PC/cm’ for A&11), Au(lOO), and Au(llO), respectively. The E peak’~at the positive sweep were plotted against Zn2+ concentrations for the main peaks in Fig. 4, and y’s were obtained as 1 on Au(ll1) and 1.2 on Au(100) and Au(llO), and nearly 1 for the other peaks. In the case of Zn2+ UPD on Pt single crystals, we found y as 2 on Pt(ll1) and Pt(llO) in phosphate solution at pH 4.5, where dE, is as high as 1 V [203. These y values indicate that the whole IJPD process is not to be

206

S. Moniwa, A. Aramata / Underpotential deposition of Zn2+ ions

understood as a simple metal ion electro-deposition on a foreign metal substrate. Recently, QCM observation [7] of Cu2+ ion UPD on A&11) showed a weight increase corresponding to a nearly full coverage of copper atoms in the region of the more positive peak of the two UPD peaks. This should be discussed in harmony with results of STM [l], FTIR [5], EXAFS [2,3], as well as the potential shift observed for changes of concentration of Zn2+ ions [8,9]. To elucidate valence state of Zn atoms of UPD processes and interface structure as a function of potentials, observation by these in situ techniques are urgently needed, and then explanation of kinetics and structure of Zn2+ ion UPD on Au, Pt, and also Pd would become possible. In summary, Zn 2+ UPD on Au single crystal electrode was observed in various anion solutions. The number of electrons transferred at the UPD process was nearly 1, irrespective of the single crystal faces. Although UPD shift potential AE, was independent of the kind of the solutions, the CV characteristics were altered in different solutions; in NaCl solution, two UPD peaks appeared in contrast to the cases of phosphate and perchlorate solutions. Acknowledgments

Authors are indebted to Dr. A. Hamelin of C.N.R.S., Meudon, France for her helpful discussion. The Special Grant-in-Aid for Promotion of Education and Science in Hokkaido University and the Grant-in-Aid for Co-operative Research (A) (No. 04303007) from the Japanese Ministry of Education, Science and Culture are acknowledged.

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