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Correlation of the underpotential deposition (upd) of zinc ions on Pt(111), Pt(100), and Pt(110) with anion specific adsorption
Journal of Electroanalytical Chemistry 457 (1998) 73 – 81
Correlation of the underpotential deposition (upd) of zinc ions on Pt(111), Pt(100), and Pt(110) with anion specific adsorption Satoshi Taguchi 1, Akiko Aramata * Catalysis Research Center, Hokkaido Uni6ersity, Sapporo 060 -0811, Japan Received 20 November 1997; received in revised form 17 July 1998
Abstract We report two roles of the adsorbed anions in the underpotential deposition (upd) of zinc ions giving cooperative and competitive interaction with the upd metals under different circumstances. The effect of specifically adsorbed anions on Zn upd was investigated systematically at Pt(111), Pt(100), and Pt(110) in solutions of pH 1 – 4.6 by cyclic voltammetry, where the anions were (bi)sulfate, phosphate, chloride, bromide, and iodide anions. Zn upd hardly occurred on Pt(111) in acidic solutions of pH 1. However, the growth of sharp Zn upd voltammetric waves was observed on Pt(111) in phosphate solutions with pH increase in the pH range of 2–4.6. In sulfate and perchlorate solutions, such behavior was not observed on Pt(111) with pH increase. On the other hand, at Pt(110), the Zn upd was clearly observed in phosphate solution of pH 1. In 0.1 M KH2PO4 (pH 4.4) with 10 − 3 M halides, the onset potential of Zn upd on Pt(111) shifted negatively according to the order of the adsorption strength of Cl − BBr − BI − . The negative shift of 0.02 V by Cl − adsorption on Pt(111) was smaller than that of 0.10 V on Pt(100). These results are discussed and correlated to the strength of anion adsorption at the onset potentials of Zn upd, in terms of cooperative and competitive interaction of the adsorbed anion with the upd Zn; adsorbed phosphate anions on Pt facilitate the Zn upd kinetically by the desorption action, but halides tightly adsorbed at the Pt surface obstruct the initiation of Zn upd. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Underpotential deposition; Zinc upd; Adsorbed anion; Pt single crystal electrodes; Phosphate solution
1. Introduction Metal underpotential deposition (upd), i.e. the formation of a metal (sub)monolayer M on a foreign substrate M% at potentials more positive than the equilibrium potential of Mn + /M, has been a subject of numerous studies in the past two decades [1 – 31]. Kolb [1], Szabo [2], and Adzic [3] reviewed upd phenomena mostly on various polycrystalline metal electrodes. Upd has been also studied on well-characterized single crystal electrodes. The upd processes of Ag + [4 – 8], Tl + [9,10], Cu2 + [11–26], Cd2 + [27], and Zn2 + [28,29] have * Corresponding author. Fax: + 81 11 7094748; e-mail: [email protected] 1 Present address: Department of Chemistry, Sapporo Campus, Hokkaido University of Education, Sapporo 002-8502, Japan.
been investigated on Pt single crystal electrodes, where the effect of specifically adsorbed anions on the processes and on the two-dimensional surface structure has attracted much attention. Upd phenomena on single crystal electrodes were reviewed recently by one of the authors [30]. Mikuni and Takamura first investigated the Zn2 + adsorption on a polycrystalline Pt electrode [31], and then our detailed study concluded that it was a upd process [32,33]. Although the standard potential of Zn2 + /Zn is very low at − 0.763V versus SHE, Zn2 + deposition and desorption currents were observed in the adsorbed hydrogen region or in the double-layer region on voltammograms of a polycrystalline Pt electrode. When we consider the Zn upd process on Pt under pseudo-equilibrium conditions, the process consists of the following three steps which were revealed by
0022-0728/98/$ - see front matter © 1998 Elsevier Science S.A. All rights reserved. PII S0022-0728(98)00297-6
S. Taguchi, A. Aramata / Journal of Electroanalytical Chemistry 457 (1998) 73–81
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voltammetric [29], radio-tracer [34,35], FTIR [36], and EQCM studies [36]; (i) removal of adsorbed anions by electron transfer number j from substrate, (ii) metal upd by electron transfer number g, and (iii) re-adsorption of the anions by electron transfer number z on the upd metals, as given below: −m /M% +j e − =Am − +M% Ajads
(1)
−g Mn + +g e − + M% =Mnupd /M%
(2)
and −g −m −g Am − +Mnupd /M% =Azads /Mnupd /M% + z e −
(3)
The overall reaction becomes: −m −m −g /M% +Mn + +(g +j −z)e − =Azads /Mnupd /M% Ajads
(4) where Am − is an anion in solution with −m ionic −m valence, Ajads /M% an adsorbed anion on the substrate surface M%, Mn + a metal ion in solution which adsorbs −g /M%, and underpotentially on M% in the form of Mnupd −m −g Azads /Mnupd /M% an adsorbed anion on the substrate deposited upd metal. For simplicity, in Eqs. (3) and (4) the anion on the upd metal is assumed to be the same as that on substrate M%, although a different anion adsorption strength on upd M and on M% was observed by the radio-tracer method when two kinds of specifically adsorbable anions are present in the solution [34,35]. Hence, Eqs. (1) – (4) show that an anion adsorbed just before the occurrence of upd, which is called ‘a pre-adsorbed anion’ in the following, is expected to have a considerable influence on the upd process. Our voltammetric study on low index single-crystal Pt electrodes in 0.1 M H2SO4 +10 − 4 M Zn2 + ions showed that the Zn upd is surface structure-sensitive [28]. Pt(111) hardly allowed the Zn upd given a very small charge density of ca. 10 mC cm − 2 corresponding to a Zn upd wave at the negative potential side of the so-called ‘anomalous wave’, which has been related to the (bi)sulfate anion adsorption – desorption process [37 – 48]. According to chronocoulometric studies by Savich et al. [48], the maximum coverage of (bi)sulfate ions on Pt(111) in 0.1 M HClO4 +10 − 3 M H2SO4 at 0.85V versus RHE is 1/3 monolayer with respect to the density of Pt atoms on the surface. In contrast, the maximum coverage by the radio-tracer method determined by Wieckowski’s group [47] was 2.5×1014 ions cm − 2, being 0.16 monolayer with respect to the density of Pt atoms on the surface. The discrepancy of (bi)sulfate coverage on Pt(111) determined by the above two methods was considered to be due to the presence of many defects at the surface of the electrode used for radiochemical studies [48,49]. Funtikov et al. reported a clear STM image of Pt(111) in 0.05 M H2SO4 at 0.5 to 0.7V versus RHE (the positive side of the sharp spike)
[49,50], which was similar to that of Au(111) in the same solution [51], and interpreted as an image of co-adsorbates containing (bi)sulfate anions and H2O or H3O + with coverage of 0.2. The Zn upd onset potential in 0.1 M H2SO4 + 10 − 4 M Zn2 + on Pt(111) was 0.4 V versus SHE [28] and most of adsorbed (bi)sulfate anions have already desorbed [41–48]. On the other hand, Pt(110) allowed clearly a charge density of ca. 90 mC cm − 2 corresponding to a Zn upd wave at the positive potential side of the so-called adsorbed hydrogen wave. The upd onset potential was 0.35V versus RHE, where the coverage of (bi)sulfate was nearly as the same as the maximum value of ca. 0.2 by the radio-tracer method [47]. We also confirmed a pH effect on Zn upd at Pt(111) [29]; in contrast to the cases of 0.1 M H2SO4 +Zn2 + ions, a sharp Zn upd wave was found in a solution of 0.1 M KH2PO4 + 10 − 3 M Zn2 + ions of pH 4.4. The corresponding charge density amounted to 180920 mC cm − 2. The upd onset potential was 0.7V versus RHE and corresponded to the ‘positive potential foot’ of the anomalous wave, where the coverage of adsorbed phosphate anion was likely to be nearly a maximum by analogy with the voltammogram with the ‘anomalous wave’ in sulfuric acid solution [37–48]. Thus, from the relation between the Zn upd and the anion adsorption at the upd onset potentials, we have presumed that the presence of pre-adsorbed anions is cooperative with the Zn upd. In the present report we studied the Zn upd system systematically to assess the validity of this interpretation. In the following discussion, we focus on the onset potential of the Zn upd and discuss this, since the relation between the upd onset potential and the degree of anion adsorption on a Pt surface seems to clarify the upd behavior. In order to change the degree of anion adsorption at the onset potential of Zn upd on a Pt substrate, the following parameters were changed; (i) the pH of base solutions with the same anion species and constant ion strength, (ii) the structure of the platinum surface, and (iii) the anion species of the base solutions of ClO4− , (H)SO4− , and H2PO4− with and without Cl − , Br − and I − . The aim of this article is to discuss the Zn upd process in terms of the controlling effect on upd by the pre-adsorbed anion.
2. Experimental Pt(111), Pt(100), and Pt(110) were prepared by Clavilier’s method [52] in our laboratory. Before each measurement, the electrode was annealed to red color in a gas + oxygen flame for a few seconds, cooled in air, and then introduced into Ar saturated ultra pure water (Millipore Milli-Q water). Then, the electrode with a pure-water droplet was inserted quickly into an elec-
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trolyte solution in the test electrode chamber of a three-compartment glass electrolytic cell. A saturated calomel electrode (SCE) was used as the reference electrode. In the text, the electrode potential is given with respect to the reversible hydrogen electrode in the same electrolyte (RHE) or the standard hydrogen electrode (SHE). The counter electrode was a Pt foil. Solutions were prepared with ultra pure water (Millipore Milli-Q water). Several kinds of electrolyte solutions were used. The pH of sulfate, perchlorate, and phosphate solutions was changed gradually between pH ca. 1–5. Sulfate solutions were 0.1 M H2SO4 (Merck Suprapur), 0.01 M H2SO4 +0.09 M K2SO4 (Merck Suprapur), and 0.1 M K2SO4 +3 ×10 − 3 M, + 10 − 3 M, and + 3×10 − 4 M H2SO4. Perchlorate solutions were 0.1 M HClO4 (Merck Suprapur), 0.01 M HClO4 +0.09 M KClO4 (Merck pro analysis), and 0.1 M KClO4 + 3×10 − 3 M, +3 ×10 − 4 M, and + 10 − 4 M HClO4. Phosphate solutions were 0.5 M H3PO4 (Kanto reagent grade), 0.1 M H3PO4, a mixture of H3PO4 and KH2PO4 (Fluka biochemica micro select), and a mixture of KH2PO4 and K2HPO4 (Fluka biochemica micro select), where the ratios between H3PO4 and KH2PO4 and between KH2PO4 and K2HPO4 were changed to change the pH at constant ion strength of 0.2. Solutions of 0.1 M KH2PO4 with and without 10 − 3 M KCl (Merck Suprapur), KBr, and KI (Wako reagent grade) were also used. Zn(ClO4)26H2O (Kishida reagent grade) was added to be 10 − 4 – 10 − 3 M to the above solutions. The zinc ionic species is not ZnOH + but Zn2 + at pH B5. The solutions were deaerated by Ar gas of 5 N purity, which was also supplied to a gas phase over the solution in the test electrode chamber during the experiments. All the measurements were carried out by the dipping method at room temperature. Cyclic voltammograms were taken with a potentiostat (Toho 2000), function generator (Toho FG-01), and X-Y recorder (Rikadenki RY-101). The voltammograms were recorded at 10 mV s − 1 in the absence and the presence of Zn2 + ions.
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sulfate (A), perchlorate (B), and phosphate (C) solutions, respectively, where the potential is expressed with respect to SHE. The voltammetric waves which are reversible in the negative- and positive-going sweeps below ca. 0.3 V are due to a hydrogen adsorption–desorption process with a coverage of 2/3 of a monolayer and shift negatively by 60 mV pH − 1 versus SHE in all the solutions studied. The reversible waves at more positive potentials than hydrogen adsorption–desorption potentials, the so-called ‘anomalous waves’ of (A) and (C), are related to sulfate and phosphate adsorption–desorption, respectively [37–48], and the ‘butterfly
3. Results and discussion
3.1. Effects of pH on anion adsorption at Pt(111) in sulfate, perchlorate, and phosphate solutions The behavior of anion adsorption was observed at various pHs by cyclic voltammograms in Zn2 + -free solutions, as shown in Fig. 1, since the Zn upd behavior in phosphate solutions was revealed to be quite different in pH dependence from that in sulfate and perchlorate solutions as shown later (Section 3.2). (Bi)sulfate and phosphate species are known as anions that adsorb specifically, and perchlorate species does not. Fig. 1 shows the characteristic voltammograms on Pt(111) for
Fig. 1. The change of voltammograms of Pt(111) at 10 mV s − 1 with pH increase in sulfate solution (A), perchlorate solution (B), and phosphate solution (C). Broken vertical lines indicate the onset potential of Zn upd.
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Fig. 2. Plots of peak potential of the so-called anomalous wave of Pt(111) in phosphate solution versus pH. The peak positions were estimated from Fig. 1 (C).
wave’ (B) is due to OH adsorption – desorption [37,38,45]. Broken vertical lines at 0.4V versus SHE in Fig. 1 indicate the onset potential of Zn upd waves on Pt(111) in sulfate, perchlorate, and phosphate solutions, being independent of pH as shown later (Section 3.2). In sulfate solutions of Fig. 1 (A), the position of the anomalous wave is found to be independent of solution pH, being in agreement with the results of Jaaf-Golze et al. [37] and Wieckowski et al. [45]. Wieckowski et al. confirmed by the radiotracer method that the coverage of adsorbed (bi)sulfate reaches a maximum value at 0.7V versus SHE positive to a sharp spike on the voltammogram [44– 47]. Faguy et al. [41], Ogasawara et al. [42], and Nart et al. [43] reported the same conclusion by FTIR. The relative position between the anomalous wave and the broken vertical line does not change with the pH increase in sulfate solutions. In perchlorate solutions of Fig. 1 (B), the butterfly wave of voltammograms in a potential range of 0.4–0.8 V, which was taken to be due to hydroxide adsorption [37,38,45], moved negatively by 62 mV pH − 1 with pH increase, as reported by other workers [37,45]. A slight overlap of the foot of the butterfly wave at negative potentials with the broken vertical line appears at pH 4.0. In phosphate solutions of Fig. 1 (C), the peak position of the anomalous wave shifted negatively by 60 mV pH − 1 [53] in the range of pH 1.9 – 3.7, as shown in Fig. 2, in contrast to that in sulfate solutions. The positions of the anomalous wave and the broken vertical line at pH 1.1 are similar to that of the sulfate solutions. However, the line begins to overlap with the foot of the anomalous wave at positive potentials at pH] 2.5. As a result, at pH 4.6, the Pt(111) surface is still saturated with the adsorbed phosphate species at the potential of the broken vertical line, as reported by Iwasita et al. [54,55] This indicates that the coverage of the adsorbed phosphate species at the potential of
the broken vertical line increases with increase of pH. Incidentally, in the case of sulfate solutions, no change in the position of the anomalous wave with the pH increase was observed, although the ratio of HSO4− and SO24 − is dependent on the pH of the solution. Faguy et al. [41] reported by in-situ FTIR that the adsorbed anion on Pt(111) was not sulfate but bisulfate in 0.05 M H2SO4. On the contrary, Nart et al. [43] concluded that the adsorbed species on Pt(111) was not bisulfate but sulfate in the solutions of HF+KF with H2SO4 of pH 0.23 and pH 2.8 with in-situ FTIR. Recently, from in-situ FTIR results, Faguy et al. proposed tentatively that the structure of the bisulfatelike adsorbate is a sulfate-hydronium ion pair; SO24 − · H3O + [56]. Tadjeddine et al. recently reported that adsorbed hydrogen was detected at the anomalous wave region by visible-infrared sum frequency generation (SFG) [57]. In the case of phosphate solution, the change in the position of the anomalous wave with pH increase suggests a different adsorption–desorption feature of the adsorbed phosphate species with the change of solution pH [55].
3.2. Effects of pH and anion adsorption on Zn upd at Pt(111) in sulfate, perchlorate, and phosphate solutions Fig. 3 shows the change of voltammograms of Pt(111) at 10 mV s − 1 with the change of pH 0.8–3.7 in
Fig. 3. The change of voltammograms of Pt(111) from pH 0.8–3.7 in sulfate solution in the absence (---) and the presence ( — ) of 10 − 4 M Zn2 + ions at 10 mV s − 1. (A) pH 0.8, (B) pH 2.1, (C) pH 2.7, (D) pH 3.2, and (E) pH 3.7.
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Fig. 4. The change of voltammograms of Pt(111) from pH 0.9 – 4.0 in perchlorate solution in the absence (---) and the presence ( — ) of 10 − 4 M Zn2 + ions at 10 mV s − 1. (A) pH 0.9, (B) pH 1.9, (C) pH 2.5, (D) pH 3.4, and (E) pH 4.0.
sulfate solutions in the absence (dotted curve) and the presence (solid curve) of 10 − 4 M Zn2 + ions. A quite small wave with increase of current was observed at the negative potential side of the anomalous wave in the solution with Zn2 + ions at pH 0.8. The onset potential of this small wave in the solutions is noticed at ca. 0.4 V (vs SHE) although the onset potential is not clear at pH 2.7. The charge corresponding to the small wave in the anomalous wave region amounted only to ca. 10 mC cm − 2 at pH 0.8 [28]. This wave resulted from Zn upd because of the partial depression of the adsorbed hydrogen wave. Such small Zn upd waves were poorly resolved at pH 2.1– 3.2, but the current of adsorbed hydrogen waves below 0.35 V versus RHE was gradually decreased by the pH increase in the solutions with Zn2 + ions. At pH 3.7, the deposition of Zn occurs sluggishly without a clear upd peak, and the desorption of upd Zn appears as a broad peak around 0.1 V versus SHE with a partial depression of the adsorbed hydrogen wave. Fig. 4 shows the change of voltammograms of Pt(111) at 10 mV s − 1 by the change of pH 0.9 –4.0 in perchlorate solutions in the absence (dotted curve) and the presence (solid curve) of 10 − 4 M Zn2 + ions. The current increase was observed by the addition of Zn2 + ions in the so-called double-layer region, and the corresponding charge of the small upd wave amounted only
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to ca. 10 mC cm − 2 at pH5 2.5, where the onset potentials stayed at ca. 0.4 V versus SHE. The upd waves were poorly resolved at 2.5 BpH5 4.0, spreading broadly over a wide potential range, where the current of adsorbed hydrogen waves decreased gradually with pH increase. The onset potential of the small Zn upd wave is also ca. 0.4 V versus SHE, although the OH adsorption–desorption waves were slightly influenced by the addition of Zn2 + ions to the solution at pH 3.4. Fig. 5 shows the change of voltammograms of Pt(111) at 10 mV s − 1 with the change of pH 1.1–4.6 in phosphate solutions in the absence (dotted curve) and the presence (solid curve) of 10 − 4 M Zn2 + ions. The voltammetric feature of Zn upd changed drastically with pH increase, being different from the aforementioned cases of Figs. 3 and 4; the enhancement of a sharp Zn upd wave with pH increase was clearly seen in the phosphate solutions. The voltammetric feature at pH 1.1 with a small Zn upd wave by 15 mC cm − 2 was similar to that of Fig. 3 (A) or Fig. 4 (A). However, well-defined Zn deposition–desorption waves were observed above pH 2. At pH 2.5–3.0, the negative-going sweep showed the growth of a sharp spike around 0.25 V versus SHE with pH increase. Such a sharp spike is often described as being due to the 2-dimensional phase
Fig. 5. The change of voltammograms of Pt(111) from pH 1.1–4.6 in phosphate solution in the absence (---) and the presence ( — ) of 10 − 4 M Zn2 + ions at 10 mV s − 1. (A) pH 1.1, (B) pH 1.9, (C) pH 2.5, (D) pH 2.8, (E) pH 3.0, (F) pH 3.4, (G) pH 3.7, and (H) pH 4.6.
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formation of upd metal and co-adsorbed anions [58]. At pH\ 3.0, the sharp spike faded into a large peak around 0.3 V versus SHE, being discussed as a result of random Zn deposition in our recent paper [58]. The positive-going sweep gave a clear sharp peak around 0.3 V versus SHE, which grew with pH increase. The charge of the sharp peak related to upd Zn desorption at pH 4.6 amounted to ca. 170 mC cm − 2, which was estimated from the area within the voltammetric curve (solid line) between 0.23 and 0.37 V versus SHE after correction of double-layer charging. The decrease of the adsorbed hydrogen wave below 0.35 V versus RHE by the addition of Zn2 + ions was also pronounced with pH increase. When the coverage of upd Zn was increased with pH increase, the hydrogen adsorption was accordingly obstructed. The onset potential of Zn upd was ca. 0.4 V versus SHE in the pH range studied, being the same as for the cases of Figs. 3 and 4, except for that of 0.5 V at pH 4.6. The onset potential of Zn upd was located around the peak of the anomalous wave in sulfate solutions of Fig. 3, being independent of pH. Accordingly, the coverage of the adsorbed (bi)sulfate on Pt(111) at the onset potential is expected to be constant in the pH range studied. In Fig. 4, a slight overlap of the negative potential foot of the butterfly wave with the upd onset potential appears even at pH 4.6 in perchlorate solutions. This indicates that the coverage of the adsorbed OH at the upd onset potential is quite low if there is any. The coverage of the phosphate species at the Zn upd onset potential increased with pH increase, as shown in Fig. 5. The enhancement of the sharp Zn deposition–desorption wave was observed with the pH increase in phosphate solutions but not in sulfate and perchlorate solutions, although the Zn coverage increased with the pH increase in all solutions investigated as judged by the gradual suppression of adsorbed hydrogen waves. Thus, the enhancement of the sharp Zn upd wave is taken to be related to the increase of pre-adsorbed anion coverage at the Zn upd onset potential from the above discussion; the presence of preadsorbed phosphate seems to co-operate with the Zn upd.
3.3. Effect of pH on Zn upd at Pt(110) in phosphate solutions Zn upd was also examined at Pt(110) in phosphate solutions in comparison with that on Pt(111) for clarification of the effect of pre-adsorbed anion. Fig. 6 (A and B) show the voltammograms of Pt(110) at 10 mV s − 1 in the absence (broken curves) and the presence (solid curves) of 10 − 4 M Zn2 + ions in the phosphate solutions of (A) pH 1.1 and (B) 3.7, respectively. The electrode potential is expressed with respect to RHE. In the absence of Zn2 + ions at pH 1.1, the charge density
Fig. 6. Cyclic voltammograms of Pt(110) in the absence (---) and the presence ( — ) of 10 − 4 M Zn2 + ions in phosphate solutions of pH 1.1 (A) and 3.7 (B) at 10 mV s − 1. Electrode potential is represented with respect to RHE. Vertical dotted line with arrow indicates the potential position of SHE.
of adsorbed hydrogen wave, 220 mC cm − 2, is identical to that in sulfuric acid on Pt(110)–(1×2) [28,52]. The voltammogram was changed drastically by the addition of Zn2 + ions to the solution. This result shows a considerable occurrence of Zn upd on Pt(110) even at pH 1.1, being in contrast to the case of Pt(111) of Fig. 5 (A). The clearness of the Zn upd wave in Fig. 6 (A) is also greater than that on Pt(111) in perchlorate and sulfate solution at pH\3; the upd waves in the case of Pt(111) being spread over a broader potential region. The features of the voltammogram in the phosphate solution of pH 3.7 were also changed by the addition of Zn2 + ions to the solution; the Pt surface seems to be wholly covered by upd Zn and co-adsorbed phosphate from the suppression of the adsorbed hydrogen wave. The charge density including the Zn upd process, Q, was estimated at pH 1.1 in Fig. 6 (A) from the area within the voltammetric curve (solid line) between 0.05 and 0.35 V at the positive-going sweep after correction for double-layer charging. The Q of ca. 360 mC cm − 2 is greater than the charge of 220 mC cm − 2 for the desorp-
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tion of a monolayer hydrogen on Pt(110) – (1× 2); unfortunately, a possibility that the Q value includes not only the charge related to the upd but also the hydrogen adsorption charge cannot be ruled out, since the Zn upd potential region overlaps with the adsorbed hydrogen potential region. However, the excess charge density of 140 mC cm − 2 ( =360 – 220 mC cm − 2), at least, is the minimum charge density of the Zn upd process including phosphate desorption from the Pt surface and its re-adsorption on the Pt surface with upd Zn. The charge amount was much larger than the 15 mC cm − 2 of the small upd wave on Pt(111) in phosphate solution at pH 1.1 in Fig. 5 (A). In the solution of pH 3.7 in Fig. 6 (B), the Q was ca. 320 mC cm − 2, which was estimated from the area within the voltammetric curve (solid line) between 0.05 and 0.60 V at the positive-going sweep after correction for double-layer charging. Wieckowski’s group found by the radiotracer method that the onset potential of (bi)sulfate adsorption on Pt(110) in an acidic solution is more negative than that on Pt(111), being in the so-called adsorbed hydrogen region. They also reported that the coverage of adsorbed (bi)sulfate on Pt(110) increased steeply at the positive limit of the adsorbed hydrogen wave and attained a nearly maximal amount of 2.2× 1014 molecules cm − 2, corresponding to ca. 0.2 of a monolayer [47], where the Zn upd begins [28]. A similar feature of the potential dependence on the amount of adsorbed phosphate anion is expected on Pt(110) from the voltammogram without Zn2 + ions shown by the dotted curve in Fig. 6 (A), since the voltammogram resembles that in sulfuric acid solution [28,52]. It is known that the shape of the so-called adsorbed hydrogen wave is sensitive to the anion adsorption isotherm. The Zn upd onset potential in Fig. 6 (A) coincides with the positive limit of the adsorbed hydrogen wave. We thus conclude that the Zn upd at Pt(110) is also enhanced by the adsorbed phosphate even in acidic phosphate solution of pH 1.1 as previously observed in 0.1 M H2SO4 [28].
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Fig. 7. (A) Cyclic voltammograms of Pt(100) in 0.1 M KH2PO4 (pH 4.4) in the absence (---) and presence ( — ) of 10 − 3 M Zn2 + ions at 10 mV s − 1. (B) Voltammetric profiles of Zn upd on Pt(100) in 0.1 M KH2PO4 (pH 4.4) with 10 − 3 M Zn2 + ions in the absence (---) and the presence ( — ) of 10 − 3 M Cl − ions at 10 mV s − 1.
Fig. 7 (B) shows the effect of chloride ions on the Zn upd at Pt(100) in 0.1 M KH2PO4. The presence of 10 − 3 M Cl − ions sharpened the Zn upd wave with a negative shift of the upd wave by 0.10 V, being identical in shift direction but a higher degree of shift in comparison with that at Pt(111) in the same solution as shown in Fig. 8.
3.4. Effect of adsorbed halides on the Zn upd at Pt(100) and Pt(111) in 0.1 M KH2PO4 of pH 4.4 Fig. 7 (A) shows cyclic voltammograms at Pt(100) in 0.1 M KH2PO4 in the absence (dotted curve) and the presence (solid curve) of 10 − 3 M Zn2 + ions. We previously reported that the Zn upd scarcely occurred on Pt(100) in 0.1 M H2SO4 [28]. In KH2PO4 of Fig. 7 (A), however, the clear Zn upd wave appeared at 0.58 V versus RHE with broad waves in the potential region of 0.5 and 0.05 V versus RHE. The total charge related to Zn upd at positive going sweep was 278 mC cm − 2, which was estimated from the area within the voltammetric curve between 0.75 and 0.05 V after correction of double-layer charging.
Fig. 8. Voltammetric profiles of Zn upd on Pt(111) in 0.1 M KH2PO4 (pH 4.4) with 10 − 3 M Zn2 + ions in the absence (---) and the presence of 10 − 3 M Cl − ( —), Br − ( —- —), and I- ions ( —---—) at 10 mV s − 1.
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Fig. 8 shows the effect of 10 − 3 M chloride, bromide, and iodide ions on the Zn upd at Pt(111) in 0.1 M KH2PO4. The Zn upd peak potentials are shifted negatively in the order of anion adsorption strength of H2PO4− BCl − B Br − BI − . This order indicates that the Zn upd competes with anion adsorption on the Pt(111) surface [29]. The halide anions obstruct the initiation of Zn upd when the upd onset potential of 0.4 V versus SHE without halide ions is located at a more positive potential than the desorption potential of the adsorbed halide, as observed in Zn2 + -free solutions, and as discussed in our previous paper [29]. The Zn deposition peak at Pt(111) in 0.1 M KH2PO4 was not sharpened by addition of Cl − ions, as is shown in Fig. 8, in contrast with the case of Pt(100) of Fig. 7 (B). Furthermore, the addition of Cl − ion shifted slightly the peak potential of the Zn upd wave at Pt(111) by − 0.02 V; the effect of Cl − ions at Pt(111) is smaller than that at Pt(100). This difference appears to be due to the weaker adsorption strength of Cl − ion on Pt(111) than that on Pt(100) [19]. Lucas et al. reported by the X-ray scattering method that the coverage of the adsorbed chloride is 0.4 at 0.2 V(Ag AgCl) with no ordered structure at a Pt(111) electrode in 0.1 MHClO4 +0.01 M KCl [59]. The addition of Br − and I − ions obstructed the initiation of Zn upd significantly at Pt(111) by 0.08 and 0.39 V, respectively, for the negative-going sweep and the voltammetric waves were sharpened significantly, as shown in Fig. 8. The bromide and iodide anions form 2-D ordered adlayers on Pt(111) [59 – 61] just before the occurrence of Zn upd. Thus the Zn upd on Pt(111) competes with these adsorbed anions with a rigidly ordered layer structure. The sharp Zn deposition–desorption wave was irreversible with respect to potential cycling at 10 mV s − 1. The sharpening and irreversibility are characteristic of a 2-D phase transition [58].
4. Summary We have examined the effect of the adsorbed anions on the Zn upd with change of pH, and the addition of halide ions in terms of the degree of the Zn upd deposition–desorption wave reversibility and sharpness under potential cycling, and the position of the upd potential. The current of the Zn upd was scarcely observed at Pt(111) in strongly acidic solutions of pH 1, being irrespective of the kind of anion in the solution. Only in the case of phosphate solutions, did the well-defined sharp and reversible upd waves grow at Pt(111) with the increase of pH in the pH range 2.5 – 4.6. The charge within the voltammetric wave related to the Zn upd process was estimated as ca. 170 mC cm − 2 at pH 4.6. In sulfate and perchlorate solutions, however, such behav-
ior was absent at Pt(111) with pH increase; the broad upd waves were poorly resolved. These facts were explained from the aspect of a co-operative effect of the pre-adsorbed anion to Zn upd. The amount of pre-adsorbed anion is relatively low at the onset potential of Zn upd in a solution of pH 1 and increases with pH only in phosphate solutions when pH] 2.5. The degree of upd Zn on Pt(110) in an acidic phosphate solution of pH 1.1 was significantly higher than that on Pt(111) in the same pH solution. The upd wave on Pt(110) was also well defined compared with that on Pt(111) in sulfate and perchlorate solutions at pH\ 3. This can also be explained in term of the co-operative effect of pre-adsorbed anions to Zn upd, since the coverage of adsorbed anions on Pt(110) in the solutions of pH 1 is expected to be high at the onset potential. Two possible effects of the presence of the pre-adsorbed anion to induce the sharp or clear upd wave are considered. One is that the pre-adsorbed anion plays the role of shielding positive charge at a positively polarized Pt electrode from the Zn2 + ions in the step of Eq. (2) and facilitates the approach of Zn2 + ions to the Pt surface. Another effect is that the pre-adsorbed anions are supplied to the electrode surface with upd Zn as soon as the anions desorb from the Pt(111) substrate with the formation of a stable co-adsorption layer of the anion and the upd Zn. The adsorption of phosphate and sulfate species on upd Zn at Pt(poly) has already been revealed by radio tracer [34,35], FTIR [36], and EQCM [36] studies. The adsorption of halide on Pt(111) in 0.1 M KH2PO4, especially the adsorption of bromide and iodide with a 2-D ordered structure [59–61], competes with the Zn upd process as shown by the negative shift of the upd potential [29]. Adsorption of chloride anion on Pt(100) competes more strongly with the Zn upd than that on Pt(111), because of the stronger interaction of Cl − ions with Pt(100) surface [19,59]. When the upd onset potential of 0.4 V versus SHE is close to the desorption potential of the adsorbed anion by itself in the case of Pt(111) in phosphate solution at pH 4.6, the upd is not obstructed by the pre-adsorbed anions. Values of the upd potential shift, i.e. potential differences between the equilibrium redox potential of Zn2 + (10 − 4 M)/Zn of − 0.883 V versus SHE and the Zn upd onset potentials, DEudps, are 1.28 and 1.18 V on Pt(111) and Pt(110) in phosphate solutions, respectively, as the upd onset potentials are 0.4 and 0.3 V on Pt(111) and Pt(110), respectively. The DEudps are close to the work function differences of 1.4 and 1.3 V between Pt(111) and Zn(polycrystal) and between Pt(110) and Zn(polycrystal), respectively. However, on the addition of halide ions, the upd potential shift on Pt(111) decreases because of obstruction of Zn upd by the pre-adsorbed halides. The relationship between the upd shift and the work function difference will be
S. Taguchi, A. Aramata / Journal of Electroanalytical Chemistry 457 (1998) 73–81
[30] A. Aramata, in: J.O.M. Bockris, et al. (Eds.), Modern Aspects of Electrochemistry, vol. 31, Plenum Press, New York, 1997, p. 181. [31] F. Mikuni, T. Takamura, Denki Kagaku 38 (1970) 113. [32] A. Aramata, M.A. Quaiyyum, W.A. Balais, T. Atoguchi, M. Enyo, J. Electroanal. Chem. 338 (1992) 367. [33] M.A. Quaiyyum, A. Aramata, S. Moniwa, S. Taguchi, M. Enyo, J. Electroanal. Chem. 373 (1994) 61. [34] G. Horanyi, A. Aramata, J. Electroanal. Chem. 434 (1997) 201. [35] G. Horanyi, A. Aramata, J. Electroanal. Chem. 437 (1997) 259. [36] A. Aramata, S. Terui, S. Taguchi, T. Kawaguchi, K. Shimazu, Electrochim. Acta 41 (1996) 761. [37] K.A. Jaaf-Golze, D.M. Kolb, D. Scherson, J. Electroanal. Chem. 200 (1986) 353. [38] R.R. Adzic, F. Feddrix, B.Z. Nikolic, E.B. Yeager, J. Electroanal. Chem. 341 (1992) 287. [39] J.M. Orts, R. Go´mez, J.M. Feliu, A. Aldaz, J. Clavilier, Electrochim. Acta 39 (1994) 1519. [40] J.M. Feliu, J.M. Orts, R. Go´mez, A. Aldaz, J. Clavilier, J. Electroanal. Chem. 372 (1994) 265. [41] P.W. Faguy, N. Markovic, R.R. Adzic, C.A. Fierro, E.B. Yeager, J. Electroanal. Chem. 289 (1990) 245. [42] H. Ogasawara, Y. Sawatari, J. Inukai, M. Ito, J. Electroanal. Chem. 358 (1993) 337. [43] F.C. Nart, T. Iwasita, M. Weber, Electrochim. Acta 39 (1994) 961. [44] E.K. Krauskopf, L.M. Rice, A. Wieckowski, J. Electroanal. Chem. 244 (1988) 347. [45] A. Wieckowski, P. Zelenay, K. Varga, J. Chim. Phys. 88 (1991) 1247. [46] M.E. Gamboa-Aldeco, E. Herrero, P.S. Zelenay, A. Wieckowski, J. Electroanal. Chem. 348 (1993) 451. [47] Y.-E. Sung, A. Thomas, M. Gamboa-Aldeco, K. Franaszczuk, A. Wieckowski, J. Electroanal. Chem. 378 (1994) 131. [48] W. Savich, S-G. Sun, J. Lipkowski, A. Wieckowski, J. Electroanal. Chem. 388 (1995) 233. [49] A.M. Funtikov, U. Stimming, R. Vogel, J. Electroanal. Chem. 428 (1997) 147. [50] A.M. Funtikov, U. Linke, U. Stimming, R. Vogel, Surf. Sci. 324 (1995) 343. [51] G.J. Edens, X. Gao, M.J. Weaver, J. Electroanal. Chem. 375 (1994) 357. [52] J. Clavilier, R. Faure, G. Guinet, R. Durand, J. Electroanal. Chem. 107 (1980) 205. [53] S. Ye, H. Kita, A. Aramata, J. Electroanal. Chem. 333 (1992) 299. [54] C. Nart, T. Iwasita, M. Weber, Ber. Bunsenges. Phys. Chem. 97 (1993) 737. [55] M. Weber, F.C. Nart, I.R.de Moraes, T. Iwasita, J. Phys. Chem. 100 (1996) 19933. [56] P.W. Faguy, N.S. Marinkovic, R.R. Adzic, J. Electroanal. Chem. 407 (1996) 209. [57] A. Tadjeddine, A. Peremans, J. Electroanal. Chem. 409 (1996) 115. [58] S. Taguchi, T. Fukuda, A. Aramata, J. Electroanal. Chem. 435 (1997) 55. [59] C.A. Lucas, L.M. Markovic, P.N. Ross, Phys. Rev. B 55 (1997) 7964. [60] G.N. Salaita, D.A. Stern, F. Lu, H. Baltruschat, B.C. Schardt, J.L. Stickney, M.P. Soriaga, D.G. Frank, A.T. Hubbard, Langmuir 2 (1986) 828. [61] N. Shinotsuka, K. Sashikata, K. Itaya, Surf. Sci. 335 (1995) 75.
discussed in a future paper. In our recent paper, we also correlated the kinetic mechanism of pre-adsorbed anion desorption with the Zn upd mechanism [58]. When the pre-adsorbed phosphate desorbs by a random desorption mechanism, the following Zn upd occurs also through the random mechanism. On the other hand, when the pre-adsorbed bromide desorbs with a 2-D phase transition, the following Zn upd occurs through the nucleation-growth mechanism but not as a random mechanism. We called this phenomenon a ‘foot print’.
References [1] D.M. Kolb, Adv. in Electrochem. and Electrochem. Eng. 11 (1978) 125. [2] S. Szabo, Int. Rev. Phys. Chem. 10 (1991) 207. [3] R.R. Adzic, Adv. Electrochem. Electrochem. Eng. 13 (1985) 159. [4] J.L. Stickney, S.D. Rosasco, D. Song, M.P. Soriaga, A.T. Hubbard, Surf. Sci. 130 (1983) 326. [5] N. Kimizuka, K. Itaya, Faraday Disc. 94 (1992) 117. [6] D.L. Taylor, H.D. Abrun˜a, J. Electrochem. Soc. 140 (1993) 3402. [7] J.F. Rodriguez, D.L. Taylor, H.D. Abrun˜a, Electrochim. Acta 38 (1993) 235. [8] P. Zelenay, M. Gamboa-Aldeco, G. Hora´nyi, A. Wieckowski, J. Electroanal. Chem. 357 (1993) 307. [9] J. Clavilier, J.P. Ganon, M. Petit, J. Electroanal. Chem. 265 (1989) 231. [10] D.R. Wheeler, J.X. Wang, R.R. Adzic, J. Electroanal. Chem. 387 (1995) 115. [11] D.M. Kolb, R. Ko¨tz, K. Yamamoto, Surf. Sci. 87 (1979) 20. [12] C.L. Scortichini, C.N. Reilley, J. Electroanal. Chem. 139 (1982) 247. [13] P.C. Andricacos, P.N. Ross, J. Electroanal. Chem. 167 (1984) 301. [14] D.M. Kolb, Ber. Bunsenges. Phys. Chem. 92 (1988) 1175. [15] J.H. White, H.D. Abruna, J. Electroanal. Chem. 300 (1991) 521. [16] T. Hachiya, H. Honbo, K. Itaya, J. Electroanal. Chem. 315 (1991) 275. [17] K. Varga, P. Zelenay, A. Wieckowski, J. Electroanal. Chem. 330 (1992) 453. [18] R. Michaelis, M.S. Zei, R.S. Zhai, D.M. Kolb, J. Electroanal. Chem. 339 (1992) 299. [19] N. Markovic, P.N. Ross, Langmuir 9 (1993) 580. [20] H.S. Yee, H.D. Abrun˜a, Langmuir 9 (1993) 2460. [21] R. Go´mez, J.M. Feliu, H.D. Abrun˜a, J. Phys. Chem. 98 (1994) 5514. [22] H. Matsumoto, J. Inukai, M. Ito, J. Electroanal. Chem. 379 (1994) 223. [23] H. Ogasawara, J. Inukai, M. Ito, Surf. Sci. 311 (1994) 665. [24] Y. Shingaya, M. Ito, J. Electroanal. Chem. 372 (1994) 283. [25] H.S. Yee, H.D. Abrun˜a, J. Phys. Chem. 98 (1994) 6552. [26] G.M. Bommarito, D. Acevedo, J.F. Rodriguez, H.D. Abrun˜a, J. Electroanal. Chem. 379 (1994) 135. [27] K. Varga, P. Zelenay, G. Hora´nyi, A. Wieckowski, J. Electroanal. Chem. 327 (1992) 291. [28] S. Taguchi, A. Aramata, M.A. Quaiyyum, M. Enyo, J. Electroanal. Chem. 374 (1994) 275. [29] S. Taguchi, A. Aramata, J. Electroanal. Chem. 396 (1995) 131.
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Correlation of the underpotential deposition (upd) of zinc ions on Pt(111), Pt(100), and Pt(110) with anion specific adsorption Satoshi Taguchi 1, Akiko Aramata * Catalysis Research Center, Hokkaido Uni6ersity, Sapporo 060 -0811, Japan Received 20 November 1997; received in revised form 17 July 1998
Abstract We report two roles of the adsorbed anions in the underpotential deposition (upd) of zinc ions giving cooperative and competitive interaction with the upd metals under different circumstances. The effect of specifically adsorbed anions on Zn upd was investigated systematically at Pt(111), Pt(100), and Pt(110) in solutions of pH 1 – 4.6 by cyclic voltammetry, where the anions were (bi)sulfate, phosphate, chloride, bromide, and iodide anions. Zn upd hardly occurred on Pt(111) in acidic solutions of pH 1. However, the growth of sharp Zn upd voltammetric waves was observed on Pt(111) in phosphate solutions with pH increase in the pH range of 2–4.6. In sulfate and perchlorate solutions, such behavior was not observed on Pt(111) with pH increase. On the other hand, at Pt(110), the Zn upd was clearly observed in phosphate solution of pH 1. In 0.1 M KH2PO4 (pH 4.4) with 10 − 3 M halides, the onset potential of Zn upd on Pt(111) shifted negatively according to the order of the adsorption strength of Cl − BBr − BI − . The negative shift of 0.02 V by Cl − adsorption on Pt(111) was smaller than that of 0.10 V on Pt(100). These results are discussed and correlated to the strength of anion adsorption at the onset potentials of Zn upd, in terms of cooperative and competitive interaction of the adsorbed anion with the upd Zn; adsorbed phosphate anions on Pt facilitate the Zn upd kinetically by the desorption action, but halides tightly adsorbed at the Pt surface obstruct the initiation of Zn upd. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Underpotential deposition; Zinc upd; Adsorbed anion; Pt single crystal electrodes; Phosphate solution
1. Introduction Metal underpotential deposition (upd), i.e. the formation of a metal (sub)monolayer M on a foreign substrate M% at potentials more positive than the equilibrium potential of Mn + /M, has been a subject of numerous studies in the past two decades [1 – 31]. Kolb [1], Szabo [2], and Adzic [3] reviewed upd phenomena mostly on various polycrystalline metal electrodes. Upd has been also studied on well-characterized single crystal electrodes. The upd processes of Ag + [4 – 8], Tl + [9,10], Cu2 + [11–26], Cd2 + [27], and Zn2 + [28,29] have * Corresponding author. Fax: + 81 11 7094748; e-mail: [email protected] 1 Present address: Department of Chemistry, Sapporo Campus, Hokkaido University of Education, Sapporo 002-8502, Japan.
been investigated on Pt single crystal electrodes, where the effect of specifically adsorbed anions on the processes and on the two-dimensional surface structure has attracted much attention. Upd phenomena on single crystal electrodes were reviewed recently by one of the authors [30]. Mikuni and Takamura first investigated the Zn2 + adsorption on a polycrystalline Pt electrode [31], and then our detailed study concluded that it was a upd process [32,33]. Although the standard potential of Zn2 + /Zn is very low at − 0.763V versus SHE, Zn2 + deposition and desorption currents were observed in the adsorbed hydrogen region or in the double-layer region on voltammograms of a polycrystalline Pt electrode. When we consider the Zn upd process on Pt under pseudo-equilibrium conditions, the process consists of the following three steps which were revealed by
0022-0728/98/$ - see front matter © 1998 Elsevier Science S.A. All rights reserved. PII S0022-0728(98)00297-6
S. Taguchi, A. Aramata / Journal of Electroanalytical Chemistry 457 (1998) 73–81
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voltammetric [29], radio-tracer [34,35], FTIR [36], and EQCM studies [36]; (i) removal of adsorbed anions by electron transfer number j from substrate, (ii) metal upd by electron transfer number g, and (iii) re-adsorption of the anions by electron transfer number z on the upd metals, as given below: −m /M% +j e − =Am − +M% Ajads
(1)
−g Mn + +g e − + M% =Mnupd /M%
(2)
and −g −m −g Am − +Mnupd /M% =Azads /Mnupd /M% + z e −
(3)
The overall reaction becomes: −m −m −g /M% +Mn + +(g +j −z)e − =Azads /Mnupd /M% Ajads
(4) where Am − is an anion in solution with −m ionic −m valence, Ajads /M% an adsorbed anion on the substrate surface M%, Mn + a metal ion in solution which adsorbs −g /M%, and underpotentially on M% in the form of Mnupd −m −g Azads /Mnupd /M% an adsorbed anion on the substrate deposited upd metal. For simplicity, in Eqs. (3) and (4) the anion on the upd metal is assumed to be the same as that on substrate M%, although a different anion adsorption strength on upd M and on M% was observed by the radio-tracer method when two kinds of specifically adsorbable anions are present in the solution [34,35]. Hence, Eqs. (1) – (4) show that an anion adsorbed just before the occurrence of upd, which is called ‘a pre-adsorbed anion’ in the following, is expected to have a considerable influence on the upd process. Our voltammetric study on low index single-crystal Pt electrodes in 0.1 M H2SO4 +10 − 4 M Zn2 + ions showed that the Zn upd is surface structure-sensitive [28]. Pt(111) hardly allowed the Zn upd given a very small charge density of ca. 10 mC cm − 2 corresponding to a Zn upd wave at the negative potential side of the so-called ‘anomalous wave’, which has been related to the (bi)sulfate anion adsorption – desorption process [37 – 48]. According to chronocoulometric studies by Savich et al. [48], the maximum coverage of (bi)sulfate ions on Pt(111) in 0.1 M HClO4 +10 − 3 M H2SO4 at 0.85V versus RHE is 1/3 monolayer with respect to the density of Pt atoms on the surface. In contrast, the maximum coverage by the radio-tracer method determined by Wieckowski’s group [47] was 2.5×1014 ions cm − 2, being 0.16 monolayer with respect to the density of Pt atoms on the surface. The discrepancy of (bi)sulfate coverage on Pt(111) determined by the above two methods was considered to be due to the presence of many defects at the surface of the electrode used for radiochemical studies [48,49]. Funtikov et al. reported a clear STM image of Pt(111) in 0.05 M H2SO4 at 0.5 to 0.7V versus RHE (the positive side of the sharp spike)
[49,50], which was similar to that of Au(111) in the same solution [51], and interpreted as an image of co-adsorbates containing (bi)sulfate anions and H2O or H3O + with coverage of 0.2. The Zn upd onset potential in 0.1 M H2SO4 + 10 − 4 M Zn2 + on Pt(111) was 0.4 V versus SHE [28] and most of adsorbed (bi)sulfate anions have already desorbed [41–48]. On the other hand, Pt(110) allowed clearly a charge density of ca. 90 mC cm − 2 corresponding to a Zn upd wave at the positive potential side of the so-called adsorbed hydrogen wave. The upd onset potential was 0.35V versus RHE, where the coverage of (bi)sulfate was nearly as the same as the maximum value of ca. 0.2 by the radio-tracer method [47]. We also confirmed a pH effect on Zn upd at Pt(111) [29]; in contrast to the cases of 0.1 M H2SO4 +Zn2 + ions, a sharp Zn upd wave was found in a solution of 0.1 M KH2PO4 + 10 − 3 M Zn2 + ions of pH 4.4. The corresponding charge density amounted to 180920 mC cm − 2. The upd onset potential was 0.7V versus RHE and corresponded to the ‘positive potential foot’ of the anomalous wave, where the coverage of adsorbed phosphate anion was likely to be nearly a maximum by analogy with the voltammogram with the ‘anomalous wave’ in sulfuric acid solution [37–48]. Thus, from the relation between the Zn upd and the anion adsorption at the upd onset potentials, we have presumed that the presence of pre-adsorbed anions is cooperative with the Zn upd. In the present report we studied the Zn upd system systematically to assess the validity of this interpretation. In the following discussion, we focus on the onset potential of the Zn upd and discuss this, since the relation between the upd onset potential and the degree of anion adsorption on a Pt surface seems to clarify the upd behavior. In order to change the degree of anion adsorption at the onset potential of Zn upd on a Pt substrate, the following parameters were changed; (i) the pH of base solutions with the same anion species and constant ion strength, (ii) the structure of the platinum surface, and (iii) the anion species of the base solutions of ClO4− , (H)SO4− , and H2PO4− with and without Cl − , Br − and I − . The aim of this article is to discuss the Zn upd process in terms of the controlling effect on upd by the pre-adsorbed anion.
2. Experimental Pt(111), Pt(100), and Pt(110) were prepared by Clavilier’s method [52] in our laboratory. Before each measurement, the electrode was annealed to red color in a gas + oxygen flame for a few seconds, cooled in air, and then introduced into Ar saturated ultra pure water (Millipore Milli-Q water). Then, the electrode with a pure-water droplet was inserted quickly into an elec-
S. Taguchi, A. Aramata / Journal of Electroanalytical Chemistry 457 (1998) 73–81
trolyte solution in the test electrode chamber of a three-compartment glass electrolytic cell. A saturated calomel electrode (SCE) was used as the reference electrode. In the text, the electrode potential is given with respect to the reversible hydrogen electrode in the same electrolyte (RHE) or the standard hydrogen electrode (SHE). The counter electrode was a Pt foil. Solutions were prepared with ultra pure water (Millipore Milli-Q water). Several kinds of electrolyte solutions were used. The pH of sulfate, perchlorate, and phosphate solutions was changed gradually between pH ca. 1–5. Sulfate solutions were 0.1 M H2SO4 (Merck Suprapur), 0.01 M H2SO4 +0.09 M K2SO4 (Merck Suprapur), and 0.1 M K2SO4 +3 ×10 − 3 M, + 10 − 3 M, and + 3×10 − 4 M H2SO4. Perchlorate solutions were 0.1 M HClO4 (Merck Suprapur), 0.01 M HClO4 +0.09 M KClO4 (Merck pro analysis), and 0.1 M KClO4 + 3×10 − 3 M, +3 ×10 − 4 M, and + 10 − 4 M HClO4. Phosphate solutions were 0.5 M H3PO4 (Kanto reagent grade), 0.1 M H3PO4, a mixture of H3PO4 and KH2PO4 (Fluka biochemica micro select), and a mixture of KH2PO4 and K2HPO4 (Fluka biochemica micro select), where the ratios between H3PO4 and KH2PO4 and between KH2PO4 and K2HPO4 were changed to change the pH at constant ion strength of 0.2. Solutions of 0.1 M KH2PO4 with and without 10 − 3 M KCl (Merck Suprapur), KBr, and KI (Wako reagent grade) were also used. Zn(ClO4)26H2O (Kishida reagent grade) was added to be 10 − 4 – 10 − 3 M to the above solutions. The zinc ionic species is not ZnOH + but Zn2 + at pH B5. The solutions were deaerated by Ar gas of 5 N purity, which was also supplied to a gas phase over the solution in the test electrode chamber during the experiments. All the measurements were carried out by the dipping method at room temperature. Cyclic voltammograms were taken with a potentiostat (Toho 2000), function generator (Toho FG-01), and X-Y recorder (Rikadenki RY-101). The voltammograms were recorded at 10 mV s − 1 in the absence and the presence of Zn2 + ions.
75
sulfate (A), perchlorate (B), and phosphate (C) solutions, respectively, where the potential is expressed with respect to SHE. The voltammetric waves which are reversible in the negative- and positive-going sweeps below ca. 0.3 V are due to a hydrogen adsorption–desorption process with a coverage of 2/3 of a monolayer and shift negatively by 60 mV pH − 1 versus SHE in all the solutions studied. The reversible waves at more positive potentials than hydrogen adsorption–desorption potentials, the so-called ‘anomalous waves’ of (A) and (C), are related to sulfate and phosphate adsorption–desorption, respectively [37–48], and the ‘butterfly
3. Results and discussion
3.1. Effects of pH on anion adsorption at Pt(111) in sulfate, perchlorate, and phosphate solutions The behavior of anion adsorption was observed at various pHs by cyclic voltammograms in Zn2 + -free solutions, as shown in Fig. 1, since the Zn upd behavior in phosphate solutions was revealed to be quite different in pH dependence from that in sulfate and perchlorate solutions as shown later (Section 3.2). (Bi)sulfate and phosphate species are known as anions that adsorb specifically, and perchlorate species does not. Fig. 1 shows the characteristic voltammograms on Pt(111) for
Fig. 1. The change of voltammograms of Pt(111) at 10 mV s − 1 with pH increase in sulfate solution (A), perchlorate solution (B), and phosphate solution (C). Broken vertical lines indicate the onset potential of Zn upd.
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S. Taguchi, A. Aramata / Journal of Electroanalytical Chemistry 457 (1998) 73–81
Fig. 2. Plots of peak potential of the so-called anomalous wave of Pt(111) in phosphate solution versus pH. The peak positions were estimated from Fig. 1 (C).
wave’ (B) is due to OH adsorption – desorption [37,38,45]. Broken vertical lines at 0.4V versus SHE in Fig. 1 indicate the onset potential of Zn upd waves on Pt(111) in sulfate, perchlorate, and phosphate solutions, being independent of pH as shown later (Section 3.2). In sulfate solutions of Fig. 1 (A), the position of the anomalous wave is found to be independent of solution pH, being in agreement with the results of Jaaf-Golze et al. [37] and Wieckowski et al. [45]. Wieckowski et al. confirmed by the radiotracer method that the coverage of adsorbed (bi)sulfate reaches a maximum value at 0.7V versus SHE positive to a sharp spike on the voltammogram [44– 47]. Faguy et al. [41], Ogasawara et al. [42], and Nart et al. [43] reported the same conclusion by FTIR. The relative position between the anomalous wave and the broken vertical line does not change with the pH increase in sulfate solutions. In perchlorate solutions of Fig. 1 (B), the butterfly wave of voltammograms in a potential range of 0.4–0.8 V, which was taken to be due to hydroxide adsorption [37,38,45], moved negatively by 62 mV pH − 1 with pH increase, as reported by other workers [37,45]. A slight overlap of the foot of the butterfly wave at negative potentials with the broken vertical line appears at pH 4.0. In phosphate solutions of Fig. 1 (C), the peak position of the anomalous wave shifted negatively by 60 mV pH − 1 [53] in the range of pH 1.9 – 3.7, as shown in Fig. 2, in contrast to that in sulfate solutions. The positions of the anomalous wave and the broken vertical line at pH 1.1 are similar to that of the sulfate solutions. However, the line begins to overlap with the foot of the anomalous wave at positive potentials at pH] 2.5. As a result, at pH 4.6, the Pt(111) surface is still saturated with the adsorbed phosphate species at the potential of the broken vertical line, as reported by Iwasita et al. [54,55] This indicates that the coverage of the adsorbed phosphate species at the potential of
the broken vertical line increases with increase of pH. Incidentally, in the case of sulfate solutions, no change in the position of the anomalous wave with the pH increase was observed, although the ratio of HSO4− and SO24 − is dependent on the pH of the solution. Faguy et al. [41] reported by in-situ FTIR that the adsorbed anion on Pt(111) was not sulfate but bisulfate in 0.05 M H2SO4. On the contrary, Nart et al. [43] concluded that the adsorbed species on Pt(111) was not bisulfate but sulfate in the solutions of HF+KF with H2SO4 of pH 0.23 and pH 2.8 with in-situ FTIR. Recently, from in-situ FTIR results, Faguy et al. proposed tentatively that the structure of the bisulfatelike adsorbate is a sulfate-hydronium ion pair; SO24 − · H3O + [56]. Tadjeddine et al. recently reported that adsorbed hydrogen was detected at the anomalous wave region by visible-infrared sum frequency generation (SFG) [57]. In the case of phosphate solution, the change in the position of the anomalous wave with pH increase suggests a different adsorption–desorption feature of the adsorbed phosphate species with the change of solution pH [55].
3.2. Effects of pH and anion adsorption on Zn upd at Pt(111) in sulfate, perchlorate, and phosphate solutions Fig. 3 shows the change of voltammograms of Pt(111) at 10 mV s − 1 with the change of pH 0.8–3.7 in
Fig. 3. The change of voltammograms of Pt(111) from pH 0.8–3.7 in sulfate solution in the absence (---) and the presence ( — ) of 10 − 4 M Zn2 + ions at 10 mV s − 1. (A) pH 0.8, (B) pH 2.1, (C) pH 2.7, (D) pH 3.2, and (E) pH 3.7.
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Fig. 4. The change of voltammograms of Pt(111) from pH 0.9 – 4.0 in perchlorate solution in the absence (---) and the presence ( — ) of 10 − 4 M Zn2 + ions at 10 mV s − 1. (A) pH 0.9, (B) pH 1.9, (C) pH 2.5, (D) pH 3.4, and (E) pH 4.0.
sulfate solutions in the absence (dotted curve) and the presence (solid curve) of 10 − 4 M Zn2 + ions. A quite small wave with increase of current was observed at the negative potential side of the anomalous wave in the solution with Zn2 + ions at pH 0.8. The onset potential of this small wave in the solutions is noticed at ca. 0.4 V (vs SHE) although the onset potential is not clear at pH 2.7. The charge corresponding to the small wave in the anomalous wave region amounted only to ca. 10 mC cm − 2 at pH 0.8 [28]. This wave resulted from Zn upd because of the partial depression of the adsorbed hydrogen wave. Such small Zn upd waves were poorly resolved at pH 2.1– 3.2, but the current of adsorbed hydrogen waves below 0.35 V versus RHE was gradually decreased by the pH increase in the solutions with Zn2 + ions. At pH 3.7, the deposition of Zn occurs sluggishly without a clear upd peak, and the desorption of upd Zn appears as a broad peak around 0.1 V versus SHE with a partial depression of the adsorbed hydrogen wave. Fig. 4 shows the change of voltammograms of Pt(111) at 10 mV s − 1 by the change of pH 0.9 –4.0 in perchlorate solutions in the absence (dotted curve) and the presence (solid curve) of 10 − 4 M Zn2 + ions. The current increase was observed by the addition of Zn2 + ions in the so-called double-layer region, and the corresponding charge of the small upd wave amounted only
77
to ca. 10 mC cm − 2 at pH5 2.5, where the onset potentials stayed at ca. 0.4 V versus SHE. The upd waves were poorly resolved at 2.5 BpH5 4.0, spreading broadly over a wide potential range, where the current of adsorbed hydrogen waves decreased gradually with pH increase. The onset potential of the small Zn upd wave is also ca. 0.4 V versus SHE, although the OH adsorption–desorption waves were slightly influenced by the addition of Zn2 + ions to the solution at pH 3.4. Fig. 5 shows the change of voltammograms of Pt(111) at 10 mV s − 1 with the change of pH 1.1–4.6 in phosphate solutions in the absence (dotted curve) and the presence (solid curve) of 10 − 4 M Zn2 + ions. The voltammetric feature of Zn upd changed drastically with pH increase, being different from the aforementioned cases of Figs. 3 and 4; the enhancement of a sharp Zn upd wave with pH increase was clearly seen in the phosphate solutions. The voltammetric feature at pH 1.1 with a small Zn upd wave by 15 mC cm − 2 was similar to that of Fig. 3 (A) or Fig. 4 (A). However, well-defined Zn deposition–desorption waves were observed above pH 2. At pH 2.5–3.0, the negative-going sweep showed the growth of a sharp spike around 0.25 V versus SHE with pH increase. Such a sharp spike is often described as being due to the 2-dimensional phase
Fig. 5. The change of voltammograms of Pt(111) from pH 1.1–4.6 in phosphate solution in the absence (---) and the presence ( — ) of 10 − 4 M Zn2 + ions at 10 mV s − 1. (A) pH 1.1, (B) pH 1.9, (C) pH 2.5, (D) pH 2.8, (E) pH 3.0, (F) pH 3.4, (G) pH 3.7, and (H) pH 4.6.
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formation of upd metal and co-adsorbed anions [58]. At pH\ 3.0, the sharp spike faded into a large peak around 0.3 V versus SHE, being discussed as a result of random Zn deposition in our recent paper [58]. The positive-going sweep gave a clear sharp peak around 0.3 V versus SHE, which grew with pH increase. The charge of the sharp peak related to upd Zn desorption at pH 4.6 amounted to ca. 170 mC cm − 2, which was estimated from the area within the voltammetric curve (solid line) between 0.23 and 0.37 V versus SHE after correction of double-layer charging. The decrease of the adsorbed hydrogen wave below 0.35 V versus RHE by the addition of Zn2 + ions was also pronounced with pH increase. When the coverage of upd Zn was increased with pH increase, the hydrogen adsorption was accordingly obstructed. The onset potential of Zn upd was ca. 0.4 V versus SHE in the pH range studied, being the same as for the cases of Figs. 3 and 4, except for that of 0.5 V at pH 4.6. The onset potential of Zn upd was located around the peak of the anomalous wave in sulfate solutions of Fig. 3, being independent of pH. Accordingly, the coverage of the adsorbed (bi)sulfate on Pt(111) at the onset potential is expected to be constant in the pH range studied. In Fig. 4, a slight overlap of the negative potential foot of the butterfly wave with the upd onset potential appears even at pH 4.6 in perchlorate solutions. This indicates that the coverage of the adsorbed OH at the upd onset potential is quite low if there is any. The coverage of the phosphate species at the Zn upd onset potential increased with pH increase, as shown in Fig. 5. The enhancement of the sharp Zn deposition–desorption wave was observed with the pH increase in phosphate solutions but not in sulfate and perchlorate solutions, although the Zn coverage increased with the pH increase in all solutions investigated as judged by the gradual suppression of adsorbed hydrogen waves. Thus, the enhancement of the sharp Zn upd wave is taken to be related to the increase of pre-adsorbed anion coverage at the Zn upd onset potential from the above discussion; the presence of preadsorbed phosphate seems to co-operate with the Zn upd.
3.3. Effect of pH on Zn upd at Pt(110) in phosphate solutions Zn upd was also examined at Pt(110) in phosphate solutions in comparison with that on Pt(111) for clarification of the effect of pre-adsorbed anion. Fig. 6 (A and B) show the voltammograms of Pt(110) at 10 mV s − 1 in the absence (broken curves) and the presence (solid curves) of 10 − 4 M Zn2 + ions in the phosphate solutions of (A) pH 1.1 and (B) 3.7, respectively. The electrode potential is expressed with respect to RHE. In the absence of Zn2 + ions at pH 1.1, the charge density
Fig. 6. Cyclic voltammograms of Pt(110) in the absence (---) and the presence ( — ) of 10 − 4 M Zn2 + ions in phosphate solutions of pH 1.1 (A) and 3.7 (B) at 10 mV s − 1. Electrode potential is represented with respect to RHE. Vertical dotted line with arrow indicates the potential position of SHE.
of adsorbed hydrogen wave, 220 mC cm − 2, is identical to that in sulfuric acid on Pt(110)–(1×2) [28,52]. The voltammogram was changed drastically by the addition of Zn2 + ions to the solution. This result shows a considerable occurrence of Zn upd on Pt(110) even at pH 1.1, being in contrast to the case of Pt(111) of Fig. 5 (A). The clearness of the Zn upd wave in Fig. 6 (A) is also greater than that on Pt(111) in perchlorate and sulfate solution at pH\3; the upd waves in the case of Pt(111) being spread over a broader potential region. The features of the voltammogram in the phosphate solution of pH 3.7 were also changed by the addition of Zn2 + ions to the solution; the Pt surface seems to be wholly covered by upd Zn and co-adsorbed phosphate from the suppression of the adsorbed hydrogen wave. The charge density including the Zn upd process, Q, was estimated at pH 1.1 in Fig. 6 (A) from the area within the voltammetric curve (solid line) between 0.05 and 0.35 V at the positive-going sweep after correction for double-layer charging. The Q of ca. 360 mC cm − 2 is greater than the charge of 220 mC cm − 2 for the desorp-
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tion of a monolayer hydrogen on Pt(110) – (1× 2); unfortunately, a possibility that the Q value includes not only the charge related to the upd but also the hydrogen adsorption charge cannot be ruled out, since the Zn upd potential region overlaps with the adsorbed hydrogen potential region. However, the excess charge density of 140 mC cm − 2 ( =360 – 220 mC cm − 2), at least, is the minimum charge density of the Zn upd process including phosphate desorption from the Pt surface and its re-adsorption on the Pt surface with upd Zn. The charge amount was much larger than the 15 mC cm − 2 of the small upd wave on Pt(111) in phosphate solution at pH 1.1 in Fig. 5 (A). In the solution of pH 3.7 in Fig. 6 (B), the Q was ca. 320 mC cm − 2, which was estimated from the area within the voltammetric curve (solid line) between 0.05 and 0.60 V at the positive-going sweep after correction for double-layer charging. Wieckowski’s group found by the radiotracer method that the onset potential of (bi)sulfate adsorption on Pt(110) in an acidic solution is more negative than that on Pt(111), being in the so-called adsorbed hydrogen region. They also reported that the coverage of adsorbed (bi)sulfate on Pt(110) increased steeply at the positive limit of the adsorbed hydrogen wave and attained a nearly maximal amount of 2.2× 1014 molecules cm − 2, corresponding to ca. 0.2 of a monolayer [47], where the Zn upd begins [28]. A similar feature of the potential dependence on the amount of adsorbed phosphate anion is expected on Pt(110) from the voltammogram without Zn2 + ions shown by the dotted curve in Fig. 6 (A), since the voltammogram resembles that in sulfuric acid solution [28,52]. It is known that the shape of the so-called adsorbed hydrogen wave is sensitive to the anion adsorption isotherm. The Zn upd onset potential in Fig. 6 (A) coincides with the positive limit of the adsorbed hydrogen wave. We thus conclude that the Zn upd at Pt(110) is also enhanced by the adsorbed phosphate even in acidic phosphate solution of pH 1.1 as previously observed in 0.1 M H2SO4 [28].
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Fig. 7. (A) Cyclic voltammograms of Pt(100) in 0.1 M KH2PO4 (pH 4.4) in the absence (---) and presence ( — ) of 10 − 3 M Zn2 + ions at 10 mV s − 1. (B) Voltammetric profiles of Zn upd on Pt(100) in 0.1 M KH2PO4 (pH 4.4) with 10 − 3 M Zn2 + ions in the absence (---) and the presence ( — ) of 10 − 3 M Cl − ions at 10 mV s − 1.
Fig. 7 (B) shows the effect of chloride ions on the Zn upd at Pt(100) in 0.1 M KH2PO4. The presence of 10 − 3 M Cl − ions sharpened the Zn upd wave with a negative shift of the upd wave by 0.10 V, being identical in shift direction but a higher degree of shift in comparison with that at Pt(111) in the same solution as shown in Fig. 8.
3.4. Effect of adsorbed halides on the Zn upd at Pt(100) and Pt(111) in 0.1 M KH2PO4 of pH 4.4 Fig. 7 (A) shows cyclic voltammograms at Pt(100) in 0.1 M KH2PO4 in the absence (dotted curve) and the presence (solid curve) of 10 − 3 M Zn2 + ions. We previously reported that the Zn upd scarcely occurred on Pt(100) in 0.1 M H2SO4 [28]. In KH2PO4 of Fig. 7 (A), however, the clear Zn upd wave appeared at 0.58 V versus RHE with broad waves in the potential region of 0.5 and 0.05 V versus RHE. The total charge related to Zn upd at positive going sweep was 278 mC cm − 2, which was estimated from the area within the voltammetric curve between 0.75 and 0.05 V after correction of double-layer charging.
Fig. 8. Voltammetric profiles of Zn upd on Pt(111) in 0.1 M KH2PO4 (pH 4.4) with 10 − 3 M Zn2 + ions in the absence (---) and the presence of 10 − 3 M Cl − ( —), Br − ( —- —), and I- ions ( —---—) at 10 mV s − 1.
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Fig. 8 shows the effect of 10 − 3 M chloride, bromide, and iodide ions on the Zn upd at Pt(111) in 0.1 M KH2PO4. The Zn upd peak potentials are shifted negatively in the order of anion adsorption strength of H2PO4− BCl − B Br − BI − . This order indicates that the Zn upd competes with anion adsorption on the Pt(111) surface [29]. The halide anions obstruct the initiation of Zn upd when the upd onset potential of 0.4 V versus SHE without halide ions is located at a more positive potential than the desorption potential of the adsorbed halide, as observed in Zn2 + -free solutions, and as discussed in our previous paper [29]. The Zn deposition peak at Pt(111) in 0.1 M KH2PO4 was not sharpened by addition of Cl − ions, as is shown in Fig. 8, in contrast with the case of Pt(100) of Fig. 7 (B). Furthermore, the addition of Cl − ion shifted slightly the peak potential of the Zn upd wave at Pt(111) by − 0.02 V; the effect of Cl − ions at Pt(111) is smaller than that at Pt(100). This difference appears to be due to the weaker adsorption strength of Cl − ion on Pt(111) than that on Pt(100) [19]. Lucas et al. reported by the X-ray scattering method that the coverage of the adsorbed chloride is 0.4 at 0.2 V(Ag AgCl) with no ordered structure at a Pt(111) electrode in 0.1 MHClO4 +0.01 M KCl [59]. The addition of Br − and I − ions obstructed the initiation of Zn upd significantly at Pt(111) by 0.08 and 0.39 V, respectively, for the negative-going sweep and the voltammetric waves were sharpened significantly, as shown in Fig. 8. The bromide and iodide anions form 2-D ordered adlayers on Pt(111) [59 – 61] just before the occurrence of Zn upd. Thus the Zn upd on Pt(111) competes with these adsorbed anions with a rigidly ordered layer structure. The sharp Zn deposition–desorption wave was irreversible with respect to potential cycling at 10 mV s − 1. The sharpening and irreversibility are characteristic of a 2-D phase transition [58].
4. Summary We have examined the effect of the adsorbed anions on the Zn upd with change of pH, and the addition of halide ions in terms of the degree of the Zn upd deposition–desorption wave reversibility and sharpness under potential cycling, and the position of the upd potential. The current of the Zn upd was scarcely observed at Pt(111) in strongly acidic solutions of pH 1, being irrespective of the kind of anion in the solution. Only in the case of phosphate solutions, did the well-defined sharp and reversible upd waves grow at Pt(111) with the increase of pH in the pH range 2.5 – 4.6. The charge within the voltammetric wave related to the Zn upd process was estimated as ca. 170 mC cm − 2 at pH 4.6. In sulfate and perchlorate solutions, however, such behav-
ior was absent at Pt(111) with pH increase; the broad upd waves were poorly resolved. These facts were explained from the aspect of a co-operative effect of the pre-adsorbed anion to Zn upd. The amount of pre-adsorbed anion is relatively low at the onset potential of Zn upd in a solution of pH 1 and increases with pH only in phosphate solutions when pH] 2.5. The degree of upd Zn on Pt(110) in an acidic phosphate solution of pH 1.1 was significantly higher than that on Pt(111) in the same pH solution. The upd wave on Pt(110) was also well defined compared with that on Pt(111) in sulfate and perchlorate solutions at pH\ 3. This can also be explained in term of the co-operative effect of pre-adsorbed anions to Zn upd, since the coverage of adsorbed anions on Pt(110) in the solutions of pH 1 is expected to be high at the onset potential. Two possible effects of the presence of the pre-adsorbed anion to induce the sharp or clear upd wave are considered. One is that the pre-adsorbed anion plays the role of shielding positive charge at a positively polarized Pt electrode from the Zn2 + ions in the step of Eq. (2) and facilitates the approach of Zn2 + ions to the Pt surface. Another effect is that the pre-adsorbed anions are supplied to the electrode surface with upd Zn as soon as the anions desorb from the Pt(111) substrate with the formation of a stable co-adsorption layer of the anion and the upd Zn. The adsorption of phosphate and sulfate species on upd Zn at Pt(poly) has already been revealed by radio tracer [34,35], FTIR [36], and EQCM [36] studies. The adsorption of halide on Pt(111) in 0.1 M KH2PO4, especially the adsorption of bromide and iodide with a 2-D ordered structure [59–61], competes with the Zn upd process as shown by the negative shift of the upd potential [29]. Adsorption of chloride anion on Pt(100) competes more strongly with the Zn upd than that on Pt(111), because of the stronger interaction of Cl − ions with Pt(100) surface [19,59]. When the upd onset potential of 0.4 V versus SHE is close to the desorption potential of the adsorbed anion by itself in the case of Pt(111) in phosphate solution at pH 4.6, the upd is not obstructed by the pre-adsorbed anions. Values of the upd potential shift, i.e. potential differences between the equilibrium redox potential of Zn2 + (10 − 4 M)/Zn of − 0.883 V versus SHE and the Zn upd onset potentials, DEudps, are 1.28 and 1.18 V on Pt(111) and Pt(110) in phosphate solutions, respectively, as the upd onset potentials are 0.4 and 0.3 V on Pt(111) and Pt(110), respectively. The DEudps are close to the work function differences of 1.4 and 1.3 V between Pt(111) and Zn(polycrystal) and between Pt(110) and Zn(polycrystal), respectively. However, on the addition of halide ions, the upd potential shift on Pt(111) decreases because of obstruction of Zn upd by the pre-adsorbed halides. The relationship between the upd shift and the work function difference will be
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