Cu + Au alloy particles formed in the underpotential deposition region of copper in acid solutions

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Journal of Eleetroanalytical Chemistry 430 (! 997) 69-76

Cu + Au alloy particles formed in the underpotential deposition region of copper in acid solutions Da-ling Lu, Ken-ichi Tanaka * The Institutefor Solid State Physics, The UniversiO,of Tokyo, 7-22-1, Roppon~i, Minato-ku, Tokyo 106, Japan Received 25 June 1996; revised 21 January 1997; accepted 24 January 1997

Abstract

Cu + Au alloy particles electrodeposited on an amorphous carbon electrode at the underpotential region of Cu in both perchloric acid and sulfuric acid solutions were investigated by means of transmission electron microscopy. The fraction of Cu in the Cu + Au alloy particles grown in both acid solutions with a concentration of 1 mM Au ion increased while the underpotential deposition (UPD) potential was decreased. However, it was independent of the concentration of Cu ion in solution. It is inferred that the composition of the Cu + Au alloy particles is dependent on the UPD potential. The fraction of Cu in the Cu + Au alloy particles grown at around the reversible Nernst potential of Cu in 0.1 mM HAuCI 4 + 50raM Cu(CIO4) 2 containing perchloric acid solution was 50%. This result suggests a layer-by-layer formation of the Cu + Au alloy particles. The fraction of Cu in the Cu + Au alloy particles formed in the presence of sulfate was lower than that in the perchloric acid solution as the UPD potential and the concentration of Cu ion were the same. This is attributed to an influence of coadsorbed sulfate ions. © 1997 Elsevier Science S.A. Kevwords: Cu + Au alloy; Alloy particle; Underpotential deposition; Anion; Layer-by-layer growth; Transmission electron microscopy

1. Introduction

Alloy formation by electrochemical codeposition of two metals has been studied by many investigators (e.g. Cu + Ni alloy [1-4], Zn + Co alloy [5,6], Ni + Zn and Ni + Fe alloys [7]) because the characteristics of the alloy, for example tensile strength, ductility [1], magnetic property [8], corrosion resistance [9] and catalytic property [ 10], are markedly influenced by alloying. So far the electrochemical codeposition has usually been done in the overpotential range for the two metals [11], and only a few investigations have dealt with the alloying in the underpotential range (Cd on Au [12], Sn on Ag [13], Cd on Ag [14] and Cu on Pt [15]). However, the alloying in the underpotential range is quite interesting because an electrochemically deposited alloy formation occurs in the underpotential deposition (UPD) range of one metal which makes no bulk deposition in this range. In this paper we report the formation of Cu + Au alloy by the codeposition of Cu and Au in the UPD range of Cu ion in acid solutions. We chose the system of Cu + Au

* Corresponding author. Tel.: (+81) 3 34786811; fax: (+81) 3 34015169. 0022-0728/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PII S0022-0728(97)00074-0

because the UPD of Cu on polycrystalline Au [16,17], Au(l 11) [18-34], Au(110) [231 and Au(100) [21,23,35-37] has been studied precisely by means of LEED, RHEED and AES [18,19], SEXAFS [20], XPS [22,35], AFM [24,31,36] STM [21,23,25-28,30,37], quartz crystal micro-balance [ 17,38], voltammetric [ 16,29,32-34] and rotating ring-disk electrode (RRDE) [29] measurements. The cyclic current-potential curves in 1 mM Cu 2+ containing sulfate solution showed that the current spikes for the UPD of Cu on Au(100) occur at about 0.22V vs. a saturated calomel electrode (SCE), and on Au(110) at about 0.27 V (SCE) [23]. In both cases the Cu adlayers give a (1 × 1) structure as observed by STM [21,23,37]. However, the negative sweep current-potential curve [23] for the deposition of Cu on Au(l 11) in the underpotential range shows two current spikes: the first one appears at about 0.215 V (SCE), the second one at about 0.04 V (SCE). By means of STM [26,28,30] the first current spike is shown to correspond to the formation of (~/3 × J3)R30 ° Cu superlattice established by the coadsorption of sulfate ions, and the second one reflects the formation of (1 × 1) Cu phase. It should be pointed out that in 1 mM Cu 2÷ containing perchlorate solution [19,24] only one current spike appears at ca. 0.35 V (SCE) for the deposition of Cu on Au(l 11) in

70

D.-L Lu, K.-i. Tanaka~Journal of Electroanalytical ChemistD"430 (1997) 69-76

the underpotential range, which gives a (1 × 1) structure of Cu adlayer. In this experiment the shape and the composition of the Cu + Au alloy particles formed in the underpotential range of Cu were studied in sulfuric acid and perchloric acid solutions. To our knowledge, it is the first report on the formation of the Cu + Au alloy particles grown in the underpotential range.

2. Experimental The codeposition of Cu + Au particles was performed in a classic three-electrode cell at room temperature. A gold mesh for the transmission electron microscope (TEM) was used as the working electrode, as described in our previous paper [39]. Before the experiment the gold mesh was covered with a collodion film on which an amorphous carbon film was evaporated. The counter electrode was a 0,5 mm diameter platinum wire and the reference electrode was an SCE. The electrolytes were prepared from HCIO4 (60% solution), H2SO 4 (97% solution) (Wako), HAuCI 4 (99%) (Kanto), CuSO 4 . 5 H 2 0 (99.999%), Cu(CIO4) 2 (98%) (Aldrich) and triply distilled water. To investigate the effect of the ratio of Au to Cu concentrations in solution on the formation of alloy, the concentration of Au ion was fixed at a constant value in the main cell while the concentration of Cu ion was varied. Therefore, the adopted solutions were 0.1M HCIO4 + I mM HAuC! 4 + x mM Cu(CIO4) ., and 0.05 M H2SO 4 + I mM HAuCI 4 + x mM CuSO 4 (x = 1, 5, 10, 50 and 100), and 0.1 M HCIO 4 + 0.1 mM HAuCI 4 + y mM Cu(CIO 4)2 (Y = 0. !, 0.5, I, 5, 10, 50, 100). The electrolyte solution in the main compartment cell was deaerated by bubbling high purity argon. The Cu + Au alloy particles formed on an amorphous carbon electrode were observed by means of a Hitachi-9000 high-resolution TEM. The lattice spacings of the Cu + Au alloy particles of different Cu content, which were formed at different deposition potentials in the acid solutions with a variety of concentrations of Cu ion, were derived from the Debye-Scherrer rings of the Cu + Au alloy particles by means of a Hitachi-700 TEM. The alloy composition of the Cu + Au alloy particles was evaluated by using a chart of Cu + Au solid solution spacings vs. composition given by Pearson [40].

3. Results and discussion 3. i. bz O.1 M HCIO 4 + 1 mM HAuCI 4 + Cu(Cl04 )2

The fraction of Au in the Cu + Au alloy particles grown in 0.1 M HCIO4 + I mM HAuCI 4 solution changed with the deposition potential and the concentration of Cu ion as shown in Fig. l(a). Fig. l(b) is replotted from the data in

Fig. l(a) against the difference of the deposition potential E from the reversible Nernst potential E r of Cu, that is E - E r. Fig. l(c) shows an electron diffraction ring image for the Cu + Au alloy particles prepared in the perchloric acid solution, which is compared to that of the Au particles prepared in Cu 2+-free solution. It can be seen clearly from Fig. l(b) that the fraction of Cu in the Cu + Au alloy particles formed in an overpotential deposition (OPD) range of Cu ion ( E < E r) depends on the concentration of Cu ion, that is the higher the concentration of Cu ion the higher the Cu fraction in the alloy particles. This phenomenon can be explained by a diffusion-controlled deposition of Au and Cu in the OPD range for both Cu and Au ions. When the ratio of Cu ion to Au ion was 10 in solution, that is in 10mM Cu(CIO4) 2 solution, the Cu + Au alloy particles formed at ca. ( E - E r) = - 0 . 0 3 6 6 V (Fig. l(b)) took the ratio of C u / A u = l. This fact indicates that the deposition rates of Cu and Au ions become almost equal at this potential in the solution of 10mM Cu 2+ and I mM AuC! 4 . Contrary to this, the fraction of Cu in the Cu + Au alloy particles formed in the UPD range ( E > E~) is independent of the concentration of Cu ion in the solution as is shown by a composition-deposition potential curve drawn in Fig. l(b). The log of the fraction of Au in the Cu + Au alloy particles formed in the UPD range of Cu ion changed linearly with l n ( E - E~) as shown by the line I in Fig. l(d) replotted from Fig. l(b), which is described by the empirical relation CAu

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where CA,, is the percentage of Au in the Cu + Au alloy particles, a is a constant depending on the coexisting anions, a is a constant smaller than unity. Under the present case, a = 0.99, a = 0.1. The meaning of a is discussed below. Htilzle et al. [34] investigated the kinetics of structural changes in Cu adlayers on A u ( l l l ) by a potential-step method and observed that transient current peaks were shifted with time when the potential was stepped down in the UPD range. It is inferred that a deposition rate of Cu ion on Au(l I 1) surface in the UPD range is dependent on the electrode potential, so that the time to complete one monolayer of the UPD of copper on A u ( l l l ) surface varies with electrode potential. Schultze and Dickertmann [41] reported that it took I min for adsorption equilibrium of Cu on gold electrodes in perchlorate solution. Shi and Lipkowski [32,33] suggested that at a potential in the UPD range of Cu 10 min is required to attain adsorption equilibrium of Cu on Au(l 11) in sulfate solution. Taking account of these statements and our experimental results, it can be supposed that the completion of the UPD of Cu on the Au layer which covers the growing alloy particles is controlled by the UPD potential. Therefore, the composition of the alloy particles depends on the deposition potential but less on the concentration of Cu ion in solution, as shown in Fig. l(b).

D.-l. Lu, K.-i. Tanaka/Journal of Electroanalytical Chemistry 430 (1997) 69-76

At the potentials shown by the filled symbols (indicated by arrows) in Fig. l(a), Cu20 particles were formed together with the Cu + Au alloy particles, where the potentials were in the OPD range of Cu. This phenomenon is consistent with our previous result [42], that is the formation of CuzO particles precedes the formation of Cu particles in the OPD range of Cu. The present result suggests that the growth of Cu20 particles and the formation of alloy particles by the deposition of Cu and Au ions proceed ir'.dependcm!y as the deposition condition, eP~er the concentration of Cu ion in solution or the deposition potential, is appropriate. Fig. 2 shows TEM images of the Cu + Au alloy particles and Au particles. Fig. 2(a) and Fig. 2(b) show the Au particles formed at 0.3 V and 0.05 V respectively, where the octahedral single crystal particles or polycrystalline particles are formed as described in our previous report [39]. However, the contour of the Cu + Au alloy particles took a round shape (Fig. 2(c)) as the fi'action of Cu in these alloy particles is less than 15%. When the fraction of Cu in the Cu + Au alloy particles increased above 20%, the shape of the alloy particles became complex (Fig.

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2(d)). This might be caused by different deposition rates of Au ions on different planes of the alloy particles, which are responsible for the crystal shape. At the potential near the reversible Nernst potential of Cu (E - Er -- 0) no large particles grew, but a coalesced film-like layer was formed (Fig. 2(e)). It was found that the decahedral and icosahedral Cu + Au alloy particles were formed at the potentials at which neither the icosahedral nor the decahedral pure Au particles are formed (see Fig. 2(a) and Fig. 2(c)). We supposed that the formation of the decahedral and icosahedral Au particles requires a certain contraction of the topmost layer for the growing particles [39]. Taking account of the OPD range of Au deposition, the topmost layer of the growing alloy particles is expected to be an Au layer. Therefore, the formation of the decahedral and the icosahedral alloy particles is caused by the contraction of the topmost Au layer, which is induced by the adsorption of Cu ions. If this is the case, the growth of icosahedral and decahedral Cu + Au alloy particles is similar to the growth of decahedral or icosahedral Au particles. That is, the contraction of the Au overlayer in either case is responsible for the

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In (E - E0 Fig. 1. (a) Pl0t of the fraction of Au in the Cu + Au alloy particles electrodeposited in 0.1 M HCIO 4 + 1 mM HAuCI 4 + Cu(CIO4) 2 solution vs. SCE. CA, is the percentage of Au in the Cu + Au alloy particles. (b) A graph replotted from (a) against ( E - Er), where E is an experimental codeposidon potential vs. SCE and E r is the reversible Nemst potential of Cu. The left side of the vertical dotted line at ( E - E,) = 0 V is the OPD range of Cu ion, the right side is the UPD of Cu ion. (c) A typical example of comparison of electron diffraction Debye-Scherrer rings of the Au particles grown in 0.1 M I-ICIO4 + 1 mM HAuCI 4 at - 0 . 5 V (left) and the Cu + An alloy particles consisting of 30% Cn formed in 0.1M HCIO 4 + I mM HAuCi4 + 50n~Vl Cu(CIO4) 2 at 0.1 V (right). (d) The data of Fig. l(b) (curve I) and Fig. 3(b) (curve II) are replotted as the relation of In CA,, VS. In ( E - E r). The symbols indicate the concentration of Cu ion in solution. The filled symbols (indicated by arrows) indicate the C u 2 0 particles grown simultaneously with the Cu + Au alloy particles.

72

D.-l. Lu, K.-i. Tanaka/Journal of Electroanalytical Chemistry 430 (1997) 69-76

Fig. 2. Images of the Au particles and the Cu + Au alloy particles taken by TEM: (a) in O. ! M HCIO4 + I mM HAuCI 4 at 0.3 V vs. SCE for 60 s; (b) at 0.05 V vs. SCE for 120s; (c) in 0.1 M HCIO4 + I mM HAuCI 4 + 100mM Cu(CIO4) 2 at 0.3 V vs. SCE for 300s, the fraction of Cu in the alloy particle is ca. 15%; (d) in 0.1 M HCIO4 + I mM HAuCI 4 + 10mM Cu(CIO4) 2 at 0.1 V vs. SCE for 300s, the fraction of Cu in the alloy particle is ca. 24%; (e) at 0V vs. SCE ( - 0 . 0 3 6 6 V vs. ( E - Er)) for 300s, the fraction of Cu in the alloy particle is ca. 50%.

growth of decahedra and icosahedra of Cu + Au alloy particles or Au particles. 3.2. in O.05M H,.SO 4 + ! m M HAuCI 4 + CuSO 4

The composition of the Cu + Au alloy particles is plotted against the deposition potential in sulfuric acid solution in Fig. 3(a) and replotted in Fig. 3(b) against the difference of the electrode potential from the reversible Nernst potential of Cu (E - Er). By comparing the result shown in Fig. 3(b) (presence of sulfate ion) to that in Fig. l(b) (absence of sulfate ion), it is known that the fraction of Cu in the Cu + Au alloy particles formed in the sulfuric acid solution was lower than that in the perchloric acid solution. This phenomenon may be explained by the coadsorption of SO4- ions during the growth of the alloy particles. The UPD of Cu ion on the Au electrode is accompanied by an adsorption of a significant amount of these anions [43]. Zei et al. [19] showed that the coadsorption of CIO4 anions with SO4- on Au(l 1 !) surface Influences the deposition of Cu ion. However, the structure of Cu deposited on Au(111) in sulfate solution takes substantially more open structure than that obtained in perchlorate solution [24]. Blum and Huchaby [44,45] proposed a structural model for the UPD of Cu on Au(111) in sulfuric acid where the bisulfate ions are coadsorbed with Cu ions by making a (~/3 × ~/3) lattice on Au(111) surface. On the other hand, it

has been known that UPD of Cu on A u ( l l l ) takes a p(l × !) structure in a perchioric acid. Our results showed that the Cu + Au alloy particles formed in perchloric acid solution apparently take a higher Cu composition than that formed in sulfuric acid solution at the same electrode potentia!~ The difference between the density of Cu in the (~/3 × ~/3) and p(l x 1) structures formed on the Au overlayer of Cu + Au alloy particles in sulfate and perchlorate solutions may be responsible for the different composition of the alloy particles. Considering the experimental error, a composition-deposition potential curve in the underpotential range was drawn in Fig. 3(b) and then replotted as the line II in Fig. l(d). The slopes of line I and line II are identical, which indicates that Eq. (1) is established not only in the perchloric acid solution but also in the sulfuric acid solution. In the sulfuric acid solution, a = 1.07, a takes the same value of 0.1. From Fig. l(d), the potentials which give the same fraction of Cu of the alloy particles are different by 0.08 V in these two solutions, which corresponds to the value of [In a(SO 2- ) - In a(CIO4)]. That is, this difference is an additional potential required for getting the same composition of Cu + Au alloy particles in the sulfuric acid solution compared to the perchloric acid solution. Therefore, a is a parameter reflecting the adsorption strength of anions.

D.-L Lu, K.-i. Tanaka~Journal of Eiectroanalytical Chemistry 430 (1997) 69-76 I00

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4(a)) and the alloy particles formed in perchloric acid solution (Fig. 2(c)). We observed the formation of the decahedral and icosahedral Cu + Au alloy particles not only in the perchloric acid solution but also in the sulfuric acid solution. It was suggested that the valence state of Cu ions adsorbed on Au(l I l) in the UPD range is different depending on the electrode potential [29,33,46,47]. The UPD Cu ions deposited on Au(111) have a positive charge at the first deposition current spike (ca. 0 . 2 1 5 V vs. SCE in I mM Ca 2+ solution [23]) and are completely discharged at the second deposition current spike (ca. 0.04V vs. SCE in I mM Cu 2+ solution [20,23]). The codeposition electrode potentials for the formation of Cu + Au alloy particles in this case wei'e chosen in the potential range of the first current spike. It is inferred that the decahedral and icosahedral Cu + Au alloy particles are given by the adsorption of incompletely discharged Cu + ion on the Au ovedayer, because the positive charge of the adsorbed Cu + ion will induce the lateral contr-z,ction of the Au overlayer, which is analogous to the reconstruction of Au surface induced by the adsorption of alkali metals. We discussed this phenomenon in another paper [48]. 3.3. In O.I M HCIO 4 + O.l m M HAuCI 4 + Cu(CI04) 2

Fig. 3. (a) Plot of the fraction of Au in the Cu + Au alloy particles electrodeposited in 0.1M HCIO4 + I mM HAuCI4 +CuSO4 solution vs. SCE. cA. is the percentage of Au in the Cu + Au alloy particles. (b) A graph replotted from (a) against ( E - Er), where E is an experimental codeposition potential vs. SCE and Er is the reversible Nernst potential of copper. The left side of the vertical dotted line at ( E - Er) = 0V is the OPD range of Cu ion, the right side is the UPD of Cu ion. The symbols indicate the concentration of Cu ion in solution. The filled symbols (indicated by an arrow) indicate the Cu_~Oparticles grown simultaneously with the Cu + Au alloy particles.

Fig. 4 shows clear evidence for an effect of sulfate ions upon the growth of the Cu + Au alloy particles. The alloy particles grew into a thorn-like shape (Fig. 4(b)) in this case. One can suppose ~hat the adsorption of sulfate ions took place much more intensely on certain faces of growing particles, so that the habit of the Cu + Au alloy particles is strongly influenced; this can be seen clearly by comparing these particles with the pure Au particles (Fig.

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When the OPD rate of Au is controlled by the diffusion rate of Au ion, the deposition rate of Au is constant at a fixed concentration of Au ion. As a result, the fraction of Cu in the alloy particles depends sensitively on the concentration of Cu ion even in the UPD range of Cu ion. In our experiments, the composition of alloy formed in 0. l mM of Au ion sensitively changed with the concentration of Cu ion, but that formed in I mM of Au ion changed not so sensitively with the concentration of Cu ion. It is known that the shift of potential of current spike, reflecting the adsorption of Cu ion on Au(111) in the UPD range, depends on the concentration of Cu ion in solution [29]. The shift coincides with the calculated reversible Nernst equation for different concentrations of Cu ion. Therefore, it, can be inferred that the UPD range of Cu is constant vs. the reversible Nemst potential. The experimental codeposition potentials for the formation of the

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Fig, 4, Images of the Au particles and the Cu + Au alloy particles taken by TEM: (a) in 0.01M H2SO 4 + I mM HAuCI4 at 0.3 V vs. SCE for 60s; (b~ in 0.05M H 2 S O 4 4- I raM HAuC! 4 4- 1 0 0 m M CuSO4 at 0.32V vs. SCE for 300s, the fraction of Cu in the Cu + Au alloy particles is ca. 6.5%.

~~ -I. Lu, K.-i. Tanaka~Journal of Electroanalytica/ Chemistm, 430 (1997) 69-76

74

and this phenomenon is more obvious than that in l mM HAuCl4 solution (Fig. 2(d)). Fig. 6 presents the fraction of Cu in the Cu + Au alloy particles vs. electrode potential formed in solutions of different concentrations of Cu ion. This result can be classified into two groups for the discussion. The first group (Fig. 6(a) and Fig. 6(b)) is the alloy particles formed in the solution of 0.1 to l mM Cu ion. In this range, the fraction of Cu in the Cu + Au alloy particles did not vaLv greatly with the deposition potential but increased as the concentration of Cu ion increased. This is attributed to the diffusion-controlled UPD rate of Cu ion from the solution onto the Au overlayer formed on the Cu + Au alloy particles by the diffusion-controlled OPD of Au ion. There was a small increase in the fraction of Au in the alloy particles as the electrode potential decreased. It is supposed that the more negative deposition potential makes the diffusion rate of Cu ion a little faster than that of Au ion. The second group (Fig. 6(c) and Fig. 6(d)) is the alloy particles tbrmed in a solution containing a concentration of Cu ion more than 100 times that of Au ion. The fraction of Cu in the alloy particles was constant on the whole at ( E - E r) > 0.01 V. Taking account of the fact that the concentration of Cu ion near the electrode is sufficiently high, the growing Au overlayer formed on the alloy particles was immediately covered by the UPD Cu layer, so that the composi-

Fig. 5. TEM image of the C u + A u alloy particles ~ o w n in 0.1M HCIO4 +0. l mM HAuCI 4 + 5 m M Cu(CI04) 2 at 0.058V vs. SCE (0.03V vs. ( E - Er)) for 300s. the fraction of Cu in the alloy particles is ca. 36%.

Cu + Au alloy particles in the case of 0.1 mM AuCI~ were adopted in the range of 0V to 0.04V vs. ( E - E,). It is worth noting that neither decahedral nor icosahedral Cu + Au alloy particles were formed in this UPD range. The fact that the fraction of Cu in the Cu + Au alloy particles formed in this range exceeds 30% may be responsible for the difficulty in forming decahedral or icosahedral particles, as discussed in Ref, [48]. The shape of the Cu + Au alloy particles formed in this potential range (Fig. 5) is quite different from that of the particles shown in Fig. 2. This may reflect different deposition rates of Au ion on different crystal planes of the alloy particles,

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D. -L Lu. K.-i. Tanaka/ Journal of Electroanalytical Chemistry 430 (1997) 69- 76

tion of the alloy particles is less dependent on the concentration of Cu ion. It is an interesting fact that the fraction of Cu in the alloy particles increased steeply to reach ca. 50% when the deposition potential approached (E - Er) = 0 V (Fig. 6(d)). Under the UPD condition for Cu ion, one layer of Cu can deposit on the Au layer over the alloy particles but no subsequent deposition of Cu occurs. When the alloy particles are covered by the overpotential-deposited Au layer, the surface is immediately covered with one layer of underpotential-deposited Cu ion when the concentration of Cu ion is sufficiently high. In this case, the growth rate of the alloy particles is equal to the OPD rate of Au ion, and the composition of the alloy can reach 50% Cu and 50% Au. In fact, as the concentration of Au ion was low (0.1 mM AuCi 4) but the concentration of Cu ion was sufficiently high (above 50mM Cu2+), an alternative layer-by-layer growth of Cu and Au layers was established at ( E - E r) = 0V, and alloy particles composed of 50% Cu and 50% Au were formed. One can also deduce that a layer-by-layer growth of Cu and Au may proceed in the case of l mM AuCl 4 if the concentration of Cu ion in solution is sufficiently high, but this was not covered in the present work.

4. Conclusion Cu + Au alloy particles were prepared by an electrochemical codeposition of Cu and Au ions in the OPD and in the UPD ranges of Cu ion in acidic solutions. The mechanism for the alloy formation is entirely different in the overpotential and the underpotential ranges of Cu ion, where, in either case the deposition of Au is at overpotentiai. The composition of Cu + Au alloy particles formed by the OPD of Cu ion was decided by the relative deposition rate of Cu and Au ions which depends on the concentration in the solution. Therefore, the composition of the alloy particles was controlled by the relative diffusion rate of Cu ion to Au ion. The formation of Cu + Au alloy particles in the UPD of Cu ion is different from that in the OPD range. When the Cu + Au alloy particles were prepared in a 0. I mM HAuC! 4 solution with lower than I mM of Cu ion, the fraction of Cu in the alloy particles varied regularly with the deposition potentials, that is the composition of the Cu + Au alloy particles was decided by the rate of the UPD of Cu ion relative to the growth of Au layers by the OPD on the alloy particles. Therefore, the composition of the aiioy particles depends also on the concentration of Au ion. In contrast, the composition of alloy particles was less sensitive to the concentration of Cu ion when the concentration of Cu ion was sufficiently high. In fact, when the concentration of Cu ion was higher than 10 mM, the composition of the alloy particles w~s independent of the concentration

75

of Cu ion. That is, the Au overlayer formed on the alloy particles by OPD may always be covered with one layer of Cu by UPD during the growth. As a result, a layer-by-layer mutual deposition of Au and Cu ions may be established at the potential of (E - Er) = 0 in 0. l mM HAuC14 + 50 mM Cu(CIO4) 2 containing perchlorate solution, which yielded 50% Cu and 50% Au alloy panicles. It was also found that the adsorption of SO42- ions during the growth of the alloy particles depended not only on the alloy composition but also on the habit of the alloy particles.

Acknowledgements We gratefully acknowledge Mr. K. Suzuki and Mr. M. Ichihara of ISSP for their help in the TEM experiments. This work was supported by a Grant-in-Aid for Science Research (05403011) of the Ministry of Education, Science and Culture of Japan.

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