Fabrication of Gold Nanoprism Thin Films and Their Applications in

Jan 9, 2009 - thin films (Au-PFs for short) on the indium tin oxide (ITO) coated glass substrates. ... The Au-PFs were further used as the substrate e...
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J. Phys. Chem. C 2009, 113, 1738–1745

Fabrication of Gold Nanoprism Thin Films and Their Applications in Designing High Activity Electrocatalysts Wenjing Li, Houyi Ma,* Jintao Zhang, Xiuyu Liu, and Xingli Feng Key Laboratory for Colloid and Interface Chemistry of State Education Ministry, School of Chemistry and Chemical Engineering, Shandong UniVersity, Jinan 250100, China ReceiVed: September 25, 2008; ReVised Manuscript ReceiVed: NoVember 23, 2008

A simple, one-step wet chemical method was developed for fabrication of closely packed gold nanoprism thin films (Au-PFs for short) on the indium tin oxide (ITO) coated glass substrates. Most Au prisms have micrometer-scale edge length and nanometer-scale thickness (herein defined as nanoprism according to the thickness). They are single crystalline, whose basal surfaces are atomically flat {111} planes and lateral surfaces are {110} planes. The Au-PFs were further used as the substrate electrodes to construct bimetallic and trimetallic Au-based catalysts. A tiny amount of Pt or Pd, the equivalent of a monolayer, was deposited onto the Au nanoprism thin film electrodes (Au-PFEs for short) by the underpotential deposition (UPD) of a Cu atomic layer on the Au surfaces, followed by redox replacement of the UPD Cu with a Pt or Pd monolayer. Interestingly, once after surface modification with a Pt monolayer, Au-PFEs exhibited greatly enhanced catalytic activity toward the electrooxidation of methanol and much better poison resistance than commercial Pt-based catalysts. The as-prepared bimetallic Au-Pt and trimetallic Au-Pd-Pt catalysts are expected to act as the promising electrocatalysts for the methanol oxidation. 1. Introduction Gold is usually viewed as an inert metal, but surprisingly it has been found that Au in nanoscale exhibits unexpectedly high catalytic activity for oxidation of CO, hydrochlorination, and epoxidation of propene.1 Because intrinsic properties of metal nanostructures can be tailored by controlling their size, shape, composition, and crystallinity,2 size- and shape-controlled synthesis of metal nanoparticles has been the focus of intensive research.3-6Among gold nanoparticles of different shapes, microand nanoscale gold prisms are a particularly interesting class of structures because of their unusual shapes and unique optical features.7 Most gold prisms prepared by using wet chemistry methods are triangular or hexagonal prisms that have submicrometer scale edge length and nanometer scale thickness.8 The perfect single-crystalline structure (atomically flat base and end)9 makes them act as quasi Au single crystal electrodes to characterize electrocatalytic activity of Au toward the methanol oxidation. Moreover, the Au-PFs can be used as substrate electrodes to fabricate bi- or multimetallic (alloy) catalysts with better catalytic performance. At the present time, Pt is the catalyst material most frequently used in low temperature PEMFC (proton exchange membrane fuel cell) systems, including DMFCs (direct methanol fuel cells).10 However, it is poisoned readily by strongly adsorbed CO species (COads), an intermediate produced in electrooxidation of small organic molecules.11 In contrast, although catalytic activity of Au toward the methanol oxidation is much lower than that of Pt in acidic media, a great advantage of Au catalysts over the Pt-based catalysts is that the poisoning intermediates are not formed.12,13 Besides, the adsorption of oxygen species (for example, OH- anions) on the Au surface has a great promoting role to the electrooxidation of methanol or other alcohol molecules.12,14 * Corresponding author. E-mail: [email protected]. Phone: +86-53188364959. Fax: +86-531-88564464.

The combination of Au and Pt provides good opportunities for making novel and high performance bimetallic catalysts. In Pt-Au catalysts, it is possible that, the CO-poisoned Pt can be regenerated through the reaction of surface CO with oxygen species associated with Au elements to form CO2. The research interest in the design and fabrication of Pt-Au nanocatalysts has grown constantly. For example, Lang et al. and Zhou et al. demonstrated that Au-Pt bimetallic nanoparticles showed the superior CO oxidation activity and CO tolerance ability in comparison with the pure Pt or Au nanoparticles.15 Kristian et al. reported that the Pt-decorated Au nanoparticles markedly promoted the direct oxidation of formic acid into CO2 by inhibiting the formation of CO-like poisoning species via “ensemble” effect.16 In this paper, we employed the underpotential deposition (UPD)-redox replacement technique17 to design and fabricate bimetallic and trimetallic noble metal catalysts based on the Au-PFs. By taking the respective advantages of Pt, Pd and Au, the as-prepared bimetallic (Au-Pt) and trimetallic (Au-Pd-Pt) catalysts exhibit much higher catalytic activity and stronger poison tolerance than those of pure Pt catalysts. These new findings are not only of fundamental importance to have a better understanding of the synergistic effect between different metal compositions on the combined catalytic ability but also helpful for the development of more effective catalysts suitable for the DMFCs. 2. Experimental Section 2.1. Chemicals. All chemicals were of analytical grade and used as received from the suppliers. The aqueous solutions were prepared with AR reagents and triply distilled water. 2.2. Fabrication of Au Nanoprism Thin Films (Au-PFs). All glassware used to fabricate Au-PFs was cleaned ultrasonically in HCl/HNO3 (3:1 v/v) and rinsed with triply distilled water. Au nanoprisms were prepared by the slow reaction of gold precursors (HAuCl4) with ethylene glycol (EG) in the

10.1021/jp8085123 CCC: $40.75  2009 American Chemical Society Published on Web 01/09/2009

Fabrication of Gold Nanoprism Thin Films presence of poly(N-vinylpyrrolidone) (PVP) at ambient temperature. A homogeneous reaction mixture was formed by mixing 20 mL of EG, 0.5 mL of 0.05 mol dm-3 HAuCl4, 19.5 mL of ultrapure water, and 10 mL of 1.7 mmol dm-3 PVP (PVP K30, MW ≈ 58 000), in which EG served as a weak reductant to reduce AuCl4- ions, and PVP worked as both stabilizer and shape-controller to induce preferential growth of Au nanoparticles. The color changes of the reaction mixture indicated the formation of Au nanoparticles. Before use, the sheets of indium-doped tin oxide coated glass (ITO glass for short) needed a thorough surface cleaning process, followed by the surface modification, to fabricate well-defined Au-PFs on the ITO glass substrates. The ITO glass sheets were cleaned ultrasonically in soapy water, deionized water, acetone, methanol, and ultrapure water for 20 min respectively, in proper order,18 and then modified with (3-aminopropyl)triethoxysilane (APTS) by immersing them into an ethanol solution containing 2 mmol dm-3 APTS for 24 h. By putting the APTS-modified ITO glass sheets into the reaction mixture used to prepare the Au nanoprisms, the preferential growth of Au on the ITO glass surfaces would spontaneously take place, which led to formation of closely packed Au-PFs. 2.3. Fabrication of Bimetallic and Trimetallic Nanocatalysts. Bimetallic and trimetallic Au-based catalysts for the methanol oxidation were fabricated on the basis of the Au-PFs using the underpotential deposition (UPD)-redox replacement technique. A UPD monolayer of copper17 was at first deposited onto the Au-PFs by means of potentiostatic polarization at a designated UPD potential (0 V) in a solution of 1 mmol dm-3 CuSO4 + 0.1 mol dm-3 H2SO4. The Cu-UPD potential region was determined according to the cyclic voltammograms (CVs) of the Au-PFE in the same solution. The Pt or Pd monolayer was prepared on a Au-PFE by galvanic replacement of the Cu UPD monolayer in a solution of 4.8 mmol dm-3 K2PtCl4 + 0.1 mol dm-3 HClO4 or in a solution of 5.6 mmol dm-3 PdCl2 + 0.1 mol dm-3 HClO4. The alternating monolayers of different metals (for example Au/Pt/Pd) were deposited layer-by-layer on an Au-PFE through repeating the Cu-UPD-redox replacement cycles. 2.4. Electrochemical Measurements and Morphology Characterization. All electrochemical measurements were performed with LK 2005 and CHI 760C electrochemical workstations at room temperature (∼22 °C). The electrochemical cell had a three-electrode configuration. A saturated calomel electrode (SCE) and a bright Pt plate with a surface area of 4 cm2 acted as the reference and counter electrodes, respectively, and the ITO glass sheets covered with Au-PFs or further modified with other noble metals were used as the working electrodes. The reference electrode was led to the surface of the working electrode through a Luggin capillary. Electrochemical properties of modified and unmodified Au-PFEs were characterized in 0.1 mol dm-3 HClO4 solutions, and their electrocatalytic activity toward the methanol oxidation was measured in 0.1 mol dm-3 HClO4 + 0.4 mol dm-3 methanol mixed solutions. The electrolyte solutions were de-aerated by N2 bubbling for 10 min prior to the experiments, and a blanket of N2 was maintained throughout the experiments. Electrochemical active surface areas of the Au-PFEs were estimated by integrating the charge needed to form gold surface oxide monolayers following the method proposed by Woods et al.19 and Trasatti and Petrii.20 The amount of the underpotentially deposited Cu on the Au-PFE may be calculated according to Faraday’s Law. Considering that the Cu UPD monolayer was completely displaced by the noble metals (Pt and Pd), the

J. Phys. Chem. C, Vol. 113, No. 5, 2009 1739 amount of Pt (or Pd) deposited on the Au-PFE in each replacement process can be estimated in terms of the amount of UPD Cu monolayer. Surface morphologies of Au-PFE before and after modification with noble metal monolayer films were observed by a JSM6700F field emission scanning electron microscope (FESEM) operating at 10 kV. 3. Results and Discussion 3.1. Formation of Au-PFs on ITO Glass Substrates. Much effort has been devoted to the synthesis of Au nanoprisms (or nanoplates) with controllable shapes, however, among the synthetic methods reported previously, only a few methods are applicable to direct construction of well-defined Au-PFs on solid substrates in aqueous solutions.21,22 Herein, a convenient method to make uniform Au-PFs on ITO glass substrates was developed on the basis of the preferential growth of Au nanocrystal seeds during the slow reduction process of AuCl4- ions with EG in the presence of PVP polymers. In the aqueous solution of HAuCl4, PVP, and EG, the PVP’s shape-directing role and the slow reduction of Au(III) ions by EG are two necessary conditions for the formation of flat, singlecrystalline Au nanoprisms with micrometer-scale edge length through the preferential growth of the Au nanoprism seeds along 〈110〉 direction.9 PVP was able to promote the preferential growth of Au nanocrystal seeds along 〈110〉 direction since the selective adsorption of PVP macromolecules on the {111} planes of Au nanocrystals significantly suppressed the growth rate of the crystal seeds along 〈111〉 direction. At the same time, the slow reaction allowed the Au(III) ions to be slowly reduced onto the surface of a growing nanoprism seed.7 In this way, the edge length of Au prisms can increase up to micrometer level without changing their thickness and crystallinity. An important finding is that the gold atoms (or nanoclusters) preferred the heterogeneous nucleation at the solid/liquid interface to the homogeneous nucleation in the bulk solution. This was confirmed by an experimental phenomenon that a pink thin film first formed on the wall and at the bottom of glass containers prior to the obvious variation of the color of the reaction mixture. In order to obtain high quality Au-PFs on the ITO glass substrates, the glass sheets must be repeatedly cleaned, followed by an appropriate surface modification. The APTSmodified ITO glass sheets were proved to fully meet the requirements for fabrication of well-defined Au-PFs. Considering that Au nanocrystals have a strong tendency to be immobilized on the APTS-modified ITO glass substrate, when placing a slide glass along the wall or at the bottom of a beaker containing the reaction mixture, the gold nanocrystals would spontaneously grow on the glass substrate, finally forming closely packed Au-PFs. A typical large-scale SEM image (Figure 1a) shows that a large number of gold prisms were close-packed on the support and almost completely covered the whole glass substrate. On closer inspection from an enlarged SEM image (Figure 1b), most Au prisms appeared in the form of regular triangles, truncated triangles and pseudohexagons, with edge lengths ranging from 0.2 to 2.5 µm. The inset in Figure 1a shows the side view of an Au nanoprism, which allows precise measurement of its thickness. From this lateral image, its thickness is determined to be ∼49.6 nm. Such Au prisms possess the larger planar sizes than those obtained with previous methods. Besides, the present method is simpler and more convenient in comparison with the previous methods.21

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Figure 1. Low- (a) and high-magnification (b) SEM images of a gold nanoprism thin film electrode (Au-PFE for short). The inset shows the side view of a gold nanoprism.

Figure 2. Cyclic voltammograms (CVs) of an Au-PFE in 0.1 mol dm-3 HClO4 solutions with and without 0.4 mol dm-3 methanol. The potential scan rate was 50 mV s-1.

Figure 3. Cyclic voltammograms (CVs) for an Au-PFE in 0.1 mol dm-3 H2SO4 + 1.0 mmol dm-3 CuSO4 solution. Scan rate: 50 mV s-1.

3.2. Voltammetric Behavior and Electrocatalytic Activity of the Au-PFEs. Electrochemical property of Au-PFs was characterized by means of cyclic voltammetry using an ITO glass sheet covered with the Au-PFs as a working electrode (i.e., Au-PFE). Figure 2 shows the cyclic voltammograms (CVs) of an Au-PFE in 0.1 mol dm-3 HClO4 solution. The CV profile (black curve) contains some characteristics of a polycrystalline Au electrode,23 including an extended electrochemical doublelayer region between 0 and 0.6 V, an anodic peak centered at 1.31 V related to the oxidation of gold surface, and a cathodic peak at ∼0.8 V corresponding to the reduction of gold oxides in the negative-going potential sweep. The electrooxidation of methanol on Au is a surface reaction catalyzed by chemisorbed OH- anions.12 Because the reaction is strongly pH dependent, Au is a poor catalyst for methanol oxidation reaction (MOR) in acidic solutions,24 as indicated by the CVs of the Au-PFE in the acidic solution with methanol (blue curve in Figure 2). It is interesting that, except for the relatively larger oxidation peak, the CV profile measured in the methanol-containing solution was almost the same with that obtained in pure HClO4 solution. It is possible that the MOR proceeds only on the Au surfaces covered with oxides. 3.3. Fabrication of Au-Pt Electrocatalysts Based on AuPFEs. Despite the relatively low catalytic activity compared with Pt catalysts, the superior poison resistance of Au catalysts and their potential applications in fuel cells have attracted great attention of many researchers. The as-prepared flat nanoprisms

are a type of single crystalline nanomaterials, whose basal surfaces are atomically flat {111} planes and lateral surfaces are {110} planes.9 Because each Au nanoprism has a large (111) plane nearly parallel to the substrate surface, the Au thin films constructed by a large amount of Au nanoprisms may been used as quasi single-crystalline Au-film electrodes to fabricate high performance bimetallic (or multimetallic) catalysts. It is feasible to prepare the Pt or other noble metal monolayers on the AuPFEs based on the redox replacement of a UPD layer of a sacrificial metal in a solution containing ions of a nobler element. This provides a simple pathway for designing the novel electrocatalysts with a monolayer level Pt loadings. In order to achieve the goal, we first electrodeposited a Cu monolayer onto an Au-PFE by means of the UPD of copper. Figure 3 shows the cyclic voltammograms (CVs) of an AuPFE in 1 mmol dm-3 CuSO4 + 0.1 mol dm-3 H2SO4 mixed solution. The small reduction peak at the potentials more positive than the bulk deposition potential is associated with the formation of the Cu UPD layers on the Au surface. By controlling the potential to be 0 V (marked with a circle in Figure 3) before the commencement of bulk Cu deposition, a monolayer of Cu atoms were deposited underpotentially on the Au surface with ease. When a clean Cu-monolayer-modified Au-PEF was immersed into an aqueous solution of 4.8 mmol dm-3 K2PtCl4 + 0.1 mol dm-3 HClO4, the galvanic replacement of the UPD Cu with Pt, driven by the large gap between standard reduction potentials of Cu2+/Cu and PtCl42-/Pt, yielded a

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Figure 4. Low- (a) and high-magnification (b) SEM images of an Au-PFE modified with a monolayer of Pt (Au-PF/Pt). Low- (c) and highmagnification (d) SEM images of an Au-PFE modified with the Au and Pt bilayer (Au-PF/Au/Pt, the Pt monolayer was on the top of a fresh Au monolayer), respectively. EDS analyses of the Au-PF/Pt (e) and Au-PF/Au/Pt (f) samples.

uniform Pt layer on the Au-PFE. Theoretically, the amount of Pt deposited by the displacement of a full UPD monolayer of Cu is limited to a coverage of one monolayer since Cu oxidation can offer two electrons per adatom and two electrons are necessary for the reduction of a Pt(II) ion. If the all-Pt clusters are one atom high, their coverage will be 100%.25 Because of ultralow Pt loading, there were no significant differences in morphology between the Au-PFE modified with the Pt clusters (defined as Au-PF/Pt) and the freshly prepared Au-PFEs. However, some small particles were observed on the flat top surface of Au prisms upon long-time potential cycling, as indicated by Figure 4a,b. This phenomenon seems more obvious for the Au and Pt bilayer deposited on the Au-PFEs (defined as Au-PF/Au/Pt) prepared with repeating underpotential deposition (UPD)-redox replacement procedure, as can be found in Figure 4c,d. In particular, some large nanoparticles with tens of nanometer appeared on the flat surfaces of Au prisms. The EDS analyses for the two film electrodes further confirmed the existence of Pt composition (Figure 4e,f). Figure 5 shows the CVs of the Au-PF/Pt and Au-PF/Au/Pt electrodes in HClO4 solutions, respectively. For the Au-PF/Pt, the voltammetric characteristics related to hydrogen adsorption/ desorption on the Pt and the reduction of platinum oxides

Figure 5. CVs of the Au-PFEs modified with a Pt monolayer (AuPF/Pt) and with the Au and Pt bilayer (Au-PF/Au/Pt, the Pt monolayer was on the top of a fresh Au monolayer) in 0.1 mol dm-3 HClO4 solutions at 50 mV s-1.

emerged although there was only a submonolayer-to-monolayer amount of Pt on the Au-PF. The modification of the Au prism

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Figure 6. CVs for the electrooxidation of methanol at the Au-PFEs modified with a Pt monolayer (Au-PF/Pt) and with the Au and Pt bilayer (Au-PF/Au/Pt) in 0.1 mol dm-3 HClO4 + 0.4 mol dm-3 methanol mixed solutions. The CVs of commercial Pt/C catalyst (20 wt %) are included for comparison. Scan rate: 50 mV s-1.

films with a submonolayer/monolayer of fresh Au did not cause obvious changes in the voltammetric profile of the top Pt layer, which exemplified the similar CVs of the Au-PF/Au/Pt (blue line). As stated earlier, Au has low catalytic activity for the MOR in acidic solutions. However, upon modification with a tiny amount (monolayer level) of Pt, the catalytic activity toward the methanol oxidation will be enhanced considerably. The CVs of the Au-PF/Pt electrode show a large primary oxidation peak centered at 0.61 V in the forward (positive-going) potential scan and a small secondary oxidation peak around 0.45 V in the backward (negative-going) potential scan (see Figure 6). The large ratio (2.11) of the forward anodic peak current (If) to the backward anodic peak current (Ib), If/Ib, suggests that the as-prepared Au-Pt bimetallic catalyst owns the better poison resistance ability than the commonly used Pt/C (If/Ib, 0.85 obtained from the CVs in Figure 6) and Pt-Ru catalysts (If/Ib, 1.1 for the carbon-supported Pt-Ru catalyst).26 The superior CO tolerance of the bimetallic Au-PF/Pt catalyst arises from the synergistic effect between Pt and Au compositions.27 It is possible that the CO-poisoned Pt can be regenerated through the reaction of surface CO with oxygen species adsorbed on the Au active sites to form CO2. By providing that a fresh Au monolayer is laid onto the surface of Au nanoprism films (AuPFs) at first, the catalytic performance of the Au-FE/Pt catalyst is expected to be enhanced to a greater degree since the oxygen species or OH- ions adsorb on the fresh Au surface more readily. A monolayer of fresh Au was first deposited on the Au-PFs by means of the electrodeposition of a Cu UPD monolayer on the Au surface, followed by the redox replacement with Au(III) ions. Subsequently, a monolayer of Pt was fabricated on the top of fresh Au layer by following the same procedure. As expected, the CVs (red curve in Figure 6) for the Au-PFE modified with the Au and Pt bilayer show that both the catalytic activity and the poison resistance of the bimetallic catalysts were promoted under otherwise identical conditions. Our recent studies have shown that the preferential adsorption of PVP macromolecules on the flat (111) surfaces of Au nanoprisms does good for directing their selective growth and for stabilizing the nanostructures28 but has a bad influence on their catalytic activity because the adsorbed PVP polymers hamper methanol molecules to approach the Au active sites.

Li et al. Therefore, once the Au-PFE is modified by a monolayer of Au atom, the synergistic effect between Pt and Au atoms naturally becomes stronger, which leads to the better catalytic performance of bimetallic Au-Pt catalysts. 3.4. Fabrication of Au-Pd and Au-Pd-Pt Electrocatalysts Based on Au-PFEs. By repeating the above-mentioned underpotential deposition (UPD)-redox replacement procedure, the alternating monolayers of Pd and Pt were prepared on an Au-PFE in proper order. The Au-PFE modified with Pd and Pt bilayers was defined as Au-PF/Pd/Pt electrode. Figure 7a,b show low- and high-magnification SEM images of the trimetallic AuPF/Pd/Pt electrode, respectively. It is clearly observed from Figure 7b that a large amount of nanoparticles with size of ∼7 nm dispersed well on the flat surfaces. EDS analysis (Figure 7c) shows that the surface composition of Pd was about 0.93 atom %; however, there were no Pt signals on the EDS spectrum. Because of low detection sensitivity, it is possible that energy-dispersive detector is unable to detect the presence of Pt in the film when the amount of Pt deposited is too low, as reported previously by Tang et al.29 Further evidence for the formation of Pt and Pd monolayers on the Au-PFE were provided by the XRD patterns of Au-PFs, Au-PF/Pt, and AuPF/Pd/Pt shown in Figure 8. In the recorded XRD spectrum of the Au-PFs, the diffraction peaks from Au{111},{200},{220}, and {311} planes of the face centered-cubic (fcc) Au were observed despite the noisy baseline. And moreover, the diffraction intensity of {111} plane was dominated with those of other planes, which implies that Au nanoprisms are preferentially oriented with their {111} planes parallel to the substrate surface. At the same time, from the XRD spectra of Au-PF/Pt and AuPF/Pd/Pt, the diffraction peaks corresponding respectively to Pt{111}and {200} planes of the fcc Pt, Pd {200}, and {220} planes of the fcc Pd were also observed, revealing the presence of Pt and Pd on the Au-PFs. The electrochemical properties of the Au-PFEs modified with Pd monolayer (Au-PF/Pd for short) and with Pd and Pt bilayers (Au-PF/Pd/Pt for short) were characterized in acidic solutions using cyclic voltammetry. The CVs of the Au-PF/Pd (blue curve in Figure 9) in HClO4 only display the reduction peak of Pd oxides at 0.5 V but do not give the Pd’s characteristic signals in hydrogen region, especially the desorption peak for the absorbed hydrogen. This should be due to the ultralow loadings and very small sizes of Pd particles. These small Pd particles deposited on the Au-PFE were unable to show the standard electrochemical behavior for hydrogen absorption/desorption on and into Pd in the acidic solution. When a monolayer of Pt was deposited on top of the Pd monolayer, the voltammetric behavior of the Au-PF/Pd/Pt (red curve in Figure 9) seems to be similar to that of Pd-Pt alloys reported previously by Grden et al.30 The large anodic peak at more negative potentials should be related to desorption of absorbed hydrogen, and two unobvious peaks at more positive potentials are attributed to the hydrogen adsorption/desorption on the surfaces of Pt or Pd nanoparticles.31 The voltammetric behavior (black curve in Figure 10) of the Au-PF/Pd in methanol-containing solution is very similar to that of Au-PF/Pd in pure HClO4 solution, clearly indicating that the Pd nanoclusters have no catalytic activity toward the MOR in acidic media. By bearing in mind that Pd is an oxophilic element, the combination of Pd and Pt elements can be used to design high performance catalysts with strong poison resistance. As expected, after depositing one monolayer of Pt on the top of the Pd monolayer, the as-prepared trimetallic Au-PF/Pd/Pt catalyst exhibited the higher catalytic activity and stronger poison resistance than the Au-PE/Au/Pt catalyst used in Figure

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Figure 7. Low- (a) and high-magnification (b) SEM images of an Au-PFE modified with the Pd and Pt bilayer (Au-PF/Pd/Pt, the Pt monolayer was on the top of a Pd monolayer). EDS analysis (c) of the Au-PF/Pd/Pt sample.

Figure 8. X-ray diffraction (XRD) patterns of the as-prepared AuPFEs (a), Au-PF/Pt (b), and Au-PF/Pd/Pt (c).

Figure 9. CVs of the Au-PFEs modified with a Pd monolayer (AuPF/Pd) and with the Pd and Pt bilayer (Au-PF/Pd/Pt) in 0.1 mol dm-3 HClO4 solutions. Scan rate: 50 mV s-1.

6. The CVs (red curve in Figure 10) shows that the electrooxidation of methanol started at 0.16 V and reached a maximum current at 0.56 V, which is 50 mV more negative than that (0.61 V) of the Au-PE/Au/Pt catalyst. In the reverse (negative-going) potential sweep, a reduction peak appeared at 0.53 V, corresponding to the reduction of Pd oxides, followed by a secondary methanol oxidation peak around 0.38 V. The larger ratio (4.45) between two oxidation peak currents indicates that the Au-PE/ Pd/Pt catalyst has the stronger ability to resist carbon monoxide poisoning than the commercial Pt/C catalyst (green curve in Figure 10) and the Au-PE/Au/Pt. The great enhancement in both catalytic activity and poison resistance should be ascribed to the possible alloy process between the Pt and the Pd monolayers. We also fabricated another kind of Pt and Pd bilayers (Pd monolayer was deposited on top of the Pt monolayer) on the Au-PFE, which is defined as the Au-PFE/Pt/Pd. Interestingly, the Au-PFE/Pt/Pd did not display any current peaks associated with the MOR in addition to a large reduction peak of Pd oxides

(see Figure 11). Obviously, the Au-PFE/Pt/Pd hardly had any catalytic activity toward the MOR after the Pt atoms were covered with the top Pd monolayer. Pd is a poor catalyst toward the methanol oxidation in acid solutions but an excellent catalyst for the formic acid oxidation at ambient temperature. The CV curve shown in Figure 12 clearly shows the characteristics of formic acid oxidation at Pd electrocatalysts. The sharp contrast in the catalytic activity between the Au-PFE/Pt/Pd and the AuPFE/Pd/Pt can help one to better understand what kind of role the Pt or Pd plays in the trimetallic Au-Pd-Pt catalysts respectively. There is no doubt that Pt is the main catalytic metal in the MOR; however, it suffers easily from the CO poisoning. The Pd component plays an important role in modifying the electronic properties of Pt32,33 and in removing the CO species adsorbed on the Pt active sites.10 With the help of Pd, the Pt component in the Au-PFE/Pd/Pt catalyst can exhibit the much higher catalytic activity than the pure Pt. Provided that the Pt atoms (or nanoclusters) are covered by the Pd monolayer, the

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Figure 10. CVs for the electrooxidation of methanol at the Au-PFEs modified with a Pd monolayer (Au-PF/Pd) and with the Pd and Pt bilayer (Au-PF/Pd/Pt) in 0.1 mol dm-3 HClO4 + 0.4 mol dm-3 methanol mixed solutions. The CVs of commercial Pt/C catalyst (20 wt %) are included for comparison. Scan rate: 50 mV s-1.

Li et al.

Figure 12. CVs for the electrooxidation of formic acid at the Au-PFE modified with the Pt and Pd bilayer (Au-PF/Pt/Pd, the Pd monolayer was on the top of a Pt monolayer) in 1.0 mol dm-3 H2SO4 + 0.4 mol dm-3 formic acid mixed solution. Scan rate: 50 mV s-1.

between Au and Pt (or Pd and Pt) components. The combination of Au, Pt, and Pd provides good opportunities for making novel and high performance bimetallic and multimetallic catalysts. Acknowledgment. This work was supported by National Natural Science Foundation of China (20673067), the National 973 Program Projects of China (2006CB605004, 2007CB936602) and the Visiting Scholar Foundation of Key Laboratory in Shandong University. References and Notes

Figure 11. CVs for the electrooxidation of methanol at the Au-PFE modified with the Pt and Pd bilayer (Au-PF/Pt/Pd, the Pd monolayer was on the top of a Pt monolayer) in 0.1 mol dm-3 HClO4 + 0.4 mol dm-3 methanol mixed solution. The potential scan rate was 50 mV s-1.

bimetallic or multimetallic catalysts will lose their catalytic activity,33 as demonstrated by the poor activity of the Au-PFE/ Pt/Pd catalyst. Therefore, it is necessary to fully consider the synergistic effect between individual metal components in designing high efficiency multimetallic catalysts. 4. Conclusions We fabricated well-defined gold nanoprism thin films (AuPFs) on ITO glass substrates by means of a simple and effective film formation method and, further, used them as the substrate electrodes to construct bimetallic Au-Pt and Au-Pd catalysts and trimetallic Au-Pt-Pd ones. Although Au and Pd catalysts are poor catalysts for the MOR, the bimetallic Au-PF/Pt and trimetallic Au-PF/Pd/Pt catalysts, which were prepared using the underpotential deposition (UPD)-redox replacement technique, exhibited greatly enhanced catalytic activity toward the MOR and much better poison resistance than the commonly used commercial Pt/C catalysts due to the good synergistic effect

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