Synthesis and Characterization of Flower-Shaped Porous Au−Pd Alloy

J. Phys. Chem. C 2008, 112, 6717-6722

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Synthesis and Characterization of Flower-Shaped Porous Au-Pd Alloy Nanoparticles Young Wook Lee, Nam Hoon Kim, Kang Yeol Lee, Kihyun Kwon, Minjung Kim, and Sang Woo Han* Department of Chemistry, Research Institute of Natural Science, and EnVironmental Biotechnology National Core Research Center, Gyeongsang National UniVersity, Jinju 660-701, Korea ReceiVed: NoVember 16, 2007; In Final Form: February 24, 2008

A facile synthesis of flower-shaped porous Au-Pd alloy nanoparticles with ascorbic acid as a reductant and PVP as a stabilizing agent is presented. The alloy nanoparticles were prepared from the aqueous solutions of HAuCl4/K2PdCl4 mixtures in molar ratios of 3:1, 1:1, and 1:3. The size, structure, optical properties, and composition distribution of the synthesized Au-Pd alloy nanoparticles were characterized by transmission electron microscopy, energy-dispersive X-ray spectroscopy, UV-vis spectroscopy, and X-ray diffraction. The experimental results for the bimetallic systems and the physical mixtures of individual monometallic nanoparticles revealed that unstable small nanoparticles aggregate into the three-dimensional flower-shaped nanoparticles and the prepared nanoparticles are Au-Pd alloys. The surfaces of Au-Pd alloy nanoparticles were characterized by cyclic voltammetry measurement in 0.1 M HClO4 and surface-enhanced Raman scattering spectra of 1,4-phenylene diisocyanide adsorbed thereon. All alloy nanoparticles have a Pd-enriched surface.

Introduction Multimetallic nanoparticles with alloy or core-shell structures are attractive materials because of their composition-dependent optical, catalytic, electronic, and magnetic properties.1 A variety of approaches have been reported on their preparation including chemical reduction,2-5 sonochemical method,6 decomposition of organometallic precursors,7 and electrolysis of bulk metal.8 Currently, further efforts are being directed toward the synthesis of alloy nanoparticles for applications in sensors,9 photonic devices,10 and catalysis.11 In fact, alloy nanoparticles often exhibit better catalytic properties than their monometallic counterparts.12 However, most of studies on the synthesis of alloy nanostructures have concentrated on isotropic spherical structures. Very few methods have been reported for making alloy structures with a non-spherical shape.13 Indeed, the synthesis of nanoparticles with controlled shapes has been a subject of intense research in recent years because their specific geometries lead to unusual physical and chemical properties14 and they can be promising building blocks for the creation of nanostructured materials.15 Among the various bimetallic nanoparticles, Au-Pd alloy nanoparticles have been widely explored by many scientists and engineers as a catalytic material for a variety of reactions.16 For instance, Au-Pd alloy nanoparticles have been proven to have a higher activity than both pure Pd nanoparticles and bulk Pd catalysts for hydrodechlorination of trichloroethane; this makes Au-Pd nanoparticles extremely effective agents for the remediation of various inorganic and organic groundwater contaminants.17 Au-Pd alloy nanoparticles have also been found to be an outstanding catalyst for the hydrogenation of naphthalene and toluene,18 which makes them important for the improvement of diesel fuels. More recently, there is also interest in the development of Au-Pd catalysts for the oxidation of alcohols to aldehydes.19 In this paper, we present a study on * Author to whom correspondence should be addressed. Tel: +82-55751-6026. Fax: +82-55-761-0244. E-mail: [email protected].

the synthesis and characterization of Au-Pd alloy nanoparticles with an unusual shape. Flower-shaped porous Au-Pd alloy nanoparticles were readily prepared from aqueous solutions of HAuCl4/K2PdCl4 mixtures with ascorbic acid as a reducing agent. Flower-shaped porous structures or multibranched structures have been observed in Au,20,21 Pt,22-25 Rh,26 Pd,27 and various metal oxide28 nanoparticles, and a number of formation mechanisms have been proposed. However, there is no report on the fabrication of flower-shaped alloy nanoparticles. The shape and composition of Au-Pd alloy nanoparticles were characterized by transmission electron microscopy (TEM), high-resolution TEM (HRTEM), UV-vis spectroscopy, energydispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), cyclic voltammetry (CV), and surface-enhanced Raman scattering (SERS). The EDS and XRD analyses showed that the compositions of alloy nanoparticles were roughly consistent with those of the feeding solutions. CV and SERS measurements, however, indicate that the Au-Pd alloy nanoparticles have a Pd-enriched surface. It is usually very difficult to analyze exclusively the outermost part of nanoparticles by using surfaceanalyzing tools such as EDS and X-ray photoelectron spectroscopy because the prove beams penetrate a finite distance into the materials.29 Herein, we clearly demonstrate that CV and SERS analyses are very useful and sensitive tools for the characterization of the nanoparticle surface. This is because CV and SERS signals obtained from the nanostructured substrates are directly dependent on the state of their surfaces. Experimental Section Chemicals. HAuCl4, K2PdCl4, ascorbic acid, poly(vinyl pyrrolidone) (PVP, MW ) 55 000), and 1,4-phenylene diisocyanide were purchased from Aldrich. Other chemicals, unless specified, were reagent grade, and triply distilled water (resistivity greater than 18.0 MΩ cm) was used when preparing aqueous solutions. Preparation of Nanoparticles. In a typical synthesis of AuPd bimetallic nanoparticles, 1 mL of a 5 mM aqueous solution

10.1021/jp710933d CCC: $40.75 © 2008 American Chemical Society Published on Web 04/08/2008

6718 J. Phys. Chem. C, Vol. 112, No. 17, 2008 of HAuCl4/K2PdCl4 mixtures in molar ratios of 3:1, 1:1, and 1:3 was added to 47 mL of highly purified water. To this solution, 1 mL of 0.1 M ascorbic acid was added. After 15 s, an aqueous solution of PVP (5 mg/mL, 1 mL) was added dropwise with vigorous stirring, and the solution was stirred further for 30 min. The resulting hydrosol was subjected to centrifugation (10 000 rpm for 5 min) to remove excess PVP. For comparison, Au and Pd monometallic nanoparticles were prepared in the same way by substituting aqueous solutions of HAuCl4/K2PdCl4 mixtures by HAuCl4 and K2PdCl4 solutions, respectively. Characterization of Nanoparticles. The extinction spectra were recorded with a UV-vis absorption spectrometer (SINCO S-3100). TEM images and EDS data were obtained with a JEOL JEM-2010 transmission electron microscope operating at 200 kV after placing a drop of hydrosol on carbon-coated Cu grids (150 mesh). HRTEM and scanning TEM (STEM)-EDS characterizations were performed with an FEI Technai G2 F30 Super-Twin transmission electron microscope operating at 300 kV. The effective electron probe size and dwell time used in STEM-EDS mapping experiments were 1.5 nm and 200 ms per pixel, respectively. Indium tin oxide (ITO) was used as the substrate for XRD, CV, and SERS measurements. The ITO substrate was washed with sonication in acetone and then pure water. The washed ITO substrates were dried in the air and then immersed in a 2-propanol solution of 1% 3-aminopropyltrimethoxysilane for 3 h, yielding an amine-terminated surface.30 The modified substrates were rinsed thoroughly with pure water prior to use. For immobilization of nanoparticles, 0.1 mL of hydrosol was dropped onto the amine-terminated ITO substrates and dried under ambient conditions. Then, the substrate was washed with triply distilled water and dried. XRD patterns were obtained with a Bruker AXS D8 DISCOVER diffractometer using Cu KR (0.1542 nm) radiation. CV measurements were carried out in a three-electrode cell using a CH Instrument model 600C potentiostat. Nanoparticle-modified ITO substrates served as working electrodes. Before CV measurements, the nanoparticle-modified substrates were cleaned again by sequentially washing with acetone, ethanol, and deionized water to remove stabilizing agents on the surface of nanoparticles. Pt wire and Ag/AgCl (in saturated KCl) were used as the counter and reference electrodes, respectively. All cyclic voltammograms were obtained at room temperature. The electrolyte solutions were purged with high-purity N2 gas before use for about 3 h. For SERS measurement, the nanoparticlemodified ITO substrate was soaked in 1 mM 1,4-phenylene diisocyanide solution in ethanol overnight. After the substrate was taken out, it was washed with ethanol and dried under ambient conditions. Raman spectra were obtained using a Jobin Yvon/HORIBA LabRAM spectrometer with the 632.8 nm line of an air-cooled He/Ne laser as an excitation source. Results and Discussion Au-Pd bimetallic nanoparticles were prepared from the aqueous solutions of HAuCl4/K2PdCl4 mixtures with ascorbic acid as a reductant in the presence of PVP as a stabilizing agent. Hereafter, we will refer to the metal nanoparticles prepared from the aqueous solutions of HAuCl4/K2PdCl4 mixtures in molar ratios of 3:1, 1:1, and 1:3 as Au(3)Pd(1), Au(1)Pd(1), and Au(1)Pd(3), respectively. Figure 1 shows typical TEM images of the prepared Au-Pd bimetallic nanoparticles. The particles have a flower-shaped porous structure, and their sizes are similar regardless of their compositions. The measured mean particle diameters for Au-

Lee et al.

Figure 1. TEM images of (a) Au(3)Pd(1), (b) Au(1)Pd(1), and (c) Au(1)Pd(3) bimetallic nanoparticles.

(3)Pd(1), Au(1)Pd(1), and Au(1)Pd(3) nanoparticles are 28, 24, and 26 nm, respectively. The average particle diameters were obtained by measuring the length of the longest axis of over 300 nanoparticles. Although the formation mechanism of flowershaped Au-Pd bimetallic nanoparticles is not clear, it can be assumed that unstable small nanoparticles aggregate into the three-dimensional flower-shaped nanoparticles. In fact, close inspection of TEM images shows that the flower-shaped nanoparticles are built up by tens of elongated primary nanoparticles with average dimensions of ∼10 nm, and boundaries and voids between the components are present (see insets of Figure 1). In addition, small particles and some aggregated structures consisting of several primary particles were also observed in the TEM images. The low concentration of metal precursor seems to induce the aggregation of small nanoparticles or seed particles due to slow addition of metal atoms, which means that the aggregation of seeds prevails over the growth of each seed particle.31 In a control experiment, spherical AuPd bimetallic nanoparticles with bigger size were obtained using higher concentrations of Au and Pd precursors (Figure 2a).

Flower-Shaped Porous Au-Pd Alloy Nanoparticles

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Figure 3. (a) High-magnification STEM image of a single Au(1)Pd(1) bimetallic nanoparticle. STEM-EDS mapping images of the Au(1)Pd(1) bimetallic nanoparticle shown in (a): (b) Au mapping image, (c) Pd mapping image, and (d) overlap image of Au and Pd mapping.

Figure 2. TEM images of Au(1)Pd(1) bimetallic nanoparticles obtained (a) using a higher concentration of Au (0.3 mM) and Pd (0.3 mM) precursors and (b) when PVP was injected into the reaction solution (low concentration of Au (0.05 mM) and Pd (0.05 mM) precursors) prior to the addition of ascorbic acid.

Moreover, when PVP was injected into the reaction solution prior to ascorbic acid for preventing the aggregation of seed particles, roughly spherical Au-Pd bimetallic nanoparticles with smaller size were obtained even in the low concentration of metal precursors (Figure 2b). A similar mechanism has also been proposed in previous studies on the formation of the nanoparticles with flower-shaped porous structures or multibranched structures. Teng et al.22 proposed a “self-organization” mechanism for the fabrication of porous Pt nanoparticles. Hoefelmeyer and co-workers proposed a “seeded growth” mechanism for flower-shaped Rh nanoparticles.26 It was also found that ascorbic acid might promote the formation of multipodal or porous particles.21,23 In our experimental conditions, PVP did not affect the morphology of Au-Pd bimetallic nanoparticles but only stabilized the formed nanoparticles because Au-Pd bimetallic nanoparticles with identical shape could be obtained without PVP. The proposed mechanism, however, cannot explain the formation of flower-shaped porous Au-Pd bimetallic nanoparticles entirely, because we could not obtain flowershaped monometallic Au and Pd nanoparticles under the same experimental conditions. The prepared Au and Pd nanoparticles have roughly spherical shapes with an average size of 20 ( 5 and 40 ( 5 nm, respectively. Coexistence of metal precursors with different reduction potentials may play a certain role in the structural evolution of the particles (vide infra). More detailed experimental studies on the formation of flower-shaped porous nanoparticles are underway. To investigate the structure of the prepared nanoparticles, flower-shaped Au-Pd particles were characterized with EDS, HRTEM, UV-vis spectroscopy, and XRD. The mole fractions

Figure 4. HRTEM image of the Au(1)Pd(1) bimetallic nanoparticle.

of Pd in Au(3)Pd(1), Au(1)Pd(1), and Au(1)Pd(3) nanoparticles obtained from EDS data were 0.31, 0.57, and 0.69, respectively. This shows that the compositions of nanoparticles were roughly consistent with those of feeding solutions. Closer inspection of one of the Au(1)Pd(1) nanoparticles by STEM-EDS analysis suggests that Au and Pd atoms are distributed uniformly over the entire nanoparticle. This is most clearly observed in the digitally colored image, which does not show a predominance of either the Au (green) or the Pd (purple) signal (Figure 3). These indicate that the flower-shaped particles are Au-Pd alloy. The HRTEM image also shows the tendency of uniformly fabricating Au-Pd alloy nanoparticles. Figure 4 shows the typical HRTEM image of an Au(1)Pd(1) nanoparticle. Lattice

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Figure 5. UV-vis spectra of (a) Au, (b) Au(3)Pd(1), (c) Au(1)Pd(1), (d) Au(1)Pd(3), (e) Pd nanoparticles, and (f) 1:1 physical mixture of Au and Pd nanoparticles.

planes of the crystal can be seen from this image. The d-spacing for adjacent lattice planes measured from five different points on a single nanoparticle was 2.30 Å, which corresponds to the mean value of the (111) planes of face-centered cubic (fcc) Au and Pd. This value is also the same as that of the (111) planes of fcc bulk Au(1)Pd(1) alloy.32 Figure 5 shows UV-vis spectra of Au, Pd, and Au-Pd bimetallic nanoparticles as well as the physical mixture of Au and Pd monometallic nanoparticles (1:1 ratio). Au nanoparticles show a characteristic surface plasmon absorption at 540 nm, and Pd nanoparticles show broad absorption over the entire spectral range. The absorbance behavior of the bimetallic nanoparticles is found to be different from those of the individual and physical mixture of monometallic nanoparticles. The characteristic absorbance band of Au nanoparticles that appears at 540 nm is absent in the case of the Au-Pd bimetallic nanoparticles, while it is present in the case of the physical mixture. This clearly indicates that dispersions of Au-Pd nanoparticle systems do not contain monometallic clusters but contain clusters with bimetallic structure. This result coincides with the early report of Wu et al.2 The absence of peaks at 310 and 407 nm corresponds to unreduced Au(III) and Pd(II), respectively, and also indicates complete reduction of the metal ions. Figure 6a shows the XRD patterns of Au, Pd, and Au-Pd bimetallic nanoparticles. Each pattern exhibits three diffraction peaks in the range of 30° < 2θ < 80° which can be indexed to diffraction from the (111), (200), and (220) of the fcc structure of metallic Au and/or Pd. In addition, the diffraction peaks of the bimetallic particles exhibit a shift from pure Au to pure Pd as the Pd content increases (Figure 6b). This suggests that the prepared nanoparticles were Au-Pd alloys. According to Vegard’s law,33 the molar compositions of Au and Pd in the alloy nanoparticles could be obtained from the lattice parameter shifts calculated from the angular position of the (111), (200), and (220) diffractions. The mole fractions of Pd of Au(3)Pd(1), Au(1)Pd(1), and Au(1)Pd(3) alloy nanoparticles are estimated to be 0.25 ( 0.03, 0.44 ( 0.03, and 0.67 ( 0.03, respectively. As revealed by EDS measurements, the composition of each bimetallic nanoparticle was proportional to that of feeding solution. It is also known that the width of the XRD peaks can provide information on the average size of singlecrystalline particles. By using the Scherrer formula, the mean sizes of the nanoparticles were calculated from the (111) peak width at half-maximum.34 The average sizes of Au, Pd, Au(3)Pd(1), Au(1)Pd(1), and Au(1)Pd(3) nanoparticles obtained from the width of the (111) diffraction are 18, 23, 12, 10, and 11 nm, respectively. The sizes of Au-Pd alloy nanoparticles are

Figure 6. (a) XRD patterns and (b) (111) diffraction peak positions of the Au, Pd, and Au-Pd bimetallic nanoparticles on ITO glass.

much smaller than those obtained by TEM measurements (Figure 1). This can be ascribed to the fact that powder XRD is sensitive to the size of coherent scattering domains that can significantly differ from the particle size in the case of mesocrystals composed of smaller primary nanoparticles.24 The characterization of the surface of nanoparticles is a very important issue in the applications of nanoparticles such as catalysts and sensors. For instance, catalytic effects are influenced strongly by the structure and composition of the surface of nanoparticles. The surface of Au-Pd alloy nanoparticles was characterized by CV measurement. Figure 7a shows a series of CV voltammograms of ITO electrodes modified with the AuPd alloy nanoparticles obtained in 0.1 M HClO4 at a scan rate of 0.3 V s-1. For preventing the preferential dissolution of Pd from the surface of Au-Pd alloy nanoparticles, potential sweeps were conducted with a rapid scan rate.35 For comparison, CV voltammograms obtained from Au and Pd monometallic particles are also shown in Figure 7b. Typical redox responses corresponding to the oxidation and reduction of Au, Pd, and Au-Pd alloy were observed at the nanoparticle-modified electrodes.35,36 As shown in Figure 7, a single oxygen desorption peak was observed at 0.90 ( 0.02, 0.35 ( 0.02, 0.45 ( 0.04, 0.44 ( 0.04, and 0.42 ( 0.04 V vs Ag/AgCl on the voltammograms of ITO electrodes modified with Au, Pd, Au(3)Pd(1), Au(1)Pd(1), and Au(1)Pd(3) nanoparticles, respectively. The observation of a single oxygen desorption peak with a potential value between those of Au and Pd for all the alloy nanoparticles demonstrates that the distribution of Au and Pd atoms on the surface of alloy nanoparticles is relatively homogeneous,35,36 although we cannot absolutely rule out the possibility that the surfaces of alloy nanoparticles consist of domains of Au and Pd. On the basis of the previous report that the position of the oxygen desorption peak of Au-Pd alloy electrodes varies in a linear manner with alloy composition,35 all Au-Pd alloy nanoparticles have Pd-enriched surfaces (see inset of Figure 7a). The Pd ratios on the surface of Au(3)Pd(1), Au(1)Pd(1), and Au(1)Pd(3) alloy nanoparticles estimated from the position of the oxygen desorption peak and the relationship between the peak position and Pd ratio, (peak position) ) -0.55 × (Pd ratio) + 0.90 (dotted line in the inset of Figure 7a), are 0.82, 0.84, and 0.87, respectively. The formation of the Pd-enriched surface of Au-Pd alloy nanoparticles may be due to the different

Flower-Shaped Porous Au-Pd Alloy Nanoparticles

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Figure 8. SERS spectra of 1,4-PDI on (a) Pd, (b) Au(1)Pd(3), (c) Au(1)Pd(1), (d) Au(3)Pd(1), (e) Au nanoparticles, and (f) 1:1 physical mixture of Au and Pd nanoparticles. All spectra were normalized with 8a mode peak at 1600 cm-1 for convenience.

Figure 7. Cyclic voltammograms of (a) Au-Pd alloy nanoparticles on ITO electrodes with a scan rate of 0.3 V s-1 in 0.1 M HClO4 and (b) Au and Pd nanoparticles on ITO electrodes with a scan rate of 0.05 V s-1 in 0.1 M HClO4. Inset in (a) shows the oxygen desorption peak position of Au, Pd, and Au-Pd alloy nanoparticle-modified ITO electrodes.

reduction rate of Au(III) and Pd(II) species. Considering the reduction potential of Au3+ (AuCl4-/Au, +1.002 V vs SHE (standard hydrogen electrode)) and Pd2+ (PdCl42-/Pd, +0.591 V vs SHE),37 it can be assumed that formation of the alloy particles should be initiated by nucleation of Au atoms followed by co-deposition of residual Au and Pd atoms on the surface of the seeds which have higher Au content.2 This makes resultant alloy particles have a Pd-enriched surface. The surface of Au-Pd alloy nanoparticles was also characterized by SERS of 1,4-phenylene diisocyanide (1,4-PDI) adsorbed thereon. Figure 8 shows normalized SERS spectra of 1,4-PDI adsorbed on Au, Pd, and Au-Pd alloy nanoparticles. It is wellknown that arylisocyanide molecules adsorbed on metal substrates via a different mechanism as the kind of metal and such structural differences can readily be detected by SERS.38 Three noticeable features were found in the SERS experiments. First, high-quality SERS spectra can be obtained from Pd and AuPd alloy nanoparticles although the SERS intensities are relatively weak compared with that obtained from Au nanoparticles. Second, the SERS spectra of 1,4-PDI adsorbed on Au and Pd are very similar except for NC peaks from 1900 to 2200 cm-1. The normal Raman spectrum of 1,4-PDI in this region contains a single sharp peak at 2128 cm-1 (data not shown here), which can be assigned to the NC stretching mode. Upon adsorption of 1,4-PDI on Au, Pd, and Au-Pd nanoparticles, significant changes were observed in the spectra. On the Au surface, the NC stretching band blue-shifted to 2177 cm-1 and broadened (Figure 8e). On the Pd surface, two peaks appeared at 2000 cm-1 (strong) and 2122 cm-1 (very weak) (Figure 8a).

From this observation, 1,4-PDI seemed to adsorb on Au and Pd by forming exclusively metal-CN bonds, through a pure σ-type interaction in the case of Au and a σ/π synergistic interaction for Pd.38 This implies that 1,4-PDI should adsorb on Au only via the on-top site, whereas not only the on-top site but also the 3-fold hollow sites can be used in the adsorption of 1,4-PDI on the Pd surface.39 Considering the relative intensities of peaks at 2000 and 2122 cm-1, the dominant biding site used for the adsorption of 1,4-PDI on Pd should be the 3-fold hollow site. Third, the Au-Pd alloy nanoparticles have a Pd-enriched surface. SERS spectra of 1,4-PDI on Au-Pd alloy nanoparticles are more similar to the SERS spectrum of 1,4PDI on Pd nanoparticles than to that of Au nanoparticles (Figure 8b-d). For quantitative analysis, the SERS spectrum of 1,4PDI adsorbed on the substrate prepared from the 1:1 physical mixture of Au and Pd monometallic nanoparticles was also obtained as a reference (Figure 8f). As shown in Figure 8f, two peaks are clearly seen at 2170 cm-1 arising from 1,4-PDI adsorbed on the Au surface and at 2000 cm-1 arising from 1,4PDI adsorbed on the Pd surface. The real areas of the Au and Pd surfaces estimated from the integration of oxygen desorption peaks for each metal in the cyclic voltammogram of the substrate prepared from the 1:1 physical mixture of Au and Pd monometallic nanoparticles in 0.1 M HClO4 were 0.025 and 0.015 cm2, respectively.35 From these values and the intensities (peak areas) of the two peaks at 2170 and 2000 cm-1 observed in the SERS spectrum (Figure 8f), we could obtain the relative SERS intensities per unit area of Au and Pd surfaces. The SERS intensity per unit area of Au nanoparticles was estimated to be 3.5 times higher than that obtained from Pd nanoparticles. Although the adsorption density and the binding nature of 1,4PDI could be different for Au and Pd, the higher value of SERS intensity per unit area for the Au nanoparticles should be the result of a higher surface enhancement factor of Au than that of Pd.40 By using the estimated values of SERS intensity per unit area for each metal and the intensities of two peaks at 2170 and 2000 cm-1 in the SERS spectra from the alloy nanoparticles (Figure 8b-d), we calculated the ratio of the Pd surface area for each alloy nanoparticle. The calculated Pd ratios on the surfaces of the Au(3)Pd(1), Au(1)Pd(1), and Au(1)Pd(3) alloy nanoparticles are about 0.80, 0.90, and 0.95, respectively. These values are consistent with the results of CV measurements,

6722 J. Phys. Chem. C, Vol. 112, No. 17, 2008 indicating the Pd-enrichment on the surfaces of the alloy nanoparticles. Because of the dependence of SERS intensity on the size and shape of nanoparticles, the different chemical interaction of the probe molecule 1,4-PDI with Au and Pd, and the different aggregation form of nanoparticles, the calculation of the real surface composition of Au-Pd alloy nanoparticles from the SERS measurements may not be accurate. However, considering the consistency with the CV investigations, we believe that the above estimation is not unreasonable. The Pdenrichment on the surface of alloy nanoparticles proved by CV and SERS measurements may seem to be inconsistent with the result of STEM-EDS mapping, which showed the uniform distribution of Au and Pd atoms over the entire alloy nanoparticle (Figure 3). With regard to this fact, it could be assumed that the Pd-enriched layer of the alloy nanoparticle is very thin. As a result, the unique surface feature of the alloy nanoparticles could not be observed in the EDS measurement. Conclusions A simple and room-temperature method for the synthesis of flower-shaped porous Au-Pd bimetallic nanoparticles has been demonstrated. The TEM, EDS, HRTEM, STEM-EDS, UVvis spectroscopy, and XRD analyses suggested the formation of alloy nanoparticles. The EDS and XRD results indicated that the compositions of alloy nanoparticles were roughly consistent with those of feeding solutions. Electrochemical analysis and SERS spectra of arylisocyanide molecules adsorbed thereon, however, indicate that the Au-Pd alloy nanoparticles have a Pd-enriched surface. These Au-Pd alloy nanoparticles with an unusual shape are expected to be useful in wide applications such as nanoelectronics, catalysis, fuel cells, and other related fields. Acknowledgment. This work was supported by the Korea Research Foundation Grant funded by the Korean Government (MOEHRD) (KRF-2006-311-C00355), by a grant from the MOST/KOSEF to the Environmental Biotechnology National Core Research Center (Grant No. R15-2003-012-01001-0), and by Technology Development Program of the Ministry of Agriculture and Forestry, Republic of Korea. References and Notes (1) Toshima, N.; Yonezawa, T. New J. Chem. 1998, 22, 1179-1201. (2) Wu, M.-L.; Chen, D.-H.; Huang, T.-C. Langmuir 2001, 17, 38773883. (3) (a) Toshima, N.; Harada, M.; Yamazaki, Y.; Asakura, K. J. Phys. Chem. 1992, 96, 9927-9933. (b) Wang, Y.; Toshima, N. J. Phys. Chem. B 1997, 101, 5301-5306. (4) Link, S.; Wang, Z. L.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 3529-3533. (5) Han, S. W.; Kim, Y.; Kim, K. J. Colloid Interface Sci. 1998, 208, 272-278. (6) Mizukoshi, Y.; Okitsu, K.; Maeda, Y.; Yamamoto, T. A.; Oshima, R.; Nagata, Y. J. Phys. Chem. B 1997, 101, 7033-7037. (7) (a) Bradley, J. S.; Hill, E. W.; Klein, C.; Chaudret, B.; Duteil, A. Chem. Mater. 1993, 5, 254-256. (b) Pan, C.; Dassenoy, F.; Casanove, M. J.; Philippot, K.; Amiens, C.; Lecante, P.; Mosset, A.; Chaudret, B. J. Phys. Chem. B 1999, 103, 10098-10101. (8) Reetz, M. T.; Helbig, W.; Quaiser, S. A. Chem. Mater. 1995, 7, 2227-2228.

Lee et al. (9) Wang, L. Y.; Shi, X.; Kariuki, N. N.; Schadt, M.; Wang, G. R.; Rendeng, Q.; Choi, J.; Luo, J.; Lu, S.; Zhong, C.-J. J. Am. Chem. Soc. 2007, 129, 2161-2170. (10) Regan, M. R.; Banerjee, I. A. Scr. Mater. 2006, 54, 909-914. (11) Ghosh, S. K.; Mandal, M.; Kundu, S.; Nath, S.; Pal, T. Appl. Catal., A 2004, 268, 61-66. (12) Liu, J.-H.; Wang, A.-Q.; Chi, Y.-S.; Lin, H.-P.; Mou, C.-Y. J. Phys. Chem. B 2005, 109, 40-43. (13) (a) Ji, C.; Searson, P. C. Appl. Phys. Lett. 2002, 81, 4437-4439. (b) Krichevski, O.; Tirosh, E.; Markovich, G. Langmuir 2006, 22, 867870. (c) Zou, X.; Ying, E.; Dong, S. J. Colloid Interface Sci. 2007, 306, 307-315. (d) Kim, M.; Lee, K. Y.; Jeong, G. H.; Jang, J.; Han, S. W. Chem. Lett. 2007, 36, 1350-1351. (14) Rodrı´guez-Gonza´lez, B.; Pastoriza-Santos, I.; Liz-Marza´n, L. M. J. Phys. Chem. B 2006, 110, 11796-11799. (15) Park, S.; Lim, J.-H.; Chung, S.-W.; Mirkin, C. A. Science 2004, 303, 348-351. (16) (a) Ferrer, D.; Torres-Castro, A.; Gao, X.; Sepu´lveda-Guzma´n, S.; Ortiz-Me´ndez, U.; Jose´-Yacama´n, M. Nano Lett. 2007, 7, 1701-1705. (b) Chen, C.-H.; Sarma, L. S.; Chen, J.-M.; Shih, S.-C.; Wang, G.-R.; Liu, D.-G.; Tang, M.-T.; Lee, J.-F.; Hwang, B.-J. ACS Nano 2007, 1, 114125. (17) Nutt, M. O.; Hughes, J. B.; Wong, M. S. EnViron. Sci. Technol. 2005, 39, 1346-1353. (18) Pawelec, B.; Cano-Serrano, E.; Campos-Martin, J. M.; Navaro, R. M.; Thomas, S.; Fierro, J. L. G. Appl. Catal., A 2004, 275, 127-139. (19) Enache, D. I.; Edwards, J. K.; Landon, P.; Solsona-Espriu, B.; Carley, A. F.; Herzing, A. A.; Watanabe, M.; Kiely, C. J.; Knight, D. W.; Hutchings, G. J. Science 2006, 311, 362-365. (20) Bakr, O. M.; Wunsch, B. H.; Stellacci, F. Chem. Mater. 2006, 18, 3297-3301. (21) Kuo, C.-H.; Huang, M. H. Langmuir 2005, 21, 2012-2016. (22) Teng, X.; Liang, X.; Maksimuk, S.; Yang, H. Small 2006, 2, 249253. (23) Lee, H.; Habas, S. E.; Kweskin, S.; Butcher, D.; Somorjai, G. A.; Yang, P. Angew. Chem., Int. Ed. 2006, 45, 7824-7828. (24) Zhong, X.; Feng, Y.; Lieberwirth, I.; Knoll, W. Chem. Mater. 2006, 18, 2468-2471. (25) Song, Y.; Yang, Y.; Medforth, C. J.; Pereira, E.; Singh, A. K.; Xu, H.; Jiang, Y.; Brinker, C. J.; van Swol, F.; Shelnutt, J. A. J. Am. Chem. Soc. 2004, 126, 635-645. (26) Hoefelmeyer, J. D.; Niesz, K.; Somorjai, G. A.; Tilley, T. D. Nano Lett. 2005, 5, 435-438. (27) Piao, Y.; Jang, Y.; Shokouhimehr, M.; Lee, I. S.; Hyeon, T. Small 2007, 3, 255-260. (28) Narayanaswamy, A.; Xu, H.; Pradhan, N.; Peng, X. Angew. Chem. Int. Ed. 2006, 45, 5361-5364. (29) Kim, K.; Kim, K. L.; Lee, S. J. Chem. Phys. Lett. 2005, 403, 7782. (30) Xue, C.; Li, Z.; Mirkin, C. A. Small 2005, 1, 513-516. (31) (a) Besson, C.; Finney, E. E.; Finke, R. G. J. Am. Chem. Soc. 2005, 127, 8179-8184. (b) Xiong, Y.; Xia, Y. AdV. Mater. 2007, 19, 33853391. (32) Tsen, S.-C. Y.; Crozier, P. A.; Liu, J. Ultramicroscopy 2003, 98, 63-72. (33) Damle, C.; Kumar, A.; Sastry, M. J. Phys. Chem. B 2002, 106, 297-302. (34) Borchert, H.; Shevchenko, E. V.; Robert, A.; Mekis, I.; Kornowski, A.; Gru¨bel, G.; Weller, H. Langmuir 2005, 21, 1931-1936. (35) Woods, R. In Electroanal. Chem.: A Series of Advances (Vol. 9); Bard, A. J., Ed.; Marcel Dekker: New York, 1974; pp 1-162. (36) Tominaga, M.; Shimazoe, T.; Nagashima, M.; Kusuda, H.; Kubo, A.; Kuwahara, Y.; Taniguchi, I. J. Electroanal. Chem. 2006, 590, 37-46. (37) Lee, K. Y.; Kim, M.; Lee, Y. W.; Lee, J.-J.; Han, S. W. Chem. Phys. Lett. 2007, 440, 249-252. (38) (a) Han, H. S.; Han, S. W.; Joo, S. W.; Kim, K. Langmuir 1999, 15, 6868-6874. (b) Kim, N. H.; Kim, K. J. Phys. Chem. B 2006, 110, 1837-1842. (39) Swanson, S. A.; McClain, R.; Lovejoy, K. S.; Alamdari, N. B.; Hamilton, J. S.; Scott, J. C. Langmuir 2005, 21, 5034-5039. (40) (a) Tian, Z.-Q.; Ren, B.; Wu, D.-Y. J. Phys. Chem. B 2002, 106, 9463-9483. (b) Tian, Z.-Q.; Ren, B.; Li, J.-F.; Yang, Z.-L. Chem. Commun. 2007, 3514-3534.