Pt-Guided Formation of Pt−Ag Alloy Nanoislands on Au Nanorods and

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Pt-Guided Formation of Pt-Ag Alloy Nanoislands on Au Nanorods and Improved Methanol Electro-Oxidation Weiwei He,†,‡ Xiaochun Wu,*,† Jianbo Liu,†,‡ Ke Zhang,†,‡ Weiguo Chu,† Lili Feng,†,‡ Xiaona Hu,†,‡ Weiya Zhou,§ and Sishen Xie*,§ National Center for Nanoscience and Technology, Beijing 100190, P. R. China, National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, P. R. China, Graduate School of the Chinese Academy of Sciences, Beijing 100190, P. R. China ReceiVed: March 27, 2009; ReVised Manuscript ReceiVed: May 10, 2009

A simple and facile method was developed to fabricate alloyed Pt-Ag nanoislands on Au nanorods (denoted as Au@Pt-Ag NRs). The island growth mode of Pt on the Au rod was employed to guide the growth behavior of Pt-Ag alloys. The formation of the Pt-Ag alloy was confirmed by scanning electron microscopy (SEM), transmission electron microscopy (TEM), XRD (X-ray diffraction), XPS (X-ray photoelectron spectroscopy), UV-visible absorption spectra, and electrochemical characterization. The alloy compositions can be tuned within the broad miscibility gap from Ag9Pt91 to Ag83Pt17 by controlling the ratios of PtCl42-/Ag+. The Au@Pt-Ag NRs show tunable surface plasmon resonance (SPR) and enhanced catalytic activity for methanol electro-oxidation, pointing to a potential future of nanometerized Pt-Ag alloys in fuel cells and other related fields. Apart from Pt-Ag, this strategy may be applied to the formation of Pt-related bimetallic (trimetallic) systems. Introduction Noble metal nanocrystals (NCs) have received great interest because their electronic, optical, magnetic, and catalytic properties can be tuned by controlling the size, shape, structure, and composition,1 thus endowing them with potential applications in catalysis, sensing, medical diagnostics and therapeutics, and imaging.2 In contrast to the size, the shape plays a more critical role in tailoring the properties. For example, for rod-shaped Au NCs, their longitudinal surface plasmon resonance (SPR) can be easily tuned from the visible to near-infrared spectral region simply by changing aspect ratios.2f Among the rod-shaped noble metal NCs, Au nanorods (NRs), due to the well-developed synthesis techniques,3 have been extensively investigated.4 The extension of the rod shape to other noble metals such as Ag,5 Pd,5d,6 and Pt7 has been realized by forming a core/shell structure with the Au nanorod as the inside core. Among the three shell metals investigated, Ag shows perfect layered growth mode on the Au rod, whereas Pd can be switched from the island growth mode5d to layered growth mode6 by tailoring the growth conditions. As for the Pt, mainly the island growth mode is observed. We have found that by finely tuning growth conditions, the locations of the Pt nanoislands can be tailored from end-caps, to edges, and finally to the whole surface of the Au rod.7d The island growth mode of Pt on gold is mainly related to the strong interaction of Pt atoms.8 Formation of an island-shaped Pt shell on the Au rod significantly improved optical responses of Pt, such as surfaceenhanced Raman scattering7c and enhanced dielectric sensitivity in visible and near-infrared region.7d Different from Au and Ag, Pt (and Pd) is mainly used as catalysts in a wide range of * To whom correspondence should be addressed. Phone: +86-10-8254 5577. Fax: +86-10-8254 5577. E-mail: [email protected] (X.W.), ssxie@ aphy.iphy.ac.cn (S.X.). † National Center for Nanoscience and Technology. ‡ Graduate School of the Chinese Academy of Sciences. § Chinese Academy of Sciences.

applications, most notably as the key component in direct methanol fuel cells (DMFCs), because it is the most efficient catalyst for methanol dehydrogenation.9 To improve the catalytic properties, the particle size and structure play very important roles. Therefore, remarkable research efforts have been made to control the size and morphology of the Pt NPs.10 Considering the size, small Pt NPs with a diameter of several nanometers are found to be highly catalytically active. As to the structure, a porous structure at nanoscale is especially desired considering the enlarged surface area. Yang’s group has reported that porous Pt nanocrystals exhibited higher catalytic activity for ethylene hydrogenation than cuboctahedra and cubic Pt NPs.10c Therefore, the structure obtained from the island growth mode of Pt on the Au rod is an optimized structure for catalysis. Apart from the size and structure, the composition of the catalyst is another important factor for catalytic activity. For instance, pure Pt nanostructures are readily poisoned by chemisorbed CO-like intermediates generated in the methanol oxidation process, which makes their catalytic performance degrade quickly. To decrease this effect, formation of bimetallic NPs with other metals (such as Ru,11 Rh,12 Pd13 and Au14) has been demonstrated to be an effective way. The other metal is proposed to provide oxygen-containing species at relative negative potential, which can oxidize CO at Pt sites. Therefore, alloyed Pt nanoparticles (NPs) are desired. As mentioned above, the well-controlled island growth mode has been realized for Pt on Au rods. Could this growth mode be extended to the formation of Pt-related bimetallic alloy? This issue is addressed herein. Recent studies indicate that proper alloying of Pt with Ag leads to a dramatic enhancement of catalytic activity.15 Therefore, Pt-Ag alloys may be a potential catalyst, especially considering the lower price of Ag in comparison with Pt. In this paper, the island growth mode of Pt is designed to guide the coreduction of Ag+ and Pt2+ ions with a weak reductant (ascorbic acid, AA). Using a modified seed-mediated procedure, the alloyed Pt-Ag shell composed

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of nanoislands is formed on Au NRs (named as Au@Pt-Ag NRs). The compositions of alloy nanoislands with diameters less than 5 nm can be tuned with a broad composition range (from Ag9Pt91 to Ag83Pt17) by controlling the added amounts of AgNO3 and K2PtCl4. We believe that the island growth mode of Pt tailors the growth behavior of Pt-Ag alloy. The Au@Pt-Ag NRs show tunable SPR in the visible and nearinfrared spectral region. Compared with pure Pt nanostructures, the alloyed nanostructures exhibit improved catalytic performance for methanol oxidation. Experimental Section Chemicals and Reagents. Sodium borohydride (NaBH4), chlorauric acid (HAuCl4 · 3H2O), cetyltrimethylammonium bromide (CTAB), potassium tetrachloroplatinate(II) (K2PtCl4), silver nitrate (AgNO3), and L-ascorbic acid were all purchased from Alfa Aesar and used as received. Milli-Q water (18 MΩ cm) was used for all solution preparations. All glassware used in the following procedures was cleaned in a bath of piranha solution (H2SO4/30%H2O2 ) 7:3 v/v) and boiling for 30 min. Au Nanorod Synthesis. Au NRs were prepared via a seedmediated growth. First, CTAB-capped Au seeds were synthesized by chemical reduction of HAuCl4 with NaBH4: 7.5 mL of CTAB (0.1M) aqueous solution was mixed with 100 µL of HAuCl4 (24 mM) and diluted with water to 9.4 mL. Then, 0.6 mL of ice-cold NaBH4 (0.01 M) was added with magnetic stirring. After 3 min, the stirring was stopped and the seed solution was kept undisturbed at room temperature for 30 min prior to any further experimentation. The seeds can be used within 2-5 h after preparation. After that, the growth solution of the Au NRs was prepared, which consisted of 100 mL of CTAB (0.1 M), 2.04 mL of HAuCl4 (0.024 M), 2 mL of H2SO4 (0.5 M), 1 mL of AgNO3 (10 mM), and 800 µL of AA (0.1 M). Then 240 µL of seed solution was added to the above growth solution to initiate the growth of the Au NRs. After 12 h, the Au NRs were purified by centrifugation (12 000 rpm for 10 min). The precipitates were collected and redispersed in deionized water. The volume is 100 mL. Precoating of a Thin Pt Layer on the Au Nanorod. PtCl42aqueous solution (2 mM) was prepared as follow: 0.0688 g of K2PtCl4 was dissolved in 2 mL of 0.2 M aqueous HCl solution and then diluted into 100 mL with deionized water. One milliliter of the above Au NR solution was mixed with 23.5 µL of 2 mM PtCl42- solution. Then, 5 µL of AA (0.1 M) was added. The mixture was shaken vigorously and placed in a 30 °C water bath. Within 5-6 h, the color of the solution changed from pink-red to gray, suggesting the formation of the Pt shell. The calculated Pt/Au molar ratio is 0.1. We denote the products as Au@Pt 0.1 NRs. Formation of a Pt-Ag Alloy Shell on the Precoated Au NRs. For the synthesis of Au@Pt-Ag core-shell NRs, a series of above solutions (2 mL) were mixed with a different volume (0, 0.8, 2.6, 8, 24, 40 µL) of 10 mM AgNO3 and 40 µL of 2 mM PtCl42-. After that, 8 µL of AA (0.1 M) was added. The mixtures were then shaken vigorously and placed in a 30 °C water bath. Within several minutes, the colors of the solutions changed from gray to darker gray. After 1 h, 1 mL of 0.1 M CTAB was added to stabilize the Au@Pt-Ag NRs. The products were purified by centrifuging (8000 rpm for 10 min) the solution. The precipitates were then redispersed in deionized water. Characterization. UV-visible absorption spectra were obtained from (Perkin-Elmer, Lamdba 950). For transmission electron microscopy (TEM, Tecnai F30) and scanning electron

Figure 1. SEM images of Au NRs (a), precoated Au@Pt NRs (b), and Au@Pt-Ag NRs with Ag/Pt molar ratios of 0 (c), 1/3 (d), 1/1 (e), and 5/1 (f). The scale bar is 100 nm for all images.

microscopy (SEM, Hitachi S-4800) measurements, one more centrifugation (8000 rpm for 10 min) was applied. XRD data were collected on a Rigaku D/max 2500 diffractometer with Cu KR radiation (45 kV, 250 mA) and a continuous scan mode was employed with a scan rate of 10° min-1. XPS experiments were studied using the synchrotron radiation XPS at the photoelectron station of Beijing Synchrotron Radiation Facility, the Chinese Academy of Sciences. Samples were deposited onto Si substrate to obtain thin films for the XPS measurements, which were carried out in an ultrahigh vacuum chamber with background pressures of 1 × 10 -9 Torr. C 1s at 285.5 eV was used to calibrate the charge effect of the samples. The experimental resolution was estimated to be 0.5 eV. Cyclic voltammetry was performed in a three-electrode glass cell at room temperature. The glassy carbon working electrode was polished with Al2O3 powders and ultrasonically washed in water. The Au@Pt dispersion was mixed with 5% Nafion solution (Aldrich). The 7.5 µL resulting dispersion was drop-cast onto the GC electrode and then dried overnight under vacuum. A saturated calomel electrode and a Pt foil of 1 × 1 cm2 were used as the reference and counter electrodes, respectively. The electrolyte solution was purged with nitrogen (99.9%) for 30 min and protected under nitrogen during the measurements. Methanol was electro-oxidized in an electrolyte containing 0.5 M H2SO4 and 2 M CH3OH in the potential range of -0.2 to 1.0 V at a sweep rate of 50 mV s-1.The catalytic oxidation of AA by nanorods was performed as follows: 10 µL of 10 mM AA is added to the aqueous nanorod suspension (Au@Pt nanorods or Au@Pt-Ag nanorods with different Pt/Ag ratio, concentrations approximately 1.1 × 1011 rod/mL). The oxidation reaction progress is checked instantly and continuously every 2 min by recording the absorption spectra with scanning kinetics mode. Results and Discussion Characterization of the Pt-Ag Alloyed Nanoislands. Figure 1 presents the SEM images of Au NRs, Au@Pt NRs with a Pt/Au ratio of 0.1, and Au@Pt-Ag NRs at a fixed Pt/ Au ratio of 0.27 but different Ag/Pt ratios (from 0 to 5). The Au NR is a single crystalline structure with four {110} and four {100} side facets, thus a cylindrical shape.16 Using the Au NRs as seeds, the Pt shell shows island growth (Volmer-Weber mode).7b,d It was found that a direct cogrowth of Ag and Pt on the Au NR led to an inhomogeneous coating: some Au NRs were not coated with Pt-Ag islands (Figure S1a, Supporting Information). In order to avoid this, a thin Pt layer (with a Pt/ Au ratio of 0.1) was first deposited on the Au NR. Small nanoislands on the Au rod verify the deposition of the Pt (Figure 1b). The Pt and Ag were subsequently codeposited on the

Pt-Guided Formation of Au@Pt-Ag NRs

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Figure 3. XRD profiles of Au NRs (a), Au@Pt NRs (b), and Au@Pt-Ag NRs with a Ag/Pt ratio of 1/1 (c).

Figure 2. TEM images of (a) Au NRs and (b) Au@Pt50Ag50 NRs. Parts c and d are HRTEM images. The insets in c and d are the FFT. The scale bar in b is 50 nm and applied to image a as well. In c and d scale bars are 5 nm.

precoated Au NR. Similar to the deposition of pure Pt, the deposition of Pt-Ag bimetallic shell also shows the island growth behavior (Figure 1d-f). In contrast, the deposition of pure Ag on the precoated Au NR shows a layered growth behavior (Frank-van Merwe mode, Figure S1b, Supporting Information). With increasing Ag/Pt ratio, the sizes of islands gradually enlarge but still keep the island morphology. Energy dispersive X-ray (EDX) analysis verifies the existence of platinum and silver in Au@Pt-Ag NRs. The calculated Ag/Pt ratios show a nice linear relationship with the measured ones, indicating that both Ag and Pt are stoichiometrically deposited on the precoated Au NR, and the Pt/Au ratios, as expected, stay constant (Figure S2, Supporting Information). Parts a and b in Figure 2 are the TEM images of Au NRs and Au@Ag50Pt50 NRs at low magnifications, respectively. Compared with the smooth surface of the Au rod, the Au@Ag50Pt50 NR has a rough surface composed of Pt-Ag nanoislands. The islands are 3.1 ( 0.4 nm in diameter (averaged from 70 dots). Figure 2c,d shows typical HRTEM images of Au@Ag50Pt50 NRs. The well-resolved lattice fringes and the fast Fourier transform (FFT, inset of Figure 2c,d) show that the Au@Ag50Pt50 NR has a single crystalline structure, indicating the epitaxial growth of the Pt-Ag shell on the Au core. The calculated [111] plane distance of the Pt-Ag shell is 0.232 nm (Figure 2d), in agreement with Vegard’s law for an Ag50Pt50 alloy. Figure 3 displays the XRD patterns of Au@Pt-Ag NRs along with the Au@Pt NRs and the Au NRs. The XRD patterns indicate that all three nanostructures have a face-centered cubic (fcc) structure. For the Au NR, it has a lattice constant of 4.08 Å. For the Au@Pt and Au@ Ag50Pt50 NRs, the Rietveld refinement with two phases of Au and Pt was carried out to derive the lattice constants. For the Au@Pt NRs with a Pt/Au ratio of 0.17, the lattice constants are 4.08 Å for the Au core and 3.97 Å for the Pt shell, respectively. The latter is a little larger than the lattice constant of pure Pt (3.92 Å), probably owing to a lattice expansion. In addition, due to the low percentage of Pt in the Au@Pt NR (ca. 15 wt %), no separate diffraction features from pure Pt are observed. For the Au@ Ag50Pt50 NRs, the lattice constants are 4.08 Å for the Au core and 4.01 Å for the Pt-Ag shell, respectively. The latter value falls in between that of the two pure metal elements, indicating that the shell is composed of the Ag-Pt alloy. This is consistent with the resulst of HRTEM.

Figure 4. (A) XPS spectra of Au NRs (a), Au@Pt NRs (b), and Au@Pt-Ag NRs with a Ag/Pt ratio of 5/1 (c). (B) High-resolution XPS spectra of Au NRs and Au@Pt-Ag NRs in the range around Ag 3d.

XPS spectra of Au NRs, Au@Pt NRs, and Au@Pt-Ag NRs are given in Figure 4. For the Au NRs, the signals from Au 4f are strongest, as expected. C 1s mainly comes from the surfactant molecules used to stabilize the rods. Signals from Ag 3d are also observed from pure Au NRs. The binding energy of Ag 3d5/2 at 368 eV indicates that Ag mainly exists in the form of Ag+ ion. In addition, Br 3d at 68.5 eV is also visible, suggesting that Ag ions mainly exist in the form of AgBr. For the Au@Pt NRs with a Pt/Au ratio of 0.1, the strong peak from Pt 4f7/2 at 71 eV indicates that Pt exists in the form of Pt0 atoms. In comparison, the intensity of Au 4f7/2 at 84 eV is decreased due to the deposition of Pt, which verifies the formation of a Pt shell on the Au NR. In the case of Au@Ag83Pt17 NRs, both Pt and Ag show strong signals. The signals from Au 4f are barely visible, indicating an increase in the shell thickness. In addition, Ag 3d5/2 shifts to 368.5 eV, a little higher than that in the Au NR, indicating that Ag mainly exists in the form of Ag0 atoms (as shown in Figure 4B). These data suggest that Pt and Ag form an alloyed shell on the Au NR.

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Figure 5. UV-vis-NIR spectra of (a) Au NRs, (b) [email protected], (c) [email protected] NRs, and (d-g) the Au@Pt-Ag NRs with Ag/Pt molar ratios of 0.1, 0.33, 1.0, and 5.0. The inset shows the relationship between LSPR peak position and Ag/Pt ratio.

Figure 6. CVs of Au@Pt NR-modifiued (a) and Au@Pt-Ag NRmodified glass carbon electrodes (b-d) in 0.5 M H2SO4 + 2 M methanol solution taken at a scan rate of 50 mV/s. The Ag/Pt molar ratio is 1/3 (b), 1/1 (c), and 5/1 (d), respectively. Inset: CVs of corresponding electrodes in 0.5 M H2SO4 at the same scan rate.

Tunable SPR Response in Visible and NIR Spectral Region. Apart from the structural features, spectral features also support the formation of an alloyed shell. Esumi et al. synthesized spherical Pt-Ag alloy NPs with sizes around 3-4 nm, similar to the sizes of our Pt-Ag nanoislands. They found that the Pt-Ag NPs with an Ag/Pt ratio of 9/1 had an obvious SPR band peaked at 373 nm. Decreasing the Ag/Pt ratio, the SPR maximum gradually shifted to shorter wavelengths with a decrease in intensity and broadening in width. At the Ag/Pt ratio of 1/1, the alloyed NPs exhibited a broad SPR band peaked at 338 nm.17 Figure 5 shows UV-vis-NIR absorption spectra of the different NRs. For Au@Pt-Ag NRs, the above features are not observable in the spectral region between 300 and 400 nm, indicating that these nanoislands are not physically separated. They are rather the nanoprotrusions of an expitaxially grown Pt-Ag shell. This is consistent with the growth mode we suggested above. In addition, pure Ag nanoislands do not exist, either. Otherwise, a strong SPR feature near 400 nm should appear. Therefore, the island-shaped Pt-Ag shell works as a whole to affect the SPR band of the inside Au core. Au NRs shows a strong longitudinal SPR (LSPR) peak at 793 nm. By coating with pure Pt, the LSPR peak shifts to longer wavelengths, accompanying a reduction in intensity and an obvious broadening in bandwidth (879 and 925 nm for Pt/Au ratio of 0.1 and 0.27, respectively).7d Interestingly, after codeposition of Ag and Pt, with increasing the amount of Ag, an initial redshift of the LSPR peak is observed, and a maximum red shift is achieved at the Ag/Pt molar ratio of 0.33. Further increasing the amount of Ag, the LSPR peak blue shifts back (Figure 5 inset). Different from a compact shell, we have herein a porous Pt-Ag alloy shell. Recent studies have shown that structure also influences the SPR features. For example, for cubic Au NPs with a size of 50 nm, the SPR maximum is 580 nm. In contrast, by forming a nanobox, the SPR red shifts to 800 nm. The red-shift is ascribed to the changes in structure (solid vs hollow).18 When depositing pure Ag on the Au@Pt NRs, a smooth Ag shell forms (Figure S1b, Supporting Information). With increasing Ag shell thickness, the LSPR band gradually blue-shifts. Additionally, at 330 nm, a transverse SPR mode of pure Ag shell appears at thicker shell thickness (Figure S3, Supporting Information). The blue-shift is due to the dielectric constant of Ag, the compact shell structure, and the decreased overall aspect ratio of the NR by depositing Ag shell. Taking all these factors together, we can divide the LSPR features of Au@Pt-Ag NRs into two cases. At lower Ag/Pt ratios, the change in structure is dominant and results in a further redshift. At higher Ag/Pt ratios, Ag plays a key role and leads to

the blue-shift. Thus, we can subtly tune the LSPR of Au@Pt-Ag NRs by changing the structure and the molar ratio of Ag/Pt. An obvious LSPR band at the visible and near-infrared region endows the possibility to study their properties by optical (spectral) means. Improved Catalytic Activity for Methanol Oxidation. The catalytic performance should benefit from the island-like Pt-Ag bimetallic alloy shell. The electrocatalytic activity was estimated from methanol electrochemical oxidation using cyclic voltammetry. In order to estimate the actual surface area that was accessible for reaction, the electrochemically active surface (EAS) was calculated from H adsorption/desorption cyclic voltammograms (CVs) taken in sulfuric acid solution (see Figure 6 inset), and the results for methanol oxidation were normalized by the corresponding EAS. Compared with Au@Pt NRs, Au@Pt-Ag NRs have larger EASs. Therefore, by forming Pt-Ag alloy, Pt catalysts own enlarged EAS, which will contribute to better catalytic activity. In addition, no electrodissolution of Ag at 0.4 V (bulk Ag oxidation) is found, indicating that alloying with Pt can greatly enhance the electrochemical stability of Ag. This is consistent with the conclusions from above structural and spectral characterizations. For Au@Pt 0.27, a shoulder peak around 0.68 V in the forward sweep indicates the electro-oxidation of methanol. In the backward sweep, no methanol oxidation peak is observed. In contrast, for three Au@Pt-Ag NRs, methanol oxidation peaks are clearly observed at 0.65 V in the forward sweep and at 0.43 V in the backward sweep (see Figure 6). The CVs of Au@Pt-Ag NRs are quite similar to that of pure Pt NPs, indicating that catalytic activities mainly come from Pt. The current at 0.65 V increases sharply at a Ag/Pt ratio of 1/3. Further increasing the ratio to 1 only leads to a small increase in the oxidation current. When the Ag/Pt ratio changes from 1 to 5, no obvious increase in current is found. After calibrating the EAS difference, the Au@Pt-Ag NRs with a Ag/Pt ratio of 5/1 exhibited ca. 4 times higher oxidation current than the Au@Pt NRs. The oxidation peak at 0.43 V in the back sweep was suggested to be associated with the removal of CO species generated in the forward scan. The ratio of the forward oxidation current peak (If) to the reverse current peak (Ib) (If/Ib) has been suggested to be an index of the catalyst’s tolerance to the poisoning species.19 The If/Ib ratios of the Au@Pt-Ag NRs with Ag/Pt ratio of 1/3, 1/1, and 5/1 were calculated as 3.1, 2.5 and 1.6, respectively. The Au@Pt-Ag NRs exhibit a compositiondependent tolerance to the poisoning CO species on the catalyst surface. Alloyed Pt-Ag nanoislands with a lower Ag ratio demonstrate better tolerance. In addition, the If/Ib ratios of

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SCHEME 1: Possible Architectures of the Pt-Ag Nanoislandsa

a

Red and blue spheres represent pure Ag and Pt islands, respectively.

Pt-Ag alloyed nanoislands are all higher than that of the commercial pure Pt catalyst E-TEK catalyst (0.74),19b showing a better performance of Pt-Ag alloyed nanoislands in tolerating poisoning species. Up to now, due to the limited experimental results from alloyed Pt-Ag NPs, the mechanism behind the improved performance was not well-understood. The interaction between Pt and Ag and possible changes in electronic structure of Pt by Ag are needed to be further investigated. During the codeposition of Pt and Ag, three possible architectures may be formed (Scheme 1): discrete Pt and Ag nanoislands, Pt@Ag and/or Ag@Pt nanoislands, and alloyed nanoislands. In bulk, Ag and Pt have a large miscibility gap at temperature below 900 K and form alloys only at very high atomic content of either Ag or Pt.20 For instance, at 400 °C, the alloys can form with the atomic composition of Age2Ptg98 or Agg95Pte5. Therefore, few reports have been involved with Pt-Ag alloys.15,17,21 Often, a core/shell structure is observed for a codeposition process.22 For instance, by using radiolytic synthesis to prepare Pt-Ag bimetallic NCs, instead of alloyed nanoparticles, the authors eventually obtain an Ag@Pt nanostructure. Up to now, it was still difficult to produce a wellmixed Pt-Ag alloy through the simple simultaneous reduction of Pt and Ag precursors. In our case, discrete Ag (or Pt) nanoislands and the core/shell structures can be excluded. Because the deposition of silver on the Pt nanoisland shows the layered growth mode, it rules out the formation of the Pt@Ag nanoislands (Figure S1b, Supporting Information). No electrodissolution of Ag gives an additional support. The exclusion of Ag@Pt nanoislands can be verified by etching experiments. Due to the existence of Ag, the Au@Pt-Ag NRs can be etched by Pt2+ ions, thus inducing an increase in Pt/Au ratio and a decrease in Ag/Au ratio (see Figure S4, Supporting Information, for the example of Au@Pt50Ag50 NRs). In contrast, Au@Pt NRs show no changes upon adding Pt2+ ions. The single crystalline structure of the nanoislands (from HRTEM), no electrodissolution of Ag, and the lack of a SPR band at 400 nm argue against the existence of discrete Ag nanoislands, thus denying the existence of discrete Pt nanoislands. Therefore, all experimental results verify that we obtained alloyed Pt-Ag nanoislands. The formation of Pt-Ag alloyed nanoislands is due to the cogrowth of Pt and Ag catalyzed by Pt and the island growth mode guided by Pt. Under the reaction conditions we employed, the Au NRs cannot effectively seed the homogeneous deposition of silver by AA molecules (Figure 7, upper part). The inhomogeneous coating starts at a Ag/Au ratio of 0.34 (corresponding to case of the [email protected] NRs). With increasing Ag/Au ratios, the situation becomes worse. The Au NRs coated with a lot of Ag show a strong absorption band between 300 and 500 nm (Figure 7, lower part, c-f), whereas the longitudinal SPR bands show fewer changes. In contrast, when a thin Pt shell is deposited on the Au NR, the scenario is quite different. Wellcontrolled deposition of Ag can be achieved by using AA as

Figure 7. (Top) SEM images of (a) Au NRs and (b-f) Au@Ag nanocrystals with Ag/Au ratios of 0.17, 0.34, 0.51, 0.68, and 0.85, respectively. The scale bar is for all images. (Bottom) UV-vis-NIR absorption spectra of corresponding (a) Au NRs and (b-f) Au@Ag nanocrystals.

Figure 8. Normalized absorption at the peak position of AA as a function of time after addition of [email protected] NRs, Au@Pt-Ag NRs, and Au@Pt@Ag NRs. For alloyed NRs, the Pt/Ag ratios are 3/1, 1/1, and 1/5, respectively. Insets show the evolutions of the absorption spectra over time for Au@Pt75Ag25 and Au@Pt50Ag50 NRs, respectively.

the reductant. We believe that the strong catalytic activity of Pt induces the controlled reduction of Ag+ with AA. Thus, under

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the guidance of Pt, Ag and Pt can co-deposit and form a wellcontrolled nanoalloy. The Au@Pt NRs can effectively catalyze the oxidation of AA by the dissolved O2 in the suspension of the nanorods. By forming alloys with Ag, a composition-dependent catalytic activity is found, as shown in Figure 8. At a Pt/Ag ratio of 3/1, the nanorods are still active for the catalysis, albeit with a decreasing rate. When the Pt/Ag ratio changes to 1/1, the catalytic activity decreases dramatically. At a Pt/Ag ratio of 1/5, the alloy nanorods show no catalytic activity any more, similar to an Ag-coated nanorod. This nonlinear catalytic behavior indicates that the electronic structure of Pt has been changed by forming the alloy with Ag. This variation in electronic structure may influence other catalytic processes and provide a route for composition-tailored catalytic control. Conclusion In summary, we have developed a simple and effective way to prepare Au@Pt-Ag alloyed NRs under a mild reaction condition. Island-shaped Pt-Ag alloy shells, with compositions (from Ag9Pt91 to Ag83Pt17) through the whole miscibility gap of the bulk, are successfully synthesized using Pt-guided codeposition of Pt and Ag. Pt plays two key roles in this strategy. First, Pt is employed to induce the island growth mode. The sizes of alloyed nanoislands are 3-5 nm and are an optimal size for catalysis. This mode may be applied to fabricate other Pt-related bimetallic catalysts. For example, our preliminary results verified the island growth mode for alloyed Pt-Au and Pt-Pd bimetallic systems as well. Second, Pt can effectively seed the deposition of Ag using a weak reductant. The metal is not limited to Ag. Other less noble metals, such as Ni and Co, might be applied. Furthermore, during the codeposition of Ag and Pt, alloyed Pt-Ag nanoislands are formed. This may provide a new route to synthesize alloyed bimetallic nanostructures for those metals that are immiscible with Pt in bulk. Finally, herein, we employed Au NRs as seeds. Other seeds, composed of other noble metals with different sizes, shapes, structures, and compositions, can be also applied, thus pointing out the great versatility and potential of this strategy for fabricating Pt-related bimetallic (trimetallic) catalysts. Acknowledgment. The work was supported by National Natural Science Foundation of China (Grant No. 20773032) and the “973” National Key Basic Research Program of China (2006CB705600 and 2006CB932602). Supporting Information Available: SEM images of Au@Pt-Ag NRs without precoated Pt layer and Ag subsequent growth on [email protected], detailed EDX analysis of Au@Pt-Ag NRs, UV-vis spectra of subsequent growth of Ag on Au@Pt, and SEM of [email protected] and Au@Pt before and after etching. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) (a) Burda, C.; Chen, X. B.; Narayanan, R.; El-Sayed, M. A. Chem. ReV. 2005, 105, 1025. (b) Xia, Y. N.; Halas, N. J. Mater. Res. Bull. 2005, 30, 338. (c) Liz-Marzan, L. M. Langmuir 2006, 22, 32. (d) Pileni, M. P. J. Phys. Chem. C 2007, 111, 9019. (2) (a) Chen, M. S.; Kumar, D.; Yi, C. W.; Goodman, D. W. Science 2005, 310, 291. (b) Rosi, N. L.; Mirkin, C. A. Chem. ReV. 2005, 105, 1547.

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