Three-Dimensional Pt-on-Au Bimetallic Dendritic Nanoparticle: One

Aug 24, 2010 - Shaojun Guo, Jing Li, Shaojun Dong, and Erkang Wang*. State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Appli...
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J. Phys. Chem. C 2010, 114, 15337–15342

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Three-Dimensional Pt-on-Au Bimetallic Dendritic Nanoparticle: One-Step, High-Yield Synthesis and Its Bifunctional Plasmonic and Catalytic Properties Shaojun Guo, Jing Li, Shaojun Dong, and Erkang Wang* State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, Jilin, China, and Graduate School of the Chinese Academy of Sciences, Beijing, 100039, People’s Republic of China ReceiVed: May 30, 2010; ReVised Manuscript ReceiVed: August 10, 2010

Pt-based bimetallic dendritic nanoparticles (NPs) with noncompact nanobranches show great potential as catalysts for reducing Pt consumption, providing a high surface area and facilitating the enhanced performance in the catalytic applications. However, developing a one-step route to synthesize such dendritic NPs with a well-defined morphology at room temperature is still a great challenge and has been rarely reported. Herein, we report a one-step, room-temperature, and aqueous route for the synthesis of Pt-on-Au bimetallic dendritic NPs with noncompact nanobranches (denoted as BDNNNs) in high yield (∼100%). These BDNNNs have many big gaps available among the Pt nanobranches, which are important for providing a high electrochemical surface area. Methanol was selected as a model molecule for studying the electrocatalytic performance of BDNNNs. Interestingly, it is found that BDNNNs have a higher electrochemically active area than Au@Pt core/shell NPs with a rough surface and thus lead to higher catalytic activity for methanol oxidation. Moreover, for BDNNNs, the surface plasmonic resonance (SPR) peak of the Au core can be observed even when many Pt nanobranches are supported on the Au core because of the noncompact Pt nanobranches, which is important for in situ spectroscopic (such as surface-enhanced Raman scattering) characterization of catalytic reactions. Introduction Bimetallic nanoparticles (NPs), either in the form of an alloy or in core-shell structures, have received increasing interest owing to their different optical, catalytic, electronic, and magnetic properties relative to their monometallic counterparts.1-7 Especially, it has been reported that adding a second metallic component has the great potential for enhancing the functionality and performance of pure metal components, such as activity, selectivity, stability, etc.8,9 At present, the shape, composition, and architecture are being recognized as important control parameters in the tailoring of new bimetallic NP systems. Prominent examples include the synthesis of Pd-Pt core-shell nanoplates with hexagonal and triangular shapes;10 Pt-Pd core-shell nanocubes, cuboctahedra, and octahedra;11,12 Au-Pd or Au-Ag core-shell nanocubes using Au nanooctahedra as cores;13 Pt-(Cu, Co, Fe, Ni) alloyed nanocubes;4-6 etc. Despite these successful demonstrations, all of these studies have been limited to bimetallic NPs with a smooth surface, which is a disadvantage for providing a high surface area from the viewpoint of the design of catalysis. Pt-based bimetallic flower-like or foamlike NPs show great potential as catalysts for reducing the Pt consumption, providing a high surface area, and facilitating enhanced performance in the catalytic applications.13-17 However, there exist some unavoidable overlaps on the gaps among the Pt nanobranches, which will reduce the efficient surface area of Pt and not facilitate the improvement of Pt activity. Recent advances reveal that Pt-based dendritic NPs with noncompact nanobranches provide new alternatives for scientists to obtain high-efficiency nanoelectrocatalysts with a high surface area for enhanced electroactivity for fuel cell reactions.18,19 However, at present, * To whom correspondence should be addressed. E-mail: ekwang@ ciac.jl.cn.

the rational design and synthesis of Pt-based bimetallic dendritic NPs with noncompact nanobranches are still a great challenge and have rarely been reported.20,21 Xia et al.20a demonstrated a two-step strategy for the preparation of Pt-on-Pd bimetallic dendritic NPs at 90 °C, which exhibited high electrocatalytic activity toward the oxygen reduction reaction. Compared with Pt-on-Pd dendritic NPs,20a,21 Pt-on-Au bimetallic dendritic NPs exhibit more obvious advantages because Au has a higher chemical stability and durability as a catalyst component than Pd,22 and Au NPs can additionally exhibit a surface plasmon resonance (SPR) property. Particularly, to the best of our knowledge, most of the Pt-based bimetallic dendritic NPs could only be prepared in the presence of as-synthesized seeds, which generally need multistep and complex procedures. A one-step strategy for the reduction of two metallic precursors usually produced a bimetallic alloy or sometimes core-shell NPs with uncontrolled spherical shapes.23,24 Ideally, one would prefer a one-step, convenient, and environmentally benign approach to synthesize bimetallic heterostructures in large scale under mild conditions. It should be noted that, during our submission period, Yamauchi et al.20b reported a one-step, room-temperature, and aqueous route for the preparation of bimetallic Pt-on-Au dendritic NPs with noncompact nanobranches (denoted as BDNNNs) under stirring. As an almost parallel result, we reported a slightly modified approach to prepare BDNNNs with a smaller size than that of Yamauchi’s work in high yield (∼100%) under sonication conditions. Different from Yamauchi’s work concentrating on the synthesis of BDNNNs, herein, we additionally highlighted two important characteristics of our as-prepared BDNNNs. (1) These BDNNNs with a high electrochemical surface area (ECSA) exhibited much higher electrocatalytic activity toward the methanol oxidation reaction than Au@Pt core/shell NPs with a rough surface. (2) Generally, for Au@Pt or Pd core/shell

10.1021/jp104942d  2010 American Chemical Society Published on Web 08/24/2010

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nanostructures, a thin shell thickness can almost lead to damp the surface plasmonic resonance (SPR) peak of the Au core. However, for the present BDNNNs, the SPR peak of Au can still be observed even when many Pt nanobranches are supported on the Au core because of the noncompact Pt nanobranches, which is important for in situ spectroscopic (such as surfaceenhanced Raman scattering) characterization of important catalytic reactions.1,25,26 Experimental Section Materials. Trisodium citrate, H2SO4, and ethanol were purchased from the Shanghai Chemical Factory (Shanghai, China) and used as received without further purification. K2PtCl4 and ascorbic acid (AA) were purchased from Alfa Aesar. Nafion (perfluorinated ion-exchange resin, 5 wt % solution in a mixture of lower aliphatic alcohols and water) and Pluronic F127 ((PEO)100(PPO)65(PEO)100 (Mw ) 12 600) were obtained from Aldrich. Water used throughout all experiments was purified with the Millipore system. Apparatus. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were obtained with a TECNAI G2 high-resolution transmission electron microscope operating at 200 kV. A XL30 ESEM scanning electron microscope equipped with an energy-dispersed X-ray spectrometer was used to determine the composition of the product. The amounts of Pt-on-Au bimetallic dendritic NPs and Au@Pt core/ shell NPs with a rough surface were determined by inductively coupled plasma-mass spectroscopy (ICP-MS, X Series 2, Thermo Scientific, U.S.A.). X-ray diffraction (XRD) analysis was carried out on a D/Max 2500 V/PC X-ray diffractometer using Cu (40 kV, 30 mA) radiation. UV-vis spectra were collected on a Cary 500 Scan UV-vis-near-infrared (UV-visNIR) spectrophotometer. Transmission infrared spectra were collected in the transmission mode on a Nicolet 560 Fourier transform infrared (FTIR) spectrometer. Thermogravimetry analyses (TGA) were carried out by using a PerkinElmer TGA-2 thermogravimetric analyzer at a heating rate of 10 °C min-1 under air from 20 to 600 °C. Cyclic voltammetry (CV) experiments were performed with a CHI 832 electrochemical analyzer (CH Instruments, Chenhua Co., Shanghai, China). A conventional three-electrode cell was used, including a Ag/AgCl (saturated KCl) electrode as a reference electrode, a platinum wire as a counter electrode, and modified glassy carbon (GC) as a working electrode. Synthesis of Pt-on-Au Bimetallic Dendritic NPs. In a typical synthesis of BDNNNs, 0.1 g of Pluronic F127 was added into 6.5 mL of water, followed by the addition of a HAuCl4 (2 mL, 25 mM)/K2PtCl4 (0.5 mL, 0.1 M) mixture and 1 mL of 0.4 M ascorbic acid (AA). After immediately being sonicated for 9 min, the solution was placed for more than 12 h at room temperature. The product was isolated, and residual Pluronic F127 was removed by centrifugation at 10 000 rpm for 20 min, followed by consecutive washing/centrifugation cycles three times with water. The collected product was dried at 60 °C under vacuum conditions and/or redispersed in water with sonicating to produce a colloidal suspension for further characterization. Synthesis of Au@Pt Core/Shell NPs with a Rough Surface. Gold NPs (13 nm) were first synthesized according to the previous report.15 Briefly, 100 mL of 1 mM HAuCl4 was brought to a reflux while stirring, and then 10 mL of a 38.8 mM trisodium citrate solution was added quickly, which resulted in a color change of the solution from pale yellow to deep red. After the color changed, the solution was refluxed for an additional 15 min. A 25 mL portion of the above solution was

Guo et al. heated to 100 °C, followed by the addition of 2 mL of 38.8 mM citrate and 1.25 mL of 1% H2PtCl6 (20 mM) under stirring. After being heated for 20 min, the Au@Pt core/shell NPs with a rough surface were obtained. Electrocatalytic Experiment. Prior to the surface coating, the GC electrode was polished carefully with 1.0, 0.3, and 0.05 µm alumina powders and rinsed with deionized water, followed by being sonicated in acetone and doubly distilled water successively. The electrode was then allowed to dry under nitrogen. For the methanol oxidation reaction, 5 µL of BDNNNs with different Pt nanobranch densities or Au@Pt core/shell NPs with a rough surface was dropped on the surface of the GC electrode and dried with an infrared lamp. Nafion (3 µL, 0.5%) was then placed on the surface of the above materials modified GC electrode and dried before electrochemical experiments. The loading amounts of Pt are 67.2 µg cm-2 (the sample obtained from 0.5 mL of the 0.1 M Pt precursor), 61.5 µg cm-2 (the sample obtained from 0.3 mL of the 0.1 M Pt precursor), 60.1 µg cm-2 (the sample obtained from 0.8 mL of the 0.1 M Pt precursor), and 60.1 µg cm-2 for BDNNNs with different Pt nanobranch densities and Au@Pt core/shell NPs with a rough surface. Results and Discussion Figure 1A shows a representative TEM image of the prepared product, where there is a strong contrast difference in all of the NPs with a dark center surrounded by a light edge, confirming their core/shell structure. The size of these NPs is 30 ( 5 nm. Figure 1B,C shows the magnified TEM images of these NPs. It is found that one metal grows from another metal core as a dendritic tendril to form a heteroaggregate structure that is architecturally distinct from bimetallic Au-Pt systems previously reported (very compact Pt nanobranches).14-17 The nucleation sites for Pt appear to be distributed over the entire surface of the Au core and do not overlap extensively (the following experimental results will reveal that these NPs are composed of a Au core and a Pt shell). For Pt nanobranches, the average diameter is about 3 nm. In addition, selected area electron diffraction recorded on the NPs (Figure 1C) shows their crystalline nature (Figure S1, Supporting Information). The highangle annular dark-field scanning TEM (HAADF-STEM) image of individual NPs shown in Figure 1D further shows the intense contrast between the core and the surrounding branches of a nanodendrite, demonstrating a three-dimensional dendritic morphology. Thus, both TEM and STEM analyses confirmed the absence of isolated Au and Pt NPs in the product. We further characterized individual NPs by HRTEM, as shown in Figure 1E-G. The magnified image (Figure 1F) reveals that the d-spacings of adjacent fringes for metal cores and metal branches ´ respectively, corresponding are 2.35 ( 0.02 and 2.25 ( 0.02 Å, to the {111} planes of face-centered cubic (fcc) Au and Pt, respectively, which indicates that bimetallic Au@Pt core/shell NPs have been obtained. The fast Fourier transform (FFT) pattern (Figure 1H and the inset of Figure S2, Supporting Information) of the HRTEM image of some Pt nanobranches (the circled part Figure 1G or Figure S2, Supporting Information) indicates that these Pt nanobranches are single-crystalline. The core/shell dendritic structure observed here is also consistent with the X-ray diffraction (XRD) pattern (Figure 2, line c) of the BDNNNs, which shows nonalloyed strong Au peaks (the same peak positions as those of Au NPs, line a) with shoulders arising from the Pt tendrils (the same peak positions as those of Pt NPs, line b). The data displays that the core is essentially pure Au, whereas Pt is concentrated at the periphery of the NPs.

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Figure 1. TEM (A-C), HAADF-STEM (D), and HRTEM (E-G) images of BDNNNs. Panel H shows the fast Fourier transform (FFT) of panel G (circled part).

Figure 2. XRD patterns of Au NPs (a), Pt NPs (b), and BDNNNs (c). “/” denotes the typical diffraction peaks of Pt.

Furthermore, an interesting observation is that a much stronger (311) diffraction peak for BDNNNs (line c, Figure 2) is observed

than that of Pt NPs (line b, Figure 2). This indicates that BDNNNs are relatively abundant in high-index {311} facets, which has been proven to be very important for improving the catalytic performance of Pt (discussed later).20a The chemical composition of BDNNNs was determined by energy-dispersive X-ray spectroscopy (EDX) (Figure S3, Supporting Information). The EDX spectrum with two main peaks (Au and Pt) was observed (Si signal is from the Si substrate), indicating that the hybrid nanostructure was made up of metallic Au and Pt. Figure S4 (Supporting Information) shows the infrared spectroscopy of Pluronic F127 (line a) and BDNNNs obtained from consecutive washing/centrifugation cycles (line b). Interestingly, it is found that BDNNNs almost did not show the typical characteristic peaks related to Pluronic F127, indicating that BDNNNs had a nearly “clean” surface after consecutive washing/centrifugation cycles. Thermogravimetric analyses (TGA) of Pluronic F127 (line a) and BDNNNs obtained from consecutive washing/ centrifugation cycles (line b) are shown in Figure S5 (Supporting

Figure 3. TEM (A, C, E, G, K, L) and HRTEM (B, D, F, H-J) images of NPs collected at different reaction times: (A, B) 9 min, (C, D) 5 h, (E, F) 5.5 h, (G-J) 8 h, (K, L) >12 h. Note that the red and green circled parts are shown in (I, J), respectively.

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Figure 4. UV-vis spectra of the reaction solution reacted for different reaction times: (a) 9 min, (b) 5 h, (c) 5.5 h, (d) 6 h, (e) 7 h, (f) 8 h, and (g) >12 h.

Information). It is observed that BDNNNs exhibit very little loss in the decomposing region of Pluronic F127 (about 200 °C), further proving that the as-prepared BDNNNs have a nearly “clean” surface.18 In our experiment, AA and Pluronic F127 were used as both reducing and directing agents, respectively. Because of the moderate reducing power of AA under the synthetic conditions, the reduction rates of different metal precursors in the presence of AA were different. On the basis of different reduction potentials of Au(III) and Pt(II) (AuCl4-/Au, +1.002 V vs SHE and PtCl42-/Pt, +0.755 V vs SHE), Au(III) would probably be preferentially reduced over the Pt(II) under our experimental conditions. We think that the nucleation of Au atoms to form the Au core at a short time and, subsequently, their acting as a nucleic center for the growth of the Pt nanobranches should be responsible for the above core/shell Au/Pt dendritic NPs. The following three facts indeed confirm our hypothesis: (1) To help understand the process of shape evolution in this system, four intermediate products were harvested at four different reaction times, which were observed by TEM and HRTEM (Figure 3). It is found that, from 9 min to 5 h, there only exist Au NPs with their size unchanged in the solution, as shown in Figure 3A-D. Particularly, HRTEM data (Figure 3B,D) show the typical lattice spacing of Au (0.235 ( 0.02 nm), indicating that the Pt precursor would not have been reduced in a time of 5 h.

Figure 6. TEM (A, B, D, E) and HRTEM (C, F) images of BDNNNs: (A-C) 0.3 mL of 0.1 M K2PtCl4, (D, E) 0.8 mL of 0.1 M K2PtCl4.

Pt nanobranches then began to deposit onto the surface of Au cores up to 5.5 h under the directing agents (Pluronic F127) (Figure 3E,F). Such growth continuously occurred as the reaction proceeded (from 5.5 to 8 h; finally, more than 12 h) until complete consumption of the Pt precursor in the reaction solution (Figure 3G-L). (2) The structural evolution also accompanied distinct changes in the UV-vis spectral features of the reaction mixtures (Figure 4). When the reaction time was 9 min, the single surface plasmon resonance (SPR) peak appeared, which could be assigned to the dipole resonance of the Au core.1 The SPR intensity was not changed in 5 h, indicating that the Au precursor has been reduced completely in 9 min. When the reaction time was further increased, the SPR peak of the Au core was gradually damped and exhibited a red shift, which is ascribed to Pt deposition.27 (3) The observed spectral changes were also reflected in the color changes of the solution during the reaction, as shown in Figure 5. The typical red color for Au NPs in less than 5 h is observed (labeled a-d), whereas the colors gradually changed to dark red (labeled e-g) and, finally, to black with the increase of time (labeled h and i). Therefore, the temporal separation of the formation of the

Figure 5. Photographs of reaction solutions reacted for different reaction times: (a) 9 min, (b) 1 h, (c) 3 h, (d) 5 h, (e) 5.5 h, (f) 6 h, (g) 7 h, (h) 8 h, (i, i’) >12 h. Note that the solutions from (a) to (e) and (i’) were diluted by 20 times of the original solution; the solutions from (f) to (i) were diluted by 40 times of the original solution in order to better observe the color change during the reaction period.

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Figure 7. (A) UV-vis spectra of gold NPs (a) and BDNNNs (b-d): (b) 0.3 mL of 0.1 M K2PtCl4, (c) 0.5 mL of 0.1 M K2PtCl4, (d) 0.8 mL of 0.1 M K2PtCl4. (B, C) CVs of BDNNNs (a-c) and Au@Pt core/shell NPs with a rough surface (d) modified GC electrodes in 0.5 M H2SO4 solution in the absence (B) and presence (C) of 1 M methanol: (a) 0.5 mL of 0.1 M K2PtCl4, (b) 0.3 mL of 0.1 M K2PtCl4, (c) 0.8 mL of 0.1 M K2PtCl4. Scan rate ) 100 (B) and 50 mV/s (C).

Au core from the formation of the Pt nanobranches owing to the slow reduction kinetics for Pt at room temperature was believed to be the key to one-step formation of the core-shell nanostructure here. The reason for growing Pt nanobranches could probably be ascribed to the structure-directing effect of Pluronic F127, as suggested by Yamauchi et al.18 Interestingly, the density of the Pt nanobranches on the Au nanosphere could be easily controlled via simply changing the molar ratio of the Pt precursor to the Au precursor. For instance, the Pt-on-Au bimetallic dendritic NPs with fewer Pt nanobranches could be obtained when 0.3 mL of the Pt precursor (0.1 M) was added into the mixture instead of 0.5 mL, as shown in the TEM images of Figure 6A-C. When the amount of the Pt precursor (0.1 M) was further increased to 0.8 mL, a higher density of Pt nanobranches supported on Au nanospheres could be observed (Figure 6D-F). Thus, the plasmonic and catalytic properties of BDNNNs can be easily tuned via changing the above parameters. Figure 7A shows the UV-vis spectra of BDNNNs with different Pt branch densities. It is observed that the SPR peaks gradually exhibit a red shift with increasing the amounts of the Pt precursor. Note that, generally, the shells of Pt or Pd can strongly damp out the dipolar plasmon oscillations of Au cores because Pd or Pt has significantly lower conductivities at the optical frequency than those of Au.30 For Au@Pt or Pd core/shell nanostructures, a thin shell thickness can almost lead to damping the SPR peak of the core. However, for the present BDNNNs, the SPR peak of Au can still be observed even when high-density Pt nanobranches are supported on the Au core because of the noncompact Pt nanobranches. The interesting characteristic will be important for in situ spectroscopic (such as surface-enhanced Raman scattering) study on the mechanism of some important Pt-related catalytic reactions. In recent years, direct methanol fuel cells (DMFCs) have been intensely studied because of their numerous advantages, such as high-energy density, the ease of handling the liquid, low operating temperatures, and their possible applications to microfuel cells.28 The performance of DMFCs is known to be strongly dependent on the electrocatalytic materials used. Herein, we investigated the potential use of these BDNNNs as an anode material for fuel cells. Figure 7B shows the electrochemical properties of BDNNNs with different Pt nanobranch densities and the Au@Pt core/shell structured reference catalyst with a similar size (its TEM image is shown in Figure S6 in the Supporting Information) in a 0.5 M H2SO4 solution. The ECSA for the BDNNNs was found to be 55.6 m2/gPt based on the CV data (trace a), which is higher than that of the reference catalyst (22.4 m2/gPt, trace d). Figure 7C shows the CVs of BDNNNs with different branch densities (traces a-c) and the reference catalyst (trace d) modified GC electrodes in a 0.5 M H2SO4 solution + 1 M methanol. Relative to the reference catalyst, the significant enhancements (e.g., about 2.73 times for trace

a) of the peak current for the methanol oxidation reaction (mass activity) can be observed on BDNNNs with different branch densities, indicating that BDNNNs have higher catalytic activity. This is probably caused by the particular structure of BDNNNs, such as the noncompact dendritic morphology and high ECSA, etc. Recently, several groups reported that Pt nanodendrites contained some high-index facets, such as the {311} facet, which could improve the electrocatalytic activity for small molecule oxidation or reduction.20a,29 We think that some high-index facets existing on BDNNNs may also contribute to the high activity for methanol oxidation here. Furthermore, from traces a-c, the BDNNNs with proper branched degrees have the optimized electrocatalytic activity. Therefore, the enhancement of the catalytic activity is expected through the optimization of both the size and the dimension of BDNNNs in such a one-step synthesis. Conclusions In summary, we have developed a one-step, environmentally benign procedure to synthesize Pt-on-Au bimetallic dendritic NPs with Pt nanobranches easily controlled at room temperature. The proposed method was unique in its simplicity. These BDNNNs exhibit bifunctional plasmonic and catalytic properties. Particularly, the Pt-on-Au bimetallic nanostructure contains noncompact dendritic-like Pt nanobranches, which exhibited much higher electrocatalytic activity toward the methanol oxidation reaction than the Au@Pt core/shell structured reference catalyst. More interestingly, these BDNNNs even with high-density Pt nanobranches have a good SPR peak, which is important for producing a strong electromagnetic field for SERS applications.30 Therefore, novel bifunctional plasmonic and catalytic properties of the present BDNNNs will probably find potential applications for in situ spectroscopic (such as surfaceenhanced Raman scattering) characterization of some important Pt-related catalytic reactions. Acknowledgment. This work was supported by the National Natural Science Foundation of China (No. 20820102037) and the 973 Project (Nos. 2009CB930100 and 2010CB933600). Supporting Information Available: Figures S1-S6. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Lee, Y. W.; Kim, M.; Kim, Z. H.; Han, S. W. J. Am. Chem. Soc. 2009, 131, 17036. (2) Huang, X. Q.; Zhang, H. H.; Guo, C. Y.; Zhou, Z. Y.; Zheng, N. F. Angew. Chem., Int. Ed. 2009, 48, 4808. (3) Camargo, P. H. C.; Xiong, Y. J.; Ji, L.; Zuo, J. M.; Xia, Y. N. J. Am. Chem. Soc. 2007, 129, 15452.

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