Journal of Materials Chemistry A

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1 Journal of Materials Chemistry A PAPER Cite this: J. Mater. Chem. A, 2013, 1, 906 One-step polyol synthesis of Rh-on-Pd bimetallic nanodendrites and their electrocatalytic properties for ethanol oxidation in alkaline media Shuiyun Shen and Tianshou Zhao* Received 9th July 2012 Accepted 30th October 2012 DOI: /c2ta00725h We report a new, facile and one-step polyol route for the synthesis of Rh-on-Pd bimetallic nanodendrites that are composed of Pd cores with Rh branches. Ethylene glycol is used as a reducing agent while hexadecyltrimethylammonium bromide (CTAB) is used as a structure-directing agent. The as-synthesized nanodendrites are characterized by transmission electron microscopy, energy-dispersive X-ray spectrometry, X-ray diffraction and X-ray photoelectron spectroscopy. It is demonstrated that the morphology and number of Rh branches can be regulated by varying, respectively, the molar ratio of Pd to Rh precursors and the CTAB content. An intriguing finding is that CTAB not only directs the growth of Rh branches but also enables the formation of uniformly-shaped Pd cores. This effective one-step polyol synthesis can be ascribed to the different reduction kinetics between Pd and Rh ions resulting in the formation of Pd cores prior to the growth of the Rh branches. The electrocatalytic properties of the carbon supported Rh-on-Pd bimetallic nanodendrites as the catalyst for ethanol oxidation in alkaline media are investigated. Cyclic voltammetry results show that the Rh-on-Pd/C catalysts display a much higher CO 2 selectivity than a Pd/C catalyst. In particular, the ratio of the forward to backward peak current density (j f /j b ) of the Rh-on-Pd (3 : 1)/C catalyst is 2.2, which is three times that of the Pd/C catalyst. 1 Introduction Over the past decade, the rational design and synthesis of bimetallic nanomaterials has attracted increasing interest because of their fascinating optical, electronic and catalytic properties relative to their monometallic counterparts. It has been recognized that compared with single metal nanomaterials, bimetallic nanomaterials can have greatly improved catalytic performances including activity, selectivity and stability. 1 3 Bimetallic nanomaterials can be in the form of either alloy or core shell structures. 4 6 Recently, special attention has been paid to the synthesis of M1-on-M2 bimetallic nanodendrites that are composed of an array of branches formed from one metal (M1) supported on a core of another metal (M2). 7,8 With respect to conventional core shell nanomaterials with smooth surfaces, the open and branched M1 shell of M1-on-M2 bimetallic nanodendrites not only exhibits a high surface area but also facilitates the mass transport of reactants to active sites, thus providing a great opportunity to improve the catalytic activity of M Bimetallic nanodendrites, including Pt-on-Au, 9,10 Pt-on-Pd and Pt-on-Ag, 16 Department of Mechanical Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong SAR, China. metzhao@ ust.hk; Fax: ; Tel: Electronic supplementary information (ESI) available. See DOI: /c2ta00725h have been obtained and are of particular interest as electrocatalysts in fuel cell applications. It has been shown by Xia et al. 12 that on the basis of an equivalent Pt mass, a Pt-on-Pd/C catalyst is two and a half times more active for the oxygen reduction reaction in acidic media than the state-of-the-art Pt/C catalyst. In addition to a higher activity, Pt-on-Pd bimetallic nanodendrites also possess a better stability than Pt nanoparticles. Yang and Peng 13 demonstrated that a er cycles of linear potential sweeps, the Pt-on-Pd/C catalyst lost 12% of its initial electrochemical surface area while the loss for the commercial E-TEK Pt/C catalyst was as high as 39%. The improved stability could be due to the favored interfacial structures between Pt and Pd as well as the larger particle size of Pt-on-Pd nanodendrites preventing the dissolution or aggregation of Pt active sites during the oxygen reduction reaction. Despite those successful demonstrations, it is noted that the above-mentioned M1-on-M2 bimetallic nanodendrites are all prepared through a two-step seed-mediated growth route, in which preformed M2 nanocrystals serve as seeds to direct the growth of M1 branches. This strategy requires multiple steps and complex procedures. Therefore, it is desirable to develop an effective one-step route for the synthesis of M1-on-M2 bimetallic nanodendrites. Up to now, to the best of our knowledge, there are only a few reports associated with the one-step synthesis of M1-on-M2 bimetallic nanodendrites In all of these cases, a block copolymer-mediated synthesis route was employed and the temporal separation of the formation of 906 J. Mater. Chem. A, 2013, 1, This journal is ª The Royal Society of Chemistry 2013

2 Paper different metals was believed to be the key to the one-step synthesis. Triblock Pluronic copolymers such as Pluronic F127 and Pluronic P123 are employed as structure-directing agents and ascorbic acid as a reducing agent An important advantage of anion-exchange membrane direct ethanol fuel cells (AEM DEFCs) is that the kinetics of the ethanol oxidation reaction (EOR) in alkaline media are much faster than those in acidic media, making it possible to use less expensive Pd catalysts However, on the Pd catalyst, ethanol is selectively oxidized to acetic acid through a 4-electron pathway and this leads to a loss of 66.7% in the faradic efficiency of the fuel cell. Recently, it has been reported that during the EOR, Rh has great potential to achieve C C bond cleavage due to the preferential formation of an oxametallacyclic conformation on Rh surfaces However, as Rh is a far less active catalyst for the EOR, it is usually combined with Pt or Pd as the electrocatalyst. In a recent achievement, Rh-on-Pd bimetallic nanodendrites were successfully synthesized through a two-step seed-mediated growth route using Pd nanocrystals with different shapes as seeds. 30 In this work, for the rst time we report a new, facile and one-step polyol route for the synthesis of Rh-on-Pd bimetallic nanodendrites. In this route, ethylene glycol (EG) is used as a reducing agent while hexadecyltrimethylammonium bromide (CTAB) is used as a structure-directing agent. We show that both the morphology and number of Rh branches can be regulated by varying, respectively, the molar ratio of Pd to Rh precursors and the CTAB content. We investigate the use of carbon supported Rhon-Pd bimetallic nanodendrites as the electrocatalyst for the EOR in alkaline media, and make comparisons with monometallic Pd/C and Rh/C catalysts. 2 Experimental 2.1 Materials Palladium(II) chloride (PdCl 2 ), rhodium(iii) chloride (RhCl 3 ), CTAB and hexadecyltrimethylammonium chloride (CTAC) were all purchased from Aldrich. EG, ethanol (CH 3 CH 2 OH) and potassium hydroxide (KOH) were from Merck KGaA. Vulcan XC- 72 carbon with an average particle size of 30 nm was purchased from E-TEK, while 5 wt% polytetra uoroethylene (PTFE) emulsion was received from Dupont. 2.2 Synthesis of Rh-on-Pd bimetallic nanodendrites In a typical synthesis of Rh-on-Pd bimetallic nanodendrites with a Pd/Rh molar ratio of 1 : 1, 1.0 ml of 56.4 mm PdCl 2, 2.95 ml of 19.1 mm RhCl 3, g of CTAB and 20 ml of EG were mixed with 20 ml of deionized water. The mixture solution was stirred for 1 h at room temperature, and then transferred to a Te onlined stainless-steel autoclave. The sealed vessel was heated at 120 C for 12 h before it was cooled to room temperature. The as-obtained precipitate was then collected by ltration, washed with ethanol and deionized (DI) water, and dried at 70 Cinan oven. For comparison, both monometallic Pd and Rh nanoparticles were synthesized using the same procedure without the other metal precursor in the starting solution. A series of comparative experiments were also carried out: (1) by lowering the CTAB content to g; (2) by replacing CTAB with the same number of moles of CTAC; (3) in the absence of the structure-directing agent. For the synthesis of Rh-on-Pd bimetallic nanodendrites with different Pd/Rh molar ratios, the Pd/Rh molar ratio in the starting solution was varied from 1.0 to 7.0. For simplicity, Rh-on-Pd bimetallic nanodendrites with Pd/Rh molar ratios of 1 : 1, 3 : 1, 5 : 1 and 7 : 1 are, respectively, labelled as Rh-on-Pd (1 : 1), Rh-on-Pd (3 : 1), Rh-on-Pd (5 : 1) and Rh-on-Pd (7 : 1). 2.3 Characterization Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were obtained using a high-resolution JEOL 2010F TEM system that was operated with a LaB6 lament at 200 kv and equipped with an energy-dispersive X-ray spectrometer (EDS). X-ray diffraction (XRD) analysis was carried out with a Philips powder diffraction system (model PW 1830) using acuka source operating at 40 kev at a scan rate of s 1. X-ray photoelectron spectroscopy (XPS) characterization was performed by a Physical Electronics PHI 5600 multi-technique system using Al monochromatic X-rays at a power of 350 W. The survey and regional spectra were obtained using passing energies of and 23.5 ev, respectively. Fourier transform infrared (FT-IR) spectroscopy was conducted with an infrared spectrometer (Bio-Rad, FTS 6000). 2.4 Electrochemical measurements For electrochemical studies, all the samples were loaded on Vulcan XC-72 carbon and a 20 wt% total metal (Pd and Rh) loading was guaranteed. Cyclic voltammetry (CV) tests were conducted using a potentiostat (EG&G Princeton, model 273A) in a conventional three-electrode cell. A glass carbon electrode (GCE) with an area of cm 2 was used as the underlying support of the working electrode, platinum foil as the counter electrode, and an Hg/HgO/KOH (1.0 mol L 1 ) (MMO (mixed metal oxide), V vs. SHE) electrode as the reference electrode. The GCE was modi ed by depositing a catalyst layer onto it and served as the working electrode. The catalyst ink was rst prepared through ultrasonically dispersing 10 mg of 20 wt% Pd/C, Rh/C or Rh-on-Pd/C catalyst in 1.9 ml of ethanol, to which 0.1 ml of 5 wt% PTFE emulsion was added. 12 ml of the ink was then pipetted onto the GCE and dried in air to yield a metal loading of 96 mg cm 2. The CV tests were performed at room temperature and in 1.0 M KOH solution containing 1.0 M ethanol, which was deaerated by bubbling nitrogen (99.9%) for 30 min in advance. The CV tests were performed in the potential range from to V at a scan rate of 50 mv s 1. All the potentials in this paper refer to the MMO electrode. 3 Results and discussion Journal of Materials Chemistry A Fig. 1a shows a representative TEM image of the Rh-on-Pd (1 : 1) bimetallic nanodendrites synthesized by the one-step polyol route in the presence of CTAB. It can be seen that for all the nanodendrites, a darker center is surrounded by a lighter This journal is ª The Royal Society of Chemistry 2013 J. Mater. Chem. A, 2013, 1,

3 Journal of Materials Chemistry A Paper Pd/Rh molar ratio is 1.06, which is also consistent with the nominal ratio of 1.0 for the precursors. The EDS analyses corresponding to the regions included in circles A and B in Fig. 1b (Fig. S3 and S4, ESI ) show that both Pd and Rh can be detected in the central region, while only Rh is found in the peripheral region, demonstrating the formation of Rh-on-Pd bimetallic nanodendrites. For comparison, the TEM images of monometallic Pd and Rh nanoparticles synthesized with the same procedure are shown in Fig. 2a and b, respectively. The insets show the corresponding HRTEM images. As can be seen in Fig. 2a, the monometallic Pd nanoparticles are highly dispersed and have de nite geometric shapes with an average diameter of 20 nm. Fig. 2b shows that interconnected small Rh nanoparticles with an average diameter of 3 nm are obtained. The as-synthesized Rh-on-Pd (1 : 1) bimetallic nanodendrites were characterized by XRD and XPS and compared with the monometallic Pd and Rh nanoparticles. Fig. 3 shows the XRD patterns of the monometallic Pd and Rh nanoparticles and the Rh-on-Pd (1 : 1) bimetallic nanodendrites. There exist ve diffraction peaks for the monometallic Pd nanoparticles that are characteristic of the face-centered cubic (fcc) crystalline Fig. 1 TEM (a) and HRTEM (b) images of Rh-on-Pd (1 : 1) bimetallic nanodendrites. branched shell, and this intense contrast demonstrates that well-de ned Rh-on-Pd bimetallic nanodendrites are obtained. The absence of isolated Pd or Rh nanoparticles indicates a high yield (100%) of Rh-on-Pd bimetallic nanodendrites. The diameter of Rh-on-Pd (1 : 1) bimetallic nanodendrites ranges from 15 to 30 nm. EDS analysis over the entire region of Fig. 1a (Fig. S1, ESI ) con rms the existence of both Pd and Rh, and the Pd/Rh molar ratio is 1.11, which is in good agreement with the nominal ratio of 1.0 in the starting solution. To examine the speci c structures of Rh-on-Pd bimetallic nanodendrites, the HRTEM image of a single Rh-on-Pd (1 : 1) bimetallic nanodendrite is shown in Fig. 1b. It can be clearly observed that a number of Rh branches are uniformly distributed on the Pd core and have an average diameter of 3 nm. The HRTEM image in Fig. 1b also reveals that the nanodendrites possess good crystallinity with well-de ned fringes. No obvious grain boundaries between the Pd core and the Rh branches are seen, indicating that the Rh branches are grown epitaxially on the surface of the Pd core. This can be attributed to the fact that there is only a small lattice mismatch of 2.3% between Pd and Rh. 30 EDS analysis of the whole single Rh-on-Pd (1 : 1) bimetallic nanodendrite in Fig. 1b (Fig. S2, ESI ) reveals that the Fig. 2 TEM images of the monometallic Pd (a) and Rh (b) nanoparticles. The insets show the corresponding HRTEM images. 908 J. Mater. Chem. A, 2013, 1, This journal is ª The Royal Society of Chemistry 2013

4 Paper Journal of Materials Chemistry A structure and assigned to the (111), (200), (220), (311) and (222) planes. For the monometallic Rh nanoparticles and the Rh-on- Pd (1 : 1) bimetallic nanodendrites, four peaks are clearly observed, but the peak for the (222) plane is not obvious. It can be observed that the Rh-on-Pd (1 : 1) bimetallic nanodendrites show strong diffraction peaks corresponding to the Pd cores with shoulders corresponding to the Rh branches, suggesting that Pd does not alloy with Rh during the simultaneous reduction process. The XPS spectra of the Rh-on-Pd (1 : 1) bimetallic nanodendrites in the Pd 3d and Rh 3d regions are shown in Fig. 4 and compared with those of the monometallic Pd and Rh nanoparticles. As seen from Fig. 4, for the Rh-on-Pd (1 : 1) bimetallic nanodendrites, the Pd 3d spectrum shows a doublet that consists of a high energy band (Pd 3d 3/2 ) at ev and a low energy band (Pd 3d 5/2 ) at ev, and the Rh 3d spectrum shows a doublet that consists of a high energy band (Rh 3d 3/2 )at ev and a low energy band (Rh 3d 5/2 ) at ev, indicating the existence of metallic Pd and Rh. The Pd 3d spectrum of the monometallic Pd nanoparticles shows a doublet that consists of a high energy band (Pd 3d 3/2 ) at ev and a low energy band (Pd 3d 5/2 ) at ev, and the Rh 3d spectrum of the monometallic Rh nanoparticles shows a doublet that consists of a high energy band (Rh 3d 3/2 ) at ev and a low energy band (Rh 3d 5/2 ) at ev. The very small differences in both the Pd and Rh binding energies between the Rh-on-Pd (1 : 1) nanodendrites and the monometallic Pd and Rh nanoparticles suggest that no electronic effect between Pd and Rh exists, which further con rms the non-alloy structure. The facile and high-quality formation of the Rh-on-Pd bimetallic nanodendrites can be ascribed to the formation of Pd cores prior to the growth of Rh branches, which is due to the different reduction kinetics of the Pd and Rh ions. 19 Because the standard reduction potential of [PdCl 4 ] 2 /Pd ( V vs. SHE) is higher than that of [RhCl 6 ] 3 /Rh ( V vs. SHE), Pd(II) species will be preferentially reduced over Rh(III) species during the reduction process. The preformed Pd nanocrystals can be used as in situ seeds, allowing for the subsequent growth of Rh branches. In this work, EG was used as a reducing agent while CTAB was used as a structure-directing agent. Owing to the Fig. 3 XRD patterns of monometallic Pd and Rh nanoparticles and Rh-on-Pd (1 : 1) bimetallic nanodendrites. Fig. 4 Pd 3d and Rh 3d XPS spectra of monometallic Pd and Rh nanoparticles and Rh-on-Pd (1 : 1) bimetallic nanodendrites. interactions between Br ions and Pd, CTAB can adsorb on the surface of the preformed Pd nanocrystals, and thus facilitate the formation of the Rh branches In the absence of the structure-directing agent, only aggregated nanoparticles are synthesized (Fig. S5-a, ESI ). Upon replacing CTAB with the same number of moles of CTAC, well-de ned Rh-on-Pd (1 : 1) bimetallic nanodendrites are also obtained (Fig. S5-b, ESI ). As shown in Fig. 2a and 5a, highly dispersed Pd nanoparticles with de nite geometric shapes are obtained in the presence of CTAB or CTAC. In contrast, as shown in Fig. 5b, only large Pd aggregates are obtained in the absence of CTAB and CTAC. Therefore, it is concluded that CTAB and CTAC not only direct the growth of the Rh branches but also enable the formation of uniformlyshaped Pd cores. By simply changing the CTAB content in the starting solution, the number of Rh branches can be easily controlled. As can be seen in Fig. 6a, when the CTAB content is decreased from to g, Rh-on-Pd (1 : 1) bimetallic nanodendrites with more Rh branches are synthesized as compared with those shown in Fig. 1a. The HRTEM image of a single Rh-on-Pd (1 : 1) bimetallic nanodendrite is shown in Fig. 6b. It is also observed that the Rhon-Pd (1 : 1) bimetallic nanodendrites in Fig. 6 possess larger Pd cores than those in Fig. 1, which is attributed to the fact that during the formation of the Pd cores, CTAB serves as a stabilizing agent, and during the reduction process, the use of less CTAB will lead to larger but fewer Pd cores. Fig. 7 shows TEM and HRTEM images of Rh-on-Pd bimetallic nanodendrites with different Pd/ Rh molar ratios. As shown in Fig. 7a, when the Pd/Rh molar ratio is increased from 1 : 1 to 3 : 1, there are only a few Rh branches on each Rh-on-Pd (3 : 1) bimetallic nanodendrite. The HRTEM image of a single Rh-on-Pd (3 : 1) bimetallic nanodendrite in the inset of Fig. 7a reveals that when the Pd/Rh molar ratio is 3 : 1, only a few Rh protrusions are formed on the Pd surface. Upon further increasing the Pd/Rh molar ratio to 5 : 1 and 7 : 1, the Rh branches become less visible in the TEM and HRTEM images, as shown in Fig. 7b and c. As can be judged from the FT-IR spectrum (Fig. S6, ESI ), a er being washed/ ltrated with ethanol and water several times, all the samples had clean surfaces and could be used as This journal is ª The Royal Society of Chemistry 2013 J. Mater. Chem. A, 2013, 1,

5 Journal of Materials Chemistry A Paper Fig. 5 TEM images of monometallic Pd nanoparticles in the presence of CTAC (a) and with no structure-directing agent (b). The inset shows the corresponding HRTEM image. Fig. 6 TEM (a) and HRTEM (b) images of Rh-on-Pd (1 : 1) bimetallic nanodendrites synthesized using a lower CTAB content. electrocatalysts. Fig. 8a compares stabilized CV curves for the EOR on the monometallic Pd/C, Rh/C and bimetallic Rh-on- Pd/C catalysts in 1.0 M KOH solution containing 1.0 M ethanol. For clear observation, scans in the positive direction are shown in Fig. 8b, and magni ed curves between and 0.3 V are shown in Fig. 8c. In Fig. 8b, it can be seen that the Pd/C catalyst is more active than Rh/C for the EOR in alkaline media. The peak current density of the EOR on Pd/C is ma mg 1 while it is only 20.6 ma mg 1 on Rh/C. However, as shown in Fig. 8c, the Rh/C catalyst has better ethanol oxidation kinetics than the Pd/C at lower potentials. The onset potential of the EOR on the Rh/C catalyst is 0.70 V, and this is 150 mv more negative than that on the Pd/C. For the EOR, the dissociative adsorption of ethanol to adsorbed CO ads and CH x species usually occurs at lower potentials, and this leads to the complete oxidation of ethanol to CO 2 when in the presence of the oxygen-containing species According to Fig. 8c, all the Rh-on-Pd/C catalysts have better ethanol oxidation kinetics at the potentials lower than 0.35 V as compared with the Pd/C. The onset potential of the EOR is 0.66 V on Rh-on-Pd (1 : 1)/C, 0.63 V on Rh-on-Pd (3 : 1)/C, 0.61 V on Rh-on-Pd (5 : 1)/C and 0.60 V on Rh-on-Pd (7 : 1)/C. Usually, the peak in the backward scan represents the removal of incompletely oxidized species formed in the forward scan, and a high ratio of the forward peak current density to the backward one (j f /j b ) can be an indication of excellent oxidation of ethanol to CO 2 and less accumulation of carbonaceous residues on the catalyst. 28,37,38 As shown in Fig. 8a, the j f /j b ratios on the Rh-on-Pd/C catalysts are much larger than those on the Pd/C. The j f /j b ratio on the Pd/C catalyst is 0.7, while it is 2.7 on Rh-on-Pd (1 : 1)/C, 2.2 on Rh-on-Pd (3 : 1)/C, 1.4 on Rh-on-Pd (5 : 1)/C and 1.2 on Rh-on-Pd (7 : 1)/C. For clear comparison, the onset potentials, peak current densities and j f /j b ratios for the Pd/C and the Rh-on-Pd/C catalysts with different Pd/Rh ratios are summarized in Table S1, ESI. Among the Rh-on-Pd/C catalysts, the Rh-on-Pd (3 : 1)/C catalyst is regarded as the best in view of its simultaneous higher peak current density, more negative onset potential and higher j f /j b ratio. The durabilities of the Pd/C, Rh/C and Rh-on-Pd (1 : 1)/C catalysts for the EOR in alkaline media were evaluated and compared through applying successive CV sweeps (Fig. S7, ESI ). A er 200 cycles, there was no decrease in the peak current density of the EOR on the Pd/C while the decrease in the peak current density of the EOR on the 910 J. Mater. Chem. A, 2013, 1, This journal is ª The Royal Society of Chemistry 2013

6 Paper Journal of Materials Chemistry A Fig. 8 CV curves (a), scans in the positive direction (b) and magnified curves between and 0.3 V (c) of the EOR on monometallic Pd/C, Rh/C and bimetallic Rh-on-Pd/C catalysts. (1.0 M KOH M ethanol; scan rate: 50 mv s 1 ). 4 Conclusions Fig. 7 TEM images of Rh-on-Pd bimetallic nanodendrites with different Pd/Rh molar ratios: 3 : 1 (a); 5 : 1 (b) and 7 : 1 (c). The insets show the corresponding HRTEM images. Rh/C was as high as 18%; there is a small decrease of 3.4% in the peak current density of the EOR on the Rh-on-Pd (1 : 1)/C, and this improved durability can be attributed to both favored interfacial structures between Rh and Pd and the larger particle size of the Rh-on-Pd bimetallic nanodendrites. 13 In this work, an effective one-step polyol route has been proposed for the synthesis of Rh-on-Pd bimetallic nanodendrites that are composed of Pd cores with Rh branches. EG is used as a reducing agent while CTAB is used as a structuredirecting agent. The number of Rh branches can be readily tuned by varying the CTAB content, while the morphology can be regulated by varying the molar ratio of Pd to Rh precursors. The underlying mechanism for the one-step polyol synthesis involves the different reduction kinetics between Pd and Rh ions resulting in the formation of Pd cores prior to the growth of Rh branches. The CV results demonstrate that for the EOR in alkaline media, the Rh-on-Pd/C catalysts result in better kinetics at lower potentials and much higher j f /j b ratios than the Pd/C catalyst, indicating that the Rh-on-Pd bimetallic nanodendrites This journal is ª The Royal Society of Chemistry 2013 J. Mater. Chem. A, 2013, 1,

7 Journal of Materials Chemistry A have much higher CO 2 selectivities during the EOR in alkaline media. Most attractively, it is found that CTAB not only directs the growth of the Rh branches but also enables the formation of uniformly-shaped Pd cores. Since both CTAB and CTAC are proven to be good capping agents for the controlled synthesis of noble metals in different shapes, we believe that the proposed polyol route offers a powerful means for the one-step synthesis of Rh-on-Pd bimetallic nanodendrites with shapecontrollable Pd cores. An effective one-step polyol route for the synthesis of Rh-on-Pd bimetallic nanodendrites with shapecontrollable Pd cores will be pursued in future work. Acknowledgements The work described in this paper was fully supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China (Project no. HKUST9/CRF/11G). Notes and references 1 S. E. Habas, H. Lee, V. Radmilovic, G. A. Somorjai and P. Yang, Nat. Mater., 2007, 6, H. Lee, S. E. Habas, G. A. Somorjai and P. Yang, J. Am. Chem. Soc., 2008, 130, C. Koenigsmann, A. C. Santulli, K. Gong, M. B. Vukmirovic, W. Zhou, E. Sutter, S. S. Wong and R. R. Adzic, J. Am. Chem. Soc., 2011, 133, J. Wu, A. Gross and H. Yang, Nano Lett., 2011, 11, H. Zhang, M. Jin, H. Liu, J. Wang, M. J. Kim, D. Yang, Z. Xie, J. Liu and Y. Xia, ACS Nano, 2011, 5, H. Zhang, M. Jin, J. Wang, M. J. Kim, D. Yang and Y. Xia, J. Am. Chem. Soc., 2011, 133, M. Jiang, B. Lim, J. Tao, P. H. C. Camargo, C. Ma, Y. Zhu and Y. Xia, Nanoscale, 2010, 2, B. Lim and Y. Xia, Angew. Chem., Int. Ed., 2011, 50, S. Wang, N. Kristian, S. Jiang and X. Wang, Nanotechnology, 2009, 20, Z. Peng and H. Yang, Nano Res., 2009, 2, B. Lim, M. Jiang, T. Yu, P. H. C. Camargo and Y. Xia, Nano Res., 2010, 3, B. Lim, M. Jiang, P. H. C. Camargo, E. Cho, J. Tao, X. Lu, Y. Zhu and Y. Xia, Science, 2009, 324, Z. Peng and H. Yang, J. Am. Chem. Soc., 2009, 131, S. Guo, S. Dong and E. Wang, ACS Nano, 2010, 4, S. Guo, S. Dong and E. Wang, Chem. Commun., 2010, 46, Z. Peng, J. Wu and H. Yang, Chem. Mater., 2010, 22, H. Esfahani, L. Wang and Y. Yamauchi, Chem. Commun., 2010, 46, Paper 18 H. Esfahani, L. Wang, Y. Nemoto and Y. Yamauchi, Chem. Mater., 2010, 22, L. Wang, Y. Nemoto and Y. Yamauchi, J. Am. Chem. Soc., 2011, 133, S. Guo, J. Li, S. Dong and E. Wang, J. Phys. Chem. C, 2010, 114, C. Bianchini and P. Shen, Chem. Rev., 2009, 109, Y. Li, T. Zhao and R. Chen, J. Power Sources, 2011, 196, Y. Li, T. Zhao, J. Xu, S. Shen and W. Yang, J. Power Sources, 2011, 196, S. Nguyen, H. Law, H. Nguyen, N. Kristian, S. Wang, S. Chan and X. Wang, Appl. Catal., B, 2009, 91, F. Ksar, G. Surendran, L. Ramos, B. Keita, L. Nadjo, E. Prouzet, P. Beaunier, A. Hagège, F. Audonnet and H. Remita, Chem. Mater., 2009, 21, R. Liu, J. Liu and G. Jiang, Chem. Commun., 2010, 46, A. Kowal, M. Li, M. Shao, K. Sasaki, M. B. Vukmirovic, J. Zhang, N. S. Marinkovic, P. Liu, A. I. Frenkel and R. R. Adzic, Nat. Mater., 2009, 8, S. Shen, T. Zhao and J. Xu, Int. J. Hydrogen Energy, 2010, 35, Y. Suo and I. Hsing, J. Power Sources, 2011, 196, H. Kobayashi, B. Lim, J. Wang, P. H. C. Camargo, T. Yu, M. J. Kim and Y. Xia, Chem. Phys. Lett., 2010, 494, L. Feng, X. Wu, L. Ren, Y. Xiang, W. He, K. Zhang, W. Zhou and S. Xie, Chem. Eur. J., 2008, 14, L. Zhang, J. Zhang, Z. Jiang, S. Xie, M. Jin, X. Han, Q. Kuang, Z. Xie and L. Zheng, J. Mater. Chem., 2011, 21, M. H. Ullah, W. Chung, I. Kim and C. Ha, Small, 2006, 2, Q. Yuan, Z. Zhou, J. Zhuang and X. Wang, Chem. Mater., 2010, 22, S. C. S. Lai, S. E. F. Kleyn, V. Rosca and M. T. M. Koper, J. Phys. Chem. C, 2008, 112, S. Shen, T. Zhao and Q. Wu, Int. J. Hydrogen Energy, 2012, 37, Y. Lee, S. Han and K. Park, Electrochem. Commun., 2009, 11, Z. Liu, X. Ling, X. Su and J. Lee, J. Phys. Chem. B, 2004, 108, J. Zhang, M. R. Langille, M. L. Personick, K. Zhang, S. Li and C. A. Mirkin, J. Am. Chem. Soc., 2010, 132, Y. Xiong, H. Cai, B. J. Wiley, J. Wang, M. J. Kim and Y. Xia, J. Am. Chem. Soc., 2007, 129, Y. Sun, L. Zhang, H. Zhou, Y. Zhu, E. Sutter, Y. Ji, M. H. Rafailovich and J. C. Sokolov, Chem. Mater., 2007, 19, J. Mater. Chem. A, 2013, 1, This journal is ª The Royal Society of Chemistry 2013