Microstructure and electronic behavior of PtPd@Pt core-shell nanowires
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Michael Murphy, Matthew Ward, and Tsun-Kong Sham Department of Chemistry, University of Western Ontario, London, Ontario, N6A 5B7 Canada
Lijun Wu and Yimei Zhu Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, New York 11973
Yongfeng Hu Canadian Light Source, University of Saskatchewan, Saskatoon, Saskatchewan, S7N 0X4 Canada
Toshihiro Aoki JEOL USA, Inc., Peabody, Massachusetts 01961 (Received 4 November 2009; accepted 4 January 2010)
PtPd@Pt core-shell ultrathin nanowires were prepared using a one-step phase-transfer approach. The diameters of the nanowires range from 2 to 3 nm, and their lengths are up to hundreds of nanometers. Line scanning electron energy loss spectra showed that PtPd bimetallic nanowires have a core-shell structure, with a PtPd alloy core and a Pt monolayer shell. X-ray absorption near edge structure (XANES) spectra reveal that a strong Pt-Pd interaction exists in this nanowire system in that there is PtPd alloying and/or interfacial interaction. Extended x-ray absorption fine structures (EXAFS) further confirms the PtPd@Pt core-shell structure. The bimetallic nanowires were determined to be face-centered cubic structures. The long-chain organic molecules of n-dodecyl trimethylammonium bromide and octadecylamine, used as surfactants during synthesis, were clearly observed using aberration-corrected TEM operated at 80 KV. The interaction of Pt and surfactants was also revealed by EXAFS. I. INTRODUCTION
Pt and Pd are two platinum group elements whose importance is due to their unique physical and chemical properties and excellent catalytic applications.1 Most previous research focused on Pt and Pd nanoparticles and more recently on one-dimensional nanostructures.2–12 Heterogeneous catalysts that contain PtPd bimetallic nanoparticles have attracted much attention because such novel nanostructures exhibit different, highly valuable physical and chemical properties and better catalytic performance compared with their monometallic counterparts.13–18 For example, Lim et al. synthesized Pd-Pt bimetallic nanodendrites consisting of dense Pt branches on a Pd core by reducing K2PtCl4 (branches) with Lascorbic acid in the presence of uniform Pd nanocrystal seeds in an aqueous solution.16 The Pd-Pt nanodendrites were several times more active on the basis of equivalent Pt mass for the oxygen reduction reaction than the statea)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2010.0090 J. Mater. Res., Vol. 25, No. 4, Apr 2010
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of-the-art Pt/C catalyst and the first-generation freestanding Pt-black catalyst.16 These heterogeneous catalysts are typically nanoparticles composed of monometallic Pt and Pd as either the core or the shell. Heterogeneous catalysts with different nanostructures and dimensions are expected to have different properties and catalytic performance. Thus, we explore a novel synthesis route for PtPd@Pt core-shell bimetallic nanowires. The Pt shell
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