Published: March 25, 2011 r2011 American Chemical Society 6078 dx.doi.org/10.1021/ja201156s | J. Am. Chem. Soc. 2011, 133, 6078–6089 ARTICLE pubs.acs.org/JACS Synthesis of PdÀPt Bimetallic Nanocrystals with a Concave Structure through a Bromide-Induced Galvanic Replacement Reaction Hui Zhang, ||,†,‡ Mingshang Jin, ||,†,§ Jinguo Wang, ^ Weiyang Li, † Pedro H. C. Camargo, † Moon J. Kim, ^ Deren Yang, ‡ Zhaoxiong Xie, § and Younan Xia* ,† † Department of Biomedical Engineering, Washington University, St. Louis, Missouri 63130, United States ‡ State Key Laboratory of Silicon Materials and Department of Materials Science and Engineering, Zhejiang University, Hangzhou, Zhejiang 310027, People’s Republic of China § State Key Laboratory for Physical Chemistry of Solid Surfaces and Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, Fujian 361005, People’s Republic of China ^ Department of Materials Science, University of Texas at Dallas, Richardson, Texas 75083, United States b S Supporting Information ’ INTRODUCTION Noble-metal nanocrystals with a concave structure on the surface have physical/chemical properties different from those enclosed by a flat or convex surface. 1À5 Owing to their unusual facets and negative curvature, nanocrystals with a concave structure have also received attention for a number of applica- tions related to catalysis, electrocatalysis, optical sensing, and localized surface plasmon resonance. 6À8 In general, overgrowth and etching represent two powerful routes to the synthesis of nanocrystals with a concave structure. For overgrowth, the formation of a concave structure is typically achieved under kinetic control to overcome the requirement of minimizing the total surface free energy. In comparison, the formation of a concave structure by etching has to rely on an anisotropic process, in which a specific set of facets of the template are preferentially dissolved over others. Compared to the remarkable success in the synthesis of nanocrystals enclosed by a flat or convex surface (with examples including cube, octahedron, tetrahedron, decahedron, icosahedron, plate, bar, and rod), 9À11 there are only a few reports on the synthesis of metal nanocrystals with a concave structure. To this end, we have demonstrated the synthesis Pt tetrahedra and octahedra with concave facets using an overgrowth approach. 12 We have demonstrated the syntheses of Au nanocages and nanoframes with cavities on the surfaces using a galvanic replacement reaction between Ag nanocrystals and a Au(I) or Au(III) salt precursor. 13 We have also demon- strated the synthesis of Pd nanocages using an oxidative etching process. 14 Zheng and co-workers reported the synthesis of tetrahedral and trigonal bipyramidal nanoframes of Pd using a solvothermal process in the presence of formaldehyde. 15 Berkovitch and co-workers demonstrated a technique based on electron beam lithography (EBL) for fabricating arrays of con- cave hyperbolic Au particles on glass substrates with controllable concavity via an etching strategy. 16 Mirkin and co-workers demonstrated the synthesis of Au concave nanocubes via seed- mediated overgrowth with cetyltrimethylammonium chloride (CTAC) as a capping agent. 17 Most recently, we reported a facile process for the formation of Rh, Pt, and Pt/Rh concave nanocubes by manipulating the reaction kinetics with a syringe pump to alter the injection rate of a salt precursor. 18,19 It still remains a grand challenge to extend these strategies to other Received: February 6, 2011 ABSTRACT: This article describes a systematic study of the galvanic replacement reaction between PtCl 6 2À ions and Pd nanocrystals with different shapes, including cubes, cuboctahe- drons, and octahedrons. It was found that Br À ions played an important role in initiating, facilitating, and directing the re- placement reaction. The presence of Br À ions led to the selective initiation of galvanic replacement from the {100} facets of Pd nanocrystals, likely due to the preferential adsorption of Br À ions on this crystallographic plane. The site-selective galvanic re- placement resulted in the formation of PdÀPt bimetallic nano- crystals with a concave structure owing to simultaneous dissolution of Pd atoms from the {100} facets and deposition of the resultant Pt atoms on the {111} facets. The PdÀPt concave nanocubes with different weight percentages of Pt at 3.4, 10.4, 19.9, and 34.4 were also evaluated as electrocatalysts for the oxygen reduction reaction (ORR). Significantly, the sample with a 3.4 wt.% of Pt exhibited the largest specific electrochemical surface area and was found to be four times as active as the commercial Pt/C catalyst for the ORR in terms of equivalent Pt mass.