Shape-Dependent Evolution of Au@Ag Core-Shell Nanocrystals by PVP-Assisted N,N-Dimethylformamide Reduction Masaharu Tsuji,* ,†,‡ Ryoichi Matsuo, ‡ Peng Jiang, †,§ Nobuhiro Miyamae, ‡ Daisuke Ueyama, ‡ Michiko Nishio, ‡ Sachie Hikino, † Hisayo Kumagae, † Khairul Sozana Nor Kamarudin, †,| and Xin-Ling Tang ‡ Institute for Materials Chemistry and Engineering, Kyushu UniVersity, Kasuga 816-8580, Japan, Graduate School of Engineering Sciences, Kyushu UniVersity, Kasuga 816-8580, Japan, National Center for Nanoscience and Technology, Beijing 100080, People’s Republic of China, and Department of Gas Engineering, Faculty of Chemical and Natural Resources Engineering, UniVersiti Teknologi Malaysia, 81310 UTM Skudai, Johor, Malaysia ReceiVed February 12, 2008; ReVised Manuscript ReceiVed March 17, 2008 ABSTRACT: Shape-dependent Au@Ag core-shell nanocrystals have been successfully synthesized by using a two-step method. First, Au nanocrystal seeds with various shapes including single-crystal octahedron, single-twinned triangle or hexagon plate, and multiple-twinned decahedron were prepared by reducing HAuCl 4 in ethylene glycol (EG) in the presence of polyvinylpyrrolidone (PVP) as a polymer surfactant under the condition of microwave heating. Subsequently, thus-obtained Au seeds were added into N,N-dimethylformamide (DMF) solution containing Ag + ions for overgrowth of Ag shells by an oil-bath heating. Transmission electron microscope (TEM) observation demonstrates that shapes of formed Ag shells strongly depend on shapes of initiated Au seeds. These newly produced triangular or hexagonal platelike, octahedral, and multiple-twinned decahedral Au@Ag nanocrystals are mainly dominated by the Ag shells having {111} facets. This case is completely different from that previously investigated by us in EG system where formed triangular-bipyramidal, cubic, and pentagonal rod/wire Au@Ag core-shell nanostructures are surrounded by the Ag shells with {100} facets. Our studies reveal that it is possible to controllably prepare the Au@Ag core/shell nanocrystal structures with optional uniform crystalline planes by the same Au seed source and different reaction solvent. Introduction In recent years, bimetallic nanoparticles have received intensive attention owing to their novel optical, electronic, magnetic, and catalytic properties different from individual metals. Since these properties strongly depend on composition, shape, and size of the nanoparticles, extensive studies have been focused on the controlled synthesis of the composition and morphology of bimetallic core-shell and alloy nanoparticles. 1–3 Noble metal gold (Au) and silver (Ag) have the same face- centered cubic (fcc) crystal structure. Their lattice constants (Au (0.408 nm), Ag (0.409 nm)) are very similar. Some studies have shown that Au and Ag can form various alloy and Au@Ag core-shell nanocrystal structures. 4–19 For the Au@Ag core-shell nanostructures, a direct approach to determining the structure is to see the core-shell structure by transmission electron microscopy (TEM), because a clear boundary between Au and Ag elements can be distinguished by bright and dark contrast in TEM imaging. 6,9 Therefore, it is possible to deduce the evolution process of the final obtained Au@Ag core-shell nanocrystal structures by analyzing their TEM images. Recently, we have succeeded in preparing Au@Ag core-shell nanocrystals by using a microwave (MW)-polyol method. 20,21 When HAuCl 4 · 4H 2 O was reduced in ethylene glycol (EG) in the presence of polyvinylpyrrolidone (PVP) as a polymer surfactant, a mixture of single-crystal octahedral, single-twinned triangular platelike, and multiple-twinned decahedral Au nanoc- rystals was obtained. Although these Au nanoparticles possess different shapes, a common feature is that they are mainly surrounded by {111}-type facets. Once Ag + ions were reduced by using these Au nanocrystals as seeds in EG solution, cubic, triangular-bipyramidal, and rod/wire Ag shells with {100}-type dominant facets were epitaxially formed around these seeds. We have concluded that morphology change between Au cores and Ag shells originates probably from the change in the adsorption selectivity of PVP molecules from Au {111}-type facets to Ag {100}-type ones in EG solvent. We have also demonstrated an important role of chloride (Cl - ) anions in the formation process of Au@Ag core-shell nanocrystals in EG. 22 The effects of Cl - ions on the Au@Ag nanocrystals have been investigated by systematically changing Cl - concentration in Au seed solutions using NaCl as a precursor. We have found that few Au@Ag core-shell nanoc- rytals can be formed in the absence of Cl - ions, while superfluous Cl - ion concentration causes the formation of AgCl, which can interfere with evolution of the Au@Ag nanocrystals. A suitable small amount of Cl - anions (0.3-3 mM) is necessary for the preparation of the Au@Ag nanocrystals. The growth mechanism of the Au@Ag nanocrystals can be reasonably explained in terms of shape selective crystal growth and oxidative etching of the Ag shells by Cl - /O 2 (dissolved in solvent). Under our experimental conditions, spherical Ag particles are etched and gradually disappear, while the Ag shells with single crystal cubic, single-twinned bipyramidal, and five- twinned nanorod/-wire structures can survive and further grow into regular Au@Ag core-shell nanocrystal structures with clear facets and sharp edges. For the Au@Ag core-shell nanocrystals, favorable facets of the Ag shells grown epitaxially by Au seeds in EG are {100}-type planes. The Ag nanocrystals surrounded by {100} facets have been widely synthesized by a polyol method. 23–25 Recently, Gao et al. 26 found that decahedral Ag nanocrystals having {111} facets could be prepared in high yields when * To whom correspondence should be addressed. E-mail: tsuji@ cm.kyushu-u.ac.jp. † Institute for Materials Chemistry and Engineering, Kyushu University. ‡ Graduate School of Engineering Sciences, Kyushu University. § National Center for Nanoscience and Technology. | Universiti Teknologi Malaysia. CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 7 2528–2536 10.1021/cg800162t CCC: $40.75  2008 American Chemical Society Published on Web 06/06/2008