Au@Ag core-shell nanocrystals have been synthesized by using a microwave-polyol method, and their growth mechanisms have been studied. When HAuCl 4 ‚4H 2 O was reduced in ethylene glycol in the presence of poly(vinylpyrrolidone) (PVP) as a polymer surfactant, a mixture of triangular twin platelike, octahedral, and multiple-twinned decahedral Au nanocrystals, having only {111} facets, was prepared. When Ag + was reduced by using these Au nanocrystals as seeds, triangular-bipyramidal, cubic, and rod/wire Ag shells, having {100} facets, were overgrown, respectively. It was concluded that morphology changes between Au cores and Ag shells arise from changes in the adsorption selectivity of PVPs from {111} facets of Au to {100} facets of Ag. Total space volumes of Ag shells overgrown on Au cores were on the order of cubes > triangular-bipyramidal crystals > rods and wires. These findings provide general information on the growth mechanism of face-centered cubic (FCC) types of single crystals in the presence of a polymer surfactant, which is difficult to obtain from monometallic crystals.

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 HAuCl4 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.