Shape-Dependent Evolution of Au@Ag Core−Shell Nanocrystals by PVP-Assisted N,N-Dimethylformamide Reduction

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.