Crystal Structures and Growth Mechanisms of Au@Ag Core−Shell Nanoparticles Prepared by the Microwave−Polyol Method

Au@Ag core−shell nanocrystals have been synthesized by using a microwave−polyol method, and their growth mechanisms have been studied. When HAuCl4·4H2O 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.