Crystal structures and growth mechanisms of octahedral and decahedral Au@Ag core-shell nanocrystals prepared by a two-step reduction method

Abstract

Octahedral and decahedral Au@Ag core-shell nanocrystals have been prepared in high yields using a two-step reduction method. In the first step, octahedral or decahedral Au core seeds were prepared by reducing HAuCl₄·4H₂O in tetraethylene glycol (TEG) under microwave (MW) heating or in diethylene glycol (DEG) under oil-bath heating, respectively, in the presence of polyvinylpyrrolidone (PVP) as a polymer surfactant. In the second step, Ag shells were overgrown on these Au seeds in N,N-dimethylformamide (DMF) in the presence of PVP under oil-bath heating for three hours or MW heating for ten minutes. Crystal structures of products were characterized using TEM, TEM–EDS, and SEM. Under oil-bath heating, octahedral or decahedral Au@Ag nanocrystals covered by uniform Ag shells were prepared. On the other hand, decahedral Au@Ag core-shell particles are formed through stepwise growth of tetrahedral units after covering thin Ag shells over decahedral Au cores under fast MW heating. Since the growth rate of Ag shells on corners having defects is slower than that on flat {111} facets, non-uniform Ag shells were formed under fast MW heating. Optical properties of each nanocrystal were determined by measuring extinction spectra.