Crystal Structures and Growth Mechanisms of Icosahedral Au@Ag Core−Shell and Au/Ag Twin Nanocrystals Prepared by PVP-Assisted N,N-Dimethylformamide Reduction

Abstract

Icosahedral Au@Ag core−shell nanocrystals have been prepared using a two-step reduction method. In the first step, icosahedral Au core seeds were prepared by reducing HAuCl4·4H2O in tetraethylene glycol (TEG) under microwave (MW) heating in the presence of polyvinylpyrrolidone (PVP) as a polymer surfactant. In the second step, Ag shells were overgrown on icosahedral Au seeds in N,N-dimethylformamide (DMF) in the presence of PVP under oil-bath heating for 3 h or MW heating for 10 min. Crystal structures of products were characterized using TEM, TEM-EDS, and SEM. Under oil-bath heating, Au@Ag icosahedral nanocrystals nearly fully covered by Ag shells were prepared, whereas those partially covered by tetrahedral units of Ag shells were formed under MW heating. On the basis of these findings, it was concluded that icosahedral Au@Ag core−shell particles are formed through stepwise growth of tetrahedral units, as observed in the crystal growth of monometallic icosahedral Ag crystals in DMF. There were minor Au/...