Epitaxial Growth of Au@Cu Core−Shell Nanocrystals Prepared Using the PVP-Assisted Polyol Reduction Method

Abstract

Au@Cu core-shell nanocrystals were prepared using a two-step polyol reduction method. First, mixtures of ochedral, triangular and hexagonal platelike, decahedral, and icosahedral Au core seeds were prepared by reducing HAuCl 4 ·4H 2 O in ethylene glycol (EG) using microwave (MW) heating in the presence of polyvinylpyrrolidone (PVP) as a polymer surfactant. Then Cu shells were overgrown on Au core seeds by reducing Cu 2 (OAc) 4 in EG with PVP using oil-bath heating. Resultant crystal structures were characterized using TEM, high-resolution (HR)-TEM, TEM-EDS, and selected area electron diffraction (SAED) measurements. A large mismatch exists in lattice constants between Au (0.4079 nm) and Cu (0.3615 nm). No monometallic Cu nanocrystals having well-defined facets were prepared by reducing Cu 2 (OAc) 4 in EG. Therefore, the epitaxial growth ofCu shells over Au cores was expected to be difficult. Nevertheless, flat {111} facets of Cu shells were grown epitaxially on {111} facets of Au cores. The SAED patterns and Moire patterns showed Cu layers parallel to Au layers. The Cu shell growth on sharp Au-core corners was slower than that on flat {111} facets and single twin facets. This report is the first describing epitaxial growth of core—shell nanocrystals despite a large lattice mismatch (11.4%). The Au@Cu nanoparticles were more antioxidative than pure Cu particles prepared under identical conditions.