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

Au@Cu core−shell nanocrystals were prepared using a two-step polyol reduction method. First, mixtures of octahedral, triangular and hexagonal platelike, decahedral, and icosahedral Au core seeds were prepared by reducing HAuCl4·4H2O 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 Cu2 (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 Cu2 (OAc) 4 in EG. Therefore, the epitaxial growth of Cu 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 Moiré 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.