Epitaxial Growth of Au@Ni Core−Shell Nanocrystals Prepared Using a Two-Step Reduction Method

Au@Ni core−shell nanocrystals were prepared using a two-step reduction method. First, mixtures of octahedral, 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, Ni shells were overgrown on Au core seeds by reducing Ni (NO 3) 2 ·6H 2 O in EG with NaOH and PVP using oil bath heating. Resultant crystal structures were characterized using transmission electron microscopic (TEM), high-resolution TEM, TEM−energy dispersed X-ray spectroscopic (EDS), selected area electron diffraction (SAED), and X-ray diffraction (XRD) measurements. Because a very large mismatch (13. 6%) exists in lattice constants between Au (0. 4079 nm) and Ni (0. 3524 nm), the epitaxial growth of Ni shells over Au cores was expected to be difficult. Nevertheless, about 40 monolayers of Ni111 shells were grown epitaxially on flat planes and sharp corners of Au111 cores after heating reagent solution at 175 °C for 2 h. The SAED patterns showed Ni layers parallel to Au layers. This result is contrasted with our recent result for Au@Cu with a smaller lattice mismatch between Au and Cu (11. 4%). In the case of Au@Cu, although the epitaxial growth of Cu111 shells over Au core was observed, the growth rate on sharp corners was slower than that on flat 111 facets. Our results and reported data show that the lattice mismatch is not a significant factor for the crystal growth of Au@Ni on sharp corners.