Epitaxial growth of Au@Pd core–shell nanocrystals prepared using a PVP-assisted polyolreduction method

Abstract

Au@Pd core–shell nanocrystals were prepared using a two-step polyol reduction method. First, decahedral Au core seeds with small amounts of octahedral, triangular-platelike, and icosahedral Au particles were prepared by reducing HAuCl₄·4H₂O in diethylene glycol (DEG) under oil-bath heating in the presence of polyvinylpyrrolidone (PVP) as a polymer surfactant. Then Pd shells were overgrown epitaxially on Au core seeds by reducing Na₂PdCl₄ in EG with PVP, KBr, and H₂O. The resultant crystal shapes were characterized using transmission electron microscopic (TEM), TEM-energy dispersed X-ray spectroscopic (EDS), and selected area electron diffraction (SAED) measurements. Effects of addition of KBr and H₂O in the second step and reaction temperature for the yield of core–shell particles and their shape, size, and composition distributions were examined. Results show that both {111} and {100} facets of Pd shells were formed in the presence of KBr, depending on the shapes of Au core seeds, whereas only {111} facets were produced in the absence of KBr. New triangular-platelike, five-twin rod, decahedral, and icosahedral shapes of Au@Pd nanocrystals were prepared using triangular-platelike, decahedral, and icosahedral Au cores. When decahedral Au particles were used as seeds, monodispersed decahedral Au@Pd nanocrystals were prepared in high yield at low temperatures of 20–100 °C in PVP/EG solution. Au acts as a catalyst for Pd²⁺ reduction in the presence of PVP at low temperatures. This work demonstrates a simple two-step technique for the epitaxial growth of various shapes of Au@Pd nanocrystals.