Synthesis of Au@Ag@Cu trimetallic nanocrystals using three-step reduction

Abstract

Au@Ag@Cu trimetallic nanocrystals were prepared using a three-step reduction method. In the first step, decahedral Au core seeds were prepared by reducing HAuCl₄·4H₂O in diethylene glycol (DEG) under oil-bath heating in the presence of polyvinylpyrrolidone (PVP) as a polymer surfactant. In the second step, Ag shells were overgrown on these Au seeds in N,N-dimethylformamide (DMF) in the presence of PVP under oil-bath heating to prepare decahedral Au@Ag nanocrystals. In the third step, Cu shells were overgrown further on Au@Ag core–shell nanocrystals in ethylene glycol (EG) in the presence of PVP under oil-bath heating. The resultant crystal shapes were characterized using transmission electron microscopic (TEM), TEM-energy dispersed X-ray spectroscopic (EDS), and X-ray diffraction (XRD) measurements. Results show that Cu shells of two kinds are grown over Au@Ag core seeds: a phase-separated major Cu component attached to one or two side edges of decahedral Au@Ag cores, and a minor Cu component that appears as thin Cu shells over decahedral Au@Ag cores. Partial reservation of pentagonal shape and appearance of Moiré patterns in Au@Ag@Cu particles suggest that epitaxial growth occurs on some parts of the Au@Ag cores despite a large lattice mismatch between Ag and Cu (11.5%). The growth mechanism of Au@Ag@Cu nanocrystals was discussed in terms of lattice mismatch, decahedral particle defects, and the favorable shape of metallic shells. Optical properties of Au@Ag@Cu nanocrystals were determined by measuring extinction spectra.