Shape changes in Au–Ag bimetallic systems involving polygonal Au nanocrystals to spherical Au/Ag alloy and excentered Au core Ag/Au alloy shell particles under oil-bath heating

Abstract

Shape changes in Au–Ag bimetallic nanoparticles involving polygonal Au particles were examined during oil-bath heating in ethylene glycol (EG). Polygonal Au and spherical Ag particles were annealed at 150 °C for 10–60 min with polyvinylpyrrolidone (PVP), yielding spherical Au-rich Au/Ag alloy particles, denoted as Au/Ag, with an average diameter of 207 ± 35 nm, and string-like Au/Ag alloy aggregates. The yield of spherical Au/Ag alloy particles increased with annealing time. Formation of Au/Ag alloy without the presence of etchants indicated that shape and size changes occur through fusion of Au and Ag particles during annealing. This method can be used as a simple technique for preparation of monodispersed Au/Ag alloy particles using mixtures of Au and Ag particles. Addition of polygonal Au seeds to AgNO ₃/EG/PVP solution preheated at 150 °C caused remarkable shape changes in products. After heating for 30 min, Au and Au@Ag core–shell particles attached to Ag-rich Ag/Au alloy nanowires were produced. They were transformed to excentered Au core Ag-rich Ag/Au alloy shell nanoparticles, denoted as Au@Ag/Au, with an average diameter of 370 ± 73 nm after further heating for 30 min. Results show that a shape change from Ag/Au alloy nanowires to spherical Ag/Au alloy shells occurs from attached parts of Au and Au@Ag particles to nanowires. The effects of HAuCl ₄ concentration used for the preparation of Au seeds and solvent temperature on the shape, sizes or composition distributions were examined. This report is the first describing new simple technique for preparation of excentered Au@Ag/Au particles during oil-bath heating.