Effects of gas bubbling for shape, size, and composition changes in Au–Ag bimetallic nanoparticles including polygonal Au seeds under oil-bath heating at 150 °C

Abstract

Effects of Ar or O₂ gas bubbling for shape, size, and composition changes in Au–Ag bimetallic nanoparticles were studied under oil-bath heating at 150 °C for 10–60 min in ethylene glycol (EG). When a mixture of polygonal Au seeds, AgNO₃, and polyvinylpyrrolidone (PVP) in EG solution was heated from room temperature to 150 °C and it was kept at that temperature for 10–60 min under Ar or O₂ gas bubbling, different shape, size, and composition changes were observed. Under Ar gas bubbling, major products after heating for 10 min were polygonal Au core Ag-rich Ag/Au alloy shell particles, denoted as Au@Ag/Au, and spherical Ag-rich Ag/Au alloy particles. They were transformed to larger excentered Au@Ag/Au particles with an average diameter of 340 ± 106 nm after further heating for 20–50 min. Under O₂ gas bubbling, major products after heating for 10 min were Au@Ag core–shell particles having a thin Ag shell, large Au-rich Au/Ag alloy particles, and many small spherical Ag particles with 4 ± 2 nm average diameter. After further heating for 20–50 min, the Au@Ag particles fuse and aggregate to form spherical Au-rich Au/Ag alloy particles with an average diameter of 259 ± 37 nm. Results show that Ag-rich or Au-rich Au–Ag bimetallic particles can be prepared easily under bubbling Ar or O₂ gas, respectively. The different structure and composition changes in Au–Ag bimetallic particles under Ar or O₂ bubbling are discussed in terms of fusion, aggregation, and oxidative etching of Au and Ag particles.