Shape and size controlled synthesis of gold nanocrystals using oxidative etching by AuCl4− and Cl− anions in microwave-polyol process

Shape and size controlled synthesis of gold nanocrystals have been studied by using a microwave (MW)-polyol method. When Au3+ in HAuCl4·4H2O was reduced in ethylene glycol (EG) in the presence of poly(vinylpyrrolidone) (PVP) at a low HAuCl4 concentration, mixtures of spherical, triangular, octahedral, and decahedral particles were prepared in a few minutes. It was found that the shape selective oxidative etching by AuCl4− + Cl− anions and crystal growth occurred simultaneously at high HAuCl4 concentrations. The addition of MCl (M = Na, K, H) was very effective to enhance the shape selective oxidative etching by AuCl4− + Cl− anions. With increasing the AuCl4− and Cl− concentrations, yields of large plate-type and ribbon-type single twin particles increased, while those of spherical and octahedral particles decreased in most cases. Possible etching and growth mechanisms are discussed from dramatic color and absorption spectral changes of Au seeds in the HAuCl4·4H2O/PVP/EG solution with increasing the solution temperature and from TEM images before and after oxidative etching. The present results provide a new simple technique for the rapid shape and size controlled synthesis of Au nanocrystals under MW heating.