Shape Selective Oxidative Etching and Growth of Single-Twin Plate-Like and Multiple-Twin Decahedral and Icosahedral Gold Nanocrystals in the Presence of Au Seeds under Microwave Heating

Shape- and size-controlled synthesis of gold nanocrystals was studied by using a microwave–polyol method in the presence of Au seeds and poly(vinylpyrrolidone) (PVP) as a polymer surfactant. Mixtures of spherical, triangular, hexagonal, octahedral, and decahedaral particles were prepared as Au seeds by reducing Au3+ in AuCl4− in ethylene glycol (EG). When Au nanoparticles were prepared from AuCl4−/Cl−/PVP/EG solutions by using these Au seeds, significant changes in shapes and sizes of the Au seeds occurred. Such changes depended strongly on the concentrations of AuCl4− and Cl−. Large single-twin plate-like particles and multiple-twin decahedral particles were the major products at high concentrations of AuCl4−, whereas icosahedral particles were preferentially produced at high Cl− concentrations. It should be noted that the octahedral and spherical particles involved in the Au seeds were completely dissolved at high AuCl4− and Cl− concentrations. The significant changes in shapes and sizes of Au nanoparticles could be explained by the oxidative etching of the Au seeds by AuCl4− + Cl− anions in the range of 60–160 °C and crystal growth of specific shapes of Au nanocrystals in the range of 160–198 °C on the basis of changes in color and UV–vis–NIR spectra of solutions and TEM images of products during etching.