Controllable optical properties of Au/SiO2 nanocomposite induced by ultrasonic irradiation and thermal annealing

Nanostructured gold/monolithic mesoporous silica assembly was synthesized by a sonochemical method. The optical absorption maximum of the as-prepared nanocomposite varies from the visible to near-infrared region, depending on the reduction rate of AuCl4- ions in solution. There exists a critical reduction rate of AuCl4- ions in solution. When the reduction rate is lower than the critical value, the absorption maximum redshifts from 560 up to 1130 nm with increase of the reduction rate; otherwise, it blueshifts. The subsequent step thermal annealing of the as-prepared nanocomposite with a maximum in the near-infrared results in a blueshift in a large region down to 550 nm. We can thus realize control of the optical absorption in a large region by ultrasonic irradiation and subsequent step thermal annealing. Further experiments reveal that the position of the absorption maximum depends on the aggregate degree of Au nanoparticles supported in the surface layer of silica and that subsequent step annealing induces disaggregation of Au nanoparticles. (C) 2003 American Institute of Physics.