Morphologic evolution and optical properties of nanostructured gold based on mesoporous silica

In this paper, we report the morphologic evolution and optical properties of nanostructured gold dispersed in monolithic mesoporous silica induced by soaking the silica into a HAuCl4 aqueous solution and subsequent treatments. It has been shown that the morphology of nanostructured Au depends on the subsequent treatments after soaking. If the HAuCl4-soaked mesoporous silica was dried at <100 deg. C for enough time (>10 h) and annealed at <300 deg. C without any special reduction treatment, Au nanowires/silica assembly can be formed. Corresponding optical-absorption spectra exhibit a broad absorption band around 1000 nm. Subsequent step annealing from 300 deg. C to 800 deg. C results in a blueshift of the absorption band down to the visible region, accompanied by a decrease of the bandwidth. The corresponding morphology of the nanostructured Au evolves from the wire, rodlike to a spherical shape. This means that we can control the optical properties of this assembly in a large region by such a simple way. Further experiments reveal that the pore walls of silica have significant reduction effect on AuCl4- ions at a low temperature (<100 deg. C). The interconnected channels in the silica host and drying at <100 deg. C for enough time after soaking are crucial to form such Au nanowire/silica assembly and hence to show tunable optical properties by subsequent step annealing. Not a single one of these conditions can be dispensed with. Otherwise, direct annealing the soaked monolithic silica at a high temperature (>300 deg. C) or treating the soaked porous silica powders only leads to nearly spherical Au nanoparticles highly dispersed in silica, accompanying a normal surface plasmon resonance of Au around 540 nm. It has been confirmed that the surface-mediated reducing groups (≡Si-OH) on the silica pore wall are responsible for the low-temperature reduction of Au3+ ions. The formation of the Au nanowires is attributed to the low nucleation rate, unidirectional diffusion of Au atoms along the pore channels and size confinement of pore channels