Physical characteristics of heat-treated nano-silvers dispersed in sol–gel silica matrix

Silica films containing various concentrations of Ag nanoparticles were deposited on glass slides using a sol–gel process and then heat-treated in air at different temperatures. The films were analysed by using UV–visible spectrophotometry, atomic force microscopy (AFM), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and x-ray photoelectron spectroscopy (XPS) for their optical, surface morphological as well as structural, and chemical properties. After heat-treatment, the optical absorption peaks of Ag nanoparticles show a blueshift and an intensity reduction due to particle size reduction and AgO x nanoparticle formation, respectively. The particle size reduction and surface morphology changes in the films were observed by AFM and TEM as a function of heat-treatment temperature and Ag concentration. Using AFM and XPS analyses, we have found that the Ag nanoparticles accumulated on the surface diffuse into the substrate as the heat-treatment temperature increases. XPS analysis also showed that the silver oxidation occurs during the heat-treatment, causing the reduction of absorption intensity. By controlling the Ag concentration and the heat-treatment temperature, we could change the silica structure, and tailor the average size of the silver nanoparticles down to less than 4 nm.