Reversible transformations of silver oxide and metallic silver nanoparticles inside SiO2 films

Reversible transformation of silver oxide and metallic nanoparticles inside a relatively porous silica film has been established. Annealing of Ag-doped films in oxidizing (air) atmosphere at 450 °C yielded colorless films containing AgOx. These films were turned yellow when heated in H2–N2 (reducing atmosphere) due to the formation of Ag nanoparticles. This yellow coloration (due to nano Ag0) and bleaching (conversion of Ag0 → Ag+) are reversible. Optical and photoluminescence spectra are well consistent with this coloration and bleaching. The soaking test of the air-annealed film in Na2S2O3 solution supports the presence of Ag+. Grazing incidence X-ray diffraction and transmission electron microscopy studies reveal the formation of Ag-oxides and Ag nanoparticles in the oxidized and reduced films, respectively.