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 AgO_x. 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.