Structure and electron spin resonance of annealed sol-gel glasses containing Ag

In this work, SiO₂ samples with silver, prepared using the sol-gel method, were analyzed after being thermally treated in air in the range of 100 to 800°C. The sol-gel starting solutions were prepared by mixing tetraethyl-orthosilicate (TEOS), water and ethanol. Samples with 4 different H₂O/TEOS molar ratios (3.3, 5, 7.5 and 11.7 respectively) and with different nominal Ag concentrations were prepared (1,2 and 4% vol. of Ag). It was found that upon annealing, different silver spices were formed, such as Ag₂⁺, Ag⁺, Ag^o, and metallic silver aggregates. The identification of these spices was carried out by means of X-ray diffraction, Electron Paramagnetic Resonance (EPR), optical emission and optical absorption. It was also found that the specific type of silver spices observed depends on the structure of the SiO₂ matrix and on the annealing temperatures. It was found that samples prepared from precursor solutions with a low H₂O/TEOS ratio have a more open structure, and therefore silver diffuses faster and forms agglomerates at lower temperatures. Samples prepared from solutions with larger H₂O/TEOS ratios have a more dense structure, which allows the formation of atomic or molecular spices in addition to silver particles. A systematic study of this system was carried out using EPR on samples prepared from solutions having different H₂O/TEOS molar ratios, various Ag concentrations and subjected to different thermal treatments.