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 800degreesC. The sol-gel starting solutions were prepared by mixing tetra-ethyl-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 Ag-2(+), Ag+, Agdegrees, 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.