OPTICAL-PROPERTIES OF CO2-DOPED SILICA-GEL MONOLITHS()

Silica gel monoliths containing Co2+ with a Co/Si molar ratio up to 1/ 20 were prepared by acid-catalyzed hydrolysis of tetraethoxysilane in the presence of cobalt nitrate and formamide. UV-visible spectra recor ded at different steps of the preparation showed important changes in the Co2+ environment. A change from octahedral to tetrahedral environm ent was observed during aging. It was attributed to the formation of b onds between cobalt ions and the gel skeleton induced by an increase o f pH. In xerogels, Co2+ was in tetrahedral coordination and the calcin ation temperature in the 300 to 700 degrees C range had no influence o n the spectra. A partial reverse evolution from tetrahedral to octahed ral was observed when calcinated gels were exposed to moisture.