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 recorded at different steps of the preparation showed important changes in the Co2+ environment. A change from octahedral to tetrahedral environment was observed during aging. It was attributed to the formation of bonds between cobalt ions and the gel skeleton induced by an increase of pH. In xerogels, Co2+ was in tetrahedral coordination and the calcination temperature in the 300 to 700 degrees C range had no influence on the spectra. A partial reverse evolution from tetrahedral to octahedral was observed when calcinated gels were exposed to moisture.