Raman spectroscopy on silica aerogels

Fluorescence-free silica aerogels obtained via CO2 hypercritical drying and baking at 600 degrees C were investigated by Raman spectroscopy. Raman spectra of silica aerogels during water adsorption and desorption processes showed characteristic changes of the intensities of the Raman bands. In thr region of the network, bands the defect line D-1 at 190 cm(-1) increases with wafer content. while the defect line D-1 at 607 cm(-1) decreases. It could be shown. that the ratio of intensities D-1/D-2 is related to the amount of chemisorbed water in the Si-O network. A similar change is observed for OH-stretching bands. The hand at 3750 cm(-1). assigned to uncoupled OH-groups. diminishes, whereas the broad band around 3470 cm(-1). related to coupled OH-groups, increases. The ratio of the Raman intensities of the OH-stretching modes is proportional to the amount of physisorbed water. Diffusion of water into the aerogel monolith is significantly hindered by the adsorption process.