Raman spectroscopy on silica aerogels

Abstract

Fluorescence-free silica aerogels obtained via CO2 hypercritical drying and baking at 600°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 the region of the network bands the defect line D1 at 490 cm-1 increases with water content, while the defect line D2 at 607 cm-1 decreases. It could be shown, that the ratio of intensities D1/D2 is related to the amount of chemisorbed water in the Si-O network. A similar change is observed for OH-stretching bands. The band 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.