Raman study of structural defects in SiO2 aerogels

The structure of the silica aerogels was studied by Raman spectroscopy. The spectra of the solid network resembles that of bulk silica with additional bands related to organic groups and a large amount of OH groups. The typical bands due to ring breathing also called defect bands D 1 and D 2 located at 490 and 610 cm−1 are present. However, the evolution of the D 2 band compared to that of OH band (980 cm−1) seems apparently, in contradiction with the results previously reported in the literature. During heat treatments between 25 and 300°C the D 2 and the OH bands increase simultaneously. Generally, in silica glass the defect band D 2 grows at the expense of the OH groups. This result is explained by the oxidation of the organic compounds which, in this temperature range, leads to the formation of the both species (OH) and those related to siloxane rings. 29Si MAS NMR results are in agreement with the Raman study.