Raman spectrum and structure of thermally treated silica aerogel

Abstract

Raman spectra have been obtained from untreated SiO2 aerogel at 25 °C, and after vacuum heating between 350 and 620 °C. The untreated aerogel displays a strong Raman peak near 478 cm−1 due to eight-membered (4-SiO) surface rings, but no measurable intensity at 600 cm−1 from six-membered (3-SiO) rings. However, an intense 600 cm−1 peak develops upon heating between 350 and 620 °C as the 478 cm−1 peak is replaced by a 490 cm−1 peak characteristic, in fused silica, of eight-membered internal rings. Vibrations involving surface CH3O groups disappear along with the fluorescence at 350 °C, well before the 478 cm−1 Raman intensity is gone. The untreated aerogel is protected from water absorption by the hydrophobic surface methoxy groups, but the thermally treated aerogel reacts readily to form Si–OH groups as shown by the 970–980 cm−1 Si vs OH and 3750 cm−1 surface OH stretches. Raman contour shapes near 800 cm−1 and between 900–1300 cm−1 are sensitive to the conditions of thermal treatment. The Hokmabadi–Walrafen vibrational correlation relating decrease of the mean Si–O–Si bridging angle to increase of the mean Si–O bond length is obeyed for both treated and untreated aerogels.