THE EVOLUTION OF TEOS TO SILICA-GEL AND GLASS BY VIBRATIONAL SPECTROSCOPY

Fourier-transform infrared and Raman spectroscopies were simultaneously used in order to study the time evolution of the acid-catalyzed hydrolysis and condensation of tetraethoxysilane (TEOS) in ethanol, to form silica xerogel. The transformation to vitreous silica by gradual densification of the gel was also monitored after different heat treatments. The extent of hydrolysis was revealed by the changes in intensity of the IR-active bands of TEOS at 793 cm-1 (Si-O asymmetric stretch), 1102 cm-1 (C-O asymmetric stretch), 1168 cm-1 (CH3 rocking) and 1400 cm-1 (CH2 wagging) and also the TEOS Raman-active bands corresponding to the strongly polarized Si-O symmetric stretch (656 cm-1) and the depolarized CH3 rocking (960 cm-1). The intensity of all these bands clearly decreased as the reactions proceeded. Good evidence for the formation of ethanol in the initial steps of hydrolysis and for the subsequent predominance of its evaporation was found in the occurrence of maximum intensities for its infrared and Raman-active bands due to deformation of CH3 or CH2 groups (881 cm-1) and CH2 bending (1275 cm-1), after a period of time. The intensity of these bands later decreased. The gel to glass conversion was followed through the disappearance of the infrared peaks at 554 cm-1 (Si-OH rocking) and 940 cm-1 (Si-OH stretching), together with the intensity increase of the peaks at 800 and 1078 cm-1. The complementary nature of the IR and Raman spectroscopies is of great value for a complete understanding of the TEOS to silica glass transformation.