Structural investigation of silica gel films by infrared spectroscopy

Abstract

Fourier transform infrared absorption spectroscopy has been utilized to characterize the structure of porous silica gel films, both deposited on c‐Si substrates and free standing. The films were either dried at room temperature or subjected to partial densification at 400–450 °C. The spectra of the gel films are compared to those of thermal SiO₂ grown on c‐Si and to Kramers–Kronig analysis of the reflection spectra of bulk SiO₂ gels and v‐SiO₂. The gel films show small frequency shifts compared to the latter spectra and they also exhibit new bands due to the presence of OH groups, although very little molecular water or residual organic species were found. The results are interpreted in terms of the gel structure. Compared to the thermal oxide, the sharp peak near 1070 cm⁻¹ is narrower for the gels and the spread in intertetrahedral angles is estimated at 24° and 27° for room temperature dried and partially densified gels, respectively, compared to 33° for the thermal oxide. This is in agreement with a state of strain in the Si—O—Si bridges of the gels near the surface of the pores. The gel films have also a stronger shoulder near 1200 cm⁻¹, whose intensity decreases with heat treatment of the gel. Its nature is related to the longitudinal optical (LO) component of the high‐frequency vibration of SiO₂ and it is suggested that the gel pores may account for the activation of the LO component.