Structure of inorganic and hybrid SiO2 sol-gel coatings studied by variable incidence infrared spectroscopy

Normal incidence infrared (IR) absorption spectroscopy is often employed for the characterization of sol-gel materials. For inorganic SiO2 gel films, the main peak at similar to1070-1080 cm(-1), due to the transverse optic component of the oxygen asymmetric stretch, has a shoulder (Sh) positioned around similar to1200 cm(-1), associated with the longitudinal optic (LO) component of the same vibration, which appears to be intensified by film porosity. In the present work, the use of oblique incidence IR absorption spectroscopy at 45degrees and 60degrees off-normal clearly shows that the Sh band evolves into two separate components of LO character, a high frequency sharp peak at similar to1225-1260 cm(-1), characteristic of the nonporous silica skeleton, plus an intermediate weaker band near similar to1140 cm(-1), tentatively associated with the gel pore network. In addition, the use of oblique incidence spectroscopy has also revealed that the hybrid SiO2 gel network is structurally different from that of the inorganic gels, such that, even after densification of both SiO2 gels at 900 degreesC, the two types of networks still differ appreciably. (C) 2002 Elsevier Science B.V. All rights reserved.