Order-disorder transitions and evolution of silica structure in self-assembled mesostructured silica films studied through FTIR spectroscopy

Silica mesoporous thin films have been synthesized with a self-assembling process using cetyltrimethylammonium bromide as an organic template and tetraethyl orthosilicate as the silica source. Mesoporous films with Pm3n cubic phase have been obtained and the films have been thermally treated in air with a progressive heating schedule from as-deposited up to 1000 degreesC. The evolution of silica network has been studied with transmission Fourier transformed infrared (FTIR) spectroscopy. The presence of cyclic species in the precursor solution has been shown by Si-29 nuclear magnetic resonance (NMR) experiments. FTIR spectra of as-deposited films have shown that some of these cyclic species are retained in the structure after film deposition, but at temperatures larger than 350 degreesC they have been no longer observed. In the 1000-1300 cm(-1) region, several overlapped absorption bands have been detected. In particular, the LO3-TO3 pair, the cyclic species absorption bands and the LO4-TO4 pair have been resolved. These last bands are associated with disorder-order transitions in the silica structure. These disorder-induced optical modes are due to the large interface area in mesoporous silica films and related to bond strains. The evolution of the bands in the 1000-1300 cm(-1) region has been followed with the Berreman configuration, performing the transmission FTIR analysis at 45degrees with respect to the normal incidence angle. The LO3 band, which in silica sol-gel films is indicative of network condensation and is enhanced by scattering of the light in the pores, has been resolved as a single sharp band from 250 degreesC.