Microstructural evolution and order-disorder transitions in mesoporous silica films studied by FTIR spectroscopy

Silica mesoporous thin films have been synthesised with a self-assembling process employing cetyltrimethylammonium bromide as the organic template and tetraethyl orthosilicate as the silica source. Mesoporous films with Pm3n cubic phase phases have been obtained and the films have been thermally treated in air with a progressive heating schedule from as-deposited up to 1000°C. The evolution of the microstructure has been studied with transmission Fourier transformed infrared (FTIR) spectroscopy. FTIR spectra of the as-deposited films have shown the presence of cyclic species, which at temperatures larger than 350°C have been no more observed. In the 1000-1300 cm^-1 region several overlapped absorption bands have been detected. In particular, the pair LO₃-TO₃, the cyclic species absorption bands and the pair LO₄-TO₄ have been resolved. These last bands, in particular, are associated with disorder-order transitions in the silica microstructure. These disorder-induced optical modes are due to the large interface area 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 45° with respect to the normal incidence angle. The LO₃ band, which in silica sol-gel films is indicative of the network condensation and is activated by scattering of the light in the pores, was resolved as a single sharp band from 250°C.