Formation of Vertically Oriented Channels during Calcination of Surfactant-Templated Titania-Doped Mesoporous Silica Thin Films

Mesoporous structure development during calcination of titania-doped mesoporous silica thin films is characterized byin situgrazing-incidence small-angle X-ray scattering (GISAXS). Varying amounts of titania (≤6 mol %) are incorporated in cetyltrimethylammonium bromide (CTAB)-templated films by two methods in this study: dispersion of titania in the silica matrix and fixing titania at the pore surface by complexation of the titania precursor with a sugar-based cosurfactant, dodecyl maltoside (C₁₂G₂). Before calcination, films templated with only CTAB display a mesoporous structure with a mixture of in-plane two-dimensional (2D) hexagonal and randomly oriented porous structure, while the C₁₂G₂-complexed film possessed randomly oriented mesostructure only. Both methods yield a final structure of hexagonal close-packed pores orthogonally oriented to the substrate after calcination at 500 °C. This is attributed to weak Si-O-Ti bonds, which allow sintering to occur at 500 °C, combined with unidirectional thermal contraction of the film in the vertical direction. Anisotropic stress and annealing of the films allow the randomly oriented pores to merge vertically and form channels with a pore diameter of 2.3 ± 0.3 nm. Titania doping greater than 1% is required for this transformation in films with no C₁₂G₂surfactant used, while transformation was observed for films with C₁₂G₂complexation even at low (0.005%) titania doping, perhaps accentuated by orientational disorder introduced by the sugar surfactant.