(574b) Mesostructure Thermal Transformation Kinetics and Mechanism for the Synthesis of SiO2-TiO2 Mixed Thin Films with Sub-3 Nanometer Vertical Pore Channels
Titania (TiO2)-containing mesoporous silica nanomaterials (thin films and particles) have been used for a range of applications including adsorption of biomolecules and environmental pollutants and in heterogeneous catalysis, which makes it possible to design reactive separation process using these materials. Although silica thin films (TFs) with accessible nanopore channels have been shown to provide tremendous opportunities for small molecule separations due to their tunable pore structure, silica thin films with small (~3nm) vertically aligned pore channels are difficult to achieve. Here, we present a unique strategy based on incorporating a small amount of TiO2 precursor (titanium IV isopropoxide) with silica precursor (tetraethyl orthosilicate) during sol-gel synthesis of cetyltrimethylammonium bromide (CTAB)-templated silica TFs to obtain vertical pore channels after calcination at 450 °C. A slightly distorted cubic mesostructure with Pm3n symmetry was observed after dip-coating and ageing at 50 °C, which transformed into an array of vertical pore channels due to anisotropic thermal contraction and vertical pore fusion during high temperature calcination. In situ and static grazing incidence small angle x-ray scattering (GISAXS) was carried out using a hot stage at Sector 8-ID-E of Argonne National Laboratory to observe the effects of thermal treatment on the mesostructure of titanosilicate films (0-13.4% TiO2). Pure silica TFs (0% TiO2) did not show a significant mesostructure transformation even after treatment at well over 500 °C, while mixed oxide thin films with 1% and 6% TiO2 transformed from cubic to vertical pore channels at around 450 °C. The mesostructure transformation kinetics were analyzed using the Avrami equation to determine the rate, order and underlying mechanism of mesostructural transformation. Corresponding in situ and static grazing incidence wide angle x-ray scattering (GIWAXS) showed no TiO2 crystallinity development up to 600 °C, suggesting that TiO2 is well dispersed in the silica matrix, and that crystallization is not responsible for the mesophase transformation. Most likely, relaxation of silica network due to the presence of well dispersed Ti heteroatoms enhance the likelihood of pore fusion by Ostwald ripening during anisotropic thermal contraction. These studies, together with complementary electron microscopy imaging, provide evidence for a novel composition-transformation relationship that can used to prepare novel nanostructured media for both separation and catalysis.