Synthesis of transparent Ti-containing mesoporous silica thin film materials and their unique photocatalytic activity for the reduction of CO ₂ with H₂O

The transparent Ti-containing mesoporous silica thin film materials having two different types of hexagonal and cubic pore structures with a thickness of about 50μm and a lateral size of a few centimeters were successfully synthesized by the solvent evaporation method from tetramethoxysilane (TMOS), vinyltrimethoxysilane (VTMOS), titanium tetra(iso-propoxide) (TIP), and octadecyltrimethylammonium chloride (C₁₈TAC). The films were converted to Ti-containing nanoporous silica thin films by subsequent calcinations in air at 823K, while the highly ordered mesostructures and macroscopic morphology were retained after the surfactant removal by calcination. The films with hexagonal and cubic symmetry were obtained by changing the composition of the starting mixtures. Various spectroscopic measurements of these films clearly showed that Ti ions are present in the silica networks as a tetrahedrally coordinated Ti-oxide species with a high dispersion state. Thus, developed transparent Ti-containing thin films were used as photocatalysts for the reduction of CO₂ with H₂O to evaluate their unique and high photocatalytic activity. UV irradiation of these films in the presence of CO₂ and H₂O led to the formation of CH₄ and CH₃OH as well as CO and O ₂ as minor products, their yields increasing linearly against irradiation time, indicating that these films operate as efficient photocatalysts to proceed such a difficult reaction catalytically at 323K. Furthermore, the films having hexagonal pore structure exhibited higher photocatalytic activity than the Ti-MCM-41 powdered catalyst even with the same pore structure.