Ligand-Assisted Fabrication of Small Mesopores in Semi-Crystalline Titanium Oxide Films for High Loading of Ru(II) Dyes

Abstract

Small mesoporous (2-3 nm) and optically uniform titania films with high surface areas were fabricated by employing the ligand-assisted templating approach using designed surfactants that contain strong N-donor sites around the hydrophilic headgroup. Two N atoms, located in a pyridine ring and in the attached alkyl chain, can strongly interact with one Ti atom via covalent ligation and reduce the reactivity (hydrolysis and condensation) of soluble Ti species. The as-made films were treated with UV-ozone irradiation, before calcination to crystallize the titania frameworks. Condensation of the titania frameworks proceeded during the UV-ozone treatment that also destroyed the binding sites between the designed surfactants and Ti atoms with partial decomposition of the surfactants. The titania surfaces were then covered with resultant charred organic species and stabilized, which was helpful for the retention of the mesostructures even after partial crystallization of the titania frameworks during calcination at 400 °C. The small mesopores were more useful for effective accommodation of Ru(II) bipyridyl dyes (∼1 nm) than were common mesopores (∼10 nm). Almost 2 times higher dye loadings were achieved over the small mesoporous titania films than other porous titania films due to the high surface areas, and consequently a high density of the photosensitive dye molecules was achieved even in the thin film fabricated in one pot.