Investigation of photocatalytically-active hydrated forms of amorphous titania, TiO2·nH2O
Low temperature preparations of hydrated forms of amorphous titania, TiO2·nH2O, have been obtained starting from Ti(n-butoxide)4 in ethanol, acetone, hexane or tetrahydrofuran solutions by either slow evaporation (TiO2–A1) or from rapid precipitation in an aqueous HCl solution using ammonia (TiO2–A2). The washed products lost/reabsorbed water up to a maximum of 19 wt.% for samples of TiO2–A1 (n ∼ 1.0) and 9.9% for TiO2–A2 (n ∼ 0.5), determined from TGA, and exhibited an optical band gap of ∼3.5 eV. Under full spectrum irradiation in aqueous methanol solutions the photocatalytic rates for H2 production reached a maximum of 314 and 1158 μmol h−1 g−1 for bare and platinized (0.5 wt.%) samples of TiO2–A1, respectively, and 210 and 170 μmol h−1 g−1 for TiO2–A2. The photocatalytic rates measured at a slightly elevated temperature of 58 °C were up to 2× greater than those measured nearer room temperature, while these rates were independent of the amount or type of solvent used in their preparation. The UV–vis diffuse reflectances, post irradiation, indicate a higher concentration of Ti3+ sites in TiO2–A1 compared to TiO2–A2, and thus a higher density of active sites and reduced electron–hole recombination.
Journal: Journal of Photochemistry and Photobiology A: Chemistry - Volume 186, Issue 1, 5 February 2007, Pages 8–13