An efficient TiO2 thin-film photocatalyst: Photocatalytic properties in gas-phase acetaldehyde degradation

A semitransparent TiO2 film with extraordinarily high photocatalytic activity was prepared on a glass substrate by sintering a TiO2 sol at 450 degrees C. Crystallographic analysis by X-ray diffraction and Raman spectroscopy showed that the film was purely anatase. The photocatalytic properties of the film were investigated by measuring the photodegradative oxidation of gaseous acetaldehyde at various concentrations under strong and weak UV light irradiation conditions. The photocatalytic activity of the film was higher than that of one of the most active commercial TiO2 powders, Degussa P-25. The kinetics of acetaldehyde degradation as catalyzed by the TiO2 film as well as by P-25 powder were analyzed in terms of the Langmuir-Hinshelwood model. It is shown that the number of adsorption sites per unit true surface area is larger with the TiO2 film, as analyzed in the powder form, than with P-25 powder. Meanwhile, the first-order reaction rate constant is also much larger with the film than with P-25 powder. Moreover, under most experimental conditions, particularly with high concentrations of acetaldehyde and weak UV illumination intensity, the quantum efficiency was found to exceed 100% on an absorbed-photon basis, assuming that only photo-generated holes play a major role in the reaction. This leads to the conclusion that the photodegradative oxidation of acetaldehyde is not mediated solely by hydroxyl radicals, generated via hole capture by surface hydroxyl ions or water molecules, but also by photocatalytically generated superoxide ion, which can be generated by the reduction of adsorbed oxygen with photogenerated electrons.