Tubular nitrogen-doped TiO2 samples with efficient photocatalytic properties based on long-lived charge separation under visible-light irradiation: synthesis, characterization and reactivity

Abstract

A nitrogen-doped TiO₂ sample was prepared at 413 K by direct hydrothermal treatment of titanium isopropoxide in an aqueous solution of NH₃. This new material has a large specific surface area of ca. 220 m² g⁻¹ because of its tubular structure and it exhibits a prominent absorption feature in the region between 400 and 650 nm. It responds strongly to light in the visible region, which is key to its potential performance as a photocatalyst that may improve the efficiency for utilization of solar energy. Actually, this sample exhibits very efficient activity in the decomposition of CH₃COOH under visible light among the samples prepared. This effective photocatalysis of the present sample was substantiated by characteristic spectroscopic features, such as: (1) an optical absorption band with λ > 400 nm because of the doped nitrogen species; (2) the formation of EPR-active, long-lived N˙ and O₂^– species, as well as N₂^– species, under visible-light irradiation in the O₂ or N₂ adsorption process at 300 K by way of the monovalent nitrogen ions in the bulk (both substitutional and interstitial); (3) the existence of IR-active O₂ species adsorbed on the nitrogen-doped TiO₂ sample even without light irradiation; and (4) an XPS N_1s band around 399.6 eV that is assignable to the N⁻ species. The amounts of N˙ and O₂⁻ species formed in the nitrogen-doped TiO₂ sample under visible-light irradiation correlated well with the levels of reactivity observed in the decomposition of CH₃COOH on the samples with varying amounts and types of doped nitrogen species. We conclude that the photoactive N˙ and O₂^– species created in the present sample are responsible for the decomposition of organic materials assisted by visible light irradiation. These features may be attributable to the interface between the sample's tubular structure and anatase with poor crystallinity, which probably causes the resistance to the recombination of electron–hole pairs formed by irradiation.