Experiments combined with the density functional theory (DFT) calculation have been performed to understand the underlying photocatalysis mechanism of the nitrogen-doped titania nanobelts. Nitrogen-doped anatase titania nanobelts are prepared via hydrothermal processing and subsequent heat treatment in NH(3). Both the nitrogen content and the oxygen vacancy concentration increase with increasing the NH(3) treatment temperature. Nitrogen doping leads to an add-on shoulder on the edge of the valence band, the localized N 2p levels above the valence band maximum, and the 3d states of Ti(3+) below the conduction band, which is confirmed by DFT calculation and X-ray photoelectron spectroscopy (XPS) measurement. Extension of the light absorption from the ultraviolet (UV) region to the visible-light region arises from the N 2p levels near the valence band and from the color centers induced by the oxygen vacancies and the Ti(3+) species. Nitrogen doping allows visible-light-responsive photocatalytic activity but lowers UV-light-responsive photocatalytic activity. The visible-light photocatalytic activity originates from the N 2p levels near the valence band. The oxygen vacancies and the associated Ti(3+) species act as the recombination centers for the photoinduced electrons and holes. They reduce the photocatalytic activity although they contribute to the visible light absorbance.

Particle size is generally considered to be the primary factor in design of nanocrystal photocatalyst, because the reduction of particle size increases the number of active sites. However, the benefit from the size reduction can be canceled by a higher electron-hole recombination rate due to the confined space in sphere-shaped nanoparticles. Here we report a mechanistic study on a novel nanobelt structure that overcomes the drawback of sphere-shaped nanoparticles. Single-crystalline anatase TiO2 nanobelts with two dominant surfaces of (101) facet exhibit enhanced photocatalytic activity than the nanosphere counterparts with an identical crystal phase and similar specific surface area. The ab initio density functional theory (DFT) calculations show that the exposed (101) facet of the nanobelts yields an enhanced reactivity with molecular O2, facilitating the generation of superoxide radical. Moreover, the nanobelts exhibit a lower electron-hole recombination rate than the nanospheres due to the following three reasons: (i) greater charge mobility in the nanobelts, which is enabled along the longitudinal dimension of the crystals; (ii) fewer localized states near the band edges and in the bandgap due to less unpassivated surface states in the nanobelts; and (iii) enhanced charge separation due to trapping of photogenerated electrons by chemisorbed molecular O2 on the (101) facet. Our results suggest that the photocatalysis efficiency of nanocrystals can be significantly improved by tailoring the shape and the surface structure of nanocrystals, which provides a new concept for rational design and development of high-performance photocatalysts.