The photocatalytic activity of TiO₂ can be mainly improved from three approaches: (1) enhancing surface energy, (2) increasing availability of visible light and (3) improving the separation efficiency of photo-induced electrons and holes. Here, we report a one-step route to obtain Mo + N codoped TiO₂ sheets with dominant {001} facets by a hydrothermal process using TiN, MoO₃, HF and HNO₃ as precursor. The XRD patterns confirmed Mo and N doping into the lattice of anatase TiO₂ and the XPS survey spectrum shows nitrogen (N) acting as interstitial N or an O–Ti–N structure and molybdenum (Mo) existed as Mo⁶⁺ in TiO₂ sheets. All Mo + N codoped TiO₂ sheets can absorb visible light, but compared with N-doped TiO₂ sheets, further Mo-doping elevates the conduction band edge and expands the band gap of anatase TiO₂ slightly. The separation efficiencies of photo-induced electrons and holes of Mo + N codoped TiO₂ sheets were enhanced compared with that of N-doped TiO₂ sheets, as confirmed by detecting the production of hydroxyl radicals (˙OH), and their photocatalytic activities on decomposition of methylene blue and methyl violet was improved. The Mo + N codoped TiO₂ sheets showed the highest photocatalyst activity when the Mo-doping ratio reached 1%, indicating an appropriate codoping ratio is necessary for synergistic effect between doped metal and nonmetal elements.

The photocatalytic process for degradation of methylene blue with P25 TiO₂ was directly observed using a high-resolution transmission electron microscopy (HRTEM). It was found that: (1) the pristine anatase TiO₂ nanoparticles exhibited a perfect crystal lattice with a clear HRTEM image; (2) after adsorption and degradation, there were many methylene blue crystals as 1 nm molecular adsorbed at the surface of TiO₂ nanoparticles, which therefore resulted in a fuzzy HRTEM image; (3) when the TiO₂ was exposed in air for a period of time, the methylene blue molecules disappeared and the TiO₂ lattice image again became integrated as the pristine one; (4) however, if the TiO₂ nanoparticles became deactivated after degradation of methylene blue for more than 20 cycles, the HRTEM lattice image was fuzzy fully and could not recover even it was exposed in air for a long time. The results reveal that the lattice distortion on the anatase TiO₂ (101) surface induced by chemical adsorption of methylene blue molecules is a crucial intermediate step during photocatalyzing. The distorted lattice atoms absorb photons under illumination of light and tend to recover and cut the molecule bond which causes the degradation of methylene blue. Furthermore, we propose that there exists a surface lattice driving force forming on the surface distortion leads to the TiO₂ lattice HRTEM image from fuzzy to integrate, and it decides the photocatalytic ability.