Nitrogen-doped black TiO2 spheres with enhanced visible light photocatalytic performance

N-doped black TiO2 spheres (N2–b-TiO2) were prepared by solvothermal reaction and calcination under a nitrogen atmosphere. N-doping introduces new impurity levels above the TiO2 valence band, enhancing the effective absorption of visible light. The presence of OV and Ti3+ in the disordered outer layer inhibits electron–hole pair recombination, and the spherical structure provides many active sites. Those were confirmed by X-ray diffraction, Fourier transform IR, X-ray photoelectron spectroscopy and High-resolution transmission electron microscopy. UV–visible absorption indicates that the nitrogen-doped black TiO2 samples has a reduced band gap and a strong visible light absorption, which is attributed to the doping of OV, Ti3+ and N. The photocatalytic efficiency of the best sample (N2–b-TiO2) for potassium dichromate and rhodamine B was 96.2% and 99.5%, respectively. The superoxide radical (•O2−) played a dominant role in photocatalysis by scavenging experiments. Thus, a photocatalytic mechanism with reduced band gap has been proposed. This study provides a simple and practical method for non-metallic doping to control the photocatalytic performance of semiconductors.