The chromium doped titanium dioxide (Cr–TiO₂) has been synthesized using a hydrothermal method. The as-prepared samples have been characterized by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), transmission electron microscopy (TEM), high resolution TEM (HR-TEM), XPS valence band spectroscopy, UV-vis diffuse reflectance spectroscopy (UV-vis DR), photoluminescence (PL) spectroscopy and time resolved PL (TR-PL) spectroscopy. The doping mechanism and related influence on the photocatalytic performance of TiO₂ are thus proposed. The doped Cr³⁺ ions can replace the Ti atoms in the lattice with oxygen vacancy compensation, distribute homogeneously in the framework of TiO₂ crystals, and may make the n-type TiO₂ less n-type or more p-type due to the resultant formation of oxygen vacancies, resulting in absorption of visible light, decrease of the intensity of PL emission and prolonged lifetime of photogenerated charge carriers. Compared with TiO₂, the doped samples exhibit an improved visible-light photocatalytic activity. The influence of nitrogen modification has also been studied. We envision that these results would afford a better understanding of the doping mechanism of TiO₂ using metal ions and, therefore, may provide a feasible way to prepare the TiO₂-based photocatalysts for real applications.

The nanosheet exhibits better visible-light photocatalytic activity than bulk g-C₃N₄ due to nanosheet nature and presence of more hydroxyl groups.