| Summary: | Two series of Sc<sup>3+</sup>- and Nb<sup>5+</sup>-doped TiO<sub>2</sub> (rutile) samples were synthesized and characterized by SEM, ICPE spectroscopy, XPS, and BET methods. Photocatalytic activity of the doped TiO<sub>2</sub> samples was tested in photocatalytic degradation of phenol. Dependences of the photocatalytic activities of the doped TiO<sub>2</sub> samples demonstrate a volcano-like behavior, indicating the existence of the optimal dopant concentrations to achieve the highest activity of photocatalysts. Remarkably, the optimal dopant concentrations correspond to the extrema observed in work function dependences on the dopant concentrations, that indicates a significant energy redistribution of the defect states within the bandgap of TiO<sub>2</sub>. Such a redistribution of the defect states is also proven by the alterations of the optical and EPR spectra of the intrinsic Ti<sup>3+</sup> defect states in TiO<sub>2</sub>. Based on the analysis of the experimental results, we conclude that both Sc<sup>3+</sup> and Nb<sup>5+</sup> doping of TiO<sub>2</sub> results in redistribution of the defect states and the optimal dopant concentrations correspond to the defect structures, which are ineffective in charge carrier recombination, that ultimately leads to the higher photocatalytic activity of doped TiO<sub>2</sub>.
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