New Insights into the Origin of Visible-Light Photocatalytic Activity in Se-Modified Anatase TiO₂ from Screened Coulomb Hybrid DFT Calculations

We report a systematic study on the optoelectronic properties of Se-modified anatase TiO 2 investigated by DFT (including the perturbation theory approach DFPT) within the screened coulomb hybrid HSE06 formalism to guarantee accurate band gap and electronic excitation predictions. Various selenium species at substitutional sites for O or Ti, at interstitial sites, as well as at mixed substitutional/interstitial sites are studied. Among the explored structures, Ti (1–2 x ) O 2 Se 2 x (containing Se 4+ species), TiO (2– x ) Se x (containing Se 2– species), and TiO (2– x ) Se 2 x (containing Se 2 2– species) reveal significant enhanced visible-light optical absorption spectra with new absorption features appearing at 500, 600, and 690 nm, respectively. Our calculated spectra are found to be in good agreement with those obtained in available experimental works. The band gap narrowing in these materials originates from incorporation of newly occupied electronic levels within 0.5–1.5 eV above the original valence band of TiO 2, leading to new narrowed band gaps of 2.5, 2.0, and 1.8 eV respectively. Our calculations also reveal suitable band positions of Ti (1–2 x ) O 2 Se 2 x and TiO (2– x ) Se x for overall water splitting, whereas TiO (2– x ) Se 2 x shows an unsuitable valence band position for the oxygen evolution reaction. In contrast, the localized electronic character of the new occupied states on the Se 4p orbitals and only on the O 2p orbitals linked to the Se species makes the holes mobility limited in this material and the recombination rate of charge carriers greatly increased in the bulk.