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 TiO2 investigated by DFT (including the perturbation theory approach DEPT) 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(1-2x)O2Se2x (containing Se4+ species), TiO(2-x)Sex (containing Se2- species), and TiO(2-x)Se2x (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 TiO2, 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(1-2x)O2Se2x and TiO(2-x)Sex for overall water splitting, whereas TiO(2-x)Se2x 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.