Screened Coulomb Hybrid DFT Study on Electronic Structure and Optical Properties of Anionic and Cationic Te-Doped Anatase TiO2

The origin of the enhanced visible-light optical absorption in Te-doped bulk anatase TiO2 is investigated in the framework of DFT and DFPT within HSE06 in order to ensure accurate electronic structure and optical transition predictions. Various oxidation states of Te species are considered based on their structural location in bulk TiO2. In fact, TiO(2-x)Tex (with isolated Te2- species at Te-Te distance of 8.28 angstrom), TiO2Tex (with isolated Te2- species at Te-Te distance of 8.28 angstrom), TiO2Te2x (with two concomitant Te2- species at Te-Te distance of 4.11 angstrom), and Ti(1-2x)O2Te2x (with two neighboring Te4+ species at nearest-neighbor Te-Te distance of 3.05 angstrom) show improved optical absorption responses in the visible range similarly as it is experimentally observed in Te-doped TiO2 powders. The optical absorption edges of TiO(2-x)Tex, TiO2Tex, and TiO2Te2x are found to be red-shifted by 400 nm compared with undoped TiO2 whereas that of Ti(1-2x)O2Te2x is red-shifted by 150 nm. On the basis of calculated valence and conduction band edge positions of Te-doped TiO2, only TiO(2-x)Tex and Ti(1-2x)O2Te2x show suitable potentials for overall water splitting under visible-light irradiation. The electronic structure analysis revealed narrower band gaps of 1.12 and 1.17 eV with respect to undoped TiO2, respectively, resulting from the appearance of new occupied electronic states in the gap of TiO2. A delocalized nature of the gap states is found to be much more pronounced in TiO(2-x)Tex than that with Ti(1-2x)O2Te2x due to the important contribution of numerous O 2p orbitals together with Te 5p orbitals.