Quasiparticle and Optical Properties of Hydrogen Titanate and Its Defective Systems: An Investigation by Density Functional Theory with Hubbard Correction, Many-Body Perturbation Theory, and Bethe-Salpeter Equation

Hydrogen-rich titanium oxides have recently attracted attention as active photocatalysts with excellent photoabsorption. Herein, the optical properties of hydrogen titanate (H2Ti3O7) are calculated both for the perfect crystal and in the presence of a high concentration of oxygen, hydroxyl, and hydrogen vacancies (V-O, V-OH, and V-H, respectively). Spin-polarized density functional theory calculations with the Hubbard correction (DFT+U) predict V-H as the dominant defect, as a consequence of the low formation energy and the insignificant accompanied structural rearrangement. By the GW approximation, it is found that V-O and V-OH create localized deep defect states below the conduction band (by 2.11 and 3.30 eV, respectively), whereas V-H leads to a shift of the Fermi level moved inside the valence band. All the vacancies result in excitonic transitions in the infrared and visible regions in the absorption spectra, as calculated by solving the Bethe-Salpeter equation (BSE). Overall, comparison between single-particle theories and BSE results shows that excitonic effects are very important in this material. Delocalized excitons are observed in the case of V-O and V-H, which can be important to suppress electron-hole recombination, thus contributing to the enhancement of the photocatalytic activity of the material.