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

Abstract

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 (VO, VOH, and VH, respectively). Spin‐polarized density functional theory calculations with the Hubbard correction (DFT+U) predict VH 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 VO and VOH create localized deep defect states below the conduction band (by 2.11 and 3.30 eV, respectively), whereas VH 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 VO and VH, which can be important to suppress electron–hole recombination, thus contributing to the enhancement of the photocatalytic activity of the material.

Modeling Excited States of Point Defects in Materials from Many-Body Perturbation Theory

“…For example, for defects in hydrogen titanate (H 2 Ti 3 O 7 ), GW/BSE predicts an exciton binding energy of ∼1 eV. 103 Even when the exciton binding energy is weak, excitonic effects can influence the line shape of absorption or emission as shown for the nitrogen vacancy in GaN. 93 For low-dimensional materials, incorporation of excitonic effects is critical because the reduced screening means that electron−hole interactions are strong even for the pristine material and that the defect poses a strong perturbation to the system.…”

“…For this class of materials, GW/BSE has resulted in reinterpretation of the defects responsible for measured low-energy luminescence. , Additionally, the presence of heavy metals can localize excitons and result in large binding energies. For example, for defects in hydrogen titanate (H 2 Ti 3 O 7 ), GW/BSE predicts an exciton binding energy of ∼1 eV . Even when the exciton binding energy is weak, excitonic effects can influence the line shape of absorption or emission as shown for the nitrogen vacancy in GaN …”