Structural and Electronic Properties of Na₂Ti₃O₇ and H₂Ti₃O₇

Abstract

Hydrogen and sodium titanates have attracted interest as possible photocatalysts for energy conversion, storage, and environmental remediation. Here, first-principles calculations based on density functional theory (DFT) have been carried out to study their crystal and electronic structures, their exfoliation behavior, and defect formation. In the hydrogen titanate, half of the hydrogen forms water in the stoichiometric compound, and the crystal cell has a lower symmetry with respect to its sodium counterpart. H 2 Ti 3 O 7 and Na 2 Ti 3 O 7 have electronic gaps of 2.96 and 3.13 eV, respectively. Hydrogen and sodium vacancies are the defects with the lowest formation energies, making these compounds p-type semiconductors. Oxygen vacancy formation is suppressed with respect to titanium dioxide. Finally, the two compounds have a low surface energy, promoting exfoliation of the bulk and the formation of 2D materials and nanotubes.

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

“…A single simulation cell was used, with 24 atoms. Relaxation of the cell resulted in lattice parameters of a = 16.30 Å, b = 3.81 Å, and 9.26 Å, in agreement with experiments and previous calculations . The use of a single cell also for defect calculations corresponds to a concentration of defects of 2.7 × 10 20 cm −3 , which can be experimentally reached by means of plasma treatment, but it can also be present in as‐synthesized materials …”

“…Here, we applied the Hubbard correction. As a consequence, our calculated cell parameters are about 1% larger than those calculated by Abass and Seriani . The creation of these defects resulted in a substantial rearrangement of the atomic positions around the vacancy.…”

“…Our starting models for perfect and defective systems (with V O , V OH , and V H vacancies) are based on the structures calculated by Abass and Seriani with DFT. We use a single cell with 24 atoms.…”

“…Relaxation of the cell resulted in lattice parameters of a ¼ 16.30 Å, b ¼ 3.81 Å, and 9.26 Å, in agreement with experiments and previous calculations. [25] The use of a single cell also for defect calculations corresponds to a concentration of defects of 2.7 Â 10 20 cm À3 , which can be experimentally reached by means of plasma treatment, but it can also be present in as-synthesized materials. [36] As implemented in the YAMBO, [33] QP corrections to the Kohn-Sham eigenvalues were calculated by adopting the single-shot G o W o approximation, and the imaginary part of the macroscopic dielectric function was computed using BSE.…”

“…As a consequence, our calculated cell parameters are about 1% larger than those calculated by Abass and Seriani. [25] The creation of these defects resulted in a substantial rearrangement of the atomic positions around the vacancy. The relaxed positions of the same atoms after removal of O, OH, and H are shown in Figure 1b-d along with the most important interatomic distances and angles of the relaxed defective structures shown in Table 1.…”