Structural and Electronic Properties of Na2Ti3O7 and H2Ti3O7

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. H2Ti3O7 and Na2Ti3O7 have electronic gaps of 2.96 and 3.13eV, 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.