Spontaneous Direct Band Gap, High Hole Mobility, and Huge Exciton Energy in Atomic-Thin TiO₂ Nanosheet

The quasi-particle band structure, carrier mobility, and optical response of atomic-thin TiO₂ nanosheets were accurately predicted with the many-body perturbation theory of G₀W₀+BSE calculations. The lepidocrocite-type TiO₂ exhibits an unexpected direct band gap of 5.3 eV, different from the retaining indirect band gap character in anatase-type TiO₂ nanosheet. Because of the dispersive valence band maxima from the strong overlap between O-2p orbitals, an extremely high hole mobility of 1069 cm² V^-1 s^-1 was proposed for the lepidocrocite-type TiO₂ nanosheet. Including the electron-hole exchange of excitonic effect, our simulations well reproduce the experimental absorption spectra implying a huge exciton binding energy. The strong anisotropic exciton originates from the crystal dependent effective mass in the lepidocrocite-type TiO₂ nanosheet due to the asymmetric orbital overlaps. The strongly bound exciton still renders a sufficiently high potential for hydrogen production from aqueous solution, although it largely decreases the photonic excitation energy.