Mono-doped (X = S²⁻, Se²⁻, and Te²⁻) and co-doped (Zr⁴⁺-X) TiO₂ monolayer nanosheet for water splitting: DFT modeling

The water splitting activity of (111) TiO₂ monolayer nanosheet and its mono and co-doped forms has been investigated by the periodic density functional theory (DFT) calculations. Upon Zr⁴⁺ mono-doping and even increasing the concentration of Zr⁴⁺ dopant, the band gap of the (111) TiO₂ monolayer becomes wider than that of the corresponding pure monolayer (3.9 eV), which reduces the photocatalytic efficiency. Fortunately, (S²⁻, Se²⁻, and Te²⁻) mono-doping and their increased concentration can effectively decrease the band gap by introducing midgap states above the valence band edge for the relevant monolayers. Moreover, the (Zr⁴⁺-S²⁻), (Zr⁴⁺-Se²⁻), and (Zr⁴⁺-Te²⁻) co-doping leads to a narrowed band gap and enhances the visible-light photoactivity of the (111) TiO₂ monolayer. Among considered monolayers, the Te²⁻-doped and (Zr⁴⁺-Te²⁻) co-doped (111) TiO₂ monolayers are the most desirable photocatalysts for hydrogen generation in this work.