The Effect of Doping TiO₂ Monolayer with Sn⁺⁴, Pb⁺⁴, and S^-2 Ions on H₂ Production by Photocatalytic Water Splitting: Periodic DFT Modeling

(Sn⁺⁴ and Pb⁺⁴) mono-doping and (Sn⁺⁴-Pb⁺⁴), (Sn⁺⁴-S^-2), and (Pb⁺⁴-S^-2) co-doping were used to investigate the performance of the water splitting activity of the (111) TiO₂ monolayer by performing periodic density functional theory (DFT) calculations. The pure (111) monolayer with a large bandgap energy (3.9 eV) limits the absorption of sunlight. However, the pure structure has appropriate conduction band (CB) and valence band (VB) energy edges, which, respectively, are above the reduction potential of the H⁺/H₂ and below the oxidation potential of H₂O/O₂ half-reactions. Upon Sn⁺⁴ and Pb⁺⁴ mono-doping and (Sn⁺⁴-Pb⁺⁴), (Sn⁺⁴-S^-2), and (Pb⁺⁴-S^-2) co-doping, the band gap energy of the (111) TiO₂ monolayer becomes smaller than that of the corresponding pure monolayer, which increases the photocatalytic efficiency. Among the considered monolayers in this work, the Sn⁺⁴ mono-doped as well as the (Sn⁺⁴-S^-2) co-doped (111) TiO₂ monolayers, which have appropriate CB energy potential for proton reduction, are the most desirable photocatalysts for hydrogen production. It is noted that all structures particularly the Pb⁺⁴ mono-doped, (Sn⁺⁴-Pb+⁴) and (Pb⁺⁴-S^-2) co-doped (111) TiO₂ monolayers with appropriate VB edges and narrow band gap energies are good candidates for O₂ evolution.