Electronic and Optical Properties of TiO₂ Solid-Solution Nanosheets for Bandgap Engineering: A Hybrid Functional Study

Herein, the electronic and optical properties of TiO₂(010) nanosheet solid solutions with transition metal oxycarbides, nitrides, and oxynitrides, (TiO₂)_2/3(M₂O₃C)_1/3 (M = Nb or Ta), (TiO₂)_2/3(MN₂)_1/3 (M = W or Mo), and (TiO₂)_2/3(MOE)_1/3 (M = W, Mo, E = C, and M = Nb, Ta, E = N) are systematically investigated. Forming a solid solution is a viable way to realize visible-light absorption and a direct band gap. The electron affinity of a solid-solution nanosheet closely depends on the energy level of the transition metal (M) d states; i.e., the hybridization of the M d states and Ti–O antibonding states introduces new bonding states, leading to a downward shift of the conduction band minimum. Meanwhile, the ionization potentials of these solid solutions are relatively low because of the introduction of high-lying occupied C/N 2p states, which lift the valence band maximum upward above that of the pristine TiO₂. The modulation of band edges effectively narrows the band gaps of the solid solutions, except for transition metal oxynitrides. Among the examined solid solutions, (TiO₂)_2/3(WOC)_1/3 was the most promising nanosheet for water splitting owing to its suitable band edges and responsive to visible and ultraviolet light.