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

Abstract

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.