Modification of NiTiO3 visible light-driven photocatalysts by Nb doping and NbOx heterojunction: Oxygen vacancy in the Nb-doped NiTiO3 structure

Highlights: • Nb-doped NiTiO₃ and NbO_x/NiTiO₃ photocatalysts are prepared with various Nb amounts. • Nb-doped NiTiO₃ exhibits higher photocatalytic activity than pure NiTiO₃. • Incorporation of Nb into the NiTiO₃ lattice induces Ti³⁺ sites and oxygen vacancies. • High Nb doping into NiTiO₃ results in formation of a triple metal oxide phase. • Inappropriate band structures of NbO_x and NiTiO₃ decrease photocatalytic activity. -- Abstract: In this study, two types of niobium-containing NiTiO₃ photocatalysts — Nb-doped NiTiO₃ and NbO_x/NiTiO₃ heterojunction — were prepared with different Nb contents to enhance photocatalytic activity for dye photodegradation under sunlight irradiation. Nb-doped NiTiO₃ exhibited higher photocatalytic activity than pure NiTiO₃, whereas the heterojunction of NbO_x with NiTiO₃ caused a decrease in photocatalytic activity. The incorporation of Nb into the NiTiO₃ lattice structure induced the formation of Ti³⁺ and Nb³⁺ sites, which considerably increased the number of oxygen vacancies in those photocatalysts; this conclusion was confirmed by the results of X-ray photoelectron spectroscopy. Eventually, the Nb-doped NiTiO₃ photocatalyst at the highest Nb content (NT-Nb-10) was transformed into a triple metal oxide phase with the highest surface area and oxygen vacancy. The abundant oxygen vacancy of NT-Nb-10 resulted in the lowest photoluminescence emission intensity, which resulted in the highest apparent photocatalytic reaction rate constant (k_app) of 14.1 min⁻¹ owing to the suppression of the recombination process. The formation of the triple metal oxide phase in this study may allow a potential way to modify low cost visible light-driven NiTiO₃ photocatalysts for sustainable photocatalytic application.