Defect engineering of two-dimensional Nb-based oxynitrides for visible-light-driven water splitting to produce H₂ and O₂

Two-dimensional (2D) Nb-based oxynitrides are promising visible-light-responsive photocatalysts for the water splitting reaction, but their photocatalytic activity is degraded by the formation of reduced Nb⁵⁺ species and O^2- vacancies. To understand the influence of nitridation on the formation of crystal defects, this study synthesized a series of Nb-based oxynitrides through the nitridation of LaKNaNb_1-TaO₅ (x = 0, 0.2, 0.4, 0.6, 0.8, 1.0). During nitridation, K and Na species volatilized, which helped transform the exterior of LaKNaNb_1-TaO₅ into a lattice-matched oxynitride shell. Ta inhibited defect formation, yielding Nb-based oxynitrides with a tunable bandgap between 1.77 and 2.12 eV, straddling the H₂ and O₂ evolution potentials. After loading with Rh and CoO cocatalysts, these oxynitrides exhibited good photocatalytic activity for H₂ and O₂ evolution in visible light (650-750 nm). The nitrided LaKNaTaO₅ and LaKNaNb_0.8Ta_0.2O₅ delivered the maximum H₂ (19.37 μmol h^-1) and O₂ (22.81 μmol h^-1) evolution rates, respectively. This work provides a strategy for preparing oxynitrides with low defect densities and demonstrates the promising performance of Nb-based oxynitrides for water splitting.