N-doped NaTaO_3 synthesized from a hydrothermal method for photocatalytic water splitting under visible light irradiation

NaTaO_3-xN_x catalysts were synthesized by a hydrothermal (H) and a solid-state (S) methods in this study. The H- and S-NaTaO_3-xN_x samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV–visible (UV–vis) diffuse reflectance spectroscopy, and photoluminescence (PL) spectroscopy. The XRD patterns of the H- and S-samples showed peaks indexed to the pure phase of perovskite NaTaO₃ and minor peaks assignable to Ta₃N₅ at various synthesis temperatures. Substitution of oxygen by nitrogen ions causes the light absorption of the H- and S-NaTaO_3-xN_x samples to be extended to the 600–650 nm region, thus making the samples visible-light active. The NaTaO_3-xN_x samples exhibited photocatalytic activity for H₂ and O₂ evolution from aqueous methanol and silver nitrate solutions under visible-light irradiation. The UV–vis and PL spectra of the H- and S-catalysts revealed the presence of cationic vacancies and reduced metallic species, which acted as recombination centers. These results demonstrated that the preparation method plays a critical role in the formation of defect states, thereby governing the photocatalytic activity of the NaTaO_3-xN_x catalysts.