Tunable Optical and Photocatalytic Performance Promoted by Nonstoichiometric Control and Site-Selective Codoping of Trivalent Ions in NaTaO<sub>3</sub>

The present work explores a solid state route to synthesis of trivalent ions (Eu<sup>3+</sup>, La<sup>3+</sup>, etc.) doped NaTaO<sub>3</sub> with controlled nonstoichiometric chemistry and lattice parameters with an aim to exploring electronic structure and photocatalytic performance. All samples were fully characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM), atomic absorption spectrophotometry, UV–vis diffuse reflectance spectroscopy, and photoluminescence measurement. By employing Eu<sup>3+</sup> as a model trivalent ion doped in NaTaO<sub>3</sub> lattice, the effects of site-selective doping and nonstoichiometric chemistry on the lattice parameters, band gap structure, photocatalytic activity toward methylene blue solution, and photocatalytic hydrogen evolution were systematically investigated. A nonstoichiometric Na/Ta molar ratio led to site-selective occupation of Eu<sup>3+</sup> ions which was changed from sole substitution to dual substitutions. Meanwhile, the nonstoichiometric Na/Ta molar ratio and site-selective occupation of Eu<sup>3+</sup> resulted in a monotonous lattice expansion and local symmetry distortion. Lattice variation, doping effects, and its relevant defect chemistry had a great impact on the ν<sub>3</sub> mode vibration of the O–Ta bond, which became asymmetric and shifted toward higher wavenumbers. Moreover, contrary to theoretical predictions, Eu<sup>3+</sup>-doped NaTaO<sub>3</sub> nanocrystals showed an abnormal narrowing of the band gap energies and weak visible light absorption with variation of Na/Ta molar ratios, which is thought to be related to doping effects, defect chemistry, and variation of lattice parameters. With well-defined lattice structure and defect centers and electronic structure via nonstoichiometric control and trivalent ions doping, the photocatalytic activity of trivalent ions-doped NaTaO<sub>3</sub> can be well regulated and optimized.