Band Gap Narrowing of Bi-Doped NaTaO₃ for Photocatalytic Hydrogen Evolution under Simulated Sunlight: A Pseudocubic Phase Induced by Doping

NaTaO3 is a promising material for the production of hydrogen fuel via photocatalytic water splitting, although the wide band gap prevents its application with solar light. In order to overcome this issue, bismuth doping has been proposed as a method for band gap narrowing by introducing midgap electron states. In this work, Bi-doped NaTaO3 nanocubes were synthesized through a facile molten salt method and the photocatalysts exhibit hydrogen evolution under simulated sunlight irradiation (AM 1.5G). X-ray diffraction, Raman, and UV-vis spectra suggest that the incorporation of Bi3+ at the Ta-site induces band gap narrowing, in addition to a structural transition, as the orthorhombic perovskite lattice becomes pseudocubic at low dopant concentrations (0.5-4 mol %). The optimal photocatalytic activity of 3 mol % Bi-doped NaTaO3 may be a result of the simultaneous presence of the pseudocubic lattice and the narrowed band gap of 3.6 eV, which in turn promote the absorption of ultraviolet light from the AM 1.5G irradiation source. Theoretical simulations based on density functional theory were used in conjunction with the experimental results to present in detail the additional contribution of the doped pseudocubic phase in the system. Furthermore, 3 mol % Bi-doped NaTaO3 was loaded with Ni cocatalysts by magnetron sputtering deposition, leading to enhanced and stable H-2 production rates for more than 100 h of reaction.