Site Specific Optical and Photocatalytic Properties of Bi-Doped NaTaO3

Synthesis, optical properties, and visible light photocatalytic activities of Bi3+-doped NaTaO3 powders have been investigated. The samples were analyzed by X-ray diffraction, X-ray photoelectron spectroscopy, diffused reflectance spectroscopy, and energy dispersive spectroscopy. It is found that the Na/Ta molar ratio of the starting materials affects the site occupancy of Bi at Na or Ta site in the lattice, and thus the optical and photocatalytic properties are significantly altered. Under Na-deficient conditions, Bi predominantly occupied Na site; the sample did not show absorption in the visible region. While under strongly Na-rich conditions, Bi occupancy at Ta site was predominant. These samples showed visible light absorption up to 450 nm. The samples prepared under mildly Na-rich condition, which leads to approximately equal occupancy of Bi at Na and Ta sites, showed visible light absorption up to 550 nm. Correspondingly, the highest photocatalytic activity for methylene blue degradation under visible light was obtained for this condition. The first principles calculation confirmed that band gap narrowing is highest when Bi is substituted at both Na and Ta sites compared with the ones at Na and Ta sites separately. The experimentally observed absorption spectra agree well with that indicated by the electronic structure calculations. The present study shows that the optical property and photocatalytic performance of Bi-doped NaTaO3 can be tuned by Bi occupancy in the lattice. The finding opens up a new venue for designing visible light sensitive ternary compounds for photocatalytical applications.