Synergic photocatalytic effects of nitrogen and niobium co-doping in TiO₂ for the redox conversion of aquatic pollutants under visible light

Nitrogen and niobium co-doped TiO₂ was synthesized by a simple sol-gel method and its visible light photocatalytic activities were compared with bare, N-, and Nb-doped TiO₂. The synthesized photocatalysts were characterized by powder X-ray diffraction (XRD), diffuse reflectance UV-Visible absorption spectroscopy, FT-IR spectroscopy, X-ray photoelectron spectroscopy, and energy-dispersive X-ray analysis. TiO₂ co-doped with N and Nb can have unique properties distinguished from the singly doped TiO₂ in the aspects of the dopant-induced charge distribution, vacancy sites, structural stability, defect energy levels, and optical absorption. The most notable is that the visible light absorption by (N,Nb)-TiO₂ was significantly higher than either of N- or Nb-TiO₂ sample. The doped samples exhibited a slight change in the band gap (by ∼0.1 eV) compared with bare TiO₂, which could be confirmed by Tauc plot, Mott- Schottky plot, and the electronic structure calculation. The co-doping of N and Nb in TiO ₂ induced the creation of the mid-gap levels and Ti³⁺ states that enhance the visible light absorption. The photocatalytic activities of bare, N-, Nb-, and (N,Nb)-TiO₂ were compared for the photocatalytic degradation of 4-chlorophenol (4-CP), oxidation of iodide, and reduction of chromate (Cr^VI) in the aqueous phase under visible light (λ > 420 nm). For all tested substrates, (N,Nb)-TiO₂ exhibited the markedly higher activities than either N-TiO₂ or Nb-TiO₂. A similar trend was also observed for the photocurrent generation under visible light irradiation.