Synthesis, Characterization, Photocatalysis, and Varied Properties of TiO₂ Cosubstituted with Nitrogen and Fluorine

TiO2 (anatase) codoped with nitrogen and fluorine, synthesized by a simple solid state route, using urea and ammonium fluoride as sources of nitrogen and fluorine, respectively, as well as by decomposition of (NH4)2TiF6 for comparison, has been characterized by various techniques. XPS analysis shows the composition to be TiO1.7N0.18F0.12 for urea-based method (N, F-TiO2-urea) and TiO1.9N0.04F0.06 for complex decomposition method (N, F-TiO2-complex). Both the materials are defect-free as revealed by photoluminescence spectroscopy. Thus, N, F-TiO2-urea exhibits smaller defect-induced magnetization compared to the nitrogen-doped sample. Cosubstitution of N and F is accompanied with an enhancement of the absorption of light in the visible region giving rise to yellow color and with a band gap of ∼2.2 eV in the case of N, F-TiO2-urea. It exhibits enhanced photocatalytic activity and also significant hydrogen evolution (400 μmol/g) on interaction with visible light in the absence of any cocatalyst, which is much higher compared to N, F-TiO2-complex and N-TiO2. First-principles calculations show significant local distortions on codoping TiO2 with N and F and a lowering of energy by 1.93 eV per N, F pair. With virtual negative and positive charges on nitrogen and fluorine, respectively, the dopants prefer pairwise clustering. Our calculations predict a reduction in the band gap in TiO2 cosubstituted with nitrogen and fluorine. The calculated band structure shows that nitrogen 2p states form a separate subband just above the valence band which is enhanced on incorporation of fluorine. Our calculations also indicate anomalous Born effective charges in N, F-TiO2 and predict enhanced photocatalytic activity on codoping of TiO2 by N and F.