Nitrogen and Fluorine Roles in Visible-Light-Driven Anion-Doped TiO 2 Photocatalysis
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Nitrogen and Fluorine roles in Visible-Light-Driven Anion-Doped TiO2 Photocatalysis Di Li*, Naoki Ohashi, Shunichi Hishita, Taras Kolodiazhnyi, and Hajime Haneda National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki, 305-0044, Japan ABSTRACT This paper describes the synthesis of N-doped, F-doped, and N-F-codoped TiO2 powders (NTO, FTO, NFTO) by spray pyrolysis (SP). An overall comparative study was carried out on these anion-doped powders in order to elucidate the roles of N and F played in visible-light (Vis)-driven photocatalysis. The comparisons in their characteristics were based on the analysis of UV-Vis, PL, NH3-TPD and ESR spectra. As the results, N-doping into TiO2 resulted in both the improvement of Vis absorption and the creation of surface oxygen vacancies (OVs). F-doping produced several beneficial effects including the creation of surface OVs, the enhancement of surface acidity and the increase of Ti3+ ions. The photocatalytic tests indicated that the NFTO demonstrated the highest Vis activity for decomposition of both acetaldehyde and trichloroethylene. This high activity was ascribed to a synergetic consequence of doped N and F. INTRODUCTION Anion-doped TiO2 photocatalysis has drawn much attention because it has good potential for the utilization of the solar energy to eliminate environmental pollutants in air or water [1-5]. However, the origin of the Vis-driven photocatalysis on these anion-doped TiO2 or the roles of these doped anions are still not clear, e.g., the Vis photocatalytic activity of N-doped TiO2 powders has been confirmed by many studies, but, the mechanism for exhibiting Vis activity is still controversial [2]. This argument maybe associated with the different preparation of samples. In our previous study, N-doped, F-doped and N-F-codoped TiO2 powders were successfully prepared by identical SP technique [2-5]. The most interesting result was that the N-F-codoped TiO2 powders demonstrated a very excellent photocatalytic activity no matter what kind of light sources (UV or Vis) was used [5]. This seems to be a consequence of the perfect combination of some beneficial effects induced by both N and F dopants. In the present study, an accurately comparative study was carried out on NTO, FTO, NFTO in order to clarify the roles of N/F atoms played in the Vis-driven photocatalysis. Attention was focused on their experimentally obtained characteristics. The findings gained in this study are helpful for the tailoring of a novel Vis photocatalysts with high performances. EXPERIMENTAL DETAILS Synthesis NTO, FTO, NFTO were synthesized by SP from a mixed aqueous solution of TiCl4 and N-precursor (H2NCONH2), F-precursor (HF), or N-F-co-precursor (NH4F). The mixed solution was first atomized by a nebulizer; then the formed droplets passed through a high-temperature tube under the suction of an aspirator. The pyrolysis proceeded quickly as droplets passed through the high-temperature tube. The produced powder was collected with a ceramic filter at the end of the
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