The Preparation, Characterization And Visible-light Catalytic Performance Of Magnetically Separable TiO₂Nanotubes

Semiconductor photocatalysis with a primary focus on TiO₂as a cheap,nontoxic, and highly efficent photocatalyst has been applied to a variety ofproblems of environmental interest in addition to water and air purification.However, titanium dioxide has a wide band gap （3.2eV）, and can only absorbs3-5%of the spectrum of the incoming sunlight in the near ultraviolet. Thus, theshift in the optical response of TiO₂from the UV to the visible spectral range willhave a profound positive effect on the practical applications of the material. Onthe other hand, since photocatalysts are often applied as suspensions, andconventional techniques （such as filtration） are not efficient because of the nanosize of the catalyst particles, so it is often difficult to isolate and separate after thereaction has completed. However, it is essential to recover and reuse thenanocatalysts, because a certain amount of residual Ti will not only affecteconomy of the wastewater treatment and the cost of the catalyst, but also causethe secondary pollution of the treated water. Thus, the chanllenging issue of easyphotocatalyst recovery and reuse is another main objective.In this paper, the synergistic effects of N doping and iron oxide couplingtitanium dioxide nanotubes by the decomposition of methyl orange （MO） in airunder visible light were investigated. The properties of these hybrids werecharacterized by XRD, TEM, BET, UV/Vis DRS, XPS and FTIR. Meanwhile, themagnetically separable photocatalyst were recycled by external magnetic field andthe renewable photocatalytic activity was evaluated. The main content andviewpiont are as follow:（1）The TiO₂nanotubes was prepared by hydrothermal method using TiO₂nanoparticles as a raw materials, and subsequently N-doped TiO₂nanotubes wassynthesized by immersed into an aqueous NH3solution and then annealed in air at573K. The XPS spectra showed that the content of nitrogen is about1.7at.%andthe state of nitrogen doped should be the formation of N–Ti–O structure,indicating that nitrogen atoms were doped into lattice oxygen TiO₂nanotubes. （2） γ-Fe₂O₃/TiO₂nanotubes catalyst was firstly prepared by an impregnationiron pentacarbonyl Fe（CO）5, vacuum thermolysis and oxidation method. Thecharacterizations revealed that iron oxide nanoparticles were decorated in/aroundnitrogen-doped TiO₂nanotubes, and most of the nanotubes keep their crystallinephase and crystal structure, in addition, the concentration of Fe is1.4%and firmlysupports the formation of γ-Fe₂O₃.（3）Magnetically separable γ-Fe₂O₃/N-TiO₂nanotubes catalyst wassynthesized for the first time by a soft and wet chemical process. The UV/VisDRS showed that the prepared photocatalysis exhibits stronger absorbance invisible area at400to600nm wavelength.（4） The photocatalytic activities of the different samples were investigated bydetecting the decomposition MO solutions under visible light irradiation. Theresults show that the order of samples’ photocatalytic activities is as follows:γ-Fe₂O₃/N-TiO₂NTs> N-doped TiO₂NTs> TiO₂NTs> P25, indicating theexistence of a synergistic effect of nitrogen codoping and iron oxide coupling,which leads to production of new states, effectively narrowing the band gapbetween the valence band and conduction band. Additionally, the iron oxide withchanging valencies in the N-doped TiO₂NTs samples could act as trapping sites,which would effectively decrease the recombination rate of photo-inducedelectrons and holes and then increase the photo-oxidation efficiency of thecatalysts.（5） The room temperature （300K） magnetic hysteresis curves of themagnetically separable catalyst samples indicates its ferromagnetic property withthe saturation magnetization （Ms） of12.52emu g1. This value is sufficiently highfor effective magnetic separation of photocatalyst. The hybrids recycled byexternal magnetic field using many times showed that the photocatalyticefficiency was almost same as that of the first time. This conclusion will help todesign more effective magnetically separable visible-light photocatalyst.