Preparation And Modification Of CoFe₂O₄@rGO@TiO₂ As A Magnetic Recovery Photocatalyst

In this paper, the S,N co-doped magnetic recoverable CoFe₂O₄@rGO@TiO₂ composites were prepared to improve the recycling and low visible-light photocatalytic activity of nano-TiO₂ catalyst. Firstly, the magnetic-graphene composites CoFe₂O₄@rGO（CFG） were prepared via a facile one-step hydrothermal method, then the TiO₂ coated CFG ternary composites CoFe₂O₄@rGO@TiO₂ （CFGT） were synthesized by a modified vapor-thermal method utilizing TBOT as Ti source. Using the degradation rate of methyl orange （MO） as the evaluation factor, the prepare conditions of CFGT such as reaction temperature and time, dosages of GO, TBOT and deionized water had been optimized.The results showed that the prepared CFGT exhibited enhanced UV-light photocatalytic activity, when the mass ratios of CoFe₂O₄ to GO, CFG to TBOT were 5:1,1:25, the dosage of deionized water was 20 ml and the reaction was conducted at 180℃ for 16 h. FETEM showed that CFGT had a core-shell structure and its diameter was about 220 nm,the shell about 40 nm. VSM displayed CFGT possessed superparamagnetism and its max saturation magnetization was 3.0 emu·g-1.To improve the visible-light photocatalytic activity of CFGT, N-doped CFGT catalysts （CFGT-N） and S,N co-doped CFGT catalysts （CFGT-S/N） were also synthesized utilizing urea and thiourea as dopant, respectively. The results showed that N doping could enhance the visible-light photocatalytic activity of CFGT to a certain extent.However S,N co-doping could greatly improve the visible-light photocatalytic activity of CFGT. The optimum prepared conditions of CFGT-S/N were that the molar ratio of sulfur to titanium was 1:3, reaction temperature was 180 ℃ and time was 16 h. The degradation rates of methyl orange were 100% both under UV irradiation for 2 h and visible light for 6 h. Particularly, its visible light photocatalytic activity was superior to that of P25. In addition, the CFGT-S/N showed excellent UV-Vis light photocatalytic activity in the degradation of organic pollutants, including methyl orange （MO）, methylene blue （MB）and Rhodamine B （Rh.B）, which was of great importance to the practical application. As a result, the obtained CFGT-S/N exhibited good magnetic response and the reusability study suggested that CFGT-S/N was stable enough and the removal rates of MO were up to 96.1% and 93.1% after five recycles, under UV and visible light illumination,respectively. XRD showed TiO₂ was anatase and CoFe₂O₄ was spinel structure. FETEM illustrated that CFGT-S/N was a spherical particle with core-shell structure and its diameter was about 160-200 nm, the shell about 15 nm. XPS and FT-IR studies confirmed that S and N were co-doped into the lattice of TiO₂ to form S-Ti and O-Ti-N, respectively.The UV-Vis DRS spectra of CFGT-S/N displayed a red shift of the absorption edge, its energy band gap was about 1.73 eV and the absorption of visible light was enhanced.VSM showed the max saturation magnetization of CFGT-S/N was 1.7 emu g-1, which can be easily recycled by an external magnetic field. The higher UV-Vis light photocatalytic activity of CFGT-S/N can attribute to the synergetic effects of strong absorption to visible-light and narrow band gap induced by S,N co-doping and narrow semiconductor CoFe₂O₄.Moreover,the rGO middle layer between TiO₂ shell and CoFe₂O₄ core can effectively prevent the photo-dissolution of catalyst, and enhance the enrichment of pollutant dye molecules.