Construction Of Fe₃O₄@SiO₂@Sn-TiO₂ Composite Photocatalyst And Its Photocatalysis On Tetracycline Hydrochloride

TiO₂ has been widely used in heterogeneous photocatalytic technology due to its no secondary pollution,strong oxidation capacity,photochemical corrosion resistance and so on.However,there are some challenges for the practical application of TiO₂owing to the high recombination rate of the photoelectron-holes and the hard separation of powder TiO₂from suspension.These limit the industrial application of heterogeneous photocatalytic technology based on nano-TiO₂.In view of the above,in order to reduce the recombination rate of the photoelectron-holes and improve the recovery rate of powder TiO₂from suspension,this study synthesized core-shell magnetically separable Sn-TiO₂composite photocatalyst.The effect of Sn dopant on the coating process of TiO₂on the surface of Fe₃O₄@SiO₂particles and the optimal heat treatment conditions of core-shell magnetically separable Sn-TiO₂composite photocatalyst were both studied by adjusting the molar ratio of Ti/Sn,calcination temperature and calcination time.The core-shell magnetically separable Sn-TiO₂composite photocatalyst was characterized by scanning electron microscope（SEM）,transmission electron microscope（TEM）,vibrating sample magnetometer（VSM）,X-ray diffractometer（XRD）,X-ray photoelectron spectroscopy（XPS）,thermogravimetric analyzer（TGA）,Fourier transform infrared spectrometer（FT-IR）and physical adsorption/desorption apparatus（BET）.Tetracycline hydrochloride was selected as the target pollutant to study the photocatalytic performance and reusability of the core-shell magnetically separable Sn-TiO₂composite photocatalyst.The experimental results are as follows:（1）The core-shell magnetically separable Sn-TiO₂composite photocatalyst was synthesized by St?ber and Sol-gel methods using tetrabutyl titanate(C₁₆H₃₆O₄Ti,TBOT),tetraethyl orthosilicate(C₈H₂₀O₄Si,TEOS)and stannous chloride dihydrate（Sn Cl₂·2H₂O）as the raw materials.The core-shell magnetically separable Sn-TiO₂composite photocatalyst is composed of Sn-TiO₂shells,uniform spherical Fe₃O₄cores and SiO₂inter layers.（2）The influence mechanism of Sn on the coating process of TiO₂layers on the surface of Fe₃O₄@SiO₂particles was investigated.The experimental results show that compared with the spherical core-shell structures of the magnetically separable Sn-undoped TiO₂composite photocatalyst（Fe₃O₄@SiO₂@TiO₂）,the core-shell structures of magnetically separable Sn-TiO₂composite photocatalyst are large Fe₃O₄@SiO₂@Sn-TiO₂clusters.This may be because some of the Sn²⁺was adsorbed on the surface of Sn²⁺-TiO₂oligomers,which changed the surface charge of Sn²⁺-TiO₂oligomers and then reduced the stability of the oligomers.As a result,Fe₃O₄@SiO₂@Sn-TiO₂particles were agglomerated together by the condensation between their Sn²⁺-TiO₂shells and the Sn²⁺-TiO₂ nanoparticles with low stability.Therefore,the TiO₂content of Fe₃O₄@SiO₂@Sn-TiO₂is higher than that of Fe₃O₄@SiO₂@TiO₂.（3）The effects of Sn doping amount on the crystal structures of TiO₂were studied.The results show that when the Ti/Sn molar ratio was 100:1 and 60:1,and the calcination condition was 500℃for 2 h,the crystallinity of anatase TiO₂increased with the increase of Sn doping amount.The continuous increase of Sn doping amount promoted the transformation of anatase into rutile TiO₂,and the transformation rate also increased with the increase of Sn doping amount.（4）The optimum heat treatment conditions of magnetically separable Sn-TiO₂composite photocatalyst were investigated.The results show that calcination temperature had effects on both the maximum saturation magnetization and specific surface area of the magnetically separable Sn-TiO₂composite photocatalyst,as well as the crystal structures and crystallinity of the TiO₂shells.With the increase of calcination temperature,anatase TiO₂gradually transformed into rutile TiO₂,and the crystallinity of anatase and rutile TiO₂increased.With the increase of calcination temperature,the Fe₃O₄content gradually decreased,leading to the decrease of the maximum saturation magnetization,and the specific surface area of magnetically separable Sn-TiO₂composite photocatalyst decreased.The effects of calcination time on the magnetic Fe₃O₄cores and the maximum saturation magnetization of the magnetically separable Sn-TiO₂composite photocatalyst were investigated.With the extension of calcination time,the content of Fe₃O₄decreased,resulting in the decrease of the maximum saturation magnetization.Based on the excellent photocatalytic activity and magnetic responsiveness of magnetically separable Sn-TiO₂composite photocatalyst prepared at 500℃for 2 h,the optimal calcination condition was determined to be 500℃for 2 h.（5）The photocatalytic activity and magnetic response of magnetically separable Sn-TiO₂composite photocatalyst were evaluated with tetracycline hydrochloride as the target pollutant.The rate constant of first-order photocatalysis to tetracycline hydrochloride of magnetically separable Sn-TiO₂composite photocatalyst with the Ti/Sn molar ratio being 60:1 was 0.103 min^-1,which was 4.75 times higher than that of magnetically separable Sn-undoped TiO₂magnetic composite photocatalyst.This is attributed to the high TiO₂content of magnetically separable Sn-TiO₂composite photocatalyst and the low recombination rate of photogenic carriers.In addition,magnetically separable Sn-TiO₂composite photocatalyst showed excellent recycling performance after6 cycles of photocatalysis,which is attributed to the good magnetic response.