| Since the industrial revolution,with mechanization popularizing and chemical industries rising,environmental issues have been beyond neglect.The conflict between human society development and environment protection has become acute.Study on how to ensure society improvement without the cost of environment destruction,is one of the most important research.Titanium dioxide photocatalyst proves a facile and efficient solution to this problem,for its stable chemical property,low fabrication cost,non-toxicity,recycle ability,and so on.Titania nanoparticles absorb light energy and degrade industrial waste like organic contamination,toxic metal ions,and pollutant gas,through redox reaction.As a typical n type semiconductor,there are four types of crystal structures of TiO2,which are anatase,rutile,brooket,and TiO2(B).Due to its large intrinsic band gap(3.2 eV),only ultraviolet light can be absorbed.There is only 5%of ultraviolet in sunlight.And nanopartiles are difficult to recycle after catalysis treatment,which may cause resource waste and possible contamination.In order to solve these drawbacks of material,improving catalyst activity through modification methods has become our study content.In order to tackle with its recycling difficulty,catalysts can be successfully collected under external field after coated on magnetic nanoparticles.Moreover,the major TiO2 modification methods conclude as doping,coupling,and facets tuning.Among these methods,doping methods are classified as homogeneous ions doping(Ti3+ doing),and heterogeneous ions doing with metal iols or lonlllal ions doping.Besides,multiple ions doping,metal and/or nonmetal ions,has also been widely investigated.Coupling TiO2 with other semiconductors,carbon materials,and noble metal is another research topic.And enhancing catalyst activity by controlling TiO2 exposed facets has been studied in last decade.Although the investigation and application of titania has a half century history,and related research has been widely reported,some of the results mutually conflicted.Because of complex function of photocatalysis and the mechanism unrevealed,further study on the effect of tuning dopant states to photocatalytic performance needs focused.The dopants Mn/V were chosen because the abundant oxidation states(Mn2+,Mn3+,Mn4+,Mn6+,Mn7+;V2+,V3+,V4+,V5+)in transition metals.The dopants’ states were tuned by post reducing of catalysts.And the research contents are listed as follows:1.Study on γ-Fe2O3(Fe,Fe/Fe2O3)@TiO2 photocatalytic performance.Fe2O3@TiO2,Fe/TiO2 and Fe@Fe2O3@TiO2 catalysts are synthesized by the hydrothermal method combined with the sol-gel method.γ-Fe2O3 nanoparticles are initially synthesized,and some are coated with MgO to obtain Fe2O3@MgO particles.MgO is used to prevent the magnetic particles from aggregating in the subsequent high temperature reduction and oxidation processes.To improve the magnetic separability,strongly magnetic Fe@MgO particles are prepared by reducing Fe2O3@MgO in a mixed H2/N2 atmosphere.To investigate the effects of the Fe2O3 intermediate layer on the photocatalytic erformance,Fe@MgO is oxidized in air at 500 and 900 ℃ to prepare Fe@Fe2O3@MgO.MgO is then dissolved in aqueous hydrochloric acid,yielding Fe and Fe@Fe2O3 magnetic particles.The magnetic Fe2O3,Fe,and Fe@Fe2O3 particles are then functionalized with TiO2 to obtain the final photocatalysts.The Fe2O3@TiO2 catalyst exhibits strong adsorption of methylene blue,and the dsorption efficiency reaches 99%within 10 min.The Fe/TiO2 catalyst has the highest degradation fficiency(94%)and apparent reaction rate constant kapp(1.19×10-2 min-1)among the tested photocatalysts.Fe/TiO2 also has the highest saturation magnetization(61.5 emu/g),which is beneficial for magnetic separation.The adsorption efficiencies,degradation efficiencies,apparent reaction rate constants,and saturation magnetizations of the Fe@Fe2O3@TiO2 catalysts are sensitive to the properties of the Fe2O3 intermediate layer.The Fe2O3 particles contain both α-Fe2O3 and y-Fe2O3.Coating TiO2 on Fe@Fe2O3 suppresses the formation of α-Fe2O3.2.The effect of different ambience treatment of γ-Fe2O3@Ti(Mn,V)O2 to photocatalytic performance.Manganese(Mn)-and vanadium(V)-doped titania(TiO2)coated on iron oxide(y-Fe2O3)nanoparticles(TM and TV)were reduced in H2/N2 mixture atmosphere at different reducing temperatures TR.Furthermore,TM and TV reduced at TR= 800 ℃(TM8 and TV8)were subsequently nitrided in the ammonia(NH3)atmosphere at different temperatures TN in order to change the chemical state(CS)of the Mn,V,Fe,Ti,O,and N elements.Samples were characterized by X-ray diffraction and X-ray photoelectron spectroscopy.For reduced samples,with increasing TR,anomalous variation of CS is as follows:Ratios of V4+ and V5+increased,while the ratio of V3+ decreased,and the ratio of oxygen vacancy Ov decreased.Data conflict with the effect of reduction reaction could be understood as follows:Ov was occupied by O from H2O(product of the reduction reaction),according to previous theoretical result.For nitrided samples,compared to TM8 and TV8 samples,as TN increased,ratios of Mn4+ and V5+ with the highest valence state exhibited an increase at TN = 400 ℃.Moreover,the ratio of TiOx decreased and the ratio of TiO2 increased,even though NH3 exhibited good reducibility.Data could be well interpreted as follows:Ov was occupied by N along with adsorbed O,possibly in the form of neutral NO radical and NO22-radical ion.Till date,these phenomena have not been reported experimentally,which are important to understand the mechanism for photocatalytic performance of Mn-,V-,N-,and self-doped TiO2.3.V-free and V-doped H2Tii2O25 hollow spheres(HTOHS)were first synthesized via the vapor-thermal method at 290 ℃ with the commonly used chemicals and solvents,over the super-critical temperature(243 ℃)of ethanol(vapor source).Then,they were annealed at 600 ℃ with different processing conditions to obtain the TiO2 photocatalysts.All catalysts were characterized by means of XRD,SEM,TEM,XPS,UV-vis DRS,FT-IR and fluorescence lifetime.(1)For V-free HTOHS,after the treatment in air,they transform to open-ended hollow tubes with the uniform length～5 μm)and diameter(～1 μm),and the wall of hollow tubes consists of aggregated nanosheets;Furthermore,it crystallizes to the anatase TiO2(white).The treatment in the N2 atmosphere breaks the long tubes into the shorter ones.(2)For V-doped HTOHS,after the treatment in the N2 atmosphere or the reduction using NaBH4 as the reductant,the resulting anatase TiO2(black)samples consist of fewer tubes and more deformed spheres.The fluorescence lifetime(r)of photo-generated carriers corresponds well with the ratio of oxygen vacancy,indicating that the oxygen vacancy dominates the lifetime even though it is very sensitive to many factors.The evolution of structure and morphology and the photocatalytic mechanism were analyzed and discussed. |
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