Investigation On The Interface Controlling And N/S Doping Modification Of TiO₂/graphene Composites

The tetragonal symmetric anatase TiO₂ has been widely investigated in the fields of degradation of organic pollutants,photocatalytic water splitting,electrocatalysis and solar cell,because of its stable physicochemical properties,nontoxicity,low-cost,environment-friendly and abundant reserve.The conduction band of anatase TiO₂ is composed of the Ti 3d state and valence band is mainly contributed by the 0 2p states and the band gap energy?Eg?is about 3.2 eV.Therefore,the excitation of electron-hole pair requires photons energy hv?3.2 eV or wavelength ??387 nm.It means that the photoresponse of TiO₂ ranges in the ultraviolet region whose energy is less than 5%of the total energy of solar spectrum.The low utilization of sunlight limits the practical application of TiO₂.Meanwhile the photo-generated electron-hole pairs will recombine and vanish away in 10?100 ns,much shorter than the time for electrons to be captured and transfer?100 ns?1 ms?,so the majority of carriers cannot migrate to the surface of the catalyst,where the photocatalytic reaction occurs,consequently reducing the photocatalytic reactivity.As a result,how to restrain the carrier recombination becomes the key issue.In addition,the TiO₂ photocatalyst usually has a small particle size;Small size is favorable to the photocatalytic reaction because of the high surface area and more reactive sites.However,small particles tend to form a suspension in the solution,which are difficult to recycle after water purification treatment,and remain in the water.This will cause the secondary pollution.The issue of catalyst recycling is also a challenging work.In order to solve these problems,researchers have made a lot of efforts in the synthesis,modification and application of TiO₂ and its composites,and obtained many meaningful research results.However,for TiO₂ composites,photocatalytic reaction is very complex,because there are many parameters that affect the photocatalytic activity.In previous reports,many conclusions are contradictory and conflicting.There are still many difficult problems to be solved,such as how to realize the intimate contact between two phases in the synthesis of TiO₂ composites,or revealing its photocatalytic reaction mechanism and so on.The etherification of polyol during vapor-thermal reaction provides water for the hydrolysis of T⁴⁺precursor,beneficial for controlling the hydrolysis rate.The research was carried out using this synthesis method,and the main research contents are as follows:1.Study on the vapor thermal synthesis and photocatalytic properties of TiO₂/graphene composites.For TiO₂/graphene,to achieve intimate contact between two phases remains a big challenge.In this work,using the self-developed vapor-thermal method,and titanium???tetraisopropanolate as the titanium source,TiO₂/graphene composites were synthesized in the isopropyl alcohol and deionized water vapor.For the samples synthesized in isopropanol vapor,the SEM and TEM results show that TiO₂ exhibits the nano-sheet morphology,and TiO₂ nanosheets uniformly and intimately grow on graphene.The lattice distortion was observed in XRD and FTIR results,further confirming the intimate contact between two phases.The valence band density of states?DOS?of TiO₂ varies with increasing the mass of graphene due to the lattice distortion and intimate contact between two phases.The results of UV-vis DRS,BET and PL showed that the combination of graphene and TiO₂ significantly enhanced the visible light absorption,increased BET specific surface area,and inhibited the recombination of photo-generated carriers,thus improved photocatalytic degradation performance.The results of ESR indicated that ·OH radicals had a greater contribution to photocatalytic degradation.For the samples synthesized in the water vapor,TiO₂ exhibits the particle morphology,and TiO₂ particles randomly scattered on graphene,similar to the results reported in literatures.The lattice distortion of TiO₂ was not observed.Although the TiO₂/graphene enhanced the visible light absorption and promoted the separation of photogenic carriers,the specific surface area decreased and the photocatalytic performance was not improved.These results show that:the intimate contact between two phases is a prerequisite for achieving synergistic enhancement effects of graphene and TiO₂,which requires the improved synthesis method.This synthesis method further improves the photocatalytic performance and provides an opportunity to discover new properties.2.Study on vapor thermal synthesis and N doping of TiO₂/graphene composites.Firstly,intimatly contacted TiO₂/graphene composites were synthesized in the vapor environment of isopropanol,and then doped with N in different N/Ti molar ratios.The existence states of doped N elements were as follows:in the interstitial sites of TiO₂ lattice,substituting O element in TiO₂,substituting C element in graphene,and adsorbed at the surface of graphene and TiO₂ particles.The effects of N doping can be summarized as follows:N doping changed TiO₂ nano-sheet to nanoparticles,accompanying with the reduction of the specific surface area of composites;The paticle size of TiO₂ increased with the increase of N doping degree,then decreased;The vibration modes of Ti-O-Ti were changed by N doping;N doping affected the chemical states of Ti and O elements in TiO₂ and introduced a variety of defects or impurity levels.The synergistic effect of these factors affects the photocatalytic performance.Only when N/Ti=2,the photocatalytic degradation performance to methylene blue was enhanced,and the degradation rate was increased from 7.7×10^-2 min^-,?without N doping?to 9.6×10^-2 min^-1.The results of ESR analysis showed that photo-generated electrons were captured by defects and impurities,and photogenic holes played a major role in the photocatalytic degradation.3.Study on the vapor thermal synthesis and S doping of TiO₂/graphene composites.Firstly,TiO₂/graphene were synthesized in the vapor environment of isopropanol by using the polyol etherification reaction to provide water for the hydrolysis of Ti sources.The intimate contact between TiO₂ and graphene was achieved,and TiO₂ presented a nano-sheet morphology.Then,TiO₂/graphene composites were doped with S in the water vapor by changing the molar ratio of S/Ti with thiourea as the S source,and a series of S-doped TiO₂/graphene composites were obtained.The composites were systematically characterized by means of XRD,TEM,FT-IR,XPS,UV-vis DRS,PL,BET and ESR to study the influence of S doping on the photocatalytic degradation performance.The changes of morphology,chemical state,crystal structure,electronic structure and photocatalytic performance depending on S doping degree were observed.The main conclusions are as follows:?1?S-doping transformed TiO₂ nano-sheets into nano-particles;?2?there were four chemical states for the doped S elements,i.e.,S^2-,S,S⁴⁺ and S⁶⁺.Additionally,the S element also had an influence on the chemical states of Ti and O in the TiO₂ lattice.When R_S/Ti?0.5,only elemental S was detected and there was no substitution of S for Ti and O in TiO₂.When R_S/Ti?1,S₂^-replaced O^2-and S⁴⁺/S⁶⁺ replaced Ti⁴⁺;?3?S-doping introduced electronic states above the valence band maximum of TiO₂;?4?the crystal structure of TiO₂ was distorted,and the lattice strain was maximum when R_S/Ti=3;?5?S doping increased the degradation rate of TiO₂ to the methylene blue?MB?pollutant from 7.7×10^-2 min^-1 to 10.7×10^-2 min^-1.The corresponding S-doped samples had the highest amount of ·OH/·O₂^-free radicals,which plays an important role in the MB degradation.4.Study on the synthesis and photocatalytic properties of CoFe₂/CoFe₂O₄/TiO₂.The purpose of this work is to study the influence of the interface between the magnetic core material and the catalyst on the photocatalytic mechanism.The CoFe₂ alloy has the highest magnetization among all binary alloys,which is beneficial to magnetic separation.First,well-dispersed uniform CoFe₂O₄ nanoparticles were prepared and coated by MgO through thermal decomposition of a metal-organic salt in organic solvent.CoFe2/CoFe₂O₄/MgO architectures were synthesized following the route from CoFe₂O₄/MgO to CoFe₂/MgO by reducing in a H₂/N₂ mixture gas and subsequently to CoFe₂/CoFe₂O₄/MgO by oxidized in air.MgO can be dissolved by the HCl solution.Next,the surfaces of CoFe₂O₄,CoFe₂/MgO,CoFe₂ and CoFe₂/CoFe₂O₄ magnetic particles were functionalized by TiO₂,and their magnetism,photocatalytic performance,separability for recycling and temperature effects were investigated.The rattle-type particles with magnetic core and TiO₂ shell were obtained without the assistance of template and etchant.The photocatalytic activity of these photocatalysts in methylene blue?MB?degradation showed that the surface supporting capacity of TiO₂ of different catalysts was different,resulting in different photocatalytic performance.The nanosheet-shaped TiO₂ and rattle-type particles exhibited good photocatalytic performance.The CoFe₂/TiO₂ sample had the highest degradation efficiency of 93%because CoFe₂ can capture the photo-generated electrons in TiO₂,inhibiting the recombination of electron and hole;meanwhile,it has the highest magnetization value of 42 emu/g,beneficial for the recovery of catalyst after degradation.Besides the photocatalytic application,these strong magnetic particles,which can be guided easily by the external magnetic field,may be potential applications in other fields,such as the cell isolation and targeting drug.