| Titanium dioxide is a widely used multifunctional semiconductor material due to its excellent stability and physicochemical properties and low cost.It is widely used in the degradation of organic pollutants,photocatalytic water decomposition,electrocatalysis and solar cells.The band gap of anatase TiO2 is Eg=3.2 eV,it means that the transition of electrons excited from the valence band to the conduction band while electron-hole pairs generate only when the photon energy hv?3.2 eV(the wavelength of light radiation X<387 nm).This means that the response wavelength of TiO2 is in the ultraviolet region,about 5%of the total energy of solar radiation.The time of electron-hole pair recombination is generally between 10 ns and 100 ns,which is much less than the time required for them to be captured and transferred(100 ns to 1 ms).Therefore,most of the photogenerated carriers disappear before they migrate to the surface of the catalyst,resulting in a decrease in photocatalytic activity.Generally,the size of the nanophotocatalyst is small due to the smaller particle size means a higher specific surface area,which is advantageous for the photocatalytic reaction.However,smaller particles tend to form a suspension in the solution,which makes it difficult to separate after sewage purification and lead to secondary pollution.In summary,how to broaden the spectral response range of TiO2 catalysts;suppress the recombination of photogenerated carriers and holes and separate and recycle catalysts become problems that researchers need to solve.According to the above problems,researchers have finished a lot of works on TiO2 modification,morphology controlling,TiO2 composites and their applications.However,there are some conflicting or contradictory conclusions due to the complicating photocatalytic reaction process and non-completely explained mechanisim.Therefore,many difficult problems still need to be solved by researchers:such as the understanding of photocatalytic reaction mechanism of TiO2 modification and composites,improving the charge transfer efficiency of TiO2,promoting photo-generated carrier separation,the difficulty of the catalyst recycling,etc.So we have carried out various methods of compounding,doping modification,crystal plane controlling and steam thermal preparation for TiO2,in order to further researching the above problems.The main research is as follows:1)The Fe3O4@TiO2 catalyst was initially synthesized by a hydrothermal method,followed by reducing in a mixed H2/N2 atmosphere at temperatures of 400,600,800 and 1000 ? in order to produce the oxygen vacancies(Ov)and Ti3+which can significantly affect the photocatalytic performance;Simultaneously,Fe3O4 was reduced to Fe,a strongly magnetic material,beneficial for the magnetic separation after the photo-degradation.The optimal catalyst was obtained at the reducing temperature of 800 ?;Its degradation efficiency De(99.6%)to the methylene blue and apparent reaction rate constant kapp(4.70×10-min-1)are both larger than those(99.0%and 4.20 × 10-2 min-1)of the commercial P25,a benchmarking material among the photocatalysts;Moreover,its saturation magnetization Ms is highest,reaching 23.8 emu/g which improves the magnetic separability.This optimal catalyst was subsequently treated in the NH3 atmosphere at temperatures of 500,600 and 700 ?,aiming to investigate the effects of N-doping in TiO2;The 600 ? treated catalyst exhibited the optimal photocatalytic performance.The factors that affect the photocatalytic performance are revealed and discussed in detail,including the ratio of Ov and N dopant in TiO2 as well as the interface states between TiO2 and the magnetic particles.2)Anatase TiO2 nanoparticles were synthesized via a self-developed ethanol vapor-thermal method at 240 ?(T240)and 250 ?(T250),respectively lower and higher than the super-critical temperature(243.5 ?C,7.0 MPa)of ethanol.Compared with T240,T250 has a higher ratio of exposed(001)facets,oxygen vacancies and concomitant TiOx.The specific area of T250 is 119.0 m2g-1,smaller than that of T240(144.2 m2g-1).During the degradation of methylene blue,T250 exhibits the high apparent rate constant(Kapp)(14.5×10-2 min-1),6.3 times larger than that of T240;Furthermore,T250 shows the more superior degradability to phenol than T240.Results of ESR and PL indicate that the photo-generated electron-hole pairs possess the higher separation efficiency in T250 than in T240.In summary,the excellent photocatalytic performance of T250 originates from the higher ratios of exposed(001)facets,oxygen vacancies or TiOx,C=O groups absorbed at the surface of particles and the higher separation efficiency of the photo-generated electron-hole pairs.Using such the self-developed vapor-thermal method,a variety of catalysts and their composites were synthesized,and they exhibited the novel morphology and properties as well as the excellent photocatalytic performance.3)TiO2/CNT composites with different masses of CNTs were synthesized via ethanol vapor-thermal method at 230 ? and 260 ??Subsequently samples were characterized by XRD,TEM,HRTEM,BET,FT-IR,UV-vis DRS,XPS,fluorescence lifetime,photocatalytic degradation and ESR.The results show that some samples are uniformly and closely grown on the CNT(TEM),while the lattice of TiO2 and CNT is distorted(XRD,FT-IR),accompanied by changes in elemental chemical states and valence band states(XPS and valence band XPS).The visible light absorptive capability of TiO2 is enhanced significantly by the combination of carbon nanotubes and TiO2(UV-vis DRS).The reduction of fluorescence lifetime means that the recombination of photo-generated carriers is inhibited.Therefore,the composite of CNT and TiO2 can realize the rapid migration of carriers,thereby inhibiting the photo-electron-hole recombination,and increasing the concentration of oxygen vacancies on the surface of TiO2,thereby improving the photocatalytic performance. |
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