Modification Of G-C₃N₄-Based Nanocomposites And Reaserch On The Product Of Photocatalvtic Reduction Of CO₂

Due to the explosive growth of the population and the over-advanced industrialization process,people are facing global energy and environmental problems,and it is urgent to explore and develop sustainable and green energy.Solar energy is the amazing gift from nature.However,due to its continuity and instability,solar energy can not be directly used as the industrial and household energy supply.Therefore,people turned their attention to the field of solar energy conversion.The use of semiconductor photocatalysts to catalytically reduce CO2 to produce CO and other hydrocarbon products is an effective way to absorb and convert solar energy.However,the photocatalysts of traditional semiconductor materials generally have the problems of low utilization efficiency of visible light,photo-corrosion,insufficient catalyst stability,and serious failure of effective separation of photogenerated carrier recombination,which greatly limits the practical application of semiconductor photocatalysts in energy.As a new type of polymer,g-C3N4 has 2D graphene structure and excellent electronic properties.Because of its suitable band gap（2.70 e V）,it has visible light response and can effectively utilize solar energy to reduce CO2.Moreover,g-C3N4does not contain metal elements and it has stable thermal stability and chemical corrosion resistance.In conclusion,it is an ideal semiconductor photocatalyst.However,the bulk g-C3N4 has small specific surface area,poor liquid phase dispersion,easy recombination of photogenerated carriers,and unsatisfied photocatalytic activity.Therefore,the main purpose of this work is to modify g-C3N4 to improve its catalytic performance for photocatalytic reduction of CO2.The research is mainly divided into the following aspects:（1）The morphology of g-C3N4 was controlled by microwave synthesis extraction instrument by using Si O2 hard templates.The hollow structure can provide larger specific surface area and more active sites for the photocatalyst spot,and light can be scattered and refracted in the cavity,which improves the utilization rate of light.The catalytic performance of the photocatalyst is optimized through nanostructure engineering.（2）On the basis of the hollow g-C3N4,a hydrothermal synthesis route was adopted to make the 2D material Cu In5S8 in situ-grow on the hollow g-C3N4,and heated with the help of a rapid heating furnace that reach to 450°C within 5 minutes.The furnace reduces the sample in H2 atmosphere to achieve the introduction of sulfur vacancies.After 6 hours of simulated visible light irradiation on the prepared photocatalyst hollow g-C3N4@Vs-Cu In5S8（CN@Vs-CIS）heterojunction,the ratio of CH4 to CO yield is 3.8:1(The yield t of CH₄ is 28.8μmol g^-1 and the yield t of CO is 7.5μmol g^-1),showing a high selectivity relative to CH4.DFT,in-situ DRIFTS technology and other photoelectric performance test results all prove that the photocatalytic performance of CN@Vs-CIS heterojunction is significantly improved compared with hollow g-C3N4.（3）By using electrostatic self-assembly method,it is successful to combine the modified Cu-In-Zn-S four-element quantum dots with hollow g-C3N4 to obtain composite hollow g-C3N4@Cu-In-Zn-S Quantom Dots（CN@CIZS QDs）.After 6hours of simulated visible light irradiation,the results showed that the yield of CO was22.4μmol g^-1,the output of CH₄ was 13.5μmol g^-1.Compared with hollow g-C₃N₄(the yield of CO was 8.52μmol g^-1,and the yield of CH₄ was 4.8μmol g^-1),the photocatalytic performance has been improved nearly three times.The reason for the optimization of the catalyst performance is not only due to the widening of the photoresponse range and the nanostructure engineering,but also because of the multiple exciton effect of the quantum dots,which induces more photogenerated electron-hole pairs after being activated,and the difference in the energy band structure the quantum dots and of hollow g-C3N4 greatly reduces the distance of carrier migration,and thus largely improves the separation efficiency of photoinduced electron-hole pairs,at the same time weakens their recombination.In-situ DRIFTS technology and other photoelectrochemical performance test results also strongly proved the significant optimization of the catalytic performance of CN@CIZS QDs nanocomposites.The research work combines the controllable construction of nanostructures,defect engineering,doping,construction of heterostructures and other modification methods to prepare a composite catalyst with greatly improved photoactivity,which can be well used for the architect,preparation and characterization of g-C3N4 photocatalyst.