Preparation Of G-C₃N₄-based Composite Materials For Photocatalytic Production Of Solar Fuel

At present,the problems of environmental pollution and energy crisis are becoming more and more serious,which have caused serious harm to the ecosystem on which human beings depend for survival.Semiconductor photocatalysis technology can use solar energy for environmental purification and fuel conversion,and has attracted wide attention of researchers.g-C₃N₄semiconductor photocatalytic material has become a research hotspot in the field of photocatalysis because of its unique stability,non-toxicity,wide source of raw materials and easy preparation.However,pure g-C₃N₄ has a low photocatalytic performance due to its easy recombination of photoelectron-holes.Therefore,it is usually necessary to prepare g-C₃N₄ composites to improve the separation efficiency of charge carriers.In order to improve the efficiency of g-C₃N₄ photocatalytic synthesis of solar fuel,three kinds of g-C₃N₄-based composite photocatalytic materials were prepared in this paper.The details are as follows:1.Step-scheme（S-scheme）heterojunctions of S-doped g-C₃N₄（SCN）and N-doped Mo S₂（NMS）were constructed by one-step thermal polycondensation method using thiourea and ammonium tetrathiomolybdate as starting materials in Ar atmosphere.The composite material was applied in photocatalytic hydrogen evolution under simulated sunlight.Within 4 h of illumination,the H₂ generation rate of the optimized NMS/SCN reached 658.5μmol/g/h,which was about 23 and 38 times higher than that of pure SCN（28.8μmol/g/h）and NMS（17.4μmol/g/h）,respectively.Material characterization combined with theoretical calculation results show that the formation of S-scheme heterojunctions not only expands the absorption range of visible light of g-C₃N_4,suppresses the combination of electron and hole,and retains the strong oxidation reduction ability of the two components,so the photocatalytic hydrogen evolution performance is greatly improved.2.Considering that the performance of traditional g-C₃N₄ photocatalytic reduction of CO₂ is not ideal,we used the diazotization reaction to convert the surface amino group to diazanyl,and at the same time,in-situ depositing SnO₂ nanoparticles,and synthesized the diazanyl and SnO₂ bi-activated g-C₃N₄（SnO₂/Hy UCN）.SnO₂/Hy UCN showed the highest photocatalytic CO₂ reduction performance,its CO generating rate reached 21.5μmol/g/h,which was 6 and 4.1 times higher than that of the pristine g-C₃N₄（UCN）and SnO₂/UCN,respectively.CO₂ adsorption-desorption test and CO₂ adsorption energy comparison based on DFT calculations revealed the enhanced CO₂ adsorption of SnO₂/Hy UCN.The time-resolved photoluminescence（PL）,surface photovoltage spectrum（SPV）and electrochemical tests revealed that SnO₂/Hy UCN suppressed the recombination of photogenerated electron-hole pairs.Furthermore,the photocatalytic mechanism was discussed at molecular level by using in-situ fourier transform infrared（FT-IR）spectroscopy.3.In-situ photodeposition was used to successfully prepare oxygen-vacancy molybdenum oxide(MoO_3-x)nanoparticle modified graphite-like carbon nitride（g-C₃N₄）photocatalyst.The efficiency of photothermal synergistic catalysis CO₂ conversion was improved by capturing the near-infrared（NIR）light photons.In addition,the existence of oxygen vacancies can expose more active sites,promote CO₂ adsorption on the catalyst surface,improve the charge carrier separation efficiency,and improve photocatalytic CO₂conversion activity.Photocatalytic CO₂ conversion experiment showed that the photocatalytic conversion activity of CO₂ was the highest with MoO_3-xloading increased to1wt%(1%-MoO_3-x/g-C₃N₄).After under UV-Vis-IR light illumination for 2 h,the CO generation rate of 1%-MoO_3-x/g-C₃N₄ reached a value of 37.19μmol g^-1,which is about3.6 times that of pure g-C₃N₄(10.29μmol g^-1).This paper provides a new idea and a new research method for the preparation of g-C₃N₄-based composite photocatalytic materials to generate solar fuel.