Preparation And Photocatalytic CO₂ Reduction Property Of G-C₃N₄-based Nanocomposites

With the acceleration of industrial development,the excessive consumption of fossil fuels has led to severe environmental problems and energy challenges.Due to its ability of transforming clean solar energy into chemical energy at room temperature and atmospheric pressure,photocatalytic CO₂ reduction technology bacomes a research hotspot in the field of environmental protection and energy production.Because of its easy availability,steady chemical properties and no secondary pollution,g-C₃N₄ has become a hot reaserch topic.However,with the deepening of research,the defects of g-C₃N₄ are gradually exposed,such as the limited light absorption capacity,the low efficiency of electron-hole separation,which result in the low photocatalytic performance.It is well known that the introduction of cocatalyst is beneficial to broaden the absorption range of photocatalyst,reduce the recombination rate of photocarriers and improve its photocatalytic activity.In addition,the latest research found that photothermal conversion can not only improve the light absorption performance,but also provide additional energy for the reaction,which is conducive to the occurrence of catalytic reaction.However,there are few related studies,especially the reduction of CO₂ by photocatalysis.Therefore,by introducing special promoter and photothermal conversion effect,a new heterojunction was constructed to optimize the photocatalytic CO₂ reduction performance of g-C₃N₄,and its related mechanism was studied in depth.The main research contents and achievements are listed as follows:（1）In this study,2D/2D Bi₂Se₃/g-C₃N₄ nanoheterojunction composite photocatalysts were prepared by direct ultrasonic method.The visible light catalytic performance of g-C₃N₄ was improved by photothermal effect.The results of temperature test showed that the narrow band gap Bi₂Se₃ can not only absorb visible and infrared light and convert it into heat and improve the temperature of the reaction system,but also reduce activation energy and promrte the reaction.According to the XPS analysis and DFT calculation results,Bi₂Se₃ played a cocatalytic role in the photocatalytic reation.Due to the potential difference between Bi₂Se₃ and g-C₃N₄,the aggregated electrons migrated from the g-C₃N₄ conduction band to the Bi₂Se₃conduction band.At the same time,there was an obvious covalent interaction between triazine C atom of g-C₃N₄ and Bi-Se layer,which constructed a charge transfer channel from g-C₃N₄ to Bi₂Se₃ nanosheet,greatly reduced the recombination rate of electrons and holes,and improved the photocatalytic CO₂ reduction performance.The experimental results of photocatalytic activity further confirmed the formation of2D/2D heterostructure,and 5wt%Bi₂Se₃/g-C₃N₄ showed the best photocatalytic activity.Under full spectrum irradiation,5wt%Bi₂Se₃/g-C₃N₄ presented the optimum photocatalytic performance for CO₂ reduction and the CO yield was about 6.3 times than that of pure g-C₃N₄.And the product of CO₂ reduction was selectable.After four times of recycling,the 5 wt%Bi₂Se₃/g-C₃N₄ composite catalyst still had high photocatalytic activity.（2）CoNiS_x modified g-C₃N₄ was prepared by one-step hydrothermal method.The DRS test confirmed that the introduction of CoNiS_x broadened the absorption range of visible light and improved the solar light utilization of g-C₃N₄.According to the XPS analysis,Ni³⁺and Co³⁺exist in the composite catalyst made it easier for electrols to migrate from the conduction band of g-C₃N₄ to that of CoNiS_x,which inhibited the recombination of electrons and holes,and improved the photocatalytic activity.Besides,the yield of CO and CH₄ could reach 11.77μmol/（g·h）and 0.904μmol/（g·h）respectively and the yield of CO was 11 times than that of single g-C₃N₄,which was because the synergistic effect of two transition metals in the CoNiS_x-CN composite can protect the redox ability of the material.Moreover,after four cycles of experiments,the CoNiS_x-CN composite still had high activity.In conclusion,the work of this paper provides a theoretical and practical reference for the development and modification of photocatalytic CO₂ reduction catalyst and recognition of the mechanism of photocatalytic CO₂ reduction.