Modification Of Carbon Nitride By Chalcogenide Compounds For Photocatalytic CO₂ Reduction

Photocatalytic CO₂ reduction is a sustainable technology that uses solar energy to convert CO₂ into CO,CH₄ and other fuels.It shows great application potential for alleviating China’s energy crisis and reducing carbon emissions in the future.Polymer Carbon Nitride（PCN）is a kind of non-metallic conjugated polymer semiconductor photocatalyst material,which has been widely studied in the field of photocatalysis CO₂reduction due to its advantages of suitable energy band structure,simple preparation,stable chemical properties,environmental protection and pollution-free.However,PCN still has problems such as insufficient absorption of visible light and easy recombination of photogenerated carriers,which leads to its low photocatalytic activity and hinders its industrial application.Based on this,this paper takes PCN as the research object,and modifies it with chalcogenide compounds.Three kinds of heterogeneous photocatalysts,Ru S₂/PCN,In₂Se₃/PCN and Co Te/PCN,were designed and synthesized respectively.Their microstructure and morphology,photophysical properties,photoelectrochemical properties,electrochemical properties,photocatalytic CO₂ reduction performance and reaction mechanism were systematically studied.The details are as follows:（1）The 3D/2D Ru S₂/PCN heterostructure photocatalyst was constructed by coating Ru S₂ nanospheres into PCN nanosheets by in-situ growth and in-situ polymerization.The modification of Ru S₂ improves the light absorption ability of PCN,and the type II heterostructure formed by Ru S₂ and PCN promotes the efficient separation of photogenerated electron-hole pairs.In addition,the special microstructure of Ru S₂ nanospheres encapsulated by mesoporous PCN nanosheets not only effectively improves the transfer and separation efficiency of photogenerated carriers,reduces the mass transfer resistance,promotes the migration of CO₂ molecules to the active sites on Ru S₂ through the mesoporous channels,and reacts in the closed space formed by PCN nanosheets,thus improving the selectivity of photocatalytic CO₂ reduction.The yield of photocatalytic CO₂ reduction to CO of the best sample Ru S₂/PCN-5 is about4.2 times that of PCN,the selectivity is close to 100%,and the activity has not been significantly reduced after 16 hours of cycle experiment.（2）PCN nanosheets modified by In₂Se₃ spheres were synthesized by solvothermal method,and 3D/2D In₂Se₃/PCN heterostructure photocatalyst was constructed.The modification of In₂Se₃ enables the composite to convert the absorbed light energy into local heat energy,generate hot electrons and reduce the activation energy of the reaction.In addition,the Z-scheme heterojunction formed between In₂Se₃ and PCN promotes the efficient separation of photogenerated electron-hole pairs,improves the utilization rate of photogenerated electrons,and further improves the photocatalytic activity.The yield of the best sample In₂Se₃/PCN-15 photocatalytic reduction of CO₂ to CO and CH₄ is12.88μmol g^-1 h^-1 and 0.53μmol g^-1 h^-1,respectively,which are 11.4 times(1.13μmol g^-1 h^-1)and 10.6 times(0.05μmol g^-1 h^-1)of PCN,respectively.At the same time,the sample can still maintain high photocatalytic activity after four groups of 16 h cycle experiments.（3）1D/2D Co Te/PCN heterostructure photocatalyst was prepared by hydrothermal method and electrostatic self-assembly method.The modification of Co Te nanorods not only expanded the absorption of PCN to visible and near-infrared light,reduced the band gap,but also reduced the resistance of interface charge transfer and accelerated the interface charge transfer due to its excellent conductivity.In addition,the Z-scheme heterostructure formed between Co Te and PCN promotes the rapid separation and efficient separation of charges,thus enhancing the photocatalytic activity of CO₂reduction.The photocatalytic CO₂ reduction experiment showed that the CO yield of the best Co Te/PCN-10 sample was 8.21μmol g^-1 h^-1,which was 3 times that of PCN,and the photocatalytic performance of the sample did not decrease significantly after16 h of cyclic operation.