Construction Of Two-dimensional Ultrathin Carbon Nitride Composite System And Study Of Its Photocatalytic CO₂ Reduction Performanc

The excessive use and rapid consumption of fossil fuels have intensified the excessive emission of carbon dioxide,leading to people’s concerns about energy crisis and the greenhouse effect.Among them,the conversion of carbon dioxide(CO2)into renewable fuels or other value-added chemicals through semiconductor photocatalysis technology offers a promising solution to the problem of energy shortage and serious global warming phenomenon.However,the CO2 conversion efficiency of many semiconductors is not high,mainly due to the weak adsorption of CO2 molecules,the high charge recombination rate of photocarriers and other limitations,so the development of efficient photocatalysts is a great challenge.Graphite carbon nitride(g-C3N4)is widely used as a metal-free photocatalyst because of its chemical stability,availability,low cost and non-toxicity.Among them,two-dimensional g-C3N4 nanosheets have attracted much attention due to their large specific surface area and more active sites.However,g-C3N4 has many disadvantages,including poor visible light response,fast photocharge recombination,and low electrical conductivity.In order to solve these shortcomings of g-C3N4,many strategies have been studied,such as heteroatomic doping,construction of heterogeneous structures and so on.Designing two semiconductor heterojunctions with appropriate band alignment is one of the effective methods to improve the photocatalytic efficiency of semiconductors.Therefore,we construct different two-dimensional ultra-thin g-C3N4 matrix composites by a simple method.By taking advantage of the composite structure,we can broaden the range of light absorption while promoting photogenerated charge separation,and further improve the photocatalytic performance.A Z-scheme CsPbBr3/g-C3N4 composite was designed and synthesized for photocatalytic CO2 reduction.g-C3N4 ultra-thin nanosheets and CsPbBr3 nanosheets were synthesized by hydrothermal method and antisolvent process,respectively.An efficient direct Z-scheme 2D/2D ultra-thin CsPbBr3/g-C3N4 nanosheet was prepared by a simple electrostatic self-assembly process.By oxidizing H2O under AM 1.5G irradiation,CO2 was effectively photocatalyzed to reduce to CH4(184.0 μmol g-1)and CO(105.2 μmol g-1).In situ X-ray photoelectron spectroscopy and work function calculation show that there is a direct Z-scheme charge transfer mechanism between CsPbBr3 and g-C3N4 interface,thus realizing efficient charge separation and high redox potential,and thus driving photocatalytic CO2 reduction and H2O oxidation simultaneously.The theoretical calculation further revealed that the p orbital of Pb atom was hybridized with the 2π*orbital of CO2 molecule,which enhanced the affinity of CO2 and weakened the binding strength of CO2.Furthermore,the overlap of the Pb-p orbital with the 5σ orbital of the CO molecule promotes the protonation of the CO*intermediate.The use of two-dimensional nanosheets in the heterostructure strategy can increase the contact area of two substances,form close contact,and reduce the advantage of charge transport distance.In this study,a 2D/2D Bi2WO6/C3N4 Z-scheme heterojunction photocatalyst was obtained by loading Bi2WO6 nanosheets onto g-C3N4 ultrathin nano-sheets with a simple electrostatic assembly technique.The material showed excellent photocatalytic CO2 reduction performance under visible light.The internal charge dynamic behavior of Bi2WO6/C3N4 was investigated by Kelvin probe test,and the Z-scheme electron transfer mode was proved.This enhanced activity is mainly attributable to the synergistic effect of adding C3N4 and Bi2WO6 and Bi2WO6/C3N4 nanosheet heterojunction in controlled proportions.In addition,the layered structure,high interfacial interaction,rich 2D coupling interface and efficient charge separation can effectively improve the photoactivity.The photocatalytic CO2 reduction results show that the reduction products of Bi2WO6/C3N4 Z-scheme heterostructure are CH4 and CO,and the yields are about 24.5 μmol g-1 and 10.6 μmol g-1,which are about 6.7 times that of C3N4 alone.Finally,from the above construction of g-C3N4 heterojunction materials,it can be found that the construction of heterojunction not only promotes the separation of photogenerated carrier,but also inhibits the charge recombination within the heterostructure.Therefore,in this experiment,Cs3Bi2Br9 nanoparticles were grown in situ on the surface of ultra-thin g-C3N4 nanosheets to prepare the heterostructure of CBB/CN and achieve efficient photocatalytic CO2 reduction.The results show that the optimized CBB/CN photocatalyst has good photoreduction activity of CO2.The yields of main products CO and CH4 are 15.2 μmol g-1 and 36.5 μmol g-1,respectively,which are 5.25 times of the original CN.The photochemical measurement results show that the heterostructure of CBB/CN can realize the effective electron transfer path from CN to CBB,inhibit the photogenerated electrons and hole coincidence,and improve the photocatalytic efficiency of g-C3N4.