Synthesis Of ZIF-8 Based Composites And Its Photoelectrocatalytic CO₂ To Formate

The increasing consumption of fossil fuels and the resulting large amount of carbon dioxide emissions have led to a serious energy crisis and climate change.Conversion of CO₂ into high value-added low-carbon chemicals has the dual significance of solving energy and environmental problems.Photoelectrocatalysis（PEC）has the advantages of both photocatalysis and electrocatalysis,and is a clean and excellent catalytic reduction technology for CO₂ reduction.Among them,the development of new catalysts with high photocatalytic activity,strong CO₂adsorption capacity and good product selectivity is the focus of current scientists’research.As an important branch of MOFs,ZIF-8 with zeolite-like structure not only has ultra-high specific surface area,adjustable pore size,ultra-high porosity,but also shows high thermal stability,chemical stability,hydrophobicity and ultra-high CO₂ capacity,so it is considered to be the best porous material for selective CO₂ capture.However,ZIF-8 material itself also has the disadvantages of poor electrical conductivity,large band gap and low energy utilization.In order to improve the performance of ZIF-8 material and expand its application in the field of photoelectrocatalysis,ZIF-8 was optimized and improved by combining some semiconductor materials with unique optical,electrical and catalytic properties,or by further calcining them at high temperature to form porous carbon materials.Based on the above ideas,this article uses Cu ion-doped modified Sn O₂ and nano-sea urchin-like Bi₂S₃ semiconductor materials to compound with ZIF-8 and ZIF-8 derived C materials,respectively,thereby further improving the catalytic performance of CO₂ and the application in the field of photoelectrocatalysis.The main contents of this paper are as follows:（1）The unique hydrophilic-hydrophobic structure Cu-Sn O₂/ZIF-8photoelectrocathode was designed and synthesized by in-situ synthesis strategy,and the photoelectrocatalytic reduction of CO₂ was carried out under mild conditions.The phase structure characterization of XRD,FT-IR,XPS,BET,SEM and TEM proved that Sn O₂ and ZIF-8 materials successfully composited to form a hydrophilic-hydrophobic structure,and Cu atoms successfully entered the Sn O₂lattice to replace Sn.The band gap of Sn O₂ is reduced to 3.49 e V,which broadens the visible light absorption range of Sn O₂.Based on the hydrophilic and hydrophobic structure of the photoelectrocathode,the excellent photoelectrocatalytic reduction of CO₂ performance of Cu-Sn O₂/ZIF-8composites was proved by the photoelectrochemical characterization of CV,LSV,EIS and Tafel.Compared with Cu-Sn O₂ alone,ZIF-8 has excellent CO₂adsorption performance and the activation of its coordination unsaturated metal center,which enables the composite material to capture and activate CO₂efficiently under the drive of visible light.Meanwhile,ZIF-8’s hydrophobicity effectively inhibits the occurrence of hydrogen evolution side reaction.Compared with Cu-Sn O₂ alone,ZIF-8 has excellent CO₂ adsorption performance and the activation of its coordination unsaturated metal center,which enables the composite material to capture and activate CO₂ efficiently under the drive of visible light.Meanwhile,ZIF-8’s hydrophobicity effectively inhibits the occurrence of hydrogen evolution side reaction.The Faraday efficiency of formic acid（HCOOH）reached 68.96%when the composite ratio of Cu-Sn O₂ and ZIF-8was 1:4.In addition,no obvious current attenuation was observed in the long-term electrolysis experiment for 10 h,indicating good stability.（2）Bi₂S₃ nanometer flowers were successfully synthesized by hydrothermal method,and ZIF-8 was successfully grown on the surface of Bi₂S₃ nanometer flowers to form a heterostructure complex catalyst by in-situ synthesis strategy,which was used as a photoelectric cathode for photoelectrocatalytic reduction of CO₂ to HCOOH.Phase characterization by XRD,FT-IR,XPS,BET and SEM confirms that ZIF-8 can be compounded on the surface of the Bi₂S₃ nanoflower structure through simple in-situ growth,significantly increasing its specific surface area.The band gap widths of Bi₂S₃ and ZIF-8 were determined by UV-Vis spectroscopic analysis and calculation,and it was further determined that Bi₂S₃ and ZIF-8 formed a"Z"type heterojunction structure,which not only accelerated the transmission in the electron catalytic system,inhibited the electron-hole pair recombination,but also improved the photoelectrocatalytic carbon sequetization performance.The unique pore structure and porosity of ZIF-8 provide an effective way for CO₂ molecule capture,activation and interfacial reaction.When the composite ratio of Bi₂S₃ to ZIF-8 was 1:10,Bi₂S₃/ZIF-8composite showed the best carbon fixation performance,with the maximum Faraday efficiency of 74.2%（-0.7V vs.RHE）and the maximum current density of 16.1 m A·cm^-2.In addition,in the stability test that lasted for 12 hours,the current density did not decrease significantly,showing extremely excellent stability.（3）On the basis of the above research,ZIF-8 is calcined at high temperature in N₂ atmosphere to form carbon nitride（NC）material rich in pyridine N and pyrrole N,and loaded on Bi₂S₃ nanoflowers to form the Bi₂S₃/NC composite material through a simple hydrothermal method,showing excellent photoelectrochemical performance and stability.Phase characterization by XRD,XPS,SEM and TEM proved that NC was successfully supported on the surface of Bi₂S₃ nanoflower,and part of C atoms entered the crystal lattice of Bi₂S₃ by replacing Bi atoms,forming a new defect energy level in Bi₂S₃,which enhanced the visible light absorption performance of Bi₂S₃.Furthermore,the photoelectrocatalytic carbon sequestration performance of the composites was improved.ZIF-8-derived C materials are rich in pyridine N and pyrrole N,which are important active sites for CO₂ reduction.The BET results show that the calcined NC material still retains the high specific surface area of the precursor ZIF-8.In addition,NC has a high degree of graphitization,which greatly promotes the electron transfer on the catalyst surface.The catalytic activity of Bi₂S₃/NC composites in CO₂ reduction to HCOOH was evaluated by CV,LSV,EIS and I-T photoelectrochemical characterization.The maximum Faraday efficiency was56.53%（-0.89 V vs.RHE）.