Preparation Of Hybrid Semiconductor Heterojunction Materials And Their Applications To Photoelectrocatalytic Reduction Of CO₂

Since the 20th century,the extensive use of fossil fuels has caused the concentration of CO₂ in the atmosphere to rise year by year,which causes global warming and also leads to a huge energy crisis.Therefore,the conversion of CO₂ into renewable carbon-based fuels can not only reduce the concentration of CO₂ in the atmosphere,but also solve the energy crisis.Combining the advantages of photocatalytic and electrocatalytic reduction of CO₂ and can simulate the process of natural photosynthesis,photoelectrocatalytic(PEC)reduction of CO₂ has been one of the most promising methods to convert CO₂ into renewable carbon-based fuels.In the field of PEC reduction of CO₂,the development of new and efficient photoelectrocatalysts is the current research hotspot.The semiconductor materials are the most common photoelectric catalysts,but most of them still have low catalytic efficiency in the PEC reduction of CO₂.To overcome this issue,doping,modifying and constructing heterojunction materials with another semiconductor might improve the absorption in visible light,adjust the band gap and enhance the separation efficiency of charge carriers,etc.In this thesis,new hybrid semiconductor heterojunction photocathode materials composed of carbon-based materials(organic conductive polymers and Zeolite Imidazole Frameworks)and semiconductor materials are constructed on the basis of semiconductor PEC materials.The photoelectric properties of the materials and their application in PEC reduction of CO₂ are comprehensively investigated.The possible mechanism of PEC reduction of CO₂ to generate carbon-based energy molecules is proposed.The detailed research and conclusions of this thesis include the following three parts:1.The heterojunction CN/Ti O2 photocathode materials for PEC reduction of CO2A series of CN/Ti O₂ semiconductor heterojunction photocathode materials based on polypyrrole(PPy)(named PTCN-X,X stands for calcination temperature)have been constructed.The research results show that as the pyrolysis temperature increases,the active sites of pyridine nitrogen in the material increase and meanwhile the heterojunction exhibits a transition from p-n to n-n junction.The test of carrier separation efficiency proves that this transformation improves the separation efficiency of photo-generated electron-hole pairs in the material,and the carrier separation efficiency of PTCN-400 is 5.5 times that of PTCN.In the best photovoltaic cell PTCN-400|KHCO₃|Bi VO₄,the apparent quantum efficiency of CO₂ reduction is the highest(0.39%),which is 26 times that of the photocathode PTCN(0.015%)under the same conditions.The system generates C₂₊products with a selectivity of up to 71.4%,and can still maintain high catalytic activity after 20 hours working.The¹³CO₂ isotope labeling experiment verified that the carbon source in the produced product is CO₂,and the possible mechanism of the PEC reduction of CO₂ in this system was proposed in combination with the H₂¹⁸O isotope labeling method.This is a simple and low-cost method for preparing semiconductor heterojunction photocathode materials,which can solve the problems of low visible light absorption efficiency and fast carrier recombination rate of Ti O₂,thereby improving the PEC reduction of CO₂.2.Metal modified polythiophene/Ti O2 heterojunction materials for PEC reduction of CO2A series of metal-modified polythiophene(PTh)/Ti O₂ composite heterojunction photocathode materials with visible light response(named M-PTCS,M stands for metal Cu,Pd or Cu Pd)have been constructed.The research results show that the conductivity and photocurrent density of M-PTCS have been significantly improved compared with PTCS.These series of photocathode materials have excellent performance in PEC reduction of CO₂ in M-PTCS|KHCO₃|Bi VO₄ photovoltaic cells.Due to the higher selectivity to C₂₊products,the photocathode Cu-PTCS has the highest apparent quantum efficiency(0.19%)under-0.6V bias potential,which is 4times that of the photocathode PTCS under the same conditions.Femtosecond transient absorption spectroscopy test shows that the photocathode Pd-PTCS has the best carrier separation efficiency,indicating that the metal Pd has the fastest extraction rate of photo-generated electrons at the interface.Therefore,due to the increase of active sites and the synergy between the metal Cu and Pd nanoparticles,the bimetal modified photocathode Cu Pd-PTCS not only reduces the overpotential of the reaction but also generates formic acid at a rate as high as 463?M h^-1 cm^-2 under a bias of-1.0V,which is 20 times that of the photocathode Pd-PTCS(23?M h^-1 cm^-2).In addition,the ¹³CO₂ isotope labeling experiment proved that the carbon source in the produced product is CO₂.Hence,this strategy of constructing a metal-modified hybrid semiconductor heterojunction improves the performance of catalytic materials to enable the excellent performance of PEC reduction of CO₂.3.CN modified p-n heterojunction of Co3O4/Zn O for PEC reduction of CO2The CN modified Co₃O₄/Zn O composite semiconductor heterojunction photocathode materials(CN/Co₃O₄/Zn O)and comparative photocathode materials have been constructed.The research results show that after the in-situ introduction of the Co₃O₄ nanolayer,the band gap of the composite material becomes narrower and the CB position shifts negatively compared to the single Zn O nanorod.Therefore,the hybrid semiconductor heterojunction CN/Co₃O₄/Zn O enhances the absorption of visible light,and the construction of the p-n heterojunction achieves a good separation efficiency of photogenerated electron-hole pairs.Applying the CN/Co₃O₄/Zn O to the PEC reduction system of CO₂ can effectively reduce CO₂ to CH₃CH₂OH-based hydrocarbons.The apparent quantum efficiency of the photocathode CN/Co₃O₄/Zn O PEC reduction of CO₂ under the-0.8V bias potential reaches 0.47%,which is approximately 5.5 times that of the Zn O photocathode under the same bias potential.Further comparative experiments show that the concentration of the electrolyte will affect the selectivity of the catalyst to different products.The test of the PEC reaction at a single wavelength shows that the catalytic efficiency of the system is related to the absorption efficiency of the material at that wavelength.The improvement in the photocatalytic performance of the best photocathode CN/Co₃O₄/Zn O can be attributed to the construction of p-n heterojunction materials to improve the separation efficiency of carriers,and the modification of Co₃O₄ and CN to improve the visible light absorption performance of the materials,and the composite materials to obtain more active sites,thereby improving the overall performance of the materials for PEC reduction of CO₂.