Structural Modulation Of Bismuth Oxyhalide-based Materials And Their Enhanced Photocatalytic CO₂ Reduction Performance

The use of semiconductor photocatalysis technology to reduce CO₂ in the environment to produce carbon-based chemicals（CO,CH₄,CH₃OH,C₂H₄,C₂H₆,etc.）is one of the effective strategies to mitigate greenhouse effect and realize the national strategy of"carbon neutrality".However,the molecular structure of CO₂ is thermodynamically stable,thus requiring high energy to activate inert CO₂ molecules.Secondly,the lack of active sites on the photocatalyst surface leads to poor CO₂ molecules adsorption/activation and low CO₂ conversion.In addition,the carrier separation efficiency of semiconductor catalysts is generally low,while CO₂ reduction involves multi-electron reactions with slow kinetics,which is not conducive to the deep reduction of CO₂molecules to hydrocarbon chemicals.Therefore,the design and preparation of high-efficiency photocatalysts are at the core of semiconductor photocatalytic CO₂ reduction technology.In this thesis,a flexible and tunable bismuth oxyhalide based material（Bi_aO_bX_c,X=Cl,Br,I）with a layered structure is used as the research object,and the photocatalytic CO₂ reduction activity is significantly enhanced by the strategies of bismuth-rich structure,surface defect structure,edge active site,metal cocatalyst loading to effectively improve the photogenerated carrier separation,transport efficiency and CO₂ molecules adsorption/activation performance of Bi_aO_bX_c material.The structure of the prepared materials was investigated by X-ray powder diffraction,X-ray photoelectron spectroscopy and Raman spectroscopy;the microscopic morphology of the materials was analyzed by scanning electron microscopy and transmission electron microscopy;the influence of structural modulation on the photocatalytic and electrochemical properties of Bi_aO_bX_cmaterials was investigated in detail by UV-vis diffuse reflectance spectroscopy,transient photocurrent response,electrochemical impedance spectroscopy,photoluminescence spectra and time-resolved fluorescence decay spectra;the evolution of CO₂ molecules was revealed by in situ Fourier transform infrared spectroscopy,and the possible mechanism of photocatalytic CO₂conversion was speculated.The main results of the thesis are as follows.1.To solve the problem on high recombination rate of photogenerated carriers,Bi₄O₅Br₂microsphere catalyst assembled with nanosheets was prepared by accurately adjusting the ratio of Bi,O and Br elements.The results show that the internal electronic structure of the catalyst is effectively optimized by composition and structure regulation,which promotes the migration of photogenerated carriers from bulk to surface.In addition,Bi₄O₅Br₂ microspheres possess large specific surface area,which is better for the adsorption/activation of CO₂ molecules.Therefore,the yields of the prepared Bi₄O₅Br₂ microspheres for the reduction of CO₂ to CO and CH₄ reached23.81 and 1.33μmol g^-1 under 300 W Xe lamp irradiation for 4 hours,which were 1.46 and 1.43times higher than those of BiOBr microspheres(CO:16.27μmol g^-1,CH₄:0.93μmol g^-1),respectively.2.In order to solve the problem of insufficient surface-active sites of catalysts,the O defect-rich BiOBr nanosheets（BiOBr-ROV）materials were prepared by the reduction of Na BH₄.Abundant coordination unsaturated active sites on the material surface effectively promote the adsorption/activation of inert CO₂ molecules.At the same time,many O defects can induce local polarization field on the catalyst surface,promote the photogenerated electrons transfer to Bi atoms near O defects,and improve the separation efficiency of photogenerated carriers.The yield of the as-prepared BiOBr-ROV for reducing CO₂ to CO under continuous 300 W Xe lamp irradiation for4 hours reached 15.66μmol g^-1,which was 2.05 folds versus hat of BiOBr nanosheets(CO:7.64μmol g^-1).3.Aiming at the problem that it is difficult to accurately construct surface active sites,ordered macroporous BiOCl materials rich in edge sites（BiOCl-P）was fabricated by assist of SiO₂ gel crystal templates.The rich edge structure provides abundant adsorption sites for CO₂ molecules.Meanwhile,the large number of dangling bonds constructed in situ can serve as local electron enrichment regions,which accelerate the photogenerated charge separation.In addition,the design of dangling bonds can effectively reduce the CO₂ activation energy barrier and promote the dissociation of C=O double bonds.As a result,BiOCl-P exhibited excellent CO₂ reduction activity(CO:78.07μmol g^-1,CH₄:3.03μmol g^-1)after 4 hours of irradiation with 300 W Xe lamp.4.To address the problem of high recombination rate of photogenerated carriers on the surface of catalysts,Bi nanoparticles loaded BiOCl nanosheet composites（Bi/BiOCl）were constructed by the solvothermal method.The introduction of Bi nanoparticles not only effectively enhances the separation and transport efficiency of photogenerated carrier on the catalyst surface,but also further improves the optical absorption of the material.The activity of the optimized composite（Bi/BiOCl-2）for CO₂ reduction to CO and CH₄ was 34.31 and 1.57μmol g^-1 for 4 hours of 300W Xe lamp irradiation,which were 2.55 and 4.76 times compared with the BiOCl nanosheets,respectively.5.Aiming at the problems of CO₂ molecules adsorption/activation,intermediate product diffusion,C–C coupling and C–H generation regulation in the process of CO₂ photoreduction were studied.In this chapter,a series of Au nanoparticles supported ordered macroporous BiOCl nanoreactors（Au/BiOCl-U）were in situ constructed by UV reduction strategy.The results show that the design of Au nanoparticles/BiOCl ordered porous nanosheets with closed interface structure can effectively reduce the carrier combination efficiency and promote the enrichment of photogenerated electrons,thus prolonging the carrier lifetime.Meanwhile,the unique local surface plasmon resonance effect of Au nanoparticles can induce generation of high-energy hot electrons,promote C–C coupling and C–H generation,and then enhance the photocatalytic CO₂ activity of the materials.The optimized composite（Au/BiOCl-U-2）reduced CO₂ to CO,CH₄,C₂H₄,and C₂H₆with yields of 123.38,22.48,3.49,and 105.46μmol g^-1 after 4 hours of irradiation with 300 W Xe lamp.The Au/BiOCl-U-2 showed significantly enhanced CO₂ reduction activity compared with the UV-treated BiOCl ordered porous nanosheets(BiOCl-U,CO:81.62μmol g^-1,CH₄:6.25μmol g^-1,C₂H₄:1.97μmol g^-1).Finally,in situ Fourier transform infrared spectroscopy was used to reveal the conversion mechanism of photocatalytic CO₂ reduction to hydrocarbon chemicals.