Design Of Heterojunctions Based On Photogenerated Charge Regulation And Enhancement Mechanism Study Of Photocatalytic Degradation Of Pollutants

Environmental pollution is one of the most important challenges of the 21st century.Among them,water pollution is a prominent problem.Emerging and refractory organic contaminants in environmental water not only adversely affect the ecological system but also threaten human health.Therefore,it is one of the urgent issues to be addressed the efficient elimination of organic pollutants from water.Semiconductor photocatalysis technology uses catalysts and low-density sunlight to generate reactive oxygen species,which can achieve effective mineralization of organic pollutants,and is considered a promising environmental remediation technology due to its excellent characteristics such as simple operation,high efficiency,and no secondary pollution.Currently,most single photocatalysts suffer from fatal drawbacks such as easy compounding of photogenerated charges and low utilization efficiency,resulting in low photocatalytic efficiency.Key factors for improving photocatalytic efficiency:（1）enhanced visible light absorption to generate more photogenerated charges;（2）inhibition of photogenerated charge compounding;and（3）redox reactions of photogenerated charges to improve utilization.Essentially,it is for the effective regulation of photogenerated charges,i.e.,the three steps of formation,separation and migration,and utilization of photogenerated charges.The effective regulation of photogenerated charges is the key to improve photocatalytic efficiency.Therefore,how to achieve effective regulation of photogenerated charges has become a hot and difficult research area in current scientific research.In this thesis,four different types of heterojunction materials were designed and constructed by adopting the regulatory strategies of constructing heterojunctions and introducing defect engineering to achieve effective separation of photogenerated charges and reveal their mechanisms as the starting point.The mechanism of enhanced photocatalytic performance was investigated for the degradation systems of organic pollutants.The mechanism of photogenerated charge separation and transport was investigated through a combination of theory and experiment to establish the constitutive relationship between photogenerated charge behavior and catalytic performance,providing a theoretical basis for the design and development of efficient and highly stable photocatalytic systems.The main research of this thesis is as follows:1.The photocatalytic performance of the catalysts was investigated using a phosphomolybdate in situ pyrolysis strategy to obtain ultrafine Mo O_xclusters anchored on g-C₃N₄substrates with the introduction of nitrogen-oxygen double defects,using tetracycline hydrochloride（TCH）as the target contaminant.A range of characterization techniques and density functional theory（DFT）calculations reveal that the introduction of the nitrogen/oxygen dual defects and Mo O_xclusters enhance the O₂adsorption energy from-2.77 e V to-2.94 e V.We find that Mo O_xclusters with oxygen vacancies（Ov）and surface Ov-mediated Mo^δ+（3≥δ≥2）possess unpaired localized electrons,which act as electron capture centers to transfer electrons to the Mo O_xclusters.These electrons can then transfer to the surface adsorbed O₂,thus promoting the photocatalytic conversion of O₂to·O₂^-and,simultaneously,realizing the efficient separation of photogenerated electron-hole pairs.Benefiting from these advantages,the best heterojunction photocatalyst exhibits excellent photocatalytic performance with a rate constant approximately 7 times higher than that of g-C₃N₄.2.A simple physical solvent evaporation strategy was used to obtain Mn,C-Ti O₂/g-C₃N₄Z-scheme heterojunction photocatalysts.The photogenerated charge transfer between Mn,C-Ti O₂and g-C₃N₄follows a Z-Scheme transfer mechanism according to the results of trapping agent experiments,electron paramagnetic resonance（ESR）and DFT calculations,i.e.,the photogenerated electrons in the CB of Mn,C-Ti O₂combine with h⁺in the VB of g-C₃N₄,retaining the photogenerated electrons in the CB of g-C₃N₄and h⁺in the VB of Mn,C-Ti O₂.Thus,the system promotes efficient separation of photogenerated charges and generates a large amount of available photogenerated charges to participate in photocatalytic reactions.The prepared heterojunctions showed the highest photocatalytic activity for the degradation of TCH with 76.0%degradation efficiency and 3.4 and 3.2 times higher apparent rate constants than those of pure g-C₃N₄and C-Ti O₂,respectively,when the mass ratio of g-C₃N₄to Mn,C-Ti O₂was 10%under visible light for 60min.3.Z-scheme heterojunctions（S-C₃N₄/Cu/C-Ti O₂）with Cu doping were successfully synthesized by a simple three-step chemical process.The experimental results show that Cu ion doping significantly improves the photocatalytic activity of S-C₃N₄/C-Ti O₂heterojunctions.The degradation efficiency of the optimal S-C₃N₄/Cu/C-Ti O₂photocatalyst for TCH was 82.6%with visible light irradiation for 30 min,and its surface rate constants were 15.4 and 7.3 times higher than those of the S-C₃N₄and C-Ti O₂samples,respectively.Experiments and theoretical calculations show that the Cu doping greatly facilitates the Z-scheme interface charge transport process and preserves the strong redox ability of the photogenerated charge.In addition,the ESR results showed that a variety of radicals（·O₂^–,¹O₂,·OH and h⁺）were generated during the photocatalytic process.4.Bi₂O₃/（Bi O）₂CO₃/Bi₂Mo O₆ternary bismuth-based heterojunctions were constructed by the solvothermal and calcination approach.By changing the concentration of Na₂CO₃,calcination temperature and time,the content and morphology of Bi₂O₃and（Bi O）₂CO₃can be fine-tuned.Bi₂O₃/（Bi O）₂CO₃/Bi₂Mo O₆ternary bismuth-based heterojunctions share a common Bi element,which facilitates the establishment of a tight contact interface for the transport of photogenerated charges.The optimized Bi₂O₃/（Bi O）₂CO₃/Bi₂Mo O₆heterojunctionshowedexcellent photocatalytic activity,with phenol removal of 98.8%and total organic carbon of about 68.0%at 180 min of visible light irradiation.Experimental and theoretical calculations show that the photogenerated charge follows a Z-scheme charge transfer mechanism,and（Bi O）₂CO₃acts as an electronic bridge to shorten the migration distance of photogenerated electrons from E_CBof Bi₂O₃to E_VBof Bi₂Mo O₆,which prolongs the lifetime of photogenerated electrons.