Ultrathin Two-dimensional Nano-materials: Surface Microstructure Modulation And Their Applications In Photocatalysis

Energy crisis and environmental pollution are two major problems faced by the society at present.The catalytic process based on semiconductor photocatalytic reactions,which is environmentally friendly,has been considered as one of most effective ways to address these problems.To develop novel photocatalytic semiconductor materials with high catalytic activities has become a research hotspot in this field.In order to deeply reveal the photocatalytic reaction mechanism,reasearchers have paid their attentions to explore new catalytic models with clearer structures,and to study the structure-activity relationships of photocatalytic materials,aming to provid theoretical guidance for the design of next generation of efficient catalysts.In recent years,ultrathin two-dimensional nanomaterials have been widely used in the field of heterogeneous catalysis due to their clear atomic structure,unique electronic structure,and large specific surface area.It not only provides an ideal reaction model for the in-depth study of the catalytic mechanism and the revealing of the structure-activity relationship,but also brings new opportunities for the development of high-efficiency catalysts for green catalytic reactions that can be applied to address energy crisis and environmental problems.The purpose of this thesis is to establish a photocatalytic reaction model with ultrathin two-dimensional nanomaterials with clear atomic structures,and investigate its applications in photocatalytic water splitting for hydrogen production,photocatalytic Fenton reaction and photocatalytic degradation of organic pollutants via oxygen activation.Various surface microstructure control strategies were used to improve the catalytic activity of related reactions.With the help of advanced characterization techniques and first-principles calculations,the structure-activity relationship between microscopic electronic structure and macroscopic photocatalytic activity was revealed.The mechanism of active sites in catalytic reactions was revealed at the atomic scale,which provided insights and references for the development of new high-performance catalysts.The research work of this thesis mainly includes the following aspects:1.First,we prepared ultrathin Zn In₂S₄nanosheets with/without Cu dopants by a simple solvothermal method,and then loaded Pt species on the surfaces of the two ultrathin 2D nanomaterials by wet impregnation,Pt₁/Cu-ZIS single-atom catalysts and Pt_nc/ZIS nanocluster catalysts were obtained,respectively.The performance of as-prepared catalysts including ZIS,Cu-ZIS,Pt₁/Cu-ZIS and Pt_nc/ZIS were compared via the photocatalytic hydrogen production.It was found that the Pt₁/Cu-ZIS catalyst exhibited the highest catalytic activity with a photocatalytic hydrogen evolution rate of5.02 mmol g^-1h^-1,which was almost 49 times of that the pristine ZIS.It was confirmed by HADDF-STEM,CO in situ infrared adsorption test,and EDS-Mapping content analysis that Pt species were highly dispersed on the surface of Cu-doped ZIS nanosheets（Cu-ZIS）in the form of single atoms.In the pristine ZIS,however,Pt species aggregated at the edge of the nanosheets as nanoclusters with a size range of0.5-1.0 nm.Experimental results and density functional theory calculations highlight the unique stabilizing effect（Pt-Cu interaction）of single Pt atoms in Cu-doped ZIS,while apparent Pt clusters are observed in pristine ZIS.Specifically,Pt-Cu interaction provides an extra coordination site except three S sites on the surface,which induces a higher diffusion barrier and makes the single atom more stable on the surface.Apart from stabilizing Pt single atoms,Pt-Cu interaction also serves as the efficient channel to transfer electrons from Cu trap states to Pt active sites,thereby enhancing the charge separation and transfer efficiency for efficient H⁺reduction.This study reveals the structure-activity relationship of the interaction between metal single atoms and metal dopants at the atomic level in the single-atom stabilization mechanism and the photocatalytic reaction process.2.Subsequently,we prepared a defect-modified Fe single-atom catalyst（Fe₁-N_v/CN,with highly dispersed Fe single atoms distributed on ultrathin carbon nitride nanosheets with abundant nitrogen vacancies）by thermal polymerization.Pristine carbon nitride,defective carbon nitride without Fe loadings,and Fe single-atom catalysts with similar loadings were also synthesized as control samples.XAFS analysis and DFT calculations indicated that the isolated Fe atom was occupied four-coordinate configurations and combined with a neighbouring N_3c type nitrogen vacancy in the 3-s-triazine ring structure of carbon nitride.This photocatalyst with single-atom and nitrogen-vacancy dual active sites exhibited high activation ability for H₂O₂ under visible light,and the optimized catalyst exhibited a much higher ciprofloxacin degradation efficiency,which was up to 18 times that of pristine CN.It also showed excellent removal performance for various antibiotic pollutants,revealing its application potential in the field of water pollution control.Based on DFT calculations and transient absorption spectroscopy results,the engineered nitrogen vacancies served as the electron trap sites,which can drive the electrons from the N vacancies to Fe atoms.The formation of highly concentrated electrons density at Fe sites significantly improved the H₂O₂ conversion efficiency.This study proposed an effective method to achieve efficient photo-Fenton process by optimizing the electron density of the active center through a defect strategy,which provided an important reference for the design of new catalysts via surface microstructure control strategies on the 2D nanomaterials.3.Finally,we promote the exfoliation of g-C₃N₄ nanosheets by coupling of a small amounts ofα-Fe₂O₃ nanosheets to formα-Fe₂O₃/g-C₃N₄ ultrathin composites,this material possessed tight interfaces and abundant nitrogen defects.Its photocatalytic performance was investigated by the antibiotics degradation via oxygen activation.The results showed that the as-prepared defect-mediatedα-Fe₂O₃/g-C₃N₄ heterojunction catalyst exhibited excellent photocatalytic oxygen activation performance due to the staggered energy band structure.The main active oxygen species generated in this process included O₂^·-,¹O₂ and·OH.O₂-TPD and XPS tests confirmed that the abundant defects on the g-C₃N₄ surface in the heterojunction catalysts provided sufficient active sites for oxygen adsorption.Moreover,in situ irradiation XPS spectroscopy,EPR and the first-principles calculations confirmed that the Z-type electron transfer mechanism in defect-mediated heterojunction catalysts,and the built-in electric field formed between the interfaces can provide the key driving force for electron migration.The excited photogenerated electrons inα-Fe₂O₃ migrated from its conduction band to the g-C₃N₄ valence band and recombined with holes,achieving efficient charge separation for g-C₃N₄,and more photogenerated electrons have the opportunity to participate in the photocatalytic reaction.Therefore,the electrons trapped in the nitrogen vacancies can efficiently react with the adsorbed oxygen,generating abundants of reactive radicals.In addition,the constructed activated oxygen system can achieve efficient removal of tetracycline,and effectively reduce its aquatic toxicity during the degradation process.In this study,an efficient photocatalytic activated oxygen system was constructed by designing defect-mediated Z-type heterostructures,which showed a great application potential in the field of water environment remediation.