Investigation Of Two-dimensional Catalysts For Visible-light-driven Degradation Of Bisphenol A From Aqueous Soultion And The Interaction Mechanisms

As a low-cost,environmentally friendly and sustainable technology,semiconductor photocatalysis has shown great potential in the treatment of organic wastewater.Two-dimensional semiconductor photocatalysts have become key materials for environmental remediation in heterogeneous catalyst systems due to their unique electronic structure,large specific surface area and adjustable energy band structure.Among them,as a two-dimensional conjugated polymer,non-metallic graphitic carbon nitride（g-C₃N₄）can be used as a highly-efficient photocatalyst to remove organic matter in water due to its visible light response ability and excellent chemical stability.Two-dimensional metallic Bi_xO_yI_z（including BiOI,Bi₅O₇I,Bi₄O₅I₂ and Bi₇O₉I₃）and bismuth oxide iodate（BiOIO₃）showed excellent photocatalytic activity due to the build-in electric field,which was favorable for the separation of photoelectron-hole（e^--h⁺）pairs.However,the fast e^--h⁺pair recombination has been a obstacle to the application of g-C₃N₄ and Bi-based semiconductor materials.Therefore,the design and synthesis of highly-efficient visible^-light-driven two-dimensional photocatalysts have attracted extensive attention.In this study,g-C₃N₄,Bi_xO_yI_z and BiOIO₃ were modified by heterojunction construction,organic matter photosensitization and formation surface defects to improve the photocatalytic activity of two-dimensional materials,and the mechanism of enhanced photocatalytic activity was further studied.The specific research contents are as follows:（1）In order to improve the shortcomings of single g-C₃N₄,such as limited absorption range and rapid photoinduced e^--h⁺pair recombination,g-C₃N₄-based composites were synthesized.In this chapter,cerium oxide（CeO₂）was selected to couple with g-C₃N₄ because of its chemical inertness,environmental friendliness and easy conversion from Ce⁴⁺ to Ce³⁺.Combined with the experimental characterization and free radical trapping experiments,the heterojunction structure and the possible eand h⁺migration path of the composite were revealed.Photochemical experiments showed that coupled with CeO₂ could effectively promote the separation of photogenerated e^--h⁺pairs.93.7%of BPA could be degraded by the optimal ratio of CeO₂/g-C₃N₄ heterojunction within 80 min under visible light（λ>420 nm）irradiation.And the degradation rate was 30.3 times and 2.7 times that of pure CeO₂ and g-C₃N₄ nanosheets,respectively.In addition,CeO₂/g-C₃N₄ heterojunction showed excellent degradation effect for organic pollutants such as phenol,chlorophenols and methyl orange（MO）,exhibiting potential application in organic wastewater treatment.（2）The low visible light absorption rate and small specific surface area of g-C₃N₄ limited its photocatalytic efficiency.Bi_xO_yI_z has good visible light response and can be used as an efficient photocatalyst to remove organic matter from water.In this chapter,oxygen-rich bismuth iodide oxide Bi₅O₇I was prepared through a facile calcination process in air by using BiOIO₃ as the precursor.The light absorption range of Bi₅O₇I was characterized by UV-vis DRS.The results showed that Bi₅O₇I could only absorb a small amount of visible light and exhibited a wide band gap structure.However,Bi₅O₇I could remove 99.3%of BPA after 60 min visible light irradiation.The excellent photocatalytic activity might be related to the interaction between semiconductor and small organic molecules.To verify the conjecture,the Bi₅O₇I-BPA sample was obtained by surface modification of Bi₅O₇I with BPA molecules via adsorption method.Bi₅O₇I-BPA exhibited enhanced light response range and stronger photocurrent.XPS revealed that there was a strong interaction between Bi₅O₇I and BPA,which might be caused by surface complexation.Therefore,it was inferred that the ligand metal charge transfer（LMCT）effect broadened the light capture range of Bi₅O₇I and made it exhibit excellent photocatalytic activity under visible light irradiation.It has important reference significance to study the interaction mechanism between semiconductor and pollutant.（3）Based on the conclusions of the above two chapters,highly-efficient photocatalysts could be synthesized by the phase transformation of BiOIO₃ through high temperature calcination process,and the coupling of the two semiconductors can effectively improve the photocatalytic activity of the semiconductor.In this chapter,BiOIO₃/Bi₄O₅I₂ heterojunction was prepared through a solvent-assisted low-temperature in situ calcination strategy in nitrogen atmosphere.Compared with the traditional heterojunction preparation method,the in-situ calcination method was regarded as time^-and effort-saving method.In the presence of ethanol,the phase transition temperature of BiOIO₃ could be reduced,and BiOIO₃ could completely transform to pure Bi₄O₅I₂ nanosheet at 300℃,which could effectively reduce the energy consumption.The BiOIO₃/Bi405I2 heteroconjugation obtained at 250℃significantly improved the degradation activity of BPA,and the degradation rate of BPA reached 99.4%after visible light irradiation for 30 min.The enhancement of photocatalytic performance was closely related to the formation of BiOIO₃/Bi₄O₅I₂ heterojunction.Well-matched heterojunction could effectively promote the transfer and separation of photogenerated e^-and h⁺.This provides a new idea for the synthesis of lower energy consumption,simple,convenient,stronger and closer interface interaction heterojunction.（4）The results of the previous chapter showed that the present of solvent could promote the phase transformation of BiOIO₃.In this chapter,using Bi（NO₃）₃·5H₂O and KIO₃ as raw materials and substituting H₂O with ethanol/glycol as solvent,to prepare oxygen-deficient Bi₇O₉I₃ by one^-step solvothermal method at 160℃.The synthesis method was simple and the resultant temperature temperature of Bi_xO_yI_z was further reduced.Under visible light irradiation,the degradation efficiency of BPA over Bi₇O₉I₃ with large number of oxygen vacancies could reach 99.6%within 20 min,and its degradation rate(0.254 min^-1)was 53.8 times and 2.2 times higher than that of BiOIO₃(0.005 min^-1)and Bi₇O₉I₃ with small number of oxygen vacancies(Bi₇O₉I₃-L,0.117 min^-1),respectively.Oxygen vacancies could serve as active centers,which could not only promote the separation of e^-and h⁺,but also facilitate the activation of oxygen molecules,and then promote the generation of highly active oxygen species,resulting the enhanced photodegradation ability of semiconductors.These attempts provide inspiration for the design of two dimensional photocatalysts regulated by surface defects.In conclusion,broadening the light absorption range of semiconductor and improving the separation efficiency of photogenerated e^-/h⁺ pairs are effective strategies to improve the performance of semiconductor photocatalysis.In this paper,two-dimensional semiconductors were modified to remove BPA and other organic pollutants in water under visible light irridiation through heterostructure construction,organic matter photosensitization and surface defect formation.Besides,the photocatalytic degradation mechanism was further studied,which has certain guiding significance for the development and design of efficient two-dimensional visible light catalyst.