Construction Of BiOIO₃-based Heterojunction And Its Photocatalytic Performance For Organic Pollutant Degradation

BiOIO₃ has been considered as a promising photocatalyst because of its earth-abundant element,low cost,non-toxicity,chemical stability,as well as its unique layered structure,intrinsic electric field and spontaneous polarization electric field,which facilitate photogenerated carrier separation and migration.Nonetheless,monosomic BiOIO₃ still suffers from insufficient utilization of solar energy,low specific surface area and high recombination efficiency of photogenerated charge carrier,which limits its application in practice.Herein,choosing BiOIO₃ as the main research object,a series of BiOIO₃-based heterojunction photocatalysts were constructed via ethanol-solvothermal,in-situ reduction and deposition,and one-pot hydrothermal methods,et al.The effects of the introduction of heterojunction components on the physical phase composition,microscopic morphology,photoelectrochemical properties as well as photocatalytic activities of the BiOIO₃-based photocatalysts were investigated.Combined with the band structure of semiconductor materials and the main active species,the migration behaviors of photocarriers at the heterojunction interface were analyzed.The reasons of enhanced photocatalytic activities of BiOIO₃-based heterojunctions were explored and the possible photocatalytic mechanisms were proposed.This work provides experimental support for the design and construction of highly efficient BiOIO₃-based heterojunction photocatalyst.The main research contents are as follows:（1）Construction of novel BiOIO₃/Mo S₂ 2D/2D heterostructures with enhanced photocatalytic activity.2D/2D BiOIO₃/Mo S₂（BM-x）heterojunction was constructed by coupling BiOIO₃ with narrow bandgap Mo S₂ semiconductor by ethanol-solvothermal method.The 2D/2D structure endows BM-x composite higher specific surface area and larger contact interface,which can provide sufficient channels for charge transport.Under the irradiation of simulated solar light,the BM-x composites show higher photocatalytic activities than BiOIO₃.The optimized BM-1.0 achieves photodegradation rhodamine B（Rh B）rate of 98%within 90min light irradiation.Its apparent rate constant k is 0.04189 min^-1,which is 5.2 times higher than that of BiOIO₃(0.00811 min^-1).According to the analysis on the energy band structure of BiOIO₃ and Mo S₂,there exists a staggered energy band arrangement and built-in electric field between them,boosting the separation and migration of electron-hole pairs follows the conventional type-Ⅱpath and enhancing the photogenerated carrier separation efficiency of BM-x composites.Radical trapping experiments showed that·O₂^-and h⁺are the main active species for the photocatalytic degradation of Rh B in this system.Thus,a possible enhanced photocatalytic mechanism of dye-sensitization with type-Ⅱheterojunction was proposed.（2）Bi₂S₃ nanorods and BiOI nanosheets co-modified BiOIO₃ nanosheets:An efficient vis-light response photocatalysts for Rh B degradation.The prepare process of BiOIO₃/Mo S₂ composite by ethanol-solvothermal method is complicated with high reaction temperature and long high time.In order to simplify the preparation process of BiOIO₃-based composite,BiOIO₃/BiOI/Bi₂S₃（BBS-x）composites were prepared via introducing of narrow bandgap BiOI and Bi₂S₃ semiconductor by in situ reduction and chemical deposition strategy.Integrating the merits of BiOI and Bi₂S₃,the BBS-x composites have widened the photo-response range and improved light absorption capacity.The tight contact interface in BBS-x improved the separation and migration efficiency of photogenerated carriers.The BiOIO₃/BiOI/Bi₂S₃ composites exhibited enhanced photocatalytic degradation activities of Rh B under the irradiation of simulated solar light and LED light.The apparent rate constant k of the optimum BBS-0.75 was 0.034 min^-1 under simulated sunlight irradiation,which was approximately 4.5 times higher than that of pure BiOIO₃(0.0076 min^-1).And complete photodegradation of Rh B over BBS-0.75 can be achieved within 30 min irradiation of LED light.According to the analysis on the energy band structures of BiOIO₃,BiOI,and Bi₂S₃,electron-hole pairs separation and migration path in the three-component interface of BiOIO₃/BiOI/Bi₂S₃ composite comply the conventional type-Ⅱpath.In combination with the results of radical scavenging experiments and ESR tests,a possible dye-sensitization acting in concert with type-Ⅱheterojunction photocatalytic mechanism was proposed.（3）Fabrication of 2D/2D BiOIO₃/BiOBr Z-scheme heterostructure with enhanced photocatalytic activity.2D/2D BiOIO₃/BiOBr（BB-x）Z-scheme heterojunctions were prepared by a simple one-pot hydrothermal method.The close face-to-face contact in 2D/2D BiOIO₃/BiOBr composite provides more migration channels for photogenerated carriers,enhancing the separation and transfer efficiency of photocarriers and facilitating the enhancement of photocatalytic activities of BB-x composites for degrading Rh B under the irradiation of Xe light and LED light.Meanwhile,IO₃^-/I^-redox couple as electron mediator further promotes the photocatalytic activities of BB-x Z-scheme heterojunctions.The apparent rate constant k of optimized BiOIO₃/BiOBr（BB-15）composite was 0.046 min^-1(or 0.17 min^-1)under irradiation of Xe light（or LED light）,which was 6.2（89.7）and 3.5（3.5）times higher than that of pure BiOIO₃ and BiOBr,respectively.The interfacial carrier behaviors of the BiOIO₃/BiOBr heterojunction were investigated and it is found that the separation and transfer behaviors of electron-hole pairs in BB-x follow a Z-scheme path under the synergistic effect of the internal electric field and energy band bending as well as IO₃^-/I^-redox couple.Therefore,a Z-scheme heterojunction photocatalytic mechanism with IO₃^-/I^-redox pairs as the mediator of charge transfer was proposed.（4）Fabrication of porous hierarchical BiOIO₃/Bi-MOF composite with enhanced photocatalytic activity.BiOIO₃/Bi-MOF（BF-x）heterojunctions with a rod-like porous hierarchical structure were prepared by in situ growth of BiOIO₃ on Bi-MOF-like rod-like template,which not only gave the BiOIO₃/Bi-MOF heterojunction a large specific surface area and increased the adsorption capacity of pollutants,but also enhanced their light-harvesting ability.Under simulated solar-light（Xe light）irradiation,BiOIO₃/Bi-MOF exhibited enhanced photocatalytic degradation efficiencies for Rh B and TC.The best BF-40composite can degrade 99%of Rh B and 78%of TC under 60 min irradiation of Xe light,and the apparent rate constant k for photodegrading Rh B（or TC）is approximately 8.1（3.9）and29.6（12.8）times higher than that by BiOIO₃ and Bi-MOF,respectively.In addition to the increase in specific surface area and light absorption capacity,the enhanced photocatalytic activity is also due to the increased separation efficiency of photogenerated carrier in the BiOIO₃/Bi-MOF heterojunction.Based on the semiconductor band structure theory and the results of radical trapping experiment,the electron-hole transfer behavior in BiOIO₃/Bi-MOF heterojunction is in accordance with the traditional type-Ⅱpath,and a possible type-Ⅱphotocatalytic degradation mechanism was proposed.