Construction Of G-C₃N₄ And Bismuth Oxyhalide-based Composite Photocatalysts For Degradation Of Organic Pollutants

The development of efficient and stable photocatalytic materials is the core of the development of photocatalytic technology.In recent years,many semiconductor photocatalysts have been developed and utilized,but the insufficient utilization of solar energy and quantum efficiency limit the practical application of photocatalysts.Construction of composite photocatalytic system via combining two or more semiconductor materials is an effective method to solve these problems.Consequently,a series of composite photocatalysts were fabricated by calcination,solvothermal,liquid-phase synthesis and chemical precipitate methods based on graphite carbon nitride?g-C₃N₄?and bismuth oxyhalide.The as-prepared photocatalysts were characterized by a variety of techniques to analyze their compositions,morphologies and photoelectrochemical properties etc.The mechanism of enhanced photocatalytic performance was investigated by exploring the transfer pathway of photogenerated carriers and main active species during the photocatalytic reaction,which provided experimental basis for the development and application of efficient composite photocatalysts.The main research contents are as follows:?1?Fabrication of hierarchical WO₃/g-C₃N₄ composite and the photocatalytic activity.The SrWO₄/g-C₃N₄ precursor was first prepared by an in-situ precipitation method and subsequently transformed into WO₃/g-C₃N₄ composite via immersing-annealing method.The results of SEM and TEM showed that WO₃ nanosheets with the thicknesses of about 30nm were erectly anchored on the surface of g-C₃N₄ nanosheets to construct a“sheet-on-sheet”three-dimensional hierarchical structure,which resulted in the improvement of the specific surface area and visible light absorption capacity.The composite with a WO₃ mass content of34.6%?WOCN-3?possessed the highest specific surface area(58.28 m² g^-1),which was 5.68and 2.61 times than that of pristine WO₃ and g-C₃N₄,respectively.And the degradation rate constant of RhB over WOCN-3 was 6.5 and 3.0 times higher than that of pristine WO₃ and g-C₃N₄,respectively.The active species trapping experiments revealed that·O₂^-and·OH played crucial roles during the photocatalytic process.Combined with the theoretical analysis of semiconductor energy band,it was speculated that Z-scheme heterojunction could be formed between g-C₃N₄ and WO₃,which effectively improves the photogenerated carrier separation efficiency and redox ability of the composite photocatalyst.?2?Ag NPs decorated WO₃/g-C₃N₄ 2D/2D composite photocatalyst and its enhanced photocatalytic activity.The WO₃ nanosheets were prepared by precipitation-calcination method,and g-C₃N₄ nanosheets?CN?were obtained by thermal exfoliation of bulk g-C₃N₄.The Ag NPs with the diameters of about 10 nm were anchored on the surface of WO₃ and CN nanosheets to construct a 2D/2D ternary nanostructure by calcination method.The face-to-face contact between WO₃ nanosheets and CN nanosheets could provide a good transport channel for the migration of the photogenerated charge carriers.The Ag NPs with surface plasmon resonance?SPR?effect acting as charge-transfer mediator,not only effectively boosted the Z-scheme electron transfer between CN and WO₃,and improved the separation efficiency of photogenerated electron-holes,but also enhanced the visible light absorption of the composite.The as-prepared WO₃/Ag/CN ternary composite exhibited remarkably enhanced photocatalytic activity and stability.Particularly,the degradation rate constants of RhB and tetracycline?TC?over WO₃/Ag/CN were 2.4 and 1.9 times than those of WO₃/CN composite,respectively.The photocatalytic degradation intermediates of RhB and TC were confirmed by HPLC-MS analysis,and the possible photocatalytic degradation pathways were analyzed.?3?Preparation of AgI/BiOBr/reduced graphene oxide?RGO?0D/2D composite photocatalyst and its photocatalytic activity for degradation of TC.The BiOBr/RGO nanosheets were synthesized by a facile solvothermal method.The AgI NPs with the diameter of 10-30 nm were located on the surface of BiOBr/RGO nanosheets to form 0D/2D nanostructure through an in-situ precipitation process.The photocatalytic degradation rate of TC over AgI/BiOBr/RGO ternary composite was 94.2%under 80 min of simulated sunlight irradiation.The degradation rate constant of ternary composite was approximately 5.9 and 2.9times higher than that of pristine BiOBr and AgI,respectively.The improved photocatalytic activity of AgI/BiOBr/RGO composite could be ascribed to the synergistic effect of AgI,BiOBr and RGO.On one hand,the band structures of AgI and BiOBr were matched to form a Z-scheme heterojunction,which could promote the separation of photogenerated electron-holes and enhanced the redox capability.On the other hand,RGO with superior electronic conductivity could accelerate the migration of photogenerated electrons between AgI and BiOBr,and improve the specific surface area and visible light absorption capacity of the composite.The possible degradation pathway of TC was illuminated through the identification of intermediate products using HPLC-MS analysis.Furthermore,the results of active species trapping experiments and ESR measurements suggested that·O₂^-and h⁺were the main active species.?4?Preparation of Bi₄O₅Br₂/AgCl/Ag composite photocatalyst and its photocatalytic activity for degradation of ciprofloxacin?CIP?.Bi₄O₅Br₂ nanosheets were obtained by liquid-phase synthesis method,and Bi₄O₅Br₂/AgCl composite was prepared by co-precipitation method.Then Bi₄O₅Br₂/AgCl/Ag ternary composite photocatalyst was obtained by photoreduction process.The composite with a AgCl/Ag mass content of 20%?BAC-2?showed the highest photocatalytic activity compared to pure Bi₄O₅Br₂ and AgCl/Ag.The photocatalytic degradation rate of ciprofloxacin?CIP?over BAC-2 composite was 75%under 150 min of simulated sunlight irradiation.The degradation rate constant of Bi₄O₅Br₂/AgCl/Ag was 4.1 and 3.2 times than that of pure Bi₄O₅Br₂ and AgCl/Ag,respectively.The SPR effect of Ag significantly enhanced the visible light absorption ability of the composites.The heterojunction between Bi₄O₅Br₂ and AgCl/Ag could promote the separation of photogenerated carriers,resulting in enhanced photocatalytic activity.Furthermore,the degradation intermediate of CIP was identified by HPLC-MS analysis,and the photocatalytic degradation pathway of CIP was illuminated.?5?Preparation of Bi₃O₄Cl/AgI composite photocatalyst and its photocatalytic activity for degradation of RhB and TC.The Bi₃O₄Cl microsphere was prepared by solvothermal-calcination method,and then AgI/Bi₃O₄Cl composite was prepared by in-situ deposition process.The compositions,morphologies and optical characteristics of the photocatalysts were determined by XRD,XPS,SEM and UV-vis.AgI particles were loaded on the surface of Bi₃O₄Cl porous microsphere with diameter of?2?m to form a heterojunction with close contact,which could improve the migration and separation ability of photogenerated carriers.The composite with the AgI mass content of 30%exhibited the optimal photocatalytic activity,and 96.5%of RhB was degraded after 80 min of simulated sunlight irradiation.The rate constant for the degradation of RhB was 25.3 times and 3.8times than that of pure Bi₃O₄Cl and AgI,respectively.The Z-scheme mechanism was illuminated according to the theoretical analysis of semiconductor energy band and the results of active species trapping experiments.