Activity Regulation Of Silver Phosphate Based Photocatalysts And Their Performance And Mechanisms For Catalytic Degradation Of Organic Pollutants From Aqueous Solutions

Energy crisis and environmental pollution have become one of the major problems that human society faces and needs to be solved urgently.Developing green,efficient and energy-saving technology to control environmental pollution is an important measure to solve this problem.Photocatalytic technology has become a promising technology in the field of energy and environment due to its advantages of mild reaction conditions and direct conversion of solar energy into chemical energy.The key of this technology is to develop efficient and stable photocatalysts,so the research and development of novel and efficient visible light response photocatalysts is the focus and hot spot in this field.Silver phosphate（Ag₃PO₄）photocatalyst stands out among many catalysts because of its excellent properties such as high quantum efficiency,simple preparation process and strong oxidation ability.However,the photo-corrosion problem of Ag₃PO₄ is serious.Besides,its photocatalytic activity and the separation efficiency of photogenerated carriers need to be further improved.Therefore,in order to solve the above scientific problems,the regulation strategies including the optimization of the preparation process,construction of Z-scheme heterojunction photocatalytic system,building photogenerated carriers spatial separation system,formation photogenerated electrons transfer layer,and construction built-in electric field enhancement system were proposed.As-prepared Ag₃PO₄-based photocatalysts composites were applied for the degradation and removal of typical organic pollutants in water.At the same time,the charge transfer mechanism and photogenerated carrier migration path of different photocatalysts were studied.The degradation performance of organic pollutants by photocatalyst was also investigated.The main research contents and results are as follows:In the first part,the morphology and activity of Ag₃PO₄ were regulated by adding MWCNTs and Cr:Sr Ti O₃,and the ternary composite photocatalyst was used to degrade malachite green（MG）.The results showed that the addition of MWCNTs can significantly change the size and morphology of Ag₃PO₄ crystal,which can change Ag₃PO₄ from polyhedral crystal with a diameter of about 30μm to spheroid like crystal with a diameter of about 0.28-0.69μm.When the dosage was 100mg/g（mass ratio of pollutants to photocatalyst）,under the condition of solar and visible light,the Ag₃PO₄catalyst composite exhibited excellent photocatalytic activity for MG,which could achieve 100%removal rate within 6 min and 12 min,respectively.The main degradation products were small molecular organic acids such as acetic acid,glyoxylic acid,fumaric acid and benzoic acid.The dominant free radicals during the photocatalytic degradation are·O₂^-and h⁺,and the migration path of photogenerated carriers is Z-scheme heterojunction transfer mechanism.（Corresponding to Chapter 2of the paper）In the second part,the type of MWCNTs was further optimized on the basis of the first part.Ag₃PO₄ was modified by the MWCNTs with higher aspect ratio and the conductive polymer of PANI,and the Ag₃PO₄@MWCNTs@PANI composite photocatalyst was synthesized by in-situ precipitation deposition method.As-prepared catalysts were used for the efficient degradation of phenol and p-nitrophenol.The results showed that with the introduction of MWCNTs and PANI,the particle size of single Ag₃PO₄ changed from 10-20μm polyhedra to 0.38～1.0μm and 0.15～0.38μm spherical crystals,respectively.The removal efficiency of phenol and p-nitrophenol by the optimized composite photocatalyst can reach 100%at 20 min,and the apparent rate constants are 21.9 times and 10 times than that of single Ag₃PO₄,respectively.The TOC removal rate of phenol could reach 83.59%within 20 min of light exposure.The synergistic effect produced by the combination of MWCNTs and PANI is the main reason for the improvement of catalytic activity and photostability of ternary composite photocatalysts.（Corresponding to Chapter 3 of the paper）In the third part,based on the first and second parts,we focus on the role of MWCNTs as photogenerated electron carriers.On the basis of the second part,the 2D conductive polymer were further optimize as PPy.At first,small-sized Ag₃PO₄ was prepared by adjusting phosphorus source and controlling droplet dropping speed.Then,the ternary composite photocatalyst was prepared by coupling with MWCNTs and PPy.At the same time,a spatial separation system of photogenerated carriers in Ag₃PO₄@MWCNTs@PPy composite was constructed.The results showed that the large Ag₃PO₄ polyhedral crystals with diameters of 10～25μm were transformed into particles with diameters of 0.2～1.4μm.The results of photochemical deposition experiments and DFT calculation demonstrated that the photogenerated electrons and holes of Ag₃PO₄ tended to migrate to MWCNTs and PPy surfaces respectively,indicating that the spatial separation system of photogenerated carriers indeed has been successfully constructed in ternary composites.The composite photocatalyst showed excellent photocatalytic activity,mineralization ability and stability under visible light.Within 20 min,the TOC removal rate of phenol was 80.12%,and tetracycline was mainly degraded into small molecular acids,fulvic acids and humic acids.（Corresponding to Chapter 4 of the paper）In the fourth part,on the basis of the previous three parts,nitrogen doping was used to further improve the performance of one-dimensional carbon nanotubes.N-doped carbon materials with hollow structure（NC）were synthesized by template etching method,and NC was combined with Ag₃PO₄ to prapared the composite photocatalyst Ag₃PO₄@NC.The results showed that the novel composite photocatalyst showed good catalytic activity for the degradation of norfloxacin（NFL）,diclofenac（DFC）and phenol.The photocatalytic degradation efficiency of the composite catalysts for NFL,DFC and phenol reached 100%after 5,8 and 12 min of visible light irradiation,and the apparent rate constant of which is 19.2,48.7,and 23.2 times than that of the pure Ag₃PO₄,respectively.The DFT calculation results,including the 3D charge density difference,band decomposed charge density,work function,and Bader charge analysis,indicated the successful construction of built-in electric field at the interface of catalyst composite.Under the effect of the built-in electric field,the photogenerated carriers are separated efficiently,and the photocatalytic performance of the catalyst is significantly improved.（Corresponding to Chapter 5 of the paper）In the fifth part,on the basis of the fourth part,the two-dimensional structure carbon materialγ-graphyne（γ-G）consists of sp-and sp²-hybrid carbon atoms was prepared by mechanical ball milling method,and the composite photocatalyst Ag₃PO₄@γ-G was prepared by electrostatic driving self-assembly method.The results showed that the composite photocatalyst has excellent visible light photocatalytic performance.After 8,5 and 16 min of visible light irradiation,the photocatalytic degradation rates of norfloxacin（NFL）,2-hydroxynaphthalene（2-HNP）and phenol over the composite photocatalyst reached 100%,respectively,and their apparent rate constants were 15.3,9.6 and 19.7 times higher than those of single Ag₃PO₄,respectively.The results of LC-MS and 3D EEMs indicated that the 2-HNP and NFL were mainly degraded into short-chain carboxylic acids.The results of DFT calculation indicated thatγ-G can be used as a photogenerated electrons transfer layer in the composite catalyst to rapidly transfer charge,thus significantly improving the photocatalytic activity and stability of the photocatalyst.（Corresponding to Chapter 6）The results of this paper provide a variety of efficient photocatalytic activity control methods,developed a variety of silver phosphate-based visible light catalysts with excellent performance,explored the migration path of photogenerated carriers in the composite photocatalysts,and clarified the enhanced treatment mechanism of modified composite photocatalyst for organic pollutants.The research content and results are expected to provide some strategies for the preparation of visible light photocatalysts and its property regulations,and provide a reference for the photocatalytic degradation of organic pollutants from aqueous solutions.