Study On Internal Electric Field Construction Of Photocatalyst And Performance In Degrading Organic Pollutants In Water

This dissertation is oriented to the demand for removing organic pollutants in an aqueous environment.The aim is to solve the critical scientific and technological problems of low charge separation in photocatalytic technology,weak absorption of visible light for photocatalysts,and difficult separation of photocatalysts in the suspension system.The strategy is to apply the internal electric field regulation.Three innovative studies are carried out to improve the visible light catalytic degradation activity in a short time.The relationship between the internal electric field and the photogenerated charge separation efficiency of photocatalysts are systematically investigated to reveal the mechanism of efficient degradation of organic pollutants through the structure design.The powder photocatalyst is further solidified to form a flexible device,which exhibited highly efficient photocatalytic degradation efficacy in a continuous flow reactor,providing a reference for the system design of photocatalytic control of water pollution.Based on bismuth halide oxide class of photocatalysts,the photocatalytic degradation and mineralization efficiency are enhanced by adjusting their crystal structures.Taking bismuth oxyiodide as an example,Bi OI,Bi₅O₇I and Bi₄O₅I₂ with decreasing iodine content were prepared,and three kinds of bismuth oxyiodide photocatalysts with obvious structures were prepared.The decrease of the number of iodine atoms in the cell induced crystal structure changes,and theoretical calculations show that the local dipole of the cell is enhanced with the iodine decreased.The internal electric field of the photocatalysts was further tested by experimental characterization,and the internal electric field of Bi₅O₇I was 1.6 and 3.4 times higher than that of Bi₄O₅I₂and Bi OI,respectively,and the enhanced electric field effectively promotes the photogenerated charge separation and migration.The photocatalytic degradation efficiency of Bi₅O₇I for BPA under UV irradiation is 4.7 and 6.6 times higher than that of Bi₄O₅I₂and Bi OI,respectively.Meanwhile,as the valence band position of bismuth oxyiodide decreases with the decrease of iodine element,the oxidation ability of photogenerated holes is significantly enhanced,thus improving the mineralization ability of organic pollutants.By modulating the crystal structure of bismuth oxide halide,the simultaneous enhancement of photocatalytic degradation and mineralization efficacy of organic pollutants is achieved.The Bi₅O₇I photocatalysts with robust internal electric field achieve efficient degradation of BPA under UV irradiation,however,the wide band gap of Bi₅O₇I limit the degradation activity inder visible light.To address this challenge,Bi₂O₂CO₃/Bi₅O₇I heterojunction is constructed by in situ conversion on Bi₅O₇I in a solvothermal environment.And the narrower band gap of Bi₂O₂CO₃broadened the response of the photocatalyst to visible light.The respective work functions of Bi₂O₂CO₃and Bi₅O₇I are different with 0.2 e V.To reach equilibrium,there is a space charge region generated by charge flow at the contact interface,resulting in a internal electric field directed from Bi₅O₇I to Bi₂O₂CO₃.The interfacial electric field drives the migration of photogenerated holes at the interface toward Bi₂O₂CO₃and the photogenerated electrons toward Bi₅O₇I.In addition,the Bi-O chemical bond at the interface provides a fast transfer channel for interfacial photogenerated charges.Adequate interfacial contacts ensure the dominant effect of these interfacial interactions,and the visible photocatalytic degradation of BPA by Bi₂O₂CO₃/Bi₅O₇I heterojunctions is enhanced by 23 times compared to Bi₅O₇I.The interfacial electric field is constructed based on the difference of the work function to enhance the separation of photogenerated charges,thus promoting the activity of visible photocatalytic degradation for organic pollutants.The problem of separating powder catalysts in suspension systems is one of the significant challenges in photocatalytic water treatment processes.The binding of inorganic photocatalysts to carriers relies on adhesives,which can adversely affect the surface reaction sites and light absorption.By contrast,the binding of organic photocatalysts to flexible carriers relies on intermolecular interactions.To obtain excellent photocatalytic degradation efficacy of organic pollutants in continuous flow systems,ternary homojunction photocatalysts formed from metal-free carbon nitride（CN）of three crystallinity levels are prepared.The Fermi energy level position increases with decreasing crystallinity,which results in the formation of a continuous built-in electric field at the interface with the direction from the relatively high Fermi energy level position to the relatively low CN.the charge complexation is suppressed by the effect of the interfacial electric field on photogenerated charge separation,which promotes the generation of degradation active species.In addition,the flexible devices prepared by combining photocatalyst and non-woven fabric overcome the challenge of powder photocatalyst recovery.The effect of the interfacial electric field on the separation of photogenerated charges enabled efficient degradation within a short contact time with organic pollutants.In a continuous flow device,under natural irradiation with low outdoor light intensity,a surface loading of 14.2 L h^-1 m^-2 for organic pollutants between 11 a.m.and 3 p.m.maintained 91.0%purification rate for wastewater containing organic matter,showing some application value.