Study On The Effect Of Surface Defect Modulation On Photocatalytic Efficiency Of Two-dimensional Layered Materials

Focusing on the surface defect control method of two-dimensional typical layered photocatalyst,in this paper,the degradation principle and mechanism of organic pollutants in water environment was systematically studied.For two typical two-dimensional nano-photocatalyst materials,metal sulfide（molybdenum disulfide,Mo S₂）and organic semiconductor（carbon nitride,g-C₃N₄）,which have similar layered structures and different active centers.Through a new method of surface defect control with simple process and green environmental protection,the effects of surface defects on the lamellar morphology,surface interface properties,crystal structure and electronic configuration of two-dimensional layered nano-photocatalytic materials were studied.And the high efficiency and universality of this series of methods for the degradation of organic pollutants in water environment were illustrated.1.Aiming at organic semiconductor photocatalyst g-C₃N₄,different kinds of atom-doped g- C₃N₄ were prepared in different solution media by microwave etching method in order to improve the crystallinity of graphite phase g-C₃N₄ materials.The influence of atomic doping type and concentration on the photocatalytic efficiency of g-C₃N₄ was investigated. These two works illustrate that high electronegative atom doping and certain atom doping concentration can promote the improvement of the crystallinity of g-C₃N₄,and at the same time peel the lamellar of g-C₃N₄,change the electronic configuration of materials,and improve the photocatalytic efficiency.It provides a new perspective on the crystallinity, lamellar morphology and electronic structure of photocatalysts that change the purification environment through the control of surface defects.（1）Specifically,the highly active g-C₃N₄ nanomaterials doped with fluorine（F）atoms on the surface were synthesized by microwave etching in the medium of low concentration hydrofluoric acid（HF）.The introduction of F atoms can effectively increase the crystallinity of the catalyst and generate surface polarization electric field,and the synergistic effect of the two plays a crucial role in charge separation,thus improving the photodegradation efficiency of organic pollutants.Under simulated solar irradiation,the removal rate of methylene blue（MB）by F-doped g-C₃N₄ reached about 100%（within 3 h）,which was 1.12 times higher than that of the bulk g-C₃N₄.The removal rate of diclofenac sodium（DCF）was also as high as 100%（within 3 h）,which was 1.7 times higher than that of the bulk g-C₃N₄.In addition,the F-doped g-C₃N₄ is universal for the degradation of organic pollutants,and also has higher photodegradation efficiency for phenol（ph OH）and bisphenol A（BPA）.（2）Secondly,the surface oxygen（O）doped g-C₃N₄ nanomaterials with high activity were synthesized by microwave etching in low concentration nitric acid medium.Preliminary demonstrated that O successfully replace pyridine nitrogen atom of g-C₃N₄ skeleton,its high electronegativity and larger size of O to enhance the degree of crystallinity and conjugate plane,greatly improves the conductivity and electron delocalization dilatation, while increasing the degree of polarization.Thus,the removal of MB by O-doped g-C₃N₄ reached about 100%under simulated solar irradiation,which was 11.3 times higher than that of the bulk g-C₃N₄（within 3 h）.The removal of DCF by O-doped g-C₃N₄ was similarly high up to 100%,and the photodegradation efficiency of ph OH and BPA was high and universal.2.For Mo S₂ with two crystal phase structures of different atomic packing configurations （semiconductor 2H phase and metal 1T phase respectively）,the increase of active sites in the nano layers conducive to the separation of electron-hole pairs,thus promoting the efficient degradation of organic pollutants by hydroxyl radicals（·OH）with strong oxidation ability.The effect on photocatalytic efficiency of Mo S₂was investigated by regulating vacancy types and interaction relationship.Specifically,Mo S₂ with the best photodegradation performance and S vacancy at the interface was prepared by temperature programmed reduction in hydrogen atmosphere（H₂-TPR）,and the high self- production rate of hydrogen peroxide（H₂O₂）in water environment was realized,thus promoting the degradation of organic pollutants.The results show that the S vacancies in the 1T phase located at the phase junction enhance the crystallinity of the material, promote the separation of photoinduced carriers,and improve the adsorption of O₂, resulting in superoxide radicals（·O₂^-）.The·O₂^-further combines with e^-in the excited state of the material surface in the system and H⁺in solution to internally produce H₂O₂. The H₂O₂ decomposes into highly reactive·OH through the transformation of the transvalent metal Mo atoms（Mo（IV）/Mo（VI））,thus improving the oxidation for organic pollutants.Among them,Mo_1-xS_2-y produced a high internal yield of H₂O₂(35μmol L^-1 h^-1, 1.5 times higher than Mo S₂)under simulated solar irradiation,which greatly contributed to the degradation efficiency of TC and DCF（two times higher than Mo S₂）.This work provides a reference for other two-dimensional catalysts with high endogenous yield of H₂O₂ and its promotion of pollutant degradation.