Study On Modification Of Photocatalytic Performances Of G-C₃N₄ And COF-based Semiconductor Photocatalysts

Environmental pollution gradually increase due to the quickly development of industry.Photocatalysis is recognized to be a green,low cost and efficient technology to mitigate environmental pollution problem.Today,various semiconductor photocatalysts have been arisen for environmental remediation.However,the large scale application of photocatalysis technology is still limited owing to their poor utilization of solar energy,fast charge recombination rate of the existed photocatalysts.Therefore,the development of new high-efficient photocatalysts or modification of existing photocatalysts to make them having high catalytic efficiency is a key scientific and technical issue that needs to be addressed urgently.In this study,two typical inorganic semiconductor photocatalysts g-C₃N₄ and Tp Pa-1-COF,which possess unique performances and good application prospect,were chosen as studied objects.Their photocatalytic performances were effectively modified through element doping and constructing heterojunctions.It is found that Fe element can be successfully doped into g-C₃N₄nanosheet skeleton through a facile thermal pyrolysis method,resulting in a significant improvement of photocatalytic performance of g-C₃N₄.In addition,it is found that the photocatalytic performance of Tp Pa-1-COF can be effectively improved through fabrication of a2D-2D heterojunction with inorganic semiconductor Mo S₂,or fabrication of a covalent-bond heterojunction with MOF(NH₂-MIL88B).The main contents of this thesis are listed below.(1).First,semiconductor photocatalyst g-C₃N₄ was modified by doping metal Fe into the framework of g-C₃N₄ nanosheets with uniform dispersion and high stability through a facile thermal pyrolysis method using Fe-EDTA as precursor.Microstructure characterizations reveal that the Fe species exist mainly at the state of Fe³⁺and chemically coordinate to g-C₃N₄nanosheets host,which alter the electronic structures,facilitate the charge transfer,increase specific surface area and light absorption,resulting in a remarkably enhanced photocatalytic activity.The experimental results showed that the TC degradation rate with the Fe-EDTA/g-C₃N₄ photocatalyst was 4 times higher than that of unmodified g-C₃N₄.This work introduces a promising precursor of Fe-EDTA and a facile method for integrating transition metal species into the frame work of g-C₃N₄ nanosheets with drastically enhanced photocatalytic activity.(2).In this work,for the first time we fabricated a novel COFs-based 2D-2D heterojunction composite Mo S₂/Tp Pa-1-COF by a facile hydrothermal method.The results of photocatalytic degradation of TC and Rh B under simulated solar light irradiation showed that the as-prepared composite exhibited outstanding catalytic efficiency compared with pristine COF and Mo S₂.The rates of photocatalytic degradation of TC over Mo S₂/COF composite with an optimized weight ratio of 4:1 were about 4 and 3 times higher than that of pristine COF and Mo S₂,respectively.The significantly enhanced catalytic efficiency can be ascribed to the formation of 2D-2D heterojunction with well-matched band position between COF and Mo S₂which can effectively restrain the recombination of charge carriers,increase light absorption as well as specific surface area.Moreover,the fabricated 2D-2D layered structure can effectively increase contact area with intimate interface contact which greatly facilitate the charge mobility and transfer in the interfaces.This study reveals that artful integration organic(COFs)and inorganic materials into a single hybrid with a 2D-2D interface is an effective strategy to fabricate highly efficient photocatalysts.(3).In this work,for the first time,a MOF material of NH₂-MIL88B was hybridized with Tp Pa-1-COF through covalent bonding to form a heterojunction which was subsequently employed for light-induced Fenton-like excitation of H₂O₂ for degradation of TC and Rh B.The results show its degradation efficiency is much higher than that of parent MOF and COF,respectively.Moreover,it is much higher than that of bare photocatalysis without Fenton-like excitation of H₂O₂.The high degradation efficiency is ascribed to two factors.One is the formation of heterojunction which facilitate the transfer and separation of the charge carriers,and increase light absorbance.Another is the Fenton-like excitation of H₂O₂ which produce more·OH radicals.This work provides a facile strategy to greatly improve photocatalytic performance of COFs materials by formation of heterojunction with MOFs and Fenton-like excitation of H₂O₂.