Preparation And Photocatalytic Properties Of Aromatic Ring Modified G-C₃N₄ And Heteroiunction

Photocatalytic hydrogen evolution and degradation of organic pollutants are considered to be promising solutions in the face of increasingly severe energy crisis and environmental pollution.In order to improve the photocatalytic performance,g-C₃N₄ was modified by grafting aromatic ring and fabricating heterojunction between g-C₃N₄ and inorganic semiconductor.This thesis contains the following three aspects:1.Tris（p-fluorophenyl）phosphine was successfully grafted into the frame of g-C₃N₄（BP-CN）by nucleophilic substitution and thermal polymerization.Compared with g-C₃N₄,the structure of HOMO and LUMO levels of BP-CN was changed,resulting in a smaller band gap and an increased absorption in visible light region.Moreover,the grafted aromatic rings brought about the distortion of g-C₃N₄ structure,which is favorable for the transfer and separation of photogenerated electrons and holes.Under visible light irradiation,the optimal BP₂₂₅-CN/Pt produced H₂ at a rate of 12.45 mmol h^-1 g^-1,which is 4.8 times of that of pure g-C₃N₄.The apparent quantum efficiency of BP₂₂₅-CN/Pt was 24.9%（λ=420 nm）.Moreover,DFT calculation displayed that BP-CN had a higher binding energy for p-chlorophenol（4-CP）.After three hours’ visible light irradiation,the degradation ratio of 4-CP over BP₂₂₅-CN was 89%,which is 3.8 times of that of pure g-C₃N₄.Cyclic photocatalytic experiments demonstrated the high stability of BP-CN.Based on the capture experiment for the active species during 4-CP degradation,EPR measurement,and HRMS/LC-MS identification of intermediates generated during the degradation of 4-CP,the mechanism of 4-CP degradation was proposed accordingly.2.Benzene sulfonyl chloride（BSC）grafted g-C₃N₄（BS-CN）was synthesized via nucleophilic substitution and thermal polycondensation.The obtained BS-CN exhibited narrower bandgap and stronger visible light absorption due to the extension of π-conjugate system.Moreover,BSC acted as electron acceptor and g-C₃N₄ acted as electron donor,therefore a quick intramolecular electron transfer was reached,and the efficiency of charge separation was thus improved.In the first part of this thesis,triethanolamine was added as sacrificial reagent for the photocatalytic H₂ production over BP-CN,and it is not enough economic.As a result,photoreforming of plastics（PET,PLA and TPU）over BSx-CNs for H₂ production was investigated.The effect of plastic substrate concentration and alkali concentration on the photoreforming of PLA to produce H₂ production was studied.When the concentration of PLA was 10 mg mL^-1 and the concentration of KOH was 1 M,the H₂ production rate over BSs-CN reached 1.89 mmol h^-1 g^-1,about 2.8 times of that of pure g-C₃N₄.The hydrolysis products of PLA and the photoreforming products were identified by ¹H NMR and ¹³C NMR,and the reforming mechanism was therefore proposed.3.The above BP_x-CNs and BS_x-CNs both needed Pt loading as co-catalyst before photocatalytic H₂ production,thus leading to an increased cost.In the last part of this thesis,ternary g-C₃N₄-CdS-NiS₂ composite nanotube was synthesized via thermal polycondensation using urea and thiourea as precursors,and subsequent two-step hydrothermal reactions.The photocatalytic activity of g-C₃N₄-CdS-NiS₂ composite without loading precious metals was investigated for H₂ generation from water using triethanolamine as sacrificial agent under visible light irradiation.The optimal g-C₃N₄-CdS-NiS₂ composite with the content of CdS 10 wt%produced H₂ at a rate of 1018 μmol h^-1 g^-1,which is 25 times and 11 times of that of pure g-C₃N₄ nanotube and g-C₃N₄-CdS（NiS₂）binary composite,respectively.Moreover,cyclic photocatalytic experiments demonstrated the high stability of g-C₃N₄-CdS-NiS₂ composite.The improvement in the photocatalytic performance for H₂ production can be mainly attributed to the formation of heterojunction between CdS,NiS₂ and g-C₃N₄ nanotubes,which is beneficial to the separation of photogenerated electron-hole pairs.