Design And Synthesis Of Graphitic Carbon Nitride Based Photocatalytic Materials And Their Performances Towards The Degradation Of Representative Antibiotics In Water

Antibiotics,as emerging organic pollutants,have drawn widespread attention in the world.Residual antibiotics in environmental waters can induce bacteria to develop drug resistance,leading to the continuous proliferation of antibiotic resistance genes,which will pose a huge threat to public health.Therefore,it is of great significance to develop environmentally friendly and highly efficient antibiotic-contaminated water remediation technologies.Photocatalytic technology is a novel advanced oxidation process,which has the advantages of simple operation,mild reaction conditions,strong purification ability,low energy consumption and no secondary pollution.Among the various photocatalytic materials,graphitic carbon nitride(g-C₃N₄)has attracted much concern due to its safety and non-toxicity,simple preparation,suitable band gap,ability to absorb and utilize visible light,and high stability.Nevertheless,the photocatalytic performance of g-C₃N₄ still suffers from the limited visible light absorption ability,low electronic conductivity and rapid recombination of photogenerated electron-hole pairs.On the basis of the above issues,this study modified g-C₃N₄ through morphology modulation,heterostructure construction,single-atom modification and molecular engineering,and detailedly investigated the performance and mechanism of g-C₃N₄-based photocatalytic materials for degradation of oxytetracycline(OTC)in water,which can not only provide new design ideas for improving the photocatalytic performance of g-C₃N₄,but also provide new options for removing OTC from water.This study is divided into the following 4 parts:In Chapter 2,Ti₃C₂/porous g-C₃N₄(TC/p CN)interfacial Schottky junction was prepared by an electrostatic self-assembly method for photocatalytic degradation of OTC in water.A series of characterization results confirmed the successful preparation of Ti₃C₂/porous g-C₃N₄ interfacial Schottky junction.Benefiting from the formation of interfacial Schottky junction and the generation of built-in electric field,the separation and transfer ability of photogenerated electron-hole pairs were significantly improved,which were in agreement with the results of transient photocurrent response curves,photoluminescence spectra,time-resolved photoluminescence spectra and electrochemical impedance spectroscopy.Under visible light irradiation(?>420 nm),the optimal sample(TC/p CN-2)could degrade 75%of OTC within 60 min.Moreover,in the process of TC/p CN-2 photocatalytic degradation of OTC,?O₂^-and h⁺played a dominated role,which?OH played a secondary role.In Chapter 3,boron nitride quantum dots/ultrathin porous g-C₃N₄(BU)heterojunction was prepared by an impregnation method for photocatalytic degradation of OTC in water.The characterization results demonstrated that boron nitride quantum dots were successfully loaded on ultrathin porous g-C₃N₄.The photoelectrochemical tests indicated that boron nitride quantum dots promoted the exciton dissociation and electron-hole pair separation of ultrathin porous g-C₃N₄.Therefore,the optimal sample(BU-3)could degrade 82%of OTC under 60 min of visible light irradiation,and its degradation rate constant was 0.0309 min^-1,which was about 4.3 and 2.1 times than that of g-C₃N₄ and ultrathin porous g-C₃N₄,respectively.Meanwhile,the BU-3 also presented excellent stability and 77%of OTC could be degraded even after 4 cycles.The major active species for BU-3 photocatalytic degradation of OTC were?O₂^-and h⁺,while?OH contributed a little to the degradation of OTC.In addition,compared with g-C₃N₄ and ultrathin porous g-C₃N₄,the average?O₂^-generation rate of BU-3 was significantly increased,which would accelerate the photocatalytic degradation reactions.In Chapter 4,single-atom cobalt modified g-C₃N₄(Co-p CN)was prepared by an in situ growth strategy for photocatalytic degradation of OTC in water.The atomic characterizations indicated that single-atom cobalt was successfully anchored on g-C₃N₄ framework by covalently forming the Co–O bond and Co–N bond,which would strengthen the interaction between single-atom cobalt and g-C₃N₄.The ultraviolet-visible diffuse reflectance spectra showed that single-atom cobalt expanded the light absorption of g-C₃N₄ in visible light region.The photoelectrochemical tests demonstrated that single-atom cobalt accelerated separation of photogenerated electron-hole pairs,increased electron density and facilitated electron transfer.Therefore,under visible light irradiation,the degradation rate constant of the optimal sample(Co(1.28%)-p CN)for OTC was 0.038 min^-1,which was about 3.7 times than that of g-C₃N₄.The analysis of degradation products showed that OTC was oxidized to produce opening ring products,and these ring-opening products were finally oxidized into CO₂ and H₂O.In addition,the electron spin resonance tests and active species trapping experiments indicated that the ¹O₂,h⁺,?O₂^-and?OH were responsible for OTC degradation,and their relative contributions to the overall OTC degradation were estimated to be 71.1%,47.4%,23.7%and 7.9%,respectively.In Chapter 5,2-hydroxy-4,6-dimethylpyrimidine(HDMP)grafted g-C₃N₄(ACN)was prepared by a keto-enol cyclization method for photocatalytic degradation of OTC in water and H₂O₂ production.Density functional theory calculations demonstrated that electrons and holes were located in the HDMP and heptazine moiety,respectively,which accelerated their separation and transfer as well as restrained their recombination.Femtosecond transient absorption spectra confirmed that the average decay lifetime of photogenerated charges increased from 222±23 ps to 289±38 ps,which was conducive to the progress of photocatalytic reactions.Therefore,the optimal sample(ACN-10)could degrade 79.3%of OTC under 60 min of visible light irradiation,and its degradation rate constant was 2.4 times than that of g-C₃N₄.The major active species for OTC degradation over ACN-10 were?O₂^-and h⁺,while the?OH was involved but not primarily.Meanwhile,the photocatalytic H₂O₂ production rate of ACN-10 was 2.9?mol/(L?min),which was also higher than that of g-C₃N₄(1.7?mol/(L?min)).Moreover,in the process of ACN-10 photocatalytic H₂O₂ production,H₂O₂ was generated from the two-step single-electron O₂ reduction.