Morphological Modulation Of Tubular Graphitic Carbon Nitride And Its Study On Efficacy And Mechanism Of Photocatalytic Removal Of Tetracycline

Since the discovery and widespread application of antibiotics in the mid21st century,the resulting environmental pollution and the emergence of drugresistant bacteria and resistance genes have become an important threat to human health and ecological balance.The development of green,efficient,economical and safe environmental remediation technology has become a research hotspot in the field of environment.Visible light photocatalytic technology driven by solar energy has the characteristics of low energy consumption,high efficiency and green sustainability,so it is regarded as the best alternative to traditional environmental remediation technology.Graphitic carbon nitride（g-C₃N₄）is an ideal photocatalyst due to its matching band gap structure with visible wavelengths and non-metallic properties.However,the shortcomings of traditional bulk g-C₃N₄ such as low carrier separation ability,high carrier recombination efficiency,slow electron transport speed and small specific surface area limit its application in the field of photocatalysis.To solve the above problems,in this paper,four kinds of tubular g-C₃N₄ photocatalysts with high specific surface area and good photogenerated carrier separation and transfer ability were prepared by morphology and structure regulating.In addition,the photocatalytic performance was further improved by cooperating with external field action and coupling with other reaction systems.Tetracycline hydrochloride（TCH）was selected as the main research object to evaluate the photocatalytic performance of the prepared photocatalyst.Density functional theory（DFT）was used to analyze the photocatalytic mechanism and pollutant degradation path.This article provided theoretical basis and support for the development of photocatalysis in practical applications.The main research contents and results were as follows:（1）In the second chapter,hollow tubular g-C₃N₄ with wall delamination（K-CN-2）were successfully prepared by potassium ion modulation of the supramolecular structure.XRD,SEM,TEM,and XPS results demonstrated the delamination effect of potassium ions,and PL fluorescence spectra,transient photocurrents,and electrochemical impedance spectra demonstrated that the delaminated wall effectively suppressed the recombination of photogenerated electron-hole pairs.The UV-vis DRS and DFT calculations showed that the tube wall delamination and the presence of K ions optimized the band gap structure of K-CN-2.Compared with bulk CN and TCN without tube wall delamination,K-CN-2 had higher photocatalytic TCH degradation and H₂O₂ production capacity,that was 83%and 133 μM,respectively.Contaminant concentration,solution pH and the presence of anions had no significant effect on the photocatalytic performance of K-CN-2,and the reusing experimental results demonstrated the high photocatalytic stability and structural stability of K-CN2.The results of radical quenching experiments and electron spin resonance（ESR）showed that superoxide radicals（·O₂-）and holes（h⁺）were the main active species of K-CN-2.（2）Considering the large diameter of hollow tubes in hollow tubular structures,which made it difficult to perform catalysis,thus leading to the problem of space idleness,and the fact that insufficient pores could limit the efficiency of catalytic reactions,modification of the hollow part of the tubes was necessary.In the third chapter,tubular g-C₃N₄（AN-TCN-4.5）with mesoporous-rich ant nest-like filled center was successfully prepared by calcination of supramolecular precursors impregnated with H₂O₂ solution.The result of SEM,TEM,and BET showed the formation of porous centers.And the UV-vis result demonstrated that the porous structure promoted the absorption of visible light.The results of fluorescence spectroscopy and electrochemical tests demonstrated that the porous filling center contributed to the good photogenerated carrier separation and transport ability of AN-TCN-4.5.Within half an hour,AN-TCN-4.5 showed 86%degradation of TCH and nearly 100%degradation of rhodamine B（RhB）and methylene blue（MB）,higher than conventional bulk g-C₃N₄ and tubular g-C₃N₄ without filled structures.The radical trapping experiments and electron spin resonance tests demonstrated that superoxide radicals and holes were the main active species for contaminant degradation.The effects of environmental factors（pH,concentration,anions）and reusing experiments proved that AN-TCN-4.5 had good photocatalytic stability and structural stability.（3）In addition to the morphology modulation can effectively improve the separation and transport of photogenerated carriers on g-C₃N₄,it can also be promoted by external field action.In addition,considering the relatively high price of H₂O₂ used in third chapter,in the forth chapter,the porous tubular gC₃N₄（PTCN）was prepared by direct calcination of wet supramolecular precursor instead of H₂O₂ as pore-making agent.Although the absence of H₂O₂ prevented the formation of the center filling structure,the porous structure was still preserved to a certain extent.The result of SEM,TEM and BET confirmed the existence of porous structure and double-layer separation of the tube wall.The UV-vis DRS proved that PTCN had good visible light response ability,and the fluorescence spectra and electrochemical test results proved that PTCN had excellent photogenerated carrier separation and transmission ability.During the photocatalytic reaction,ultrasound was applied to induce the PTCN to generate a piezoelectric polarization electric field,which further promoted the separation of photogenerated carriers.The piezoelectric properties of PTCN were demonstrated by the comparison of photocurrent with/without ultrasound conditions and the free radical quantization.Finite element simulations also confirmed that the tubular g-C₃N₄ had higher piezoelectric properties than the bulk structure,and the thin tube wall further improved the piezoelectric properties of tubular g-C₃N₄.DFT calculations also showed that the applied pressure conditions were more favorable for the electron transport of PTCN.The degradation rate of TCH by PTCN piezoelectric-photocatalysis was 89%and the mineralization rate was 79.29%within 30 min.The degradation of RhB and MB by PTCN was 100%within 10 min and 20 min,respectively.The PTCN still maintained good photocatalytic performance under outdoor sunlight as well as actual wastewater conditions.ECOSAR analysis showed that most degradation intermediates of TCH were less toxic than TC molecules.（4）Although the photocatalytic system has shown considerable pollutant degradation capacity,the catalytic efficiency can be further improved by cooperating with the persulfate system.In Chapter 5,porous g-C₃N₄ nanoparticles were grown on the sulfur-doped tubular g-C₃N₄ to successfully prepare Type Ⅱ homojunction composite photocatalyst（UCN1-STCN1）.Sulfur doping destroyed the highly delocalized π-conjugated electron structure on the tubular g-C₃N₄ to promote the separation of photogenerated electrons.At the same time,the type Ⅱ electron transport path inhibited photogenerated carrier recombination and promotes the separation of photogenerated electrons and holes.The catalytic performance was promoted by photocatalytic synergistic peroxymonosulfate（PMS）activation.SEM and TEM showed that the prepared UCN1-STCN1 showed a tubular structure with nanoparticles growing on the outer layer of the tube wall.The photoelectric performance test showed that UCN1-STCN1 had excellent visible light capturing ability and photogenerated electron-hole pair separation and transmission ability.DFT calculations also confirmed that sulfur doping optimizes the bandgap structure and electron transport performance of STCN1.The degradation efficiency of UCN1-STCN1 photocatalytic-PMS coupled system for TCH and ciprofloxacin（CIP）under visible light was 93.3%and 44.8%,respectively,and the degradation rate for RhB and MB was 100%and 53%,respectively.The radical capture experiments and ESR tests showed that superoxide radicals（·O₂-）,photogenerated holes（h⁺）and singlet oxygen（¹O₂）were the main active species.The electron donoracceptor relationship between UCN1-STCN1 and PMS was an important reason for the improved catalytic efficiency.