Construction Of Highly Efficient G-C₃N₄-based Photocatalyst For The Degradation Of Tetracycline Antibiotics And Mechanism Study

Photocatalysis is a solar energy conversion technology based on semiconductor photocatalyst,which can excite photogenerated holes and electrons with strong redox ability under the drive of solar energy to achieve efficient removal of pollutants.Because of the advantages of low energy consumption,low cost,mild reaction conditions and strong sustainability,it has a wide application prospect in the field of environmental water pollution control.In view of the existing limitations of photocatalytic technology,how to further develop low-cost and efficient photocatalytic materials is still the key issue to promote the application of photocatalytic pollution treatment technology.Graphite phase carbon nitride,as a metal-free semiconductor material,has many advantages such as wide raw material,low cost and environmental friendliness,and is extremely suitable as an environmental photocatalytic material.In this paper,the optical absorption,carrier separation and surface mass transfer properties of g-C₃N₄were optimized by means of heterogeneous structure construction,element doping,defect modification and homogeneous structure construction,and the microstructures,catalytic mechanism and activity promotion mechanism of the catalysts were investigated by combining the multispectral characterization and density functional（DFT）theoretical simulation.Center on the catalytic degradation of tetracycline antibiotics in water,the study on the optimization of reaction conditions,the influence of environmental factors,the analysis of degradation path and the treatment of actual water samples were carried out.Specific research contents and experimental results are as follows:（1）g-C₃N₄/Ta Na O₃composite heterojunction was prepared by evaporative solvent method,and then Ag nanoparticles were deposited on its surface by photoreduction deposition to form Ag/g-C₃N₄/Ta Na O₃ternary composite photocatalyst.The results show that the ternary photocatalyst owns excellent charge separation efficiency,and its charge migration path change fromⅡinto Z mechanism under the induction of Ag nanoparticles,which effectively solved the drawback of the reduction of redox capacity caused by the migration of photogenerated carriers to the lower potential energy band.The SPR effect of Ag nanoparticles can also effectively enhance the light absorption efficiency of the composites in the visible region.Under visible light irradiation（λ﹥420nm）,the removal rate of tetracycline（20mg/L）reached 95.47%within 1h reaction over the ternary photocatalyst.The active substances in the catalytic reaction were studied by using free radical trapping agent and electron spin resonance spectroscopy.The degradation pathway of tetracycline was analyzed by liquid mass spectrometry.（2）For the first time,a N-GQDs modified Ag₂Cr O₄@g-C₃N₄core-shell structured composite was fabricated to achieve full-spectrum response from UV to near-infrared region.In the Ag₂Cr O₄/N-GQDs@g-C₃N₄composites,Ag₂Cr O₄and g-C₃N₄can be excited by UV and visible light,and N-GQDs can absorb NIR light to emit visible light,which greatly enhances the utilization of solar light.Moreover,compared with the normal hybrid heterojunctions,the core-shell structure provides larger contact area between Ag₂Cr O₄and g-C₃N₄.The large contact area and highly conductive N-GQDs effectively promote the photoelectron transfer from Ag₂Cr O₄to g-C₃N₄,which not only restrains the charge combination,but also greatly inhibits the photocorrosion of Ag₂Cr O₄.As a result,the optimized composites exhibit excellent photocatalytic degradation of doxycycline under full-spectrum light.The cycle experimental results showed that the performance of AN@CN did not decrease even after eight cycles of usage,and the XRD and EIS results also confirmed the stable nature of AN@CN composites.（3）In order to simplify the composition of the full-spectrum responsive photocatalyst and verify the protective effect of the core-shell structure on the corrosion-prone semiconductor materials,core-shell W₁₈O₄₉@g-C₃N₄composite photocatalyst was constructed.The results show that W₁₈O₄₉@g-C₃N₄composite has the ability to absorb light from ultraviolet and visible to infrared,and significantly improves the degradation ability of minocycline under the simulated sunlight（AM1.5）.The kinetic constants of its degradation reaction were 0.0278,which were 2.8times that of g-C₃N₄and 5.7 times that of W₁₈O₄₉,respectively.For W₁₈O₄₉,the oxygen vacancies that provide LSPR effect will be oxidized and disappear after being used or exposed in the air for a long time,leading to the losing of NIR absorption ability.Therefore,an oxygen vacancy inactivation experiment was designed to test the protective effect of g-C₃N₄shell on W₁₈O₄₉.The results showed that exposed W₁₈O₄₉would gradually lose LSPR activity after long-term use in aerobic environment,and the presence of g-C₃N₄shell could effectively inhibit this process.The protective mechanism of g-C₃N₄shell for oxygen vacancy on W₁₈O₄₉was further revealed by a series of characterization methods and DFT theoretical analysis.（4）In order to adjust the band structure of g-C₃N₄,sulfur doped g-C₃N₄nanosheets were prepared by a novel sulfur-assisted annealing method.The results show that the introduced S replaces the N_2Catom in the framework of heptazine ring in the form of covalent binding and can adjust the band structure of g-C₃N₄to narrow the band gap.The band gap and photocatalytic activity of SS-CN could be controlled by adjusting the amount of S,and the optimal amount of S was determined based on the photocurrent test results.The mechanism of S doping regulating the band structure of g-C₃N₄and the promoting effect of S site on the charge separation and surface charge transfer are revealed by DFT calculation.The prepared SS-CN photocatalyst showed excellent degradation efficiency for chlorotetracycline and oxytetracycline under visible light,and the complete removal of aureomycin and oxytetracycline（20mg/L）could be achieved within 70 min.（5）In order to investigate the effect of defect modification on the band structure and catalytic performance of g-C₃N₄,B-doped defect g-C₃N₄photocatalyst was prepared by one-step KBH₄-assisted annealing method.The results show that the prepared material has a leaf-vein-like morphology,which greatly increases the specific surface area.By adjusting the amount of KBH₄in the preparation process,the concentration of B and defect site in the catalyst can be directly affected,so as to regulate the energy band structure and catalytic performance.Based on DFT calculation,it is found that B-doping site and defect site can not only directly regulate the band structure,optical response,carrier separation and transport capacity of g-C₃N₄,but also have synergistic effect,which can promote the directional separation of photogenerated carriers.It was found that the catalyst not only could efficiently remove metacycline within 50 min,but also can produce H₂O₂under visible light.The quantum efficiency of H₂O₂production under 420 nm light is27.8%,which is higher than most of the reported catalytic materials.（6）Based on the above studies,it can be seen that the construction of composite photocatalyst can effectively improve the carrier separation efficiency of g-C₃N₄,but the preparation method and composition are relatively complex.Element doping and defect state modification can change the band structure of g-C₃N₄to improve the catalytic activity,but the promotion effect of carrier separation still needs to be improved.Therefore,in combination with the advantages of the above modification methods,and in order to reduce the preparation cost and maintain the environment friendly characteristics of g-C₃N₄,an innovative strategy was proposed to prepare the homogeneous junction photocatalyst by combining the defective g-C₃N₄and ordinary g-C₃N₄.The results show that the prepared g-C₃N_4-X/g-C₃N₄homojunction not only enhances the light absorption capacity,but also has the capability of efficient carrier separation at the interface.The prepared photocatalyst can effectively degrade tetracycline antibiotics under visible light.In addition,the effects of different influencing factors and water samples on the degradation efficiency were investigated,the reactive substances and corresponding formation process in the degradation process were also studied,so that revealing the mechanism of photocatalytic degradation of pollutants.