Synthesis Of Graphitic Carbon Nitride-based Composites And Study On Their Photocatalytic Activity For The Degradation Of Antibiotic In Aqueous Solution

The discovery and use of antibiotics has made great contributions to the health of human beings and the growth of animals and plants.However,due to the extreme reliance on antibiotics in clinical medicine and aquiculture and live stocking,the phenomenon of abuse of antibiotics is becoming more and more serious.Excessive use of antibiotics not only causes damage to human and animal organs,but also produces the multiple antimicrobial resistances.In addition,most antibiotics will be discharged in the form of prototypes or metabolites into various water bodies,which brings a serious threat to human health and ecological equilibrium.How to efficiently remove antibiotics from water environment has become the focus of researchers.Photocatalytic technology has many advantages,such as high reaction activity,simple operation,pollution-free and can make full use of solar energy.It provides an efficient and convenient green way to solve the problem of water environment pollution.Among many photocatalysts,graphite-like carbon nitride?g-C₃N₄?is favored by scholars for its unique advantages.G-C₃N₄ is composed of carbon and nitrogen elements,so the raw materials for the preparation of g-C₃N₄ are wide source,non-toxic,rich and cheap.In addition,g-C₃N₄ has its moderate band gap of 2.7 eV,which can stimulate the generation of light-generated carriers under visible light.The good thermal stability and chemical stability also make g-C₃N₄ one of the most promising visible light-responsive photocatalytic materials.However,the application of g-C₃N₄ in photocatalysis is greatly limited due to the low utilization of visible light and the high recombination rate of photogenerated carriers.To solve the above problems,five different types of g-C₃N₄ based heterojunction photocatalysts were designed and prepared,including Type II g-C₃N₄/ZnIn₂S₄,p-n CuBi₂O₄/g-C₃N₄,complementary p-n CoO/g-C₃N₄,Z-scheme CuInS₂/g-C₃N₄ and ternary CDs/g-C₃N₄/ZnO.The crystal phase,structure,morphology and elemental composition of the synthesized materials were characterized by various methods,including X-ray powder diffraction,scanning electron microscopy,transmission electron microscopy,X-ray photoelectron spectroscopy,UV-Vis absorption spectrum,fluorescence spectrum,energy spectrum,etc.Using tetracycline as the target antibiotic pollutant,the synthesized g-C₃N₄ heterojunction photocatalysts were used to degrade it to investigate their performance.At the same time,the reaction mechanism of diverse types of g-C₃N₄ based heterojunction photocatalysts is also discussed based on the reactive active substances determined in the trapping experiment.The main contents and results of this thesis are as follows:?1?The g-C₃N₄/ZnIn₂S₄ composite photocatalyst was prepared by a hydrothermal method,which the layered g-C₃N₄ was tightly encapsulated on the ZnIn₂S₄ microspheres.The photocatalytic degradation experiment results exhibited that the photocatalytic activity of g-C₃N₄/ZnIn₂S₄ composite photocatalyst was significantly higher than that of pure g-C₃N₄ and ZnIn₂S₄.The reaction rate constants of the degradation of TC were 39 times and 22 times of g-C₃N₄ and ZnIn₂S₄,respectively.The photocatalytic performance of g-C₃N₄/ZnIn₂S₄ was improved.It is mainly attributed to the formation of band-matched Type II heterostructures between g-C₃N₄ and ZnIn₂S₄,which promotes the separation efficiency of photogenerated charges.?2?The pure p-type CuBi₂O₄ and n-type g-C₃N₄ photocatalysts were prepared by the hydrothermal route and thermal polymerization method,respectively.CuBi₂O₄/g-C₃N₄?CBO-CN?p–n heterojunctions with different mass ratios were synthesized via a simple calcining method.The degradation experiments demonstrated that the CuBi₂O₄/g-C₃N₄ p-n composite photocatalyst showed better photocatalytic activity in comparison with pure CuBi₂O₄ and g-C₃N₄.It was found that the increase of photocatalytic activity was due to the formation of p-n heterostructures in CuBi₂O₄/g-C₃N₄ composites.Furthermore,an internal electric field is formed at the interface of p-n junction,which improves the separation efficiency of photogenerated carriers.?3?The complementary CoO/g-C₃N₄ p-n type heterojunction photocatalyst was prepared through a simplely solvothermal method,which the CoO nanoparticles were uniformly embedded in the layered structure of g-C₃N₄.The experimental result demonstrated that compared with pure g-C₃N₄ and CoO,CoO/g-C₃N₄ composites not only exhibited higher photocatalytic activity,but also possessed excellent stability.And the highest photocatalytic activity is obtained by 30%CoO/g-C₃N₄ composite.There are three reasons which may explain this highly efficient and the stable photocatalytic activity over complementary p-n heterojunction:?i?CoO can increase the photoabsorption of g-C₃N₄,which is beneficial to the production of more photogenerated electron holes in the visible light;?ii?g-C₃N₄ with with large specific surface area and a flexible 2D structure could effectively prevent the agglomeration of CoO nanoparticles from inactivation,improving their stability;?iii?the internal electric field established by p-n junction can further improve the separation efficiency of photogenerated electrons and holes in the CoO/g-C₃N₄ composite system.?4?Z-scheme CuInS₂/g-C₃N₄ heterojunction photocatalyst was synthesized by a simple solvothermal method after CuCl₂,InCl₃,sulfur powder and g-C₃N₄ were sonicated in the triethylene glycol solution.The result of photocatalytic degradation experiment showed that50%CuInS₂/g-C₃N₄ composite exhibited the highest photoactivity for the degradation of TC?100 mL,20 mg/L,83%degradation within 60 min?under visible light irradiation.The results of capture experiments showed that h⁺and·O₂^-were the main active species in the reaction process,indicating that the electron transfer path in CuInS₂/g-C₃N₄ composite photocatalyst is carried out through Z-scheme mechanism.The Z-scheme CuInS₂/g-C₃N₄ heterojunction photocatalyst can effectively separate photogenerated carriers in the composite system.In addition,electrons and holes with strong redox ability can be retained in thermodynamics to participate in photocatalytic reaction.?5?Firstly,carbon dots?CDs?were synthesized by an electrochemical method,which the sizes were aroun 5 nm.Then the g-C₃N₄/ZnO?CZ?was synthesized by a chemical deposition method.Finally,a ternary CDs/g-C₃N₄/ZnO heterojunction photocatalyst was formed by a facile impregnation-thermal method.On the basis of experimental analysis,the introduction of CDs can effectively increase the specific surface area of g-C₃N₄/ZnO composites and provide more reactive sites.Moreover,CDs could not only further extend the optical absorption of g-C₃N₄/ZnO,but also effectively enhance the photogenerated charge separation efficiency of g-C₃N₄/ZnO composites.Therefore,the CDs/g-C₃N₄/ZnO heterojunction photocatalyst displayed more outstanding photocatalytic activity than pure g-C₃N₄,ZnO and binary g-C₃N₄/ZnO composites.