Preparation And Photocatalytic Activity Of Graphite Carbon Nitride And Its Composites

In recent decades,the increase in global population and the rapid development of modern industry have resulted in pressing environmental pollution issues.Semiconductor photocatalysis is considered an effective and green strategy to address these problems.Researchers have been dedicating themselves to developing low-cost,stable,non-toxic,visible light-responsive photocatalysts for practical applications.Graphitic carbon nitride（g-C₃N₄）has emerged as a promising visible light-responsive photocatalyst for photodegradation of organic pollutants due to its moderate band gap,suitable electronic band structure,non-toxicity,good physical and chemical stability,low cost,and simple preparation.This paper presents a novel synthesis strategy and heterostructure based on low-cost urea and melamine precursors to regulate the structure and performance of bulk g-C₃N₄.The synthesized photocatalyst was characterized using various methods to investigate its crystal structure,microscopic morphology,chemical element composition,photocatalytic activity,and mechanism.The specific research contents are as follows:（1）Preparation and photocatalytic properties of porous thin-layer graphitic carbon nitrideA green and simple synthesis method has been developed for the direct high-temperature polymerization of urea solution as a precursor,and optimized the reaction parameters of the thermal polymerization process to successfully prepare a small-sized porous thin g-C₃N₄nanosheet.The details regarding the relationship among the polymerization reaction,structure-characteristics correlation,and photocatalytic activity of new-developed g-C₃N₄ have been comprehensively analyzed.The developed g-C₃N₄ demonstrates outstanding structural properties,including the enhanced oxidizability of the holes on the valence band,which enables rapid transportation of charge carriers and leads to a substantial enhancement of photocatalytic activity.Compared to conventionally synthesized bulk g-C₃N₄ catalyst from urea powder,the photocatalytic performance of the WCN-600 catalyst is 16.8 times greater for the rhodamine B photodegradation under visible light.Moreover,the photocatalytic performance is equivalent or even better to that of previously reported g-C₃N₄ photocatalysts,with a substantially reduced catalyst usage,which highlights its tremendous potential for practical photocatalytic water treatment.（2）Preparation and photocatalytic properties of gas-phase SiO₂/graphitic carbon nitride compositesA cost-effective synthesis strategy was developed using melamine and gas-phase SiO₂ as precursors,in combination with solvent evaporation and high-temperature thermal polymerization methods,to successfully fabricate a series of gas-phase SiO₂/g-C₃N₄（SCN-x）composite materials.This composite exhibited remarkable visible-light catalytic performance for RhB degradation,owing to the larger specific surface area and hierarchical porous structure introduced by the gas-phase SiO₂.This structure provided more active sites for the adsorption and degradation of RhB molecules,along with shorter molecular and electron diffusion paths.Under visible-light irradiation,the excited-state electrons from the g-C₃N₄ conduction band migrated to the SiO₂ surface,thereby inhibiting the rapid recombination of photo-generated electrons and holes and enhancing the photocatalytic performance of g-C₃N₄.Among the synthesized composite materials,when the mass ratio of melamine to SiO₂ is 2:1,SCN-3exhibited the highest photocatalytic activity,with a RhB degradation rate of 98.7%within 15min and a degradation kinetic constant of 0.2708 min^-1,14.8 times higher than the original g-C₃N₄(0.0182 min^-1).The composite material demonstrated good stability and recyclability for visible-light photocatalytic RhB degradation.（3）Preparation and photocatalytic properties of boron nitride/graphitic carbon nitride compositesA series of BN/g-C₃N₄（BCN-x）composite materials was successfully synthesized using a simple combination of solvent evaporation and high-temperature thermal polymerization methods.BCN-2 exhibited a higher specific surface area(35.15 m²·g^-1)and larger pore volume(0.2702 cm²·g^-1).The optimized pore structure and specific surface area are advantageous for better adsorption of dye molecules by the catalyst and providing more active sites.Additionally,the combination of BN with g-C₃N₄ nanosheets facilitates the rapid transport of charge carriers.The hole-extracting properties of BN nanosheets provide a hole transfer channel to promote the separation of holes and photo-generated electron pairs,thereby extending the lifetime of the charge carriers and further improving the photocatalytic performance of g-C₃N₄ When the mass ratio of melamine to BN is 5:0.25,the BCN-2 composite material exhibits the highest photocatalytic activity,with a RhB degradation rate of 99%within 25 min.The composite material also demonstrates good stability and recyclability.Free radical trapping experiments indicate that h⁺and generated·O₂^-are the main active species of the composite photocatalyst system for RhB degradation.This study provides new insights for designing g-C₃N₄-based photocatalysts with efficient carrier separation capabilities.