Study On Design,Synthesis And Properties Of Graphite Carbon Nitride Based Materials For Photocatalytic Degradation Of Environmental Organic Pollutants

With the rapid development of industry and economy,various environmental pollution problems affect human health and ecological environment,and the removal of pollutants in the environment has become a common problem.Semiconductor photocatalysis technology uses clean and regenerable sunlight as a driving force to degrade environmental pollutants.It can completely mineralize various organic pollutants at room temperature to form non-toxic small molecules such as H₂O and CO₂,which will not cause secondary environmental pollution.Therefore,photocatalytic technology has great application prospects in the environmental field.Graphite carbon nitride?g-C₃N₄?,a metal-free polymer semiconductor with a tri-s-triazine structure unit,have attracted much attention due to its unique layer structure,suitable band structure,metal-free properties,excellent thermal stability and chemical stability,facile synthesis and bandgap adjustability.However,insufficient absorption of visible light,low specific surface area and rapid recombination of photogenerated electron-hole pairs severely limit the application of g-C₃N₄ in the photocatalytic degradation of organic pollutants.In this study,g-C₃N₄was modified by element doping,nanosheet construction,heterostructure construction and Z-scheme system construction to improve its visible light absorption,specific surface area and reduce the recombination of photogenerated electron-hole pair.Meanwhile,we studied the performance and mechanism of the removal of antibiotics and dyes in water under visible light conditions.It provides new methods and new ideas for constructing high-efficiency photocatalysts based on g-C₃N₄ materials.The main contents and main conclusions of this study are as follows:?1?In order to regulate the energy band structure of g-C₃N₄,metal-free phosphorus and sulfur co-doped g-C₃N₄ was synthesized by one-step calcination of hexachlorotriphosphazene and thiourea.In this study,hexachlorotriphosphazene was used as the phosphorus source.While,thiourea served as self-doped sulfur source and g-C₃N₄ precursor.The results showed that the introduction of P and S atoms can significantly change the energy band structure of g-C₃N₄ and inhibit the recombination of photogenerated charges.UV-visible diffuse reflection indicated that the doping of P and S can broaden the visible light response region of the photocatalyst,thereby improving the efficiency of solar energy utilization.In addition,The much enhanced surface area of P,S co-doped g-C₃N₄ was owing to the P atoms and S atoms being doped into the g-C₃N₄ lattices inhibiting the crystal growth.Defects in the g-C₃N₄ framework caused by the doping of P and S can serve as a center for capturing photoelectrons,thereby suppressing the charge recombination and improving its photocatalytic performance.The best photocatalytic activity of P,S co-doped g-C₃N₄ for the degradation of tetracycline?TC?and methyl orange?MO?was 5.9 times and 7.1 times than that of g-C₃N₄,respectively.Recycle experiments showed that P,S co-doped g-C₃N₄ has well recyclability and stability.Radical species trapping experiments and ESR analysis indicated that h⁺and·O₂^-are the dominant active species.?2?In order to regulate the micro-nano structure and band structure of g-C₃N₄,metal-free nitrogen self-doped g-C₃N₄ was synthesized by thermally polymerization method.In this study,N,N-dimethylformamide?DMF?is used as the self-doped nitrogen source,and dicyandiamide is the precursor of g-C₃N₄.Thermal exfoliation was further used to prepare nitrogen self-doped g-C₃N₄ nanosheets.Compared with g-C₃N₄,nitrogen self-doped g-C₃N₄ nanosheets have a higher specific surface area(74.79 m² g^-1),enhanced visible light absorption,better photogenerated electron-hole separation ability and prolonged charges lifetime.Therefore,nitrogen self-doped g-C₃N₄ nanosheets exhibited the best photocatalytic efficiency?81.72%?for TC degradation under visible light irradiation in 60 minutes.Firstly,the introduction of nitrogen doping and midgap states are conductive to the narrowing of the band gap of DCN and the enhancement of optical absorption.Secondly,the morphology change from multi-layer structureto ultrathin nanosheets would shorten the distance for the photogenerated charges to reach the surface,thus enhancing the utilize efficiency of charges and reducing the charge recombination.In addition,the midgap states can also temporarily trap the excited electrons in conduction band and thus enhance the separation of photogenerated charge carries.Finally,the higher specific surface for accommodating massive absorption and reactive active sites,as well as facilitating the rapid transfer of reactants and products,were beneficial to the photocatalytic oxidation of TC.?3?In order to promote the separation of photogenerated electron-hole pairs of g-C₃N₄,a metal-free hexagonal boron nitride?h-BN?nanosheet/g-C₃N₄ heterojunction photocatalyst was prepared by in-situ growth method.The commercialized h-BN and dicyandiamide were uniformly mixed and then calcined in a muffle furnace to obtain a h-BN/g-C₃N₄ composite photocatalyst.SEM and TEM results showed that h-BN is uniformly distributed on g-C₃N₄ material,and a close contact interface is established.Negatively charged h-BN nanosheets attract photogenerated holes,thereby increasing the separation efficiency of photogenerated electron-hole pairs.Under visible light irradiation,the photocatalytic degradation efficiencies of TC and rhodamine B?RhB?by h-BN/g-C₃N₄ under optimal conditions were 79.7%and 99.5%in 60 minutes and40 minutes,respectively.Photocatalytic degradation is mainly through h⁺and·O₂^-oxidation.The photocatalytic ability of h-BN/g-C₃N₄ is enhanced mainly caused by the larger specific surface area and the formation of a heterojunction between h-BN and g-C₃N₄ which promotes efficient separation of photogenerated carriers.?4?In order to more effectively promote the separati on of photogenerated electron-hole pairs of g-C₃N₄,a phosphorus-doped g-C₃N₄/g-C₃N₄?PCN/CN?isotype heterojunction composite photocatalyst was prepared by a simple calcination method.Phosphorus doping can change the band structure and result in minor band difference between CN and PCN.Under the drive of inner electric field,photogenerated electrons will be transferred from PCN to CN,and holes will move from CN to PCN,which help to reduce the recombination and prolong the lifetime of photoinduced charges.Meanwhile,the PCN/CN isotype heterojunction also exhibited improved visible light absorption and larger specific surface area,leading to enhanced photocatalytic activity for TC degradation.The optimum photocatalytic activity of PCN/CN composite for TC degradation was approximately 3.8,2.8 and 2.9 times higher than that of pure CN,PCN and physically mixed counterpart,respectively.It also exhibited remarkable stability and repeatability.Radical species trapping experiments and ESR experiments demonstrated that h⁺and·O₂^-are the main active species for photocatalytic degradation of TC.?5?In order to promote the photogenerated electron-hole separation and retain strong redox ability,a direct double Z-scheme WO₃/g-C₃N₄/Bi₂O₃ photocatalyst was synthesized by one-step co-calcination.The photocatalytic activity of WO₃/g-C₃N₄/Bi₂O₃ composites for TC degradation is better than that of g-C₃N₄,WO₃,Bi₂O₃ and its binary composites under visible light irradiation.The enhanced photocatalytic properties of the WO₃/g-C₃N₄/Bi₂O₃ composites can be attributed to enhanced visible light absorption,increased surface area and improved separation efficiency of photogenerated electron-hole pairs.Radical species trapping experiments and ESR analysis confirmed that·O₂^-,h⁺and·OH are the main active species in the photocatalytic degradation of TC in WO₃/g-C₃N₄/Bi₂O₃ samples.The order of the influence is·O₂^->h⁺>·OH.The mechanism analysis showed that the photoinduced charge transfer of WO₃/g-C₃N₄/Bi₂O₃ composites follows a direct a direct solid-state Z-scheme mechanism rather than the type II heterojunction.Further,the composite photocatalyst exhibited high stability and recyclability.