Preparation And Photocatalytic Performance Of Carbon Nitride Based Composite Materials

With the rapid development of modern industry,environmental problems are becoming more and more prominent due to the excessive use of energy and improper use of the resources,especially the pollution of water resources,which poses a great threat to human life and health.In order to actively protect and improve the environment,we must develop green technologies,use clean energy,and deal with existing environmental pollutants.Semiconductor photocatalytic technology has the advantages of high efficiency,low price and environmental friendliness.Besides these,it also has outstanding advantages in environmental protection and water treatment technology.Therefore,it is an effective solution for improve environmental problems by using semiconductor photocatalytic technology to treat environmental pollutants.Recently,graphitic carbon nitride（g-C₃N₄）has attracted extensive attention from researchers due to its metal-free,unique electrical,optical,physical and chemical properties,especially its photocatalytic properties.However,g-C₃N₄ has a high photogenerated carrier recombination rate and a low charge mobility,which largely limits its photocatalytic activity.In order to improve the photocatalytic activity of g-C₃N₄,researchers have proposed many effective strategies,such as elemental doping,semiconductor recombination and morphology conversion and so on.This study took g-C₃N₄ as the research object,and aimed to combine g-C₃N₄ with another semiconductor with matching energy band structure to form the heterostructure,so as to reduce the recombination rate of photoinduced electron hole pairs and improve its photocatalytic efficiency.First of all,g-C₃N₄ powder was prepared using melamine as a precursor by pyrolysis polycondensation method,and exfoliated into g-C₃N₄ nanosheets（NSs）by liquid-phase ultrasonication.Then,a series of different proportions of AgX（X=Cl,Br,I）/g-C₃N₄ composites were prepared by the deposition precipitation method.The morphologies of AgX/g-C₃N₄ composites were characterized and AgX/g-C₃N₄composites were applied to the selective photocatalytic oxidation of organic compounds1,4-dihydro-2,6-dimethyl-3,5-dicarboxylate（1,4-DHP）and photodegradation of the organic pollutant rhodamine（RhB）.When the ratio of Ag I to g-C₃N₄ was 30:100,1,4-DHP was almost completely selectively dehydrogenated into its pyridine derivatives within 30 min of visible light irradiation without further deep oxidation or degradation.The photocatalytic degradation efficiency of the optimized AgX/g-C₃N₄ for Rh B reached more than 97%within 20 min of visible light irradiation.AgX/g-C₃N₄ composites exhibited stronger photocatalytic activity duo to the unique heterostructure promoted the transfer and separation of photoinduced charge carriers.Secondly,sulfur doped g-C₃N₄（referred to as S-g-C₃N₄ or SCN）was prepared by calcining thiourea several times at high temperature,and then Bi OX（X=Cl,Br,I）were deposited onto the surface of S-g-C₃N₄ by hydrothermal method,respectively.A range of new type BiOX（X=Cl,Br,I）/SCN composite photocatalysts with good structure were constructed successfully.The morphologies of BiOX/SCN were explored by various characterization methods,and the photocatalytic activities of the composite materials for 1,4-DHP and RhB were tested,respectively.The experimental results showed that the photocatalytic activity of S-g-C₃N₄ has been improved compared with g-C₃N₄,and the combination of S-g-C₃N₄ with BiOBr or BiOCl was contributed to the photocatalytic degradation of RhB and photocatalytic oxidation of1,4-DHP under visible-light irradiation.In addition,the BiOX/SCN catalyst showed good repeatability in 1,4-DHP photocatalytic oxidation,and the photocatalytic degradation efficiency remained above 80%after five cycles.The improved photocatalytic activities were attributed to the excellent carrier separation efficiency as well as the heterojunction formed at the interface of the composite photocatalyst.Finally,A series of of Ag₃PO₄/SCN composites with different ratios were synthesized by loaded Ag₃PO₄ onto the S-g-C₃N₄ used a simple precipitation method.The composition and morphology of the Ag₃PO₄/SCN composites were explored and the photocatalytic activities of the Ag₃PO₄/SCN composites for 1,4-DHP and RhB were also investigated,respectively.The experimental results reveal that the introduction of Ag₃PO₄ not only obtained a broader spectral response,but also had a good photoinduced separation efficiency of electrons and holes,thereby improved the photochemical properties of S-g-C₃N₄.The oxidation rate of 1,4-DHP over the best Ag₃PO₄/SCN reached 91%in 20 minutes under visible-light irradiation.Moreover,the degradation rate of RhB over the optimal Ag₃PO₄/SCN sample under visible-light irradiation within 10 minutes reached 98%.The Ag₃PO₄/SCN sample maintained stable photocatalytic activities during photocatalytic oxidation of 1,4-DHP.The results of free radical trapping experiments showed that the hole（h⁺）was the main active species in the reaction system.At the same time,we proposed possible photocatalytic pathways for 1,4-DHP and Rh B.