Preparation And Modification Of Graphitic Carbon Nitride For Enhanced Photocatalytic Activity

In recent years,the rapid development of society has not only caused excessive use of energy,but also caused various environmental pollution.Developing clean,efficient and renewable new energy sources has become one of the top priorities for researchers and semiconductor photocatalysis technology was considered an effective strategy.As a visible light response semiconductor photocatalyst,graphitic carbon nitride?g-C₃N₄?has attracted much attention due to its outstanding characteristics of cheap raw materials,simple synthesis methods,good thermal stability,chemical stability,narrow bandgap?2.7eV?and appropriate position of energy band.However,the as-obtained of g-C₃N₄ by thermal polycondensation possesses fatal shortcoming,which small specific surface area,few reactive sites and high recombination rate of photogenerated electron-hole pairs,and thus leading to unsatisfactory photocatalytic activity.Therefore,it is imperative to prepare a highly efficient g-C₃N₄-based photocatalyst to improve the photocatalytic activity under visible light.In this paper,in order to solve the shortcomings of g-C₃N₄,various modification approaches have been proposed to improve the photocatalytic performance of g-C₃N₄,such as preparation of various nanostructures and construction of a composite with another co-catalyst or semiconductor.g-C₃N₄ obtained by various modification approaches possesses more active sites,higher specific surface area and more effective separation efficiency of photogenerated carriers,and thus improve photocatalytic activity.The morphology and structure of as-obtained photocatalysts were discussed by different analysis methods.Meanwhile,the photocatalytic activity and mechanism of as-obtained photocatalysts were analyzed by photocatalytic degradation and active species trapping experiments.The mainly three parts in the paper are as follow:?1?Ultrathin g-C₃N₄ nanosheets?U-CNs-4?were prepared via short time sonication-assisted exfoliation and solvothermal treatment method in ethylene glycol solvent.It was found that the obtained ultrathin g-C₃N₄ nanosheets with a thickness of ranges from 4 to 5 nm.Result show that the U-CNs-4 exhibited obviously enhanced photocatalytic activity in comparison with other samples.Meanwhile,the degradation efficiency of RhB was 4.4 times higher than that of the bulk g-C3N4?B-CN?.The enhanced photocatalytic activity was mainly due to that the U-CNs-4 photocatalyst increased its specific surface area,provided more reactive sites,enhanced oxidation ability of holes?h⁺?and improved transport ability of photo-charges,which improving separation efficiency of photogenerated electron-hole pairs and photocatalytic activity.Finally,the possible for the enhancing photocatalytic mechanism was revealed based on the active species trapping and electron spin resonance?ESR?experiments.?2?Graphene as a cocatalyst in the photocatalyst can not only reduce the recombination of photogenerated electron-hole pairs,but also retain prominent redox ability.Various 2D/2D hybrid heterojunction photocatalysts with Graphene?including GO and RGO?and g-C₃N₄ nanosheets?CN-S?were fabricated by an electrostatic self-assembly strategy.The obtained 2D nanostructures of CN-S/GR were explored by electron microscopy analysis.Meanwhile,based on the DRS,PL and photoelectrochemical analyses,the introduction of graphene in various photocatalysts can greatly improve the light absorption and photogenerated charge electron-hole pairs separation and migration of CN-S.Compared to other samples,the CN-S/RGO-C nanocomposite,including RGO obtained by reduction of GO using NaHSO₃ as a reducing agent,showed the best photocatalytic performance toward RhB degradation,which is 5.8 times higher than that of CN-S.Lastly,a visible-light photocatalytic degradation mechanism associated with CN-S/GR nanocomposites was also proposed.?3?A ternary Z-scheme photocatalytic heterojunction of g-C₃N₄ nanosheet/WO₃/GO?CWG?was prepared by in-situ growth of WO₃ during thermal exfoliation bulk g-C₃N₄ to g-C₃N₄ nanosheet,followed by electrostatic self-assembly of GO and g-C₃N₄nanosheet in the HNO₃ solution.Compared to other photocatalysts,the CWG heterojunction exhibited outstanding photocatalytic efficiency for RhB degradation.The enhanced photocatalytic activity of CWG photocatalyst was mainly due to synergistic effect of the three components:the formation of Z-scheme heterojunction between g-C₃N₄ nanosheet and WO₃ inhibited the rapid recombination of photogenerated carries and preserved excellent redox ability,GO as co-catalyst not only promoted the charge transfer but also provided abundant reactive sites and broadened light absorption.In addition,the possible for the enhancing photocatalytic mechanism was revealed based on the active species trapping and ESR analysis.