Modification Of Graphitic Carbon Nitride For Enhanced Photocatalytic Activity

Graphitic carbon nitride?g-C₃N₄?,containing only carbon and nitrogen elements,has been widely applied in the fields of photocatalysis.As a metal-free polymer semiconductor,g-C₃N₄ has drawn much attention due to its numerous outstanding features,such as easy preparation,fairly good chemical and thermal stability,suitable band gap?E_g=2.70 eV?with response to visible light.However,g-C₃N₄ prepared by thermal polymerization displays relatively low surface area,limited response to visible light and low electron-hole pair separation rates,further restricting its practical application.In this paper,the surface area of g-C₃N₄ was enhanced by nitric acid exfoliation and gas bubble exfoliation,and then Ag nanoparticles were doped on the g-C₃N₄ nanosheets by photo-reduction method to enhance the separation rates of photogenerated electrons-holes,resulting in the improvement of photocatalytic performance.?1?g-C₃N₄ nanosheets?CN-N-X?were prepared by ultrasonication assisted nitric acid exfoliation method from bulk g-C₃N₄?b-CN?,which was prepared by thermal polymerization of melamine.The obtained products were characterized by scanning electron microscopy?SEM?,solution adsorption method and X-ray diffraction spectrometry?XRD?.And the photocatalytic performance of samples were investigated.The results exhibit that,the CN-N-2 possesses a special surface area of 16.12 m²/g when the concentration of HNO₃ was 2 M,and shows the best photocatalytic activity.The enhanced photocatalytic performance is attributed to the increased active sites of the nanosheets owing to the increased specific surface area after acid treatment.?2?g-C₃N₄ nanosheets?CNNs?were fabricated by gas bubble exfoliation method from b-CN.The morphology,thickness and special surface area were mearsured by transmission electron miccroscopy?TEM?,atomic force microscopy?AFM?and solution adsorption method.The photoelectric and photocatalytic properties of the samples were tested.The results show that the thickness of CNNs is about 7.24 nm,the specific surface area is 16.22 m²/g,and the photoelectric and photocatalytic properties is improved.It is caused by the generation and expansion of gas bubbles obtained from the electrolysis of water,which provide essential and dominant driving force to overcome the interlayer van der Waals force of b-CN and achieve the exfoliation.?3?Ag/g-C₃N₄ composite photocatalysts were synthesized by photo-reduction method to dope Ag nanoparticles on CNNs.Moreover,the effects of composite Ag contents on the photocatalytic activity were assessed.The results show that with the introduction of Ag nanoparticles,the absorption of visible light and the separation efficiency of photogenerated electron-holes are enhanced,resulting in the increasement of photocatalytic preformance.Among them,when the mass fraction of Ag is 6%,the 6%Ag/CN exhibit the best photocatalytic activity.The enhancement of the photocatalytic performance is due to the synergistic effect of Ag and g-C₃N₄ promoting the absorption of visible light and the separation of photogenerated electron-hole.