Graphite Carbon Nitride Nanomaterials:Synthesis And Photocatalytic Performance

With the development of economy and the continuous growth of population,the energy consumption on the earth is increasing daily.The burning of traditional fossil fuels produce a large number of greenhouse gases that affect climate change which fossil fuels can not regenerate.The continuous consumption will lead to the energy crisis.Therefore,it is necessary to develop a new type of clean energy instead of fossil fuels.Solar energy is considered as the ideal energy source for the future society due to its wide distribution,easy access and renewable.Photocatalytic technology is driven by solar energy to stimulate semiconductors to produce photogenerated carriers,which participate in the following redox reactions.The key of photocatalytic technology is to design highly efficient and stable photocatalyst semiconductor catalysts.Graphite carbon nitride（g-C₃N₄）is a new type of organic non-metallic semiconductor material with a band gap of 2.7 e V,which is easy to prepare,non-toxic and easy to recover.It has been widely used in the field of photocatalysis.However,in general,the bulk g-C₃N₄（BCN）has small specific surface area,few surface active sites and limited absorption and utilization of visible light which limits the development of g-C₃N₄.In addition,high carrier recombination rate and low electron mobility seriously affect its photocatalytic activity.Therefore,the bulk g-C₃N₄ needs to be modified to improve its photocatalytic performance.Therefore,in this work g-C₃N₄ is modified by introducing functional groups,element doping and high temperature heat treatment,the structure property and photocatalytic performance of the modified g-C₃N₄ have also been investigated.The specific works are follows:（1）Cyano group（-C≡N）modified g-C₃N₄nanosheets（MCN）was prepared by an alkali-assisted method,in which melamine and KOH were mixed evenly and then calcined.The successful introduction of cyano group was confirmed by Fourier transform infrared spectroscopy（FT-IR）.After the introduction of cyano groups,the conduction band（CB）of g-C₃N₄ moves from-1.34 V to-1.55 V,leading to a decreased band gap and an expanded absorption in visible light region.In addition,the existence of cyano group help to enhance the photogenerated carrier mobility of g-C₃N₄,reduce the charge transfer impedance,and improve the photoelectrochemical properties of the bulk g-C₃N₄.Moreover,as a strong electron-absorbing group,-C≡N can also be used as the active site for N₂adsorption and activation after being introduced into the g-C₃N₄ structure.The experimental results show that MCN has a better photocatalytic nitrogen fixation performance than that of the bulk g-C₃N₄（BCN）,the NH₄⁺production rate is about 3.8 times higher than that of the BCN.（2）Using ammonium molybdate as a molybdenum source,dicyandiamide as the precursor of g-C₃N₄,the g-C₃N₄nanosheets doped with Mo（MOCN）was successfully prepared by calcination under a high temperature.The X-ray photoelectron spectroscopy（XPS）and energy dispersive X-ray element surface scanning（EDX-elemental mapping）shows that Mo atoms have been successfully doped into the structure of g-C₃N₄.X-ray powder diffraction（XRD）shows that element Mo is uniformly dispersed in g-C₃N₄.After the Mo doping,the band gap of g-C₃N₄ is significantly reduced,resulting in an enhanced visible light absorption.Compared with the bulk g-C₃N₄（BCN）,MOCN exhibits a superior performance in the photocatalytic degradation of RhB.（3）Porous g-C₃N₄ nanosheets with enhanced visible light utilization（CN-800）were prepared by one-step rapid thermal treatment（800℃）of the bulk g-C₃N₄（BCN）.The structure and properties of CN-800 were characterized.Compared with BCN,CN-800 possess a better light response and carrier migration ability,as well as a larger specific surface area.It is worth mentioning that high temperature treatment makes the conduction band（CB）of g-C₃N₄ moves from-1.32 V to-1.45 V,thus the electrons reduction ability of CN-800 is enhanced.Therefore,CN-800 exhibits an improved photocatalytic degradation of RhB.