Study On Preparation Of Two-dimensional Materials By Liquid Nitrogen Assisted Shear Exfoliation Of Layered Material

Two-dimensional layered materials such as graphite and black phosphorus are connected by van der Waals forces.By weakening the van der Waals forces between layers,it can be stripped into a two-dimensional material with fewer layers.The results show that this kind of two-dimensional material has a very good application prospect in energy storage,catalysis,medical,sensor,semiconductor and other fields.The key to the application of two-dimensional materials lies in their efficient preparation.Aiming at the problems of low efficiency,high cost and the use of organic solvents in the preparation of two-dimensional materials,this paper explores the advantages of liquid nitrogen assisted stripping by improving the disadvantages of traditional shear stripping method（the use of organic solvents and surfactants）,and develops a low-cost,time-saving,efficient and environmentally friendly method-liquid nitrogen assisted shearing stripping method.Taking graphite as the main object of study,the stripping research was carried out.Firstly,the pretreatment of graphite was studied,and the effect of temperature and atmosphere on the graphite layer spacing was discussed.On this basis,a control experiment was set up to verify the superiority of liquid nitrogen assisted shear stripping method,and the stripping mechanism of this method was preliminarily clarified.Finally,the universality of the method was verified by the preparation of high-quality nano black phosphorus.Specific research work is as follows:（1）XRD,Raman,SEM and XPS were used to characterize the graphite before and after pretreatment.The results show that the higher the heating temperature and the larger the inert atmosphere（N₂）can effectively increase the graphite layer spacing during the pretreatment process.The inert atmosphere（N₂）and liquid nitrogen in the flowing state can cooperate to enhance the initial stripping degree of graphite.After being treated with high temperature and inert atmosphere（N₂）,the oxygen content of graphite decreased from 9.82%（high temperature,air atmosphere）to 5.32%,indicating that the inert atmosphere can effectively prevent the oxidation of graphite due to high temperature and reduce the generation of defects.The results laid a foundation for the preparation of graphene by liquid nitrogen assisted shear stripping.（2）Through comparative experiments,the advantages of liquid nitrogen assisted shear stripping method were proved,and the stripping mechanism of two-dimensional materials prepared by liquid nitrogen assisted shear stripping method was clarified.The structure and morphology of graphene prepared by different methods were characterized by XRD,Raman,SEM and UV-vis.The results show that compared with liquid nitrogen static and liquid nitrogen oscillation stripping,the preparation efficiency of liquid nitrogen assisted shear stripping is higher,which verifies the superiority of liquid nitrogen assisted shear stripping.The graphene was characterized by XRD,Raman,SEM,TEM,AFM and UV-vis at different peel times.The results show that there is an extreme value in the relationship between the peel time and the graphene concentration.Under the system used in this study,when the peel time is 2.5 h,the concentration of the graphene dispersion is no longer significantly increased with the extension of the peel time.The graphene produced by this method has the characteristics of ultra-thin and few layers,the thickness of the lamellar is about 1.12nm（2 layers）,and the degree of oxidation of the graphene is very low,realizing the efficient preparation of low-cost,defect-free and high-quality graphene.On this basis,the stripping mechanism of two-dimensional materials prepared by liquid nitrogen assisted shear stripping was preliminarily clarified.At the same time,the method has a certain universality and can be used for the stripping of other two-dimensional layered materials,which lays a foundation for the preparation of other two-dimensional materials.（3）Using liquid nitrogen as solvent and intercalating agent,black phosphorus nanosheets were successfully prepared by high-speed shear stripping method,which verified that liquid nitrogen assisted shear stripping of layered materials to prepare two-dimensional materials has a certain universality.The black phosphorus nanosheets were characterized by XRD,Raman,SEM,TEM,XPS and UV-vis.The results show that the transverse size of black phosphorus nanosheets is about 50 nm,and the oxidation degree is very low,and the high-quality black phosphorus nanosheets can be prepared efficiently.It provides a new idea for the batch preparation of high-quality black phosphorus nanosheets.