The Exfoliation Of Graphene-analogous Two-dimensional Materials（BN, MoO₃）with The Assistant Of Supercritical CO₂ Andfurther Application

Two-dimensional（2D） materials shows many outstanding physical and chemical properties, having great potential in the progress of new material science. The discovery and development of graphene have sparked the research on graphene-analogous two-dimensional materials, such as hexagonal boron nitride（h-BN）, molybdenum disulfide（MoS2）, tungsten disulfide（WS2） and molybdenum trioxide（MoO₃）. In order to make full use of the fabulous 2D materials, it is of great importance to fabricate them with defined structure and property. Graphene-analogous two-dimensional materials can be prepared via bottom-up and top-down approaches, including chemical vapor deposition（CVD）, micromechanical cleavage, sonication-assistant solvent exfoliation, et al. Unfortunately, these approaches may induce some drawbacks in fabrication or application of the materials. Therefore, searching for a method of low-cost, easily manageable and scalable to fabricate 2D nanomaterials still remains a challenge.Supercritical carbon dioxide（SC CO₂） is regarded as alternative to traditional organic solvents, with several advantages. In particular, SC CO₂ is quite soluble in many liquids, making it possible to change the properties of liquids continuously by controlling the pressure or temperature of SC CO₂. In addition, CO₂ molecules can intercalate into the interlayer of 2D materials due to its high diffusivity and compressibility, expanding the space between the interlayer and thus resulting in a decrease of the interaction forces between layers, and thus achieve the exfoliation and functionalization. In this thesis, we exfoliate BN and MoO₃ nanosheets by the behaviors of emulsions produced by SC CO₂, water and ethanol. Then the applications of reinforcement of polymer and photocatalytic degradation are investigated respectively. The research contents are shown as follows.（1） The exfoliation of BN nanosheets and further application in polymer reinforcementBN nanosheets can be exfoliated with efficiency in emulsions produced by SC CO₂/surfactant/water system, and the obtained nanosheets are structural integrity. The good diffusivity and compressibility of SC CO₂ make it possible to penetrate and intercalate into the interlayer of BN, expanding the space between layers and thus decreasing the interaction forces. The further increase in pressure can induce the the emulsion transition from micelles into reverse micelles and result in the exfoliation. Further we use the exfoliated BNNS as the filler to its effect on the mechanical properties of polyvinylalcohol（PVA） substrate. With a small amount loading, the mechanical properties of the nanocomposites are largely enhanced.（2） Fabrication of MoO₃ nanosheets and its application in photocatalytic degradationIn SC CO₂/water/ethanol system, MoO₃ nanosheets are successfully fabricated by Pickering emulsions and strong shear forces induced by magnetic stirring. The characterizations of the morphology and structure indicate the structural integrity and the decrease of the number of the layers. In addition, some of the nanosheets are certified to have crystal transition from α-MoO₃ to h-MoO₃ during the exfoliation process. It may be induced by the lattice twisting arising from the absorption of CO₂ and water molecules. Compared with bulk material, MoO₃ nanosheets showed higher photocatalytic degradation activity, indicating potential application in degradation of organic dyes and environment protection.