Exfoliation and characterization of 2D layered nanomaterials

The objective of this research was to exfoliate 2D layered materials in liquid phase. The layers of the nanosheets are stacked together by van der Waals attraction forces in the parent materials. Liquid phase exfoliation is necessary to overcome the attraction forces in the parent materials. Various techniques are applied to synthesize these nanomaterials such as chemical vapor deposition (CVD) on a substrate, or liquid phase exfoliation in a solvent. However, there are difficulties associated with the use of CVD grown graphene for composite production. Mixing of CVD grown graphene powder with polymer melts is hard due to aggregation of graphene. On the other hand, the nanomaterials exfoliated in the liquid phase can be easily mixed in different polymers. Some organic solvents and ionic liquids have both been candidates to disperse graphene directly without dispersant. However, the yield of graphene in organic solvent is comparatively lower than in ionic liquids. Ionic liquids, also described as designer solvent, can be synthesized with preferred functional groups which are able to stabilize graphene. The phenyl groups in the cation of ionic liquids non-covalently π-π stack with the graphene surface and prevent aggregation by electrostatic repulsion. Hence, high quality and high yield production of graphene is possible by the aid of these designer solvents. We have utilized a dissolved polymer to disperse inorganic 2D layered nanomaterials such as boron nitride nanosheets, molybdenum disulfide, and tungsten disulfide in water and a range of organic solvents, which are resistant to aggregation. The aqueous dispersions were characterized by microscopy and dynamic light scattering (DLS) that helped to determine the lateral size, number of layers of nanosheets, and structure of powdered samples. The dispersions were freeze-dried and redispersed in water to check dispersion stability against centrifugation. Finally, a spray drying technique was utilized to convert 2D nanosheets to 3D crumpled nanosheets.