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Two-dimensional (2D) nanomaterials, such as Graphene, hexagonal Boron Nitride (h-BN) and Molybdenum Disulfide (MoS2), consisting of a single layer of atoms are finding numerous applications because of their unique properties such as high mechanical strength, high electrical and thermal conductivities, high surface areas effects, etc. These attributes necessitate developing mass production of 2D nanomaterials. In this study, the newly conceived compressible flow exfoliation (CFE) method for producing 2D h-BN nanosheets and the underlying mechanism behind the CFE method have been investigated. In the CFE method, high-pressure gas carrying the 2D nanomaterials passes through a partially open needle valve. The adjustment of the needle valve makes a smaller channel which acts as a converging-divergent nozzle where supersonic flow (Ma>4) is achieved that eventually creates multiple shock waves. This gas-solid mixture is sprayed in a suitable solvent, where a significant portion (up to 10%) of the initial hexagonal boron nitride (h-BN) material is found to be exfoliated with a mean thickness of 4.2 nm. Moreover, 43% of the exfoliated h-BN was found to be 10 layers or less thick while having a mean flake length of 276 nm and a concentration of 0.22 mg/mL in Isopropyl Alcohol (IPA) after centrifugation. The residence time of the materials was calculated to be 4 microseconds. This supersonic flow velocity and the shock structure around the flow impacts the nanomaterials by creating a drag force. To understand the standing shock wave and flow structure within, a simple commercial convergent-divergent (CD) nozzle and for creating traveling shock, an experimental set up of a shock tube is used. It has been found that traveling shock wave does not exfoliate materials while some accelerating flow is needed. From a standing shock wave, it has been found that a drag force is required to exfoliate the material by acceleration at nozzle throat and work done on nanoparticles is roughly proportional to how much exfoliation occurs. The approximate cut-off force to initiate exfoliation is about 30μN. The fast and continuous nature of the CFE process makes it more advantageous over current 2D material exfoliation methods by offering environmentally friendly processing, reduced occurrence of defects, and versatility to be applied to any 2D layered material using gaseous medium. |
