Exploring graphene exfoliation using thin film and Taylor Couette flow in a rotating cylinder device
Creators: name: Farooq, Usmaan
Publisher: name: Imperial College London
Year: 2021

description: Graphene is a 2D material with exceptional properties and a wide range of applications. Production techniques are varied and can generate graphene with a wide range of quality and cost. Liquid phase exfoliation is a production technique that has the potential to generate graphene on an industrial scale. In this thesis, we examine one such technique where exfoliation takes place in a device comprising two concentric cylinders, the inner one of which rotates at constant rotational speed while the outer remains stationary, which is typical of a so-called Taylor Couette configuration. Experimental work is conducted to identify the key parameters which influence exfoliation. These are coupled with detailed numerical simulations of the flow dynamics within the device, in order to understand the role of the complex flow field on the rate of exfoliation. Analysis of the influence of rotational speed, initial graphite particle size, and the choice of solvent is conducted. Whilst the quality of the graphene produced is largely dictated by the centrifugation strategy, the concentration varies significantly with a change in these parameters. UV-vis and Raman spectroscopy are used to determine concentration and quality of the graphene, and the concentration of graphene is in the region of 0.2 mg/mL which compares favourably to common liquid phase exfoliation techniques. An increase in the rotational speed of the inner cylinder resulted in a significant rise in the concentration, due to an increase in the shear rate associated with the narrow gap between the cylinders. Use of a larger initial particle size results in higher concentrations, attributed to the increase in the number of exfoliation sites available. Finally, a change in solvent from N-Methyl-2-pyrrolidone is explored, which has excellent performance but drawbacks from a health and safety perspective. Use of binary solvents, IPA-water and ethanol-water, lead to lower concentrations, mitigated by the potential for significant scale up. In order to understand the exfoliation mechanism in more detail, the two flow regimes, Taylor-Couette flow in the gap region, and a film region on the outer surface of the inner, rotating cylinder are studied in detail using numerical simulations in the absence of graphite/graphene particles. For the case of the film, a study of a general case of a thin film adhering to the surface of a rotating cylinder is conducted, followed by simulations of the flow configuration which matches the experimental one. These studies demonstrated that an increase in the rotational speed causes a stabilising effect due to the increased centrifugal force, leading to a reduction in the rate of exfoliation. This is in contrast to the gap region wherein an increase in the rotational speed enhances exfoliation, and the vortical structures associated with Taylor-Couette flow have a significant influence on maximising the exfoliation rates. Finally, simulations of particle-laden flows, designed to mimic the presence of the graphite/graphene particles, show that larger particles preferentially migrate towards the cylinder walls, where the shear rates are highest, maximising exfoliation.