Utilising the hydrodynamics of thin film flow over a spinning disc for graphene exfoliation

Abstract

Graphene, a 2D nanomaterial, has remarkable mechanical, electrical, thermal and optical properties, which has the potential to transform the technological landscape. Methods currently available for graphene production are not at an industrial scale, with rates rarely exceeding 0.04 g/h. This thesis investigates the use of the spinning disc as a vehicle for the scale-up of graphene production by linking how the hydrodynamics of the spinning disc impacts liquid phase exfoliation. High speed imaging results identifi ed the flow regime showing that it is as a direct result from the balance of the key forces (inertia, surface tension and viscosity). The regimes identi ed were smooth, spiral, transition to 3D and fully 3D waves. Increasing the body force relative over surface tension and viscosity stipulates a change in regime suggesting a critical Reynolds number is needed to be exceeded locally on the disc for each of the flow regime. Both 2D and 3D direct numerical simulations further reinforced this interplay between the forces and established that the spiral wave regime could not generate enough local shear rate to exceed the critical needed for exfoliation of 104 s-1 which will be predominantly governed by the 3D regime. Graphene exfoliation was governed by the hydrodynamics in particular related to the shear rate and residence time which determined the yield of about 0.25 wt % with natural graphite precursor for the highest shear rate after 6 hours of processing. Shear rate was also found to impact lateral size and selectivity with the highest shear giving sizes between 0.1 - 5 micro m and selectivity of 90 % to 1 - 4 layers. The Raman ID/IG ratio of 0.14 and ID/ID' of 1.8 suggested it was mostly edge defects that were present implying high quality graphene was produced. To further reduce the interlayer cohesive energy to expedite the exfoliation process, pre-processing the graphite either via pre-sonication or ball-milling was carried which led to an increase of the yield to 1.1 wt % (production rate of 0.01 g=h) however some of the graphene sheets were wrinkled due to the introduction of the ball milling.