3. Top-Down Nanomaterial Transformation
The pre-eminence of utilizing a VFD for top-down nanomaterial fabrication is evident by the high shear stress within dynamic the thin film in the device being effective for controlled exfoliating of 2D laminar materials, in gaining access to material for use in a wide range of applications. For instance, hexagonal boron nitride (h-BN) and single-layered graphene sheets can be effectively exfoliated from bulk material by using VFD processing23. For specific applications it is essential to develop a simple approach to generate feasible amounts of pristine 2 D material without any chemical degradation or imparting defects, and for this the VFD can be effective. Graphene synthesis from graphite oxide or graphite, for example, using solution-based methods, such as high energy wet ball milling30 or high power sonication31, 32, can deleteriously affect the properties of the resulting graphene. To this end, Chen et al . developed the VFD as device for imparting tunable mechanical energy simply by varying the rotational speed, ω, of the tube, to control the exfoliation of oxide-free graphene and also h-BN sheets from graphite and bulk h-BN respectively, in N-methyl-pyrrolidone, Figure 4. This work is the first report on using the VFD to essentially disassemble material, and indeed for the VFD in general, and is effectively a paradigm shift for the top-down fabrication of nanomaterials23.
The high shear stress (mechanical energy) in the VFD is also effective in exfoliating graphene from graphite as spheres confining self-assembled fullerene C6033. The spheres form in high yield in the VFD through micro-mixing ao -xylene solution of C60 and a dispersion of graphite in DMF at room temperature, without the need for auxiliary substances and surfactants33. Interestingly their formation involves both top-down (graphene exfoliation) and bottom up (C60 self-assembly and simultaneous confinement) processes. The spheres are uniform in shape and have a size distribution of 1.5 to 3.5 µm with the ability to control their diameters by varying the VFD operating parameters, Figures 4e – 4j. As an electrode, the composite material has high cycle capacitance stability, with capacitance maintained at a high scan rate of 100 mV s-1 at 86.4 mF cm-2 (83.5%) and 24.7 F g-1, and high areal capacitance of 103.4 mF cm-2. These findings augur well in developing a range of all carbon material for energy storage applications. Moreover, the ability to prepare such material provides tantalizing possibilities for making composites of fullerene aggregates shrouded by other 2D material, with different properties in general. In this context, a composite of graphene oxide shrouding self assembled fullerene C70has recently been reported34.