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.