Figure 4. TEM of (a) exfoliated graphene sheets with the inset SAED
pattern, (b) a single sheet of h-BN, (c) adjacent h-BN layers, post
processing, (d) BN sheets SAED pattern for (b) and intensity scan for
the dashed line23. (e) The proposed mechanism for
forming graphene spheres confining fullerene C60 within
the VFD, (f-j) SEM images of composite spheres, with an inset size
distribution33. (a-d) Reproduced under the terms of CC
BY 3.0 license23. Copyright 2012, Royal Society of
Chemistry. (e-j) Reproduced with permission33.
Copyright 2019, American Chemical Society.
The precise slicing of carbon nanotubes to obtain a narrow length
distribution with minimized defects except at the tube ends, holds
tremendous potential for various applications, such as in solar cell
technologies, sensors35,36, electronic devices, and
biomedical sciences.37 Several methods have been
reported for producing short carbon nanotubes using different
physical,38 electrical or chemical
strategies39. The dynamic thin film of liquid in a VFD
can laterally slice micron length BN and carbon nanotubes (BNNTs and
CNTs respectively) with17,21,37 and without the use of
lasers40, independent of the number of concentric
rings making up the tubes. Notably, the process reduces the level of
defects for sliced CNTs and occurs without the need for chemical
stabilizers and surfactants, with scalability of the process using the
continuous mode of operation of the VFD. In contrast, controlled lengths
of shorter nanotubes were found to be more substantial in the confined
mode for single-walled carbon nanotubes (SWCNTs), with potential for
drug delivery applications due to the suitable length scale with narrow
size distributions, Figures 5a-5c21. Indeed, the
availability of short single, double, or multi-walled carbon nanotubes
(MWCNTs) using VFD processing, where the side wall defects are minimal,
further highlights the unique capabilities of the VFD and the prospect
of uptake of the material in applications when specificity of the length
scale is imperative.
As well as slicing CNTs, the VFD is also effective in fabricating coiled
SWCNT nano-rings in high yield (80%) under continuous flow, with the
processing devoid of surfactants, and without toxic
chemicals41. The coiled nano-rings were fabricated
with wall thicknesses from 3 to 70 nm, with 300 nm being the average
diameter, Figures 5d-5j. Importantly the rings are formed with more
uniformity and without variation relative to the use of batch-to-batch
processing. Magnetic force microscopy established that the thickness of
the VFD fabricated SWCNT nano-rings affects their magnetic properties
with the magnetic interactions stronger for the thicker nano-rings due
to them being more efficiently packed, thus allowing electrons to tunnel
more efficiently. Compared to straight SWCNTs, the curvature of coiled
SWCNT nanoring influences their properties, endowing the coiled
structures with potential in a range of applications, for example,
electronic circuits and polymer composites as sensing devices, noting
the scalability of their VFD mediated fabrication. In recent
developments, there has been significant progress in understanding the
high-shear topological fluid flow within submicron dimensions in the
VFD, Figure 5o29,14. This understanding has enabled
the control of the ring size of coiled single-walled carbon nanotube
(SWCNT) toroids based on the solvent systems (water with toluene,m -xylene or p -xylene), diameter of the VFD tube (10, 15
and 20 mm outside diameter) and the shape characteristics of its reactor
base (hemispherical or flat-base), all achieved under confined mode VFD
processing42. By carefully manipulating these
parameters, selective control over the diameter of the resulting toroids
of SWCNTs down to about 35 nm in diameter has been established.
Aside from SWCNT transformations, the VFD thin film microfluidic
platform can also decorate MWCNTs with superparamagnetic magnetite
(Fe3O4) NPs43. This
process was a three-step-in-one operation: (i) Bulk iron metal was
ablated using a pulsed laser at 1064 nm to generate iron oxide
superparamagnetic NPs in situ , (ii) MWCNTs were sliced, and
finally, (iii) the surface of the MWCNTs was decorated with the NPs, as
shown in Figures 5k-5n. Generating this composite material directly from
stock MWCNTs with minimal processing time and without the use of harsh
chemicals further demonstrates the versatility of the VFD. The composite
material was utilized for supercapacitor measurements as an electrode.
At a scan rate of 10 mV s-1, high areal capacitances
and gravimetric of 1317.7 mF cm-2 and 834 F
g-1, respectively, was established, being superior to
those reported using similar materials prepared using other methods.
Moreover, at the specific power of 2085 W kg-1, the
VFD fabricated material also had a substantially higher specific energy
of 115.8 W h kg-1, thus demonstrating potential as a
material for next-generation energy storage devices.