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.