4. Bottom-Up Nanomaterial Transformation
Further advantages of the VFD include its ability to create
self-assembly structures such as micelles44,
mesoporous nanomaterials45,46,
hydrogels47, fullerene and its
composites48,33,29 and vesicles49and the potential to control the shape and size of NPs during formation
and for regulating the thickness and strength of coating at the
nanoscale.
The VFD has also been utilized as an innovative strategy for preparing
mesoporous silica prior to the required calcining to remove the polymer
template44. Tong et al . reported that the
processing time of the VFD was less than an hour, with 5 hours being the
overall duration for the synthesis involving 60 mL of solution,
contrasting to the 48 hours required for the traditional materials
involving hydrothermal batch processing45. Moreover,
the VFD process operated at ambient pressure and temperature,
significantly reducing energy consumption, and remarkably the pore size
can now be controlled to fine-tune the properties of mesoporous silica,
Figure 6. This is under continuous flow VFD processing such that there
is potential for scaling up the synthesis of the pre-calcined material
with uniformity of the product along with controlled pore wall thickness
and pore size. The latter relates to the micelles being stretched by the
shear forces in the thin film within the VFD. Consequentially, the
increase in pore wall thickness increased the thermal stability of the
calcines material, which is essential for high-temperature applications.
Tong et al . also developed an effective method for the nucleation
and growth of palladium NPs in situ in the above micelle
templated mesoporous silica synthesis, also as a continuous flow VFD
process, then for calcining under batch processing46.
The Pd/SBA-15 composite calcined material is effective in a wastewater
treatment process for removing nitrate-nitrogen, Figure 6, with the
maximum adsorption value for nitrate-nitrogen at 9.5
mgL-1 after 16 hours, upwards of 41% removal
efficiency, with 36% removal efficiency upon recycling. Because
Pd/SBA-15 is easily separated from the effluent, the relatively low
nitrate-nitrogen adsorption is significant. Of particular note is that
the Pd NPs in the composite material are entirely embedded within the
pores rather than on the surface of the mesoporous silica particles,
which is important in avoiding leaching of the NPs and for taking
advantage of any catalytic process being enhanced within the confined
space within the pores.
Silica hydrogel is readily formed in the VFD in water under continuous
flow as a benign process, without the need for organic solvents or any
other acids or bases, with significantly reduced processing times
relative to batch processing47. Curcumin nanoparticles
were incorporated into the silica in situ through the same
processing, with the silica hydrogel converting to an xerogel 3.5 hours
post VFD processing. The functional composite material showed improved
inhibition of bacterial growth compared to bulk curcumin, Figure 6. Here
the xerogel silica network provides shielding for the curcumin particles
for then slow leaching of the curcumin which becomes bioavailable. The
xerogel silica has potential for drug delivery applications, noting that
the overall process of incorporating drugs in situ is simple, without
the need for auxiliary substances.