Six general categories can be identified for a multitude of promising application areas. Examples of such are presented in the graphic below. The first application is photocatalytic water splitting of water into H2 and O2. Synthesizing BiVO4-decorated TiO2 nanorods allows a renewable source of chemical energy to be harnessed, especially in sunny climates with little existing infrastructure.
Secondly, in regions of the world where there is critical infrastructure such as bridges, pipelines etc. scattered across remote locations, the use of inexpensive, reliable self-powered sensors can be exploited to monitor and diagnose the status of such structures. MEMs can be used in this regard, and can be self-powered through nanostructured piezoelectric materials.
Thirdly, energy can be stored in electric form using supercapacitors. Such devices compete with batteries in the rate at which they can be charged and discharged. The Coulombic capacity can be enhanced through interdigitated electrodes with a high degree of dispersion, and therefore raising the energy density.
The last three applications shown above are more exotic demonstrations of nanotech – under the broad umbrella of optoelectronics. The Field Effect Transistor (FET), for example, can be miniaturized to the nanoscale such that the FET packing density can be massively increased on integrated chips, as well as rendered more efficient through ballistic conduction.
On the use of light, tailor-made therapeutics can be activated using electromagnetic radiation (e.g., visible light) either passively or through surgical intervention. This works analogously to photocatalytic water splitting, and could help to cure specific cancerous growths.
Finally, light can also be harvested via the antenna effect. Due to the reduced length scales, compact antennae can be constructed to directly transform relatively low wavelength light into electrical signals. This is another route to harvest energy from the surrounding environment towards self-powered devices.
A case study → Desalination of widely available seawater is increasingly the only option to derive copious quantities of potable water fit for human consumption and agriculture in many arid regions of the world. With the depletion of traditional sources of water, such as aquifers, and population pressures there is a need for finding more sustainable methods to desalinate with greater energy efficiency.
Reverse osmosis (RO) is the most widely used approach; however, there remains plenty of potential to improve the process through better design of membrane materials. Typically, composite materials have been explored to increase water permeability to reduce operating pressures, and hence running costs. An aim is to improve the filtration performance through redesign of conventional RO membrane materials, to ultimately increase energy efficiency.
The filtration performance can be increased either by improving the water permeability or the water-salt selectivity. A typical RO membrane comprises a polysulfone support with a very thin saline-rejecting polyamide layer. This thin layer can be replaced with a polyamide-VACNT composite, that promises to massively enhance filtration performance.
We can explore various aspects of such VACNT composites such as: type of polyamide, polymerization techniques, CNT functionalization, areal density and diameter of the CNTs. Such novel VACNT-polyamide composite membranes can be prepared via CVD synthesis of the VACNTs, their functionalization, and by interfacial polymerization to derive mixed-matrix architectures.
With subsequent testing in a standard filtration apparatus, performance parameters of water permeability and selectivity can be plotted and compared with competing membranes. A goal is to maximize the numerical product of these two performance parameters – preferentially maintaining a high selectivity similar to current commercial values, whilst maximizing the water permeability.
The novelty of incorporating VACNTs into active saline-rejecting polyamide layers offer an order-of-magnitude potential increase over current attempts that use randomly-oriented CNT composites.
Lesson → an integrated approach – how intended application drives the nanomaterial to be used and consequent likely synthesis routes.
From this, we sense the opportunities of nanotechnology that lay ahead, hampered nonetheless by issues relating to reliable synthesis. The top-down synthesis paradigm, commonly used to create small samples for testing a novel technical application, suffers from inherently slow rates of production. Bottom-up synthesis overcomes this major obstacle, and at the same time can encode desired modulation of composition into the nanomaterial with relative ease.
In view of bottom-up synthesis being suited to mass production, the long-term vision is: (i) to promote new nanotechnologies, and (ii) to expedite semi-established concepts towards commercialization. This can be facilitated by introducing nanomaterials that are straightforward to manufacture whilst retaining effectiveness at performing their target application, devising synthesis methods that preferably obviate subsequent assembly, and quality control through appropriate process characterization.