Material of the month – Carbon nanotubes
At the laboratory scale, they have the potential to impact countless corners of materials science. So why don’t we have our space elevator yet? Anna Ploszajski investigates.
Infinitesimally small, hollow tubes made from carbon were reported as early as 1952, in the Soviet Journal of Physical Chemistry in Russian, but crept under the radar of the Western scientific community because of the limited communications with the Soviets during the Cold War. Subsequent publications in the 1970s and 80s still failed to bring carbon nanotubes (CNTs) into the mainstream. It wasn’t until 1991, when Sumio Iijima’s paper was published in Nature, that carbon nanotubes were catapulted into the scientific spotlight. At the time of writing, Iijima’s paper has more than 32,000 citations, and to this day, most sources quote it as the true discovery of CNTs.
When carbon atoms form layered structures, such as in many-layered graphite or single-layered graphene, they are arranged in a flat 2D hexagonal pattern. When single-layered graphene is rolled up, it forms a single-walled nanotube (SWNT). There are different angles at which the graphene layer can be rolled, called its chirality, and this, together with the diameter of the carbon nanotube, dictates its electrical properties. SWNTs can be metallic or semiconducting in behaviour. Graphene can also be rolled up like a scroll, or nanotubes can be inserted one into another to form multi-walled nanotubes (MWNTs).
As the name suggests, carbon nanotubes exist at the nanoscale – their diameters are around 0.8–20nm, although they can often be many million times longer. Nanoscale effects bestow record-breaking properties on carbon nanotubes. But, despite their astounding promise at the laboratory scale, potential applications are limited by the difficulty of producing unbundled nanotubes with distinct chirality.
Most current industrial applications of carbon nanotubes are those that can tolerate unorganised bundles of them. These tend to exploit the phenomenal strength, hardness and stiffness of carbon nanotubes properties which arise from their chemical bonds. A tensile strength of 100GPa has been measured for an individual MWNT, beating other industrial fibres tenfold, although the bundling reduces this in practice. Incorporating MWNT powders in polymer composites enhances their mechanical properties, including damping, making these materials useful in high-end sporting goods such as tennis rackets, baseball bats and bicycle frames, in turbine blades and in hulls for boats.
Nanotubes can also be added to metals to increase tensile strength. Aluminium-MWNT composites exhibit a tensile strength comparable with stainless steel (0.7–1GPa), at one-third the density (2.6g/cm). This makes them cost-competitive with aluminium-lithium alloy alternatives. Carbon nanotube anticorrosion coatings on metals provide enhanced robustness to the coating and an electrical pathway to deliver cathodic protection.
Yet carbon nanotube-metal composites are actually not a new innovation. Damascus steel was used in Indian and Middle Eastern sword making during the 16–18th Centuries, and produced tough, shatter-resistant, superplastic and sharp blades. In 2006, it was discovered that carbon nanotube inclusions, probably arising from the forging and annealing processes, may account for the legendary strength of the swords.
The mechanical properties of carbon nanotubes are extremely anisotropic – nanotubes are weak under compression and even Van der Waals forces can deform two adjacent nanotubes. Therefore, nanotube orientation must be taken into consideration to optimise composite materials, and any structural defects, such as atomic vacancies should be minimised, since these can significantly lower the tensile strength.
It is the unsurpassed strength of carbon nanotubes that caught the eye of the space elevator enthusiasts. The space elevator, a concept invented and reinvented since 1895 as a theoretical space transportation system, would rely on the specific strength of carbon nanotubes to tether a counterweight in geostationary orbit to the Earth. Space explorers could also use the electrical conductivity of a carbon nanotube-based tether to power a climber vehicle, enabling them to reach space without the use of rockets.
Nanotubes are considered to be 1D electrical conductors, since electrons are confined to propagate only along the tube axis. This opens up the possibility for nanoscale electronic components, such as electrical cables and wires, transistors and integrated memory circuits, which are 10 times smaller than those used today. The conduction and band-gap of the nanotube depends on its diameter and chirality and as such the control of these factors, as well as the density and placement of the nanotubes, remains insufficient for microelectronics production, particularly over large areas. Nevertheless, disordered MWNT-polymer composites have exhibited conductivities as high as 10,000S/m at 10wt% loading.
In just a few years, we could be answering our phones courtesy of transparent conducting carbon nanotube coatings instead of indium tin oxide films. Improvements in the resistivity and transparency of the CNT-based films would be needed before we see this transition away from globally scarce indium in touchscreens.
Carbon nanotubes are already making great waves in the energy sector. MWNTs are blended with active materials and a polymer binder to increase electrical connectivity and mechanical integrity in battery electrodes, which are widely used in lithium-ion batteries for mobile devices.
They have enabled the reimagining of the battery altogether. Making batteries out of paper seems like science fiction, but it has been achieved in real life, thanks to carbon nanotubes. Flexible, ultra-thin, non-toxic, biodegradable and operable at temperatures between -74°C and 150°C, these batteries are made by infusing aligned SWNTs into a thin sheet of cellulose. The nanotubes form the electrodes of the battery, and discharge when in contact with an ionic liquid. All in a single device, they can supply long, steady power output like a conventional battery, and are able to give quick bursts of power like a supercapacitor. A battery the size of a postage stamp can produce 2.4V of electricity. Once the production of carbon nanotubes becomes more economical, these devices could be used in aerospace, hybrid vehicles and biological applications.
Single-walled carbon nanotubes exhibit strong absorption of UV, visible and near infrared light, due to their wide range of direct band gaps, which match parts of the electromagnetic spectrum. They also exhibit high carrier mobility. Incorporating SWCNT in conjugated polymers could produce more efficient organic photovoltaic devices. These devices promise cheap, flexible solar energy conversion, although their low efficiencies, low strength and instability mean that they remain uncompetitive with their inorganic counterparts.
The field of hydrogen power will soon be making use of carbon nanotubes. Currently, expensive platinum catalysts are decorated onto the surface of porous carbon black to catalyse redox reactions at the electrodes of these devices. Minimising the amount of platinum required will significantly reduce the cost of hydrogen fuel cells. Replacing the carbon black with carbon nanotubes could reduce platinum loading by up to 60%. Nitrogen-doped carbon nanotubes could replace the platinum altogether, with the added benefit of being immune to poisoning by carbon monoxide contaminants.
Given the green light
And the green credentials of carbon nanotubes don’t stop there. Nano-sponges containing CNTs, sulphur and iron are very effective at soaking up water contaminants such as oil, fertilisers, pesticides and pharmaceuticals. The combination of high surface area, superhydrophobic and oleophilic properties of the carbon nanotubes mean that they selectively absorb oil over water, and the iron inclusions make for simple magnetic recovery.
The geometry and chemistry of carbon nanotubes renders them suitable for probing the smallest features of the human body in medical imaging, sensing and drug delivery. Products under development include inkjet-printed test strips for hormone detection and microarrays for DNA and protein detection. Carbon nanotubes can interact with individual receptors on cell membranes and transfer the drugs contained inside on demand.
Concerns surrounding the retention and toxicity of carbon nanotubes in the body are not unfounded. It is known that carbon nanotubes can cross membrane barriers, enter human cells and ultimately kill them. In the lungs, chronic exposure to carbon nanotubes causes similar ailments to asbestos fibres (see Material of the Month, September 2014). However, although there are historical concerns surrounding exposure to carbon nanotubes, there is evidence to suggest that the risks can be eliminated by using shorter nanotubes where surface chemistry has been altered to make them safer for use.
Carbon nanotubes are all set to realise their ‘wonder material’ potential, and once commercially pure, unbundled sources are created, the final piece of the puzzle will be solved. Then we should see them everywhere from clean energy devices and wearable electronics, to biomedicine and space.