Applications of carbon nanotubes

Graphite carbonaceous materials and carbon fiber electrodes are commonly used in fuel cells, batteries and other electrochemical applications. Advantages of considering nanotubes for energy storage are their small dimensions, smooth surface topology and perfect surface specificity. The efficiency of fuel cells is determined by the electron transfer rate at carbon electrodes, which is fastest on nanotubes following the ideal Nernstian behavior [48]. Electrochemical energy storage and gas phase intercalation will be described more thoroughly in the following.

1.1.2.2 Hydrogen storage

The advantage of hydrogen as energy source is water as the combustion product. In addition, hydrogen can be easily regenerated. For this reason, a suitable hydrogen storage system is necessary to satisfy both volume and weight limitations. Two common means to store hydrogen are gas phase and electrochemical adsorption.

Because of their cylindrical and hollow geometry and nanometre-scale diameters, it has been predicted that carbon nanotubes can store liquids or gases in inner cores through capillary effects. As a threshold for economical storages, the storage requirements of 6.5 % by weight as the minimum level for hydrogen fuel cells has been set. It is reported that SWNTs were able to meet and sometimes exceed this level by using gas phase adsorption (physisorption). Yet, most experimental reports of high storage capacities are rather controversial so that it is difficult to assess the application potential. What lacks is a detailed understanding of the hydrogen storage mechanism and the effect of material processing on this mechanism. Another possibility for hydrogen storage is electrochemical storage. In this case H atoms instead of hydrogen molecules are adsorbed. This is called chemisorption.

1.1.2.3 Lithium intercalation

The basic principle of rechargeable lithium batteries is the electrochemical intercalation and deintercalation of lithium in both electrodes. An ideal battery requires high-energy capacity, fast charging and long cycle time. The capacity is determined by the lithium saturation concentration of the electrode materials. For Li, this is highest in nanotubes if all interstitial sites (inter-shell van der Waals spaces, inter-tube channels and inner cores) are accessible for Li intercalation. SWNTs have shown to possess both highly reversible and irreversible capacities. Because of the large voltage hysteresis observed, Li-intercalation in nanotubes is still unsuitable for battery application. This feature can potentially be reduced or eliminated by

processing, i.e. cutting nanotubes to short segments.

1.1.2.4 Electrochemical supercapacitors

Supercapacitors have high capacitance and potentially applicable in electronic devices. Typically, they are comprised of two electrodes separated by an insulating material that is ionically conducting in electrochemical devices. The capacity of the electrochemical supercap inversely depends on the separation between the charge on the electrode and the counter charge in the electrolyte. Because this separation is about a nanometre for nanotubes in electrodes, very large capacities result from the high nanotube surface area accessible to the electrolyte. In this way, a large amount of charge injection may occur if only a small voltage is applied. This charge injection is used for energy storage in nanotube supercapacitors [49]. Generally speaking, most interest is laid upon the double-layer supercapacitors and redox supercapacitors with different charge-storage modes.

1.1.2.5 Field emitting devices

If a solid is subjected to a sufficiently high electric field, electrons near the Fermi level can be extracted from the solid by tunneling through the surface potential barrier.

This emission current depends on the strength of the local electric field at the emission surface and its work function, which denotes the energy necessary to extract an electron from its highest bounded state into the vacuum level. The applied electric field must be very high in order to extract an electron. This condition is fulfilled for carbon nanotubes, because their elongated shape ensures a very large field amplification [48].

For technological applications, the emissive material should have a low threshold emission field and large stability at high current density. Furthermore, an ideal emitter is required to have a diameter in nanometer size , a structural integrity, a high electrical conductivity, a small energy spread and a large chemical stability. Carbon

nanotubes possess all these properties. However, the application bottleneck of nanotubes is the dependence of conductivity and emission stability on fabrication processes and synthesis conditions. Examples of potential applications of nanotubes as field emitting devices are flat panel displays, gas discharge tubes in telecom networks, electron guns for electron microscopes, AFM tips and microwave amplifiers.

1.1.2.6 Transistors

The field-effect transistor – a three-terminal switching device – can be constructed of only one semiconducting SWNT. By applying a voltage to a gate electrode, nanotubes can be switched from conducting to insulating state [49]. A schematic representation of such a transistor is given in Fig. 1-5. Such carbon nanotube transistors can be coupled together to work as a logical switch, which is the basic component of computers [50].

Fig. 1-5 A single semi-conducting nanotube is contacted by two electrodes. Si substrate covered by a layer of SiO2 300nm thick acts as a back-gate.

1.1.2.7 Nanoprobes and sensors

Because of their flexibility, nanotubes can also be used in scanning probe instruments. Since MWNT tips are conducting, they can be used in STM and AFM instruments (Fig. 1-6). Advantages are the improved resolution in comparison with conventional Si or metal tips. Tips do not suffer from crashes with surfaces because of their high elasticity. However, nanotube vibration, due to their large length, is still an important issue unless shorter nanotubes can be grown under control.

Fig. 1-6 Use of a MWNT as AFM tip. VGCF stands for Vapour Grown Carbon Fibre. At the centre of this fibre MWNT forms the tip [48].

Nanotube tips can be modified chemically by the attachment of functional groups.

Nanotubes can be used as molecular probes with potential applications in chemistry and biology. Described below are the further applications. A pair of nanotubes can be used as tweezers to move nanoscale structures on surfaces [49]. Sheets of SWNTs can be used as electromechanical actuators, mimicking the actuator mechanism present in natural muscles SWNTs may be used as miniaturised chemical sensors. On exposure to environments containing NO2, NH3 or O2, the electrical resistance changes.

1.1.2.8 Composite materials [48]

Because of the stiffness of carbon nanotubes, they are ideal for structural applications. For example, they may be used as reinforcement composites of high strength, low weight and high performance. Theoretically, SWNTs have Young’s Modulus of 1 TPa. MWNTs are weaker because the individual cylinders slide with respect to each other. Ropes of SWNTs are also less strong. The individual tubes can pull out by shearing and at last the whole rope breaks. This happens at stresses far below the tensile strength of individual nanotubes. Nanotubes also sustain large

strains in tension without fracture. In other directions, nanotubes are highly flexible [48]. One of the most important applications of nanotubes based on their properties is the reinforcement in composite materials. However, there have not been enough successful experiments to prove the better filler performance over traditional carbon fibers. The main problem is to create a good interface between nanotubes and the polymer matrix, because nanotubes are too smooth and too small in diameter, which is nearly the same as that of a polymer chain. Next, nanotubes are quite different from the individual nanotube in mechanical properties because of the easy aggregation.

Limiting factors for good load transfer are sliding of cylinders in MWNTs and shearing of tubes in SWNT ropes. To solve this problem the aggregates need to be broken up and dispersed or cross-linked to prevent slippage. A main advantage of using nanotubes for structural polymer composites is that nanotube reinforcements increase the toughness of the composites by absorbing energy due to their highly flexible elasticity. Other advantages are the low density of nanotubes, increased electrical conduction and better performance during compressive load. Another possibility, which is an example of a non-structural application, is the filling of photoactive polymers with nanotubes. PPV (Poly-p-phenylenevinylene) filled with MWNTs and SWNTs is a composite, which has been used in several experiments.

These composites show a large increase in conductivity with only a little loss in photoluminescence and electro-luminescence yields. Another benefit is that the composite is more robust than pure polymers. Of course, nanotube-polymer composites could also be used in other areas. For instance, they could be used in the biochemical field as membranes for molecular separations or for osteointegration (growth of bone cells). However, these areas are less explored. The most important thing we have to know about nanotubes for their efficient usage as reinforcing fibers is the knowledge on how to manipulate surfaces chemically to enhance interfacial

behavior between individual nanotubes and the matrix material.

1.1.2.9 Templates [48]

Because of the small channels, strong capillary forces exist in nanotubes. These forces are strong enough to hold gases and fluids in nanotubes and it is possible to fill cavities in nanotubes to build nanowires. The critical issue is the wetting characteristics of nanotubes. Because of their smaller pore sizes, filling of SWNTs is more difficult than filling of MWNTs. If it becomes possible to keep fluids inside nanotubes, it could also be possible to run chemical reactions inside cavities. Though special organic solvents wet nanotubes easily to make nanoreactor available, that nanotubes are normally closed cannot meet the application requirement. This is accessible through a simple chemical reaction, oxidation. Pentagons in the end cap of nanotubes are more reactive than sidewalls amd during oxidation, caps are easily removed while sidewalls stay intact.