Double-Walled Carbon Nanotubes

What are Double-Walled Carbon Nanotubes?

DWCNTs consist of two individual carbon nanotubes, with one embedded inside the other. The differences in diameters and the chirality of the two different nanotubes lead to a varying degree of interaction between the two, while at the same time the properties of the individual nanotubes being separate from each other. It is this wide variety of possibilities that have made DWCNTs a focus of interest for carbon nanotube research. Varying chirality allows a range of inner-wall outer-wall interactions to occur, because the chirality determines whether the nanotube will be semiconducting or metallic. It is possible to achieve metallic-metallic, semiconducting-metallic, metallic-semiconducting or semiconducting-semiconducting interactions. In addition to this, the metallic and semiconducting properties can vary depending upon the exact lattice parameters, which enables a wide range of possible property combinations.

DWCNTs also have a large advantage over single-walled carbon nanotubes, as it is possible to modify the outer nanotube without changing the properties of the inner nanotube. This modification could be either through functionalisation (to add solubilising groups), or the doping of the structure (to alter the properties). This allows the double-walled system to maintain functionality of a single-walled nanotube whilst simultaneously having the solubility of functionalised nanotubes. This combination makes double-walled systems attractive for use as additives in composite materials as it allows high doping concentrations without affecting the properties of the nanotube overall.

The biggest barriers for DWCNTs - with regards to further research and commercialisation - are their synthesis and purification. The yields produced by various synthesis techniques can vary from around 50% to 90% for arc discharge. Similarly, for catalytic chemical vapour deposition the yields can vary from 70% to 85%. The remainder of the nanotubes synthesised using these techniques are a mixture of single-walled and multi-walled nanotubes which then need to be purified to obtain individual double-walled nanotubes. The process of purification is much more difficult. Methods such as high-temperature oxidation result in preferential oxidation of single-walled nanotubes over double-walled. However, the process can damage the remaining nanotubes and will leave residual multi-walled contaminants behind. Other processes, such as ultra-centrifugation, can be used to obtain high-purity DWCNT samples and sort double-walled samples by outer diameter. However this process is labour and time intensive making commercialisation and large scale production of high purity DWCNTs difficult.

Just like with single-walled carbon nanotubes, there are many different areas in which DWCNT's can be applied due to their impressive mechanical and electrical properties. In addition double-walled nanotubes show an increase in the mechanical strength, thermal stability, and also chemical stability over that of single-walled nanotubes. However, the ability to combine different nanotube types have the potential to result in interesting optical, electronic and mechanical properties that are not possible with single-walled nanotubes, and could result in the most interesting research in the coming years.

Dispersion Guide

Similarly to single-walled carbon nanotubes, DWCNTs are insoluble. But by using a combination of surfactants and ultrasonic vibration, it is possible to disperse and suspend small concentrations of nanotubes. For dispersing in aqueous solutions, we recommend the use of sodium dodecylbenzene sulfonate if an ionic surfactant is suitable. If a non-ionic surfactant is needed, we recommend surfactants with high molecular weights.

• Weigh out the desired amount of carbon nanotubes.
• Mix together your solvent and surfactant of choice at the desired surfactant concentration. This should be below the critical micelle concentration of the surfactant.
• Add the solvent-surfactant mix to the dry powder and shake vigorously to mix.
• Either place an ultrasonic probe into the solution or place the solution into an ultrasonic bath. Be careful about the length of time and power used - as damage to the carbon nanotubes can occur, shortening their average length.
• The resulting solution will be a mixture of suspended single walled nanotubes and bundles of single walled nanotubes; further sonication will help break up the bundles.
• To separate out the individual nanotubes in solution from the bundles, the solution should be placed into a centrifuge. If the solution is centrifuged for a longer time and/or at a higher speed, the smaller bundles will be removed narrowing the distribution of suspended nanotubes.

Functionalized DWCNT's can be dispersed without the use of surfactants, a maximum of 0.1mg/ml can be achieved for COOH and OH.