Both carbon nanotubes and graphene have many unique properties which make them of great interest for their use in a wide range of applications but in order to access some of these properties the structure must first be modified. Using chemical or thermal modification of the surface or edges of these carbon nanomaterials it is possible to obtain many new functionalities and properties.
High temperature treatment
First type of modification is an annealing of as-produced carbon nanotube materials. This high-temperature treatment (under high vacuum or inert atmosphere) aims to decrease the level of defects/imperfections within the graphitic structures of CNTs. Using such process we are able to achieve the higher crystallographic order within nanotubes by means of thermal graphitisation combined with nearly 100% removal of metal iron-based catalyst residues. The characterisation techniques associated with the quality of CNTs after this high-temperature treatment include Raman Spectroscopy, TGA, TEM, SEM and EDX analysis. By studying various parameters such as treatment temperature, time of annealing, inert gas flow rate and heating rate we have been able to optimise the purification process of CNTs. This process enables scalable purifications for example of aligned multiwall carbon nanotube arrays under atmospheric pressure within a reasonable timescale and could be used for a wide range of applications.
Often, where a carbon nanotube application requires good compatibility with other materials, functional groups need to be introduced onto their surface and one way to achieve this is through exposing carbon nanotubes to reactive gases such as halogens. Resistively heating the carbon nanotubes by a few hundred centigrade significantly speeds up the rate of reaction. Using this method we have demonstrated the potential for grafting chlorine, bromine and iodine onto carbon nanotubes and found that the introduction of these halogen atoms improved the conductivity under certain conditions.
However this process is not just restricted to using gaseous reactants, functionalization agents as simple as hydrogen peroxide can also be deployed. Here, the resistively heated submerged carbon nanotubes are cleaned and functionalised within a minute.
We have developed a simple method for removing unwanted forms of carbon produced in the synthesis process. This method has the added benefit of enabling the introduction of functional groups onto the otherwise inert surface of carbon nanomaterials to ensure good compatibility with other materials such as polymer matrices or metals. Many techniques involve multi-step preparation using hazardous chemicals such as concentrated acids and sophisticated equipment. However our approach introduces functional groups using air as the reactant. In this process carbon nanotubes are exposed to air flow at 400°C for just one hour in a conventional furnace.
This simple treatment is successful in modification of the chemistry of the surface of carbon nanotubes.