23/05/2007

The solubilisation of carbon nanotubes by an appropriate chemical modification was achieved by a research team of the Theoretical and Physical Chemistry Institute of the National Hellenic Research Foundation (NHRF) under scientific director Dr Nikos Tagmatarhis. This success has already attracted international recognition and appears to be opening up key prospects for technological applications with the composition of new functional nano-based hybrid materials.

Specifically, carbon nanohorns (Carbon Nanohorns - CNHs) are a new innovative form of nano-based carbon which was discovered in 1999. CNHs are produced by the method of laser ablation of common graphite in inactive conditions, with high yields, without metal admixtures and, since 2004, in macroscopic quantities capable of being studied analytically. CNHs differ from the known carbon nanotubes (the hollow and incredibly thin graphite cylinders with a diameter of around one billionth of a metre), not only in their structural form, since one of their ends results in the shape of a cone, but also due to the fact that they form secondary spherical supramolecular structures in the form of aggregations of a typical diameter of 80 nanometres which resemble the flowers of the dahlia.

In contrast to carbon nanotubes, catalysts from modified metals are not used during the production process of CNHs, resulting in CNHs being produced entirely free of admixtures. This fact is of particular importance, as they do not need further cleansing treatment, a process which is known to affect not only the mechanical and electronic attributes of carbon nanotubes, but also their unique morphological characteristics. In any case however, a restrictive factor in the use of CNHs, as in carbon nanotubes, is that they remain insoluble in organic solvents and/or in aqueous solutions.

Recently, Dr Nikos Tagmatarhis and the postgraduate student Georgia Pagona at the Theoretical and Physical Chemistry Institute of the National Hellenic Research Foundation in Athens, in collaboration with members of the research team of Professor Sumio Iijima at the research laboratories of the Japanese company NEC Corp, chemically modified CNHs, rendering them soluble in organic solvents, as well as in aqueous solutions.

As described in the research paper published in the journal Chemistry of Materials (Vol. 18, Issue 17, p. 3918, DOI: 10.1021/cm0604864) of the American Chemical Society, Dr N. Tagmatarhis and his colleagues removed the conical end of the CNHs while simultaneously introducing carboxylic groups to their open ends. The mild conditions of modification maintained the high purity of the CNHs and their unique innovative structure. Afterwards, the carboxylic groups were activated through their modification on the respective acyl chlorides of the CNHs, which in the end reacted with an abundance of amines, alcohols and thiols, introducing large and small hydrophobic and hydrophilic alkyl chains, chromophoric molecules, as well as protected active groups for further chemical modification.

The new hybrid materials of the chemically modified CNHs indicated their anticipated solubilisation in various organic solvents. For example, polar hybrid materials are soluble in polar solvents, while respectively non-polar hybrid materials are soluble in non-polar solvents. The researchers used the High-Resolution Transmission Electron Microscopy (HR-TEM) technique to observe the new hybrid materials, verifying that the modified CNHs maintained their characteristic morphology in the solution. In addition, analytical phasmatoscopic techniques aided in the more extensive characterization of the modified CNHs and the study of their attributes in the solution. There are indications that in the case of modification of the CNHs with groups of electron donors, endomolecular electronic communication is observed between the CNHs and the electron rich molecule.

As Dr N. Tagmatarhis emphasizes, "The procedure of modification of the CNHs opens up new horizons for the composition of a plethora of nanohybrid materials based on CNHs. These materials will be suitable for nanotechnological applications, particularly in energy transformation systems as well as in systems which mimic photosynthesis, through procedures that are based on the transfer of electronic charge and/or energy, such as the exploitation of solar energy and the creation of a new generation of photovoltaic cells as well as fuel cells, where CNHs will assume the role of the ideal electronic receiver."

Source: NHRF