Direct solvent-free amination of closed-cap carbon nanotubes:: A link to fullerene chemistry

We attempted the direct solvent-free amination of closed caps of multiwalled carbon nanotubes (MWNTs) with octadecylamine (ODA), which is essentially similar to the amination of spherical fullerenes. Thermogravimetric analysis revealed a relatively high content of organics in the product of derivatization (ODA-MWNTs), suggesting that a large ODA fraction is distributed over MWNT sidewalls through chemical attachment. This was confirmed by high-resolution transmission electron microscopy observations. Quantum chemical calculations showed that the presence of pyracylene units in the closed caps is not crucial for the amine addition, although the site specificity of the reaction does depend on the mutual position of five-membered rings. If the caps contain pyracylene units, then the addition preferentially takes place on their 6,6 bonds; if they do not, then the preferential reaction sites are C-C bonds of the pentagons. Whereas ideal nanotube sidewalls composed of solely benzene rings were found to be inert with respect to amines, the real nanotube sidewalls must contain numerous reactive five-membered rings as defects. ODA-MWNTs exhibited enhanced dispersibility/solubility in propanol. The proposed amination reaction is the most direct link between carbon nanotube and fullerene chemistry, contrary to all derivatization methods designed previously.