Stoichiometric control of single walled carbon nanotubes functionalization

Covalent modifications of single-walled carbon nanotubes, while being useful for their manipulation and functionalization, alter the electronic properties through disruption of the nanotube pi electronic system. To avoid such negative impact, we demonstrate here that carbon nanotubes can be alkylated in a controlled manner by first preparing and isolating reduced nanotube salts with varied charge/C ratios. The reduction reaction, in the present work performed in THF with a K/naphthalene salt, is almost quantitative and the KC(x) salt can be isolated with a wide range of x values ranging from 10 to 370, while remaining soluble in DMSO. The reaction of these salt solutions with two alkyl bromide reagents yields functionalized SWCNTs, as demonstrated by combined IR, UV-vis-NIR and Raman spectroscopies, XPS and thermogravimetric measurements. In particular, quantification of the number of functional groups grafted shows a direct correlation with the charge/C ratio of the initial salts. Detailed analysis of Raman spectra confirms this control over the extent of covalent functionalization and shows it is not selective towards any type of SWCNT. This latter observation is ascribed to the isolation of the intermediate salts as solids, resulting in homogenization of charge densities on the reduced CNTs.