Structure-Dependent Thermal Defunctionalization of Alkylated Single-Walled Carbon Nanotubes

Covalent sidewall functionalization of SWCNTs with alkyl groups is an important process that allows nanotubes to be dissolved in organic solvents for further chemical processing, fundamental research studies, and applications development. Because of structure-dependent strain and reactivity, it is expected that the stability of functionalized SWCNTs may depend on nanotube diameter and roll-up angle. We report here a detailed study of variations in the rates of thermally induced defunctionalization for alkylated SWCNTs with a range of structures. Raw HiPco SWCNT samples were functionalized via reductive alkylation using lithium and alkyl halides in liquid ammonia. This gave high degrees of side-wall functionalization, as evidenced by the solubility and high Raman D to G peak ratios of the products. The alkylated samples were then heated for 30 min in an inert atmosphere at specific temperatures in the range of 100 to 500 °C and characterized by Raman, NIR fluorescence, and absorption spectroscopies, plus thermogravimetric analysis. It was found that the thermolysis treatment fully defunctionalized the nanotubes, but in a strongly structure dependent fashion, with larger diameter species becoming dealkylated at lower temperatures than smaller diameter species. This structure selectivity was especially evident from changes in the samples’ NIR fluorescence spectra. A quantitative interpretation of the results will be presented.