Degree of functionalization of carbon nanofibers with benzenesulfonic groups in an acid medium

Benzene sulfonic groups have been successfully attached to a carbon nanofiber surface by reaction of diazonium benzenesulfonic salt in sulfuric acid. The extent of the functionalization reaction was determined by X-ray photoelectron spectroscopy, energy dispersive X-ray analysis, elemental analysis, and thermogravimetric analysis complemented with temperature-programmed desorption experiments. Good agreement between the degrees of functionalization provided by these techniques was observed. The results pointed to a higher extent of anchorage of –SO3H groups when the nanofibers were treated in fuming sulfuric acid, for which a surface S/C (%) atomic ratio of 2.4 was obtained. Raman spectroscopy revealed that the D-band does not fully disappear after CNF treatment, indicating that a certain degree of structural disorder is maintained. However, a decrease in the D-band was observed after the diazotization reaction and this was attributed to the chemical change occurring at the edges. No significant changes to the morphological and textural characteristics of the CNFs by surface treatment were observed. This study may offer an important guideline in the application of CNFs modified with benzenesulfonic groups in polymeric membranes for fuel cells.