DETERMINATION OF FUNCTIONAL GROUPS AFTER NITRIC ACIDS OXIDATION OF MULTI-WALLED CARBON NANOTUBES AND THE SIGNIFICANCE OF FULVIC ACIDS

摘要

Here we present results to show that multi-walled carbon nanotubes (MWCNTs) undergoes fragmentation in nitric acid oxidation, generating small highly polar, water soluble organic species, classified as fulvic acids (FAs), which also possess surface acidic groups. These in-situ generated FAs are immobilized on the surface of MWCNTs in acidic solution, but can be removed in alkaline solution. Therein, the surface acidic group concentration of the carbon nanotubes was shown to decrease after FAs removal and, depending on the degree of oxidation, can account for up to a 43% reduction. The concentration of surface acidic functional groups on MWCNTs was determined after FAs had been removed, to reveal the continued presence of carboxyl, lactone and phenol groups which were covalent bonded with MWCNTs main structure. Moreover, FAs derived from the nitric acid oxidised MWCNTs can be re-adsorbed to the surface of either oxidised or purified MWCNTs, which assists the stability of carbon nanotubes suspensions in the aqueous phase.