The surface acidity of acid oxidised multi-walled carbon nanotubes and the influence of in-situ generated fulvic acids on their stability in aqueous dispersions

The oxidation of multi-walled carbon nanotubes (MWCNTs) with nitric acid was studied. In addition to the formation of oxygen-containing surface functional groups, the oxidative digestion of graphene caps and layers generated polycyclic aromatic substances, classified as fulvic acids (FAs). The latter remained immobilised on the MWCNT surface in acidic and neutral solutions but were released in basic pH solutions, which were subsequently separated, purified and characterised by high-performance liquid chromatography and mass spectrometry. Using acid–base titrations, the number of surface acidic functional groups was determined, which was shown to significantly decrease after FAs were removed. Depending on the length of oxidation, FAs account for up to 43% of the surface acidity of MWCNTs. The protonated solubilised fulvic acids can be readsorbed on the surface of oxidised or unfunctionalised MWCNTs, which assists the stability of carbon nanotube suspensions in the aqueous phase.