Multi-walled carbon nanotubes functionalized with a ultrahigh fraction of carboxyl and hydroxyl groups by ultrasound-assisted oxidation

In this study, multifunctionalized multi-walled carbon nanotubes (MWCNTs) were functionalized through a novel three-step oxidation approach assisted by ultrasonication. We chose an oxidation system with three oxidation agents, concentrated sulfuric acid, potassium permanganate, and hydrogen peroxide. In comparison to non-ultrasonic processes, the effect of our ultrasound-assisted process on the carboxyl and hydroxyl content of the modified MWCNTs (m-MWCNTs-2) was discussed using Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. The results verified that the ultrasound-assisted oxidation process is much more effective than those without ultrasonication; a higher hydroxyl content of 11.88 at.% and a carboxyl content of 8.82 at.% in the m-MWCNTs-2 were estimated. A Raman spectrum showed an increased value of I _D/I _G, which proved the presence of defect sites or the functional groups on the surface of the MWCNTs. Thermogravimetric analysis was found to semiquantitatively evaluate the amount of carboxyl and hydroxyl groups in the MWCNTs. A better dispersion of the m-MWCNTs-2 in ethanol or water was expected from a photograph, a particle size analysis, and field emission scanning electron microscopy. High-resolution transmission electron microscopy revealed that the diameter of the m-MWCNTs-2 was reduced to a lesser degree using the ultrasonication assistance than of those generated without ultrasonication.