Oxidative acid treatment of carbon nanotubes

This study investigated the oxidative acid treatment of multi-walled carbon nanotubes (MWCNT). Experiments were conducted by refluxing MWCNT with HNO3 and H2SO4/HNO3 (3:1 v/v) at two different molar concentrations. Treated MWCNT were characterized for their weight loss, surface chemistry, surface morphology, structural integrity, thermal stability and dispersion stability using a precision scale, X-ray Photoelectron Spectroscopy (XPS), Transmission Electron Microscopy (TEM), Thermogravimetric Analysis (TGA), and visual observation. The results showed that treatment with 15 M HNO3 introduced the highest atomic % of oxygen-containing chemical moieties to MWCNT surface with less sample weight loss. The TEM micrographs showed that during treatment with 15 M HNO3, MWCNT fragmented into smaller nanotubes of an average length of 900 nm. MWCNT experienced the most severe structural degradation after treatment with 15 M H2SO4/HNO3, where the nanotubes were completely dissolved during the refluxing process. Carboxyl and carbonyl groups were identified as the predominant functional groups present on treated MWCNT surfaces. TGA study revealed the preservation of graphitic MWCNT structure, increased water sorption capacity, and newly formed functional groups after the treatments. Dispersion of MWCNT was improved after all treatments. However, 10 M HNO3-treated sample showed the weakest stability. According to the experimental results, MWCNT treatment with 15 M HNO3, yielded a formation of 5.2 at% surface oxygen with only 6.8% sample weight loss. The extent of surface oxidation in the case of 10 M H2SO4/HNO3 treatment was 3.2 at% and the weight loss was about 17%.