Oxidation of multiwalled carbon nanotubes by nitric acid

The oxidation of MWCNTs in nitric acid was monitored using sample weight, Raman spectrum, solubility, morphology and alignment. The influence of the acid concentration, temperature and oxidation duration on the monitored parameters was assessed. A new method, based on optical microscopy is proposed for the determination of MWCNT solubility in concentrated aqueous-suspensions. The investigations revealed that the solubility is determined not only by the functional groups on the MWCNT, but also by the functionalized amorphous carbon generated during the digestion of the nanotubes. High solubility (20–40 mg/ml) is obtained only after prolonged exposure (24–48 h) in concentrated acid (⩾60%). But in these conditions 60–90% of the MWCNTs are lost. Furthermore the MWCNTs are strongly fragmented and covered by amorphous carbon after 48 h of oxidation. It was found that the solubility correlates well with the area ratio of the G and D bands from the Raman spectrum. SEM examination of the MWCNT films showed extended alignment after 24 h of oxidation.