Reducing defects on multi-walled carbon nanotube surfaces induced by low-power ultrasonic-assisted hydrochloric acid treatment

A simple method to prepare multi-walled carbon nanotubes (MWCNTs) with high purity, high-aspect ratio and especially minimal defects is presented by low-power ultrasonic-assisted treatment of raw carbon nanotubes in hot concentrated hydrochloric acid under continuous reflux conditions. The evolution of surface morphology of MWCNTs during the treatment is evaluated by transmission electron microscopy, and microstructure is characterised by Fourier transform infrared and micro-Raman spectroscopy. The results show that with increasing the treatment time, the external layers with defects are peeled off piece by piece from the surface of MWCNTs until the inner graphite layers with good chemical resistance are exposed. The MWCNTs treated for less than 15 h by this method have been demonstrated to have minimal microstructural defects and large aspect ratios as compared to those treated in mixed concentrated nitric acid and sulphuric acid, which is further confirmed that almost no manganese dioxide particles can be anchored on the surface of the resulting MWCNTs due to their lack of the reactive defect sites.