A simple method was presented for the preparation of multi-walled carbon nanotubes of high purity, high-aspect ratio and minimal defect content by the low-power ultrasonic-assisted treatment of raw carbon nanotubes in hot concentrated hydrochloric acid under continuous reflux conditions. The evolution of surface morphology of multi-walled carbon nanotubes during the treatment was evaluated by transmission electron microscopy, and the microstructure was characterized using Fourier transform infra-red and micro-Raman spectroscopy. The results showed that, with increasing , treatment time, the external layers with defects , peeled off piece by piece from the surface of multi-walled carbon nanotubes until , inner graphite layers with good chemical resistance were exposed. Multi-walled carbon nanotubes treated for less than 15h using this method were shown to contain minimal microstructural defects and large aspect ratios as compared to those treated in mixed concentrated nitric acid and sulphuric acid. It was further confirmed that almost no manganese dioxide particles could be anchored on the surface of the resulting multi-walled carbon nanotubes, due to the lack of reactive defect sites.

Reducing Defects on Multi-Walled Carbon Nanotube Surfaces Induced by Low-Power Ultrasonic-Assisted Hydrochloric Acid Treatment. Q.Fan, Z.Qin, X.Liang, L.Li, W.Wu, M.Zhu: Journal of Experimental Nanoscience, 2010, 5[4], 337-47