An easy method for purifying multi-walled carbon nanotubes (MWCNTs), involving chlorine water and ammonia water treatments, is described. Transmission electron microscopy, X-ray diffraction, energy dispersive X-ray analysis and Fourier-transform infrared spectroscopy were used to characterize the purified MWCNTs. It was shown that catalyst particles, amorphous carbon, and carbon particles were almost completely eliminated after purification and the tips of the purified MWCNTs were opened. During purification, the crystal structure of the MWCNTs was unchanged and some carboxyl groups and C-Cl bonds were introduced. Sedimentation experiments showed that the purified MWCNTs could be easily dispersed in polar solvents such as water, ethanol, acetone, chloroform and N,N-dimethylformamide.
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