Purification of multi-wall carbon nanotubes produced over supported catalysts

Catalytic synthesis and some characterization of multi- and single-wall carbon nanotubes are presented. Supported transition metal catalysts were prepared by different methods and were used to produce nanotubes by decomposition of acetylene at 600 degrees C, using a fixed bed flow reactor. The quantities of deposited carbon were measured and the quality of the nanotubes was characterized by means of transmission electron microscopy. The multi-wall straight and coiled nanotubes were found quite regular with an average inner (outer) diameter of 4-7 nm (15-25 nm) and with lengths up to 60 mu m. Separation of nanotubes and catalyst particles was carried out by repeated dissolution of the support in hydrofluoric acid and filtering. On the TEM images of purified carbon nanotubes after the second HF treatment, only carbon nanotubes and amorphous carbon released from the inner pores were seen. It was also possible to see lots of regular nanotubes and some open tips. These open tips were formed by the dissolution of the catalyst active particles in acidic media. The HF treated samples - nanotubes free of catalyst and support - were submitted to different oxidation (KMnO4; air) and reduction (H-2/N-2) treatments to get rid of the amorphous carbon. The three different purification procedures yielded long and short nanotubes, in the same diameter range. A larger proportion of short nanotubes was observed in the purified products resulting from the cleavage of the nanotubes at the defect sites. The latter cleavages am more effective, giving larger proportion of short nanotubes, applying hydrogenation.