Disappearance of Inner Tubes and Generation of Double-wall Carbon Nanotubes from Highly Dense Multiwall Carbon Nanotubes by Heat Treatment

We previously reported that highly dense multiwall carbon nanotubes (hd-MWNTs) with the innermost tube diameters of about 0.4 nm and outside diameters of 5−20 nm were obtained easily with yields higher than 90% by evaporation of graphite with radio frequency (RF) plasma (Koshio et al. Chem. Phys. Lett. 2002, 356, 595). This report shows that vacuum heat-treatment at 2200 °C or higher is useful to preferentially remove single to three inside tubes from hd-MWNTs depending on the heat-treatment temperatures. At the same time, we found that double-wall carbon nanotubes were newly generated, the mechanism of which is unclear.