Abstract
The d-CNTs were produced by direct-vaporization of graphite without metal catalyst, using radio-frequency (RF) plasma. They were quite high in purity. The d-CNTs are multi-wall CNTs packed densely toward the center, and the innermost tubes have a diameter of 0.4nm, the smallest diameter possible for a single-wall carbon nanotube. All of them had acute tips with a cone angle of 19.2°. We observed that most of these d-CNTs became hollow after heat treatment at more than 2200℃ in vacuum. The inner diameter of the hollow multi-wall CNTs formed by heat treatment of the d-CNTs was about 3nm. We deduce that the inner nanotubes narrower than about 3 nm decomposed faster than the outer nanotubes did because the inner tubes had more structural stress due to their small diameters. This enables us to strictly control the outer and inner diameters of CNTs.
