Dimensional control of tubular-type carbon nanofibers via pyrolytic carbon coating

A posttreatment was carried out to control the dimensional and surface properties of tubular-type carbon nanofibers (TCNFs) via a pyrolytic carbon coating derived from the thermal decomposition of methane at 950 °C. The pyrolytic carbon had a highly graphitic structure, oriented with the carbon layers parallel to the TCNF axis. The diameter and wall thickness of the TCNF increased linearly as the deposition time increased, while the inner tube diameter remained relatively constant. The surface defects that were induced in the preparation step of the catalytic pyrolysis improved steadily with further heat treatment. The diameter of the TCNF was easily adjusted in the range of 20–90 nm. In the initial 1-h deposition, the thickened TCNFs had a smooth surface. In the following 2-h deposition, some pyrolytic carbon granules were decorated on the surfaces of the thickened TCNFs. Introduction of hydrogen could suppress the formation of granules, but lowered the thickening efficiency. The TCNFs could also be thickened via the thermal decomposition of ethylene and propane at low temperatures. The resulting pyrolytic carbon exhibited a less graphitic structure. The homogenous thickening of the TCNFs was associated with the relatively low decomposition temperature, at which small polyaromatic hydrocarbons were formed in the gas phase and then adsorbed and rearranged on the hot surfaces of the TCNFs.