Effects of hydrogen on carbon nanotube formation in CH 4/H 2 plasmas

Abstract

Carbon nanotubes (CNTs) were synthesized using CH 4/H 2 plasmas and plasmas simulated using a one-dimensional fluid model. The thinnest and longest CNTs with the highest number density were obtained using CH 4/H 2 = 27/3 sccm at 10 Torr. These conditions allowed CNTs to grow for 90 min without any meaningful loss of catalyst activity. However, an excess H 2 supply to the CH 4/H 2 mixture plasma made the diameter distribution of the CNTs wider and the yield lower. Hydrogen concentration is considered to affect catalyst particle size and activity during the time interval before starting CNT growth (=incubation period). With CH 4/H 2 = 27/3 sccm for a growth time of 10 min efficient CNT growth was achieved because the amount of carbon atoms in the CNTs and that calculated from simulation showed good agreement. The effect of hydrogen etching on CNTs was analyzed by scanning electron microscopy and X-ray photoelectron spectroscopy by observing CNTs treated by H 2 plasma after CNT growth. It was confirmed that (a) multi-walled CNTs were not etched by the H 2 plasma, (b) the C 1s XPS spectra of the CNTs showed no chemical shift after the treatment, and (c) C–H bonds were produced in CNTs during their growth.