Effects of hydrogen on the formation of aligned carbon nanotubes by chemical vapor deposition

Well-aligned carbon nanotubes with controllable properties were grown on porous silicon substrates by thermal chemical vapor deposition. The morphologies of the carbon nanotubes were varied with the introduction of H2 during the catalyst activation and/or carbon nanotube growth processes. It was found that H2 promotes the growth of carbon nanotubes while preventing the formation of spherical amorphous carbon particles. Without the introduction of H2 during the C2H2 thermal decomposition, aligned carbon nanotubes mixed with spherical carbon particles were formed on the substrate. However, with the introduction of H2, pure carbon nanotubes were synthesized. These nanotubes also had uniform diameters of 10-20 nm, which is much smaller than nanotubes synthesized without H2. The average growth rate of nanotubes was also affected by the introduction of hydrogen into the reaction chamber during nanotube growth. With the addition of hydrogen, the average growth rate changed from 78 nm/s to 145 nm/s. A possible growth mechanism, including the effect of a high ratio of H2 to C2H2, is suggested for the growth of these well-aligned carbon nanotubes with uniform diameters.