Effect of hydrogen on catalyst nanoparticles in carbon nanotube growth

Abstract

The structures of carbon nanotubes grown from catalytic nanoparticles via plasma-enhanced chemical vapor deposition in CH4 / H2 mixtures show a strong dependence on the H2 -to- CH4 ratio in the feed gas. A suite of characterization techniques, including optical emission, infrared, and Raman spectroscopies combined with convergent-beam and selected-area electron diffraction, and high-resolution (scanning) transmission electron microscopy imaging were used to systematically investigate the interrelation among plasma gas phase composition, catalysts morphology, catalyst structure, and carbon nanotube structure. Hydrogen plays a critical role in determining the final carbon nanotube structure through its effect on the catalyst crystal structure and morphology. At low H2 -to- CH 4 ratios (∼1), iron catalyst nanoparticles are converted to Fe3 C and well-graphitized nanotubes grow from elongated Fe 3 C crystals. High (>5) H2 -to- CH4 ratios in the feed gas result in high hydrogen concentrations in the plasma and strongly reducing conditions, which prevents conversion of Fe to Fe3 C. In the latter case, poorly-graphitized nanofibers grow from ductile bcc iron nanocrystals that are easily deformed into tapered nanocrystals that yield nanotubes with thick walls. © 2010 American Institute of Physics.