Effect of hydrogen on catalyst nanoparticles in carbon nanotube growth

The structures of carbon nanotubes grown from catalytic nanoparticles via plasma-enhanced chemical vapor deposition in CH(4)/H(2) mixtures show a strong dependence on the H(2)-to-CH(4) 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 H(2)-to-CH(4) ratios (similar to 1), iron catalyst nanoparticles are converted to Fe(3)C and well-graphitized nanotubes grow from elongated Fe(3)C crystals. High (>5) H(2)-to-CH(4) ratios in the feed gas result in high hydrogen concentrations in the plasma and strongly reducing conditions, which prevents conversion of Fe to Fe(3)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. (C) 2010 American Institute of Physics. [doi : 10.1063/1.3467971]