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-CH4 ratios (∼1), iron catalyst nanoparticles are converted to Fe3C and well-graphitized nanotubes grow from elongated Fe3C 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 Fe3C. 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.
- Publication:
-
Journal of Applied Physics
- Pub Date:
- September 2010
- DOI:
- 10.1063/1.3467971
- Bibcode:
- 2010JAP...108e3303B
- Keywords:
-
- carbon nanotubes;
- catalysis;
- catalysts;
- infrared spectra;
- iron;
- luminescence;
- nanofabrication;
- nanoparticles;
- plasma CVD;
- transmission electron microscopy;
- visible spectra;
- 81.16.Hc;
- 81.15.Gh;
- 81.07.De;
- 78.67.Ch;
- 61.48.De;
- 82.65.+r;
- Catalytic methods;
- Chemical vapor deposition;
- Nanotubes;
- Nanotubes;
- Structure of carbon nanotubes boron nanotubes and closely related graphitelike systems;
- Surface and interface chemistry;
- heterogeneous catalysis at surfaces