Journal
CHEMISTRY OF MATERIALS
Volume 24, Issue 24, Pages 4633-4640Publisher
AMER CHEMICAL SOC
DOI: 10.1021/cm301402g
Keywords
carbon nanotube; CVD; Fe; carbide; catalyst; XRD; ETEM
Funding
- ERC grant InsituNANO [279342]
- EC project Technotubes
- Marshall Aid Commemoration Commission
- National Science Foundation
- EPSRC
- Hughes Hall, Cambridge
- European Research Council (ERC) [279342] Funding Source: European Research Council (ERC)
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We study the Fe-catalyzed chemical vapor deposition of carbon nanotubes by complementary in situ grazing-incidence X-ray diffraction, in situ X-ray reflectivity, and environmental transmission electron microscopy. We find that typical oxide supported Fe catalyst films form widely varying mixtures of bcc and fcc phased Fe nanoparticles upon reduction, which we ascribe to variations in minor commonly present carbon contamination levels. Depending on the as-formed phase composition, different growth modes occur upon hydrocarbon exposure: For gamma-rich Fe nanoparticle distributions, metallic Fe is the active catalyst phase, implying that carbide formation is not a prerequisite for nanotube growth. For alpha-rich catalyst mixtures, Fe3C formation more readily occurs and constitutes part of the nanotube growth process. We propose that this behavior can be rationalized in terms of kinetically accessible pathways, which we discuss in the context of the bulk iron-carbon phase diagram with the inclusion of phase equilibrium. lines for metastable Fe3C. Our results indicate that kinetic effects dominate the complex catalyst phase evolution during realistic CNT growth recipes.
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