The Phase of Iron Catalyst
Nanoparticles during Carbon
Nanotube Growth
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Abstract
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 γ-rich Fe nanoparticle
distributions, metallic Fe is the active catalyst phase, implying
that carbide formation is not a prerequisite for nanotube growth.
For α-rich catalyst mixtures, Fe<sub>3</sub>C 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
Fe<sub>3</sub>C. Our results indicate that kinetic effects dominate
the complex catalyst phase evolution during realistic CNT growth recipes