Abstract
Nonequilibrium quantum chemical molecular dynamics (QM/MD) simulation of
early stages in the nucleation process of carbon nanotubes from acetylene feedstock on an
Fe38 cluster was performed based on the density-functional tight-binding (DFTB) potential.
Representative chemical reactions were studied by complimentary static DFTB and density
functional theory (DFT) calculations. Oligomerization and cross-linking reactions between
carbon chains were found as the main reaction pathways similar to that suggested in previous
experimental work. The calculations highlight the inhibiting effect of hydrogen for the
condensation of carbon ring networks, and a propensity for hydrogen disproportionation, thus
enriching the hydrogen content in already hydrogen-rich species and abstracting hydrogen
content in already hydrogen-deficient clusters. The ethynyl radical C2H was found as a
reactive, yet continually regenerated species, facilitating hydrogen transfer reactions across
the hydrocarbon clusters. The nonequilibrium QM/MD simulations show the prevalence of
a pentagon-first nucleation mechanism where hydrogen may take the role of one “arm” of an
sp2 carbon Y-junction. The results challenge the importance of the metal carbide formation
for SWCNT cap nucleation in the VLS model and suggest possible alternative routes
following hydrogen-abstraction acetylene addition (HACA)-like mechanisms commonly
discussed in combustion synthesis.
