First-Principles-Based Multiscale, Multiparadigm Molecular Mechanics and Dynamics Methods for Describing Complex Chemical Processes - Multiscale Molecular Methods in Applied Chemistry - page 19

As reported previously [ 85 , 86 ], we have developed a set of ReaxFF parameters
describing hydrocarbon chemistry catalyzed by nickel and nickel carbide catalyst
particles. This ReaxFF potential is capable of treating the adsorption and decom-
position of both saturated and unsaturated hydrocarbon species on several different
nickel surfaces. Of particular relevance for studying CNT growth is that a single
set of ReaxFF parameters accurately describes carbon in all hybridization states and
a variety of chemical environments. These states include sp, sp 2 , and sp 3 hybridized
carbon in various hydrocarbon molecules, carbon binding at and migrating between
interstitial sites in bulk nickel, and carbon bonded to nickel surfaces strongly as
an adsorbed lone adatom or in a small hydrocarbon molecule, or weakly as part of
a graphene layer.
While the vast majority of theoretical studies of CNT growth starts with lone
carbon atoms, assuming that decomposition has already taken place, there are
conditions (e.g., low temperature growth) under which decomposition is believed
to be the rate-limiting step [ 87 ]. Thus we have utilized this ReaxFF force field in
a reactive dynamics (RD) study of the early stages of CNT growth. In [ 85 ]we
reported on the chemisorption and decomposition of various hydrocarbon species
on a nickel nanoparticle. Over the course of 100 ps of RD simulations performed,
we were able map out the preferred reaction pathways for the decomposition of
each hydrocarbon species studied.
The synthesis of CNTs can be broken down into three or four distinct stages. The
first stage is feedstock decomposition, as discussed above. Under low temperature
growth conditions, experiments suggest that feedstock decomposition is the rate-
limiting step [ 87 ]. Thus our analysis of hydrocarbon decomposition pathways on
nickel nanoparticles shows how the selection of different hydrocarbon species for
the feedstock influences the chemisorption rate, surface coverage, and extent of
carbide formation during the nanotube growth process. In particular, because we
find that chemisorption is the rate limiting decomposition step for saturated hydro-
carbons, the selection of unsaturated hydrocarbon species, with very small chemi-
sorption barriers, for the feedstock, is expected to improve the growth rate where
feedstock decomposition is rate limiting.
Following feedstock decomposition is the carbon transport stage, in which a
hydrocarbon or carbon species is either transported along the catalyst surface or else
diffuses through the catalyst bulk as carbide. Because a constant supply of carbon is
needed for both nucleation and growth, carbon transport likely occurs during
both the nucleation and growth stages and so is most naturally treated as a part of
each of these stages taken separately. It is also possible that a partially decomposed
species migrates to the nucleation or growth site where it further decomposes
into the activated species. In any case, experiments indicate that there are growth
conditions under which surface diffusion is the rate-limiting step [ 88 ]. ReaxFF RD
simulations demonstrate the formation of nickel carbide following acetylene che-
misorptions and decomposition, lending plausibility to either mechanism.
It is believed that nucleation occurs when enough carbon material accumulates
on the surface for the formation of surface ring structures. The ring structures
develop into a graphene island on the particle which, when it becomes large enough,