We present a model for the process of the growth of carbon nanotubes (CNTs)
obtained by chemical vapour deposition in the presence of transition metal
nanoparticles (Me-NPs) which act as a catalyst. We have deduced that the growth
of a CNT occurs in the presence of two forces: (i) a viscous force, due to the
surrounding hot gas, which opposes and slows down the growth of the CNT, and
(ii) an extrusive force that causes the growth and that in the steady-state stage of
the growth is completely balanced by the viscous force. We believe that it is the
great decrease in free energy in the assembling reaction that occurs at the
interface of the Me-NP catalyst that causes the extrusive force for the
growth of a CNT. Moreover, the process of chemisorption of a C2
fragment, through the interaction of the C2–π
system with the 3d metal orbitals, has been considered as well as
the coordination action of the Fe, Ni and Co metal surfaces. The
structural properties of the Fe, Co and Ni surfaces show that the (1,
− 1,
0) planes of Fe and the (1, 1, 1) planes of Co and Ni exhibit the symmetry and
distances required to overlap with the lattice of a graphene sheet. This
gives us information about the coordination mechanism responsible for
assembling the CNTs. In fact, we show that it is possible to cleave an Me-NP
in such a way as to match the correct symmetry and dimension of the
armchair structure of a single-walled nanotube. The mechanism of C2
addition at the edge of the growing CNT has also been considered in relation to
the highest occupied molecular orbital–lowest unoccupied molecular orbital
(HOMO–LUMO) symmetry. We demonstrate that the action of d orbitals
of the metal atoms forming the Me-NP makes possible the thermally
forbidden reaction, which involves the C2–π
system.