Biomedical Engineering Reference
In-Depth Information
showed more significant distortion when more than two hydrogen
atoms were adsorbed. As indicated in Fig. 7.8, the (9,0) nanotube has
adopted a slightly egg-shaped cross-section and some buckling in the
structure of the wall around the C-H bonds. This distortion is more
pronounced in the 3(R) (Fig. 7.8e) than in the 3(P) configuration
(Fig. 7.8d). This is interesting given the iso-surfaces again show that
the rehybridization is localized in the vicinity of the adsorbates.
An important aspect that is difficult to deduce from Figs. 7.7
and 7.8, is that dangling bonds are being formed on the CNT wall
when three H atoms are adsorbed in the 3(P), but not in the 3(R)
configuration. If formed on a CNT wall, an sp
3
defect containing
a dangling bond would (under the right conditions) represent
a preferred site for subsequent gas adsorption, with lower
characteristic adsorption energies.
The adsorption energy (
E
) of a single H atom is simply defined
ad
by:
=
1
)
___
(
E
N
E
+
N
m
-
E
(7.1)
ad
CNT
H
H
CNT, H
where
E
is the total energy of the clean CNT,
E
is the total
CNT
CNT,H
energy of the CNT with H adsorbates,
is the chemical potential of
atomic hydrogen (usually taken as half that of an H
μ
H
dimer) and
N
2
H
is the number of adsorbed H atom. This expression is independent
of the configuration or clustering of the adsorbates, but can be
used on any clustering configuration to determine which (if any) is
energetically preferable. Based on the DFT simulations discussed
above, the adsorption energy for H on the (6,6) structure changes
from 3.60 eV for a single adsorbed H atom, 3.47 eV for two adsorbed
H atoms at neighboring sites, and 3.38 eV and 3.42 eV for the
3(P) and 3(R) configurations, respectively. In the case of the (9,0)
structure the results were 3.69 eV, 3.65 eV, 3.58 eV, and 3.63 eV for
the same configurations, respectively [55]. Overall the absorption
energy was found to decrease with increasing number of H atoms,
which is consistent with the notion that the rehybridized regions
represent preferred binding sites. However, the most interesting
aspect is that the 3(P) configuration is lower in energy than the 3(R)
configuration, even though it is accompanied by the formation of a
dangling carbon bond.
The addition of more H adsorbates would logically extend
the defective region, and increase the probability of dangling sp
3
bond formation. Extrapolating this trend to a monolayer coverage,
