Biomedical Engineering Reference
In-Depth Information
where all available sites are passivated, and (the probability of) the
pyramid-type and ring-type configurations are equivalent, the
E
ad
(Θ = 1) = 2.73 and 3.34 eV for the armchair and zigzag geometries,
respectively. This highlights, along with the results above, that
is
lower for the nanotube with a larger diameter, which is consistent
with the in study conducted by Gülseren
E
ad
. [43] The adsorption
energy and the strain energy associated with the curvature of the
CNT are intrinsically linked.
Based on these results we can identify that the efficiency of gas
adsorption on CNTs is related to the diameter and chirality of the
CNT, the number of adsorbates (total coverage) and the configuration
of the adsorbates with respect to one another. We also find that,
irrespective of the above, the rehybridization associated with gas
adsorption will likely remain localized around the adsorption site,
unless the coverage is sufficient to introduce enough stored elastic
energy to overcome the sp
et al
2
2 +
x
3
to sp
or sp
barrier and invoke a
cascade.
Since more than one adsorbate cannot occupy the same C
adsorption site simultaneously, we can assume that these effects
are additive provided that the differences between isolated and
clustered defects are included. Based on these defined rules, this
represents an ideal problem for analytical multiscale modeling.
7.3
Multiscale Model
Before constructing such a model, a number of important pieces
of information must be distilled from the results mentioned above.
Firstly, that the strain energy associated with curvature of the CNT
is a function of the diameter and gas coverage [43]. Secondly, the
adsorption energy of gases on the outer walls of CNTs is dependent
upon the chirality of the nanotube, as well as the “clustering” of the
adsorbates (that is the number of gas adsorbates is a rehybridized
region, and the pattern of these adsorbates) [55]. And finally, that
the rehybridized region surrounding each adsorption site which
extends only out to the first nearest neighbors of the adsorption site
[55-57].
Given these points, the energetics of this system can be described
in terms of the formation enthalpy, written as a sum of contributions
from the cohesive energy of the carbon atoms of the CNT (
E
),
C
