Biomedical Engineering Reference
In-Depth Information
for the third adsorption pattern illustrated in Fig. 7.11c, where
Θ
= 1/8 and
N
= 4
N
, but
N
=
N
- 4
N
. This pattern is the first
3
3
max
sp
H
sp
C
H
2
3
to include sp
-hybridized
atoms, making each rehybridized region isolated from the others. In
this case (due to this isolation) the coverage independent adsorption
energy
-hybridized C atoms surrounding the sp
) may be used, as defined in Eq. (7.1). The effect of this
is minimal, and the main difference between Figs. 7.12b and 7.12c
can be attributed to the reduction of Θ
E
(
H
ad
.
The final pattern illustrated in Fig. 7.11d is the most experimentally
realistic pattern selected for presentation here. The sparsely adsorbed
H atoms (indicated by the dark spheres) are randomly distributed,
so that this pattern contains elements of all three of the other (more
ideal) adsorption patterns described above. In this case Θ
max
=
1/7,
max
N
= 3.7
N
, and
N
=
N
- 3.7
N
. Approximately ~30% of the
3
3
sp
H
sp
C
H
3
rehybridized sp
regions intersect or share a common carbon neighbor,
thereby requiring the use of
), while the remaining ~70%
are isolated and (the coverage independent)
E
(Θ,
R
,
H
ad
may be used. The
results for this pattern are shown in Fig. 7.12d, and are very similar to
those for the third pattern in Fig. 7.12c. The primary difference, which
is not easily discerned from the graph in Fig. 7.12d, is a shift in the
crossover point of +0.03 nm. This crossover point is dependent on the
gas species, and may shift if different adsorbates are investigated, as
demonstrated in Fig. 7.13, where the same adsorption configurations
are tested for the case of oxygen. Within the uncertainties of the
model, the crossover no longer exists, and only very sparse coverage
of isolated C-O-C bonds are thermodynamically favorable, and only
at large diameters.
When undertaking modeling of this sort, it is important to
remember that the characteristics of rehybridized regions on CNTs
grown using different methods may vary significantly. For example,
it is unlikely that using atomic H alone will be sufficient to give a
complete description of CNTs such as those in the vertically aligned
arrays grown using the plasma enhanced chemical vapor deposition
(PECVD) method [63]. Differences in precursor gases, catalysts and
experimental conditions (such as temperature, pressure, and the
electric field) will influence the concentration, pattern, and type of
adsorbates on the CNTs, and hence alter the minimum diameter and
relative stability. In order to construct a more complete description
of PECVD nanotubes the model outlined here will need to be
parameterized for a larger range of adsorbates (such as various
E
(
H
)
ad
