Biomedical Engineering Reference
In-Depth Information
mixed gas system on earth is air. Air is ubiquitous, and eventually
every component of new technologies (including nanomaterials in
nanotechnology) are exposed to it. Devices break, seals degrade,
and air pervades our laboratories and factories. The chemical
composition of dry air is dominated by 78.084% N (from N
) and
2
20.9476% O (from O
), with a small fraction of 0.0001% H or more
2
(from H
Although it may not seem that way on cloudy days, water
vapor comprises only ~0.40% over the full atmosphere, and is
typically 1-4% in the troposphere and at the surface of the earth.
).
1
2
7.5.1
Atmospheric Gases
Using the model outlined in Eq. (7.12) and the parameterization
described in Section 1.4 and Table 7.1, the formation enthalpy for a
range of CNTs in the presence H, O, N, and H
O is displayed in Fig. 7.14.
These results are provided for Θ = 2%, 4%, 6%, and 8%, respectively,
and compared to pristine CNTs (in a vacuum). In each case a random
adsorption pattern is assumed, with 20% of adsorbates adjacent to
another adsorbate, and 80% isolated with respect to one another.
We can see from Fig. 7.14 that the adsorption of water is
considerably more favorable than oxygen or nitrogen, even at low
Θ. It can also be said that the formation of small diameter nanotubes
is improved in the presence of small quantities of water, and to a
lesser extent, hydrogen. This is consistent with recent reports of
various (efficient) methods of water-assisted synthesis of CNTs via
chemical vapor deposition [65-71] and pyrolysis of organometallic
precursors [72]. In addition to this, these results indicate that the
formation of ultra-fine nanotubes (such as (4,0) which is included in
Fig. 7.14) may become more favorable if N or O were present during
synthesis, provided that they do not undergo other reactions that
have not been explicitly considered here. Quantifying the role of N
or O chemisorption in the formation of small diameter nanotubes
is therefore an ideal topic for further work, either with or without a
catalyst particle.
From Fig. 7.14 we see that at low gas coverage (Fig. 7.14a) pristine
CNTs are thermodynamically preferred over 1 nm in diameter, but
a sparse coating of all of these species is still stable over the same
CNT diameter. Oxygen and nitrogen, both adopting bidentate (bond-
2
1
The remaining constituents are in low concentrations, and in this model would not
be statistically significant.
