Biomedical Engineering Reference
In-Depth Information
hydrogen adsorption decreases with increasing local curvature. This
indicates that CNTs are more susceptible to chemisorption of gases,
accompanied by a re-hybridization from sp
2
3
of the underlying
C atom, than extended surfaces or planar materials such as graphene
[34]. In addition to atomic H adsorption, Sun
to sp
et al.
have also treated
CNTs in an H
atmosphere, and have annealed nanotubes in a vacuum
[35]. In both cases, no changes were reported in the structure of the
nanotubes, thereby demonstrating that
2
atomic hydrogen is
necessary, in agreement with the results of Ruffieux [33].
The re-hybridization of C atoms upon exposure to gases can
be useful. It was used to great advantage by Rossi
reactive
[36] who
successfully grew uniform nanocrystalline diamond coatings onto
commercial vapor grown carbon fibers. The coatings had a grain size
of 5-50 nm and an intermediate layer of turbostratic graphite was
identified at the film/fiber interface [36]. Following from this result,
Terranova
et al.
[37] have reported on the growth and characterization
of hybrid nanocarbon systems, coupling nanodiamonds and CNTs
in one step. The nanodiamonds (with diameters in the range
20-100 nm) were nucleated and grown directly on single walled
CNTs, and bundles of CNTs up to 15 μm long [37]. In contrast, Sun
et al
et al.
. [35] reported a two-step process, where the degradation
of multi-walled CNTs into amorphous carbon nanorods was
observed before the hydrogen plasma induced transformation into
nanocrystalline diamond was observed [35].
A common feature of these studies is that “defective” regions/
sites on the sp
2
-bonded nanotubes, formed as a result of gas
adsorption, act as ignition points for the nucleation (and subsequent
growth) of diamond nanoparticles or the formation of amorphous
structures [35, 37]. Locally the properties of the CNTs change in a very
fundamental way when gases are adsorbed. Although it is controlled
during the experiments mentioned above, if the same event occurred
in an uncontrolled way such changes could dramatically alter the
properties of the material, and may have an important impact on
possible failure of devices. It could also impact the bioavailability
and bioactivity of CNT samples and the sensitivity of CNT-based
gas sensors (since the number of active sites is consequentially
reduced). Clearly, the integrity of future CNT-based technologies,
and our ability to anticipate failure of these technologies, all require
a detailed understanding of the stability and properties of CNTs
exposed to a
full range of environmental conditions
.
