Biomedical Engineering Reference
In-Depth Information
on graphene, attributing pairing stability from their magnetic
properties [32]. Considering the high stress due to strained C-C
bonds, changes in surface electronics and the presence of magnetism,
in a rather simplified statement even pairs create dangling bonds
whereas odd pairs do not, which is directly manifested in the cluster
stabilities.
5.6
Graphene Two-Face Hydrogenation
and Saturation
When compared with the graphite terraces that have been
emphasized so far, the edges of graphite show much more
reactivity to hydrogen atoms. On the experimental side mechanical
alloying has been used to prepare defective structures of graphite
under hydrogen atmosphere [33, 34], and the reported 7.4 wt.%
hydrogen stored in the resulting nanocrystalline (finely ground, in
other words) graphite was partly attributed to the formation of a
large number of edge-dangling bonds resulting from the rupture
of graphite intralayer linkages. A previous work [8] performed
by the same research group on hydrogen molecule dissociative
adsorption on the zigzag edge of graphite, was in agreement with
the aforementioned observations, as the potential energy surface
for H2 indicates a nonactivated process. Adsorption onto bare
armchair edges, however, has been shown to be a slightly less
favored route [9], suggesting a possible utility of the zigzag edge
over the armchair edge as a reaction channel.
A clear trend is as follows: the more defects you have, the more
chances you get for hydrogen chemisorbing onto a C atom. An
alternative to increasing the number of dangling bonds by creating
more edges is to increase the sp
3
character of the substrate atoms
on the sheet itself, which way should be a viable option to achieve
a high hydrogen uptake in carbon-based materials. As a reference,
it is known that hydrogenated tetrahedral amorphous carbon (ta-C:
H), a diamond-like carbon material with a high sp
3
2
ratio, is able
to reach a hydrogen/carbon atomic ratio of about 60% (4.6 wt.%).
A higher uptake value (>6.5 wt.%) is, however, desired for practical
use and the explicit control of the uptake and release of hydrogen is
necessary in actual situations. A fully advantageous use of graphite
for this specific purpose should involve the exploitation of every
/sp
