Biomedical Engineering Reference
In-Depth Information
possible single section, hence a need to promote adsorption on the
faces of each sheet of the graphite lattice.
While one-face hydrogenation of graphene is itself a fertile ground
for academic discussion, the problem with one-face hydrogenation,
considering practical applications, is the impossibility to achieve
a full monolayer hydrogen coverage, since there is no way for the
carbon atoms to be in sp
3
configurations all at the same time. This is
disadvantageous, for example, if one needs to maximize the hydrogen
content on graphene. The situation, however, changes drastically
when both faces of graphene can have access to hydrogen [35]. This
is best illustrated by a pair of hydrogen atoms adsorbed closely, but
on opposite faces of graphene, i.e., in a way forming the two-face
analogue of the ortho pair, discussed earlier.
Results for relevant adsorption pathways are shown in Fig. 5.15,
which unambiguously show that atomic hydrogen adsorption on a
graphene sheet with preadsorbed hydrogen on the opposite side
can proceed much more easily compared to adsorption onto a plain
graphene sheet, since the difficulty in overcoming the energy barrier
to chemisorption on graphene is removed.
Figure 5.15
Potential energy curves showing the increased substrate
reactivity brought about by adsorption of the first hydrogen.
Adsorption of a single hydrogen atom on frozen (relaxed)
graphene is shown as filled (open) squares, while that for
the second hydrogen atom is shown in circles.
The final state for this pair is also a very stable one, significantly
lower in energy as compared with any of the same-face pairs
