Biomedical Engineering Reference
In-Depth Information
for a deuterium coverage that favors formation of deuterium pairs
on the surface show three key features, two peaks (500 K and
580 K) with a shoulder between them (540 K). Assuming that the
numbers of each specific pair type, simultaneously existing on
the carbon surface, do not vary greatly, on the basis solely on the
adsorption energies pair the features at 580, 540, and 500 K can
be tentatively ascribed to p1, p2, and the combination of p3 and
p4, respectively. A more detailed treatment of the simultaneous
dynamics of two adatoms desorbing from graphene would be
needed to confirm these assignments. Nonetheless, the initial rise
in sticking probability with hydrogen exposure [13] and reduced
barriers to hydrogen sticking [24] are in agreement with the lower
energy reported for clustering.
Figure 5.9
Odd and even adsorbed hydrogen pairs.
We expect that this trend should hold for pairs with adatom
separations beyond that covered by two graphite rings, but should
be weakened with the increase of H-H interatomic separation. The
results do not imply that the p5, p6, and p7 pairs cannot be formed
— they can. However, given sufficient energy, the hydrogen atoms
may diffuse out to form the lower energy pair configurations, if not
moving into isolated positions. Barriers for diffusion between H
atom chemisorptions sites on graphite are relatively large [26], but
we expect that the moving out of the metastable states should be
generally easier, as in I
→
B transition of Ref. [14].
These results also suggest that the first adsorbed hydrogen
can induce nearby neighbor reactivity. It is clearly shown here: the
presence of adsorbed hydrogen on graphene has a significant effect
on the adsorption of a second one, even to a relatively distant H-H
separation of 5 Å. Energetics of multiple adsorbates on surfaces can
