Biomedical Engineering Reference
In-Depth Information
The more comprehensive tool for looking at the motion of
H atom on graphene is shown in Fig. 5.3, which is a two-
dimensional potential energy surface for H atom motion along a
plane perpendicular to the graphene sheet. The arrow indicates
the effective adsorption pathway of H on graphene, showing that a
hydrogen atom on its way to penetrate the graphene hollow would
rather change its course and attach via the top site. Similar to the
direct top-site pathway, this process has a small energy barrier (0.18 eV).
From the same plot, the activation barrier for H atom diffusion
on graphene is found to be 0.1 eV. These results suggest that H atom
absorption and desorption through the hexagonal center of graphene
hardly occur within the thermal energy region. A study on stable
hydrogen configurations inside bilayer graphene [6] has furthermore
shown the bonding energies and geometries that are very similar
to the aforementioned results on a single sheet, demonstrating that
the weak interaction between graphene sheets does not strongly
influence the hydrogen adsorption on the individual carbon sheets.
5.3
Hydrogen Molecule Dissociative Adsorption
On one face of graphene, the main difference between the reaction
with impinging H atoms and H
molecules is that a stable hydrogen
molecule adsorption [7] requires the energy for a molecular
dissociation before the stable chemisorption states are reached.
The breaking up of the incoming molecules requires a large amount
of energy (in particular, from our calculations, the H
2
binding
energy is 4.48 eV). So it is not surprising to find out that the overall
dissociative adsorption reaction is at least 3.7 eV endothermic,
not mentioning an activation barrier of at least 4.3 eV when the
carbon atoms have not got time to relax. Fortunately, graphene
reconstruction reduces the barrier to reach stable adsorbed states
of about 3.3 eV (Fig. 5.4), and reduces the final endothermicity
state of about 1.3 eV (Fig. 5.4).
The configuration with atoms attaching to opposite corners
of a graphene hexagon (i.e., in the “para” configuration) has been
found to be the most stable configuration for an adsorbed pair, and
has been additionally found, from the constructed potential energy
surface, to be the most accessible. A deeper discussion of these
final adsorbed states is presented in the proceeding sections.
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