Biomedical Engineering Reference
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E
=
E
+
- (
E
+
E
), where the terms
E
,
E
,
p
gr
3H(ads)
gr+2H(para)
H(g)
gr
H(g)
E
are the total energies for a graphene sheet,
a gas phase H atom, a system formed by adsorbed H trimer
and graphene, and a system formed by an adsorbed H pair and
graphene, respectively. The adsorption energy of an isolated H atom
on graphene
, and
E
gr+3H(ads)
gr+2H
) is -0.77 eV, a value that
differs slightly from previous calculations [5, 16] due to the different
adsorbate coverage used in these studies. For the same reason, pair
and trimer adsorption energies reported here also slightly differ
from the corresponding values reported in Refs. [24, 25].
The closest pairing, commonly called in related literature as
“ortho pair,” given that the hydrogen atoms are directly attached to
the adjacent carbon atoms, is found to be the most stable pairing, in
contrast to findings at higher coverage [5], hence suggesting a specific
coverage effect on pairing stability. The para pair, however, does
not differ too much in energy. A comparison with analogous, even if
more restricted calculations (very localized relaxation), suggests that
the increased stability of p1 over p2 arises from the carbon atoms
inclusion beyond the hydrogen's immediate neighbors in the substrate
reconstruction. The two most stable pairs are followed by the one
with the largest atomic separation among the considered pairs, and
then by a not immediately foreseeable sequence of the remaining
pairs. Despite this apparent irregularity with H-H separation, the
results noticeably indicate that hydrogen pair attachment is favored
at locations involving carbon atoms from different sublattices, i.e.,
when the two adatoms are separately attached to an
E
=
E
- (
E
+
E
iso
gr+H(ads)
gr
H(g)
α
β
carbon
atom. The preference arises from unavoidable disruption of the stable
sp
and a
2
π
-bonding of the neighboring carbon atoms induced by the
tetrahedrization of receiving carbon, required for H attaching.
As seen in Fig. 5.6, the four most stable adsorbed pair
configurations are more stable than the state of two isolated H
adatoms, while the remaining pairings are not. Bound pairs are
all chemically stable with respect to free hydrogen atoms, but the
results support the greater likelihood of odd-neighbored pairs
(Fig. 5.9) being present on hydrogenated graphite surfaces.
This completely agrees with experiments involving different
deuterium atom doses. More specifically, pairs p2, p3, and p4 of the
current study correspond exactly to the observed pairs c, e, and d
of Andree
/
[13], respectively, while pairs p1 and p2 correspond
to the dimers A and B of Hornekaer
et al.
et al.
[14]. TDS data [11, 12]
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