Environmental Engineering Reference
In-Depth Information
expression for the interface stability:
h
GT,
g(T, a
n
, a
me
, a
ac
, a
c
, Df)
¼
1
A
ð
a
fg
,
N
fg
Þ
N
n
m
n
(T, a
n
)
N
me
g
bulk
me
(T, a
me
)
q
e
(Df
F
=
e)
N
a
m
ac
(T, a
ac
)
N
c
y
m
c
(T, a
c
)
i
x
N
a
(5
:
19)
The interfacial free energy g(T, a
n
, a
me
, a
ac
, a
c
, Df) depends only on temperature,
the activities, and the difference between the electrostatic potentials of electrode and
electrolyte, Df, which, apart from a
c
, are all well-defined and experimentally acces-
sible quantities. Therefore, the accurate calculation of gdepends on the accuracy of
evaluating a
c
or the corresponding activity coefficient f
-
.
Finally, it should be remarked that, as long as the interfacial region is extended suf-
ficiently to include all structural and electronic deviations from the reservoirs, (5.18)
and (5.19) are valid for any type of connection between a metallic electrode and an
electrolyte. They also include the cases of nonspecific and specific adsorption on
the electrode.
5.3 APPLICATIONS
On the basis of (5.19), in this section we will evaluate the importance of different
contributions to the overall interfacial free energy by applying the extended ab initio
atomistic thermodynamics approach to two examples.
5.3.1 Potential-Dependent Surface Reconstruction of Au(100)
Surface reconstruction and relaxation can be understood as a deviation from the bulk-
truncated structure on the atomic level, by which the surface minimizes its free energy
[Kolb, 1996]. In particular, surface reconstruction usually involves a change in the
periodicity of the surface and in some cases a change in symmetry as well, whereas
surface relaxation is a (small) rearrangement of surface layers.
Under ultrahigh vacuum (UHV) conditions, some low index surfaces of the late 5d
metals Au, Pt, and Ir show reconstruction of the first surface layer. In order to maxi-
mize the number of surface bonds, the ground state of the (100) face corresponds to a
quasihexagonal (hex) close-packed structure, which results in an overall lower surface
free energy. The reconstruction of Au(100) has been particularly well studied, both
experimentally and theoretically. As nicely summarized in [Kolb, 1996] and [Feng
et al., 2005], from low energy electron diffraction (LEED) and helium diffraction
experiments, first a (1
5) and later a (20
15) reconstruction have been proposed
[Fedak and Gjostein, 1966, 1967; Rieder et al., 1983]. However, further LEED studies
[Wendelken and Zehner, 1978; Van Hove et al., 1981] suggested a c(26
68) recon-
struction, while even more complex structures were found by scanning tunneling
Search WWH ::
Custom Search