4.3 Changes in Surface Stress during UPD
123
(SHE) results from the co-adsorption of Cu atoms and SO 4
2− ions. The decrease
in bond charge density between surface atoms due to the co-adsorption would
bring the decrease in g. Moreover, the atomic configuration of co-adsorbed Cu
atoms and SO 4
2− ions would contribute to the decrease in g since the honeycomb
√
3 ×
√
3
R30
◦ structure is thermodynamically stabilized by the occupation of
SO 4
2− ions at the centers of the honeycomb. The further decrease in g in the potential region between 0.36 and 0.25 V (SHE) is associated with the formation of pseudomorphic Cu-(1 × 1) monolayer on the Au (111) electrode. As the lattice constant
(a Cu = 0.3615 nm) of bulk Cu is less than that (a Au = 0.4079 nm) of bulk Au, the
pseudomorphic Cu-(1 × 1) monolayer on Au (111) has a positive lattice mismatch
of +12.8%, which would provide the increase in g, opposed to the experimental
result.
The molecular dynamic simulations [17] indicated the increase in g due to
the biaxial stretching of a Cu (111) slab caused by a surface strain of +12.8% is
(g) = 2.78 J m
−2 . Leiva et al. [51], using the embedded-atom method, calculated
that the net change in surface stress due to the adsorption of pseudomorphic Cu-(1
× 1) monolayer on Au (111) is (g) Cu(1×1)/Au(111) = −0.28 J m
−2 , the content of
which can be divided into the following two different contributions:
(g) Cu(1×1)/Au(111) =
dE ads
dA
+
dU mono
dA
,
(4.22)
where the first contribution
dE ads
d A
= −6.38 J m
−2 corresponds to the change of adsorption energy of the Cu-(1 × 1) monolayer with an elastic change (d A) brought by
the substrate Au (111) surface, and the second contribution
dU mono
d A
= 6.10 J m
−2
corresponds to the energy change due to the expansion of the isolated Cu-(1 ×
1) monolayer in vacuum caused by a positive lattice mismatch. The first term in
Eq. (4.22) should be negative since the formation of metallic bonds between Cu and
Au becomes more favorable when the surface is expanded [51].
However, (g) Cu(1×1)/Au(111) = −0.28 J m
−2 is significantly small as compared
to (g) = −1.10 J m
−2 obtained experimentally at 0.25 V (SHE) corresponding
to the formation of the Cu-(1 × 1) monolayer on the Au (111) electrode as shown in
Fig. 4.9b. The calculation by Leiva et al. [51] has been made for the uncharged surface
corresponding to E pzc ; thus, any changes of surface stress caused by the change of
surface charge density are disregarded. Besides, the change of surface stress due
to the adsorption of SO 4
2− ions on Cu-(1 × 1) monolayer/Au (111) is not taken
into consideration for the calculation of (g) Cu(1×1)/Au(111) . The large difference
between the values of (g) Cu(1×1)/Au(111) obtained theoretically and experimentally
may result from the neglect of the above two factors.
It is known that a
√
3 ×
√
7 monolayer is formed by the adsorption of SO 4
2−
ions on Cu (111) electrode at 0.175 V(SHE) [52] as well on Au (111) electrode at
1.15 V (SHE) [53]. The potential difference of about 1.0 V between the formation
of the
√
3 ×
√
7 monolayer on Cu (111) and Au (111) electrodes may result from
the difference in E pzc since E pzc = −0.20 V (SHE) of the Cu (111) electrode in
123
(SHE) results from the co-adsorption of Cu atoms and SO 4
2− ions. The decrease
in bond charge density between surface atoms due to the co-adsorption would
bring the decrease in g. Moreover, the atomic configuration of co-adsorbed Cu
atoms and SO 4
2− ions would contribute to the decrease in g since the honeycomb
√
3 ×
√
3
R30
◦ structure is thermodynamically stabilized by the occupation of
SO 4
2− ions at the centers of the honeycomb. The further decrease in g in the potential region between 0.36 and 0.25 V (SHE) is associated with the formation of pseudomorphic Cu-(1 × 1) monolayer on the Au (111) electrode. As the lattice constant
(a Cu = 0.3615 nm) of bulk Cu is less than that (a Au = 0.4079 nm) of bulk Au, the
pseudomorphic Cu-(1 × 1) monolayer on Au (111) has a positive lattice mismatch
of +12.8%, which would provide the increase in g, opposed to the experimental
result.
The molecular dynamic simulations [17] indicated the increase in g due to
the biaxial stretching of a Cu (111) slab caused by a surface strain of +12.8% is
(g) = 2.78 J m
−2 . Leiva et al. [51], using the embedded-atom method, calculated
that the net change in surface stress due to the adsorption of pseudomorphic Cu-(1
× 1) monolayer on Au (111) is (g) Cu(1×1)/Au(111) = −0.28 J m
−2 , the content of
which can be divided into the following two different contributions:
(g) Cu(1×1)/Au(111) =
dE ads
dA
+
dU mono
dA
,
(4.22)
where the first contribution
dE ads
d A
= −6.38 J m
−2 corresponds to the change of adsorption energy of the Cu-(1 × 1) monolayer with an elastic change (d A) brought by
the substrate Au (111) surface, and the second contribution
dU mono
d A
= 6.10 J m
−2
corresponds to the energy change due to the expansion of the isolated Cu-(1 ×
1) monolayer in vacuum caused by a positive lattice mismatch. The first term in
Eq. (4.22) should be negative since the formation of metallic bonds between Cu and
Au becomes more favorable when the surface is expanded [51].
However, (g) Cu(1×1)/Au(111) = −0.28 J m
−2 is significantly small as compared
to (g) = −1.10 J m
−2 obtained experimentally at 0.25 V (SHE) corresponding
to the formation of the Cu-(1 × 1) monolayer on the Au (111) electrode as shown in
Fig. 4.9b. The calculation by Leiva et al. [51] has been made for the uncharged surface
corresponding to E pzc ; thus, any changes of surface stress caused by the change of
surface charge density are disregarded. Besides, the change of surface stress due
to the adsorption of SO 4
2− ions on Cu-(1 × 1) monolayer/Au (111) is not taken
into consideration for the calculation of (g) Cu(1×1)/Au(111) . The large difference
between the values of (g) Cu(1×1)/Au(111) obtained theoretically and experimentally
may result from the neglect of the above two factors.
It is known that a
√
3 ×
√
7 monolayer is formed by the adsorption of SO 4
2−
ions on Cu (111) electrode at 0.175 V(SHE) [52] as well on Au (111) electrode at
1.15 V (SHE) [53]. The potential difference of about 1.0 V between the formation
of the
√
3 ×
√
7 monolayer on Cu (111) and Au (111) electrodes may result from
the difference in E pzc since E pzc = −0.20 V (SHE) of the Cu (111) electrode in