124
4 Changes in Surface Stress Associated with Underpotential …
perchloric acid [54] is more negative by about 0.7 V than E pzc = 0.48 V (SHE) of
the unreconstructed Au (111) electrode in perchloric acid [55]. Wilms et al. [52]
implied from the Moiré pattern that the formation of the
√
3 ×
√
7 monolayer on
the Cu (111) electrode expands the top Cu layer by 4%. Moreover, Trimble et al.
[17] reported that the surface stress of the Cu (111) electrode varies by 0.3 J m
−2
toward compressive direction, i.e., (g) SO
2−
4
= −0.3 J m
−2 , due to the adsorption
of SO 4
2− ions at 0.165 V (SHE) in pH 2, 0.1 M Na 2 SO 4 solution. Accordingly, the
negative value of (g) SO
2−
4
should be also brought by the adsorption of SO 4
2− ions
on the Cu-(1 × 1) monolayer/Au (111) electrode.
The adsorption of SO 4
2− ions on the Cu-(1 × 1) monolayer/Au (111) electrode
would be more favorable than that on the top Cu layer of the Cu (111) electrode
since the Cu-(1 × 1) monolayer is more expanded than the top Cu layer of the
Cu (111) electrode. The value of (g) SO
2−
4
due to the adsorption of SO 4
2− ions
on the Cu-(1 × 1) monolayer/Au (111) electrode may be more negative than that
due to the adsorption of SO 4
2− ions on the Cu (111) electrode. The calculation of
(g) Cu(1×1)/Au(111) in Eq. (4.22) was made for the bare and unreconstructed Au
(111) surface without surface charge. The value of (g) = −1.10 J m
−2 at 0.25 V
(SHE) in Fig. 4.9b is referred to g = 0 for the Au (111) electrode in the absence of
Cu
2+ . Therefore, in addition to the term of (g) SO
2−
4
, the correction for the changes
of surface stress caused by the potential shift (i.e., the change in surface charge) from
E pzc to 0.25 V (SHE) for the Au (111) electrode in the absence of Cu
2+ is needed for
Eq. (4.22).
The potential of zero charge E pzc of the unreconstructed Au (111) electrode in
perchloric acid is 0.48 V (SHE) [55]. The addition of 10
−3 M SO 4
2− in perchloric acid
shifts E pzc of the unreconstructed Au (111) electrode toward negative direction at
least by about 50 mV [56], which leads to E pzc = 0.43 V (SHE) of the unreconstructed
Au (111) surface in sulfuric acid. The level of surface stress for the unreconstructed
Au (111) electrode at 0.43 V (SHE) corresponds to that for the reference Au (111)
electrode without surface charge, which is lower by 0.13 J m
−2 than that at 0.25 V
(SHE) as seen from the dotted line in Fig. 4.9b. As a result, the correction term
(g) Au(111) for the potential shift is −0.13 J m
−2 . Provided that each term has an
additive property, (g) at 0.25 V (SHE) may be
(g) = (g) Cu(1×1)/Au(111) + (g) SO
2−
4
+ (g) Au(111) .
(4.23)
The substitution of (g) = −1.10 J m
−2 , (g) Cu(1×1)/Au(111) = −0.28 J m
−2 ,
and (g) Au(111) = −0.13 J m
−2 into Eq. (4.23) provides (g) SO
2−
4
= −0.69 J m
−2
for the Cu-(1 × 1) monolayer/Au (111) electrode, which is 2.3 times as much as
(g) SO
2−
4
= −0.30 J m
−2 for the Cu (111) electrode, suggesting that the adsorption
of SO 4
2− on the Cu-(1 × 1) monolayer/Au (111) electrode is more energetically
favorable that that on the top Cu layer of the Cu (111) electrode. Although the
Cu species in the Cu-(1 × 1) monolayer on Au (111) electrode in sulfuric acid is
regarded as completely discharged, X-ray absorption near-edge structure (XANES)
4 Changes in Surface Stress Associated with Underpotential …
perchloric acid [54] is more negative by about 0.7 V than E pzc = 0.48 V (SHE) of
the unreconstructed Au (111) electrode in perchloric acid [55]. Wilms et al. [52]
implied from the Moiré pattern that the formation of the
√
3 ×
√
7 monolayer on
the Cu (111) electrode expands the top Cu layer by 4%. Moreover, Trimble et al.
[17] reported that the surface stress of the Cu (111) electrode varies by 0.3 J m
−2
toward compressive direction, i.e., (g) SO
2−
4
= −0.3 J m
−2 , due to the adsorption
of SO 4
2− ions at 0.165 V (SHE) in pH 2, 0.1 M Na 2 SO 4 solution. Accordingly, the
negative value of (g) SO
2−
4
should be also brought by the adsorption of SO 4
2− ions
on the Cu-(1 × 1) monolayer/Au (111) electrode.
The adsorption of SO 4
2− ions on the Cu-(1 × 1) monolayer/Au (111) electrode
would be more favorable than that on the top Cu layer of the Cu (111) electrode
since the Cu-(1 × 1) monolayer is more expanded than the top Cu layer of the
Cu (111) electrode. The value of (g) SO
2−
4
due to the adsorption of SO 4
2− ions
on the Cu-(1 × 1) monolayer/Au (111) electrode may be more negative than that
due to the adsorption of SO 4
2− ions on the Cu (111) electrode. The calculation of
(g) Cu(1×1)/Au(111) in Eq. (4.22) was made for the bare and unreconstructed Au
(111) surface without surface charge. The value of (g) = −1.10 J m
−2 at 0.25 V
(SHE) in Fig. 4.9b is referred to g = 0 for the Au (111) electrode in the absence of
Cu
2+ . Therefore, in addition to the term of (g) SO
2−
4
, the correction for the changes
of surface stress caused by the potential shift (i.e., the change in surface charge) from
E pzc to 0.25 V (SHE) for the Au (111) electrode in the absence of Cu
2+ is needed for
Eq. (4.22).
The potential of zero charge E pzc of the unreconstructed Au (111) electrode in
perchloric acid is 0.48 V (SHE) [55]. The addition of 10
−3 M SO 4
2− in perchloric acid
shifts E pzc of the unreconstructed Au (111) electrode toward negative direction at
least by about 50 mV [56], which leads to E pzc = 0.43 V (SHE) of the unreconstructed
Au (111) surface in sulfuric acid. The level of surface stress for the unreconstructed
Au (111) electrode at 0.43 V (SHE) corresponds to that for the reference Au (111)
electrode without surface charge, which is lower by 0.13 J m
−2 than that at 0.25 V
(SHE) as seen from the dotted line in Fig. 4.9b. As a result, the correction term
(g) Au(111) for the potential shift is −0.13 J m
−2 . Provided that each term has an
additive property, (g) at 0.25 V (SHE) may be
(g) = (g) Cu(1×1)/Au(111) + (g) SO
2−
4
+ (g) Au(111) .
(4.23)
The substitution of (g) = −1.10 J m
−2 , (g) Cu(1×1)/Au(111) = −0.28 J m
−2 ,
and (g) Au(111) = −0.13 J m
−2 into Eq. (4.23) provides (g) SO
2−
4
= −0.69 J m
−2
for the Cu-(1 × 1) monolayer/Au (111) electrode, which is 2.3 times as much as
(g) SO
2−
4
= −0.30 J m
−2 for the Cu (111) electrode, suggesting that the adsorption
of SO 4
2− on the Cu-(1 × 1) monolayer/Au (111) electrode is more energetically
favorable that that on the top Cu layer of the Cu (111) electrode. Although the
Cu species in the Cu-(1 × 1) monolayer on Au (111) electrode in sulfuric acid is
regarded as completely discharged, X-ray absorption near-edge structure (XANES)