4.3 Changes in Surface Stress during UPD
121
showed that the Cu-UPD layer after the C 2 peak has a pseudomorphic Cu-(1 × 1)
structure (see Fig. 4.10b). Moreover, the SEXAFS [43, 47] revealed that sulfate or
bisulfate ions are adsorbed on top of the Cu-(1 × 1) monolayer.
Figure 4.10 represents schematically the top views of (a)
√
3 ×
√
3
R30
◦ and
(b) Cu-(1 × 1) structures on Au (111) electrode. The cathodic charge density q c =
−4.9 C m
−2 at 0.25 V (SHE), referred to q c = 0 at 0.60 V (SHE), is calculated from
the cyclic voltammogram (Fig. 4.9a) in the cathodic potential scan [19]. Assuming
the surface roughness S r = 1.1 of the Au (111) electrode, the value of q c = −
4.9 C m
−2 is close to −4.4 C m
−2 required for a fully discharged Cu monolayer
on a smooth Au (111) electrode surface [48]. The difference between g in the
presence and absence of 10
−3 M Cu
2+ at the cathodic potential limit of 0.25 V (SHE)
corresponding to the Cu (1 × 1) monolayer on Au (111) electrode is (g) = −
1.10 J m
−2 , which is comparable to that ((((g) = −1.07 J m
−2
) at the cathodic limit
potential of −0.24 V (SHE) corresponding to the incommensurate hcp Pb monolayer
for the Pb-UPD on Au (111). Trimble et al. [17] reported that the difference between
the changes in surface stress of the Au (111) in acidic sodium sulfate solutions (pH 2)
with and without 10
−3 M Cu
2+ is (g) = −0.6 J m
−2 at the potential corresponding
to the Cu-(1 × 1) monolayer, which is about 0.5 times as much as that in Fig. 4.9b.
On the other hand, Kongstein et al. [18] reported the difference in changes in surface
stress of the Au (111) electrode between 0.1 M H 2 SO 4 solutions with and without
10
−2 M Cu
2+ is (g) = −1.0 J m
−2 at the potential corresponding to the Cu-(1 ×
1) monolayer, which is close to that in Fig. 4.9b.
Shi and Lipkowski [49] measured the cyclic voltammograms of the Au (111)
electrode in 0.1 M HClO 4 solutions containing 1 × 10
−5 M ∼ 5 × 10
−3 M Cu(ClO 4 ) 2
and 10
−3 M K 2 SO 4 to calculate the electrocapillary curves from which the surface
excesses Γ of Cu adatom and SO 4
2− ion co-adsorbed on the Au (111) electrode were
obtained as a function of potential as shown in Fig. 4.11. It is remarked that Γ =
2
3
monolayer (ML) of Cu adatom and Γ =
1
3
monolayer (ML) of SO 4
2− ion at 0.36 V
(SHE) in Fig. 4.11 correspond just to the honeycomb
√
3 ×
√
3
R30
◦ monolayer
with SO 4
2− ions co-adsorbed at the centers of the honeycomb (see Fig. 4.10a).
The following complicated features of the relationship between g and Γ in the
cathodic potential scan can be drawn from the comparison between Figs. 4.9 and
4.11: (1) g increases toward tensile direction with decreasing Γ of SO 4
2− ions in
the potential region between 0.80 and 0.60 V (SHE), (2) g increases up to the C 1
peak at 0.44 V (SHE) with increasing Γ of Cu adatom and with decreasing Γ of
SO 4
2− ion in the potential region between 0.60 and 0.44 V (SHE), (3) g decreases
toward compressive direction with increasing both Γ of Cu adatom and of SO 4
2−
ion in the potential region between 0.44 and 0.36 V (SHE), and (4) g decreases
further with increasing Γ of Cu adatom and with decreasing Γ of SO 4
2− ion in the
potential region between 0.36 and 0.25 V (SHE) corresponding to the transition from
the
√
3 ×
√
3
R30
◦ to the Cu-(1 × 1) structure. The above features demonstrate
that the co-adsorption of SO 4
2− ions plays a vital role in the Cu-UPD on the Au
(111) electrode, which reflects strongly on the magnitude and sign of g.
121
showed that the Cu-UPD layer after the C 2 peak has a pseudomorphic Cu-(1 × 1)
structure (see Fig. 4.10b). Moreover, the SEXAFS [43, 47] revealed that sulfate or
bisulfate ions are adsorbed on top of the Cu-(1 × 1) monolayer.
Figure 4.10 represents schematically the top views of (a)
√
3 ×
√
3
R30
◦ and
(b) Cu-(1 × 1) structures on Au (111) electrode. The cathodic charge density q c =
−4.9 C m
−2 at 0.25 V (SHE), referred to q c = 0 at 0.60 V (SHE), is calculated from
the cyclic voltammogram (Fig. 4.9a) in the cathodic potential scan [19]. Assuming
the surface roughness S r = 1.1 of the Au (111) electrode, the value of q c = −
4.9 C m
−2 is close to −4.4 C m
−2 required for a fully discharged Cu monolayer
on a smooth Au (111) electrode surface [48]. The difference between g in the
presence and absence of 10
−3 M Cu
2+ at the cathodic potential limit of 0.25 V (SHE)
corresponding to the Cu (1 × 1) monolayer on Au (111) electrode is (g) = −
1.10 J m
−2 , which is comparable to that ((((g) = −1.07 J m
−2
) at the cathodic limit
potential of −0.24 V (SHE) corresponding to the incommensurate hcp Pb monolayer
for the Pb-UPD on Au (111). Trimble et al. [17] reported that the difference between
the changes in surface stress of the Au (111) in acidic sodium sulfate solutions (pH 2)
with and without 10
−3 M Cu
2+ is (g) = −0.6 J m
−2 at the potential corresponding
to the Cu-(1 × 1) monolayer, which is about 0.5 times as much as that in Fig. 4.9b.
On the other hand, Kongstein et al. [18] reported the difference in changes in surface
stress of the Au (111) electrode between 0.1 M H 2 SO 4 solutions with and without
10
−2 M Cu
2+ is (g) = −1.0 J m
−2 at the potential corresponding to the Cu-(1 ×
1) monolayer, which is close to that in Fig. 4.9b.
Shi and Lipkowski [49] measured the cyclic voltammograms of the Au (111)
electrode in 0.1 M HClO 4 solutions containing 1 × 10
−5 M ∼ 5 × 10
−3 M Cu(ClO 4 ) 2
and 10
−3 M K 2 SO 4 to calculate the electrocapillary curves from which the surface
excesses Γ of Cu adatom and SO 4
2− ion co-adsorbed on the Au (111) electrode were
obtained as a function of potential as shown in Fig. 4.11. It is remarked that Γ =
2
3
monolayer (ML) of Cu adatom and Γ =
1
3
monolayer (ML) of SO 4
2− ion at 0.36 V
(SHE) in Fig. 4.11 correspond just to the honeycomb
√
3 ×
√
3
R30
◦ monolayer
with SO 4
2− ions co-adsorbed at the centers of the honeycomb (see Fig. 4.10a).
The following complicated features of the relationship between g and Γ in the
cathodic potential scan can be drawn from the comparison between Figs. 4.9 and
4.11: (1) g increases toward tensile direction with decreasing Γ of SO 4
2− ions in
the potential region between 0.80 and 0.60 V (SHE), (2) g increases up to the C 1
peak at 0.44 V (SHE) with increasing Γ of Cu adatom and with decreasing Γ of
SO 4
2− ion in the potential region between 0.60 and 0.44 V (SHE), (3) g decreases
toward compressive direction with increasing both Γ of Cu adatom and of SO 4
2−
ion in the potential region between 0.44 and 0.36 V (SHE), and (4) g decreases
further with increasing Γ of Cu adatom and with decreasing Γ of SO 4
2− ion in the
potential region between 0.36 and 0.25 V (SHE) corresponding to the transition from
the
√
3 ×
√
3
R30
◦ to the Cu-(1 × 1) structure. The above features demonstrate
that the co-adsorption of SO 4
2− ions plays a vital role in the Cu-UPD on the Au
(111) electrode, which reflects strongly on the magnitude and sign of g.