126
4 Changes in Surface Stress Associated with Underpotential …
Fig. 4.12a, is only 50% of q c = −4.9 C m
−2 (calculated from Fig. 4.9a) required
for the formation of the Cu-(1 × 1) monolayer on Au (111) electrode, indicating
that the Cu-UPD on Au (111) electrode is strongly hindered in perchloric acid media
without SO 4
2− ions. In response to the cyclic voltammogram, the change in surface
stress in the potential range from 0.40 to 0.25 V (SHE) in the cathodic potential scan
in Fig. 4.12b is (g) = −0.17 J m
−2 , which is significantly small as compared to
(g) = −0.90 J m
−2 in the same potential range in Fig. 4.9b, indicating that the
formation of Cu-(1 × 1) monolayer is not accomplished. The feature of the cyclic
voltammogram in Fig. 4.12a is consistent with that of the cyclic voltammogram
obtained for the Cu-UPD on Au (111) in 0.1 M HClO 4 solution containing 10
−2 M
Cu(ClO 4 ) 2 [57].
Hotlos et al. [57] pointed out that the broad cathodic and anodic peaks in the cyclic
voltammogram are associated with trace amounts of Cl
− ions (≤10
−6 M) contained
as impurity in the solution. The STM images [57] of the Au (111) electrode surface
in 0.1 M HClO 4 solution containing 10
−2 M Cu(ClO 4 ) 2 with small amounts of Cl
−
ions (10
−6 M) showed that a “(5 × 5)” structure is transformed into a (2 × 2)
superstructure upon changing the potential from 0.55 to 0.38 V (SHE). Moreover,
the careful measurements revealed that the “(5 × 5)” structure is not a real (5 × 5)
but an incommensurate structure between (4 × 4) and (5 × 5). On the other hand, in
the case where the concentration of Cl
− ions exceeds a critical level (>10
−5 M), the
“(5 × 5)” phase remains stable in the Cu-UPD potential range up to the onset of the
Cu bulk deposition, while the (2 × 2) phase is unstable and dissolves. Hotlos et al.
[57] proposed that the “(5 × 5)” structure consists of a bilayer of Cu and Cl, in which
each Cl
− ion is adsorbed in a hollow site between three Cu adatoms with Cu-Cu and
Cl–Cl interatomic distances of 0.367 nm. The results of SEXAS by Wu et al. [58]
supported the above Cu-Cl bilayer model. The interatomic distance of 0.367 nm in
the Cu-Cl bilayer model is shorter by 4% than that (0.382 nm) in the (111) plane of
CuCl crystal with a zinc blende lattice.
Figure 4.13 shows (a) the cyclic voltammogram and (b) the g versus E curve
measured at a potential scan rate of 5 mV s
−1 for the (111)-textured Au thin-film
(with a thickness of 220 nm) electrode in 0.1 M HClO 4 solution containing 10
−3
M Cu(ClO 4 ) 2 and 10
−3 M KCl [19]. It is distinct from the comparison between
Figs. 4.12a and 4.13a that the Cu-UPD process on the Au (111) electrode is significantly enhanced by an addition of 10
−3 M Cl
− ions in perchloric acid media. Two
pairs of cathodic and anodic peaks (C 1 /A 1 and C 2 /A 2 ) in Fig. 4.13a are characteristic
of the Cu-UPD on the Au (111) electrode in perchloric acid or acidic perchlorate
media containing 10
−4 M or 10
−3 M Cl
− ions, which are consistent with the previous
results [57, 58]. The potential at the C 1 peak in Fig. 4.13a is 0.52 V (SHE) which is
more positive by about 80 mV than that (0.44 V (SHE)) at the corresponding C 1 peak
in Fig. 4.9a, suggesting that the co-adsorption of Cu with Cl
− ions is stronger than
that with SO 4
2− ions as reported by Shi et al. [59]. In response to the enhancement
of the Cu-UPD on the Au (111) electrode due to the addition of Cl
− ions, the surface
stress varies significantly depending on the potential region as shown in Fig. 4.13b.
The value of g increases toward tensile direction with decreasing potential from
0.80 (SHE) to 0.65 V (SHE), which is associated with the desorption of Cl
− ions
4 Changes in Surface Stress Associated with Underpotential …
Fig. 4.12a, is only 50% of q c = −4.9 C m
−2 (calculated from Fig. 4.9a) required
for the formation of the Cu-(1 × 1) monolayer on Au (111) electrode, indicating
that the Cu-UPD on Au (111) electrode is strongly hindered in perchloric acid media
without SO 4
2− ions. In response to the cyclic voltammogram, the change in surface
stress in the potential range from 0.40 to 0.25 V (SHE) in the cathodic potential scan
in Fig. 4.12b is (g) = −0.17 J m
−2 , which is significantly small as compared to
(g) = −0.90 J m
−2 in the same potential range in Fig. 4.9b, indicating that the
formation of Cu-(1 × 1) monolayer is not accomplished. The feature of the cyclic
voltammogram in Fig. 4.12a is consistent with that of the cyclic voltammogram
obtained for the Cu-UPD on Au (111) in 0.1 M HClO 4 solution containing 10
−2 M
Cu(ClO 4 ) 2 [57].
Hotlos et al. [57] pointed out that the broad cathodic and anodic peaks in the cyclic
voltammogram are associated with trace amounts of Cl
− ions (≤10
−6 M) contained
as impurity in the solution. The STM images [57] of the Au (111) electrode surface
in 0.1 M HClO 4 solution containing 10
−2 M Cu(ClO 4 ) 2 with small amounts of Cl
−
ions (10
−6 M) showed that a “(5 × 5)” structure is transformed into a (2 × 2)
superstructure upon changing the potential from 0.55 to 0.38 V (SHE). Moreover,
the careful measurements revealed that the “(5 × 5)” structure is not a real (5 × 5)
but an incommensurate structure between (4 × 4) and (5 × 5). On the other hand, in
the case where the concentration of Cl
− ions exceeds a critical level (>10
−5 M), the
“(5 × 5)” phase remains stable in the Cu-UPD potential range up to the onset of the
Cu bulk deposition, while the (2 × 2) phase is unstable and dissolves. Hotlos et al.
[57] proposed that the “(5 × 5)” structure consists of a bilayer of Cu and Cl, in which
each Cl
− ion is adsorbed in a hollow site between three Cu adatoms with Cu-Cu and
Cl–Cl interatomic distances of 0.367 nm. The results of SEXAS by Wu et al. [58]
supported the above Cu-Cl bilayer model. The interatomic distance of 0.367 nm in
the Cu-Cl bilayer model is shorter by 4% than that (0.382 nm) in the (111) plane of
CuCl crystal with a zinc blende lattice.
Figure 4.13 shows (a) the cyclic voltammogram and (b) the g versus E curve
measured at a potential scan rate of 5 mV s
−1 for the (111)-textured Au thin-film
(with a thickness of 220 nm) electrode in 0.1 M HClO 4 solution containing 10
−3
M Cu(ClO 4 ) 2 and 10
−3 M KCl [19]. It is distinct from the comparison between
Figs. 4.12a and 4.13a that the Cu-UPD process on the Au (111) electrode is significantly enhanced by an addition of 10
−3 M Cl
− ions in perchloric acid media. Two
pairs of cathodic and anodic peaks (C 1 /A 1 and C 2 /A 2 ) in Fig. 4.13a are characteristic
of the Cu-UPD on the Au (111) electrode in perchloric acid or acidic perchlorate
media containing 10
−4 M or 10
−3 M Cl
− ions, which are consistent with the previous
results [57, 58]. The potential at the C 1 peak in Fig. 4.13a is 0.52 V (SHE) which is
more positive by about 80 mV than that (0.44 V (SHE)) at the corresponding C 1 peak
in Fig. 4.9a, suggesting that the co-adsorption of Cu with Cl
− ions is stronger than
that with SO 4
2− ions as reported by Shi et al. [59]. In response to the enhancement
of the Cu-UPD on the Au (111) electrode due to the addition of Cl
− ions, the surface
stress varies significantly depending on the potential region as shown in Fig. 4.13b.
The value of g increases toward tensile direction with decreasing potential from
0.80 (SHE) to 0.65 V (SHE), which is associated with the desorption of Cl
− ions