128
4 Changes in Surface Stress Associated with Underpotential …
ordered CuCl bilayer would lead to the decrease in g toward compressive direction
which is caused by a lateral expansion due to the strong bond between Cu and Cl
atoms. The stagnant value of g in the cathodic potential scan from 0.52 to 0.40 V
(SHE) may be brought by balancing the formation of the disordered adlayer with
the transformation to the ordered CuCl bilayer. The continuous decrease in g in
the cathodic potential scan from 0.40 to 0.25 V (SHE) results from the growth of the
CuCl bilayer.
4.3.4 Pd-UPD on Au (111)
Figure 4.14 shows (a) the cyclic voltammograms and (b) the g vs. E curves
measured by changing the cathodic limit potential and fixing the anodic limit potential (0.53 V (SSE)) at a potential scan rate of 5 mV s
−1 for the (111)-textured Au
thin-film (with a thickness of 250 nm) electrode in 0.1 M H 2 SO 4 solution containing
10
−3 M H 2 PdCl 4 [20]. In Fig. 4.14a, the increase in cathodic current density from
about 0.28 V (SSE) in the cathodic potential scan corresponds to the progress in PdUPD on Au (111) electrode. In the case where the cathodic limit potential is more
positive than 0.08 V (SSE), the anodic current peak appears at about 0.23 V (SSE)
in the anodic potential scan, which results from the anodic stripping of the Pd-UPD
layer. When the cathodic limit potential is more negative than 0.08 V, the additional
anodic current peak appears in the potential range of 0.12 – 0.16 V (SSE), which
results from the anodic stripping of the bulk-deposited Pd layer.
A rapid increase in cathodic current density from 0.08 V (SSE) in the cathodic
potential scan corresponds to the progress in Pd-bulk deposition (overpotential deposition of Pd: Pd-OPD). The boundary potential between Pd-UPD and Pd-OPD is not
clearly distinct, but it is located at about 0.08 V (SSE). The potential region between
0.08 and 0.28 V (SSE) where the Pd-UPD proceeds on the Au (111) electrode is
consistent with that reported by Baldauf and Kolb [60]. An adsorbed [PdCl 4 ]
2− layer
would be present on the surface in the Pd-UPD potential region since E pzc for the
Au (111) electrode in sulfuric acid is about −0.2 V (SSE) [61]. The deposition of Pd
is presumed to occur via reduction of the adsorbed tetrachloro palladate ([PdCl 4 ]
2−
+ 2e
−
→ Pd + 4Cl
− ). The small coupled anodic/cathodic peaks at about 0.49 V
(SSE) in Fig. 4.14 are associated with the replacement of adsorbed [PdCl 4 ]
2− by Cl
−
[61, 62].
In Fig. 4.14b, the anodic limit potential is fixed at 0.38 V (SSE), while the cathodic
limit potential is varied in turn toward negative direction. The surface stress increases
toward tensile direction, i.e., g > 0, in the cathodic potential scan from 0.38 V
(SSE) at which g is referred to zero. The significant increase in g is observed at
potentials more negative than 0.08 V (SSE), suggesting that the tensile stress in the
bulk-deposited Pb layer is larger than that in the Pd-UPD monolayer. The surface
stress transients in the anodic potential scan from the cathodic limit potential exhibit
a large hysteresis with negative shift of the cathodic limit potential, which is caused
by a slow growth process of the Pd-bulk deposition. Nonetheless, the g vs. E curves
4 Changes in Surface Stress Associated with Underpotential …
ordered CuCl bilayer would lead to the decrease in g toward compressive direction
which is caused by a lateral expansion due to the strong bond between Cu and Cl
atoms. The stagnant value of g in the cathodic potential scan from 0.52 to 0.40 V
(SHE) may be brought by balancing the formation of the disordered adlayer with
the transformation to the ordered CuCl bilayer. The continuous decrease in g in
the cathodic potential scan from 0.40 to 0.25 V (SHE) results from the growth of the
CuCl bilayer.
4.3.4 Pd-UPD on Au (111)
Figure 4.14 shows (a) the cyclic voltammograms and (b) the g vs. E curves
measured by changing the cathodic limit potential and fixing the anodic limit potential (0.53 V (SSE)) at a potential scan rate of 5 mV s
−1 for the (111)-textured Au
thin-film (with a thickness of 250 nm) electrode in 0.1 M H 2 SO 4 solution containing
10
−3 M H 2 PdCl 4 [20]. In Fig. 4.14a, the increase in cathodic current density from
about 0.28 V (SSE) in the cathodic potential scan corresponds to the progress in PdUPD on Au (111) electrode. In the case where the cathodic limit potential is more
positive than 0.08 V (SSE), the anodic current peak appears at about 0.23 V (SSE)
in the anodic potential scan, which results from the anodic stripping of the Pd-UPD
layer. When the cathodic limit potential is more negative than 0.08 V, the additional
anodic current peak appears in the potential range of 0.12 – 0.16 V (SSE), which
results from the anodic stripping of the bulk-deposited Pd layer.
A rapid increase in cathodic current density from 0.08 V (SSE) in the cathodic
potential scan corresponds to the progress in Pd-bulk deposition (overpotential deposition of Pd: Pd-OPD). The boundary potential between Pd-UPD and Pd-OPD is not
clearly distinct, but it is located at about 0.08 V (SSE). The potential region between
0.08 and 0.28 V (SSE) where the Pd-UPD proceeds on the Au (111) electrode is
consistent with that reported by Baldauf and Kolb [60]. An adsorbed [PdCl 4 ]
2− layer
would be present on the surface in the Pd-UPD potential region since E pzc for the
Au (111) electrode in sulfuric acid is about −0.2 V (SSE) [61]. The deposition of Pd
is presumed to occur via reduction of the adsorbed tetrachloro palladate ([PdCl 4 ]
2−
+ 2e
−
→ Pd + 4Cl
− ). The small coupled anodic/cathodic peaks at about 0.49 V
(SSE) in Fig. 4.14 are associated with the replacement of adsorbed [PdCl 4 ]
2− by Cl
−
[61, 62].
In Fig. 4.14b, the anodic limit potential is fixed at 0.38 V (SSE), while the cathodic
limit potential is varied in turn toward negative direction. The surface stress increases
toward tensile direction, i.e., g > 0, in the cathodic potential scan from 0.38 V
(SSE) at which g is referred to zero. The significant increase in g is observed at
potentials more negative than 0.08 V (SSE), suggesting that the tensile stress in the
bulk-deposited Pb layer is larger than that in the Pd-UPD monolayer. The surface
stress transients in the anodic potential scan from the cathodic limit potential exhibit
a large hysteresis with negative shift of the cathodic limit potential, which is caused
by a slow growth process of the Pd-bulk deposition. Nonetheless, the g vs. E curves