120
4 Changes in Surface Stress Associated with Underpotential …
presence of Cu
2+ are characteristic of the Cu-UPD on Au (111) electrode in sulfuric
acid media, which are consistent with the results reported so far [18, 37, 38]. The
g vs. E curve in the presence of Cu
2+ , although some hysteresis is observed, has
maxima at 0.44 V (SHE) in the cathodic potential scan and at 0.47 V (SHE) in the
anodic potential scan, which correspond to the C 1 and A 1 current peaks in the cyclic
voltammogram, respectively. Moreover, the g vs. E curve in the presence of Cu
2+
has bending points at 0.28 V (SHE) in the cathodic potential scan and at 0.33 V
(SHE) in the anodic potential scan, which correspond to the C 2 and A 2 current peaks
in the cyclic voltammogram, respectively. In contrast, the g vs. E curve in the
absence of Cu
2+ has no maxima of g and no bending points; that is, g increases
and decreases monotonously with cathodic and anodic potential scans, respectively.
The comparison between the cyclic voltammograms in the presence and absence
of Cu
2+ indicates that the Cu-UPD on Au (111) electrode starts at 0.60 V (SHE) in
the cathodic potential scan. The surface stress still varies toward tensile direction,
i.e., (g) > 0 during the cathodic potential scan from 0.60 to 0.44 V (SHE) where
the Cu-UPD proceeds. In the potential region between 0.60 and 0.44 V (SHE), the
STM images lacked in atomic resolution [37, 39, 40], indicating that the Cu adatoms
are mobile and randomly deposited on the electrode surface. After the C 1 peak in
the cathodic potential scan, the surface stress varies toward compressive direction,
i.e., (g) < 0. The STM images [37, 38, 41] showed that the Cu-UPD layer
formed at 0.40 V (SHE) in the potential region between C 1 and C 2 peaks has a
honeycomb
√
3 ×
√
3
R30
◦ structure with sulfate or bisulfate ions occupying the
centers of the honeycomb (see Fig. 4.10a), which was confirmed by SXS [42] and
by surface-extended X-ray fine structure (SEXAFS) [43] and was also supported
by electrochemical quartz crystal microbalance (EQCM) [44, 45]. The slope of the
g vs. E curve in the presence of Cu
2+ varies sharply toward compressive direction
after the C 2 peak in the cathodic potential scan. The STM and AFM images [24, 46]
Fig. 4.10 Schematic top views of a
√
3 ×
√
3
R30 ◦ and b Cu-(1 × 1) structures on Au (111)
electrode
4 Changes in Surface Stress Associated with Underpotential …
presence of Cu
2+ are characteristic of the Cu-UPD on Au (111) electrode in sulfuric
acid media, which are consistent with the results reported so far [18, 37, 38]. The
g vs. E curve in the presence of Cu
2+ , although some hysteresis is observed, has
maxima at 0.44 V (SHE) in the cathodic potential scan and at 0.47 V (SHE) in the
anodic potential scan, which correspond to the C 1 and A 1 current peaks in the cyclic
voltammogram, respectively. Moreover, the g vs. E curve in the presence of Cu
2+
has bending points at 0.28 V (SHE) in the cathodic potential scan and at 0.33 V
(SHE) in the anodic potential scan, which correspond to the C 2 and A 2 current peaks
in the cyclic voltammogram, respectively. In contrast, the g vs. E curve in the
absence of Cu
2+ has no maxima of g and no bending points; that is, g increases
and decreases monotonously with cathodic and anodic potential scans, respectively.
The comparison between the cyclic voltammograms in the presence and absence
of Cu
2+ indicates that the Cu-UPD on Au (111) electrode starts at 0.60 V (SHE) in
the cathodic potential scan. The surface stress still varies toward tensile direction,
i.e., (g) > 0 during the cathodic potential scan from 0.60 to 0.44 V (SHE) where
the Cu-UPD proceeds. In the potential region between 0.60 and 0.44 V (SHE), the
STM images lacked in atomic resolution [37, 39, 40], indicating that the Cu adatoms
are mobile and randomly deposited on the electrode surface. After the C 1 peak in
the cathodic potential scan, the surface stress varies toward compressive direction,
i.e., (g) < 0. The STM images [37, 38, 41] showed that the Cu-UPD layer
formed at 0.40 V (SHE) in the potential region between C 1 and C 2 peaks has a
honeycomb
√
3 ×
√
3
R30
◦ structure with sulfate or bisulfate ions occupying the
centers of the honeycomb (see Fig. 4.10a), which was confirmed by SXS [42] and
by surface-extended X-ray fine structure (SEXAFS) [43] and was also supported
by electrochemical quartz crystal microbalance (EQCM) [44, 45]. The slope of the
g vs. E curve in the presence of Cu
2+ varies sharply toward compressive direction
after the C 2 peak in the cathodic potential scan. The STM and AFM images [24, 46]
Fig. 4.10 Schematic top views of a
√
3 ×
√
3
R30 ◦ and b Cu-(1 × 1) structures on Au (111)
electrode