116
4 Changes in Surface Stress Associated with Underpotential …
thickness of 220 nm) electrode in 0.1 M HClO 4 solutions with and without 2.5 ×
10
−3 M Bi 2 O 3 [14]. The solid and dotted curves represent the results in the solutions
with and without Bi
3+ , respectively. The cathodic limit potential of 0.26 V (SHE) in
Fig. 4.7 corresponds to the equilibrium potential of 5.0 × 10
−3 M Bi
3+ /Bi electrode.
The anodic limit potential of 0.80 V (SHE) is chosen to avoid the onset of surface
oxygenation reaction on Au. Three pairs of cathodic and anodic current peaks (C 1 /A 1 ,
C 2 /A 2 , and C 3 /A 3 ) in the presence of Bi
3+ are characteristic of the Bi-UPD on the
Au (111) electrode. These current peaks are associated with the structural changes
of the Bi-UPD layer on the Au (111) electrode.
The structural changes of the Bi-UPD layer on Au (111) in 0.1 M HClO 4 solution
containing 10
−3 M Bi
3+ have been investigated by SXS [31]. The structure of the
Bi-UPD layer changes from disorder to (2 × 2) and then to
p ×
√
3
− 2Bi with
decreasing potential. The stable region of each structure is represented by the dashed
vertical lines in Fig. 4.7a. The above structures of the Bi-UPD layer on the Au (111)
electrode have been also confirmed by atomic force microscope (AFM) [32] and
STM [33, 34]. Furthermore, no surface alloy formation has been reported for the
Bi-UPD on the Au (111) electrode [31–34]. The (2 × 2) structure is commensurate
with the underlying Au (111) surface, while the
p ×
√
3
− 2Bi structure is uniaxially incommensurate with the underlying Au (111) surface. The latter structure is
subjected to electro-compression in the incommensurate direction when θ Bi changes
from 0.61 to 0.67 with decreasing potential [31]. The pseudomorphic growth of the
Bi-UPD layer such as the hcp Pb-UPD layer on the closed-packed Au (111) surface
seems difficult since Bi has a rhombohedral crystal structure.
In Fig. 4.7b, the value of g is referred to zero at the anodic limit potential of
0.8 V (SHE). The surface stress levels of the Au (111) electrode in the solutions with
and without Bi
3+ are the same at 0.80 V since the Bi-UPD does not proceed at 0.80 V
(SHE). As the potential is scanned from 0.80 V (SHE) to the cathodic direction, g
for the Au (111) electrode in the absence of Bi
3+ varies monotonously toward tensile
direction (g > 0) until the cathodic limit potential of 0.26 V (SHE), while g in
the presence of Bi
3+ takes a maximum at about 0.55 V(SHE) and then varies toward
compressive direction (g < 0). The value of g in the presence of Bi
3+ decreases
rapidly with decreasing potential in the narrow potential region between C 2 and
C 3 peaks where the (2 × 2) structure is stable. Subsequently, g decreases linearly
with decreasing potential in the potential region between 0.42 V and 0.26 V (SHE)
where the
p ×
√
3
− 2Bi structure is stable. There is no hysteresis of g between
cathodic and anodic potential scans in the absence of Bi
3+ , while some hysteresis in
the presence of Bi
3+ is observed in the potential region between 0.26 V and 0.60 V
(SHE) where the Bi-UPD proceeds. The difference between g in the presence and
absence of 5 × 10
−3 M Bi
3+ at −0.26 V (SHE) is (g) = −1.35 J m
−2 which is
more negative by −0.28 J m
−2 than that ((g) = −1.07 J m
−2 ) for the Pb-UPD
on the Au (111) electrode, indicating that the
p ×
√
3
− 2Bi monolayer on the
Au (111) electrode is subjected to large compressive stress as compared to the hcp
Pb-UPD monolayer on the Au (111) electrode.
4 Changes in Surface Stress Associated with Underpotential …
thickness of 220 nm) electrode in 0.1 M HClO 4 solutions with and without 2.5 ×
10
−3 M Bi 2 O 3 [14]. The solid and dotted curves represent the results in the solutions
with and without Bi
3+ , respectively. The cathodic limit potential of 0.26 V (SHE) in
Fig. 4.7 corresponds to the equilibrium potential of 5.0 × 10
−3 M Bi
3+ /Bi electrode.
The anodic limit potential of 0.80 V (SHE) is chosen to avoid the onset of surface
oxygenation reaction on Au. Three pairs of cathodic and anodic current peaks (C 1 /A 1 ,
C 2 /A 2 , and C 3 /A 3 ) in the presence of Bi
3+ are characteristic of the Bi-UPD on the
Au (111) electrode. These current peaks are associated with the structural changes
of the Bi-UPD layer on the Au (111) electrode.
The structural changes of the Bi-UPD layer on Au (111) in 0.1 M HClO 4 solution
containing 10
−3 M Bi
3+ have been investigated by SXS [31]. The structure of the
Bi-UPD layer changes from disorder to (2 × 2) and then to
p ×
√
3
− 2Bi with
decreasing potential. The stable region of each structure is represented by the dashed
vertical lines in Fig. 4.7a. The above structures of the Bi-UPD layer on the Au (111)
electrode have been also confirmed by atomic force microscope (AFM) [32] and
STM [33, 34]. Furthermore, no surface alloy formation has been reported for the
Bi-UPD on the Au (111) electrode [31–34]. The (2 × 2) structure is commensurate
with the underlying Au (111) surface, while the
p ×
√
3
− 2Bi structure is uniaxially incommensurate with the underlying Au (111) surface. The latter structure is
subjected to electro-compression in the incommensurate direction when θ Bi changes
from 0.61 to 0.67 with decreasing potential [31]. The pseudomorphic growth of the
Bi-UPD layer such as the hcp Pb-UPD layer on the closed-packed Au (111) surface
seems difficult since Bi has a rhombohedral crystal structure.
In Fig. 4.7b, the value of g is referred to zero at the anodic limit potential of
0.8 V (SHE). The surface stress levels of the Au (111) electrode in the solutions with
and without Bi
3+ are the same at 0.80 V since the Bi-UPD does not proceed at 0.80 V
(SHE). As the potential is scanned from 0.80 V (SHE) to the cathodic direction, g
for the Au (111) electrode in the absence of Bi
3+ varies monotonously toward tensile
direction (g > 0) until the cathodic limit potential of 0.26 V (SHE), while g in
the presence of Bi
3+ takes a maximum at about 0.55 V(SHE) and then varies toward
compressive direction (g < 0). The value of g in the presence of Bi
3+ decreases
rapidly with decreasing potential in the narrow potential region between C 2 and
C 3 peaks where the (2 × 2) structure is stable. Subsequently, g decreases linearly
with decreasing potential in the potential region between 0.42 V and 0.26 V (SHE)
where the
p ×
√
3
− 2Bi structure is stable. There is no hysteresis of g between
cathodic and anodic potential scans in the absence of Bi
3+ , while some hysteresis in
the presence of Bi
3+ is observed in the potential region between 0.26 V and 0.60 V
(SHE) where the Bi-UPD proceeds. The difference between g in the presence and
absence of 5 × 10
−3 M Bi
3+ at −0.26 V (SHE) is (g) = −1.35 J m
−2 which is
more negative by −0.28 J m
−2 than that ((g) = −1.07 J m
−2 ) for the Pb-UPD
on the Au (111) electrode, indicating that the
p ×
√
3
− 2Bi monolayer on the
Au (111) electrode is subjected to large compressive stress as compared to the hcp
Pb-UPD monolayer on the Au (111) electrode.