78
3 Potential- or Adsorbate-Induced Changes in Surface Stress …
Au. Furthermore, the value of ε = −0.042 is estimated from the uniaxial compression
along the [110] direction in the Au (111)3 × 22
surface. As a result, g = −
0.59 J m
−2 is eventually calculated by substituting the above-estimated values into
Eq. (3.2), which is in good agreement with the experimental data (g = −0.60 J m
−2 ),
supporting that the reconstruction of the Au (111)-(1 × 1) surface is driven by the
surface stress relaxation [8].
In contrast to the reconstruction of the Au (111)-(1 × 1) surface, the surface
stress relaxation involved in the reconstruction of the Au (100)-(1 × 1) surface is
significantly small (g = −0.25 J m
−2 ). The gain in elastic energy due to the small
surface stress relaxation would not be sufficient to compensate for the extra energy
to put the surface atoms in the less stable sites (i.e., on top of the substrate atoms).
Therefore, it is deduced that the surface stress relaxation is not the driving force for the
reconstruction of the Au (100)-(1 × 1) surface. The reconstruction of the Au (100)-(1
× 1) surface would be driven by the increase in number of the nearest-neighbor atom
due to the formation of the Au (100)-(hex) surface [8].
3.3 Adsorption of Electrolyte Anions
3.3.1 Au (111) Electrode in Acid Solutions Containing
ClO 4
− , SO 4
2− , and Cl −
Haiss et al. [22] measured the voltammogram and the changes in surface stress for
a (111)-textured Au thin-film electrode in acid solutions containing ClO 4
− , SO 4
2− ,
and Cl
− ions. A cantilever in STM [21, 26] on which a (111)-textured Au thin film
was evaporated is used for the measurement of the surface stress change. During
the measurement, the (111)-textured Au (111) thin-film surface was kept in the
unreconstructed state by using somewhat higher potential scan rate (0.20 V s
−1 )
because of a slow reconstruction process.
Figure 3.7 shows (a) the voltammogram of the (111)-textured Au thin-film electrode in 1.0 M H 2 SO 4 solution, (b) the concomitant changes in surface stress g,
and (c) the first derivative
∂g
∂ E
of g with respect to potential [22]. The small anodic
and cathodic current peaks at about 0.7 V (SCE) in Fig. 3.7a are associated with the
formation and disappearance of an ordered sulfate overlayer [27]. In Fig. 3.7b, the
value of g is referred to zero at −0.2 V (SCE) and the surface stress shifts toward
compressive direction as the potential becomes more positive, i.e., with increasing
surface coverage of adsorbed sulfate ions. It is remarked that the
∂g
∂ E
versus E curve in
Fig. 3.7c has a close resemblance to the voltammogram in Fig. 3.7a. Particularly, the
potentials (0.4 and 0.3 V (SCE), respectively) at anodic current and cathodic current
peaks in Fig. 3.7a coincide with those at the corresponding
∂g
∂ E
peaks in Fig. 3.7c.
Therefore, the following relationship holds approximately:
3 Potential- or Adsorbate-Induced Changes in Surface Stress …
Au. Furthermore, the value of ε = −0.042 is estimated from the uniaxial compression
along the [110] direction in the Au (111)3 × 22
surface. As a result, g = −
0.59 J m
−2 is eventually calculated by substituting the above-estimated values into
Eq. (3.2), which is in good agreement with the experimental data (g = −0.60 J m
−2 ),
supporting that the reconstruction of the Au (111)-(1 × 1) surface is driven by the
surface stress relaxation [8].
In contrast to the reconstruction of the Au (111)-(1 × 1) surface, the surface
stress relaxation involved in the reconstruction of the Au (100)-(1 × 1) surface is
significantly small (g = −0.25 J m
−2 ). The gain in elastic energy due to the small
surface stress relaxation would not be sufficient to compensate for the extra energy
to put the surface atoms in the less stable sites (i.e., on top of the substrate atoms).
Therefore, it is deduced that the surface stress relaxation is not the driving force for the
reconstruction of the Au (100)-(1 × 1) surface. The reconstruction of the Au (100)-(1
× 1) surface would be driven by the increase in number of the nearest-neighbor atom
due to the formation of the Au (100)-(hex) surface [8].
3.3 Adsorption of Electrolyte Anions
3.3.1 Au (111) Electrode in Acid Solutions Containing
ClO 4
− , SO 4
2− , and Cl −
Haiss et al. [22] measured the voltammogram and the changes in surface stress for
a (111)-textured Au thin-film electrode in acid solutions containing ClO 4
− , SO 4
2− ,
and Cl
− ions. A cantilever in STM [21, 26] on which a (111)-textured Au thin film
was evaporated is used for the measurement of the surface stress change. During
the measurement, the (111)-textured Au (111) thin-film surface was kept in the
unreconstructed state by using somewhat higher potential scan rate (0.20 V s
−1 )
because of a slow reconstruction process.
Figure 3.7 shows (a) the voltammogram of the (111)-textured Au thin-film electrode in 1.0 M H 2 SO 4 solution, (b) the concomitant changes in surface stress g,
and (c) the first derivative
∂g
∂ E
of g with respect to potential [22]. The small anodic
and cathodic current peaks at about 0.7 V (SCE) in Fig. 3.7a are associated with the
formation and disappearance of an ordered sulfate overlayer [27]. In Fig. 3.7b, the
value of g is referred to zero at −0.2 V (SCE) and the surface stress shifts toward
compressive direction as the potential becomes more positive, i.e., with increasing
surface coverage of adsorbed sulfate ions. It is remarked that the
∂g
∂ E
versus E curve in
Fig. 3.7c has a close resemblance to the voltammogram in Fig. 3.7a. Particularly, the
potentials (0.4 and 0.3 V (SCE), respectively) at anodic current and cathodic current
peaks in Fig. 3.7a coincide with those at the corresponding
∂g
∂ E
peaks in Fig. 3.7c.
Therefore, the following relationship holds approximately: