88
3 Potential- or Adsorbate-Induced Changes in Surface Stress …
0.15 V (SSE). As shown in Fig. 3.12,
ζ g,q
varies sensitively with potential and takes
maxima at about 0.15 V (SSE) in the anodic potential scan and at about 0.1 V (SSE) in
the cathodic potential scan. The maximum value of
ζ g,q
= 2.4 V is somewhat higher
than that of
ζ g,q
= 2.0 V in the cathodic potential scan. The cyclic voltammogram
of the (111)-textured Au thin-film electrode in 0.1 M HClO 4 solution [39] exhibited
the abrupt increase in anodic current due to oxidation at potentials more positive
than 0.65 V (SSE) and the cathodic current peak due to reduction of the oxide film
at 0.45 V (SSE) in the cathodic potential scan from 0.90 V (SSE). The hysteresis
of
ζ g,q
in anodic and cathodic potential scans may be associated with the oxide
formation/reduction on Au electrode.
The value of ψ g,q keeps constant, taking exactly −180
o in the potential region
between −0.1 and 0.5 V (SSE), while ψ g,q fluctuates by ±5
o around −195
o at
potentials more negative than −0.2 V (SSE) and by ±6
o around −186
o at potentials
more positive than 0.6 V (SSE). The slight deviation of ψ g,q from −180
o at potentials
more negative than −0.2 V (SSE) and at potentials more positive than 0.6 V (SSE)
may result from the reduction of oxygen in solution and from the oxidation of the
Au electrode, respectively. Nevertheless, ψ g,q ≈ −180
o means that the sign of ζ g,q
is minus over the entire range of potentials. Consequently, ζ g,q =
∂g
∂q
takes −2.4 V
at about 0.15 V (SSE) in the anodic potential scan and −2.0 V at about 0.1 V (SSE)
in the cathodic potential scan. The potential dependence of ζ g,q in Fig. 3.12 is quite
similar to the potential dependence of ζ E,ε obtained from the potential variation due
to cyclic strain by Smetanin et al. [42] (compare Fig. 3.12 with Fig. 3.13b).
3.4.3 Potential–Surface Elastic Strain Coefficient ζ E,ε
It has been derived by Gokhshtein [34] that surface stress–surface charge density
coefficient ζ g,q =
∂g
∂q
is equivalent to potential–surface elastic strain coefficient
ζ E,ε =
∂ E
∂ε
(see Eq. (1.124) in Sect. 1.8 of Chap. 1). Smetanin et al. [43] tried to
measure ζ E,ε from the potential variation of a (111)-textured Au thin-film electrode
subjected to cyclic elastic strain under open-circuit condition in 0.01 M HClO 4
solution. The experimental value of ζ E,ε exhibited a frequency dependence due to
Faraday loss current. However, the frequency dependence became negligible beyond
30 Hz. The experimental value of ζ E,ε = −1.83 V at higher frequency is close to ζ g,q
= −2.0 V obtained by a cantilever bending method [35] and is in good agreement
with ζ g,q = −1.86 V predicted by ab initio calculations for Au (111) in vacuum [37].
Furthermore, Smetanin et al. [42] developed a new technique for measuring the
current elastic strain or potential elastic strain response as a function of potential during cyclic voltammetry. This technique is named “dynamic electro-chemomechanical analysis (DECMA).” The instrumentation and experimental setup of
DECMA have been briefly described in Sect. 2.4 of Chap. 2. Nevertheless, detailed
explanation of its principle is needed for better understanding of the results obtained
by DECMA. In DECMA, surface elastic strain ε is changed sinusoidally with time
3 Potential- or Adsorbate-Induced Changes in Surface Stress …
0.15 V (SSE). As shown in Fig. 3.12,
ζ g,q
varies sensitively with potential and takes
maxima at about 0.15 V (SSE) in the anodic potential scan and at about 0.1 V (SSE) in
the cathodic potential scan. The maximum value of
ζ g,q
= 2.4 V is somewhat higher
than that of
ζ g,q
= 2.0 V in the cathodic potential scan. The cyclic voltammogram
of the (111)-textured Au thin-film electrode in 0.1 M HClO 4 solution [39] exhibited
the abrupt increase in anodic current due to oxidation at potentials more positive
than 0.65 V (SSE) and the cathodic current peak due to reduction of the oxide film
at 0.45 V (SSE) in the cathodic potential scan from 0.90 V (SSE). The hysteresis
of
ζ g,q
in anodic and cathodic potential scans may be associated with the oxide
formation/reduction on Au electrode.
The value of ψ g,q keeps constant, taking exactly −180
o in the potential region
between −0.1 and 0.5 V (SSE), while ψ g,q fluctuates by ±5
o around −195
o at
potentials more negative than −0.2 V (SSE) and by ±6
o around −186
o at potentials
more positive than 0.6 V (SSE). The slight deviation of ψ g,q from −180
o at potentials
more negative than −0.2 V (SSE) and at potentials more positive than 0.6 V (SSE)
may result from the reduction of oxygen in solution and from the oxidation of the
Au electrode, respectively. Nevertheless, ψ g,q ≈ −180
o means that the sign of ζ g,q
is minus over the entire range of potentials. Consequently, ζ g,q =
∂g
∂q
takes −2.4 V
at about 0.15 V (SSE) in the anodic potential scan and −2.0 V at about 0.1 V (SSE)
in the cathodic potential scan. The potential dependence of ζ g,q in Fig. 3.12 is quite
similar to the potential dependence of ζ E,ε obtained from the potential variation due
to cyclic strain by Smetanin et al. [42] (compare Fig. 3.12 with Fig. 3.13b).
3.4.3 Potential–Surface Elastic Strain Coefficient ζ E,ε
It has been derived by Gokhshtein [34] that surface stress–surface charge density
coefficient ζ g,q =
∂g
∂q
is equivalent to potential–surface elastic strain coefficient
ζ E,ε =
∂ E
∂ε
(see Eq. (1.124) in Sect. 1.8 of Chap. 1). Smetanin et al. [43] tried to
measure ζ E,ε from the potential variation of a (111)-textured Au thin-film electrode
subjected to cyclic elastic strain under open-circuit condition in 0.01 M HClO 4
solution. The experimental value of ζ E,ε exhibited a frequency dependence due to
Faraday loss current. However, the frequency dependence became negligible beyond
30 Hz. The experimental value of ζ E,ε = −1.83 V at higher frequency is close to ζ g,q
= −2.0 V obtained by a cantilever bending method [35] and is in good agreement
with ζ g,q = −1.86 V predicted by ab initio calculations for Au (111) in vacuum [37].
Furthermore, Smetanin et al. [42] developed a new technique for measuring the
current elastic strain or potential elastic strain response as a function of potential during cyclic voltammetry. This technique is named “dynamic electro-chemomechanical analysis (DECMA).” The instrumentation and experimental setup of
DECMA have been briefly described in Sect. 2.4 of Chap. 2. Nevertheless, detailed
explanation of its principle is needed for better understanding of the results obtained
by DECMA. In DECMA, surface elastic strain ε is changed sinusoidally with time