3.4 Surface Stress Versus Surface Charge Density or Potential …
93
Seo and Serizawa [48] measured the changes in surface stress for a (111)textured Pt thin-film electrode in sulfate solutions with different pH values by using a
cantilever bending method. Figure 3.14 shows (a) the cyclic voltammogram, (b) the
g versus E curve, and (c) the
∂g
∂ E
versus E curve obtained at a potential scan rate
of 20 mV s
−1 for the (111)-textured Pt thin-film electrode in pH 2.4, 0.5 M sulfate
solution [48]. The
∂g
∂ E
versus E curve in Fig. 3.14c is obtained by differentiating
g with respect to E for the g versus E curve in the anodic potential scan of −
0.26 to 1.15 V (SHE) in Fig. 3.14b. The value of g decreases gradually toward
compressive direction with increasing potential in the electric double-layer region
where a capacitive charging current flows mainly and it decreases rapidly in the
oxide region where oxide formation/reduction takes place. On the other hand, g
decreases rapidly with decreasing potential in the hydrogen region where hydrogen
adsorption/desorption takes place on Pt. The
∂g
∂ E
versus E curve in Fig. 3.14c indicates that the sign of
∂g
∂ E
or ζ g,q is plus in the hydrogen region, while it becomes
minus in both the electric double-layer and oxide regions. The sign-reversal of
∂g
∂ E
or
ζ g,q in the hydrogen region has been also observed for the (111)-textured Pt thin-film
electrode in pH 12.3, 0.5 M NaF solution [48]. The sign-reversal of
∂g
∂ E
or ζ g,q in the
hydrogen region is consistent with the results obtained with the piezoelectric technique [44, 45]. Moreover, the sign-reversal of
∂g
∂ E
or ζ g,q from minus to plus in the
oxide region has been observed by the piezoelectric technique [44, 45]. However, the
sign-reversal was not observed in the oxide region by the cantilever bending method
[48].
Viswanath et al. [49, 50] measured the changes in surface stress of a consolidated
cylindrical nano-porous Pt electrode (particle size: 6 ± 1 nm and mass-specific
surface area: α m = 25.3 m
2 g
−1 ) in 0.7 M NaF solution by in situ dilatometry (see
Sect. 2.5 of Chap. 2) and found that the sign-reversal of ζ g,q takes place, depending
on the potential region and the surface condition of the nano-porous Pt electrode. In
principle, the relative length change
l
l o
of the nano-porous Pt electrode measured
by a dilatometer can be converted to the mean value of surface stress change g A
as represented by Eq. (2.40) in Sect. 2.5 of Chap. 2: g A = −
9Y b
2α m ρ
l
l o
, where Y b
is the bulk modulus, α m is the mass-specific surface area, and ρ is the mass density.
Figure 3.15 shows the relationship between
l
l o
or −g and net surface charge Q
obtained during cyclic voltammetry in different potential regions between (a) −0.95
and 1.20 V (SHE), (b) −0.30 and 1.10 V (SHE), and (c) 0.30 and 1.20 V (SHE) at a
potential scan rate of 1 mV s
−1 by in situ dilatometry for the nano-porous Pt electrode
in 0.7 M NaF solution [50]. In Fig. 3.15a, the arrows from HA and HD represent
the progress directions of hydrogen adsorption and desorption, respectively, while
the arrows from OA and OD represent the progress directions of oxygen (or OH)
adsorption and desorption, respectively. The relationship between
l
l o
or −g and
Q in the HA/HD region has an opposite slope as compared to that in the OA/OD
region, i.e., ζ g,q =
g
Q
> 0 in the HA/HD region, while ζ g,q =
g
Q
< 0 in the OA/OD
region, from which the sign-reversal of ζ g,q in the hydrogen region is confirmed.
93
Seo and Serizawa [48] measured the changes in surface stress for a (111)textured Pt thin-film electrode in sulfate solutions with different pH values by using a
cantilever bending method. Figure 3.14 shows (a) the cyclic voltammogram, (b) the
g versus E curve, and (c) the
∂g
∂ E
versus E curve obtained at a potential scan rate
of 20 mV s
−1 for the (111)-textured Pt thin-film electrode in pH 2.4, 0.5 M sulfate
solution [48]. The
∂g
∂ E
versus E curve in Fig. 3.14c is obtained by differentiating
g with respect to E for the g versus E curve in the anodic potential scan of −
0.26 to 1.15 V (SHE) in Fig. 3.14b. The value of g decreases gradually toward
compressive direction with increasing potential in the electric double-layer region
where a capacitive charging current flows mainly and it decreases rapidly in the
oxide region where oxide formation/reduction takes place. On the other hand, g
decreases rapidly with decreasing potential in the hydrogen region where hydrogen
adsorption/desorption takes place on Pt. The
∂g
∂ E
versus E curve in Fig. 3.14c indicates that the sign of
∂g
∂ E
or ζ g,q is plus in the hydrogen region, while it becomes
minus in both the electric double-layer and oxide regions. The sign-reversal of
∂g
∂ E
or
ζ g,q in the hydrogen region has been also observed for the (111)-textured Pt thin-film
electrode in pH 12.3, 0.5 M NaF solution [48]. The sign-reversal of
∂g
∂ E
or ζ g,q in the
hydrogen region is consistent with the results obtained with the piezoelectric technique [44, 45]. Moreover, the sign-reversal of
∂g
∂ E
or ζ g,q from minus to plus in the
oxide region has been observed by the piezoelectric technique [44, 45]. However, the
sign-reversal was not observed in the oxide region by the cantilever bending method
[48].
Viswanath et al. [49, 50] measured the changes in surface stress of a consolidated
cylindrical nano-porous Pt electrode (particle size: 6 ± 1 nm and mass-specific
surface area: α m = 25.3 m
2 g
−1 ) in 0.7 M NaF solution by in situ dilatometry (see
Sect. 2.5 of Chap. 2) and found that the sign-reversal of ζ g,q takes place, depending
on the potential region and the surface condition of the nano-porous Pt electrode. In
principle, the relative length change
l
l o
of the nano-porous Pt electrode measured
by a dilatometer can be converted to the mean value of surface stress change g A
as represented by Eq. (2.40) in Sect. 2.5 of Chap. 2: g A = −
9Y b
2α m ρ
l
l o
, where Y b
is the bulk modulus, α m is the mass-specific surface area, and ρ is the mass density.
Figure 3.15 shows the relationship between
l
l o
or −g and net surface charge Q
obtained during cyclic voltammetry in different potential regions between (a) −0.95
and 1.20 V (SHE), (b) −0.30 and 1.10 V (SHE), and (c) 0.30 and 1.20 V (SHE) at a
potential scan rate of 1 mV s
−1 by in situ dilatometry for the nano-porous Pt electrode
in 0.7 M NaF solution [50]. In Fig. 3.15a, the arrows from HA and HD represent
the progress directions of hydrogen adsorption and desorption, respectively, while
the arrows from OA and OD represent the progress directions of oxygen (or OH)
adsorption and desorption, respectively. The relationship between
l
l o
or −g and
Q in the HA/HD region has an opposite slope as compared to that in the OA/OD
region, i.e., ζ g,q =
g
Q
> 0 in the HA/HD region, while ζ g,q =
g
Q
< 0 in the OA/OD
region, from which the sign-reversal of ζ g,q in the hydrogen region is confirmed.