96
3 Potential- or Adsorbate-Induced Changes in Surface Stress …
0.45–1.3 V (SHE), more positive than the reduction peak of surface oxide. The net
anodic charge transferred after 12 cycles is about 6 C which corresponds to about
1.5 monolayers of PtO. Afterward, the cyclic voltammetry of the oxygenated nanoporous Pt electrode was performed in the potential range of 0.3–1.20 V (SHE) to
measure simultaneously the changes in surface stress by in situ dilatometry as shown
in Fig. 3.15c. The capacitive charging (CC) or discharging (CD) current flows mainly
for the oxygenated Pt surface in the above potential range. The average value of ζ g,q
= 1.60 V is obtained from the slope of the relationship between
l
l o
or −g and Q
in the CC and CD directions in Fig. 3.15c.
In order to prepare the clean (oxide-free) nano-porous Pt electrode as the next step,
the cyclic voltammetry was performed in the potential range of −0.35 to 0.1 V (SHE)
until the cathodic current of oxide reduction became negligible. Afterward, the cyclic
voltammetry of the clean nano-porous Pt electrode was performed in the potential
range of 0.0–0.70 V (SHE) to measure simultaneously the changes in surface stress.
The capacitive charging (CC) or discharging (CD) current flowed mainly for the
clean Pt surface in the above potential range. Although not shown in Fig. 3.15, the
average value of ζ g,q = −1.06 V for the clean Pt surface [50] was obtained from the
slope of the relationship between
l
l o
or −g and Q in the CC and CD directions.
It is remarked that the sign of ζ g,q for the oxygenated Pt surface is opposite to the
sign of ζ g,q for the clean Pt surface, which supports the results of the polycrystalline
Pt foil electrodes obtained by the piezoelectric detection [44, 45].
The sign-reversal of ζ g,q has been also observed for a (111)-textured Pt thin-film
electrode in 0.1 M HClO 4 solution by using the DSA/EIS method [40]. Figure 3.16
shows (a) the real component ζ r and (b) the phase angle ψ g,q of ζ g,q as a function of
potential for the (111)-textured Pt thin-film electrode, responding to the input signal
amplitude of 50 mV at a frequency of 1 Hz. In the dotted curve of Fig. 3.16, the
applied dc potential E dc is sequentially stepped by 25 mV, starting from −0.1 to
0.4 V (SSE), then from 0.4 to −0.6 V (SSE), and finally back to −0.1 V (SSE), while
in the solid curve, E dc is sequentially stepped by 30 mV, starting from 0.0 to 0.6 V
(SSE), then from 0.6 to −0.6 V (SSE), and finally back to 0.0 V (SSE). The value of
the imaginary component ζ i remained around zero over the entire potential region
between −0.6 and 0.6 V (SSE) although not shown in Fig. 3.16 [40].
The feature of ζ r versus E curve in Fig. 3.16a is quite similar to that of the
∂g
∂ E
versus E curve in Fig. 3.14c. The potential regions of hydrogen, electric double layer,
and oxide described in Fig. 3.16b were determined from the cyclic voltammogram
of the (111)-textured Pt thin-film electrode measured in 0.1 M HClO 4 solution at
a potential scan rate of 50 mV s
−1 [40]. The positive value of ζ r in the hydrogen
region for the (111)-textured Pt in 0.1 M HClO 4 solution is consistent with the results
by in situ dilatometry for the nano-porous Pt electrode in 0.7 M NaF solution [50].
Furthermore, the positive value of ζ r decreases with increasing potential and the sign
of ζ r changes from plus to minus, accompanying the gradual change in ψ g,q from 0
o
to 180
o in the electric double-layer region. The negative value of ζ r in the electric
double-layer region is also consistent with the results by in situ dilatometry for the
nano-porous Pt electrode [50].
3 Potential- or Adsorbate-Induced Changes in Surface Stress …
0.45–1.3 V (SHE), more positive than the reduction peak of surface oxide. The net
anodic charge transferred after 12 cycles is about 6 C which corresponds to about
1.5 monolayers of PtO. Afterward, the cyclic voltammetry of the oxygenated nanoporous Pt electrode was performed in the potential range of 0.3–1.20 V (SHE) to
measure simultaneously the changes in surface stress by in situ dilatometry as shown
in Fig. 3.15c. The capacitive charging (CC) or discharging (CD) current flows mainly
for the oxygenated Pt surface in the above potential range. The average value of ζ g,q
= 1.60 V is obtained from the slope of the relationship between
l
l o
or −g and Q
in the CC and CD directions in Fig. 3.15c.
In order to prepare the clean (oxide-free) nano-porous Pt electrode as the next step,
the cyclic voltammetry was performed in the potential range of −0.35 to 0.1 V (SHE)
until the cathodic current of oxide reduction became negligible. Afterward, the cyclic
voltammetry of the clean nano-porous Pt electrode was performed in the potential
range of 0.0–0.70 V (SHE) to measure simultaneously the changes in surface stress.
The capacitive charging (CC) or discharging (CD) current flowed mainly for the
clean Pt surface in the above potential range. Although not shown in Fig. 3.15, the
average value of ζ g,q = −1.06 V for the clean Pt surface [50] was obtained from the
slope of the relationship between
l
l o
or −g and Q in the CC and CD directions.
It is remarked that the sign of ζ g,q for the oxygenated Pt surface is opposite to the
sign of ζ g,q for the clean Pt surface, which supports the results of the polycrystalline
Pt foil electrodes obtained by the piezoelectric detection [44, 45].
The sign-reversal of ζ g,q has been also observed for a (111)-textured Pt thin-film
electrode in 0.1 M HClO 4 solution by using the DSA/EIS method [40]. Figure 3.16
shows (a) the real component ζ r and (b) the phase angle ψ g,q of ζ g,q as a function of
potential for the (111)-textured Pt thin-film electrode, responding to the input signal
amplitude of 50 mV at a frequency of 1 Hz. In the dotted curve of Fig. 3.16, the
applied dc potential E dc is sequentially stepped by 25 mV, starting from −0.1 to
0.4 V (SSE), then from 0.4 to −0.6 V (SSE), and finally back to −0.1 V (SSE), while
in the solid curve, E dc is sequentially stepped by 30 mV, starting from 0.0 to 0.6 V
(SSE), then from 0.6 to −0.6 V (SSE), and finally back to 0.0 V (SSE). The value of
the imaginary component ζ i remained around zero over the entire potential region
between −0.6 and 0.6 V (SSE) although not shown in Fig. 3.16 [40].
The feature of ζ r versus E curve in Fig. 3.16a is quite similar to that of the
∂g
∂ E
versus E curve in Fig. 3.14c. The potential regions of hydrogen, electric double layer,
and oxide described in Fig. 3.16b were determined from the cyclic voltammogram
of the (111)-textured Pt thin-film electrode measured in 0.1 M HClO 4 solution at
a potential scan rate of 50 mV s
−1 [40]. The positive value of ζ r in the hydrogen
region for the (111)-textured Pt in 0.1 M HClO 4 solution is consistent with the results
by in situ dilatometry for the nano-porous Pt electrode in 0.7 M NaF solution [50].
Furthermore, the positive value of ζ r decreases with increasing potential and the sign
of ζ r changes from plus to minus, accompanying the gradual change in ψ g,q from 0
o
to 180
o in the electric double-layer region. The negative value of ζ r in the electric
double-layer region is also consistent with the results by in situ dilatometry for the
nano-porous Pt electrode [50].