98
3 Potential- or Adsorbate-Induced Changes in Surface Stress …
oxygen (or OH) would reside on the Pt surface. However, if the potential exceeds
0.4 V (SSE), a place-exchange of chemisorbed oxygen with Pt surface atoms takes
place to form a surface PtO lattice [44]. The positive value of ζ r in the potential
range from 0.6 to 0.3 V (SSE) is inherent in the surface PtO layer formed with the
place-exchange mechanism, which is supported by the in situ dilatometry results of
the oxygenated porous Pt electrode in Fig. 3.15c.
The sign-reversal of ζ g,q has been observed for Pd [51–54] as well as Pt. The
positive value of ζ g,q = 1.7 ±0.3 V in the hydrogen adsorption/desorption region
and the negative value of ζ g,q = −1.07 ± 0.07 V in the capacitive charging region or
ζ g,q = −0.86 ± 0.15 V in the oxygen adsorption/desorption region were measured
by in situ dilatometry for a nano-porous Pd electrode in 0.7 M NaF solution [51, 53].
In addition, ζ E,ε = 1.1 V in the hydrogen adsorption/desorption region, ζ E,ε = −
1.3 V in the electric double layer or oxygen adsorption/desorption region, and ζ E,ε =
0.5 V in the PdO formation/reduction region have been reported for a (111)-textured
Pd thin-film electrode in 0.01 M H 2 SO 4 solution [52, 54]. Particularly, the feature of
the ζ E,ε versus E curve is quite similar to the results obtained by the piezoelectric
detection for the polycrystalline Pd foil electrode in 0.5 M sulfate solutions with pH
values of 1.87, 8.8, and 12.8 [46].
3.4.5 Origin of Sign-Reversal of ζ g,q
Feibelman [55] made first-principles calculations of the effects of hydrogen and
oxygen adsorption on surface stress for a Pt (111) surface to indicate that both
hydrogen and oxygen adsorptions reduce tensile stress on Pt (111) terraces despite
the fact that hydrogen adsorption reduces the work function of the substrate Pt,
while oxygen adsorption increases the work function. The calculated surface stress
for hydrogen adsorption on Pt (111) decreases linearly from 6.27 to 1.70 J m
−2 with
increasing hydrogen coverage θ H from zero to monolayer, i.e., g = −4.57 J m
−2
for the H (1 × 1)/Pt (111) system. Since the cathodic charge density required for
the formation of hydrogen monolayer on Pt (111) surface is q c = −2.4 C m
−2 ,
the positive value of ζ g,q =
g
q c
= 1.9 V is estimated for the Pt (111) surface in the
hydrogen region, which is close to ζ g,q = 1.51 V obtained in the hydrogen region
for the nano-porous Pt electrode in 0.7 M NaF solution [50].
The decrease in work function due to hydrogen adsorption means that adsorbed
hydrogen atoms are electron-donating adsorbates. If the donated electrons enter the
unfilled bonding states of Pt, the bonds in the uppermost surface of Pt would be
reinforced, increasing the tensile stress. In contrast to the above speculation, the
unfilled 5d-bands of Pt are antibonding, and thereby the donation of electrons into
the unfilled antibonding states of Pt should weaken the inter-Pt bonds to reduce the
tensile stress [55]. The plus sign of ζ g,q in the hydrogen region for Pt may result from
the electron donation into the unfilled antibonding states of Pt. Pd as well as Pt takes
the positive value of ζ g,q in the hydrogen region. As suggested by Weissmüller [54],
the extra electrons donated from hydrogen would contribute to filling the antibonding
3 Potential- or Adsorbate-Induced Changes in Surface Stress …
oxygen (or OH) would reside on the Pt surface. However, if the potential exceeds
0.4 V (SSE), a place-exchange of chemisorbed oxygen with Pt surface atoms takes
place to form a surface PtO lattice [44]. The positive value of ζ r in the potential
range from 0.6 to 0.3 V (SSE) is inherent in the surface PtO layer formed with the
place-exchange mechanism, which is supported by the in situ dilatometry results of
the oxygenated porous Pt electrode in Fig. 3.15c.
The sign-reversal of ζ g,q has been observed for Pd [51–54] as well as Pt. The
positive value of ζ g,q = 1.7 ±0.3 V in the hydrogen adsorption/desorption region
and the negative value of ζ g,q = −1.07 ± 0.07 V in the capacitive charging region or
ζ g,q = −0.86 ± 0.15 V in the oxygen adsorption/desorption region were measured
by in situ dilatometry for a nano-porous Pd electrode in 0.7 M NaF solution [51, 53].
In addition, ζ E,ε = 1.1 V in the hydrogen adsorption/desorption region, ζ E,ε = −
1.3 V in the electric double layer or oxygen adsorption/desorption region, and ζ E,ε =
0.5 V in the PdO formation/reduction region have been reported for a (111)-textured
Pd thin-film electrode in 0.01 M H 2 SO 4 solution [52, 54]. Particularly, the feature of
the ζ E,ε versus E curve is quite similar to the results obtained by the piezoelectric
detection for the polycrystalline Pd foil electrode in 0.5 M sulfate solutions with pH
values of 1.87, 8.8, and 12.8 [46].
3.4.5 Origin of Sign-Reversal of ζ g,q
Feibelman [55] made first-principles calculations of the effects of hydrogen and
oxygen adsorption on surface stress for a Pt (111) surface to indicate that both
hydrogen and oxygen adsorptions reduce tensile stress on Pt (111) terraces despite
the fact that hydrogen adsorption reduces the work function of the substrate Pt,
while oxygen adsorption increases the work function. The calculated surface stress
for hydrogen adsorption on Pt (111) decreases linearly from 6.27 to 1.70 J m
−2 with
increasing hydrogen coverage θ H from zero to monolayer, i.e., g = −4.57 J m
−2
for the H (1 × 1)/Pt (111) system. Since the cathodic charge density required for
the formation of hydrogen monolayer on Pt (111) surface is q c = −2.4 C m
−2 ,
the positive value of ζ g,q =
g
q c
= 1.9 V is estimated for the Pt (111) surface in the
hydrogen region, which is close to ζ g,q = 1.51 V obtained in the hydrogen region
for the nano-porous Pt electrode in 0.7 M NaF solution [50].
The decrease in work function due to hydrogen adsorption means that adsorbed
hydrogen atoms are electron-donating adsorbates. If the donated electrons enter the
unfilled bonding states of Pt, the bonds in the uppermost surface of Pt would be
reinforced, increasing the tensile stress. In contrast to the above speculation, the
unfilled 5d-bands of Pt are antibonding, and thereby the donation of electrons into
the unfilled antibonding states of Pt should weaken the inter-Pt bonds to reduce the
tensile stress [55]. The plus sign of ζ g,q in the hydrogen region for Pt may result from
the electron donation into the unfilled antibonding states of Pt. Pd as well as Pt takes
the positive value of ζ g,q in the hydrogen region. As suggested by Weissmüller [54],
the extra electrons donated from hydrogen would contribute to filling the antibonding