4.3 Changes in Surface Stress during UPD
131
where E Pd(111) is Young’s modulus for the Pd (111) layer, ν Pd(111) is Poisson’s ratio,
ε mf = 0.049 is the misfit strain, d Pd(111) = 0.225 nm is the thickness of the Pd-UPD
monolayer, and n is the number of the monolayers on the surface. The values of
E Pd(111) = 136 GPa and ν Pd(111) = 0.528 are calculated from the elastic compliances
of Pd [28]. The value of g = 3.17 J m
−2 is estimated as the contribution of the
lattice mismatch by inserting these values into Eq. (4.24) for the Pd-UPD monolayer,
which is larger by about one order of magnitude than that (g = 0.4 J m
−2 ) obtained
experimentally in the Pd-UPD region in Fig. 4.15.
The formation of metallic bonds between Pd and Au atoms contributes to the
surface stress change toward compressive direction (g < 0) which counterbalances the surface stress change toward tensile direction due to the positive lattice
mismatch. Equation (4.22) derived by Leiva et al. [51] can be applied to the (1 × 1)
Pd/Au (111) system since the Pd-UPD layer on the Au (111) electrode has the same
pseudomorphic structure as the Cu-UPD layer on the Au (111) electrode. The value
of (g) pd(1×1)/Au(111) = − 0.1 J m
−2 calculated by Leiva et al. [51] is close to the
experimental value of g = 0.4 J m
−2 as compared to that (g = 3.17 J m
−2 ) due to
the lattice mismatch calculated from Eq. (4.24). In the case of the Cu-UPD on the Au
(111) electrode, the desorption of anion species and followed by the co-adsorption
of anion species with Cu atoms influences the magnitude and direction of surface
stress change. Particularly, the co-adsorption of SO 4
2− ion on the Cu adlayer or the
co-adsorption of Cl
− ion followed by the formation of the CuCl bilayer on the Au
(111) electrode induces the compressive surface stress, which provides the surface
stress maximum during the Cu-UPD process.
On the other hand, in the case of the Pd-UPD on the Au (111) electrode, no
surface stress maximum is observed during the Pd-UPD process. Since E pzc of Pd
electrode is more negative than that of Au electrode [5], the adsorbed [PdCl 4 ]
2− is
not desorbed in the Pd-UPD potential region and remains on the Pd-UPD monolayer.
The adsorbed [PdCl 4 ]
2− on the Pd-UPD monolayer may contribute to the decrease
in surface stress toward compressive direction. However, its contribution would not
be enough to induce a surface stress maximum. The surface stress change of g =
0.62 J m
−2 for the Pd-OPD monolayer (corresponding to q c = −9.0 C m
−2 ) is
obtained from the slope of −0.14 V in the Pd-OPD region of Fig. 4.15. The value
of g for the Pd-OPD monolayer is larger by about 50% than that (0.4 J m
−2 ) for
the Pd-UPD monolayer, suggesting that the contribution of the decrease in surface
stress toward compressive direction due to the bonding between Pd and Au atoms
diminishes with growth of the Pd-OPD layer [20].
The changes in surface stress during Pd deposition on the Au (111) electrode in
0.1 M H 2 SO 4 solution containing 10
−3 M PdSO 4 were also measured to examine
the contribution of [PdCl 4 ]
2− to the changes in surface stress [20]. The surface stress
response during Pd deposition in the solution containing PdSO 4 was similar to that
in the solution containing [PdCl 4 ]
2− . The adsorbed SO 4
2− as well as [PdCl 4 ]
2− is not
desorbed in the Pd-UPD potential region and remains on the Pd-UPD monolayer.
Nevertheless, the anodic stripping of the deposited Pd is kinetically hindered in
chloride-free solution [20]. The in situ gravimetry by EQCM combined with cyclic
131
where E Pd(111) is Young’s modulus for the Pd (111) layer, ν Pd(111) is Poisson’s ratio,
ε mf = 0.049 is the misfit strain, d Pd(111) = 0.225 nm is the thickness of the Pd-UPD
monolayer, and n is the number of the monolayers on the surface. The values of
E Pd(111) = 136 GPa and ν Pd(111) = 0.528 are calculated from the elastic compliances
of Pd [28]. The value of g = 3.17 J m
−2 is estimated as the contribution of the
lattice mismatch by inserting these values into Eq. (4.24) for the Pd-UPD monolayer,
which is larger by about one order of magnitude than that (g = 0.4 J m
−2 ) obtained
experimentally in the Pd-UPD region in Fig. 4.15.
The formation of metallic bonds between Pd and Au atoms contributes to the
surface stress change toward compressive direction (g < 0) which counterbalances the surface stress change toward tensile direction due to the positive lattice
mismatch. Equation (4.22) derived by Leiva et al. [51] can be applied to the (1 × 1)
Pd/Au (111) system since the Pd-UPD layer on the Au (111) electrode has the same
pseudomorphic structure as the Cu-UPD layer on the Au (111) electrode. The value
of (g) pd(1×1)/Au(111) = − 0.1 J m
−2 calculated by Leiva et al. [51] is close to the
experimental value of g = 0.4 J m
−2 as compared to that (g = 3.17 J m
−2 ) due to
the lattice mismatch calculated from Eq. (4.24). In the case of the Cu-UPD on the Au
(111) electrode, the desorption of anion species and followed by the co-adsorption
of anion species with Cu atoms influences the magnitude and direction of surface
stress change. Particularly, the co-adsorption of SO 4
2− ion on the Cu adlayer or the
co-adsorption of Cl
− ion followed by the formation of the CuCl bilayer on the Au
(111) electrode induces the compressive surface stress, which provides the surface
stress maximum during the Cu-UPD process.
On the other hand, in the case of the Pd-UPD on the Au (111) electrode, no
surface stress maximum is observed during the Pd-UPD process. Since E pzc of Pd
electrode is more negative than that of Au electrode [5], the adsorbed [PdCl 4 ]
2− is
not desorbed in the Pd-UPD potential region and remains on the Pd-UPD monolayer.
The adsorbed [PdCl 4 ]
2− on the Pd-UPD monolayer may contribute to the decrease
in surface stress toward compressive direction. However, its contribution would not
be enough to induce a surface stress maximum. The surface stress change of g =
0.62 J m
−2 for the Pd-OPD monolayer (corresponding to q c = −9.0 C m
−2 ) is
obtained from the slope of −0.14 V in the Pd-OPD region of Fig. 4.15. The value
of g for the Pd-OPD monolayer is larger by about 50% than that (0.4 J m
−2 ) for
the Pd-UPD monolayer, suggesting that the contribution of the decrease in surface
stress toward compressive direction due to the bonding between Pd and Au atoms
diminishes with growth of the Pd-OPD layer [20].
The changes in surface stress during Pd deposition on the Au (111) electrode in
0.1 M H 2 SO 4 solution containing 10
−3 M PdSO 4 were also measured to examine
the contribution of [PdCl 4 ]
2− to the changes in surface stress [20]. The surface stress
response during Pd deposition in the solution containing PdSO 4 was similar to that
in the solution containing [PdCl 4 ]
2− . The adsorbed SO 4
2− as well as [PdCl 4 ]
2− is not
desorbed in the Pd-UPD potential region and remains on the Pd-UPD monolayer.
Nevertheless, the anodic stripping of the deposited Pd is kinetically hindered in
chloride-free solution [20]. The in situ gravimetry by EQCM combined with cyclic