132
4 Changes in Surface Stress Associated with Underpotential …
voltammetry and surface stress measurement in the potential range covering from
Pd-UPD to Pd-OPD on Au (111) electrode in the solution containing PdSO 4 [20]
confirmed that the anodic stripping of the deposited Pd is incomplete in the absence
of chloride in solution and some part of the Pd layer remains on the surface, indicating
that the adsorbed [PdCl 4 ]
2− species promotes the anodic stripping of the deposited
Pd.
4.4 Surface Alloying
In spite of the immiscibility of foreign metal in bulk metal, the surface alloy formation
limited to surface monolayer has been observed in many adsorbate/substrate systems
[64] such as Pb, Sn, or Sb on Cu (hkl) and Pb or Sn on Ni (hkl) that were prepared
with vapor deposition in vacuum. In addition, the surface alloy formation has been
also observed in several UPD systems such as Pb or Tl on both Au (111) and Ag
(111) [25, 65]. Various ion scattering studies by low-energy alkali ion (500 eV Li
+ )
[66–69], coaxial impact-collision ion (3 keV Ne
+ ) [70], and medium-energy ion
(100 keV H
+ ) [71] revealed that the magnitude of surface rumpling for the surface
alloy phases prepared with vapor deposition in vacuum is smaller than that predicted
from the atomic radii in bulk metals based on a hard sphere model in the case where
the alloying adsorbate atoms are larger than the substrate atoms in the surface alloy
phases such as Ni (111)
√
3 ×
√
3
R30
◦ -Pb and Cu (111)
√
3 ×
√
3
R30
◦ -Sn.
The exceptional case is the surface alloy of W (100) c(2 × 2)-Cu for which the
alloying adsorbate Cu atom is smaller than the substrate W atom.
The nearest-neighbor interatomic distance d B−A in the surface alloy (i.e., the B–
A distance within the surface alloy phase formed by deposition of B on A) can
be determined from the magnitude of the surface rumpling by taking the atomic
configuration of the uppermost surface alloy monolayer and the underneath substrate
layer into account. The calculated d B−A for the surface alloys in which the alloying
adsorbate atoms are larger than the substrate atoms is also shorter than that predicted
from the hard sphere model, indicating that the effective radius of the adsorbate B
atom is reduced by surface alloying if the radius of the substrate A atom is assumed to
be unchanged. For example, d Pb−Ni in the surface alloy of Ni (111)
√
3 ×
√
3
R30
◦ -
Pb is 0.258 ± 0.001 nm [71, 72] which is significantly shorter than that (d Pb−Ni = 0.
300 nm) estimated from the sum of atomic radii (0.1246 nm for Ni and 0.1750 nm
for Pb) in the respective bulk metals based on a simple hard sphere model, and the
effective radius of Pb in the surface alloy obtained under the assumption that the
atomic radius of Ni is unchanged, is 0.133 nm, which is smaller by about 25% than
the atomic radius of Pb in bulk Pb.
The single-bond covalent radii of Ni and Pb atoms are 0.115 nm and 0.154 nm,
respectively, as defined by Pauling [73]. The value of d Pb−Ni = 0.258 nm in the
surface ally is close to d Pb−Ni = 0.269 nm estimated from the covalent radii of Pb
and Ni atoms, suggesting that the Pb-Ni bond in the surface alloy has a covalent
4 Changes in Surface Stress Associated with Underpotential …
voltammetry and surface stress measurement in the potential range covering from
Pd-UPD to Pd-OPD on Au (111) electrode in the solution containing PdSO 4 [20]
confirmed that the anodic stripping of the deposited Pd is incomplete in the absence
of chloride in solution and some part of the Pd layer remains on the surface, indicating
that the adsorbed [PdCl 4 ]
2− species promotes the anodic stripping of the deposited
Pd.
4.4 Surface Alloying
In spite of the immiscibility of foreign metal in bulk metal, the surface alloy formation
limited to surface monolayer has been observed in many adsorbate/substrate systems
[64] such as Pb, Sn, or Sb on Cu (hkl) and Pb or Sn on Ni (hkl) that were prepared
with vapor deposition in vacuum. In addition, the surface alloy formation has been
also observed in several UPD systems such as Pb or Tl on both Au (111) and Ag
(111) [25, 65]. Various ion scattering studies by low-energy alkali ion (500 eV Li
+ )
[66–69], coaxial impact-collision ion (3 keV Ne
+ ) [70], and medium-energy ion
(100 keV H
+ ) [71] revealed that the magnitude of surface rumpling for the surface
alloy phases prepared with vapor deposition in vacuum is smaller than that predicted
from the atomic radii in bulk metals based on a hard sphere model in the case where
the alloying adsorbate atoms are larger than the substrate atoms in the surface alloy
phases such as Ni (111)
√
3 ×
√
3
R30
◦ -Pb and Cu (111)
√
3 ×
√
3
R30
◦ -Sn.
The exceptional case is the surface alloy of W (100) c(2 × 2)-Cu for which the
alloying adsorbate Cu atom is smaller than the substrate W atom.
The nearest-neighbor interatomic distance d B−A in the surface alloy (i.e., the B–
A distance within the surface alloy phase formed by deposition of B on A) can
be determined from the magnitude of the surface rumpling by taking the atomic
configuration of the uppermost surface alloy monolayer and the underneath substrate
layer into account. The calculated d B−A for the surface alloys in which the alloying
adsorbate atoms are larger than the substrate atoms is also shorter than that predicted
from the hard sphere model, indicating that the effective radius of the adsorbate B
atom is reduced by surface alloying if the radius of the substrate A atom is assumed to
be unchanged. For example, d Pb−Ni in the surface alloy of Ni (111)
√
3 ×
√
3
R30
◦ -
Pb is 0.258 ± 0.001 nm [71, 72] which is significantly shorter than that (d Pb−Ni = 0.
300 nm) estimated from the sum of atomic radii (0.1246 nm for Ni and 0.1750 nm
for Pb) in the respective bulk metals based on a simple hard sphere model, and the
effective radius of Pb in the surface alloy obtained under the assumption that the
atomic radius of Ni is unchanged, is 0.133 nm, which is smaller by about 25% than
the atomic radius of Pb in bulk Pb.
The single-bond covalent radii of Ni and Pb atoms are 0.115 nm and 0.154 nm,
respectively, as defined by Pauling [73]. The value of d Pb−Ni = 0.258 nm in the
surface ally is close to d Pb−Ni = 0.269 nm estimated from the covalent radii of Pb
and Ni atoms, suggesting that the Pb-Ni bond in the surface alloy has a covalent