4.4 Surface Alloying
135
experimental results of changes in surface stress for the formation of the surface alloys
with a large difference in atomic radius between adsorbate and substrate atoms such
as Cu (111)
√
3 ×
√
3
R30
◦ -Sn and Ni (111)
√
3 ×
√
3
R30
◦ -Pb, although the
experimental value of g ≈ −5 J m
−2 [50] has been obtained at the deposition of
Ag monolayer (but the formation of surface alloy was not confirmed) on Pt (111) in
which the radius of Ag atom is larger by 4.3% than that of Pt atom.
The corrosion inhibition of ferrous metals such as steels, nickel, and iron in the
presence of Pb
2+ or Sn
2+ in perchloric acid or acidic perchlorate solution has been
ascribed to the UPD of Sn or Pb on the substrate metal [76]. The in situ EXAFS
analysis of the Pb-UPD on surface-roughened (sr-) polycrystalline Ni electrode in
acidic perchlorate solution containing Pb
2+ [77] suggested that the Pb-UPD layer
consists of a mixture of the surface alloy and overlayer, and indicated that the average
interatomic distance d Pb−Ni in the Pb-UPD layer is 0.264 nm, which is significantly
small as compared to that (d Pb−Ni = 0.300 nm) estimated from a hard sphere model.
If the atomic radius (0.1246 nm) of Ni is unchanged, the effective radius of Pb atom
in the Pb-UPD layer is 0.139 nm, which is smaller by 21% than that (0.175 nm) of
Pb atom in bulk Pb.
Similarly, in situ EXAFS analysis of the Sn-UPD on the sr-polycrystalline Ni
electrode in perchloric acid containing Sn
2+ [78] suggested that Ni atoms at facecentered cubic (fcc) sites in the first Ni layer are substituted by Sn atoms like in a
surface alloy, and indicated that the average interatomic distance d Sn−Ni in the SnUPD layer is 0.256 nm, which is significantly small as compared to that (d Sn−Ni =
0.265 nm or 0.276 nm) estimated from the atomic radii (r α−Pb = 0.145 nm or r β−Pb =
0.1551 nm) of metal atoms in the corresponding bulk metals based on a hard sphere
model. The effective radius of Sn atom in the Sn-UPD layer is 0.131 nm, which is
smaller by 6.7% or 13.3% than that (0.1405 nm or 0.1551 nm) of Sn atom in bulk αor β-Sn. The significant reduction in the effective radius of Pb or Sn adsorbate atom
has been observed in the Pb- or Sn-UPD layer on the sr-polycrystalline Ni electrode
in aqueous solution as well as the surface alloys prepared with vapor deposition in
vacuum.
Although the measurement of changes in surface stress during the Pb-UPD or SnUPD on a smooth Ni electrode has not been achieved, a large compressive surface
stress should arise during the UPD process. The compressive surface stress may be
relaxed by the reduction in effective radius of the adsorbate atom in the UPD layer
due to surface alloying, which may contribute to the minimization of total energy for
the equilibrium structure of the UPD layer.
References
1. Kolb DM (1978) Physical and electrochemical properties of metal monolayers on metallic
substrates. In: Gerischer H, Tobias CW (eds) Advances in electrochemistry and electrochemical
engineering. vol 11. Wiley, New York, pp 125–271
135
experimental results of changes in surface stress for the formation of the surface alloys
with a large difference in atomic radius between adsorbate and substrate atoms such
as Cu (111)
√
3 ×
√
3
R30
◦ -Sn and Ni (111)
√
3 ×
√
3
R30
◦ -Pb, although the
experimental value of g ≈ −5 J m
−2 [50] has been obtained at the deposition of
Ag monolayer (but the formation of surface alloy was not confirmed) on Pt (111) in
which the radius of Ag atom is larger by 4.3% than that of Pt atom.
The corrosion inhibition of ferrous metals such as steels, nickel, and iron in the
presence of Pb
2+ or Sn
2+ in perchloric acid or acidic perchlorate solution has been
ascribed to the UPD of Sn or Pb on the substrate metal [76]. The in situ EXAFS
analysis of the Pb-UPD on surface-roughened (sr-) polycrystalline Ni electrode in
acidic perchlorate solution containing Pb
2+ [77] suggested that the Pb-UPD layer
consists of a mixture of the surface alloy and overlayer, and indicated that the average
interatomic distance d Pb−Ni in the Pb-UPD layer is 0.264 nm, which is significantly
small as compared to that (d Pb−Ni = 0.300 nm) estimated from a hard sphere model.
If the atomic radius (0.1246 nm) of Ni is unchanged, the effective radius of Pb atom
in the Pb-UPD layer is 0.139 nm, which is smaller by 21% than that (0.175 nm) of
Pb atom in bulk Pb.
Similarly, in situ EXAFS analysis of the Sn-UPD on the sr-polycrystalline Ni
electrode in perchloric acid containing Sn
2+ [78] suggested that Ni atoms at facecentered cubic (fcc) sites in the first Ni layer are substituted by Sn atoms like in a
surface alloy, and indicated that the average interatomic distance d Sn−Ni in the SnUPD layer is 0.256 nm, which is significantly small as compared to that (d Sn−Ni =
0.265 nm or 0.276 nm) estimated from the atomic radii (r α−Pb = 0.145 nm or r β−Pb =
0.1551 nm) of metal atoms in the corresponding bulk metals based on a hard sphere
model. The effective radius of Sn atom in the Sn-UPD layer is 0.131 nm, which is
smaller by 6.7% or 13.3% than that (0.1405 nm or 0.1551 nm) of Sn atom in bulk αor β-Sn. The significant reduction in the effective radius of Pb or Sn adsorbate atom
has been observed in the Pb- or Sn-UPD layer on the sr-polycrystalline Ni electrode
in aqueous solution as well as the surface alloys prepared with vapor deposition in
vacuum.
Although the measurement of changes in surface stress during the Pb-UPD or SnUPD on a smooth Ni electrode has not been achieved, a large compressive surface
stress should arise during the UPD process. The compressive surface stress may be
relaxed by the reduction in effective radius of the adsorbate atom in the UPD layer
due to surface alloying, which may contribute to the minimization of total energy for
the equilibrium structure of the UPD layer.
References
1. Kolb DM (1978) Physical and electrochemical properties of metal monolayers on metallic
substrates. In: Gerischer H, Tobias CW (eds) Advances in electrochemistry and electrochemical
engineering. vol 11. Wiley, New York, pp 125–271