104
4 Changes in Surface Stress Associated with Underpotential …
to reveal the formation processes and structural changes of various UPD layers on
noble metal electrodes. The correlation between the changes in surface stress and
the structural changes of UPD layers is one of the most interesting topics from the
electro-chemo-mechanical viewpoint.
This chapter deals mainly with the changes in surface stress induced by the formation and structural changes of various UPD layers on a (111)-textured Au thin-film
electrode. In some UPD systems, co-adsorption of electrolyte anions with UPD atoms
stabilizes the UPD layers. We discuss the main UPD factors influencing the changes
in surface stress including co-adsorption of electrolyte anions with UPD atoms.
Furthermore, surface alloying/dealloying processes are often involved in structural
changes of UPD layers. The changes in surface stress due to surface alloying are
closely linked to the stability of the UPD layers. We also discuss the relationship
between the changes in surface stress due to surface alloying and the stability of the
UPD layers from the viewpoint of energetics.
4.2 Underpotential Deposition (UPD)
4.2.1 UPD and Work Function
The term of underpotential deposition (UPD) is applied to the reaction for metallic
cations M
z+ in solution being electrodeposited up to the monolayer level on foreign
metal electrode M
at potentials more positive than the equilibrium potential of M
z+ /M
electrode [1, 3]. Kolb et al. [1, 3] found empirically that the UPD potential window
E UPD is proportional to the difference in work function between electrodeposited
metal M and substrate metal M
:
E UPD ≈ 0.5(Φ M
− Φ M ),
(4.1)
where Φ M and Φ M
are the work functions of electrodeposited metal M and of
substrate metal M
, respectively. Equation (4.1) holds approximately on many UPD
systems in which noble metals such as Pt, Au, and Ag are used as substrate metals.
The UPD phenomena are originated from the excess binding energy of metal atoms
M on substrate metal M
with respect to metal atoms M on bulk metal M.
As shown schematically in Fig. 4.1, Kolb et al. [1, 3] determined experimentally
E UPD as the difference between two characteristic potentials of E
a
M,b and E
a
M,s in
potentiodynamic polarization curve of the substrate metal M
, measured in solution
containing metallic cations M
z+ by anodic potential scan from a potential more
negative than the equilibrium potential of M
z+ /M electrode:
E UPD = E
a
M,s − E
a
M,b ,
(4.2)
4 Changes in Surface Stress Associated with Underpotential …
to reveal the formation processes and structural changes of various UPD layers on
noble metal electrodes. The correlation between the changes in surface stress and
the structural changes of UPD layers is one of the most interesting topics from the
electro-chemo-mechanical viewpoint.
This chapter deals mainly with the changes in surface stress induced by the formation and structural changes of various UPD layers on a (111)-textured Au thin-film
electrode. In some UPD systems, co-adsorption of electrolyte anions with UPD atoms
stabilizes the UPD layers. We discuss the main UPD factors influencing the changes
in surface stress including co-adsorption of electrolyte anions with UPD atoms.
Furthermore, surface alloying/dealloying processes are often involved in structural
changes of UPD layers. The changes in surface stress due to surface alloying are
closely linked to the stability of the UPD layers. We also discuss the relationship
between the changes in surface stress due to surface alloying and the stability of the
UPD layers from the viewpoint of energetics.
4.2 Underpotential Deposition (UPD)
4.2.1 UPD and Work Function
The term of underpotential deposition (UPD) is applied to the reaction for metallic
cations M
z+ in solution being electrodeposited up to the monolayer level on foreign
metal electrode M
at potentials more positive than the equilibrium potential of M
z+ /M
electrode [1, 3]. Kolb et al. [1, 3] found empirically that the UPD potential window
E UPD is proportional to the difference in work function between electrodeposited
metal M and substrate metal M
:
E UPD ≈ 0.5(Φ M
− Φ M ),
(4.1)
where Φ M and Φ M
are the work functions of electrodeposited metal M and of
substrate metal M
, respectively. Equation (4.1) holds approximately on many UPD
systems in which noble metals such as Pt, Au, and Ag are used as substrate metals.
The UPD phenomena are originated from the excess binding energy of metal atoms
M on substrate metal M
with respect to metal atoms M on bulk metal M.
As shown schematically in Fig. 4.1, Kolb et al. [1, 3] determined experimentally
E UPD as the difference between two characteristic potentials of E
a
M,b and E
a
M,s in
potentiodynamic polarization curve of the substrate metal M
, measured in solution
containing metallic cations M
z+ by anodic potential scan from a potential more
negative than the equilibrium potential of M
z+ /M electrode:
E UPD = E
a
M,s − E
a
M,b ,
(4.2)