4.2 Combination of UPD with Other Surface-Area-Limited Processes
105
metal atoms, n S /n G . If this ratio is less than one (for instance, Pb replacement by a
Pt(IV) compound as solute), the direct exchange mechanism is more likely, whereas
for n S /n G < 1 (e.g., for Pb + 2Ag
+
2 Ag + Pb
2+ ), the growing layer is rendered to
be uneven. The clarification of the stoichiometry of the system sometimes requires
a great care. For instance, Cu displacement by Pt(IV) that is present as a [PtCl 6 ]
2−
complex was shown to lead to the formation of Cu(I) as a chloride complex [137].
Nevertheless, with a different anion that does not stabilize Cu(I) but provides that
Cu(II) is the sole soluble species, copper behaves as a divalent sacrificial metal.
Not only does the morphology of the layer deposited in the SLRR process depends
on the nature of the sacrificial/depositing metal pair but the technical implementation
and the kinetic parameters of a displacement system are also important. These factors
determine together which mechanism depicted in Fig. 4.11 takes place in a particular
reaction. If the displacement reaction is very fast and the fill-up of the flow cell with
the reactant is relatively slow, an overplating takes place at the cell ingress [16,
146]. By tuning the composition of the solution of the growing metal, especially by
applying a suitable complexing agent for slowing down the displacement process,
this discrepancy could be eliminated [142, 146]. This draws the attention to a complex
view of the SLRR process and to the importance of the optimization of the chemistry
of the reactions. It was also shown that the nucleation density and the cluster surface
area are a complicated function of the technical parameters of the SLRR process
[132], in which the initial UPD coverage of the sacrificial metal and the concentration
of the ions of the growing metal play a crucial role.
For monolayer-wise growth, a general question is the commensurability of the
atomic structures of the substrate and the deposit. If they have the same structure and
the lattice misfit is small, heteroepitaxial growth can take place, at least for the first
few monolayers like for Pd on Pt [140].
Although the general view on the SLRR process is that the first monolayer is
already a fully reduced metal layer, this point is questioned by some studies. The
doubt concerning the reduction is typical for Pt whose ions appear to react in a
rather sluggish way during the replacement half-cycle of the SLRR process. The
partial reduction of Pt(IV) in the Cu replacement reaction [127] was explained with
a partial reduction of the Pt(IV) species. This leads to the presence of both O and
Cl in the surface layer, which was evidenced by EXAFS data on the basis of bond
lengths. The results of another polarization-dependent total-reflection fluorescence
X-ray absorption fine structure study of a Pt monolayer on Au(111) prepared with Cu
monolayer-mediated SLRR showed that the structural data are consistent with the
presence of [PtCl 4 ]
2− species at the surface [136]. The planar tetrachloro complexes
appeared to lay on the gold surface, each atoms of the complex being in touch with
the surface gold atoms at a Pt coverage equivalent to 0.45 as referred to surface
gold atoms. Since the electrolyte solutions used in this study did not contain excess
chloride ions but only as a component of the H 2 PtCl 6 precursor material of the Ptlayer, the impact of the insufficient rinsing of the electrode prior to the transfer to the
surface investigation system could be ruled out. The results indicate that the simple
view on the replacement process is to be scrutinized. The finding shown above is
in agreement with another study on Pt SLRR in the sense that the adsorption of Pt
105
metal atoms, n S /n G . If this ratio is less than one (for instance, Pb replacement by a
Pt(IV) compound as solute), the direct exchange mechanism is more likely, whereas
for n S /n G < 1 (e.g., for Pb + 2Ag
+
2 Ag + Pb
2+ ), the growing layer is rendered to
be uneven. The clarification of the stoichiometry of the system sometimes requires
a great care. For instance, Cu displacement by Pt(IV) that is present as a [PtCl 6 ]
2−
complex was shown to lead to the formation of Cu(I) as a chloride complex [137].
Nevertheless, with a different anion that does not stabilize Cu(I) but provides that
Cu(II) is the sole soluble species, copper behaves as a divalent sacrificial metal.
Not only does the morphology of the layer deposited in the SLRR process depends
on the nature of the sacrificial/depositing metal pair but the technical implementation
and the kinetic parameters of a displacement system are also important. These factors
determine together which mechanism depicted in Fig. 4.11 takes place in a particular
reaction. If the displacement reaction is very fast and the fill-up of the flow cell with
the reactant is relatively slow, an overplating takes place at the cell ingress [16,
146]. By tuning the composition of the solution of the growing metal, especially by
applying a suitable complexing agent for slowing down the displacement process,
this discrepancy could be eliminated [142, 146]. This draws the attention to a complex
view of the SLRR process and to the importance of the optimization of the chemistry
of the reactions. It was also shown that the nucleation density and the cluster surface
area are a complicated function of the technical parameters of the SLRR process
[132], in which the initial UPD coverage of the sacrificial metal and the concentration
of the ions of the growing metal play a crucial role.
For monolayer-wise growth, a general question is the commensurability of the
atomic structures of the substrate and the deposit. If they have the same structure and
the lattice misfit is small, heteroepitaxial growth can take place, at least for the first
few monolayers like for Pd on Pt [140].
Although the general view on the SLRR process is that the first monolayer is
already a fully reduced metal layer, this point is questioned by some studies. The
doubt concerning the reduction is typical for Pt whose ions appear to react in a
rather sluggish way during the replacement half-cycle of the SLRR process. The
partial reduction of Pt(IV) in the Cu replacement reaction [127] was explained with
a partial reduction of the Pt(IV) species. This leads to the presence of both O and
Cl in the surface layer, which was evidenced by EXAFS data on the basis of bond
lengths. The results of another polarization-dependent total-reflection fluorescence
X-ray absorption fine structure study of a Pt monolayer on Au(111) prepared with Cu
monolayer-mediated SLRR showed that the structural data are consistent with the
presence of [PtCl 4 ]
2− species at the surface [136]. The planar tetrachloro complexes
appeared to lay on the gold surface, each atoms of the complex being in touch with
the surface gold atoms at a Pt coverage equivalent to 0.45 as referred to surface
gold atoms. Since the electrolyte solutions used in this study did not contain excess
chloride ions but only as a component of the H 2 PtCl 6 precursor material of the Ptlayer, the impact of the insufficient rinsing of the electrode prior to the transfer to the
surface investigation system could be ruled out. The results indicate that the simple
view on the replacement process is to be scrutinized. The finding shown above is
in agreement with another study on Pt SLRR in the sense that the adsorption of Pt