40
2 Electrochemistry and Electrodeposition
depends, beside the quality of the atoms themselves, on the surface atomic configuration. It is possible that one crystal face of a substrate can accommodate a
monoatomic layer of a deposit, but another crystal plane of the same substrate
cannot.
• UPD can be studied with substrates that are stable in contact with the solution and
themselves do not dissolve (typically noble metals). The deposit which makes
the UPD layer is less noble than the substrate. The UPD layer should form
in the stability range of the solvent, otherwise voltammetric observations are
not possible because of the background current of the solvent decomposition.
Although it is theoretically possible that the substrate–deposit interaction itself
provides favourable condition of the UPD with non-noble substrates, such cases
have not been identified because of the technical difficulties.
• A UPD layer is always ordered. The special periodicity of the UPD layer along
the substrate surface is at most a few times of the nearest neighbour distance of
the substrate surface atoms.
• The maximum theoretical coverage of a UPD layer is 1 (as referred to the number
of the surface atoms of the substrate), but the theoretical maximum cannot be
achieved in each single system. However, partial UPD layers may also form. In
some systems, UPD layers with more and more densely-packed adatom configurations can form at more and more negative electrode potentials prior to the
formation of a bulk deposit.
• The modification of the nature of the atoms at the solid surface by the UPD process
is often accompanied with a change of the coverage with adsorbed anions. The
change in the surface coverage with adsorbed anions leads to a change of the
surface charge, too (see Sect. 2.5 on the capacitive effects). Hence, the charge
passing through an electrode during the formation of an UPD layer is often not
indicative of the total surface coverage. Instead, the charge corresponding to the
reduction of the ions of the resulting UPD layer must be corrected with the charge
corresponding to the change in surface coverage with co-adsorbed (or desorbed)
anions, too. The electrode potential where a UPD layer can form may also depend
on the nature of the anions present in the solution since the co-adsorbed anion
layer has an energy contribution to the stabilization of the UPD layer.
• As the coulometric data referring to the atomic density of a UPD layer may misestimate the real coverage due to the charge corresponding to anion co-adsorption,
local imaging methods (like scanning probe techniques) also show the topmost
species only, should they be the UPD atoms of the co-adsorbed anions. Therefore,
the identification of the coverage and the complete structure of a UPD layer is
often a very involved task.
A detailed description of the theoretical background of the UPD phenomenon
and a collection of the substrate plane–deposit pairs exhibiting UPD can be found
in Ref. [6]. A former issue of this book series was fully devoted to the discussion
of UPD [16]. The interested readers are advised to study these works. Below, a
few important practical aspects of the study of UPD are summarized only that can
facilitate the understanding of the UPD-based nanostructure formation.
2 Electrochemistry and Electrodeposition
depends, beside the quality of the atoms themselves, on the surface atomic configuration. It is possible that one crystal face of a substrate can accommodate a
monoatomic layer of a deposit, but another crystal plane of the same substrate
cannot.
• UPD can be studied with substrates that are stable in contact with the solution and
themselves do not dissolve (typically noble metals). The deposit which makes
the UPD layer is less noble than the substrate. The UPD layer should form
in the stability range of the solvent, otherwise voltammetric observations are
not possible because of the background current of the solvent decomposition.
Although it is theoretically possible that the substrate–deposit interaction itself
provides favourable condition of the UPD with non-noble substrates, such cases
have not been identified because of the technical difficulties.
• A UPD layer is always ordered. The special periodicity of the UPD layer along
the substrate surface is at most a few times of the nearest neighbour distance of
the substrate surface atoms.
• The maximum theoretical coverage of a UPD layer is 1 (as referred to the number
of the surface atoms of the substrate), but the theoretical maximum cannot be
achieved in each single system. However, partial UPD layers may also form. In
some systems, UPD layers with more and more densely-packed adatom configurations can form at more and more negative electrode potentials prior to the
formation of a bulk deposit.
• The modification of the nature of the atoms at the solid surface by the UPD process
is often accompanied with a change of the coverage with adsorbed anions. The
change in the surface coverage with adsorbed anions leads to a change of the
surface charge, too (see Sect. 2.5 on the capacitive effects). Hence, the charge
passing through an electrode during the formation of an UPD layer is often not
indicative of the total surface coverage. Instead, the charge corresponding to the
reduction of the ions of the resulting UPD layer must be corrected with the charge
corresponding to the change in surface coverage with co-adsorbed (or desorbed)
anions, too. The electrode potential where a UPD layer can form may also depend
on the nature of the anions present in the solution since the co-adsorbed anion
layer has an energy contribution to the stabilization of the UPD layer.
• As the coulometric data referring to the atomic density of a UPD layer may misestimate the real coverage due to the charge corresponding to anion co-adsorption,
local imaging methods (like scanning probe techniques) also show the topmost
species only, should they be the UPD atoms of the co-adsorbed anions. Therefore,
the identification of the coverage and the complete structure of a UPD layer is
often a very involved task.
A detailed description of the theoretical background of the UPD phenomenon
and a collection of the substrate plane–deposit pairs exhibiting UPD can be found
in Ref. [6]. A former issue of this book series was fully devoted to the discussion
of UPD [16]. The interested readers are advised to study these works. Below, a
few important practical aspects of the study of UPD are summarized only that can
facilitate the understanding of the UPD-based nanostructure formation.