326
10 Electrochemical Manufacturing Methods Based on Surface …
Finally, pulse D in the above scheme is highly optional. It is applied when a
post-treatment is indispensable, like for the electrochemical oxidation of a metal to
a metal oxide.
It has to be noted that the role of the surface inhomogeneity is not always evidenced
directly. The indirect evidence for the role of the importance of the site-specific
nucleation is the immiscibility of the substrate material with the deposit, their lattice
mismatch and the resulting particle formation. Nevertheless, nanoparticles can be
produced with many substrate–deposit pairs with small enough deposition times.
10.2 Electrodeposition of Nanocrystals
10.2.1 General Aspects of the Electrochemical Nucleation
of Nanoparticles
Whenever the Volmer–Weber-type nucleation and growth process takes place,
nanocrystals can be deposited onto the electrode surface, provided that the deposition process is short enough and the particles formed do not coalesce. Although
the role of surface defects in the nucleation process often remains hidden, there are
a large number of studies for substrate–deposit pairs that reveal which cases are
to be considered as inhomogeneity-influenced deposition processes. In most of the
cases, the Volmer–Weber-type nucleation process involves incommensurable atomic
distances of the substrate and the deposit.
The direct evidence for the influence of the surface inhomogeneity can be revealed
by the study of the atomic-scale surface structure of the substrate surface (e.g.,
with STM). This is by far not always affordable. An indirect evidence for the role
of some specific surface position in the nucleation process is that during repeated
experiments with the same substrate, the deposit nuclei form on identical positions.
This method can be applied for deposits only that can be removed from the surface
without modifying the substrate, either by anodic dissolution or with a chemical
etching method. However, in this case one cannot exclude that the nucleation itself
causes some irreversible change in the substrate. Beside this uncertainty, the repeated
nucleation method also requires the imaging of the surface at the nanoscale.
If the goal is the deposition of monodispersed particles on a surface, it is indispensible to make the nucleation and growth process quasi-instantaneous. This is
carried out by the application of a nucleation pulse (pulse B in Fig. 10.1). As it
was demonstrated for a large number of metals deposited as random nanoparticle
assemblies on HOPG surface, a nucleation overvoltage of −500 mV for 5 ms and
a growth overvoltage of −5 to −300 mV (depending on the metal to be deposited)
can lead to a nearly monodisperse particle diameter distribution [4]. Note that no
anodic activation was applied here for the HOPG substrate and the deposit formed
particles on the HOPG basal plane (unlike in anodically activated processes where
the preferred nucleation spots were the step edges; see Sect. 10.3).
10 Electrochemical Manufacturing Methods Based on Surface …
Finally, pulse D in the above scheme is highly optional. It is applied when a
post-treatment is indispensable, like for the electrochemical oxidation of a metal to
a metal oxide.
It has to be noted that the role of the surface inhomogeneity is not always evidenced
directly. The indirect evidence for the role of the importance of the site-specific
nucleation is the immiscibility of the substrate material with the deposit, their lattice
mismatch and the resulting particle formation. Nevertheless, nanoparticles can be
produced with many substrate–deposit pairs with small enough deposition times.
10.2 Electrodeposition of Nanocrystals
10.2.1 General Aspects of the Electrochemical Nucleation
of Nanoparticles
Whenever the Volmer–Weber-type nucleation and growth process takes place,
nanocrystals can be deposited onto the electrode surface, provided that the deposition process is short enough and the particles formed do not coalesce. Although
the role of surface defects in the nucleation process often remains hidden, there are
a large number of studies for substrate–deposit pairs that reveal which cases are
to be considered as inhomogeneity-influenced deposition processes. In most of the
cases, the Volmer–Weber-type nucleation process involves incommensurable atomic
distances of the substrate and the deposit.
The direct evidence for the influence of the surface inhomogeneity can be revealed
by the study of the atomic-scale surface structure of the substrate surface (e.g.,
with STM). This is by far not always affordable. An indirect evidence for the role
of some specific surface position in the nucleation process is that during repeated
experiments with the same substrate, the deposit nuclei form on identical positions.
This method can be applied for deposits only that can be removed from the surface
without modifying the substrate, either by anodic dissolution or with a chemical
etching method. However, in this case one cannot exclude that the nucleation itself
causes some irreversible change in the substrate. Beside this uncertainty, the repeated
nucleation method also requires the imaging of the surface at the nanoscale.
If the goal is the deposition of monodispersed particles on a surface, it is indispensible to make the nucleation and growth process quasi-instantaneous. This is
carried out by the application of a nucleation pulse (pulse B in Fig. 10.1). As it
was demonstrated for a large number of metals deposited as random nanoparticle
assemblies on HOPG surface, a nucleation overvoltage of −500 mV for 5 ms and
a growth overvoltage of −5 to −300 mV (depending on the metal to be deposited)
can lead to a nearly monodisperse particle diameter distribution [4]. Note that no
anodic activation was applied here for the HOPG substrate and the deposit formed
particles on the HOPG basal plane (unlike in anodically activated processes where
the preferred nucleation spots were the step edges; see Sect. 10.3).