330
10 Electrochemical Manufacturing Methods Based on Surface …
during the negative shift in the electrode potential from the double-layer region to
the potential interval of electroreduction of the adlayer. Although the deposit along
the step edge formed a labyrinth-like network (see Fig. 10.4b), it was anticipated that
such polymerization processes can be used for surface nanopatterning.
10.3.2 Deposition on the Step Edges of Graphite
Among the common substrates applied in electrodeposition, carbonaceous materials and particularly HOPG play an important role. The reason for this importance
is multifold. First, the interatomic distance of the carbon atoms in the graphene
plane differs significantly from that of any metal lattice, which results in an incommensurable surface structure of the graphene plane and the metal deposit forming
thereon. This makes the deposition of separated grains favoured over a layer formation. Secondly, carbonaceous substrates withstand to anodic treatments, which allows
the oxidative post-treatment of the deposit. Furthermore, the atomic position of the
surface carbon atoms can be taken as being fixed due to the covalent bond within
the graphene planes, which makes unwanted intermixing of the deposit atoms with
substrate ones much hindered.
It has long been known that the areal density of appropriate nucleation sites on
carbon surface for metal deposition is quite small regardless of the nature of the
carbon substrate. This is why various early studies on electrochemical nucleation
applied carbonaceous substrates. Even before the discovery of atomic-scale imaging
methods, it was speculated that the step edge of the basal plane plays a crucial role
in the deposition process. Later, this was evidenced and taken advantage of in the
process called electrochemical step edge decoration (ESED).
In the simplest case, the electrodeposition process from aqueous solutions leads
to metallic nanowires. The three-pulse deposition process (steps A, B and C as seen
in Fig. 10.1) seems to be typical [21]. Pulse A with a length of about 5 s ensures the
appropriate chemical environment at the step edges. The nucleation pulse initiates the
growth of nanoparticles, most of them being formed along the step edges. The length
of this pulse varies between 5 and 100 ms, depending on the metal to be deposited and
the concentration of the precursor ions. Then, the individual particles coalesce during
the growth pulse lasting up to several hundreds of seconds, which lends a pearl chainlike appearance to the nanowires obtained. Examples of morphological features of
various nanowires are presented in Fig. 10.5. Depending on the deposit composition,
the nanowire formation can take place without either the initial oxidation pulse or
the nucleation pulse [22], although this method can be taken rather as an exception.
Regardless of the number of the components, the concentration of the precursor metal
cations was at most a few tens of mM. The length of the nanowires is independent
of the electrochemical parameters of the process but is determined by the length of
the appropriate step edges of the substrate. In an ideal case, even millimeter-long
nanowires can grow. Such nanowires form a class of nanoscopic objects with an
extremely high aspect ratio.
10 Electrochemical Manufacturing Methods Based on Surface …
during the negative shift in the electrode potential from the double-layer region to
the potential interval of electroreduction of the adlayer. Although the deposit along
the step edge formed a labyrinth-like network (see Fig. 10.4b), it was anticipated that
such polymerization processes can be used for surface nanopatterning.
10.3.2 Deposition on the Step Edges of Graphite
Among the common substrates applied in electrodeposition, carbonaceous materials and particularly HOPG play an important role. The reason for this importance
is multifold. First, the interatomic distance of the carbon atoms in the graphene
plane differs significantly from that of any metal lattice, which results in an incommensurable surface structure of the graphene plane and the metal deposit forming
thereon. This makes the deposition of separated grains favoured over a layer formation. Secondly, carbonaceous substrates withstand to anodic treatments, which allows
the oxidative post-treatment of the deposit. Furthermore, the atomic position of the
surface carbon atoms can be taken as being fixed due to the covalent bond within
the graphene planes, which makes unwanted intermixing of the deposit atoms with
substrate ones much hindered.
It has long been known that the areal density of appropriate nucleation sites on
carbon surface for metal deposition is quite small regardless of the nature of the
carbon substrate. This is why various early studies on electrochemical nucleation
applied carbonaceous substrates. Even before the discovery of atomic-scale imaging
methods, it was speculated that the step edge of the basal plane plays a crucial role
in the deposition process. Later, this was evidenced and taken advantage of in the
process called electrochemical step edge decoration (ESED).
In the simplest case, the electrodeposition process from aqueous solutions leads
to metallic nanowires. The three-pulse deposition process (steps A, B and C as seen
in Fig. 10.1) seems to be typical [21]. Pulse A with a length of about 5 s ensures the
appropriate chemical environment at the step edges. The nucleation pulse initiates the
growth of nanoparticles, most of them being formed along the step edges. The length
of this pulse varies between 5 and 100 ms, depending on the metal to be deposited and
the concentration of the precursor ions. Then, the individual particles coalesce during
the growth pulse lasting up to several hundreds of seconds, which lends a pearl chainlike appearance to the nanowires obtained. Examples of morphological features of
various nanowires are presented in Fig. 10.5. Depending on the deposit composition,
the nanowire formation can take place without either the initial oxidation pulse or
the nucleation pulse [22], although this method can be taken rather as an exception.
Regardless of the number of the components, the concentration of the precursor metal
cations was at most a few tens of mM. The length of the nanowires is independent
of the electrochemical parameters of the process but is determined by the length of
the appropriate step edges of the substrate. In an ideal case, even millimeter-long
nanowires can grow. Such nanowires form a class of nanoscopic objects with an
extremely high aspect ratio.