332
10 Electrochemical Manufacturing Methods Based on Surface …
zero until the nucleation of the deposit takes place. Then, a typical voltammetric peak
arises due to the combined impact of the growth and solution depletion with respect to
the reactant. At high negative potential, the decomposition of the solvent takes place,
which involves hydrogen evolution in aqueous solutions. In the anodic-going scan,
the current depends on the quality of the deposit. For a hydrogen-absorbing metal
like Pd, the hydrogen oxidation takes place and the current becomes anodic prior
to reaching the onset potential of metal dissolution. This anodic peak is obviously
missing for metals exhibiting no hydrogen absorption. Should the deposit absorb
hydrogen or not, the negative current is maintained to potentials more positive than the
onset of the nucleation-related current rise in the cathodic-going scan. This hysteresis
is taken advantage of for choosing the growth potential without the influence of
further nucleation events. In the potential range of nucleation-free growth, the current
is much less than the diffusion-limited current.
The range of the metallic element from which ESED-synthesized nanowires were
obtained directly from aqueous solutions includes Pd [21, 28, 30], Ag [21], Au [21],
Cu [21] and Cd [23]. For Ag nanowire deposition, acetonitrile was also used as solvent
[24]. If the nanowire is a homogeneous alloy, the composition is hardly predictable but
an empirical relationship between the experimental conditions (such as the concentrations of the metal ions and the deposition potential) and the nanowire composition
has to be established. All metal pairs used so far for ESED-based nanowire deposition belong to systems exhibiting regular codeposition (see Chap. 2), which has
the general consequence that the increase in the deposition overvoltage results in an
enrichment of the less noble component. This was well seen for Pd–Ag [22] and
Pd–Ni [25, 31] nanowires. Interestingly, it was found that the potential during the
nucleation pulse also has a great impact on the nanowire composition [31], and the
trend concerning the nucleation potential and the composition is the same as for
the deposition pulse. This raises the question whether the initial nuclei and the shell
connection between them grown later are of the same composition or different. Since
the nucleation density along the step edge is typically in the range of 4–15 μm
−1 ,
it is obvious that a simple EDS composition analysis is unable to resolve internal
inhomogeneities in a particular nanowire. This problem might be addressed with a
TEM investigation, but no such study was published so far.
Binary nanowires can be obtained also by tailoring two nanowire sections with
different composition; however, this requires the combination of ESED with lithographic techniques [32]. In the first step, about the half of the HOPG substrate has
to be covered by lithography with a mask whose edge is perpendicular to the step
edges on the substrate. Then, the step edges on the uncovered half of the substrate
are decorated with nanowires. After removing the insulation applied in the first step
and protecting the already deposited nanowires with another masking layer, the step
edge decoration can be performed at the second half of the substrate. Hence, the two
segments of the wires can be deposited sequentially by applying a protective layer
for insulating the zone that is not used in a particular step.
Electrodeposition of non-noble transition metals as step edge-decorating
nanowires is possible from nonaqueous environments only, in particular, from ionic
liquids. Titanium nanowires were successfully produced from 1-butyl-3-methyl
10 Electrochemical Manufacturing Methods Based on Surface …
zero until the nucleation of the deposit takes place. Then, a typical voltammetric peak
arises due to the combined impact of the growth and solution depletion with respect to
the reactant. At high negative potential, the decomposition of the solvent takes place,
which involves hydrogen evolution in aqueous solutions. In the anodic-going scan,
the current depends on the quality of the deposit. For a hydrogen-absorbing metal
like Pd, the hydrogen oxidation takes place and the current becomes anodic prior
to reaching the onset potential of metal dissolution. This anodic peak is obviously
missing for metals exhibiting no hydrogen absorption. Should the deposit absorb
hydrogen or not, the negative current is maintained to potentials more positive than the
onset of the nucleation-related current rise in the cathodic-going scan. This hysteresis
is taken advantage of for choosing the growth potential without the influence of
further nucleation events. In the potential range of nucleation-free growth, the current
is much less than the diffusion-limited current.
The range of the metallic element from which ESED-synthesized nanowires were
obtained directly from aqueous solutions includes Pd [21, 28, 30], Ag [21], Au [21],
Cu [21] and Cd [23]. For Ag nanowire deposition, acetonitrile was also used as solvent
[24]. If the nanowire is a homogeneous alloy, the composition is hardly predictable but
an empirical relationship between the experimental conditions (such as the concentrations of the metal ions and the deposition potential) and the nanowire composition
has to be established. All metal pairs used so far for ESED-based nanowire deposition belong to systems exhibiting regular codeposition (see Chap. 2), which has
the general consequence that the increase in the deposition overvoltage results in an
enrichment of the less noble component. This was well seen for Pd–Ag [22] and
Pd–Ni [25, 31] nanowires. Interestingly, it was found that the potential during the
nucleation pulse also has a great impact on the nanowire composition [31], and the
trend concerning the nucleation potential and the composition is the same as for
the deposition pulse. This raises the question whether the initial nuclei and the shell
connection between them grown later are of the same composition or different. Since
the nucleation density along the step edge is typically in the range of 4–15 μm
−1 ,
it is obvious that a simple EDS composition analysis is unable to resolve internal
inhomogeneities in a particular nanowire. This problem might be addressed with a
TEM investigation, but no such study was published so far.
Binary nanowires can be obtained also by tailoring two nanowire sections with
different composition; however, this requires the combination of ESED with lithographic techniques [32]. In the first step, about the half of the HOPG substrate has
to be covered by lithography with a mask whose edge is perpendicular to the step
edges on the substrate. Then, the step edges on the uncovered half of the substrate
are decorated with nanowires. After removing the insulation applied in the first step
and protecting the already deposited nanowires with another masking layer, the step
edge decoration can be performed at the second half of the substrate. Hence, the two
segments of the wires can be deposited sequentially by applying a protective layer
for insulating the zone that is not used in a particular step.
Electrodeposition of non-noble transition metals as step edge-decorating
nanowires is possible from nonaqueous environments only, in particular, from ionic
liquids. Titanium nanowires were successfully produced from 1-butyl-3-methyl