10.3 Electrodeposition on Surfaces with Step Edges
333
imidazolium bis(trifluoromethylsulfonyl) imide containing TiCl 4 [33]. Since the
ionic liquid is non-reactive, the oxidative treatment of the HOPG substrate was not
necessary, and the entire deposition process could be performed with one single
potential pulse. The STM images of the edge-aligned nanowires were composed of
a series of coalesced grains, similar to those obtained from aqueous solutions; therefore, this growth characteristic of metal nanowires along HOPG step edges seems to
be general. It was confirmed later with the same type of ionic liquid that the oxidation pulse has either no impact on the ESED process or, in the worst case, disfavours
the process reproducibility [34]. In the latter study, the deposition of Zn, Ti and Si
nanowires was demonstrated, all of them having the same morphological properties
as those obtained from aqueous electrolyte solutions.
It is also possible to produce metal oxide nanowires, typically from solutions of
the oxoanions of transition metals. In these cases, the common characteristic of the
reduction process of the oxoanions is that it takes place in several steps. The nanowire
growth can be driven at a potential where an intermediate oxide is formed without
much interference of the solvent reduction process. This usually occurs near to the
onset potential of the first reduction wave of the anion.
In the pioneering study of this field [35], MoO 2 nanowires were deposited on
HOPG at a fixed electrode potential. Besides using MoO 2 nanowires as they are, they
can also be transformed to either metallic Mo [27, 35] or MoS 2 [36] upon annealing
in a hydrogen or hydrogen sulphide atmosphere, respectively. Molybdenum forms
oxoanions with the Mo(VI) oxidation state (whose speciation ranges from MoO 4
2– at
pH > 6.5 to Mo 7 O 24
6– for 6.5 > pH > 2.5 and Mo 8 O 26
4– for pH < 2.5; the protonation
degree of the latter two forms varies with pH). The reduction of the Mo(VI) species
to metal from aqueous solution in the absence of another metal ion is hindered, and
the process stops at intermediate oxidation state, even though the stability range
of metallic Mo is just slightly more negative than the stability regime of water.
Molybdenum can be reduced to metal in an induced codeposition process together
with the deposition of iron group metals only. This is why pure metallic molybdenum
nanowires laying along HOPG step edges could be performed in a two-step process
only. It is apparent that Mo nanowires as well as their MoO 2 precursor deposits thus
produced were quite smooth, in contrast to other metallic nanowires electrodeposited
in one single step (see Fig. 10.5). It is possible that for the production of MoO 2 , the
linear density of the growth centres along the step edge is much larger than for metals,
which makes the individual grains indiscernible after their coalescence. Concerning
the growth site preference, it was shown [27] that nanowire growth can be expected
if the overvoltage of the deposition is small. The more negative deposition potential
was applied, the more MoO x “parasitic particles” at spots other than step edges were
obtained and the smaller was the probability of nanowire-wise growth along the
HOPG step edges.
Another oxide produced with ESED was MnO 2 [37]. Here, KMnO 4 was applied
as precursor material in a solution of pH = 6.5 which tends to prevent from the full
reduction of the central metal anion to Mn
2+ but stabilizes the intermediate-valency
oxide. The fully crystalline MnO 2 nanowires were achieved with a heat treatment
333
imidazolium bis(trifluoromethylsulfonyl) imide containing TiCl 4 [33]. Since the
ionic liquid is non-reactive, the oxidative treatment of the HOPG substrate was not
necessary, and the entire deposition process could be performed with one single
potential pulse. The STM images of the edge-aligned nanowires were composed of
a series of coalesced grains, similar to those obtained from aqueous solutions; therefore, this growth characteristic of metal nanowires along HOPG step edges seems to
be general. It was confirmed later with the same type of ionic liquid that the oxidation pulse has either no impact on the ESED process or, in the worst case, disfavours
the process reproducibility [34]. In the latter study, the deposition of Zn, Ti and Si
nanowires was demonstrated, all of them having the same morphological properties
as those obtained from aqueous electrolyte solutions.
It is also possible to produce metal oxide nanowires, typically from solutions of
the oxoanions of transition metals. In these cases, the common characteristic of the
reduction process of the oxoanions is that it takes place in several steps. The nanowire
growth can be driven at a potential where an intermediate oxide is formed without
much interference of the solvent reduction process. This usually occurs near to the
onset potential of the first reduction wave of the anion.
In the pioneering study of this field [35], MoO 2 nanowires were deposited on
HOPG at a fixed electrode potential. Besides using MoO 2 nanowires as they are, they
can also be transformed to either metallic Mo [27, 35] or MoS 2 [36] upon annealing
in a hydrogen or hydrogen sulphide atmosphere, respectively. Molybdenum forms
oxoanions with the Mo(VI) oxidation state (whose speciation ranges from MoO 4
2– at
pH > 6.5 to Mo 7 O 24
6– for 6.5 > pH > 2.5 and Mo 8 O 26
4– for pH < 2.5; the protonation
degree of the latter two forms varies with pH). The reduction of the Mo(VI) species
to metal from aqueous solution in the absence of another metal ion is hindered, and
the process stops at intermediate oxidation state, even though the stability range
of metallic Mo is just slightly more negative than the stability regime of water.
Molybdenum can be reduced to metal in an induced codeposition process together
with the deposition of iron group metals only. This is why pure metallic molybdenum
nanowires laying along HOPG step edges could be performed in a two-step process
only. It is apparent that Mo nanowires as well as their MoO 2 precursor deposits thus
produced were quite smooth, in contrast to other metallic nanowires electrodeposited
in one single step (see Fig. 10.5). It is possible that for the production of MoO 2 , the
linear density of the growth centres along the step edge is much larger than for metals,
which makes the individual grains indiscernible after their coalescence. Concerning
the growth site preference, it was shown [27] that nanowire growth can be expected
if the overvoltage of the deposition is small. The more negative deposition potential
was applied, the more MoO x “parasitic particles” at spots other than step edges were
obtained and the smaller was the probability of nanowire-wise growth along the
HOPG step edges.
Another oxide produced with ESED was MnO 2 [37]. Here, KMnO 4 was applied
as precursor material in a solution of pH = 6.5 which tends to prevent from the full
reduction of the central metal anion to Mn
2+ but stabilizes the intermediate-valency
oxide. The fully crystalline MnO 2 nanowires were achieved with a heat treatment