8.3 Porous Structures Electrodeposited from Dilute Solutions
281
centres makes it possible to obtain parallel columns perpendicular to the substrate
surface. The production rate of the hydroxide ions is usually much larger than the
growth rate allowed by the Zn
2+ ion transport; therefore, the deposition efficiency is
mostly below 50%.
Nevertheless, the morphology of the deposit changes with deposition time if no
seed layer is applied. At the beginning, the near-substrate part of the coating appears
to be somewhat irregular, and a morphologically stable columnar structure can be
observed after a long enough deposition time only (see, e.g., [154, 155]).
The zinc salt applied is usually either Zn(NO 3 ) 2 [155, 159] or ZnCl 2 [156, 160–
162] with nitrate ion or oxygen reduction as the hydroxide ion source, respectively.
The concentration of the zinc salt varies between 0.1 and 5 mM. The minimum
total Zn concentration is determined by the solubility of ZnO; therefore, the further
decrease in the Zn
2+ concentration does not lead to deposit growth. In contrast, with
Zn
2+ concentration higher than about 20 mM, the nanocolumns start to coalesce and
the resulting deposits are composed of platelets instead of columns, although the
deposit remains somewhat porous [155].
The supporting electrolyte can be chosen in accord with the zinc compound
applied. In the case of the nitrate-based solution, the addition of another nitrate salt
(e.g., NaNO 3 [163]) can increase the concentration of the hydroxide ion precursor,
hence opening a way to tune the Zn
2+ and OH
– concentrations independently of each
other. For chloride-based media, KCl as supporting electrolyte is not a reactant but
a growth-regulating agent. It improves the deposit quality in the concentration range
of about 0.1 M; however, if c(KCl) > 0.5 M, the flat-capped ZnO nanorods adopt a
pencil-like peaky shape [164].
Apart from ZnO, various other materials have been synthesized as porous
columnar structures with the template-free approach by applying a relatively small
solution concentration (usually 1–12 mM). Lanthanum(III) hydroxide plated from
nitrate solution with NH 4 Cl as additive showed a fairly regular columnar structure with unimodal column diameter distribution [154], while the application of an
additive-free La(NO 3 ) 3 solution led to a less regular column structure under essentially the same deposition conditions [165]. Cadmium oxide nanocolumn assembly
can be plated from oxygen-saturated dimethyl sulfoxide solution of 0.01 M CdCl 2
at 150 °C [158].
Finally, a few examples will be mentioned where the oxidation state of some of
the structure-forming ion also changes during the nanocolumn deposition. In these
cases, the reasons for the formation of nanocolumns as morphological units remain
the same, but the process becomes more complicated. The first example is the nitratebased electrochemical production coupled with the oxidation of the manganese (II)
ions, leading to the deposition of Mn 3 O 4 (hausmannite) nanocolumns [166]. In this
process, dissolved oxygen must be present for the oxidation of a part of the Mn
2+
ions to Mn
3+ ; otherwise the Mn 3 O 4 composition could not be achieved. Hence, the
nitrate ions and oxygen have a synergetic effect in the overall reaction. The second
example is the formation of CuSCN nanocolumns from CuSO 4 solution [157]. Here,
the reduction of the Cu
2+ ions takes place without any side reaction that forms the
desired compound with the SCN
– ions present. The third example is the formation of
281
centres makes it possible to obtain parallel columns perpendicular to the substrate
surface. The production rate of the hydroxide ions is usually much larger than the
growth rate allowed by the Zn
2+ ion transport; therefore, the deposition efficiency is
mostly below 50%.
Nevertheless, the morphology of the deposit changes with deposition time if no
seed layer is applied. At the beginning, the near-substrate part of the coating appears
to be somewhat irregular, and a morphologically stable columnar structure can be
observed after a long enough deposition time only (see, e.g., [154, 155]).
The zinc salt applied is usually either Zn(NO 3 ) 2 [155, 159] or ZnCl 2 [156, 160–
162] with nitrate ion or oxygen reduction as the hydroxide ion source, respectively.
The concentration of the zinc salt varies between 0.1 and 5 mM. The minimum
total Zn concentration is determined by the solubility of ZnO; therefore, the further
decrease in the Zn
2+ concentration does not lead to deposit growth. In contrast, with
Zn
2+ concentration higher than about 20 mM, the nanocolumns start to coalesce and
the resulting deposits are composed of platelets instead of columns, although the
deposit remains somewhat porous [155].
The supporting electrolyte can be chosen in accord with the zinc compound
applied. In the case of the nitrate-based solution, the addition of another nitrate salt
(e.g., NaNO 3 [163]) can increase the concentration of the hydroxide ion precursor,
hence opening a way to tune the Zn
2+ and OH
– concentrations independently of each
other. For chloride-based media, KCl as supporting electrolyte is not a reactant but
a growth-regulating agent. It improves the deposit quality in the concentration range
of about 0.1 M; however, if c(KCl) > 0.5 M, the flat-capped ZnO nanorods adopt a
pencil-like peaky shape [164].
Apart from ZnO, various other materials have been synthesized as porous
columnar structures with the template-free approach by applying a relatively small
solution concentration (usually 1–12 mM). Lanthanum(III) hydroxide plated from
nitrate solution with NH 4 Cl as additive showed a fairly regular columnar structure with unimodal column diameter distribution [154], while the application of an
additive-free La(NO 3 ) 3 solution led to a less regular column structure under essentially the same deposition conditions [165]. Cadmium oxide nanocolumn assembly
can be plated from oxygen-saturated dimethyl sulfoxide solution of 0.01 M CdCl 2
at 150 °C [158].
Finally, a few examples will be mentioned where the oxidation state of some of
the structure-forming ion also changes during the nanocolumn deposition. In these
cases, the reasons for the formation of nanocolumns as morphological units remain
the same, but the process becomes more complicated. The first example is the nitratebased electrochemical production coupled with the oxidation of the manganese (II)
ions, leading to the deposition of Mn 3 O 4 (hausmannite) nanocolumns [166]. In this
process, dissolved oxygen must be present for the oxidation of a part of the Mn
2+
ions to Mn
3+ ; otherwise the Mn 3 O 4 composition could not be achieved. Hence, the
nitrate ions and oxygen have a synergetic effect in the overall reaction. The second
example is the formation of CuSCN nanocolumns from CuSO 4 solution [157]. Here,
the reduction of the Cu
2+ ions takes place without any side reaction that forms the
desired compound with the SCN
– ions present. The third example is the formation of