8.3 Porous Structures Electrodeposited from Dilute Solutions
279
[147], stacks of interconnected Pd nanoplatelets could be synthesized with a potential
scan. The result is very similar for Ag if the concentration of the silver salt is increased
from the 10
–5 M range to 10
–3 M [148], as shown in Fig. 8.8b. Here, the detailed
data of the original work reveal that a nanorod-to-platelet transformation happens as
a prolonged deposition of several hours is carried out.
In contrast to the relatively noble metals, the deposition of non-noble metals is
always accompanied by hydrogen evolution that necessitates a different approach
to template-free nanowire deposition. As it was demonstrated for cobalt [149],
successful nanorod deposition can be carried out in electrolyte solutions with
orders of magnitude larger metal ion concentrations than for similar noble metal
deposits (0.05 M CoSO 4 with 0.4 M H 3 BO 3 ). The relatively large current density of
−150 mA cm
–2 , although much less than that used for the DHBT method, indicates
an intense hydrogen evolution. It was probably due to the combined impact of the
electric field, mass transport and gas evolution that the nanorod diameter distribution was less homogeneous than for noble metals. The Co nanorod system could be
successfully transformed to Co 3 O 4 without any appreciable morphological change
by annealing in air.
8.3.2 Non-metallic Nanocolumnar Deposits
The most important nanostructured material obtained with electrodeposition from
dilute solution is zinc oxide. It is an inexpensive n-type semiconductor with a direct
band gap of 3.37 V, and its application in photoelectrochemical devices is emerging
fast. The literature of this field is very rich, and a large variety of deposition conditions
have been tested, whose summary can be found in a recent review article [150].
The chemical background of the formation of ZnO nanorods is similar to the
coatings discussed in Sect. 8.2.4 in the sense that the charge transfer reaction does
not modify the oxidation number of the metal ion. Instead, a reactant is produced in an
electrochemical process which acts as a co-reactant in the precipitation of the product.
The solubility of ZnO is the lowest between pH values of about 9–12, and the optimum
pH range of the ZnO deposition changes slightly with temperature [151, 152]. When
the synthesis of zinc oxide is carried out in aqueous media, the temperature has to be
above 40 °C so that zinc oxide is produced instead of zinc hydroxide. The increase
in deposition temperature strongly improves both the crystallinity and the texture of
the deposits [152] and leads to a slight increase in the nanocolumn diameter. The
reaction is as follows:
Zn
2+
+ 2OH
−
ZnO + H 2 O.
(8.3)
Concerning the source of the hydroxide ions, various reactants are possible. The
simplest way is to saturate the solution with oxygen by bubbling air through the
solution and reducing the dissolved oxygen:
279
[147], stacks of interconnected Pd nanoplatelets could be synthesized with a potential
scan. The result is very similar for Ag if the concentration of the silver salt is increased
from the 10
–5 M range to 10
–3 M [148], as shown in Fig. 8.8b. Here, the detailed
data of the original work reveal that a nanorod-to-platelet transformation happens as
a prolonged deposition of several hours is carried out.
In contrast to the relatively noble metals, the deposition of non-noble metals is
always accompanied by hydrogen evolution that necessitates a different approach
to template-free nanowire deposition. As it was demonstrated for cobalt [149],
successful nanorod deposition can be carried out in electrolyte solutions with
orders of magnitude larger metal ion concentrations than for similar noble metal
deposits (0.05 M CoSO 4 with 0.4 M H 3 BO 3 ). The relatively large current density of
−150 mA cm
–2 , although much less than that used for the DHBT method, indicates
an intense hydrogen evolution. It was probably due to the combined impact of the
electric field, mass transport and gas evolution that the nanorod diameter distribution was less homogeneous than for noble metals. The Co nanorod system could be
successfully transformed to Co 3 O 4 without any appreciable morphological change
by annealing in air.
8.3.2 Non-metallic Nanocolumnar Deposits
The most important nanostructured material obtained with electrodeposition from
dilute solution is zinc oxide. It is an inexpensive n-type semiconductor with a direct
band gap of 3.37 V, and its application in photoelectrochemical devices is emerging
fast. The literature of this field is very rich, and a large variety of deposition conditions
have been tested, whose summary can be found in a recent review article [150].
The chemical background of the formation of ZnO nanorods is similar to the
coatings discussed in Sect. 8.2.4 in the sense that the charge transfer reaction does
not modify the oxidation number of the metal ion. Instead, a reactant is produced in an
electrochemical process which acts as a co-reactant in the precipitation of the product.
The solubility of ZnO is the lowest between pH values of about 9–12, and the optimum
pH range of the ZnO deposition changes slightly with temperature [151, 152]. When
the synthesis of zinc oxide is carried out in aqueous media, the temperature has to be
above 40 °C so that zinc oxide is produced instead of zinc hydroxide. The increase
in deposition temperature strongly improves both the crystallinity and the texture of
the deposits [152] and leads to a slight increase in the nanocolumn diameter. The
reaction is as follows:
Zn
2+
+ 2OH
−
ZnO + H 2 O.
(8.3)
Concerning the source of the hydroxide ions, various reactants are possible. The
simplest way is to saturate the solution with oxygen by bubbling air through the
solution and reducing the dissolved oxygen: