294
8 Porous Nanostructured Materials
dealloying potential (Fig. 8.14b), the increase in the alloying ratio results in a twostep curve. The first and fast step accounts for the stripping of the pure deposit layer
and the second plateau-like dissolution period is related to the dealloying when the
small anodic current is due to the relatively slow transport of the dealloyed metal
from the bulk to the surface.
Although Zn as a dealloyed metal is by far the most popular one in the depositioncoupled dealloying studies, there are other possibilities, too. For instance, Li as a
dealloyed metal can lead to high surface area aluminium [264]. This reaction differs
from Zn alloying/dealloying in various aspects. The Li insertion requires a strictly
oxygen- and water-free environment, and can be carried out in the solution of a Li
salt in an organic carbonate, a typical medium for Li ion batteries. In contrast to the
Zn insertion, Li uptake by Al takes place at room temperature.
All the processes described above involved a deposition step (at sufficiently negative electrode potentials) followed by a dealloying step (oxidation). However, the
formation of porous layers is also possible with an oxidation—reduction sequence.
This is an analogy of the deposition-combined dealloying procedure, even though it
can be debated whether the final process truly has the dealloying character. Here we
cite a work related to the formation of an AgCl layer on Ag sheets in an aqueous solution containing NaOH and NaCl [265]. The electrode potential used for the reduction
step was the determining factor in tuning the pore size: at the more negative potential the AgCl layer was reduced, the smaller pore size was obtained. The reduced
nanoporous Ag electrodes were shown to exhibit a high catalytic activity towards
both oxygen reduction and oxidation of organic compounds.
8.5 Combination of Porosity-Related Electrochemical
Methods
The combination of the methods described in the earlier parts of Chap. 8 makes
it possible to prepare porous materials with porosity levels that are not available
with one single method. Electrodeposition by using a dynamic bubble template is
a bottom-up method that results in a hierarchical structure with a primary pore size
above 10 µm and a secondary pore size around 100 nm. In contrast, dealloying, which
is a top-down method, leads to pore sizes between 5 and 100 nm. It is expected that a
combination of these methods, i.e., dealloying of alloys produced with the dynamic
bubble template method, can lead to a porous material with three levels of porosity.
A few studies will be presented below in which the combinatorial application of the
dynamic bubble template method and dealloying was exploited.
Images recorded for dealloyed bubble-templated alloys verify the above described
expectation concerning the morphology of the resulting structures, as shown in
Fig. 8.15. The high-magnification images indicate that the dendrites constituting
the spongy walls of the primary pores developed internal pores after dealloying.
8 Porous Nanostructured Materials
dealloying potential (Fig. 8.14b), the increase in the alloying ratio results in a twostep curve. The first and fast step accounts for the stripping of the pure deposit layer
and the second plateau-like dissolution period is related to the dealloying when the
small anodic current is due to the relatively slow transport of the dealloyed metal
from the bulk to the surface.
Although Zn as a dealloyed metal is by far the most popular one in the depositioncoupled dealloying studies, there are other possibilities, too. For instance, Li as a
dealloyed metal can lead to high surface area aluminium [264]. This reaction differs
from Zn alloying/dealloying in various aspects. The Li insertion requires a strictly
oxygen- and water-free environment, and can be carried out in the solution of a Li
salt in an organic carbonate, a typical medium for Li ion batteries. In contrast to the
Zn insertion, Li uptake by Al takes place at room temperature.
All the processes described above involved a deposition step (at sufficiently negative electrode potentials) followed by a dealloying step (oxidation). However, the
formation of porous layers is also possible with an oxidation—reduction sequence.
This is an analogy of the deposition-combined dealloying procedure, even though it
can be debated whether the final process truly has the dealloying character. Here we
cite a work related to the formation of an AgCl layer on Ag sheets in an aqueous solution containing NaOH and NaCl [265]. The electrode potential used for the reduction
step was the determining factor in tuning the pore size: at the more negative potential the AgCl layer was reduced, the smaller pore size was obtained. The reduced
nanoporous Ag electrodes were shown to exhibit a high catalytic activity towards
both oxygen reduction and oxidation of organic compounds.
8.5 Combination of Porosity-Related Electrochemical
Methods
The combination of the methods described in the earlier parts of Chap. 8 makes
it possible to prepare porous materials with porosity levels that are not available
with one single method. Electrodeposition by using a dynamic bubble template is
a bottom-up method that results in a hierarchical structure with a primary pore size
above 10 µm and a secondary pore size around 100 nm. In contrast, dealloying, which
is a top-down method, leads to pore sizes between 5 and 100 nm. It is expected that a
combination of these methods, i.e., dealloying of alloys produced with the dynamic
bubble template method, can lead to a porous material with three levels of porosity.
A few studies will be presented below in which the combinatorial application of the
dynamic bubble template method and dealloying was exploited.
Images recorded for dealloyed bubble-templated alloys verify the above described
expectation concerning the morphology of the resulting structures, as shown in
Fig. 8.15. The high-magnification images indicate that the dendrites constituting
the spongy walls of the primary pores developed internal pores after dealloying.