290
8 Porous Nanostructured Materials
electrochemical etching was also shown for an amorphous Fe 60 Pt 10 B 30 precursor
alloy [240]. Here, B and Fe were removed selectively, but the residual Fe content
was limited by the formation of an fcc Fe–Pt alloy with an approximately 2:1 molar
ratio.
Selective dissolution of the multicomponent alloys leads to various compositions
in term of the completeness and the selectivity of the removal of the components.
Dealloying of an Au 30 Si 20 Cu 33 Ag 7 Pd 10 master alloy at room temperature led to
a gold-rich Au–Pd porous structure with enhanced Au-to-Pd ratio as compared to
the master alloy composition [232]. However, the dealloying of another alloy with
similar composition, Au 40 Si 20 Cu 28 Ag 7 Pd 5 , at 70 °C resulted in a complete dealloying
of all four accompanying element beside the most noble one (i.e., Au) [180, 234],
indicating the importance of the thermal activation.
A very interesting secondary structural transformation may happen when the pore
wall-forming components of the dealloyed material are not miscible in equilibrium.
This was demonstrated for an Mg–Ag–Cu–Y precursor alloy with equimolar Ag and
Cu ratio (12.5%) [238]. As the dealloying process took place, the initially amorphous
structure turned into a crystalline one, and in parallel, the Ag and Cu crystals started
segregating. The segregation leads to a primarily Ag-rich pore wall and Cu crystals at
the sample surface with much larger diameter than the feature size of the bicontinuous
pore/ligament system.
The morphology of the porous dealloyed materials strongly depends on the homogeneity of the master alloy. If there are “pre-existing features” (grain boundaries,
composition inhomogeneities etc.) in the sample, the etching process is likely to
take place preferentially along these parts [229]. Trenches at the surface of meltspun ribbons also influence the dealloying morphology. For amorphous alloys, the
increase in the number of the components and the corresponding increase in the
homogeneity of the precursor alloy have a beneficial impact on the morphology of
the dealloyed porous structure.
8.4.5 Dealloying of Electrochemically Produced Layers
Dealloying procedures discussed in the previous chapters result in a porous material
where the resulting structure is open at all surfaces. These materials can be used as
filters or flow-through catalysts. If the porous material must form a layer, the alloy
to be etched has to be grown on an existing surface, and the dealloying procedure
has to take place on this surface only. Therefore, the dealloyed material is obtained
as a supported porous layer that is capped at the bottom side (which means that the
specimen is not permeable). These materials can be used, e.g., as porous electrodes
with high surface area. The procedures based on deposition combined with dealloying
have the advantage that the thickness of the porous layer can be easily regulated by
an appropriate choice of the thickness of the alloyed layer.
In the simplest case, the coating to be dealloyed can be electroplated, and the
resulting deposit can be dealloyed in the same solution. This procedure can be
8 Porous Nanostructured Materials
electrochemical etching was also shown for an amorphous Fe 60 Pt 10 B 30 precursor
alloy [240]. Here, B and Fe were removed selectively, but the residual Fe content
was limited by the formation of an fcc Fe–Pt alloy with an approximately 2:1 molar
ratio.
Selective dissolution of the multicomponent alloys leads to various compositions
in term of the completeness and the selectivity of the removal of the components.
Dealloying of an Au 30 Si 20 Cu 33 Ag 7 Pd 10 master alloy at room temperature led to
a gold-rich Au–Pd porous structure with enhanced Au-to-Pd ratio as compared to
the master alloy composition [232]. However, the dealloying of another alloy with
similar composition, Au 40 Si 20 Cu 28 Ag 7 Pd 5 , at 70 °C resulted in a complete dealloying
of all four accompanying element beside the most noble one (i.e., Au) [180, 234],
indicating the importance of the thermal activation.
A very interesting secondary structural transformation may happen when the pore
wall-forming components of the dealloyed material are not miscible in equilibrium.
This was demonstrated for an Mg–Ag–Cu–Y precursor alloy with equimolar Ag and
Cu ratio (12.5%) [238]. As the dealloying process took place, the initially amorphous
structure turned into a crystalline one, and in parallel, the Ag and Cu crystals started
segregating. The segregation leads to a primarily Ag-rich pore wall and Cu crystals at
the sample surface with much larger diameter than the feature size of the bicontinuous
pore/ligament system.
The morphology of the porous dealloyed materials strongly depends on the homogeneity of the master alloy. If there are “pre-existing features” (grain boundaries,
composition inhomogeneities etc.) in the sample, the etching process is likely to
take place preferentially along these parts [229]. Trenches at the surface of meltspun ribbons also influence the dealloying morphology. For amorphous alloys, the
increase in the number of the components and the corresponding increase in the
homogeneity of the precursor alloy have a beneficial impact on the morphology of
the dealloyed porous structure.
8.4.5 Dealloying of Electrochemically Produced Layers
Dealloying procedures discussed in the previous chapters result in a porous material
where the resulting structure is open at all surfaces. These materials can be used as
filters or flow-through catalysts. If the porous material must form a layer, the alloy
to be etched has to be grown on an existing surface, and the dealloying procedure
has to take place on this surface only. Therefore, the dealloyed material is obtained
as a supported porous layer that is capped at the bottom side (which means that the
specimen is not permeable). These materials can be used, e.g., as porous electrodes
with high surface area. The procedures based on deposition combined with dealloying
have the advantage that the thickness of the porous layer can be easily regulated by
an appropriate choice of the thickness of the alloyed layer.
In the simplest case, the coating to be dealloyed can be electroplated, and the
resulting deposit can be dealloyed in the same solution. This procedure can be