8.4 Electrochemical Dealloying
291
performed, e.g., for Au–Cu alloys deposited from a solution containing KAuCl 4 ,
CuSO 4 and H 2 SO 4 [241]. The suggested procedure involved one single deposition step at constant negative potential followed with one dealloying step where Cu
dissolved selectively. Although the composition of the primary Au–Cu layer was
not reported, it seems that the dealloying process exhibits the same features as the
dealloying of metallurgically processed bulk Au–Cu alloys. The opportunity that
the primary layer composition can be regulated with the composition of the solution and the deposition current density for tuning the pore size was not exploited
for Au–Cu alloys but was demonstrated in a set of experiments related to Ni–Cu
alloys deposited from a sulphamate bath [190]. For dealloyed Ni obtained from Ni–
Cu alloys, a detailed magnetization study confirmed the formation of an isotropic
nanoporous structure [190].
Here, the attention must be drawn to the fact that Ni–Cu alloys tend to segregate
under some deposition conditions because the unlimited miscibility observed at high
temperature is followed by segregation at lower temperature. This low-temperature
miscibility gap was not confirmed directly by structural studies but was concluded
from model calculations [242–244]. In accord with the theoretical calculations, some
physical properties of the Ni–Cu alloys indicate that bulk Ni–Cu alloys exhibit a
tendency for the formation of Ni-rich segregations. The same tendency for segregation was also confirmed in electrodeposition experiments [245–249]. The formation
of Ni-rich and Cu-rich zones is suitable to obtain porous structures that are otherwise
not available. As demonstrated by Chang et al. [209], dealloying of electrodeposited
Ni–Cu alloys with columnar structure leads to an irregular nanochannel system with
channel axes perpendicular to the substrate and with Ni wall thickness around 20 nm.
As shown in Fig. 8.12, the structure of the dealloyed inhomogeneous Ni–Cu alloy is
not conformal to the usual bicontinuous pore/ligament systems obtained for dealloyed
homogeneous alloys.
The segregation tendency of the alloys containing Ni and Cu can be also exploited
when other components are also present, as it was exemplified for Ni–Co–Cu alloys
[250]. The mole fraction of the alloying element beside Ni has to be limited in this
case because the release of the Co content of the alloys is accompanied with some Ni
loss, too. On the one hand, this makes the process difficult to design quantitatively,
and, on the other hand, may lead to a collapse of the residual Ni network.
When a pure metal layer is plated onto the surface of another one, the spontaneous diffusion of the adlayer into the substrate usually does not take place to the
desired extent at room temperature. Therefore, essentially all interdiffusion-related
dealloying processes require some thermal activation, either by performing the electrodeposition step itself at an elevated temperature or treating the coated substrate at
high temperature ex situ. The interdiffusion temperature may vary between 60 and
150 °C. Electroplating at room temperature and interdiffusion at higher temperature
may be followed also by a chemical etching, as it was shown for nanoporous Au and
Cu plated with a Zn layer [251].
Zinc was used as a much preferred plated adlayer and etched metal in many
other deposition-combined dealloying studies because its relatively large diffusion
coefficient in other metals, the availability of alloying at a relatively low temperature
291
performed, e.g., for Au–Cu alloys deposited from a solution containing KAuCl 4 ,
CuSO 4 and H 2 SO 4 [241]. The suggested procedure involved one single deposition step at constant negative potential followed with one dealloying step where Cu
dissolved selectively. Although the composition of the primary Au–Cu layer was
not reported, it seems that the dealloying process exhibits the same features as the
dealloying of metallurgically processed bulk Au–Cu alloys. The opportunity that
the primary layer composition can be regulated with the composition of the solution and the deposition current density for tuning the pore size was not exploited
for Au–Cu alloys but was demonstrated in a set of experiments related to Ni–Cu
alloys deposited from a sulphamate bath [190]. For dealloyed Ni obtained from Ni–
Cu alloys, a detailed magnetization study confirmed the formation of an isotropic
nanoporous structure [190].
Here, the attention must be drawn to the fact that Ni–Cu alloys tend to segregate
under some deposition conditions because the unlimited miscibility observed at high
temperature is followed by segregation at lower temperature. This low-temperature
miscibility gap was not confirmed directly by structural studies but was concluded
from model calculations [242–244]. In accord with the theoretical calculations, some
physical properties of the Ni–Cu alloys indicate that bulk Ni–Cu alloys exhibit a
tendency for the formation of Ni-rich segregations. The same tendency for segregation was also confirmed in electrodeposition experiments [245–249]. The formation
of Ni-rich and Cu-rich zones is suitable to obtain porous structures that are otherwise
not available. As demonstrated by Chang et al. [209], dealloying of electrodeposited
Ni–Cu alloys with columnar structure leads to an irregular nanochannel system with
channel axes perpendicular to the substrate and with Ni wall thickness around 20 nm.
As shown in Fig. 8.12, the structure of the dealloyed inhomogeneous Ni–Cu alloy is
not conformal to the usual bicontinuous pore/ligament systems obtained for dealloyed
homogeneous alloys.
The segregation tendency of the alloys containing Ni and Cu can be also exploited
when other components are also present, as it was exemplified for Ni–Co–Cu alloys
[250]. The mole fraction of the alloying element beside Ni has to be limited in this
case because the release of the Co content of the alloys is accompanied with some Ni
loss, too. On the one hand, this makes the process difficult to design quantitatively,
and, on the other hand, may lead to a collapse of the residual Ni network.
When a pure metal layer is plated onto the surface of another one, the spontaneous diffusion of the adlayer into the substrate usually does not take place to the
desired extent at room temperature. Therefore, essentially all interdiffusion-related
dealloying processes require some thermal activation, either by performing the electrodeposition step itself at an elevated temperature or treating the coated substrate at
high temperature ex situ. The interdiffusion temperature may vary between 60 and
150 °C. Electroplating at room temperature and interdiffusion at higher temperature
may be followed also by a chemical etching, as it was shown for nanoporous Au and
Cu plated with a Zn layer [251].
Zinc was used as a much preferred plated adlayer and etched metal in many
other deposition-combined dealloying studies because its relatively large diffusion
coefficient in other metals, the availability of alloying at a relatively low temperature