282
8 Porous Nanostructured Materials
CdSe from 0.01 M solution of both CdSO 4 and Na 2 SeO 3 [167]. Here, the reduction
of the selenate ions takes place. The CdSe columnar deposits showed more frequent
branching than ZnO.
8.4 Electrochemical Dealloying
8.4.1 Background of Dealloying
Dealloying is a process in which an alloy is selectively dissolved so that the more
noble (MN) component is enriched at the surface while the less noble (LN) component dissolves preferentially. Dealloying can be a simple chemical process and it
can be controlled electrochemically by applying a desired electrode potential. For
alloys having several components, dealloying may lead to the dissolution of either
one single or several components, leaving behind either an alloy as a partly dealloyed
material or a metal composed of solely the most noble element of the original alloy,
respectively. Topical reviews of various aspects of dealloying are available in the
literature [168–170].
Electrochemical dealloying has been known historically from essentially the
beginning of the electrochemical corrosion studies, and the spongy nature of the dealloyed material was also recognized already in the 1920s for the corrosion/dissolution
of several alloys (see, e.g., the references in [171]). The polarization behaviour of
the alloys undergoing dealloying was also studied with a great accuracy [171, 172],
and three electrode potential zones were identified. For dealloying, there must be a
difference between the standard electrode potentials (E
0 ) of the Me
z+ /Me systems
for the constituents, and there must be a comparable difference between the onset
potentials of the dissolution (E D ) of the constituent metals. While E
0 is a relevant
parameter if the LN metal dissolves reversibly, E D can be used only if the LN metal
can be passive; hence, even the sign of the two parameters can be different (as will
be shown later for the Ni–Cu system).
The relative position of the anodic parts of the polarization curves is shown in
Fig. 8.10a. For dealloying, the prerequisite is that at least for a range of intermediate alloy composition, the polarization curve of the alloy must run in between the
polarization curves of the pure metals. There is a monotonous dependence of the
critical dealloying potential (E crit ) on the alloy composition (see Fig. 8.10b). The
composition range in which dealloying can take place is the narrower, the smaller is
E
0 for the alloy constituents [172], as indicated with the trend line in Fig. 8.10c.
In line with the research on dealloying, it was recognized that atomic-scale component redistribution is necessary which involves surface diffusion. In the potential
region where the surface enrichment of the MN takes place without inducing porosity,
the surface diffusion was observed real-time with STM [175, 176]. The in situ STM
study also revealed that at small overpotential, dealloying-related surface diffusion of
the MN metal leads to a smooth surface. Dealloying in this potential range influences
8 Porous Nanostructured Materials
CdSe from 0.01 M solution of both CdSO 4 and Na 2 SeO 3 [167]. Here, the reduction
of the selenate ions takes place. The CdSe columnar deposits showed more frequent
branching than ZnO.
8.4 Electrochemical Dealloying
8.4.1 Background of Dealloying
Dealloying is a process in which an alloy is selectively dissolved so that the more
noble (MN) component is enriched at the surface while the less noble (LN) component dissolves preferentially. Dealloying can be a simple chemical process and it
can be controlled electrochemically by applying a desired electrode potential. For
alloys having several components, dealloying may lead to the dissolution of either
one single or several components, leaving behind either an alloy as a partly dealloyed
material or a metal composed of solely the most noble element of the original alloy,
respectively. Topical reviews of various aspects of dealloying are available in the
literature [168–170].
Electrochemical dealloying has been known historically from essentially the
beginning of the electrochemical corrosion studies, and the spongy nature of the dealloyed material was also recognized already in the 1920s for the corrosion/dissolution
of several alloys (see, e.g., the references in [171]). The polarization behaviour of
the alloys undergoing dealloying was also studied with a great accuracy [171, 172],
and three electrode potential zones were identified. For dealloying, there must be a
difference between the standard electrode potentials (E
0 ) of the Me
z+ /Me systems
for the constituents, and there must be a comparable difference between the onset
potentials of the dissolution (E D ) of the constituent metals. While E
0 is a relevant
parameter if the LN metal dissolves reversibly, E D can be used only if the LN metal
can be passive; hence, even the sign of the two parameters can be different (as will
be shown later for the Ni–Cu system).
The relative position of the anodic parts of the polarization curves is shown in
Fig. 8.10a. For dealloying, the prerequisite is that at least for a range of intermediate alloy composition, the polarization curve of the alloy must run in between the
polarization curves of the pure metals. There is a monotonous dependence of the
critical dealloying potential (E crit ) on the alloy composition (see Fig. 8.10b). The
composition range in which dealloying can take place is the narrower, the smaller is
E
0 for the alloy constituents [172], as indicated with the trend line in Fig. 8.10c.
In line with the research on dealloying, it was recognized that atomic-scale component redistribution is necessary which involves surface diffusion. In the potential
region where the surface enrichment of the MN takes place without inducing porosity,
the surface diffusion was observed real-time with STM [175, 176]. The in situ STM
study also revealed that at small overpotential, dealloying-related surface diffusion of
the MN metal leads to a smooth surface. Dealloying in this potential range influences