8.4 Electrochemical Dealloying
285
is little as compared to the difference between the onset of the dissolution potential of the LN metal and the dealloying potential applied in practice for essentially
all alloys, the exact determination of E crit is not crucial from the preparative point
of view. Nowadays, the application of various computational methods to simulate
dealloying phenomena is rather common [186–188].
It must be stressed that the volume fraction of the structure-forming (i.e., MN)
and leaving (i.e., LN) metal in the dealloyed master alloy is not indicative of the
ligament-to-pore diameter ratio in the dealloyed porous structure. This is because a
volume change also takes place during dealloying, which can lead to a compression
up to 8 vol.% [189]. Beside the change of the specimen shape, another discrepancy is
that the purity of the dealloyed samples is limited since the slow solid-state diffusion
does not allow the removal the LN component entirely. A 2–5 at.% impurity of the LN
metal in the porous MN structure is typical for both chemically and electrochemically
dealloyed samples. Concerning the mean ligament diameter, the general trend is that
the smaller the mole fraction of the porous structure-forming (i.e., MN) metal in the
master alloy, the smaller the ligament diameter and the most fragile porous structure
is obtained upon the dealloying process (see, e.g., [190–194]).
8.4.2 Dealloying of Binary Alloys
Typical binary alloys processed by electrochemical dealloying and leading to
nanoporous materials are listed in Table 8.2. The master alloys were synthesized
with a variety of methods but mostly with classical metallurgical procedures.
Special care has to be taken when the dealloying potential is beyond the stability
range of water when gas evolution may accompany the dealloying process. This
usually occurs when both constituents are noble metals like Au and Ag [217]. In
such cases, the solution is acidified near the dealloying front, which has an impact on
the dealloying process. The pH of the solution chosen for the dealloying experiments
also has a role in the entire dealloying process. In acidic solutions, the surface pH
change is insignificant, while in unbuffered neutral solution it is substantial. Another
phenomenon to be taken into account is that the mechanism of dealloying may
change if the dealloying potential is high and is close to the oxidation of the MN
metal. Namely, the dealloying process may take place with a dissolution–redeposition
mechanism. In this case, the oxidation of the MN metal is possible strictly at the
dissolution front where the LN metal leaves the alloy, but later these MN metal
ions are redeposited onto the pure parent metal surface. The mode of dealloying
(e.g., simple or redeposition-mediated) can be chosen by changing the dealloying
potential, as it was shown for the Au–Sn system [204]. The consequence of the
redeposition process on the sample morphology is that nanoparticle-like additional
deposit is formed on the top of the dealloyed structure.
The pore (and ligament) size of the electrochemically dealloyed porous structure
cannot be related merely to the composition of the master alloy but is a function of the
dealloying conditions, including the electrolyte solution and the dealloying potential.
285
is little as compared to the difference between the onset of the dissolution potential of the LN metal and the dealloying potential applied in practice for essentially
all alloys, the exact determination of E crit is not crucial from the preparative point
of view. Nowadays, the application of various computational methods to simulate
dealloying phenomena is rather common [186–188].
It must be stressed that the volume fraction of the structure-forming (i.e., MN)
and leaving (i.e., LN) metal in the dealloyed master alloy is not indicative of the
ligament-to-pore diameter ratio in the dealloyed porous structure. This is because a
volume change also takes place during dealloying, which can lead to a compression
up to 8 vol.% [189]. Beside the change of the specimen shape, another discrepancy is
that the purity of the dealloyed samples is limited since the slow solid-state diffusion
does not allow the removal the LN component entirely. A 2–5 at.% impurity of the LN
metal in the porous MN structure is typical for both chemically and electrochemically
dealloyed samples. Concerning the mean ligament diameter, the general trend is that
the smaller the mole fraction of the porous structure-forming (i.e., MN) metal in the
master alloy, the smaller the ligament diameter and the most fragile porous structure
is obtained upon the dealloying process (see, e.g., [190–194]).
8.4.2 Dealloying of Binary Alloys
Typical binary alloys processed by electrochemical dealloying and leading to
nanoporous materials are listed in Table 8.2. The master alloys were synthesized
with a variety of methods but mostly with classical metallurgical procedures.
Special care has to be taken when the dealloying potential is beyond the stability
range of water when gas evolution may accompany the dealloying process. This
usually occurs when both constituents are noble metals like Au and Ag [217]. In
such cases, the solution is acidified near the dealloying front, which has an impact on
the dealloying process. The pH of the solution chosen for the dealloying experiments
also has a role in the entire dealloying process. In acidic solutions, the surface pH
change is insignificant, while in unbuffered neutral solution it is substantial. Another
phenomenon to be taken into account is that the mechanism of dealloying may
change if the dealloying potential is high and is close to the oxidation of the MN
metal. Namely, the dealloying process may take place with a dissolution–redeposition
mechanism. In this case, the oxidation of the MN metal is possible strictly at the
dissolution front where the LN metal leaves the alloy, but later these MN metal
ions are redeposited onto the pure parent metal surface. The mode of dealloying
(e.g., simple or redeposition-mediated) can be chosen by changing the dealloying
potential, as it was shown for the Au–Sn system [204]. The consequence of the
redeposition process on the sample morphology is that nanoparticle-like additional
deposit is formed on the top of the dealloyed structure.
The pore (and ligament) size of the electrochemically dealloyed porous structure
cannot be related merely to the composition of the master alloy but is a function of the
dealloying conditions, including the electrolyte solution and the dealloying potential.