5.3 The Single-Bath Method
147
to the salt of the MN metal. In practice, this means that the concentration of the ions
of the MN metal is between a few and a few tens of millimole/litre.
The structure, the mean composition and the occurrence of the composition modulation in pulse-plated materials depends on the following parameters: (i) The amount
of the material deposited during a single pulse; (ii) the kinetics of the displacement
process, including the mass transport of the ions of the displacing metal in the solution; (iii) the off-time between the pulses. For achieving a composition modulation
along the growth direction, the following criteria are to be met: (i) The deposit thickness achieved in a single pulse should be at least several atomic layers, and the deposit
must indeed form a layer. If the deposit forms segregated grains, the displacement
may result in a composition modulation, but the composition gradient does not coincide with the growth direction; instead, it will result in a granular material. (ii) The
displacement reaction must be kinetically possible, which can be ensured with an
appropriate choice of the anions present. Additives often impact the kinetics of the
displacement, while they may also incorporate into the deposit [46]. (iii) The off-time
should be long enough so that a layer accumulation can take place, even in the case
when the concentration of the displacing material is small in the electrolyte solution
(which is the typical case). This means that, unlike in the case of the pulse plating
processes of alloys when the off-time is usually at most a few tens of milliseconds, the
off-time for displacement can range up to several tens of second. (iv) The displacement process has to lead to a nearly even consumption of the displaced metal and
an even accumulation of the displacing metal. This condition is the harder to fulfil
because both the local cell effects and the nucleation barrier may favour the separation of the dissolution and the deposition sites. It is always a concern whether
the LN metal slowly dissolves at the pinholes of the MN metal cover layer [47]. If
the coverage is even and no pinholes occur, the displacement process may become
self-terminating, and the electrode potential returns during the off-time to the rest
potential of the MN metal in the plating solution [48]. The morphological features
of the displacement process may depend on the bath pH, too [49]. Cases when the
pulse plating process combined with displacement lead to granular materials will be
discussed in Chap. 7.
The displacement-based nanostructure synthesis processes always apply metals
among which at least the most noble one (which will then displace the other(s)) is
regularly codeposited with the rest. This makes it possible to measure the diffusionlimited deposition rate of this metal and compare it with the rate of displacement.
When the displacement process is kinetically not hindered, the displacement
process can be taken as a superposition of the diffusion-limited deposition of the
displacing (MN) metal and the dissolution of the displaced (LN) metal at the same
rate [50]. This mechanism can be verified with the independent measurement of the
diffusion-limited current density of the LN metal. A diffusion-limited mechanism
was found for Cu displacement by Ag [47, 51] and Ni displaced by Cu [50]. When the
LN metal is an alloy, the displacement can take place in a diffusion-limited manner
concerning the MN metal deposition, but the displacement modifies the composition of the LN layer due to the selective displacement of the least noble component.
This was found for the Ni–Co–Fe–Cu quaternary system where Cu
2+ displaced Fe
147
to the salt of the MN metal. In practice, this means that the concentration of the ions
of the MN metal is between a few and a few tens of millimole/litre.
The structure, the mean composition and the occurrence of the composition modulation in pulse-plated materials depends on the following parameters: (i) The amount
of the material deposited during a single pulse; (ii) the kinetics of the displacement
process, including the mass transport of the ions of the displacing metal in the solution; (iii) the off-time between the pulses. For achieving a composition modulation
along the growth direction, the following criteria are to be met: (i) The deposit thickness achieved in a single pulse should be at least several atomic layers, and the deposit
must indeed form a layer. If the deposit forms segregated grains, the displacement
may result in a composition modulation, but the composition gradient does not coincide with the growth direction; instead, it will result in a granular material. (ii) The
displacement reaction must be kinetically possible, which can be ensured with an
appropriate choice of the anions present. Additives often impact the kinetics of the
displacement, while they may also incorporate into the deposit [46]. (iii) The off-time
should be long enough so that a layer accumulation can take place, even in the case
when the concentration of the displacing material is small in the electrolyte solution
(which is the typical case). This means that, unlike in the case of the pulse plating
processes of alloys when the off-time is usually at most a few tens of milliseconds, the
off-time for displacement can range up to several tens of second. (iv) The displacement process has to lead to a nearly even consumption of the displaced metal and
an even accumulation of the displacing metal. This condition is the harder to fulfil
because both the local cell effects and the nucleation barrier may favour the separation of the dissolution and the deposition sites. It is always a concern whether
the LN metal slowly dissolves at the pinholes of the MN metal cover layer [47]. If
the coverage is even and no pinholes occur, the displacement process may become
self-terminating, and the electrode potential returns during the off-time to the rest
potential of the MN metal in the plating solution [48]. The morphological features
of the displacement process may depend on the bath pH, too [49]. Cases when the
pulse plating process combined with displacement lead to granular materials will be
discussed in Chap. 7.
The displacement-based nanostructure synthesis processes always apply metals
among which at least the most noble one (which will then displace the other(s)) is
regularly codeposited with the rest. This makes it possible to measure the diffusionlimited deposition rate of this metal and compare it with the rate of displacement.
When the displacement process is kinetically not hindered, the displacement
process can be taken as a superposition of the diffusion-limited deposition of the
displacing (MN) metal and the dissolution of the displaced (LN) metal at the same
rate [50]. This mechanism can be verified with the independent measurement of the
diffusion-limited current density of the LN metal. A diffusion-limited mechanism
was found for Cu displacement by Ag [47, 51] and Ni displaced by Cu [50]. When the
LN metal is an alloy, the displacement can take place in a diffusion-limited manner
concerning the MN metal deposition, but the displacement modifies the composition of the LN layer due to the selective displacement of the least noble component.
This was found for the Ni–Co–Fe–Cu quaternary system where Cu
2+ displaced Fe