6.1 General Considerations Concerning Nanocrystalline Deposits
185
the nanocrystalline nature of the samples. The principles of the electrodeposition of
nanocrystalline materials have been clarified for more than two decades. Therefore,
the reviews dealing with the properties of electrodeposited nanomaterials written
before 2010 can be taken as fully relevant even nowadays [6–9], also when application of nanomaterials is concerned [10], although the topic is revisited time by time
[11]. Nevertheless, the number of papers in which electrodeposited nanocrystalline
materials are dealt with keeps increasing since electroplating remained the simplest
method to produce nanocrystalline materials; in particular, metals.
6.1.3 Electrochemical Techniques in the Deposition
of Nanocrystalline Materials
Electrodeposition of bulk nanocrystalline materials can be performed for various
substrate–deposit pairs regardless of the nucleation mode of the deposit (see
Chap. 2.10.1). For relatively thick layers, the grain size distribution of the deposit
becomes independent of the nucleation mode (as it was also shown in Fig. 2.20),
and the near-substrate zone of the deposit is negligible when the properties of thick
nanocrystalline deposits are concerned. When the lattice of the substrate and that of
the deposit is incommensurable, the deposition starts with a layer in which the crystallite size scales with the deposit thickness until a stable growth mode is achieved.
The latter example holds for microcrystalline deposits, while in this chapter, we deal
with deposition processes in which the deposit is nanocrystalline throughout the
entire deposit thickness.
The trends governing the deposition conditions and the grain size of the resulting
deposits have been summarized in Chap. 2.11; hence, the most important factors
are repeated here only. All experimental factors that contribute to the enhancement
of the adatom concentration on the deposit surface lead to grain refinement. This is
due to the increase of the nucleation rate.
1 All other experimental conditions that
1 Although it is not in the scope of a monograph to deal with misinterpretations, it is important to
mention here that the small grain size occurring during either high-current density d.c. plating or
pulse plating is often explained with the electrochemical analogy of the Kelvin equation, i.e., with
ρ crit = 2V m σ /ze|η| (ρ crit is the critical nuclei radius for a hemispherical grain, V m is the molar
volume of the deposit, and σ is the interfacial specific energy at the deposit–solution interface). The
equation is the analogy of the vapour pressure change of a small-radius droplet as compared to a
flat surface, and the intensive parameter analogous to the vapour pressure is the electrode potential
(or the overvoltage). For more details, see Ref. [12].
It is to be clarified that the Kelvin equation refers to a situation when a deposit nucleus is at the
surface of an inert foreign substrate. This state is the typical case of unstable equilibrium. Particles
that are smaller than the critical nucleus size are expected to dissolve, while larger particles can
grow; hence, any fluctuation drives the system away from its actual state to which the system will
never return. However, plating with a high current density always represents a far-from-equilibrium
experiment where the application of equations valid for equilibrium is strictly forbidden. While the
Kelvin equation predicts a 5–25 mV equilibrium potential correction for grains of a few tens of
atoms, plating modes applied for electrodeposition of nanocrystalline materails usually work in the
185
the nanocrystalline nature of the samples. The principles of the electrodeposition of
nanocrystalline materials have been clarified for more than two decades. Therefore,
the reviews dealing with the properties of electrodeposited nanomaterials written
before 2010 can be taken as fully relevant even nowadays [6–9], also when application of nanomaterials is concerned [10], although the topic is revisited time by time
[11]. Nevertheless, the number of papers in which electrodeposited nanocrystalline
materials are dealt with keeps increasing since electroplating remained the simplest
method to produce nanocrystalline materials; in particular, metals.
6.1.3 Electrochemical Techniques in the Deposition
of Nanocrystalline Materials
Electrodeposition of bulk nanocrystalline materials can be performed for various
substrate–deposit pairs regardless of the nucleation mode of the deposit (see
Chap. 2.10.1). For relatively thick layers, the grain size distribution of the deposit
becomes independent of the nucleation mode (as it was also shown in Fig. 2.20),
and the near-substrate zone of the deposit is negligible when the properties of thick
nanocrystalline deposits are concerned. When the lattice of the substrate and that of
the deposit is incommensurable, the deposition starts with a layer in which the crystallite size scales with the deposit thickness until a stable growth mode is achieved.
The latter example holds for microcrystalline deposits, while in this chapter, we deal
with deposition processes in which the deposit is nanocrystalline throughout the
entire deposit thickness.
The trends governing the deposition conditions and the grain size of the resulting
deposits have been summarized in Chap. 2.11; hence, the most important factors
are repeated here only. All experimental factors that contribute to the enhancement
of the adatom concentration on the deposit surface lead to grain refinement. This is
due to the increase of the nucleation rate.
1 All other experimental conditions that
1 Although it is not in the scope of a monograph to deal with misinterpretations, it is important to
mention here that the small grain size occurring during either high-current density d.c. plating or
pulse plating is often explained with the electrochemical analogy of the Kelvin equation, i.e., with
ρ crit = 2V m σ /ze|η| (ρ crit is the critical nuclei radius for a hemispherical grain, V m is the molar
volume of the deposit, and σ is the interfacial specific energy at the deposit–solution interface). The
equation is the analogy of the vapour pressure change of a small-radius droplet as compared to a
flat surface, and the intensive parameter analogous to the vapour pressure is the electrode potential
(or the overvoltage). For more details, see Ref. [12].
It is to be clarified that the Kelvin equation refers to a situation when a deposit nucleus is at the
surface of an inert foreign substrate. This state is the typical case of unstable equilibrium. Particles
that are smaller than the critical nucleus size are expected to dissolve, while larger particles can
grow; hence, any fluctuation drives the system away from its actual state to which the system will
never return. However, plating with a high current density always represents a far-from-equilibrium
experiment where the application of equations valid for equilibrium is strictly forbidden. While the
Kelvin equation predicts a 5–25 mV equilibrium potential correction for grains of a few tens of
atoms, plating modes applied for electrodeposition of nanocrystalline materails usually work in the