112
4 Ultrathin Layers
produce an effective protective hydroxide layer, where the alkalization of the vicinity
of the cathode is due to the hydrogen evolution from the unbuffered solution of pH
= 3. When the experiment involves a potential scan, a regular cyclic voltammetric
peak of the metal ion reduction can be seen, then the decay of the negative current
is reversed as the onset potential of the water decomposition is reached. This is
the potential where the growth of the metal layer stops, as shown by the EQCM
signal observed in parallel to the voltammetric scan. Therefore, we encounter the
paradox situation that at relatively small cathodic overvoltage a continuous growth
of a metal layer can be seen, while at more negative potential, where the driving force
of the reduction of the metal ion is higher, the process stops. The resolution of the
controversy is possible if the blocking effect of the hydroxide adlayer on the electrode
is taken into account. The self-limitation is clearly a consequence of a finely-tuned
system where the transport of the protons/hydroxide ions towards/from the cathode
surface, respectively, is too slow to eliminate the blocking layer of hydroxide ions
from the freshly-deposited metal surface. The thickness of the thus produced nickel
layers proved to be a function of the Ni
2+ concentration in the solution.
Water can be a key component of the self-limiting metal electrodeposition process
also in the case when it is not a major component of the electrolyte solution. In a
common deep eutectic solvent, the 2:1 ratio mixture of choline chloride and urea,
the passivation of the nickel deposition was found [165]. Beside the observation
of the cathodic passivation itself, the common origin of the self-limitation with the
previously discussed process was evidenced by the necessity of water for the blocking
of the metal deposition, the occurrence of the self-limitation at a sufficiently negative
potential only and the transformation of the deposit morphology from granular to
surface-conformal. The regulation of the solution composition and especially the
buffer capacity of the solvent was unimportant here since the dissolution of the
nickel(II) hydroxide in the deep eutectic solvent does not take place. An indirect
evidence for the passivation process is that upon the switch of the electrode potential
to the Ni dissolution regime, the anodic process is retarded due to the presence of
the passivating layer that was previously formed in the cathode process.
4.3.2 Atomic-Scale Observation of the Initial Phase
of the Layer Growth
As it was discussed in Chap. 2.10.1, nucleation of a new phase on a foreign substrate
can take place through various deposition modes. Although the Frank–van der Merwe
type nucleation and growth mode would theoretically be the most suitable layer
formation pathway to deposit ultrathin layers, this seldom takes place in practical
systems. The Volmer–Weber type nucleation can also lead to layer-like deposits,
though with less uniform thickness distribution, if the growth of the newly-formed
nuclei is not isotropic but the lateral growth is highly preferred. Detailed analysis
of the role of the anisotropy on the coalescence of the grains and the formation of
4 Ultrathin Layers
produce an effective protective hydroxide layer, where the alkalization of the vicinity
of the cathode is due to the hydrogen evolution from the unbuffered solution of pH
= 3. When the experiment involves a potential scan, a regular cyclic voltammetric
peak of the metal ion reduction can be seen, then the decay of the negative current
is reversed as the onset potential of the water decomposition is reached. This is
the potential where the growth of the metal layer stops, as shown by the EQCM
signal observed in parallel to the voltammetric scan. Therefore, we encounter the
paradox situation that at relatively small cathodic overvoltage a continuous growth
of a metal layer can be seen, while at more negative potential, where the driving force
of the reduction of the metal ion is higher, the process stops. The resolution of the
controversy is possible if the blocking effect of the hydroxide adlayer on the electrode
is taken into account. The self-limitation is clearly a consequence of a finely-tuned
system where the transport of the protons/hydroxide ions towards/from the cathode
surface, respectively, is too slow to eliminate the blocking layer of hydroxide ions
from the freshly-deposited metal surface. The thickness of the thus produced nickel
layers proved to be a function of the Ni
2+ concentration in the solution.
Water can be a key component of the self-limiting metal electrodeposition process
also in the case when it is not a major component of the electrolyte solution. In a
common deep eutectic solvent, the 2:1 ratio mixture of choline chloride and urea,
the passivation of the nickel deposition was found [165]. Beside the observation
of the cathodic passivation itself, the common origin of the self-limitation with the
previously discussed process was evidenced by the necessity of water for the blocking
of the metal deposition, the occurrence of the self-limitation at a sufficiently negative
potential only and the transformation of the deposit morphology from granular to
surface-conformal. The regulation of the solution composition and especially the
buffer capacity of the solvent was unimportant here since the dissolution of the
nickel(II) hydroxide in the deep eutectic solvent does not take place. An indirect
evidence for the passivation process is that upon the switch of the electrode potential
to the Ni dissolution regime, the anodic process is retarded due to the presence of
the passivating layer that was previously formed in the cathode process.
4.3.2 Atomic-Scale Observation of the Initial Phase
of the Layer Growth
As it was discussed in Chap. 2.10.1, nucleation of a new phase on a foreign substrate
can take place through various deposition modes. Although the Frank–van der Merwe
type nucleation and growth mode would theoretically be the most suitable layer
formation pathway to deposit ultrathin layers, this seldom takes place in practical
systems. The Volmer–Weber type nucleation can also lead to layer-like deposits,
though with less uniform thickness distribution, if the growth of the newly-formed
nuclei is not isotropic but the lateral growth is highly preferred. Detailed analysis
of the role of the anisotropy on the coalescence of the grains and the formation of