6.2 Nanocrystalline Deposits of Metallic Elements
195
[39, 40]. Beside its practical importance, it is worthwhile of paying attention to this
field because it is a prominent example for the difference between the properties of
bulk and nanocrystalline materials. The first and most obvious observation is that
hydrogen permeation through nanocrystalline palladium foils takes place at a much
higher rate than in conventional polycrystalline Pd [39]. From these experiments,
a 30-fold increase in hydrogen diffusivity can be concluded for nanocrystalline Pd.
While it can be speculated from the permeation experiment that the greatly enhanced
diffusivity is due to the high grain boundary density in the nanocrystalline specimens,
this conclusion obtained direct evidence from a nuclear magnetic resonance study
[40]. This revealed that the majority of dissolved hydrogen can be found at the grain
boundaries where the packing of the Pd atoms is less dense than in perfect crystals.
Due to the high portion of the grain boundary zones, the hydrogen solubility in the
so-called α Pd–H phase increased. In contrary, the critical hydrogen concentration
of the β Pd–H phase formation decreased, hence contributing to the narrowing of
the two-phase regime.
Although palladium is a popular hydrogen-transfer catalyst, bulk nanocrystalline
Pd is seldom used for this purpose since nanocrystals (as separate entities) are
preferred for providing a high surface area. However, nanocrystalline Pd was also
tested as catalyst for the oxygen reduction reaction [41]. The study of electrodeposited films with a grain size range from 13 to 30 nm showed that the most effective
catalyst behaviour can be expected from the layers of highest roughness, and other
parameters are of secondary importance.
6.2.2 Copper
A basic work in this field was published in 1996 by Natter and Hempelmann [42]
who studied many aspects of the electrodeposition of nanocrystalline copper. In this
work, several trends whose validity is much beyond the deposition of copper were
demonstrated: (i) The application of pulse plating in combination with a suitable
additive in the bath lead to a significant grain refinement. (ii) If the current efficiency
is close to unity, the increase in temperature has an adverse effect, resulting in grain
coarsening. Results related to the above-mentioned trends can be seen in Fig. 6.4.
The smallest grain size achieved by Natter and Hempelmann was smaller than
10 nm. A surprising finding was that in the presence of citric acid in an equimolar
concentration with Cu
2+ , the increase in pH lead to microcrystalline deposits, while
a pH of as low as 1.5 gave the smallest grain size where a weak complex can be
formed from the metal ion and the chelating ligand.
Electrodeposition of nanocrystalline copper is a relatively simple process that
can be carried out with d.c. current, too [43–50]. Reports on works performed with
traditional acidic copper plating baths with about 0.5 mol dm
−3 CuSO 4 concentration
are controversial. While some authors claim that such baths, even when additives
are present, lead to polycrystalline deposits [46], others could successfully obtain
nanocrystalline deposits even in the absence of additives [43]. The decisive factor may
195
[39, 40]. Beside its practical importance, it is worthwhile of paying attention to this
field because it is a prominent example for the difference between the properties of
bulk and nanocrystalline materials. The first and most obvious observation is that
hydrogen permeation through nanocrystalline palladium foils takes place at a much
higher rate than in conventional polycrystalline Pd [39]. From these experiments,
a 30-fold increase in hydrogen diffusivity can be concluded for nanocrystalline Pd.
While it can be speculated from the permeation experiment that the greatly enhanced
diffusivity is due to the high grain boundary density in the nanocrystalline specimens,
this conclusion obtained direct evidence from a nuclear magnetic resonance study
[40]. This revealed that the majority of dissolved hydrogen can be found at the grain
boundaries where the packing of the Pd atoms is less dense than in perfect crystals.
Due to the high portion of the grain boundary zones, the hydrogen solubility in the
so-called α Pd–H phase increased. In contrary, the critical hydrogen concentration
of the β Pd–H phase formation decreased, hence contributing to the narrowing of
the two-phase regime.
Although palladium is a popular hydrogen-transfer catalyst, bulk nanocrystalline
Pd is seldom used for this purpose since nanocrystals (as separate entities) are
preferred for providing a high surface area. However, nanocrystalline Pd was also
tested as catalyst for the oxygen reduction reaction [41]. The study of electrodeposited films with a grain size range from 13 to 30 nm showed that the most effective
catalyst behaviour can be expected from the layers of highest roughness, and other
parameters are of secondary importance.
6.2.2 Copper
A basic work in this field was published in 1996 by Natter and Hempelmann [42]
who studied many aspects of the electrodeposition of nanocrystalline copper. In this
work, several trends whose validity is much beyond the deposition of copper were
demonstrated: (i) The application of pulse plating in combination with a suitable
additive in the bath lead to a significant grain refinement. (ii) If the current efficiency
is close to unity, the increase in temperature has an adverse effect, resulting in grain
coarsening. Results related to the above-mentioned trends can be seen in Fig. 6.4.
The smallest grain size achieved by Natter and Hempelmann was smaller than
10 nm. A surprising finding was that in the presence of citric acid in an equimolar
concentration with Cu
2+ , the increase in pH lead to microcrystalline deposits, while
a pH of as low as 1.5 gave the smallest grain size where a weak complex can be
formed from the metal ion and the chelating ligand.
Electrodeposition of nanocrystalline copper is a relatively simple process that
can be carried out with d.c. current, too [43–50]. Reports on works performed with
traditional acidic copper plating baths with about 0.5 mol dm
−3 CuSO 4 concentration
are controversial. While some authors claim that such baths, even when additives
are present, lead to polycrystalline deposits [46], others could successfully obtain
nanocrystalline deposits even in the absence of additives [43]. The decisive factor may