208
6 Nanocrystalline Deposits
as the concentration of the platinum metal ions is typically a few mmol dm
−3 only,
the Ig ion concentration is only somewhat larger than this level. With the appropriate choice of the metal ion ratio and the bath components, a wide range of the
deposit composition can be scanned by plating the alloys at different potentials (or,
with different current densities) from one single solution [202–205, 212, 217], while
other bath type lead to nearly invariant composition as the deposition potential is
varied [213, 214]. For the latter case, it seems to be likely that the plating process is
near to the pure mass transport control at the deposition potentials tested, which is
due to the unusually small metal ion concentrations.
Regardless of the solution type, the purity of the deposits is a crucial problem.
Since the platinum group metals are good electrocatalysts of the hydrogen evolution,
the current efficiency of the deposition process is relatively small (10–60%), and the
concomitant alkalination of the solution near the cathode leads to significant oxygen
incorporation into the deposits [213]. The oxygen incorporation level is often the
largest in the potential range of the formation of near-equimolar alloys [204]. The
hydrogen evolution and the relatively small precursor ion concentration often lead
to the development of a high roughness and porosity [207, 209, 215].
From the structural point of view, no general trends can be extracted from the
available literature data. What can be outlined quite uniformly is that the increase
in either the deposition current density or the overvoltage of the deposition leads to
a grain refinement. The available grain size is between 5 and 20 nm. However, the
impact of the pH on the grain size is controversial since grain refinement can be
favoured by either the increase [201] or the decrease [209] in pH.
6.3.5 Alloys of 4d Transition Metals with Metalloid
Element(s)
The direct electrodeposition of the nanocrystalline alloys of transition metals with
either phosphorus or boron is the modified version of the electroplating of the amorphous alloys containing the same elements. The major difference is that the concentration of the precursor compound responsible for the metalloid content of the alloy
is smaller than in the case of the production of the amorphous alloy, hence achieving
a lower level of metalloid incorporation. For instance, the Ni–P alloys are nanocrystalline with a phosphorus content higher than about 4 at.%, the 10–16 at.% P range
can be characterized with a mixed nanocrystalline–amorphous structure, and fully
amorphous materials are obtained with electroplating at P concentrations larger than
about 16 at.% [219, 220]. In studies dealing with electroplated nanocrystalline alloys
with metalloid elements, the metalloid concentration is usually less than the half of
the lower limit of the crystalline–amorphous transition zone.
Several baths are analogous to those developed for plating of amorphous coatings.
Ni–P [221–223] and Co–P [224–226] baths contain H 3 PO 3 as phosphorus source,
while Fe–P baths are based on NaH 2 PO 2 [227, 228]. Although NaH 2 PO 2 would also
6 Nanocrystalline Deposits
as the concentration of the platinum metal ions is typically a few mmol dm
−3 only,
the Ig ion concentration is only somewhat larger than this level. With the appropriate choice of the metal ion ratio and the bath components, a wide range of the
deposit composition can be scanned by plating the alloys at different potentials (or,
with different current densities) from one single solution [202–205, 212, 217], while
other bath type lead to nearly invariant composition as the deposition potential is
varied [213, 214]. For the latter case, it seems to be likely that the plating process is
near to the pure mass transport control at the deposition potentials tested, which is
due to the unusually small metal ion concentrations.
Regardless of the solution type, the purity of the deposits is a crucial problem.
Since the platinum group metals are good electrocatalysts of the hydrogen evolution,
the current efficiency of the deposition process is relatively small (10–60%), and the
concomitant alkalination of the solution near the cathode leads to significant oxygen
incorporation into the deposits [213]. The oxygen incorporation level is often the
largest in the potential range of the formation of near-equimolar alloys [204]. The
hydrogen evolution and the relatively small precursor ion concentration often lead
to the development of a high roughness and porosity [207, 209, 215].
From the structural point of view, no general trends can be extracted from the
available literature data. What can be outlined quite uniformly is that the increase
in either the deposition current density or the overvoltage of the deposition leads to
a grain refinement. The available grain size is between 5 and 20 nm. However, the
impact of the pH on the grain size is controversial since grain refinement can be
favoured by either the increase [201] or the decrease [209] in pH.
6.3.5 Alloys of 4d Transition Metals with Metalloid
Element(s)
The direct electrodeposition of the nanocrystalline alloys of transition metals with
either phosphorus or boron is the modified version of the electroplating of the amorphous alloys containing the same elements. The major difference is that the concentration of the precursor compound responsible for the metalloid content of the alloy
is smaller than in the case of the production of the amorphous alloy, hence achieving
a lower level of metalloid incorporation. For instance, the Ni–P alloys are nanocrystalline with a phosphorus content higher than about 4 at.%, the 10–16 at.% P range
can be characterized with a mixed nanocrystalline–amorphous structure, and fully
amorphous materials are obtained with electroplating at P concentrations larger than
about 16 at.% [219, 220]. In studies dealing with electroplated nanocrystalline alloys
with metalloid elements, the metalloid concentration is usually less than the half of
the lower limit of the crystalline–amorphous transition zone.
Several baths are analogous to those developed for plating of amorphous coatings.
Ni–P [221–223] and Co–P [224–226] baths contain H 3 PO 3 as phosphorus source,
while Fe–P baths are based on NaH 2 PO 2 [227, 228]. Although NaH 2 PO 2 would also