7.2 Composite Deposition from Particle Suspensions
251
7.2.14 Suspension Plating in Anodic Processes
The suspension plating in an anodic process is similar to the inversion of its cathodic
counterpart. The particles suspended have to be inert both in the starting solution
and under the anodization conditions after the incorporation into the coating. The
precursor metal ions are present in the solution in an intermediate oxidation state in
the form of an ionic salt that is well soluble. In contrast, the product of the oxidation
of the precursor metal ion must lead to an essentially insoluble oxide that is stable
in contact with the solution. The latter condition is not fulfilled automatically since the
anodization process nearly always takes place in an aqueous solution at considerably
more positive potential than the stability regime of water; hence, the oxygen evolution
as a result of water decomposition is preferred to be hindered on the surface of the
resulting oxide. The suspended particles incorporate the same way as in cathodic
processes involving metal deposition. When some of the theories developed for
cathodic particle incorporation was applied to anodic processes, it was found that
the evaluation of the incorporation ratio as a function of the particle concentration is
possible on the basis of the Guglielmi model [208, 209]. The significant adsorption
of the metal ions (Pb
2+ ) on the surface of various particles [210] indicates that the
approach of Celis, Roos and Buelens may potentially be applied for anodic processes,
too.
The big majority of the relevant works is based on the formation of PbO 2 from
Pb
2+ -containing solutions. The starting solution can be either acidic [208, 209] or
alkaline [208, 209]. Acidic solutions are usually more concentrated, containing Pb
2+
species up to about 1 M, while alkaline baths are somewhat more dilute with a typical
concentration of 0.1 M. Regardless of the nature of the particles, the d.c.-plated PbO 2 -
based composites have a fine-grained structure as opposed to the coarse-grained
columnar structure of their particle-free counterpart, and the texture of the PbO 2
matrix is strongly decreased as a result of the composite formation. Figure 7.7 shows
a typical series of SEM images on PbO 2 deposits containing colloidal particles. The
increased active surface area of the composite PbO 2 electrodes is widely considered
as a key factor for the enhancement of the activity of theses electrodes as compared
to pure PbO 2 .
Pulse plating was seldom reported for the anodic codeposition of colloidal particles. When this was tested, the experience was that the decrease in duty cycles
and the increase of the pulse frequency is favourable for the enhancement of the
concentration of incorporated particles [211, 212]. The application of reverse pulse
electrodeposition can further enhance the incorporation ratio of the nanoparticles,
indicating that the cathodic pulse does not lead to the release of particles, but can
generate adsorption sites for more particles to be incorporated [213]. The literature
background does not yield unambiguous information about the effect of solution
agitation on the particle incorporation ratio. Apparently, the codeposition of fairly
large particles with d > 100 nm on downward-facing electrodes can be stimulated
with solution agitation or electrode rotation [208, 209], the stable suspension of very
small particles with d ≈ 5 nm stirring is rather counterproductive. However, there is
251
7.2.14 Suspension Plating in Anodic Processes
The suspension plating in an anodic process is similar to the inversion of its cathodic
counterpart. The particles suspended have to be inert both in the starting solution
and under the anodization conditions after the incorporation into the coating. The
precursor metal ions are present in the solution in an intermediate oxidation state in
the form of an ionic salt that is well soluble. In contrast, the product of the oxidation
of the precursor metal ion must lead to an essentially insoluble oxide that is stable
in contact with the solution. The latter condition is not fulfilled automatically since the
anodization process nearly always takes place in an aqueous solution at considerably
more positive potential than the stability regime of water; hence, the oxygen evolution
as a result of water decomposition is preferred to be hindered on the surface of the
resulting oxide. The suspended particles incorporate the same way as in cathodic
processes involving metal deposition. When some of the theories developed for
cathodic particle incorporation was applied to anodic processes, it was found that
the evaluation of the incorporation ratio as a function of the particle concentration is
possible on the basis of the Guglielmi model [208, 209]. The significant adsorption
of the metal ions (Pb
2+ ) on the surface of various particles [210] indicates that the
approach of Celis, Roos and Buelens may potentially be applied for anodic processes,
too.
The big majority of the relevant works is based on the formation of PbO 2 from
Pb
2+ -containing solutions. The starting solution can be either acidic [208, 209] or
alkaline [208, 209]. Acidic solutions are usually more concentrated, containing Pb
2+
species up to about 1 M, while alkaline baths are somewhat more dilute with a typical
concentration of 0.1 M. Regardless of the nature of the particles, the d.c.-plated PbO 2 -
based composites have a fine-grained structure as opposed to the coarse-grained
columnar structure of their particle-free counterpart, and the texture of the PbO 2
matrix is strongly decreased as a result of the composite formation. Figure 7.7 shows
a typical series of SEM images on PbO 2 deposits containing colloidal particles. The
increased active surface area of the composite PbO 2 electrodes is widely considered
as a key factor for the enhancement of the activity of theses electrodes as compared
to pure PbO 2 .
Pulse plating was seldom reported for the anodic codeposition of colloidal particles. When this was tested, the experience was that the decrease in duty cycles
and the increase of the pulse frequency is favourable for the enhancement of the
concentration of incorporated particles [211, 212]. The application of reverse pulse
electrodeposition can further enhance the incorporation ratio of the nanoparticles,
indicating that the cathodic pulse does not lead to the release of particles, but can
generate adsorption sites for more particles to be incorporated [213]. The literature
background does not yield unambiguous information about the effect of solution
agitation on the particle incorporation ratio. Apparently, the codeposition of fairly
large particles with d > 100 nm on downward-facing electrodes can be stimulated
with solution agitation or electrode rotation [208, 209], the stable suspension of very
small particles with d ≈ 5 nm stirring is rather counterproductive. However, there is