11.3 Templates Obtained Through the Self-assembly of Particles
393
i.e., the areas not completely covered by the colloidal templates. These cracks are
formed due to the shrinkage of the colloidal template upon drying, and their width
is several times larger than the diameter of the template-forming particles. It was
demonstrated that various metals either tend or do not tend to fill up these gaps,
while the interparticle space could be filled up continuously in both cases [260]. If
the metal overgrows the opal structure, its starts developing crystal shapes that are
characteristics of its deposition onto a flat substrate.
The controlled parameter during the electroplating of metals into a colloidal
template can be either the current or the electrode potential, and both are suitable for
tuning the deposition preference. As far as it can be established from the occasionally
published data, the deposition rate has to be chosen so that the deposition remains in
the kinetically controlled range and not diffusion-limited. Since the diffusion length
between the colloidal particles can be quite long for multilayered templates, the
kinetic control practically means low current densities. Hence, the fill-up time of a
multilayered template with a handful colloidal particle layers may range from hours
to tens of hours [240, 261, 262], a period about two orders of magnitude longer
than that normally required for the formation of equally thick continuous coatings.
Although a few works are available that present I(t) curves for potentiostatic metal
deposition into colloidal templates [240, 244, 255, 256, 262–264], their results are
quite controversial, so much that even the classification of the I(t) curves published
is challenging. For multilayered colloidal templates, some works claim that the I(t)
curves exhibit an oscillatory character, corresponding to the actual active surface
area of the growing metal in the interparticle space [244, 262, 263]. Even though a
very regular periodicity of the current maxima and minima was presented [263], the
variation of the mean current makes it very dubious that the oscillation can be quantitatively related to the growth phase. Another typical observation is that the current
tends to increase slightly as the growth front passes a colloid particle layer [255,
256]. However, even the slowly growing mean current cannot be taken as a general
observation since decreasing currents [263], followed by a constant-current period
after a long time [240], have also been reported. The character of the current transient is rather clear for monolayer templates [264, 265] where the smallest cathodic
current can be associated with the growth phase when the deposit reaches the radius
of the colloidal particles.
Below, a short summary is presented with metals (elements and alloy families)
whose inverse opal structure was extensively synthesized. The specific features that
make the deposit useful are discussed together with the metal types.
Gold. Due to its high stability, gold is a very popular material for colloidal templatebased synthesis. Its deposition can be carried out with the help of various baths:
slightly acidic HAuCl 4 solution [239, 252], complex bath with K 2 HPO 4 either as a
single additive [266] or combined with Na 2 SO 3 and other chelating agents [239], and
commercial bath formulations [264, 265]. The surface area of the inverse opal Au
structure can be conveniently measured by determining the charge for the reduction
of the oxide layer on Au formed during the anodic-going part of the CV curves [267].
393
i.e., the areas not completely covered by the colloidal templates. These cracks are
formed due to the shrinkage of the colloidal template upon drying, and their width
is several times larger than the diameter of the template-forming particles. It was
demonstrated that various metals either tend or do not tend to fill up these gaps,
while the interparticle space could be filled up continuously in both cases [260]. If
the metal overgrows the opal structure, its starts developing crystal shapes that are
characteristics of its deposition onto a flat substrate.
The controlled parameter during the electroplating of metals into a colloidal
template can be either the current or the electrode potential, and both are suitable for
tuning the deposition preference. As far as it can be established from the occasionally
published data, the deposition rate has to be chosen so that the deposition remains in
the kinetically controlled range and not diffusion-limited. Since the diffusion length
between the colloidal particles can be quite long for multilayered templates, the
kinetic control practically means low current densities. Hence, the fill-up time of a
multilayered template with a handful colloidal particle layers may range from hours
to tens of hours [240, 261, 262], a period about two orders of magnitude longer
than that normally required for the formation of equally thick continuous coatings.
Although a few works are available that present I(t) curves for potentiostatic metal
deposition into colloidal templates [240, 244, 255, 256, 262–264], their results are
quite controversial, so much that even the classification of the I(t) curves published
is challenging. For multilayered colloidal templates, some works claim that the I(t)
curves exhibit an oscillatory character, corresponding to the actual active surface
area of the growing metal in the interparticle space [244, 262, 263]. Even though a
very regular periodicity of the current maxima and minima was presented [263], the
variation of the mean current makes it very dubious that the oscillation can be quantitatively related to the growth phase. Another typical observation is that the current
tends to increase slightly as the growth front passes a colloid particle layer [255,
256]. However, even the slowly growing mean current cannot be taken as a general
observation since decreasing currents [263], followed by a constant-current period
after a long time [240], have also been reported. The character of the current transient is rather clear for monolayer templates [264, 265] where the smallest cathodic
current can be associated with the growth phase when the deposit reaches the radius
of the colloidal particles.
Below, a short summary is presented with metals (elements and alloy families)
whose inverse opal structure was extensively synthesized. The specific features that
make the deposit useful are discussed together with the metal types.
Gold. Due to its high stability, gold is a very popular material for colloidal templatebased synthesis. Its deposition can be carried out with the help of various baths:
slightly acidic HAuCl 4 solution [239, 252], complex bath with K 2 HPO 4 either as a
single additive [266] or combined with Na 2 SO 3 and other chelating agents [239], and
commercial bath formulations [264, 265]. The surface area of the inverse opal Au
structure can be conveniently measured by determining the charge for the reduction
of the oxide layer on Au formed during the anodic-going part of the CV curves [267].