394
11 Templated Systems
If the diameter of the colloidal template particles is in the order of the wavelength
of the light, the reflectance spectrum of the inverse opal metal structure mostly
depends on the filling ratio of the monolayered template [268]. Inverse opal Au
layer is often applied in surface-enhanced Raman spectroscopy (SERS) combined
with electrochemistry. When the substrate is ITO and the strike layer (usually also
from gold, evaporated onto the ITO substrate) is thin enough, the electrode of high
surface area is transparent and can be conveniently studied with a light beam passed
through it. The adsorption of various organic materials can be effectively studied
with this technique. Besides, the resonance mode of SERS can be determined by the
filling ratio of the colloidal monolayer, which makes its deposition a versatile tool
to produce optically active materials [264, 266].
The transferability of the inverse opal metal layers from the electrode onto which
it was deposited to another substrate was demonstrated with gold. If the deposition
is carried out with an ITO substrate not coated with an adhesive layer prior to the
colloidal template formation, the inverse opal layer can detach spontaneously [239]
(the same effect was found for Ni deposits [254]). If a less noble metal is used first for
partly filling the colloidal template and then the deposition is continued with gold, the
first metal layer can be used as a sacrificial one after removing the colloidal particles
[267]. This procedure leads to self-supporting electrodeposited inverse opal layers.
Since the electrical resistance of such macroporous Au layers is highly determined by
the finite size effect, mostly by the “neck” parts of the porous layer, the resistance of
the layer is sensitive to adsorbed chemical species. For this reason, such free-standing
structures can be used as chemisorption-based resistive sensors [267].
Platinum group metals. Both Pt [265, 269] and Pd [257] macroporous layers
were deposited from relatively simple baths containing H 2 PtCl 6 and PdCl 2 , respectively (occasionally with some supporting electrolyte). While Pd tends to cover the
nanospheres conformally and to form hollow spheres, Pt seems to form an open structure. The primary application of the macroporous platinum metal structures is the
electrochemical oxidation of organic compounds, taking advantage of the enlarged
activity due to the enhanced electrode surface area.
Nickel, cobalt and various ferromagnetic alloys. Electrodeposition of nickel into
colloidal templates has been carried out with solutions of highly varying metal salt
concentrations. Application of conventional (i.e., Watts type) solution with c(Ni
2+ )
> 0.5 mol dm
–3 is widespread [240, 255, 256, 270, 271], but custom-made lowconcentration baths proved to be equally sufficient [254, 262]. Commercial nickel
baths also appear in a few works [261, 272]. Some properties of the macroporous Ni
films are similar to that of Au films of similar structure (i.e., filling ratio-dependent
reflectivity [272]). An advantage of Ni as compared to Au is that the macroporous
structure can be etched after the removal of the template-forming particles, which
allows one to apply thinner metal ligaments that would naturally belong to the original
template [272]. A mechanical study of macroporous Ni film showed that the hardness
is nearly proportional to the filling ratio [271].
11 Templated Systems
If the diameter of the colloidal template particles is in the order of the wavelength
of the light, the reflectance spectrum of the inverse opal metal structure mostly
depends on the filling ratio of the monolayered template [268]. Inverse opal Au
layer is often applied in surface-enhanced Raman spectroscopy (SERS) combined
with electrochemistry. When the substrate is ITO and the strike layer (usually also
from gold, evaporated onto the ITO substrate) is thin enough, the electrode of high
surface area is transparent and can be conveniently studied with a light beam passed
through it. The adsorption of various organic materials can be effectively studied
with this technique. Besides, the resonance mode of SERS can be determined by the
filling ratio of the colloidal monolayer, which makes its deposition a versatile tool
to produce optically active materials [264, 266].
The transferability of the inverse opal metal layers from the electrode onto which
it was deposited to another substrate was demonstrated with gold. If the deposition
is carried out with an ITO substrate not coated with an adhesive layer prior to the
colloidal template formation, the inverse opal layer can detach spontaneously [239]
(the same effect was found for Ni deposits [254]). If a less noble metal is used first for
partly filling the colloidal template and then the deposition is continued with gold, the
first metal layer can be used as a sacrificial one after removing the colloidal particles
[267]. This procedure leads to self-supporting electrodeposited inverse opal layers.
Since the electrical resistance of such macroporous Au layers is highly determined by
the finite size effect, mostly by the “neck” parts of the porous layer, the resistance of
the layer is sensitive to adsorbed chemical species. For this reason, such free-standing
structures can be used as chemisorption-based resistive sensors [267].
Platinum group metals. Both Pt [265, 269] and Pd [257] macroporous layers
were deposited from relatively simple baths containing H 2 PtCl 6 and PdCl 2 , respectively (occasionally with some supporting electrolyte). While Pd tends to cover the
nanospheres conformally and to form hollow spheres, Pt seems to form an open structure. The primary application of the macroporous platinum metal structures is the
electrochemical oxidation of organic compounds, taking advantage of the enlarged
activity due to the enhanced electrode surface area.
Nickel, cobalt and various ferromagnetic alloys. Electrodeposition of nickel into
colloidal templates has been carried out with solutions of highly varying metal salt
concentrations. Application of conventional (i.e., Watts type) solution with c(Ni
2+ )
> 0.5 mol dm
–3 is widespread [240, 255, 256, 270, 271], but custom-made lowconcentration baths proved to be equally sufficient [254, 262]. Commercial nickel
baths also appear in a few works [261, 272]. Some properties of the macroporous Ni
films are similar to that of Au films of similar structure (i.e., filling ratio-dependent
reflectivity [272]). An advantage of Ni as compared to Au is that the macroporous
structure can be etched after the removal of the template-forming particles, which
allows one to apply thinner metal ligaments that would naturally belong to the original
template [272]. A mechanical study of macroporous Ni film showed that the hardness
is nearly proportional to the filling ratio [271].