404
11 Templated Systems
in the double layer capacitance) when potassium ferricyanide was reduced [328], the
porous microelectrodes proved to be excellent amperometric sensors for a number of
compounds with much larger linearity range than either flat or even platinized platinum electrodes of identical geometric surface area. Examples range to hydrogen
peroxide [330], ascorbic acid [331] and formic acid [332]. The unique combination
of the high temporal stability of the mesoporous structure, the high accuracy and
the good reproducibility of the measurements makes such electrode an outstanding
candidate for commercial amperometric sensors.
While pure Pt and Pt–Ru [334] mesoporous structures can be deposited and retain
their morphology, stable mesoporous Pt–Au alloys can be synthesized if Pt is the
majority component [333]. Hence, the composition of mesoporous deposits is not
arbitrary. The experience with the Pt–Au alloys is much in accord with that reported
for Cu, Ag, Au and Zn [335], namely, that the electrodeposition of the mesoporous
form of these metals was unsuccessful. The reason behind the instability of the
mesoporous structure of these metals can lie in their high self-diffusion coefficient
(see also Sect. 6.2 about the self-annealing process related to nanocrystalline Ag and
Cu).
The deposition of H 1 -e Pd is very similar to that of Pt and so is its stability.
However, in contrast to Pt, the peaks in the voltammogram of H 1 -e Pd is H 2 SO 4
solutions is much better resolved in the H-adsorption region than for bulk Pd samples
[336, 337], which was explained by the nanostructured morphology of H 1 -e Pd. The
formation of both the α and β hydride phases was observed for H 1 -e Pd at the corresponding cathodic potentials, and the hydrogen absorption took place much faster
than planar films due to the shorter solid phase diffusion pathways in the nanostructured electrodes. With a suitable in situ pre-filling of the porous Pd structure with
hydrogen so that the two hydride phases are in equilibrium, the Pd microelectrodes
can be used as low-response-time pH sensors [337]. H 1 -e Pd was shown to be suitable for combustion-based gas sensor in micromachined planar pellistors with high
temporal stability at 500 °C [338].
An interesting approach was applied to deposit multilayered metal coating with
the porosity of the H 1 lyotropic phase and by applying the double-bath method
[339]. A Ru layer was deposited first, and it was followed by a Pd layer after a
solution change. Interestingly, the nanoporosity of the deposit could be retained and
a morphologically homogeneous structure was achieved even if the template-forming
solution was very carefully removed from the nanopores, indicating that the Ru pores
served as seeding points for the nucleation of molecular columns once the second
solution was applied.
Oxides and hydroxides of various metals. The pursuit of obtaining mesoporous
metal hydroxides and oxides stems from the supercapacitor properties of these materials. The high porosity increases the accessibility of the entire active layer by the
electrolyte solution and the cyclability of the layer at a high rate. All supercapacitor
materials to be mentioned below proved to be stable under repeated cycling.
The synthesis of H 1 -e metal hydroxides was performed with the usual nitrate ionbased method with cathodic polarization in which the alkalization of the media leads
11 Templated Systems
in the double layer capacitance) when potassium ferricyanide was reduced [328], the
porous microelectrodes proved to be excellent amperometric sensors for a number of
compounds with much larger linearity range than either flat or even platinized platinum electrodes of identical geometric surface area. Examples range to hydrogen
peroxide [330], ascorbic acid [331] and formic acid [332]. The unique combination
of the high temporal stability of the mesoporous structure, the high accuracy and
the good reproducibility of the measurements makes such electrode an outstanding
candidate for commercial amperometric sensors.
While pure Pt and Pt–Ru [334] mesoporous structures can be deposited and retain
their morphology, stable mesoporous Pt–Au alloys can be synthesized if Pt is the
majority component [333]. Hence, the composition of mesoporous deposits is not
arbitrary. The experience with the Pt–Au alloys is much in accord with that reported
for Cu, Ag, Au and Zn [335], namely, that the electrodeposition of the mesoporous
form of these metals was unsuccessful. The reason behind the instability of the
mesoporous structure of these metals can lie in their high self-diffusion coefficient
(see also Sect. 6.2 about the self-annealing process related to nanocrystalline Ag and
Cu).
The deposition of H 1 -e Pd is very similar to that of Pt and so is its stability.
However, in contrast to Pt, the peaks in the voltammogram of H 1 -e Pd is H 2 SO 4
solutions is much better resolved in the H-adsorption region than for bulk Pd samples
[336, 337], which was explained by the nanostructured morphology of H 1 -e Pd. The
formation of both the α and β hydride phases was observed for H 1 -e Pd at the corresponding cathodic potentials, and the hydrogen absorption took place much faster
than planar films due to the shorter solid phase diffusion pathways in the nanostructured electrodes. With a suitable in situ pre-filling of the porous Pd structure with
hydrogen so that the two hydride phases are in equilibrium, the Pd microelectrodes
can be used as low-response-time pH sensors [337]. H 1 -e Pd was shown to be suitable for combustion-based gas sensor in micromachined planar pellistors with high
temporal stability at 500 °C [338].
An interesting approach was applied to deposit multilayered metal coating with
the porosity of the H 1 lyotropic phase and by applying the double-bath method
[339]. A Ru layer was deposited first, and it was followed by a Pd layer after a
solution change. Interestingly, the nanoporosity of the deposit could be retained and
a morphologically homogeneous structure was achieved even if the template-forming
solution was very carefully removed from the nanopores, indicating that the Ru pores
served as seeding points for the nucleation of molecular columns once the second
solution was applied.
Oxides and hydroxides of various metals. The pursuit of obtaining mesoporous
metal hydroxides and oxides stems from the supercapacitor properties of these materials. The high porosity increases the accessibility of the entire active layer by the
electrolyte solution and the cyclability of the layer at a high rate. All supercapacitor
materials to be mentioned below proved to be stable under repeated cycling.
The synthesis of H 1 -e metal hydroxides was performed with the usual nitrate ionbased method with cathodic polarization in which the alkalization of the media leads