11.4 Soft Templates Formed by Molecular Self-assembly Processes
405
to the hydroxide precipitation. This route was feasible for the preparation of mesoporous Ni(OH) 2 [339, 340] and Co(OH) 2 [341–343]. Due to the precipitation-based
deposit formation, the process proved to be quite sensitive to the deposition conditions, most prominently, the electrode potential. The mesoporous Ni(OH) 2 deposits
were morphologically stable for moderate annealing that was a part of the optimization of supercapacitor performance. The annealing of Co(OH) 2 led to the formation
of mesoporous Co 3 O 4 with good electrochromic performance [343]. Supercapacitor
properties similar to H 1 -e Ni(OH) 2 can be achieved with H 1 -e Ni after oxidation in
alkaline solutions [344].
Mesoporous RuO x supercapacitor can be obtained with a combination of two
electrochemical steps, the deposition of mesoporous Ru and its subsequent oxidation
to RuO x [345, 346]. The specific surface area of the H 1 -e RuO x was more than
an order of magnitude larger than Ru-black obtained with conventional methods.
Another supercapacitor material, MnO 2 could be deposited anodically in a single
step from Mn
2+ salts [347].
11.4.3 Semiconductors Obtained with Hexagonal Lyotropic
Phases
The electrodeposition of several mesoporous semiconductor materials from the H 1
lyotropic phase has been reported (CdS [348], CdTe [348–350], PbTe [325], Cu 2 O
[351] and ZnO [351]). For CdTe, the band gap of the mesoporous form was found to
be exactly the same as the bulk one, while birefringence and photoelectrochemical
conversion efficiency improved as a result of the mesoporous structure. The surface
of the mesoporous CdTe obtained with the template-assisted deposition was smooth
and defect-free. However, this behaviour cannot be generalized since the band gap
of mesoporous Cu 2 O was higher than that of its bulk form, although similar to flat
electrodeposited Cu 2 O layers [351]. A much more pronounced band gap enhancement (“blue shift”) due to the mesoporous nature of the deposit was found for PbTe
[325]. All mesoporous semiconductor materials had a direct band gap. For ZnO, the
optical properties were strongly influenced by the cracked nature of the mesoporous
films formed [351], in contrast to the optimal smoothness observed for CdTe. The
explanation behind this difference may lay in the deposition potential of the specific
materials. CdTe is deposited from CdSO 4 and TeO 2 at potentials where no hydrogen
evolution takes place, and the gas evolution may be a significant factor where the ZnO
deposit formed (−1 V vs. Ag/AgCl) by precipitation from cathodically alkalinized
nitrate-containing media.
The H 2 phase of lyotropic liquid crystals was seldom used in electrodeposition
experiments. A study for the deposition of Cu 2 O applied sodium bis(2-ethylhexyl)
sulphosuccinate as the continuous phase with p-xylene and a CuCl 2 solution for the
inverse hexagonal phase [352]. Since no other component was applied (e.g., lactic
acid that is customary in Cu 2 O deposition processes to provide OH
− ions during its
405
to the hydroxide precipitation. This route was feasible for the preparation of mesoporous Ni(OH) 2 [339, 340] and Co(OH) 2 [341–343]. Due to the precipitation-based
deposit formation, the process proved to be quite sensitive to the deposition conditions, most prominently, the electrode potential. The mesoporous Ni(OH) 2 deposits
were morphologically stable for moderate annealing that was a part of the optimization of supercapacitor performance. The annealing of Co(OH) 2 led to the formation
of mesoporous Co 3 O 4 with good electrochromic performance [343]. Supercapacitor
properties similar to H 1 -e Ni(OH) 2 can be achieved with H 1 -e Ni after oxidation in
alkaline solutions [344].
Mesoporous RuO x supercapacitor can be obtained with a combination of two
electrochemical steps, the deposition of mesoporous Ru and its subsequent oxidation
to RuO x [345, 346]. The specific surface area of the H 1 -e RuO x was more than
an order of magnitude larger than Ru-black obtained with conventional methods.
Another supercapacitor material, MnO 2 could be deposited anodically in a single
step from Mn
2+ salts [347].
11.4.3 Semiconductors Obtained with Hexagonal Lyotropic
Phases
The electrodeposition of several mesoporous semiconductor materials from the H 1
lyotropic phase has been reported (CdS [348], CdTe [348–350], PbTe [325], Cu 2 O
[351] and ZnO [351]). For CdTe, the band gap of the mesoporous form was found to
be exactly the same as the bulk one, while birefringence and photoelectrochemical
conversion efficiency improved as a result of the mesoporous structure. The surface
of the mesoporous CdTe obtained with the template-assisted deposition was smooth
and defect-free. However, this behaviour cannot be generalized since the band gap
of mesoporous Cu 2 O was higher than that of its bulk form, although similar to flat
electrodeposited Cu 2 O layers [351]. A much more pronounced band gap enhancement (“blue shift”) due to the mesoporous nature of the deposit was found for PbTe
[325]. All mesoporous semiconductor materials had a direct band gap. For ZnO, the
optical properties were strongly influenced by the cracked nature of the mesoporous
films formed [351], in contrast to the optimal smoothness observed for CdTe. The
explanation behind this difference may lay in the deposition potential of the specific
materials. CdTe is deposited from CdSO 4 and TeO 2 at potentials where no hydrogen
evolution takes place, and the gas evolution may be a significant factor where the ZnO
deposit formed (−1 V vs. Ag/AgCl) by precipitation from cathodically alkalinized
nitrate-containing media.
The H 2 phase of lyotropic liquid crystals was seldom used in electrodeposition
experiments. A study for the deposition of Cu 2 O applied sodium bis(2-ethylhexyl)
sulphosuccinate as the continuous phase with p-xylene and a CuCl 2 solution for the
inverse hexagonal phase [352]. Since no other component was applied (e.g., lactic
acid that is customary in Cu 2 O deposition processes to provide OH
− ions during its