396
11 Templated Systems
11.3.3 Electrodeposition of Non-metallic Materials
into Particulate Templates
The principles of template formation and electrodeposition of non-metallic materials
are essentially the same as those of metals. Different features of the overall sample
preparation procedure may originate from the redox reaction taking place, the media
used and the post-electroplating treatment. However, for all system in which the
deposition of a metal oxide or hydroxide is the initial step, the maximum achievable
thickness may be limited by the relatively small conductivity of the metal hydroxide
plated. Hence, the cell voltage increases with time if the deposition is carried out
galvanostatically (and, vice versa, the deposition rate decreases with time at constant
electrode potential).
Cathodic deposition of metal oxides and hydroxides. The common feature of the
direct electrodeposition of several sorts of metal hydroxides and oxides is that the
reduction process does not modify the oxidation number of the metal ions; instead,
a reactant (mostly hydroxide ion) is produced for a precipitation reaction.
ZnO can be produced in colloidal template with the usual method of generating
Zn(OH) 2 that is spontaneously dehydrated and yields ZnO. The procedures used
differ in what reactant provides the alkalization of the solution (reduction of nitrate
to nitrite ions [285, 286], H 2 O 2 [287] or O 2 [288]). Nanostructured ZnO exhibits
unique optical emission and reflectance properties [285, 288].
The cathodic deposition of macroporous WO 3 can be carried out with acidic
Na 2 WO 4 solution containing H 2 O 2 [289, 290]. Hydrogen peroxide and hydroxonium
ions are reduced, and the WO 3 is finally deposited onto the cathode in a process of
complicated kinetics, involving also oxygen evolution. Macroporous WO 3 exhibits
improved electrochromic properties as compared to flat films.
The reduction of sodium permanganate provides an example when a metal oxide
is deposited cathodically with the oxidation number change of the metal ion. If MnO 2
is deposited in macroporous form from a KMnO 4 solution [291], the electrochemical capacitance of the resulting film is much larger than that of the corresponding
continuous film, and the capacity improvement is accompanied with an outstanding
cycling stability.
Another typical material that is deposited with the partial reduction of the metal ion
is Cu 2 O [292–295]. All works cited above adopted the well-known bath used for the
deposition of free-standing Cu 2 O (0.4 mol dm
–3 CuSO 4 , 3 mol dm
–3 lactic acid, pH
= 9–12). While the photoelectrochemical properties of thus obtained layers are noteworthy [293], a phenomenon not seen for other materials was observed for 3DOM
Cu 2 O materials. Namely, two different growth modes could be distinguished [292,
294]. On the one hand, especially for well-ordered templates, the deposit exhibits a
typical inverse opal structure. On the other hand, it may occur that the envelope shape
of the growing particle is identical to that observed for free growth, with its internal
structure being characteristic of the usual colloid-related porosity. An example for
the latter phenomenon is shown in Fig. 11.16. The size of the internally porous parti-
11 Templated Systems
11.3.3 Electrodeposition of Non-metallic Materials
into Particulate Templates
The principles of template formation and electrodeposition of non-metallic materials
are essentially the same as those of metals. Different features of the overall sample
preparation procedure may originate from the redox reaction taking place, the media
used and the post-electroplating treatment. However, for all system in which the
deposition of a metal oxide or hydroxide is the initial step, the maximum achievable
thickness may be limited by the relatively small conductivity of the metal hydroxide
plated. Hence, the cell voltage increases with time if the deposition is carried out
galvanostatically (and, vice versa, the deposition rate decreases with time at constant
electrode potential).
Cathodic deposition of metal oxides and hydroxides. The common feature of the
direct electrodeposition of several sorts of metal hydroxides and oxides is that the
reduction process does not modify the oxidation number of the metal ions; instead,
a reactant (mostly hydroxide ion) is produced for a precipitation reaction.
ZnO can be produced in colloidal template with the usual method of generating
Zn(OH) 2 that is spontaneously dehydrated and yields ZnO. The procedures used
differ in what reactant provides the alkalization of the solution (reduction of nitrate
to nitrite ions [285, 286], H 2 O 2 [287] or O 2 [288]). Nanostructured ZnO exhibits
unique optical emission and reflectance properties [285, 288].
The cathodic deposition of macroporous WO 3 can be carried out with acidic
Na 2 WO 4 solution containing H 2 O 2 [289, 290]. Hydrogen peroxide and hydroxonium
ions are reduced, and the WO 3 is finally deposited onto the cathode in a process of
complicated kinetics, involving also oxygen evolution. Macroporous WO 3 exhibits
improved electrochromic properties as compared to flat films.
The reduction of sodium permanganate provides an example when a metal oxide
is deposited cathodically with the oxidation number change of the metal ion. If MnO 2
is deposited in macroporous form from a KMnO 4 solution [291], the electrochemical capacitance of the resulting film is much larger than that of the corresponding
continuous film, and the capacity improvement is accompanied with an outstanding
cycling stability.
Another typical material that is deposited with the partial reduction of the metal ion
is Cu 2 O [292–295]. All works cited above adopted the well-known bath used for the
deposition of free-standing Cu 2 O (0.4 mol dm
–3 CuSO 4 , 3 mol dm
–3 lactic acid, pH
= 9–12). While the photoelectrochemical properties of thus obtained layers are noteworthy [293], a phenomenon not seen for other materials was observed for 3DOM
Cu 2 O materials. Namely, two different growth modes could be distinguished [292,
294]. On the one hand, especially for well-ordered templates, the deposit exhibits a
typical inverse opal structure. On the other hand, it may occur that the envelope shape
of the growing particle is identical to that observed for free growth, with its internal
structure being characteristic of the usual colloid-related porosity. An example for
the latter phenomenon is shown in Fig. 11.16. The size of the internally porous parti-