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Fig. 5
The three-step process of “nanocasting”. First, a condensed LLC phase is formed,
with the inorganic precursors occupying the liquid (aqueous) phase. Second, the inor-
ganic molecules are solidified by some chemical or electrochemical means. Third, the LLC
template is removed, leaving a mesoporous structure. Partially reproduced with permis-
sion from [32].
©
2002 by the Royal Society of Chemistry
it has since been applied to other inorganic materials with functional proper-
ties, including metals and semiconductors, using a variety of surfactants and
LLC phases, including polymerized LLC phases [34-48].
Bulk semiconductor nanostructured networks of CdS have been prepared
in L and H
I
phases [33, 34]. The L phase was formed by a polyol amphiphile
shown in Fig. 6. This LLC phase was doped with Cd
2+
ions and then CdS was
precipitated by diffusion of H
2
S into the gel. This nanocomposite was found
to incorporate the amphiphile and metal in alternating layers (Fig. 6), closely
resembling the structure of biominerals (e.g., mother of pearl). Similarly, H
I
phases were prepared from an oligo(ethylene oxide) oleyl ether amphiphile
(Fig. 7) mixed with a solution of Cd
2+
.Again,H
2
S was added to precipitate
CdS and the resulting composite preserved the hexagonal symmetry (Fig. 7).
Thin films of nanostructured metals and semiconductors (e.g., Pt, Sn,
CdTe) can be prepared by electrodeposition of the metal ions doped into
the H
I
LLC phase [40, 47, 48]. Similar to the precipitation of CdS, these films
can retain the symmetry of the LLC template during the deposition. These
materials allow one to combine well-defined porous nanostructures, high
specific surface areas, electrical connectivity, fast electrolyte diffusion, and
good mechanical and electrochemical stability. With this approach, hexago-
nally structured semiconductor films of uniform thickness can be prepared.
Nanostructured thin films of this type are proposed to have relevance in catal-
ysis,batteries,fuelcells,capacitors,andsensors.
A recent example of a templated thin film is shown in Fig. 8 [48]. Ini-
tial experiments showed that the templated CdTe film generated a higher
closed-circuit current and open-circuit voltage than the non-templated film,
although the reasons for this were not entirely clear. Also, the role of the
surfactant, beyond simply aiding in organization, is not clear. However, the
surfactant used in templating the semiconductor is of special interest (Fig. 8),
given the phase stability required to retain the nanoscale features and orien-
tation of the replica. While oligo(ethylene oxide)-based surfactants undergo
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