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Figure 7.4
Two possible pathways for the LC templating mechanism. [Adapted from
Ying et al., (1999) .]
elevated temperatures (
100°C) for long periods (24-144 h), thereby reaching
solid precipitation. The inorganic components (which were negatively charged
at the high pH used) mostly interact with the positively charged surfactant
ammonium head group and condense into solid organic-inorganic nanorods.
Then, after the product is fi ltered, washed, and dried, it is calcined at
≥
500 ° C
to burn off the surfactant and to produce the mesoporous material (Ying
et al., 1999).
The mechanistic pathway raised by the authors is that the inorganic materi-
als mediated the ordering of the surfactants into the hexagonal arrangement,
especially since the surfactant concentration was lower than the amount
required for the formation of a hexagonal LC. Chen et al. (1993) used in situ
nitrogen-14 nuclear magnetic resonance (
14
N - NMR) spectroscopy to evaluate
a similar product, MCM-41 silicate mesoporous material arranged in a hex-
agonal symmetry. According to their postulation, in the fi rst step, two to three
silicate precursor monolayers are deposited onto isolated surfactant micellar
rods (Fig. 7.4; Ying et al., 1999). The randomly oriented silicate-encapsulated
rods were further packed into the hexagonal structure. Finally, heating and
aging completed the condensation of the silicate nanorods. It was also dem-
onstrated that the structures and pore dimensions of mesoporous materials
can be controlled by the properties of the surfactants, such as surfactant chain
length and the solution chemical properties (Kresge et al., 1992).
Additional reports, using surfactant as a phase transfer agent, that is, as an
assistant in 1D growth that is not based on silicate materials, are also available
such as selenium and tellurium nanowires and nanorods, respectively (Gates
et al., 2000; Liu et al., 2003). Generally, by using this approach, the resulting
pore size is barely predictable, although it is highly reproducible. This is due
to the fact that the fi nal structural properties, including pore size, are develop-
ing during the fi nal condensation process.
∼
7.2.2
LC Structures as Direct Templates and Reverse Templates
Nanoscale particles usually do not induce signifi cant distortions in the LC
phases. Therefore, various nanomaterials have been dispersed and studied in
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