Synthesis and Alignment of Nanostructured Materials Using Liquid Crystals - Self-Assembled Supramolecular Architectures: Lyotropic Liquid Crystals - page 197

Finally, upon calcination a porous nanostructure is obtained retaining the
structure of the original liquid crystal. Taken together, this technique allows
rational prediction of the pore properties since it starts from an organized, LC
phase, in contrast to the former approach that utilized LC surfactant assembly
as a directing agent.
The production of silica-based mesoporous material was fi rst introduced by
Attard and co-workers in 1995. They synthesized a silica mesostructure based
on the H I LC templating process using polyethylene oxide surfactant (as was
generally described in Fig. 7.5). The tetramethylorthosilicate (TMOS) in the
continuous phase was gelled in the H I system, and then the LC was destabi-
lized and removed by methanol to produce the fi nal hexagonal mesoporous
material. It was further shown that the formation of Ia 3 and lamellar silica-
based materials was also possible by changing the tail chain length of the
surfactant (Antonietti, 2006; Attard et al., 1995).
Many types of silica-based (Doshi et al., 2003; Kresge et al., 1992; Thomas
et al., 2003) as well as nonsiliceous mesoporous nanomaterials (Kresge et al.,
1992; Lyons et al., 2002)—for example, metal oxides, CdS and CdSe compos-
ites, and Pt/Ru alloys—have been successfully synthesized through this method
using polymer or oligomer surfactant systems.
It has been shown that this technique offers the possibility to create an exact
1 : 1 negative copy of the origin LC phase. It has been suggested to replace a
majority of the solvent by a metal or metal oxide precursor with similar polar-
ity and condensing this precursor around the aggregates. For instance, the
precursor hydrated silicic acid shows a very similar polarity and proton-
bridging behavior as water, and therefore the fi nal structure is very similar to
that found in water (Antonietti, 2006). X-ray measurements performed during
the process showed that the fi nal structure of the mesoporous material pre-
served all structural features during the solidifi cation process, shrinking only
slightly on condensation (Attard et al., 1995, 1997).
Additionally, comparing transmission electron microscopy (TEM) images
of original branched lamellar LC phase with its replicate (Figs. 7.6a and 7.6b,
respectively; Hentze et al., 1999) reveals that one is the “negative” of the
other, while it is almost impossible to differentiate them without a priori
knowledge.
To conclude, it has been demonstrated that in this approach, the pores size
and shape are determined solely by the size and shape of the LC template that
preexists in solution prior to the solidifi cation process.
It was further demonstrated that upon addition of salt to the synthesis
mixture, the mesoporous silicates can be modifi ed from the original LC phase
(McGrath et al., 1997). McGrath et al. revealed that after the addition of
sodium chloride the mesoporous material deviated from the ideal hexagonal
structure to form a sponge phase (L 3 , randon bicontinuous cubic phase) of
mesoporous silicate (McGrath et al., 1997). Ethylenediaminetetraacetic acid
(EDTA) was also found to induce the formation of L 3 at the expense of hex-
agonal structures (McGrath et al., 2000).