Synthesis and Alignment of Nanostructured Materials Using Liquid Crystals - Self-Assembled Supramolecular Architectures: Lyotropic Liquid Crystals - page 196

LC medium, while the hosting systems act as tunable solvents for the nano-
materials, and, being anisotropic media, they can provide a controlled align-
ment and self-assembly for the nanomaterials into larger organized structures
in multiple dimensions (Hegmann et al., 2007; Kumar, 2007).
Furthermore, since the LC medium responds to external stimuli, the dis-
persed nanomaterials can be forced to track the order of the host medium,
which develops the functionality of the host system. Diverse LC phases have
been successfully used as templates for the synthesis of nanostructure materi-
als, either by producing porous inorganic replicates of LLC origin (termed as
direct templating) or by using LC mesophases as nanoreactors to produce the
corresponding morphologies (reverse templating) (Attard et al., 1995, 1997;
Ding et al., 2000).
7.2.2.1 Direct Templating Method (Nanocasting) The direct templating
procedure enables the formation of porous materials exhibiting uniform pore
size, morphology, and three-dimensional (3D) distribution. Additionally, these
structural characteristics as well as their macroscopic shape can be adjusted
by modifying the composition of the liquid crystalline systems. However, it
should be noted that this process is restricted to the formation of frameworks
that can remain stable even after surfactant removal and when exposed to air
(Schü th 2001 ).
This technique is termed true LC templating and nanocasting and is widely
used in the production of porous materials used in catalysis or adsorption
techniques (Antonietti and Ozin, 2004; Attard et al., 1995, 1997; Landskron
and Ozin, 2004).
Generally, in this approach, the continuous phase of the direct liquid crystal-
line structure (Fig. 7.5; Antonietti, 2006) may either be composed of a siliceous
material or an aqueous metal salt. In the next step, the metal salt will be
reduced, thus resulting in an ordered network of the surrounding host medium.
Figure 7.5 Illustration of a typical templating process using lyotropic liquid crystal-
line systems (in this case with a hexagonal array). The LLC phase is surrounded with
a high concentration of inorganic precursor (left), and the continuous phase is solidi-
fi ed (middle). After solidifi cation of the inorganic phase, the template is removed,
and a solid negative replica of the original phase is obtained (right). [Adapted from
Antonietti (2006) .]