Figure 6. Schematic illustration of the ice-assisted
hierarchical self-assembly approach of CNCs in isotropic and anisotropic
phases, respectively.
To further explore the feasibility of this fabrication approach,
attempts were made to fabricate silica aerogels using the chiral nematic
liquid crystalline CNCs as template. The preparation method was similar
to that used to prepare the CNC aerogel (see Supporting Information for
details). The CNC template was removed by calcination, resulting in a
pure free-standing silica aerogel, with affordable collapse. The
conversion yield of hybrid aerogel into silica aerogel was approximately
20%, as determined by thermogravimetric analysis (TGA, Figure S7). POM
imaging showed strong birefringence, indicative of the anisotropic
property of the silica aerogel (Figure 7a). The long-range orientational
ordering of CNCs was essentially retained in the silica aerogel (Figure
7b and Figure S8), which featured smooth surfaces combined with
repeating layered structures perpendicular to the surface, as observed
in the pure CNC aerogel. The mesoporosity of the silica aerogel was
evidenced in the high magnification TEM image (Figure 7c), and further
confirmed by the nitrogen adsorption-desorption isotherms (Figure 7d).
The mesopores were locally aligned (Figure 7c), which was consistent
with the local nematic organization of the CNC template. The silica
aerogel also showed a type IV adsorption-desorption isotherm with a H2
hysteresis loop in the range of P/P0 = 0.4-0.7 (Figure
7d). The corresponding Brunauer-Emmett-Teller (BET) surface area and
Barret-Joyner-Halenda (BJH) average pore diameter of the aerogel were
300 m2·g−1 and 7.2 nm, respectively.
The t-plot external surface area was approximately 480
m2·g−1, indicating both micro- and
mesoporosity.50