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