Cellulose-silica nanocomposite aerogels by in situ formation of silica in cellulose gel.

Aerogels with their low density (0.004–0.500 gcm ), large internal surface area, and large open pores are promising candidates for various advanced applications. The utilization of inorganic aerogels, however, has been hampered by their poor mechanical properties. A prominent example is silica aerogel, which is prepared by an organic sol–gel process, and has unique features, such as ultralow density (the lightest silica aerogel has a density that is similar to the density of air, which is 0.00129 gcm ), near transparency, and low thermal conductivity. However, the extreme fragility of this aerogel necessitates its reinforcement for practical uses. A typical method is hybridization with organic polymers, such as polyurea, polyurethane, poly(methyl methacrylate), polyacrylonitrile, and polystyrene. Other candidates for the reinforcement of inorganic aerogels are insoluble polysaccharides, which are abundantly available and show wide varieties in structure and properties. The useful features of these compounds are hydrophilicity, biocompatibility, hydroxy reactivity, and reasonable thermal and mechanical stabilities. For example, nanofibrillar bacterial cellulose and microfibrillated cellulose gel have been proposed as templates for cobalt ferrite nanoparticles and titanium dioxide. While in the above-mentioned work native cellulose with cellulose I crystallinity was used, cellulose can be prepared as a hydrogel with cellulose II crystallinity through dissolution and coagulation. Some of the resulting aerogels have remarkable mechanical strength and light transmittance. They have high porosity with open structures and thus provide an effective substrate for the synthesis of metallic nanoparticles. To further utilize the regenerated cellulose gel, we herein attempted in situ synthesis of silica in cellulose gels. While a similar attempt has been reported, in which the cellulose gel was obtained from solution in N-methylmorpholine-N-oxide monohydrate, the development of the nanostructure (nitrogen BET surface area of 220–290 mg ) and the level of silica loading (less than 13% w/w) were rather limited. By using the aqueous alkali-based solvent, we obtained the cellulose aerogel with a surface area of 356 mg , and a silica loading of more than 60% w/w resulted in surface areas that exceeded 600 mg . We used the sol–gel synthesis method toward nanostructured silica, which typically starts from tetraethyl orthosilicate (TEOS). The resulting composite gels were dried with supercritical CO2 to give cellulose–silica aerogels with low density, moderate light transmittance, a large surface area, high mechanical integrity, and excellent heat insulation. This method can also lead to fabrication of silica-only aerogels through the removal of cellulose by calcination, that is, the use of cellulose aerogel as sacrificial template. Figure 1 shows the preparation of the aerogel. The cellulose hydrogel is a transparent material that has a water content of 92% and a porosity of 95%. The sol–gel process catalyzed by ammonia converts TEOS to SiO2, which is deposited on the cellulose network (Figure 1b). The composite is converted to an aerogel by drying with supercritical CO2 to maintain the porous structure (Figure 1c), thus resulting in a flexible and translucent cellulose–silica aerogel. Subsequent calcination removes the cellulose matrix to give a silica-only aerogel (Figure 1d and g). The cellulose aerogel is composed of regenerated cellulose fibrils, which are typically less than 10 nm wide (Figure 2a). The BET surface area of 356 mg 1 (determined by