Supercritical CO₂ for Making Nanoscale Materials

Nanoporous metals and oxides were synthesized by the templating process using supercritical solvents, which is called nanoscale casting using supercritical fluids (NC−SCF). Precursors dissolved in supercritical CO 2 were attached to and coated on activated carbon templates. After removing the activated carbon templates, nanoporous materials replicating both the porous structures on a nanometer scale and the macroscopic shapes of the templates can be obtained. The surface areas of these nanoporous materials were much higher than those of the materials synthesized by a templating process using liquid solvents. The SCFs were also used for the synthesis of nanoparticles in microporous and mesoporous silicas with uniform pore sizes, FSM-16 (1.6−3.5 nm diam). Nanoparticles could be impregnated into the micropores and mesopores of FSM-16 by impregnation in a supercritical fluid solvent. On the other hand, nanoparticles could not be introduced into the smaller pores of FSM-16 by impregnation using liquid solvents. These results are based on the penetration difference into the nanoporous structures by supercritical solvents and liquid solvents.