Direct synthesis of zirconia aerogel nanoarchitecture in supercritical CO2

The objective of the present study was to synthesize porous ZrO2 aerogels with a nanostructure via a direct sol-gel route in the green solvent supercritical carbon dioxide (seCO(2)). The synthesis involved the coordination and polycondensation of a zirconium alkoxide using acetic acid in CO2, followed by scCO(2) drying and calcination. Either a translucent or opaque monolith was obtained, which was subsequently characterized by electron microscopy, X-ray diffraction, thermal analysis, N-2 physisorption, and infrared spectroscopy analysis. The electron microscopy results showed that the translucent monolithic ZrO2 exhibited a well-defined mesoporous structure, while the opaque monolith, formed using added alcohol as a cosolvent, was composed of loosely compacted nanospherical particles with a diameter of ca. 20 nm. After calcination at 400 and 500 C, X-ray diffraction results indicated that the ZrO2 exhibited tetragonal and/or monoclinic phases. In situ infrared spectroscopy results showed the formation of a Zr-acetate coordinate complex at the initial stage of the polycondensation, followed by further condensation of the complex into macromolecules.