Synthesis of Nanostructured Titania Powders via Hydrolysis of Titanium Isopropoxide in Supercritical Carbon Dioxide

Abstract

Titania powders were synthesized via hydrolysis of titanium(IV) isopropoxide (TIP) in supercritical carbon dioxide (SCCD). Injection of TIP into water-in-CO2 (w/c) dispersions resulted in precipitation of spherical titania particles, and free-flowing white titania powders were isolated in 65−70% yield by slow isothermal depressurization. Qualitatively similar results were obtained with and without the addition of an anionic phosphate fluorosurfactant (DuPont Zonyl FSP) to stabilize the w/c dispersions. The titania powders had broad particle size distributions (20−800 nm) and specific surface areas in the 100−500 m2/g range. Addition of Zonyl FSP resulted in a decrease in specific surface area at a given water-to-alkoxide molar ratio (hydrolysis level). The specific surface area increased as the hydrolysis level was increased, irrespective of the presence of surfactant. The surface area is associated primarily with internal porosity of the spherical titania particles, as evidenced by scanning transmission electron microscopy and N2 porosimetry. Calcination of a surfactant-free titania powder at 300 °C in air decreased the specific surface area from ∼300 to 65 m2/g and increased the mean cylindrical pore diameter from 2.6 to 4.9 nm, consistent with collapse of micropores. Titania nanoparticle synthesis via TIP hydrolysis in SCCD was attempted using w/c microemulsions formed with ammonium carboxylate perfluoropolyether (PFPE-NH4); however, injection of TIP into PFPE-NH4-stabilized microemulsions resulted in precipitation of 0.3−2 μm titania particles. The use of other CO2-soluble titanium(IV) alkoxides gave qualitatively similar results.