Dynamic phase behavior during sol-gel reaction in supercritical carbon dioxide for morphological design of nanostructured titania

•Nanostructured titania is fabricated by sol-gel reaction in supercritical CO2.•Dynamic phase behavior during sol-gel reaction in supercritical CO2 is studied.•Swelling of reaction liquid phase affects consequent morphology of titania.•Hollow urchin titania is fabricated by phase separation of CO2 and water.

Dynamic phase behavior during sol-gel reaction in supercritical carbon dioxide is investigated for a morphological design of titania. The sol-gel reactions are conducted using acetic acid (HAc) and titanium tetrabutoxide (TBO) as the precursor of titania at 313–353 K and 20.0 MPa. It is demonstrated the effects of the precursor concentration and temperature on the swelling ratio of the reaction phase and the formation of the homogeneous phase during the reaction in supercritical carbon dioxide. Nano-spherical titania is formed at the condition of lower precursor concentration and temperature 313 K that leads to the larger swelling ratio of the reaction liquid phase. The titania with a hollow urchin-shaped is obtained at 333 K with the higher precursor concentration. It is implied that this unique structured titania is produced by the phase separation between supercritical carbon dioxide and water phases which is the byproduct on the reaction.

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