A detailed insight into the preparation of nanocrystalline TiO₂ powders in supercritical carbon dioxide

This work reports detailed investigations for the preparation of nanostructured titania powders by a solvent-free sol-gel-derived process, operated in supercritical CO₂ (SC-CO₂) at high pressures (10-30 MPa) and large range of temperatures (373-823 K). Depending on the processing temperature, the reaction between Ti(OiPr)₄ and water performed in a single supercritical phase led to the formation of either amorphous (Ti(OH)₄ - titanium hydroxide) or crystalline (TiO₂ - titanium dioxide) nanostructured particles. Crystalline (anatase) mesoporous powders with high specific surface area were obtained directly in CO₂ solvent under supercritical conditions at temperatures as low as 523 K. The effect of hydrodynamic key process parameters such as stirring and water injection rate on both powder morphology and aggregation degree was also investigated in details. The optimized TiO₂ anatase powders exhibited attractive photocatalytic activity, with high potential for the degradation of water pollutants. (authors)