Surfactant removal from mesoporous TiO2 nanocrystals by supercritical CO2 fluid extraction

In this study, mesoporous TiO2 crystals were synthesized with a sol–gel process in the presence of hexadecylamine which was used as a structure-directing agent. The neutral template was removed by either supercritical CO2 extraction (SCE) or conventional calcinations to produce mesopores, and the pore characteristics of mesoporous TiO2 were investigated with TGA, XRD, FT-IR, SEM and N2 sorption analysis. The results showed that hexadecylamine was completely combusted by a thermal treatment above 400 °C, whereas SCE successfully removed more than 62% of the surfactant with methanol used as a modifier. However, in contrast to SCE, calcination at 400 °C accompanied a reduction in surface area and pore volume, as well as the partial collapse of the mesostructure. The combination of SCE and subsequent heat treatment at 400 °C not only improved the porosity but also retarded the structure destruction due to much less exothermal behavior. The SCE approach provided mesoporous TiO2 nanocrystals with remarkably high mesoporosity. The SCE technique can result in numerous advantages such as high mesoporosity, short processing time, low energy consumption, and environmental benefit.