Direct continuous hydrothermal synthesis of high surface area nanosized titania

Nanosized TiO2 powder of high surface area was prepared from an aqueous solution of titanium(IV) bis(ammonium lactato) dihydroxide using a continuous hydrothermal flow synthesis (CHFS) reactor which uses superheated water at 400 °C and 24.1 MPa as a crystallizing medium. Freeze-dried nano-TiO2 was heat-treated in air over a range of temperatures and then the resulting powders were characterized using X-ray diffraction (XRD), high-resolution transmission electron microscopy (HR-TEM), BET surface area measurement, and Raman spectroscopy. The particle size of the ‘as-prepared’ TiO2 made using CHFS was ca. 4.8 nm (by HR-TEM), and grew with increasing heat-treatment temperature. It was found that the onset of the anatase–rutile transition of heat-treated nano-TiO2 in air occurred at 500 °C and reached 100% rutile at 900 °C. The Raman band (Eg) at ∼150 cm−1 of anatase (nano-TiO2) softens as the particle size increases with heat-treatment temperature (up to 600 °C). The photocatalytic activity of the TiO2 powders for the decolourisation of methylene blue dye was assessed. The effects of nano-TiO2 anatase-rutile phase composition, crystallinity, and crystallite size on the catalytic activity were investigated.