Rapid Hydrothermal Preparation of Rutile TiO2 Nanoparticles by Simultaneous Transformation of Primary Brookite and Anatase: An in Situ Synchrotron PXRD Study

The formation mechanism and crystal growth of TiO2 in high-temperature high-pressure fluids were studied using HCl or H2SO4 as additives. In situ synchrotron radiation powder X-ray diffraction reveals that phase-pure rutile TiO2 nanoparticles can be formed using HCl as additive, whereas phase-pure anatase TiO2 is obtained when H2SO4 is used as additive. The supercritical (or near-critical) conditions provide a fast, one-step synthesis of rutile TiO2 nanoparticles and when using a 1:1 volume ratio of isopropanol-water as solvent at a temperature of 300 degrees C and a pressure of 25 MPa particles with an average particle size of about 22 nm are obtained in 20 min. A detailed analysis by sequential Rietveld refinements shows that the formation of rutile TiO2 occurs by a combined transformation of anatase and brookite TiO2. Analysis of the unit cell dimensions of the nanoparticles shows a lattice expansion with decreasing particle size for anatase prepared with H2SO4 medium and this may explain the stability of anatase particles that are significantly larger than their critical size.