Supercritical Propanol–Water Synthesis and Comprehensive Size Characterisation of Highly Crystalline anatase TiO 2 Nanoparticles

Abstract

Highly crystalline anatase TiO 2 nanoparticles have been synthesised in less than 1 min in a supercritical propanol–water mixture using a continuous flow reactor. The synthesis parameter space ( T, P, concentration) has been explored and the average particle size can be accurately controlled within 10–18 nm with narrow size distributions (2–3 nm). At subcritical conditions amorphous products are obtained, whereas a broad range of T and P in the supercritical regime gives 11–14 nm particles. At high temperature and pressure, the particles size increase to 18 nm. The nanoparticles have been extensively characterised with powder X-ray diffraction (PXRD), transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS) with excellent agreement on size and size distribution parameters. The SAXS analysis suggests disk-shaped particles with diameters that are approximately double the height. For comparison, a series of conventional autoclave sol–gel syntheses have been carried out. These also produce phase-pure anatase nanoparticles, but with much broader size distributions and at much longer synthesis times (hours). The study demonstrates that synthesis in supercritical fluids is a very promising method for manipulating the size and size distribution of nanoparticles, thus removing one of the key limitations in many applications of nanomaterials.