The growth of anatase bipyramidal crystals during hydrothermal synthesis

The morphology, size and growth of anatase TiO2 crystals have been studied using a transmission electron microscope (TEM) and X-ray powder diffraction (XRD). The material was prepared by hydrothermal synthesis from aqueous suspension of amorphous TiO2 at temperatures from 40 to 200 degrees C and times up to 200 h. The size of the crystals increased with the synthesis temperature and time from around 6 to 20 nm. From high-resolution TEM images shape of crystals was reconstructed. It was found that in the initial stages of growth anatase crystals adopt uncommon morphologies, which were combination of basic crystallographic forms typical for anatase, however these forms were not developed symmetrically as determined by the surface energy values for these particular planes. The asymmetry and abnormal growth into rod-like and wedge-shaped crystals was most probably a consequence of inhomogeneous supply of building material due to high competition among numerous (relatively) rapidly growing randomly oriented nanocrystals. This leads to highly non-equilibrium conditions in the initial growth stages. Close to equilibrium at higher temperatures and times all crystals tend to adopt simple bipyramidal morphology. (C) 2012 Elsevier B.V. All rights reserved.