Author's personal copy The growth of anatase bipyramidal crystals during hydrothermal synthesis Barbara Horvat a,b,n , Aleksander Rec ˇnik a,b , Goran Draz ˇic ´ a,b a Department for Nanostructured Materials, Joz ˇef Stefan Institute, Ljubljana, Slovenia b Joz ˇef Stefan International Postgraduate School, Ljubljana, Slovenia article info Article history: Received 13 January 2012 Received in revised form 16 February 2012 Accepted 13 March 2012 Communicated by D.P. Norton Available online 18 March 2012 Keywords: A1. Electron diffraction A1. Transmission electron microscopy A2. Hydrothermal crystal growth B1. Anatase B1. TiO 2 abstract The morphology, size and growth of anatase TiO 2 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 TiO 2 at temperatures from 40 to 200 1C 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. & 2012 Elsevier B.V. All rights reserved. 1. Introduction Nanocrystalline TiO 2 is one of the most studied oxides due to its extensive use in photocatalysis, solar-energy conversion, sensors, mesoporous membranes, food, cosmetics, pigments, etc. Physical properties of TiO 2 can be controlled by crystal modifica- tion, size and morphology. In nature TiO 2 exists in three different crystal modifications, i.e., as anatase (tetragonal crystal structure with 3.2 eV energy gap [1,2]), rutile (tetragonal with 3.02 eV energy gap [1,2]) and brookite (orthorhombic with 2.96 eV energy gap [1,2]). Among the three modifications rutile represents the most stable phase [3], as the other two modifications usually transform into rutile upon heating. It has been reported, however, that anatase pre- pared via hydrothermal synthesis at 200 1C remains as a stable crystal form during heating in air at least up to 850 1C [4], at least up to 800 1C and below 900 1C [5], when hydrothermally treated at 160 1C anatase is stable at least up to 700 1C and below 750 1C, when doped with Si, it is stable at least up to 800 1C [6]. If the thermal energy is too high for anatase crystal modification, TiO 2 will crystallize in form of brookite or even rutile. Using suitable conditions (i.e., when the minimal free energy for anatase is still higher than that for rutile [7]) it is possible to grow fairly large anatase crystals, similar to those observed in nature. During the hydrothermal synthesis the final crystal size depends on the available amount of building material (TiO 2 ) and the total dura- tion of growth conditions including: p  T product (pressure times temperature), pH of the solution, and the type of solvents and dopants that assist the crystal growth. When preparing TiO 2 via hydrothermal synthesis from Ti (IV) isopropoxide in water or ethanol, the product is always in anatase form with a small amount of brookite due to the growth in acidic conditions [8]. There are numerous papers concerning the crystal growth of TiO 2 via hydrothermal synthesis for which various growth models were proposed. Cho et al. [9] proposed a growth model for bipyramidal anatase crystals, synthesized in alkaline conditions at 200 1C for 2–16 h and at 240 1C for 32–64 h. They suggested that anatase crystals grow by oriented attachment along the [001] direction which is followed by thickening with Ostwald’s ripening. They observed that the longer the time and the higher the temperature of hydrothermal synthesis, the more defined and larger the final bipyramidal crystals are. Deng et al. [10] described the growth of large bipyramidal anatase single crystals in the range of a few microns produced by hydrothermal synthesis at 200 1C for 48 h at pH values between 5.6 and 11.0. They observed that formed anatase nuclei had truncated bipyr- amidal morphology. The fastest growth rate was observed on the high-energy {001} faces, and the crystal elongation was observed along the /001S directions, which eventually resulted in the Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/jcrysgro Journal of Crystal Growth 0022-0248/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jcrysgro.2012.03.027 n Corresponding author at: Department for Nanostructured Materials, Joz ˇef Stefan Institute, Ljubljana, Slovenia. Tel.: þ386 1 477 39 31; fax: þ386 1 477 32 21. E-mail address: barbara.horvat@ijs.si (B. Horvat). Journal of Crystal Growth 347 (2012) 19–24