Materials Chemistry and Physics 95 (2006) 275–279 Effects of aging on the kinetics of nanocrystalline anatase crystallite growth Hsing-I Hsiang ∗ , Shih-Chung Lin Department of Resources Engineering, National Cheng Kung University, Tainan, Taiwan, ROC Received 1 December 2004; received in revised form 2 June 2005; accepted 7 June 2005 Abstract In this study, the effects of aging on crystalline phases, crystallite sizes and agglomeration of TiO 2 gels were investigated using thermo- gravimetry (TG), differential thermal analysis (DTA), X-ray diffraction (XRD), BET specific surface area analyzer and transmission electron microscopy (TEM). The activation energies of anatase crystallite growth for aged and unaged gels were calculated to determine and explain the growth mechanism. TiO 2 aging treatment decreased the number for hydroxyl ions existing in the anatase, resulting in lowering the rate of surface area reduction and decreasing the amount of agglomeration after calcination. The activation energy values for crystallite growth in samples with aging treatment were greater than that found in unaged samples. In nanocrystalline anatase samples without aging treatment, the oriented attachment controlled the crystallite growth. However, grain boundary diffusion controlled the crystallite growth for nanocrystalline anatase samples with aging treatment. © 2005 Elsevier B.V. All rights reserved. Keywords: Crystallite growth; Titania oxide; Kinetics; Oriented attachment; Anatase 1. Introduction TiO 2 is widely used as a catalyst, pigment and in electronic materials [1–3]. However, these applications demand high purity titania with controlled crystallite size, definite phase composition and surface properties. Therefore, it is important to control the particle size, morphology and texture of the TiO 2 system. Recently, many different methods have been used to prepare nanosized TiO 2 powders [4–7]. Among these vari- ous methods, the thermal treatment of precursors obtained using sol–gel [4,5], TiCl 4 precipitation [6] and Ti(SO 4 ) 2 precipitation [7] have been widely investigated because of the easily controlled process. There are three well- known TiO 2 crystalline types: anatase, rutile and brookite. The phase transition of a-TiO 2 → r-TiO 2 takes place dur- ing calcination [8]. The phase transformation, which occurs during calcination, gives rise to a transformed ∗ Corresponding author. Tel.: +886 6 2757575x62821; fax: +886 6 2380421. E-mail address: hsingi@mail.ncku.edu.tw (H.-I. Hsiang). rutile powder that has undergone considerable aggrega- tion and grain growth [9]. The above characteristics are detrimental to the formation of nanosized TiO 2 pow- ders. The kinetics of anatase crystallite growth has been studied extensively. The activation energy values reported in the liter- ature for anatase crystallite growth are very diverse, ranging from 5 to 230 kJ mol -1 [10–15]. However, to the best of our knowledge, the disagreement between the previous reports has not been discussed. Aging is a process in which the gel physical properties can be changed as the result of polymerization, coarsen- ing and phase transformation [16]. The changes in the gel structure and properties that occur during aging have a pro- found effect on the subsequent sintering process [10]. Various studies have shown that the amorphous TiO 2 gels crystallize into anatase through the aging process [17,18]. These reports focused mainly on the aging effects the crystallization, spe- cific surface area and crystallite size of the TiO 2 powders. However, the aging treatment mechanism effects on the crys- tallite growth of TiO 2 powders during calcination have not been well understood. 0254-0584/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2005.06.019