Effect of Nuclei on the Formation of Rutile Titania

Abstract

It is well known that titania (TiO2) can exist in the following three crystalline modi®cations: rutile (tetragonal), anatase (tetragonal), and brookite (orthorhombic). Rutile is the only stable form, whereas anatase and brookite are metastable and transform to rutile when heated. All of these crystalline forms of TiO2 occur in nature as minerals, but only the TiO2 in anatase form is synthesized at low temperatures. Various studies on the anatase-to-rutile transformation with an emphasis on the transformation temperature have been discussed [1±4]. Anatase-torutile transformation is a metastable-to-stable transformation, and there is no real phase transformation temperature as there is for an equilibrium-reversible transformation. The temperature at which this transformation occurs depends on many factors: the impurities present in the powders, the method of preparation, the oxygen-to-metal coordination in the precursor, and even particle size or surface area of the powders [5±15]. The anatase-to-rutile transformation temperature reported in the literature ranges from 400 to 1200 8C. During the process of transformation, the particle size of the powders in anatase form increases on calcining at ®rst, the anatase-to-rutile transformation occurs until the particle size of powders reaches a certain size. In this paper, TiO2 in rutile form was directly obtained by adding SnO2 as rutile-forming nuclei, avoiding the appearance of TiO2 in anatase form. In the process of cohydrolysis, SnO2 will hydrolyze ®rst and act as nuclei for the growth of the rutile phase of titania. The a and c values of the unit-cell parameters of SnO2 and R-TiO2 are not too different. For SnO2, a ˆ 0:4737 and c ˆ 0:3186 nm; and for TiO2 (rutile), a ˆ 0:4593 and c ˆ 0:2958 nm [16]. SnCl2 and SnCl4 are both selected as raw materials of SnO2. A solution of Ti(SO4)2 and SnCl2 (or SnCl4) was added to a vessel. The concentration of Ti(SO4)2 in the solution was 1 mol ly1. The amount of SnCl2 (or SnCl4) in the solution was varied to investigate the effect on the rutile phase formation. Ammonia with concentration of 2 mol ly1 was added dropwise into the vessel while stirring at room temperature. At the end of the precipitation, the pH of the mixing liquor was adjusted to 8 9. The resulting precipitates were centrifugally separated from the mother liquor and washed several times with distilled water to remove SO2y 4 and Cl y. Then the precipitates were divided into two parts: one was dried at 80 8C in a vacuum oven for further analysis and comparison; the other was redispersed in an aqueous solution of HNO3 for peptizing, resulting in a Ti 4‡=H‡ ratio of ca. 0.4. This dispersion was re uxed at 80 8C for 5 h. Rutile TiO2 was obtained by peptized precipitate drying in an oven. TiO2 in rutile form was achieved in the asprecipitated stage by the addition of SnO2 as rutileforming nuclei. The amount of SnO2 added in the powders directly in uences the structure of resulting powders. Fig. 1 and Fig. 2 show XRD patterns of peptized TiO2 powders containing different amounts of SnO2, with SnCl2 and SnCl4 as raw materials, respectively (denoted by TS-AN and TS-BN, where N is the molar percent of SnO2 in the samples). The peaks correspond to the anatase form of titania at a low amount of SnO2, while all the peaks correspond to the rutile form of titania when the level of SnO2 reaches a certain amount. The range of the amount of SnO2 in this change is 1y10 mol % of TS-AN and 4y8 mol % of TS-BN. Second, the line width of the XRD patterns of TS-BN becomes wider with the increase of the addition of SnO2, indicating that the particle size of powders decreases with an increase of the amount of SnO2; however, the particle sizes of TS-AN change little. We have concluded that crystalline nuclei of the rutile form of TS-BN appeared in the course of