Crystallization Kinetics of Rutile Formation from Amorphous Titania Films

Abstract

ABSTRACTTitania films with two types of phase composition were sputter deposited on fused silica substrates: Type (I) amorphous+anatase+rutile and Type (II) amorphous+rutile. These films were subjected to various annealing procedures in air. We studied three relevant transitions: 1) amorphous→crystalline (anatase and rutile) at temperature <800 °C, 2) amorphous→mitile at an temperatures, and 3) anatase→rutile in the 750–800 °C temperature range.X-ray diffraction was used for phase identification and crystallographic orientation. The films had a preferred orientation, with (101) anatase and/or (110) rutile planes parallel to the substrate. The activation energy for the amorphous-to-rutile transformation was 0.3 eV. The anatase-to-rutile transformation occurring at 750 °C was modelled using the Avrami relation, which yielded an exponent of unity.From the results of this study, we propose a model for titania crystallization in which site saturation of rutile and/or anatase nuclei already exists in the as-deposited film. Further crystallization upon annealing occurs by one dimensional growth of these nuclei into the amorphous material. If no anatase seeds exist in the as-deposited material, then no anatase will form upon annealing, indicating again that no new nuclei are formed during annealing. In this manner, highly oriented rutile titania films can be produced at temperature below the bulk anatase-to-rutile transformation temperature (750 °C).