Polymorphism of nanocrystalline TiO 2 prepared in a stagnation flame : formation of the TiO 2 -II phase

• Main Authors: Manuputty, Manoel Y., Dreyer, Jochen A. H., Sheng, Yuan, Bringley, Eric J., Botero, Maria L., Akroyd, Jethro, Kraft, Markus
• Other Authors: School of Chemical and Biomedical Engineering
• Format: Article
• Language: English
• Published: 2019
• Subjects: Stagnation Flame; DRNTU::Engineering::Chemical engineering; TiO2
• Online Access: https://hdl.handle.net/10356/88848; http://hdl.handle.net/10220/47638
• Institution: Nanyang Technological University

A metastable “high-pressure” phase known as α-PbO2-type TiO2 or TiO2-II is prepared via a single-step synthesis using a laminar premixed stagnation flame. Three other TiO2 polymorphs, namely anatase, rutile and TiO2-B phases, can also be obtained by tuning the oxygen/fuel ratio. TiO2-II is observed as a mixture with rutile under oxygen-lean flame conditions. To the best of our knowledge, this is the first time that this phase has been identified in flame-synthesised TiO2. The formation of TiO2-II in an atmospheric pressure flame cannot be explained thermodynamically and is hypothesised to be kinetically driven through the oxidation and solid-state transformation of a sub-oxide TiO2−x intermediate. In this scenario, rutile is nucleated from the metastable TiO2-II phase instead of directly from a molten/amorphous state. Mixtures containing three-phase heterojunctions of anatase, rutile, and TiO2-II nanoparticles as prepared here in slightly oxygen-lean flames might be important in photocatalysis due to enhanced electron–hole separation.