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

A metastable “high-pressure” phase known as α-PbO(2)-type TiO(2) or TiO(2)-II is prepared via a single-step synthesis using a laminar premixed stagnation flame. Three other TiO(2) polymorphs, namely anatase, rutile and TiO(2)-B phases, can also be obtained by tuning the oxygen/fuel ratio. TiO(2)-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 TiO(2). The formation of TiO(2)-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 TiO(2–x) intermediate. In this scenario, rutile is nucleated from the metastable TiO(2)-II phase instead of directly from a molten/amorphous state. Mixtures containing three-phase heterojunctions of anatase, rutile, and TiO(2)-II nanoparticles as prepared here in slightly oxygen-lean flames might be important in photocatalysis due to enhanced electron–hole separation.