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

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