Authors: Manoel Y. Manuputty, Jochen Dreyer, Yuan Sheng, Eric J. Bringley, Maria Botero, Jethro Akroyd, and Markus Kraft*

• Anatase, brookite, rutile, and TiO₂-II were synthesised with a stagnation flame by tuning the oxygen/fuel ratio.
• An anatase/TiO₂-II/rutile three-phase heterojunction is formed in slightly oxygen-lean flames (Φ=1.1-1.3), and is potentially of interest for photocatalytic applications.
• The formation of the metastable TiO₂-II phase cannot be explained thermodynamically. This suggests a kinetically driven transformation, possibly with a sub-oxide intermediate.

A metastable "high-pressure" phase known as the 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. The TiO₂-II is observed as a mixture with rutile in oxygen-lean flame conditions. To the best of our knowledge, this is the first time 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 TiO_2-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 heterojunction 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.

• This paper draws from preprint 200: Polymorphism of nanocrystalline TiO₂ prepared in a stagnation flame: Formation of TiO₂-II phase
• Access the article at the publisher: DOI: 10.1039/C8SC02969E

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