Polymorphism of nanocrystalline TiO2 prepared in a stagnation flame: formation of the TiO2-II phase

Manoel Y Manuputty; Jochen A H Dreyer; Yuan Sheng; Eric J Bringley; Maria L Botero; Jethro Akroyd; Markus Kraft

doi:10.1039/c8sc02969e

Polymorphism of nanocrystalline TiO₂ prepared in a stagnation flame: formation of the TiO₂-II phase

Chem Sci. 2018 Nov 14;10(5):1342-1350. doi: 10.1039/c8sc02969e. eCollection 2019 Feb 7.

Authors

Manoel Y Manuputty^{1

2}, Jochen A H Dreyer^{1

2}, Yuan Sheng^{2

3}, Eric J Bringley¹, Maria L Botero^{2

4}, Jethro Akroyd^{1

2}, Markus Kraft^{1

2

3}

Affiliations

¹ Department of Chemical Engineering and Biotechnology , University of Cambridge , West Site, Philippa Fawcett Drive , Cambridge , CB3 0AS , UK . Email: mk306@cam.ac.uk.
² Cambridge Centre for Advanced Research and Education in Singapore (CARES) , CREATE Tower, 1 Create Way , 138602 , Singapore.
³ School of Chemical and Biomedical Engineering , Nanyang Technological University , 62 Nanyang Drive , 637459 , Singapore.
⁴ Department of Mechanical Engineering , National University of Singapore , 9 Engineering Drive 1 , 117575 , Singapore.

Abstract

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 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 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.