Flame Synthesized Blue TiO2- x with Tunable Oxygen Vacancies from Surface to Grain Boundary to Bulk.

Fabrication of nonstoichiometric metal oxides containing oxygen vacancies (OVs) has been an effective strategy to modulate their (photo)catalytic or (photo)electrochemical performances which are all affected by charge transfer at the interface and in the bulk. Considerable efforts are still needed to achieve tunability of OVs, as well as their quantitative characterization. Herein, a one-step flame synthesis method is reported for the first time for fast fabrication of blue TiO2- x with controllable defect content and location. Temperature-programmed oxidation (TPO) analysis is applied for the first time and found to be an excellent technique in both differentiating and quantifying OVs at the surface, grain boundary (GB), and bulk of TiO2- x . The results indicate that a moderate level of OVs can greatly enhance the charge transfer. Importantly, the OVs locked at GBs due to the thermal sintering of nanoparticles during the synthesis can facilitate the anchoring and reduction of Pt species.

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

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.