From Titanium Sesquioxide to Titanium Dioxide: Oxidation-Induced Structural, Phase, and Property Evolution

In contrast to Ti⁴⁺-containing titanium dioxide (TiO₂), which has a wide bandgap (∼3.0 eV) and has been widely explored for catalysis and energy applications, titanium sesquioxide (Ti₂O₃) with an intermediate valence state (Ti³⁺) possesses an ultranarrow bandgap (∼0.1 eV) and has been much less investigated. Although the importance of Ti³⁺ to the applications of TiO₂ is widely recognized, the connection between TiO₂ and Ti₂O₃ and the transformation pathway remain unknown. Herein, we investigate the oxidation-induced structural, phase, and property evolution of Ti₂O₃ using a complementary suite of microscopic and spectroscopic tools. Interestingly, transformation pathways to both rutile and anatase TiO₂ are identified, which sensitively depend on oxidation conditions. Unique Ti₂O₃/TiO₂ core–shell structures with annealing-controlled surface nanostructure formation are observed for the first time. The compositional and structural evolution of Ti₂O₃/TiO₂ particles is accompanied by continuously tuned optical and electrical properties. Overall, our work reveals the connection between narrow-bandgap Ti³⁺-containing Ti₂O₃ and wide-bandgap Ti⁴⁺-containing TiO₂, providing a versatile platform for exploring photoelectrocatalytic applications in valence- and structure-tailored oxide materials