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 (similar to 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 (similar to 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.