Effects of Oxygen Vacancies and Crystal Distortions on the Oxygen Reduction Reactivity of Titanium Oxide Model Electrodes

This study demonstrates the effects of oxygen vacancies and crystal distortion on the oxygen reduction reaction (ORR) activity of titanium oxide nanosheet (TiO2ns) electrodes in acidic environments. The TiO2ns were obtained by treating exfoliated Ti0.91□0.09O20.36– nanosheets, and their oxygen vacancies and crystal distortion were modified via heat treatment under hydrogen gas flow conditions. The morphology and crystal structure of the TiO2ns were evaluated by using X-ray, microscopic, and spectroscopic characterization techniques. At reduction temperatures below 600 °C, the TiO2ns crystal structure retained a lepidocrocite phase and oxygen vacancies were introduced; a phase transition occurred above 600 °C, accompanied by crystal distortion. A relatively high onset potential for the ORR was observed in samples treated at reduction temperatures between 500 and 700 °C. This trend differed from the variation in Ti valence, which can reflect oxygen vacancies, and from the variation in the degree of crystal distortion with increasing temperature. Overall, both oxygen vacancies and crystal distortion enhanced ORR activity to a similar degree. Notably, the changes in the ORR currents at 0.05 V were closely correlated with changes in the degree of crystal distortion, suggesting that the number of active sites on the TiO2ns surface depends on the reduction temperature. Another key factor influencing the ORR activity of TiO2ns was the onset of the phase transition to an anatase structure. The active sites likely shifted from oxygen vacancies to crystal distortions near this phase transition point, leading to the high activity observed between 500 and 700 °C. However, the phase transition triggered a loss of oxygen vacancies, making it challenging to maintain uniform control across all TiO2ns on an electrode. This report proposes a design strategy for titanium dioxide catalysts based on maximizing the number of oxygen vacancies while avoiding crystal phase transitions.