Elucidating the Role of Interstitial Oxygen in Transparent Conducting Anatase TiO₂ by Polarized X-ray Absorption Spectroscopy Study

In titanium dioxide (TiO2), oxygen vacancies- and cation interstitials-related functionalities have widely been studied in various fields, such as catalysis, sensors, and electronic devices. Anatase Nb-doped TiO2, a promising transparent conducting oxide material, shows a reversible metal–insulator transition by annealing in an oxidizing and reducing atmosphere. Theoretical and experimental studies have proposed that interstitial oxygen is key to this behavior. However, the chemical state of interstitial oxygen and its impact on the crystal lattice, which governs the electrical conduction mechanism, have not been elusive for over a decade. Herein, we reveal that superoxo-type (M–OO–) interstitial oxygen hinders electrical conductivity. Polarized X-ray absorption spectroscopy is combined with multimodal experiments, such as X-ray diffraction, Raman spectroscopy, and electrical transport measurements. The results show that the superoxo-type interstitial oxygens introduced in the vicinity of the dopant Nb ions trap conduction electrons. Furthermore, the interstitial oxygens induce off-centered [NbO6] and rutile-like [TiO6] octahedral distortions, potentially reducing carrier mobility. This study showcases the potential for expanding the oxygen-related functionalities of oxide materials from transparent conductors to battery materials, catalysts, and oxygen conductors.