Oxygen Vacancy Ordering and Electron Localization in CeO2: Hybrid Functional Study

Oxygen vacancies in bulk CeO2 have been investigated using the Heyd-Scuseria-Ernzerhof (HSE) hybrid functional method. Results show that oxygen vacancies tend to linearly order in the < 111 > direction of CeO2, yielding much more dispersive gap states with the weakened electron localization compared to the case of a single vacancy. Such vacancy ordering and electron localization give rise to a profound influence on material properties. First, the dispersive gap states are expected to act as stepping stones to facilitate the electron excitation from valence band to conduction band, contributing to extended optical absorption in the longer wavelengths and thus enhancing photovoltaic and photocatalytic functionalities. Also, the linear ordering of oxygen vacancies leads to the electron localization on Ce ions and oxygen vacancy sites, inducing the polarization of electrons on vacancy sites which effectively enhances stability of ferromagnetism. The fundamental understanding of these functional mechanisms is presented in detail. Additionally, the kinetic analysis of the oxygen-vacancy cluster has also been performed, and its high kinetic stability suggests its physical existence in bulk CeO2. The outcome of this work offers great promise for practical application of CeO2 in visible-light photocatalysis and photovoltaics as well as magneto-optic and spintronic devices.