Theoretical study on the arrangement of oxygen vacancies and excess charges in reduced bulk and CeO 2 surface

G. E. MURGIDA; M. V. GANDUGLIA PIROVANO; V. FERRARI; A. M. LLOIS

La Plata

Congreso; VI Reunión Nacional Sólidos; 2015

Ceria (CeO 2 ) stands out among other oxides due to its singular ability to store, release, and transport oxygen, which is extensively exploited in catalysis and other applications, such as nonvolatile memories, logic gates, and fuel cells. The reversible CeO 2 ↔CeO 2−δ ↔ Ce 2 O 3reactions as well as the localization of the electrons left behind upon vacancy formation, driving the Ce 4+ → Ce 3+ reduction, are essential to such applications. In order to optimize ceria-baseddevices it is crucial to know the ordering of both the oxygen vacancies and excess charge, but there is still no wide consensus about this issue. To tackle this problem, we use density functional theory with the DFT+U approach in combination with statistical thermodynamics to elucidate the structure of defective bulk ceria [1], and that of CeO 2 (111) with near-surface oxygen vacancies [2].We find correlations between vacancy-vacancy as well as vacancy-Ce 3+ relative positions and total energies, providing clear indication of the proneness of vacancies and Ce 3 + to adopt cer-tain configurations [1]. Such correlations are at the basis of a proposed simple model to predict the ordering of excess charge and vacancies in bulk CeO 2 that enables the separation of the different contributions to the total energy. For the CeO 2 (111) surface, it is shown that for a wide range of near-surface vacancy concentrations, the energetically most stable structures have all the vacancies localized in the second oxygen layer. Moreover, we found that that under a wide range of reducing conditions, a (2×2) ordered subsurface vacancy structure is stable, providing support for recent experimental results [3]. Defect-induced lattice relaxations are used to explain the findings.[1] G. E. Murgida, V. Ferrari, M. V. Ganduglia-Pirovano, and A. M. Llois, Phys. Rev. B 90,115120 (2014).[2] G. E. Murgida and M.V. Ganduglia-Pirovano, Phys. Rev. Lett. 110, 246101, (2013).[3] S. Torbrügge, M. Reichling, A. Ishiyama, S. Morita, and O. Custance, Phys. Rev. Lett.90,056101 (2007).