Stability and morphology of cerium oxide surfaces in an oxidizing environment: A first-principles investigation

We present density functional theory investigations of the bulk properties of cerium oxides (Ce O₂ and Ce₂ O₃) and the three low index surfaces of Ce O₂, namely, (100), (110), and (111). For the surfaces, we consider various terminations including surface defects. Using the approach of "ab initio atomistic thermodynamics," we find that the most stable surface structure considered is the stoichiometric (111) surface under "oxygen-rich" conditions, while for a more reducing environment, the same (111) surface, but with subsurface oxygen vacancies, is found to be the most stable one, and for a highly reducing environment, the (111) Ce-terminated surface becomes energetically favored. Interestingly, this latter surface exhibits a significant reconstruction in that it becomes oxygen terminated and the upper layers resemble the Ce₂ O₃ (0001) surface. This structure could represent a precursor to the phase transition of Ce O₂ to Ce₂ O₃.