Predominance of Subsurface and Bulk Oxygen Vacancies in Reduced Manganese Oxide

Oxygen vacancies (Vos) play a vital role in the physical and chemical properties of oxide materials and their applications. Because Vos are first produced on oxide surfaces, it has been often recognized that they are richer at the top surface than at the subsurface or bulk region. Here the depth distribution of Vos in reduced manganese oxide (MnOx) films has been studied by a combination of ion-sputtering treatment and variable-angle X-ray photoelectron spectroscopy measurement. The density of Vos on the top surface is determined to be ∼8% lower than that in the subsurface region after reduction treatment of the oxide in CO at 573 K. Density functional theory calculations show that lattice oxygen atoms in the bulk tend to migrate and fill into the Vos on the top surface with a barrier of 0.94 eV at most. The stability of Vos in the bulk is higher than that on the surface via a reduced Mn3O4 skeleton, which can be reflected by the energy of oxygen filling at the Vo.