Electronic properties and surface reactivity of SrO-terminated SrTiO and SrO-terminated iron-doped SrTiO.

Surface reactivity and near-surface electronic properties of SrO-terminated SrTiO and iron doped SrTiO were studied with first principle methods. We have investigated the density of states (DOS) of bulk SrTiO and compared it to DOS of iron-doped SrTiO with different oxidation states of iron corresponding to varying oxygen vacancy content within the bulk material. The obtained bulk DOS was compared to near-surface DOS, i.e. surface states, for both SrO-terminated surface of SrTiO and iron-doped SrTiO. Electron density plots and electron density distribution through the entire slab models were investigated in order to understand the origin of surface electrons that can participate in oxygen reduction reaction. Furthermore, we have compared oxygen reduction reactions at elevated temperatures for SrO surfaces with and without oxygen vacancies. Our calculations demonstrate that the conduction band, which is formed mainly by the d-states of Ti, and Fe-induced states within the band gap of SrTiO, are accessible only on TiO terminated SrTiO surface while the SrO-terminated surface introduces a tunneling barrier for the electrons populating the conductance band. First principle molecular dynamics demonstrated that at elevated temperatures the surface oxygen vacancies are essential for the oxygen reduction reaction.