Oxygen Activation and Dissociation on Transition Metal-Free Perovskite Surfaces. Isights on the Surface Reactivity of SrTiO3

Abstract

Density functional theory and low energy ion scattering spectroscopy were applied to study the mechanism of oxygen dissociation on the SrO-terminated surfaces of strontium titanate (SrTiO3) and iron-doped strontium titanate (SrTi1–x FexO3−δ). Our study reveals that while O2 dissociation is not favored on the SrO-terminated perovskite surface, oxygen vacancies can act as active sites and catalyze the O–O bond cleavage. Electron transfer from lattice oxygen atoms to the O2 molecule, mediated by the subsurface transition metal cations, plays an important role in the resulting formation of surface superoxo species. The O2 molecule dissociates to produce oxygen ions, which are incorporated into the perovskite lattice, and highly active oxygen radicals on the perovskite surface, which further recombine to O2 molecules. Our focus on the SrO-terminated surface, rather than the TiO2 layer, which is presumed to be more catalytically active, was driven by experimental observation using low energy ion scattering spectroscopy, which reveals that the surface of SrTiO3 after high temperature heat treatment is SrO-terminated, and hence this is the surface that is technologically relevant for devices such as solid oxide fuel cells (SOFCs). Our study demonstrates that although the more active BO2-perovskite layer is not exposed at the gas–solid interface, the SrO-terminated surfaces also actively participate in oxygen exchange reaction. Further calculations were performed using the non-equilibrium Green's function method combined with density functional theory (NEGF-DFT) to study the ballistic electron transport through a few layers of iron-doped strontium titanate. This computational method can provide critical information for the surface reactivity and surface active sites which is comparable with the STM experiments. Our study shows that the electron transport is enhanced if oxygen vacancies are present in the surface layed and the electron transport is significantly suppressed by overlayers of SrO. We believe that this study provides useful information for the control over the surface activity of SrTiO3 and explains the mechanisms of the material degradation with SrO surface segregation. Finally, we investigate the properties of the SrTiO3 / SrO interface to elucidate the mechanism of oxygen ion transport between the perovskite and the oxide phase. Figure 1