Impact of Sr segregation on the electronic structure and oxygen reduction activity of SrTi1-xFexO3 surfaces

The correlation between the surface chemistry and electronic structure is studied for SrTi1-xFexO3 (STF), as a model perovskite system, to explain the impact of Sr segregation on the oxygen reduction activity of cathodes in solid oxide fuel cells. Dense thin films of SrTi0.95Fe0.05O3 (STF5), SrTi0.65Fe0.35O3 (STF35) and SrFeO3 (STF100) were investigated using a coordinated combination of surface probes. Composition, chemical binding, and valence band structure analysis using angle-resolved X-ray photoelectron spectroscopy showed that Sr enrichment increases on the STF film surfaces with increasing Fe content. In situ scanning tunnelling microscopy/spectroscopy results proved the important and detrimental impact of this cation segregation on the surface electronic structure at high temperature and in an oxygen environment. While no apparent band gap was found on the STF5 surface due to defect states at 345 degrees C and 10(-3) mbar of oxygen, the surface band gap increased with Fe content, 2.5 +/- 0.5 eV for STF35 and 3.6 +/- 0.6 eV for STF100, driven by a down-shift in energy of the valence band. This trend is opposite to the dependence of the bulk STF band gap on the Fe fraction, and is attributed to the formation of a Sr-rich surface phase in the form of SrOx on the basis of the measured surface band structure. The results demonstrate that Sr segregation on STF can deteriorate oxygen reduction kinetics through two mechanisms - inhibition of electron transfer from bulk STF to oxygen species adsorbing onto the surface and the smaller concentration of oxygen vacancies available on the surface for incorporating oxygen into the lattice.