Ultrathin SrTiO3 Capping Stabilizes the Surface Chemistry of Brownmillerite Ferrites during Reduction

Abstract Redox processes in oxides are central to materials synthesis and functionality, but their control is often hindered by surface instabilities. Using ambient-pressure X-ray photoelectron spectroscopy, we show that a nominally 1 nm-thick SrTiO3 (STO) cap markedly enhances the surface stability of La0.05Sr0.95FeO2.5-δ (LSFO) thin films during H2 reduction by suppressing the formation of a secondary metallic Fe0 phase. Kinetic analysis indicates that the STO cap reduces the effective Fe0 formation rates by more than 6-fold at both 250 and 350 °C. These findings highlight ultrathin capping layers as an effective strategy for stabilizing oxide surfaces during redox.