Structural and electronic characterization of fluorine-doped La_0.5Sr_0.5CoO₃₋ using electron energy-loss spectroscopy

Perovskite oxides, ABO₃, are potential catalysts for the oxygen evolution reaction, which is important in the production of hydrogen as a sustainable energy resource. Optimizing the chemical composition of such oxides by substitution or doping with additional elements is an effective approach to improving the activity of such catalysts. Here, we characterized the crystal and electronic structures of fluorine-doped La_0.5Sr_0.5CoO₃₋ particles using scanning transmission electron microscopy (STEM) and electron energy-loss spectroscopy (EELS). High-resolution STEM imaging demonstrated the formation of a disordered surface phase caused by fluorine doping. In addition, spatially resolved EELS data showed that fluorine anions were introduced into the interiors of the particles and that Co ions near the surfaces were slightly reduced by fluorine doping in conjunction with the loss of oxygen ions. Peak fitting of energy-loss near-edge structure data demonstrated an unexpected nanostructure in the vicinity of the surface. An EELS characterization comprising elemental mapping together with an energy-loss near-edge structure analysis indicated that this nanostructure could not be assigned to Co-based materials but rather to the solid electrolyte BaF₂. Complementary structural and electronic characterizations using STEM and EELS as demonstrated herein evidently have the potential to play an increasingly important role in elucidating the nanostructures of functional materials.