Origin of spin-state crossover and electronic reconstruction at the surface of a LaCoO3 nanoparticle

Combining atomic-resolution scanning transmission electron microscopy (STEM) images and electron-energy-loss spectroscopy spectra in aberration-corrected STEM, first-principle calculations, and spectra simulations, we investigate the origin of the surface state in a $\mathrm{LaCo}{\mathrm{O}}_{3}$ nanoparticle. We show that a $\mathrm{LaCo}{\mathrm{O}}_{3}$ nanoparticle appears as the core-shell structure, where the core has the mixture of low- and high-spin states and the shell has increased concentration of the high-spin state. Based on experimental observations and theoretical calculations, we propose that it is neither the oxygen vacancy nor strain but the suppression of oxygen octahedral tilting due to surface reconstruction that plays a dominant role in the spin-state crossover to the higher-spin state at the surface. These results provide great insights into understanding the origin of ferromagnetism as well as the enhanced electrocatalytic activity in this compound.