Reversible Water-Induced Phase Changes of Cobalt Oxide Nanoparticles.

Abstract

Cobalt oxides have been identified as highly active catalysts for the electrochemical water splitting and oxygen evolution reaction. Using near-ambient pressure resonant photoelectron spectroscopy, we studied changes in the metal-oxygen coordination of size-selected core-shell CoO_<i>x</i> nanoparticles induced by liquid water. In dry conditions, the nanoparticles exhibit an octahedrally coordinated Co²⁺ core and a tetrahedrally coordinated Co²⁺ shell. In the presence of liquid water, we observe a reversible phase change of the nanoparticle shell into octahedrally coordinated Co²⁺ as well as partially oxidized octahedrally coordinated Co³⁺. This is in contrast to previous findings, suggesting an irreversible phase change of tetrahedrally coordinated Co²⁺ after the oxygen evolution reaction conditioning. Our results demonstrate the appearance of water-induced structural changes different from voltage-induced changes and help us to understand the atomic scale interaction of CoO_<i>x</i> nanoparticles with water in electrochemical processes.