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Updated: Dec 3, 2025

Phase Diagram Characterization Using Magnetic Beads as Liquid Carriers
Published on: September 4, 2015
Reversible Water-Induced Phase Changes of Cobalt Oxide Nanoparticles
Dennis Hein1,2, Garlef Wartner1,2, Arno Bergmann3
1Helmholtz-Zentrum Berlin für Materialien und Energie (HZB), Albert-Einstein-Straße 15, 12489 Berlin, Germany.
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 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 nanoparticles with water in electrochemical processes.
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