• Hein, D.; Wartner, G.; Bergmann, A.; Bernal, M.; Cuenya, B.R.; Seidel, R.: Reversible water-induced phase changes of cobalt oxide nanoparticles. ACS Nano 14 (2020), p. 15450-15457

Open Access Version

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 CoOx nanoparticles induced by liquid water. Indry 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 CoOx nanoparticles with water in electrochemical processes.