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Core‐shell Co3O4@CoO nanoparticles were prepared by calcination of rock‐salt CoO and fully characterized by PXRD, TEM and XPS measurements. XPS studies showed that the Co(III) cation concentration steadily increased and the OER activity of the resulting core‐shell Co3O4@CoO nanoparticles decreased with increasing calcination time, which was attributed to the increasing presence of catalytically inactive Co(III) cations. The decrease in OER activity was also observed for untreated CoO nanoparticles (1) after chronopotentiometric (CP) treatment at 10 mA cm−2 for 18 h. The IR and EEL spectra of the resulting nanoparticles are comparable to those of a fully oxidized Co3O4 sample (1–300), that was prepared by calcining of the CoO sample (1) for 1 h at 300 °C. [ABSTRACT FROM AUTHOR]