The formation of the Co₃O₄ cobalt oxide within CoO substrate

Highlights: • The mechanism of the formation of Co₃O₄ oxide inside of CoO phase has been studied. • During CoO oxidation, Co₃O₄ is formed in the substrate as isolated islands. • Co₃O₄ islands are formed as a result of decomposition of CoO. - Abstract: The kinetics and mechanism of the Co₃O₄ formation in the interior of oxidized CoO substrate have been studied. It has been shown that the Co₃O₄ oxide is growing not only as a continuous layer on the surface of CoO, but it can also be detected within the CoO substrate in the form of isolated islands. The formation kinetics of Co₃O₄ islands are virtually independent of oxidation time. Two stage oxidation experiments using the ¹⁸O isotope have demonstrated that the Co₃O₄ phase is formed within the CoO substrate not as a result of internal CoO oxidation but through substrate decomposition.