Failure analysis of hydrothermal synthesis for spinel manganese–cobalt oxide

Abstract

Spinel Co₃O₄ is one of the important functional materials applied in many fields such as electrochemical catalysis and energy storage. Compared with pure Co₃O₄, the activity of the catalytic sites increases when the Co²⁺/Co³⁺ located in tetra-/octahedral sites was substituted by Mn²⁺/Mn³⁺. In this work, we designed a molar feed ratio of 1 : 2 of bivalent Mn and Co sources and utilized a hydrothermal method to synthesize a spinel manganese–cobalt oxide (Mn–Co–O), however, we found that the atomic ratio of the Mn–Co–O precursor after hydrothermal synthesis was just 0.08 instead of 0.5. Moreover, synchrotron radiation X-ray diffraction analyses combined with the electron diffraction data confirm that the final synthetic sample is neither MnCo₂O₄ nor MnCo₂O_4.5, and the sample after calcining the precursor is identified as a two-phase coexistence of pure cubic Co₃O₄ and Mn-doped cubic Co₃O₄. Furthermore, X-ray absorption spectroscopy (XAS) results reveal that Mn²⁺ has been oxidized to Mn^3+/4+ after hydrothermal processing, while Co²⁺ still remains unchanged. Finally, XAS and Raman spectra jointly illustrate that Co²⁺ hydrate transforms to a spinel Co₃O₄ structure consisting of Co²⁺ and Co³⁺ after calcining the precursor. This work indicates that the inconsistent and incomplete reaction between Mn²⁺ and Co²⁺ during hydrothermal processing leads to the failure of synthesizing a spinel manganese–cobalt oxide using a Mn : Co molar feed ratio.