Morphology-controlled fabrication of Co₃O₄ nanostructures and their comparative catalytic activity for oxygen evolution reaction

Rod-like, mixed rod-like and sheet-like, and sheet-like cobalt oxide (Co₃O₄) nanostructures were synthesized by a two-step process of microwave-hydrothermal and calcination using cobalt hydroxide carbonate as precursor, whose morphology could be controlled by adjusting the urea concentration in microwave-hydrothermal. With the increasing urea concentration, the cobalt hydroxide carbonate precursors transformed from rods to sheets, and their crystal structures changed from orthorhombic to monoclinic. After the calcination process, the cobalt hydroxide carbonate converted to Co₃O₄ without obvious change of their morphology. Meanwhile, a large number of mesopores appeared in the Co₃O₄ nanostructures. Of the three morphologies, analysis of catalytic activity for oxygen evolution reaction (OER) demonstrated that the Co₃O₄ nanosheets exhibited highest current density and lowest overpotential. The superior electrochemical performance of Co₃O₄ nanosheets might be attributed to their high surface areas and porous structure, which could efficiently accelerate electron transport and improve the catalytic activity for OER in alkaline solution.