Codoping-Induced, Rhombus-Shaped Co₃O₄Nanosheets as an Active Electrode Material for Oxygen Evolution

Nanostructured Co3O4 doped with Zn(2+), Ni(2+), and both were directly grown on an ITO substrate by an easily available hydrothermal method. The doped Co3O4 showed a unique structural morphology evolution upon controlling the doping elements and the doping ratio of the cations. For the codoped samples, the novel rhombus-shaped Co3O4 nanosheets doped with Zn(2+) and Ni(2+) (concentration ratio of 1:2) exhibited the optimal electrocatalytic performance. The sample showed a current density of 165 mA cm(-2) at 1.75 V, approximately 1.6 and 4 times higher than that of samples doped with Zn(2+) and Ni(2+) at a concentration ratio of 1:1 and 1:3. The unique architecture and its corresponding modified physical properties, such as high active-site density created by codoping, large structural porosity, and high roughness, are together responsible to its superior performance. For codoped Co3O4 nanostructures, Zn(2+) facilitates the creation of Co cations in their high oxidation state as active centers, while Ni(2+) contributed to the new active sites with lower activation energy. The synergistic effect of Zn(2+) and Ni(2+) doping can explain the improved physicochemical properties of codoped Co3O4 nanostructures.