Enhancing Multifunctionality through Secondary Phase Inclusion by Self-Assembly of Mn3O4 Nanostructures with Superior Exchange Anisotropy and Oxygen Evolution Activity

While altering physical properties by self-assembly is a common phenomenon, controlled inclusion of a secondary phase that in turn enhances the properties of the ensemble is a rare occurrence. Herein monodisperse Mn3O4 spherical nanoparticles were self-assembled into hierarchical flakes and cubes by regulating the surfactant–metal precursor molar ratio, reaction atmosphere, and time. The secondary phase of Mn2O3 was incorporated differently, depending on the type of self-assembly as 2, 3.5, and 6.5 wt % in the flake, spherical, and cubic morphologies, respectively. The highest percentage of Mn2O3 in the cubes boosts its multifunctionality in terms of enhanced magnetic exchange coupling and oxygen evolution reaction (OER) activity. With a 2 T cooling field, the hysteresis loop shift corresponding to coupling between antiferromagnetic Mn2O3 and ferrimagnetic Mn3O4 reached 3813 ± 2 Oe for the cubes, which is a record high for any reported Mn3O4–Mn2O3 system. The presence of a eg 1 electron due to a higher Mn2O3 fraction in the cubes facilitated high structural flexibility for optimum strength of interaction between the catalyst and intermediate ions during OER. Likewise, a current density 10 mA cm–2 was reached at an overpotential of 0.946 ± 0.02 V for the cubes, which is to superior those of the spherical morphology and flakes.