Enhancing Multifunctionality through Secondary Phase Inclusion by Self-Assembly of Mn 3 O 4 Nanostructures with Superior Exchange Anisotropy and Oxygen Evolution Activity

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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 Mn 3 O 4 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 Mn 2 O 3 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 Mn 2 O 3 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 Mn 2 O 3 and ferrimagnetic Mn 3 O 4 reached 3813 ± 2 Oe for the cubes, which is a record high for any reported Mn 3 O 4 –Mn 2 O 3 system. The presence of a e g 1 electron due to a higher Mn 2 O 3 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.