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

Debnath, Bharati, Kumar, Ashwani, Salunke, Hemant G., Bhattacharyya, Sayan
The Journal of Physical Chemistry C 2017 v.121 no.45 pp. 25594-25602
catalysts, ferrimagnetic materials, hysteresis, ions, magnetism, manganese oxides, nanoparticles, oxygen production, physical chemistry
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 eg¹ 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.