Stabilization of Mn(IV) in nanostructured zinc manganese oxide and their facile transformation from nanospheres to nanorods

Abstract

Nanostructured ternary manganese(IV) oxides are of importance as electrode materials. A low-temperature, precursor mediated route has been designed to obtain ternary oxides containing Mn(IV) and Zn(II) at ambient pressure. The defect spinel, Zn_0.83(Mn_1.42Zn_0.34)O₄ was obtained by annealing at optimal conditions after the thermal decomposition of oxalates of zinc and manganese co-precipitated at room temperature. This is the first report of a low temperature (250 °C) and normal atmospheric pressure synthesis of a ternary zinc manganese oxide containing purely tetravalent manganese. Rietveld refinement indicates zinc occupancy in both tetrahedral and octahedral sites with the refined composition of (Zn_0.83)_tet(Mn_1.42Zn_0.34)_octO₄. Also, we show for the first time that refluxing with acetic acid transforms the oxide nanospheres (5-20 nm) to nanorods with diameter of 10-15 nm and length varying from 60 to 150 nm. The stoichiometry of Zn and Mn, as well as the oxidation state of manganese has been confirmed by SEM-EDX, PXRD, AAS, XPS and magnetic studies. Zn_0.83(Mn_1.42Zn_0.34)O₄ is antiferromagnetic with a Néel temperature of ~5-10 K. The rods of 10 nm diameter (aspect ratio = 6) show higher susceptibility values (5 fold enhancement) compared to spherical nanoparticles. This low temperature route can be extended for the design of other material phases.