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.