Structural, optical and magnetic properties of α- and β-MnO2 nanorods

Highlights: • α-, β- and αβ-MnO₂ nanorods are synthesised through hydrothermal technique. • Rietveld refinement and SEM confirms the presence of α- and β-phase in αβ-MnO₂. • Among α-, β- and αβ-MnO₂, β-MnO₂ possess high capacitance. • High concentration of Mn³⁺ contributes high bandgap in α-MnO₂ nanorods. • SG or CG behaviour in MnO₂ is critical towards the concentration of Mn³⁺ ions. Herein, we synthesise various polymorphs of MnO₂ through hydrothermal technique. Formation of pure tetragonal structured α-, β- and mixture of α and β phase of MnO₂ are identified through Fourier transform infrared spectroscopy (FTIR) and X-Ray diffraction (XRD).Rietveld refinement reviles 73% of α-MnO₂ and 27% of β-MnO₂ present in αβ-MnO₂. From Field emission scanning electron micrographs (FESEM), average diameter of α- and β-MnO₂ nanorods is found to be 36 nm and 23 nm, respectively, while in αβ-MnO₂, nanorods of two different sizes, 28 and 116 nm are observed. Among all polymorphs, small size of nanorods in β-MnO₂ results high specific capacitance. Through UV–visible absorption spectra, the optical band gap of α-, β- and αβ-MnO₂ nanorods are estimated to be 1.53, 1.30 and 1.45 eV, respectively. XPS confirms the presence of large concentration of Mn³⁺ in α- and αβ-MnO₂ nanorods results high effective magnetic moment in comparison to the theoretical one which is almost same in β-MnO₂. Henceforth, we evaluate the accurate composition of MnO₂ as α-MnO_1.48, β-MnO_1.86 and αβ-MnO_1.15. Spin-glass/cluster-glass behavior is confirmed from ac susceptibility and remanent magnetization measurements in β-MnO₂ and αβ-MnO₂ which is absent in α-MnO₂ depending on a critical concentration of Mn³⁺ ions.