Study of structural, optical and dielectric properties of α-MnO₂ nanotubes (NTS)

In this work, the synthesis of α-MnO₂ nanotubes (NTs) via hydrothermal route is reported. XRD investigation confirms the tetragonal crystalline structure of α-MnO₂ NTs, where the calculated grain size is equal to 29.3205 ± 4.3227 nm. The observed morphology of α-MnO₂ in SEM and TEM images confirm the formation of nanotubes. EDX data confirms that the sample contains Mn and O only. In FTIR, Mn–O stretching mode at 530 cm⁻¹ is observed. In addition, the O–O stretching mode at 715 cm⁻¹ also confirms the presence of MnO₆ units. Raman spectrum provides the fingerprints at 569 cm⁻¹ and 635 cm⁻¹ of α-MnO₂ originating due to breathing vibrations of MnO₆ units. The calculated band gap is 2.7 eV, which is blue-shifted as compared to the bulk (2.1 eV). The PL results indicate prominent emission peaks in blue region (467 nm) and in green region (566 nm). The dielectric constant and dielectric loss of α-MnO₂ NTs are high at low frequencies due to conducting grain volume and resistive grain boundaries, while both are low at higher frequencies due to the lagging of charge carriers behind the field. The conductivity of α-MnO₂ NTs increases with increasing applied field frequency due to the hopping process between Mn³⁺/Mn⁴⁺ and liberation of trapped charges. Magnetic properties show ferromagnetic nature of α-MnO₂ NTs which is attributed to coupling of Mn⁴⁺–O²⁻–Mn⁴⁺. The magnetic properties of α-MnO₂ are affected by Mn ion interaction which in turn depends upon valence division of Mn in the Mn₂O₄ framework.