Structural and morphological evolution of β-MnO2 nanorods during hydrothermal synthesis

β-MnO2 nanorods were synthesized via a redox reaction of (NH4)2S2O8 and MnSO4 under hydrothermal conditions. In situ and ex situ x-ray diffraction and scanning electron microscopy were employed to follow the structural and morphological evolution during growth. It was found that the crystallization of β-MnO2 nanorods proceeds through two steps: γ-MnO2 nanorods form first via a dissolution-recrystallization process and then transform topologically into β-MnO2 with increasing temperature. The phase transformation was associated with a short-range rearrangement of MnO6 octahedra. Vibrational spectroscopic studies showed that the β-MnO2 nanorods had four infrared absorptions at 726, 552, 462 and 418 cm-1 and four Raman scattering bands at 759 (B2g), 662 (A1g), 576 (Ramsdellite impurity) and 537 (Eg) cm-1, which are in agreement with Mn-O lattice vibrations within a rutile-type MnO6 octahedral matrix.