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

beta-MnO2 nanorods were synthesized via a redox reaction of (NH4)(2)S2O8 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 beta-MnO2 nanorods proceeds through two steps: gamma-MnO2 nanorods form first via a dissolution-recrystallization process and then transform topologically into beta-MnO2 with increasing temperature. The phase transformation was associated with a short-range rearrangement of MnO6 octahedra. Vibrational spectroscopic studies showed that the beta-MnO2 nanorods had four infrared absorptions at 726, 552, 462 and 418 cm(-1) and four Raman scattering bands at 759 (B-2g), 662 (A(1g)), 576 (Ramsdellite impurity) and 537 (E-g) cm(-1), which are in agreement with Mn-O lattice vibrations within a rutile- type MnO6 octahedral matrix.