Controllable synthesis of alpha-MnO2 nanostructures and phase transformation to beta-MnO2 microcrystals by hydrothermal crystallization.

Single-crystalline alpha-MnO2 nanowires and beta-MnO2 microcrystals have been controllably synthesized by a common hydrothermal method based on the redox reaction between MnO4- and Cr3+. The products were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), energy dispersive spectroscopy (EDS), transmission electron microscopy (TEM), selected area electron diffraction (SAED), and high-resolution transmission electron microscopy (HRTEM). The effects of various experimental conditions on the morphologies and crystalline structures of the final products were investigated. It was found that alpha-MnO2 nanowires were converted to beta-MnO2 microcrystals by prolonging the hydrothermal treatment under a dissolution-recrystallization process. Higher hydrothermal temperature and lower pH value were beneficial for the phase transformation from alpha-MnO2 to beta-MnO2.