Synthesis and characterization of α-MnO 2 nanowires: Self-assembly and phase transformation to β-MnO 2 microcrystals

Abstract

Single-crystalline α-MnO 2 nanowires and β-MnO 2 microcrystals have been synthesized by a hydrothermal method. The products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM) and simultaneous thermogravimetry–mass spectrometry (TG–MS). The effects of varying the hydrothermal reaction times and the initial reactant concentrations on the morphology and crystalline structures of the final products have been investigated. It was found that at high reactant concentrations, the α-MnO 2 nanowires formed initially became self-assembled into bundles and subsequently underwent a phase transformation to β-MnO 2 microcrystals on prolonging the hydrothermal treatment. At low reactant concentrations, however, β-MnO 2 microcrystals were formed even after short hydrothermal reaction times. A possible mechanism for the oriented attachment of the α-MnO 2 nanowires and subsequent phase transformation to β-MnO 2 microcrystals has been suggested.