Fluoride anions-assisted hydrothermal preparation and growth process of β-MnO2 with bipyramid prism morphology

Abstract

β-MnO₂ well-dispersed single crystals (average length ∼20 μm) with growth shape composed by bipyramid prism, are prepared by a F⁻ anions-assisted hydrothermal method in a reaction system of Mn²⁺ and MnO₄⁻ at 180 °C for 36 h, and their formation process has been discussed. The as-prepared materials are characterized by XRD, SEM, TEM, XPS, and TG–DTA analyses, and their phase and morphology depend on the amount of F⁻ anions, hydrothermal treatment temperature, and the hydrothermal treatment time. The F⁻ anions play an important role in controlling the crystal phase and morphology of the obtained materials. A phase transformation change from the nanofiber α-MnO₂ to tetragonal bipyramid, and finally, bipyramid prism β-MnO₂ is observed with the increasing of the hydrothermal treatment time at 180 °C. The free F⁻ anions in the reaction system are adsorbed preferentially on the {111} faces of the tetragonal bipyramid, which inhibits the growth rate of the [111] direction and favors growth of the {110} faces. The F⁻ anions-assisted hydrothermal method can be potentially extended to prepare other metal oxides with particular crystallographic faces and novel morphologies.