Controllable hydrothermal synthesis of manganese dioxide nanostructures: shape evolution, growth mechanism and electrochemical properties

Abstract

In this work, four well-defined morphologies, including nanorod, nanowire, nanoflower and nanowall, of MnO₂ nanostructures with different crystal phases (α-, β-, and δ-MnO₂) have been synthesized employing a simple hydrothermal process. The samples are characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), energy-dispersive X-ray spectroscopy (EDS), and Brunauer–Emmett–Teller (BET) spectrometry. Our experimental results demonstrate that the concentration of KMnO₄ plays a key role in forming different shapes and phases of MnO₂ nanostructures. Specifically, the K⁺ concentration can affect the crystal phase of MnO₂ seeds in the nucleation processes and the decomposition rate of MnO₄⁻ can influence the number of MnO₂ nuclei at the initial nucleating stage and also can affect the subsequent crystal growth process. Moreover, the effects of reaction temperature on the morphology of the δ-MnO₂ nanowall are systematically studied. The electrochemical performances of the as-prepared MnO₂ as the positive material of rechargeable Li-ion batteries have also been researched. It is found that the δ-MnO₂ nanowall possesses largely enhanced electrochemical activity compared to α-MnO₂ nanowires and β-MnO₂ nanorods. The vast difference in electrochemical activity is discussed in terms of the morphology, crystal phase and specific surface area of MnO₂ nanostructures. It is highly expected that these findings are useful in understanding the formation of MnO₂ nanocrystals with different morphologies, which are also applicable to other metal oxides nanocrystals.