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

In this work, four well-defined morphologies, including nanorod, nanowire, nanoflower and nanowall, of MnO2 nanostructures with different crystal phases (alpha-, beta-, and delta-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 delta-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 alpha-MnO2 nanowall possesses largely enhanced electrochemical activity compared to alpha-MnO2 nanowires and beta-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.