The directional structure transition of MnO2 during the drying process

Abstract

MnO₂ as a versatile transition metal oxide has multiple crystalline forms, such as δ-, α-, γ-, and β-phases, and they have important applications in miscellaneous domains, but there remains a huge challenge to simultaneously manage the crystal and morphology structure of MnO₂ by a facile approach. Herein, we report a simple post-treatment method for the synthesis of MnO₂ with different structures. The transition from δ-MnO₂ nanosheets to α-MnO₂ nanowires or γ-MnO₂ nanoflakes can be achieved by simply varying the water evaporation rate of the wet synthesized samples during the drying process at a temperature of 60 °C. KMnO₄ or NaMnO₄ was used to prepare layered δ-MnO₂, and the drying conditions were altered to adjust the evaporation rate of water, so that the K⁺ or Na⁺ ions in the layered structure were redistributed and served as templates for the evolution of the tunnel structure. Impressively, δ-MnO₂ is enabled to achieve a high specific capacitance of 586 F g⁻¹. Our findings greatly reduce the energy required for the transition between different phases of MnO₂, unveil the directional assembling of the [MnO₆] unit under a controlled drying environment as well as the pivotal role of the drying conditions in determining the crystalline phase of MnO₂, and provide a novel strategy for the structural engineering of metal oxides.