The directional structure transition of MnO₂ during the drying process

MnO2 as a versatile transition metal oxide has multiple crystalline forms, such as delta-, alpha-, gamma-, and beta-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 delta-MnO2 nanosheets to alpha-MnO2 nanowires or gamma-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 degrees C. KMnO4 or NaMnO4 was used to prepare layered delta-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, delta-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.