Simultaneous phase- and morphology-controlled synthesis of MnO2 crystals through controlled release of cuprous ions in hydrothermal condition

The alpha-, beta-, and delta-MnO2 with various morphologies have been synthesized by a novel redox system of KMnO4 and CuCl with HCl added under a hydrothermal condition. The resultant MnO2 products have been characterized by x-ray diffraction, scanning electron microscopy, and transmission electron microscopy. Upon control of reaction temperature and duration, it was observed that MnO2 polymorphs of different morphology (e.g., flowery delta-MnO2, beta-MnO2 nanowires and octahedrons, alpha-MnO2 nanowires) can be prepared in an adjustable manner. The phenomenon is mainly attributed to the effect of cuprous ions controllably released from CuCl by the action of HCl at different experimental conditions. The corresponding formation mechanism for the MnO2 crystals will also be proposed and discussed.