α-MnO2 nanowires transformed from precursor δ-MnO2 by refluxing under ambient pressure: The key role of pH and growth mechanism

α-MnO2 nanowires were obtained by reflux treatment of precursor δ-MnO2 in acidic medium under ambient pressure. The great effects of pH on the transformation of δ-MnO2 to α-MnO2 and the concentration of coexistent cations (K+, Mn2+) was investigated in systematically designed experiments by using powder X-ray diffraction and atomic absorption spectrometry analysis. The specific surface area of the products could be simply controlled by adjusting the initial pH value of the suspension. The micro-morphologies during the transition process from the precursors to final products were characterized by SEM and TEM. A dissolution-recrystallization mechanism was proposed to describe the growth process of the one-dimensional nanowire. MnOx units or MnO6 octahedra was formed firstly from the dissolution of outmost surfaces of δ-MnO2, followed by a rearrangement/crystallization to form one-dimensional α-MnO2 nanowire. In addition, the time-dependent process of dissolution would take place gradually from the external to internal of the precursor.