Mechanistic study on phase and morphology conversion of MnO₂ nanostructures grown by controlled hydrothermal synthesis

Morphology and phase of MnO₂ nanostructure were successfully governed by controlling reaction time and temperature in a simple hydrothermal synthesis. δ-MnO₂ nanoflowers were initially formed at a growth condition of 80°C and 12 hr and were transformed into α-MnO₂ nanorods with increasing reaction temperature and time through an intermediate stage where the nanoflowers became sea urchins like structure with δ-MnO₂ core and α-MnO₂ spikes coexisting before they completely formed into α-MnO₂ nanorods. Longer reaction times and higher temperatures were favorable for the growth of α-MnO₂ nanorods. The thickness of α-MnO₂ nanorods increased with increasing reaction temperature and time. The crystalline phase and morphology conversion mechanism of MnO₂ nanostructures was proposed by examining the grown MnO₂ nanostructures with various analytic techniques such as XRD, SEM, HRTEM and TEM analyses. Electrochemical properties of the synthesized nanomaterials were also extracted for the application to lithium-air battery as cathode catalyst.