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. delta-MnO2 nanoflowers were initially formed at a growth condition of 80 degrees C and 12 hr and were transformed into alpha-MnO2 nanorods with increasing reaction temperature and time through an intermediate stage where the nanoflowers became sea urchins like structure with delta-MnO2 core and alpha-MnO2 spikes coexisting before they completely formed into alpha-MnO2 nanorods. Longer reaction times and higher temperatures were favorable for the growth of alpha-MnO2 nanorods. The thickness of alpha-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.