Crystallization Pathways and Evolution of Morphologies and Structural Defects of alpha-MnO2 under Air Annealing

Manganese dioxide nanomaterials have wide applications in many areas from catalysis and Li-ion batteries to gas sensing. Understanding the crystallization pathways, morphol-ogies, and formation of defects in their structure is particularly important but still a challenging issue. Herein, we employed an arsenal of X-ray diffraction (XRD), scanning electron microscopy (SEM), neutron diffraction, positron annihilation spectroscopies, and ab initio calculations to investigate the evolution of the morphology and structure of alpha-MnO2 nanomaterials prepared via reduction of KMnO4 solution with C2H5OH prior to being annealed in air at 200-600 degrees C. We explored a novel evolution that alpha-MnO2 nucleation can be formed even at room temperature and gradually developed to alpha-MnO2 nanorods at above 500 degrees C. We also found the existence of H+ or K+ ions in the [1 x 1] tunnels of alpha-MnO2 and observed the simultaneous presence of Mn and O vacancies in alpha-MnO2 crystals at low temperatures. Increasing the temperature removed these O vacancies, leaving only the Mn vacancies in the samples.