Crystallization Pathways and Evolution of Morphologies and Structural Defects of α-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, morphologies, 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 α-MnO₂ nanomaterials prepared via reduction of KMnO₄ solution with C₂H₅OH prior to being annealed in air at 200-600 °C. We explored a novel evolution that α-MnO₂ nucleation can be formed even at room temperature and gradually developed to α-MnO₂ nanorods at above 500 °C. We also found the existence of H⁺ or K⁺ ions in the [1 × 1] tunnels of α-MnO₂ and observed the simultaneous presence of Mn and O vacancies in α-MnO₂ crystals at low temperatures. Increasing the temperature removed these O vacancies, leaving only the Mn vacancies in the samples.