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

Abstract

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-MnO 2 nanomaterials prepared via reduction of KMnO 4 solution with C 2 H 5 OH prior to being annealed in air at 200−600 °C. We explored a novel evolution that α-MnO 2 nucleation can be formed even at room temperature and gradually developed to α-MnO 2 nanorods at above 500 °C. We also found the existence of H + or K + ions in the [1 × 1] tunnels of α-MnO 2 and observed the simultaneous presence of Mn and O vacancies in α-MnO 2 crystals at low temperatures. Increasing the temperature removed these O vacancies, leaving only the Mn vacancies in the samples.

De-Ammonium Ba0.18V2O4.95/NH4V4O10 Film Electrodes as High-Performance Cathode Materials for Magnesium-Ion Batteries

“…50-1715), and the lattice parameters are a = 9.845 Å, b = 3.495 Å, c = 17.11 Å, and β = 121.73° (Table S1). This is the result of the effect of different annealing temperatures on phase transformation and crystallinity . In addition, the weaker diffraction peak at 9.28° is consistent with monoclinic-phase NH 4 V 4 O 10 (JCPDS no.…”

“…This is the result of the effect of different annealing temperatures on phase transformation and crystallinity. 37 In addition, the weaker diffraction peak at 9.28°is consistent with monoclinic-phase NH 4 V 4 O 10 (JCPDS no. 31-0075), and the lattice parameters are a = 11.71 Å, b = 3.66 Å, c = 9.72 Å, and β = 101.0°(Table S1).…”

Effects of Sodium and Magnesium Ions on the Photochemically Induced Heterogeneous Formation of Manganese Oxides and Their Structural Evolution

“…For the Mn oxides heterogeneously formed on quartz substrates without Na + or Mg 2+ , romanechite (3 × 2) was initially formed at 6 h and cryptomelane (2 × 2) and groutite (2 × 1) were newly formed at 12 and 20 h. This is probably a consequence of one large tunnel splitting into two smaller tunnels of Mn oxides, by migrating the Mn atom from the intralayer to the interlayer to construct tunnel walls. , Mn–O bonds can be partially broken or weakened to enable a Mn migration . The structural stability and defects of Mn oxides also affected the phase evolution process. , On the other hand, in the presence of Na + , cryptomelane (2 × 2) was produced first, and gradually became romanechite (3 × 2) and groutellite (2 × 1). Two (2 × 2) tunnels were evolved into one bigger (3 × 2) and one smaller (2 × 1) tunnel.…”

“…60 The structural stability and defects of Mn oxides also affected the phase evolution process. 62,63 On the other hand, in the presence of Na Mn(III) can be formed via the comproportionation reaction between adsorbed Mn(II) and Mn(IV) in Mn oxides, 64 and then adsorb at the vacant sites of Mn oxides. The Mn(III) can serve as a precursor for rapid structural rearrangement.…”