In-Situ Transmission Electron Microscopy Investigation Of MnO₂ Nanowires With High Density Of Defects

Manganese oxides have been widely used in catalysis,capacitors,ion batteries and other fields due to their abundant polymorphisms and multivalent states of manganese ions.Most of their functionalities are connected to transitions among the various polymorphisms and Mn valences.However,the effects of complex multiphase coexistence and defects in nanomanganese oxides on the phase transition mechanism are still unclear;especially direct observation of their dynamic structural transitions at the atomic scale remains a great challenge.In this thesis,a series of γ-MnO₂nanowires were synthesized by traditional hydrothermal method at 140℃,160℃,180℃and 200℃.In the first part of the experiment,phases structures were identified by powder X-ray diffraction（PXRD）and confirmed to be mainlyγ-MnO₂（1×1+1×2 tunnel composed of manganese-oxygen octahedra）.In order to study the internal structure of nanowires,a single nanowire cross-section sample was prepared by focused ion beam（FIB）.Combined with atomic-resolution high-angle annular dark-field scanning transmission electron microscopy（HAADF-STEM）imaging,it reveals that there are a large number of defects in the nanowires with various impurity phases（such asα-Mn O₂,β-Mn O₂）at the interface.The defects and impurities degrade the electrochemical performance ofγ-Mn O₂nanowires,resulting in their lower charge-discharge capacity and poor cycling performance.In the second part of the experiment,the structure changes ofγ-Mn O₂nanowires with high density of defects during in-situ heating were investigated by high resolution spherical aberration corrected transmission electron microscopy combined with in-situ heating holder.Upon heating,the Mn O₂nanowires underwent a series of crystal structure changes:fromγ-Mn O₂toβ-Mn O₂（shown as a change in the size of the tunnel composed of manganese-oxygen octahedra,from 1×1+1×2 tunnel to 1×1tunnel）through Mn ion migration driven by dynamics,and then to spinel Mn₃O₄（with reduced oxygen content）and finally to the rocksalt Mn O（with further reduced oxygen content）.The third part of the experiment revealed the stability of the defects during heating.We found that the large tunnel（2×2 tunnel composed of manganese-oxygen octahedron,K⁺in the middle of the tunnel as a stabilizer）embedded in the matrix of small tunnel（1×1+1×2 tunnel composed of manganese-oxygen octahedron）showed abnormally high dynamic stability.The large tunnel maintains the original 2×2structure under low-temperature,which is attributed to the fact that K⁺in the small tunnel tends to migrate to the larger tunnel,enhancing its structural stability.When the temperature continues to rise,the 2×2 tunnel structure begean to lose lattice oxygen and gradually transforms into spinel phase and rock salt phase.Our results have revealed the complex coexistence of polycrystalline impurity phases and the kinetic and thermodynamic processes of phase transitions in Mn O₂.It sheds light on the multiphase dynamics,thermodynamics and ion transport properties of tunnel-based oxides.