Manganese
oxides are attracting great interest owing to their rich
polymorphism and multiple valent states, which give rise to a wide
range of applications in catalysis, capacitors, ion batteries, and
so forth. Most of their functionalities are connected to transitions
among the various polymorphisms and Mn valences. However, their atomic-scale
dynamics is still a great challenge. Herein, we discovered a strong
heterogeneity in the crystalline structure and defects, as well as
in the Mn valence state. The transitions are studied by in situ transmission
electron microscopy (TEM), and they involve a complex ordering of
[MnO6] octahedra as the basic building tunnels. MnO2 nanowires synthesized using solution-based hydrothermal methods
usually exhibit a large number of multiple polymorphism impurities
with different tunnel sizes. Upon heating, MnO2 nanowires
undergo a series of stoichiometric polymorphism changes, followed
by oxygen release toward an oxygen-deficient spinel and rock-salt
phase. The impurity polymorphism exhibits an abnormally high stability
with interesting small-large-small tunnel size transition, which is
attributed to a preferential stabilizer (K+) concentration,
as well as a strong competition of kinetics and thermodynamics. Our
results unveil the complicated intergrowth of polymorphism impurities
in MnO2, which provide insights into the heterogeneous
kinetics, thermodynamics, and transport properties of the tunnel-based
building blocks.
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