Large-Scale Synthesis and Comprehensive Structure Study of δ-MnO₂

Abstract

Layered δ-MnO (birnessites) are ubiquitous in nature and have also been reported to work as promising water oxidation catalysts or rechargeable alkali-ion battery cathodes when fabricated under appropriate conditions. Although tremendous effort has been spent on resolving the structure of natural/synthetic layered δ-MnO in the last few decades, no conclusive result has been reached. In this Article, we report an environmentally friendly route to synthesizing homogeneous Cu-rich layered δ-MnO nanoflowers in large scale. The local and average structure of synthetic Cu-rich layered δ-MnO has been successfully resolved from combined Mn/Cu K-edge extended X-ray fine structure spectroscopy and X-ray and neutron total scattering analysis. It is found that appreciable amounts (∼8%) of Mn vacancies are present in the MnO layer and Cu occupies the interlayer sites above/below the vacant Mn sites. Effective hydrogen bonding among the interlayer water molecules and adjacent layer O ions has also been observed for the first time. These hydrogen bonds are found to play the key role in maintaining the intermediate and long-range stacking coherence of MnO layers. Quantitative analysis of the turbostratic stacking disorder in this compound was achieved using a supercell approach coupled with anisotropic particle-size-effect modeling. The present method is expected to be generally applicable to the structural study of other technologically important nanomaterials.

Structural water and disordered structure promote aqueous sodium-ion energy storage in sodium-birnessite

“…It is also possible that water molecules in the solvation shell of Na-ion could form hydrogen bonding in the interlayer region. This argument is also supported by a recent X-ray and neutron total scattering study of the birnessite materials, where hydrogen bonding among the interlayer water molecule and adjacent Mn–O layer oxygen ion was found to play an important role in maintaining the intermediate and long-range stacking of Mn–O octahedral layer 53 . Therefore, hydrogen bonding between structural water could also stabilize the Na-ions inside the interlayer region, thus extraction these hydrated Na-ion out of interlayer region (charging) might become more difficult.…”

There's no place like real-space: elucidating size-dependent atomic structure of nanomaterials using pair distribution function analysis

“…196 Many layered oxides show a high degree of inter-and intralayer disorder, and several structures have been extensively studied with PDF. 77,[197][198][199][200][201][202] The complex challenges involved in structural characterization of disordered layered oxides are addressed in an extensive study of the structure of d-MnO 2 by Liu et al 199 Layered metal oxides are oen small nanoparticles, contain turbostatic stacking faults and exhibit in-plane cation disorder. Liu et al address all three aspects of the structural complexity in a study of Cu-rich d-MnO 2 (ref.…”

Hierarchical porosityvialayer-tunnel conversion of macroporous δ-MnO2nanosheet assemblies

“…For context, we note δ-MnO 2 nanosheet floccs produced in acidic environments typically possess 10–20 at% in-plane manganese vacancies, 70,74 where 1/7 [ V Mn ] represents the limit for which neighboring V Mn are completely screened by occupied Mn L sites. Meanwhile T 2,2 MnO 2 contain ordered arrays of 1 in 3 vacant Mn columns in comparison to the ideal δ-MnO 2 motif.…”

“…The defective δ-MnO 2 layer was represented by a chalcophanite-structured layer model, analogous to the work of Liu et al 70 This model fixes the concentration of vacant Mn L sites at 1/7, assuming all vacancies are coordinated by occupied Mn L O 6 . The doubly capped in-plane vacancy sites ( i.e.…”

“…A suitable model must account for a number of factors, including (i) the point defect content and relaxation of the layer motif, (ii) the position uctuation of adjacent layers, (iii) the inuence of highly anisotropic particle shapes and sizes, and (iv) the inuence of bending and other distortions. This has been done rigorously for d-MnO 2 nanosheet ensembles and other 2D materials on relatively few occasions, 64,[67][68][69][70] and is not pursued in this work. Alternatively, PDF analysis is performed to provide insight into the local and intermediate range structure evolution, detailed below.…”