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Structure of HEPES-Reduced δ‑MnO2 Nanosheets
Alain Manceau1,2, Yan Li1, Jianlin Liao1
1European Synchrotron Radiation Facility (ESRF), 38043 Grenoble, France.
Chemistry of Materials : a Publication of the American Chemical Society
|May 18, 2026
Summary
HEPES buffer reduces manganese dioxide (δ-MnO₂) structure, creating distinct forms at low and high ionic strengths. This research clarifies δ-MnO₂ structures, aiding understanding of its redox reactivity and electrochemical applications.
Area of Science:
- Materials Science
- Environmental Chemistry
- Electrochemistry
Background:
- The reduction of tetravalent manganese (Mn-(IV)) to trivalent manganese (Mn-(III)) by HEPES Good's buffer is crucial for modifying δ-MnO₂ reactivity.
- Understanding the structure of HEPES-reacted δ-MnO₂ is essential for elucidating interfacial electron transfer mechanisms.
- Previous structural characterization of HEPES-reacted δ-MnO₂ has been limited, hindering detailed analysis.
Purpose of the Study:
- To characterize the structure of δ-MnO₂ reacted with HEPES under varying pH and ionic strength conditions.
- To investigate the impact of HEPES reaction on the average manganese oxidation state (AMOS) and crystallite morphology.
- To resolve structural ambiguities in HEPES-reacted δ-MnO₂ for improved understanding of its properties.
Main Methods:
- Utilized chemical analysis, high-energy X-ray diffraction, pair distribution function (PDF), and EXAFS spectroscopy.
- Employed high-resolution transmission electron microscopy (HRTEM) coupled with selected area electron diffraction (SAED).
- Analyzed samples reacted at pH 6 and 8 under low and high NaCl ionic strengths.
Main Results:
- The average Mn oxidation state decreased significantly after HEPES addition, varying with pH and ionic strength.
- Two distinct structures of HEPES-reacted δ-MnO₂ were identified: small crystallites at low ionic strength and large, nanodomain-rich crystals at high ionic strength.
- Structural disorder in anionic sheets, Na positions, and Mn-(IV)-Mn-(III) distribution was observed, particularly at high ionic strength.
Conclusions:
- The study successfully identified two distinct structural forms of HEPES-reacted δ-MnO₂, clarifying previous literature discrepancies.
- The findings provide a foundation for understanding the redox reactivity and electrochemical performance of modified manganese oxides.
- This structural insight is critical for applications involving manganese oxides in catalysis, energy storage, and environmental remediation.
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