Structure of HEPES-Reduced δ‑MnO₂ Nanosheets

The reduction of tetravalent manganese (Mn-(IV)) to trivalent manganese (Mn-(III)) by HEPES Good's buffer is often used to modify the reactivity of δ-MnO₂ and to distinguish between the Mn-(III) and Mn-(IV) oxidants in redox reactions. However, the structure of HEPES-reacted δ-MnO₂ has remained elusive, hindering a detailed understanding of interfacial electron transfer between adsorbed species and structural Mn. Here, we characterized the structure of δ-MnO₂ reacted with HEPES at pH 6 and 8 under low and high NaCl ionic strength, using chemical analysis, high-energy X-ray diffraction, pair distribution function (PDF), extended X-ray absorption fine structure (EXAFS) spectroscopy, and high-resolution transmission electron microscopy (HRTEM) coupled with selected area electron diffraction (SAED). The average Mn oxidation state (AMOS) decreases from 3.92-3.87 to 3.71-3.59 after HEPES addition, depending on pH and ionic strength. HEPES-reacted δ-MnO₂ has a distinctly different structure at low and high ionic strength. At low ionic strength, the δ-MnO₂ ^HE crystallites are 3-6 nm across, and the MnO₂ layers have approximately 23% vacant sites capped with mainly Mn-(III) and some Mn-(II). At high ionic strength and pH 8, δ-MnO₂ ^HE contains large crystals, several hundred nanometers across, made up of crystallographically oriented nanodomains. Most SAED patterns show streaks along the [100]* direction, indicating a high degree of disorder in the close packing of the anionic sheets, in the Na position within the interlayer, and in the Mn-(IV)-Mn-(III) distribution within the layer. Some nanodiffraction patterns show distinct superstructure reflections along the streaks with A* = 3a*, as seen in well-crystallized triclinic birnessite, and A* = 6a*. High-ionic-strength δ-MnO₂ ^HE has no interlayer Mn-(III), and the Na-(I) ions, along with the layer Mn-(III) and Mn-(IV) cations, are semiordered at the short- to medium-range scales and essentially disordered over longer distances. Identifying the two distinct structures of HEPES-reacted δ-MnO₂ clarifies structural ambiguities reported in the literature and provides a solid foundation for exploring its redox reactivity and electrochemical performance.