Structural Characterization of Self-Assembled MnO₂Nanosheets from Birnessite Manganese Oxide Single Crystals

Abstract

We report in this paper the studies on protonation, exfoliation, and self-assembly of birnessite-type manganese oxide single crystals. The protonation was carried out by extracting K + ions from the potassium manganese oxide single crystals in a (NH 4 ) 2 S 2 O 8 aqueous solution heated at 60 °C, exfoliation to nanosheets by the intercalation of TMA + ions followed by water-washing, and the self-assembly of MnO 2 nanosheets in a dilute NaCl solution. The structures of the samples at these stages were systematically investigated using X-ray diffraction, scanning electron microscopy, transmission electron microscopy, atomic force microscopy, X-ray photoelectron spectroscopy, Fourier transform infrared, thermogravimetric analysis-differential thermal analysis, and chemical compositional analysis. Electron density distribution in the protonated single crystal was visualized by whole-pattern fitting based on the maximum entropy method. The results indicated that the protonated single crystals can be exfoliated to MnO 2 unilamellar nanosheets. The selfassembly yields layered crystals with basal spacing of 0.72 nm and with a composition of H 0.18 Na 0.089 MnO 2 ‚0.47H 2 O. The layered crystals had a textured polycrystalline structure, where c-axes of the nanosheets aligning along a certain direction constitute a fiber axis with azimuthal orientations of a-or b-axes about the fiber axis. Moreover, the azimuthal orientations of a-or b-axes are probably arranged at particular angles to one another, rather than randomly. The mean oxidation state of manganese exhibits no marked change at the various stages of the protonation, exfoliation, and self-assembly.

Hydration-induced spin-glass state in a frustrated Na-Mn-O triangular lattice

“…The changes in this temperature regime may be caused by a transformation of birnessite to other layered polymorphs such as γ-MnO 2 , and its subsequent partial conversion to Mn 2 O 3 , inferring reduction of the tetravalent manganese correlated also with the release of oxygen [18]. Due to the disproportionation of manganese valence state, analogous observation have been made in the TG-DTA measurements for Nadeficient α-Na 0.7 MnO 2.25 [22], as well as other birnessite-like systems including [36], and Na 4 Mn 14 O 27 9H 2 O [14]. The additional weight loss (∼2%) up to 1000 °C, likely relates to redox/extraction reactions due to the increased mobility of alkali ions in forming high-temperature manganese polymorphs.…”

Synthesis of nanosized MnO2 prepared by the polyol method and its application in high power supercapacitors

“…Taking into account the Na content, the calculated ratio between structural H 2 O and Mn (H 2 O:Mn) was 0.26:1, that is, the final ratio among Na, H 2 O and Mn was found to be 0.32:0.26:1. Above 500°C, a third weight loss of 4.4 % occurs, most probably caused by the oxygen release during the partial reduction of Mn(IV) as also reported for similar type of compounds [33]. Overall, the results suggest that the formula of our compound is Na 0.32 MnO 2 Á0.26 H 2 O and the average oxidation state of Mn is 3.68.…”

Green synthesis of manganese oxide nanoparticles for the electrochemical sensing of p-nitrophenol

“…Careful interpretation of various peaks in the FTIR spectrum of MnO NPs gave an idea of stabilizing moieties (Supplementary figure S4a ). FTIR peaks around 504, 554, 758 and 827 cm -1 were due to MnO NPs [36][37][38][39][40]. However, peaks around 603 (R-CH group), 922 (-C-O bond), 1220 (CH 2 group or C-O stretching), 1319, 1368 (C-O vibrations), 1475 (bending frequency methylene group), 1618 (aromatic C = C bond stretching), 1707 (C = O stretching vibrations), 2925 (-C = C bond) and 3393 (OH bond) cm -1 were observed mainly due to the presence of eugenol, caryophyllene, humulene and eugenol acetate in CE [41][42][43].…”