Structural Characterization of Self-Assembled MnO2 Nanosheets from Birnessite Manganese Oxide Single Crystals

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 (NH4)2S2O8 aqueous solution heated at 60 °C, exfoliation to nanosheets by the intercalation of TMA+ ions followed by water-washing, and the self-assembly of MnO2 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 MnO2 unilamellar nanosheets. The self-assembly yields layered crystals with basal spacing of 0.72 nm and with a composition of H0.18Na0.089MnO2·0.47H2O. 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.