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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.

The details of the ion exchange properties of layered H2TiO3, derived from the layered Li2TiO3 precursor upon treatment with HCl solution, with lithium ions in the salt lake brine (collected from Salar de Uyuni, Bolivia) are reported. The lithium adsorption rate is slow, requiring 1 d to attain equilibrium at room temperature. The adsorption of lithium ions by H2TiO3 follows the Langmuir model with an adsorptive capacity of 32.6 mg g(-1) (4.7 mmol g(-1)) at pH 6.5 from the brine containing NaHCO3 (NaHCO3 added to control the pH). The total amount of sodium, potassium, magnesium and calcium adsorbed from the brine was <0.30 mmol g(-1). The H2TiO3 was found capable of efficiently adsorbing lithium ions from the brine containing competitive cations such as sodium, potassium, magnesium and calcium in extremely large excess. The results indicate that the selectivity order Li(+) ≫ Na(+), K(+), Mg(2+), Ca(2+) originates from a size effect. The H2TiO3 can be regenerated and reused for lithium exchange in the brine with an exchange capacity very similar to the original H2TiO3.

A thin layer electrode of birnessite-type manganese oxide was prepared by brushing a mixed solution of KOCOCH3 and Mn(OCOCH3)2 on a platinum substrate, followed by heating at 1073 K. The chemical composition of the electrode was K x MnO y (x = 0.33 and y ∼ 2) with an interlayer spacing of c 0 = 0.697 nm. The positive-potential going sweep on the electrode in an aqueous phase caused the deintercalation of K+ with an increase in c 0. The quasi-reversible intercalation of K+ occurred with a subsequent negative-potential going sweep in a 0.2 mol/dm3 KCl solution. The electrochemical measurements suggested that K+ is not electrochemically active in the deintercalation/intercalation reaction but H+ is. The reaction proceeds based on a mechanism consisting of an electrochemical reaction (the redox reaction between Mn3+ and Mn4+) and an ion-exchange reaction between K+ and H+. The intercalation experiments in various alkali-metal chloride solutions showed the intercalation capacity to be in the order of Na ∼ K > Li > Rb > Cs.