Single-Crystal Growth of Layered Birnessite-Type Manganese Oxides and Their Delamination into MnO₂ Nanosheets

We have successfully grown hexagonally developed KxMnO2 single crystals (50 μm in lateral size and 10 μm in thickness) by slow cooling from 1273 to 1173 K at a rate of 0.5 K/h using K2MoO4 as a flux source. To obtain large-sized MnO2 nanosheets from these KxMnO2 single crystals, we investigated an appropriate process to suppress cracking in the soft-chemical delamination process. Crystals with fewer cracks were obtained upon reacting KxMnO2 with (NH4)2S2O8. The protonated crystals were found to undergo massive swelling upon contact with a tetramethylammonium hydroxide solution. The swollen crystals were exfoliated into unilamellar MnO2 nanosheets by subsequent treatment with tetrabutylammonium ions. The nanosheets were several micrometers in lateral sizes, larger than those obtained from polycrystalline birnessite samples, and they had a torn texture due to etching defects.