Synthesis and Substitution Chemistry of Redox-Active Manganese/Cobalt Oxide Nanosheets

We report the synthesis and electrochemical properties of Co-substituted manganese oxide nanosheets (Mn1–xCoxO2). Polycrystalline samples of layered Na0.6Mn1–xCoxO2 (x = 0.2–0.5) were synthesized as starting materials. A linear decrease in the lattice constant a with increasing Co content supported the successful substitution of Co3+ ions for Mn3+ ions in the host layers. Acid-exchange treatment of the Na0.6Mn1–xCoxO2 powders resulted in the formation of H–Mn1–xCoxO2 while preserving the Mn/Co ratio and layered structure. Exfoliation of H–Mn1–xCoxO2 was achieved by reaction with tetra–n–butylammonium ions, yielding unilamellar Mn1–xCoxO2 (x = 0.2–0.5) nanosheets with a thickness of 0.8 nm. The optical absorption peak of the obtained Mn1–xCoxO2 nanosheets was continuously blueshifted as the Co content increased. The Mn1–xCoxO2 nanosheets exhibited well-defined redox peaks, which were shifted to a negative potential with increasing Co content. These results suggest that the 3d orbitals of Mn and Co are mixed owing to their statistical distribution in the nanosheets. The Mn1–xCoxO2 nanosheet electrodes showed a capacitance of 700–1000 F g–1 and improved cycle performance compared to MnO2 nanosheets.