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.