Single-Step Synthesis, Characterization, and Application of Nanostructured K_xMn_1-yCo_yO_2-δ with Controllable Chemical Compositions and Crystal Structures

Cobalt-substituted manganese oxide one-dimensional (1D) nanowires have been synthesized through one-pot hydrothermal treatment of the ion pair solution of Co 2+ and MnO 4 -, along with their three-dimensional (3D) hierarchically assembled microspheres. The stabilization of cobalt ion in the manganese sites of α- and δ-MnO 2 structures was clearly evidenced by Mn K-edge and Co K-edge X-ray absorption spectroscopy as well as by powder X-ray diffraction and elemental analyses. The chemical composition, crystal structure, and morphology of the resultant nanostructures can be easily tailored through the control of reaction condition and precursor composition. That is, 1D nanowires with the tunnel-type α-MnO 2 structure can be prepared by the reaction at 140 °C, whereas the lowering of reaction temperature or the increase of Co content gives rise to the synthesis of 3D hierarchical nanostructured microspheres with the layered δ-MnO 2 structure. It has been obviously demonstrated that the partial Co substitution for the nanostructured manganese oxides causes the improvement of their catalytic activity with respect to olefin oxidation as well as affects significantly their electrode performances. The present study provides an effective way of controlling the chemical properties and functionalities of nanostructured manganese oxides through cation substitution.