Improved capacitive properties of layered manganese dioxide grown as nanowires

Birnesite-type MnO(2) nanowires were prepared by direct reaction of NaMnO(4) with ethanol under hydrothermal conditions. Changing the pressure vessel load allowed a layered oxide in the form of nanoflakes to be obtained. The resulting nanowires were 10-20 nm wide and several micrometers long. Interestingly, the nanowires were poorly crystalline as revealed by their XRD patterns; also, they exhibited especially strong (1 0 0) reflections that were assigned to 1D growth of the particles. By contrast, the XRD pattern for the nanoflakes was consistent with an increased crystallinity as a result of 2D particle growth. The electrochemical properties of the nanowires and nanoflakes in 0.5 M Na(2)SO(4) were evaluated by cyclic voltammetry and galvanostatic charge/discharge testing in two- and three-electrode cells. The nanowires exhibited an ideal capacitive behavior and a higher specific capacitance than the nanoflakes (191 vs. 157 Fg(-1)). In our opinion, the improved performance of the nanowires is a result of their high structural disorder and lattice strain rather than their higher specific surface area. (C) 2010 Elsevier B.V. All rights reserved.