Effects of vanadium- and iron-doping on crystal morphology and electrochemical properties of 1D nanostructured manganese oxides

One-dimensional (1D) nanostructures of vanadium- and iron-doped manganese oxides, Mn_1-xM_xO₂ (M = V and Fe), are synthesized via one-pot hydrothermal reactions. The results of X-ray diffraction studies and electron microscopic analyses demonstrate that all the present 1D nanostructured materials possess α-MnO₂-type structure. While the vanadium dopants produce 1D nanorods with a smaller aspect ratio of ∼3-5, iron dopants produce 1D nanowires with a high aspect ratio of >20. X-ray absorption spectroscopy clearly shows that the dopant vanadium ions are stabilized in tetravalent oxidation state with distorted octahedral symmetry, while the iron ions are stabilized in trivalent oxidation state with regular octahedral symmetry. Significant local structural distortion and size mismatch of dopant vanadium ions are responsible for the low aspect ratio of the vanadium-doped nanorods through the less effective growth of a 1D nanostructure. According to electrochemical measurements, doping with Fe and V can improve the electrode performance of 1D nanostructured manganate and such a positive effect is much more prominent for the iron dopant. The present study clearly indicates that doping with Fe and V provides an effective way of tailoring the crystal dimension and electrochemical properties of 1D nanostructured manganese oxides.