Effects of vanadium- and iron-doping on crystal morphology and electrochemical properties of 1D nanostructured manganese oxides
Introduction
Over the past decades, manganese oxides have received considerable attention because of their promising functionality as electrodes for lithium-ion batteries and supercapacitors, redox catalysts, bio-sensors, and so on [1], [2], [3], [4], [5]. Research on manganese oxides has been extended to one-dimensional (1D) nanostructured homologues and three-dimensional (3D) assemblies [6], [7], [8], [9], [10], [11]. It is well known that the functionality of bulk manganese oxide can be optimized by the partial substitution of Mn with other metal ions [1]. In light of this, several attempts have been made to substitute a fraction of manganese ions in the nanostructured manganate with other transition metal ions [12], [13], [14]. However, in general, the chemical substitution is quite difficult for nanostructured manganese oxides compared with their bulk counterparts whose compositions can be easily tailored by changing the mixing ratio of solid-state reactants. Recently, we were successful [14], [15], [16], [17], [18] in preparing Cr-, Al-, Ni-, and Co-substituted manganese oxide nanowires through the redox reactions of solid-state precursors or ion-adduct precursors under hydrothermal or non-hydrothermal condition. It was found that the partial replacement of Mn with transition metal ions could improve the electrode performance of 1D nanostructured manganates [15], [16], [17]. According to X-ray absorption spectroscopic (XAS) studies, manganese ions in these nanostructured compounds are commonly stabilized in octahedral sites with a trivalent or tetravalent oxidation state. Considering the preference of transition metal ions for a specific oxidation state, iron and vanadium ions are expected to be as suitable dopants for 1D nanostructured manganate. Moreover, iron ions have a strong preference for octahedral symmetry, whereas vanadium ions can be stabilized in various local symmetries from octahedra to square planar geometry [19]. In this regard, it would be very interesting to investigate the effects of the local symmetry of dopant metal ions on the crystal morphology and electrochemical properties of 1D nanostructured manganese oxides. To date, however, there have been no reports concerning the doping of 1D nanostructured manganates with vanadium and iron ions.
In this study, we have synthesized successfully V- and Fe-doped manganese oxide nanowires via a one-pot hydrothermal reaction of a mixed solution of metal components and oxalate ligands. The effects of vanadium- and iron-doping on the chemical bonding nature and crystal morphology of the 1D nanostructured manganates have been investigated by means of a combination of microscopic and spectroscopic tools. The nanostructured materials are evaluated as intercalation electrodes for lithium secondary batteries to examine the influence of cation doping on the electrochemical properties of manganese oxide nanostructures.
Section snippets
Synthesis
The 1D nanostructures of the vanadium- and iron-doped manganese oxides, Mn1−xMxO2·yH2O (M = V and Fe), were prepared by the hydrothermal treatment of a mixture solution of manganese(II) sulfate (4 × 10−3 mol), iron(III) nitrate hemipentahydrate or vanadium(III) chloride (4 × 10−4 mol), oxalic acids (2 × 10−2 mol), and ammonium persulfate (2.5 × 10−2 mol) at 180 °C for 40 h. The pH of the reactant solution was adjusted to pH 5. To promote the incorporation of dopant ions into the lattice of manganese oxide,
Powder XRD, EDS, and TGA measurements
The powder XRD patterns of vanadium- and iron-doped manganese oxides are presented in Fig. 1. Both the cation-doped manganates showed a series of sharp Bragg reflections, which can be well indexed to α-MnO2 structure containing 2 × 2 pores [15]. Between the two compounds, the Fe-doped manganate displays diffraction peaks with higher intensities than the V-doped sample. This reflects the better crystallinity of the former phase. According to least squares fitting analysis, the unit cell volume of
Conclusions
The 1D nanostructures of vanadium- and iron-doped α-Mn1−xMxO2 (M = V and Fe) have been synthesized by one-pot hydrothermal reactions. The results of FE-SEM and TEM analyses show that the aspect ratio of the 1D nanostructured manganese oxides is effectively controlled through the doping of vanadium and iron ions. This can be attributed to a limitation in the crystal growth of manganese oxide by the incorporation of vanadium ions with different local symmetries and dissimilar ionic sizes. XANES
Acknowledgments
This work was supported by a grant (20070401034003) from the BioGreen 21 Program and partly by a Korean Science and Engineering Foundation (KOSEF) grant funded by the Korea government (MEST) (grant: R11-2005-008-00000-0). The experiments at Pohang Accelerator Laboratory (PAL) were supported in part by MOST and POSTECH.
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