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Synthesis and Characterization of Nanocrystalline Manganese Oxide

Manganese oxides are important materials in many applications such as catalyst, electrode, high-density magnetic storage media, ion exchange, sensor, molecular adsorption and electronics.1-4 Particularly Mn3O4 (hausmannite) is known to be an efficient catalyst in the decomposition of waste gas NOx, reduction of nitrobenzene and oxidation of methane. MnO is ferromagnetic in the form of nanoparticles although its bulk form is antiferromagnetic. In this preliminary overview we present the synthesis and characterization of MnO and Mn3O4 nanoparticles together with their magnetic properties. A novel nonaqueous route has been applied for the preparation of nanocrystalline manganese oxide compounds. The applied procedure is based on the benzyl alcohol route5 using KMnO4 as precursor for the synthesis of Mn3O4 and Mn(acac)2 for MnO. Rietveld refinement of the XRD powder patterns, combined high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED) and energy dispersive X-ray (EDX) analysis as well as electron energy loss spectroscopy (EELS) were employed for the structural characterization of the as-synthesized compounds. Magnetic measurements, M(H, T), were performed using a SQUID magnetometer. The XRD analysis revealed that the as-prepared samples are not phase pure. In the case of KMnO4 as precursor, the products consists of around 80 wt.% of Mn3O4 and 20 wt.% of MnO, while in the Mn(acac)2 case the dominant phase is MnO with only traces of Mn3O4. The analysis of positions and shape of the manganese L2, 3-edge spectra confirmed the mixed valence of Mn (2+ and 3+) and the simultaneous presence of both oxides in the investigated samples. TEM images revealed multi-morphology of MnO, Mn3O4 nanoparticles including spherules, ellipsoids and rhombohedra. HRTEM images showed zone-axis oriented nanocrystals, supercell formation in MnO structure. Image simulation based on the multislice approach shows variation of the final HRTEM image with thickness and defocus value. Finally, the SQUID measurement of the MnO nanoparticles gave evidence for their superparamagnetic behaviour which is different from previous reports that predicted a ferromagnetic character of the MnO nanoparticles. (1) Armstrong, A. R. ; Bruce, P. G. Nature 1996, 381, 499-500. (2) Shen, Y. F. ; P., Z. R. ; N., D. R. ; L., S. S. ; L., M. ; Potter, D. I. ; O` Young, C. L. Science 1993, 260, 511-515. (3) Seo, W. S. ; Jo, H. H. ; Lee, K. ; Kim, B. ; Oh, S. J. ; Park, J. T. Angew. Chem. Int. Ed. 2004, 43, 1115-1117. (4) Anilkumar, M. ; Ravi, V. Mater. Res. Bull. 2005, 40, 605-609. (5) Niederberger, M. ; Garnweitner, G. ; Pinna, N. ; Neri, G. Prog. Solid State Chem. 2005, 33, 59-70.

Nonaqueous sol-gel; Electron microscopy; Oxides

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