Iron oxide films of a spinel structure from thermal decomposition of metal ion citrate complex
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Iron oxide films of a spinel structure from thermal decomposition of metal ion citrate complex Naofumi Uekawa Department of Applied Chemistry, Faculty of Engineering, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba-shi 263 Japan
Katsumi Kaneko Department of Chemistry, Faculty of Science, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba-shi 263, Japan (Received 7 October 1998; accepted 8 January 1999)
Iron oxide films were prepared by the polymer precursor method with alkaline metal (Li, Na, and K) ion doping. Alkaline metal ions were used to regulate the thermal decomposition process of the cation-citrate complex, that is, the precursor of the film. The spinel iron oxide films were obtained by firing the precursor with the alkaline ion doping [NayFe (atomic ratio) ^ 0.2 and KyFe (atomic ratio) ^ 0.2] at 773 K for 5 min in air. The formation mechanism of the spinel iron oxide films was investigated by differential thermal analysis (DTA) and x-ray photoelectron spectroscopy (XPS) measurements. Formation of the carbon-iron oxide complex was observed, and this reduction atmosphere induced the formation of the spinel iron oxide films. This method gave the spinel iron oxide films of which nanostructures are controlled.
I. INTRODUCTION
The polymeric precursor method that was first investigated by Pechini is a simple preparation method which produces oxide powders by firing the polymeric precursor made of cations, citric acid, and ethylene glycol.1,2 The most remarkable feature of this method is that the homogeneous dispersion of cations in the aqueous solution is reflected into the cation-citrate complex. Oxides are formed by firing the complex so that the highly homogeneous multicomponent oxide can be obtained. Tai and Lessing applied this method to the preparation of perovskite powders.3,4 Further, high Tc superconductors were successfully prepared at lower firing temperature than by the solid phase reaction.5 In a previous paper, we showed that the polymeric precursor method can be applied to the preparation of thin oxide films.6,7 The homogeneity of the resulting oxides and the low temperature synthesis have been noted in most studies on the polymeric precursor method. However, the formation process of oxides through a decomposition of the cation-citrate complex is also very important because this process contains very drastic structural change compared with the other wet-chemical synthesis of oxides. It is important to investigate the relation between the decomposition process of the metal-citrate complex and the resulting oxide structure.
Iron oxides have three representative crystal structures of a –Fe2 O3 , g –Fe2 O3 , and Fe3 O4 .8 a –Fe2 O3 has the colundum structure, while g –Fe2 O3 and Fe3 O4 have the spinel structure. In particular, the thin films of the spinel iron oxides (g –Fe2 O3 ) are very important as the magnetic recording materials. In general, the yield of these iron oxides strongly depends on the conditions of the thermal decomposition of precurso
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