Amorphous iron oxide prepared by microwave heating
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Israel Felner Racah Institute of Physics, Hebrew University, Jerusalem, Israel
Gina Kataby and Aharon Gedanken Department of Chemistry, Bar-Ilan University, Ramat-Gan, 52900, Israel (Received 11 January 2000; accepted 20 July 2000)
Amorphous iron oxide (Fe2O3) was prepared by the pyrolysis of iron pentacarbonyl [Fe(CO)5] in a modified domestic microwave oven in refluxing chlorobenzene as a solvent under air. The reaction time was 20 min. Partially separated particles of iron oxide, 2–3 nm in diameter, were obtained. The other part showed aggregated spheres with a diameter of 25–40 nm. Differential scanning calorimetry measurements showed an amorphous/crystalline phase transition at about 250 °C.
I. INTRODUCTION
Iron oxide is one of the most abundant compounds on earth. Generally, iron oxide is a product of the oxidation of iron, and many resources are devoted to stop or slow down the process of rusting. The widespread study of iron oxides is due to their importance to many scientific disciplines.1 In the chemical industry, specifically, they are used as catalysts, pigments, and recording materials. Amorphous metal oxides have important applications, such as solar energy transformation, magnetic storage media, in the electronics industry, and also as catalysts.2–6 Much research has been carried out on the preparation of amorphous metal oxides and their characteristics over the last 20 years. These materials have been synthesized by rapidly quenching the molten mixture of metal oxides and a glass former such as P2O5, V2O5, Bi2O3, and SiO2,2,7–9 or by the thermal decomposition of some easily decomposed metal compounds.5,10 Amorphous metal oxide thin films on a substrate can be prepared by means of ion-beam sputtering, electron-beam evaporation, and thermal evaporation.11 So far, only a few pure amorphous metal oxides, such as Cr2O3, V2O5, MnO2, and PbO2 powders, have been successfully prepared by means of thermal decomposition,5,10 but it is difficult to control their purity. Other amorphous metal oxides are usually obtained in the form of hydrous oxides.6,12 Cooling rates of about 105 to 107 Ks−1 are generally required to prepare amorphous metals.13 The thermal conductivities of metal oxides are usually much lower than those of metals. It is therefore more difficult to prepare amorphous metal oxides than amorphous metals, because larger cooling rates are required for their preparation. Recently, a new method has been developed for the preparation of amorphous metal oxides.14,15 The 2176
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J. Mater. Res., Vol. 15, No. 10, Oct 2000 Downloaded: 15 Jun 2014
method uses ultrasonic waves passing through the ambient solution of a metal carbonyl or other precursors in decane or decalin; for example, Fe2O3,14 Mo2O5,15 Cr2O3, and Mn2O316 were prepared in nanophase form by this method. The sonochemical preparation of nanophased metal oxides is based on a previous experiment by Suslick and co-workers,17 who obtained nanophased amorphous iron from the sonication of Fe(CO)5 under argon. The reaso
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