Effect of the preparation route, PEG and annealing on the phase stability of Fe3O4 nanoparticles and their magnetic properties

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Fe3O4 nanoparticles are synthesised via two different methods: (1) co-precipitation of Fe2+ and Fe3+ ions and (2) oxidative alkaline hydrolysis of Fe2+ ions under atmospheric pressure using different protective agents (PEG 200 and PEG 3000) and urea as a base. The preparation method and the polyethylene glycol (PEG) used are concurrently affecting the phase stability of the formation of the iron oxides: the co-precipitation method using PEG 200 (E4a) or PEG 3000 (E4b) leads to the formation of different ratios of Fe2O3 and Fe3O4, whereas the oxidative hydrolysis of Fe2+ using PEG 200 gives Fe3O4 (E2) powder as a major product. The average crystallites size of E4a and E4b is almost identical, i.e. around 19 nm but the saturation magnetisation of E4b is three times larger than that of E4a. The sample E2 shows the highest saturation magnetisation value 74 emu/g, with an average crystallites size of 71 nm. Transmission electron microscopy analysis confirmed that the E2 sample shows the presence of needles crystals with typical sizes around 10 and 50 nm and its selected area diffraction (SAD) shows a typical diffraction of the spinel structure of magnetite. On the other hand, E4b sample shows elongated nanoparticles with typical sizes around 24 nm and its SAD confirmed the presence of a mixture of Fe2O3 and Fe3O4 as many dispersed spots were obtained.