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Local structure and water cleaning ability of iron oxide nanoparticles prepared by hydro-thermal reaction

Nanoparticles (NPs) of Fe3O4 and γFe2O3 synthesized by hydrothermal reaction were characterized by X-ray diffractometry (XRD), 57Fe-Mössbauer spectroscopy and field emission scanning electron microscopy (FE-SEM). A decrease in concentration of methylene blue (MB) aqueous solution due to bulk Fe0-NP γFe2O3 mixture with the mass ratio of 3:7 was measured by ultraviolet-visible light absorption spectroscopy (UV-Vis). The Mössbauer spectrum of NP Fe3O4 prepared from hydrothermal reaction was composed of two sextets with absorption area (A), isomer shift (δ) and internal magnetic field (H int) of 56.3 %, 0.34±0.03mm s-1 and 49.0±0.30 T for tetrahedral (T d) FeIII, and 43.7 %, 0.66±0.11mm s-1 and 44.0±0.71 T for octahedral (O h) Fe(II + III). The FeII/FeIII ratio was determined to be 0.280 for NP Fe3O4, giving 'x' of 0.124 in Fe3-xO4. These results show that NP Fe3O4 prepared by hydrothermal reaction was not regular but nonstoichiometric Fe3O4. Consistent results were observed for XRD patterns of NP Fe3-xO4 indicating sharp intense peaks at 2Θ of 30.2, 35.7 and 43.3° with a large linewidth of 0.44°, yielding the crystallite size of 29-37 nm from the Scherrer's equation. Iso-thermal annealing of NP Fe3-xO4 at 250 °C for 30 min resulted in the precipitation of NP γFe2O3 with δ of 0.33±0.03mms-1 and H into f 46.4±0.27T due to magnetic tetrahedral FeIII. The Debye temperature of NP Fe3-xO4 was respectively estimated to be 267±5.45 K for FeIII(Td) and 282±7.17 K for FeII+III(Oh), both of which were smaller than that obtained for bulk Fe3O4 of 280±4.15 K and 307±5.70 K, indicating that the chemical environment of iron of NPs is less rigid than that of the bulk compounds. A leaching test using methylene blue (MB) and mixture of bulk Fe0-NP γFe2O3(3:7) showed a remarkable decrease in MB concentration from 1.90 × 10-2to 9.49 × 10-4 mM for 24 h with the first order rate constant (k MB) of 2.1 × 10-3min-1. This result verifies that MB decomposing ability is enhanced by using NP γFe2O3 compared with the k MB of 1.1 × 10-4 min-1 previously obtained from the leaching test using MB and bulk mixture of Fe0-γFe2O3 (3:7).

Debye temperature ; Maghemite ; Magnetite ; Methylene blue ; Nano particle

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