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

Abstract

Nanoparticles (NPs) of Fe3O4 and γFe2O3 synthesized by hydrothermal reaction were characterized by X-ray diffractometry (XRD), 57Fe-Mossbauer 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 Mossbauer spectrum of NP Fe3O4 prepared from hydrothermal reaction was composed of two sextets with absorption area (A), isomer shift (δ) and internal magnetic field (Hint) of 56.3 %, 0.34±0.03 mm s − 1 and 49.0±0.30 T for tetrahedral (Td) FeIII, and 43.7 %, 0.66±0.11 mm s − 1 and 44.0±0.71 T for octahedral (Oh) FeII + 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±0.03 mm s − 1 and Hintof 46.4±0.27 T due to magnetic tetrahedral FeIII. The Debye temperature of NP Fe3 − xO4 was respectively estimated to be 267±5.45 K for Fe\(^{\mathrm{III}}(T_{\mathrm{d}})\) and 282±7.17 K for Fe\(^{\mathrm{II+III}}(O_{\mathrm{h}})\), 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 − 2 to 9.49 × 10 − 4 mM for 24 h with the first order rate constant (kMB) of 2.1 × 10 − 3 min − 1. This result verifies that MB decomposing ability is enhanced by using NP γFe2O3 compared with the kMB of 1.1 × 10 − 4 min − 1 previously obtained from the leaching test using MB and bulk mixture of Fe0 − γFe2O3 (3:7).