Structural characteritics of iron oxide nanoparticles synthesized by co-precipitation method in different conditions

Abstract. This study aimed to investigate the structural characteristics of iron oxide nanoparticles (IONPs) prepared in different conditions. IONPs were synthesized as aqueous magnetic fluids by co-precipitation of ferrous and ferric salts, then analyzed by transmission electron microscopy (TEM), X-ray diffraction (XRD), and Raman scattering spectroscopy. Results showed that as-prepared IONPs revealed as a mixture of hematite (α-Fe2O3), maghemite (γ-Fe2O3), and magnetite (Fe3O4) composition. At 900C of reaction temperature, IONPs showed a good monodispersity and stability, and sizes range from 9.2 to 12 nm. The Raman spectra also showed the appearance of moderate laser heating in the hematite structure. The model of IONPs formation was found with a magnetite nucleus and maghemite that formed the outer layers after the oxidation process.

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100 HNUE JOURNAL OF SCIENCE DOI: 10.18173/2354-1059.2018-0034 Natural Sciences 2018, Volume 63, Issue 6, pp. 100-105 This paper is available online at STRUCTURAL CHARACTERITICS OF IRON OXIDE NANOPARTICLES SYNTHESIZED BY CO-PRECIPITATION METHOD IN DIFFERENT CONDITIONS Nguyen Thi Luyen 1 , Tran Quang Huy 2,3 and Pham Van Vinh 4 1 Faculty of Physics and Technology, Thai Nguyen University of Science 2 National Institute of Hygiene and Epidemiology, 3 Hung Yen University of Technology and Education 4 Faculty of Physics, Hanoi National University of Education Abstract. This study aimed to investigate the structural characteristics of iron oxide nanoparticles (IONPs) prepared in different conditions. IONPs were synthesized as aqueous magnetic fluids by co-precipitation of ferrous and ferric salts, then analyzed by transmission electron microscopy (TEM), X-ray diffraction (XRD), and Raman scattering spectroscopy. Results showed that as-prepared IONPs revealed as a mixture of hematite (α-Fe2O3), maghemite (γ-Fe2O3), and magnetite (Fe3O4) composition. At 90 0 C of reaction temperature, IONPs showed a good monodispersity and stability, and sizes range from 9.2 to 12 nm. The Raman spectra also showed the appearance of moderate laser heating in the hematite structure. The model of IONPs formation was found with a magnetite nucleus and maghemite that formed the outer layers after the oxidation process. Keywords: Iron oxide nanoparticles, IONPs, co-precipitation, Raman spectra. 1. Introduction Iron oxide nanoparticles (IONPs) have been widely studied for different applications in the fields of biomedicine [1, 2], and water treatment [3-5]. IONPs can be effectively prepared by several methods, in which the co-precipitation method of ferric and ferrous salts has been popularly used. Interestingly, IONPs exhibit super paramagnetic behavior with sizes of less than 35 nm [6, 7]. Magnetite (Fe3O4) nanoparticles have been proved with many unique magnetic properties, such as super paramagnetic, low Curie temperature, and high magnetic susceptibility [1, 6], so they can be applied for various purposes. However, the broad size distribution of IONPs can result in limitations for further applications. Among four different crystalline polymorphs of Fe2O3 including γ-Fe2O3, β-Fe2O3, ε-Fe2O3 and α-Fe2O3, the structure of γ-Fe2O3 and α-Fe2O3 phases has been considered rather than the rest [8-10]. γ-Fe2O3, a ferromagnetic material exhibits the cubic spinel structure at room temperature with a Curie temperature (TC) of 928 K. By contrast, α-Fe2O3 crystallizes in the corundum structure reveals as the most thermodynamically stable phase. It is weakly ferromagnetic at room temperature due to the Dzyaloshinsky-Moriya mechanism and anti-ferromagnetic below 263 K [11-14]. At elevated temperatures of more than 950 K, α-Fe2O3 shows paramagnetic behavior [15]. Moreover, there are some models available to explain Received May 23, 2018. Revised August 13, 2018. Accepted August 20, 2018. Contact Nguyen Thi Luyen, Email: luyennt@tnus.edu.vn.com. Structural characteristics of iron oxide nanoparticles synthesized by co-precipitation method 101 the formation of IONPs. For examples, Sousa [16] and Wang [17] also have suggested models with a maghemite nucleus and magnetite forming the outer layers after oxidation. Chroupa et al. [18] have used Raman confocal multispectral imaging to investigate the chemical and structural properties of ferrite-based nanoparticles, precursors for magnetic drug targeting, the results affirmed that oxidized nanoparticles have partly conversed of magnetite into maghemite, and a model of IONPs formation is also suggested by a magnetite nucleus and maghemite layers. In fact, the molecular composition of IONPs can vary considerably, depending on experimental conditions. Through a well-known magnetite oxidation, antiferromagnetic hematite (α-Fe2O3) can be found [19, 20] or reveals as an amorphous non-stoichiometric oxyhydroxide [FeOx(OH)3-2x, x < 1] and Fe(OH)3 in magnetite and maghemite nanoparticles [16], resulting in the changes of IONPs formation. Furthermore, the structural characteristics of IONPs synthesized in different conditions could be influential to the monodispersity and stability in the solution as well as magnetic properties leading to advantage or limitations in various applications [18]. In this work, three methods of TEM, XRD and Raman analyses were employed to investigate the structural characteristics of IONPs synthesized by the co-precipitation method in different conditions, particularly, under the systematic changes of reaction temperature, and precursors. The results would be expected to give supplementary information related to the morphology, size, crystallinity and the magnetic behavior of Fe3O4, γ-Fe2O3 and α-Fe2O3 nanoparticles synthesized. 2. Content 2.1. Experiments 2.1.1. Materials Ferric chloride (FeCl3.6H2O), ferrous chloride (FeCl2.4H2O), and ammonia solution (NH4OH), were purchased from Sigma Aldrich, Merck Chemical Company. Deionized water was used to prepare all solutions. 2.1.2. Preparation of iron oxide nanoparticles IONPs were prepared by the co-precipitation of ferric and ferrous ions in deionized water. A series of two samples was prepared by changing the reaction temperature from 30 o C to 90 o C (denoted by F1 and F5) and concentrations of NH4OH from 0.1 M to 0.23 M (denoted by F5 and F9). Firstly, by changing the reaction temperature, a mixture of 0.6 g of FeCl2.4H2O and 1.6 g of FeCl3.6H2O were dissolved in 25 mL of deionized water with the range of temperatures changed from 30, 45, 60, and 75 to 90 o C. Then, 5 mL of NH4OH was added drop wise to the mixture with vigorous stirring. After that, the solution was vigorously stirred for another 30 min. Finally, the resultant synthesized IONPs were centrifuged and washed with deionized water for several times. To change concentrations of NH4OH, the reaction temperature was fixed at 75 o C. 2.1.3. Characterization Surface morphology, size and dispersion of IONPs were characterized by transmission electron microscopy (TEM) (JEM 1010, JEOL), operated at 80 kV. Structure of IONPs were investigated by a D2 X-ray diffractometer equipped with a Cu Kα tube and a Ni filter (λ = 1.542 Ao). Raman spectra of these samples were recorded by using a high-resolution confocal Raman microscope (Horiba, LabRam HR) and a 538.14 nm Ar laser source, at the room temperature under the same experimental condition (18 mW, 20x microscopy objective) with acquisition time of 10 s. Nguyen Thi Luyen, Tran Quang Huy and Pham Van Vinh 102 2.2. Results and discussions 2.2.1. Morphology of IONPs TEM images of IONPs prepared at different temperatures (60 o C and 90 o C), for 30 min, are given in Figure 1. There was no different in size and dispersity of IONPs at the time point of synthesis and after three months. It can be observed that they are all sphere-like structures. IONPs prepared at 60 o C, exhibit serious agglomeration (Figure 1a), while the IONPs prepared at 90 o C show a good dispersion (Figure 1b). A histogram of their size distribution has been obtained by the particle size measurement at different reaction temperatures, which are shown in Figure 1c, and 1d, respectively. TEM images of IONPs samples at 60 o C and 90 o C revealed that the average particle sizes are about 9.2 nm and 12 nm, respectively, with narrow size distributions. The size of IONPs increases with the increase of reaction temperature in good dispersion. Figure 1. TEM images of IONPs obtained at different temperatures: (a) 60 o C; (b) 90 o C and (c), (d) histograms of IONPs size distribution, respectively 2.2.2. XRD patterns and Raman spectra of IONPs In this study, XRD was performed with IONPs synthesized at different temperatures and concentrations of NH4OH to investigate the change in crystal structures (Figure 2). Obviously, the diffraction peaks are indexed by cubic structure of γ-Fe2O3 phase, meet the reference data for cubic close packed γ-Fe2O3 phase (JCPDS card 39-1356). Both the magnetite and maghemite phases have the same spinel structure as well as very similar lattice parameters. As seen at 2 theta, all characteristic peaks are found at 30.58 o , 36.03 o , 43.66 o , 54 o , 57.55 o and 63.10 o , corresponding to the lattice planes of (220), (311), (400), (422), (511) and (440) of either γ-Fe2O3 phase or Fe3O4 cubic structure. No specific peaks are observed in the samples for any impurities, so it also reveals Structural characteristics of iron oxide nanoparticles synthesized by co-precipitation method 103 a high phase purity of γ-Fe2O3 or Fe3O4. According to Debye – Scherrer equation, the average crystallite sizes of two samples at 60 o C (F3) and 90 o C (F5) are calculated to be 9.5 and 11.2 nm, respectively [21]. Figure 2. X-ray power diffraction patterns of IONPs prepared with (a) different temperature ranging from 30 o C to 90 o C (F1-F5); and (b) different concentration of NH4OH from 0.1 to 0.23 M (F5-F9) As discussed in the XRD patterns, we found that IONPs still contain a dominant fraction of magnetite. A model is suggested with a magnetite nucleus, since the magnetite can be preserved from oxidation only in the inner layers of each nanoparticle. Similar to iron oxidation films, nanoparticles consist of an inner Fe3O4 and an outer, passive layer of Fe2O3. Figure 3. Raman spectra of IONPs prepared with (a) different reaction temperature from 30 o C to 90 o C (F1-F5); and (b) different concentrations of NH4OH from 0.1 to 0.23 M (F5-F9) Raman spectroscopy was used to distinguish the γ-Fe2O3, α-Fe2O3 and Fe3O4. Figure 3 (a, b) shows the Raman spectrum of two samples series with different reaction temperatures and concentrations of NH4OH, respectively. The peaks with strong intensity are at about 383 cm -1 , 582 cm -1 and 806 cm -1 , corresponding to γ-Fe2O3 nanoparticles. The sharp and intense peak at around 806 cm -1 , which characteristic of magnetite, can be assigned to A1g vibrational mode of magnetite [18, 22, 23]. In fact, it is not easy to see the difference in crystal structures of IONPs from XRD patterns, for example, Fe3O4 and γ-Fe2O3 nanoparticles have some diffraction peaks overlapped, so Raman spectroscopy becomes an advantageous to differ these structures. Nguyen Thi Luyen, Tran Quang Huy and Pham Van Vinh 104 Figure 4. Spectral changes in the 100 - 900 cm -1 region for IONPs (F8 sample) Raman spectra fitted with Lorentz function Table 1. Raman frequencies (cm -1 ) of as-prepared IONPs Bulk hematite Bulk magnetite Bulk maghemite Nanophase in this study Assignment in the spectra nanoparticles s 703 s 806 Maghemite s 662 w 610 m 411 vw 528 vw 508 vw 456 s 502 s 582 s 484 Maghemite hematite w 303 s 330 s 383 Maghemite ss 292 vw 247 ss 225 w 194 ss 212 ss 275 Hematite Hematite The peaks at 383 cm -1 and 582 cm -1 can be attributed to T2g vibrational modes of magnetite. Generally, upon laser-stimulated, hematite structure was formed from pre-initialized oxidation sites. The remaining peaks with very strong intensity at 212 cm -1 , 275 cm -1 and strong intensity at 484 cm -1 , corresponding to α-Fe2O3 nanoparticles, can be assigned to A1g, E1g and A1g vibrational modes of hematite [18, 24]. There is no significant Raman peaks assignable to iron oxyhydroxide goethite (α-FeOOH) or lepidocrocite (γ-FeOOH) [18, 20, 24]. Through the information of structural characteristics, it is clear that IONPs formed by a magnetite nucleus and outer layers of maghemite after the oxidation process, and it can be influential to their stability, monodispersity, and also magnetic properties. In this study, the magnetic characteristics of as-synthesized IONPs have not investigated yet, but it should be done according to the structural changes of IONPs synthesized in different conditions, and also the purpose of particular applications. Structural characteristics of iron oxide nanoparticles synthesized by co-precipitation method 105 3. Conclusion In summary, iron oxide nanoparticles have been successfully synthesized using the co- precipitation method in different conditions, and their structural characteristics are investigated. IONPs are found with good monodispersity, and stability at the reaction temperature of 90 o C, and their sizes are in the range of 9.2 to 12 nm. As-synthesized IONPs have a mixed magnetite- maghemite structures under XRD investigation, while Raman spectroscopy reveals the presence of hematite structure in the samples. The model of IONPs formation is also confirmed with a magnetite nucleus and maghemite layers. Together with the information of structural characteristics of IONPs investigated by Raman confocal multispectral imaging, the results would be helpful for various applications, particularly in drug targeting and delivery. Acknowledgements: This research was supported by the bilateral project between Italy and Vietnam, coded NĐT.05.ITA/15. REFERENCES [1] Y. L. Chueh, M. W. Lai, J. Q. Liang, L. J. Chou and Z. L. Wang, 2006. Adv. Funct. Mater. 16 2243. [2] P. P. Yang, S. L. Gai and J. Lin, 2012. Chem. Soc. Rev. 413679. [3] T. Q. Huy, P. V. Chung, N. T. Thuy, C. 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