Synthesis and Characterization of Monodisperse Iron Oxides

Jul 25, 2008 - Monodisperse. Fe3O4 microspheres have also been prepared, and their magnetization saturation value is about 40.1 emu/g. 1. Introduction...
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J. Phys. Chem. C 2008, 112, 12710–12715

Synthesis and Characterization of Monodisperse Iron Oxides Microspheres Debao Wang,*,† Caixia Song,‡ Yihong Zhao,† and Maoli Yang† College of Chemistry and Molecular Engineering, College of Materials Science and Engineering, Qingdao UniVersity of Science & Technology, Qingdao 266042, People’s Republic of China ReceiVed: April 12, 2008; ReVised Manuscript ReceiVed: June 9, 2008

Monodisperse R-Fe2O3 microspheres of different shapes have been selectively synthesized through the hydrothermal process. The products were characterized by powder X-ray diffraction and scanning electron microscopy. All the R-Fe2O3 microspheres have good dispersibility, uniform size, and well-defined shapes. The ferromagnetic investigation of the egg-like R-Fe2O3 microspheres gave extremely high coercivity of 3800 Oe at room temperature. The formation mechanism of these microspheres was preliminarily discussed. Nanorods and branched nanorods of a hydroxide precursor Fe8O8(OH)8Cl1.35 have been captured. Monodisperse Fe3O4 microspheres have also been prepared, and their magnetization saturation value is about 40.1 emu/g. 1. Introduction It has been accepted that the dimensional and structural characteristics of inorganic nanostructures endow them with potential applications in catalysis, medicine, electronics, cosmetics, etc. Thus the synthesis of magnetic particles with specific size and well-defined morphologies has become a hot topic in material research fields. Among magnetic particles, iron oxides (Fe2O3 and Fe3O4) have been extensively investigated. Fe2O3, as one of the most important transition magnetic metal oxides, has received increasing attention due to its extensive applications, such as magnetic recording media, catalysts, pigments, water treatment, gas sensors, optical devices, and electromagnetic devices.1 Fe3O4 particles have long been considered as an ideal candidate for applications in magnetic data storage, biological application, drug delivery, and ferrofluids.2 The synthesis of iron oxides nanocrystals of different size and shapes has attracted considerable interesting in recent years. Various iron oxide nanocrystal building blocks, such as nanoparticles,3 nanocubes,4 nanodiscs,5 nanorods,6 nanowires,7 nanotubes,8 and nanobelts,9 have been successfully prepared by a variety of methods. Recent synthesis studies on iron oxides particles have mainly focused on the preparation of hierarchical and complex structures of iron oxides, including peanutlike,10 shuttlelike,11 cantaloupe-like,12 urchinlike,13 dendritic structures,14 hollow spheres,15 nanopropellers,16 and so on. Although the fabrication of monodiperse magnetic particles have been pioneered by Matijevic’s group in the early 1980s,17 less attention has been paid to the synthesis of monodisperse spherical microparticles compared to the synthesis of nanoparticles. To meet the ever increasing technological demand, the controllable synthesis of monodisperse micro/nanospheres still needs to be greatly explored. In this paper, we reported the morphology controllable synthesis of monodisperse R-Fe2O3 microspheres with higher coercivity and Fe3O4 microspheres low magnetization saturation. * Corresponding author. E-mail: [email protected]. Tel: +86-5324022787. † College of Chemistry and Molecular Engineering. ‡ College of Materials Science and Engineering.

Figure 1. XRD patterns of the iron oxides samples synthesized with 8.6 × 10-2 mol/L of Fe3+ at 150 °C for (a) 16 h, (b) 5 h, (c) 2 h, and (d) 0.5 h.

2. Experimental Section In a typical experiment, 3 mmol FeCl3 · 6H2O and 0.06 mmol poly-(vinylpyrrolidone) (PVP) were dissolved in 35 mL of deionized water to get an orange solution of 8.6 × 10-2 mol/L of Fe3+ ions. The precursor solution was then sealed into a 50 mL Teflon-lined autoclave, followed by hydrothermal treatment at 150 °C for 16 h. The products were harvested by washing several times using alcohol and deionized water, respectively. The sample was dispersed in ethanol for further characterization. The Fe3O4 samples were obtained through similar route using ethyl glycol (EG) as solvent and in the presence of 15 mmol urea. X-ray diffraction (XRD) patterns were recorded on a Rigaku (Japan) D/Max r-A X-ray diffractometer with CuKR radiation. The morphologies and structures of the as-synthesized products were observed with a JSM-6700F field emission scanning electron microscopy (SEM). All the samples for the SEM characterization were prepared by directly transferring the suspended products to the silicon substance. Magnetic measurements for the as-synthesized samples in the powder form were carried out at room temperature using a vibrating sample magnetometer (VSM, Lakeshore 7307, U.S.A.) with a maximum magnetic field of 15 kOe.

10.1021/jp8031644 CCC: $40.75  2008 American Chemical Society Published on Web 07/25/2008

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Figure 2. SEM images of the Fe2O3 sample in different magnification (8.6 × 10-2 mol/L of Fe3+, 150 °C, 16 h).

3. Results and Discussion Figure 1a shows XRD pattern of the Fe2O3 sample prepared by hydrothermal reaction at 150 °C for 16 h. All the diffraction peaks are labeled and can be indexed to hexagonal phase of R-Fe2O3 according to the reported data (JCPDS card no. 33664), and the calculated cell parameters (a ) 0.5012 nm, c ) 1.346 nm) are comparable with the reported values. No obvious XRD peaks arising from impurities are found. Figure 2 displays SEM images of the R-Fe2O3 sample. Figure 2a reveals that the precipitate consists of a large amount of submicrometer scale particles. All these particles are monodisperse particles with a narrow size distribution. The highmagnification SEM image shown in Figure 2b indicates that these microparticles are egglike in shape and uniform in size. The diameter of the microspheres is mainly centered at 500 nm. The high-magnified SEM image in Figure 2c reveals the surface configuration of the microspheres. It is evident that the surface of the egglike microsphere is not smooth but rough, implying that these microspheres may be comprised of smaller subunits. To investigate the formation process of the R-Fe2O3 microspheres, samples subjected to different reaction durations were characterized through SEM observation. SEM image in Figure 3a reveals that samples consist of large amount of nanorods and multipod-like structures in the earlier stage of the reaction (0.5 h). An individual nanorod has length of about 300 nm and diameters of 30 nm on average. Corresponding XRD pattern is shown in Figure 1d. The sample can be identified as iron oxide hydroxide chloride (Fe8O8(OH)8Cl1.35) according to the reported data (JCPDS card no. 80-1770), which indicates the formation of a precursor for Fe2O3 in the earlier reaction stage. As the reaction proceeded (2 h), these precursor nanorods tend to aggregate into more complex assemblies (Figure 3b), and the precursor was confirmed by the XRD pattern shown in Figure 1c. R-Fe2O3 was obtained after hydrothermal reaction was conducted for 5 h. The transformation of the precursor to pure

phase R-Fe2O3 at present stage was identified by XRD pattern shown in Figure 1b. As shown in figure 3c, the obtained R-Fe2O3 microspheres are nearly monodisperse and have a narrow size distribution. The diameters of the microspheres are 1.0 µm on average. These microsphers are larger than those obtained in the final product. Figure 3d displays the SEM image of an individual R-Fe2O3 microsphere in high-magnification. It reveals the surface configuration of the microsphere, and the surface is not smooth but somewhat rough. On the basis of the SEM and TEM observation and the XRD results, a plausible formation process of the R-Fe2O3 microspheres was proposed. The overall hydrolysis reaction of FeCl3 in hydrothermal process can be expressed as follows

Fe3+ f Fe(H2O)63+ f Fe8O8(OH)8Cl1.35 f Fe2O3

(1)

There are many reports concerning the formation mechanism of inorganic microspheres.18 Generally, the formation of microstructure features after fast nucleation in solution relates to two primary mechanisms: the aggregation growth process and the Ostwald ripening process. Crystal growth by aggregation can occur by random aggregation and/or the oriented attachment mechanism, while the Ostwald ripening process involves the growth of larger crystals at expense of smaller ones. In the present work, both of the mechanisms may be involved in the formation of Fe2O3 microspheres. First, hydrated Fe3+ ions hydrolyzed under hydrothermal conditions form Fe8O8(OH)8Cl1.35 nanorods and multipod nanorods (Figure 3a). As the hydrothermal reaction proceeded, the precursor nanorods would act as primary building units to self-assemble and produce large aggregates (Figure 3b). These aggregates will continue to grow through oriented attachment of newly formed nanorods, resulting in increasing the size of the microspheres. When the hydrothermal process was further prolonged, a phase transformation took place, and the precursor Fe8O8(OH)8Cl1.35 aggregates would further hydrolyze into Fe2O3 microspheres (Figure 3c,d). Then,

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Figure 3. SEM images of iron oxides samples synthesized with 8.6 × 10-2 mol/L of Fe3+ at 150 °C for (a) 0.5 h, (b) 2 h, and (c,d) 5 h; (e,f) 170 °C, 16 h; (g,h) 4.3 × 10-2 mol/L of Fe3+, 150 °C, 16 h.

Ostwald ripening process would gradually take the place of the aggregation-based crystal growth. In this stage, the Fe2O3 subunits would recrystallize into Fe2O3 microspheres at the cost of the aggregated smaller subunits, and these microspheres might shrink in size. In addition, slight shape alteration took place along with the size shrinkage, which might relate to the

anisotropy growth of Fe2O3 in the Ostwald ripening process. Scheme 1 illustrates the schematic sketch of the formation process of Fe2O3 microspheres. Hematite has a corundum structure in the direction perpendicular to the basal plane (001) of Fe2O3, and layers of closely packed negative oxygen atoms alternate with two one third layers of positive iron atoms

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SCHEME 1: Schematic Sketch of the Formation Process of Fe2O3 Microspheres

(bilayers),19 which favors one-dimensional growth of Fe2O3 nanocrystals in hydrothermal process. As a result, the Fe2O3 microspheres were built up with one-dimensional subunits and resulted in structure anisotropy. Capped by PVP molecules, these Fe2O3 microspheres were kept monodisperse from each other. Of course, the exact formation mechanism of the Fe2O3 microspheres, especially the shape alteration, needs to be further investigated. Several other experiment parameters have also been investigated to explore the morphology control of R-Fe2O3 microspheres. When the reaction was conducted with lower concentration of iron ions of 4.3 × 10-2 mol/L, R-Fe2O3 microspheres can also be obtained, and the corresponding SEM images are shown in Figure 3e,f. The spherical particles are nearly monodisperse, and the size is rather uniform (Figure 3e). The high-magnified SEM image is shown in Figure 3f. Most of the microspheres have a linelike dent on semispheres and thus exhibit peachlike shapes. The diameters of these spheres are about 1.2 µm although two or three smaller particles can also be observed. The broken sphere reveals that these microspheres are composed of bundles of rodlike aggregates (indicated by a arrow in Figure 3f), although the surfaces of these R-Fe2O3 microspheres are rather smooth. Microspheres obtained with a lower concentration of Fe3+ ions are bigger than those with a higher concentration of Fe3+ and can be understood by the fact that fewer nuclei are formed due to the lower supersaturation

in the media. Figure 3g shows the SEM image corresponding to the sample prepared at 170 °C, which clearly indicates these R-Fe2O3 microspheres own narrow size distribution and have diameters of about 800 nm. High-magnification SEM image in Figure 3h shows that the surface of the microspheres is rather rough. Higher reaction temperature would result in higher reaction velocity and then larger Fe2O3 microspheres. When the reaction was carried out in ethyl glycol, Fe3O4 nanoparticles, instead of Fe2O3, were obtained through a similar process in the presence of urea. Figure 4a shows the XRD pattern of the sample prepared at 170 °C for 16 h. All the diffraction peaks can be indexed to cubic phase of Fe3O4 according to the reported data (JCPDS card no.19-629) confirming the formation of pure phase Fe3O4. The cell parameter was calculated to be a ) 0.8421 nm, consisting with the reported value. Figure 4b exhibits a representative SEM image of the samples. It can be observed that particles of the Fe3O4 products are spherical and quasi-monodisperse, they are uniform in size, and the average size of these particles is about 150 nm. The structural details are revealed in high-magnification SEM (Figure 4c). It can be seen clearly that the larger Fe3O4 spherical particles have a rough surface and are built up with even smaller Fe3O4 primary particles as building blocks, indicating that the Fe3O4 nanoparticles have self-assembled into Fe3O4 spherical aggregated particles.

Figure 4. (a) XRD pattern, (b,c) SEM images of Fe3O4 sample synthesized with 8.6 × 10-2 mol/L of Fe3+ in ethyl glycol at 170 °C in the presence of urea.

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Figure 5. Room-temperature magnetization curves of the (a) Fe2O3 sample and (b) Fe3O4 sample.

It is well known that the magnetization of ferromagnetic materials is very sensitive to the morphologis and structures of the as-synthesized samples.20 The magnetic hysteresis measurements of the egglike R-Fe2O3 microspheres were carried out at room temperature in the applied magnetic field sweeping from -15 to 15 kOe. Figure 5a is the hysteresis loop of the as-synthesized R-Fe2O3 microspheres. It can be seen that no saturation of the magnetization as a function of the field is observed up to the maximum applied magnetic field, which is similar to the reported cases in the literature.6c,11a,12,13,21 The hysteresis loop indicates that these R-Fe2O3 microspheres have a ferromagnetic behavior with remanent magnetization of 0.1 emu/g and extremely high coercivity of 3800 Oe at room temperature. The value of remanent magnetization is somewhat lower than that of cantaloupe-like R-Fe2O3 microparticles (0.2182 emu/g)12 and is much higher than those of urchinlike R-Fe2O3 microparticles (4.6783 × 10-3 emu/g)13 and rodlike R-Fe2O3 nanoparticles (2.754 × 10-3 emu/g).6c While the coercivity is much higher than those of other morphologies of R-Fe2O3. Although the detailed reasons are not clear, it is well accepted that the magnetic properties of magnetic materials are influenced by many factors, such as size, structure, and morphologies, and so forth. The higher remanent magnetization and coercivity behavior of R-Fe2O3 microspheres in this work may be mainly caused by their unique structure and shape. Obviously, further investigations are needed to clarify the physical origin of the abrupt step observed in the central part of Figure 5a. Figure 5b shows the room temperature magnetic hysteresis curves for the Fe3O4 sample. The hysteresis loop of the product shows ferromagnetic behavior with saturation magnetization, remanent magnetization, and coercivity values of about 40.1 emu/g, 3.3 emu/g, and 60.0 Oe, respectively. The saturation magnetization is quite close to those of the porous Fe3O4 nanoparticles (42.8 emu/g)22 and the Fe3O4 nanoparticles protected by (N(CH3)4OH) (40 emu/g)23 and is smaller than those reported for Fe3O4 nanodiscs (70 emu/g),5 nanoparticles (46-86 emu/g),24 nanorods complex (82.6 emu/g),25 and singlecrystalline microspheres (81.9 emu/g).26 The coercivity values are larger than that of the porous Fe3O4 nanoparticles (40 Oe)22 but is obviously smaller than that of the bulk maghemite (115-150 Oe). As we know, the saturation magnetization of nanoparticles was lower than that of correspondent bulk sample and decreased with the reduction of the particle size. As compared to the saturation magnetization of the bulk maghemite (92 emu/g),27 the as-obtained Fe3O4 nanoparticles exhibit much lower saturation magnetization. It could be explained by the

small particle surface effect and the internal cation disorder,28 considering these spherical aggregates are composed of smaller Fe3O4 primary particles. 4. Conclusion In conclusion, Fe2O3 microspheres of different shapes were prepared by employing suitable experiment conditions. The particles have uniform size and well dispersity. No saturation of the magnetization was observed for egglike Fe2O3 microspheres up to the maximum applied magnetic field, but extremely high coercivity of 3800 Oe was recorded. The asprepared Fe3O4 microspheres showed saturation magnetization and coercivity of 40.1 emu/g and 60.0 Oe, respectively. For technological advances in the future, further improvements of their magnetic properties are desired. Acknowledgment. This work was financially supported by the Found of Education Department of Shandong Province (J05D07) and the Found for Excellent Young and Middle Aged Scientist (2007BS04029) in Shandong Province in China. References and Notes (1) (a) Kesavan, V.; Sivanand, P. S.; Chandrasekaran, S.; Koltypin, Y.; Gedanken, A. Angew. Chem., Int. Ed. 1999, 38, 3521. (b) Faust, B. C.; Hoffmann, M. R.; Bahnemann, D. W. J. Phys. Chem. 1989, 93, 6371. (c) Matijevic, E.; Scheiner, P. J. Colloid Interface Sci. 1978, 63, 509. (2) (a) Pascal, C.; Pascal, J. L.; Favier, F.; Moubtassim, M. L. E.; Payen, C. Chem. Mater. 1999, 11, 141. (b) Berggren, K. F.; Yakimenko, I. I. Phys. ReV. B 2002, 66, 085353. (c) Hyeon, T.; Lee, S. S.; Park, J.; Chung, Y.; Na, H. B. J. Am. Chem. Soc. 2001, 123, 12798. (3) (a) Song, Q.; Ding, Y.; Wang, Z. L.; Zhang, Z. J. J. Phys. Chem. B 2006, 110, 25547. (b) Sun, S.; Zeng, H. J. J. Am. Chem. Soc. 2002, 124, 8204. (c) Sahu, K. K.; Rath, C.; Mishra, N. C.; Anand, S.; Das, R. P. J. Colloid Interface Sci. 1997, 185, 402. (d) Jia, B. P.; Gao, L. J. Phys. Chem. C 2008, 112, 666. (e) Xu, L. Q.; Zhang, W. Q.; Ding, Y. W.; Peng, Y. Y.; Zhang, S. Y.; Yu, W. C.; Qian, Y. T. J. Phys. Chem. B 2004, 108, 10859. (4) (a) Zhao, L.; Zhang, H. J.; Xing, Y.; Song, S. Y.; Yu, S. Y.; Shi, W. D.; Guo, X. M.; Yang, J. H.; Lei, Y. Q.; Cao, F. Chem. Mater. 2007, 20, 198. (b) Zheng, Y. H.; Cheng, Y.; Wang, Y. S.; Bao, F.; Zhou, L. H.; Wei, X. F.; Zhang, Y. Y.; Zheng, Q. J. Phys. Chem. B 2006, 110, 3093. (5) Zou, G. F.; Xiong, K.; Jiang, C. L.; Li, H.; Wang, Y.; Zhang, S. Y.; Qian, Y. T. Nanotechnology 2005, 16, 1584. (6) (a) Tang, B.; Wang, G. L.; Zhuo, L. H.; Ge, J. H.; Cui, L. J. Inorg. Chem. 2006, 45, 5196. (b) Lian, S.; Wang, E.; Kang, Z.; Bai, Y.; Gao, L.; Jiang, M.; Hu, C.; Xu, L. Solid State Commun. 2004, 129, 485. (c) Jing, Z.; Wu, S. Mater. Lett. 2004, 58, 3637. (7) (a) Pascal, C.; Pascal, J. L.; Favier, F.; Moubtassim, M. L. E.; Payen, C. Chem. Mater. 1999, 11, 141. (b) Fu, Y.; Chen, J.; Zhang, H. Chem. Phys. Lett. 2001, 350, 491. (c) Fu, Y.; Wang, R.; Xu, J.; Chen, J.; Yan, Y.; Narlikar, A.; Zhang, H. Chem. Phys. Lett. 2003, 379, 373. (d) Chueh, Y.L.; Lai, M.-W.; Liang, J.-Q.; Chou, L.-J.; Wang, Z. L. AdV. Funct. Mater. 2006, 16, 2243.

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