A FeCO3 Precursor-Based Route to Microsized Peanutlike Fe3O4 Shouhu Xuan,†,‡ Lingyun Hao,‡ Wanquan Jiang,‡ Lei Song,† Yuan Hu,*,† Zuyao Chen,‡ Lingfeng Fei,§ and Tanwei Li§ State Key Laboratory of Fire Science, Department of Chemistry, and Structure Research Laboratory, Department of Chemistry, The UniVersity of Science and Technology of China (USTC), Hefei, Anhui 230026, China
CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 2 430-434
ReceiVed July 16, 2006; ReVised Manuscript ReceiVed NoVember 22, 2006
ABSTRACT: It is reported in this paper that a microsized magnetic particle Fe3O4 with peanutlike morphology is successfully prepared from FeCO3 with the same morphology via direct-sealed thermal decomposition. The peanutlike FeCO3 is obtained by hydrothermal reaction of the mixed aqueous solution containing Fe3+ and CO32- in the presence of ascorbic acid. The dynamic formation process of the FeCO3 precursor and influence of reaction conditions on the FeCO3 morphology are discussed in detail. The results show that both concentrations of ascorbic acid and Fe3+ in the reactive system play critical roles in obtaining peanutlike morphology. With this method, other microsized Fe3O4 or Fe2O3 with ellipsoidal, ellipsoidal rods, or rodlike morphologies could also be developed. 1. Introduction Magnetic materials have attracted a great deal of attention for technological application and fundamental studies.1-7 The design and synthesis of various magnetic architectural structures have been important research subjects due to their unique size and shape-dependent properties. Therefore, studies on the shape-control synthesis of magnetic materials are of great interest and are actively being pursued. Among magnetic materials, iron oxide is becoming more and more attractive because of its intrinsic half-metallic ferromagnetic nature, which can be widely used in ferrofluids,8 catalysts,9,10 biological assays,11 chemical sensors,12 and electrophotographic developers.13 Different approaches such as coprecipitation,14 the reverse micelle method,15 microwave plasma synthesis,16 sol-gel techniques,17 freeze drying,18 ultrasound irradiation,19 hydrothermal methods,20 and laser pyrolysis techniques21 are usually applied for the fabrication of magnetite. In recent years, certain studies have focused on their morphology as nanoparticles,22 nanorods,23 plate-shaped,24 wirelike,25 and tubelike26 nanostructures. However, little work has been done on other unique morphologies of iron oxides in micrometer sizes. Furthermore, it is necessary to develop a new route for the synthesis of novel magnetic microstructures and to investigate their properties. Ferrous carbonate is a special chemical material, which can be invited as the precursor to prepare Fe3O4. However, little work has been reported to fabricate magnetite particles by this method. Different morphologies for various metal carbonate minerals such as CaCO3, BaCO3, CdCO3, MnCO3, and PbCO3 have been successfully prepared.27,28 Nevertheless, FeCO3 with various morphologies has not been reported. Therefore, both the synthesis of FeCO3 with various morphologies and the synthesis of magnetite particles by using FeCO3 with certain morphologies as precursors have pressing needs not only for the fundamental interest but also for their high efficiency and facility. * To whom correspondence should be addressed. Tel: +86 551 3601664. Fax: +86 551 3601664. E-mail:
[email protected]. † State Key Laboratory of Fire Science. ‡ Department of Chemistry. § Structure Research Laboratory.
In this work, we report a novel method to synthesis Fe3O4 microcrystallites with peanutlike morphology. The synthetic procedure can be divided into two steps. First, peanutlike FeCO3 microcrystallites are fabricated on the basis of controlled selfassembly by an ascorbic acid-assisted hydrothermal process. Then, the Fe3O4 microcrystallites with peanutlike morphology are obtained by direct thermal decomposition of ferrous carbonate under controlled conditions. To the best of our knowledge, this is the first report on the synthesis of peanutlike magnetic microcrystallites. This work has resulted in an important method for further obtaining magnetic particles with various morphologies such as ellipsoidal and rods and has provided an opportunity to further apply these promising materials. 2. Experimental Section All chemicals were analytical grade and used as received without further purification. 2.1. Synthesis of FeCO3. In a typical experiment, FeCl3‚6H2O (0.002 mol) was dissolved in 25 mL of H2O with continuous stirring to form a red solution. Then, 10 mL of 0.6 M Na2CO3 was added to the solution. Five minutes later, 0.5 g of ascorbic acid was added into the abovementioned solution. After it was stirred for 10 min, the solution was transferred and sealed in a 45 mL Teflon-sealed autoclave. The autoclave was kept at 160 °C for 3 h before being cooled in air naturally. The final products were separated from the reaction medium by centrifugation and washed by deionized water and ethanol several times. Then, the products were dried at 50 °C under a vacuum oven for 12 h. A summary of all crystallization experiments that we will discuss is shown in Table 1. 2.2. Synthesis of Fe3O4 and Fe2O3. A 25 mg amount of the asprepared product was sealed in a quartz tube with 4 mL of air. Then, the tube was heated to 500 °C for 2 h at a heating rate of 2 °C/min. The black Fe3O4 powder obtained was rinsed with distilled water and absolute alcohol several times. After it was dried in a vacuum at 50 °C for 4 h, the product was collected for characterization. For the synthesis of Fe2O3, these peanutlike FeCO3 were heat-treated at 500 °C for 2 h at a heating rate of 2 °C/min under air. 2.3. Characterization. X-ray powder diffraction (XRD) patterns of the products were obtained on a Japan Rigaku D/Max-γA rotation anode X-ray diffractometer equipped with graphite monochromatized Cu KR radiation (λ ) 1.541 78 Å).Transmission electron microscopy (TEM) photographs were taken on a Hitachi model H-800 transmission electron microscope at an accelerating voltage of 200 kV. Scanning electron microscopy (SEM) images were explored on a KYKY 1010B microscope. The field emission scanning electron microscopy (FE-SEM) images were taken on a JEOL JSM-6700F SEM. Their magnetic
10.1021/cg060459q CCC: $37.00 © 2007 American Chemical Society Published on Web 01/18/2007
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Table 1. Parameters of the Representative Experiment temperature time trial (°C) (h) 1 2 3 4 5 6 7 8 9 10 11 12
160 160 160 160 160 160 160 160 160 160 160 160
3 3 3 3 3 1 1.5 2 2.5 3 3.5 3
AA (g) 0 0.12 0.4 0.5 1.0 0.5 0.5 0.5 0.5 0.5 0.5 0.5
FeCl3 Na2CO3 (mmol) (mmol) 2 2 2 2 2 2 2 2 2 1.5 1.5 4
6 6 6 6 6 6 6 6 6 6 6 6
morphology nanoparticle nanoparticle ellipsoid peanut nonuniform particle gel nanoparticle spindle oval rod rod dumbbell nonuniform particle
Figure 2. SEM image of the peanutlike FeCO3 structures with (a,b) lower magnification and (c,d) higher magnification.
Figure 1. XRD pattern of hexagonal FeCO3 with peanutlike morphology. properties (M-H curve) were measured at room temperature on a MPMS XL magnetometer made by Quantum Design Corp.
3. Results and Discussions 3.1. Characterization of the Peanutlike FeCO3. The crystal structure and phase composition of FeCO3 products were characterized by X-ray measurements. Figure 1displays the representative XRD pattern of the as-prepared FeCO3 sample, suggesting their high crystallinity. All of the peaks in this figure can be readily indexed to a pure hexagonal phase of FeCO3 (JCPDS no. 12-0531) with lattice constants a ) 4.69 Å and c ) 15.37 Å. The morphology of the product prepared by the procedure described in the Experimental Section is characterized by SEM, as shown in Figure 2. At lower magnification, the SEM image (Figure 2a,b) reveals that the FeCO3 products consist almost entirely of micrometer-scale structures with uniform sizes and well-defined peanutlike shapes. Clearly, these peanutlike structures had well-dispersed diameters in the range of 2-3 µm at the middle and 6-8 µm in the length. Moreover, a higher resolution SEM image (Figure 2c,d) shows that the surface of an individual peanutlike particle is composed of nanoparticles with sizes of 50-200 nm, which are built on the peanut via oriented attachment. 3.2. Possible Mechanism for the Formation of the Peanutlike FeCO3. To understand the formation mechanism of such peanutlike FeCO3, the detailed growth process of the peanutlike microcrystallites was carefully followed by time-dependent experiments. Figure 3a shows TEM images of the samples that were fabricated after the hydrothermal reaction was performed for 1 h and just water-soluble gel product was obtained. After 1.5 h of reaction, nearly spherical nanoparticles were formed
Figure 3. TEM and SEM image of the products prepared under different times: (a) 1.0, (b) 1.5, (c) 2, (d) 2.5, and (e) 3.0 h, respectively.
Figure 4. TEM image of the spindle FeCO3 structures.
as shown in Figure 3b. These small nanoparticles were loosely aggregated, and these aggregations may act as ellipsoid backbones for the further development of the FeCO3 spindle structures. The reason for this morphology evolution from small nanoparticles to spindle is still not clear, and the detailed growth mechanism will be discussed in the future. Figure 3c shows the SEM image of the spindle products obtained at a reaction time of 2 h. At this intermediate stage, the as-obtained sample is homogeneous in shape. It can be seen in Figure 4a that the spindle FeCO3 particles are sized ∼0.3-0.6 µm in width and
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Figure 5. TEM and SEM image of the peanutlike FeCO3 microcrystallites.
∼0.8-1.2 µm in length, whose surfaces are composed of many aggregated nanoparticles with sizes in the range of 50-200 nm. The higher magnification TEM microscope provides further insight into the morphology of the products, and Figure 4b is a typical TEM image showing spindle structures. Parts e and f of Figure 4 show the selected area electron diffraction (SAED) patterns of points 1 and 2 in Figure 4b-d. From them, it can be concluded that each nanoparticle on the spindle surface might be built on the spindle particles via an oriented attachment. The electron diffraction at Figure 4e,f shows that the spindle FeCO3 microcrystallites should be a single crystal. There were still some weak diffraction rings at the SAED, which notes that there may be little amorphous nanoparticles at the end of this structure. The growth of microcrystallites must occur by the aggregation of the colloidal amorphous precursors onto the surface and their subsequent crystallization along the vectorial system of the structure. When the reaction time was 2.5 h, oval rods were obtained (Figure 3d). This indicates that the crystal growth of the FeCO3 has the tendency to orient attachment toward larger and grow at their ends, which was confirmed by the further formation of peanutlike FeCO3 microcrystallites when the heating treatment was performed for 3.0 h. The ED pattern taken from the peanutlike structure shows that the as-prepared product is a single crystal, as shown in Figure 3e. Such FeCO3 microcrystallites, with a peanutlike morphology, have not been reported previously. A series of contrastive experiments were done and indicated that the concentration of Fe3+ ions and ascorbic acid significantly affect the morphology of the FeCO3. For example, to examine the effect of the Fe3+ ions concentration on the crystallization, the crystallization was repeated at lower and higher concentrations (0.0015 and 0.004 mol, as compared with the standard of 0.002 mol). At the lower Fe3+ concentration, rodlike particles with a rather rough surface are seen and are shown in Figure 6a,b,e. The SAED (Figure 6c) shows that these rods are single crystals. When the reaction time was 3.5 h, dumbbell-like particles were obtained (Figure 6d). The obtained dumbbell-like particles are smaller, and the outgrowth extent of both ends of the peanuts seems to be smoother than the particles obtained under the standard conditions. While the Fe3+ concentration was increased to twice the standard concentration, nonuniform FeCO3 nanoparticles were obtained. In our synthetic method, ascorbic acid is a key factor to prepare peanutlike FeCO3 microstructures, which is confirmed by the results of experiments that did not use an ascorbic acidassisted hydrothermal process. Only irregular FeCO3 particles were obtained in hydrothermal process when glucose was employed instead of ascorbic acid (Figure 7a). It should be mentioned that the concentration of ascorbic acid has a
Figure 6. TEM and SEM image of FeCO3 particles prepared under different Fe3+ concentrations.
Figure 7. SEM image of the products prepared. (a) Glucose was employed instead of ascorbic acid; (b) the amount of ascorbic acid was 0.4 g.
significant effect on the morphology of the product. When the amount of ascorbic acid is lower than 0.3 g, Fe3+ does not reduce completely and only the mixture of Fe3O4 and FeCO3 nanoparticles with several nanometers is obtained. When the amount of ascorbic acid was increased to 0.4 g, the products are ellipsoidal FeCO3 structures (Figure 7b). Peanutlike FeCO3 structures (Figure 2) were formed when the amount of ascorbic acid was more than 0.5 g while keeping other conditions unchanged. However, too much ascorbic acid was not good for the formation of peanutlike FeCO3 microcrystallites for the carbonization of the ascorbic acid at the temperature of 160 °C. Both the Fe3+ and the ascorbic acid concentration experiments indicate that the relative proportion between the
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Figure 9. XRD patterns of Fe3O4 with peanutlike morphology.
Figure 8. XRD patterns (a) and SEM image (b) of the peanutlike Fe2O3.
ascorbic acid and the Fe3+ concentrations is more relevant for determining the polymorph and morphology of the produced particles. These results suggest that it is possible to control and turn the shape of FeCO3 microcrystallites by controlling the kinetic parameters, such as the reaction time and concentration. However, our present understanding of formation of the mechanism of the peanutlike FeCO3 is still limited, and more in-depth studies are in progress. 3.3. Fabrication and Characterization of the Peanutlike Fe2O3 and Fe3O4. It has been reported that the conversion of Fe3O4 and FeCO3 into hematite is a topotactic reaction,29,30 which means that the size and shape of the starting material are preserved during the conversion. On the basis of this point, reactions were designed to obtain Fe2O3 and Fe3O4 with peanutlike morphology by direct thermal decomposition of ferrous carbonate under controlled conditions. In our experiment, these FeCO3 particles could be transformed to Fe2O3 by further annealing them at 500 °C in air for 2 h. Figure 8a shows the XRD pattern of Fe2O3 structures. All of the observed peaks of the pattern in the figure can be indexed to the hexagonal phase of Fe2O3 (JCPDS no. 86-0550). No other impurities were observed, and the diffraction peaks are strong and sharp, which indicates that the crystallization is good. Figure 8b displays the typical SEM images of the as-synthesized peanutlike morphology for Fe2O3 microcrystallites with a uniform size of 2-3 µm at the middle and 6-8 µm in the length. Although FeCO3 was transformed to Fe2O3 during the heating treatment process, the peanutlike morphology of Fe2O3 was almost maintained. Hydrogen reduction technology is often employed to convert the nonmagnetic materials to magnetic ones.31 However, little work has been reported about the synthesis of Fe3O4 by using controlled oxidation method. Herein, the direct-sealed thermal decomposition of FeCO3 is introduced. The formation of Fe3O4 can be explained on the basis that ferrous carbonate undergoes thermal decomposition to produce wustite that is oxidized to
Figure 10. SEM and TEM image of the peanutlike Fe3O4 microcrystallites.
magnetite in the presence of a controllable amount of oxygen.
6FeCO3 + O2 f 2Fe3O4 + 6CO2 4FeCO3 + O2 f 2Fe2O3 + 4CO2 It is easy to know that 6 mol of FeCO3 could be deoxidized by 1 mol of O2 to give 2 mol of Fe3O4. As calculated, there are about 0.0036 mmol of O2 in the sealed 4 mL tube, which could exactly react to 0.0216 mmol (25 mg) of FeCO3 to give Fe3O4. While the reaction was conducted in air, the excess O2 will further react to the Fe3O4 or FeO to give Fe2O3. Therefore, in our system, the molar ration between the FeCO3 and the O2 is carefully controlled, and pure magnetic Fe3O4 product was successfully obtained. Figure 9 shows the XRD patterns of the obtained Fe3O4 product. All of the strong and sharp diffraction peaks in the patterns can be indexed as the face center-cubic phase of Fe3O4 with cell constants a ) 8.396 Å, which are consistent with the values in the literature (JCPDS no. 79-0419). Figure 10a displays the typical SEM images of the as-synthesized Fe3O4 microcrystallites. From the image, one can see that the product consists of peanutlike morphology with a uniform size of 2-3 µm at the middle and 6-8 µm in the length, which is similar to the FeCO3 and Fe2O3 that we have described above. In order to determine the detailed crystalline structure, TEM measurements were employed to investigate the sample. A typical TEM image of Fe3O4 peanutlike microcrystallites is shown in Figure 10b. The size of the peanutlike particles is in good agreement with the above SEM image. Although the FeCO3 was transformed to Fe3O4 during the annealing process, the peanut morphology of the Fe3O4 was almost maintained and just a little Fe3O4 nanoparticle formed during the decomposition progress. Furthermore, many pores have been observed in the Fe3O4
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of the precursor formation and influence of reaction conditions on the FeCO3 morphology is discussed in detail. Both concentrations of Fe3+ and ascorbic acid in the reactive system play critical roles in obtaining peanutlike morphology. With this method, other microsized Fe3O4 or Fe2O3 with ellipsoidal-, ellipsoidal rod-, or rodlike shapes will also be developed. Acknowledgment. This work is supported by the Specialized Research Fund for the Doctoral Program of Higher Education (SRFDP, 20040358056) and the Postdoctoral Fund of Chinese Academy of Sciences (No. 20060390689), China. References
Figure 11. Room temperature magnetization curves of obtained Fe3O4 microcrystallites.
microstructures due to the decomposition of FeCO3 and release of CO2. This result was further proven by the following high magnification TEM image of the peanutlike Fe3O4 (Figure 10c). The SAED pattern that is performed on a peanutlike Fe3O4 is shown in Figure 10d, and these clear diffraction spots can be indexed to the cubic structure of Fe3O4, indicating that the FeCO3 transformed to peanutlike particles via a decompositionmelting-recrystallization progress. The magnetic properties of the magnetic materials have been believed to be highly dependent on the sample shape, crystallinity, magnetization direction, etc. Thus, an enhanced ferromagnetic property of the as-synthesized peanutlike magnetite microcrystallites, comparable to that of Fe3O4 nanoparticles, should be supposed. The hysteresis loop (Figure 11) of the peanutlike Fe3O4 microcrystallites shows a ferromagnetic behavior with high saturation magnetization (Ms) of 116.5 emu/g and coercivity (Hc) of 162.5 Oe. It is reported that onedimensional nanostructures have increased anisotropies in both the shape anisotropy and the magnetocrystalline, which exert influence on their magnetic properties. Shape anisotropy can increase the coercivity. Enhance anisotropy induces large magnetic coercivity, where the magnetic spins are preferentially aligned to the long axis and their reversal to the opposite direction requires higher energies than that for spheres.32,33 Here, as compared to the Hc value of the bulk Fe3O4 (115-150 Oe), the peanutlike Fe3O4 microcrystallites exhibit higher values, which may be attributed to their peanutlike structures. 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. However, for these microsized peanutlike Fe3O4 particles, the Ms value is a little higher than the bulk Fe3O4 (85-100 emu/g). Although the reasons are not clear, it is well-known that the effects of size, structure, and morphologies are related to the magnetic properties of the products. Further work should be done to clarify the physical origin of the differences. 4. Conclusion A microsized magnetic particle Fe3O4 with peanutlike was successfully prepared from FeCO3 with the same morphology as a precursor via direct-sealed thermal decomposition for 2 h at 500 °C. The precursor was obtained by hydrothermal reaction of the mixed solution containing Fe3+ and CO32- for 3 h at 160 °C in the presence of ascorbic acid. The dynamic process
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