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Study of Self-Assembly of Octahedral Magnetite under an External Magnetic Field Haiping Qi,† Qianwang Chen,*,‡,§ Mingsheng Wang,‡ Minhua Wen,‡ and Jie Xiong‡ Department of Chemistry, UniVersity of Science and Technology of China, Hefei, Anhui 230026, P. R. China; Department of Materials Science & Engineering, UniVersity of Science and Technology of China, Hefei, Anhui 230026, P. R. China; and Hefei National Laboratory for Physical Sciences at Microscale, UniVersity of Science and Technology of China, Hefei, Anhui 230026, P. R. China ReceiVed: April 28, 2009; ReVised Manuscript ReceiVed: August 18, 2009
In order to study the self-assembly of octahedral magnetite (Fe3O4) under external magnetic fields, the octahedral magnetite with high crystallization was synthesized through a solvothermal process. When the concentration of NaOH was adjusted from 0 to 3.0 mol/L, the shape of Fe3O4 particles was transformed from 3-D selfassembly sphere to truncated octahedron and finally to octahedron. Previous studies have showed that magnetite chains can be formed when an external magnetic field was applied during the reaction process. Here we study the self-assembly of magnetite octahedrons under an external magnetic field and find that these octahedrons can also form chains, mainly along the 〈111〉 directionsone of its two easy magnetic axessrather than the other axis 〈110〉 direction. This fact indicates that particle morphology can significantly influence their assembly mechanism besides the directions of easy-magnetic axis. Introduction Magnetic field is an important parameter for studying the physical properties of materials.1-4 In recent years, magnetic field, similar to the conventional reaction parameters such as temperature and pressure, has been introduced as a new tool to control chemical reactions, materials synthesis, and especially to prepare ordered nanoarrays in several systems, such as electrochemical process,5,6 solid state reactions,7 hydrothermal synthesis,8 and chemical vapor deposition.9 It is well-known that magnetic fields can induce self-assembly of magnetic particles. For example, our group has first introduced magnetic fields into a hydrothermal system for the synthesis of onedimensional magnetic wires or chains8,10,11 and found that magnetic particles would align and assemble into 1D ordered pattern along the easy-magnetic axis under an external magnetic field. Whitesides et al. have described the three-dimensional (3D) self-assembly of metallic rods into high stable microstructures using magnetic interactions to guide the assembly process.12 The alignment of cobalt ferrite nanoparticles (CoFe2O4) microtubules under magnetic fields has also been reported by Williams et al.13 However, those works mainly focused on the study of spheres or near-spheres, while the self-assembly of the magnetic polyhedrons under magnetic fields has not been reported yet. The self-assembly of single-crystal magnetic particles remains a significant research subject, especially particles with specific crystallographic shape, which is supposed to be very helpful in understanding the essence of the selfassembly process under magnetic fields. From a crystallographic point of view, Fe3O4 has a spinel structure with oxygen anions forming a face-centered-cubic (FCC) closed packing. The crystal structure of magnetite comprises two different iron sites: tetrahedral sites that are occupied by Fe(III) and octahedral sites that are occupied by * Corresponding author: e-mail
[email protected]; Tel (+86)5513607292; Fax (+86)551-3607292. † Department of Chemistry. ‡ Department of Materials Science & Engineering. § Hefei National Laboratory for Physical Sciences at Microscale.
both Fe(II) and Fe(III).14 Over the past decades, numerous attentions have been paid to the study of Fe3O4 due to its potential applications in high-density magnetic recording media,15 magnetic sensors,16 ferrofluid,17 printing ink,18 magnetic resonance imaging,19 especially biomedical field,20,21 etc. As a FCC crystal, Fe3O4 possesses a general sequence of surface energies, γ{111} < γ{100} < γ{110},22 which means that the Fe3O4 crystals usually exist with an octahedron surrounded by {111} lattice planes. Naturally, Fe3O4 with an octahedral shape can best exhibit its intrinsic properties. Fe3O4 has two easymagnetic axes: the 〈111〉 direction and the 〈110〉 direction. Safely, it can be concluded that the octahedral magnetite will form chains along the easy-magnetic axes under an external magnetic field.10,11,23 However, for the magnetite polyhedrons, the competition of magnetic interactions along two easymagnetic axes is not clearly understood and still needs to be investigated. Recently, our group has synthesized octahedral magnetite microcrystals with high crystallinity.24 However, applying an external magnetic field is restricted because of the high pressure in the reaction process, so it is still necessary to design a new reaction system. Herein, we report a simple solvothermal process to prepare Fe3O4 octahedrons. Keeping other conditions constant, chains of magnetite octahedrons could formed under an external magnetic field in the reaction process. On the basis of the novel morphology of chains, we discussed the possible assembly mechanism of Fe3O4 octahedrons under external magnetic fields. Experimental Section The chemical reagents used in the work were potassium ferrocene ((η-C5H5)2Fe), sodium borohydride (NaBH4), sodium hydroxide (NaOH), polyvinylpyrrolidone (PVP, K30), and alcohol (g99%). All the chemicals were of analytical grade and were used as received. In a typical procedure of synthesizing Fe3O4 octahedrons, 0.3 g of ferrocene, 0.3 g of sodium borohydride, and 3.0 g of PVP were dissolved in 1 mol/L of NaOH alcohol solution. After being vigorously stirred for 10 min and sonication for 20 min (250 W ultrasonic power), the
10.1021/jp904928s CCC: $40.75 2009 American Chemical Society Published on Web 09/11/2009
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Figure 1. XRD patterns of as-prepared samples. The concentrations of NaOH are as follows: S1, 0; S2, 0.5 mol/L; S3, 1.0 mol/L. Sxm (x ) 1, 2, 3) denotes the sample prepared by applying an external magnetic field of 0.4 T in the reaction process of Sx sample while keeping other conditions constant.
solution was transferred into a Teflon-lined stainless steel autoclave with 40 mL capacity (without external magnetic field). The autoclave was sealed tightly and maintained at 180 °C for 30 h subsequently. After the reaction was over, the autoclave was allowed to cool to room temperature naturally. The resulting black solid precipitates were separated easily from the reaction system by applying a permanent magnet; the supernatant was discarded by decantation. Then, distilled water and alcohol were added to wash the precipitates several times. Finally, the products were dried in a vacuum oven at 40 °C. Other reference experiments of different concentration of NaOH were conducted to synthesize magnetite with different morphologies. In order to study the self-assembly of octahedral magnetite (Fe3O4) under external magnetic fields, a magnetic field of 0.4 T was introduced during the reaction process. The as-synthesized samples were checked in sequence by X-ray diffraction (XRD, Philips X’Pert PRO SUPER X-ray diffractometer with Cu KR radiation (λ ) 1.541 78 Å)), transmission electron microscopy (TEM, JEOL-2010), field emission scanning electron microscopy (FESEM, JEOL JSM6700M), and a vibrating sample magnetometer (VSM, BHV55).
Fe2+ + BH4- f Fe V +H2v
(2)
Fe2+ + BH4- + OH- f Fe V +BO2-
(3)
3Fe + 2O2 f Fe3O4
(4)
As shown in Figure 2, Fe3O4 particles form chains under an external magnetic field. Parts a and b of Figure 2 show representative TEM and SEM images of S1 obtained without NaOH, respectively. It can be found that the sample has a spherical shape with the average diameter of about 450 nm. The Fe3O4 spherical particles have a rough appearance and are composed of large numbers of Fe3O4 nanoparticles (average diameter of 5 nm). Figure 2c-h shows the morphologies of Fe3O4 synthesized with different concentrations of NaOH. We find that the amount of NaOH plays an important role in the morphologies of Fe3O4 particles, based on the SEM images. Fe3O4 particles first transformed from aggregated sphere (Figure 2a,b) to truncated octahedron (Figure 2d,e) and finally to octahedron (Figure 2f,h) as the concentration of NaOH increases. It is known that single-crystal particles usually have to be enclosed by crystallographic facets that have a lower energy. Octahedrons are enclosed by {111} planes which have the
Results and Discussion The XRD patterns of the as-prepared samples are shown in Figure 1. S1, S2, and S3 denote that the concentration of NaOH are 0, 0.5, and 1.0 mol/L, respectively; Sxm (x ) 1, 2, 3) denotes that the sample prepared by applying an external magnetic field of 0.4 T in the reaction process of Sx sample while keeping other condition constant. From Figure 1, it can be clearly found that all the diffraction peaks, except the one at about 2θ ) 44.7°, can be perfectly indexed to the standard card (JCPDS 19-0629), which can be identified as face-centered-cubic (FCC) magnetite with a spinel structure, and a space group of Fd3m[227]. The peak at 2θ ) 44.7° is the finger peak of iron (Fe) (JCPCD 060696), indicating that the Fe3O4 dominated the as-prepared samples with trace amount of element Fe. S2, S2m, S3, and S3m, compared with S1 and S1m, possess much stronger peaks, which means that the addition of NaOH will favor the increase of the crystallinity of Fe3O4. Sodium borohydride (NaBH4) is a strong reducing agent. Its reductive capacity can be enhanced when dissolved in NaOH solution. The suggested reaction pathway in our system is as follows: 2+
Fe(η-C5H5)2TFe
+ 2C5H5
-
(1)
Figure 2. Morphology of as-prepared samples. The concentrations of the NaOH (mol/L) are (a, b) 0, (c) 0.1, (d) 0.25, (e) 0.5, (f) 0.75, (g) 1.0, and (h) 3.0.
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Figure 3. Magnetization curve for as-prepared samples measured at room temperature. The inset shows an expanded low field hysteresis curve. S1: sphere; S2: truncated octahedron; S3: octahedron.
TABLE 1: Magnetic Parameters of As-Prepared Samples (S1: Sphere; S2: Truncated Octahedron; S3: Octahedron) sample
Ms (emu/g)
Mr (emu/g)
Mr/Ms
Hc (Oe)
S1 S2 S3
65 102 98
0 3.3 5.2
0 0.032 0.053
0 37 58
lowest surface energy for the face-centered cubic crystal structure. The theoretical growth habit of the magnetite crystal is the octahedron because the {111} surfaces have the lowest energy.24-27 In our system, the raw material Fe(C5H5)2 plays a crucial role in the formation of Fe3O4 octahedrons. The images above provide much support to the fact that the increase of the concentration of NaOH should be responsible for the transformation of Fe3O4 shape from assembled sphere to truncated octahedron and to octahedron. The formation mechanism of assembled spheres was discussed from the viewpoint of the diffusion growth.28 The addition of NaOH not only enhanced the reduction capacity of NaBH4 but also changed the chemical potential of reaction system. It has been illustrated that the chemical potential of reaction system favored the anisotropic growth of the magnetite.27,29 From the point of view of crystal growth kinetics, if the crystal adsorbs the hydroxyl on some areas of its surface, the growth rate of this surface will be significantly hindered. Therefore, the concentration of NaOH can modify the growth kinetics of the growing crystal, which results in anisotropic growth of the crystals. Magnetic hysteresis loops (M-H loop) (Figure 3) of those samples obtained without external magnetic field were measured at room temperature (300 K). The inset of Figure 3 shows an enlarged part of expanded low-field hysteresis curve. Magnetic parameters of each sample are shown in Table 1. Recent investigations have showed that the magnetic properties of materials are greatly affected by their morphology and particle size.30-32 Generally speaking, the saturation magnetization of magnetic particles increases with the increase in particle size or with the increase in crystallinity. From the curve of S1 (the assembled Fe3O4 spherical particles), we can see that the saturation magnetization (Ms) of S1 is 64.6 emu/g, which is lower than that of bulk magnetic (85-100 emu/g).33-35 The decrease in the saturation magnetization is most likely attributed to the low crystallinity of the product, which has been proved by the XRD pattern (S1 of Figure 1). The coercive force (Hc) and remnant magnetization (Mr) of the self-assembled spheres are zero, which is a characteristic of superparamagnetic material that happens when the particle size of Fe3O4 particles decreases under a critical value, generally around 54 nm.36 In our case, the small primary particles size of about 5 nm is responsible for the superparamagnetic behavior. The values of the truncated octahedrons and octahedrons are somewhat different, which may
Figure 4. Images of magnetite chains. The concentrations of NaOH are (a) 0, (b) 0.5, and (c, d) 1 mol/L. Intensity of magnetic field are (a, b, d) 0.40 T and (c) 0.25 T.
Figure 5. STM images of magnetite chains: (a, b) face-to-face; (c) edge-to-edge; (d) apex-to-apex.
be ascribed to the shape anisotropy. The high Ms values of both samples are resulted from the high crystallinity while the low Hc and Mr/Ms values could be possibly caused by the low magnetocrystalline anisotropy.37 As expected, the Fe3O4 particles form chains under the induction of external magnetic fields, shown in Figure 4. When the concentration of NaOH was zero, the lines are composed with large numbers of Fe3O4 nanoparticles (Figure 4a), which was similar to the results reported in ref 11. As shown in the figures, octahedron chains obtained when the concentration of NaOH is 0.5 mol/L (Figure 4b) and 1.0 mol/L (Figure 4d); there is no obvious differences between the two products. Peng et al.38 reported that magnetic fields would enhance the diffusion of O2-; as a result, the reaction rate of magnetite formation was increased. In our experiment, the magnetic field would also induce the increasing of reaction rate of magnetite octahedrons that was thought to be responsible for the change from truncated octahedron to octahedron of chains. From the figures, we found that only the length of chains increased when the intensity of magnetic field varied from 0.25 T (Figure 4c) to 0.40 T (Figure 4d), while the morphologies of the chains remain unchanged. The detailed images of octahedral chains are shown in Figure 5. As shown in figures, octahedral magnetite particles connect with each other in chains by three modes: face-to-face (Figure 5a,b), edge-to-edge (Figure 5c), and apex-to-apex (Figure 5d). Because of their ferromagnetism, it is reasonable to assume that Fe3O4 particles form chains along their two easy-magnetic axes, the 〈111〉 direction and the 〈110〉 direction equally. However, in our experiment, we found that face-to-face connection happens more frequently than the other two modes. There is an interesting phenomenon. When several columniform magnets are placed disorderly, they could arrange orderly
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Qi et al. magnetic axis, the minimum energy principle will determine which easy-magnetic axis will be adopted and hence determine the corresponding connection mode. The easy-magnetic axis, which is perpendicular to crystal face, will dominate the direction of chain growth. In this way, the particles connect by face-to-face, which is the steadiest structure. If the particles connect by edge-to-edge or apex-to-apex, the chain will shrink by sharing partial body of basal particles to make the chain steady. This assumption also holds for the spheral magnetic particles11 which formed chain or line by sharing their partial body. Conclusion
Figure 6. Illustration of growth model of chains with octahedral magnetite: (a) a typical octahedron; (b) face-to-face; (c, d) edge-toedge; (e, f) apex-to-apex.
with one top surface cover with another bottom surface. The structure has the lowest energy and is the steadiest. A similar phenomenon is shown in this work. An octahedral magnetite is a magnet under magnetic field. So, magnetite octahedrons tend to formed chains by face-to-face connection under magnetic field. Figure 6 is a schematic diagram that was supposed to illustrate the phenomenon. An octahedron is enclosed by eight {111} planes, as shown in Figure 6a; the crystal orientations are marked. There are two {111} facets, (111) facet, and (-1-1-1) facet, perpendicular to the [111] direction in an octahedron. When an external magnetic field is applied during the reaction process, magnetic octahedrons formed chains along mainly two easy-magnetic axes. The minimum energy principle will determine the way of octahedron assembly. When magnetite octahedrons formed a chain along the [111] direction, the (111) face of an octahedron connected with (111) face of another octahedron, which was the so-called face-to-face connection. The ideal model is shown in Figure 6b. When chains formed along [110] direction, two magnetite octahedrons contact with each other via only one 〈110〉 edge (the edge-to-edge contact (Figure 6c)), which is instable and therefore less frequently observed. Since 〈100〉 is not one of the easy-magnetic axes of magnetite, less attention has been paid to the assembly of magnetite along 〈100〉 under a magnetic field; therefore, the chains along [100] reported in our work can be a novel supplement of the magnetic field assisted assembly. When the octahedral magnetite grew along the 〈100〉 direction, the octahedrons connected via only one apex (apex-to-apex contact (Figure 6d)). This abnormal crystal was unstable, which almost has not been observed. In order to form final chains, two neighboring octahedral magnetite particles need to share their partial bodies with each other to increase their interface and to reduce the total energy. The representative SEM images and models of these two types of connection are shown as Figure 5c, Figure 5d, Figure 6d, and Figure 6f, respectively. On the basis of the Figures 5 and 6, a possible growth mechanism is tentatively proposed to interpret the mechanism by which magnetic materials self-assemble under magnetic fields. When an external magnetic field is applied during the reaction process, magnetic particles form chains or lines along the easy-magnetic axis direction. If there is more than one easy-
In this study, a new solvothermal process was designed to obtain octahedron magnetic chains under external magnetic fields. As expected, magnetite with chain structure could be found when an external magnetic field was applied in the reaction process. The study shows that the octahedral magnetite formed chains along the 〈111〉 direction by face-to-face connection more frequently than along the 〈110〉 direction by edgeto-edge connection. On the basis of the experimental results, it is concluded that both the direction of easy-magnetic axis and the morphology of magnetic particles can significantly determine the mode how magnetic particles connect to form chains. The minimum energy principle determines the way of octahedron assembly. If there is more than one easy-magnetic axis, the axis which is perpendicular to crystal surfaces will become the primary chain growth direction. In this way, the particles can form the steadiest structure with the lowest energy by connecting each other by face-to-face. If the particles connect by edge-toedge or apex-to-apex, the chain will shrink by sharing parts of bodies to reduce energy. On the basis of previous works of synthesizing chains of magnetic particles under an external magnetic field, our work further shows the essence of selfassembly under magnetic fields. At the same time, further efforts are still needed to establish the correlation between magnetic filed and chemical synthesis. Acknowledgment. This work was supported by the National Natural Science Foundation of China (No. 10774138). References and Notes (1) Pankhurst, Q. A.; Pollard, R. J. Phys. ReV. Lett. 1991, 67, 248. (2) Levy, F.; Sheikin, I.; Grenier, B.; Huxley, A. D. Science 2005, 309, 1343. (3) Na, C. W.; Han, D. S.; Park, J.; Jo, Y.; Jung, M. H. Chem. Commun. 2006, 21, 2251. (4) Wolf, M.; Muller, K. H.; Skourski, Y.; Eckert, D.; Georgi, P.; Krause, M.; Dunsch, L. Angew. Chem., Int. Ed. 2005, 44, 3306. (5) Hinds, G.; Coey, J. M. D.; Lyons, M. E. G. J. Appl. Phys. 1998, 83, 6447. (6) Bodea, S.; Ballou, R.; Pontonnier, L.; Molho, P. Phys. ReV. B: Condens. Matter Mater. Phys. 2002, 66, 224104. (7) Affleck, L.; Aguas, M. D.; Pankhurst, Q. A.; Parkin, I. P.; Steer, W. A. AdV. Mater. 2000, 12, 1359. (8) Wang, J.; Chen, Q. W.; Zeng, C.; Hou, B. Y. AdV. Mater. 2004, 16, 137. (9) Kornev, K. G.; Halverson, D.; Korneva, G.; Gogotsi, Y.; Friedman, G. Appl. Phys. Lett. 2008, 92, 233117. (10) Niu, H. L.; Chen, Q. W.; Ning, M.; Jia, Y. S.; Wang, X. J. J. Phys. Chem. B 2004, 108, 3996–3999. (11) Sun, L. X.; Chen, Q. W.; Tang, Y.; Xiong, Y. Chem. Commun. 2007, 844–2846. (12) Christopher Love, J.; Urbach, A. R.; Prentiss, M. G.; Whitesides, G. M. J. Am. Chem. Soc. 2003, 125, 12696. (13) Platt, M.; Muthukrishnan, G.; Hancock, W. O.; Williams, M. E. J. Am. Chem. Soc. 2005, 127, 15686. (14) Black, C. T.; Murray, C. B.; Sandstrom, R. L.; Sun, S. Science 2000, 290, 1131.
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