Morphology-Controlled Synthesis of Magnetites with Nanoporous

Key Laboratory of Rare Earth Chemistry and Physics, Changchun Institute of Applied Chemistry, and Graduate School of the Chinese Academy of Sciences, ...
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Chem. Mater. 2008 , 20, 198–204

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Morphology-Controlled Synthesis of Magnetites with Nanoporous Structures and Excellent Magnetic Properties Lijun Zhao,† Hongjie Zhang,*,† Yan Xing,†,‡ Shuyan Song,†,‡ Shiyong Yu,†,‡ Weidong Shi,†,‡ Xianmin Guo,†,‡ Jianhui Yang,†,‡ Yongqian Lei,†,‡ and Feng Cao†,‡ Key Laboratory of Rare Earth Chemistry and Physics, Changchun Institute of Applied Chemistry, and Graduate School of the Chinese Academy of Sciences, Changchun 130022, P. R. China ReceiVed August 19, 2007. ReVised Manuscript ReceiVed October 10, 2007

Different morphological single-crystal magnetites (Fe3O4) with a nanoporous structure, which exhibit excellent magnetic properties, have been synthesized by a polyol process. Both the type of polyol and the concentration of KOH play important roles in the formation of various morphologies. Cubic, truncatedoctahedral, and octahedral shapes can be prepared by changing the concentration of the KOH solution in ethylene glycol. Sequentially, a series of submicro magnetites have been successfully obtained in the shape of spheres, truncated cubes, and equilateral octahedra by incrementally adjusting the concentration of KOH in glycerol. All the samples are ferromagnetic at 2 K but nearly paramagnetic at 300 K except the cubic sample. These Fe3O4 crystals may have a potential application as magnetic carriers for drug targeting because of their excellent soft-magnetic properties.

Introduction Magnetic crystals with well-defined superstructures have attracted considerable attention for their structure characteristics that endow them for a wide range of potential applications.1–4 Furthermore, researchers in various disciplines have also attached importance to their many technological applications, including magnetic storage media, ferrofluids, contrast agents for magnetic resonance imaging, and magnetic carriers for drug targeting.5–14 However, it is still a great challenge to develop simple and reliable synthetic methods for the synthesis of magnetic materials with * To whom correspondence should be addressed. Tel: +86-431-85262127. Fax: +86-431-85698041. E-Mail: [email protected]. † Changchun Institute of Applied Chemistry, Chinese Academy of Sciences. ‡ Graduate School of the Chinese Academy of Sciences.

(1) Rodrigues, V.; Ugarte, D. Nanotechnology 2002, 13, 404. (2) Mbindyo, J. K. N.; Mallouk, T. E.; Mattzela, J. B.; Kratochvilova, J. B.; Razavi, B.; Jackson, T. N.; Mayer, T. S. J. Am. Chem. Soc. 2002, 124, 4020. (3) Pan, H.; Chen, W. Z.; Feng, Y. P.; Ji, W.; Lin, J. Y. Appl. Phys. Lett. 2006, 88, 223106. (4) Reiss, B. D.; Mao, C. B.; Solis, D. J.; Ryan, K. S.; Thomson, T.; Belcher, A. M. Nano Lett. 2004, 4, 1127. (5) Schmid, G. Nanoparticles: From Theory to Application; Wiley-VCH: Weinheim, 2004. (6) Klabunde, K. J. Nanoscale Materials in Chemistry; Wiley-Interscience: New York, 2001. (7) Fendler, J. H. Nanoparticles and Nanostructured Films; Wiley-VCH: Weinheim, 1998. (8) Fertman, V. E. Magnetic Fluids Guide Book: Properties and Applications; Hemisphere Publishing Corp.: New York, 1990. (9) Berkovsky, B. M.; Medvedev, V. F.; Krakov, M. S. Magnetic Fluids: Engineering Applications; Oxford University Press: Oxford, 1993. (10) Ziolo, R. F.; Giannelis, E. P.; Weinstein, B. A.; O’Horo, M. P.; Ganguly, B. N.; Mehrotra, V.; Russel, M. W.; Huffman, D. R. Science 1992, 257, 219. (11) Alivisatos, A. P. Science 1996, 271, 933. (12) Hyeon, T. Chem. Commun. 2003, 927. (13) Zhang, J. Z.; Wang, Z. L.; Liu, J.; Chen, S.; Liu, G. Y. Self-Assembled Nanostructures; Kluwer Academic/Plenum Publishers: New York, 2003. (14) Pacholski, C.; Kornowski, A.; Weller, H. Angew. Chem., Int. Ed. 2002, 41, 1188.

designed chemical components and controlled morphologies, which strongly affect the properties of magnetic materials. Now, numerous and diverse synthetic methodologies, surface analytical techniques, and materials fabrication methods exist for building structures of many compositions and shapes on the nanometer and micrometer length scale while wetchemical techniques are still extensively used. These methods are being used to create isotropic particles as well as novel anisotropic structures, such as spheres,15 octahedra,16–18 wires,19 fractals,20 hollow structures,21 and nanosheets.22 In fact, these special morphologies of magnetic materials usually possess a high anisotropy, which directly cause a relatively high coercive force (Hc) because the microstructure of a magnetic material plays an important role in its physical properties. Herein, variously morphological submicro magnetites (Fe3O4) with a high magnetization and very low coercive force have been synthesized by a simple polyol method. Among the magnetic crystals, magnetite shows excellent soft magnetism because of its higher saturation magnetization (Ms) and lower coercive force. So magnetite has recently been considered as an ideal candidate for biological applications, both as a tag for sensing and imaging and as an activity agent for antitumor therapy.23–27 For high performance in function-specific biological applications, (15) Deng, H.; Li, X. L.; Peng, Q.; Wang, X.; Chen, J. P.; Li, Y. D. Angew. Chem., Int. Ed. 2005, 44, 2782. (16) Liu, X. M.; Fu, S. Y.; Zhu, L. P. J. Solid State Chem. 2007, 180, 461. (17) Zhang, D. E.; Zhang, X. J.; Ni, X. M.; Song, J. M.; Zheng, H. G. J. Magn. Magn. Mater. 2006, 305, 68. (18) Liu, X. M.; Fu, S. Y.; Xiao, H. M. Mater. Lett. 2006, 60, 2979. (19) Chueh, Y. L.; Lai, M. W.; Liang, J. Q.; Chou, L. J.; Wang, Z. L. AdV. Funct. Mater. 2006, 16, 2243. (20) Zou, G. F.; Xiong, K.; Jiang, C. L.; Li, H.; Li, T. W.; Du, J.; Qian, Y. T. J. Phys. Chem. B 2005, 109, 39. (21) Yu, D. B.; Sun, X. Q.; Zou, J. W.; Wang, Z. R.; Wang, F.; Tang, K. J. Phys. Chem. B 2006, 110, 21667. (22) Chin, K. C.; Chong, G. L.; Poh, C. K.; Van, L. H.; Sow, C. H.; Lin, J. Y.; Wee, A. T. S. J. Phys. Chem. C 2007, 111, 9136.

10.1021/cm702352y CCC: $40.75  2008 American Chemical Society Published on Web 12/11/2007

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Table 1. Synthesis Conditions for the Preparation of Fe3O4 Crystals with Various Morphologiesa

a Cube (C), truncated octahedron (TO), octahedron (O), sphere (S), truncated cube (TC), equilateral octahedron (EO), ethylene glycol (EG), and glycerol (Glyc.).

magnetic particles must be spherical and have smooth surfaces, narrow size distributions, large surface areas (for maximal protein or enzyme binding), high saturation magnetization to provide maximum signal, and good dispersion in liquid media.28–32 In this contribution, we demonstrate the successful synthesis of magnetic submicro spheres with a uniform size distribution and larger surface area (318.9 m2/g) and a high Ms value of 92 emu/g, which is comparable to the theoretical value of bulk magnetite.33 To our knowledge, there have been few reports about the transformation of microstructural magnetic materials. In this paper, we study the morphological transformation of magnetites by controlling the reaction conditions carefully. Experimental Section Preparation. All chemicals used in this experiment were of analytical grade and were used without further purification. Two milliliters of FeSO4 solution (0.5 mol · L-1) was dissolved in 30 mL of ethylene glycol (EG) or glycerol (Glyc.) to form a homogeneous solution. Then, a KOH solution was quickly added into the solution at room temperature by intensive stirring (30 min) in a nitrogen gas protective atmosphere; the mixture was then transferred into a 50 mL Teflon lined stainless steel autoclave, sealed, and maintained at 200 °C for 24 h. After completion of the reaction, the black solid products were collected by magnetic separation and washed several times with water and ethanol. The final products were dried in a vacuum oven at 40 °C for 6 h. Characterization. X-ray diffraction (XRD) patterns were recorded on a Rigaku-D/max 2500 V X-ray diffractometer equipped with a source of Cu KR radiation (λ ) 1.54178 Å) at a step width of 0.02°. The morphologies and structures of the as-synthesized ferrite products were observed with field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). N2 adsorption–desorption isotherms were measured at the liquid nitrogen temperature (77 K) using a Quantachrome NONA (23) Perez, J. M.; O’Loughlin, T.; Simeone, F. J.; Weissleder, R.; Josephson, L. J. Am. Chem. Soc. 2002, 124, 2856. (24) Perez, J. M.; Simeone, F. J.; Tsourkas, A.; Josephson, L.; Weissleder, R. Nano Lett. 2004, 4, 119. (25) Perez, J. M.; Josephson, L.; O’Loughlin, T.; Hogemann, D.; Weissleder, R. Nat. Biotechnol. 2002, 20, 816. (26) Louie, A. Y.; Huber, M. M.; Ahrens, E. T.; Rothbacher, U.; Moats, R.; Jacobs, R. E.; Fraser, S. E.; Meade, T. J. Nat. Biotechnol. 2000, 18, 321. (27) H(a)feli, U.; Schütt, W.; Teller, J.; Zborowski, M. Scientific and Clinical Application of Magnetic Carriers; Plenum: New York, 1997. (28) Sun, Y.; Xia, Y. Science 2002, 298, 2176. (29) Lu, Y.; Yin, Y. B.; Mayers, T.; Xia, Y. Nano Lett. 2002, 2, 183. (30) Wang, Y. L.; Xia, Y. N. Nano Lett. 2004, 4, 2047. (31) Gee, S. H.; Hong, Y. K.; Erickson, D. W.; Park, M. H. J. Appl. Phys. 2003, 93, 7560. (32) Woo, K.; Hong, J.; Choi, S.; Lee, H.; Ahn, J.; Kim, C. S.; Lee, S. W. Chem. Mater. 2004, 16, 2814. (33) Han, D. H.; Wang, H. L.; Luo, J. J. Magn. Magn. Mater. 1994, 136, 176.

1000 instrument. Samples were degassed at 120 °C overnight before measurements. Specific surface areas were calculated using the Brunauer–Emmett–Teller (BET) model, and the pore size distributions were evaluated from the desorption branches of the nitrogen isotherms using the Barrett–Joyner–Halenda (BJH) model. Hysteresis loops were collected on a Quantum Design superconducting quantum interference device (SQUID) magnetometer (LakeShore 7307) at 300 and 2 K, respectively. The SQUID measurements for all the samples were done on the pure and dried powders.

Results and Discussion Studies of Structure and Morphology. Magnetites with various morphologies can be successfully obtained by incremental adjustment of the concentration of KOH in polyols when FeSO4 · 6H2O is used as the iron source. The different synthetic conditions for magnetites are listed in Table 1, and the transformation of magnetite microstructure is commented on in the following paragraphs. XRD patterns of the cubes, truncated octahedra, and octahedra are shown in Figure 1A in which all the peaks can be indexed to a pure face-centered cubic phase (fcc, space group Fd3jm (No. 227)) of magnetite. All the diffraction peaks in the XRD patterns can be indexed to face-centered cubic structure of magnetite according to JCPDS card no. 19-0629. SEM images of the magnetites derived from different KOH concentrations in EG are shown in Figure 1B (cubes), 1C (truncated octahedra), and 1D (octahedra). With the increase of the KOH quantity, the transformation from cubes to truncated octahedra to octahedra was observed. Weller et al. reported that with the particles growing bigger, the formation of truncated octahedra is easier than that of cubic structures to reach a more stable state.34 In our experiment, the cubes with a fairly stable structure have been successfully prepared except for the truncated octahedra. The KOH quantity in the precursor solution has been found to be very important for the microstructure. With the further increase in the concentration of KOH, good crystallization and regular octahedral morphology can be discerned from Figure 1D. It is known that single-crystal particles usually have specific shapes because a single-crystal particle has to be enclosed by crystallographic facets that have a lower energy.35–37 Octahedra are enclosed by (111) planes which have the (34) Shevchenko, E. V.; Talapin, D. V.; Schnablegger, H.; Kornowski, A.; Festin, O.; Svedlindh, P.; Haase, M.; Weller, H. J. Am. Chem. Soc. 2003, 125, 9090. (35) Wu, M.; Xiong, Y.; Jia, Y.; Niu, H.; Qi, H.; Ye, J.; Chen, Q. Chem. Phys. Lett. 2005, 401, 374. (36) Deng, H.; Li, X.; Peng, Q.; Wang, X.; Chen, J.; Li, Y. Angew. Chem., Int. Ed. 2005, 44, 2782. (37) Wang, Z. L.; Feng, X. J. Phys. Chem. B 2003, 107, 13563.

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Figure 1. (A) XRD patterns of the products; SEM images of the magnetites derived from different concentrations of KOH in EG: (B) 0.5 mol · L-1, (C) 3 mol · L-1, and (D) 5 mol · L-1.

Figure 2. (A) XRD patterns of the products; SEM images of the magnetite derived from different concentrations of KOH in Glyc.: (B) 0.5 mol · L-1, (C) 3 mol · L-1, and (D) 5 mol · L-1.

lowest surface energy for the face-centered cubic crystal structure. The anisotropy of crystal structure or the crystal surface reactivity is identified as the main driving force of the growth of the anisotropic structure. The influence of the chemical potential on the shape evolution has been elucidated by Peng and Peng.38,39 In the case of crystal growth, it is beneficial to have a higher chemical potential, which is mainly determined by the concentration of KOH. Octahedral magnetite with a high quality and crystallinity will be obtained in concentrated KOH solutions because a higher OH- ion concentration and a higher chemical potential in (38) Peng, Z. A.; Peng, X. J. Am. Chem. Soc. 2001, 123, 1389. (39) Peng, Z. A.; Peng, X. J. Am. Chem. Soc. 2002, 124, 3343.

the solution favor the growth of octahedral structures over other possible iron oxides crystal forms. Figure 2A presents the XRD patterns of spheres, truncated cubes, and equilateral octahedra. All the diffraction peaks in the XRD patterns can be indexed to the face-centered cubic structure of magnetite according to JCPDS card no. 19-0629. It was observed that the intensities of the diffraction peaks of (111), (222), and (511) for the equilateral octahedron were stronger than that of the bulk, suggesting a different oriented growth direction of the equilateral octahedra. For other structures, such as the cubes, truncated octahedra, octahedra, spheres, and truncated cubes, the intensities of the diffraction peaks are almost consistent with those of the bulk sample. The high alkali concentration and the adsorption of the

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Figure 3. TEM images of the cubes, spheres, and octahedra synthesized in (A) EG, [KOH] ) 0.5 mol · L-1; (B) Glyc., [KOH] ) 0.5 mol · L-1; and (C) EG, [KOH] ) 5 mol · L-1.

H-bonding of Glyc. on the crystal facets of iron hydroxide could be responsible for the strong diffraction peaks of (111), (222), and (511). The concentration of OH- ions and the types of polyols determined the crystal growth rates along some special directions. Crystal growth rates in the direction perpendicular to a high-index plane are usually much faster than those along the normal direction of a low-index plane, so high-index planes were rapidly eliminated during the formation of particles.40 When Glyc. is used as a reaction medium, the KOH quantities cause the transformation from spheres to truncated cubes to octahedra. The SEM images of the spheres, truncated cubes, and octahedra are shown in Figure 2B,C,D. For the synthesis of submicrometer spheres, the apparently spherical shape of the resulting particles indicated that several different families of slow-growing facets grew at a similar rate. In a low alkali concentration condition, Glyc. played a key role in determining the morphology by forming Hbonding on the crystal surfaces while, with the increase of alkali concentration, OH– ions were responsible for the shape evolution. Figure 2B shows that the formed submicrometer spheres have both a uniform morphology and a narrow size distribution, which are very important for their future application in the biological field. Truncated cubes are the intergradations for spheres and octahedra (Figure 2C). Monodispersed equilateral octahedra with the crystallite sizes of 1–5 µm are observed in Figure 2D. The chemical potential of octahedra should be generally higher than that of other morphologies if the unit cell of the corresponding crystal structure is not highly distorted along a certain axis. We believe that KOH behaves not only as a mineralizer but also as a surfactant in the hydrothermal process. As a result, the

growth of such anisotropic structures should require a relatively high chemical potential environment in the solution, that is, a relatively high concentration of KOH; hence, the nucleation will be accelerated. The quick nucleation leads directly to the size distribution in a broader scope. The size distribution plots for samples are shown in Figure S1 of the Supporting Information. The microstructures of the magnetites have been further examined by TEM. Figure 3A demonstrates that the cubes are connected with one another, which may be due to the magnetic dipole–dipole interaction between neighboring particles and their electrostatic function. It is the same with the spheres (Figure 3B) and the octahedra (Figure 3C). Larger area TEM images were provided in Figure S2 of the Supporting Information. The projections of octahedra are parallelograms or hexagons because of the different view angles. We could measure the angles of octahedra from the projections. The isosceles-triangular crystal planes (Figure 3C) had a protogonia of 65 ( 1°. Theoretical values are 54.1 and 71.8°,41 respectively; the observed deviation is probably due to the presence of disoriented crystallites and crystal defects in the triangular sheets that result in mismatched planes. Electron diffraction patterns taken from individual Fe3O4 submicro crystals reveal the single-crystalline nature of these samples. The above discussions prove that the polyols have effect on the morphologies of the magnetites and that the KOH quantity causes the shape transformation. To understand the function of the solvent, water and the mixture of EG and Glyc. were investigated. When water was used as the solvent, octahedra and irregular particles could be seen in Figure 4A. On the basis of Ostwald ripening, the ripening process

(40) Buckley, H. E. Crystal Growth; Wiley: New York, 1951.

(41) Yang, H. G.; Zeng, H. C. Angew. Chem., Int. Ed. 2004, 43, 5930.

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Figure 4. SEM images of the magnetites derived from different solvents: (A) water and (B) EG and Glyc.

Figure 5. Schematic illustration of the evolution process from cubes to truncated octahedra to octahedra (B-D); from spheres to truncated octahedra to equilateral octahedra (E-G, bar ) 100 nm). (A) Ideal cube structure; (H) ideal octahedral structure.

involves the growth of larger crystals from those smaller sized crystals that have a higher solubility than the larger ones, which has been commonly observed in general crystal growth for more than a century.42 The depletion of the OHions by the growth of crystals will eventually make the smaller crystals in the solution soluble because of the strongly size-dependent solubility in the nanometer regime. This means that the smaller crystals in the solution shrink and the bigger ones continue to grow. In water, the smaller crystals do not completely grow into big ones because of the quick nucleation and growth. This serves as direct evidence that the polyols slow down the growth rate of the magnetite crystals because the dissolution–crystallization process of the precursor is inhibited. At the same time, we can observe octahedra and spheres in Figure 4B. All the octahedra and spheres are fairly perfect; consequently, we can obtain different proportions of octahedra and spheres at one time by varying the volume ratio of EG to Glyc. Polyols may serve as stabilizing reagents and bind to the surface of submicrometer crystals. While the crystallite sizes of the submicrometer crystals obtained from EG or water were not even, this indicates that the viscosity of the solvent has an important function on the control of the growth rate. When Glyc. was considered as

solvent, homogeneous size distribution could be observed at a low KOH quantity. The concentration of KOH controls the nucleation rate while hydroxyl numbers of polyols control the growth rate. The viscosity ranking of the three kinds of solvents is water < EG < Glyc., thereby, to some extent, higher viscosity can slow down the growth rate and be more advantageous for the isotropic growth. Hence, the narrow distribution of crystallite sizes could be obtained by increasing the hydroxyl numbers of polyol, which means that the desired size distributions could be obtained by a diffusioncontrolled process of the larger crystals in solution. To clarify the transformations of the microstructural magnetites, the evolution processes are shown in Figure 5. The crystal structure analysis showed that magnetite adopts a face-centered cubic crystal structure, and the general sequence for the surface energies of the magnetite structure was γ{111} < γ{100} < γ{110}.43 On the basis of the surface free energy minimization principle, the cubic structure with six {100} faces (Figure 5A), which had relatively high surface energies, was a less stable morphology. The growth rate along the (100) plane is quicker than along the (111) plane, so the (100) plane will be gradually decrescent. Therefore, the submicrometer octahedra are enclosed by (111) planes of which the structure model is presented in

(42) Liu, B.; Zeng, H. C. Small 2005, 1, 566.

(43) Zhang, L.; He, R.; Gu, H. C. Mater. Res. Bull. 2006, 41, 260.

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Figure 6. N2 adsorption (b) and desorption (O) isotherms for the magnetites with different morphologies. The inset shows the BJH pore size distributions of the magnetites.

Figure 7. Hysteresis loops of the magnetites with special morphologies tested at 300 K (black line) and 2 K (red line).

Figure 5H. Figure 5B,C,D shows that the magnetite gradually transforms from a cube to a truncated octahedron to an octahedron with the increase of the KOH quantity in EG. By measuring the octahedron in Figure 5 D, we obtained that the triangular sheets were isosceles triangles. The angles, which are consistent with the TEM result (Figure 3C), are shown. The cubes are enclosed by (100) planes, and the truncated octahedra are enclosed by both (111) and (100) planes while the octahedron is enclosed by (111) planes. Figure 5E,F,G shows that the morphologies of the magnetites gradually transform from a sphere to a truncated cube to an octahedron with the increase of KOH quantity in Glyc. By measuring the octahedron in Figure 5G, it was determined that the triangular sheets were equilateral triangles. The spheres yielded no crystal facets, or the truncated cubes yielded solids with both the (100) and the (111) facets while the equilateral octahedra were enclosed by the (111) planes. The external conditions of crystal growth, for example, the solution ingredient, solvent medium, crystal growth temperature, viscosity, and so on, directly affect the adoption of various crystal faces and the crystal relative surface free enthalpy. Thus, the factors above influence the relative stability of the crystal surface, its growth rate, and the crystal morphology. It suggests that the driving forces such as the surface tension, the inherent crystal structure of the magnetite, and its chemical potential in a concentrated KOH solution may favor the anisotropic growth of the magnetite. From the point of view of the kinetics of crystal growth, if the crystal adsorbs the hydroxyl on some areas of its surface, then the growth rate of the crystal in a certain direction will be confined. Therefore, the concentration of the KOH can modify the growth kinetics of the growing crystal, which in turn leads to the anisotropic growth of the crystals. In addition, Swaminathan et al. pointed out that the ratio R, the growth rate along {100} to that along {111}, plays a

key role in determining the final morphology of the particles.44 With the increasing of R, the shape of the particles undergoes an evolution from a cube (R ) 0.53) to a truncated octahedron (R ) 0.87) and finally to an octahedron (R ) 1.73). From the experimental results, we concluded that the concentration of the KOH and the solvent properties were important in determining the R values. The critical nucleus shape was predicted as either a perfect cube or a perfect octahedron, while the equilibrium growth types were the truncated shapes. To investigate the difference of the surface areas among the cubes, spheres, and octahedra, nitrogen adsorption and desorption experiments were carried out. From the adsorption and desorption isotherms in Figure 6, we concluded that it could be categorized as a type IV with an inconspicuous hysteresis loop observed in the range of 0.2–0.6 P/P0 for cubes, 0.4–0.8 P/P0 for spheres, and 0.0–1.0 P/P0 for octahedra. The BET model surface areas of these morphologies were calculated to be 291.8, 318.9, and 389.6 m2 g-1. BJH calculations for the pore size distribution revealed that the distribution for the cubes, spheres, and octahedra were centered at approximately 2.4, 2.3, and 2.4 nm (Figure 6, inset). The high surface areas of the magnetites might make them show a strong binding force with maximal biomolecules. In addition, the existence of nanopores on the surfaces makes them a kind of special magnetic material which may be used as a magnetic carrier for drug targeting. Magnetic Properties. Figure 7 shows the hysteresis loops of the magnetite with special morphologies. Room-temperature SQUID testing results indicated that the cubic magnetite had an Hc value of 27 Oe and an Mr value of 2 emu/g, while the Hc and Mr values were zero for other morphological (44) Swaminathan, R.; Willard, M. A.; Mchenry, M. E. Acta Mater. 2006, 54, 807.

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Table 2. Magnetic Parameters of the Cube (C), Truncated Octahedron (TO), Octahedron (O), Sphere (S), Truncated Cube (TC), and Equilateral Octahedron (EO) Tested by SQUID at 300 and 2 K 300 K sample

Ms (emu/g)

Mr (emu/g)

C TO O S TC EO

89 90 91 91 93 95

2 0 0 0 0 0

2K Hc (Oe)

Ms (emu/g)

Mr (emu/g)

Hc (Oe)

27 0 0 0 0 0

95 97 98 98 100 105

20 2 2 3 6 2

242 34 36 40 94 30

magnetites. Shape anisotropy might be responsible for the experimental results. The particles with an ellipse morphology had the lowest shape anisotropy. We speculated that cubes and truncated cubes had the higher shape anisotropy energy. The truncated octahedron, octahedron, and sphere were close to the ellipse, so they had similar and much lower shape anisotropy. Magnetic parameters of the magnetites with various morphologies are listed in Table 2. The Hc values of the magnetites synthesized in this article were even lower than those reported.45,46 The very low Hc values might be caused by the surface coordination with the hydroxyl. Moreover, the magnetic structure at the surface layer could be drastically different from the one in the core magnetite particles. The coordination symmetry was greatly reduced for the metal cations at the surface because of missing coordination oxygen atoms. The surface usually exhibited some degree of spin disorder and pinning. The adsorbed hydroxyls could be viewed as effectively taking the positions of the missing oxygen atoms, which made the symmetry and crystal field of the surface metal ions more closely resemble those of the core and therefore reduced the spin disorder and pinning. Such changes certainly caused the decrease of the surface anisotropy. Néel’s early calculations as well as several recent theoretical studies have suggested that coercivity decreases with decreasing surface anisotropy.47–49 The existence of hydroxyls on the surface of magnetite not only leads to the decrease of coercivity but also makes the magnetite biocompatible. Furthermore, the surface anisotropy is important (45) Zhu, Y. F.; Zhao, W. R.; Chen, H. R.; Shi, J. L. J. Phys. Chem. C 2007, 111, 5283. (46) Xuan, S. H.; Hao, L. Y.; Jiang, W. Q.; Song, L.; Hu, Y.; Chen, Z. Y.; Fei, L. F.; Li, T. W. Cryst. Growth Des. 2007, 7, 430. (47) Néel, L. J. Phys. Radium 1954, 15, 225. (48) Dimitrov, D. A.; Wysin, G. M. Phys. ReV. B 1994, 50, 3077. (49) Kodoma, R. H.; Berkowitz, A. E. Phys. ReV. B 1999, 59, 6321.

in the determination of the final particle shape. In summary, the magnetic properties depend on the morphologies of the magnetites. The Ms and Hc values of the magnetite at 300 K were lower than those of the magnetite at 2 K. To understand the low temperature magnetic properties, the theory of low temperature spin wave is proposed. The spin of the atom system is completely parallel at absolute zero. Because of the increase of temperature, the inverse numbers of the spin would be increased. The thermal decrease of the magnetization between 2 and 300 K was fitted to a spin-wave type dependence Ms(T) ) Ms(0)(1 - βT b), where Ms(0), β, and b are the saturation magnetization estimated at 0 K, the Bloch constant, and the Bloch exponent, respectively. Therefore, the value of Ms increases with the decreasing temperature. The coercivity of the nanoparticles at low temperature determined by irreversible domain rotation could be defined by the equation Hc ) (2K1/(µ0Ms), where K1 is the magnetocrystalline anisotropy constant and µ0 is the vacuum susceptibility. In general, the magnetocrystalline anisotropy constant increases with the decreasing temperature, so the coercivity increases rapidly at 2 K. In the meantime, the change of the cation distribution at low temperature may cause the variation of Ms and Hc. Conclusions In summary, a simple straightforward synthetic method has been developed for the preparation of multiappearance magnetites. The transformation of the microstructural magnetite is determined by both types of polyols and the KOH quantity. The concentration of KOH controls the nucleation rate, while the hydroxyl numbers of the polyols control the growth rate. The magnetic properties of the magnetites depended on their morphologies. All samples showed outstanding soft-magnetic properties at room temperature; the spherical particles, especially, display widespread application potentials in the biological field. Acknowledgment. The authors are grateful to the financial aid from the National Natural Science Foundation of China (Grants 20490210, 206301040, 20602035, and 20771099) and the MOST of China (Grants 2006CB601103 and 2006DFA42610). Supporting Information Available: Additional TEM images and size distribution plots for samples. This material is available free of charge via the Internet at http://pubs.acs.org. CM702352Y