Morphology-Controlled Synthesis and Magnetic Property of

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Morphology-Controlled Synthesis and Magnetic Property of Pseudocubic Iron Oxide Nanoparticles Lili Wang and Lian Gao* State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, People’s Republic of China ReceiVed: June 1, 2009; ReVised Manuscript ReceiVed: July 19, 2009

Pseudocubic iron oxide nanoparticles with a narrow size distribution have been synthesized. These pseudocubic nanocrystals are found to be bound by {012} facets, whose surface energy is more than those of other common planes such as {014} planes. The morphology transformation was successfully achieved by adjusting the ratio of the concentration of the reagent from irregular spherical, quasi-rhombic, and truncated cubic, to finally pseudocubic. The magnetic property of these nanocrystals is found to be influenced by varying the concentration of sodium oleate and oleic acid. It is attributed to the generation of Fe2+ from Fe3+ reduced by the reductive ambience provided by the surfactant. It may become a simple route to prepare magnetic iron oxide without the addition of Fe2+ or reductive reagent to some extent. Experimental Section

Introduction In the past two decades, the synthesis of nanoparticles has been greatly motivated by fundamental scientific interest and also their potential applications. Therefore, magnetic metal oxide nanoparticles have received enormous attention for their broad applications in many fields.1-5 Hematite (R-Fe2O3), as one of the most important iron oxides, has been studied extensively due to its nontoxicity, stability, and corrosion-resistance. Also, it is an n-type semiconductor with a wide bandgap (Eg) of 2.1 eV and is widely used in gas sensors,6 nonlinear optics,7 catalysts,8,9 and so on. The corresponding property of nanoparticles is found to be intensely affected by morphology and structure. For example, the catalytic activities of terminated facets are different from one another for various atom densities in their hematite structure.10 Thus, the fabrication of hematite nanoparticles with controlled shape and size has been a most attractive and well-documented goal.11-14 Since the structure of hematite belongs to the corundum-type (space group R3cj, No. 167), pseudocubic morphology is rather unusual, as it is a trigonal crystal system.15 To the best of our knowledge, few reports have referred to the synthesis of cube-like hematite nanoparticles.10,16-19 Sugimoto et al. have prepared pseudocubic particles by “gel-sol” methods20 and systematically studied the influence of synthetic parameters on shape.21 Yu et al. have synthesized uniform hematite nanocubes through hydrothermal reaction, yet no nanoparticles bound by special planes have been found.22 Wang et al. have prepared R-Fe2O3 nanoparticles enclosed by six {110} planes, in which the uniformity of morphology should be further improved.10 Here, we report the synthesis of uniform pseudocubic iron oxide nanoparticles. They are bound by {012} facets of hematite, which possess higher atom densities and corresponding surface energies than those of {110} and {104} planes. We discuss the growth mechanism and influence of surfactant concentration on their magnetic properties and morphologies. * To whom correspondence should be addressed. Phone: +86-2152412718. Fax: +86 21 52413122. E-mail: [email protected].

All of the chemicals were of analytical grade and used as received without any further purification (commercially obtained from Shanghai Chemical Reagent Co., Ltd., Shanghai, China). According to the reference, a typical experimental procedure with some alternation is described as follows:22 A 1.92-g portion (6 mmol) of sodium oleate (C18H33NaO2), 40 mL of ethanol, and 20 mL of oleic acid (C18H34O2) were mixed by vigorously stirring, to which 10 mL of iron(III) chloride hexahydrate aqueous solution (0.2 M) was added at room temperature. The dark red solution mixture was transferred into a Teflon-lined autoclave of 100 mL capacity, and maintained at 180 °C for 20 h. After spontaneously cooling to room temperature, the dark precipitates were obtained by removing the upper flaxen mixture. The products were repeatedly washed with cyclohexane and ethanol and finally vacuum-dried. Controlled experiments were carried out to better understand the growth mechanism and the influence of the surfactant concentration. The crystalline structure of the samples was analyzed by X-ray diffraction (XRD, D/max 2550 V, Rigaku, Tokyo, Japan) using Cu KR radiation (λ ) 1.5406 Å) and an instrument resolution of 0.02° in 2θ. The powder sample for XRD analysis was prepared by drying the cyclohexane dispersion of hematite nanoparticles at room temperature on a single-crystalline Si (100) wafer as substrate. Nanoparticle size, morphology, and structure were investigated by transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and selected area electron diffraction (SAED) on a field emission transmission electron microscope (JEM-2100F, JEOL, Tokyo, Japan; accelerating voltage: 200 kV). The TEM samples were prepared by dropping as-synthesized nanoparticle dispersions in cyclohexane onto carbon-coated Cu-grids, evaporating them for 90 s, and wicking away the excessive solvent with filter paper. The Raman spectrum was recorded using a Super LabRam microscopic Raman spectrometer (Labram, Jobin Yvon, France, an He-Ne laser with an excitation wavelength of 632.8 nm). The organic groups absorbed on the surface of nanoparticles were analyzed by Fourier transform infrared spectroscopy (FTIR, Nicolet 7000-C with 4 cm-1 resolution). The sample was prepared as follows: the sample was ground

10.1021/jp9051243 CCC: $40.75  2009 American Chemical Society Published on Web 08/18/2009

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Figure 1. XRD diffraction spectra of the as-synthesized sample.

peak (cm-1)

assignment

228 292 411 496 545 615 669 1050-1100 1319

A1 g (hematite) Eg (hematite) Eg (hematite) A1 g (hematite) T2 g (magnetite) Eg (hematite) A1 g (magnetite) hematite hematite

concluded that the weak peaks mentioned above in the XRD patterns are attributed to the magnetite phase in the assynthesized nanoparticles.24 The TEM image of the as-obtained nanocrystals is shown in Figure 3(A). They appear to be pseudocubic, and the size distribution is narrow (a statistical result is displayed in Figure 3(B)). The average size of the nanocrystals is 25 nm. From the HRTEM image shown in Figure 3(C), it can be found that the edges of the nanoparticle are clear and the lattice spacing of the crystal planes is similar to 0.3682 nm, which corresponds to a dihedral angle of ∼86°. The fast Fourier transform (FFT) image indicates that the nanoparticle is single crystalline (shown in the bottom inset of Figure 3(C)). The corresponding inverse

Figure 2. Raman spectra of the as-synthesized sample.

into KBr until it was uniformly distributed throughout the KBr; and then the mixture was transferred to a die to prepare a pellet. The magnetic properties were measured by a commercial Physical Properties Measurement System (PPMS Model 6000, Quantum Design, Inc.). The residual mixture of the reaction was treated with separation process and the upper and lower layers were analyzed by gas chromatography-mass spectrometry (Aligent 6890N-5973). The thermogravimetric-diffierential thermal analysis was taken on an STA 449C system (Netzsch, Germany) in air to investigate the contents of oleic group absorbed on the surface of the products. Results and Discussion 1. Observation on the Morphology Transformation. Figure 1 shows the XRD pattern of as-prepared nanocrystals. The 2θ value of the major peaks located in the range from 20° to 70° reveals a good match with the characteristic diffraction of the rhombohedral R-Fe2O3 (JCPDS No. 33-0664). However, two weak peaks respectively located at 30° and 43° seem to correspond to the diffraction of Fe3O4 as well as the left shoulder peak of the double-peak located at 57°. To verify this, a Raman spectrum study was taken, and the result is shown in Figure 2. Above 119 K, magnetite belongs to the inverse spinel cubic structure under the Oh7 (P4332) crystal space group.23 And five Raman bands are expected: three T2 g, one Eg, and one A1 g. The 545 and 669 cm-1 bands observed in the Raman spectrum are characteristically assigned to its T2 g and A1 g modes, respectively. The other peaks shown in Figure 2 are all attributed to hematite.24-26 And the intense feature at 1319 cm-1 is derived from a two-magnon scattering.27 Table 1 summarizes the peak positions and their assignments. Since the characteristic broad features of maghemite around 350, 500, and 700 cm-1 are not present in the Raman spectrum, it can be

Figure 3. (A) Low-magnification TEM image; (B) a statistical size distribution histogram; (C) HRTEM images, the bottom inset is the FFT pattern, and the upper inset is the corresponding inverse FFT pattern; (D) SAED pattern; (E) HRTEM image of a nanoparticle with dislocation; (F) the FFT pattern (bottom inset), and the corresponding inverse FFT pattern of the selected area, marked by white box in (E).

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Wang and Gao TABLE 2: Synthetic Parameters Used for the Preparation of Iron Oxide Nanoparitcles Observed by TEMa sample no.

sodium oleate (g)

oleic acid (mL)

ethanol (mL)

a b c d e f g h

1.92 2.88 1.68 1.44 1.92 1.68 0 0

20 20 20 20 14 14 20 17

40 40 40 40 46 46 40 43

a A 10-mL portion of iron(III) chloride hexahydrate aqueous solution (0.2 M) was added.

Figure 4. (A) Low-magnification TEM image of 3-h sample; (B) A typical aggregate in 3-h sample; (C) Low-magnification TEM image of 4-h sample; and (D) Low-magnification TEM image of 5-h sample.

FFT pattern (shown in the upper inset of Figure 3(C)) displays a clear lattice with no defects. The corresponding SAED pattern (shown in the inset of Figure 3(D)) exhibits sharp diffraction spots. It can be concluded that the nanoparticle is bound by sharp {012} planes.28,29 A typical dislocation was formed and is marked by the white box shown in Figure 3(E). The FFT and corresponding inverse FFT images of the selected area are employed to clearly exhibit the dislocation in Figure 3(F). The position of the dislocation runs diagonally across the particles, while there is no change in lattice distance along the dislocation. It seems attributed to the disorientation in the attachment process of the primary particles. To study the growth mechanism in our case, a series of samples obtained in different reaction times were observed. As shown in Figure 4(A), the sample obtained after a 3-h reaction appears spherical with a size of 4-15 nm. Some of them tended to form bigger aggregates (shown in the lower left corner of Figure 4(A) and a typical aggregate is marked by the white box in Figure 4(B)). The particles of the 4-h sample (shown in Figure 4(C)) seem bigger than those of the 3-h sample and some particles with pseudocubic appearance were demonstrated. When the reaction time was prolonged to 5 h, more pseudocubic particles were produced (shown in Figure 4(D)). This indicates that the formation of the final pseudocubic particles was dominated by the attachment of primary crystals. To clarify the effect of surfactant on the morphology transformation, a group of parallel experiments were carried out whose reactant proportion is listed in Table 2. When the sodium oleate was increased to 2.88 g (sample (b) in Table 2), the as-synthesized product shows clear edges and corners (shown in Figure 5(A)). And the corresponding FFT image (shown in the inset of Figure 5(A)) also indicates the good crystallinity of the nanoparticle. The lattice spacing is nearly 0.35 nm, which is smaller than the distance between (012) planes in the structure of the pure-phase hematite. And there were some small nanoparticles with irregular morphology and size of about 10 nm (shown in Figure 5(B)), which can be removed by performing a simple size-selective process.22 The

Figure 5. (A) HRTEM image, the inset is the FFT pattern; (B) small nanoparticles with irregular morphology.

Figure 6. (A) Low-magnification TEM image of sample (c); (B) HRTEM image of sample (c), the bottom inset is the FFT pattern; (C) HRTEM image of sample (c), the growth direction is indicated by the white arrow and the botton inset is the FFT pattern; (D) Lowmagnification TEM image of sample (d).

wide size distribution is attributed to excessive sodium oleate that accelerated the growth rate of primary nanocrystals.30 By decreasing the concentration of sodium oleate with the amount of oleic acid fixed (sample (c) in Table 2), a quasirhombic morphology was formed (shown in Figure 6(A)). The HRTEM image (shown in Figure 6(B)) indicates that the lattice distance is also consistent with the d-spacing value of (012) plane. The FFT pattern (shown in the inset of Figure 6(B)) is similar to those of samples (a) and (b). Further observation is taken in the HRTEM image and its FFT pattern (shown in Figure

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Figure 8. Fourier transform infrared spectra of as-synthesized nanoparticles.

TABLE 3: GC-MS Analysis Result of the Upper Layer Collected from the Residual Mixture

Figure 7. (A) Low-magnification TEM image of sample (e); (B) HRTEM image of sample (e), the upper inset is the FFT pattern; (C) Low-magnification TEM image of sample (f); (D) HRTEM image of sample (f).

6(C)) to find that the dominant growth direction of the nanocrystals was parallel to the [104j] direction (marked by a white arrow), which is one of the preferential growth directions of hematite.31 It indicates that the stoichiometric ratio of sodium oleate to Fe3+ is necessary to gain the nanocrystals of pseudocubic morphology. And it is obviously demonstrated in sample (d) shown in Figure 6(D). The morphology transforms from quasi-rhombic to pseudocubic with an increasing amount of sodium oleate. However, it is also influenced by the concentration of oleic acid. Figure 7(A) shows the as-prepared nanocrystals of sample (e) in Table 2 with truncated cubic morphology when decreasing the concentration of oleic acid compared to sample (a). Close observation on the truncated cubic nanocrystal (shown in Figure 7(B)) shows that the intersecting faces of the truncated polyhedral form a dihedral angle of 133°, which is consistent with the angle between (104j) and (1j12) planes. It indicates that the growth of nanocrystals along the [104j] direction was restrained. When a smaller amount of sodium oleate was added into the mixture (sample (f) in Table 2), nanocrystals with irregular spherical morphology appeared (shown in Figure 7(C)). And the HRTEM image confirms that partial nanocrystals with pseudocubic appearance are also bound by {012} facets, while their corners are smooth (shown in Figure 7(D)). These parallel experimental results suggest that the morphologies of as-synthesized nanoparticles are influenced by the concentration of the oleic acid and sodium oleate. It is wellknown that the chemisorption of surfactant on the surface of nanocrystals changes the growth rate of certain crystal faces, which is dominated by the surface enengy.32 An FTIR spectrum was taken to study the surface of these nanoparticles obtained in our experiments, and the result is shown in Figure 8 (only the FTIR spectrum of sample (a) in Table 2 is shown here since the spectra of other samples in Table 2 are similar to this). The bands at 1627 and 1395 cm-1 are assigned to the asymmetrical and symmetrical vibration of carboxylate groups, respectively. And the separation of ν (COO-) bands is 232 cm-1, which indicates that the carboxylate coordination mode is unidentate.33,34 This confirms that the oleic group was chemisorbed onto the

real time (min)

area percent (%)

substance

11.9726 13.2071 13.3271 13.8358 14.4359 14.4873 14.573

2.79878 3.49249 5.00374 0.947081 75.074 9.87685 2.80704

tetradecanoic acid, ethyl ester E-11-hexadecenoic acid, ethyl ester hexadecanoic acid, ethyl ester (E)-9-octadecenoic acid ethyl ester ethyl oleate ethyl oleate octadecanoic acid, ethyl ester

surface by coordination of oxygen atoms in carboxylate and iron atoms of nanoparticles asymmetrically in the growth process of nanoparticles. According to the literature, the final shape of the particles is determined as a function of either the ratio of the growth rates of the individual facets or the ratio of their surface energies, depending on the growth mechanisms.35-38 In our experiments, the pseudocubic nanoparticles (samples (a) and (b) in Table 2) were bound by {012} facets. This may arise from the different surface energies among the preferential growth planes of hematite, {012}, {104}, and {110}. In recent research, fatty acid salts have been proven to be good shape controllers, such as linoleate, oleate, stearate, and so on.22,39,40 Generally, dilute surfactant results in irregular morphology for poor control on the growth rate of crystal planes. When sufficient sodium oleate was added, the nanocrystals with pseudocubic morphology were obtained (samples (a) and (b) in Table 2). While nanocrystals with poor shape uniformity were produced when the sodium oleate added originally was less than stoichiometric (samples (d) and (f) in Table 2). It may be attributed to the ion exchange between Na+ and Fe3+ controlling the amount of reactive free Fe3+ in solution.40,41 According to the report of Li et al.,40 the primary crystals were produced through the liquid-solid-solution (LSS) process. In this process, a phase transfer process based on ion exchange was prerequisite for the preparation of the nanocrystals. Therein, the carboxylates played an important role. To clarify the effect of sodium oleate in our case, mixtures without addition of sodium oleate (samples (g) and (h) in Table 2) were used and it was found that no products were finally obtained. It seems that the formation of iron oleate is a key procedure during the reaction.22 And the result also revealed that the ferric ion failed to react with oleic acid directly to form iron oleate. If the stability constant of iron oleate was higher than that of oleic acid, then it may be attributed to the pH drop caused by the released protons from oleic acid, which was not conducive to the formation of iron oleate. The production of hematite particles was inhibited in such an acidic environment.20 In contrast, if

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the stability constant of oleic acid were higher than that of iron oleate, then the ferric ion would remain free from oleic acid without sodium oleate. And the hematite was not produced by the hydrolysis of ferric ion with hydroxide ion in such conditions. This can be explained by the LSS process. It is assumed that the ferric ion reacted with hydroxide directly in the solution phase. As the freshly generated nuclei owned high surface energy, the oleic group would bond with the iron atom exposed on the surface. And then the nuclei were endowed with a hydrophobic nature. They were stable in the ethanol-oleic acid liquid phase, and further growth was restrained by the absence of ferric ion transfer from the solution to the liquid phase. The protons released from the hydrolysis reaction made the pH drop large enough to prevent the further hydrolysis and dissolve the generated nuclei. It seemed that even if the nucleation step occurred by direct reaction between the ferric ion and hydroxide in the solution phase, no particles were found as a result of the inhibition of the growth step. Meanwhile, it was found that the solution was separated into two layers along with the reaction. To clarify the reaction in our case, GC-MS analysis was taken to determine the substances in the two layers. No organic substances were found in the underlayer. It changed the color of allochroic silica gel and became muddy after dropping AgNO3 solution. It seems that the underlayer solution was mainly composed of water and sodium chloride. And the result of the upper layer is shown in Table 3. This indicates that the ethyl oleate was the main substance in the upper layer generated by the esterification between ethanol and oleic acid, which was similar to that reported by Peng et al.42 It is worth noting that there was some octadecanoic acid, ethyl ester which could be obtained by the hydrogenation of the ethyl oleate, which reveals that the reaction was undergone in a reductive ambience. Since no ethanol was found either in the upper or underlayer, it can safely be assumed that the ethanol not only played a role as solvent to help the mixture form a heterogeneous and transparent system, but was also involved in the reaction. According to the recent research, oleic acid is expected to act as an inhibiting reagent to retard the growth of nanoparticles in the reaction process. In contrast, ethanol has a positive influence on accelerating the crystal growth.42,43 As mentioned above, there was an esterification reaction between ethanol and oleic acid along with the reaction. The oleic group absorbed on the surfaces of primary crystals and hindered their attachment and, subsequently, their further growth, while the ethanol promoted desorption of the oleic group from the surface. Since the total volume of oleic acid and ethanol was fixed in our experiments, there were two opposite factors involved in slowing down the decomposition of oleate by increasing the amount of oleic acid. When the oleic acid was deficient, the decomposition rate of oleate was too high. Hence, the oleate was exhausted too early for the nanocrystals to grow completely (samples (e) and (f) in Table 2). Simultaneously, the decrease of oleic acid also weakened the shape control on the pseudocubic nanoparticles. However, the primary nanocrystals capped with the oleic group had less time to select the right position to form a pseudocubic morphology by decreasing the amount of oleic acid.10 In contrast, the addition of sufficient oleic acid effectively slowed down the decomposition rate of oleate. Comparing samples (c) and (e), it can be found that the high decomposition rate leads to nanocubes truncated along [104j] direction, owing to deficient oleic acid and stoichiometric sodium oleate. In contrast, quasi-rhombic nanoparticles, preferentially growing along the [104j] direction, were produced at a low decomposition

Wang and Gao

Figure 9. The morphology transformation, in which the character “r” represents the molar ratio of sodium oleate to ferric ion.

TABLE 4: Synthetic Parameters Used for the Preparation of Iron Oxide Nanoparticles Determined by XRDa sample number sodium oleate (g) oleic acid (mL) ethanol (mL) a b c

1.92 1.92 1.68

20 17 20

40 43 40

a A 10-mL portion of iron(III) chloride hexahydrate aqueous solution (0.2 M) was added.

Figure 10. XRD diffraction spectra of samples listed in Table 4.

rate. Note that when the ratio of sodium oleate to oleic acid was too high, the mixture was muddy and less uniform nanocrystals were obtained which were not enclosed by {012} planes. Figure 9 shows the schematic diagram of morphology transformation. As illustrated, the molar ratio of sodium oleate to ferric ion was more than or equal to the stoichiometric in routes 1 and 2, whereas in routes 3 and 4, the ratio was less than stoichiometric. Moreover, sufficient oleic acid was added in routes 1 and 3, and the amounts in routes 2 and 4 were deficient. The spherical objects drawn on the left side represent the primary particles produced at an early stage in our experiments, which is displayed in Figure 4. The yields of samples (a)-(f) listed in Table 2 reached as high as ∼80% (detail shown in the Supporting Information). 2. Study on the Magnetic Properties. Meanwhile, the corresponding changes of the structure and magnetic property were also studied. As mentioned above, the magnetite phase is found in the as-synthesized nanocrystals. However, there was no Fe2+ ion originally added in our experiment. In other words, a portion of Fe3+ ion was reduced to Fe2+ in the synthesis process to produce the magnetite. According to previous research,44,45 this may be attributed to the organic surfactants used for shape-control. They worked as a reductive reagent at high temperature and provided the reductive ambience in which

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Figure 11. (A) A series of room-temperature hysteresis loops for samples listed in Table 4. A closer look at traces (b) and (c) are displayed in two insets, respectively. (B) Photograph of sample (a) before and after using an applied magnetic field.

the reaction proceeded. So the influence of surfactants on the reducing action was studied by altering the reactant ratio (shown in Table 4). Figure 10 shows the XRD patterns of samples listed in Table 4. When the oleic acid was decreased to 17 mL (sample (b) in Table 4), the peaks located at 30°, 43°, and 57°, respectively, became weaker (shown in curve (b) of Figure 10) than those of sample (a). It seems that the magnetite phase was subject to the amount of oleic acid under the same reaction conditions. A slight decrease of sodium oleate (sample (c) in Table 4) led to the disappearance of peaks corresponding to the diffraction of Fe3O4 (shown in curve (c) of Figure 10). The as-synthesized product was phase-pure hematite. Noteworthily, a gradual shift of the peak nearly located at 57° from a double-peak to a singlepeak accompanies the composition variation. This indicates that the concentration of sodium oleate and oleic acid both had a significant effect on the reducing reaction between Fe3+ and Fe2+. The Fe3+ was deduced to be reduced in such a reductive ambience provided by the surfactant, as mentioned above. However, magnetite belongs to a spinel-type cubic system. different from the crystal structure of hematite in the hexagonal system. According to the work of Sugimoto et al.,46 if singlecrystal hematite particles are totally reduced to magnetite in hydrogen atmosphere, then the resulting magnetite particles become polycrystals consisting of much smaller subcrystals of magnetite, but with the external particle shape completely retained. So there would have been many defects in the crystal body if the pseudocubic hematite particles were entirely reduced. Yet this is different from our observations. Thus, a possible position of the magnetite phase might only be in the surface zones of the pseudocubic hematite particles where the lattice structure is rather irregular. Some distinct surface layers with their lattices oriented in a direction different from that of the host hematite appear to be observed in the HRTEM of Figure 5A. Figure 11(A) displays a series of room-temperature hysteresis loops for these samples. Since hematite is weakly magnetic at room temperature, the saturation magnetization is mainly due to the magnetite phase, which is subject to the concentration of the surfactant. As shown in Figure 11(A), the saturation magnetization of sample (a) is obviously higher than that of others, which is consistent with the XRD pattern shown in Figure 10. This suggests that the saturation magnetization is influenced by the concentration of sodium oleate and oleic acid. The magnetic property of nanoparticles seems to be controllable by adjusting the concentration of the surfactant without the addition of Fe2+

or reductive agent to some extent. Figure 11(B) shows a demonstration photograph of sample (a) by using a magnetic field. Conclusions In summary, uniformly distributed iron oxide nanoparticles with tunable morphology have been synthesized in ethanol/water media with oleic acid/sodium oleate used as the surfactants. The influence of the surfactant on the morphology of as-prepared nanocrystals was studied by altering their amount. From the results, it could be concluded that the morphology was mainly dominated by the actual concentration of sodium oleate in the solution. And the morphology was also greatly subject to the amount of the oleic acid adsorbed on the surface of the nanoparticles. In our experiments, morphology transformation was successfully achieved from irregular spherical, rhombic, and truncated cubic to finally pseudocubic. Meanwhile, the changes of structure and magnetic property are also studied. It is found that the phases of the products gradually transformed from pure hematite into mixed-phase iron oxide by adjusting the concentration of oleic acid and sodium oleate. It was attributed to the reductive ambience provided by the surfactant in our experiments. The phase composition of magnetite was proportional to the concentration of sodium oleate, which was proven by the XRD patterns and corresponding Raman spectra. Thus, the saturation magnetization, which is subject to the composition of the magnetite phase, is influenced by the concentration of surfactant. Acknowledgment. This work was supported by the National Key Project of Fundamental Research (Grant No.2005CB623605), the National Natural Science Foundation of China (No. 50572116, 5060249), and Shanghai Nanotechnology Promotion Center (No. 0652 nm022). Special thanks are given to Dr. S. W. Yang for his guidance in this research. Supporting Information Available: The calculated yields of the samples synthesized in our experiments. This information is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Raj, K.; Moskowitz, R. J. Magn. Magn. Mater. 1990, 85, 233. (2) McMichael, R. D.; Shull, R. D.; Swartzendruber, L. J.; Bennett, L. H.; Watson, R. E. J. Magn. Magn. Mater. 1992, 111, 29. (3) Zeng, H.; Li, J.; Liu, J. P.; Wang, Z. L.; Sun, S. Nature 2002, 420, 395. (4) Bae, Y.; Fukushima, S.; Harada, A.; Kataoka, K. Angew. Chem., Int. Ed. 2003, 42, 4640. (5) Hermanek, M.; Zboril, R.; Medrik, I.; Pechousek, J.; Gregor, C. J. Am. Chem. Soc. 2007, 129, 10929.

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