DOI: 10.1021/cg900920q
Shape-Controlled Synthesis and Magnetic Properties of Monodisperse Fe3O4 Nanocubes
2010, Vol. 10 2888–2894
Guanhua Gao,† Xiaohe Liu,*,†,‡ Rongrong Shi,† Kechao Zhou,‡ Youguo Shi,§ Renzhi Ma,§ Eiji Takayama-Muromachi,§ and Guanzhou Qiu*,† †
Department of Inorganic Materials, Central South University, Changsha, Hunan 410083, PR China, State Key Laboratory of Powder Metallurgy, Central South University, Changsha, Hunan 410083, PR China, and §International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science, Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan ‡
Received August 4, 2009; Revised Manuscript Received May 11, 2010
ABSTRACT: Monodisperse Fe3O4 nanocubes have been successfully synthesized by a facile solvothermal method at 260 C in the presence of oleic acid and oleylamine. Well-defined assembly of uniform Fe3O4 nanocubes with an average size of 12 nm could be obtained without a size-selection process. The shape of as-prepared Fe3O4 nanoparticles could be reversibly interchanged between spheres and cubes by adjusting the reaction parameters. The phase structures, morphologies, and sizes of as-prepared products were investigated in detail by X-ray diffraction (XRD), transmission electron microscopy (TEM), and high-resolution transmission electron microscopy (HRTEM). The magnetic properties of Fe3O4 nanocubes were measured by using a quantum design superconducting quantum interference device (SQUID). The magnetic study reveals that the assynthesized nanocubes are ferromagnetic at 2 K while they are superparamagnetic at 300 K.
Introduction Monodisperse magnetite (Fe3O4) nanoparticles with controlled shape and size have currently aroused enormous interest due to their unique electric and magnetic properties based on the transfer of electrons between Fe2þ and Fe3þ in the octahedral sites and potential applications in various fields, e.g., ferrofluids,1 biocatalysts,2 magnetic resonance imaging (MRI),3 high-density data storage,4 sensing,5 magnetic separation,6 etc. In particular, they have also been considered as promising candidates for biological applications, such as drug-delivery carrier for antitumor therapy and activity agent for medical diagnostics.7-10 During the past decades, various synthetic methods have continually been developed for fabricating monodisperse Fe3O4 nanoparticles; among these methods, thermal decomposition of organometallic precursors and solvothermal methods are versatile strategies in the synthesis of monodisperse Fe3O4 nanoparticles with controlled shape and size.11-17 Recently, some progress was made in the synthesis of monodisperse Fe3O4 nanocubes. Yang et al. have synthesized monodisperse Fe3O4 nanocubes by a modified polyol process in which oleic acid and oleylamine serve as surfactants, while 1,2-hexdecandiol and benzyl ether act as reducing agent and solvent, respectively.18 Hyeon and coworkers have reported the synthesis of monodisperse Fe3O4 nanocubes ranging in size from 20 to 160 nm via thermal decomposition of iron(III) acetylacetonate (Fe(acac)3, acac = acetylacetonate, CH3COCHCOCH3) precursors in the mixture solution of oleic acid and benzyl ether at 290 C.19 Nanocubes exposed a specific surface out, which provided an ideal model for the study of surface related properties.20 In order to improve the shape-dependent functional properties, such as electrics or magnetism, it remains a highly sophisticated *To whom correspondence should be addressed. E-mail:
[email protected]. edu.cn;
[email protected]. Telephone: þ86-731-88830543. Fax: þ86731-88879815. pubs.acs.org/crystal
Published on Web 06/08/2010
challenge to correlate the relation between synthetic parameters and shape of products, to rationally tune the shapedependent functional properties. Alternative synthetic procedures promising to reveal the correlation between reaction parameters and shape of products have to be developed. Herein, we demonstrate that monodisperse magnetite Fe3O4 nanocubes with a mean size of 12 nm could be successfully synthesized through a simple and improved solvothermal method via the reaction of Fe(acac)3 in octadecene solvent in the presence of oleic acid and oleylamine. It is worthy to note that neither polyalcohol nor benzyl ether was used, which makes this process attractive due to its mild conditions. The shape of the final products could be readily controlled by adjusting the experimental parameters such as reaction time, temperature, and surfactants. After the evaporation of the solvents from dilute hexane suspensions, two-dimensional (2D) self-assembly of nanocubes spontaneously formed on the copper TEM grid. The magnetic properties of Fe3O4 nanocubes were investigated at both 2 and 300 K. The synthetic strategy presented here may have a good prospect for large scale application and provide an effective route to synthesize different spinel ferrite and other metal oxide nanoparticles with tuned shape. Experimental Section Materials. The chemicals used in this work, such as iron acetylacetonate (Fe(acac)3), n-hexane, and ethanol absolute, were of analytical grade and used as received. Tri-n-octylphosphine oxide (TOPO, 99%), oleic acid, oleylamine, and octadecene (90%) were purchased from ACROS. Synthesis of Monodisperse Fe3O4 Nanocubes. In a typical synthesis of 12 nm Fe3O4 nanocubes, Fe(acac)3, oleic acid, oleylamine, and TOPO (in molar ratios of 1:4:10:0.1) were mixed in octadecene (20 mL) under vigorous stirring at room temperature. Then the mixture was put into a Teflon-lined stainless steel autoclave with a capacity of 40 mL. Before sealing the autoclave, argon was bubbled into the solution to remove the air. The autoclave was first maintained at 200 C for 30 min and then heated to 260 C for 2 h. After the reaction finished, the autoclave was cooled to room temperature r 2010 American Chemical Society
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Table 1. Synthesis Conditions of the Decomposition of Fe(acac)3 (1 mmol) in the Solvent of ODEa (20 mL) TOPOb (mmol)
OAc (mL)
0.1 0.3 0.5 1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1
4 4 4 4 4 4 4 4 4 4 4 1 10
OAmd temp (mL) (C) 10 10 10 10 10 10 10 10 10 4 16 10 10
260 260 260 260 260 220 240 260 260 260 260 260 260
time (h)
shapee
2 2 2 2 2 2 2 0.5 1 2 2 2 2
spherical cubic triangular irregular spheres nearly spherical spherical nearly cubic spherical, cubic roughly cubic roughly cubic irregular spherical or triangular nearly hexagonal
a ODE: octadecene. b TOPO: tri-n-octylphosphine oxide. c OA: oleic acid. d OAm: oleylamine. e Obtained by means of TEM.
naturally. The samples were washed with hexane and ethanol several times and finally dried in vacuum at 60 C for 4 h. All the synthesis conditions (surfactant TOPO, the molar ratio of oleic acid to oleylamine, temperatures, and reaction times) surveyed for the preparation of Fe3O4 samples are summarized in Table 1. Characterization. The obtained samples were characterized on a D/max2550 VBþ X-ray powder diffractometer (XRD) with Cu KR radiation (λ = 1.54178 A˚). The operation voltage and current were kept at 40 kV and 40 mA, respectively. The size and morphology of the as-prepared products were determined at 160 kV with a JEM200CX transmission electron microscope (TEM) and a JEOL JEM2010F high-resolution transmission electron microscope (HRTEM). Magnetic measurements were conducted using a Quantum Design MPMS XL-5 superconducting quantum interference device (SQUID).
Results and Discussion Transmission electron microscopy (TEM) was used to characterize the morphology, size, and size distribution of as-prepared products. Parts A and B of Figure 1are the lowand high-magnification TEM images of as-prepared Fe3O4 nanocubes obtained at 260 C for 2 h using 0.1 mmol TOPO with a molar ratio of oleic acid to oleylamine of 4:10. They reveal that a large number of well-defined Fe3O4 nanocubes have been successfully synthesized under current conditions. TEM images suggest that the resulting nanocubes display a narrow size distribution, which spontaneously forms a 2D self-assembly. A detailed crystal structure analysis on these nanocubes was performed by means of selected area electron diffraction (SAED) and high-resolution TEM (HRTEM). A characteristic SAED pattern inserted in Figure 1A illustrates the spinel crystalline structure of magnetite (Fe3O4). A representative HRTEM image of a single Fe3O4 nanocube with a mean size of about 12 nm was shown in Figure 1C; it reveals that the two fringe spacing was measured to be 0.298 nm, agreeing well with the interplanar spacing of (220) planes of face-centered cubic (fcc) Fe3O4. The 2D fast Fourier transform (FFT) pattern of the high-resolution image in the inset of Figure 1C reveals a 4-fold symmetry, consistent with the fcc structure projected from the [001] direction. A model structure of the nanocube is also provided in Figure 1C. Figure 1D shows the XRD pattern of the as-synthesized products. The position and relative intensity of all diffraction peaks can be indexed to a pure fcc phase [space group: Fd3m (227)] of Fe3O4 ˚ , which match well with that of with cell constants a = 8.405 A magnetite (JCPDS card no. 89-0688). No other impurity phase was observed, indicating pure Fe3O4 nanocubes have been synthesized under current mild experimental conditions.
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The size distribution histogram shown in Figure 1E illustrates the mean size of the nanocubes is around 12 nm, which agrees well with the TEM results. The yield of the cubic nanoparticles is estimated to be over 87% based on the count of over 800 particles (Figure 1F), indicating that the majority of the particles are cubic. For a better understanding of the growth process of Fe3O4 nanocubes, the effect of various experimental conditions on the synthesis has been investigated in detail. Reaction parameters such as the amount of TOPO, temperature, reaction time, and molar ratio of oleic acid to oleylamine have been studied for the shape control of nanoparticles. The morphologies deduced from TEM results for all the samples under current conditions have been summarized in Table 1. Figure 2 displays the influence of surfactant TOPO on the shape of Fe3O4 nanoparticles. Figure 2A is the TEM image of Fe3O4 nanoparticles prepared in the absence of TOPO while keeping the other parameters constant. It reveals only spherical Fe3O4 nanoparticles were produced. The shape of Fe3O4 with a 0.3 mmol dose of TOPO changes spherical nanoparticles into inhomogeneous trigonal ones (Figure 2B). When the amount of TOPO increased to 0.5 mmol (Figure 2C), the shape of triangles transformed into irregular spherical nanoparticles. Further increasing the amount of TOPO to 1.0 mmol (Figure 2D), the shape of as-prepared Fe3O4 nanoparticles remains spherical, but the uniformity of the particle size was improved greatly. These results indicated that the shape and uniformity of Fe3O4 nanoparticles can be controlled by altering the amount of surfactant TOPO. The impact of reaction temperature and time on the formation of Fe3O4 nanocubes was significant in the synthesis. TEM images of the nanoparticles prepared under different reaction temperatures are shown in Figure 3A and B, and those prepared under different reaction times are given in Figure 3C and D. In Figure 3A, the shapes of the Fe3O4 product obtained at 220 C for 2 h are roughly spherical. The spherical nanoparticles are arranged in a closed packed array, demonstrating the uniformity of the particle size. No cubes appeared under the condition of 220 C for 2 h. But plenty of nanocubes occurred at the temperature of 240 C, as shown in Figure 3B. Parts C and D of Figure 3 display the monodisperse Fe3O4 nanocubes prepared at 260 C for different reaction times. The Fe3O4 nanocubes prepared for 0.5 h (Figure 3C) display almost irregular nanoparticles, with nanocubes occasionally observed. Prolonging the reaction time from 0.5 to 1 h, the number of nanocubes has increased remarkably, indicating the generation of nanocubes needs an extended period of reaction time. In Figure 3D, as-prepared nanocubes with uniform size also formed large-area ordered arrays. The well-organized self-assemblies have a promising future in applications such as high density data storage. These results reveal that the high temperature or long reaction time seems to be desirable to get monodisperse Fe3O4 nanocubes. The molar ratio of oleic acid to oleylamine is also a crucial factor for the production of monodisperse Fe3O4 nanocubes. Through a careful adjustment of the molar amount of stabilizer as well as the reaction temperature and time, shapecontrolled monodisperse magnetite nanocubes could be prepared. Figure 4 shows the Fe3O4 nanoparticles obtained using different molar ratios of oleic acid to oleylamine. On the one hand, a reaction mixture with a 4:4 molar ratio of oleic acid to oleylamine was applied in the synthesis; Figure 4A shows an interesting short-range order in the nanocube assembly. By increasing the amount of oleylamine to 16 mmol, large
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Figure 1. (A) Low- and (B) higher-magnification TEM images of the as-prepared Fe3O4 nanocubes prepared at 260 C for 2 h using 0.1 mmol TOPO with a molar ratio of 4:10 for oleic acid to oleylamine. The inset of part A shows the SAED pattern of the Fe3O4 nanocubes acquired from sample A. (C) HRTEM image of an individual Fe3O4 nanocube. The insets are the FFT of the high-resolution TEM image looking down the [001] direction and a model depiction of the nanocube structure consistent with the TEM results. (D) XRD pattern of the as-prepared Fe3O4 nanocubes. (E) Size distribution of the as-prepared Fe3O4 nanocubes. (F) Relative amount of cubic and irregular particles.
numbers of monodispersed triangular nanoparticles were obtained; however, other shapes such as nanospheres and irregular nanoparticles also appeared (Figure 4B). On the other hand, a TEM image of Fe3O4 nanoparticles is shown in Figure 4C when the molar ratio of oleic acid to oleylamine was reduced to 1:10. The products display irregular spherical nanoparticles accompanying triangular ones. Contrastively, a TEM image of as-prepared nanoparticles obtained in the case of a 10:10 molar ratio of oleic acid to oleylamine is shown in Figure 4D. These products were arranged in a 2D hexagonal array. These results demonstrate the shape of as-prepared Fe3O4 nanoparticles can be controlled by changing the molar ratio of oleic acid to oleylamine.
Combined with the above experimental results, a possible formation mechanism of Fe3O4 nanocubes is proposed. We consider that thermolysis of precursors in a high-boiling-point solvent can result in the controlled synthesis of monodisperse nanocrystals. This mainly comprises two processes: (i) the formation of Fe3O4 crystal nuclei and (ii) the subsequent crystal growth from these nuclei to form cubic nanocrystals. On the basis of the results shown in Figures 2-4, we guess that both thermodynamic and kinetic factors dictate the shape evolution of the Fe3O4 nanocrystals at different stages of the reaction, and the organic surfactants also play important roles in the synthesis. At the relatively low temperature of 220 C, the nanocrystals have spherical shapes; this may be because
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Figure 2. TEM images of Fe3O4 nanoparticles prepared using different moles of the surfactant TOPO: (A) 0; (B) 0.3 mmol; (C) 0.5 mmol; (D) 1.0 mmol.
Figure 3. TEM images of Fe3O4 nanocubes prepared at different reaction conditions: (A) 220 C, 2 h; (B) 240 C, 2 h; (C) 260 C, 0.5 h; (D) 260 C, 1 h.
the monomers attach and grow isotropically from the nucleating seeds at a slow growth rate with no specificity in the growth direction, leading to the thermodynamically favored spherical morphology. Under these conditions, the surface energy contribution primarily determines the shape of the nanocrystals, since the spherical shape has the lowest surface
area. With increasing the temperature from 220 to 240 C, it results in intermediate spherical-to-cubic shape nanocrystals. When the temperature was further elevated to 260 C, relatively fast growth along the Æ111æ crystal directions results in the eventual formation of cubic-shaped nanocrystals. In contrast, relatively higher temperature and longer reaction time,
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Figure 4. TEM images of Fe3O4 nanocubes prepared using different molar ratios of oleic acid to oleylamine: (A) 4:4; (B) 4:16; (C) 1:10; (D) 10:10.
as compared with the synthetic condition of the spherical shape product, are necessary for the growth of cubic-shaped product. With prolonging the reaction time at 260 C to 0.5, 1, and 2 h, more and more spherical nanocrystals continue growing to form nanocubes, as can be seen by comparing the TEM images in Figure 3C, 3D, and 1A. After the longest reaction time of 2 h at 260 C, it results in near-complete transformation to the cubic-shaped product. Additionally, oleic acid, oleylamine, and TOPO were used as capping surfactants to stabilize the growth of monodisperse Fe3O4 nanocrystals. They may play important roles in the crystal growth process. The exact binding nature between capping surfactants and the crystal facets is not known, but we note that the shape-controlled synthesis can be achieved through selective stabilization of crystal facets. TOPO usually acts as a particle stabilizer, and the surface group may preferentially bind to certain crystal faces of the particles for controlling the shape of nanoparticles. Selective adsorption of TOPO on certain growing faces of nanoparticles may lead to an anisotropic structure. In the absence of TOPO, formation of spherical Fe3O4 particles is preferred, since oleic acid and oleylamine adsorbed on the surface iron atoms of the formed Fe3O4 nuclei, forming a steric stabilizing layer that prevented their anisotropic growth. Finally, it resulted in the formation of uniform oleic acid/oleylamine-capped Fe3O4 nanoparticles. When TOPO is used as a cosurfactant, however, Fe3O4 nanocubes are obtained. The formation of nanocubes demonstrating TOPO selectively stabilizes the (100) faces of Fe3O4 and decreases its relative growth rate. Furthermore, when increasing the concentration of TOPO, the mixed morphologies including triangle and other irregular shapes were found in the products (Figure2B and C). This may be attributed to the nonuniform distribution of excess TOPO molecules on the growing faces of nanoparticles inducing the growth of nanoparticles into various shapes. When the amount of TOPO
increased significantly, up to 1 mmol, the product (Figure 2D) exhibited nearly spherical-like shapes, owing to all crystal faces being coordinated by TOPO capping ligands and showing saturated surface coverage, thus directing the nuclei to grow isotropically along the three crystallographic axes, resulting in 0D spheres. Oleylamine ligand used in this study seems to bind weakly to iron centers on the surface of nanocrystals, while oleic acid ligand forms a much stronger bond to the crystal surface due to its high oxophilicity.21 Thus, the reduction of the concentration of oleylamine shows an inapparent effect on the morphology of the product, remaining nanocubes (Figure 4A). A weakly binding ligand can reversibly coordinate to the iron sites on the surface, and further growth or ripening processes are possible when sufficient amounts of oleylamine ligand are available. In fact, by increasing the amount of oleylamine to 16 mmol, a mixture of faceted nanocrystals is observed. The large nanocrystals grow to about 25 nm, while some smaller ones observed are very small in size (
