Oriented Assembly of Fe3O4 Nanoparticles into Monodisperse Hollow

Oct 6, 2006 - Synthesis of High Saturation Magnetization Superparamagnetic Fe3O4 Hollow Microspheres for Swift Chromium Removal. Yubiao Liu , Yongqian...
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J. Phys. Chem. B 2006, 110, 21667-21671

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Oriented Assembly of Fe3O4 Nanoparticles into Monodisperse Hollow Single-Crystal Microspheres Dabin Yu,* Xiaoquan Sun, Jiwei Zou, Zirong Wang, Feng Wang, and Kun Tang Laboratory of Optical and Nano-Scale Functional Materials, State Key Laboratory of Pulsed Power Laser Technology, Electronic Engineering Institute, 460 Huangshan Road, Hefei, Anhui 230037, People’s Republic of China ReceiVed: July 24, 2006; In Final Form: August 30, 2006

Magnetite nanoparticles of Fe3O4 were found to assemble into monodisperse hollow Fe3O4 microspheres with tunable diameters ranging from 200 to 400 nm and open pores on the shells in ethylene glycol in the presence of dodecylamine (DDA). The oriented assembly of nanoparticles conferred the individual hollow Fe3O4 microspheres a remarkable feature of single crystals. The morphologies of the products could be easily manipulated by varying the synthesis parameters. Increasing the concentration of DDA led to an obvious shape evolution of the products from rhombic nanoparticles to hollow microspheres, solid microspheres, and finally irregular nanoparticles, which were mainly attributed to the special self-assembly phenomenon of Fe3O4 nanoparticles in the solvothermal process.

1. Introduction Monodisperse magnetic microspheres with sizes of hundreds of nanometers are currently attracting considerable interest because of their technical use in biomedical fields, especially applications in vivo,1 in addition to their wide potential applications in ferrofluids, catalysts, colored pigments, highdensity magnetic recording media, etc.2 Although magnetic particles with narrow size distribution have been synthesized by various methods since the fabrication of monodisperse magnetic nanoparticles reported by Matijeviæ in the early 1980s,3 the products were overwhelmingly dominated by particles with sizes of tens of nanometers.1,4 It is therefore an interesting challenge to develop approaches to monodisperse magnetic particles with suitable sizes to meet the special need of biomedical fields.1 Among various magnetic nano- or microparticles, monodisperse magnetite (Fe3O4) particles have been considered as an ideal candidate for biological applications such as a tag for sensing and imaging,5 a drug-delivery carrier for antitumor therapy,6 and an activity agent for medical diagnostics,7 due to their good hydrophilic and biocompatible properties. Moreover, as for the applications in biomedical fields, the hollow microspheres are of particular importance because they can act as promising drug-delivery carriers owing to their structures with hollow interiors,8 while monodisperse Fe3O4 hollow microspheres, to the best of our knowledge, have not been reported so far. In this paper, we demonstrate a simple “one-pot” approach to Fe3O4 hollow microspheres with tunable diameters ranging from 200 to 400 nm in ethylene glycol by using dodecylamine as template agent to control their morphologies. Although developing approaches to hollow nano- or microspheres has been, in fact, pursued for quite a few years because of both aesthetic beauty and scientific attractions when they are coupled with functional materials with their inner “nanospace”,9 it is still desirable to explore other wet-chemical means, aiming at such a simple “one-pot” synthetic approach for important metal * Address correspondence to this author. E-mail address: [email protected].

oxides.10 This method allows the large-scale synthesis of monodisperse Fe3O4 hollow microspheres, which resulted from an oriented assembly of small nanoparticles and exhibited a remarkable feature of single crystals. It is worth further noting that the oriented assembly of nanoparticles attracts much attention as a promising method for creating advanced artificial materials with distinct micro- and nanostructures and as clues for the explanations of the growth mechanism of crystals.11 Up to now, oriented assembly of primary oxide nanoparticles was mainly carried out in aqueous media in the presence of suitable surfactants or other shape controller, while there are few routes that have been reported for the oriented assembly of oxide nanocrystals in nonaqueous media.12 Therefore, this method may not only provide a potential method for the control over microor nanostructures, morphologies, and particle size, but also contribute to the development of functional magnetic materials, in particular biocompatible magnetic materials for applications in biology and medicine as diagnostic and therapeutic tools. 2. Experimental Section Materials. Analytical FeCl3‚6H2O, ethylene glycol, dodecylamine (DDA), and solvents were purchased from Shanghai Chemical Reagents Company and used as received. Synthesis. In a typical synthesis, 2 mmol of FeCl3‚6H2O was dissolved in 20 mL of ethylene glycol under stirring. Next 2 mmol of DDA was added into the solution under continuous stirring until it was dissolved completely. Separately, 4 mmol of NaOH was dissolved in 10 mL of ethylene glycol in another flask. The solution of NaOH was added to the solution of iron chloride under stirring, causing an immediate color change from colorless to yellow. The mixture was then transferred into an autoclave, sealed, and heated at 220 °C for 12 h. The black solid product was obtained by cooling the reaction mixture to room temperature and centrifuging, and sequentially washed with water and ethanol twice to remove ethylene glycol, DDA, and byproducts. Characterization. The XRD patterns were recorded on a Philips X’Pert PROSUPER X-ray powder diffractometer (λ )

10.1021/jp0646933 CCC: $33.50 © 2006 American Chemical Society Published on Web 10/06/2006

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Figure 1. Schematic illustration of the formation of Fe3O4 hollow microspheres.

1.54178 Å) in the 2θ range of 10-70°. TEM (HRTEM) images and the corresponding selected area electron diffraction (SAED) patterns were obtained on a JEM2010F microscope operated at an acceleration voltage of 200 kV. The scanning electron microscopy (SEM) images were taken on a LEO1530 scanning electron microscope. The magnetic properties were investigated with a BHV-55 vibrating sample magnetometer at room temperature. 3. Results and Discussion Ethylene glycol has been reported as an effective reaction medium due to its special physical and chemical properties.13 The high permittivity enables it to dissolve highly polar inorganic and organic compounds and to make the synthesis reactions achieved by the routes of highly polar or ionic intermediates, and the strong chelating ability makes it easy to form complexes with transition metals by using its hydroxyls as ligands so that some inorganic materials such as chlorides can be dissolved in it to form a uniform solution.14 Besides, ethylene glycol is a strong reducing agent that can react with FeCl3 to form Fe3O4 at a high temperature.1 Therefore, we designed a novel route to monodisperse Fe3O4 hollow microspheres by using ethylene glycol as the reaction medium and

Yu et al. DDA as the template agentsthe latter has been used as surfactant to control growth of nanocrystals because of the long nonpolar carbon chain.15 Since the morphology of the surfactant micelles may range from spherical shape to prolate or rodlike, and lamellar phase, depending on the concentration of the surfactant and additives,16 our strategy is therefore to control DDA micelles in a spherical shape (I), on the basis of which Fe3O4 nanoparticles gradually formed and assembled into shells (II). With the removal of the DDA cores, hollow Fe3O4 microspheres were finally obtained (III), as schematically illustrated in Figure 1. The important roles of DDA in the formation of hollow Fe3O4 microspheres will be discussed in the following sections. Figure 2a shows a typical SEM image of Fe3O4 products, indicating that the products consisted of a large quantity of spheres with average diameter of ca. 210 nm. As indicated by the black arrows in Figure 1b, many of the spheres have open pores and some broken spheres have a hemispherical or bowllike shape, indicating that the spheres are of hollow structures. Although the spheres with thick walls (∼30 nm) were not very easy for the electron beam to transmit, the contrast of TEM images, as shown in panels c and d of Figure 2, also confirmed the hollow sphere structure of the products. Moreover, both the TEM and SEM images exhibited that the microspheres were, in fact, formed by the assembly of small Fe3O4 nanoparticles with average diameter of tens of nanometers, in particular the ones with high magnifications as shown in Figure 2b,d. Figure 3 shows a typical XRD pattern of the final products, in which all the peaks can be indexed to the cubic Fe3O4 (JCPDS 861354) with lattice parameter of a ) 8.441 Å, in good agreement

Figure 2. SEM images of Fe3O4 hollow microspheres with (a) low magnification and (b) with high magnification; TEM images of Fe3O4 hollow microspheres with (c) low magnification and (d) high magnification. Some broken microspheres and the ones with open pores on the shells, as indicated by black arrows in panel b, show the hollow structure of the particles.

Oriented Assembly of Fe3O4 Nanoparticles

Figure 3. A typical XRD pattern of Fe3O4 hollow microspheres.

with the reported value of a ) 8.432 Å. It is noted that, according to the Scherer equation,17 the average size of the nanoparticles is approximately 30.8 nm, while the size of the Fe3O4 microspheres is as large as 200 nm, due to the facts that the products are of hollow structures and the crystal growth may be greatly inhibited during the formation process.12 To obtain more detailed information of the crystal structure of the microspheres, a broken Fe3O4 hollow microsphere with hemispherical shape, as shown in Figure 4a, was used as a candidate for further investigations so as to make the electron beam easily transmit only one side of the shell. Of particular interest is to note that, when the whole particle was taken as

J. Phys. Chem. B, Vol. 110, No. 43, 2006 21669 the selected area for ED measurement, the diffraction pattern exhibited a remarkable single-crystal feature as shown in Figure 4b, in which the spots were assigned to (220) and (311) of cubic Fe3O4, respectively. Panels c and d of Figure 4 show the HRTEM images of the Fe3O4 microspheres obtained from the areas marked with black panes 1 and 2 as shown in Figure 4a, respectively, where there are obvious boundaries of the assembled small Fe3O4 nanoparticles. It can be seen that the nanoparticles organized so well that they assembled into a single crystal by sharing identical lattices though some open pores and defects were clearly seen in the particle. As shown in Figure 4c,d, lattice plane spacings calculated from the HRTEM images are ca. 2.5 Å hkl (311) and are typical for cubic Fe3O4 structures. As one of the important crystal growth mechanisms, oriented assembly has been extensively investigated in aqueous solutions. It was said that surface hydroxyls were the main reason for the aggregation of metal oxide nanoparticles,18 because water acted as a binder to control the nanocrystal aggregation. Chen’s group has proved that addition of a small amount of water was crucial for the oriented aggregation of Co3O4 nanocrystals in nhexanol.12 However, in our synthesis process, there was no water added to the solvent, but the small amount of water resulting from the crystal water of FeCl3‚6H2O and a trace amount of water contained in ethylene glycol (0.02%, wt) probably played an important role in the oriented aggregation of Fe3O4 nanoparticles. Compared to the fast nucleation and aggregation growth in aqueous solution, the nanocrystals aggregating in the nonaqueous solution are kinetically slower due to fewer surface

Figure 4. (a) Enlarged TEM image of broken Fe3O4 hollow microspheres with a hemispherical shape; (b) ED pattern of the corresponding particle as shown in panel a by taking the whole particle as the selected area with the electron beam projecting along the [1h14] zone axis; (c and d) HRTEM images, obtained from the areas marked with black panes 1 and 2 as shown in panel a, respectively.

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Yu et al.

Figure 5. Various Fe3O4 products synthesized at different concentrations of DDA: (a) 20, (b) 40, (c) 100, and (d) 150 mM.

hydroxyls and greater viscosity, thus allowing the nanocrystals to rotate adequately to find the low-energy configuration interface and form perfectly oriented aggregations.19 As a result, the individual Fe3O4 nanoparticles organized so well that they exhibited the feature of single crystals. Furthermore, when the surface-capped nanocrystals were used as building blocks, they were difficult to assemble into compact single-crystal structure so that the products with mesoporous-like structure were always obtained.12 Similarly, the open pores observed on the shell of Fe3O4 hollow microspheres (Figures 2 and 4) probably could be attributed to the surface-capped action of DDA. Due to the obvious difference of polarity between DDA and ethylene glycol, DDA acted as the surfactant in ethylene glycol to template the growth of Fe3O4 crystals, similar to those surfactants used in the hydrothermal system to control the growth of nanostructures, which can be confirmed by its dramatic effect on the morphologies of the products. With other synthetic conditions held constant, adjusting the concentration of DDA led to an obvious shape evolution of Fe3O4 particles from rhombic nanoparticles to solid microspheres, hollow microspheres, and large microspheres as shown in Figure 5. It has been proved that the shape of surfactant micelles can be dramatically affected by its concentration, thus resulting in the obvious evolution of the shape of inorganic crystals. When the concentration of DDA was near 20 mM, rhombic nanoparticles were obtained (Figure 5a), and when the concentration increased up to 40 mM, the Fe3O4 nanocrystals began to exhibit a strong tendency to assemble into large particles under the experimental conditions, and large-scale monodisperse solid spheres that resulted from the assembly of nanoparticles were obtained (Figure 5b, see the Supporting Information). The hollow Fe3O4

microspheres were mainly synthesized with the concentration of DDA ranging from 50 to100 mM, which most probably could be attributed to the spherical micelles acting as the “soft template” for the formation of the structures, as illustrated in Figure 1. Interestingly, when the concentration of DDA was more than 100 mM, the hollow structure of the products disappeared, and large-scale solid spheres that resulted from the assembly of Fe3O4 nanoparticles were also obtained (Figure 5c), but the Fe3O4 nanoparticles were much smaller than those that formed when the concentration of DDA was less than 50 mM, which can be seen from the contrast of TEM images as shown in Figure 5b,c. Further increasing the concentration of DDA, the size of the nanoparticles decreased but they tended to assemble into even large spheres (Figure 5d). When the concentration of DDA was more than 200 mM, the products were dominated by irregular particles. Besides the concentration of DDA, the shape of Fe3O4 particles was also affected by other synthesis parameters such as the concentration of reagents, synthesis temperature, and time; among them NaOH was found to be the most important factor. As one of additives, NaOH could affect the shape of DDA thus further affecting the shape of products. For example, when the molar ratio of DDA to NaOH was constant, adjusting their concentrations could control the diameters of the microspheres between 200 and 400 nm. Of course, similar to the previous reports in the literature, all the parameters for the controlled synthesis of nanocrystals under solvothermal conditions should be interdependent, thus resulting in interesting combinations for the shape-selective synthesis of various Fe3O4 particles. Figure 6 shows the magnetization curves of Fe3O4 products measured at room temperature. The magnetic saturation values

Oriented Assembly of Fe3O4 Nanoparticles

J. Phys. Chem. B, Vol. 110, No. 43, 2006 21671 hollow structure, and good hydrophilic and biocompatible properties, the magnetic hollow Fe3O4 microspheres will have wide applications in biomedical fields such as targeted drug delivery, biomolecular separations, cancer treatment, and magnetic resonance imaging. Acknowledgment. The authors acknowledge the financial support from the Natural Science Foundation of Anhui Province (03044903). Supporting Information Available: Additional HRTEM, TEM, and SEM images of the products. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes

Figure 6. Room temperature magnetization curves of various Fe3O4 products: (a) hollow microspheres; (b) solid spheres; and (c) rhombic nanoparticles.

are 68.2, 68.9, and 79.3 emu g-1 for Fe3O4 hollow microspheres (Figure 2), solid spheres, and rhombic nanoparticles (see the Supporting Information), respectively, which are near the magnetic saturation values of Fe3O4 nanocrystals reported in the literature.1 It can be seen that the hollow and solid spheres have almost the same magnetic saturation values, while the rhombic nanoparticles have a higher value, which is probably due to the fact that the anisotropic features of rhombic nanoparticles have made them obvious dipole-dipole interactions (see the Supporting Information), favoring a head-to-tail orientation, thus resulting in a relatively higher magnetic saturation value. The magnetic properties allow them to be easily manipulated by an external magnetic field, which is important for their applications in the biological field. More detailed investigations on the magnetic properties are still in progress. 4. Conclusion In summary, we developed a convenient and effective “onepot” route to high-yield monodisperse hollow single-crystal Fe3O4 microspheres with tunable diameters in the range of 200400 nm. It was found that Fe3O4 nanocrystals have a strong tendency to self-assemble into large particles in the synthesis process, and DDA was the crucial factor that determined the morphologies of the products. The hollow Fe3O4 microspheres, resulting from the oriented assembly of small nanoparticles, exhibited the feature of single crystals, and had a magnetic saturation value as high as 68.2 emu g-1. In particular, the hollow Fe3O4 microspheres with open pores on their shells may meet the special need of biomedical fields and are favorable for drugs to be loaded and released when they are used as drugdelivery carriers. Taking into account the suitable size, special

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