J. Phys. Chem. C 2007, 111, 5281-5285
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A Simple One-Pot Self-Assembly Route to Nanoporous and Monodispersed Fe3O4 Particles with Oriented Attachment Structure and Magnetic Property Yufang Zhu, Wenru Zhao, Hangrong Chen, and Jianlin Shi* State Key Laboratory of High Performance Ceramics and Superfine Microstructures, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Ding-xi Road, Shanghai 200050, People’s Republic of China ReceiVed: NoVember 19, 2006; In Final Form: February 7, 2007
Nanoporous and monodispersed Fe3O4 aggregated spheres with high surface area and oriented attachment structure have been successfully prepared by a polyol reduction process. The structure and morphology of the Fe3O4 particles were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and N2 adsorption-desorption technique. The spherical aggregates are formed by the assembling among the Fe3O4 primary nanoparticles (5 nm), and the average size of the spherical particles is around 100 nm. The nanopores are less than 3 nm in the aggregated spheres. Besides, the magnetic properties of these nanoporous particles are also investigated and the magnetization saturation value is about 42.8 emu/g.
Introduction Self-assembly of nanomaterials with highly controlled structures, uniform morphology, and novel property have attracted much attention, because assembling nanoparticles into highly ordered materials is a potential bottom-up approach to construct electronic, optical, or magnetic nanodevices with high performances.1 In the past decade, a variety of methods have been developed to form the highly structure-controlled materials of functionalized metal, semiconductor, and copolymer nanoparticles on the nano- or microscale. One of the more efficient methods is the “brick-and-motor” strategy, it is necessary to use the multifunctional organic ligands, polymers, or biomolecules as the cohesion “motor”.2-3 Many efforts have also been made to design self-assembled nanomaterials at solid-liquid, liquid-liquid, and liquid-gas interfaces based on the LangmuirBodgett technique, layer-by-layer technique, and templating approaches.4-6 Besides, Matijevic´ et al. demonstrated a forced hydrolysis route to prepare monodispersed CeO2 colloids with uniform size and shape.7-8 Yu9 and Zhong10 developed a sonochemical approach to prepare hierarchically porous titania spheres and linear or spherical Au aggregates, respectively. Fan et al.11 presented a facile method for assembling Pd nanoparticles into spherical, core-shell aggregates by visible laser decomposition of [Pd(PPh3)4]. In recent years, the synthesis of nanostructured magnetic materials, especially magnetite (Fe3O4) nanoparticles, has become a particularly important research field and is attracting growing interest.12-14 It is well-known that these nanostructured magnetic materials have shown great potential for many useful applications ranging from information storage and electronic devices to medical diagnostics and drug delivery, for example, magnetic recording media, magnetic resonance imaging (MRI), and targeted drug delivery.15-17 To date, monodispersed Fe3O4 nanopoarticles have been prepared by various chemistry-based synthetic methods, such as coprecipitation, the reverse micelle method, ultrasound irradiation, hydrothermal method, laser pyrolysis techniques, thermal decomposition of organometallic * To whom correspondence should be addressed. Fax: +86-2152413122. Phone: +86-21-52412712. E-mail:
[email protected].
compounds, and so on.18-22 However, most of these approaches were focused on the synthesis of Fe3O4 nanoparticles with sizes below 30 nm. In addition, Sugimoto and Matijevic´ reported a method to prepare magnetic spheres with mean diameters ranging between 0.03 and 1.1 µm by aging ferrous hydroxide gels at elevated temperatures.8,23 Recently, Li et al. demonstrated a solvothermal reduction method to synthesize monodispersed Fe3O4 nanospheres with their diameters in a range of 200800 nm.24 Gao et al. reported highly ordered superlattice structures of the Fe3O4 nanoparticles with large surface area by self-assembling the as-synthesized Fe3O4 colloids.12 To our knowledge, self-assembling preparation of monodispersed spherical Fe3O4 nanoparticles with high surface area and highly porous structure have not been reported until now although such a porous aggregate structure has been reported for noble metals (Au, Pd, and Pt), metal oxides, and semiconductors.6,9-10,25-31 The porous Fe3O4 nanostructures with strong enough magnetization strength will be especially interesting for high capacity drug loading and targeted drug delivery as well as other biomedical and catalytic applications. Herein, we report a facile one-pot method to prepare highly porous and monodispersed Fe3O4 nanoparticles with high surface area, oriented attachment structure and high magnetization strength by a polyol reduction process. Iron acetylacetonate [Fe(acac)3] was used as the metal ion source, ethylene glycol (EG) was taken to reduce this metal salt in situ to Fe3O4 in the presence of poly(vinylpyrrolidone) (PVP), while PVP here plays an important role for the formation of monodispersed spheres and oriented attachment structure. This is the first report on the synthesis of self-assembled Fe3O4 nanoporous particles with oriented attachment structure. Besides, the magnetic properties of these nanoporous particles are also investigated and found to have high enough magnetization strength. Experimental Preparation of [Fe(acac)3]. FeCl3‚6H2O (6.75 g) was dissolved in 25 mL of H2O. A solution containing 5 g of acetylacetonate and 10 mL of methanol was added into the above solution at room temperature under stirring. After several minutes, another solution containing 4.1 g of sodium acetate
10.1021/jp0676843 CCC: $37.00 © 2007 American Chemical Society Published on Web 03/20/2007
5282 J. Phys. Chem. C, Vol. 111, No. 14, 2007
Figure 1. XRD pattern of the Fe3O4 spherical nanoporous particles synthesized at the amount of 0.45 g of [Fe(acac)3].
dissolved in 15 mL of H2O was added into the above mixed solution. Subsequently, the solution was heated for several minutes, then cooled, filtered, and washed with H2O. At last the red product was dried at 100 °C. Preparation of Monodispersed Fe3O4 Spherical Nanoporous Particles. In a typical synthesis, 0.45 g of [Fe(acac)3] and 1.0 g of PVP (K30) were added to 72 mL of ethylene glycol. The mixture was stirred with a magnetic stirrer bar in room temperature for 4 h to give a wine solution. Then the solution was sealed in Teflon-lined autoclaves, heated to and maintained at 140-145 °C for 36 h. The black products were washed and separated by centrifugation-redispersion cycles with acetone, alcohol, and distilled water after cooling to room temperature. Finally, the black products were dried under vacuum at room temperature. Characterization. The powder XRD pattern was recorded on Rigaku D/MAX-2550V at 40 kV and 40 mA (Cu KR radiation). Scanning electron microscopy (SEM) images were obtained on JSM-6700F field emission scanning electron microscope at 10.0 kV. Transmission electron microscopy (TEM) and high resolution (HR) TEM were performed using a JEOL 2100F electron microscope operated at 200 kV. N2 adsorption-desorption isotherm was obtained on Micromeritics ASAP 2020 at 77 K under a continuous adsorption condition. Magnetic property was carried out on a vibrating sample magnetometer (VSM) at room temperature. Results and Discussion The crystalline structure of the black products was characterized by the powder X-ray diffraction (XRD) technique. As shown in Figure 1, the pattern can be easily indexed to Fe3O4 (JCPDS 75-1609) according to the reflection peak positions and relative intensities, which confirms the magnetite structure of this material. This indicates that Fe3O4 materials can be obtained by such a simple polyol reduction method. The average size of the Fe3O4 nanoparticles deduced from Sherrer’s formula is about 5 nm, which is consistent with the result obtained from the TEM observation of the same sample. The morphology and structure of the black Fe3O4 products were investigated by SEM and TEM. Figure 2a shows a representative SEM image of the products. It can be found that particles of the Fe3O4 products are spherical, they are uniform in size, and the average size of the spherical particles is around 100 nm. The structural details are revealed in high-magnification SEM (Figure 2b). It can be observed clearly that the larger Fe3O4 spherical particles have rough appearance and are composed of many small Fe3O4 nanoparticles, which indicates that the Fe3O4 nanoparticles have self-assembled into Fe3O4 spherical ag-
Zhu et al. gregated particles. Figure 2c is a TEM image of the Fe3O4 aggregated particles. This feature also shows that the spherical aggregates are formed by the assembling among the Fe3O4 primary nanoparticles. It also can be seen that the disordered pores exist among the primary nanoparticles but within the spherical aggregates. Therefore, these aggregated particles have a nanoporous structure. The selected area electron diffraction (SAED, the inset of Figure 2c) gives the characteristic spot pattern of a Fe3O4 single crystal. HRTEM analysis provides more detailed structural information. A representative HRTEM image took from the edge of an aggregated particle is shown in Figure 2d. The parallel lattice fringes across almost all the primary nanoparticles are clearly visible. The above information confirm the perfectly oriented aggregation (oriented attachment) between nanoparticles.32 In addition, in the typical synthesis process, with the temperature being held constant at 140145 °C, the size of the Fe3O4 aggregated particles could be readily regulated by changing the amount of [Fe(acac)3]. At the amount of 0.45 g of [Fe(acac)3], the size of the Fe3O4 aggregates is around 100 nm (Figure 2a), while at the amount of 0.30 g of [Fe(acac)3], the sizes of the Fe3O4 aggregates are observed to be ∼50 nm (parts e and f of Figure 2, differences in sperical particle sizes can be found by comparing bewteen parts b and e of Figure 2 and between parts d and f of Figure 2). However, in spite of the different particle demensions, the near monodispersed spherical morphology and nanoporous structure of the Fe3O4 aggregates, as well as the size of Fe3O4 nanoparticles, remain almost unchanged. Figure 3 shows the N2 adsorption-desorption isotherm of the Fe3O4 nanoporous aggregated particles. It can be clearly seen that two distinct capillary condensation steps appear on the isotherm. The first step is at 0.01 < P/P0
