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Apr 27, 2010 - With further increasing hydrothermal duration, hollow magnetite microspheres can evolve into solid durian-like architectures with compa...
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Formation of Hollow Magnetite Microspheres and Their Evolution into Durian-like Architectures Xiao-Fei Qu,† Qi-Zhi Yao,‡ Gen-Tao Zhou,*,† Sheng-Quan Fu,§ and Jian-Liu Huang§ CAS Key Laboratory of Crust-Mantle Materials and EnVironments, School of Earth and Space Sciences, UniVersity of Science and Technology of China, Hefei 230026, People’s Republic of China, School of Chemistry and Materials, UniVersity of Science and Technology of China, Hefei 230026, People’s Republic of China, and Hefei National Laboratory for Physical Sciences at Microscale, UniVersity of Science and Technology of China, Hefei 230026, People’s Republic of China ReceiVed: December 31, 2009; ReVised Manuscript ReceiVed: March 28, 2010

Hollow magnetite microspheres with a diameter of ca. 1 µm have been successfully synthesized in aqueous medium by use of triblock copolymer F127 (PEO106PPO70PEO106) as capping and assembly reagents and aspartic acid (Asp, HOOCCH(NH2)CH2COOH) as a reductant. The products were characterized by X-ray diffraction (XRD), Raman spectroscopy, field emission scanning electron microscopy (FE-SEM), FT-IR spectroscopy, transmission electron microscopy (TEM), high-resolution TEM (HRTEM), Brunauer-EmmettTeller (BET) gas sorptometry, and vibrating sample magnetometer (VSM). These hollow microspheres are hierarchically assembled by hundreds of tiny magnetite nanoparticles. The time-dependent experiments unveil that the magnetite nanoparticles first aggregate into spherolites, then the spherolites develop into hollow microspheres. The in-depth investigations, based on control experiments and FT-IR spectrum analyses, reveal that at the stage of nanopaticles assembling, F127 molecules capping to the surface of individual nanoparticle play a crucial role in inhibiting nanoparticles regrowth and promoting nanopaticles aggregation. At the subsequent stage Ostwald ripening contributes to the formation of the hollow microspheres. With further increasing hydrothermal duration, hollow magnetite microspheres can evolve into solid durian-like architectures with compact surface and numerous octahedral vertexes, which can be attributed to the further growth of magnetite nanocrystals on the wall. Moreover, we also measured the magnetic properties of the synthesized products. The saturation magnetizations (Ms) of hollow and durian-like microspheres are 45.2 and 62.3 emu/ g, respectively. Introduction As one of the most basic and important magnetic materials, magnetite (Fe3O4) has aroused extensive attention in recent years because of its potential physicochemical applications in electronic devices,1 information storage,2 ferrofluid,3 catalysis,4 and promising biomedical utilizations such as magnetically tagging biomolecules,5 bioseparation,6 biosensing,7 magnetic resonance imaging (MRI) contrast enhancement,8 and drug-delivery technology.9 As far as the application in biological fields is concerned, the hollow magnetite microspheres are of specific importance because they can act as available delivery carriers and provide maximum signal owing to their hollow interior chambers and high magnetic saturation.10 Since Deng et al. synthesized spherical magnetic nanoparticles, including Fe3O4, MnFe2O4, ZnFe2O4, and CoFe2O4, by use of ethylene glycol (EG) as solvent and reductant,11 a wealth of sphere-like magnetite nanostructures have been prepared in the EG solvent system.12-14 For example, hollow magnetite microspheres with open pores on the shells self-assembled by magnetite nanoparticles were obtained in the presence of EG and dodecylamine (DDA).12 In EG, Cao et al. first obtained hollow ferrous alkoxide microspheres by a microwave-assisted hydrothermal method, * To whom correspondence should be addressed. Phone: 86 551 3600533. Fax: 86 551 3603554. E-mail: [email protected]. † CAS Key Laboratory of Crust-Mantle Materials and Environments, School of Earth and Space Sciences. ‡ School of Chemistry and Materials. § Hefei National Laboratory for Physical Sciences at Microscale.

and then magnetic hollow microspheres assembled with magnetite or maghemite nanosheets via alternately heat-treating these spherical precursors under the flowing nitrogen or air atmosphere.13 Utilizing polyethylene glycol (PEG-1000) and EG, Gao and Jia obtained spherical magnetite solid aggregates composed of tiny nanomagnetite, and hollow oriented architectures constructed by rod-like subcrystals.14 Likewise, Li et al. prepared magnetite/PEO-PPO-PEO hybrid hollow submicrospheres in the poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) (PEO-PPO-PEO) and EG system.15 Up to now, the formation of magnetite nano- or microspheres was mainly carried out in EG solvent in addition to essential surfactants or morphology directors, while few routes for the preparation in aqueous medium have been involved. It appears that it is still a challenge to explore simple and effective aqueous strategies to synthesize magnetite with hierarchical structures. The self-assembly of nanoparticles mediated by polymers has recently attracted increasing interest because this strategy can lead to various morphologically controlled or highly ordered structures.16 Silver nanocubes with a truncated shape, for instance, were synthesized in the presence of poly(vinylpyrrolidone) (PVP), and could serve as sacrificial templates to generate hollow gold nanoboxes.17 Hollow CaCO3 microrings were assembled by CaCO3 nanoparticles under the modification of double-hydrophilic block copolymers (DHBC), poly(ethylene glycol)-block-poly(ethylene imine)-poly(acetic acid) (PEGPEIPA).18 Unusual CaCO3 pancakes were also constructed by the DHBC-directing self-assembly of nanoparticles.19 It has been

10.1021/jp912278r  2010 American Chemical Society Published on Web 04/27/2010

Formation of Hollow Magnetite Microspheres well established that the self-assembly of the subunits can be promoted by the binding affinities between organic ligands with inorganic nanoparticles, or influenced by the interactions between the surface-anchored ligands. In this paper, we describe a simple aqueous medium method for synthesizing magnetite hollow microspheres by using copolymer F127 as capping and assembly reagents. F127 is a pluronic triblock copolymer with the structural formula of HO(CH2CH2O)106(CH2CH(CH3)O)70(CH2CH2O)106H (PEO106PPO70PEO106), and has been successfully applied to both drug delivery and tissue engineering due to its biocompatibility.20,21 Moreover, it has been reported that alkylene oxide segments in the pluronic triblock copolymer are capable of forming crownether-type complexes with metal ions, resulting in coordination bonds between copolymer and inorganic nanoparticles.22 Thus, in our route F127 is designed as the assembling reagent via the coordination bonding with magnetite nanoparticles. Besides, aspartic acid (Asp, HOOCCH(NH2)CH2COOH) is introduced to serve as the reducing reagent instead of EG owing to its potential reductive ability.23 This F127-mediated method for the synthesis of magnetite hollow microspheres can be successfully carried out in aqueous medium. Therefore, such aqueous synthetic strategy may potentially be applicable to the fabrication of other metal oxides with assembled or hierarchical structures.

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Figure 1. XRD pattern of the product synthesized at 220 °C for 10 h.

Experimental Section Preparation of the Magnetite Hollow Microspheres. F127 (MW ≈ 12 000) was purchased from BASF. All chemical reagents were of analytical grade and were used as received without any further purification. In a typical synthesis procedure, 0.54 g of FeCl3 · 6H2O (2 mmol), 3.0 g of F127 (0.25 mmol), and 0.5 g of aspartic acid (Asp, 3.76 mmol) were dissolved in 60 mL of deionized water, and the mixture was stirred for 1 h to form a homogeneous solution. Then the solution was transferred into a Teflon-lined stainless steel autoclave. The autoclave was sealed, maintained at 220 °C for 10 h, and finally cooled to room temperature naturally. The black product was isolated by decantation, and washed several times with deionized water and absolute ethanol. Finally, the well-prepared product was dried in a 40 °C vacuum oven for 12 h. Characterization. The phase of the products was characterized by X-ray powder diffraction (XRD), using an 18 kW advanced X-ray diffractometer (MXPAHF, Japan) with Cu KR radiation (λ ) 1.54056 Å) in the 2θ range of 10-70°. Raman spectrum was taken on a LABRAM-HR Confocal Laser MicroRaman spectrometer, using an Ar+ laser with 514.5 nm at room temperature. Scanning electron microscopy imaging was carried out on a JEOL JSM-6700 M Field emission scanning electron microscope (FESEM). Infared (IR) spectra analyses were operated on samples palletized with KBr powders in the range 4000-400 cm-1, using an infared Fourier transform spectrophotometer (Nicolet, ZOSX). Transmission electron microscopy (TEM), high-resolution TEM (HRTEM) images, and corresponding selected area electron diffraction (SAED) pattern were obtained on a JEM-2010F microscope operated at an acceleration voltage of 200 kV. Nitrogen adsorption-desorption isotherms at the temperature of liquid nitrogen were measured with a Micromeritics Coulter (USA) instrument. Magnetic properties of the synthesized products were measured on a BHV-55 vibrating sample magnetometer (VSM) at room temperature. Results and Discussion Figure 1 shows the XRD pattern of the product synthesized at 220 °C for 10 h. All of the reflection peaks can be well

Figure 2. Typical Raman spectrum of the magnetite product.

assigned to a spinel structure with the characteristic reflections of iron oxides (magnetite Fe3O4, JCPDS 79-0419, or maghemite γ-Fe2O3, JCPDS 39-1346).24 No diffraction peaks from hematite or other impurities can be detected in the XRD pattern. The diffraction peaks from our product are slightly broadened, implying that the prepared product consists of nanocrystals. Estimated in terms of Scherrer equation, the average crystal size is ca. 82.6 nm based on the measurement of the full width at half-maximum (fwhm) of the (311) peak (2θ ) 35.5°). Nevertheless, because magnetite and maghemite have the same structure and a similar lattice parameter a (8.396 Å for magnetite and 8.351 Å for maghemite), a further characterization should be carried out in order to identify the exact phase of the product. Raman spectroscopy has been widely used to differentiate the iron oxides, especially magnetite and maghemite.25 A representative Raman spectrum of the synthesized product is presented in Figure 2. The characteristic bands at 662 and 530 cm-1 can be assigned to the A1g and T2g transitions of magnetite, respectively, which are consistent with the values of magnetite reported in the literature.23,26 The characteristic vibration bands belonging to magnetite can be easily distinguished from characteristic bands of maghemite at 720, 500, and 350 cm-1. Therefore, the black product can be ascribed to a pure magnetite phase. The size and morphology of the obtained magnetite products were observed by SEM and TEM, as shown in Figure 3. Figure

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Figure 3. SEM (A, C, E) and TEM (D) images of the hollow microspheres synthesized for 10 h, and histogram (B) of particle size distribution corresponding to image A.

Figure 4. TEM images of a typical hollow microsphere synthesized for 10 h (A), and of the boxed region (B) in image A; HRTEM image of the boxed region (C) in image B, and fast Fourier transforms (FFTs) (insets) of the selected areas in image C.

3A shows a representative low-magnification SEM image of the magnetite sample derived from 220 °C and 10 h hydrothermal reaction, which indicates that the F127-mediated synthetic route can result in a good yield of magnetite microspheres. The histogram of particle size distribution (Figure 3B) exhibits that the magnetite microspheres have a narrow size distribution around 1 µm. Figure 3C is the magnified image of an individual microsphere, displaying that numerous magnetite nanoparticles aggregate into the spherolite. TEM image of a random spherolite is shown in Figure 3D. The distinct contrasts between the black margin and the light center unveil that the magnetite spherolite has a hollow interior chamber, and the diameter of the hollow chamber and the shell thickness can be determined as 200 and 400 nm, respectively. Moreover, the SEM image of the broken spherolites presented in Figure 3E further reveals that the hollow microspheres are hierarchically assembled by hundreds of tiny magnetite nanoparticles, and the sizes of the subunits (magnetite nanoparticles) are in the range of 80 to 90 nm, which

corresponds with the XRD result. For the hollow microspheres, HRTEM observations further ascertain that the magnetite nanoparticles on the wall are well crystallized owing to their clearly resolved lattice fringes of {111} faces and corresponding fast Fourier transform (FFT) dots, as depicted in Figure 4. To understand the formation process of hollow magnetite spherolites, we conducted time-dependent experiments. Figure 5 displays SEM and TEM images of the magnetite products synthesized for different reaction durations. It can be seen from panels A and B of Figure 5 that a 3 h hydrothermal reaction only yields magnetite nanoparticles with a diameter of ca. 80 nm. When the hydrothermal time is prolonged to 6 h, a large amount of spherical aggregates have formed although they have a wide size distribution from 0.3 to 1.3 µm (Figure 5C). Further SEM and TEM observations confirm that the spherical aggregates are of loose spherolite-like structure (Figure 5D,E). Nevertheless, it is noteworthy that all magnetite nanoparticles obtained in either 3 or 6 h are ca. 80 nm in size, indicating that

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Figure 6. SEM images of the products synthesized without F127 for 3 (A) and 6 h (B, C).

Figure 5. SEM and TEM images of the products synthesized for 3 (A, B) and 6 h (C, D, E).

no further growth occurs. These results unambiguously demonstrate that the aggregation of primary nanocrystals leads to the formation of spherolites. The driving force for magnetite nanoparticles assembly into spherolites may originate from the inherent magnetic interaction or other physical or energetic means,27 e.g., Brownian motion. But in the presence of organic additives, it has been reputed that the subunits aggregation can be driven through the binding affinities between organic ligands with inorganic nanoparticles or promoted by the interactions between surface-anchored ligands.16,18,19,28 In this context, in order to examine the contribution of F127 to the aggregation, we also carried out control experiments without F127 while other conditions remained unchanged. Figure 6 depicts the SEM images of the products obtained without F127. After 3 h of reaction massive magnetite nanoparticles are produced, and some even aggregate into octahedron-like embryos (Figure 6A), while a 6 h hydrothermal reaction only produces regular magnetite microoctahedrons with sharp-cut edges and smooth surfaces, and no nanosized particles can be observed (Figure 6B,C). These results indicate that in the absence of F127 fcc magnetite nanoparticles grow spontaneously into thermodynamically stable octahedral form. It appears that F127 plays a crucial role in mediating the magnetite nanoparticle assembly into the spherolites.

Figure 7. FT-IR spectra of the magnetite products synthesized for 3 (A) and 6 h (B) and the commercial F127 (C). The ν(CdC)* may result from the dehydration of terminal -OH in F127 molecules.

FT-IR analyses provide further supporting evidence. Panels A and B in Figure 7 depict the FT-IR spectra of the magnetite products synthesized for 3 and 6 h. Paralleled with the FT-IR spectrum of reagent F127 (Figure 7C), it can be observed that in addition to the characteristic Fe-O band at 586 cm-1, the products have clearly the C-O-C band at 1080 cm-1 and the CdC band at 1643 cm-1. These are characteristic stretching vibration bands of F127 molecules. That is to say, the backbone structure of F127 molecules remains attached to the surface of magnetite particles. Therefore, we reasonably believe that at the formation stage of the nanoparticles, a capping layer of F127 molecules has formed by the binding affinities of pluronic groups in F127 with the surface of magnetite nanoparticles. The experiments of Yang et al. have confirmed that many transition

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Figure 8. Schematic illustration for the formation of hollow magnetite microspheres.

metal ions can form crown-ether-type complexes with alkylene oxide segments in pluronic triblock copolymer.22 Moreover, in comparison with the C-O-C band in reagent F127 (Figure 7C) and the Fe-O band in pure magnetite,29 our FT-IR results show that the C-O-C band shifts from 1113 cm-1 to 1080 cm-1, and the Fe-O band shifts from 548 cm-1 to 586 cm-1. These shifts can be assigned to the interaction between F127 molecules and magnetite nanoparticles, confirming the existence of binding affinities. Because F127 molecules anchor to the surfaces of isolated nanoparticles, the further growth of the magnetite nanoparticles would be effectively inhibited. As a result, the magnetite nanoparticles remain transiently stable rather than grow into octahedral form. Meanwhile, it is the capping F127 molecules that may induce the magnetite nanoparticles aggregation into the spherolites by their interactions with one another. It is not difficult to find from panels A and B of Figure 7 that the intensities of vibration bands related to H, such as the O-H band at 3445 cm-1 and C-H bands at 2885, 1468, 1351, 1278, and 1239 cm-1, significantly decline, which may result from the thermolysis of F127 during the hydrothermal process. Nevertheless, the backbone structure of F127, based on the C-O-C band at 1080 cm-1 and the CdC band at 1643 cm-1, vigorously remains. Thus, van der Waals force and/or hydrogen bonds among the capping F127 molecules, and coupling with inherent magnetic interactions between the nanoparticles could be the most probable candidates for the driving force of magnetite nanopartcile assembling. The subsequent development from the spherolite assemblies to hollow microspheres is probably due to Ostwald ripening. According to the Gibbs-Thompson equation and Fick’s first law, the chemical potential of particle increases with the decrease in particle size, meaning that the equilibrium solute concentration near a small particle is higher than that near a larger one. The resulting concentration gradients would lead to the diffusion of molecular-scale species from smaller particles to larger particles through solution.30 Usually, the inner crystallites in spherolite-like aggregates can be visualized as smaller spheres because they have higher curvature (i.e., higher surface energies and thus easily dissolved).31 Therefore, they are capable of diffusing to the outer shell by the dissolution-recrystallization process. In our case, because F127 molecules capping to magnetite nanocrystals gradually decompose with the hydrothermal reaction proceeding, the transient stabilization of the magnetite nanopartciles imposed by the capping F127 molecules would be broken. As a consequence, this facilitates the dissolution and outward diffusion of inner magnetite nanoparticles. The successive outward migration of dissolved species results in continuing expansion of the interior chamber within the original spherolites. Finally, the hollow spherolites form after long hydrothermal duration. The schematic illustration for the formation of hollow magnetite microspheres is shown in Figure 8. Interestingly, with further prolonged hydrothermal duration the hollow microspheres can evolve into durian-like architec-

Figure 9. SEM and TEM images of the products synthesized for 20 (A, B), 30 (C, D), and 40 h (E, F).

Figure 10. XRD patterns of the products synthesized for 20 (A), 30 (B), and 40 h (C).

tures. Panels A and b of Figure 9 display that the nanoparticles on the spherical surface grow into tapered vertexes after 20 h. With prolonged reaction to 30 h, as shown in Figure 9C,D, the tapered vertexes further develop. After 40 h, it can be clearly seen that the hollow spherolite has evolved into a durian-like morphology with compact surface and numerous pyramid vertexes; typical SEM and TEM images are exhibited in Figure 9E,F. Corresponding XRD patterns are shown in Figure 10. Similarly, these XRD patterns can be indexed to fcc magnetite, and all of the diffraction peaks become much sharper and stronger with the increase in hydrothermal duration in com-

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Figure 11. TEM image (A) of the local surface of a typical durian-like microsphere and TEM image (B) of the boxed region in panel A; HRTEM image (C) and corresponding SAED pattern (D) of the boxed region in panel B.

parison with Figure 1, pointing to the long hydrothermal durations (20-40 h) leading to highly crystallized and more perfect magnetite. From SEM, TEM observations (Figure 9), and XRD patterns (Figures 1 and 10), it is reasonably concluded that the further growth of magnetite nanocrystals on the sphere wall or surface is responsible for the evolution of the hollow microspheres into durian-like structures. More detailed structure information about the vertexes is presented in HRTEM analyses. Figure 11 shows the TEM (Figure 11A) and HRTEM (Figure 11B,C) images of a vertex on a durian-like structure and the corresponding SAED pattern (Figure 11D). From the HRTEM image in Figure 11B,C the atomic lattice fringes can be clearly observed, suggesting that the vertex is highly crystallized. The resolved interplanar spacings are 0.489 and 0.491 nm, respectively, which are very close to the {111} lattice planes of magnetite crystal. The HRTEM image and SAED pattern reveal that the surface of the vertex is terminated by {111} crystallographic planes. In combination with the crystallographic habit of magnetite, it can be safely concluded that the vertex is converged by the octahedral {111} planes of magnetite. The emergence of octahedral vertexes decreases entire surface energies because octahedral {111} planes are the lowest energetic planes for the magnetite crystal,23 and hence the evolution process of surface texture is energetically favorable. Additionally, when the reaction time is longer (20-40 h) the interior chambers of magnetite products are invisible, which may result from the solid structure or the thicker wall of the spherolite. Brunauer-Emmett-Teller (BET) gas adsorption-desorption was carried out to acquire a deeper insight into the interior structure. Nitrogen adsorption-desorption isotherms of the magnetite products synthesized for different durations are shown in Figure 12. The samples obtained after 10 and 20 h contain type IV isotherms with hysteresis loops. This type of hysteresis loop is traditionally associated with textural slit-like pores, but recently has also been observed in structures consisting of voids surrounded by a mesoporous matrix,32 or hollow particles with mesoporous walls.33 Accordingly, the hysteresis loops in our case could be assigned to the hollow chambers and mesoporous walls assembled by the nanoparticles. The samples synthesized after 30 and 40 h have type I isotherms, indicating that these products only have surface micropores

Figure 12. Nitrogen adsorption-desorption isotherms of the magnetite products obtained after different hydrothermal durations.

rather than mesopores. The specific surface areas of the products synthesized at the different times are calculated to be 135 (10 h), 78 (20 h), 46 (30 h), and 37 cm2/g (40 h), respectively. For the synthesized octahedral magnetite, we also determined its adsorption-desorption isotherm (type I, data not shown), and calculated its specific surface area (32 cm2/g). From these results, it is not difficult to conclude that magnetite microspheres synthesized after much longer durations (>30 h) are most probably the solid structures without interior chambers or cavities. The formation of solid durian-like architectures may be due to the further growth of interior magnetite nanocrystals concomitant with F127 capping molecules decomposition, so as to minimize entire system energies. The magnetic properties of hollow microspheres (10 h) and durian-like architectures (40 h) were also measured at room temperature from -10 000 to 10 000 Oe. Figure 13 displays the hysteresis loops and the expanded low-field hysteresis (inset) of magnetite samples. The saturation magnetization (Ms) of hollow microspheres is 45.2 emu/g. The Ms value is smaller than 71.6, 102.57, or 93.3 emu/g for micro-octahedrons,23,26,29 90.5 emu/g for nanorods,34 84.4 emu/g for nanocubes,35 68.2 emu/g for hollow nanospheres of magnetite,12 but close to the value of hollow magnetite microspheres (31.4 emu/g).13 It is

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Figure 13. Magnetization hysteresis loops of hollow microspheres (1) and durian-like (2) architectures measured at room temperature.

well-known that magnetic properties of materials are significantly influenced by many factors, such as size, structure, shape, and surface disorder.23,26,29 For magnetite, Dunlop suggested that the critical size of the single domain is 54 nm,36 and thus the subunit nanoparticles (ca. 80 to 90 nm) in our hollow spheres should be of multiple domain. On the other hand, the anisotropy, including shape anisotropy and easy-magnetization anisotropy, can affect the saturation magnetization of magnetite crystals.23,37 Here, the anisotropies of the subunits or the spherolites are low relative to octahedron-, cube-, and rod-like magnetite. Therefore, the low Ms of hollow microspheres may result from multiple magnetic domains of the magnetite nanoparticle and the low anisotropies of the subunits or the spherolites. However, the Ms of durian-like architectures is 62.3 emu/g, and larger than that of the hollow microspheres. This is probably because the magnetite particles on the durian-like spheres have developed into octahedral vertexes, and the high anisotropies lead to the high Ms. The in-depth investigation is still in progress. Conclusions In summary, hollow magnetite microspheres can be manipulatively synthesized through a facile F127-mediated method. These hollow microspheres with a chamber diameter of 200 nm and a shell thickness of 400 nm are hierarchically assembled by hundreds of tiny magnetite nanoparticles, and have a narrow size distribution around 1 µm. The formation process can be described as the spherolite aggregation and subsequent development into hollow microsphere. F127 molecules play a crucial role in mediating the nanoparticles assembling. Ostwald ripening is responsible for the development of the hollow microsphere. With further prolonged reaction time, magnetite hollow microspheres can evolve into more stable solid durian-like structures. The magnetic measurements show that the Ms values of the hollow and durian-like microspheres are 45.2 and 62.3 emu/g, respectively. Acknowledgment. This work was partially supported by the Natural Science Foundation of China (NSFC) under Grant No.

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