Ligand-Assisted Hydrothermal Synthesis of Hollow Fe2O3 Urchin-like

Facile Fabrication of Rare-Earth-Doped Gd2O3 Hollow Spheres with Upconversion Luminescence, Magnetic Resonance, and Drug Delivery Properties...
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10754

J. Phys. Chem. C 2008, 112, 10754–10758

Ligand-Assisted Hydrothermal Synthesis of Hollow Fe2O3 Urchin-like Microstructures and Their Magnetic Properties Dejuan Du and Minhua Cao* Department of Chemistry, Northeast Normal UniVersity, Changchun, 130024, P. R. China ReceiVed: March 5, 2008; ReVised Manuscript ReceiVed: April 25, 2008

Novel urchin-like Fe2O3 hollow microstructures were synthesized via a simple ligand-assisted hydrothermal route, without any templates in the reaction system. X-ray diffraction pattern (XRD), transmission electron microscope (TEM), scanning electron microscope (SEM), N2 adsorption-desorption, and magnetic measurements were carried out to characterize the composition, morphology, adsorption, and magnetic properties. Various experimental parameters such as concentration, temperature, and reaction time were discussed in detail to obtain their influences on the morphology of Fe2O3 microstructures. Introduction Recently, hollow structures have attracted great attention due to their low effective density, high specific surface area, and potential scale-dependent applications in catalysis, drug delivery, active material encapsulation, ionic intercalation, lightweight filler, acoustic insulation, surface functionalization, photonic crystal, energy storage, and so on. So far, numerous methods have been developed to synthesize hollow structures of many materials, including template-assisted synthesis, direct evacuation with Ostwald ripening, and the Kirkendall effect, among which the template-assisted method is the most universally adopted for the preparation of hollow structures. Templates are usually classified as two types, one is the hard template, such as polystyrene latex spheres, spherical silica, carbon spheres, colloidal particles etc. The other is the soft template, such as liquid droplet, vesicle, microemulsion, polymer, or macromolecule micelles and so on. For example, gold nanoboxes and nanocages with controllable pores on the surface have been obtained using silver cubes as sacrificial template.1,2 Silver hollow nanostructures3,4 and semiconductor CdX (X ) Se, Te, S) hollow structures5 were generated using Ag3PO4 and Cd(OH)Cl precursors as the sacrificial templates, respectively, and all of these templates can be considered as hard-templates; semiconductor PbS hollow spheres were achieved by a sonochemical method in the presence of sodium dodecylbenzene sulfonate (DBS);6 Co3O4 hollow cubes7 and ZnO-based hollow microspheres8 were fabricated using surfactant as template; cage-like polymer microspheres9 and In2O3 hollow spheres10 were synthesized using emulsion droplets and vesicles as templates, respectively. Additionally, Cu4S7 hollow structures have been prepared based on the Kirkendall effect,11 whereas cadmium molybdate hollow microspheres12 and Cu2O/Cu hollow nanocubes13 have been synthesized based on the Ostwald ripening process. Although many hollow structures have been effectively achieved by the above-mentioned methods, these strategies seem to be inconvenient because complete template removal is needed, which means a much more complicated process including the selection of appropriate solvent or calcination at elevated temperature. Furthermore, the removal of the core will always bring a negative effect on the shell structure. Thus, the search for template-free, simple, mild, high* Corresponding author e-mail: [email protected].

yield, and environmentally friendly methods to synthesize different inorganic hollow structures is still a big challenge.12 Fe2O3 with a band gap of 2.2 eV is a very attractive material in materials field because of its wide applications in photoanode for photoassisted electrolysis of water,14 gas sensors,15–18 catalysis,19–23 magnetic recording materials, pigments, paints,24–29 and so on. Its nontoxicity, low cost, and relatively good stability are definitely very attractive features for these applications. Many Fe2O3 nanostructures with various morphologies have been fabricated by different methods. For example, our group had synthesized dendritic Fe2O3 micropines by K3Fe(CN)6 decomposition under hydrothermal conditions;30 airplane-like FeOOH and Fe2O3 nanostructures31 and R-FeOOH and Fe2O3 nanorods32 were obtained via template-free hydrothermal processes; Chu Y. et al.33 have fabricated novel Fe2O3 hollow spheres by a facile hydrothermal treatment of the K4Fe(CN)6 and (NH4)2S2O8 mixture in the presence of cetyltrimethyl ammonium bromide (CTAB); Mao B. D. et al.34 have synthesized hollow Fe2O3 spheres by a polyoxometalate-assisted hydrolysis process of Fe3+ under hydrothermal conditions. Hematite nanobelt and nanowire35 arrays were obtained on iron substrate. Zeng S. U. et al. have synthesized hollow hematite spindles and microspheres using FeCl3, oxalic acid, and bases as reactants at first, and then calcined the precursors in air at 400 °C for 2 h, during which they speculated that the urea is crucial to the formation of hollow microspheres;36 however, the iron(III) precursor was not discussed. Herein we reported ligand (oxalic acid)-assisted synthesis of Fe2O3 hollow urchins by hydrothermal treatment of a Fe(NO3)3-oxalic acid coordination compound without the presence of any other regents such as surfactant, bases, and so on. Furthermore, there is no need for calcination. It is noticeable here that when FeCl3 was used as iron source, no precipitate was obtained. This result suggests that FeCl3 is not appropriate to produce hematite hollow urchins, which maybe results from its strong lewis base properties. Through controlling the concentration of Fe3+, reacting temperature, and reaction time, Fe2O3 with different morphologies could be obtained. Li B. X. et al.37 have recently obtained similar hollow urchin morphology of R-MnO2 via a facile hydrothermal synthesis and explained that the Ostwald ripening process was ascribed to the formation of this kind of morphology. We also applied this famous theory here to illustrate in detail the formation of hollow hematite

10.1021/jp8019599 CCC: $40.75  2008 American Chemical Society Published on Web 06/27/2008

Synthesis of Hollow Fe2O3 Urchin-like Microstructures

J. Phys. Chem. C, Vol. 112, No. 29, 2008 10755

Figure 1. XRD patterns of (a) hollow urchine-like microstructures and (b) solid urchin-like microstructures.

urchins. N2 adsorption-desorption and magnetic measurements were carried out to reveal their potentially excellent applications. Experimental Section Synthesis. Fe2O3 hollow urchin-like microstructures were synthesized by a hydrothermal treatment of Fe3+-oxalic acid coordination compound solution at suitable temperatures. In a typical synthesis, 0.3 mmol of Fe(NO3)3 and 1.0 mmol of H2C2O4 were dissolved in 60 mL of distilled water to form a transparent yellow solution, which was then transferred to a 80 mL Teflon-lined autoclave and maintained at 150 or 180 °C for 10 h. The obtained red precipitation was isolated by centrifugation, repeatedly washed with distilled water and absolute ethanol, and then dried at room temperature naturally. If the reaction temperature was decreased to 120 °C while keeping other reaction conditions constant, solid Fe2O3 urchins were formed. Characterization. X-ray diffraction pattern (XRD) measurements were performed on a Japan Rigaku D/max γA X-ray diffractometer with graphite-monochromatized Cu Ka radiation (λ ) 1.5406 nm). A JEOL JEM-2010F transmission electron microscope operating at 200 kv accelerating voltage was used for transmission electron microscopy (TEM) analysis. Scanning electron microscopy (SEM) was performed with an Amray 1910FE microscope. Magnetic measurements were carried out on a quantum Design MPMS-XL5 SQUID magnetometer with the field sweeping from -50 000 to +50 000 Oe. N2 adsorption-desorption characterization was carried out on a Nova 1000 analyzer at liquid nitrogen temperature (77 K). Results and Discussions XRD Results of the Samples. Figure 1 shows typical XRD patterns of the solid (Figure 1b) and hollow (Figure 1a) structured urchin-like Fe2O3 samples, in which all the peaks are sharp and can be indexed to the rhomb-centered Fe2O3 (JCPDS 86-0550) with lattice parameters a ) 0.5035 nm and c ) 1.374 nm, indicating high purity and good crystallinity of the final samples. Morphology and Microstructure Characterization. Figure 2a shows the scanning electron microscope (SEM) image of the sample, which is composed of uniform microspheres. A higher magnification SEM image shown in Figure 2b indicates that these microspheres look like urchins with an average diameter of 0.9 µm. Moreover, these urchins display attracting hollow structures as shown clearly by Figure 2c. The transmission electron microscope (TEM) image of a single urchin as shown in Figure 2d further confirms the hollow urchin structure with a shell thickness of about 150 nm around. The higher

Figure 2. SEM images at (a) low magnification and (b) high magnification and of (c) typical broken hollow urchins of the Fe2O3 sample. (d) TEM image of a single hollow urchin. (e) Higher magnification TEM image taken on the shell of the hollow sphere. (f) HRTEM image taken on the shell of the hollow spheres. Inset shows the corresponding SAED pattern.

magnification TEM (Figure 2e) clearly discloses that the shell consists of somewhat round flakes with average diameter of about 15 nm around. Figure 2f is a high resolution transmission electron microscopy (HRTEM) image taken on the single flake of a hollow sphere, in which two-dimensional lattice fringes can be clearly seen. The typical lattice fringe spacings were determined to be 0.249 and 0.278 nm, corresponding to the (11-20) and (10-14) lattice planes of R-Fe2O3, respectively, which clearly demonstrates that the hollow urchins consist of the single crystalline flakes. The selected area electron diffraction (SAED) pattern indicates that Fe2O3 hollow urchins have a polycrystalline structure. Influence of Experimental Conditions on Morphology. In our synthesis process, neither template nor surfactant was used. To learn the influence of experimental parameters on Fe2O3 morphology, a series of experiments were carried out. The reaction temperature, reaction time, concentration of Fe(NO3)3 and H2C2O4, and the ligand all significantly affect the morphology of the final product. First, the temperature was adjusted from 120 to 180 °C while keeping the molar of Fe(NO3)3 and H2C2O4 constant at 0.6 and 2.0 mmol, respectively, and the reaction time was fixed at 10 h, the morphology of the sample changed from urchin-like solid microstructures at 120 °C (Figure 3a) to hollow urchins with thick shell at 150 °C (Figure 3b) and hollow urchins with thinner shell at 180 °C (Figure 3c). Second, the molar of Fe(NO3)3 was varied from 0.3 to 3.6 mmol (the molar of H2C2O4 changed accordingly in ratio) while the reaction temperature and time were fixed at 150 °C and 10 h, respectively, the morphology of the product varied from urchinlike hollow microstructures (same as the case for 0.6 mmol) at 0.3 mmol to the a mixture of urchin-like solid and hollow microstructures at 1.8 mmol (Figure 3d) and eventually irregular nanoflakes at 3.6 mmol, which suggested that the lower concentration facilitate the formation of hollow microstructures. Third, ethylenediaminetetraacetic acid (EDTA) and sodium citrate instead of H2C2O4 were used to investigate the influence of ligand on the Fe2O3 morphology. As a result, irregular nanoparticles for both cases were obtained (Figure 3, panels e

10756 J. Phys. Chem. C, Vol. 112, No. 29, 2008

Du and Cao

Figure 3. TEM images of Fe2O3 urchins obtained at different conditions. (a) Solid urchins obtained at 120 °C. (b) Hollow urchins obtained at 150 °C. (c) Hollow urchins obtained at 180 °C. (d) A mixture of solid and hollow urchins obtained when the molar of Fe(NO3)3 is 1.8 mmol. (e) TEM image of sample obtained with EDTA as ligand. (f) TEM image of sample obtained with sodium citrate as ligand.

Figure 4. TEM images of Fe2O3 samples obtained at different reacting times. (a) 2 h, (b) 4 h, and (c) 6 h.

and f), which illustrates that H2C2O4 is crucial for the formation of urchin-like morphology. Formation Mechanism. Hollow structures are of great interest because of their diverse applications. To investigate the formation mechanism of hollow Fe2O3 microstructures, additional time-dependent experiments were carried out. The reacting period is varied as 1, 2, 4, 6, and 10 h, maintaining the reaction temperature at 180 °C and the amount of Fe(NO3)3 at 0.6 mmol. It is noteworthy that no product was observed after hydrothermal treatment for 1 h. When the reaction time prolonged to 2 h, flake-like particles were obtained (Figure 4a). After reacting for 4 h, semihollow urchins were formed (Figure 4b), which may result from the aggregation of the flake-like particles around bubbles. As reaction time reached to 6 h, welldefined and uniform hollow urchins were obtained (Figure 4c). When it finally came to 10 h, the shell thickness of the hollow urchins became thinner (Figure 3c). On the basis of the above temperature- and time-dependent experiment results, the bubble template combined with an Ostwald ripening process was proposed to be responsible for the formation of the urchin-like hollow microstructures. As described above, when the reaction temperature was 120 °C, only solid urchins were formed. Therefore, we speculate that no bubbles were formed because of the low temperature. When the reaction temperature was fixed over 150 °C, which has reached the decomposition temperature of H2C2O4, CO and CO2 bubbles were generated. The bubbles may serve as templates, and the freshly crystalline nanoflakes

Figure 5. Magnetic characterization of Fe2O3 hollow urchins. Magnetic hysteresis curves measured at 5 K (a) and 300 K (b). (c) Zero-field cooling/field cooling (ZFC/FC) curves at the applied magnetic field of 1000 Oe.

aggregated together around the surface of bubbles to minimize the high surface energy, resulting in the formation of the hollow urchins. With the prolongation of the reaction time, the shell thickness of the hollow urchins gradually became thinner. The reason may be that many small crystals initially formed slowly disappear except for a few that grow larger, at the expense of the small crystals, which is a typical Ostwald ripening process.39 As for the mixture of solid and few hollow urchins obtained with the temperature of 120 °C (the molar of Fe(NO3)3 and H2C2O4 at 1.8 and 6.0 mmol, respectively), it is speculated that only solid urchins were obtained at first because the H2C2O4 can not decompose, so there were no bubbles formed to supply the template. Therefore, the freshly crystalline nanoflakes aggregated together to form only solid urchins. As time elongated, few hollow urchins appeared because of the Ostwald ripening process.

Synthesis of Hollow Fe2O3 Urchin-like Microstructures

J. Phys. Chem. C, Vol. 112, No. 29, 2008 10757 The ligand (oxalic acid), regent concentration, temperature, and reaction time all play important roles in the formation of the hollow structures. On the basis of a series of experiments, a possible formation mechanism was proposed. The magnetic property and N2 adsorption property were investigated in detail. It is our wish that this simple and mild synthesis method could be a general approach in the future for the large-scale growth of hollow structures of a wide range of materials.

Figure 6. Nitrogen adsorption/desorption isotherms of Fe2O3 hollow urchins.

Magnetic Properties. The magnetic properties of the hollow Fe2O3 urchins was investigated in Figure 5. Panels a and b are the magnetic hysteresis curves measured at 5 and 300 K, respectively, with the field sweeping from -50 000 to +50 000 Oe, which clearly confirms the fact that hematite behaves antiferromagnetic at low temperature and weak ferromagnetic above the Morin transition temperature (TM). The hysteresis loop at 5 K shows no saturation of the magnetization as a function of the field up to the maximum applied magnetic field. Furthermore, no remnant is observed, and the coercivity force is determined to be 2750 Oe. The hysteresis loop measured at 300 K exhibits typical weak ferromagnetic behavior, indicating the value of coercivity to be 395 Oe and that of the remnant magnetization to be 0.048 emu/g. To further study the magnetic property, zero-field-cooled (ZFC) and field-cooled (FC) measurements were carried out from 4 to 300 K under an applied field of 1000 Oe as shown in Figure 5c. The characteristics of the ZFC and FC curves are similar, but the ZFC decreases more rapidly than FC, and the ZFC transformation region is larger than the FC, which results in a significant split during the whole temperature region. Normally, bulk hematite has a Morin transition from the low-temperature antiferromagnetic phase to a weakly ferromagnetic phase at 263 K, but here it should be noted that the TM value in differential ZFC curve (inset in Figure 5c) is found to be 181 K, which is much lower than the bulk transition temperature. This obvious decrease can be attributed to the size effect and defects in the hollow urchins obtained by the low-temperature hydrothermal synthesis. It is known that the Morin transition temperature increases with the increase of the particle size, lattice strain, and defects.40–44 Here the confined dimensions of crystalline R-Fe2O3 in the hollow urchins maybe play an important role in suppressing TM. N2 Adsorption Property. N2 adsorption isotherms were presented in Figure 6, which indicate that these hollow urchins exhibit a macroporous structure. The calculated BrunauerEmmett-Teller (BET) surface area is 30.68 m2/g, which is much higher than those of the hematite submicron particles previously reported.36,38 However, this value is much lower than the 135.77 m2/g of hollow hematite spheres34 constructed by smaller nanoparticles with diameter of 600-700 nm and shell thickness lower than 100 nm. It is suggested that, in a comparable size range, it is the pores that increase specific surface area.38 Therefore, the large cavum and well-porous surface of hollow spheres may be attributed to such large surface area, whereas our hollow urchins have smaller cavum. Conclusion In summary, uniform hollow hematite urchins were successfully synthesized by a simple ligand-assisted hydrothermal route.

Acknowledgment. The author thanks the National Natural Science Foundation of China (NSFC, 20401005 and 20771022), the Jilin Distinguished Yong Scholars Program Foundation, and the Huo Yingdong Foundation for financial support. This work also was supported by the analysis and testing foundation of Northeast Normal University. References and Notes (1) Sun, Y. G.; Xia, Y. N. Science 2002, 298, 2176. (2) Chen, J. Y.; McLellan, J. M.; Siekkinen, A.; Xiong, Y. J.; Li, Z. Y.; Xia, Y. N. J. Am. Chem. Soc. 2006, 128, 14776. (3) Yang, J. H.; Qi, L. M.; Lu, C. H.; Ma, J. M.; Cheng, H. M. Angew. Chem., Int. Ed. 2005, 44, 598. (4) Chen, M. H.; Gao, L. Inorg. Chem. 2006, 45, 5145. (5) Miao, J. J.; Jiang, L. P.; Liu, C.; Zhu, J. M.; Zhu, J. J. Inorg. Chem. 2007, 46, 5673. (6) Wang, S. F.; Gu, F.; Lu, M. K. Langmuir 2006, 22, 398. (7) He, T.; Chen, D. R.; Jiao, X. L.; Wang, Y. L. AdV. Mater. 2006, 18, 1078. (8) Gao, S. Y.; Zhang, H. J.; Wang, X. M.; Deng, R. P.; Sun, D. H.; Zhen, G. L. J. Phys. Chem. B 2006, 110, 15847. (9) He, X. D.; Ge, X. W.; Liu, H. R.; Wang, M. Z.; Zhang, Z. C. Chem. Mater. 2005, 17, 5891. (10) Li, B. X.; Xie, Y.; Jing, M.; Rong, G. X.; Tang, Y. C.; Zhang, G. Z. Langmuir 2006, 22, 9380. (11) Cao, H. L.; Qian, X. F.; Wang, C.; Ma, X. D.; Yin, J.; Zhu, Z. K. J. Am. Chem. Soc. 2005, 127, 16024. (12) Wang, W. S.; Zhen, L.; Xu, C. Y.; Zhang, B. Y.; Shao, W. Z. J. Phys. Chem. B 2006, 110, 23154. (13) Teo, J. J.; Chang, Y.; Zeng, H. C. Langmuir 2006, 22, 7369. (14) Kennedy, J. H.; Anderman, M. J. Electronchem. Soc. 1983, 130, 848. (15) Chauhan, P.; Annapoorni, S.; Trikha, S. K. Thin Solid Films 1999, 346, 266. (16) Fukazawa, M.; Matuzaki, H.; Hara, K. Sens. Actuators, B 1993, 13, 521. (17) Han, J. S.; Bredow, T.; Davey, D. E.; Yu, A. B.; Mulcahy, D. E. Sens. Actuators, B 2001, 75, 18. (18) Comini, E.; Guidi, V.; Frigeri, C.; Ricco, I.; Sberveglieri, G. Sens. Actuators B 2001, 77, 16. (19) Ohmori, T.; Takahashi, H.; Mametsuka, H.; Suzuki, E. Phys. Chem. Chem. Phys. 2000, 2, 3519. (20) Frank, S. N.; Bard, A. J. J. Phys. Chem. 1977, 81, 484. (21) Faust, B. C.; Hoffmann, M. R.; Bahnemann, D. W. J. Phys. Chem. 1989, 93, 6371. (22) Weiss, W.; Zscherpel, D.; Schlogl, R. Catal. Lett. 1988, 52, 215. (23) Gues, J. W. Appl. Catal. 1986, 25, 313. (24) Gong, C. R.; Chen, D. R.; Jiao, X. L.; Wang, Q. L. J. Mater. Chem. 2002, 12, 1844. (25) Morales, M. P.; Gonzalez-Carreno, T.; Serna, C. J. J. Mater. Res. 1992, 7, 2538. (26) Kiwi, J.; Cartzel, M. J. Chem. Soc., Faraday Trans. 1 1987, 83, 1101. (27) Siroky, K.; Jiesova, J.; Hudec, L. O. Thin Solid Films 1994, 245, 211. (28) Neri, G.; Bonavita, A.; Galvagno, S.; Siciliano, P.; Capone, S. Sens. Actuators, B 2002, 82, 40. (29) Matijevic, E.; Scheiner, P. J. Colloid Interface Sci. 1978, 63, 509. (30) Cao, M. H.; Liu, T. F.; Gao, S. Angew. Chem., Int. Ed. 2005, 44, 4197. (31) Li, S. Z.; Zhang, H.; Wu, J. B. Cryst. Growth Des. 2006, 6, 351. (32) Tang, B.; Wang, G. L.; Zhuo, L. H. Inorg. Chem. 2006, 45, 5196. (33) Li, L. L.; Chu, Y.; Liu, Y. J. Phys. Chem. C 2007, 111, 2123. (34) Mao, B. D.; Kang, Z. H.; Wang, E. B. J. Solid State Chem. 2007, 180, 489. (35) Wen, X. G.; Wang, S. H.; Ding, Y. J. Phys. Chem. B 2005, 109, 215. (36) Zeng, S. Y.; Tang, K. B.; Li, T. W.; Liang, Z. H.; Wang, D.; Wang, Y. K.; Zhou, W. W. J. Phys. Chem. C 2007, 111, 10217.

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