15218
J. Phys. Chem. B 2006, 110, 15218-15223
Surfactant-Assisted Synthesis of r-Fe2O3 Nanotubes and Nanorods with Shape-Dependent Magnetic Properties Lu Liu,*,† Hui-Zhong Kou,*,‡ Wenling Mo,§ Huajie Liu,| and Yuqiu Wang† Tianjin Key Laboratory of EnVironmental Remediation and Pollution Control, Nankai UniVersity, Tianjin 300071, China, Department of Chemistry, Tsinghua UniVersity, Beijing 100084, China, College of Information, Polytechnology UniVersity of Hebei, Tangshan 063009, China, and National Center for Nanoscience and Technology, Beijing 100080, China. ReceiVed: May 5, 2006; In Final Form: June 14, 2006
R-Fe2O3 nanorods and nanotubes have been synthesized and characterized by high-resolution transmission electron microscopy and X-ray diffraction. By means of different surfactant assistance, the high-quality onedimensional products were obtained, respectively, with aqueous butanol solution as the solvent and carbamide as the base, giving rise to single-crystalline products at 150 °C. The formation mechanism has been presented. Significantly, the magnetic investigations show that the magnetic properties are strongly shape-dependent; i.e., the nanorods have a Morin transition at 166 K from canted antiferromagnetic state to antiferromagnetic state, while the nanotubes exhibit a three-dimensional magnetic ordering above 300 K that has been attributed to the presence of small particles in a few regions of the tubes.
Introduction Nanocrystals, either quantum dots or nanorods and nanowires, have the quantum-confined effect,1 which leads to the unique size dependence of their optical, electronic, and electrochemical properties.2-4 However, the shapes of the nanomaterials have a considerable influence on their physical properties and are important in many potential applications, for instance, solar cells, light-emitting diodes, as well as scanning microscopy probes.5-7 Currently, one-dimensional (1D) nanostructures such as wires, rods, belts, and tubes have become the focus of intensive research owing to their unique applications in mesoscopic physics and the fabrication of nanoscale devices.8,9 At present, various methods have been used for the synthesis of 1D nanocrystals such as hydrothermal/solvothermal processes,10-16 organometallic precursors,17,18 emulsion liquid membrane systems,19 reverse micelle methods,20,21 solid template methods,22,23 vapor-phase-transport patterned-catalyst growth techniques,24 chemical vapor deposition,25 vapor-liquid-solid methods,26-30 polymer assembly,31,32 and so on. In recent years, the preparation of magnetic nanomaterials has gradually attracted much interest, since these materials have many potential applications in information storage, color imaging, bioprocessing, magnetic refrigeration, gas sensors, ferrofluids, and so on.33 In particular, hematite (R-Fe2O3) with n-type semiconducting properties under ambient conditions is of great scientific and technological importance. Nanowires, nanotubes, and nanorods of R-Fe2O3 represent a class of 1D magnetic materials, in which carrier motion is restricted in two directions so that they are expected essentially to improve photochemical, photophysical, and electron-transport properties. Until now, a few synthetic methodologies for R-Fe2O3 1D nanostructured * Authors to whom correspondence should be addressed. E-mail:
[email protected];
[email protected]. † Nankai University. ‡ Tsinghua University. § Polytechnology University of Hebei. | National Center for Nanoscience and Technology.
materials have been reported: Mann et al. used biomacromolecules as templates to synthesize R-Fe2O3 nanotubular materials;34 Wang et al. used a gas-solid reaction process under 700 and 800 °C to synthesize R-Fe2O3 nanobelts and nanowires materials;35 Vayssieres et al. used a solution method to synthesize hematite nanorod films;36 Antonelli et al. used ligandassisted templating with a chelating triol surfactant to synthesize mesoporous iron oxide, and some researchers synthesized R-Fe2O3 nanotubes with porous alumina membranes with ordered and vertical 1D channel structures as the synthetic templates.37-42 However, relatively high temperatures or long reaction times were needed in these methods. Herein, we report an efficient and convenient method, in which carbamide decomposition (>90 °C) provides OH- at the rate of diffusion and surfactants are used as templates, to synthesize R-Fe2O3 nanotubes and monocrystalline nanorods . We also compared the magnetic properties of the tubular structures and the rod structures of R-Fe2O3 nanoproducts with those of the bulk and nanoparticles. To the best of our knowledge, it is the first report in which nanorods and nanotubes of R-Fe2O3 were fabricated by surfactant-assisted means in solution. Experimental Section Approximately 0.2 g of FeCl3 and an appropriate amount carbamide were put into a Teflon-lined stainless steel autoclave of 30 mL capacity, then the autoclave was filled with 10 mL of distilled water and 1.5 mL of the surfactant polyisobutylene bissuccinimide (L113B) (or surfactant span80) as well as 10 mL of butanol. The autoclave was maintained at 150 °C for 12-15 h without shaking or stirring during the heating period and allowed to cool to room temperature. A red-brown precipitate was collected and then washed with distilled water and absolute ethanol. The size and shape of all the samples were examinated using a JEOL2010 high-resolution transmission electron microscope operated at an acceleration voltage of 200 kV and a Hitachi 3500 scanning electron microscope as well as a Sirion 200
10.1021/jp0627473 CCC: $33.50 © 2006 American Chemical Society Published on Web 07/19/2006
Synthesis of R-Fe2O3 Nanotubes and Nanorods
J. Phys. Chem. B, Vol. 110, No. 31, 2006 15219
Figure 1. XRD patterns of as-prepared products using different surfactants: (a) L113B (sample A); (b) span80 (sample B).
Figure 2. TEM images of the products prepared at 150 °C for 15 h using different surfactants: (a) L113B (sample A); (b) HRTEM of sample A; (c) span80 (sample B); (d) HRTEM of sample B.
scanning electron microscope. The crystal structure of the Fe2O3 nanocrystals was characterized by a Rigaku D/Max-2500 X-ray diffractometer employing Cu KR radiation, λ ) 1.54056 Å. X-ray photoelectron spectra were recorded using a Kratos Axis Ultra DLD spectrometer employing a monochromated Al KR X-ray source (hV ) 1486.6 eV), hybrid (magnetic/electrostatic) optics, and a multichannel plate and delay line detector (DLD). All X-ray photoelectron spectra were recorded using an aperture slot of 300 × 700 µm2, survey spectra were recorded with a pass energy of 160 eV, and high-resolution spectra were recorded with a pass energy of 40 eV. Magnetic measurements were performed on a Quantum Design MPMS-5 superconducting quantum interference device (SQUID) magnetometer. Results and Discussions Figure 1 shows X-ray diffraction (XRD) patterns of asprepared products using either L113B (Figure 1a) or span80 (Figure 1b) as the surfactant. It can be seen that both XRD patterns conform with rhombohedral R-Fe2O3 (a ) 5.038 Å, c ) 13.772 Å, JCPDS Card No. 33-0664). No characteristic peaks were observed for other impurities such as γ-Fe2O3 and Fe3O4.
X-ray photoelectron spectroscopy (XPS) profiles of the Fe2p regions for the nanorods are shown in the Supporting Information. The Fe2p peaks at binding energies of 711.10 and 724.77 eV, with a satellite at 719.25 eV, closely correspond to peaks expected for Fe2O3, also consistent with XRD results. Figure 2 displays the morphologies of the products determined by high-resolution transmission electron microscopy (HRTEM). The TEM images in Figure 2a show that sample A (surfactant L113B as the template) was solid and smoothly rodlike with diameters of 30-50 nm and lengths of 500-1100 nm. Figure 2c shows that sample B (surfactant span80 as the template) was smoothly tubelike with diameters of 18-29 nm, wall thicknesses of 3-7 nm, and lengths of 110-360 nm. The HRTEM images of the nanorods and selected area electron diffraction (SAED) indicate that they are monocrystalline, as shown in Figure 2b. The distances of adjacent lattice fringes, measured as 2.78 Å, are the interplanar distances of Fe2O3(104), in excellent agreement with the literature value of the (104) d-space (JCPDS Card No. 33-0664). The HRTEM image of the Fe2O3 nanotubes (Figure 2d) shows clearly the multiwalled structure. The interlayer spacing is ca. 2.76 Å (approximately the separation
15220 J. Phys. Chem. B, Vol. 110, No. 31, 2006
Liu et al.
Figure 3. SEM images of the products prepared at 150 °C for 15 h: (a and b) using L113B as the template; (c and d) using span80 as the template.
Figure 4. (a-g) TEM images under different conditions.
between (104) planes) and agrees well with that calculated from the literature value of the (104) d-space (JCPDS Card No. 330664); SAED indicates that the nanotubes grow along a certain direction, as shown in Figure 2d. Figure 3 shows the SEM images of samples A and B. Figure 3a revealed that the rodlike features of sample A were clearly produced, however, containing a few tubular products (Figure 3b); the reason will be discussed as follows. Figures 3c and 3d show the SEM images of sample B. To research the role of the surfactant span80 or L113B in the nanotube or nanorod formation process, the preparation of Fe2O3 particles without span80 or L113B was also carried out. Granular particles closely packed together were observed. In particular, we investigated the relationship between Fe2O3 morphology and added surfactant volume. When 0.5 mL of L113B was used, only Fe2O3 particles appeared (Figure 4a), When 1.0 mL of L113B was used, Fe2O3 particles and nanorods were simultaneously obtained (Figure 4b, nanoparticles and a
few nanorods being shown). The morphologies of the pure Fe2O3 nanorodlike structures were obtained in the presence of 1.5 mL of the surfactant L113B (Figure 2a), which suggests that the surfactants could play an indispensable role in this synthetic process. The relationship between the Fe2O3 morphology and the synthetic time was also investigated. The synthetic times were 1, 2, 4, 6, and 15 h; the images are shown in Figures 4c-f and 2a (15 h). Rodlike morphologies are produced for a synthetic time of 1 h (Figure 4c); well-structured nanorods are formed for a synthetic time of 2 h (Figure 4d). As the reaction time increased, the diameters and lengths of the well-structured nanorods increased (Figure 4e, 4 h; Figure 4f, 6 h). The investigation revealed that the synthetic time is the main parameter in the formation of 1D nanostuctures. However, the use of n-butanol is also crucial to the synthetic process of shapecontrolled Fe2O3 1D nanorods. Only Fe2O3 nanoparticles are produced in the reaction if n-butanol is replaced with kerosene
Synthesis of R-Fe2O3 Nanotubes and Nanorods
Figure 5. Proposed synthetic mechanism of as-prepared 1D products.
(Figure 4g). However, pure Fe2O3 1D nanotubes, either nbutanol or kerosene as the solvent, were obtained. On the basis of the information that we have gathered, a growth process of the Fe2O3 1D nanoproductions can be proposed. It is well-known that the surfactant molecules spontaneously organize into rod-shaped micelles (or inverse micelles) when their concentrations reach a critical value.43 These anisotropic structures can be used as soft templates to promote the formation of the 1D nanostructured materials when coupled with an appropriate chemical reaction. In this synthetic process, the appropriate surfactant L113B or span80 forms rodshaped micelles between n-butanol and water solution. There are a few functional groups such as CdO and -NH on the surfactant molecules, which are hydrophilic groups and can provide coordination sites.44 When Fe3+ ions enter into aqueous solution, the sites provide the necessary heterogeneous nucleation sites, and Fe3+ forms complexes with the hydrophilic functional groups. Herein, when the temperature is more than 90 °C, these carbamide reagents in aqueous solution are decomposed to form NH4OH, then the NH4OH provides OH-,
Figure 6. XRD of the sample with a synthetic time of 1 h.
J. Phys. Chem. B, Vol. 110, No. 31, 2006 15221 which with Fe3+ forms the FeOOH crystalline nucleus on the coordination sites, the FeOOH molecules with FeOOH monomers are connected by hydrogen bonds along rod-shaped micelles, then dehydrate among the molecules at 150 °C and in the end lead to one-dimensional Fe2O3 nanowires or nanorods with a simple crystalline structure. However, when Fe3+ forms FeOOH crystalline nucleus on the coordination sites, the FeOOH molecules with FeOOH monomers are connected by hydrogen bonds around a molecule of the surfactant or along exterior surface of rod-shaped micelles, then dehydrate between the molecules at 150 °C. When the surfactant molecules are removed by organic solvent, single-crystalline 1D tubular nanostructured products are obtained. Our experiment has proven that when using surfactant L113B as the template its 1D products contain about 95% nanorods and 5% nanotubes and when using surfactant span80 as the template its 1D products contain about 90% nanotubes and 10% nanorods. At 150 °C, the as-obtained product powder through 1 h of synthesis was determined by the XRD (Figure 6, JCPDS Card No. 751594), showing that the powder was pure FeOOH, indicating that R-Fe2O3 comes from FeOOH. A comparison between the curves of the as-prepared samples and bulk FeOOH powder indicated that the pattern of the FeOOH also shows one strongest peak of (130), while the pattern of the standard FeOOH powder exhibits the strongest peak of (110), which suggest that the FeOOH displays preferential orientations of (130). The experimental facts reveal that our inference is reasonable. The equations of the chemical reaction in the synthetic process were listed as fallows
NH2CONH2 + 3H2O f 2NH3‚H2O + CO2
(1)
FeCl3 + 3NH3‚H2O f FeOOH + 3NH4Cl + H2O (2) 2nFeOOH f nFe2O3 + nH2O
(3)
It is of great interest to investigate the magnetic properties of R-Fe2O3 with two different shapes and nanosizes. It has been reported that R-Fe2O3 orders antiferromagnetically below TN ) 955 K.45 For rhombohedral crystalline bulk R-Fe2O3 the magnetic phase transition from the canted ferromagnetic phase to another antiferromagnetically ordered state has been reported
15222 J. Phys. Chem. B, Vol. 110, No. 31, 2006
Figure 7. FCM measured in 100 Oe for sample A. Inset: Hysteresis loop at 300 K for sample A.
Figure 8. FCM measured in 100 Oe for sample B.
to be at ca. 260 K.46 A sharp decrease in magnetization should be observed at this transition, termed as the Morin transition temperature (Tm). The field-cooled magnetization (FCM) curve under 100 Oe for sample A (Figure 7) displays a regular decrease, corresponding to a Morin temperature (Tm) of ca. 166 K derived from the extremum of the dM/dT curve. The noticeable hysteresis at 300 K clearly shows that sample A is in a canted antiferromagnetic (or weak ferromagnetic) state with a coercive field of 280 Oe, as shown in the inset of Figure 7. However, the Tm value for sample A is lower than that of the bulk samples. A similar phenomenon has been previously reported on R-Fe2O3 dendrite (more than 50 nm in size) with Tm of 216 K.49 This phenomenon may be related to the shape of the nanorods. It has been recently shown that the Tm value strongly depends on the size of R-Fe2O3 particles; i.e., the smaller the size, the lower the Tm.50 Thus, the comparatively low Tm value for sample A should be due to the small diameter of the nanorod (30-50 nm). However, the magnetic behavior of sample B is completely different; as shown in Figure 8, the FCM plot measured in an applied field of 100 Oe shows a constant increase in χm and no maximum down to 5 K. For a paramagnetic compound, the FCM value should be nearly zero even down to 2 K in a small applied magnetic field, e.g., 100 Oe. The occurrence of an abrupt increase in magnetization at ca. 300 K indicates the presence of magnetic spontaneous magnetization above room temperature.51 The hysteresis loop (Figure 9) measured at 5 K reveals the presence of the coercive field of 280 Oe, indicative of a soft magnet. The results show that a long-range magnetic ordering occurs, which suppresses the Morin transition. Considering that the sample was pure, the possibility that the feature is due to impurities can be precluded. Similar phenomena have been recently observed in mesoporous R-Fe2O3 with disordered
Liu et al.
Figure 9. Hysteresis loop at 5 K for sample B.
walls, and this abnormality has been assigned to be due to the presence of small crystalline particles in a few regions of the sample.51 It is noteworthy that sample B (nanotubes) and the mesoporous sample with disordered walls are similar in structure. Therefore, such regions containing small crystalline particles are present in sample B, giving rise to spontaneous magnetization above 300 K. Although to reach a clear conclusion requires further investigations on different sizes and shapes of R-Fe2O3, we emphasize that the curl of layers might bring about significant defects in the nanotubes. Such defects are the origin of the magnetic phase transition. Overall, the two samples of R-Fe2O3 display different magnetic behavior. This intriguing phenomenon might be due to the different shapes of two samples. Obviously, further work should be performed to understand better the effects of the shapes on magnetism of R-Fe2O3. Conclusion We have found a route to obtain R-Fe2O3 nanorods and nanotubes by means of different surfactants. This route can allow facile preparation of monodisperse R-Fe2O3 nanorods and nanotubes, respectively. The investigation provided obvious proof for the possible formation mechanism, which revealed that our inference is reasonable. Magnetic measurements showed that two samples of R-Fe2O3 nanotubs and nanorods display shape-dependent magnetic behavior. Meanwhile, our synthetic method could be extended further to the preparation of various oxide 1D nanocrystals (e.g., TiO2). Acknowledgment. This work is supported by the National Natural Science Foundation of China (Grant No. 50272034), the Ministry of Science Technology of China through a 973 project (Grant No. 2002CB613301), and the Support Foundation of Nankai University. Supporting Information Available: X-ray photoelectron spectra for as-prepared R-Fe2O3. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Arnim, H. Chem. ReV. 1989, 89, 1861. (2) Alivisatos, A. P. Science 1996, 271, 933. (3) Alivisatos, A. P. J. Phys. Chem. 1996, 100, 13226. (4) Nirmal, M.; Brus, L. E. Acc. Chem. Res. 1999, 32, 407. (5) Huynh, W. U.; Dittmer, J. J.; Alivisatos, A. P. Science 2002, 295, 2425. (6) Ahmadi, T. S.; Wang, Z. L.; Green, T. C.; Henglein, A.; Elsayed, M. A. Science 1996, 272, 1924. (7) Mattoussi, H.; Radzilowski, L. H.; Dabbousi, B. O.; Thomas, E. L.; Bawendi, M. G.; Rubner, M. F. J. Appl. Phys. 1998, 83, 7965. (8) Wang, Z. L. AdV. Mater. 2000, 12, 1295.
Synthesis of R-Fe2O3 Nanotubes and Nanorods (9) Odom, J.; Hu, T. W.; Lieber, C. M. Acc. Chem. Res. 1999, 32, 435. (10) Wang, X.; Zhuang, J.; Chen, J.; Zhou, K. B.; Li, Y. D. Angew. Chem., Int. Ed. 2004, 43, 2017. (11) Chen, S. C.; Hidajat, K.; Yu, L. Y. E.; Kawi, S. AdV. Mater. 2004, 16, 541. (12) Yang, J.; Xue, C.; Yu, S. H.; Zeng, J. H.; Qian, Y. T. Angew. Chem., Int. Ed. 2004, 41, 4697. (13) Wang, X.; Li, Y. D. J. Am. Chem. Soc. 2002, 124, 2880. (14) Yan, P.; Xie, Y.; Wang, W.; Liu, F.; Qian, Y. J. Am. Chem. Soc. 1999, 121, 4062. (15) Yu, S. H.; Shu, L.; Yang, J.; Han, Z. H.; Qian, Y. T.; Zhang, Y. H. J. Mater. Res. 1999, 14, 4157. (16) Yang, J.; Zeng, J. H.; Yu, S. H.; Yang, L.; Zhang, Y. H.; Qian, Y. T. Chem. Mater. 2000, 12, 2924. (17) Chang, K. W.; Wu, J. J. AdV. Mater. 2004, 16, 545. (18) Micic, O. I.; Sprague, J. R.; Curtis, C. J.; Jones, K. M.; Machol, J. L.; Nozik, A. J.; Giessen, H.; Fluegel, B.; Mohs, G.; Peyghambarian, N. J. Phys. Chem. 1995, 99, 7754. (19) Liu, L.; Wu, Q. S.; Ding, Y. P.; Liu, H. J.; Qi, J. Y.; Liu, Q. Aust. J. Chem. 2004, 57, 219. (20) Zheng, N. W.; Wu, Q. S.; Ding, Y. P.; Li, Y. D. Chem. Lett. 2000, 6, 638. (21) Wu, Q. S.; Zheng, N. W.; Ding, Y. P.; Li, Y. D. Inorg. Chem. Commun. 2002, 5, 671. (22) Cao, H.; Xu, Z.; Sang, H.; Sheng, D.; Tie, C. AdV. Mater. 2001, 13, 121. (23) Masuda, H.; Yanagishita, T.; Yasui, K.; Nishio, K.; Yagi, I.; Rao, T. N.; Fujishima, A. AdV. Mater. 2001, 13, 247. (24) Huang, M. H.; Mao, S.; Feick, H.; Yan, H.; Wu, Y.; Kind, H.; Weber, E.; Russo, R.; Yang, P. D. Science 2001, 292, 1897. (25) Yazawa, M.; koguchi, M.; Muto, A.; Ozama, M.; Hiruma, K. Appl. Phys. Lett. 1992, 61, 2051. (26) Duan, X. F.; Lieber, C. M. AdV. Mater. 2000, 12, 298. (27) Duan, X. F.; Lieber, C. M. J. Am. Chem. Soc. 2000, 122, 188. (28) Wu, Y.; Yang, P. Chem. Mater. 2000, 12, 605. (29) Wang, Y. W.; Zhang, L. D.; Liang, C. H.; Wang, G. Z.; Peng, X. S. Chem. Phys. Lett. 2002, 357, 314.
J. Phys. Chem. B, Vol. 110, No. 31, 2006 15223 (30) Morales, A. M.; Lieber, C. M. Science 1998, 279, 208. (31) Wang, Y. L.; Herrick, T.; Xia, Y. N. Nano. Lett. 2003, 3, 1163. (32) Yu, S. H.; Colfen, H.; Antonietti, M. Chem.sEur. J. 2002, 8, 2937. (33) Pascal, C.; Pascal, J. L.; Favier, F.; Moubtassim, M. L. E.; Payen, C. Chem. Mater. 1999, 11, 141. (34) Archibald, D. D.; Mann, S. Nature 1993, 364, 430. (35) Wen, X. G.; Wang, S. H.; Ding, Y.; Wang, Z. L.; Yang, S. H. J. Phys. Chem. B 2005, 109, 215. (36) Vayssieres, L.; Hagfeldt, A.; Lindquist, S. E. Pure Appl. Chem. 2000, 72, 47. (37) Martin, C. R. Science 1994, 266, 266. (38) Steinhart, M.; Wendorff, J. H.; Greiner, A.; Wehrspohn, R. B. Science 2002, 296, 1997. (39) Sone, E. D.; Zubarev, E. R.; Stupp, S. V. Angew. Chem., Int. Ed. 2002, 41, 1706. (40) Schuth, F. Angew. Chem., Int. Ed. 2003, 42, 3604. (41) Lahav, M.; Sehayek, T.; Vaskeich, A.; Rubinstein, I. Angew. Chem., Int. Ed. 2003, 42, 5576. (42) Steinhart, M.; Wehrspohn, R. B.; Gosele, U.; Wendorff, J. H. Angew. Chem., Int. Ed. 2004, 43, 1334. (43) Ringsdorf, H.; Schlarb, B.; Venzmer, J. Angew. Chem., Int. Ed. Engl. 1988, 27, 113. (44) Lezau, A.; Trudeau, M.; Tsoi, G. M.; Wenger, L. E.; Antonelli, D. J. Phys. Chem. B 2004, 108, 5211. (45) Gronvold, F.; Samuelsen, E. J. J. Phys. Chem. Solids 1975, 36, 249. (46) Morin, F. J. Phys. ReV. 1950, 78, 819. (47) Smith, T. T. Phys. ReV. 1916, 8, 721. (48) Jing, Z.; Wu, S.; Zhang, S.; Huang, W. Mater. Res. Bull. 2004, 39, 2057. (49) Cao, M.; Liu, T.; Gao, S.; Sun, G.; Wu, X.; Hu, C.; Wang, Z. L. Angew. Chem., Int. Ed. 2005, 44, 4197. (50) Zysler, R. D.; Fiorani, D.; Testa, A. M.; Suber, L.; Agnostinelli, E.; Godinho, M. Phys. ReV. B 2003, 68, 212408. (51) Jiao, F.; Harrison, A.; Jumas, J.-C.; Chadwick, A. V.; Kockelmann, W.; Bruce, P. G. J. Am. Chem. Soc. 2006, 128, 5468.
