18108
2006, 110, 18108-18111 Published on Web 08/31/2006
Growth and Magnetic Properties of Oriented r-Fe2O3 Nanorods Jih-Jen Wu,* Ya-Lien Lee, Hsuen-Han Chiang, and Daniel Kwan-Pang Wong Department of Chemical Engineering, National Cheng Kung UniVersity, Tainan 701, Taiwan ReceiVed: July 14, 2006; In Final Form: August 20, 2006
R-Fe2O3 nanorods have been deposited on Si substrates using the metal-organic chemical vapor deposition method. Structural analyses indicated that R-Fe2O3 nanorods are preferentially oriented in the [104] direction on Si(100) substrates, and the nanorod possesses the single-crystalline structure. MFM image suggests that a spin domain is formed in the R-Fe2O3 nanorod. Anisotropic magnetic property of the R-Fe2O3 nanorods, i.e., the discrepancy of the saturation magnetization, is observed from SQUID measurements when the magnetic field are applied parallel and perpendicular to the substrate. A lower Morin temperature than that of the macroscopically crystalline hematite is observed when the magnetic field is applied parallel to the substrate. R-Fe2O3 (hematite), which is the most stable ion oxide under ambient conditions, exhibits a rhombohedral structure with an indirect band gap of 2.2 eV.1 It is a versatile and environmentally friendly material and has been used considerably for its photochemical, catalytic, and sensing properties.1-3 Apart from the aforementioned excellent properties, magnetic properties of R-Fe2O3 crystals also attract much attention.4-6 Crystalline R-Fe2O3 possesses antiferromagnetic characteristic with a Neel temperature (TN) at 960 K. A magnetic phase transition at the Morin temperature (TM) of 263 K, characterized by reorientation of the magnetization of the two spin sublattices from parallel to perpendicular to the [111] axis, is observed from the macroscopically crystalline hematite. Owing to the anisotropic superexchange interaction, the spins are slightly canted (∼0.13°) out of the basal planes, resulting in a weak ferromagnetic state of the R-Fe2O3 between TN and TM.4 In the case of R-Fe2O3 nanostructures, it has been demonstrated that the Morin temperature is influenced by particle size, lattice strain, and defects.7-10 The Morin temperature was found to decrease with decreasing particle size and to vanish for the particle size smaller than 8-20 nm. Recently, research concerning one-dimensional (1D) nanostructures has attracted considerable interest due to their contribution to the fundamental studies and their potential for future technological applications.11-15 1D R-Fe2O3 nanostructures (nanotubes, nanorods, nanowires, and nanobelts) have been synthesized using various methods, including the hydrothermal method,16,17 the thermal oxidation method,18,19 and the templatebased method.20-22 Excellent gas sensing properties and electrochemical activity have been demonstrated in R-Fe2O3 nanotubes.21 Magnetic properties of the R-Fe2O3 nanowires have also been investigated.20,22 However, anisotropic characteristics of the magnetic properties of the 1D R-Fe2O3 nanostructures were not reported in the previous studies. Herein, we report the growth of oriented R-Fe2O3 nanorods on Si substrates using metalorganic chemical vapor deposition (MOCVD). Magnetic proper* To whom correspondence
[email protected].
should
be
addressed.
E-Mail:
10.1021/jp0644661 CCC: $33.50
ties of the oriented R-Fe2O3 nanorods were investigated using magnetic force microscopy (MFM) and superconducting quantum interference device magnetometer (SQUID). Anisotropic magnetic property of the R-Fe2O3 nanorods was observed from SQUID measurements when the magnetic fields were applied perpendicular and parallel to the substrates. R-Fe2O3 nanorods were grown in a 1 in quartz tube insert to a two-temperature-zone furnace. Iron acetylacetonate (Fe(C5H7O2)3 placed on a cleaned Pyrex glass container was loaded into the low-temperature zone of the furnace. The temperature was controlled to be at 125-130 °C to vaporize the solid reactant. The vapor was carried by a 500 sccm N2/O2 flow into the higher-temperature zone of the furnace, which was heated to 550 °C at a pressure of 3 Torr. Si(100) substrates were employed for nanorod growth. It should be noted here that there is no catalytic metal film precoated on these substrates. The morphology of the nanorods was examined using scanning electron microscopy (SEM, JEOL, JSM-7000F). The crystal structure of the nanorods was analyzed using X-ray diffraction (XRD, Rigaku D/MAX-2000 and D/MAX-2500), micro-Raman (Jobin Yvon, Labram HR), and high-resolution transmission electron microscopy (HRTEM, JEOL, JEM-3010). The magnetic properties of the nanorods were examined using MFM (DI NS3a-MMAFM) and SQUID (Quantum Design, MPMS-7). Figure 1 shows that high-density and oriented nanorods are formed over the entire Si substrate using the catalytic-free MOCVD method. A top-view SEM image, Figure 1a, reveals that the nanorods, which possess a rectangular cross section perpendicular to the growth direction, naturally assemble to form flowerlike bundles in a range of ∼1 µm. Figure 1b shows that the nanorods with an average length of 700 nm are grown on the Si substrate after 3 h deposition. The average dimension of the rectangular cross section of the nanorods is 120 × 40 nm2. In addition, both tip- and flat-end nanorods are obtained as shown in Figure 1b. The crystal structure of the nanorods was examined by XRD and Raman. Figure 2a shows a typical glancing-angle-mode XRD pattern of the oriented nanorods. All the diffraction peaks © 2006 American Chemical Society
Letters
J. Phys. Chem. B, Vol. 110, No. 37, 2006 18109
Figure 1. SEM images of iron oxide nanorods on Si substrates: (a) top view, (b) 45° tilted view.
in the XRD pattern can be indexed to those of the rhombohedral structure of R-Fe2O3 according to JCPDS file no. 84-0307. The powder-diffraction-mode XRD pattern was taken further to investigate the oriented characteristic of the iron oxide nanorods. As shown in the insert of Figure 2a, higher intensity ratios of (104) to other planes in comparison with those in JCPDS file no. 84-0307 are observed, revealing the R-Fe2O3 nanorods preferentially oriented in the [104] direction on the Si(100) substrate. As shown in Figure 1, the nanorods are not all wellaligned but naturally form flowerlike bundles on the substrates. The preferential orientation obtained from XRD measurement does not necessarily represent the growth direction of every nanorod. The Raman spectrum of the R-Fe2O3 nanorods is shown in Figure 2b. The peaks appearing in the spectrum at 225, 245, 288, 409, 500, 610, and 819 cm-1 are all corresponding to the Raman peaks of the R-Fe2O3 structure.23,24 The absence of the characteristic peaks of other iron oxide phases in both XRD and Raman analyses indicates the pure R-Fe2O3 nanorods were formed on the Si substrate. The upper insert of Figure 2c shows a TEM image of the segments of two nanorods scratched off from the Si substrate. The high-resolution (HR) TEM image and the corresponding electron diffraction pattern of the region denoted in the upper insert are illustrated in Figure 2c and the insert at the bottom, respectively. They both demonstrate that the nanorod possesses the single crystalline structure. The electron diffraction pattern confirms the XRD analysis that R-Fe2O3 nanorods were formed on the Si substrate. The lattice spacing of 0.22 nm shown in Figure 2c corresponds to the d spacing of (113) crystal planes of the rhombohedral structure of R-Fe2O3. It implies that this nanorod may be located at the outer region of the flowerlike bundle due to the inconsistency of the longitudinal direction of the nanorod and the preferential orientation observed from XRD measurement. MFM was employed to image the state of individual nanorods. The MFM image reflects the magnetic polarization at the top end of each nanorod. Figure 3a,b shows the roomtemperature AFM and the corresponding MFM images of the
Figure 2. (a) Typical glancing-angle-mode and powder-diffractionmode (insert) XRD patterns of the R-Fe2O3 nanorods. (b) Raman spectrum of the R-Fe2O3 nanorods. (c) HRTEM image and the corresponding electron diffraction of the denoted region in the insert.
R-Fe2O3 nanorods formed on the Si substrate, respectively. The sample is slightly tilted in order to resolve the nanorod feature. The flowerlike bundles are also observed from the AFM image. The appearance of a dark and bright dot pattern in the MFM image corresponds to the top of each nanorod shown in the AFM image, suggesting that a spin domain is formed in the R-Fe2O3 nanorod. The magnetic properties of the R-Fe2O3 nanorods grown on the Si substrate were further investigated using SQUID. The magnetic fields were applied parallel and perpendicular to the substrate to examine the anisotropic magnetic characteristic of the R-Fe2O3 nanorods. Figure 4a shows the field dependences of magnetization (M-H curves) of the R-Fe2O3 nanorods on the Si substrate measured at 300 K, in which the magnetic contribution of the Si substrate has been subtracted. Weak ferromagnetism in the oriented R-Fe2O3 nanorods is observed at 300 K. The M-H curves reveal that hysteresis curves with the coercive field (Hc) of ∼200 Oe are observed in the R-Fe2O3 nanorods in both applied magnetic field directions. However, the saturation magnetization (Ms) per unit volume of the R-Fe2O3 nanorods is dependent on the directions
18110 J. Phys. Chem. B, Vol. 110, No. 37, 2006
Letters
Figure 3. (a) AFM and (b) the corresponding MFM images of the R-Fe2O3 nanorods on Si substrate.
hematite is ascribed to the small cross-sectional dimensions of the R-Fe2O3 nanorods. In summary, the formation of R-Fe2O3 nanorods with anisotropic magnetic characteristic has been demonstrated in the letter. Structural analyses indicated that R-Fe2O3 nanorods are preferentially oriented in the [104] direction on Si(100) substrates, and the nanorod possesses the single-crystalline structure. MFM image suggests that a spin domain is formed in the R-Fe2O3 nanorod. Anisotropic magnetic property of the R-Fe2O3 nanorods was observed from SQUID measurements. A higher Ms at 300 K is obtained when the magnetic field is applied perpendicular to the long axis of most R-Fe2O3 nanorods. Morin temperatures of the nanorods are examined to be ∼125 K in the magnetic fields applied parallel to the substrate. Acknowledgment. The authors would like to thank the Center for Micro/Nano Technology Research, National Cheng Kung University, Tainan, Taiwan, for equipment access. Financial support from the National Science Council in Taiwan under contract nos. NSC 93-2815-C-006-037-E and NSC 932214-E-006-022 is gratefully acknowledged. References and Notes
Figure 4. (a) M-H curves of the R-Fe2O3 nanorods on the Si substrate measured at 300 K. 9 and b represent the magnetic fields applied parallel and perpendicular to the substrate, respectively. (b) ZFC and FC M-T curves of the R-Fe2O3 nanorods in a magnetic field of 2000 Oe applied parallel to the substrate.
of the applied magnetic field. A higher Ms was obtained when the magnetic field was applied parallel to the substrate, demonstrating the anisotropic magnetization of the R-Fe2O3 nanorods. The discrepancy of the saturation magnetization is suggested to be ascribed to the anisotropic characteristic of the oriented R-Fe2O3 nanorods grown on the substrates. The temperature dependences of magnetization for the R-Fe2O3 nanorods followed zero-field cooling (ZFC) and field cooling (FC) procedures in a magnetic field of 2000 Oe applied parallel to the substrate are shown in Figure 4b. The ZFC M-T curve reveals that the magnetization possesses two changes of slope at T ≈ 40 and 125 K. It has been suggested that the lower change may be attributed to blocking processes of superparamagnetic particles.25 The steeper increase of the magnetization at 125 K is suggested to be due to the appearance of the weak ferromagnetic moments, i.e., the Morin transition occurring, within the R-Fe2O3 nanorods. The lower TM than that of microcrystalline
(1) Anderman, M.; Kennedy, J. H. Semiconductor Electrodes; Finklea, H. O., Ed., Elsevier: Amsterdam, 1988. (2) Geus, J. W. Appl. Catal. 1986, 25, 313. (3) Huo, L.; Li, W.; Lu, L.; Cui, H.; Xi, S.; Wang, J.; Zhao, B.; Shen, Y.; Lu, Z. Chem. Mater. 2000, 12, 790. (4) Besser, P. J.; Morrish, A. M.; Searle, C. W. Phys. ReV. 1967, 153, 632. (5) Morin, F. J. Phys. ReV. 1950, 78, 819. (6) Shull, C. G.; Stranser, W. A.; Wollan, E. O. Phys. ReV. 1951, 83, 333. (7) Amin, N.; Arajs, S. Phys. ReV. B 1987, 35, 4810. (8) Dang, M. Z.; Rancourt, D. G.; Dutrizac, J. E.; Lamarche, G.; Provencher, R. Hyperfine Interact. 1998, 117, 271. (9) Dormann, J. L.; Cui, J. R.; Sella, C. J. Appl. Phys. 1985, 57, 4283. (10) Zysler, R.; Fiorani, D.; Dormann, J. L.; Testa, A. M. J. Magn. Magn. Mater. 1994, 133, 71. (11) Hu, J.; Ouyang, M.; Yang, P.; Leiber, C. M. Nature (London) 1999, 399, 48. (12) Lauhon, L. J.; Gudiksen, M. S.; Wang, D.; Leiber, C. M. Nature (London) 2002, 420, 57. (13) Gudilsen, M. S.; Lauhon, L. J.; Wang, J.; Smith, D. C.; Leiber, C. M. Nature (London) 2002, 415, 617. (14) He, R.; Law, M.; Fan, R.; Kim, F.; Yang, P. Nano Lett. 2002, 2, 1109. (15) Bjork, M. T.; Ohlsson, B. J.; Sass, T.; Persson, A. I.; Thelander, C.; Magnusson, M. H.; Depperk, K.; Wallenberg, L. R.; Samuelson, L. Appl. Phys. Lett. 2002, 80, 1058. (16) Wang, X.; Chen, X.; Gao, L.; Zheng, H.; Ji, M.; Tang, C.; Shen, T.; Zhang, Z. J. Mater. Chem. 2004, 14, 905. (17) Jia, C.-J.; Sun, L.-D.; Yan, Z.-G.; You, L.-P.; Luo, F.; Han, X.-D.; Pang, Y.-C.; Zhang, Z.; Yan, C.-H. Angew. Chem., Int. Ed. 2005, 44, 4328.
Letters (18) Fu, Y. Y.; Wang, R. M.; Xu, J.; Chen, J.; Yan, Y.; Narlikar, A. V.; Zhang, H.; Chem. Phys. Lett. 2003, 379, 373. (19) Wen, X.; Wang, S.; Ding, Y.; Wang, Z. L.; Yang, S. J. Phys. Chem. B 2005, 109, 215. (20) Zhang, L. Y.; Xue, D. S.; Xu, X. F.; Gui, A. B.; Gao, C. X. J. Phys.: Condens. Matter 2004, 16, 4541. (21) Chen, J.; Xu, L.; Li, W.; Gou, X. AdV. Mater. 2005, 17, 582.
J. Phys. Chem. B, Vol. 110, No. 37, 2006 18111 (22) Suber, L.; Imperatori, P.; Ausanio, G.; Fabbri, F.; Hofmeister, H. J. Phys. Chem. B 2005, 109, 7103. (23) Bersani, D.; Lotticil, P. P.; Montenero, A. J. Raman Spectrosc. 1999, 30, 355. (24) Ruby, C.; Humber, B.; Fusy, J. Surf. Interface Anal. 2000, 29, 377. (25) Suber, L.; Fiorani, D.; Imperatori, P.; Foglia, S.; Montone, A.; Zysler, R. Nanostruct. Mater. 1999, 11, 797.
