CRYSTAL GROWTH & DESIGN
Controlled Growth of Aligned Arrays of Cu-Ferrite Nanorods Zhongbing Huang,*,† Ying Zhu,† Shutao Wang,† and Guangfu Yin*,‡ Institute of Chemistry, Chinese Academy of Sciences, Beijing, China, and School of Materials Sciences and Engineering, Sichuan UniVersity, Chengdu, China
2006 VOL. 6, NO. 8 1931-1935
ReceiVed October 15, 2005; ReVised Manuscript ReceiVed March 28, 2006
ABSTRACT: The aligned arrays of Cu-ferrite nanostructures, which consisted of magnetic Cu0.2Fe2.5O4 nanorods, have been prepared by an ethylene glycol (EG) and hexamethylenetetramine (HMTA)-assisted sol-gel process at 85 °C on a Cu substrate. The X-ray diffraction (XRD) pattern indicated that the sword-like Cu-ferrite nanorods were tetragonal phase. Furthermore, the high-resolution transmission electron microscopy (HRTEM) and the selected area electron diffraction (SAED) revealed that they had good crystal structure. The mechanism for the EG-HMTA-assisted sol-gel synthesis of tetragonal Cu-ferrite nanorods has been preliminarily explained. Introduction
Experimental Section
There is no doubt that one-dimensional (1D) nanomaterials have drawn special attention because of their properties quite different from the bulk and their potential applications in the construction of a variety of state-of-the-art nanodevices1. In particular, the one-dimensional magnetic nanocrystal not only exhibits novel optical and magnetic properties but also is one of crucial components in the fabrication of nanodiveces.2 The oriented alignment of nanowires on a substrate is paramount to the realization of integrated electronic or photonic nanotechnology. Many methods have been developed for the fabrication of nanowire arrays, such as template methods,3 catalytic growth,4 Langmuir-Blodgett and fluidic alignment techniques,5 electrospinning,6 etc. These methods often suffer from the difficulty to form single crystals or the need for tedious post-treatment. Because the magnetic easy axis of a particle correlates closely with its crystal structure,7 the controlled growth of nanocrystals in an assembly would lead to an aligned magnetic easy axis. Such an alignment is often a necessity for a nanostructured assembly that is suitable for various magnetic applications in as areas of data storage, magneto-optical readers, micro-electron motors, and advanced magnets.8 Architectural control of the anisotropic nanocrystals including 1D wires is imperative for the success of bottom-up smart-device applications, not to mention the novel scientific features that accompany them. However, the unique geometries of 1D nanomaterials are not easy to fabricate in magnetic fields. Until now, to the best of our knowledge, no magnetic 1D nanomaterial arrays have been synthesized in solution. Therefore, to explore a simple and controllable synthetic method in these matters for cell populations is significant for the preparation of artificial organs. Tetragonal CuFe2O4 is an inverse spinel in which Cu2+ ions occupy mainly octahedral B-sites, whereas Fe3+ ions are found on B-sites and tetrahedral A-sites with approximately equal occupancy. The oxygen tetrahedral and octahedral with the central Fe3+ each contribute in a different way to magnetooptic Kerr effect (MOKE) spectra.9 In this paper, we demonstrate controlled growth of large-area arrays of Cu-ferrite nanorods (NRs) on a Cu substrate without use of any template. Our further experiments indicate that the large-area arrays of Cu-ferrite NRs can be controlled more conveniently through a one-pot reaction by changing the concentration of the reactants.
To grow well-aligned Cu-ferrite NRs, it is necessary that a thin film of Cu-ferrite nanoparticles is grown on a substrate by a sol-gel method. Virtually any substrate can be used as long as a Cu-ferrite film can be grown on its surface, such as a copper slide, a Si wafer, and glass. In the typical procedure, a copper slide was used as substrate, and a good Cu-ferrite film grew on it due to its conductivity. The copper substrate was cleaned in an aqueous 1.0 N HCl solution for about 2 min, followed by repeated rinsing three times with distilled water. Further we have extended coprecipitation for synthesis of Cuferrite NRs by adopting hexamethylenetetramine (HMTA) as the nucleation-control reagent and ethylene glycol (EG), a strong bidentate chelating agent to Cu2+ and Fe2+ ions with a high stability constant, as the capping reagent. After being dried under a N2 gas flow, a Cu slide covered with Cu-ferrite nanoparticles was vertically placed in the reaction solution. Cu-ferrite NRs were grown in 3 mmol of FeCl2‚ 2H2O, 1.5 mmol of CuCl2‚5H2O, 18 mmol of HMTA, 45 mmol of EG, and 30 mL of deionized water in a conventional reaction flask with a reflux condenser. The reaction temperature was 85 °C. The growth time was 10 h for growth of NRs on a Cu substrate. The obtained Cu-ferrite rods were typically 200-400 nm in width and about 4-6 µm long. It is believed that the following reactions are involved in the formation of Cu-ferrite rods:
† ‡
Chinese Academy of Sciences. Sichuan University.
(CH2)6N4 + 6H2O f 6HCHO + 4NH3
(1)
NH3 + H2O T NH4+ + OH-
(2)
Fe2+ + HCHO + 2H2O f Fe3+ + HCH2OH + HO-
(3)
Cu2+ + 2Fe3+ + 8OH- f CuFe2O4 + 4H2O
(4)
Results and Discussion Role of the CuFe2O4 Seed Layer in Nanorod Alignment. Figure 1b shows SEM images of CuFe2O4 NRs fabricated on a Cu substrate that was covered with CuFe2O4 nanoparticles and on a bare Cu substrate. It can be seen that alignment of NRs grown on the CuFe2O4-covered substrate (Figure 1b) is significantly better than that grown on the bare Cu region (Figure 1a). It is suggested that the superior alignment on CuFe2O4 nanoparticles is due to the matching lattice structure and due to the polar nature of the CuFe2O4 nanoparticle surface. The budding crystals can be formed at the junction interface of ferrites. The budding crystal’s surface is either positively charged or negatively charged. In either case, the surface will attract ions of opposite charges (OH- or Cu2+, Fe3+) onto itself, and this new surface covered with ions will in turn attract ions with
10.1021/cg0505517 CCC: $33.50 © 2006 American Chemical Society Published on Web 06/23/2006
1932 Crystal Growth & Design, Vol. 6, No. 8, 2006
Figure 1. SEM images of Cu-ferrite NRs fabricated on a Cu film: (a) NRs on bare Cu have poor alignment; (b) NRs on the Cu-ferrite are uniformly aligned.
opposite charges to cover the surface next and react to form Cu-ferrite. Thus, Cu-ferrite NRs could grow continuously leading to alignment. Morphology of Nanorods. Figure 2 shows the representative SEM images of the Cu-ferrite NRs on the Cu substrate. The aligned Cu-ferrite NRs consisted of rhombic Cu-ferrite NRs with a diameter of 0.4-0.5 µm, exhibiting a highly uniform and densely packed array of NRs. Figure 2b is a cross sectional view, and NRs with the length of 4-5 µm almost grow vertically onto the substrates, which originate the divaricated growth of some NRs from the Cu substrates. Chemical Composition and Crystal Structure of NRs. When the Fe/Cu ratio of the two precursors is smaller than 2:1, the Cu-ferrite is an irregular particle with very small aspect ratio and no well-defined facets. Increasing the ratio near to 2:1 yields rodlike particles. Energy-dispersive X-ray spectroscopy (EDX) analysis of the NRs showed that Fe, Cu, and O were the main elemental components (Figure 3). With use of the two precursors at 2:1 (Fe/Cu), a typical array of the Cuferrite NRs analyzed as Fe ) 33.66, Cu ) 6.16, O ) 58.53, and C ) 1.65 atom %. The Fe/Cu ratio of as-produced rods is only 5:1; the NRs in the array are the Cu0.5Fe2.5O4 initial formula. Thus, although the Cu2+ is required to initiate precipitation, the ferrite growth appears more efficient for iron ions under the experimental conditions used. The superfluous oxide and carbon impurity in Cu-ferrite NRs showed that there were EG and hydroxide ions on the surface of the NRs. As shown in Figure 4, X-ray diffraction patterns of the asdeposited sample on the Cu substrate indicate that the asdeposited NRs mainly possess a structure with good tetragonal crystallinity and also some small diffraction peaks of any other minerals were detected. The Cu-ferrite NRs deposited on the Cu substrate exhibit four obvious diffraction peaks in the (112),
Huang et al.
Figure 2. SEM images of (a) the aligned arrays of Cu-ferrite rods prepared on Cu substrate and (b) cross sectional view of the aligned Cu-ferrite rods.
Figure 3. Energy-dispersive X-ray spectroscopy (EDX) of the Cuferrite nanorods.
(202), (220), and (204) planes, with higher intensity in the (204) plane, revealing the (204) oriented growth of the Cu-ferrite NRs. We consider that several small peaks not according with the card file may be ones of γ-Fe2O3 due to the oxidation of HCHO from (CH2)6N4. These results show that the as-deposited arrays are ones of Cu-ferrite NRs with a little of ferric oxide nanoparticles. Figure 5a shows the TEM images of several small NRs obtained from the array Cu-ferrite nanostructures treated by long time ultrasound. The selected area electron diffraction (SAED) pattern inserted in Figure 5a indicates that the nanorod is well crystallized and can be indexed as the tetragonal Cuferrite phase, which corresponds to XRD results in Figure 4.
Aligned Arrays of Cu-Ferrite Nanorods
Crystal Growth & Design, Vol. 6, No. 8, 2006 1933
Figure 4. XRD pattern of the arrays of CuFe2O4 nanorods prepared by EG-assisted coprecipitation process. The solid lines indicate the XRD pattern of powder CuFe2O4 (JCPDS Card File 34-0425). The / and ∧ symbols represent CuFe2O4 and γ-Fe2O3, respectively.
Figure 5b,c shows the typical high-resolution transmission electron microscopy (HRTEM) images of the individual Cuferrite NRs. The images clearly reveal that only the fringes of the (204) planes with a lattice spacing of about 1.80 Å can be observed, indicating that the Cu-ferrite NR has a good crystal structure, which is consistent with the SAED pattern inserted in Figure 5a. The presence of the hydroxide and ether groups of EG on the surface of NRs was also verified by IR spectra in the OH or -O- stretching region. A broad absorption band centered at ca. 3400 cm-1 and a small absorption peak at 1080 cm-1 was observed for all of the material (data not shown). The hydrophilic outer shell of EG could enhance the cellular absorption on the surface of nanomaterials.7 Mechanism for Nanorod Growth. It is found that, for the (iron-EG)-(copper-HMTA) complex system, an induction period is required for the development of a brown-yellow precipitate when the reactants have been mixed together. To examine the function of HMTA in the formation of Cu-ferrite nanorods, we used urea instead of HMTA to hydrolyze and release OH-. Figure 6a illustrates SEM images of these samples. When no HMTA was used, many Cu-ferrite microspheres composed of sheets were grown on the substrate. The CuFe2O4 particles may originate from the heterogeneous deposition of Cu-ferrite on the substrate at the initial stage of thermolysis of the iron-EG complex. When HMTA was added into the Cu2+ solution, the color of system changed to brown, showing that the copper-HMTA complex was formed. During the heating process to 85 °C, HMTA will hydrolyze and release OH- into the solution, leading to the increase of both Fe(OH)42and Cu(OH)42-. So the heterogeneous precipitation of Fe(OH)2‚ Cu(OH)2 on the substrate can be effectively prevented in the initial stage of the induced process. Formaldehyde in the reaction system released by HAMT can oxidize some Fe2+ to Fe3+ ions during the coprecipitation. It is considered that the Fe(OH)3‚ Cu(OH)2 complex can form chains of [Cu(HMTA)OH2]n‚ [Fe(EG)OH3]m by di-dentate coordination linking tetrahedral ferric ions. The structure can act as the nucleation center and serve as the organic template. Furthermore, HMTA-H4 molecules may also help to form twin crystal Fe3O4 nuclei during nucleation process10 on the surface of glass substrate, and the budding crystals can be formed at the junction interface. Then, these budding crystals grow and form Cu-ferrite NRs, resulting in the formation of the well-aligned arrays of Cu-ferrite NRs. In our case, we also observed some NRs (see Figure 7) that are formed out of bundling of smaller diameter NRs. It is
Figure 5. (a) TEM image of the NRs, (b) low-magnification HRTEM image of a NR, and (c) HRTEM image of the NR from the boxed portion of the image in panel b. The lower left inset in panel a corresponds to the SAED pattern of the relative Cu-ferrite nanorod.
believed that thin diameter NRs grow as a bundle individually initially and eventually coalesce to form large-diameter rods that can lower their surface energy. Since the reaction temperature was low (80 °C) for the experiment corresponding to Figure 7, a high concentration of alkali (due to lower evaporation at lower temperature) might have resulted in Cu-ferrite rods with large width on the order of 500 nm. According to our experimental results, the proposed growth mechanism of crystalline rhombic rods of Cu-ferrite is shown in Scheme 1. Step a is the growth process of fine Cu-ferrite rods that bundle together. Steps b and c are the growth steps of rhombic rods of Cu-ferrite via the coalescence mode. Effect of EG. The iron-EG complex forms when the molar ratio of EG to Fe2+ is higher than 8. To examine the function of EG in the deposition process of Cu-ferrite NRs, the morphologies of two control samples without EG and with lower iron-EG ratio (1:8) were compared with the typical sample. When no EG was added, sheet-like microparticles instead of
1934 Crystal Growth & Design, Vol. 6, No. 8, 2006
Huang et al. Scheme 1. Growth Mechanism of Cu-Ferrite Rodsa
a (a) NRs grow as a bundle but are independent, (b) NRs begin to coalesce, and (c) a single NR results.
Figure 8. Magnetic hysteresis curves of NRs for field applied parallel, H| (s), or perpendicular, H⊥ (- - -), to the NR axis. The inset is low-field enlargements of the hysteresis curves.
Figure 6. SEM images of CuFe2O4 structures with different precursors: (a) Fe-Cu-urea-EG ratio of 1:0.5:5:16 (the inset is a closeup view of the microsphere); (b) Fe-Cu-HMTA ratio of 1:0.5:10 without EG; (c) Fe-Cu-HMTA-EG ratio of 1:0.5:5:8.
Figure 7. SEM image of Cu-ferrite rods obtained after reaction at 80 °C for 1 h. Here bundles of NRs are clearly visible.
the NRs were obtained (Figure 6b). When EG was used with an iron-EG ratio of 1:8, which means that the amount of EG is not enough to complex all the iron ions in the precursor, arrays of Cu-ferrite sheet-like and dagger-like microparticles with 2-3
aspect ratio were obtained, as illustrated Figure 5c. All the obtained Cu-ferrite particles in Figure 6b exhibit larger diameter and smaller aspect ratio. These results indicate that the chelating agent EG and proper iron-EG ratio are key factors for the formation of Cu-ferrite NRs. The Cu-ferrite crystal is a polar crystal whose positive plane is rich in iron and copper ions and negative polar plane is rich in O2-. In the reaction, the negative nature of the [Fe(OH)4]2- will lead to the different growth rates of planes. When there is no organic additive in solution, spherical Cu-ferrite particles will be easily developed because of the Ostwald ripening process. When EG is present in the aqueous solution, EG will hydrate and form EG-H2, which bears two positive charges. Thus, by the coulomb interaction,11 EG-H2 molecules will adsorb on the negative polar planes retarding the growth rate of these planes. However, the budding crystals from the junction interface do not adsorb enough EGH2 and develop into the top surface of the NR, while other surfaces of the NRs have been covered by enough EG-H2 molecules, thus enhancing the growth along the (204) plane greatly, and their hydrophilicity results in the formation of NRs with uniform thickness and high aspect ratios. Therefore, the concentration of EG in the precursor has significant effects on the morphology of Cu-ferrite crystals. Magnetic Measurements. Shown in Figure 8 are magnetic hysteresis curves of NRs for fields applied parallel and perpendicular to the NR axis, which were measured at room temperature. It can be seen that when the field is applied perpendicular to the rods, the hysteresis curves exhibit a small coercivity (HC), a small remanence (MR), and large saturation field (i.e., the field necessary to reach the saturation magnetization, MS). Contrarily, when the field is applied parallel to the wires, HC and MR become larger, and the different systems reach MS at rather low fields. This is the behavior typical of a hard and easy axis hysteresis loop, respectively. Hence, this indicates that the easy axis of the system is along NRs of the Cu-ferrite,
Aligned Arrays of Cu-Ferrite Nanorods
which is characteristic of polycrystalline NRs where the shape anisotropy dominates over the intrinsic magnetocrystalline anisotropy and thus dictates the magnetic behavior of the system. MS of 10.2 emu/g in the parallel field is lower than the 35.6 emu/g of the spinel ferrite CuFe2O4 nanoparticles12. This may result from the significant amount of organics, which lowers the volume ratio of the magnetic phase. Conclusions In summary, the array of rhombic magnetic Cu-ferrite nanorods has been prepared by an EG and HMTA-assisted solgel process at 85 °C on a Cu substrate. The rhombic Cu-ferrite nanorods were tetragonal phase and well crystallized in nature. It is considered that HMTA can interact with growth units of Cu-ferrite to generate active sites on the interface of twin crystal Fe3O4 nuclei so that rhombic Cu-ferrite nanorods could grow into the formation of the arrays. A lower saturation magnetization was observed in Cu-ferrite nanorods due to the significant amount of organics coating the NRs. The EG-coated nanorods are suited for conjugation to a target cell or enzyme for the preparation artificial organs. Acknowledgment. We gratefully acknowledge China Postdoctor Science Foundation (Grant No. 2004036305) and Chinese Academy of Sciences K. C. Wong Postdoctoral Fellowships for financial support of this project. We thank Professor L. Jiang of ICCAS for his constant instruction and support. Thanks also to Professor F. Tang of Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, for her support and encouragement.
Crystal Growth & Design, Vol. 6, No. 8, 2006 1935
References (1) (a) Wang, J. F.; Gudiksen, M. S.; Duan, X. F.; Cui, Y.; Lieber, C. M.; Science 2001, 293, 1445. (b) Kind, H.; Yan, H. Q.; Messer, B.; Law, M.; Yang, P. D. AdV. Mater. 2002, 14, 158. (2) (a) Peng, X.; Manna, L.; Wang, W.; Wichham, J.; Scher, E.; Kadavanich, A.; Alivisatos, A. P. Nature 2000, 404, 59. (b) Hu, J.; Li, L. S.; Yang, W.; Manna, L.; Wang, L.-W.; Alivisatos, A. P. Science 2001, 292, 2060. (3) (a) Melosh, N. A.; Boukai, A.; Diana, F.; Gerardot, B.; Badolato, A.; Petroff, P. M.; Heath, J. R. Science 2003, 300, 112. (b) Fan, R.; Wu, Y. Y.; Li, D. Y.; Yue, M.; Majumdar, A.; Yang, P. D. J. Am. Chem. Soc. 2003, 125, 5254. (4) (a) Ren, Z. F.; Huang, Z. P.; Xu, J. W.; Wang, J. H.; Bush, P.; Siegal, M. P.; Provencio, P. N. Science 1998, 282, 1105. (b) Wang, X. D.; Summers, C. J.; Wang, Z. L. Nano Lett. 2004, 4, 423. (5) Yang, P. D. Nature 2003, 425, 243. (6) (a) Li, D.; Xia, Y. N. AdV. Mater. 2004, 16, 1151. (b) Li, D.; Wang, Y. L.; Xia, Y. N. Nano Lett. 2003, 3, 1167. (7) (a) Zeng, H.; Rice, P. M.; Wang, S. X.; Sun, S. H. J. Am. Chem. Soc. 2004, 126, 11458. (b) Kumar, A.; Gupta, M. Biomaterials 2005, 26, 1565. (8) (a) Sun, S.; Zeng, H. J. Am. Chem. Soc. 2002, 124, 8204. (b) Sun, S.; Zeng, H.; Robinson, D. B.; Raoux, S.; Rice, P. M.; Wang, S. X.; Li, G. J. Am. Chem. Soc. 2004, 126, 273. (9) (a) Vies, M.; Kolinsky´, V.; Visˇnˇovsky´, Sˇ .; Kulkarni, P. D.; Desai, M.; Venkataramani, N.; Prasad, S. Krishnan, R. J. Magn. Magn. Mater. 2004, e885-e886, 272. (b) Kim, K. J.; Lee, J. H.; Lee, H. J. Magn. Magn. Mater. 2004, 279, 173. (10) Huang, Z. B.; Zhang, Y. Q.; Tang, F. Q. Chem. Commun. 2005, No. 3, 342. (11) Cao, M. H.; Hu, C. W.; Peng, G.; Qi, Y. J.; Wang, E. B. J. Am. Chem. Soc. 2003, 125, 4982. (12) Goya, G. F.; Rechenberg, H. R.; Jiang, J. Z. J. Magn. Magn. Mater. 2000, 218, 221.
CG0505517
