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Preparation of One-Dimensional CoFe2O4 Nanostructures and Their Magnetic Properties Zhongli Wang, Xiaojuan Liu, Minfeng Lv, Ping Chai, Yao Liu, Xianfeng Zhou, and Jian Meng* State Key laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, 130022, People’s Republic of China and Graduate UniVersity of Chinese Academy of Sciences, Beijing, 100049, People’s Republic of China ReceiVed: March 26, 2008; ReVised Manuscript ReceiVed: July 4, 2008
Cobalt ferrite one-dimensional nanostructures (nanoribbons and nanofibers) were prepared by electrospinning combined with sol-gel technology. The nanoribbons and nanofibers were formed through assembling magnetic nanoparticles with poly(vinyl pyrrolidone) (PVP) as the structure-directing template. Nanoribbons and nanofibers were obtained after calcining the precursor nanoribbons at different temperatures. Successive Ostwald ripening processes occur during the formation of CoFe2O4 nanoribbons and nanofibers. The sizes of nanoparticles varied with calcination temperatures, which leads to different one-dimensional structures and variable magnetic properties. These novel magnetic one-dimensional structures can potentially be used in nanoelectronic devices, magnetic sensors, and flexible magnets. 1. Introduction In the past decade, more and more attention has been drawn to the fabrication of one-dimensional (1D) nanomaterials, including nanotubes, nanowires, nanofibers, and nanobelts, because they show some distinctive properties compared with their bulk or particle counterparts. Particularly, ordered 1D magnetic nanomaterials have been extensively exploited for their potential utilization as active components for ultrahigh-density data storage, as well as in the fabrication of sensors and spintronic devices.1-4 For example, nanoscale magnetic logic junctions have recently been fabricated with ferromagnetic nanowires as building blocks.1 Many attempts have been made to the design and synthesis of such 1D magnetic nanomaterials in order to explore their properties and potential applications. Magnetic metal nanowires such as Fe, Co, and CoPt have been synthesized by various methods including template-assisted electrodeposition,5 catalyzed high-temperature growth via the vapor-liquid-solid mechanism,6 and solvothermal synthesis.7 One-dimensional Fe3O4 nanostructures were also obtained by magnetic fieldinduced self-assembly8 and pulsed laser deposition.9 As a kind of important magnetic material, 1D cobalt ferrite has attracted considerable attention due to its large magnetocrystalline anisotropy, high coercivity, moderate saturation magnetization, large magnetostrictive coefficient, chemical stability, and mechanical hardness. For example, CoFe2O4 nanowires and nanotubes have been synthesized with templates such as anodic aluminum oxide (AAO)10 and carbon nanotubes.11,12 Onedimensional chains of CoFe2O4 nanoparticles have also been obtained using a DNA guide.13 However, developing new and simple preparation techniques is still of great scientific and commercial interest. Electrospinning is a simple and effective method for fabricating ultrathin nanofibers. In a typical procedure, a high voltage is applied to a droplet of polymer solution that rests on a sharp conduction tip. With a sufficiently high electrical field, the * Corresponding author. E-mail:
[email protected]. Phone: +86-43185262030. Fax: +86-431-85698041.
electrostatic forces can overcome the surface tension of the polymer solution and thus cause the ejection of a thin jet from the tip. The charged jet then undergoes a stretching and whipping processing, resulting in the formation of many continuous fibers.4,14 Initially, this technique was utilized for the electrospinning of polymers. Recently, such technique was extended to the synthesis of nanofibers containing inorganic constituents such as ZnO, TiO2, SnO2, and WO3 oxide nanofibers.15-20 Electrospinning of nanofibers with magnetic properties has also met with some success. For example, magnetic nanofibers of ferrites MFe2O4 (M ) Co, Ni) have been prepared by electrospinning.3,15 In this study, we synthesized 1D CoFe2O4 nanostructures by applying simpler electrospinning conditions and investigated their variation of magnetic properties with calcination temperature in detail. Two different morphologies (nanoribbons and nanofibers) were also obtained by controlling the calcination temperature of the precursor. 2. Experimental Section 2.1. Preparation. The electrospinning solution is obtained from cheap inorganic salts based on a simple sol-gel method. In a typical procedure, 1 g of Fe(NO3)3 · 9H2O and 0.3052 g of Co(CH3COO)2 · 4H2O were dissolved into a mixed solvent containing 10 mL of absolute ethanol and 4 mL of H2O under magnetic stirring, and then 1 g of poly(vinyl pyrrolidone) (PVP, Mw ) 1 300 000) was added into the above solution. A transparent solution could be formed after 12 h of stirring. The viscous solution thus obtained was drawn into a hypodermic syringe. The positive terminal of a variable highvoltage power supply was connected to the needle tip of the syringe while the other was connected to the mirror-looking collector plate. The positive voltage applied to the tip was 30 kV, and the distance between the needle tip and the collector was 15 cm.21 When the spinning was completed, the as-prepared fibers were dried at 80 °C for 12 h. Then they were calcined in air at different temperatures from 500 to 900 °C with a heating rate of 2 °C/min. 2.2. Structure and Magnetic Characterization. Thermogravimetric (TG) and differential thermal analysis (DTA) were
10.1021/jp802614v CCC: $40.75 2008 American Chemical Society Published on Web 08/27/2008
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Figure 2. XRD patterns for CoFe2O4 nanoribbons and nanofibers calcined in air at different temperatures: (a) 500, (b) 600, (c) 700, and (d) 900 °C.
Figure 1. SEM image (a) and TG-DTA curve (b) of precursor nanoribbons PVP/Fe(NO)3, Co(Ac)2.
carried out on a TA SDT 2960 simultaneous thermal analyzer in air with a heating rate of 10 °C/min. X-ray diffraction (XRD) patterns were recorded on a Rigaku-D/max 2500 V X-ray diffractometer equipped with a source of Cu KR radiation (λ ) 1.54178 Å) at a step width of 0.02°. The morphologies and structures of the as-synthesized ferrite products were observed with field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). The magnetic properties of the samples were investigated by a superconducting quantum interference device (SQUID) magnetometer at 300 and 2 K, respectively. 3. Results and Discussion 3.1. Morphological Characteristic and Thermal Property of the Precursor. The SEM image of precursor nanoribbons is shown in Figure 1a. The ribbons collected aligned in random orientation because of the bending instability associated with the spinning jet. The length of the ribbons can reach to several hundred micrometers. As observed, each individual ribbon was uniform in cross section with the average width of 500 nm and thickness of 50 nm (inset of Figure 1a). The surface of the ribbons is smooth due to the amorphous nature of the PVP. The result of TGA-DTA of the as-spun composite nanoribbons is shown in Figure 1b. Most of the organic material (PVP), NO3-, CH3COO- groups of inorganic salts, and the other
Figure 3. SEM images of CoFe2O4 nanoribbons and nanofibers calcined in air at different temperatures: (a) 500, (b) 600, (c) 700, and (d) 900 °C.
volatiles (H2O, ethanol, etc.) were removed below 400 °C. The exothermic peak at 268 °C in the DTA curve corresponded to the decomposition of inorganic salts and the degradation of PVP, which had two degradation mechanisms involving both intraand intermolecular transfer reactions.22 3.2. Structure Analysis and Morphological Characteristics. The XRD patterns of composite nanoribbons calcined at different temperatures are shown in Figure 2. The diffraction peaks and relative intensities of all patterns match well with a cubic spinel structure (JCPDS 03-0864, space group: Fd3jm). No impurity phase was found in all the patterns. The peak intensity increases, and the full width of the peak decreases, with the increase of calcination temperature, which indicates the crystallinity is improved and the crystallites become large. Figure 3 shows the SEM images of the synthesized CoFe2O4 nanoribbons and nanofibers (the four samples are denoted nanoribbons500, nanoribbons600, nanoribbons700, and nanofibers900). At different calcination temperatures, 1D structures remained but morphologies changed a lot. The nanoribbons calcined at 500 and 600 °C have a smooth surface, with reduced width of 300 nm and thickness of 30 nm. When the calcination
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Figure 5. Dependence of the crystallite sizes on the calcination temperature.
Figure 4. TEM images of CoFe2O4 nanoribbons and nanofibers calcined in air at different temperatures: (a) 500, (b) 600, (c) 700, and (d) 900 °C.
temperature increased to 700 °C, small particles gradually grow into big particles and individual particle is clearly observed (Figure 3c). So the width of nanoribbons exhibits shrinkage, and it decreases to 200-250 nm. The surface of them also becomes rough due to the irregular growth of the particles at this temperature. The most obvious change arises at 900 °C. In Figure 3d, the nanoribbons were shrunk into nanofibers with formation of bigger particles at high temperature, and the diameters of such nanofibers are in the range of 100-150 nm. The microstructures of the samples were further investigated by TEM. Figure 4 shows TEM images of CoFe2O4 nanoribbons and nanofibers after calcination at different temperatures, indicating that the 1D CoFe2O4 nanostructures were formed through the agglomeration of nanoparticles. The sizes of particles increased, and the distribution of size became broad, with increasing calcination temperature. Figure 4a shows nanoribbons that were calcined at 500 °C. Clearly, the particle size is very small and relatively uniform with average size of 15 nm. Figure 4b shows nanoribbons calcined at 600 °C. Although these nanoribbons are similar to those annealed at 500 °C, the grain size has grown larger and increases to 40-50 nm. At 700 °C (Figure 4c), the nanoparticles are much larger and the number of particles is much lower, indicating a ripening effect. The distribution of particle size becomes broad, which ranges from 50 to 80 nm. In order to maintain the continuous structure, some small particles exist among big ones (Supporting Information Figure S2). The selected area electron diffraction patterns (insets of Figure 4a-c) of the synthesized nanoribbons showed polycrystalline rings, and the rings became faint with calcination temperature, which were all indexed for cubic spinel structures. At 900 °C, the size of nanoparticles increases to
around 100 nm. As clearly seen from Figure 4d, the annealed nanofibers consist of individual CoFe2O4 nanocrystals arranged linearly. The individual crystals were bound to each other to form a stable 1D structure. From the inset of Figure 4d, the electron diffraction pattern from one individual crystal was observed: the indexed diffraction pattern for face-centered cubic (fcc) crystals in the [011] beam direction. The growth process can be viewed as a morphologically templated nucleation process, which allows seamless transformation of amorphous precursor into the corresponding crystalline 1D structures with conformity of the precursor morphology. Successive Ostwald ripening processes occur during the formation of CoFe2O4 nanoribbons and nanofibers.23 Figure 5 shows the effect of calcination temperature on the main crystallite sizes. This is in agreement with the results of XRD. Different calcination temperatures lead to different sizes of nanoparticles which formed different morphologies. 3.3. Magnetic Characterization. The magnetic behavior of a nanoparticles assembly varies with the morphology of the particles (particle size and shape) and is strongly affected also by interparticle interactions. The magnetic interactions can be due to dipolar coupling and exchange coupling among nanoparticles surface atoms and play a fundamental role in the physics of these systems.24,25 Figure 6 shows the hysteresis loops of CoFe2O4 nanoribbons and nanofibers at room temperature (300 K) and low temperature (2 K). The variation of saturation magnetization (Ms) with calcination temperature is shown in Figure 7a. The Ms of CoFe2O4 nanofibers calcined at 900 °C is 76.2 emu/g, and it is 85.2 emu/g at 300 and 2 K, respectively, close to the bulk values of 80.8 and 93.9 emu/g.26 The saturation magnetization decreases with decreasing calcination temperature. CoFe2O4 1D structures are assembled from nanoparticles, so their magnetic properties depend on the properties of CoFe2O4 nanoparticles and the interaction among them. The size of nanoparticles decreases with decreasing calcination temperature (Figure 5). The reduction of the saturation magnetization for 1D polycrystalline structures could be attributed to noncolliner magnetic structure of CoFe2O4 nanoparticles consisting of a core with the usual spin arrangement and a boundary surface layer with disordered atomic moments.27,28 The existence of a spinglass shell in ferrite nanoparticles can be rationalized by the broken exchange bonds at its surface and variations in coordination of surface cations.29 As the particle size decreases, the contribution from the surface is more important giving rise to a decrease in the Ms. Figure 7b shows the variation of reduced remanence, Mr/Ms, at 300 and 2 K with calcination temperature. The remanence versus saturation magnetization ratio drastically decreases with measure temperature from 2 to 300 K. At 300
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Figure 6. Magnetic properties of the electrospun CoFe2O4 nanoribbons and nanofibers: (a) at room temperature (300 K); (b) at low temperature (2 K).
K, the values of Mr/Ms are very low. This consists with the previous results.30,31 At 2 K, the Mr/Ms for nanoribbons500 is 0.75, close to the theoretical value 0.83. The Mr/Ms increases with calcination temperature, and interestingly, the Mr/Ms for nanoribbons600 is equal to the theoretical value and the Mr/Ms for nanoribbons700 and nanofibers900 are 0.86 and 0.89, respectively, which are higher than the theoretical value. This indicates that such 1D nanostuctures consist of randomly oriented equiaxial particles with cubic magnetic anisotropy.32 The evolution of the coercivity with calcination temperature is shown in Figure 7c. At room temperature, a maximum coercivity is observed in nanoribbons700 corresponding to the particle size of around 70 nm, which is bigger than the critical size of 40 nm for single-domain to multidomain transition behavior of cobalt ferrite.33-35 Usually, the coercivity should reach a maximum at the critical size. But in such preparation method, it is hard to control the uniformity of the sizes of particles, especially for the nanoribbons calcined at the medium temperatures of 600 and 700 °C. In order to maintain the continuous 1D structures, some small particles exist among big ones (Supporting Information Figure S4, parts d and f). At the same time, the interaction between particles plays a fundamental role. So the coercivity changes observed for the different samples at room temperature could be due to the shape, size, and aggregation effect of particles in the 1D structures. At 2 K, the evolution of coercivity is different since it deceases monotonically with increasing calcination temperature. The largest coercivity (1.69 T) observed for nanoribbons500 consisting of the smallest particles indicates an increase in the effective anisotropy, which should be attributed to surface effects. The
Figure 7. Variation of saturation magnetization (Ms) (a), reduced remanence (Mr/Ms) (b), and coercivity (c) with calcination temperature.
strong increase in the anisotropy with the decrease in size is due to surface anisotropy. As the particle size increases, the surface anisotropy decreases, which leads to the decrease of coercivity at low temperature. Interestingly, the obvious change happens at the nanoribbons calcined at 400 °C (Supporting Information Figures S1-3). For the nanoribbons400, the coercivity reaches 895 Oe, which is higher than the other samples at room temperature, but at low temperature, the value of coercivity decreases much compared with the nanoribbons500. From the TEM images (Supporting Information Figure S4), it can be clearly observed that the particles within the nanoribbons400 are very small and the sizes are very uniform (mainly in the range of 7-9 nm), which leads to relatively close-grained aggregation. Such structure enhances the interaction between nanoparticles at room temperature and reduces surface anisotropy at low temperature. The saturation magnetization of
One-Dimensional CoFe2O4 Nanostructures nanoribbons400 is also higher than that of nanoribbons500. Therefore, the size and uniformity of particles play an important role in the magnetic properties. 4. Conclusion Cobalt ferrite 1D nanostructures (nanoribbons and nanofibers) were prepared by electrospinning combined with sol-gel technology. The nanoribbons and nanofibers were formed through assembling magnetic nanoparticles with PVP as the structure-directing template. The effects of the calcination temperature on the 1D structures of the ferrite samples were investigated. Nanoribbons were obtained after calcining the precursor nanoribbons at 400-700 °C. At high temperature (900 °C), nanoribbons were shrunk into nanofibers. Particles sizes in the 1D structures increased with the calcination temperature. The magnetic properties of nanoribbons and nanofibers were strongly affected by the shape, size, and packaging effect of particles in the 1D structures. Recently, it is reported that 1D chains of CoFe2O4 nanoparticles have a magnetoresistance effect.13 This effect could be better in cobalt ferrite 1D nanostructures. So these novel magnetic 1D structures can potentially be used in nanoelectronic devices, magnetic sensors, and flexible magnets. Acknowledgment. This project was supported by the National Natural Science Foundation of China (Grant Nos. 20671088, 20601026, and 20771100). Supporting Information Available: TEM images of CoFe2O4 nanoribbons calcined in air at different temperatures from 400 to 700 °C, XRD patterns and SEM images for CoFe2O4 nanoribbons calcined in air at 400 °C, and variation of coercivity with calcination temperature from 400 to 900 °C. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Allwood, D. A.; Xiong, G.; Cooke, M. D.; Faulkner, C. C.; Atkinson, D.; Vernier, N.; Cowburn, R. P. Science 2002, 296, 2003. (2) Thurn-Albrecht, T.; Schotter, J.; Kastle, G. A.; EmLey, N.; Shibauchi, T.; Krusin-Elbaum, L.; Guarini, K.; Black, C. T.; Tuominen, M. T.; Russell, T. P. Science 2000, 290, 2126. (3) Li, D.; Herricks, T.; Xia, Y. Appl. Phys. Lett. 2003, 83, 4586. (4) Wu, H.; Zhang, R.; Liu, X.; Lin, D.; Pan, W. Chem. Mater. 2007, 19, 3506.
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