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Polymer-controlled synthesis of Fe3O4 single-crystal nanorods
Journal of Colloid and Interface Science 278 (2004) 372–375 www.elsevier.com/locate/jcis
Polymer-controlled synthesis of Fe3 O4 single-crystal nanorods Lin Feng a,b , Lei Jiang b , Zhenhong Mai a,∗ , Daoben Zhu b a Soft Material Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100080, People’s Republic of China b Center for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, People’s Republic of China
Received 11 December 2003; accepted 2 June 2004 Available online 31 July 2004
Abstract In this study, we describe a simple approach to preparing single-crystal Fe3 O4 nanorods in the presence of poly(vinylpyrrolidone) (PVP). The morphologies of the nanorods are characterized by transmission electron microscopy (TEM), which indicates that these nanorods are formed by nucleation and growth process in situ in aqueous solution. A superconducting quantum interference device (SQUID) magnetometer characteristic of the as-synthesized Fe3 O4 nanorods shows superparamagnetic properties. 2004 Elsevier Inc. All rights reserved.
1. Introduction The fabrication or synthesis of nanometer-sized onedimensional (1D) materials has been the focus of considerable interest because of their potential applications in nanoconnectors and nanodevices [1]. To possess enhanced electrical, magnetic, optical, and mechanical properties, these 1D materials need to be of small diameter, high aspect ratio, and uniform orientation [2]. Since the discovery of carbon nanotubes in 1991 [3], many attempts have been made to fabricate 1D nanostructure materials such as carbon nanotubes, metals, semiconductors, and conductive polymer nanorods or nanowires [4–7]. Among these, we also succeeded in preparing aligned 1D carbon nanotubes and polymer nanofibers, whose surfaces show special wettability [8–12]. As Fe3 O4 is one of the most important magnetic materials, having vital applications in magnetic recording media, soft magnetic materials, color imaging, magnetic refrigeration, and ferrofluids [13], the preparation of Fe3 O4 nanoparticles has been investigated. However, no attention has been paid to the synthesis of 1D Fe3 O4 nanorods. Recently, a number of chemical approaches have been successfully explored to prepare 1D nanostructures, among * Corresponding author. Fax: +861082640224.
E-mail address: [email protected] (Z. Mai). 0021-9797/$ – see front matter 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2004.06.019
which the template-based synthetic method has been proved to be a powerful one [14]. This technique entails preparing the desired material within various types of 1D structures. The use of physical templates has proved able to ensure good control over the morphology of the final products. However, it may complicate the synthetic procedure and limit the scale at which a material can be processed in each synthesis, and dissolution of the templates in corrosive media is often required to retrieve the desired 1D nanostructures. To overcome these difficulties, solution-phase methods that involve no physical templates are developed. For example, Murphy et al. [15] reported a seed-mediated growth approach to make metallic nanorods and nanowires in aqueous solution. Xia and co-workers [16] developed a solution-phase method to generate silver nanowires by introducing exotic seeds (e.g., platinum nanoparticles) or using a self-seeding process. Park’s group demonstrated the synthesis of singlecrystalline barium titanate nanowires using a solution-based method and investigated their ferroelectric properties by scanning probe microscopy [17]. Peng discussed monomer concentration in the growth solution as the determining factor in shape control and shape evolution of colloidal semiconductor nanocrystals [18]. Here, we describe a simple approach to the preparation of Fe3 O4 nanorods in the presence of poly(vinylpyrrolidone) (PVP). The nucleation and growth process are in situ in
L. Feng et al. / Journal of Colloid and Interface Science 278 (2004) 372–375
373
aqueous solution, and no other solvents are needed. Fe3 O4 short nanorods with lower aspect ratio were initially formed by typical chemical coprecipitation of FeIII and FeII salts in alkaline solution [19]. In the process of refluxing, some of the larger short nanorods will grow at the expense of smaller ones, i.e., Ostwald ripening [20]. With the help of PVP, which can probably control the growth rates, the Fe3 O4 short nanorods can be directed to grow into Fe3 O4 nanorods with higher aspect ratio. The as-synthesized Fe3 O4 nanorods have superparamagnetic properties.
2. Experimental A mixture of 0.050 g FeCl2 · 4H2 O and 0.135 g FeCl3 · 6H2 O were dissolved in 100 ml Milli-Q water containing 1 g poly(vinylpyrrolidone). Then 0.634 g NaOH was dissolved in 100 ml water to form a solution with pH 13.2, which was dropwise added to 30 ml mixture solution of Fe3+ and Fe2+ in N2 . The reaction was allowed to proceed for 2 h at 80 ◦ C under constant and vigorous stirring in reflux equipment. Subsequently, the reactant was cooled slowly to room temperature and the pH decreased to 6.8. Precipitates and supernatant fluid were separated by magnet and the precipitations were washed several times with deionized water. The precipitations were dried in air for the purpose of characterization by XRD measurements. TEM studies are performed with transmission electron microscopy (JEOL JEM 200 CX) operating at 200 eV by placing a drop of the suspension solution carefully onto a carbon-coated copper grid and letting the water evaporate slowly in air. XRD pattern is collected using a M18 AHFX CuKα radiation X-ray diffractometer over the range of 20◦ 2θ 80◦ . Magnetic property measurement is carried out on a SQUID magnetometer (Quantum Design MPMS-7).
3. Results and discussion Fig. 1 is the X-ray diffraction (XRD) pattern of the asprepared nanorods. All the peaks in this figure are in good agreement with the standard taken from powder diffraction file 19-629 of the Database of the International Center for Diffraction Data (ICDD), which confirms that the nanorods prepared in this study are the magnetite nanocrystals. The Bragg reflection peaks are all relatively broad because of the small dimensions of the Fe3 O4 nanocrystals. The transmission electron microscopy (TEM) image in Fig. 2 demonstrated the time evolution of the morphology of the Fe3 O4 nanostructures with different aspect ratios. The aspect ratio of a shape is defined as the length of the major axis divided by the width of the minor axis [15]. Spheres have an aspect ratio of 1. Murphy et al. defined nanorods as materials that have a width of ∼1–100 nm and aspect ratios greater than 1 but less than 20, and
Fig. 1. X-ray diffraction pattern of Fe3 O4 nanorods.
nanowires as materials that have aspect ratios greater than 20. In addition, most people would call “short nanorods” analogous materials that have aspect ratio greater than 1 but less than 5. Accordingly, Fig. 2a shows the initial product, Fe3 O4 short nanorods that have an average width of about 11.8 nm and a length of about 47.1 nm. The aspect ratio can therefore be calculated to be 4. These short nanorods would serve as seeds on which to grow more anisotropic nanostructures. Herein, the presence of PVP macromolecules could chemically absorb onto the surfaces of Fe3 O4 short nanorods to prevent them from aggregating [21]. After 30 min with the solution constantly refluxed at 80 ◦ C, some of the larger Fe3 O4 short nanorods grow into rod-shaped structures based on the mechanism of Ostwald ripening in the presence of PVP (Fig. 2b). The seeds of Fe3 O4 short nanorods here serve as nucleation sites for nanorod growth. Typical product dimensions of the asgrown nanorods are about 18.2-nm short axes and 310.6-nm long axes; the aspect ratio is up to 17. The function of PVP in this process was to kinetically control the growth rates of various faces by interacting with these faces through adsorption and desorption [22]. After 2h, all the Fe3 O4 short nanorods are found to convert into nanorods (Fig. 2c). The electron diffraction pattern (the inset of Fig. 2c) obtained from the individual nanorods indicates that these nanorods of Fe3 O4 are single-crystalline. Fig. 2d shows a typical highresolution electron microscopy (HRTEM) image of the single nanorod in Fig. 2c. This image reveals that the {311} lattice fringes with lattice spacing around 0.25 nm are parallel to the axis of the nanorod and the growth plane is one of the {311} planes. The result further confirms that the as-synthesized Fe3 O4 nanorods are single-crystalline. The Fe3 O4 growth is achieved by the atom-by-atom growth of individual crystallites and not by agglomeration of particles formed in the solution phase. Previously, we demonstrated the individual monolayer-coated magnetic nanoparticles self-assemble into uniform spherical aggregates through π –π interaction, the interaction between individual particles is relatively weak, and the spherical aggregates can be
374
L. Feng et al. / Journal of Colloid and Interface Science 278 (2004) 372–375
Fig. 3. Magnetization vs applied field plots at 300 and 5 K for Fe3 O4 nanorods.
The magnetic properties of as-synthesized Fe3 O4 nanorods are investigated using a superconducting quantum interference device (SQUID) magnetometer. Typical magnetization curves as a function of applied field at room (300 K) and low temperature (5 K) are shown in Fig. 3, respectively. At 300 K, no hysteresis loop is seen, which indicates that both the retentivity and the coercivity of the particles are zero. This observation is consistent with superparamagnetic behavior [24,25]. According to many authors, the property is typical of nanostructures and is due to surface anisotropy and the presence of a dead magnetic layer [26]. As the temperature is lowered to 5 K, the magnetization of the nanorods increases and exhibits a symmetric hysteresis loop, indicating the transition from superparamagnetic to ferromagnetic behavior. In summary, single-crystal Fe3 O4 nanorods with uniform diameters have been successfully synthesized by a PVPcontrolled process. The nucleation and growth process are in situ in aqueous solution, and the aspect ratio of the assynthesized nanorods increased with time evolution. This formation process might be extendible as a model to other systems, which may prove useful in fabricating a variety of functional nanoscale devices.
Acknowledgments
Fig. 2. TEM images of three as-synthesized samples, showing different stages of rod growth: (a) 5 min; (b) 30 min; (c) 2 h; (d) HRTEM image of a single nanorod.
destroyed by sonication to leave monodispersed nanoparticles [23]. In comparison, the as-prepared Fe3 O4 nanorods in the present study are relatively stable; their structure cannot be destroyed by outside force such as sonication and stirring.
The authors thank the State Key Project Fundamental Research (G1999064504) and the Special Research Foundation of the National Nature Science Foundation of China (29992530, 20125102, 50072046, and 10274096) for continuing financial support. L. Feng thanks the China Postdoctoral Science Foundation (2003034218) and the K.C. Wong Education Foundation, Hong Kong (20031008155638).
References [1] G. Fasol, Science 280 (1998) 545.
L. Feng et al. / Journal of Colloid and Interface Science 278 (2004) 372–375
[2] J.D. Holmes, K.P. Johnston, R.C. Doty, B.A. Korgel, Science 287 (2000) 1471. [3] S. Iijima, Nature 354 (1991) 56. [4] D.D. Archibald, S. Mann, Nature 364 (1993) 430. [5] A.M. Morales, C.M. Lieber, Science 279 (1998) 208. [6] Z. Tang, N.A. Kotov, M. Giersig, Science 297 (2002) 237. [7] J. Ma, Y. Gu, L. Shi, L. Chen, Z. Yang, Y. Qian, Chem. Phys. Lett. 381 (2003) 194. [8] H. Li, X. Wang, Y. Song, Y. Liu, Q. Li, L. Jiang, D. Zhu, Angew. Chem. Int. Ed. 40 (2001) 1743. [9] L. Feng, S. Li, J. Zhai, Y. Song, L. Jiang, D. Zhu, Angew. Chem. Int. Ed. 41 (2002) 1221. [10] L. Feng, Y. Song, J. Zhai, J. Xu, L. Jiang, D. Zhu, Angew. Chem. Int. Ed. 42 (2003) 800. [11] L. Feng, Z. Yang, J. Zhai, Y. Song, B. Liu, Y. Ma, Z. Yang, L. Jiang, D. Zhu, Angew. Chem. Int. Ed. 42 (2003) 4217. [12] L. Feng, S. Li, Y. Li, H. Li, L. Zhang, J. Zhai, Y. Song, B. Liu, L. Jiang, D. Zhu, Adv. Mater. 14 (2002) 1857.
375
[13] J.L. Dormann, D. Fiorani, in: Magnetic Properties of Fine Particles, North-Holland, Amsterdam, 1991, pp. 309–423. [14] C.R. Martin, Science 266 (1994) 1961. [15] C.J. Murphy, N.R. Jana, Adv. Mater. 14 (2002) 80. [16] Y. Sun, B. Gates, B. Mayers, Y. Xia, Nano Lett. 2 (2002) 165. [17] J.J. Urban, J.E. Spanier, L. Ouyang, W.S. Yun, H. Park, Adv. Mater. 15 (2003) 423. [18] X. Peng, Adv. Mater. 15 (2003) 459. [19] L. Shen, P.E. Laibinis, T.A. Hatton, Langmuir 15 (1999) 447. [20] A.R. Roosen, W.C. Carter, Physica A 261 (1998) 232. [21] Z. Zhang, B. Zhao, L. Hu, J. Solid State Chem. 121 (1996) 105. [22] F. Bonet, K. Tekaia-Elhsissen, K.V. Sarathy, Bull. Mater. Sci. 23 (2000) 165. [23] J. Jin, T. Iyoda, C. Cao, Y. Song, L. Jiang, T.J. Li, Angew. Chem. Int. Ed. 40 (2001) 2135. [24] T. Nguyen, A.F. Diaz, Adv. Mater. 6 (1994) 858. [25] O. Jarjayes, P.H. Fries, G. Bidan, Synth. Met. 69 (1995) 343. [26] J.A.L. Pérez, M.A.L. Quintela, J. Mira, J. Rivas, S.W. Charles, J. Phys. Chem. B 101 (1997) 8045.
Polymer-controlled synthesis of Fe3 O4 single-crystal nanorods Lin Feng a,b , Lei Jiang b , Zhenhong Mai a,∗ , Daoben Zhu b a Soft Material Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100080, People’s Republic of China b Center for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, People’s Republic of China
Received 11 December 2003; accepted 2 June 2004 Available online 31 July 2004
Abstract In this study, we describe a simple approach to preparing single-crystal Fe3 O4 nanorods in the presence of poly(vinylpyrrolidone) (PVP). The morphologies of the nanorods are characterized by transmission electron microscopy (TEM), which indicates that these nanorods are formed by nucleation and growth process in situ in aqueous solution. A superconducting quantum interference device (SQUID) magnetometer characteristic of the as-synthesized Fe3 O4 nanorods shows superparamagnetic properties. 2004 Elsevier Inc. All rights reserved.
1. Introduction The fabrication or synthesis of nanometer-sized onedimensional (1D) materials has been the focus of considerable interest because of their potential applications in nanoconnectors and nanodevices [1]. To possess enhanced electrical, magnetic, optical, and mechanical properties, these 1D materials need to be of small diameter, high aspect ratio, and uniform orientation [2]. Since the discovery of carbon nanotubes in 1991 [3], many attempts have been made to fabricate 1D nanostructure materials such as carbon nanotubes, metals, semiconductors, and conductive polymer nanorods or nanowires [4–7]. Among these, we also succeeded in preparing aligned 1D carbon nanotubes and polymer nanofibers, whose surfaces show special wettability [8–12]. As Fe3 O4 is one of the most important magnetic materials, having vital applications in magnetic recording media, soft magnetic materials, color imaging, magnetic refrigeration, and ferrofluids [13], the preparation of Fe3 O4 nanoparticles has been investigated. However, no attention has been paid to the synthesis of 1D Fe3 O4 nanorods. Recently, a number of chemical approaches have been successfully explored to prepare 1D nanostructures, among * Corresponding author. Fax: +861082640224.
E-mail address: [email protected] (Z. Mai). 0021-9797/$ – see front matter 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2004.06.019
which the template-based synthetic method has been proved to be a powerful one [14]. This technique entails preparing the desired material within various types of 1D structures. The use of physical templates has proved able to ensure good control over the morphology of the final products. However, it may complicate the synthetic procedure and limit the scale at which a material can be processed in each synthesis, and dissolution of the templates in corrosive media is often required to retrieve the desired 1D nanostructures. To overcome these difficulties, solution-phase methods that involve no physical templates are developed. For example, Murphy et al. [15] reported a seed-mediated growth approach to make metallic nanorods and nanowires in aqueous solution. Xia and co-workers [16] developed a solution-phase method to generate silver nanowires by introducing exotic seeds (e.g., platinum nanoparticles) or using a self-seeding process. Park’s group demonstrated the synthesis of singlecrystalline barium titanate nanowires using a solution-based method and investigated their ferroelectric properties by scanning probe microscopy [17]. Peng discussed monomer concentration in the growth solution as the determining factor in shape control and shape evolution of colloidal semiconductor nanocrystals [18]. Here, we describe a simple approach to the preparation of Fe3 O4 nanorods in the presence of poly(vinylpyrrolidone) (PVP). The nucleation and growth process are in situ in
L. Feng et al. / Journal of Colloid and Interface Science 278 (2004) 372–375
373
aqueous solution, and no other solvents are needed. Fe3 O4 short nanorods with lower aspect ratio were initially formed by typical chemical coprecipitation of FeIII and FeII salts in alkaline solution [19]. In the process of refluxing, some of the larger short nanorods will grow at the expense of smaller ones, i.e., Ostwald ripening [20]. With the help of PVP, which can probably control the growth rates, the Fe3 O4 short nanorods can be directed to grow into Fe3 O4 nanorods with higher aspect ratio. The as-synthesized Fe3 O4 nanorods have superparamagnetic properties.
2. Experimental A mixture of 0.050 g FeCl2 · 4H2 O and 0.135 g FeCl3 · 6H2 O were dissolved in 100 ml Milli-Q water containing 1 g poly(vinylpyrrolidone). Then 0.634 g NaOH was dissolved in 100 ml water to form a solution with pH 13.2, which was dropwise added to 30 ml mixture solution of Fe3+ and Fe2+ in N2 . The reaction was allowed to proceed for 2 h at 80 ◦ C under constant and vigorous stirring in reflux equipment. Subsequently, the reactant was cooled slowly to room temperature and the pH decreased to 6.8. Precipitates and supernatant fluid were separated by magnet and the precipitations were washed several times with deionized water. The precipitations were dried in air for the purpose of characterization by XRD measurements. TEM studies are performed with transmission electron microscopy (JEOL JEM 200 CX) operating at 200 eV by placing a drop of the suspension solution carefully onto a carbon-coated copper grid and letting the water evaporate slowly in air. XRD pattern is collected using a M18 AHFX CuKα radiation X-ray diffractometer over the range of 20◦ 2θ 80◦ . Magnetic property measurement is carried out on a SQUID magnetometer (Quantum Design MPMS-7).
3. Results and discussion Fig. 1 is the X-ray diffraction (XRD) pattern of the asprepared nanorods. All the peaks in this figure are in good agreement with the standard taken from powder diffraction file 19-629 of the Database of the International Center for Diffraction Data (ICDD), which confirms that the nanorods prepared in this study are the magnetite nanocrystals. The Bragg reflection peaks are all relatively broad because of the small dimensions of the Fe3 O4 nanocrystals. The transmission electron microscopy (TEM) image in Fig. 2 demonstrated the time evolution of the morphology of the Fe3 O4 nanostructures with different aspect ratios. The aspect ratio of a shape is defined as the length of the major axis divided by the width of the minor axis [15]. Spheres have an aspect ratio of 1. Murphy et al. defined nanorods as materials that have a width of ∼1–100 nm and aspect ratios greater than 1 but less than 20, and
Fig. 1. X-ray diffraction pattern of Fe3 O4 nanorods.
nanowires as materials that have aspect ratios greater than 20. In addition, most people would call “short nanorods” analogous materials that have aspect ratio greater than 1 but less than 5. Accordingly, Fig. 2a shows the initial product, Fe3 O4 short nanorods that have an average width of about 11.8 nm and a length of about 47.1 nm. The aspect ratio can therefore be calculated to be 4. These short nanorods would serve as seeds on which to grow more anisotropic nanostructures. Herein, the presence of PVP macromolecules could chemically absorb onto the surfaces of Fe3 O4 short nanorods to prevent them from aggregating [21]. After 30 min with the solution constantly refluxed at 80 ◦ C, some of the larger Fe3 O4 short nanorods grow into rod-shaped structures based on the mechanism of Ostwald ripening in the presence of PVP (Fig. 2b). The seeds of Fe3 O4 short nanorods here serve as nucleation sites for nanorod growth. Typical product dimensions of the asgrown nanorods are about 18.2-nm short axes and 310.6-nm long axes; the aspect ratio is up to 17. The function of PVP in this process was to kinetically control the growth rates of various faces by interacting with these faces through adsorption and desorption [22]. After 2h, all the Fe3 O4 short nanorods are found to convert into nanorods (Fig. 2c). The electron diffraction pattern (the inset of Fig. 2c) obtained from the individual nanorods indicates that these nanorods of Fe3 O4 are single-crystalline. Fig. 2d shows a typical highresolution electron microscopy (HRTEM) image of the single nanorod in Fig. 2c. This image reveals that the {311} lattice fringes with lattice spacing around 0.25 nm are parallel to the axis of the nanorod and the growth plane is one of the {311} planes. The result further confirms that the as-synthesized Fe3 O4 nanorods are single-crystalline. The Fe3 O4 growth is achieved by the atom-by-atom growth of individual crystallites and not by agglomeration of particles formed in the solution phase. Previously, we demonstrated the individual monolayer-coated magnetic nanoparticles self-assemble into uniform spherical aggregates through π –π interaction, the interaction between individual particles is relatively weak, and the spherical aggregates can be
374
L. Feng et al. / Journal of Colloid and Interface Science 278 (2004) 372–375
Fig. 3. Magnetization vs applied field plots at 300 and 5 K for Fe3 O4 nanorods.
The magnetic properties of as-synthesized Fe3 O4 nanorods are investigated using a superconducting quantum interference device (SQUID) magnetometer. Typical magnetization curves as a function of applied field at room (300 K) and low temperature (5 K) are shown in Fig. 3, respectively. At 300 K, no hysteresis loop is seen, which indicates that both the retentivity and the coercivity of the particles are zero. This observation is consistent with superparamagnetic behavior [24,25]. According to many authors, the property is typical of nanostructures and is due to surface anisotropy and the presence of a dead magnetic layer [26]. As the temperature is lowered to 5 K, the magnetization of the nanorods increases and exhibits a symmetric hysteresis loop, indicating the transition from superparamagnetic to ferromagnetic behavior. In summary, single-crystal Fe3 O4 nanorods with uniform diameters have been successfully synthesized by a PVPcontrolled process. The nucleation and growth process are in situ in aqueous solution, and the aspect ratio of the assynthesized nanorods increased with time evolution. This formation process might be extendible as a model to other systems, which may prove useful in fabricating a variety of functional nanoscale devices.
Acknowledgments
Fig. 2. TEM images of three as-synthesized samples, showing different stages of rod growth: (a) 5 min; (b) 30 min; (c) 2 h; (d) HRTEM image of a single nanorod.
destroyed by sonication to leave monodispersed nanoparticles [23]. In comparison, the as-prepared Fe3 O4 nanorods in the present study are relatively stable; their structure cannot be destroyed by outside force such as sonication and stirring.
The authors thank the State Key Project Fundamental Research (G1999064504) and the Special Research Foundation of the National Nature Science Foundation of China (29992530, 20125102, 50072046, and 10274096) for continuing financial support. L. Feng thanks the China Postdoctoral Science Foundation (2003034218) and the K.C. Wong Education Foundation, Hong Kong (20031008155638).
References [1] G. Fasol, Science 280 (1998) 545.
L. Feng et al. / Journal of Colloid and Interface Science 278 (2004) 372–375
[2] J.D. Holmes, K.P. Johnston, R.C. Doty, B.A. Korgel, Science 287 (2000) 1471. [3] S. Iijima, Nature 354 (1991) 56. [4] D.D. Archibald, S. Mann, Nature 364 (1993) 430. [5] A.M. Morales, C.M. Lieber, Science 279 (1998) 208. [6] Z. Tang, N.A. Kotov, M. Giersig, Science 297 (2002) 237. [7] J. Ma, Y. Gu, L. Shi, L. Chen, Z. Yang, Y. Qian, Chem. Phys. Lett. 381 (2003) 194. [8] H. Li, X. Wang, Y. Song, Y. Liu, Q. Li, L. Jiang, D. Zhu, Angew. Chem. Int. Ed. 40 (2001) 1743. [9] L. Feng, S. Li, J. Zhai, Y. Song, L. Jiang, D. Zhu, Angew. Chem. Int. Ed. 41 (2002) 1221. [10] L. Feng, Y. Song, J. Zhai, J. Xu, L. Jiang, D. Zhu, Angew. Chem. Int. Ed. 42 (2003) 800. [11] L. Feng, Z. Yang, J. Zhai, Y. Song, B. Liu, Y. Ma, Z. Yang, L. Jiang, D. Zhu, Angew. Chem. Int. Ed. 42 (2003) 4217. [12] L. Feng, S. Li, Y. Li, H. Li, L. Zhang, J. Zhai, Y. Song, B. Liu, L. Jiang, D. Zhu, Adv. Mater. 14 (2002) 1857.
375
[13] J.L. Dormann, D. Fiorani, in: Magnetic Properties of Fine Particles, North-Holland, Amsterdam, 1991, pp. 309–423. [14] C.R. Martin, Science 266 (1994) 1961. [15] C.J. Murphy, N.R. Jana, Adv. Mater. 14 (2002) 80. [16] Y. Sun, B. Gates, B. Mayers, Y. Xia, Nano Lett. 2 (2002) 165. [17] J.J. Urban, J.E. Spanier, L. Ouyang, W.S. Yun, H. Park, Adv. Mater. 15 (2003) 423. [18] X. Peng, Adv. Mater. 15 (2003) 459. [19] L. Shen, P.E. Laibinis, T.A. Hatton, Langmuir 15 (1999) 447. [20] A.R. Roosen, W.C. Carter, Physica A 261 (1998) 232. [21] Z. Zhang, B. Zhao, L. Hu, J. Solid State Chem. 121 (1996) 105. [22] F. Bonet, K. Tekaia-Elhsissen, K.V. Sarathy, Bull. Mater. Sci. 23 (2000) 165. [23] J. Jin, T. Iyoda, C. Cao, Y. Song, L. Jiang, T.J. Li, Angew. Chem. Int. Ed. 40 (2001) 2135. [24] T. Nguyen, A.F. Diaz, Adv. Mater. 6 (1994) 858. [25] O. Jarjayes, P.H. Fries, G. Bidan, Synth. Met. 69 (1995) 343. [26] J.A.L. Pérez, M.A.L. Quintela, J. Mira, J. Rivas, S.W. Charles, J. Phys. Chem. B 101 (1997) 8045.