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Preparation and magnetic properties of magnetite nanoparticles
Materials Letters 68 (2012) 112–114
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Preparation and magnetic properties of magnetite nanoparticles Fangyuan Zhao, Baolin Zhang ⁎, Lingyun Feng State Key Laboratory Breeding Base of Nonferrous metals and specific Materials Processing, Guilin University of Technology, Guilin 541004, China
a r t i c l e
i n f o
Article history: Received 2 August 2011 Accepted 16 September 2011 Available online 20 October 2011 Keywords: Fe3O4 nanoparticles Magnetic materials Nanomaterials Superparamagnetism
a b s t r a c t Water-soluble superparamagnetic Fe3O4 nanoparticles with an average diameter of 9.5 ± 1.7 nm were synthesized by thermal decomposition of Fe(acac)3 in MPEG. MPEG was used as solvent, reducing agent, and modifying agent in this reaction. An obvious advantage of this approach is that no further reducing agent and surfactants are required. The products were characterized by powder X-ray diffraction, transmission electron microscopy and fourier transform infrared spectroscopy. The magnetic properties were evaluated using a superconducting quantum interference device. The elevating temperature procedures influence the sizes and the size distribution of the magnetite (Fe3O4) nanoparticles. A tentative mechanism of the formation of Fe3O4 nanoparticles is proposed to explain the formation of the magnetite nanoparticles. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Magnetic nanoparticles have attracted a great attention due to its unique physical, chemical and structural properties when the particle sizes approach to nanoscale. It is these unique features that endow magnetic nanoparticles with widely applications, such as magnetic storage, catalysis, microwave absorption, magnetic resonance contrast, cancer hyperthermia, cell separation and drug delivery [1–7]. Therefore, more and more efforts have been devoted to the synthesis of magnetic nanoparticles recently. Many preparation routes, including coprecititation, sonochemistry, colloidal method, combustion synthesis, solvothermal synthesis, hydrothermal method, microemulsion method and thermal decomposition [8–15], have been reported. Different from other methods, the thermal decomposition is the most promising one for the formation of pretty small and uniform magnetic nanoparticles. This method is a novel non-hydrolytic synthetic route. Generally, the thermal decomposition involves decomposition of iron precursor in a high-boiling temperature solvent in presence of stabilizing surfactants such as trioctylamine, oleic acid, oleyamine and so on. This method was first introduced by Alivisatos and latter developed by Hyeon, Cheon, Sun and Peng [16–20]. However, the produced magnetic nanoparticles are only soluble in nonpolar solvents due to the capped hydrophobic surfactant ligand, which limits their applications in the biomedical field. In order to successfully solve these issues, Gao and his coworkers [21] synthesize the biocompatible magnetic iron oxide nanoparticles by thermal decomposition of ferric triacetylacetonate (Fe(acac)3) in 2pyrrolidone in the presence of MPEG-COOH. Here, we have prepared MPEG modified magnetite nanoparticles in a simple way by reacting
⁎ Corresponding author. Tel.: + 86 7735896671; fax: + 86 7735896436. E-mail address: [email protected] (B. Zhang). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.09.116
Fe(acac)3 in methoxy polyethylene glycol (MPEG) at elevated temperature and no further surfactants are required. MPEG in this reaction acts as high-boiling point solvent, reducing agent and modifying agent at the same time. 2. Experimental All the reagents were of analytical purity, and were used without further purification. Magnetite nanocrystal sample was prepared by the thermal decomposition method. 2 mmol ferric triacetylacetonate (Fe(acac)3,98%) was dissolved in 20 mL of methoxy polyethylene glycol (MPEG-1200, 99%) media which was magnetically stirred under a flow of argon. One batch of the above components was dehydrated at 120 °C for 1 h, and then heated to 200 °C for 2 h. The second batch of the components was dehydrated at 120 °C for 1 h, and then heated to 240 °C for 2 h. And the third batch of the components was dehydrated at 120 °C for 1 h, and then first heated to 200 °C for 30 min, followed by being heated to 240 °C for another 90 min. Afterwards, the black solutions were cooled to room temperature by removing the heat source. The magnetite nanoparticles were precipitated by addition of 40 mL ethanol and then separated by centrifugation. The black precipitate was washed with ethanol for additional two times and then dried for characterization. 3. Results and discussion The XRD patterns of the prepared products are shown in Fig. 1. Curves a and b represents the magnetite nanoparticles synthesized at 240 °C for 2 h, and 240 °C for 90 min after an initial treatment at 200 °C for 30 min, respectively. Six characteristic peaks can be indexed as the cubic structure Fe3O4, which is in accordance with the reported data (JCPDS No.01-085-1436). The peaks with 2θ values of 30.09°, 35.44°, 43.07°, 53.43°, 56.96°, 62.54° correspond to the
F. Zhao et al. / Materials Letters 68 (2012) 112–114
113
Fig. 1. XRD patterns of Fe3O4 samples obtained via thermal decomposition of Fe(acac)3 in MPEG: (a) at 240 °C for 2 h; (b) 240 °C for 90 min after an initial treatment at 200 °C for 30 min.
crystal planes (220), (311), (400), (422), (511), (440) of crystalline Fe3O4, respectively. The peak width obviously was broaden in curve b as compared with curve a. The average crystallite sizes are calculated using Scherrer's equation, which gives 12.8 nm for (311) peak in curve a, 9.3 nm for (311) peak in curve b, respectively. Fig. 2 shows typical TEM images of the magnetite nanoparticles synthesized at (a) 200 °C and (b) 240 °C for 2 h, (c) 240 °C for 90 min after an initial treatment at 200 °C for 30 min. It is clearly observed that the magnetite nanoparticles exhibit aggregation in Fig. 2a. Fig. 2b indicates that the nanoparticles have a semispherical shape and are monodispersed, the nanoparticles' diameter is 9.5 ± 3.2 nm, but the size distribution is rather wide. Fig. 2c shows that the magnetite nanoparticles have semispherical shapes and are also monodispersed with an average diameter of 9.5 ± 1.7 nm. In contrast to the products prepared by being directly heated to a certain temperature, the magnetite nanoparticles prepared at separated elevating temperatures have a relatively narrow size distribution. It indicates that the separated elevating temperature facilitates the narrow size distribution. So, we assumed the growth mechanism of the magnetite nanoparticles in Fig. 3. The magnetite nanoparticle formation contains two process, nucleation and grain growth. Firstly, nucleation at 200 °C is the dominant process for the formation of Fe3O4 nucleus and it is initiated by the reaction between Fe3 + and OH functional group of the MPEG. Secondly, magnetite nucleus grows at 240 °C, at the same time, the modification process of magnetite nanoparticles by MPEG takes place. The separation of nucleation and growth processes is critical for the formation of nanoparticles with narrower size distribution. The surface chemical structure of the magnetite nanocrystals was characterized by fourier-transform infrared (FTIR) spectroscopy. A number of changes are clearly shown in the upper frame of Fig. 4, which shows a comparison of the spectra of Fe3O4 prepared by first heated to 200 °C for 30 min, followed by being heated to 240 °C for another 90 min (spectrum b), with that of pure MPEG (spectrum a). Three additional vibration bands at around 1615 cm − 1, 1384 cm − 1 and 580 cm − 1 in spectrum b appeared. The peaks at 1615 cm − 1, 1384 cm − 1 can be ascribed to the bidentate asymmetric and symmetric mode of COO − [22]. The presence of the COO − group could be resulted from the partial oxidation of the terminal OH group of the MPEG during the high temperature synthesis of magnetite nanoparticles and COO − are covalently bound to the surface of magnetite nanoparticles [23]. The modification makes the nanoparticles to have a good dispersion property in deionized water. The strong absorption band at about 580 cm − 1 is due to Fe–O stretching vibration for the Fe3O4 nanoparticles. The peaks at about 2865 and 1100 cm − 1 are due to C–H asymmetric stretching, and C–O–C bending vibration. The broad band 3410 cm − 1 is due to the O–H stretching vibration
Fig. 2. TEM images of magnetite nanoparticles obtained via thermal decomposition of Fe(acac)3 in MPEG: at (a) 200 °C and (b) 240 °C for 2 h; (c) 240 °C for 90 min after an initial treatment at 200 °C for 30 min.
Fig. 3. The illustration of the magnetite nanoparticles prepared by MPEG.
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F. Zhao et al. / Materials Letters 68 (2012) 112–114
Fig. 4. FT-IR spectra of (a) magnetite nanoparticles coated with MPEG, (b) MPEG.
attributed for water and MPEG molecules adsorbed to the nanoparticles surface. FTIR spectroscopic results indicate that MPEG covalently attaches to the surface of the magnetite nanoparticles. Fig. 5 shows the magnetization curves measured at 300 K for the magnetite nanoparticles obtained in the synthesis procedure with the separated elevating temperatures (i.e. 240 °C for 90 min after an initial treatment at 200 °C for 30 min), Zero coercivity and zero remanence on the magnetization curve indicate superparamagnetic behavior of the magnetite nanoparticles, meaning that the thermal energy can overcome the anisotropy energy barrier of a single particle, and the saturation magnetization of the magnetite nanoparticles is 45 emu/g. Thermogravimetric analysis of the magnetite nanoparticles synthesized in MPEG showed the presence of 19% of polymer, so the saturation magnetization of Fe3O4 can be adjusted to 56 emu/g. 4. Conclusion Water-soluble Fe3O4 nanoparticles have been successfully synthesized by a simple route by thermal decomposition of Fe(acac)3 in MPEG, MPEG were used as the solvent, reducing agent and modifying agent. The heating procedures had an important impact on both the morphology and the particle size of the magnetic nanoparticles. The FTIR experimental results indicated that MPEG molecules are covalently bound to the surface of the magnetite nanoparticles via the partial oxidation of the terminal OH group of the MPEG. The magnetite nanoparticles performed superparamagnetic behavior and the saturation magnetization of the nanoparticles with the modification layer of MPEG is 45 emu/g. These water-soluble magnetic nanoparticles may have great potential in biomedical application. Acknowledgments We thank the National Natural Science Foundation of China (50962005) for financial support.
Fig. 5. M–H curve of the magnetite nanoparticles synthesized by thermal decomposition of Fe(acac)3 in MPEG at 240 °C for 90 min after an initial treatment at 200 °C for 30 min.
References [1] Chakraborty A. J Magn Magn Mater 1999;204:57–60. [2] Rodríguez EM, Fernández G, Álvarez PM, Hernández R, Beltrán FJ. Appl Catal B: Environmental 2011;102:572–83. [3] Jia K, Zhao R, Zhong JC, Liu XB. J Magn Magn Mater 2010;322:2167–71. [4] Hu FQ, Wei L, Zhou Z, Ran YL, Li Z, Gao MY. Adv Mater 2006;18:2553–6. [5] Li ZX, Kawashita M, Araki N, Mitsumori M, Hiraoka M, Doi M. Mater Sci Eng C 2010;30:990–6. [6] Jing Y, Moore LR, Williams PS, Chalmers JJ, Farag SS, Bolwell B, et al. Biotechnol Bioeng 2007;96:1139–54. [7] Mejías R, Costo R, Roca AG, Arias CF, Veintemillas-Verdaguer S, González-Carreño T, et al. J Control Release 2008;130:168–74. [8] Wu JH, Ko SP, Liu HL, Kim S, Ju JS, Kim YK. Mater Lett 2007;61:3124–9. [9] Kim EH, Lee HS, Kwak BK, Kim BK. J Magn Magn Mater 2005;289:328–30. [10] Martínez-Mera I, Espinosa-Pesqueira ME, Pérez-Hernández R, Arenas-Alatorre J. Mater Lett 2007;61:4447–51. [11] Costa ACFM, Lula RT, Kiminami RHGA, Gama LFV, de Jesus AA, Andrade HMC. J Mater Sci 2006;41:4871–5. [12] Liu XM, Kim JK. Mater Lett 2009;63:428–30. [13] Duan LF, Jia SS, Wang YJ, Chen J, Zhao LJ. J Mater Sci 2009;44:4407–12. [14] Chin AB, Yaacob II. J Mater Process Technol 2007;191:235–7. [15] Sun SH, Zeng H, Robinson DB, Raoux S, Rice PM, Wang SX, et al. J Am Chem Soc 2004;126:273–9. [16] Rockenberger J, Scher EC, Alivisatos AP. J Am Chem Soc 1999;121:11595–6. [17] Hyeon T, Lee SS, Park J, Chung Y, Na HB. J Am Chem Soc 2001;123:12798–801. [18] Cheon JW, Kang NJ, Lee SM, Lee JH, Yoon JH, Oh SJ. J Am Chem Soc 2004;126: 1950–1. [19] Sun SH, Zeng H. J Am Chem Soc 2002;124:8204–5. [20] Jana NR, Chen Y, Peng XG. Chem Mater 2004;16:3931–5. [21] Li Z, Wei L, Gao MY, Lei H. Adv Mater 2005;17:1001–5. [22] Liu QX, Xu ZH. Langmuir 1995;11:4617–22. [23] Gonçalves RH, Cardoso CA, Leite ER. J Mater Chem 2010;20:1167–72.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Preparation and magnetic properties of magnetite nanoparticles Fangyuan Zhao, Baolin Zhang ⁎, Lingyun Feng State Key Laboratory Breeding Base of Nonferrous metals and specific Materials Processing, Guilin University of Technology, Guilin 541004, China
a r t i c l e
i n f o
Article history: Received 2 August 2011 Accepted 16 September 2011 Available online 20 October 2011 Keywords: Fe3O4 nanoparticles Magnetic materials Nanomaterials Superparamagnetism
a b s t r a c t Water-soluble superparamagnetic Fe3O4 nanoparticles with an average diameter of 9.5 ± 1.7 nm were synthesized by thermal decomposition of Fe(acac)3 in MPEG. MPEG was used as solvent, reducing agent, and modifying agent in this reaction. An obvious advantage of this approach is that no further reducing agent and surfactants are required. The products were characterized by powder X-ray diffraction, transmission electron microscopy and fourier transform infrared spectroscopy. The magnetic properties were evaluated using a superconducting quantum interference device. The elevating temperature procedures influence the sizes and the size distribution of the magnetite (Fe3O4) nanoparticles. A tentative mechanism of the formation of Fe3O4 nanoparticles is proposed to explain the formation of the magnetite nanoparticles. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Magnetic nanoparticles have attracted a great attention due to its unique physical, chemical and structural properties when the particle sizes approach to nanoscale. It is these unique features that endow magnetic nanoparticles with widely applications, such as magnetic storage, catalysis, microwave absorption, magnetic resonance contrast, cancer hyperthermia, cell separation and drug delivery [1–7]. Therefore, more and more efforts have been devoted to the synthesis of magnetic nanoparticles recently. Many preparation routes, including coprecititation, sonochemistry, colloidal method, combustion synthesis, solvothermal synthesis, hydrothermal method, microemulsion method and thermal decomposition [8–15], have been reported. Different from other methods, the thermal decomposition is the most promising one for the formation of pretty small and uniform magnetic nanoparticles. This method is a novel non-hydrolytic synthetic route. Generally, the thermal decomposition involves decomposition of iron precursor in a high-boiling temperature solvent in presence of stabilizing surfactants such as trioctylamine, oleic acid, oleyamine and so on. This method was first introduced by Alivisatos and latter developed by Hyeon, Cheon, Sun and Peng [16–20]. However, the produced magnetic nanoparticles are only soluble in nonpolar solvents due to the capped hydrophobic surfactant ligand, which limits their applications in the biomedical field. In order to successfully solve these issues, Gao and his coworkers [21] synthesize the biocompatible magnetic iron oxide nanoparticles by thermal decomposition of ferric triacetylacetonate (Fe(acac)3) in 2pyrrolidone in the presence of MPEG-COOH. Here, we have prepared MPEG modified magnetite nanoparticles in a simple way by reacting
⁎ Corresponding author. Tel.: + 86 7735896671; fax: + 86 7735896436. E-mail address: [email protected] (B. Zhang). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.09.116
Fe(acac)3 in methoxy polyethylene glycol (MPEG) at elevated temperature and no further surfactants are required. MPEG in this reaction acts as high-boiling point solvent, reducing agent and modifying agent at the same time. 2. Experimental All the reagents were of analytical purity, and were used without further purification. Magnetite nanocrystal sample was prepared by the thermal decomposition method. 2 mmol ferric triacetylacetonate (Fe(acac)3,98%) was dissolved in 20 mL of methoxy polyethylene glycol (MPEG-1200, 99%) media which was magnetically stirred under a flow of argon. One batch of the above components was dehydrated at 120 °C for 1 h, and then heated to 200 °C for 2 h. The second batch of the components was dehydrated at 120 °C for 1 h, and then heated to 240 °C for 2 h. And the third batch of the components was dehydrated at 120 °C for 1 h, and then first heated to 200 °C for 30 min, followed by being heated to 240 °C for another 90 min. Afterwards, the black solutions were cooled to room temperature by removing the heat source. The magnetite nanoparticles were precipitated by addition of 40 mL ethanol and then separated by centrifugation. The black precipitate was washed with ethanol for additional two times and then dried for characterization. 3. Results and discussion The XRD patterns of the prepared products are shown in Fig. 1. Curves a and b represents the magnetite nanoparticles synthesized at 240 °C for 2 h, and 240 °C for 90 min after an initial treatment at 200 °C for 30 min, respectively. Six characteristic peaks can be indexed as the cubic structure Fe3O4, which is in accordance with the reported data (JCPDS No.01-085-1436). The peaks with 2θ values of 30.09°, 35.44°, 43.07°, 53.43°, 56.96°, 62.54° correspond to the
F. Zhao et al. / Materials Letters 68 (2012) 112–114
113
Fig. 1. XRD patterns of Fe3O4 samples obtained via thermal decomposition of Fe(acac)3 in MPEG: (a) at 240 °C for 2 h; (b) 240 °C for 90 min after an initial treatment at 200 °C for 30 min.
crystal planes (220), (311), (400), (422), (511), (440) of crystalline Fe3O4, respectively. The peak width obviously was broaden in curve b as compared with curve a. The average crystallite sizes are calculated using Scherrer's equation, which gives 12.8 nm for (311) peak in curve a, 9.3 nm for (311) peak in curve b, respectively. Fig. 2 shows typical TEM images of the magnetite nanoparticles synthesized at (a) 200 °C and (b) 240 °C for 2 h, (c) 240 °C for 90 min after an initial treatment at 200 °C for 30 min. It is clearly observed that the magnetite nanoparticles exhibit aggregation in Fig. 2a. Fig. 2b indicates that the nanoparticles have a semispherical shape and are monodispersed, the nanoparticles' diameter is 9.5 ± 3.2 nm, but the size distribution is rather wide. Fig. 2c shows that the magnetite nanoparticles have semispherical shapes and are also monodispersed with an average diameter of 9.5 ± 1.7 nm. In contrast to the products prepared by being directly heated to a certain temperature, the magnetite nanoparticles prepared at separated elevating temperatures have a relatively narrow size distribution. It indicates that the separated elevating temperature facilitates the narrow size distribution. So, we assumed the growth mechanism of the magnetite nanoparticles in Fig. 3. The magnetite nanoparticle formation contains two process, nucleation and grain growth. Firstly, nucleation at 200 °C is the dominant process for the formation of Fe3O4 nucleus and it is initiated by the reaction between Fe3 + and OH functional group of the MPEG. Secondly, magnetite nucleus grows at 240 °C, at the same time, the modification process of magnetite nanoparticles by MPEG takes place. The separation of nucleation and growth processes is critical for the formation of nanoparticles with narrower size distribution. The surface chemical structure of the magnetite nanocrystals was characterized by fourier-transform infrared (FTIR) spectroscopy. A number of changes are clearly shown in the upper frame of Fig. 4, which shows a comparison of the spectra of Fe3O4 prepared by first heated to 200 °C for 30 min, followed by being heated to 240 °C for another 90 min (spectrum b), with that of pure MPEG (spectrum a). Three additional vibration bands at around 1615 cm − 1, 1384 cm − 1 and 580 cm − 1 in spectrum b appeared. The peaks at 1615 cm − 1, 1384 cm − 1 can be ascribed to the bidentate asymmetric and symmetric mode of COO − [22]. The presence of the COO − group could be resulted from the partial oxidation of the terminal OH group of the MPEG during the high temperature synthesis of magnetite nanoparticles and COO − are covalently bound to the surface of magnetite nanoparticles [23]. The modification makes the nanoparticles to have a good dispersion property in deionized water. The strong absorption band at about 580 cm − 1 is due to Fe–O stretching vibration for the Fe3O4 nanoparticles. The peaks at about 2865 and 1100 cm − 1 are due to C–H asymmetric stretching, and C–O–C bending vibration. The broad band 3410 cm − 1 is due to the O–H stretching vibration
Fig. 2. TEM images of magnetite nanoparticles obtained via thermal decomposition of Fe(acac)3 in MPEG: at (a) 200 °C and (b) 240 °C for 2 h; (c) 240 °C for 90 min after an initial treatment at 200 °C for 30 min.
Fig. 3. The illustration of the magnetite nanoparticles prepared by MPEG.
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F. Zhao et al. / Materials Letters 68 (2012) 112–114
Fig. 4. FT-IR spectra of (a) magnetite nanoparticles coated with MPEG, (b) MPEG.
attributed for water and MPEG molecules adsorbed to the nanoparticles surface. FTIR spectroscopic results indicate that MPEG covalently attaches to the surface of the magnetite nanoparticles. Fig. 5 shows the magnetization curves measured at 300 K for the magnetite nanoparticles obtained in the synthesis procedure with the separated elevating temperatures (i.e. 240 °C for 90 min after an initial treatment at 200 °C for 30 min), Zero coercivity and zero remanence on the magnetization curve indicate superparamagnetic behavior of the magnetite nanoparticles, meaning that the thermal energy can overcome the anisotropy energy barrier of a single particle, and the saturation magnetization of the magnetite nanoparticles is 45 emu/g. Thermogravimetric analysis of the magnetite nanoparticles synthesized in MPEG showed the presence of 19% of polymer, so the saturation magnetization of Fe3O4 can be adjusted to 56 emu/g. 4. Conclusion Water-soluble Fe3O4 nanoparticles have been successfully synthesized by a simple route by thermal decomposition of Fe(acac)3 in MPEG, MPEG were used as the solvent, reducing agent and modifying agent. The heating procedures had an important impact on both the morphology and the particle size of the magnetic nanoparticles. The FTIR experimental results indicated that MPEG molecules are covalently bound to the surface of the magnetite nanoparticles via the partial oxidation of the terminal OH group of the MPEG. The magnetite nanoparticles performed superparamagnetic behavior and the saturation magnetization of the nanoparticles with the modification layer of MPEG is 45 emu/g. These water-soluble magnetic nanoparticles may have great potential in biomedical application. Acknowledgments We thank the National Natural Science Foundation of China (50962005) for financial support.
Fig. 5. M–H curve of the magnetite nanoparticles synthesized by thermal decomposition of Fe(acac)3 in MPEG at 240 °C for 90 min after an initial treatment at 200 °C for 30 min.
References [1] Chakraborty A. J Magn Magn Mater 1999;204:57–60. [2] Rodríguez EM, Fernández G, Álvarez PM, Hernández R, Beltrán FJ. Appl Catal B: Environmental 2011;102:572–83. [3] Jia K, Zhao R, Zhong JC, Liu XB. J Magn Magn Mater 2010;322:2167–71. [4] Hu FQ, Wei L, Zhou Z, Ran YL, Li Z, Gao MY. Adv Mater 2006;18:2553–6. [5] Li ZX, Kawashita M, Araki N, Mitsumori M, Hiraoka M, Doi M. Mater Sci Eng C 2010;30:990–6. [6] Jing Y, Moore LR, Williams PS, Chalmers JJ, Farag SS, Bolwell B, et al. Biotechnol Bioeng 2007;96:1139–54. [7] Mejías R, Costo R, Roca AG, Arias CF, Veintemillas-Verdaguer S, González-Carreño T, et al. J Control Release 2008;130:168–74. [8] Wu JH, Ko SP, Liu HL, Kim S, Ju JS, Kim YK. Mater Lett 2007;61:3124–9. [9] Kim EH, Lee HS, Kwak BK, Kim BK. J Magn Magn Mater 2005;289:328–30. [10] Martínez-Mera I, Espinosa-Pesqueira ME, Pérez-Hernández R, Arenas-Alatorre J. Mater Lett 2007;61:4447–51. [11] Costa ACFM, Lula RT, Kiminami RHGA, Gama LFV, de Jesus AA, Andrade HMC. J Mater Sci 2006;41:4871–5. [12] Liu XM, Kim JK. Mater Lett 2009;63:428–30. [13] Duan LF, Jia SS, Wang YJ, Chen J, Zhao LJ. J Mater Sci 2009;44:4407–12. [14] Chin AB, Yaacob II. J Mater Process Technol 2007;191:235–7. [15] Sun SH, Zeng H, Robinson DB, Raoux S, Rice PM, Wang SX, et al. J Am Chem Soc 2004;126:273–9. [16] Rockenberger J, Scher EC, Alivisatos AP. J Am Chem Soc 1999;121:11595–6. [17] Hyeon T, Lee SS, Park J, Chung Y, Na HB. J Am Chem Soc 2001;123:12798–801. [18] Cheon JW, Kang NJ, Lee SM, Lee JH, Yoon JH, Oh SJ. J Am Chem Soc 2004;126: 1950–1. [19] Sun SH, Zeng H. J Am Chem Soc 2002;124:8204–5. [20] Jana NR, Chen Y, Peng XG. Chem Mater 2004;16:3931–5. [21] Li Z, Wei L, Gao MY, Lei H. Adv Mater 2005;17:1001–5. [22] Liu QX, Xu ZH. Langmuir 1995;11:4617–22. [23] Gonçalves RH, Cardoso CA, Leite ER. J Mater Chem 2010;20:1167–72.