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Preparation and characterization of hollow glass microspheres coated by CoFe2O4 nanoparticles using urea as precipitator via coprecipitation method
Materials Research Bulletin 44 (2009) 360–363
Contents lists available at ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Preparation and characterization of hollow glass microspheres coated by CoFe2O4 nanoparticles using urea as precipitator via coprecipitation method Xiaofen Pang a, Wuyou Fu a, Haibin Yang a,*, Hongyang Zhu a, Jing Xu a, Xiang Li b, Guangtian Zou a a b
National Laboratory of Superhard Materials, Jilin University, Changchun 130012, PR China Institute of Materials Science and Engineering, Henan Polytechnic University, Jiaozuo 454003, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 4 January 2007 Received in revised form 4 May 2008 Accepted 8 May 2008 Available online 15 May 2008
The composite of hollow glass microspheres coated by CoFe2O4 nanoparticles has been successfully prepared using urea as precipitator via coprecipitation method. The resultant composites were characterized by X-ray diffraction, field emission scanning electron microscope and vibrating sample magnetometer. The results showed that the slow decomposition of urea could be beneficial to form uniform and entire cobalt ferrite coating layer on the surface of hollow glass microspheres. The smoothest morphology was obtained for the sample prepared from 0.7 M urea, while the sample prepared from 1.0 M urea had the thickest shell. This indicated that there was a competition between the morphology and thickness of the coated microspheres. A possible formation mechanism of hollow glass microspheres coated with cobalt ferrite was proposed. The magnetic properties of the samples were also investigated. ß 2008 Published by Elsevier Ltd.
Keywords: A. Magnetic materials A. Glasses B. Chemical synthesis D. Magnetic properties
1. Introduction CoFe2O4 is an interesting material owing to its magnetic properties such as strong anisotropy, high coercivity at room temperature, moderate saturation magnetization, good mechanical and chemical stabilities [1,2]. Therefore, it is widely applied in various areas such as electronic devices, ferrofluids, magnetic drug delivery, microwave devices, high-density information storage, catalysts and gas sensors [3–14]. However, CoFe2O4 is also a highdensity material, which restricts its usefulness in applications requiring lightweight mass [15–17]. Coating CoFe2O4 nanoparticles on hollow microspheres of low density might be a feasible way to overcome the problem. Hollow glass microspheres are exactly suitable for this purpose due to their low density, excellent chemical and thermal stabilities and low price. Hence, they have been widely used in paint and coating systems. Coating hollow glass microspheres with metal and alloy by electroless plating has been reported [18,19]. However, coating hollow glass microspheres with CoFe2O4 nanoparticles is seldom reported. It is expected that the CoFe2O4-coated hollow glass microspheres composites could have potential uses in many fields, such as magnetic materials, biomedicine and catalysts [14,20–23]. In this paper, we prepared CoFe2O4-coated hollow glass microspheres samples using urea as precipitator via coprecipita-
* Corresponding author. Tel.: +86 431 85168763; fax: +86 431 85168816. E-mail address: [email protected] (H. Yang). 0025-5408/$ – see front matter ß 2008 Published by Elsevier Ltd. doi:10.1016/j.materresbull.2008.05.009
tion technique, and investigated the effect of the urea concentration on the CoFe2O4 coating layer on hollow glass microspheres. Since the decomposition of urea can accelerate the hydroxylation process [24], we found that this is beneficial to the homogeneous precipitation of metal cations on the surface of hollow glass microspheres. It is also found that the magnetic properties of hollow glass microspheres/CoFe2O4 composites can be controlled by altering the shell thickness of the CoFe2O4 coating, which in turn could be controlled by adjusting the concentration of urea. 2. Experimental 2.1. Preparation of CoFe2O4-coated hollow glass microspheres composites The hollow glass microspheres supplied by Beijing Lucida Chemical Corporation have a low density of about 0.4 g/cm3 and the particles size varies from 10 to 100 mm with a mean diameter of 50 mm. All the other reagents were of analytical grade and used without further purification. The hollow glass microspheres were washed with NaOH solution (0.3 mol/L) in ultrasonic vessel for 30 min, and then dried at 80 8C. 3.0 g pre-treated hollow glass microspheres were dispersed in each 100 mL water solution of different urea concentrations (0.2, 0.4, 0.7 and 1.0 M). 3.0 g (10 mmol) FeCl36H2O and 1.3 g (5 mmol) CoCl26H2O were dissolved into 100 mL distilled water to achieve a Fe/Co mole ratio of 2. Then the mixed solution was added drop by drop into the solution above under stirring with a constant speed.
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They were refluxed in a water bath of a temperature 90 8C for 6 h. The precursors of CoFe2O4-coated hollow glass microspheres were washed with distilled water and dried at 80 8C in the air, and finally calcined at 200, 400 and 600 8C for 2 h in a muffle furnace, respectively. 2.2. Characterization Several techniques have been used to characterize as-synthesized powders. The phase structure analysis of products was identified (within 2u range of 27–678) using an X-ray diffractometer (XRD, Rigaku D/max-rA) utilizing Cu Ka X-radiation of wavelength 1.5418 A˚. Field emission scanning electron microscope (FESEM, JSM-6700F) was employed to examine the morphology of the samples. The magnetization loops of the samples were measured at room temperature using a vibrating sample magnetometer (VSM, JDM-13), and the field reached up to 104 Oe. Bulk densities of different samples were obtained by pouring the material in a 10 mL measuring cylinder and tapping it to a constant volume of the material. The bulk density is calculated by the mass of the microspheres to the volume. 3. Results and discussion 3.1. XRD analysis The XRD patterns of spinel CoFe2O4 nanoparticles, hollow glass microspheres and the precursor of CoFe2O4-coated hollow glass microspheres composite synthesized from 0.7 M urea heated at different temperatures are shown in Fig. 1. As seen in Fig. 1b, the hollow glass microspheres are mainly composed of the mixture of mullite and quartz. It can be seen from Fig. 1c and d that there are no other peaks except the peaks of hollow glass microspheres. This indicates that the coating layer is amorphous composites and the CoFe2O4 phase is not formed. When the composite precursor sample was calcined at 600 8C for 2 h (Fig. 1e), the characteristic peaks of CoFe2O4 appearing at 30.18 (with index number of 2 2 0), 35.48 (3 1 1), 43.18 (4 0 0), 56.98 (5 1 1) and 62.68 (4 4 0) (JCPDS, No. 22-1086) reveal the formation of CoFe2O4. Except the peaks of CoFe2O4 and hollow glass microspheres, there are no other impurity peaks in Fig. 1e.
Fig. 1. XRD patterns of: (a) CoFe2O4 nanoparticles heated at 600 8C for 2 h; (b) hollow glass microspheres; (c), (d) and (e) precursor of hollow glass microspheres/ CoFe2O4 synthesized under 0.7 M urea heated for 2 h at 200, 400 and 600 8C, respectively.
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3.2. FESEM analysis Fig. 2 shows the FESEM micrographs of hollow glass microspheres and hollow glass microspheres/CoFe2O4 composites synthesized at different urea concentrations. Fig. 2a shows that most hollow glass microspheres with smooth surface are intact. The diameters of hollow glass microspheres are about 10–100 mm. It is clear that hollow glass microspheres were successfully coated with CoFe2O4 nanoparticles using 0.7 M urea as precipitator by coprecipitation technique in Fig. 2b. Moreover, the coating layer was made up of the nanoparticles with average size about 30 nm (Fig. 2d). When the urea concentration was increased up to 1.0 M, the surface of hollow glass microspheres/CoFe2O4 composites was very uneven and some CoFe2O4 nanoparticles were not coated on hollow glass microspheres, as shown in Fig. 2c. It can be observed that the products synthesized at the urea concentration of 0.7 M are coated more completely and uniformly than that of concentration 1.0 M. This result will be discussed later in the analysis of the coating mechanism and shell thickness. 3.3. Analysis of the coating mechanism and shell thickness According to analysis about XRD patterns and FESEM pictures above, we would like to propose a surface reaction and the CoFe2O4 coating layer nucleation-growth model on the surface of hollow glass microspheres about how this coating layer was formed, as shown schematically by Fig. 3A. First, the smooth decomposition of urea in hot aqueous solution would produce a lot of OH . Hollow glass microspheres mainly consist of mullite and quartz. The mullite and quartz easily absorb hydroxyl in alkaline solution. When hollow glass microspheres are added into urea aqueous solution, large numbers of OH react with the surface of hollow glass microspheres to form the amount of OH function groups as the nucleation sites on their surface. Furthermore, the hydroxylation process can be accelerated by the slow decomposition of urea [24]. Then the added CoCl26H2O and FeCl36H2O provide Co2+ and Fe3+ would be absorbed by OH functional groups through electrostatic interaction to form precursors Co(OH)2 and Fe(OH)3 on the surface of the modifiedhollow glass microspheres. In the late period, further Co(OH)2 and Fe(OH)3 are continuously formed on the prior Co(OH)2 and Fe(OH)3 nucleation layer from Co2+, Fe3+ and OH to form a uniform and entire amorphous Co(OH)2 and Fe(OH)3 coating. After the thermal treatment, CoFe2O4 nanocrystals are obtained by the dehydration of Co(OH)2 and Fe(OH)3. Thus, CoFe2O4 coating layer on the surface of hollow glass microspheres is achieved. To examine the shell thickness of CoFe2O4 coating layer, hollow glass microspheres/CoFe2O4 composites are crushed by force. Changes in the CoFe2O4 shell thickness with adjusting the concentration of urea are shown in Fig. 3B: (a) 0.2 M, (b) 0.4 M, (c) 0.7 M, and (d) 1.0 M, respectively. Repeated experiments indicate that at the given condition, the bulk density of these composite microspheres measured [25] by the mass of the microspheres to the volume and the CoFe2O4 shell thickness values determined by Fig. 3B are as listed in Table 1, which increase with the increasing of urea concentration. This may be explained that with the increasing of the urea concentration, the amount of the OH function groups formed on the surface of hollow glass microspheres increases. As a result, the quantities of the nucleation sites increase which led to the increasing quantities of Co(OH)2 and Fe(OH)3 nucleation. So the thickness of the coating layer gradually increases. While the urea concentration increases from 0.7 to 1.0 M, the acceleration of the rate of nucleation leads to the formation of free deposition and uneven coating layer as showed in
X. Pang et al. / Materials Research Bulletin 44 (2009) 360–363
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Fig. 2. FESEM micrographs of: (a) hollow glass microspheres, (b) and (c) hollow glass microspheres/CoFe2O4 composites synthesized under 0.7 and 1.0 M urea concentrations heated at 600 8C, (d) the CoFe2O4 nanoparticles of CoFe2O4 coating.
Fig. 3. (A) The model about the formation mechanism of hollow glass microspheres (HGMS)/CoFe2O4 composite; (B) FESEM micrographs of the thickness of CoFe2O4 coating prepared at different urea concentrations: (a) 0.2 M, (b) 0.4 M, (c) 0.7 M and (d) 1.0 M.
Table 1 The densities, shell thickness, saturation magnetizations and coercivities of different samples Samples no.
Concentration of urea (mol/L)
Density (g/cm3)
Thickness (nm)
Ms (emu/g)
Hc (Oe)
1 2 3 4
0.2 0.4 0.7 1.0
0.41 0.42 0.47 0.50
10–50 60–100 200–300 250–500
0.58 3.2 9.5 15.5
272 273 638 583
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363
Figs. 4 and 5. These results suggest that it is possible to control the magnetization value of the CoFe2O4-coated hollow glass microspheres samples by controlling the thickness of the CoFe2O4 coating layer, which in turn can be controlled by adjusting the urea concentration. 4. Conclusions
Fig. 4. Magnetization loops of samples synthesized at 600 8C under different the urea concentrations for (1) 0.2 M, (2) 0.4 M, (3) 0.7 M, (4) 1.0 M and (5) pure CoFe2O4 nanoparticles.
Figs. 2c and 3B-d. As expected, this comparison tells us that there is a competition between a smooth morphology and a thick shell. 3.4. Analysis of magnetic properties of hollow glass microspheres/ CoFe2O4 samples Magnetic properties of the different samples were measured at room temperature with a VSM. The hysteresis loops are shown in Fig. 4. The saturation magnetization (Ms) and the coercivity (Hc) are listed in Table 1. It is clear that the hysteresis loops of the samples exhibit ferromagnetic behavior. The saturation magnetization for sample 1 is very weak (0.58 emu/g) (inset a of Fig. 4), it may be considered that the CoFe2O4 coating layer is too thin on the surface of hollow glass microspheres (Fig. 3B-a). For other samples (2–4), the saturation magnetization values increase with the thickness of the CoFe2O4 coating layer increasing (Fig. 3B-b, c and d). Because the weights of all samples used for measurement of magnetic properties are constant, the increase of saturation magnetization is due to the increased CoFe2O4 content that depends on the thickness of the CoFe2O4 coating layer. The Ms values of the samples are significantly lower than the pure CoFe2O4 nanoparticles (Figs. 4 and 5). This behavior may be attributed mainly to the contribution of the volume of the non-magnetic hollow glass microspheres to the total sample volume. The coercivity of samples first increases with the increasing thickness of the CoFe2O4 coating layer, reaches a maximum value of 638 Oe for sample 3 in Table 1 and then decreases for sample 4. Since the coercivity is determined by the strength and number of the magnetic dipole in magnetic domain and relations between adjacent magnetic domain [26], the comparison suggests that even though the sample 4 has a thicker coating, its CoFe2O4 is not well crystallized, as could be expected from it is much unevener CoFe2O4 coating layer than sample 3. Also, the coercivity of all samples is smaller than the pure CoFe2O4 sample’s indicated by
CoFe2O4-coated hollow glass microspheres samples with low density (0.4–0.5 g/cm3) have been successfully synthesized with urea as precipitator by coprecipitation technique. The decomposition of urea can accelerate the hydroxylation process on the surface of hollow glass microspheres. The most uniform and entire CoFe2O4 coating layer has been synthesized under the urea concentration of 0.7 M. The thickness of the CoFe2O4 coating layer is controlled by adjusting the urea concentration. The results of VSM indicate that the saturation magnetization of hollow glass microspheres/CoFe2O4 compounds increases with the increase of the coating thickness. The variation of coercivity is dependent on the thickness of the CoFe2O4 coating layer and its morphology. Thus, the magnetization values of the samples can be adjusted by altering the thickness of the CoFe2O4 coating layer, which in turn can be controlled by altering the urea concentration. Acknowledgements The authors are grateful to Professors Z.X. Guo and Y. Xu for their help of the FESEM and XRD measurement. They are also thankful to Doctors N. Yang, J.J. Jia and Q.J. Yu for their useful advice. References [1] P.C. Dorsey, P. Lubitz, K.B. Chrisey, J.S. Horwitz, J. Appl. Phys. 79 (1996) 6338. [2] J.G. Lee, J.Y. Park, Y.J. Oh, C.S. Kim, J. Appl. Phys. 84 (1998) 2801. [3] Y. Kitamoto, S. Kantake, F. Shirasaki, M. Abe, M. Naoe, J. Appl. Phys. 85 (1999) 4708. [4] M.H. Sousa, F.A. Tourinho, J. Phys. Chem. B 105 (2001) 1168. [5] E. Dennis, Speliotis, J. Magn. Magn. Mater. 193 (1999) 29. [6] F. Mazaleyrat, L.K. Varga, J. Magn. Magn. Mater. 215–216 (2000) 253. [7] C.V. Gopal Reddy, S.V. Manorama, V.J. Rao, J. Mater. Sci. Lett. 19 (2000) 775. [8] K. Yamaguchi, K. Matsumoto, T. Fujii, J. Appl. Phys. 67 (1990) 4493–4495. [9] V.T. Peikove, K.S. Jeon, A.M. Lane, J. Magn. Magn. Mater. 193 (1999) 307–310. [10] R.D. McMichael, R.D. Shull, L.J. Swartzendruber, J. Magn. Magn. Mater. 111 (1992) 29–33. [11] D.K. Kim, Y. Zhang, W. Voit, J. Magn. Magn. Mater. 225 (2001) 30–36. [12] Y. Shahoo, A. Goodarzi, M.T. Swihart, J. Phys. Chem. B 109 (2005) 3879–3885. [13] H. Lee, E. Lee, D.K. Kim, J. Am. Chem. Soc. 128 (2006) 7383–7389. [14] C.G. Ramankutty, S. Sugunan, Appl. Catal. A: Gen. 218 (2001) 39–51. [15] M. Matsumoto, Y. Miyata, IEEE Trans. Magn. 33 (1997) 4459–4464. [16] K. Hatakeyama, T. Inui, IEEE Trans. Magn. 20 (1984) 1261. [17] M. Matsumoto, Y. Miyata, IEEE Trans. Magn. 33 (1994) 4459. [18] Q.Y. Zhang, M. Wu, W. Zhao, Surf. Coat. Technol. 192 (2005) 213–219. [19] Z. Aixiang, X. Weihao, X. Jian, Surf. Coat. Technol. 197 (2005) 142–147. [20] Y.D. Deng, X. Liu, B. Shen, J. Magn. Magn. Mater. 303 (2006) 181–184. [21] S.S. Kim, S.T. Kim, J.M. Ahn, J. Magn. Magn. Mater. 271 (2004) 39–45. [22] J.B. Silva, C.F. Diniz, R.M. Lago, J. Non-Cryst. Sol. 348 (2004) 201–204. [23] Y. Zhang, Z. Huang, F. Tang, J. Ren, Thin Solid Films 515 (2006) 2555–2561. [24] M. Hirano, M. Inagaki, J. Mater. Chem. 10 (2000) 475. [25] I.N. Bhattacharya, P.K. Gochhayat, P.S. Mukherjee, Mater. Chem. Phys. 88 (2004) 34. [26] W. Fu, H. Yang, Q. Yu, Mater. Lett. 61 (2007) 2187–2190.
Contents lists available at ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Preparation and characterization of hollow glass microspheres coated by CoFe2O4 nanoparticles using urea as precipitator via coprecipitation method Xiaofen Pang a, Wuyou Fu a, Haibin Yang a,*, Hongyang Zhu a, Jing Xu a, Xiang Li b, Guangtian Zou a a b
National Laboratory of Superhard Materials, Jilin University, Changchun 130012, PR China Institute of Materials Science and Engineering, Henan Polytechnic University, Jiaozuo 454003, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 4 January 2007 Received in revised form 4 May 2008 Accepted 8 May 2008 Available online 15 May 2008
The composite of hollow glass microspheres coated by CoFe2O4 nanoparticles has been successfully prepared using urea as precipitator via coprecipitation method. The resultant composites were characterized by X-ray diffraction, field emission scanning electron microscope and vibrating sample magnetometer. The results showed that the slow decomposition of urea could be beneficial to form uniform and entire cobalt ferrite coating layer on the surface of hollow glass microspheres. The smoothest morphology was obtained for the sample prepared from 0.7 M urea, while the sample prepared from 1.0 M urea had the thickest shell. This indicated that there was a competition between the morphology and thickness of the coated microspheres. A possible formation mechanism of hollow glass microspheres coated with cobalt ferrite was proposed. The magnetic properties of the samples were also investigated. ß 2008 Published by Elsevier Ltd.
Keywords: A. Magnetic materials A. Glasses B. Chemical synthesis D. Magnetic properties
1. Introduction CoFe2O4 is an interesting material owing to its magnetic properties such as strong anisotropy, high coercivity at room temperature, moderate saturation magnetization, good mechanical and chemical stabilities [1,2]. Therefore, it is widely applied in various areas such as electronic devices, ferrofluids, magnetic drug delivery, microwave devices, high-density information storage, catalysts and gas sensors [3–14]. However, CoFe2O4 is also a highdensity material, which restricts its usefulness in applications requiring lightweight mass [15–17]. Coating CoFe2O4 nanoparticles on hollow microspheres of low density might be a feasible way to overcome the problem. Hollow glass microspheres are exactly suitable for this purpose due to their low density, excellent chemical and thermal stabilities and low price. Hence, they have been widely used in paint and coating systems. Coating hollow glass microspheres with metal and alloy by electroless plating has been reported [18,19]. However, coating hollow glass microspheres with CoFe2O4 nanoparticles is seldom reported. It is expected that the CoFe2O4-coated hollow glass microspheres composites could have potential uses in many fields, such as magnetic materials, biomedicine and catalysts [14,20–23]. In this paper, we prepared CoFe2O4-coated hollow glass microspheres samples using urea as precipitator via coprecipita-
* Corresponding author. Tel.: +86 431 85168763; fax: +86 431 85168816. E-mail address: [email protected] (H. Yang). 0025-5408/$ – see front matter ß 2008 Published by Elsevier Ltd. doi:10.1016/j.materresbull.2008.05.009
tion technique, and investigated the effect of the urea concentration on the CoFe2O4 coating layer on hollow glass microspheres. Since the decomposition of urea can accelerate the hydroxylation process [24], we found that this is beneficial to the homogeneous precipitation of metal cations on the surface of hollow glass microspheres. It is also found that the magnetic properties of hollow glass microspheres/CoFe2O4 composites can be controlled by altering the shell thickness of the CoFe2O4 coating, which in turn could be controlled by adjusting the concentration of urea. 2. Experimental 2.1. Preparation of CoFe2O4-coated hollow glass microspheres composites The hollow glass microspheres supplied by Beijing Lucida Chemical Corporation have a low density of about 0.4 g/cm3 and the particles size varies from 10 to 100 mm with a mean diameter of 50 mm. All the other reagents were of analytical grade and used without further purification. The hollow glass microspheres were washed with NaOH solution (0.3 mol/L) in ultrasonic vessel for 30 min, and then dried at 80 8C. 3.0 g pre-treated hollow glass microspheres were dispersed in each 100 mL water solution of different urea concentrations (0.2, 0.4, 0.7 and 1.0 M). 3.0 g (10 mmol) FeCl36H2O and 1.3 g (5 mmol) CoCl26H2O were dissolved into 100 mL distilled water to achieve a Fe/Co mole ratio of 2. Then the mixed solution was added drop by drop into the solution above under stirring with a constant speed.
X. Pang et al. / Materials Research Bulletin 44 (2009) 360–363
They were refluxed in a water bath of a temperature 90 8C for 6 h. The precursors of CoFe2O4-coated hollow glass microspheres were washed with distilled water and dried at 80 8C in the air, and finally calcined at 200, 400 and 600 8C for 2 h in a muffle furnace, respectively. 2.2. Characterization Several techniques have been used to characterize as-synthesized powders. The phase structure analysis of products was identified (within 2u range of 27–678) using an X-ray diffractometer (XRD, Rigaku D/max-rA) utilizing Cu Ka X-radiation of wavelength 1.5418 A˚. Field emission scanning electron microscope (FESEM, JSM-6700F) was employed to examine the morphology of the samples. The magnetization loops of the samples were measured at room temperature using a vibrating sample magnetometer (VSM, JDM-13), and the field reached up to 104 Oe. Bulk densities of different samples were obtained by pouring the material in a 10 mL measuring cylinder and tapping it to a constant volume of the material. The bulk density is calculated by the mass of the microspheres to the volume. 3. Results and discussion 3.1. XRD analysis The XRD patterns of spinel CoFe2O4 nanoparticles, hollow glass microspheres and the precursor of CoFe2O4-coated hollow glass microspheres composite synthesized from 0.7 M urea heated at different temperatures are shown in Fig. 1. As seen in Fig. 1b, the hollow glass microspheres are mainly composed of the mixture of mullite and quartz. It can be seen from Fig. 1c and d that there are no other peaks except the peaks of hollow glass microspheres. This indicates that the coating layer is amorphous composites and the CoFe2O4 phase is not formed. When the composite precursor sample was calcined at 600 8C for 2 h (Fig. 1e), the characteristic peaks of CoFe2O4 appearing at 30.18 (with index number of 2 2 0), 35.48 (3 1 1), 43.18 (4 0 0), 56.98 (5 1 1) and 62.68 (4 4 0) (JCPDS, No. 22-1086) reveal the formation of CoFe2O4. Except the peaks of CoFe2O4 and hollow glass microspheres, there are no other impurity peaks in Fig. 1e.
Fig. 1. XRD patterns of: (a) CoFe2O4 nanoparticles heated at 600 8C for 2 h; (b) hollow glass microspheres; (c), (d) and (e) precursor of hollow glass microspheres/ CoFe2O4 synthesized under 0.7 M urea heated for 2 h at 200, 400 and 600 8C, respectively.
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3.2. FESEM analysis Fig. 2 shows the FESEM micrographs of hollow glass microspheres and hollow glass microspheres/CoFe2O4 composites synthesized at different urea concentrations. Fig. 2a shows that most hollow glass microspheres with smooth surface are intact. The diameters of hollow glass microspheres are about 10–100 mm. It is clear that hollow glass microspheres were successfully coated with CoFe2O4 nanoparticles using 0.7 M urea as precipitator by coprecipitation technique in Fig. 2b. Moreover, the coating layer was made up of the nanoparticles with average size about 30 nm (Fig. 2d). When the urea concentration was increased up to 1.0 M, the surface of hollow glass microspheres/CoFe2O4 composites was very uneven and some CoFe2O4 nanoparticles were not coated on hollow glass microspheres, as shown in Fig. 2c. It can be observed that the products synthesized at the urea concentration of 0.7 M are coated more completely and uniformly than that of concentration 1.0 M. This result will be discussed later in the analysis of the coating mechanism and shell thickness. 3.3. Analysis of the coating mechanism and shell thickness According to analysis about XRD patterns and FESEM pictures above, we would like to propose a surface reaction and the CoFe2O4 coating layer nucleation-growth model on the surface of hollow glass microspheres about how this coating layer was formed, as shown schematically by Fig. 3A. First, the smooth decomposition of urea in hot aqueous solution would produce a lot of OH . Hollow glass microspheres mainly consist of mullite and quartz. The mullite and quartz easily absorb hydroxyl in alkaline solution. When hollow glass microspheres are added into urea aqueous solution, large numbers of OH react with the surface of hollow glass microspheres to form the amount of OH function groups as the nucleation sites on their surface. Furthermore, the hydroxylation process can be accelerated by the slow decomposition of urea [24]. Then the added CoCl26H2O and FeCl36H2O provide Co2+ and Fe3+ would be absorbed by OH functional groups through electrostatic interaction to form precursors Co(OH)2 and Fe(OH)3 on the surface of the modifiedhollow glass microspheres. In the late period, further Co(OH)2 and Fe(OH)3 are continuously formed on the prior Co(OH)2 and Fe(OH)3 nucleation layer from Co2+, Fe3+ and OH to form a uniform and entire amorphous Co(OH)2 and Fe(OH)3 coating. After the thermal treatment, CoFe2O4 nanocrystals are obtained by the dehydration of Co(OH)2 and Fe(OH)3. Thus, CoFe2O4 coating layer on the surface of hollow glass microspheres is achieved. To examine the shell thickness of CoFe2O4 coating layer, hollow glass microspheres/CoFe2O4 composites are crushed by force. Changes in the CoFe2O4 shell thickness with adjusting the concentration of urea are shown in Fig. 3B: (a) 0.2 M, (b) 0.4 M, (c) 0.7 M, and (d) 1.0 M, respectively. Repeated experiments indicate that at the given condition, the bulk density of these composite microspheres measured [25] by the mass of the microspheres to the volume and the CoFe2O4 shell thickness values determined by Fig. 3B are as listed in Table 1, which increase with the increasing of urea concentration. This may be explained that with the increasing of the urea concentration, the amount of the OH function groups formed on the surface of hollow glass microspheres increases. As a result, the quantities of the nucleation sites increase which led to the increasing quantities of Co(OH)2 and Fe(OH)3 nucleation. So the thickness of the coating layer gradually increases. While the urea concentration increases from 0.7 to 1.0 M, the acceleration of the rate of nucleation leads to the formation of free deposition and uneven coating layer as showed in
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Fig. 2. FESEM micrographs of: (a) hollow glass microspheres, (b) and (c) hollow glass microspheres/CoFe2O4 composites synthesized under 0.7 and 1.0 M urea concentrations heated at 600 8C, (d) the CoFe2O4 nanoparticles of CoFe2O4 coating.
Fig. 3. (A) The model about the formation mechanism of hollow glass microspheres (HGMS)/CoFe2O4 composite; (B) FESEM micrographs of the thickness of CoFe2O4 coating prepared at different urea concentrations: (a) 0.2 M, (b) 0.4 M, (c) 0.7 M and (d) 1.0 M.
Table 1 The densities, shell thickness, saturation magnetizations and coercivities of different samples Samples no.
Concentration of urea (mol/L)
Density (g/cm3)
Thickness (nm)
Ms (emu/g)
Hc (Oe)
1 2 3 4
0.2 0.4 0.7 1.0
0.41 0.42 0.47 0.50
10–50 60–100 200–300 250–500
0.58 3.2 9.5 15.5
272 273 638 583
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363
Figs. 4 and 5. These results suggest that it is possible to control the magnetization value of the CoFe2O4-coated hollow glass microspheres samples by controlling the thickness of the CoFe2O4 coating layer, which in turn can be controlled by adjusting the urea concentration. 4. Conclusions
Fig. 4. Magnetization loops of samples synthesized at 600 8C under different the urea concentrations for (1) 0.2 M, (2) 0.4 M, (3) 0.7 M, (4) 1.0 M and (5) pure CoFe2O4 nanoparticles.
Figs. 2c and 3B-d. As expected, this comparison tells us that there is a competition between a smooth morphology and a thick shell. 3.4. Analysis of magnetic properties of hollow glass microspheres/ CoFe2O4 samples Magnetic properties of the different samples were measured at room temperature with a VSM. The hysteresis loops are shown in Fig. 4. The saturation magnetization (Ms) and the coercivity (Hc) are listed in Table 1. It is clear that the hysteresis loops of the samples exhibit ferromagnetic behavior. The saturation magnetization for sample 1 is very weak (0.58 emu/g) (inset a of Fig. 4), it may be considered that the CoFe2O4 coating layer is too thin on the surface of hollow glass microspheres (Fig. 3B-a). For other samples (2–4), the saturation magnetization values increase with the thickness of the CoFe2O4 coating layer increasing (Fig. 3B-b, c and d). Because the weights of all samples used for measurement of magnetic properties are constant, the increase of saturation magnetization is due to the increased CoFe2O4 content that depends on the thickness of the CoFe2O4 coating layer. The Ms values of the samples are significantly lower than the pure CoFe2O4 nanoparticles (Figs. 4 and 5). This behavior may be attributed mainly to the contribution of the volume of the non-magnetic hollow glass microspheres to the total sample volume. The coercivity of samples first increases with the increasing thickness of the CoFe2O4 coating layer, reaches a maximum value of 638 Oe for sample 3 in Table 1 and then decreases for sample 4. Since the coercivity is determined by the strength and number of the magnetic dipole in magnetic domain and relations between adjacent magnetic domain [26], the comparison suggests that even though the sample 4 has a thicker coating, its CoFe2O4 is not well crystallized, as could be expected from it is much unevener CoFe2O4 coating layer than sample 3. Also, the coercivity of all samples is smaller than the pure CoFe2O4 sample’s indicated by
CoFe2O4-coated hollow glass microspheres samples with low density (0.4–0.5 g/cm3) have been successfully synthesized with urea as precipitator by coprecipitation technique. The decomposition of urea can accelerate the hydroxylation process on the surface of hollow glass microspheres. The most uniform and entire CoFe2O4 coating layer has been synthesized under the urea concentration of 0.7 M. The thickness of the CoFe2O4 coating layer is controlled by adjusting the urea concentration. The results of VSM indicate that the saturation magnetization of hollow glass microspheres/CoFe2O4 compounds increases with the increase of the coating thickness. The variation of coercivity is dependent on the thickness of the CoFe2O4 coating layer and its morphology. Thus, the magnetization values of the samples can be adjusted by altering the thickness of the CoFe2O4 coating layer, which in turn can be controlled by altering the urea concentration. Acknowledgements The authors are grateful to Professors Z.X. Guo and Y. Xu for their help of the FESEM and XRD measurement. They are also thankful to Doctors N. Yang, J.J. Jia and Q.J. Yu for their useful advice. References [1] P.C. Dorsey, P. Lubitz, K.B. Chrisey, J.S. Horwitz, J. Appl. Phys. 79 (1996) 6338. [2] J.G. Lee, J.Y. Park, Y.J. Oh, C.S. Kim, J. Appl. Phys. 84 (1998) 2801. [3] Y. Kitamoto, S. Kantake, F. Shirasaki, M. Abe, M. Naoe, J. Appl. Phys. 85 (1999) 4708. [4] M.H. Sousa, F.A. Tourinho, J. Phys. Chem. B 105 (2001) 1168. [5] E. Dennis, Speliotis, J. Magn. Magn. Mater. 193 (1999) 29. [6] F. Mazaleyrat, L.K. Varga, J. Magn. Magn. Mater. 215–216 (2000) 253. [7] C.V. Gopal Reddy, S.V. Manorama, V.J. Rao, J. Mater. Sci. Lett. 19 (2000) 775. [8] K. Yamaguchi, K. Matsumoto, T. Fujii, J. Appl. Phys. 67 (1990) 4493–4495. [9] V.T. Peikove, K.S. Jeon, A.M. Lane, J. Magn. Magn. Mater. 193 (1999) 307–310. [10] R.D. McMichael, R.D. Shull, L.J. Swartzendruber, J. Magn. Magn. Mater. 111 (1992) 29–33. [11] D.K. Kim, Y. Zhang, W. Voit, J. Magn. Magn. Mater. 225 (2001) 30–36. [12] Y. Shahoo, A. Goodarzi, M.T. Swihart, J. Phys. Chem. B 109 (2005) 3879–3885. [13] H. Lee, E. Lee, D.K. Kim, J. Am. Chem. Soc. 128 (2006) 7383–7389. [14] C.G. Ramankutty, S. Sugunan, Appl. Catal. A: Gen. 218 (2001) 39–51. [15] M. Matsumoto, Y. Miyata, IEEE Trans. Magn. 33 (1997) 4459–4464. [16] K. Hatakeyama, T. Inui, IEEE Trans. Magn. 20 (1984) 1261. [17] M. Matsumoto, Y. Miyata, IEEE Trans. Magn. 33 (1994) 4459. [18] Q.Y. Zhang, M. Wu, W. Zhao, Surf. Coat. Technol. 192 (2005) 213–219. [19] Z. Aixiang, X. Weihao, X. Jian, Surf. Coat. Technol. 197 (2005) 142–147. [20] Y.D. Deng, X. Liu, B. Shen, J. Magn. Magn. Mater. 303 (2006) 181–184. [21] S.S. Kim, S.T. Kim, J.M. Ahn, J. Magn. Magn. Mater. 271 (2004) 39–45. [22] J.B. Silva, C.F. Diniz, R.M. Lago, J. Non-Cryst. Sol. 348 (2004) 201–204. [23] Y. Zhang, Z. Huang, F. Tang, J. Ren, Thin Solid Films 515 (2006) 2555–2561. [24] M. Hirano, M. Inagaki, J. Mater. Chem. 10 (2000) 475. [25] I.N. Bhattacharya, P.K. Gochhayat, P.S. Mukherjee, Mater. Chem. Phys. 88 (2004) 34. [26] W. Fu, H. Yang, Q. Yu, Mater. Lett. 61 (2007) 2187–2190.