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Salt-assisted combustion synthesis of highly dispersed superparamagnetic CoFe2O4 nanoparticles
Journal of Alloys and Compounds 475 (2009) L34–L37
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Letter
Salt-assisted combustion synthesis of highly dispersed superparamagnetic CoFe2 O4 nanoparticles Xiaojuan Zhang, Wei Jiang, Dan Song, Huijuan Sun, Zhendong Sun, Fengsheng Li ∗ National Special Superfine Powder Engineering Research Center of China, Nanjing 210094, China
a r t i c l e
i n f o
Article history: Received 26 May 2008 Received in revised form 27 July 2008 Accepted 29 July 2008 Available online 17 September 2008 Keywords: Oxide materials Chemical synthesis Magnetization TEM
a b s t r a c t Highly dispersed spinel CoFe2 O4 nanoparticles with average size of about 10 nm were successfully prepared by a novel salt-assisted combustion process. The effects of KCl and calcination temperature on the properties of the products were investigated by X-ray diffraction, transmission electron microscopy and Brunnauer–Emmet–Teller method. The results indicated that the obtained CoFe2 O4 nanoparticles had small particle size, well crystallinity and dispersibility, and large BET surface area. Moreover, the magnetic property of CoFe2 O4 was measured by superconducting quantum interference device. Magnetization studies at room temperature showed superparamagnetic behavior of the obtained CoFe2 O4 nanoparticles. Crown Copyright © 2008 Published by Elsevier B.V. All rights reserved.
1. Introduction Magnetic nanoparticles offer great potential applications in a variety of biomedical fields, such as improved contrast agents for magnetic resonance imaging (MRI) [1], cell separation [2], hyperthermia tumor treatment [3] and as magnetic field-guided carriers for localizing drugs or radioactive therapies [4]. Superparamagnetic CoFe2 O4 nanoparticles are considered as promising materials for above biomedical application, due to high magnetocrystalline anisotropy, moderate saturation magnetization, high chemical stability and high biocompatibility [5,6]. Additionally, superparamagnetic CoFe2 O4 nanoparticles do not retain magnetization before and after exposure to an external magnetic field, reducing the possibility of particle aggregation. Since well-dispersed CoFe2 O4 nanoparticles benefit many applications, many synthetic methods have been extensively investigated, such as sol–gel method [7], microemulsion [8], coprecipitation method [9], hydrothermal method [10], combustion [11], and forced hydrolysis in a polyol medium [12]. Among the various routes, self-propagating combustion reaction has attracted considerable attention due to its convenient processing, simple experimental setup, significant saving in time and energy, and homogeneous products [13]. However, self-propagating combustion reaction of welldispersed spinel ferrites faces great difficulty of inhibiting
∗ Corresponding author. Tel.: +86 25 84315942; fax: +86 25 84315042. E-mail address: [email protected] (F. Li).
agglomeration. In the past, great efforts have been made to solve this problem by adjusting the molar ratio of fuel/oxidant and choosing the fuel which can release a large amount of heat and gas in the combustion process [14,15]. In this paper, we develop a salt-assisted combustion synthesis (SCS) to prepare CoFe2 O4 nanoparticles and found that the addition of KCl resulted in the formation of welldispersed superparamagnetic CoFe2 O4 nanoparticles. 2. Experimental 2.1. Preparation of CoFe2 O4 nanoparticles All reagents were of analytical grade and used without further purification. In a typical procedure, firstly, 0.005 mol Co(NO3 )2 ·6H2 O and 0.01 mol Fe(NO3 )3 ·9H2 O were dissolved in 100 ml aqueous solution of glycine (NH2 CH2 COOH). The molar ratio of glycine/NO3 − (G/NO3 − ) is 1.0. Secondly, 0.01 mol KCl was added to the above solution and then the mixture was placed in the magnetic mixer and heated for 2 h at 60 ◦ C. During this procedure, the mixture evolved into brown transparent one and then viscous gel. Thirdly, the reaction temperature was adjusted to 110 ◦ C and the viscous gel bubbled up and autoignited, with the rapid evolution of a large volume of gases to produce black powder. In order to remove salt, the as-burnt powder was boiled in deionized water, filtered and washed with deionized water and ethanol. Finally, the product was dried in an oven at 60 ◦ C for 2 h. For comparison, CoFe2 O4 nanoparticles were prepared by the convention combustion synthesis (CCS). 2.2. Characterization The crystalline phase structure was determined by Bruker D8 Advance X-ray diffractometer (XRD, D/max 18 kV) using Cu K␣ radiation. Transmission electron microscopy (TEM) images were recorded on a Tecnai 12 transmission electron microscope. The BET surface areas were measured on Bechman Coulter SA3100 Plus instrument using N2 absorption at −196 ◦ C. And the magnetic properties of as-burnt
0925-8388/$ – see front matter. Crown Copyright © 2008 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.07.131
X. Zhang et al. / Journal of Alloys and Compounds 475 (2009) L34–L37
Fig. 1. XRD patterns of CoFe2 O4 samples prepared at different conditions: in the absence of salt (a) no heat treatment, (b) 400 ◦ C, (c) 600 ◦ C and in the presence of KCl, (d) no heat treatment, (e) 400 ◦ C and (f) 600 ◦ C.
powder and the samples annealed at different temperature were detected by MPMS XL7SQUID superconducting quantum interference device (SQUID).
3. Results and discussion
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Fig. 2. XRD patterns of CoFe2 O4 samples prepared under the conditions of G/NO3 − = 1 and different KCl/M: (a) KCl/M = 0, (b) KCl/M = 2/3, (c) KCl/M = 4/3 and (d) KCl/M = 2.
specific surface area from 13.443 to 124.57 m2 /g. It shows that KCl plays a vital role in forming small crystallite size and high specific surface area of CoFe2 O4 nanoparticles. The possible mechanism is to be discussed later.
3.1. XRD and BET analyses of CoFe2 O4 nanoparticles
3.2. TEM micrograph of CoFe2 O4 nanoparticles
Fig. 1 shows the XRD patterns of samples prepared at different conditions: in the absence of salt (a) no heat treatment, (b) 400 ◦ C, (c) 600 ◦ C and in the presence of KCl, (d) no heat treatment, (e) 400 ◦ C and (f) 600 ◦ C. From Fig. 1a, a single phase of CoFe2 O4 with spinel structure (JCPDS: 22-1086) is formed. The peaks can be assigned to (2 2 0), (3 1 1), (2 2 2), (4 0 0), (4 2 2), (5 1 1) and (4 4 0) plane of spinel structure CoFe2 O4 . When the G/NO3 − is 1.0, the convention combustion reaction can directly produce spinel structure CoFe2 O4 , as confirmed in Fig. 1a. In comparison, as shown in Fig. 1d, it is very significant that the introduction of KCl in the combustion synthesis results in a drastic decrease of crystallite size. Moreover, Fig. 1 reveals that both heat treatment and higher calcination temperature promote the formation of spinel structure and accelerate the growth of crystallite size. Fig. 2 shows XRD patterns of CoFe2 O4 prepared by SCS as a function of the amount of KCl. It is apparent that increasing KCl/M from 0 to 2/3 results in a drastic decrease in the crystallite size. However, at the same G/NO3 − = 1.0, further increasing KCl/M leads to an increase in the crystallite size and the possible formation of impurity phase (shown in Fig. 2d). When KCl/M exceeds a limit, the auto-propagating combustion reaction cannot occur, which was observed in our experiment. Therefore, on the basis of the experiment results, the optimal molar ratio of KCl/M is fixed on 2/3 in this paper. In addition, the effects of KCl and subsequent heat treatment temperature on the properties of CoFe2 O4 are summarized in Table 1. It is remarkable that in the case of equal G/NO3 − and annealing at 400 ◦ C, an addition of KCl into the redox mixture solution greatly reduces crystallite size from 36 to 7 nm, and enhances BET
The size, shape, and agglomeration state of the CoFe2 O4 nanoparticles obtained by combustion process are shown in Fig. 3. Fig. 3a reveals that the typical morphology of the particles obtained in the CCS, i.e. the three-dimensional network fractals composed of tightly bundled square-like nanoparticles. The ring-type diffraction patterns insert in Fig. 3 are indexed to polycrystalline CoFe2 O4 . As shown in Fig. 3b, the introduction of KCl into CCS breaks up the network structure of agglomerated nanocrystallites and results in the formation of the dispersed square-like CoFe2 O4 nanoparticles. The particle size is in the range of 10–12.5 nm and consistent with the crystallite size calculated using the Scherrer’s equation (listed in Table 1), which indicates that CoFe2 O4 agglomerates are well separated into nanocrystallites. Fig. 3c shows that the calcination of the KCl-containing combustion resultants leads to an increase in particle size from 10–12.5 nm to 13–17 nm. Due to the agglomeration inhibition of KCl during annealing, no apparent agglomeration between particles is observed. 3.3. Magnetic properties of CoFe2 O4 nanoparticles To clarify the magnetic properties of CoFe2 O4 nanoparticles, the hysteresis loops of the samples obtained under different conditions are measured by SQUID at room temperature (shown in Fig. 4). The magnetic parameters, including saturation magnetization (Ms ), residual magnetization (Mr ), and coercivity field (Hc ) are given in Table 2. Fig. 4a reveals that the magnetic properties of CoFe2 O4 nanoparticles by CCS show high dependence upon the annealing
Table 1 Effects of KCl and different calcination temperatures on the properties of CoFe2 O4 samples (G/NO3 − = 1.0) Sample
KCl/NO3 − (molar ratio)
Annealed temperature (◦ C)
Crystallite size (nm)
BET surface area (m2 /g)
a b c d e f
0 0 0 2/3 2/3 2/3
– 400 600 – 400 600
34 36 42 10 7 20
13.31 13.443 11.600 115.00 124.57 112.23
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X. Zhang et al. / Journal of Alloys and Compounds 475 (2009) L34–L37
Fig. 4. Hysteresis loops of CoFe2 O4 samples prepared at different conditions: in the absence of salt (a) no heat treatment; (c) 400 ◦ C; (e) 600 ◦ C and in the presence of KCl (b) no heat treatment; (d) 400 ◦ C; (f) 600 ◦ C.
temperature adopted. The Ms increases with annealing temperature, with a increment occurring between no calcination and 600 ◦ C from 14.3 to 47.49 emu/g, and similar evolution trend is also found in Mr , from 6.17 to 23.06 emu/g. However, Hc reaches its maximum of 1810 Oe at 400 ◦ C and then drastically decreases to 802 Oe at 600 ◦ C. It is easy to deduce that the evolution behaviors of Ms and Mr are highly depending upon the growth of CoFe2 O4 nanocrystallites. What should be especially noted is that the variation of Hc is different from that of Ms and Mr , which hinds that the size of CoFe2 O4 nanocrystallites is not the unique factor in deciding Hc . As have been reported by other groups [8], Hc increases with decreasing grain size (D) down to values of about 40 nm, independent of the kind of material. The increase of Hc is proportional to 1/D. Combining all the magnetic properties of described above, it could be concluded that CoFe2 O4 nanopowder with moderate Ms and relatively high Hc could be prepared by CCS. For example, the Ms and Hc of as-burnt CoFe2 O4 nanopowder after annealing at 400 ◦ C are 16.39 emu/g and 1810 Oe, repectively. However, further improvements on these magnetic parameters of CoFe2 O4 nanopowders are still wanted for satisfying the practical requirements of biomedical application. As shown in Fig. 4 Table 2 Magnetic parameters of CoFe2 O4 samples obtained under different conditions
Fig. 3. TEM micrographs of combustion-synthesized CoFe2 O4 (G/NO3 = 1): (the insert shows the diffraction patterns) (a) in the absence of salt, (b) in the presence of KCl (KCl/M = 2/3) and (c) after 3 h calcination of the KCl-containing products under the condition of (b) at 400 ◦ C.
Samples
Ms (emu/g)
Mr (emu/g)
a-As-burnt CoFe2 O4 powder by CCS c-CoFe2 O4 by CCS annealed at 400 ◦ C e-CoFe2 O4 by CCS annealed at 600 ◦ C b-As-burnt CoFe2 O4 powder by SCS d-CoFe2 O4 by SCS annealed at 400 ◦ C f-CoFe2 O4 by SCS annealed at 600 ◦ C
14.3 34.94 47.49 51.95 7.5 19.79
6.17 16.39 23.06 0 0 0
Hc (Oe) 802 1810 802 0 0 0
X. Zhang et al. / Journal of Alloys and Compounds 475 (2009) L34–L37
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Fig. 5. Schematic diagram of the possible formation processes of well-dispersed CoFe2 O4 nanoparticles in the salt-assisted combustion synthesis.
and Table 2, the coercivity and remanence of CoFe2 O4 nanopowder synthesized by SCS are almost zero, which is characteristic of superparamagnetism [16]. The as-burnt CoFe2 O4 powder by SCS has the highest Ms , and exhibits superparamagnetic phenomena. As listed in Table 1, the average crystallite sizes of CoFe2 O4 powders prepared by SCS are below 20 nm and the specific surface areas of them are more than 110 m2 /g. The quantum size effects and the large surface area of CoFe2 O4 nanoparticles dramatically change the magnetic properties and exhibit superparamagnetic phenomena and quantum tunneling of magnetization, because each particle can be considered as a single magnetic domain.
combustion synthesis. It was found that the instant salt precipitation in situ inhibits the formation of hard agglomerates and results in drastic increase in surface area. This procedure was suitable for the synthesis of well-dispersed CoFe2 O4 nanoparticles and could be potentially applied to the preparation of other spinel ferrites nanoparticles. TEM results indicated the obtained CoFe2 O4 nanoparticles by SCS were polycrystals, and the largest specific BET surface area of that was 124.57 m2 /g. Superparamagnetic behavior was obtained in the CoFe2 O4 nanoparticles by SCS and the highest saturation magnetization value reached 51.95 emu/g.
3.4. The possible mechanism of well-dispersed CoFe2 O4 prepared by SCS method
Acknowledgments
The possible formation processes of well-dispersed CoFe2 O4 nanoparticles by SCS are presented in Fig. 5. As is well known, in the process of solvent evaporation, the solute concentration will eventually reach a supersaturated state and begin to nucleate and precipitate especially on the crystal seeds such as some impurities. Since the loss of solvent occurs at the solution surface, it is here that the salt will be in highest concentration. In our SCS process, the nature of the salt precipitation and its location are intimately connected to the prevention of the particles from sintering and the generation of the well-dispersed nanoparticles. We suggest that since the self-propagating combustion reaction releases large amount of heat in the very short time, resulting in instant high temperature of the reaction system, the salt precipitation in situ is completed in an instant to form a thin layer of salt crust on the surface of the newly formed nanoparticles [17]. After the rapid cooling, the salt-coated CoFe2 O4 nanoparticles are trapped into the salt matrix, since the frozen salt matrix is no longer able to move, which prevents the re-agglomeration of the newly formed crystallites and stabilize the derived nanoparticles [18]. Followed by removing the soluble salt by aqueous wash and drying, the welldispersed CoFe2 O4 nanoparticles can be obtained. 4. Conclusions Highly dispersed CoFe2 O4 nanoparticles were successfully prepared via the facile introduction of salt in the conventional
We gratefully acknowledge the preliminary research project of China general equipment ministry (41328030507) and National Natural Science Foundation of China (No: 50602024). References [1] D.K. Kim, Y. Zhang, J. Kehr, T. Klason, B. Bjelke, M. Muhammed, J. Magn. Magn. Mater. 225 (2001) 256–261. [2] S. Roath, J. Magn. Magn. Mater. 122 (1993) 329–334. [3] S.W. Lee, S. Bae, Y. Takemura, I.B. Shim, T.M. Kim, J. Kim, et al., J. Magn. Magn. Mater. 310 (2007) 2868–2870. [4] J.Q. Cao, Y.X. Wang, J.F. Yu, J.Y. Xia, C.F. Zhang, D.Z. Yin, et al., J. Magn. Magn. Mater. 277 (2004) 165–174. [5] S. Rana, A. Gallo, R.S. Srivastava, R.D.K. Misra, Acta Biometal. 3 (2007) 233–242. [6] G.S. Rajan, S.L. Stromeyer, K.A. Mauritz, G.X. Miao, P. Mani, M. Shamsuzzoha, et al., J. Magn. Magn. Mater. 299 (2006) 211–218. [7] I.H. Gul, A. Maqsood, J. Alloys Compd 465 (2008) 227–231. [8] D.S. Mathew, R.S. Juang, Chem. Eng. J. 129 (2007) 51–65. [9] X.H. Yang, X. Wang, Z.D. Zhang, J. Cryst. Growth 277 (2005) 467–470. [10] L.J. Zhao, H.J. Zhang, Y. Xing, S.Y. Song, S.Y. Yu, W.D. Shi, J. Solid State Chem. 181 (2008) 245–252. [11] S.D. Bhame, P.A. Joy, Sens. Actuators A 137 (2007) 256–261. [12] N. Hanh, O.K. Quy, N.P. Thuy, L.D. Tung, L. Spinu, Phys. B 327 (2003) 382–384. [13] N.N. Mallikarjuna, A. Lagashetty, A. Venkataraman, J. Therm. Anal. Calorim. 74 (2003) 819–826. [14] C.H. Yan, Z.G. Xu, F.X. Cheng, Z.M. Wang, L.D. Sun, C.S. Liao, J.T. Jia, Solid State Commun. 111 (1999) 287–291. [15] A.F. Junior, E.C. de Oliveira Lima, M.A. Novak, P.R. Wells Jr., J. Magn. Magn. Mater. 308 (2007) 198–202. [16] P. Deb, A. Basumallick, P. Chatterjee, S.P. Sengupta, Scripta Mater. 45 (2001) 341–346. [17] W.F. Chen, F.S. Li, J.Y. Yu, L.L. Liu, H.L. Gao, Mater. Res. Bull. 41 (2006) 2318–2324. [18] Y.X. Li, W.F. Chen, X.Z. Zhou, Z.Y. Gu, C.M. Chen, Mater. Lett. 59 (2005) 48–52.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Letter
Salt-assisted combustion synthesis of highly dispersed superparamagnetic CoFe2 O4 nanoparticles Xiaojuan Zhang, Wei Jiang, Dan Song, Huijuan Sun, Zhendong Sun, Fengsheng Li ∗ National Special Superfine Powder Engineering Research Center of China, Nanjing 210094, China
a r t i c l e
i n f o
Article history: Received 26 May 2008 Received in revised form 27 July 2008 Accepted 29 July 2008 Available online 17 September 2008 Keywords: Oxide materials Chemical synthesis Magnetization TEM
a b s t r a c t Highly dispersed spinel CoFe2 O4 nanoparticles with average size of about 10 nm were successfully prepared by a novel salt-assisted combustion process. The effects of KCl and calcination temperature on the properties of the products were investigated by X-ray diffraction, transmission electron microscopy and Brunnauer–Emmet–Teller method. The results indicated that the obtained CoFe2 O4 nanoparticles had small particle size, well crystallinity and dispersibility, and large BET surface area. Moreover, the magnetic property of CoFe2 O4 was measured by superconducting quantum interference device. Magnetization studies at room temperature showed superparamagnetic behavior of the obtained CoFe2 O4 nanoparticles. Crown Copyright © 2008 Published by Elsevier B.V. All rights reserved.
1. Introduction Magnetic nanoparticles offer great potential applications in a variety of biomedical fields, such as improved contrast agents for magnetic resonance imaging (MRI) [1], cell separation [2], hyperthermia tumor treatment [3] and as magnetic field-guided carriers for localizing drugs or radioactive therapies [4]. Superparamagnetic CoFe2 O4 nanoparticles are considered as promising materials for above biomedical application, due to high magnetocrystalline anisotropy, moderate saturation magnetization, high chemical stability and high biocompatibility [5,6]. Additionally, superparamagnetic CoFe2 O4 nanoparticles do not retain magnetization before and after exposure to an external magnetic field, reducing the possibility of particle aggregation. Since well-dispersed CoFe2 O4 nanoparticles benefit many applications, many synthetic methods have been extensively investigated, such as sol–gel method [7], microemulsion [8], coprecipitation method [9], hydrothermal method [10], combustion [11], and forced hydrolysis in a polyol medium [12]. Among the various routes, self-propagating combustion reaction has attracted considerable attention due to its convenient processing, simple experimental setup, significant saving in time and energy, and homogeneous products [13]. However, self-propagating combustion reaction of welldispersed spinel ferrites faces great difficulty of inhibiting
∗ Corresponding author. Tel.: +86 25 84315942; fax: +86 25 84315042. E-mail address: [email protected] (F. Li).
agglomeration. In the past, great efforts have been made to solve this problem by adjusting the molar ratio of fuel/oxidant and choosing the fuel which can release a large amount of heat and gas in the combustion process [14,15]. In this paper, we develop a salt-assisted combustion synthesis (SCS) to prepare CoFe2 O4 nanoparticles and found that the addition of KCl resulted in the formation of welldispersed superparamagnetic CoFe2 O4 nanoparticles. 2. Experimental 2.1. Preparation of CoFe2 O4 nanoparticles All reagents were of analytical grade and used without further purification. In a typical procedure, firstly, 0.005 mol Co(NO3 )2 ·6H2 O and 0.01 mol Fe(NO3 )3 ·9H2 O were dissolved in 100 ml aqueous solution of glycine (NH2 CH2 COOH). The molar ratio of glycine/NO3 − (G/NO3 − ) is 1.0. Secondly, 0.01 mol KCl was added to the above solution and then the mixture was placed in the magnetic mixer and heated for 2 h at 60 ◦ C. During this procedure, the mixture evolved into brown transparent one and then viscous gel. Thirdly, the reaction temperature was adjusted to 110 ◦ C and the viscous gel bubbled up and autoignited, with the rapid evolution of a large volume of gases to produce black powder. In order to remove salt, the as-burnt powder was boiled in deionized water, filtered and washed with deionized water and ethanol. Finally, the product was dried in an oven at 60 ◦ C for 2 h. For comparison, CoFe2 O4 nanoparticles were prepared by the convention combustion synthesis (CCS). 2.2. Characterization The crystalline phase structure was determined by Bruker D8 Advance X-ray diffractometer (XRD, D/max 18 kV) using Cu K␣ radiation. Transmission electron microscopy (TEM) images were recorded on a Tecnai 12 transmission electron microscope. The BET surface areas were measured on Bechman Coulter SA3100 Plus instrument using N2 absorption at −196 ◦ C. And the magnetic properties of as-burnt
0925-8388/$ – see front matter. Crown Copyright © 2008 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.07.131
X. Zhang et al. / Journal of Alloys and Compounds 475 (2009) L34–L37
Fig. 1. XRD patterns of CoFe2 O4 samples prepared at different conditions: in the absence of salt (a) no heat treatment, (b) 400 ◦ C, (c) 600 ◦ C and in the presence of KCl, (d) no heat treatment, (e) 400 ◦ C and (f) 600 ◦ C.
powder and the samples annealed at different temperature were detected by MPMS XL7SQUID superconducting quantum interference device (SQUID).
3. Results and discussion
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Fig. 2. XRD patterns of CoFe2 O4 samples prepared under the conditions of G/NO3 − = 1 and different KCl/M: (a) KCl/M = 0, (b) KCl/M = 2/3, (c) KCl/M = 4/3 and (d) KCl/M = 2.
specific surface area from 13.443 to 124.57 m2 /g. It shows that KCl plays a vital role in forming small crystallite size and high specific surface area of CoFe2 O4 nanoparticles. The possible mechanism is to be discussed later.
3.1. XRD and BET analyses of CoFe2 O4 nanoparticles
3.2. TEM micrograph of CoFe2 O4 nanoparticles
Fig. 1 shows the XRD patterns of samples prepared at different conditions: in the absence of salt (a) no heat treatment, (b) 400 ◦ C, (c) 600 ◦ C and in the presence of KCl, (d) no heat treatment, (e) 400 ◦ C and (f) 600 ◦ C. From Fig. 1a, a single phase of CoFe2 O4 with spinel structure (JCPDS: 22-1086) is formed. The peaks can be assigned to (2 2 0), (3 1 1), (2 2 2), (4 0 0), (4 2 2), (5 1 1) and (4 4 0) plane of spinel structure CoFe2 O4 . When the G/NO3 − is 1.0, the convention combustion reaction can directly produce spinel structure CoFe2 O4 , as confirmed in Fig. 1a. In comparison, as shown in Fig. 1d, it is very significant that the introduction of KCl in the combustion synthesis results in a drastic decrease of crystallite size. Moreover, Fig. 1 reveals that both heat treatment and higher calcination temperature promote the formation of spinel structure and accelerate the growth of crystallite size. Fig. 2 shows XRD patterns of CoFe2 O4 prepared by SCS as a function of the amount of KCl. It is apparent that increasing KCl/M from 0 to 2/3 results in a drastic decrease in the crystallite size. However, at the same G/NO3 − = 1.0, further increasing KCl/M leads to an increase in the crystallite size and the possible formation of impurity phase (shown in Fig. 2d). When KCl/M exceeds a limit, the auto-propagating combustion reaction cannot occur, which was observed in our experiment. Therefore, on the basis of the experiment results, the optimal molar ratio of KCl/M is fixed on 2/3 in this paper. In addition, the effects of KCl and subsequent heat treatment temperature on the properties of CoFe2 O4 are summarized in Table 1. It is remarkable that in the case of equal G/NO3 − and annealing at 400 ◦ C, an addition of KCl into the redox mixture solution greatly reduces crystallite size from 36 to 7 nm, and enhances BET
The size, shape, and agglomeration state of the CoFe2 O4 nanoparticles obtained by combustion process are shown in Fig. 3. Fig. 3a reveals that the typical morphology of the particles obtained in the CCS, i.e. the three-dimensional network fractals composed of tightly bundled square-like nanoparticles. The ring-type diffraction patterns insert in Fig. 3 are indexed to polycrystalline CoFe2 O4 . As shown in Fig. 3b, the introduction of KCl into CCS breaks up the network structure of agglomerated nanocrystallites and results in the formation of the dispersed square-like CoFe2 O4 nanoparticles. The particle size is in the range of 10–12.5 nm and consistent with the crystallite size calculated using the Scherrer’s equation (listed in Table 1), which indicates that CoFe2 O4 agglomerates are well separated into nanocrystallites. Fig. 3c shows that the calcination of the KCl-containing combustion resultants leads to an increase in particle size from 10–12.5 nm to 13–17 nm. Due to the agglomeration inhibition of KCl during annealing, no apparent agglomeration between particles is observed. 3.3. Magnetic properties of CoFe2 O4 nanoparticles To clarify the magnetic properties of CoFe2 O4 nanoparticles, the hysteresis loops of the samples obtained under different conditions are measured by SQUID at room temperature (shown in Fig. 4). The magnetic parameters, including saturation magnetization (Ms ), residual magnetization (Mr ), and coercivity field (Hc ) are given in Table 2. Fig. 4a reveals that the magnetic properties of CoFe2 O4 nanoparticles by CCS show high dependence upon the annealing
Table 1 Effects of KCl and different calcination temperatures on the properties of CoFe2 O4 samples (G/NO3 − = 1.0) Sample
KCl/NO3 − (molar ratio)
Annealed temperature (◦ C)
Crystallite size (nm)
BET surface area (m2 /g)
a b c d e f
0 0 0 2/3 2/3 2/3
– 400 600 – 400 600
34 36 42 10 7 20
13.31 13.443 11.600 115.00 124.57 112.23
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X. Zhang et al. / Journal of Alloys and Compounds 475 (2009) L34–L37
Fig. 4. Hysteresis loops of CoFe2 O4 samples prepared at different conditions: in the absence of salt (a) no heat treatment; (c) 400 ◦ C; (e) 600 ◦ C and in the presence of KCl (b) no heat treatment; (d) 400 ◦ C; (f) 600 ◦ C.
temperature adopted. The Ms increases with annealing temperature, with a increment occurring between no calcination and 600 ◦ C from 14.3 to 47.49 emu/g, and similar evolution trend is also found in Mr , from 6.17 to 23.06 emu/g. However, Hc reaches its maximum of 1810 Oe at 400 ◦ C and then drastically decreases to 802 Oe at 600 ◦ C. It is easy to deduce that the evolution behaviors of Ms and Mr are highly depending upon the growth of CoFe2 O4 nanocrystallites. What should be especially noted is that the variation of Hc is different from that of Ms and Mr , which hinds that the size of CoFe2 O4 nanocrystallites is not the unique factor in deciding Hc . As have been reported by other groups [8], Hc increases with decreasing grain size (D) down to values of about 40 nm, independent of the kind of material. The increase of Hc is proportional to 1/D. Combining all the magnetic properties of described above, it could be concluded that CoFe2 O4 nanopowder with moderate Ms and relatively high Hc could be prepared by CCS. For example, the Ms and Hc of as-burnt CoFe2 O4 nanopowder after annealing at 400 ◦ C are 16.39 emu/g and 1810 Oe, repectively. However, further improvements on these magnetic parameters of CoFe2 O4 nanopowders are still wanted for satisfying the practical requirements of biomedical application. As shown in Fig. 4 Table 2 Magnetic parameters of CoFe2 O4 samples obtained under different conditions
Fig. 3. TEM micrographs of combustion-synthesized CoFe2 O4 (G/NO3 = 1): (the insert shows the diffraction patterns) (a) in the absence of salt, (b) in the presence of KCl (KCl/M = 2/3) and (c) after 3 h calcination of the KCl-containing products under the condition of (b) at 400 ◦ C.
Samples
Ms (emu/g)
Mr (emu/g)
a-As-burnt CoFe2 O4 powder by CCS c-CoFe2 O4 by CCS annealed at 400 ◦ C e-CoFe2 O4 by CCS annealed at 600 ◦ C b-As-burnt CoFe2 O4 powder by SCS d-CoFe2 O4 by SCS annealed at 400 ◦ C f-CoFe2 O4 by SCS annealed at 600 ◦ C
14.3 34.94 47.49 51.95 7.5 19.79
6.17 16.39 23.06 0 0 0
Hc (Oe) 802 1810 802 0 0 0
X. Zhang et al. / Journal of Alloys and Compounds 475 (2009) L34–L37
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Fig. 5. Schematic diagram of the possible formation processes of well-dispersed CoFe2 O4 nanoparticles in the salt-assisted combustion synthesis.
and Table 2, the coercivity and remanence of CoFe2 O4 nanopowder synthesized by SCS are almost zero, which is characteristic of superparamagnetism [16]. The as-burnt CoFe2 O4 powder by SCS has the highest Ms , and exhibits superparamagnetic phenomena. As listed in Table 1, the average crystallite sizes of CoFe2 O4 powders prepared by SCS are below 20 nm and the specific surface areas of them are more than 110 m2 /g. The quantum size effects and the large surface area of CoFe2 O4 nanoparticles dramatically change the magnetic properties and exhibit superparamagnetic phenomena and quantum tunneling of magnetization, because each particle can be considered as a single magnetic domain.
combustion synthesis. It was found that the instant salt precipitation in situ inhibits the formation of hard agglomerates and results in drastic increase in surface area. This procedure was suitable for the synthesis of well-dispersed CoFe2 O4 nanoparticles and could be potentially applied to the preparation of other spinel ferrites nanoparticles. TEM results indicated the obtained CoFe2 O4 nanoparticles by SCS were polycrystals, and the largest specific BET surface area of that was 124.57 m2 /g. Superparamagnetic behavior was obtained in the CoFe2 O4 nanoparticles by SCS and the highest saturation magnetization value reached 51.95 emu/g.
3.4. The possible mechanism of well-dispersed CoFe2 O4 prepared by SCS method
Acknowledgments
The possible formation processes of well-dispersed CoFe2 O4 nanoparticles by SCS are presented in Fig. 5. As is well known, in the process of solvent evaporation, the solute concentration will eventually reach a supersaturated state and begin to nucleate and precipitate especially on the crystal seeds such as some impurities. Since the loss of solvent occurs at the solution surface, it is here that the salt will be in highest concentration. In our SCS process, the nature of the salt precipitation and its location are intimately connected to the prevention of the particles from sintering and the generation of the well-dispersed nanoparticles. We suggest that since the self-propagating combustion reaction releases large amount of heat in the very short time, resulting in instant high temperature of the reaction system, the salt precipitation in situ is completed in an instant to form a thin layer of salt crust on the surface of the newly formed nanoparticles [17]. After the rapid cooling, the salt-coated CoFe2 O4 nanoparticles are trapped into the salt matrix, since the frozen salt matrix is no longer able to move, which prevents the re-agglomeration of the newly formed crystallites and stabilize the derived nanoparticles [18]. Followed by removing the soluble salt by aqueous wash and drying, the welldispersed CoFe2 O4 nanoparticles can be obtained. 4. Conclusions Highly dispersed CoFe2 O4 nanoparticles were successfully prepared via the facile introduction of salt in the conventional
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