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Preparation of cobalt ferrite nanoparticles via a novel solvothermal approach using divalent iron salt as precursors
Materials Research Bulletin 48 (2013) 214–217
Contents lists available at SciVerse ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Preparation of cobalt ferrite nanoparticles via a novel solvothermal approach using divalent iron salt as precursors Jie Ma a,b,*, Jiantao Zhao a, Wenlie Li a, Shuping Zhang a,b, Zhenran Tian a, Sergey Basov a a b
College of Science, University of Shanghai for Science and Technology, China Green Bio- & Eco-Chem. Eng. Lab, University of Shanghai for Science and Technology, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 19 July 2012 Received in revised form 14 September 2012 Accepted 29 September 2012 Available online 15 November 2012
Cobalt ferrite (CoFe2O4) nanoparticles are synthesized by a facile novel solvothermal method. The reactions are firstly performed in water–glycol system and Fe2+ salt is used as iron source without oxidant help. Some factors influenced the reactions, including temperature, reaction time, additives, are investigated. The samples are characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and scanning electron microscopy (SEM), respectively. The magnetic properties of some samples are detected by vibrating sample magnetometry techniques (VSM). It is firstly found that the magnetism of cobalt ferrites nanomaterials can be modified by some additives. The coercivity of CoFe2O4 nanoparticles evidently decreases from 600 to 50 Oe in the presence of PEG-4000 in the system. ß 2012 Elsevier Ltd. All rights reserved.
Keywords: A. Magnetic materials A. Nanostructures A. Oxides C. Magnetic properties
1. Introduction Spinel ferrites CoFe2O4, nanomaterials are paid more attentions due to their stability and striking magnetic, electrical and optical properties, with potential applications in various fields. They can be applied in high-density information storage, magnetic resonance imaging (MRI) enhancement, magnetically guided drug delivery [1– 5], ferrofluid technology [6], electric catalysis, chemical sensors, photoelectric devices, and so on. The magnetic characteristics of the ferrite are strongly influenced by the particle size, and superparamagnetic phenomenon may be generated at special conditions [7]. Ayyappan et al. reported that metal ion concentration plays an important role in the size, magnetic property and purity of cobalt ferrite nanoparticles [8]. Vestal et al. found that interactions among particles have significant effect on their magnetic properties [9]. Many methods have been developed for synthesizing the functional nanomaterials, including sol–gel method [10,11], chemical coprecipitation [12–15], microemulsion, hydrothermal method [16– 18], mechano-chemical method, solvothermal approach [19–21], and self-propagating combustion reaction [22], and so on. Among these methods, hydrothermal/solvothermal method is an ideal approach for preparing CoFe2O4 nanoparticles with excellent magnetism, stability and special shapes. As we well known, although this ferrite is commonly synthesized by trivalent iron resource, it is
not relative report about that a divalent iron salt is used as resource for preparing CoFe2O4 nanoparticles without any ex-oxidants via a solvothermal approach. In this study, it is firstly demonstrated that cobalt ferrite nanoparticles can be synthesized via solvothermal approach using Fe2+ as iron source without extraneous oxidant help. For controlling the size and morphologies of nanoparticles, some influence factors of products are investigated, including the reaction temperature, the reaction time, and additives. The study demonstrates that the crucial factor, which influences the magnetic properties of sample, is not reaction time or temperature, but some additives. Compared to some reported approaches, this solvothermal method may be a facile, environmentally and economically alternative approach to prepare other ferrite nanomaterials. 2. Experimental Ferrous chloride (FeCl24H2O), cobalt nitrate (Co(NO3)26H2O), absolute ethanol, glycol, hexamethylenetetramine (HMTA), sodium citrate, cetyltrimethyl ammonium bromide (CTAB) and polyethylene glycol 4000 (PEG-4000) are all purchased from Shanghai Chemical Reagent Ltd., and used without further purification. De-ionized water used in all experiment is self-made with the electric conductivity round 5 mS. 2.1. Synthesized process
* Corresponding author at: Jungong Road 334#, 200093 Shanghai, China. Tel.: +86 21 65665041x110. E-mail address: [email protected] (J. Ma). 0025-5408/$ – see front matter ß 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2012.09.072
The typical procedure is described as follows: 0.5 mmol Co(NO3)26H2O, 1 mmol FeCl24H2O, 0.1 mmol hexamethylene
J. Ma et al. / Materials Research Bulletin 48 (2013) 214–217
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tetramine (HMTA), 50 ml glycol and 20 ml deionized water are placed into a 100 ml Teflon-lined stainless-steel autoclave. After the mixture is agitated vigorously about 20 min, the autoclaves are sealed and settled in a digital-controlled constant temperature oven at 100–160 8C for 3–24 h. The autoclave is took out and cooled to room temperature naturally. The products are washed with deionized water and absolute ethanol three times to remove excess electrolytes and organics. Some additives, such as 0.5 g PEG4000, 0.5 g CTAB and 0.5 g sodium citrate are respectively added into some reaction systems to control the properties of CoFe2O4 nanoparticles during the reaction. 2.2. Characterized method and instruments The crystal phase and purity of the product are characterized by X-ray powder diffraction (XRD, Bruker, D8 Focus, Germany) equipped with graphite monochromatized Cu Ka radiation (l = 1.54056). The XRD patterns are acquired in a 2u ranges from 5 8C to 70 8C at scanning rate of 0.028 s 1. The morphologies of the as-synthesized products are inspected by scanning electron microscopy (SEM, Philip XL30, Holland) at an accelerating voltage of 20 kV, and transmission electron microscopy (TEM, Hitachi, H800EM, Japan) at an accelerating voltage of 200 kV. The roomtemperature M–H curves of typical samples are obtained by vibrating sample magnetometer (VSM, Lake Shore 735VSM, USA) in the range from 8000 to 8000 Oe. 3. Results and discussion The crystalline structure of CoFe2O4 samples are characterized by XRD and relative results shown in Fig. 1. The diffraction peaks of all samples can easily be indexed as (1 1 1), (2 2 0), (3 1 1), (4 0 0), (5 1 1) and (4 4 0) planes of a cubic structure and matches well with the standard data of spinel-type CoFe2O4 (JCPDS No. 221086). The crystallite sizes of as-obtained samples, which are calculated by Scherrer equation, only slightly change from 28 nm to 33 nm with change in reaction time. According to some
Fig. 1. The XRD patterns of CoFe2O4 powder obtained at reaction time of 6 h (a), 12 h (b), and 24 h (c) with the same temperature of 160 8C, respectively.
references [19,20], the crystallinity of sample should be improved in the procedure with extending reaction time. In the synthesis system, some glycol and its oligomer can be absorbed on the surface of nanoparticles and form an organic layer, which inhibits the growth of crystal size. So the size of samples does not increase obviously. The typical SEM images of samples obtained at different time are listed in Fig. 2. Fig. 2A and B indicates that the samples obtained in less than 6 h consists of clumps of particles with various sizes. Contrarily, when the reaction time is over 12 h, the sample is component with the narrow diameter region around 30 nm shown in Fig. 2C and D. Meanwhile, compared images in Fig. 2C and D, the similar size of two samples further confirms that the size of CoFe2O4 is not affected by the reaction time when it exceeds an
Fig. 2. SEM images of CoFe2O4 obtained at 160 8C with different reaction time (A) 3 h, (B) 6 h, (C) 12 h, and (D) 24 h.
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Fig. 3. SEM pictures of cobalt ferrite samples synthesized at 160 8C for 12 h with various organic agent types (a) CTAB, (b) phenol, and (c) PEG-4000.
opportune value. It is in accordance with the analyzed Xdiffraction. With the change in reaction time, the crystallization degree of the samples is improved, and more and more glycol molecules are polymerized. The low-polymerized organic substances are absorbed on the surface of nanoparticles, which inhibit the growth of nanoparticles. Some glycols are polymerized in the system, which can be confirmed evidently by increased viscosity of the solution further after the reaction. Contrarily, when the reaction time is less than the proper value, the nanoparticles easily aggregate and form diverse clumps via the strong interaction among nanoparticles due to the low absorbance of organic polymer on the surface of them. For demonstrating the above assumption, some reactions are readjusted and some additives are presented in the system, all reaction conditions are same as in Fig. 2B. The SEM images of typical samples are shown in Fig. 3. The images display that the morphologies and size of the samples have been significantly improved with the help of additives, which is accordant with Ref. [23]. This result is also proved by a typical TEM image of the sample shown in Fig. 4. Above results suggest that the grown and agglomeration of nanoparticles should be modified by organics absorbed on the surface gains. According to the above experimental results and some references [24], some reaction procedures had to be performed in the synthesis system as shown in Fig. 5. HMTA is hydrolyzed into ammonia and formaldehyde in warm aqueous solution (see Eq. (1)). In the hydrothermal setting, the Fe2+ and Co2+ ions react with ammonium hydroxide to form iron(II) and cobalt(II) hydroxide at firstly stage, respectively (Eqs. (2) and (3)). In alkaline conditions, Fe(OH)2 is sufficiently oxidized into Fe(OH)3 by O2 dissolved in the mixture system via hydrothermal procedure. Finally, Fe(OH)3 and Co(OH)2 are transformed into CoFe2O4 via dehydration process in
Fig. 4. TEM image of PEG-4000 added CoFe2O4 nanoparticles sample obtained at 160 8C with reaction time 12 h.
Fig. 5. The main reaction equations supposed in the solvothermal procedure.
the system as shown in Eq. (5). During the procedures, some glycol molecules can be condensed into low-clustering PEG. PEG is absorbed on the surface of the nanoparticles or dissolved into the solution, which can retain the growth of CoFe2O4 nanoparticles [23]. The room-temperature magnetic properties of CoFe2O4 nanoparticles obtained at 160 8C with the different time are studied by VSM with an applied magnetic field of 8000 Oe. Fig. 6 shows the room-temperature hysteresis loop of CoFe2O4 nanoparticles and the corresponding evolution of magnetic parameters of saturation magnetization (Ms) and coercivity (Hc), respectively. Their curves prove that the Ms of the samples increases with increase in reaction time. Meanwhile, the similar Ms value of samples a and b show that the ferromagnetic CoFe2O4 nanomaterials with the maximum value of Ms about 73 emu/g at this synthesis system can be obtained at an
Fig. 6. The hysteresis loop of CoFe2O4 nanoparticles obtained at 160 8C for 12 h (a), 6 h (b), 3 h (c), respectively.
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salt as iron resource without the help of any oxidizing additives. The morphology of the samples can be modified by adjusting reaction time or by adding additives. Its saturation magnetization can be improved, but the coercive force of them hardly changes with increasing reaction time. It is firstly found that the coercive force of samples can be adjusted from 600 to 50 Oe by the addition of PEG-4000. It will be further investigated how additives control the magnetic properties of ferrite nanoparticles in the synthesis system.
Acknowledgements The authors are thankful to the National Natural Science Foundation of China (No. 20906061), the Shanghai International Cooperation Foundation (No. 073458014) for financial supports. Fig. 7. The hysteresis loop of CoFe2O4 nanoparticles obtained at 160 8C for 12 h with PEG-4000.
appropriate reaction time, after that the Ms value of sample hardly changes with increase in time. One the other hand, the Hc of the samples changes in range of 600 and 900 Oe with the increasing reaction time, which should be attributed to the different crystal degree and interaction among particles. Fig. 7 shows the magnetic properties of sample obtained in the PEG-4000 system. Compared to the magnetic curves, shown in Fig. 6b, of the sample obtained at same conditions without PEG, the Ms value of the sample changes from 70 to 60 emu/g, meanwhile, the coercivity of the sample has enormously decreased from 600 to 50 Oe, and the remanence of the particles has decreased from 40 to 2.5 emu/g. The above obvious difference should be attributed to PEG-4000. For PEG-4000 is linear two hydroxyterminated molecular, it is easily absorbed at the surface of metal oxide colloid by hydrogen bonds, chemical bonds, or Van der Waals force. When the surface of the colloid adsorbs PEG, the colloidal activities will decrease greatly and the growth rate of the colloids will be confined [23]. Therefore, the growing process kinetics of the nanoparticles can be modified by the addition of PEG-4000, which leads to the crystallinity and the morphology change of nanoparticles in the synthesis system. These results indicate that the magnetic properties of samples can be modified by additives. It can be predicted that super-paramagnetic cobalt ferrites nanomaterials might be obtained by the addition of an appropriate additive, and this synthesis system can be evolved into an alternative method for preparing the super-paramagnetic ferrites in future. 4. Conclusions In summary, a novel solvothermal process is achieved to synthesize monodispersed cobalt ferrite nanoparticles using Fe2+
References [1] H. Wu, G. Liu, X. Wang, J. Zhang, Y. Chen, J. Shi, H. Yang, H. Hu, S. Yang, Acta Biomater. 7 (2011) 3496–3504. [2] M.D. Shultz, S. Calvin, P.P. Fatouros, S.A. Morrison, E.E. Carpenter, J. Magn. Magn. Mater. 311 (2007) 464–468. [3] A.N. Ay, D. Konuk, B. Zu¨mreog˘lu-Karan, Mater. Sci. Eng. C 31 (2011) 851–857. [4] H.M. Joshi, Y.P.L. Aslam, P.V. Prasad, E.A. Schultz-Sikma, R. Edelman, V.P. Dravid, J. Phys. Chem. C 113 (2009) 17761–17767. [5] S. Rana, A. Gallo, R.S. Srivastava, R.D.K. Misra, Acta Biomater. 3 (2007) 233–242. [6] I. Sharifi, H. Shokrollahi, S. Amiri, J. Magn. Magn. Mater. 324 (2012) 903–915. [7] C. Va´zquez-Va´zquez, M.A. Lo´pez-Quintela, M.C. Buja´n-Nu´nez, J. Rivas, J. Nanopart. Res. 13 (2011) 1663–1676. [8] S. Ayyappan, S. Mahadevan, P. Chandramohan, M.P. Srinivasan, J. Philip, B. Raj, J. Phys. Chem. C 114 (2010) 6334–6341. [9] C.R. Vestal, Q. Song, Z.J. Zhang, J. Phys. Chem. B 108 (2004) 18222–18227. [10] M. Sajjia, M. Oubaha, T. Prescott, A.G. Olabi, J. Alloys Compd. 506 (2010) 400–406. [11] S.H. Xiao, K. Luo, L. Zhang, Mater. Chem. Phys. 123 (2010) 385–389. [12] R.D. Desautels, J. van Lierop, J.M. Cadogan, J. Phys.: Conf. Ser. 217 (2010) 012105. [13] S.S. More, R.H. Kadam, A.B. Kadam, A.R. Shite, D.R. Mane, K.M. Jadhav, J. Alloys Compd. 502 (2010) 477–479. [14] M.M. El-Okr, M.A. Salem, M.S. Salim, R.M. El-Okr, M. Ashoush, H.M. Talaat, J. Magn. Magn. Mater. 323 (2011) 920–926. [15] Y. Zhang, Z. Yang, D. Yin, Y. Liu, C.L. Fei, R. Xiong, J. Shi, G.L. Yan, J. Magn. Magn. Mater. 322 (2010) 3470–3475. [16] V. Cabuil, V. Dupuis, D. Talbot, S. Neveu, J. Magn. Magn. Mater. 323 (2011) 1238– 1241. [17] J. Peng, M. Hojamberdiev, Y. Xu, B. Cao, J. Wang, H. Wu, J. Magn. Magn. Mater. 323 (2011) 133–138. [18] R.M. Mohamed, M.M. Rashad, F.A. Haraz, W. Sigmund, J. Magn. Magn. Mater. 322 (2010) 2058–2064. [19] L. Ajroudi, S. Villain, V. Madigou, N. Mliki, Ch. Leroux, J. Cryst. Growth 312 (2010) 2465–2471. [20] S. Zhao, D. Ma, Hindawi Publishing Corporation, J. Nanomater. (2010) (article ID 842816). [21] S. Yanez-Vilar, M. Sanchez-Andujar, C. Gomez-Aguirre, J. Mira, M.A. SenarsRodrguez, S. Castro-Garcla, J. Solid State Chem. 182 (2009) 2685–2690. [22] C. Fei, Y. Zhang, Z. Yang, Y. Liu, R. Xiong, J. Shi, X. Ruan, J. Magn. Magn. Mater. 323 (2011) 1811–1816. [23] Z.T. Chen, L. Gao, Mater. Sci. Eng. B 141 (2007) 82–86. [24] J. Ma, W. Liu, S.P. Zhang, J.T. Zhao, W.L. Li, J. Alloys Compd. 509 (2011) 7895–7899.
Contents lists available at SciVerse ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Preparation of cobalt ferrite nanoparticles via a novel solvothermal approach using divalent iron salt as precursors Jie Ma a,b,*, Jiantao Zhao a, Wenlie Li a, Shuping Zhang a,b, Zhenran Tian a, Sergey Basov a a b
College of Science, University of Shanghai for Science and Technology, China Green Bio- & Eco-Chem. Eng. Lab, University of Shanghai for Science and Technology, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 19 July 2012 Received in revised form 14 September 2012 Accepted 29 September 2012 Available online 15 November 2012
Cobalt ferrite (CoFe2O4) nanoparticles are synthesized by a facile novel solvothermal method. The reactions are firstly performed in water–glycol system and Fe2+ salt is used as iron source without oxidant help. Some factors influenced the reactions, including temperature, reaction time, additives, are investigated. The samples are characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and scanning electron microscopy (SEM), respectively. The magnetic properties of some samples are detected by vibrating sample magnetometry techniques (VSM). It is firstly found that the magnetism of cobalt ferrites nanomaterials can be modified by some additives. The coercivity of CoFe2O4 nanoparticles evidently decreases from 600 to 50 Oe in the presence of PEG-4000 in the system. ß 2012 Elsevier Ltd. All rights reserved.
Keywords: A. Magnetic materials A. Nanostructures A. Oxides C. Magnetic properties
1. Introduction Spinel ferrites CoFe2O4, nanomaterials are paid more attentions due to their stability and striking magnetic, electrical and optical properties, with potential applications in various fields. They can be applied in high-density information storage, magnetic resonance imaging (MRI) enhancement, magnetically guided drug delivery [1– 5], ferrofluid technology [6], electric catalysis, chemical sensors, photoelectric devices, and so on. The magnetic characteristics of the ferrite are strongly influenced by the particle size, and superparamagnetic phenomenon may be generated at special conditions [7]. Ayyappan et al. reported that metal ion concentration plays an important role in the size, magnetic property and purity of cobalt ferrite nanoparticles [8]. Vestal et al. found that interactions among particles have significant effect on their magnetic properties [9]. Many methods have been developed for synthesizing the functional nanomaterials, including sol–gel method [10,11], chemical coprecipitation [12–15], microemulsion, hydrothermal method [16– 18], mechano-chemical method, solvothermal approach [19–21], and self-propagating combustion reaction [22], and so on. Among these methods, hydrothermal/solvothermal method is an ideal approach for preparing CoFe2O4 nanoparticles with excellent magnetism, stability and special shapes. As we well known, although this ferrite is commonly synthesized by trivalent iron resource, it is
not relative report about that a divalent iron salt is used as resource for preparing CoFe2O4 nanoparticles without any ex-oxidants via a solvothermal approach. In this study, it is firstly demonstrated that cobalt ferrite nanoparticles can be synthesized via solvothermal approach using Fe2+ as iron source without extraneous oxidant help. For controlling the size and morphologies of nanoparticles, some influence factors of products are investigated, including the reaction temperature, the reaction time, and additives. The study demonstrates that the crucial factor, which influences the magnetic properties of sample, is not reaction time or temperature, but some additives. Compared to some reported approaches, this solvothermal method may be a facile, environmentally and economically alternative approach to prepare other ferrite nanomaterials. 2. Experimental Ferrous chloride (FeCl24H2O), cobalt nitrate (Co(NO3)26H2O), absolute ethanol, glycol, hexamethylenetetramine (HMTA), sodium citrate, cetyltrimethyl ammonium bromide (CTAB) and polyethylene glycol 4000 (PEG-4000) are all purchased from Shanghai Chemical Reagent Ltd., and used without further purification. De-ionized water used in all experiment is self-made with the electric conductivity round 5 mS. 2.1. Synthesized process
* Corresponding author at: Jungong Road 334#, 200093 Shanghai, China. Tel.: +86 21 65665041x110. E-mail address: [email protected] (J. Ma). 0025-5408/$ – see front matter ß 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2012.09.072
The typical procedure is described as follows: 0.5 mmol Co(NO3)26H2O, 1 mmol FeCl24H2O, 0.1 mmol hexamethylene
J. Ma et al. / Materials Research Bulletin 48 (2013) 214–217
215
tetramine (HMTA), 50 ml glycol and 20 ml deionized water are placed into a 100 ml Teflon-lined stainless-steel autoclave. After the mixture is agitated vigorously about 20 min, the autoclaves are sealed and settled in a digital-controlled constant temperature oven at 100–160 8C for 3–24 h. The autoclave is took out and cooled to room temperature naturally. The products are washed with deionized water and absolute ethanol three times to remove excess electrolytes and organics. Some additives, such as 0.5 g PEG4000, 0.5 g CTAB and 0.5 g sodium citrate are respectively added into some reaction systems to control the properties of CoFe2O4 nanoparticles during the reaction. 2.2. Characterized method and instruments The crystal phase and purity of the product are characterized by X-ray powder diffraction (XRD, Bruker, D8 Focus, Germany) equipped with graphite monochromatized Cu Ka radiation (l = 1.54056). The XRD patterns are acquired in a 2u ranges from 5 8C to 70 8C at scanning rate of 0.028 s 1. The morphologies of the as-synthesized products are inspected by scanning electron microscopy (SEM, Philip XL30, Holland) at an accelerating voltage of 20 kV, and transmission electron microscopy (TEM, Hitachi, H800EM, Japan) at an accelerating voltage of 200 kV. The roomtemperature M–H curves of typical samples are obtained by vibrating sample magnetometer (VSM, Lake Shore 735VSM, USA) in the range from 8000 to 8000 Oe. 3. Results and discussion The crystalline structure of CoFe2O4 samples are characterized by XRD and relative results shown in Fig. 1. The diffraction peaks of all samples can easily be indexed as (1 1 1), (2 2 0), (3 1 1), (4 0 0), (5 1 1) and (4 4 0) planes of a cubic structure and matches well with the standard data of spinel-type CoFe2O4 (JCPDS No. 221086). The crystallite sizes of as-obtained samples, which are calculated by Scherrer equation, only slightly change from 28 nm to 33 nm with change in reaction time. According to some
Fig. 1. The XRD patterns of CoFe2O4 powder obtained at reaction time of 6 h (a), 12 h (b), and 24 h (c) with the same temperature of 160 8C, respectively.
references [19,20], the crystallinity of sample should be improved in the procedure with extending reaction time. In the synthesis system, some glycol and its oligomer can be absorbed on the surface of nanoparticles and form an organic layer, which inhibits the growth of crystal size. So the size of samples does not increase obviously. The typical SEM images of samples obtained at different time are listed in Fig. 2. Fig. 2A and B indicates that the samples obtained in less than 6 h consists of clumps of particles with various sizes. Contrarily, when the reaction time is over 12 h, the sample is component with the narrow diameter region around 30 nm shown in Fig. 2C and D. Meanwhile, compared images in Fig. 2C and D, the similar size of two samples further confirms that the size of CoFe2O4 is not affected by the reaction time when it exceeds an
Fig. 2. SEM images of CoFe2O4 obtained at 160 8C with different reaction time (A) 3 h, (B) 6 h, (C) 12 h, and (D) 24 h.
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Fig. 3. SEM pictures of cobalt ferrite samples synthesized at 160 8C for 12 h with various organic agent types (a) CTAB, (b) phenol, and (c) PEG-4000.
opportune value. It is in accordance with the analyzed Xdiffraction. With the change in reaction time, the crystallization degree of the samples is improved, and more and more glycol molecules are polymerized. The low-polymerized organic substances are absorbed on the surface of nanoparticles, which inhibit the growth of nanoparticles. Some glycols are polymerized in the system, which can be confirmed evidently by increased viscosity of the solution further after the reaction. Contrarily, when the reaction time is less than the proper value, the nanoparticles easily aggregate and form diverse clumps via the strong interaction among nanoparticles due to the low absorbance of organic polymer on the surface of them. For demonstrating the above assumption, some reactions are readjusted and some additives are presented in the system, all reaction conditions are same as in Fig. 2B. The SEM images of typical samples are shown in Fig. 3. The images display that the morphologies and size of the samples have been significantly improved with the help of additives, which is accordant with Ref. [23]. This result is also proved by a typical TEM image of the sample shown in Fig. 4. Above results suggest that the grown and agglomeration of nanoparticles should be modified by organics absorbed on the surface gains. According to the above experimental results and some references [24], some reaction procedures had to be performed in the synthesis system as shown in Fig. 5. HMTA is hydrolyzed into ammonia and formaldehyde in warm aqueous solution (see Eq. (1)). In the hydrothermal setting, the Fe2+ and Co2+ ions react with ammonium hydroxide to form iron(II) and cobalt(II) hydroxide at firstly stage, respectively (Eqs. (2) and (3)). In alkaline conditions, Fe(OH)2 is sufficiently oxidized into Fe(OH)3 by O2 dissolved in the mixture system via hydrothermal procedure. Finally, Fe(OH)3 and Co(OH)2 are transformed into CoFe2O4 via dehydration process in
Fig. 4. TEM image of PEG-4000 added CoFe2O4 nanoparticles sample obtained at 160 8C with reaction time 12 h.
Fig. 5. The main reaction equations supposed in the solvothermal procedure.
the system as shown in Eq. (5). During the procedures, some glycol molecules can be condensed into low-clustering PEG. PEG is absorbed on the surface of the nanoparticles or dissolved into the solution, which can retain the growth of CoFe2O4 nanoparticles [23]. The room-temperature magnetic properties of CoFe2O4 nanoparticles obtained at 160 8C with the different time are studied by VSM with an applied magnetic field of 8000 Oe. Fig. 6 shows the room-temperature hysteresis loop of CoFe2O4 nanoparticles and the corresponding evolution of magnetic parameters of saturation magnetization (Ms) and coercivity (Hc), respectively. Their curves prove that the Ms of the samples increases with increase in reaction time. Meanwhile, the similar Ms value of samples a and b show that the ferromagnetic CoFe2O4 nanomaterials with the maximum value of Ms about 73 emu/g at this synthesis system can be obtained at an
Fig. 6. The hysteresis loop of CoFe2O4 nanoparticles obtained at 160 8C for 12 h (a), 6 h (b), 3 h (c), respectively.
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salt as iron resource without the help of any oxidizing additives. The morphology of the samples can be modified by adjusting reaction time or by adding additives. Its saturation magnetization can be improved, but the coercive force of them hardly changes with increasing reaction time. It is firstly found that the coercive force of samples can be adjusted from 600 to 50 Oe by the addition of PEG-4000. It will be further investigated how additives control the magnetic properties of ferrite nanoparticles in the synthesis system.
Acknowledgements The authors are thankful to the National Natural Science Foundation of China (No. 20906061), the Shanghai International Cooperation Foundation (No. 073458014) for financial supports. Fig. 7. The hysteresis loop of CoFe2O4 nanoparticles obtained at 160 8C for 12 h with PEG-4000.
appropriate reaction time, after that the Ms value of sample hardly changes with increase in time. One the other hand, the Hc of the samples changes in range of 600 and 900 Oe with the increasing reaction time, which should be attributed to the different crystal degree and interaction among particles. Fig. 7 shows the magnetic properties of sample obtained in the PEG-4000 system. Compared to the magnetic curves, shown in Fig. 6b, of the sample obtained at same conditions without PEG, the Ms value of the sample changes from 70 to 60 emu/g, meanwhile, the coercivity of the sample has enormously decreased from 600 to 50 Oe, and the remanence of the particles has decreased from 40 to 2.5 emu/g. The above obvious difference should be attributed to PEG-4000. For PEG-4000 is linear two hydroxyterminated molecular, it is easily absorbed at the surface of metal oxide colloid by hydrogen bonds, chemical bonds, or Van der Waals force. When the surface of the colloid adsorbs PEG, the colloidal activities will decrease greatly and the growth rate of the colloids will be confined [23]. Therefore, the growing process kinetics of the nanoparticles can be modified by the addition of PEG-4000, which leads to the crystallinity and the morphology change of nanoparticles in the synthesis system. These results indicate that the magnetic properties of samples can be modified by additives. It can be predicted that super-paramagnetic cobalt ferrites nanomaterials might be obtained by the addition of an appropriate additive, and this synthesis system can be evolved into an alternative method for preparing the super-paramagnetic ferrites in future. 4. Conclusions In summary, a novel solvothermal process is achieved to synthesize monodispersed cobalt ferrite nanoparticles using Fe2+
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