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Ionic liquid-assisted solvothermal synthesis of hollow CoFe2O4 microspheres and their absorbing performances
Accepted Manuscript Ionic liquid-assisted solvothermal synthesis of hollow CoFe2O4 microspheres and their absorbing performances Xiang Ni, Zhuangzhang He, Xi Liu, Qingze Jiao, Hansheng Li, Caihong Feng, Yun Zhao PII: DOI: Reference:
S0167-577X(17)30126-X http://dx.doi.org/10.1016/j.matlet.2017.01.109 MLBLUE 22062
To appear in:
Materials Letters
Received Date: Revised Date: Accepted Date:
8 August 2016 13 January 2017 23 January 2017
Please cite this article as: X. Ni, Z. He, X. Liu, Q. Jiao, H. Li, C. Feng, Y. Zhao, Ionic liquid-assisted solvothermal synthesis of hollow CoFe2O4 microspheres and their absorbing performances, Materials Letters (2017), doi: http:// dx.doi.org/10.1016/j.matlet.2017.01.109
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Ionic liquid-assisted solvothermal synthesis of hollow CoFe2O4 microspheres and their absorbing performances Xiang Ni a, Zhuangzhang He a, Xi Liu a, Qingze Jiao a, b, Hansheng Li a, Caihong Feng a, Yun Zhao a, * a
School of Chemical Engineering and the Environment, Beijing Institute of Technology, Beijing 100081,
China b
School of Chemical Engineering and Material Science, Beijing Institute of Technology, Zhuhai,
Zhuhai 519085, China
ABSTRACT Hollow CoFe2O4 microspheres were successfully synthesized in high yield using a solvothermal method in the presence of long-chain ionic liquid [C18MIM][Br]. Uniform CoFe2O4 hollow spheres with a diameter of 100-200 nm and a shell thickness of approximately 10-50 nm are assembled by nanoparticles of 5-10 nm and have cubic spinel structures. A minimum reflection loss of -15.23 dB was observed at 16.98 GHz for hollow CoFe2O4 microspheres with a thickness of 5.5mm, and the effective absorption frequency ranged from 15.28 to 18 GHz. Hollow CoFe2O4 microspheres exhibited better microwave absorbing performance than solid microspheres. A possible formation mechanism of hollow CoFe2O4 microspheres was discussed. Keywords: nanocomposites; hollow sphere; ionic liquid; structural; wave absorption
1. Introduction Spinel ferrites (CoFe2O 4, ZnFe2O4, etc.) are applied in many aspects such as biology, absorbing materials, sensitive elements, catalysts and high-density magnetic memories [1] due to their superior magnetic and physical properties. The substaintial properties of spinel ferrites are primarily dependent on their compositions, structures, morphologies and sizes [2, 3]. Therefore it is extremely important to explore strategies for synthesis of nanoscaled spinel ferrites with specific compositions. Hollow ferrite spheres have drawn a lot of attention due to their excellent properties, such as high surface area, low density as well as special optical property. A hydrothermal/solvothermal method [4-6], template technique [7, 8], gas-phase diffusion method [9] can be used to prepare hollow ferrite spheres. Upon most occasions, a hydrothermal/solvothermal method has proven to be a versatile and economic route, which is appropriate to prepare monodispersed hollow ferrite crystals. Meng et al [10] prepared hollow CoFe2O4 spheres using glucose as a template by a hydrothermal method in combination with calcination. Yang et al [11] prepared hollow M-ferrite (M=Zn, Co) magnetic spheres with the introduction of dodecylamine in ethylene glycol solution. However, low yield and high cost of the product have limited the large-scale production and application. Hence, further development of a novel and high–yield synthetic method is very significant. Ionic liquids (ILs), which are environmentally friendly solvents, have shown excellent properties such as low melting point, high polarity, low vapor pressure as well as good thermal and dissolving properties [12-14]. Many inorganic materials have been prepared in ILs. Xu et al [15] prepared hollow α-Fe2O3
microspheres
in
the
presence
of
metal
ion-containing
reactable
ionic
liquid
1-octyl-3-methylimidazolium tetrachlorideferrate(III) ([Omim]FeCl4) under the solvothermal condition. 4
Di et al [16] synthesized hollow BiOI microspheres through a facile reactable ionic liquid [Bmim]I-assisted microemulsion method at room temperature. Herein, hollow CoFe2O4 microspheres with a diameter of 100-200 nm were obtained by a very facile one-step solvothermal method in the presence of long-chain IL [C18MIM][Br]. As-prepared CoFe 2O4 hollow spheres showed enhanced wave absorbing properties.
2. Experimental 2.1. Synthesis of hollow CoFe2O4 microspheres 3.33 mmol FeCl3.6H2O, 1.67 mmol CoCl2.6H2O, 15.6 mmol NaAc were dissolved in 50 mL ethylene glycol in the presence of [C18MIM][Br]. The mixture was stirred for 2 h at room temperature to acquire a homogeneous solution. The solution was then transferred to a 75 mL Teflon-lined autoclave. The autoclave was then sealed, heated and kept at 200℃ for 24 h. The product was separated by centrifugation, washed with distilled water and absolute ethanol and dried at 80℃ for 10 h. By varying the content of [C18MIM][Br], a series of CoFe2O4 microspheres were prepared. 2.2. Characterization Field emission scanning electron microscopy (FESEM) was performed on Hitachi S-4800 microscope. Transmission electron microscopy (TEM) was carried out using a Hitachi HT7700 microscope. X-ray diffraction (XRD) was collected by X-ray diffractometer (Rigaku Ultima IV). The nitrogen adsorption-desorption isotherms were measured at 77K using a Quantachrome Autosorb iQ porosimeter. A vector network analyzer (HP 8722ES) was used to measure electromagnetic parameter, the composites were mixed with paraffin at a certain mass filling ratio (60%).
3. Results and discussion The XRD patterns of the CoFe2O4 samples prepared using different content of [C18MIM][Br] are shown in Fig. 1. There are no peaks of other impurities, indicating a high purity of as-synthesized products.
(311)
(220)
(400)
d
(511) (440) (422)
(533)
Intensity(a.u.)
(111)
c
b
a
10
20
30
40
50
60
70
80
2θ(degree)
Fig. 1. XRD patterns of CoFe2O4 prepared with different content of [C18MIM][Br] (a) 0 mmol/L (b) 0.1 mmol/L (c) 0.2 mmol/L (d) 0.3 mmol/L. Fig. 2 shows the typical SEM and TEM images of CoFe2O4 prepared using different content of [C18MIM][Br]. CoFe2O4 prepared without [C18MIM][Br] are ununiform spheres with sizes in the range of 130-330 nm (Fig. 2a). A typical TEM image in Fig. 2b reveals that they are solid in nature. In the case of 0.1 mmol/L [C18MIM][Br], the diameter of CoFe2O 4 microspheres is decreased to 70-250 nm 5
(Fig. 2c). As shown in Fig. 2d, most of spheres are hollow but there are still some solid spheres. In the case of 0.2 mmol/L [C18MIM][Br], homogeneous CoFe2O4 microspheres with diameter of about 100-200 nm and a shell thickness of 10-50 nm are observed (Fig. 2e). It can be seen from Fig. 2f that almost all CoFe2O4 microspheres are hollow. CoFe2O4 synthesized using 0.3 mmol/L [C18MIM][Br] exhibits hollow structure and similar size to those prepared using 0.2 mmol/L [C18MIM][Br] (Fig. 2g, h). The surface area of solid CoFe2O 4 microspheres prepared with no [C18MIM][Br] is 17.49 m2/g, while surface areas of hollow CoFe2O4 microspheres obtained using 0.1, 0.2 and 0.3 mmol/L [C18MIM][Br] are 24.62, 33.95 and 30.97 m2/g, respectively.
6
Fig. 2. SEM and TEM images of CoFe2O4 synthesized with different content of [C18MIM][Br]: (a, b) 0 mmol/L (c, d) 0.1 mmol/L (e, f) 0.2 mmol/L (g, h) 0.3 mmol/L. On the basis of observations above, [C18MIM][Br] plays an important role. In this ethylene glycol system, Co2+ and Fe3+ react with NaAc to form CoFe2O4 nanoparticles for the first step. CoFe2O4 nanoparticles with small particle size are high surface active. Ionic liquid [C18MIM][Br] used in this 7
process, consisting of a large, amphiphilic and asymmetric imidazolium cation and Br anion, interact with the CoFe2O4 nanoparticles forming protecting layer. On the one hand, [C18MIM]+ consisting of hydrophilic cation adhere to the surface of CoFe2O4 nanoparticles by the hydrogen bond formed between the hydrogen atom at C 2 position of the imidazole ring and oxygen atoms of O-Co and O-Fe, which prevent smaller particles adhesion and aggregation, preparing spheres with smaller size. On the other hand, [C18MIM][Br] has a strong tendency to self-aggregate into ordered structures which possess a large steric hindrance [17]. Therefore, they can provide spaces for the oriented arrangement of CoFe2O4 nanoparticles into spheres. After washing, [C18MIM][Br] adhering to the surface of CoFe2O 4 nanoparticles can be cleaned up and hollow spheres are obtained.
4. Absorbing performance The reflection loss curves of solid CoFe2O 4 spheres prepared with no [C18MIM][Br] and hollow CoFe2O4 spheres synthesized using 0.2 mmol/L [C18MIM][Br] with different thickness are shown in Fig. 3. The minimum RL of solid CoFe2O4 spheres reaches -8.56 dB at 18 GHz, and the effective bandwidth (RL < -10dB) is 0. However, for hollow CoFe2O4 spheres, the minimum RL is -15.23 dB while the effective bandwidth achieves 2.72 GHz (from 15.28 to 18 GHz) with a matching thickness of 5.5mm. When the thickness increases to 6.5 mm, the minimum RL is as high as -15.0 dB with the effective bandwidth of 3.4 GHz (from 12.56 to 15.96 GHz). When the thickness decreases to 2.5mm, the minimum RL is as high as -11.43 dB with the effective bandwidth of 2.89 GHz (from 11.37 to 14.26 GHz). All the above analyses apparently demonstrate that hollow CoFe2O4 microspheres exhibit much better microwave absorbing ability than solid CoFe2O4 microspheres. CoFe2O4 microspheres synthesized using [C18MIM][Br] possess unique hollow structure and higher surface area than solid spheres, which will induce more electromagnetic energy attenuation in the hollow sphere sample.
(a)
0
Reflection Loss(dB)
-2
-4
-6
2.5mm 3.5mm 4.5mm 5.5mm 6.5mm
-8
-10 2
4
6
8
10
12
14
16
18
14
16
18
Frequency (GHz)
0
(b)
-2
Reflection Loss(dB)
-4 -6 -8 -10 -12 -14
2.5mm 3.5mm 4.5mm 5.5mm 6.5mm
-16 -18 -20 2
4
6
8
10
12
Frequency (GHz)
8
Fig. 3. The calculated reflection loss of (a) solid CoFe2O4 (b) hollow CoFe2O4 spheres.
5. Conclusions Hollow CoFe2O 4 microspheres with a diameter of 100-200 nm and a shell thickness of 10-50 nm were successfully synthesized through a simple solvothermal method in the presence of long-chain ionic liquid [C18MIM][Br]. [C18MIM][Br] plays a key role for formation of hollow spheres. Uniform CoFe2O4 hollow spheres are assembled by nanoparticles of 5-10 nm and have cubic spinel structures. The minimum RL of hollow CoFe2O 4 microspheres at a thickness of 5.5 mm reaches -15.23 dB, and the effective absorption bandwidth is 2.72 GHz. The absorbing performance of hollow CoFe2O4 microspheres is better than that of solid CoFe2O4 microspheres.
Acknowledgements Financial support came from National Natural Science Foundation of China (no. 21376029). The authors would like to acknowledgement the National Natural Science Foundation of China for sponsoring this research.
References
[1] Y.W. Wang, L.D. Zhang, G.W. Meng, X.S. Peng, Y.X. Jin, J. Zhang, Fabrication of ordered ferromagnetic-nonmagnetic alloy nanowire arrays and their magnetic property dependence on annealing temperature, J. Phys. Chem. B 106 (2002) 2502-2507.
[2] S.H. Sun, H. Zeng, D.B. Robinson, S. Raoux, P.M. Rice, S.X. Wang, G.X. Li, Monodisperse MFe2O4 (M = Fe, Co, Mn) nanoparticles, J. Am. Chem. Soc. 126 (2004) 273-279.
[3] Q. Song, Z.J. Zhang, Shape control and associated magnetic properties of spinel cobalt ferrite nanocrystals, J. Am. Chem. Soc. 126 (2004) 6164-6168.
[4] Y. Wang, D.W. Su, A. Ung, J.H. Ahn, G.X. Wang, Hollow CoFe2O4 nanospheres as a high capacity anode material for lithium ion batteries, Nanotechnology 23 (2012) 055402.
[5] W. Li, X. Qiao, Q. Zheng, T. Zhang, One-step synthesis of MFe2O4 (M= Fe, Co) hollow spheres by template-free solvothermal method, J. Alloys. Compd. 509 (2011) 6206-6211.
[6] F. Wang, J. Liu, J. Kong, Z. Zhang, X. Wang, M. Itoh, K. Machida, Template free synthesis and electromagnetic wave absorption properties of monodispersed hollow magnetite nano-spheres, J. Mater. Chem. 21 (2011) 4314.
9
[7] Y. Zhang, Z. Huang, F. Tang, J. Ren, Ferrite hollow spheres with tunable magnetic properties, Thin Solid Films 515 (2006) 2555–2561.
[8] M.M. Titirici, M. Antonietti, A. Thomas, A generalized synthesis of metal oxide hollow spheres using a hydrothermal approach, Chem. Mater. 18 (2006) 3808-3812.
[9] J. Liu, Q. Jiao, W. Cao, Y. Zhao, H. Li, Preparation and magnetic properties of hollow ferrite microspheres by a gas-phase diffusion method in an ionic liquid/H2O mixed solution, J. Mater. Sci. 49 (2014) 3795–3804.
[10] Y. Meng, D. Chen, X. Jiao, Synthesis and characterization of CoFe2O4 hollow spheres, Eur. J. Inorg. Chem. (2008) 4019–4023.
[11] L.X. Yang, Y. Bin Xu, R.C. Jin, F. Wang, P. Yin, G.H. Li, C.P. Xu, L.B. Pan, Nonstoichiometric M-ferrite porous spheres: preparation, shape evolution and magnetic properties, Ceram. Int. 41 (2015) 2309–2317.
[12] N.L. Mai, K. Ahn, Y.M. Koo, Methods for recovery of ionic liquids—A review, Process Biochem. 49 (2014) 872–881.
[13] X. Wang, Y. Chi, T. Mu, A review on the transport properties of ionic liquids, J. Mol. Liq. 193 (2014) 262–266.
[14] M.P. Singh, R.K. Singh, S. Chandra, Ionic liquids confined in porous matrices: Physicochemical properties and applications, Prog. Mater. Sci. 64 (2014) 73–120.
[15] L. Xu, J. Xia, K. Wang, L. Wang, H. Li, H. Xu, L. Huang, M. He, Ionic liquid assisted synthesis and photocatalytic properties of α-Fe2O3 hollow microspheres, Dalt. Trans. 42 (2013) 6468–6477. [16] J. Di, J. Xia, Y. Ge, L. Xu, H. Xu, M. He, Q. Zhang, H. Li, Reactable ionic liquid-assisted rapid synthesis of BiOI hollow microspheres at room temperature with enhanced photocatalytic activity, J. Mater. Chem. A. 2 (2014) 15864–15874.
[17] J. Ma, X. Liu, J. Lian, X. Duan, W. Zheng, Ionothermal synthesis of BiOCl nanostructures via a long-chain ionic liquid precursor route, Cryst. Growth Des. 10 (2010) 2522-2527.
10
Highlights: 1.
Hollow CoFe2O4 spheres were obtained in the presence of long-chain ionic liquid.
2.
Uniform CoFe2O4 hollow spheres are assembled by nanoparticles of 5-10 nm.
3.
Hollow CoFe2O4 spheres showed better absorbing performance than solid spheres.
11
S0167-577X(17)30126-X http://dx.doi.org/10.1016/j.matlet.2017.01.109 MLBLUE 22062
To appear in:
Materials Letters
Received Date: Revised Date: Accepted Date:
8 August 2016 13 January 2017 23 January 2017
Please cite this article as: X. Ni, Z. He, X. Liu, Q. Jiao, H. Li, C. Feng, Y. Zhao, Ionic liquid-assisted solvothermal synthesis of hollow CoFe2O4 microspheres and their absorbing performances, Materials Letters (2017), doi: http:// dx.doi.org/10.1016/j.matlet.2017.01.109
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Ionic liquid-assisted solvothermal synthesis of hollow CoFe2O4 microspheres and their absorbing performances Xiang Ni a, Zhuangzhang He a, Xi Liu a, Qingze Jiao a, b, Hansheng Li a, Caihong Feng a, Yun Zhao a, * a
School of Chemical Engineering and the Environment, Beijing Institute of Technology, Beijing 100081,
China b
School of Chemical Engineering and Material Science, Beijing Institute of Technology, Zhuhai,
Zhuhai 519085, China
ABSTRACT Hollow CoFe2O4 microspheres were successfully synthesized in high yield using a solvothermal method in the presence of long-chain ionic liquid [C18MIM][Br]. Uniform CoFe2O4 hollow spheres with a diameter of 100-200 nm and a shell thickness of approximately 10-50 nm are assembled by nanoparticles of 5-10 nm and have cubic spinel structures. A minimum reflection loss of -15.23 dB was observed at 16.98 GHz for hollow CoFe2O4 microspheres with a thickness of 5.5mm, and the effective absorption frequency ranged from 15.28 to 18 GHz. Hollow CoFe2O4 microspheres exhibited better microwave absorbing performance than solid microspheres. A possible formation mechanism of hollow CoFe2O4 microspheres was discussed. Keywords: nanocomposites; hollow sphere; ionic liquid; structural; wave absorption
1. Introduction Spinel ferrites (CoFe2O 4, ZnFe2O4, etc.) are applied in many aspects such as biology, absorbing materials, sensitive elements, catalysts and high-density magnetic memories [1] due to their superior magnetic and physical properties. The substaintial properties of spinel ferrites are primarily dependent on their compositions, structures, morphologies and sizes [2, 3]. Therefore it is extremely important to explore strategies for synthesis of nanoscaled spinel ferrites with specific compositions. Hollow ferrite spheres have drawn a lot of attention due to their excellent properties, such as high surface area, low density as well as special optical property. A hydrothermal/solvothermal method [4-6], template technique [7, 8], gas-phase diffusion method [9] can be used to prepare hollow ferrite spheres. Upon most occasions, a hydrothermal/solvothermal method has proven to be a versatile and economic route, which is appropriate to prepare monodispersed hollow ferrite crystals. Meng et al [10] prepared hollow CoFe2O4 spheres using glucose as a template by a hydrothermal method in combination with calcination. Yang et al [11] prepared hollow M-ferrite (M=Zn, Co) magnetic spheres with the introduction of dodecylamine in ethylene glycol solution. However, low yield and high cost of the product have limited the large-scale production and application. Hence, further development of a novel and high–yield synthetic method is very significant. Ionic liquids (ILs), which are environmentally friendly solvents, have shown excellent properties such as low melting point, high polarity, low vapor pressure as well as good thermal and dissolving properties [12-14]. Many inorganic materials have been prepared in ILs. Xu et al [15] prepared hollow α-Fe2O3
microspheres
in
the
presence
of
metal
ion-containing
reactable
ionic
liquid
1-octyl-3-methylimidazolium tetrachlorideferrate(III) ([Omim]FeCl4) under the solvothermal condition. 4
Di et al [16] synthesized hollow BiOI microspheres through a facile reactable ionic liquid [Bmim]I-assisted microemulsion method at room temperature. Herein, hollow CoFe2O4 microspheres with a diameter of 100-200 nm were obtained by a very facile one-step solvothermal method in the presence of long-chain IL [C18MIM][Br]. As-prepared CoFe 2O4 hollow spheres showed enhanced wave absorbing properties.
2. Experimental 2.1. Synthesis of hollow CoFe2O4 microspheres 3.33 mmol FeCl3.6H2O, 1.67 mmol CoCl2.6H2O, 15.6 mmol NaAc were dissolved in 50 mL ethylene glycol in the presence of [C18MIM][Br]. The mixture was stirred for 2 h at room temperature to acquire a homogeneous solution. The solution was then transferred to a 75 mL Teflon-lined autoclave. The autoclave was then sealed, heated and kept at 200℃ for 24 h. The product was separated by centrifugation, washed with distilled water and absolute ethanol and dried at 80℃ for 10 h. By varying the content of [C18MIM][Br], a series of CoFe2O4 microspheres were prepared. 2.2. Characterization Field emission scanning electron microscopy (FESEM) was performed on Hitachi S-4800 microscope. Transmission electron microscopy (TEM) was carried out using a Hitachi HT7700 microscope. X-ray diffraction (XRD) was collected by X-ray diffractometer (Rigaku Ultima IV). The nitrogen adsorption-desorption isotherms were measured at 77K using a Quantachrome Autosorb iQ porosimeter. A vector network analyzer (HP 8722ES) was used to measure electromagnetic parameter, the composites were mixed with paraffin at a certain mass filling ratio (60%).
3. Results and discussion The XRD patterns of the CoFe2O4 samples prepared using different content of [C18MIM][Br] are shown in Fig. 1. There are no peaks of other impurities, indicating a high purity of as-synthesized products.
(311)
(220)
(400)
d
(511) (440) (422)
(533)
Intensity(a.u.)
(111)
c
b
a
10
20
30
40
50
60
70
80
2θ(degree)
Fig. 1. XRD patterns of CoFe2O4 prepared with different content of [C18MIM][Br] (a) 0 mmol/L (b) 0.1 mmol/L (c) 0.2 mmol/L (d) 0.3 mmol/L. Fig. 2 shows the typical SEM and TEM images of CoFe2O4 prepared using different content of [C18MIM][Br]. CoFe2O4 prepared without [C18MIM][Br] are ununiform spheres with sizes in the range of 130-330 nm (Fig. 2a). A typical TEM image in Fig. 2b reveals that they are solid in nature. In the case of 0.1 mmol/L [C18MIM][Br], the diameter of CoFe2O 4 microspheres is decreased to 70-250 nm 5
(Fig. 2c). As shown in Fig. 2d, most of spheres are hollow but there are still some solid spheres. In the case of 0.2 mmol/L [C18MIM][Br], homogeneous CoFe2O4 microspheres with diameter of about 100-200 nm and a shell thickness of 10-50 nm are observed (Fig. 2e). It can be seen from Fig. 2f that almost all CoFe2O4 microspheres are hollow. CoFe2O4 synthesized using 0.3 mmol/L [C18MIM][Br] exhibits hollow structure and similar size to those prepared using 0.2 mmol/L [C18MIM][Br] (Fig. 2g, h). The surface area of solid CoFe2O 4 microspheres prepared with no [C18MIM][Br] is 17.49 m2/g, while surface areas of hollow CoFe2O4 microspheres obtained using 0.1, 0.2 and 0.3 mmol/L [C18MIM][Br] are 24.62, 33.95 and 30.97 m2/g, respectively.
6
Fig. 2. SEM and TEM images of CoFe2O4 synthesized with different content of [C18MIM][Br]: (a, b) 0 mmol/L (c, d) 0.1 mmol/L (e, f) 0.2 mmol/L (g, h) 0.3 mmol/L. On the basis of observations above, [C18MIM][Br] plays an important role. In this ethylene glycol system, Co2+ and Fe3+ react with NaAc to form CoFe2O4 nanoparticles for the first step. CoFe2O4 nanoparticles with small particle size are high surface active. Ionic liquid [C18MIM][Br] used in this 7
process, consisting of a large, amphiphilic and asymmetric imidazolium cation and Br anion, interact with the CoFe2O4 nanoparticles forming protecting layer. On the one hand, [C18MIM]+ consisting of hydrophilic cation adhere to the surface of CoFe2O4 nanoparticles by the hydrogen bond formed between the hydrogen atom at C 2 position of the imidazole ring and oxygen atoms of O-Co and O-Fe, which prevent smaller particles adhesion and aggregation, preparing spheres with smaller size. On the other hand, [C18MIM][Br] has a strong tendency to self-aggregate into ordered structures which possess a large steric hindrance [17]. Therefore, they can provide spaces for the oriented arrangement of CoFe2O4 nanoparticles into spheres. After washing, [C18MIM][Br] adhering to the surface of CoFe2O 4 nanoparticles can be cleaned up and hollow spheres are obtained.
4. Absorbing performance The reflection loss curves of solid CoFe2O 4 spheres prepared with no [C18MIM][Br] and hollow CoFe2O4 spheres synthesized using 0.2 mmol/L [C18MIM][Br] with different thickness are shown in Fig. 3. The minimum RL of solid CoFe2O4 spheres reaches -8.56 dB at 18 GHz, and the effective bandwidth (RL < -10dB) is 0. However, for hollow CoFe2O4 spheres, the minimum RL is -15.23 dB while the effective bandwidth achieves 2.72 GHz (from 15.28 to 18 GHz) with a matching thickness of 5.5mm. When the thickness increases to 6.5 mm, the minimum RL is as high as -15.0 dB with the effective bandwidth of 3.4 GHz (from 12.56 to 15.96 GHz). When the thickness decreases to 2.5mm, the minimum RL is as high as -11.43 dB with the effective bandwidth of 2.89 GHz (from 11.37 to 14.26 GHz). All the above analyses apparently demonstrate that hollow CoFe2O4 microspheres exhibit much better microwave absorbing ability than solid CoFe2O4 microspheres. CoFe2O4 microspheres synthesized using [C18MIM][Br] possess unique hollow structure and higher surface area than solid spheres, which will induce more electromagnetic energy attenuation in the hollow sphere sample.
(a)
0
Reflection Loss(dB)
-2
-4
-6
2.5mm 3.5mm 4.5mm 5.5mm 6.5mm
-8
-10 2
4
6
8
10
12
14
16
18
14
16
18
Frequency (GHz)
0
(b)
-2
Reflection Loss(dB)
-4 -6 -8 -10 -12 -14
2.5mm 3.5mm 4.5mm 5.5mm 6.5mm
-16 -18 -20 2
4
6
8
10
12
Frequency (GHz)
8
Fig. 3. The calculated reflection loss of (a) solid CoFe2O4 (b) hollow CoFe2O4 spheres.
5. Conclusions Hollow CoFe2O 4 microspheres with a diameter of 100-200 nm and a shell thickness of 10-50 nm were successfully synthesized through a simple solvothermal method in the presence of long-chain ionic liquid [C18MIM][Br]. [C18MIM][Br] plays a key role for formation of hollow spheres. Uniform CoFe2O4 hollow spheres are assembled by nanoparticles of 5-10 nm and have cubic spinel structures. The minimum RL of hollow CoFe2O 4 microspheres at a thickness of 5.5 mm reaches -15.23 dB, and the effective absorption bandwidth is 2.72 GHz. The absorbing performance of hollow CoFe2O4 microspheres is better than that of solid CoFe2O4 microspheres.
Acknowledgements Financial support came from National Natural Science Foundation of China (no. 21376029). The authors would like to acknowledgement the National Natural Science Foundation of China for sponsoring this research.
References
[1] Y.W. Wang, L.D. Zhang, G.W. Meng, X.S. Peng, Y.X. Jin, J. Zhang, Fabrication of ordered ferromagnetic-nonmagnetic alloy nanowire arrays and their magnetic property dependence on annealing temperature, J. Phys. Chem. B 106 (2002) 2502-2507.
[2] S.H. Sun, H. Zeng, D.B. Robinson, S. Raoux, P.M. Rice, S.X. Wang, G.X. Li, Monodisperse MFe2O4 (M = Fe, Co, Mn) nanoparticles, J. Am. Chem. Soc. 126 (2004) 273-279.
[3] Q. Song, Z.J. Zhang, Shape control and associated magnetic properties of spinel cobalt ferrite nanocrystals, J. Am. Chem. Soc. 126 (2004) 6164-6168.
[4] Y. Wang, D.W. Su, A. Ung, J.H. Ahn, G.X. Wang, Hollow CoFe2O4 nanospheres as a high capacity anode material for lithium ion batteries, Nanotechnology 23 (2012) 055402.
[5] W. Li, X. Qiao, Q. Zheng, T. Zhang, One-step synthesis of MFe2O4 (M= Fe, Co) hollow spheres by template-free solvothermal method, J. Alloys. Compd. 509 (2011) 6206-6211.
[6] F. Wang, J. Liu, J. Kong, Z. Zhang, X. Wang, M. Itoh, K. Machida, Template free synthesis and electromagnetic wave absorption properties of monodispersed hollow magnetite nano-spheres, J. Mater. Chem. 21 (2011) 4314.
9
[7] Y. Zhang, Z. Huang, F. Tang, J. Ren, Ferrite hollow spheres with tunable magnetic properties, Thin Solid Films 515 (2006) 2555–2561.
[8] M.M. Titirici, M. Antonietti, A. Thomas, A generalized synthesis of metal oxide hollow spheres using a hydrothermal approach, Chem. Mater. 18 (2006) 3808-3812.
[9] J. Liu, Q. Jiao, W. Cao, Y. Zhao, H. Li, Preparation and magnetic properties of hollow ferrite microspheres by a gas-phase diffusion method in an ionic liquid/H2O mixed solution, J. Mater. Sci. 49 (2014) 3795–3804.
[10] Y. Meng, D. Chen, X. Jiao, Synthesis and characterization of CoFe2O4 hollow spheres, Eur. J. Inorg. Chem. (2008) 4019–4023.
[11] L.X. Yang, Y. Bin Xu, R.C. Jin, F. Wang, P. Yin, G.H. Li, C.P. Xu, L.B. Pan, Nonstoichiometric M-ferrite porous spheres: preparation, shape evolution and magnetic properties, Ceram. Int. 41 (2015) 2309–2317.
[12] N.L. Mai, K. Ahn, Y.M. Koo, Methods for recovery of ionic liquids—A review, Process Biochem. 49 (2014) 872–881.
[13] X. Wang, Y. Chi, T. Mu, A review on the transport properties of ionic liquids, J. Mol. Liq. 193 (2014) 262–266.
[14] M.P. Singh, R.K. Singh, S. Chandra, Ionic liquids confined in porous matrices: Physicochemical properties and applications, Prog. Mater. Sci. 64 (2014) 73–120.
[15] L. Xu, J. Xia, K. Wang, L. Wang, H. Li, H. Xu, L. Huang, M. He, Ionic liquid assisted synthesis and photocatalytic properties of α-Fe2O3 hollow microspheres, Dalt. Trans. 42 (2013) 6468–6477. [16] J. Di, J. Xia, Y. Ge, L. Xu, H. Xu, M. He, Q. Zhang, H. Li, Reactable ionic liquid-assisted rapid synthesis of BiOI hollow microspheres at room temperature with enhanced photocatalytic activity, J. Mater. Chem. A. 2 (2014) 15864–15874.
[17] J. Ma, X. Liu, J. Lian, X. Duan, W. Zheng, Ionothermal synthesis of BiOCl nanostructures via a long-chain ionic liquid precursor route, Cryst. Growth Des. 10 (2010) 2522-2527.
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Highlights: 1.
Hollow CoFe2O4 spheres were obtained in the presence of long-chain ionic liquid.
2.
Uniform CoFe2O4 hollow spheres are assembled by nanoparticles of 5-10 nm.
3.
Hollow CoFe2O4 spheres showed better absorbing performance than solid spheres.
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