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Topochemical synthesis and magnetic properties of BaFe12O19 nanorods using α-FeOOH nanowires as templates
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CERAMICS INTERNATIONAL
Ceramics International 40 (2014) 8593–8597 www.elsevier.com/locate/ceramint
Topochemical synthesis and magnetic properties of BaFe12O19 nanorods using α-FeOOH nanowires as templates Cheng-Yan Xua,b, Li-Shun Fua, Xin Caia, Xue-Yin Suna, Liang Zhena,b,n a School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China MOE Key Laboratory of Micro-systems and Micro-structures Manufacturing, Harbin Institute of Technology, Harbin 150080, China
b
Received 19 August 2013; received in revised form 16 December 2013; accepted 15 January 2014 Available online 26 January 2014
Abstract We present a topochemical route for the synthesis of BaFe12O19 nanorods by using hydrothermal synthesized mixture of α-FeOOH nanowires and BaCO3 as precursors. The α-FeOOH nanowires and in-situ formed BaCO3 transformed to BaFe12O19 nanorods by a two-step annealing process. The crystal structure and morphology changes of the products with annealing temperatures were investigated. BaFe12O19 nanorods with diameters of about 250 nm and lengths of about 1 μm were obtained at annealing temperature of 1000 1C. A comparison of magnetic properties between BaFe12O19 nanorods and granules was performed and discussed. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: A. Powders: chemical preparation; C. Magnetic properties; D. Ferrites
1. Introduction As an important hard magnetic material, barium hexaferrite (BaFe12O19) is widely used in high density perpendicular recording media [1–3], magnetic fluids [4,5], electromagnetic wave absorber [6–8] and high frequency devices [9–11] due to its excellent physical and chemical properties, like large magnetocrystalline anisotropy, high Curie temperature, relatively large saturation magnetization, as well as excellent chemical stability and good corrosion resistivity [12,13]. In recent years, the fabrication of one-dimensional (1D) nanostructured magnetic materials has aroused considerable interest due to their applications in high density recording media with promising giant magnetoresistance (GMR) properties [14]. So far, various chemical routes were utilized to synthesize 1D nanostructured barium ferrite, such as the inverse microemulsion method [15], sol–gel synthesis [7,16], the hydrothermal method [17] and electrospinning technique [18], etc. Mu et al. prepared barium ferrite (BaFe12O19) nanorods with diameters of about 60 nm and lengths n Corresponding author at: School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China. Tel.: þ 86 451 86412133; fax: þ 86 451 86413922. E-mail address: [email protected] (L. Zhen).
of about 300 nm via a sol–gel process [16]. By a hydrothermal and annealing process, Wang et al. fabricated rod-like BaFe12O19 nanoparticles with coercivity of 4511 Oe [17]. Nano-sized BaFe12O19 fibers with diameter of 150 nm were obtained using electrospinning technique [18]. Although these works provided pathways to prepare the BaFe12O19 nanorods, the practical applications are still limited due to the relatively long reaction time or the requirements for unique instruments to prepare the material. In addition, the formation mechanisms are always the bottle neck problem restraining the development of the nanosized BaFe12O19. The topochemical method is a synthesis process that can control the shape of final products by selecting the morphology of precursors with mixing components in ionic states and precisely measuring the chemical activity and optimization of the experiment parameters of all components. α-FeOOH nanowires could be easily obtained and were thus used as templates for preparing a wealth of 1D nanostructured magnetic oxides, such as α-Fe2O3 and Fe3O4 [19,20]. Cao and co-workers synthesized BaFe12O19 nanorods with the topochemical reaction method using α-FeOOH nanowires as templates. They first prepared α-FeOOH nanowires with the chemical precipitation method, then coated with BaCO3 via the solution chemical method, and followed calcination at 1000 1C to obtain the BaFe12O19 nanorods [21].
0272-8842/$ - see front matter & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved. http://dx.doi.org/10.1016/j.ceramint.2014.01.075
C.-Y. Xu et al. / Ceramics International 40 (2014) 8593–8597
In this work, we prepared α-FeOOH nanowires and in-situ formed BaCO3 which were coated on the nanowires by the hydrothermal method, then converted them into BaFe12O19 nanorods through a two-step annealing process. The microstructure and morphology of barium ferrite nanorods and precursory nanowires were investigated. The magnetic properties of BaFe12O19 nanorods and granules were compared and the mechanism for this difference was discussed.
Intensity (a.u.)
8594
∇
∇∇
∇
JCPDS 29-0713, α-FeOOH
JCPDS 45-1471, BaCO3
2. Experimental BaFe12O19 nanorods were synthesized by a topochemical reaction method, namely, annealing the hydrothermal synthesized α-FeOOH nanowires at high temperature. In a typical process, barium nitrate [Ba(NO3)2] and ferric nitrate [Fe(NO3)2 9H2O] were dissolved in 20 ml distilled water, and then 20 ml sodium hydroxide (NaOH) solution and 2.0 g sodium dodecyl benzene sulfonate (SDBS) was added to the mixture solution under vigorous stirring. The molar ratio of Ba(NO3)2, Fe(NO3)2 9H2O and NaOH is 1:12:60, and the concentrations of Ba(NO3)2, Fe(NO3)2 9H2O and NaOH are 0.017, 0.2 and 1.0 M, respectively (stoichiometrically for BaFe12O19, and in the product Fe is in form of α-FeOOH and Ba in form of BaCO3). Then, the mixture was transferred to a Teflon-lined stainless-steel autoclave for hydrothermal reaction at 220 1C for 24 h. The yellow brown precursors were collected by centrifugation, washed several times with distilled water and ethanol. After being dried at 60 1C, the precursors were heat-treated at 450 1C for 3 h in air, aiming to convert αFeOOH into γ-Fe2O3 [22]. The obtained precursory powders were subsequently annealed in air for 3 h at temperatures of 800, 1000 and 1100 1C, respectively. The products were named in turn as A800, A1000 and A1100, accordingly. The crystalline structures of products were characterized by X-ray diffraction (XRD, Rigaku D/max-γB, Cu Kα). The morphology of products was observed on a scanning electron microscope (SEM, FEI Quanta 200F). Static magnetic properties were measured by a vibrating sample magnetometer (VSM, Lakeshore 7410) with an applied magnetic field of 20 kOe at room temperature. 3. Results and discussion Fig. 1 shows the X-ray diffraction pattern of the precursors obtained by the hydrothermal method. The main peaks can be indexed to α-FeOOH (JCPDS 29-0713) and BaCO3 (JCPDS 45-1471), and the other peaks (marked with triangles) are assigned to Ba(OH)2 H2O (JCPDS 26-0154) as minor constituent. The strong and narrow diffraction peaks indicates that the as-prepared product was well crystallized. Fig. 2a shows the SEM image of the precursor product. As demonstrated in this figure, the precursors are nanowires with diameters of about 100 nm and lengths of about 2 μm. It is also found in Fig. 2b that the surfaces of nanowires are not smooth due to adhesion of small particles, indicating that BaCO3 phases are likely to be distributed on the surface of α-FeOOH nanowires
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2θ (°) Fig. 1. XRD pattern of the hydrothermal synthesized products. The main peaks can be indexed to α-FeOOH and BaCO3, and the other peaks (marked with triangles) are assigned to Ba(OH)2 H2O as minor constituent.
in the form of small particles for the hydrothermally synthesized product. The precursors containing both α-FeOOH and BaCO3 were first heat-treated at 450 1C for 3 h in air, followed by annealing in air for another 3 h at different temperatures to obtain BaFe12O19. After heat treatment at 450 1C for 3 h, α-FeOOH can be converted into γ-Fe2O3 [23]. The formation process of BaFe12O19 is as follows: γ-Fe2O3 þ BaCO3-BaFe2O4 þ CO2↑
(1)
BaFe2O4 þ 5γ-Fe2O3-BaFe12O19
(2)
The crystal structure of γ-Fe2O3 is similar to that of BaFe2O4, which makes the transformation from γ-Fe2O3 to BaFe2O4 much easier than that from α-Fe2O3. As reported before, without additional annealing at 450 1C for 3 h, γ-Fe2O3 would be easily converted into α-Fe2O3 as temperature increased, which might hinder the formation of BaFe12O19 [22]. XRD patterns of the products annealed at different temperatures are shown in Fig. 3. All the diffraction peaks of products prepared at 1000 1C are indexed to a pure hexagonal structured BaFe12O19 with lattice constants of a¼ 5.895 Å, and c¼ 23.215 Å (JCPDS 39-1433), indicating complete transformation to barium ferrite. For the products annealed at 800 and 1100 1C, as shown in Fig. 3a and c, the peaks of products are indexed to BaFe12O19 (JCPDS 39-1433) and α-Fe2O3 (JCPDS 33-0664), indicating that the majority of precursory powders was transformed to BaFe12O19 whereas a small amount of α-Fe2O3 residue was also formed after the annealing. Fig. 4 shows the SEM images of products annealed at different temperatures. The SEM image of sample A800 shown in Fig. 4a demonstrates that the products maintained the morphology of nanowires with an increase in diameters of about 50 nm after annealing at 800 1C. With the increase of annealing temperature to 1000 1C, the diameters of products increase and the lengths decrease, resulting in the variation of the morphology from nanowires to short rods. Typical size of
C.-Y. Xu et al. / Ceramics International 40 (2014) 8593–8597
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Fig. 2. SEM images of the precursors obtained by the hydrothermal method. (a) Low magnification and (b) high magnification.
BaFe12O19 α-Fe2O3
Intensity (a.u.)
(c)
(b)
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2θ (°) Fig. 3. XRD patterns of the products annealed at different temperatures: (a) 800 1C; (b) 1000 1C; and (c) 1100 1C.
the BaFe12O19 nanorods is about 250 nm in diameter and about 1 μm in length, as shown in Fig. 4b. When the annealing temperature is further increased to 1100 1C, the morphology of products changed dramatically from short rods to granules with typical particle size of about 1 μm, as displayed in Fig. 4c. Such morphology change may be due to the greatly enhanced diffusion process arising from the elevated annealing temperature. It has been reported that the coercivity (Hc) of BaFe12O19 nanorods was higher than that of plate-like barium hexaferrites because the rod-like shape increases the effective anisotropy due to its large aspect ratio [16]. Room-temperature static magnetic properties of BaFe12O19 obtained at different annealing temperatures were measured by VSM, as shown in Fig. 5. The magnetic hysteresis loop of BaFe12O19 nanorods obtained at 1000 1C exhibits a saturation magnetization (Ms) of 62.5 emu/g and a high Hc of 3811 Oe. Compared with BaFe12O19 nanorods, BaFe12O19 granules possess a higher Ms (72.2 emu/g) and a much lower Hc (3171 Oe). It is noted that the Ms of BaFe12O19 granules reaches the theoretical value (72 emu/g) of bulk BaFe12O19 [24]. The larger Ms value for BaFe12O19 granules may be attributed to the lower oxygen vacancy concentration compared to BaFe12O19 nanorods. The annealing temperature
of barium ferrite granules is higher than that of barium ferrite nanorods, which may reduce the concentration of oxygen vacancies and thus increase the number of nearest neighbor oxygen atoms of Fe. This effect can favorably improve the superexchange of Fe–O–Fe [17] and lead to a higher Ms value, as observed above. There are at least three reasons for the enhanced coercivity of BaFe12O19 nanorods comparing to that of BaFe12O19 granules. One reason is the effect of particle size. The critical diameter of single magnetic domain for spherical barium ferrite was reported to be 460 nm [25]. With particle sizes below this critical value, the barium ferrite nanorods tend to form single magnetic domains, which would exhibit a high value of Hc. Another reason could be attributed to the preferential orientation of BaFe12O19 nanorods during the measurement. In addition, the presence of a small amount of α-Fe2O3 residue which is paramagnetic can reduce the coercivity of BaFe12O19 granules [26]. However, the contributions related to shape anisotropy are negative in this work. As we know, the shape anisotropy (Ks) contributes to the effective anisotropy together with the magnetocrystalline anisotropy (Km). It is possible to estimate the shape anisotropy according to Ks ¼ δNM2s /2, where δN is the anisotropy contribution of the demagnetizing factor due to the deviation from sphericity [27]. In this work, considering that BaFe12O19 nanorods have small aspect ratio compared with BaFe12O19 granular platelets, the rod-like shape decreases the overall magnetic anisotropy of the barium ferrite. 4. Conclusions BaFe12O19 nanorods were prepared by a topochemical reaction method including hydrothermal and annealing process. The precursor nanowires obtained by the hydrothermal method are the mixture of α-FeOOH and BaCO3. With the increase of annealing temperature, the phase of the products transformed to BaFe12O19 while the morphology changed from nanowires to nanorods, and further converted granules. When the annealing temperature is 1000 1C, BaFe12O19 nanorods were successfully synthesized with diameters of about 250 nm and lengths of about 1 μm, which possess lower Ms and much higher Hc than those of BaFe12O19 granules obtained at 1100 1C.
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C.-Y. Xu et al. / Ceramics International 40 (2014) 8593–8597
Fig. 4. SEM images of the products annealed at different temperatures: (a) 800 1C; (b) 1000 1C; and (c) 1100 1C.
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Fig. 5. Magnetic hysteresis loops of barium ferrite obtained at different temperatures. Insert is the enlarged plot around the zero field.
Acknowledgments This work was supported by the Fundamental Research Funds for the Central Universities (HIT.KLOF.2010004). C.Y.X. acknowledges the Fundamental Research Funds for the Central Universities (HIT.BRETIII.201203). The authors appreciate Dr. Ning Xie and Dr. Kai He for language polishing.
References [1] T. Fujiwara, Barium ferrite media for perpendicular recording, IEEE Trans. Magn. 21 (1985) 1480–1485.
[2] K. Yamamori, T. Suzuki, T. Fujiwara, High-density recording characteristics for ba-ferrite flexible disks, IEEE Trans. Magn. 22 (1986) 1188–1190. [3] V.V. Pankov, M. Pernet, P. Germi, P. Mollard, Fine hexaferrite particles for perpendicular recording prepared by the coprecipitation method in the presence of an inert component, J. Magn. Magn. Mater. 120 (1993) 69–72. [4] D. Primc, D. Makovec, D. Lisjak, M. Drofenik, Hydrothermal synthesis of ultrafine barium hexaferrite nanoparticles and the preparation of their stable suspensions, Nanotechnology 20 (2009) 315605. [5] Q.A. Pankhurst, J. Connolly, S.K. Jones, J. Dobson, Applications of magnetic nanoparticles in biomedicine, J. Phys. D: Appl. Phys. 36 (2003) R167–R181. [6] V.B. Bregar, Advantages of ferromagnetic nanoparticle composites in microwave absorbers, IEEE Trans. Magn. 40 (2004) 1679–1684. [7] G.H. Mu, N. Chen, X.F. Pan, H.G. Shen, M.Y. Gu, Preparation and microwave absorption properties of barium ferrite nanorods, Mater. Lett. 62 (2008) 840–842. [8] J.X. Qiu, Y. Wang, M.Y. Gu, Microwave absorption properties of substituted BaFe12O19/TiO2 nanocomposite multilayer film, J. Mater. Sci. 42 (2007) 166–169. [9] V.G. Harris, Z.H. Chen, Y.J. Chen, S. Yoon, T. Sakai, A. Gieler, A. Yang, Y.X. He, K.S. Ziemer, N.X. Sun, C. Vittoria, Ba-hexaferrite films for next generation microwave devices (invited), J. Appl. Phys. 99 (2006) 08M911. [10] Y.J. Chen, T. Sakai, T.Y. Chen, S.D. Yoon, A.L. Geiler, C. Vittoria, V.G. Harris, Oriented barium hexaferrite thick films with narrow ferromagnetic resonance linewidth, Appl. Phys. Lett. 88 (2006) 062516. [11] M.C. Dimri, S.C. Kashyap, D.C. Dube, Electrical and magnetic properties of barium hexaferrite nanoparticles prepared by citrate precursor method, Ceram. Int. 30 (2004) 1623–1626. [12] H.M. Lee, S.Y. Bae, J.H. Yu, Y.J. Kim, Preparation of unsintered spherical BaFe12O19 nanoparticles via an alcohol-assisted spray-pyrolysis route, J. Am. Ceram. Soc. 91 (2008) 2856–2861. [13] Y.C. Du, H.B. Gao, X.R. Liu, J.Y. Wang, P. Xu, X.J. Han, Solvent-free synthesis of hexagonal barium ferrite (BaFe12O19) particles, J. Mater. Sci. 45 (2010) 2442–2448.
C.-Y. Xu et al. / Ceramics International 40 (2014) 8593–8597 [14] A. Fert, L. Piraux, Magnetic nanowires, J. Magn. Magn. Mater. 200 (1999) 338–358. [15] P.F. Yang, M.G. Wang, F.J. Meng, J.H. Zhang, A.J. Sun, C.X. Lu, Preparation of rodlike barium ferrite particles by inverse microemulsion method, Chin. J. Inorg. Chem. 21 (2005) 607–611. [16] G.H. Mu, X.F. Pan, N. Chen, K.K. Gan, M.Y. Gu, Preparation and magnetic properties of barium hexaferrite nanorods, Mater. Res. Bull. 43 (2008) 1369–1375. [17] J. Wang, Y.J. Wu, Y.J. Zhu, P.Q. Wang, Formation of rod-shaped BaFe12O19 nanoparticles with well magnetic properties, Mater. Lett. 61 (2007) 1522–1525. [18] Y.Z. Guo, C.J. Li, J.N. Wang, Fabrication and magnetic properties of nano-BaFe12O19 fibre by electrospinning, Chin. J. Inorg. Chem. 25 (6) (2009) 1018–1021. [19] Z. Xiao, Y. Xia, Z.H. Ren, Z.Y. Liu, G. Xu, C.Y. Chao, X. Li, G. Shen, G.R. Han, Facile synthesis of single-crystalline mesoporous α-Fe2O3 and Fe3O4 nanorods as anode materials for lithium-ion batteries, J. Mater. Chem. 22 (2012) 20566–20573. [20] B. Tang, G.L. Wang, L.H. Zhuo, J.C. Ge, L.J. Cui, Facile route to α-FeOOH and α-Fe2O3 nanorods and magnetic property of α-Fe2O3 nanorods, Inorg. Chem. 45 (2006) 5196–5200.
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[21] X.H. Cao, Y.Z. Tan, J.H. Meng, J. Sun, Synthesis process of Ba-ferrites from α-FeOOH and γ-FeOOH and investigation of magnetic properties, Chin. Sci. Bull. 55 (2010) 1952–1956. [22] J.F. Xu, F.F. Guo, Z. Xu, Sythesis and characterization of hexagonal Ba-ferrite nanocrystals, J. Tongji Univ. (Nat. Sci.) 32 (2004) 929–932 (in Chinese). [23] Q. Gao, F.H. Chen, J.L. Zhang, G.Y. Hong, J.Z. Ni, X. Wei, D.J. Wang, The study of novel Fe3O4@γ-Fe2O3 core/shell nanomaterials with improved properties, J. Magn. Magn. Mater. 321 (2009) 1052–1057. [24] B.T. Shirk, W.R. Buessem, Temperature dependence of Ms and K1 of BaFe12O19 and SrFe12O19 single crystals, J. Appl. Phys. 40 (1969) 1294–1296. [25] L. Rezlescu, E. Rezlescu, P.D. Popa, N. Rezlescu, Fine barium hexaferrite powder prepared by the crystallisation of glass, J. Magn. Magn. Mater. 193 (1999) 288–290. [26] R. Doni W, A. Manaf, P. Sardjono, Physical characteristics and magnetic properties of barium hexaferrites (BaFe12O19) derived from mechanical alloying, Int. J. Basic Appl. Sci. 13 (2013) 65–68. [27] N. Cordente, M. Respaud, F. Senocq, M.J. Casanove, C. Amiens, B. Chaudret, Synthesis and magnetic properties of nickel nanorods, Nano Lett. 1 (2001) 565–568.
CERAMICS INTERNATIONAL
Ceramics International 40 (2014) 8593–8597 www.elsevier.com/locate/ceramint
Topochemical synthesis and magnetic properties of BaFe12O19 nanorods using α-FeOOH nanowires as templates Cheng-Yan Xua,b, Li-Shun Fua, Xin Caia, Xue-Yin Suna, Liang Zhena,b,n a School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China MOE Key Laboratory of Micro-systems and Micro-structures Manufacturing, Harbin Institute of Technology, Harbin 150080, China
b
Received 19 August 2013; received in revised form 16 December 2013; accepted 15 January 2014 Available online 26 January 2014
Abstract We present a topochemical route for the synthesis of BaFe12O19 nanorods by using hydrothermal synthesized mixture of α-FeOOH nanowires and BaCO3 as precursors. The α-FeOOH nanowires and in-situ formed BaCO3 transformed to BaFe12O19 nanorods by a two-step annealing process. The crystal structure and morphology changes of the products with annealing temperatures were investigated. BaFe12O19 nanorods with diameters of about 250 nm and lengths of about 1 μm were obtained at annealing temperature of 1000 1C. A comparison of magnetic properties between BaFe12O19 nanorods and granules was performed and discussed. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: A. Powders: chemical preparation; C. Magnetic properties; D. Ferrites
1. Introduction As an important hard magnetic material, barium hexaferrite (BaFe12O19) is widely used in high density perpendicular recording media [1–3], magnetic fluids [4,5], electromagnetic wave absorber [6–8] and high frequency devices [9–11] due to its excellent physical and chemical properties, like large magnetocrystalline anisotropy, high Curie temperature, relatively large saturation magnetization, as well as excellent chemical stability and good corrosion resistivity [12,13]. In recent years, the fabrication of one-dimensional (1D) nanostructured magnetic materials has aroused considerable interest due to their applications in high density recording media with promising giant magnetoresistance (GMR) properties [14]. So far, various chemical routes were utilized to synthesize 1D nanostructured barium ferrite, such as the inverse microemulsion method [15], sol–gel synthesis [7,16], the hydrothermal method [17] and electrospinning technique [18], etc. Mu et al. prepared barium ferrite (BaFe12O19) nanorods with diameters of about 60 nm and lengths n Corresponding author at: School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China. Tel.: þ 86 451 86412133; fax: þ 86 451 86413922. E-mail address: [email protected] (L. Zhen).
of about 300 nm via a sol–gel process [16]. By a hydrothermal and annealing process, Wang et al. fabricated rod-like BaFe12O19 nanoparticles with coercivity of 4511 Oe [17]. Nano-sized BaFe12O19 fibers with diameter of 150 nm were obtained using electrospinning technique [18]. Although these works provided pathways to prepare the BaFe12O19 nanorods, the practical applications are still limited due to the relatively long reaction time or the requirements for unique instruments to prepare the material. In addition, the formation mechanisms are always the bottle neck problem restraining the development of the nanosized BaFe12O19. The topochemical method is a synthesis process that can control the shape of final products by selecting the morphology of precursors with mixing components in ionic states and precisely measuring the chemical activity and optimization of the experiment parameters of all components. α-FeOOH nanowires could be easily obtained and were thus used as templates for preparing a wealth of 1D nanostructured magnetic oxides, such as α-Fe2O3 and Fe3O4 [19,20]. Cao and co-workers synthesized BaFe12O19 nanorods with the topochemical reaction method using α-FeOOH nanowires as templates. They first prepared α-FeOOH nanowires with the chemical precipitation method, then coated with BaCO3 via the solution chemical method, and followed calcination at 1000 1C to obtain the BaFe12O19 nanorods [21].
0272-8842/$ - see front matter & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved. http://dx.doi.org/10.1016/j.ceramint.2014.01.075
C.-Y. Xu et al. / Ceramics International 40 (2014) 8593–8597
In this work, we prepared α-FeOOH nanowires and in-situ formed BaCO3 which were coated on the nanowires by the hydrothermal method, then converted them into BaFe12O19 nanorods through a two-step annealing process. The microstructure and morphology of barium ferrite nanorods and precursory nanowires were investigated. The magnetic properties of BaFe12O19 nanorods and granules were compared and the mechanism for this difference was discussed.
Intensity (a.u.)
8594
∇
∇∇
∇
JCPDS 29-0713, α-FeOOH
JCPDS 45-1471, BaCO3
2. Experimental BaFe12O19 nanorods were synthesized by a topochemical reaction method, namely, annealing the hydrothermal synthesized α-FeOOH nanowires at high temperature. In a typical process, barium nitrate [Ba(NO3)2] and ferric nitrate [Fe(NO3)2 9H2O] were dissolved in 20 ml distilled water, and then 20 ml sodium hydroxide (NaOH) solution and 2.0 g sodium dodecyl benzene sulfonate (SDBS) was added to the mixture solution under vigorous stirring. The molar ratio of Ba(NO3)2, Fe(NO3)2 9H2O and NaOH is 1:12:60, and the concentrations of Ba(NO3)2, Fe(NO3)2 9H2O and NaOH are 0.017, 0.2 and 1.0 M, respectively (stoichiometrically for BaFe12O19, and in the product Fe is in form of α-FeOOH and Ba in form of BaCO3). Then, the mixture was transferred to a Teflon-lined stainless-steel autoclave for hydrothermal reaction at 220 1C for 24 h. The yellow brown precursors were collected by centrifugation, washed several times with distilled water and ethanol. After being dried at 60 1C, the precursors were heat-treated at 450 1C for 3 h in air, aiming to convert αFeOOH into γ-Fe2O3 [22]. The obtained precursory powders were subsequently annealed in air for 3 h at temperatures of 800, 1000 and 1100 1C, respectively. The products were named in turn as A800, A1000 and A1100, accordingly. The crystalline structures of products were characterized by X-ray diffraction (XRD, Rigaku D/max-γB, Cu Kα). The morphology of products was observed on a scanning electron microscope (SEM, FEI Quanta 200F). Static magnetic properties were measured by a vibrating sample magnetometer (VSM, Lakeshore 7410) with an applied magnetic field of 20 kOe at room temperature. 3. Results and discussion Fig. 1 shows the X-ray diffraction pattern of the precursors obtained by the hydrothermal method. The main peaks can be indexed to α-FeOOH (JCPDS 29-0713) and BaCO3 (JCPDS 45-1471), and the other peaks (marked with triangles) are assigned to Ba(OH)2 H2O (JCPDS 26-0154) as minor constituent. The strong and narrow diffraction peaks indicates that the as-prepared product was well crystallized. Fig. 2a shows the SEM image of the precursor product. As demonstrated in this figure, the precursors are nanowires with diameters of about 100 nm and lengths of about 2 μm. It is also found in Fig. 2b that the surfaces of nanowires are not smooth due to adhesion of small particles, indicating that BaCO3 phases are likely to be distributed on the surface of α-FeOOH nanowires
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2θ (°) Fig. 1. XRD pattern of the hydrothermal synthesized products. The main peaks can be indexed to α-FeOOH and BaCO3, and the other peaks (marked with triangles) are assigned to Ba(OH)2 H2O as minor constituent.
in the form of small particles for the hydrothermally synthesized product. The precursors containing both α-FeOOH and BaCO3 were first heat-treated at 450 1C for 3 h in air, followed by annealing in air for another 3 h at different temperatures to obtain BaFe12O19. After heat treatment at 450 1C for 3 h, α-FeOOH can be converted into γ-Fe2O3 [23]. The formation process of BaFe12O19 is as follows: γ-Fe2O3 þ BaCO3-BaFe2O4 þ CO2↑
(1)
BaFe2O4 þ 5γ-Fe2O3-BaFe12O19
(2)
The crystal structure of γ-Fe2O3 is similar to that of BaFe2O4, which makes the transformation from γ-Fe2O3 to BaFe2O4 much easier than that from α-Fe2O3. As reported before, without additional annealing at 450 1C for 3 h, γ-Fe2O3 would be easily converted into α-Fe2O3 as temperature increased, which might hinder the formation of BaFe12O19 [22]. XRD patterns of the products annealed at different temperatures are shown in Fig. 3. All the diffraction peaks of products prepared at 1000 1C are indexed to a pure hexagonal structured BaFe12O19 with lattice constants of a¼ 5.895 Å, and c¼ 23.215 Å (JCPDS 39-1433), indicating complete transformation to barium ferrite. For the products annealed at 800 and 1100 1C, as shown in Fig. 3a and c, the peaks of products are indexed to BaFe12O19 (JCPDS 39-1433) and α-Fe2O3 (JCPDS 33-0664), indicating that the majority of precursory powders was transformed to BaFe12O19 whereas a small amount of α-Fe2O3 residue was also formed after the annealing. Fig. 4 shows the SEM images of products annealed at different temperatures. The SEM image of sample A800 shown in Fig. 4a demonstrates that the products maintained the morphology of nanowires with an increase in diameters of about 50 nm after annealing at 800 1C. With the increase of annealing temperature to 1000 1C, the diameters of products increase and the lengths decrease, resulting in the variation of the morphology from nanowires to short rods. Typical size of
C.-Y. Xu et al. / Ceramics International 40 (2014) 8593–8597
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Fig. 2. SEM images of the precursors obtained by the hydrothermal method. (a) Low magnification and (b) high magnification.
BaFe12O19 α-Fe2O3
Intensity (a.u.)
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80
2θ (°) Fig. 3. XRD patterns of the products annealed at different temperatures: (a) 800 1C; (b) 1000 1C; and (c) 1100 1C.
the BaFe12O19 nanorods is about 250 nm in diameter and about 1 μm in length, as shown in Fig. 4b. When the annealing temperature is further increased to 1100 1C, the morphology of products changed dramatically from short rods to granules with typical particle size of about 1 μm, as displayed in Fig. 4c. Such morphology change may be due to the greatly enhanced diffusion process arising from the elevated annealing temperature. It has been reported that the coercivity (Hc) of BaFe12O19 nanorods was higher than that of plate-like barium hexaferrites because the rod-like shape increases the effective anisotropy due to its large aspect ratio [16]. Room-temperature static magnetic properties of BaFe12O19 obtained at different annealing temperatures were measured by VSM, as shown in Fig. 5. The magnetic hysteresis loop of BaFe12O19 nanorods obtained at 1000 1C exhibits a saturation magnetization (Ms) of 62.5 emu/g and a high Hc of 3811 Oe. Compared with BaFe12O19 nanorods, BaFe12O19 granules possess a higher Ms (72.2 emu/g) and a much lower Hc (3171 Oe). It is noted that the Ms of BaFe12O19 granules reaches the theoretical value (72 emu/g) of bulk BaFe12O19 [24]. The larger Ms value for BaFe12O19 granules may be attributed to the lower oxygen vacancy concentration compared to BaFe12O19 nanorods. The annealing temperature
of barium ferrite granules is higher than that of barium ferrite nanorods, which may reduce the concentration of oxygen vacancies and thus increase the number of nearest neighbor oxygen atoms of Fe. This effect can favorably improve the superexchange of Fe–O–Fe [17] and lead to a higher Ms value, as observed above. There are at least three reasons for the enhanced coercivity of BaFe12O19 nanorods comparing to that of BaFe12O19 granules. One reason is the effect of particle size. The critical diameter of single magnetic domain for spherical barium ferrite was reported to be 460 nm [25]. With particle sizes below this critical value, the barium ferrite nanorods tend to form single magnetic domains, which would exhibit a high value of Hc. Another reason could be attributed to the preferential orientation of BaFe12O19 nanorods during the measurement. In addition, the presence of a small amount of α-Fe2O3 residue which is paramagnetic can reduce the coercivity of BaFe12O19 granules [26]. However, the contributions related to shape anisotropy are negative in this work. As we know, the shape anisotropy (Ks) contributes to the effective anisotropy together with the magnetocrystalline anisotropy (Km). It is possible to estimate the shape anisotropy according to Ks ¼ δNM2s /2, where δN is the anisotropy contribution of the demagnetizing factor due to the deviation from sphericity [27]. In this work, considering that BaFe12O19 nanorods have small aspect ratio compared with BaFe12O19 granular platelets, the rod-like shape decreases the overall magnetic anisotropy of the barium ferrite. 4. Conclusions BaFe12O19 nanorods were prepared by a topochemical reaction method including hydrothermal and annealing process. The precursor nanowires obtained by the hydrothermal method are the mixture of α-FeOOH and BaCO3. With the increase of annealing temperature, the phase of the products transformed to BaFe12O19 while the morphology changed from nanowires to nanorods, and further converted granules. When the annealing temperature is 1000 1C, BaFe12O19 nanorods were successfully synthesized with diameters of about 250 nm and lengths of about 1 μm, which possess lower Ms and much higher Hc than those of BaFe12O19 granules obtained at 1100 1C.
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Fig. 4. SEM images of the products annealed at different temperatures: (a) 800 1C; (b) 1000 1C; and (c) 1100 1C.
80 60
M (emu/g)
40 20 0 -20
1000 °C 1100 °C
-40 -60 -80 -20
-15
-10
-5
0
5
10
15
20
H (kOe)
Fig. 5. Magnetic hysteresis loops of barium ferrite obtained at different temperatures. Insert is the enlarged plot around the zero field.
Acknowledgments This work was supported by the Fundamental Research Funds for the Central Universities (HIT.KLOF.2010004). C.Y.X. acknowledges the Fundamental Research Funds for the Central Universities (HIT.BRETIII.201203). The authors appreciate Dr. Ning Xie and Dr. Kai He for language polishing.
References [1] T. Fujiwara, Barium ferrite media for perpendicular recording, IEEE Trans. Magn. 21 (1985) 1480–1485.
[2] K. Yamamori, T. Suzuki, T. Fujiwara, High-density recording characteristics for ba-ferrite flexible disks, IEEE Trans. Magn. 22 (1986) 1188–1190. [3] V.V. Pankov, M. Pernet, P. Germi, P. Mollard, Fine hexaferrite particles for perpendicular recording prepared by the coprecipitation method in the presence of an inert component, J. Magn. Magn. Mater. 120 (1993) 69–72. [4] D. Primc, D. Makovec, D. Lisjak, M. Drofenik, Hydrothermal synthesis of ultrafine barium hexaferrite nanoparticles and the preparation of their stable suspensions, Nanotechnology 20 (2009) 315605. [5] Q.A. Pankhurst, J. Connolly, S.K. Jones, J. Dobson, Applications of magnetic nanoparticles in biomedicine, J. Phys. D: Appl. Phys. 36 (2003) R167–R181. [6] V.B. Bregar, Advantages of ferromagnetic nanoparticle composites in microwave absorbers, IEEE Trans. Magn. 40 (2004) 1679–1684. [7] G.H. Mu, N. Chen, X.F. Pan, H.G. Shen, M.Y. Gu, Preparation and microwave absorption properties of barium ferrite nanorods, Mater. Lett. 62 (2008) 840–842. [8] J.X. Qiu, Y. Wang, M.Y. Gu, Microwave absorption properties of substituted BaFe12O19/TiO2 nanocomposite multilayer film, J. Mater. Sci. 42 (2007) 166–169. [9] V.G. Harris, Z.H. Chen, Y.J. Chen, S. Yoon, T. Sakai, A. Gieler, A. Yang, Y.X. He, K.S. Ziemer, N.X. Sun, C. Vittoria, Ba-hexaferrite films for next generation microwave devices (invited), J. Appl. Phys. 99 (2006) 08M911. [10] Y.J. Chen, T. Sakai, T.Y. Chen, S.D. Yoon, A.L. Geiler, C. Vittoria, V.G. Harris, Oriented barium hexaferrite thick films with narrow ferromagnetic resonance linewidth, Appl. Phys. Lett. 88 (2006) 062516. [11] M.C. Dimri, S.C. Kashyap, D.C. Dube, Electrical and magnetic properties of barium hexaferrite nanoparticles prepared by citrate precursor method, Ceram. Int. 30 (2004) 1623–1626. [12] H.M. Lee, S.Y. Bae, J.H. Yu, Y.J. Kim, Preparation of unsintered spherical BaFe12O19 nanoparticles via an alcohol-assisted spray-pyrolysis route, J. Am. Ceram. Soc. 91 (2008) 2856–2861. [13] Y.C. Du, H.B. Gao, X.R. Liu, J.Y. Wang, P. Xu, X.J. Han, Solvent-free synthesis of hexagonal barium ferrite (BaFe12O19) particles, J. Mater. Sci. 45 (2010) 2442–2448.
C.-Y. Xu et al. / Ceramics International 40 (2014) 8593–8597 [14] A. Fert, L. Piraux, Magnetic nanowires, J. Magn. Magn. Mater. 200 (1999) 338–358. [15] P.F. Yang, M.G. Wang, F.J. Meng, J.H. Zhang, A.J. Sun, C.X. Lu, Preparation of rodlike barium ferrite particles by inverse microemulsion method, Chin. J. Inorg. Chem. 21 (2005) 607–611. [16] G.H. Mu, X.F. Pan, N. Chen, K.K. Gan, M.Y. Gu, Preparation and magnetic properties of barium hexaferrite nanorods, Mater. Res. Bull. 43 (2008) 1369–1375. [17] J. Wang, Y.J. Wu, Y.J. Zhu, P.Q. Wang, Formation of rod-shaped BaFe12O19 nanoparticles with well magnetic properties, Mater. Lett. 61 (2007) 1522–1525. [18] Y.Z. Guo, C.J. Li, J.N. Wang, Fabrication and magnetic properties of nano-BaFe12O19 fibre by electrospinning, Chin. J. Inorg. Chem. 25 (6) (2009) 1018–1021. [19] Z. Xiao, Y. Xia, Z.H. Ren, Z.Y. Liu, G. Xu, C.Y. Chao, X. Li, G. Shen, G.R. Han, Facile synthesis of single-crystalline mesoporous α-Fe2O3 and Fe3O4 nanorods as anode materials for lithium-ion batteries, J. Mater. Chem. 22 (2012) 20566–20573. [20] B. Tang, G.L. Wang, L.H. Zhuo, J.C. Ge, L.J. Cui, Facile route to α-FeOOH and α-Fe2O3 nanorods and magnetic property of α-Fe2O3 nanorods, Inorg. Chem. 45 (2006) 5196–5200.
8597
[21] X.H. Cao, Y.Z. Tan, J.H. Meng, J. Sun, Synthesis process of Ba-ferrites from α-FeOOH and γ-FeOOH and investigation of magnetic properties, Chin. Sci. Bull. 55 (2010) 1952–1956. [22] J.F. Xu, F.F. Guo, Z. Xu, Sythesis and characterization of hexagonal Ba-ferrite nanocrystals, J. Tongji Univ. (Nat. Sci.) 32 (2004) 929–932 (in Chinese). [23] Q. Gao, F.H. Chen, J.L. Zhang, G.Y. Hong, J.Z. Ni, X. Wei, D.J. Wang, The study of novel Fe3O4@γ-Fe2O3 core/shell nanomaterials with improved properties, J. Magn. Magn. Mater. 321 (2009) 1052–1057. [24] B.T. Shirk, W.R. Buessem, Temperature dependence of Ms and K1 of BaFe12O19 and SrFe12O19 single crystals, J. Appl. Phys. 40 (1969) 1294–1296. [25] L. Rezlescu, E. Rezlescu, P.D. Popa, N. Rezlescu, Fine barium hexaferrite powder prepared by the crystallisation of glass, J. Magn. Magn. Mater. 193 (1999) 288–290. [26] R. Doni W, A. Manaf, P. Sardjono, Physical characteristics and magnetic properties of barium hexaferrites (BaFe12O19) derived from mechanical alloying, Int. J. Basic Appl. Sci. 13 (2013) 65–68. [27] N. Cordente, M. Respaud, F. Senocq, M.J. Casanove, C. Amiens, B. Chaudret, Synthesis and magnetic properties of nickel nanorods, Nano Lett. 1 (2001) 565–568.