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SiO2 nanocomposites with core–shell structure
Materials Letters 61 (2007) 2187 – 2190 www.elsevier.com/locate/matlet
Preparation and magnetic properties of SrFe12O19/SiO2 nanocomposites with core–shell structure Wuyou Fu ⁎, Haibin Yang, Qingjiang Yu, Jing Xu, Xiaofen Pang, Guangtian Zou National Laboratory of Superhard Materials, Jilin University, Changchun, 130012, P.R. China Received 26 June 2006; accepted 17 August 2006 Available online 15 September 2006
Abstract SrFe12O19/SiO2 nanocomposites with a core–shell structure have been obtained. The core SrFe12O19 nanoparticles were synthesized by a citrate precursor technique with Fe/Sr ratios of 10.8, and silica was coated on SrFe12O19 forming complete coverage by the controlled hydrolysis and condensation of tetraethyl orthosilicate (TEOS). The composition, morphology and structure of the products were characterized by EDS, XRD, TEM, and IR spectroscopy, respectively. The results indicate that the product has a core–shell structure, which is combined through the chemical bond of Fe–O–Si. The magnetic measurements were carried out on a vibrating sample magnetometer (VSM), and the measurement results indicate the reduction of the magnetization of the SiO2 coated strontium ferrite nanoparticles compared with the uncoated ferrite nanoparticles. High coercivity also shows that the prepared uncoated and coated ferrite nanoparticles are not superparamagnetic. © 2006 Elsevier B.V. All rights reserved. Keywords: Core–shell; SrFe12O19/SiO2; Magnetic; Nanoparticles
1. Introduction The M type hexaferrites, MFe12O19 (M = Ba, Sr, Pb) have been studied for a long time because of their technical applications [1–3]. The magnetic properties of these materials make them excellent materials for use as permanent magnets, components of microwave and higher frequency devices, and perpendicular magnetic recording media [4,5]. Studies on protective layer-coated permanent magnets nanoparticles are of great interest for both fundamental magnetic investigations and practical engineering applications. In fundamental studies, the coated nanoparticles are of interest as the coating prevents the nanocomposites from coarsening and agglomeration. In practical engineering applications, for example, magnetic applications, the coating not only works as an insulate phase to achieve high electric resistivity, but also behaves as a binder to ease the consolidation of the nanoparticles. Coating magnetic nanoparticles with silica is becoming a promising and important
⁎ Corresponding author. Tel.: +86 431 5168763; fax: +86 431 5168816. E-mail address: [email protected] (W. Fu). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.08.059
approach in the development of magnetic nanocomposites for both fundamental study and technology application [6,7]. First, silica formed on the surface of magnetic nanoparticles could screen the magnetic dipolar attraction between magnetic nanoparticles, which favors the dispersion of magnetic nanocomposites in liquid media and protects them from leaching in an acidic environment. Second, due to the existence of abundant silanol groups on the silica layer, silica-coated magnetic nanoparticles could be easily activated to provide the surface of silica-coated magnetic nanoparticles with various functional groups. Finally, the most important is that the silica layer provides a chemically inert surface for magnetic nanoparticles. However, up to now, the reports of magnetic nanoparticles coated with SiO2 focused on the preparation and the magnetic behavior of magnetic metal or spinel ferrite coated with SiO2 [8–17]. The M type hexaferrites SrFe12O19 are an excellent magnetic material, it is well known that the report of SrFe12O19/SiO2 has not been identified. In this paper, we first report a synthetic route to prepare a new material which is nanometer-size strontium ferrite particles coated with uniform silica layer based on a combined citrate precursor technique [1] with Stöber method [18].
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2. Experimental section 2.1. Materials Strontium nitrate (Sr(NO3)2), citric acid (C6H8O7·H2O), pentahydrate ferric citrate (C6H5O7Fe·5H2O), anhydrous ethanol (C2H5OH), tetraethyl orthosilicate (TEOS) and ammonia (NH3H ˙ 2O) were obtained from Beijing Chemical Corporation (Beijing, China). All reagents were of analytical grade and were used without further purification. Deionized water (18.2 MΩ cm) was used in all the experiments. 2.2. Preparation of sample 2.2.1. Preparation of strontium ferrite nanoparticles In the citrate precursor technique [19], first a strontium citrate solution is formed from stoichiometric amounts of strontium nitrate and citric acid in the molar ratio 1:1. This was
Fig. 2. The XRD patterns of (a) pure SrFe12O19 nanoparticles and (b)SrFe12O19/ SiO2 core–shell structure nanocomposites.
further mixed with a ferric citrate solution in proportion for a set of compositions with Fe/Sr ratio of 10.8. The resultant homogeneous solution with pH = 2.6 was refluxed at 100 °C for 12 h and further dehydrated from ethanol (1:10 ratio) to obtain the citrate precursor. By following the solid state reactivity studies [20] the dehydrated precursors were thermally decomposed at 600 °C for 1 h to form crystalline SrFe12O19 materials and were further heat treated at 800 °C for 1 h to control the fine particle nature of the SrFe12O19 phase. 2.2.2. Preparation of SrFe12O19/SiO2 nanocomposites The shell SiO 2 was derived via sol–gel hydrolysis precipitation of tetraethyl orthosilicate (TEOS) at 45 °C. A certain amount of as-prepared strontium ferrite nanoparticles
Fig. 1. EDS analysis image of (a) the pure SrFe12O19 nanoparticles and (b) core– shell structure of SrFe12O19/SiO2 nanocomposites.
Fig. 3. TEM micrograph of the SrFe12 O19/SiO2 core–shell structure nanocomposites.
W. Fu et al. / Materials Letters 61 (2007) 2187–2190
were dispersed in the mixture solution of water–ethanol with a molar ratio of 1:5. The suspension was sonicated for 30 min in an ultrasonic cleaning bath and subsequently vigorously stirred at 45 °C for 1 h and a certain amount of 1.4 M NH3H2O was added to the above suspension. After the temperature was stabilized to 45 °C, a certain amount of TEOS was rapidly injected into the reaction system and the reaction was continued. A slow and gradual supersaturation is essential to achieve the heterogenous nucleation of silicon oxide onto the strontium ferrite nanoparticles. The stock suspension was stirred for at least 6 h to ensure the complete coating. The precipitates were separated from the mother by centrifugation and then redispersed in ethanol in order to minimize particle agglomeration by hydrogen bonding. This washing process was repeated five times. The precipitate was separated from the mother liquid and dried at 50 °C for 8 h in a vacuum oven to obtain the core–shell structure of the SrFe12O19/SiO2 nanocomposites.
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Fig. 5. Hysteresis loop at room temperature of (1) pure SrFe12O19 nanoparticles and (2) SrFe12O19/SiO2 core–shell structure nanocomposites.
2.3. Characterization of sample The phase structure of the powder was identified using a Rigaku-2400 X-ray diffractometer (Cu–Kα radiation, λ = 1.5418 Å). The size and morphology of the composites were observed using (JEM1200EX) transmission electron microscope (TEM). A JEOL JEM-6700F field emission scanning electron microscope (FESEM) equipped with EDS was employed for the composition analysis. Mid-infrared spectra, from 4000 to 400 cm− 1, were recorded using a Nicolet-510 spectrophotometer on pellets obtained dispersing the samples in KBr. The magnetization measurements were carried out at room temperature by using a Vibrating Sample Magnetometer (Model TMVSM1230-HHHS) with a maximum applied field of 10 kOe. 3. Results and discussion 3.1. Confirmation of SrFe12O19/SiO2 core–shell structure nanocomposites The EDS patterns of pure SrFe12O19 and SrFe12O19/SiO2 nanocomposites are presented in Fig. 1. It is clearly seen that the
Fig. 4. IR spectra of the SrFe12O19/SiO2 nanocomposites.
SrFe12O19 consists of Sr, Fe and O elements (Fig. 1a), the ratio of Sr, Fe and O in the strontium ferrite obtained was determined to be 1:12:19, while for SrFe12O19/SiO2 nanocomposites (Fig. 1b), except for Sr, Fe, O (elements of SrFe12O19), another one such as Si (elements of SiO2) also exists. The results indicate that the as-prepared nanocomposite has two phases of SrFe12O19 and SiO2. Typical X-ray diffraction patterns for the SrFe12O19 nanoparticles before and after silica coating are shown in Fig. 2. As seen in Fig. 2a, the peaks appeared at 19.34, 23.22, 30.93, 31.13, 32.61, 34.2, 36.05, 37.80, 41.06, 57.46 and 64.3° attributed to (102), (006), (110), (008), (107), (201), (203), (205), (206), (2011) and (220) etc reflections for the SrFe12O19 nanoparticles which can be indexed to a pure M type hexaferrite structure ferrite that is in agreement with JCPDS data of PDF #720739. No impurity peaks were detected indicating that the powders had high purity. The core–shell structure SrFe12O19/SiO2 nanocomposites (Fig. 2b) had similar diffraction peaks, but the intensity of the peaks decreased and there was a broad weak peak around 2 theta 23° which indicated that the shell layer of SiO2 was amorphous and the crystallinity of the magnetic nanoparticles core is retained after the coating procedure. The TEM image of typical SrFe12O19/SiO2 nanocomposites is shown in Fig. 3. It can be clearly seen that a SiO2 coating is enwrapped on the SrFe12O19 surface forming a core–shell structure of SrFe12O19/ SiO2 nanocomposites. The SrFe12O19 is a kind of magnetic material that has more scattered electrons than that of SiO2 which is a kind of non-magnetic oxide, so the core region is dark, while the shell region is bright in SrFe12O19/SiO2 nanocomposites. The core of strontium ferrite is spherical or elliptical and the diameter of the core is in the range of 40–65 nm. The shell of SiO2 enwraps closely around the core and the thickness of the shell is about 9 nm. The infrared spectra of pure SiO2, SrFe12O19/SiO2 and pure SrFe12O19 are shown in Fig. 4(a), (b) and (c), respectively. In Fig. 4(a), these characteristic absorption peaks of the samples are according to those of the standard spectrum of SiO2X ˙ H2O [21]. These spectra confirm the presence of Si–O–Si bonds because of the absorption peaks at 470.1, 800.4 and 1103.4 cm− 1. They are ascribed to bond bending vibration, symmetric bond stretching vibration of Si–O–Si and asymmetric bond stretching vibration [22–24]. The presence of Si–OH is confirmed by the peak at 955.7 cm− 1, which is associated with its stretching mode vibration. The peak at 1649.1 cm− 1 is due to the deformation mode of absorbed molecular water and that around 3400.1 cm− 1 is due to O–H
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Table 1 Magnetic properties and the thickness of SiO2 shell of SrFe12O19 nanoparticles coated with silica Sample no.
Hc (Oe)
Ms (emu/g)
Thickness of shell (nm)
Mr/Ms
1 2 3 4
4946.6 4947.1 4945.9 4947.4
85.7 56.5 40.3 20.0
0 4 9 15
0.628 0.604 0.609 0.598
stretching of the same [25]. In Fig. 4(b), strong absorptions at 1094.4, 800.6 and 470.3 cm− 1 indicate the formation of a silica shell [26]. The band at 860.0 cm− 1 is assigned to Si–O–Fe. The presence of Si–O–Fe vibrations reflects some interaction between the highly isolated Fe3+ ions and the nearest silica shell. The Si–O–Fe bond is also evident by the presence of another band at 586.6 cm− 1, which is associated with the Fe– O stretching in Si–O–Fe bonds [27]. The results indicate that the product has a core–shell structure, which is combined through the chemical bond of Fe–O–Si. From Figs. 1, 2, 3 and 4, we can draw a conclusion that the SrFe12O19 nanoparticles were well-coated with SiO2. The magnetic properties of the pure SrFe12O19 and SrFe12O19/SiO2 core–shell structure nanoparticles were measured by VSM, as shown in Fig. 5. From VSM experiments, the magnetic parameters such as saturation magnetization (Ms), coercivity (Hc) and remnant magnetization (Mr) were given in Table 1. It is clearly seen that the value of saturation magnetization decreases with an increasing content of SiO2 shell and is lower than the pure SrFe12O19 in all cases, attributed mainly to the contribution of the volume of the non-magnetic coating layer to the total sample volume, while the coercivity of silicon dioxide-coated SrFe12O19 nanoparticles does not show any change after coating, because coercivity represents the property of a magnetic material and is determined by the strength and number of the magnetic dipole in magnetic domain and relations between adjacent magnetic domain. All in all, the coating of SiO2 doesn't change the magnetic property of SrFe12O19 particles. In addition, the non-magnetic coating layer can be considered as a magnetically dead layer at the surface, thus affecting the uniformity or magnitude of magnetization due to quenching of surface moments [28,29]. Note that, although the particles are nanoscale, they are not superparamagnetic as confirmed from the high Hc values.
4. Conclusions SrFe12O19/SiO2 core–shell nanoparticles have been prepared using a sequential synthetic method, in which core SrFe12O19 nanoparticles were prepared by a citrate precursor technique and the average particle size was 50 nm, shell silica was produced by the hydrolysis and condensation of tetraethyl orthosilicate
(TEOS). The analysis of XRD, TEM IR and EDS results indicates that the product has a core–shell structure which is combined through the chemical bonding of Fe–O–Si. The magnetic properties (the magnetization Ms) of SrFe12O19/SiO2 nanocomposites can be adjusted by the controlling the thickness of the SiO2 coating layer. The core–shell structure of SrFe12O19/ SiO2 can also been used as precursors to prepare hollow structure silica by complete removal of the core through chemical etching, or further grafted by other functional groups to prepare novel multifunctional nanomaterials. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29]
G.K. Thompson, B.J. Evans, J. Appl. Phys. 73 (1993) 6295. P. Sharrock, IEEE Trans. Magn. MAG. 25 (1989) 4374. H. Kryder, J. Magn. Magn. Mater. 83 (1990) 1. O. Kubo, T. Ido, H. Yokoyama, IEEE Trans. Magn. MAG. 18 (1982) 1122. Tohoku Metal Industries Ltd., Magnetic Recording Powders, Jpn. Patent. 59207 C4 (1984) 6. W. Fu, H. Yang, H. Bala, S. Liu, M. Li, G. Zou, Mater. Lett. 60 (2006) 1728. H. Nathani, R.D.K. Misra, Mater. Sci. Eng., B 113 (2004) 228. C. Wang, A. Harrison, J. Colloid Interface Sci. 217 (1999) 203. D.G. Shchukin, A.I. Kulak, D.V. Sviridov, Photochem. Photobiol. Sci. 1 (2002) 742. M. Wu, Y. Zhang, S. Hui, T. Xiao, S. Ge, W. Hines, J.I. Budnick, J. Appl. Phys. 92 (2002) 1. M. Aslam, L. Fu, S. Li, J. Colloid Interface Sci. 290 (2005) 444. M. Ohmori, E. Matuević, J. Colloid Interface Sci. 160 (1993) 288. C.R. Vestal, Z.J. Zhang, Nano Lett. 3 (2003) 12. M. Bonini, A. Wiedenmann, P. Baglioni, Physica A 339 (2004) 86. Y. Deng, C. Wang, J. Hu, W. Yang, S. Fu, Colloids Surf., A Physicochem. Eng. Asp. 262 (2005) 87. M. Iijima, Y. Yonemochi, M. Kimata, M. Hasegawa, M. Tsukada, H. Kamiya, J. Colloid Interface Sci. 287 (2005) 526. Q. Liu, Z. Xu, J.A. Finch, Egerton Chem. Mater. 10 (1998) 3936. W. Stöber, A. Fink, A. Bohm, J. Colloid Interface Sci. 26 (1968) 62. Vijayalakshmi, N.S. Gajbhiye, J. Appl. Phys. 83 (1998) 1. N.S. Gajbhiye, A. Vijayalakshim, J. Therm. Anal. 52 (1998) 595. R.A. Nyquisst, R.O. Kagel, Infrared Spectra on Inorganic Compounds, Academic Press, New York, 1971, p. 94. A. bertoluzza, J. Non-Cryst. Solids 147–148 (1992) 232. F. Orgaz, J. Non-Cryst. Solids 82 (1986) 57. L.L. Hench, Chem. Rev. 90 (1990) 33. S. Roy, S. Bandyopadhyay, J. Mater. Sci. Lett. 15 (1996) 1872. F.D. Monte, M.P. Morales, D. Levy, Langmuir 13 (1997) 3627. C. Chaneac, E. Tronc, J.P. Jolivet, J. Mater. Chem. 6 (1996) 1905. M. Ma, Y. Zhang, X. Li, D. Fu, H. Zhang, N. Gu, Colloids Surf., A Physicochem. Eng. Asp. 262 (2005) 87. R. Kaiser, G. Miskolezy, J. Appl. Phys. 41 (1970) 1064.
Preparation and magnetic properties of SrFe12O19/SiO2 nanocomposites with core–shell structure Wuyou Fu ⁎, Haibin Yang, Qingjiang Yu, Jing Xu, Xiaofen Pang, Guangtian Zou National Laboratory of Superhard Materials, Jilin University, Changchun, 130012, P.R. China Received 26 June 2006; accepted 17 August 2006 Available online 15 September 2006
Abstract SrFe12O19/SiO2 nanocomposites with a core–shell structure have been obtained. The core SrFe12O19 nanoparticles were synthesized by a citrate precursor technique with Fe/Sr ratios of 10.8, and silica was coated on SrFe12O19 forming complete coverage by the controlled hydrolysis and condensation of tetraethyl orthosilicate (TEOS). The composition, morphology and structure of the products were characterized by EDS, XRD, TEM, and IR spectroscopy, respectively. The results indicate that the product has a core–shell structure, which is combined through the chemical bond of Fe–O–Si. The magnetic measurements were carried out on a vibrating sample magnetometer (VSM), and the measurement results indicate the reduction of the magnetization of the SiO2 coated strontium ferrite nanoparticles compared with the uncoated ferrite nanoparticles. High coercivity also shows that the prepared uncoated and coated ferrite nanoparticles are not superparamagnetic. © 2006 Elsevier B.V. All rights reserved. Keywords: Core–shell; SrFe12O19/SiO2; Magnetic; Nanoparticles
1. Introduction The M type hexaferrites, MFe12O19 (M = Ba, Sr, Pb) have been studied for a long time because of their technical applications [1–3]. The magnetic properties of these materials make them excellent materials for use as permanent magnets, components of microwave and higher frequency devices, and perpendicular magnetic recording media [4,5]. Studies on protective layer-coated permanent magnets nanoparticles are of great interest for both fundamental magnetic investigations and practical engineering applications. In fundamental studies, the coated nanoparticles are of interest as the coating prevents the nanocomposites from coarsening and agglomeration. In practical engineering applications, for example, magnetic applications, the coating not only works as an insulate phase to achieve high electric resistivity, but also behaves as a binder to ease the consolidation of the nanoparticles. Coating magnetic nanoparticles with silica is becoming a promising and important
⁎ Corresponding author. Tel.: +86 431 5168763; fax: +86 431 5168816. E-mail address: [email protected] (W. Fu). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.08.059
approach in the development of magnetic nanocomposites for both fundamental study and technology application [6,7]. First, silica formed on the surface of magnetic nanoparticles could screen the magnetic dipolar attraction between magnetic nanoparticles, which favors the dispersion of magnetic nanocomposites in liquid media and protects them from leaching in an acidic environment. Second, due to the existence of abundant silanol groups on the silica layer, silica-coated magnetic nanoparticles could be easily activated to provide the surface of silica-coated magnetic nanoparticles with various functional groups. Finally, the most important is that the silica layer provides a chemically inert surface for magnetic nanoparticles. However, up to now, the reports of magnetic nanoparticles coated with SiO2 focused on the preparation and the magnetic behavior of magnetic metal or spinel ferrite coated with SiO2 [8–17]. The M type hexaferrites SrFe12O19 are an excellent magnetic material, it is well known that the report of SrFe12O19/SiO2 has not been identified. In this paper, we first report a synthetic route to prepare a new material which is nanometer-size strontium ferrite particles coated with uniform silica layer based on a combined citrate precursor technique [1] with Stöber method [18].
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2. Experimental section 2.1. Materials Strontium nitrate (Sr(NO3)2), citric acid (C6H8O7·H2O), pentahydrate ferric citrate (C6H5O7Fe·5H2O), anhydrous ethanol (C2H5OH), tetraethyl orthosilicate (TEOS) and ammonia (NH3H ˙ 2O) were obtained from Beijing Chemical Corporation (Beijing, China). All reagents were of analytical grade and were used without further purification. Deionized water (18.2 MΩ cm) was used in all the experiments. 2.2. Preparation of sample 2.2.1. Preparation of strontium ferrite nanoparticles In the citrate precursor technique [19], first a strontium citrate solution is formed from stoichiometric amounts of strontium nitrate and citric acid in the molar ratio 1:1. This was
Fig. 2. The XRD patterns of (a) pure SrFe12O19 nanoparticles and (b)SrFe12O19/ SiO2 core–shell structure nanocomposites.
further mixed with a ferric citrate solution in proportion for a set of compositions with Fe/Sr ratio of 10.8. The resultant homogeneous solution with pH = 2.6 was refluxed at 100 °C for 12 h and further dehydrated from ethanol (1:10 ratio) to obtain the citrate precursor. By following the solid state reactivity studies [20] the dehydrated precursors were thermally decomposed at 600 °C for 1 h to form crystalline SrFe12O19 materials and were further heat treated at 800 °C for 1 h to control the fine particle nature of the SrFe12O19 phase. 2.2.2. Preparation of SrFe12O19/SiO2 nanocomposites The shell SiO 2 was derived via sol–gel hydrolysis precipitation of tetraethyl orthosilicate (TEOS) at 45 °C. A certain amount of as-prepared strontium ferrite nanoparticles
Fig. 1. EDS analysis image of (a) the pure SrFe12O19 nanoparticles and (b) core– shell structure of SrFe12O19/SiO2 nanocomposites.
Fig. 3. TEM micrograph of the SrFe12 O19/SiO2 core–shell structure nanocomposites.
W. Fu et al. / Materials Letters 61 (2007) 2187–2190
were dispersed in the mixture solution of water–ethanol with a molar ratio of 1:5. The suspension was sonicated for 30 min in an ultrasonic cleaning bath and subsequently vigorously stirred at 45 °C for 1 h and a certain amount of 1.4 M NH3H2O was added to the above suspension. After the temperature was stabilized to 45 °C, a certain amount of TEOS was rapidly injected into the reaction system and the reaction was continued. A slow and gradual supersaturation is essential to achieve the heterogenous nucleation of silicon oxide onto the strontium ferrite nanoparticles. The stock suspension was stirred for at least 6 h to ensure the complete coating. The precipitates were separated from the mother by centrifugation and then redispersed in ethanol in order to minimize particle agglomeration by hydrogen bonding. This washing process was repeated five times. The precipitate was separated from the mother liquid and dried at 50 °C for 8 h in a vacuum oven to obtain the core–shell structure of the SrFe12O19/SiO2 nanocomposites.
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Fig. 5. Hysteresis loop at room temperature of (1) pure SrFe12O19 nanoparticles and (2) SrFe12O19/SiO2 core–shell structure nanocomposites.
2.3. Characterization of sample The phase structure of the powder was identified using a Rigaku-2400 X-ray diffractometer (Cu–Kα radiation, λ = 1.5418 Å). The size and morphology of the composites were observed using (JEM1200EX) transmission electron microscope (TEM). A JEOL JEM-6700F field emission scanning electron microscope (FESEM) equipped with EDS was employed for the composition analysis. Mid-infrared spectra, from 4000 to 400 cm− 1, were recorded using a Nicolet-510 spectrophotometer on pellets obtained dispersing the samples in KBr. The magnetization measurements were carried out at room temperature by using a Vibrating Sample Magnetometer (Model TMVSM1230-HHHS) with a maximum applied field of 10 kOe. 3. Results and discussion 3.1. Confirmation of SrFe12O19/SiO2 core–shell structure nanocomposites The EDS patterns of pure SrFe12O19 and SrFe12O19/SiO2 nanocomposites are presented in Fig. 1. It is clearly seen that the
Fig. 4. IR spectra of the SrFe12O19/SiO2 nanocomposites.
SrFe12O19 consists of Sr, Fe and O elements (Fig. 1a), the ratio of Sr, Fe and O in the strontium ferrite obtained was determined to be 1:12:19, while for SrFe12O19/SiO2 nanocomposites (Fig. 1b), except for Sr, Fe, O (elements of SrFe12O19), another one such as Si (elements of SiO2) also exists. The results indicate that the as-prepared nanocomposite has two phases of SrFe12O19 and SiO2. Typical X-ray diffraction patterns for the SrFe12O19 nanoparticles before and after silica coating are shown in Fig. 2. As seen in Fig. 2a, the peaks appeared at 19.34, 23.22, 30.93, 31.13, 32.61, 34.2, 36.05, 37.80, 41.06, 57.46 and 64.3° attributed to (102), (006), (110), (008), (107), (201), (203), (205), (206), (2011) and (220) etc reflections for the SrFe12O19 nanoparticles which can be indexed to a pure M type hexaferrite structure ferrite that is in agreement with JCPDS data of PDF #720739. No impurity peaks were detected indicating that the powders had high purity. The core–shell structure SrFe12O19/SiO2 nanocomposites (Fig. 2b) had similar diffraction peaks, but the intensity of the peaks decreased and there was a broad weak peak around 2 theta 23° which indicated that the shell layer of SiO2 was amorphous and the crystallinity of the magnetic nanoparticles core is retained after the coating procedure. The TEM image of typical SrFe12O19/SiO2 nanocomposites is shown in Fig. 3. It can be clearly seen that a SiO2 coating is enwrapped on the SrFe12O19 surface forming a core–shell structure of SrFe12O19/ SiO2 nanocomposites. The SrFe12O19 is a kind of magnetic material that has more scattered electrons than that of SiO2 which is a kind of non-magnetic oxide, so the core region is dark, while the shell region is bright in SrFe12O19/SiO2 nanocomposites. The core of strontium ferrite is spherical or elliptical and the diameter of the core is in the range of 40–65 nm. The shell of SiO2 enwraps closely around the core and the thickness of the shell is about 9 nm. The infrared spectra of pure SiO2, SrFe12O19/SiO2 and pure SrFe12O19 are shown in Fig. 4(a), (b) and (c), respectively. In Fig. 4(a), these characteristic absorption peaks of the samples are according to those of the standard spectrum of SiO2X ˙ H2O [21]. These spectra confirm the presence of Si–O–Si bonds because of the absorption peaks at 470.1, 800.4 and 1103.4 cm− 1. They are ascribed to bond bending vibration, symmetric bond stretching vibration of Si–O–Si and asymmetric bond stretching vibration [22–24]. The presence of Si–OH is confirmed by the peak at 955.7 cm− 1, which is associated with its stretching mode vibration. The peak at 1649.1 cm− 1 is due to the deformation mode of absorbed molecular water and that around 3400.1 cm− 1 is due to O–H
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Table 1 Magnetic properties and the thickness of SiO2 shell of SrFe12O19 nanoparticles coated with silica Sample no.
Hc (Oe)
Ms (emu/g)
Thickness of shell (nm)
Mr/Ms
1 2 3 4
4946.6 4947.1 4945.9 4947.4
85.7 56.5 40.3 20.0
0 4 9 15
0.628 0.604 0.609 0.598
stretching of the same [25]. In Fig. 4(b), strong absorptions at 1094.4, 800.6 and 470.3 cm− 1 indicate the formation of a silica shell [26]. The band at 860.0 cm− 1 is assigned to Si–O–Fe. The presence of Si–O–Fe vibrations reflects some interaction between the highly isolated Fe3+ ions and the nearest silica shell. The Si–O–Fe bond is also evident by the presence of another band at 586.6 cm− 1, which is associated with the Fe– O stretching in Si–O–Fe bonds [27]. The results indicate that the product has a core–shell structure, which is combined through the chemical bond of Fe–O–Si. From Figs. 1, 2, 3 and 4, we can draw a conclusion that the SrFe12O19 nanoparticles were well-coated with SiO2. The magnetic properties of the pure SrFe12O19 and SrFe12O19/SiO2 core–shell structure nanoparticles were measured by VSM, as shown in Fig. 5. From VSM experiments, the magnetic parameters such as saturation magnetization (Ms), coercivity (Hc) and remnant magnetization (Mr) were given in Table 1. It is clearly seen that the value of saturation magnetization decreases with an increasing content of SiO2 shell and is lower than the pure SrFe12O19 in all cases, attributed mainly to the contribution of the volume of the non-magnetic coating layer to the total sample volume, while the coercivity of silicon dioxide-coated SrFe12O19 nanoparticles does not show any change after coating, because coercivity represents the property of a magnetic material and is determined by the strength and number of the magnetic dipole in magnetic domain and relations between adjacent magnetic domain. All in all, the coating of SiO2 doesn't change the magnetic property of SrFe12O19 particles. In addition, the non-magnetic coating layer can be considered as a magnetically dead layer at the surface, thus affecting the uniformity or magnitude of magnetization due to quenching of surface moments [28,29]. Note that, although the particles are nanoscale, they are not superparamagnetic as confirmed from the high Hc values.
4. Conclusions SrFe12O19/SiO2 core–shell nanoparticles have been prepared using a sequential synthetic method, in which core SrFe12O19 nanoparticles were prepared by a citrate precursor technique and the average particle size was 50 nm, shell silica was produced by the hydrolysis and condensation of tetraethyl orthosilicate
(TEOS). The analysis of XRD, TEM IR and EDS results indicates that the product has a core–shell structure which is combined through the chemical bonding of Fe–O–Si. The magnetic properties (the magnetization Ms) of SrFe12O19/SiO2 nanocomposites can be adjusted by the controlling the thickness of the SiO2 coating layer. The core–shell structure of SrFe12O19/ SiO2 can also been used as precursors to prepare hollow structure silica by complete removal of the core through chemical etching, or further grafted by other functional groups to prepare novel multifunctional nanomaterials. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29]
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