- Email: [email protected]
magnetite nanocomposite particles
Materials Letters 112 (2013) 153–157
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Fabrication of spherical silica aerogel/magnetite nanocomposite particles Jun Sung Lee a, Sun Ki Hong a,b, Nam Jung Hur c, Won-Seon Seo b, Hae Jin Hwang a,n a
School of Materials Science and Engineering, Inha University, Incheon 402-751, Republic of Korea Korea Institute of Ceramic Engineering and Technology, Seoul, Republic of Korea c Department of Physics, Inha University, Incheon 402-751, Republic of Korea b
art ic l e i nf o
a b s t r a c t
Article history: Received 11 March 2013 Accepted 8 September 2013 Available online 14 September 2013
Fe3O4/SiO2 nanocomposite particles were synthesized from a hydrophobic Fe3O4 suspension, prepared by precipitating Fe (II) and Fe (III) chlorides and using a hydrophobic SiO2 wet gel from a silicic acid solution derived from a sodium silicate (water glass) solution. Fe3O4 nanoparticles having a crystallite size of 10 nm embedded in SiO2 aerogel can be successfully fabricated by mixing the Fe3O4 suspension and silica wet gel in n-hexane. It was clearly observed that the Fe3O4 nanoparticles were well crystallized and homogeneously dispersed in the SiO2 aerogel. The Fe3O4/SiO2 nanocomposite particles exhibited superparamagentic behavior. The saturation magnetization was determined to 34.5, 18.2, and 8.3 emu/ g for 90 wt% Fe3O4, 70 wt% Fe3O4, and 50 wt% Fe3O4/SiO2 nanocomposite particles, respectively. & 2013 Elsevier B.V. All rights reserved.
Keywords: Fe3O4 Silica aerogel Nanocomposites Superparamagnetism
1. Introduction Because magnetite (Fe3O4) nanoparticles show superparamagnetism with high saturation magnetization, biocompatibility, and chemical stability, they have attracted much attention in biomedicine and bioengineering applications such as in drug delivery systems, magnetic resonance imaging contrast agents, and cancer therapies [1,2,3]. However, the anisotropic dipolar attraction of Fe3O4 nanoparticles often results in large aggregates and a loss of their superparamagentic characteristics [4]. Thus, Fe3O4 nanoparticles are dispersed within a nonmagnetic matrix in order to avoid agglomeration and/or to protect the magnetic nanoparticles against corrosion or oxidation [5]. In some cases, the nonmagnetic matrix can provide various surface sites for attaching a dedicated functional group for a specific reaction [6]. Silica (SiO2) is the most suitable material for the matrix because of its nontoxicity, inertness to magnetic fields, and ease to form cross-linked network structures [7,8]. Several methods are used to fabricate silica-coated Fe3O4 nanoparticles. One is the sol–gel technique, in which silica is coated on colloidal Fe3O4 nanoparticles in an alkoxide/alcohol/ water mixture via hydrolysis and condensation [9,10]. Another one is based on the formation of Fe3O4 nanoparticles inside the pores of silica nanoparticles using iron salts such as iron chlorides or nitrates [11]. Emulsion polymerization was recently proposed, in which aqueous droplets containing magnetic nanoparticles
n
Corresponding author. Tel.: þ 82 32 860 7521; fax: þ 82 32 862 4482. E-mail address: [email protected] (H. Jin Hwang).
0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.09.038
were dispersed in an organic solvent by using a surfactant; next, tetraethylorthosilicate (TEOS) is added to the microemulsion to form a silica coating around the magnetic nanoparticles [12]. In this study, nanocomposite of Fe3O4 nanoparticles embedded in a spherical silica (SiO2) aerogel particle were synthesized via a novel synthesis route that comprises incorporation of hydrophobic Fe3O4 nanoparticles in a spherical silica wet gel, which is modified by hydrophobic surface methyl groups and subsequent drying at ambient pressure. It is considered that emulsion polymerization is the best way to synthesize mono-dispersed spherical silica aerogel particles. The silica aerogel particles can serves as a magnetite nano particle support for and also as surface sites for various functional groups. The morphology, microstructure, and magnetic properties of the nanocomposite were investigated in terms of the content of Fe3O4 nanoparticles.
2. Experimental procedure The Fe3O4 nanoparticles were synthesized by coprecipitation of Fe2 þ and Fe3 þ by adding ammonia. Iron (III) chloride hexahydrate (FeCl3 6H2O) and iron (II) chloride tetrahydrate (FeCl2 4H2O) with a molar ratio of 1:2 were dissolved in water. The solution was stirred at 90 1C for 1 h before adding an aqueous ammonia solution (29%). Black Fe3O4 nanoparticles precipitated immediately after ammonia was added to the mixture. The solution was further stirred at 90 1C for 40 min. The obtained Fe3O4 nanoparticles were repeatedly washed with water and ethanol and then redispersed with oleic acid in n-hexane.
154
J. Sung Lee et al. / Materials Letters 112 (2013) 153–157
A sodium silicate solution (water glass) was used as a precursor to prepare silicic acid because it is cheap and easy to handle compared to alkoxide-based precursors such as tetraethyl orthosilicate (TEOS) and tetramethyl orthosilicate (TMOS). The water glass solution (Young Il Chemical, South Korea) was diluted with distilled water to make an 8 wt% silicate solution. The solution was then passed through a column filled with an ion-exchange resin (Amberite, IR120H, H. Rohm & Haas Co., PA, USA). The sodium concentration in the silicic acid solution was estimated to be in the range between 40 and 90 ppm. Silicic acid droplets were produced by mixing the obtained silicic acid solution and a surfactant (span 80) with n-hexane using a homogenizer (T25 D, IKA, Germany). The stirring speeds were 3200, 4000, 6000, 9000, and 14,000 rpm. Subsequently, a base catalyst (NH4OH) was used to adjust the pH of the silicic acid to 6.0. The silicic acid droplets were gelated at room temperature. The obtained spherical hydrogels were surface-modified in a 10% (trimethylchlorosilane (TMCS), Si(CH3)3Cl, 98% ACROS)/n-hexane solution for 10 h at room temperature. The surface-modification process was repeated seven times. Finally, the surface-modified wet gels were washed with n-hexane to remove residual agents and reaction products such as hydrochloric acid (HCl). Finally, the spherical wet gels were mixed with n-hexane containing Fe3O4 nanoparticles for 4 h and then dried at 80 1C in an electric oven at ambient pressure. X-ray diffraction (XRD) patterns were taken using a diffractometer (DMAX-2500, Rigaku, Japan) with Ni-filtered Cu-Kα radiation to perform a crystallographic study on the Fe3O4 and Fe3O4/ SiO2 composite nanoparticles. The crystallite size of the synthesized particles was estimated using the Scherrer equation [13]: D ¼ 0:9λ=β cos θ
ð1Þ
Here λ, θ, and β are the X-ray wavelength (0.15418 nm for Cu-Kα), Bragg diffraction angle, and full width at half maximum (FWHM) of the diffraction peak, respectively. The morphology and microstructure of the Fe3O4 and Fe3O4/SiO2 composite nanoparticles were examined by scanning (SEM, S-4300 Hitachi, Japan) and transmission (TEM, JEM2100F, JEOL, Japan) electron microscopy. A vibrating sample magnetometer (VSM, Physical Property Measurement System (PPMS), Quantum Design, USA) was used at room temperature to measure the magnetization of the Fe3O4 and Fe3O4/SiO2 composite nanoparticles.
3. Results and discussion Fig. 1 shows typical XRD patterns of Fe3O4 and Fe3O4/SiO2 composite nanoparticles. The obtained particle was found to be Fe3O4 with cubic inverse spinel structure (XRD pattern (a)). The peaks correspond to the (111), (220), (311), (400), (422), (511), (440), and (533) reflection of the Fe3O4 crystal structure. The presence of the broad diffraction peak at 2θ¼231 in the Fe3O4/ SiO2 nanocomposite particles indicates that SiO2 has a typical amorphous structure that has been established in various silica aerogels [14]. For the nanocomposite samples, the intensity of the peaks corresponding to Fe3O4 increased with increasing Fe3O4 content. The crystallite sizes of Fe3O4 and nanocomposite samples were estimated using the Scherrer equation and the results are listed in Table 1. The crystallite size of Fe3O4 and 90 wt% Fe3O4/ SiO2 nanocomposite sample were 9.7 and 10.0 nm, respectively. This result suggests that the Fe3O4 nanoparticles are well dispersed within the pore structure of SiO2 aerogel particles. On the other hand, the crystallite size gradually decreased as the Fe3O4 content was decreased. Fig. 2 shows SEM images and energy dispersive X-ray spectroscopy (EDS) analysis result for the SiO2 aerogel and Fe3O4/SiO2 nanocomposite particles. The size of the spherical SiO2 aerogel particle was estimated to be 150 mm. The SEM images (Fig. 2 (a)) show the typical mesoporous microstructure that can be observed in a silica aerogel. The pore diameter of the silica aerogel determined by Brunauer–Emmett–Teller (BET) isotherm was 14 nm [15], which is larger than the crystallite size of the Fe3O4 nanoparticles, as specified in Table 1. The EDS analysis confirmed that the particles inside the pores consist of Fe3O4, while the matrix particles were composed of SiO2. Figs. 1 and 2 clearly indicate that the Fe3O4 nanoparticles are homogeneously dispersed in the pore structure of the SiO2 aerogel particles. Fig. 3(a)–(d) shows TEM images of Fe3O4 and Fe3O4/SiO2 nanocomposite particles at low magnification. In Fig. 3(b)–(d), the dark areas correspond to Fe3O4 particles while the gray matrix originates from the SiO2 aerogel. It can be observed that Fe3O4 nanoparticles are quasispherical with an average diameter of 10 nm. In the case of Fe3O4/SiO2 nanocomposite samples, the Fe3O4 nanoparticles were homogeneously dispersed in the silica aerogel structure. In addition, it seems that the Fe3O4 nanoparticles were embedded in the mesopores of the SiO2 aerogel as Fig. 3(b) and (c) shows. The highly magnified TEM image (Fig. 3(e)) and the selected area electron diffraction (SAED) pattern (Fig. 3(f)), aligned with the electron beam parallel to the 〈110〉 axis, reveal that the Fe3O4 nanoparticle in the SiO2 aerogel has a single crystalline structure. As can be seen in Fig. 3 (e), the distance between two adjacent planes was measured to be 0.484 and 0.295 nm, corresponding to the (111) and (220) planes of the cubic Fe3O4 structure, respectively. In addition, it was determined that spots 1, 2, 3, 4, and 5 in Fig. 3(f) corresponds to (220), (311), (400), (511), and (440) plane, respectively, and the obtained result is good agreement with previous XRD measurements (Fig. 1). Fig. 4 shows the room-temperature magnetization curves of the Fe3O4 and Fe3O4/SiO2 nanocomposite particles. Similar to pure
Table 1 The crystallite size of Fe3O4 and Fe3O4/SiO2 nanocomposite particles.
Fig. 1. XRD patterns of Fe3O4 (a), 50 wt% Fe3O4/SiO2 (b), 70 wt% Fe3O4/SiO2 (c), and 90 wt% Fe3O4/SiO2 (d) nanocomposite particles.
Sample
Crystallite size (nm)
Hydrophilic Fe3O4 Hydrophobic Fe3O4 90Fe3O4/SiO2 70Fe3O4/SiO2 50Fe3O4/SiO2
11.2 9.7 10 9.7 9.3
J. Sung Lee et al. / Materials Letters 112 (2013) 153–157
155
Fig. 2. SEM images of SiO2 aerogel (a) and SEM images and EDS analysis of a 90 wt% Fe3O4/SiO2 nanocomposite particle (b).
Fe3O4 particles, the Fe3O4/SiO2 nanocomposites exhibited superparamagentic behavior. On decreasing the magnetic field intensity, the magnetization decreased from the saturation value to zero. There was no hysteresis loop or remnant magnetization for all examined samples. This means that all these samples form a single magnetic domain. In addition, it appears that the SiO2 matrix has almost no effect on the magnetic behavior; for example, the coercive force of the Fe3O4 nanoparticles dispersed in the SiO2 aerogel matrix remained unchanged. The saturation magnetization (Ms) values at 20000 Oe and calculated Ms values are summarized in Table 2. The Ms were calculated from the Ms of hydrophobic Fe3O4 and silica diamagnetic susceptibility, 0.277 10 6 emu/g [16]. The Ms values were 50.2 emu/g for the hydrophobic Fe3O4 nanoparticles. These values are similar to those of the Fe3O4 nanoparticles, as reported previously [4,17]. As can be seen in Fig. 4, the higher Ms of hydrophilic Fe3O4 might be associated with the size effect of Fe3O4 nanoparticles [15,18,19]. In general, the Ms strongly depends on the size and crystallinity of the Fe3O4 particles and gradually increases as the particle size increases. On the other hand, the Ms values of the Fe3O4/SiO2 nanocomposite particles decreased on increasing the silica content, which results from the decrease in the mass proportion of Fe3O4. The Ms
values were 34.5, 18.2, 8.3 emu/g for 90 wt% Fe3O4, 70 wt% Fe3O4, and 50 wt% Fe3O4/SiO2 nanocomposite particles, respectively. An interesting feature observed in Fig. 4 is that the experimentally determined Ms values of the nanocomposite particles are much lower than those calculated on the basis of the saturation magnetization of hydrophobic Fe3O4 nanoparticles and their weight fraction in the composites. In addition, it appears that this tendency is more pronounced in the nanocomposites having a high weight fraction of the SiO2 aerogel. Xu et al. pointed out that nonmagnetic materials such as silica influence the magnitude of the saturation magnetization of Fe3O4 because of the quenching of surface moments [20]. Because the Fe3O4 nanoparticles are incorporated in the pore structure of the SiO2 aerogel, decrease in the saturation magnetization of the nanocomposite particles observed in the present study can be explained by a similar mechanism.
4. Conclusions Spherical Fe3O4/SiO2 nanocomposite particles were synthesized from a Fe3O4 suspension dispersed by oleic acid and spherical hydrophobic SiO2 wet gels in n-hexane, derived from a sodium silicate solution. Fe3O4 nanoparticles with size 10 nm were
156
J. Sung Lee et al. / Materials Letters 112 (2013) 153–157
f
Fig. 3. TEM images of Fe3O4 and Fe3O4/SiO2 nanocomposite particles; (a) Fe3O4, (b) 90 wt% Fe3O4/SiO2, (c) 90 wt% Fe3O4/SiO2, (d) 50 wt% Fe3O4/SiO2, (e) high-resolution TEM image selected from (b), and (f) selected diffraction pattern image of (b).
60
embedded in the pores of SiO2 aerogel particles. It was observed that the particle size of Fe3O4 slightly decreased as the SiO2 content increased. From XRD analysis and TEM observations, the single crystalline nature of the Fe3O4 nanoparticles and the amorphous structure of the SiO2 aerogel matrix were established. The Fe3O4/ SiO2 nanocomposite particles showed superparamagentic behavior and the saturation magnetization values were determined to 34.5, 18.2, and 8.3 emu/g for 90 wt% Fe3O4, 70 wt% Fe3O4, and 50 wt% Fe3O4/SiO2 nanocomposite particles, respectively.
Hydrophilic Fe3O4 Hydrophobic Fe3O4 90Fe3O4/SiO2
40
70Fe3O4/SiO2 50Fe3O4/SiO2
M (emu/g)
20
0
-20
Acknowledgment
40
This study was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (No. 2012-0005578).
60 -20000
-10000
0
10000
20000
H(Oe) Fig. 4. Magnetization of Fe3O4 and Fe3O4/SiO2 nanocomposite particles.
Table 2 The observed and calculated saturation magnetization (Ms) values of Fe3O4 and Fe3O4/SiO2 nanocomposite particles. Sample
Observed Ms
Calculated Ms
Hydrophobic Fe3O4 90Fe3O4/SiO2 70Fe3O4/SiO2 50Fe3O4/SiO2
50.2 34.5 18.2 8.3
50.2 45.2 40.2 35.1
References [1] Arruebo M, Galan M, Navascues N, Tellez C, Marquina C, Ibarra MR, et al. Chemistry of Materials 2006;18:1911–9. [2] Berry CC, Curtis ASG. Journal of Physics D: Applied Physics 2003;36:R198–206. [3] Xia JH, Shen H, Shu BF, Zhang W. Materials Research Bulletin 2008;43:2213–9. [4] Liu B, Wang D, Huang W, Yao A, Kamitakahara M, Ioku K. Journal of the Ceramic Society of Japan 2007;115:877–81. [5] Ruuge EK, Rusetski AN. Journal of Magnetism and Magnetic Materials 1993;122:335–9. [6] Deng YH, Wang CC, Hu JH, Yang WL, Fu SK. Colloids and Surfaces A: Physicochemical and Engineering Aspects 2005;262:87–93. [7] Pope NM, Alsop RC, Chang YA, Smith AK. Journal of Biomedical Materials Research 1994;2:449–57. [8] Sohn BH, Cohen RE, Papaefthymiou GC. Journal of Magnetism and Magnetic Materials 1998;182:216–24. [9] Kim YJ, Pee JH, Chang JH, Choi K, Kim KJ, Jung DY. Chemistry Letters 2009;38:842–3.
J. Sung Lee et al. / Materials Letters 112 (2013) 153–157
[10] He YP, Wang SQ, Li CR, Miao YM, Wu ZY, Zou BS. Journal of Physics D: Applied Physics 2005;38:1342–50. [11] Chaneac C, Tronc E, Jolivet J. Nanostructured Materials 1995;6:715. [12] Vestal CR, Zhang J. Nano Letters 2003;3:1739–43. [13] B.D. Cullity. Elements of X-ray Diffractions. Reading, MA: Addition-Wesley; 102. [14] Folgar C, Folz D, Suchicital C, Clark D. Journal of Non-Crystalline Solids 2007;353:1483–90. [15] Hong SK, Yoon MY, Hwang HJ. Journal of Ceramic Processing Research 2012;13:s145–8.
157
[16] Smirnov VM, Bobrysheva NP, Osmolowsky MG, Semenov VG, Murin IV. Surface Review and Letters 2001;8:295–302. [17] Shi R, Liu X, Gao G, Yi R, Qiu G. Journal of Alloys and Compounds 2009;485:548–53. [18] Lu HM, Zheng WT, Jiang Q. Journal of Physics D: Applied Physics 2007;40:320–5. [19] Li Z, Sun Q, Gao M. Angewandte Chemie International Edition 2005;44:123–6. [20] Xu H, Tong N, Cui L, Lu Y, Gu H. Journal of Magnetism and Magnetic Materials 2007;311:125–30.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Fabrication of spherical silica aerogel/magnetite nanocomposite particles Jun Sung Lee a, Sun Ki Hong a,b, Nam Jung Hur c, Won-Seon Seo b, Hae Jin Hwang a,n a
School of Materials Science and Engineering, Inha University, Incheon 402-751, Republic of Korea Korea Institute of Ceramic Engineering and Technology, Seoul, Republic of Korea c Department of Physics, Inha University, Incheon 402-751, Republic of Korea b
art ic l e i nf o
a b s t r a c t
Article history: Received 11 March 2013 Accepted 8 September 2013 Available online 14 September 2013
Fe3O4/SiO2 nanocomposite particles were synthesized from a hydrophobic Fe3O4 suspension, prepared by precipitating Fe (II) and Fe (III) chlorides and using a hydrophobic SiO2 wet gel from a silicic acid solution derived from a sodium silicate (water glass) solution. Fe3O4 nanoparticles having a crystallite size of 10 nm embedded in SiO2 aerogel can be successfully fabricated by mixing the Fe3O4 suspension and silica wet gel in n-hexane. It was clearly observed that the Fe3O4 nanoparticles were well crystallized and homogeneously dispersed in the SiO2 aerogel. The Fe3O4/SiO2 nanocomposite particles exhibited superparamagentic behavior. The saturation magnetization was determined to 34.5, 18.2, and 8.3 emu/ g for 90 wt% Fe3O4, 70 wt% Fe3O4, and 50 wt% Fe3O4/SiO2 nanocomposite particles, respectively. & 2013 Elsevier B.V. All rights reserved.
Keywords: Fe3O4 Silica aerogel Nanocomposites Superparamagnetism
1. Introduction Because magnetite (Fe3O4) nanoparticles show superparamagnetism with high saturation magnetization, biocompatibility, and chemical stability, they have attracted much attention in biomedicine and bioengineering applications such as in drug delivery systems, magnetic resonance imaging contrast agents, and cancer therapies [1,2,3]. However, the anisotropic dipolar attraction of Fe3O4 nanoparticles often results in large aggregates and a loss of their superparamagentic characteristics [4]. Thus, Fe3O4 nanoparticles are dispersed within a nonmagnetic matrix in order to avoid agglomeration and/or to protect the magnetic nanoparticles against corrosion or oxidation [5]. In some cases, the nonmagnetic matrix can provide various surface sites for attaching a dedicated functional group for a specific reaction [6]. Silica (SiO2) is the most suitable material for the matrix because of its nontoxicity, inertness to magnetic fields, and ease to form cross-linked network structures [7,8]. Several methods are used to fabricate silica-coated Fe3O4 nanoparticles. One is the sol–gel technique, in which silica is coated on colloidal Fe3O4 nanoparticles in an alkoxide/alcohol/ water mixture via hydrolysis and condensation [9,10]. Another one is based on the formation of Fe3O4 nanoparticles inside the pores of silica nanoparticles using iron salts such as iron chlorides or nitrates [11]. Emulsion polymerization was recently proposed, in which aqueous droplets containing magnetic nanoparticles
n
Corresponding author. Tel.: þ 82 32 860 7521; fax: þ 82 32 862 4482. E-mail address: [email protected] (H. Jin Hwang).
0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.09.038
were dispersed in an organic solvent by using a surfactant; next, tetraethylorthosilicate (TEOS) is added to the microemulsion to form a silica coating around the magnetic nanoparticles [12]. In this study, nanocomposite of Fe3O4 nanoparticles embedded in a spherical silica (SiO2) aerogel particle were synthesized via a novel synthesis route that comprises incorporation of hydrophobic Fe3O4 nanoparticles in a spherical silica wet gel, which is modified by hydrophobic surface methyl groups and subsequent drying at ambient pressure. It is considered that emulsion polymerization is the best way to synthesize mono-dispersed spherical silica aerogel particles. The silica aerogel particles can serves as a magnetite nano particle support for and also as surface sites for various functional groups. The morphology, microstructure, and magnetic properties of the nanocomposite were investigated in terms of the content of Fe3O4 nanoparticles.
2. Experimental procedure The Fe3O4 nanoparticles were synthesized by coprecipitation of Fe2 þ and Fe3 þ by adding ammonia. Iron (III) chloride hexahydrate (FeCl3 6H2O) and iron (II) chloride tetrahydrate (FeCl2 4H2O) with a molar ratio of 1:2 were dissolved in water. The solution was stirred at 90 1C for 1 h before adding an aqueous ammonia solution (29%). Black Fe3O4 nanoparticles precipitated immediately after ammonia was added to the mixture. The solution was further stirred at 90 1C for 40 min. The obtained Fe3O4 nanoparticles were repeatedly washed with water and ethanol and then redispersed with oleic acid in n-hexane.
154
J. Sung Lee et al. / Materials Letters 112 (2013) 153–157
A sodium silicate solution (water glass) was used as a precursor to prepare silicic acid because it is cheap and easy to handle compared to alkoxide-based precursors such as tetraethyl orthosilicate (TEOS) and tetramethyl orthosilicate (TMOS). The water glass solution (Young Il Chemical, South Korea) was diluted with distilled water to make an 8 wt% silicate solution. The solution was then passed through a column filled with an ion-exchange resin (Amberite, IR120H, H. Rohm & Haas Co., PA, USA). The sodium concentration in the silicic acid solution was estimated to be in the range between 40 and 90 ppm. Silicic acid droplets were produced by mixing the obtained silicic acid solution and a surfactant (span 80) with n-hexane using a homogenizer (T25 D, IKA, Germany). The stirring speeds were 3200, 4000, 6000, 9000, and 14,000 rpm. Subsequently, a base catalyst (NH4OH) was used to adjust the pH of the silicic acid to 6.0. The silicic acid droplets were gelated at room temperature. The obtained spherical hydrogels were surface-modified in a 10% (trimethylchlorosilane (TMCS), Si(CH3)3Cl, 98% ACROS)/n-hexane solution for 10 h at room temperature. The surface-modification process was repeated seven times. Finally, the surface-modified wet gels were washed with n-hexane to remove residual agents and reaction products such as hydrochloric acid (HCl). Finally, the spherical wet gels were mixed with n-hexane containing Fe3O4 nanoparticles for 4 h and then dried at 80 1C in an electric oven at ambient pressure. X-ray diffraction (XRD) patterns were taken using a diffractometer (DMAX-2500, Rigaku, Japan) with Ni-filtered Cu-Kα radiation to perform a crystallographic study on the Fe3O4 and Fe3O4/ SiO2 composite nanoparticles. The crystallite size of the synthesized particles was estimated using the Scherrer equation [13]: D ¼ 0:9λ=β cos θ
ð1Þ
Here λ, θ, and β are the X-ray wavelength (0.15418 nm for Cu-Kα), Bragg diffraction angle, and full width at half maximum (FWHM) of the diffraction peak, respectively. The morphology and microstructure of the Fe3O4 and Fe3O4/SiO2 composite nanoparticles were examined by scanning (SEM, S-4300 Hitachi, Japan) and transmission (TEM, JEM2100F, JEOL, Japan) electron microscopy. A vibrating sample magnetometer (VSM, Physical Property Measurement System (PPMS), Quantum Design, USA) was used at room temperature to measure the magnetization of the Fe3O4 and Fe3O4/SiO2 composite nanoparticles.
3. Results and discussion Fig. 1 shows typical XRD patterns of Fe3O4 and Fe3O4/SiO2 composite nanoparticles. The obtained particle was found to be Fe3O4 with cubic inverse spinel structure (XRD pattern (a)). The peaks correspond to the (111), (220), (311), (400), (422), (511), (440), and (533) reflection of the Fe3O4 crystal structure. The presence of the broad diffraction peak at 2θ¼231 in the Fe3O4/ SiO2 nanocomposite particles indicates that SiO2 has a typical amorphous structure that has been established in various silica aerogels [14]. For the nanocomposite samples, the intensity of the peaks corresponding to Fe3O4 increased with increasing Fe3O4 content. The crystallite sizes of Fe3O4 and nanocomposite samples were estimated using the Scherrer equation and the results are listed in Table 1. The crystallite size of Fe3O4 and 90 wt% Fe3O4/ SiO2 nanocomposite sample were 9.7 and 10.0 nm, respectively. This result suggests that the Fe3O4 nanoparticles are well dispersed within the pore structure of SiO2 aerogel particles. On the other hand, the crystallite size gradually decreased as the Fe3O4 content was decreased. Fig. 2 shows SEM images and energy dispersive X-ray spectroscopy (EDS) analysis result for the SiO2 aerogel and Fe3O4/SiO2 nanocomposite particles. The size of the spherical SiO2 aerogel particle was estimated to be 150 mm. The SEM images (Fig. 2 (a)) show the typical mesoporous microstructure that can be observed in a silica aerogel. The pore diameter of the silica aerogel determined by Brunauer–Emmett–Teller (BET) isotherm was 14 nm [15], which is larger than the crystallite size of the Fe3O4 nanoparticles, as specified in Table 1. The EDS analysis confirmed that the particles inside the pores consist of Fe3O4, while the matrix particles were composed of SiO2. Figs. 1 and 2 clearly indicate that the Fe3O4 nanoparticles are homogeneously dispersed in the pore structure of the SiO2 aerogel particles. Fig. 3(a)–(d) shows TEM images of Fe3O4 and Fe3O4/SiO2 nanocomposite particles at low magnification. In Fig. 3(b)–(d), the dark areas correspond to Fe3O4 particles while the gray matrix originates from the SiO2 aerogel. It can be observed that Fe3O4 nanoparticles are quasispherical with an average diameter of 10 nm. In the case of Fe3O4/SiO2 nanocomposite samples, the Fe3O4 nanoparticles were homogeneously dispersed in the silica aerogel structure. In addition, it seems that the Fe3O4 nanoparticles were embedded in the mesopores of the SiO2 aerogel as Fig. 3(b) and (c) shows. The highly magnified TEM image (Fig. 3(e)) and the selected area electron diffraction (SAED) pattern (Fig. 3(f)), aligned with the electron beam parallel to the 〈110〉 axis, reveal that the Fe3O4 nanoparticle in the SiO2 aerogel has a single crystalline structure. As can be seen in Fig. 3 (e), the distance between two adjacent planes was measured to be 0.484 and 0.295 nm, corresponding to the (111) and (220) planes of the cubic Fe3O4 structure, respectively. In addition, it was determined that spots 1, 2, 3, 4, and 5 in Fig. 3(f) corresponds to (220), (311), (400), (511), and (440) plane, respectively, and the obtained result is good agreement with previous XRD measurements (Fig. 1). Fig. 4 shows the room-temperature magnetization curves of the Fe3O4 and Fe3O4/SiO2 nanocomposite particles. Similar to pure
Table 1 The crystallite size of Fe3O4 and Fe3O4/SiO2 nanocomposite particles.
Fig. 1. XRD patterns of Fe3O4 (a), 50 wt% Fe3O4/SiO2 (b), 70 wt% Fe3O4/SiO2 (c), and 90 wt% Fe3O4/SiO2 (d) nanocomposite particles.
Sample
Crystallite size (nm)
Hydrophilic Fe3O4 Hydrophobic Fe3O4 90Fe3O4/SiO2 70Fe3O4/SiO2 50Fe3O4/SiO2
11.2 9.7 10 9.7 9.3
J. Sung Lee et al. / Materials Letters 112 (2013) 153–157
155
Fig. 2. SEM images of SiO2 aerogel (a) and SEM images and EDS analysis of a 90 wt% Fe3O4/SiO2 nanocomposite particle (b).
Fe3O4 particles, the Fe3O4/SiO2 nanocomposites exhibited superparamagentic behavior. On decreasing the magnetic field intensity, the magnetization decreased from the saturation value to zero. There was no hysteresis loop or remnant magnetization for all examined samples. This means that all these samples form a single magnetic domain. In addition, it appears that the SiO2 matrix has almost no effect on the magnetic behavior; for example, the coercive force of the Fe3O4 nanoparticles dispersed in the SiO2 aerogel matrix remained unchanged. The saturation magnetization (Ms) values at 20000 Oe and calculated Ms values are summarized in Table 2. The Ms were calculated from the Ms of hydrophobic Fe3O4 and silica diamagnetic susceptibility, 0.277 10 6 emu/g [16]. The Ms values were 50.2 emu/g for the hydrophobic Fe3O4 nanoparticles. These values are similar to those of the Fe3O4 nanoparticles, as reported previously [4,17]. As can be seen in Fig. 4, the higher Ms of hydrophilic Fe3O4 might be associated with the size effect of Fe3O4 nanoparticles [15,18,19]. In general, the Ms strongly depends on the size and crystallinity of the Fe3O4 particles and gradually increases as the particle size increases. On the other hand, the Ms values of the Fe3O4/SiO2 nanocomposite particles decreased on increasing the silica content, which results from the decrease in the mass proportion of Fe3O4. The Ms
values were 34.5, 18.2, 8.3 emu/g for 90 wt% Fe3O4, 70 wt% Fe3O4, and 50 wt% Fe3O4/SiO2 nanocomposite particles, respectively. An interesting feature observed in Fig. 4 is that the experimentally determined Ms values of the nanocomposite particles are much lower than those calculated on the basis of the saturation magnetization of hydrophobic Fe3O4 nanoparticles and their weight fraction in the composites. In addition, it appears that this tendency is more pronounced in the nanocomposites having a high weight fraction of the SiO2 aerogel. Xu et al. pointed out that nonmagnetic materials such as silica influence the magnitude of the saturation magnetization of Fe3O4 because of the quenching of surface moments [20]. Because the Fe3O4 nanoparticles are incorporated in the pore structure of the SiO2 aerogel, decrease in the saturation magnetization of the nanocomposite particles observed in the present study can be explained by a similar mechanism.
4. Conclusions Spherical Fe3O4/SiO2 nanocomposite particles were synthesized from a Fe3O4 suspension dispersed by oleic acid and spherical hydrophobic SiO2 wet gels in n-hexane, derived from a sodium silicate solution. Fe3O4 nanoparticles with size 10 nm were
156
J. Sung Lee et al. / Materials Letters 112 (2013) 153–157
f
Fig. 3. TEM images of Fe3O4 and Fe3O4/SiO2 nanocomposite particles; (a) Fe3O4, (b) 90 wt% Fe3O4/SiO2, (c) 90 wt% Fe3O4/SiO2, (d) 50 wt% Fe3O4/SiO2, (e) high-resolution TEM image selected from (b), and (f) selected diffraction pattern image of (b).
60
embedded in the pores of SiO2 aerogel particles. It was observed that the particle size of Fe3O4 slightly decreased as the SiO2 content increased. From XRD analysis and TEM observations, the single crystalline nature of the Fe3O4 nanoparticles and the amorphous structure of the SiO2 aerogel matrix were established. The Fe3O4/ SiO2 nanocomposite particles showed superparamagentic behavior and the saturation magnetization values were determined to 34.5, 18.2, and 8.3 emu/g for 90 wt% Fe3O4, 70 wt% Fe3O4, and 50 wt% Fe3O4/SiO2 nanocomposite particles, respectively.
Hydrophilic Fe3O4 Hydrophobic Fe3O4 90Fe3O4/SiO2
40
70Fe3O4/SiO2 50Fe3O4/SiO2
M (emu/g)
20
0
-20
Acknowledgment
40
This study was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (No. 2012-0005578).
60 -20000
-10000
0
10000
20000
H(Oe) Fig. 4. Magnetization of Fe3O4 and Fe3O4/SiO2 nanocomposite particles.
Table 2 The observed and calculated saturation magnetization (Ms) values of Fe3O4 and Fe3O4/SiO2 nanocomposite particles. Sample
Observed Ms
Calculated Ms
Hydrophobic Fe3O4 90Fe3O4/SiO2 70Fe3O4/SiO2 50Fe3O4/SiO2
50.2 34.5 18.2 8.3
50.2 45.2 40.2 35.1
References [1] Arruebo M, Galan M, Navascues N, Tellez C, Marquina C, Ibarra MR, et al. Chemistry of Materials 2006;18:1911–9. [2] Berry CC, Curtis ASG. Journal of Physics D: Applied Physics 2003;36:R198–206. [3] Xia JH, Shen H, Shu BF, Zhang W. Materials Research Bulletin 2008;43:2213–9. [4] Liu B, Wang D, Huang W, Yao A, Kamitakahara M, Ioku K. Journal of the Ceramic Society of Japan 2007;115:877–81. [5] Ruuge EK, Rusetski AN. Journal of Magnetism and Magnetic Materials 1993;122:335–9. [6] Deng YH, Wang CC, Hu JH, Yang WL, Fu SK. Colloids and Surfaces A: Physicochemical and Engineering Aspects 2005;262:87–93. [7] Pope NM, Alsop RC, Chang YA, Smith AK. Journal of Biomedical Materials Research 1994;2:449–57. [8] Sohn BH, Cohen RE, Papaefthymiou GC. Journal of Magnetism and Magnetic Materials 1998;182:216–24. [9] Kim YJ, Pee JH, Chang JH, Choi K, Kim KJ, Jung DY. Chemistry Letters 2009;38:842–3.
J. Sung Lee et al. / Materials Letters 112 (2013) 153–157
[10] He YP, Wang SQ, Li CR, Miao YM, Wu ZY, Zou BS. Journal of Physics D: Applied Physics 2005;38:1342–50. [11] Chaneac C, Tronc E, Jolivet J. Nanostructured Materials 1995;6:715. [12] Vestal CR, Zhang J. Nano Letters 2003;3:1739–43. [13] B.D. Cullity. Elements of X-ray Diffractions. Reading, MA: Addition-Wesley; 102. [14] Folgar C, Folz D, Suchicital C, Clark D. Journal of Non-Crystalline Solids 2007;353:1483–90. [15] Hong SK, Yoon MY, Hwang HJ. Journal of Ceramic Processing Research 2012;13:s145–8.
157
[16] Smirnov VM, Bobrysheva NP, Osmolowsky MG, Semenov VG, Murin IV. Surface Review and Letters 2001;8:295–302. [17] Shi R, Liu X, Gao G, Yi R, Qiu G. Journal of Alloys and Compounds 2009;485:548–53. [18] Lu HM, Zheng WT, Jiang Q. Journal of Physics D: Applied Physics 2007;40:320–5. [19] Li Z, Sun Q, Gao M. Angewandte Chemie International Edition 2005;44:123–6. [20] Xu H, Tong N, Cui L, Lu Y, Gu H. Journal of Magnetism and Magnetic Materials 2007;311:125–30.