Preparation of low-density superparamagnetic microspheres by coating glass microballoons with magnetite nanoparticles

Materials Science and Engineering B 135 (2006) 38–43

Preparation of low-density superparamagnetic microspheres by coating glass microballoons with magnetite nanoparticles Xiang Li a , Haibin Yang a,b,∗ , Wuyou Fu b , Cunxi Wu a , Shikai Liu b , Hongyang Zhu b , Xiaofen Pang b a

Institute of Materials Science and Engineering, Henan Polytechnic University, Jiaozuo 454003, PR China b National Laboratory of Superhard Materials, Jilin University, Changchun 130012, PR China Received 26 June 2006; accepted 14 August 2006

Abstract A novel low-density (0.4–0.5 g/cm3 ) hollow composite material with the superparamagnetic character has been synthesized by coating glass microballoons (GMBs) with magnetite (Fe3 O4 ) nanoparticles via chemical deposition process. The products were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), energy dispersion X-ray spectroscopy (EDS) and vibrating sample magnetometer (VSM). A uniform and continuous Fe3 O4 -coating with thickness ca. 400–500 nm was obtained under the given conditions. The size of Fe3 O4 particles of the coating was below 25 nm. The magnetization of the Fe3 O4 -coated GMBs can be controlled by changing the GMB content and the reaction temperature. These low-density magnetic microspheres are expected to have many advantages in applications such as microwave absorbing materials and catalysts. © 2006 Elsevier B.V. All rights reserved. Keywords: Fe3 O4 nanoparticles; Bulk density; Superparamagnetism; Reduction–precipitation; Surfaces

1. Introduction Magnetite (Fe3 O4 ) is an important member of ferrite that has a cubic inverse spinel structure with Fe cations occupying interstitial tetrahedral sites and octahedral sites of the fcc stack of oxygen [1]. The electrons can hop between Fe2+ and Fe3+ ions in the octahedral sites at room temperature renders Fe3 O4 an important class of half-metallic materials [2–4]. Fe3 O4 nanoparticles are mainly found useful in various applications for its unique magnetic and electronic properties [5–7], such as magnetic storage media, printing inks, magnetic refrigeration, photoelectric devices, ferrofluids, magnetic resonance imaging (MRI), magnetically guided drug delivery, magnetic bioseparation, biosensors and bioprocessing [8–17]. However, the density of the Fe3 O4 (ca. 4.9–5.2 g/cm3 ) restricts its applications in fields requiring light-weight mass, for instance, microwave absorbing materials of stealthy defense system for aircraft

∗ Corresponding author at: Institute of Materials Science and Engineering, Henan Polytechnic University, Jiaozuo 454003, PR China. Tel.: +86 431 5168763; fax: +86 431 5168258. E-mail address: [email protected] (H. Yang).

0921-5107/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2006.08.036

[18,19]. Coating Fe3 O4 nanoparticles on hollow microspheres of low density might be a feasible way to overcome the problem. By virtue of the very low bulk density and excellent chemical stability, glass microballoons (GMBs), which are hollow microspheres with dimension of ca. 40–60 ␮m, are highlighted in recent years as a light-weight filling material in various kinds of metallic materials, ceramics, concretes, plastics, etc. It is a good idea that GMBs could be imparted with magnetic property by coating the microspheres with Fe3 O4 nanoparticles, in order to obtain the low-density magnetic microspheres. However, as far as we know, little work has been done to combine Fe3 O4 nanoparticles with GMBs. In this study, we coated Fe3 O4 nanoparticles on the surfaces of the pretreated GMBs by reduction–precipitation method to provide the magnetic hollow microspheres with low density (0.4–0.5 g/cm3 ). The Fe3 O4 -coated GMBs exhibit the superparamagnetic character and their magnetization can be adjusted by altering the GMB content and the reaction temperature. It can be expected that these composite microspheres could have potential applications in some fields, such as microwave absorbing materials and catalysts [20–22].

X. Li et al. / Materials Science and Engineering B 135 (2006) 38–43

39

2. Experimental The GMBs (diameter: 40–60 ␮m) were supplied by Beijing Lucid Ltd. of China. Chemicals used in our experiment, hexahydrate ferric chloride (FeCl3 ·6H2 O), anhydrous sodium sulfite (Na2 SO3 ), sodium hydroxide (NaOH), ammonia (NH3 ·H2 O, 25 wt.%) and ethanol (CH3 CH2 OH), were analytical reagent grade and used without further purification. Deionized water with conductivity larger than 10 M cm was used during the sample preparation. All the GMBs were pretreated to improve the surface activity by two steps: (1) washing a proper quantity of GMBs with deionized water and ethanol for several times, then filtering and drying at 80 ◦ C; (2) immersing the GMBs in NaOH solution (0.5 mol/l) and supersonic cleaning for 30 min, then selecting the floating GMBs and drying at 80 ◦ C. The coating procedure was as follows: 1.620 g (6 mmol) FeCl3 ·6H2 O and 0.252 g (2 mmol) Na2 SO3 were dissolved in 10 ml deionized water, respectively. Then the two solutions were mixed under stirring. It was observed that the color of the mixed solution changed from light yellow to red, indicating formation of complex ions [23]. When the color altered from red to light yellow again, a proper quantity (2, 3, 4 g) of the pretreated GMBs were placed into above mixed solution, subsequently, a predetermined amount of NH3 ·H2 O was added into it under intensive stirring (pH > 12). The reactions were processed at different temperatures (20, 40, 60, 80 ◦ C) and all continuing for 60 min. When the reaction temperature reached 40 ◦ C or higher, some black resultants were formed. After coating, the products were filtered off, washed with deionized water to remove excess ions, and dried at room temperature for 12 h. The phase structure analysis of products was identified (within 2θ range of 25–70◦ ) using a X-ray diffractometer (XRD; Rigaku D/max-rA) utilizing Cu K␣ X-radiation of wave˚ A field emission scanning electron microscopy length 1.5418 A. (FESEM; JSM-6700F) equipped with an energy dispersion Xray spectroscopy (EDS; INCA X-sight 7421) was used to analyze the surface morphology and the size distribution of the microspheres, while EDS analysis was performed to understand their chemical constituents. To avoid any charging, the samples were coated with a thin layer of gold using an auto fine coater (JFC-1600) for FESEM analysis. Their magnetic properties were evaluated on a vibrating sample magnetometer (VSM; JDM-13), and the field reached up to 104 Oe. Bulk densities of different samples were obtained by pouring the material in a 10 ml measuring cylinder and tapping it to a constant volume of the material. The bulk density is calculated by the mass of the microspheres to the volume. 3. Results and discussion The XRD patterns of bare GMBs and the products synthesized at different reaction temperatures are shown in Fig. 1. XRD analysis of GMBs shows that these spheres are mainly composed of mixture of mullite and quartz. After coating, and the reaction temperature reaches 40 ◦ C or higher, some more peaks at 2θ = 30.1◦ , 35.5◦ , 43.2◦ , 57.1◦ and 62.7◦ appear in the spectra.

Fig. 1. XRD patterns of (a) bare GMBs and products synthesized at: (b) 20 ◦ C, (c) 40 ◦ C, (d) 60 ◦ C and (e) 80 ◦ C.

They tally with the PDF card (No. 88-0315) of PCPDFWIN software. The corresponding diffraction indices are (2 2 0), (3 1 1), (4 0 0), (5 1 1) and (4 4 0). Although there is little difference between the XRD patterns of Fe3 O4 and ␥-Fe2 O3 (maghemite) for the same inverse spinel structure and nearly the same lattice parameter [24], the characteristic reflection of (2 2 1) plane corresponding to ␥-Fe2 O3 [25] is found to be absent in Fig. 1. It indicates that the coating of GMBs is Fe3 O4 instead of ␥-Fe2 O3 . Furthermore, the black color of these samples suggests the formation of Fe3 O4 -coating on account of the red–brown color of ␥-Fe2 O3 . These Fe3 O4 peaks become sharper with the reaction temperature elevating, which indicates that the Fe3 O4 -coating is better crystallized and the particle size of Fe3 O4 increases. The FESEM analysis of GMBs before pretreatment shows that these microspheres have spherical smooth surface morphology (Fig. 2(a)). But the spheres which were pretreated can be seen many eroded speckles on their surfaces (Fig. 2(b)). The chemical constituents of these spheres are revealed by EDS analysis. It shows that they are mainly composed of a mixture of SiO2 and Al2 O3 , while other oxides of trace elements such as K, Fe and Ca are also present (Fig. 2(c)). Typical FESEM micrographs of the Fe3 O4 -coated GMBs, synthesized under different reaction temperatures, are presented in Fig. 3(a–e), while EDS analysis is presented in Fig. 3(f). In Fig. 3(a–c), GMBs appear to be successfully coated using the precipitation coating process. It can be observed that the products synthesized at 80 ◦ C are coated most uniformly and continuously of the three samples. Further, the coating appears to be made up of the nanoparticles having average size below 25 nm (Fig. 3(d)). The hollow structure can be clearly observed from the image of the broken microsphere (Fig. 3(e)). The thickness of the coating is within a range from 400 to 500 nm. The EDS analysis of one of the Fe3 O4 -coated GMBs (Fig. 3(f)) shows that there are more Fe element and O element appearance. Thus, FESEM and EDS analyses suggest the uniform and continuous Fe3 O4 -coating coated on the surfaces of GMBs by the reduction–precipitation coating process.

40

X. Li et al. / Materials Science and Engineering B 135 (2006) 38–43

Fig. 2. FESEM micrographs of GMBs: (a) before and (b) after pretreatment. (c) EDS analysis of GMBs.

The bulk density (ρB ) of these composite microspheres measured [26] by the mass of the spheres to the volume is 0.4–0.5 g/cm3 as listed in Table 1. In the reduction–precipitation reaction of synthesizing the Fe3 O4 -coating, we started from Fe3+ ions, which are stable in

air. Fe2+ ions are not added, but instead are formed from the Fe3+ ions by partial reduction with SO3 2− ions before NH3 ·H2 O is added. The reason that reoxidation of Fe2+ as formed through reduction can be avoided is Fe3+ can form complex ions with SO3 2− [23]. The Fe3 O4 particles of the coating were synthesized as following processes:

Table 1 Bulk densities and saturation magnetizations of different samples

2Fe3+ + SO3 2− → [Fe2 (SO3 )]4+

(1)

[Fe2 (SO3 )]4+ + H2 O → 2Fe2+ + SO4 2− + 2H+

(2)

Sample S1 S2 S3 S4 S5 a

Addition mass of GMBs (g)

Reaction temperature (◦ C)

ρB a (g/cm3 )

2 2 2 3 4

40 60 80 80 80

0.49 0.49 0.50 0.45 0.42

Ms (emu/g)

2Fe 18.5 22.6 30.0 20.5 14.2

The bulk density of bare GMBs measured by the same method is 0.39 g/cm3 .

3+

+ Fe

2+

−

+ 8OH → Fe3 O4 + 4H2 O

(3)

According to above processes, the theoretical initial molar ratio R = [Fe3+ ]/[SO3 2− ] should be six in order that [Fe3+ ]/[Fe2+ ] = 2 after reduction to obtain a pure Fe3 O4 -coating. However, varied factors can affect the equilibrium of the reduction reaction, such as the concentration of FeCl3 and Na2 SO3 .

X. Li et al. / Materials Science and Engineering B 135 (2006) 38–43

41

Fig. 3. FESEM micrographs of Fe3 O4 -coated GMBs synthesized at: (a) 40 ◦ C, (b) 60 ◦ C and (c) 80 ◦ C. (d) The Fe3 O4 nanoparticles of Fe3 O4 -coating and (e) the cross-section of the Fe3 O4 -coated GMBs synthesized at 80 ◦ C. (f) EDS analysis of Fe3 O4 -coated GMBs.

42

X. Li et al. / Materials Science and Engineering B 135 (2006) 38–43

Fig. 4. The process of how Fe3 O4 -coating is formed.

Table 2 Experimental results of coatings obtained at different initial ratios: R = [Fe3+ ]/[SO3 2− ] MFeCl3 ·6H2 O (mmol)a

MNa2 SO3 (mmol)a

R

Coating

6 6 6 6

1 1.2 1.5 2

6 5 4 3

Fe(OH)3 Fe3 O4 + Fe(OH)3 Fe3 O4 + Fe(OH)3 Fe3 O4

a

FeCl3 and Na2 SO3 were dissolved in 10 ml deionized water, respectively.

Experiments at different R-values were carried out and the results are summarized in Table 2. It shows that the ideal initial ratio is R = 3. The formation of Fe3 O4 -coating is illustrated in Fig. 4. When GMBs were added into the NaOH solution for pretreatment, many –OH function groups got adsorbed on their surfaces. There were also lots of –OH function groups on the surfaces of the Fe3 O4 nanoparticles which were formed in the alkaline solution. Then these Fe3 O4 nanoparticles combined with the GMBs by the condensation reaction. In the late period, further Fe3 O4 was continuously formed on the prior Fe3 O4 nucleation layer to form a uniform and continuous Fe3 O4 -coating on the surface of GMB. The magnetic properties of different samples, which were measured at room temperature, are shown in Fig. 5. It can be observed that the hysteresis loops of all samples demonstrate a typical superparamagnetic behavior with zero coercivity

and zero remanence. The superparamagnetism of these samples should be attributed to the size of the Fe3 O4 nanoparticles coated on GMBs below the superparamagnetic critical size. It is because that fine particles are easier to be thermally activated to overcome the magnetic anisotropy [27]. If the size of Fe3 O4 particle is smaller than the superparamagnetic critical size (DP ) [28], above the blocking temperature (TB ) [29], the magnetization intensity becomes zero with the applied magnetic field decreasing to zero and aggrandizes fleetly with the aggrandizement of the applied magnetic field. The saturation magnetization (Ms ) is listed in Table 1. With the same GMB content (S1–S3), the saturation magnetization values increase with the reaction temperatures elevating. It is due to the Fe3 O4 -coating better crystallizing and larger crystal size as indicated by XRD (Fig. 1). At the same reaction temperature (S3–S5), the saturation magnetization values decrease with increased GMB content. Because the weights of all samples used for measurement of magnetic properties are constant, the decrease of saturation magnetization is due to the increased quantity of GMBs incorporated in the Fe3 O4 -coated GMBs. These results suggest that it is possible to control the magnetization value of the magnetic microspheres by controlling the reaction temperature and the GMB content. 4. Conclusion In summary, low-density (0.4–0.5 g/cm3 ) composite microspheres with the superparamagnetic character have been successfully synthesized by coating GMBs with Fe3 O4 nanoparticles. These GMBs appear to be coated most uniformly and continuously and their saturation magnetization intensity is the strongest of all the samples when the reaction temperature reaches 80 ◦ C. The thickness of the Fe3 O4 -coating is ca. 400–500 nm and the size of Fe3 O4 particles of the coating is below 25 nm. The magnetization value of the Fe3 O4 -coated GMBs can be adjusted by controlling the GMB content and the reaction temperature. Furthermore, the low-density and the magnetic properties of these composite microspheres make them a good candidate in microwave absorbing materials and catalysts. Acknowledgements

Fig. 5. Magnetization curves of different samples.

The authors are grateful to Professors Z.X. Guo and G. Peng of Jilin University, who made the FESEM and XRD analyses possible.

X. Li et al. / Materials Science and Engineering B 135 (2006) 38–43

References [1] R.M. Cornell, U. Schwertmann, The Iron Oxides: Structure, Properties, Reactions, Occurrences and Uses, Wiley-VCH, New York, 1996, pp. 28–29. [2] E.J.W. Verwey, Nature 144 (1939) 327–328. [3] J.M.D. Coey, A.E. Berkowitz, L. Balcells, F.F. Putris, F.T. Parker, Appl. Phys. Lett. 72 (1998) 734–736. [4] S. Soeya, J. Hayakawa, H. Takahashi, K. Ito, C. Yamamoto, A. Kida, H. Asano, M. Matsui, Appl. Phys. Lett. 80 (2002) 823–825. [5] D.G. Rethwisch, J.A. Dumesic, Appl. Catal. 21 (1986) 97–109. [6] J.P. Jolivet, E. Tronc, J. Colloid Interface Sci. 125 (1988) 688–701. [7] U. H¨afeli, W. Sch¨utt, J. Teller, M. Zborowski, Scientific and Clinical Applications of Magnetic Carriers, Plenum, New York, 1997. [8] K. Yamaguchi, K. Matsumoto, T. Fujii, J. Appl. Phys. 67 (1990) 4493–4495. [9] V.T. Peikove, K.S. Jeon, A.M. Lane, J. Magn. Magn. Mater. 193 (1999) 307–310. [10] R.D. McMichael, R.D. Shull, L.J. Swartzendruber, L.H. Bennett, R.E. Walson, J. Magn. Magn. Mater. 111 (1992) 29–33. [11] D.K. Kim, Y. Zhang, W. Voit, K.V. Rao, M. Muhammed, J. Magn. Magn. Mater. 225 (2001) 30–36. [12] Y. Shahoo, A. Goodarzi, M.T. Swihart, T.Y. Ohulchanskyy, N. Kaur, E.P. Furlani, P.N. Prasad, J. Phys. Chem. B 109 (2005) 3879–3885. [13] H. Lee, E. Lee, D.K. Kim, N.K. Jang, Y.Y. Jeong, S. Jon, J. Am. Chem. Soc. 128 (2006) 7383–7389.

43

[14] N. Kohler, C. Sun, J. Wang, M. Zhang, Langmuir 21 (2005) 8858– 8864. [15] T. Sen, A. Sebastianelli, I.J. Bruce, J. Am. Chem. Soc. 128 (2006) 7130–7131. [16] J.M. Perez, F.J. Simeone, Y. Saeki, L. Josephson, R. Weissleder, J. Am. Chem. Soc. 125 (2003) 10192–10193. [17] L. Nixon, C.A. Koval, R.D. Noble, G.S. Slaff, Chem. Mater. 4 (1992) 117–121. [18] K. Hatakeyama, T. Inui, IEEE Trans. Magn. 20 (1984) 1261–1263. [19] M. Matsumoto, Y. Miyata, IEEE Trans. Magn. 33 (1997) 4459–4464. [20] Y.D. Deng, X. Liu, B. Shen, L. Liu, W.B. Hu, J. Magn. Magn. Mater. 303 (2006) 181–184. [21] F. Pinna, T. Fantinel, G. Strukul, A. Benedetti, N. Pernicone, Appl. Catal. A 149 (1997) 341–351. [22] D.L. Wilcox, T. Bernat, D. Kellerman, J.K. Cochran, Materials Research Society Proceedings, vol. 375, MRS, Pittsburgh, 1995. [23] E. Danilczuk, A. Swinarski, Roczn. Chem. 35 (1961) 1563–1572. [24] D.O. Smith, Phys. Rev. 102 (1956) 959–963. [25] S.Z. Wang, H.W. Xin, Y.T. Qian, Mater. Lett. 33 (1997) 113–116. [26] I.N. Bhattacharya, P.K. Gochhayat, P.S. Mukherjee, S. Paul, P.K. Mitra, Mater. Chem. Phys. 88 (2004) 32–40. [27] X.B. Gao, L. Gu, Nanotechnology 16 (2005) 180–185. [28] J. Lee, T. Isobe, M. Senna, J. Colloid Interface Sci. 177 (1996) 490– 494. [29] Y.L. Hou, J.F. Yu, S. Gao, J. Mater. Chem. 13 (2003) 1983–1987.