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Sol–gel preparation and characterization of CoFe2O4–SiO2 nanocomposites
Solid State Communications 132 (2004) 845–850 www.elsevier.com/locate/ssc
Sol–gel preparation and characterization of CoFe2O4–SiO2 nanocomposites Xiang-Hui Huang*, Zhen-Hua Chen College of Materials Science and Engineering, Hunan University, Changsha 410082, China Received 18 June 2004; accepted 20 September 2004 by T.T.M. Palstra Available online 20 October 2004
Abstract Magnetic nanocomposites formed by cobalt ferrite particles dispersed in a silica matrix were prepared by a sol–gel process. The effects of the thermal treatment temperature and the salt concentration on the structural and magnetic properties of the composites were investigated. By controlling these parameters, CoFe2O4/SiO2 nanocomposites with different crystallite size and magnetic properties were obtained. By increasing the annealing temperature and salt concentration, composites with a progressive increase in the coercive field and of the density of magnetization were produced. In particular, a nanocomposite, with a Fe/Si molar concentration of 21%, obtained by drying the gel at 150 8C and further annealing at 800 8C, has a coercivity of 2000 Oe, which is more than twice higher than the coercivity of bulk cobalt ferrite. q 2004 Elsevier Ltd. All rights reserved. PACS: 71.55.Jv; 72.80.Tm; 74.25.Ha; 75.50.Dd Keywords: B. Chemical synthesis; A. Silica; B. Magnetic properties; A. Ferrites
1. Introduction There is an increasing interest in magnetic ferrite nanoparticles, because of their broad applications in several technological fields including permanent magnets, magnetic fluids, magnetic drug delivery, microwave devices and highdensity information storage. Though g-Fe2O3 and Co-doped magnetites are already being used in recording media, renewed interest is being shown to cobalt ferrite (CoFe2O4), which is considered as a potential candidate for high-density recording. This is because of its magnetic properties such as strong anisotropy and hence, high coercivity at room temperature and moderate saturation magnetization, along with good mechanical hardness and chemical stability. These characteristics and also the fact that the magnetic properties of the ferrite particles are strongly dependent on * Corresponding author. Tel./fax: C86 731 882 1648. E-mail address: [email protected] (X.-H. Huang). 0038-1098/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2004.09.060
their size, justify any effort to produce size-tuned cobalt ferrite particles with diameters ranging from the superparamagnetic threshold of 10 nm to the critical singledomain size of 70 nm [1,2]. A lot of synthetic strategies for preparing nanosized cobalt ferrite have been presented [3–10]. While the nanoparticles obtained usually have a strong tendency to aggregate, which makes it very difficult to exploit their unique physical properties. Dispersion of the nanoparticles in a matrix is one method for reducing particle agglomeration and this technique allows one to stabilize the particles and to study their formation reactions. Sol–gel process has some advantages in making inorganic composite materials containing highly dispersed magnetic fine particles; the process facilitates a good and homogeneous dispersion of the particles into the inorganic matrix. The sol–gel derived amorphous silica matrix is an excellent host for supporting different types of guest nanoparticles, the porous nature of the amorphous silica matrix provides nucleation sites for nanomagnetic particles and minimizes the aggregation
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phenomena imposing an upper limit to the size of the particles. The large variety of parameters affecting the process opens up the possibility of versatile control of the structure and chemical properties of these products. Various magnetic systems like Fe2O3/SiO2 [11–15], Ni/ SiO2 [16] and NiFe2O4/SiO2 [17] dispersed in silica matrix with applications in areas such as catalysis and electronics have been studied. The properties of these materials depend strongly on the particle size, the particle–matrix interactions and the degree of dispersion of the nanoparticles in the matrix. The ability to control these parameters by chemical modifications of the method of preparation is of crucial importance now-a-days from both a fundamental and an industrial point of view [11]. We deal in this paper with the structure and magnetic study by infrared spectroscopy, X-ray diffraction, transmission electron micrographs and differential scanning calorimetry of xerogels mixed with iron, cobalt salts added as precursors to form cobalt ferrite embedded in a silica matrix. Special attention is given to the correlation between the structure and magnetic properties of cobalt ferrite embedded in a silica matrix, for different salt concentration and after-heat treatment at elevated temperatures.
2. Experimental Nanocomposites of cobalt ferrite dispersed in a silica matrix were prepared by sol–gel process using tetraethylorthosilicate (TEOS) as a precursor of silica and metallic nitrates as precursors of the ferrite. The TEOS/EtOH/H2O and Fe/Co molar ratios were controlled at 1:4:8 and 2:1, respectively. To determine the effect of the salt concentration, samples with Fe/Si molar concentration between 3 and 21% were prepared. The sols were prepared by
Fig. 1. X-ray diffraction patterns of the sample with Fe/Si molar concentration of 21% dried at 150 8C and then calcined at different temperatures.
dissolving Fe and Co nitrates in deionized water, adding the alcoholic solution of Si (OC2H5)4 and drops of HCl. After vigorous stirring for 1 h, the sols were allowed to gel at room temperature for 5 days in partially closed glass vessels. The obtained gels were put into ovens for further drying at 150 8C for 24 h to obtain xerogels. The xerogels were thermally treated at temperatures between 400 and 800 8C for 2 h in air to form CoFe2O4/SiO2 nanocomposites. The CoFe2O4/SiO2 nanocomposites were characterized for phases using a powder X-ray diffractometer (Cu Ka, Siemens D500 diffractometer). The mid-infrared (IR) spectra, from 4000 to 400 cmK1, were recorded using a Nicolet-510 spectrophotometer on pellets obtained dispersing the samples in KBr. The average crystallite diameters (DXRD) were calculated from the X-ray peak broadening of the (311) diffraction peak using Scherrer’s formula. A transmission electron microscope (TEM, Hitachi-800 microscopy) was employed for studies of particle and crystallite characteristics of cobalt ferrite in silica. Thermal behaviors of the samples were carried out in air at a heating rate of 10 8C/min, using a Netzsch STA 449C thermal analysis system. The magnetic properties of the nanocomposites were measured using a vibrating sample magnetometer. The hysteresis loops of the nanocomposites were collected using a vibrating sample magnetometer with a maximum applied magnetic field of 20 KOe. Ms values at room temperature were obtained by extrapolation to infinite field in M versus 1/H2 plot [18].
3. Results and discussion X-ray diffraction patterns of the sample with Fe/Si molar concentration of 21% fired at various temperatures are shown in Fig. 1. The very broad peak at 2q of around 27 in these XRD traces for all the four samples is attributed to the amorphous nature of the SiO2 matrix. The results demonstrate that the xerogel is amorphous. Weak peaks assigned to CoFe2O4 appear at 400 8C, suggesting that CoFe2O4 have nucleated in silica matrix. An increase in the intensity and a narrowing in the diffraction band are observed with increasing temperature. The changes in the matrix microstructure and the pore environment before and after heat treatment at various temperatures were followed by IR spectroscopy on the CoFe2O4/SiO2 nanocomposites with Fe/Si molar concentration of 21% (Fig. 2). For the xerogel, the broad bands centered at 1638.85 cmK1 are assigned to the H–O–H bending vibration of the absorbed water. Obviously, there are certain amounts of micropores that exist in the present xerogel, which must contain physical absorbed water molecules. The band at 1449.92 cmK1 is associated with the anti-symmetric NOK stretching vibration directly 3 arising from the residual nitrate groups in the xerogel. Strong absorptions at 1083.16, 802.03 and 464.01 cmK1 indicate the formation of silica network [13]. The band at
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860.09 cmK1 is assigned to Si–O–Fe. The presence of Si– O–Fe vibrations reflects some interaction between the highly isolated Fe3C ions and the nearest silica matrix. The Si–O–Fe bond is also evident by the presence of another band at 586.57 cmK1, which is associated with the Fe–O stretching in Si–O–Fe bonds [20]. The faint absorption band at 667.05 cmK1 is assigned to the stretching vibrating mode of Co–O band [21]. The presence of Si–O–Fe and Co–O bonds sufficiently reflects the chemical nature of the transition metals involved in the xerogel. That is, these transition metal ions do not participate directly in the sol– gel chemistry even though they were introduced into the starting solutions in the form of soluble inorganic salts. For samples obtained at treatment temperature of 400 8C, the intensities for the broad bands associated with the absorbed water are drastically weakened. The absorption at 1449.92 cmK1 disappears, which is a consequence of complete decomposition of the nitrate species as confirmed
Fig. 2. IR spectra of the nanocomposite samples treated at different temperatures.
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by DSC analysis. The characteristic absorptions for the silica network remain nearly the same as those of the xerogel, while the bands at 568.14 and 861.72 cmK1 slightly increase in intensity, which can be ascribed to the enhanced interactions between the CoFe2O4 clusters and silica matrix. For the samples obtained at a treatment temperature of 600 8C, the absorption band at 1079.71 cmK1 for bSi–O– Sib of the SiO4 tetrahedron is further broadened, while that for Si–O–Si or O–Si–O bending mode at 465.70 cmK1 is much weaker, which corresponds to a rearrangement process of silica network [22]. It should be noted that a new band appears at 847.53 cmK1. Correspondingly, the absorption of the Fe–O stretching band in Fe–O–Si bonds increases in intensity. These facts reflect the formation of CoFe2O4 clusters that is accompanied with the rearrangement of silica network and with the enhancement of the Si– O–Fe bonds between the CoFe2O4 clusters and the surrounding silica network. For samples heat treated at 800 8C, the IR spectrum changes greatly compared with that for samples heat treated at 600 8C. The absorption at 1080.18 cmK1 for bSi–O–Sib of the SiO4 tetrahedron grows narrower and stronger, while the band at 860.21 cmK1 becomes very weak with the absence of the Si–O–Fe bonds. These results reflect the broken Si–O–Fe bond, which coincides with the disappearance of Fe–O stretching band at 574.79 cmK1 for Si–O–Fe bonds. The band intensity of samples heat treated at 800 8C at 800.01 cmK1 for the Si–O– Si symmetric stretch increases and the stronger absorption band at 457.94 cmK1 for Si–O–Si or O–Si–O bending mode reappears. The absorption band at 678.59 cmK1 becomes very strong, which can be associated with the characteristic Co–O stretching modes in CoFe2O4 phase [23]. The breakage of the Fe–O–Si bonds in the interface between the clusters and matrix and the formation of CoFe2O4 clusters are probably results of the transformation from FeO6 octahedron to FeO4 tetrahedron [24]. Hysteresis loops at 77 K (Fig. 3) show a drastic change in shape from the initial gel, which is paramagnetic to that of the sample heated at 400 8C, which is clearly superparamagnetic at this temperature. An increase in the initial susceptibility (slope M/H) with the temperature takes place up to 800 8C, indicating a gradual increase in the particle size. From 600 to 800 8C, the sample exhibits hysteresis at 77 K and both coercivity and remanent magnetization increase with the annealing temperature. This type of behavior is entirely consistent with a model of particle growth in the system in such a way that the differences in the magnetic parameters are associated with changes in particle size [24]. X-ray diffraction patterns for CoFe2O4/SiO2 nanocomposites prepared at different Fe/Si molar concentrations are presented in Fig. 4. Samples with less than an 8% Fe/Si molar concentration give only broad peaks, characteristic of amorphous silica (data not shown). The (311) reflection band narrows as the Fe/Si molar concentration increases from 8 to 21%, indicating an increase in crystallite size. The
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Fig. 3. Hysteresis loops at 77 K of the evolution with temperature from xerogel to the final CoFe2O4/SiO2 nanocomposite.
Fig. 4. X-ray diffraction patterns for CoFe2O4/SiO2 nanocomposites with different Fe/Si molar concentrations.
average diameters of the crystallite determined through the (311) reflection of the spinel structure are listed in Table 1. All of the nanocomposites consist of well-dispersed CoFe2O4 particles, nearly spherical, embedded in the silica network as shown in Fig. 5 for the sample with Fe/Si molar concentration of 21%.
Fig. 5. TEM micrograph of the sample with Fe/Si molar concentration of 21% calcined at 800 8C for 2 h following drying 150 8C.
The hysteresis loops of the nanocomposites were collected at RT using a vibrating sample magnetometer with a maximum applied magnetic field of 20 KOe (Fig. 6). Ms values at room temperature were obtained by extrapolation to infinite field in M versus 1/H2 plot (Table 1). The increase in the density of magnetization with the increase in the Fe/Si molar concentration can be observed very clearly. The changes in the magnetic properties of the nanocomposites can be accounted for by the modification of the average size of nanocrystallites with the Fe/Si molar concentration (Table 1). For nanocomposites with Fe/Si molar concentration of 8%, the Ms value is measured to be 1.21 emu gK1 at 20 KOe. The magnetization does not show saturation even at a field of 20 KOe for this sample. The sample is found to be superparamagnetic from the complete reversibility of the M–H curve recorded at room temperature. The superparamagnetic behavior of this sample is caused by its small crystallite size (6.2 nm) as revealed by XRD characterization. As previously reported [25], when the crystallite size is less than 14 nm, CoFe2O4 will present a predominantly superparamagnetic behavior at room temperature, therefore, sample with an average crystallite size of about 6 nm display superparamagnetic property. Ms increases with the increasing of the diameters of CoFe2O4 nanocrystallites and its maximum value is 16.6 emu gK1. Considering only the ferrite mass and assuming that the final
Table 1 Crystalline size and magnetic parameters of NiFe2O4/SiO2 nanocomposites Samples
Fe/Si molar concentration (%)
Crystalline size DXRD (311) (nm)
Ms (emu gK1) RT
Hc (Oe) RT
A B C D
8 13 18 21
6.2 14.5 17.4 21.8
1.21 8.08 12.4 16.6
0 900 1300 2000
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Fig. 6. Hysteresis loops at RT for the CoFe2O4/SiO2 nanocomposites with different Fe/Si molar concentrations.
composition was equal to that of the as-prepared gel, the maximum value of density of magnetization is about 55 emu gK1. The density of magnetization is lower than the reported values for bulk cobalt ferrite (80 emu gK1) [2], but it is in fairly good agreement with the values measured in CoFe2O4 particles of similar size [26]. The decrease in the density of magnetization with the decrease in the average diameter of the nanocrystallites can be attributed to surface effects [22,27] and core-shell morphology. The surface effects are the result of finite-size scaling of nanocrystallites, which in turn leads to a non-collinearity of magnetic moments on their surface. These effects are more intense in ferromagnetic system, where the exchange interaction occurs through the oxygen ion O2K (superexchange) [28]. The absence of the oxygen ion at the surface or the presence of another atom (ion) in the form of an impurity leads to a break of the superexchange bonds between the magnetic cations which induce surface spin disorder. Due to the above-mentioned effects, the density of magnetization in the nanocrystallites is lower than that of bulk cobalt ferrite. The decrease is more pronounced with the increase in the surface–volume ratio of the nanocrystallites and the decrease in the average diameter of nanocrystallites, respectively. The coercivity field (Hc) (Table 1) decreases to zero with the decreasing of the Fe/Si molar concentration. The observed behavior is conditioned by the magnetic structure that corresponds to a single-domain configuration
of the crystallites. The maximum value of Hc is as high as 2000 Oe, which is much higher than the coercivity of bulk cobalt ferrite (980 Oe) [29].
4. Conclusion Superparamagnetic and ferromagnetic CoFe2O4/SiO2 nanocomposites were synthesized by sol–gel processing and subsequently thermal treatment. A wide range of cobalt ferrite particle sizes within the nanometer scale is obtained by this method. By changing the concentration of nitrate salts, particles with an average diameter from 6.2 to 21.8 nm can be prepared. The magnetic properties exhibit a strong dependence on the crystalline size; Ms values increase with the increase in crystalline sizes ranging from 1.21 to 16.6 emu gK1. The maximum value of Hc is as high as 2000 Oe, which is much higher than the coercivity of bulk cobalt ferrite.
References [1] D.J. Dunlop, Philos. Mag. 19 (1969) 329. [2] A.E. Berkowitz, W.J. Schuele, J. Appl. Phys. 30 (1959) 134S. [3] N. Moumen, P. Veillet, M.P. Pileni, J. Magn. Magn. Mater. 149 (1995) 67.
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[4] C. Liu, B. Zou, A.J. Rondinone, Z.J. Zhang, J. Am. Chem. Soc. 122 (2000) 6263. [5] V. Pillai, D.O. Shah, J. Magn. Magn. Mater. 163 (1996) 243. [6] Y. Ahn, E.J. Choi, S. Kim, H.N. Ok, Mater. Lett. 50 (2001) 47. [7] C.H. Yan, Z.G. Xu, F.X. Cheng, Z.M. Wang, L.D. Sun, C.S. Liao, J.T. Jia, Solid State Commun. 111 (1999) 287. [8] J.G. Lee, J.Y. Park, C.S. Kim, J. Mater. Sci. 33 (1998) 3965. [9] K.I. Yeong, K. Don, L.S. Choong, Physica B 337 (2003) 42. [10] P.C. Morais, V.K. Garg, A.C. Oliveira, L.P. Silva, R.B. Azevedo, A.M.L. Silva, E.C.D. Lima, J. Magn. Magn. Mater. 225 (2001) 37. [11] E.M. Moreno, M. Zayat, M.P. Morales, Langmuir 18 (2002) 4972. [12] G. Ennas, M.F. Casula, G. Piccaluga, S. Solinas, J. Mater. Res. 17 (2002) 590. [13] F.D. Monte, M.P. Morales, D. Levy, Langmuir 13 (1997) 3627. [14] S. Solinas, G. Piccaluga, M.P. Morales, C.J. Serna, Acta Mater. 49 (2001) 2805. [15] C. Cannas, D. Gatteschi, A. Musinu, G. Piccaluga, C. Sangregorio, J. Phys. Chem. B 102 (1998) 7721. [16] C. Estourne`s, T. Lutz, J. Happich, T. Quaranta, P. Wissler, J.L. Guille, J. Magn. Magn. Mater. 173 (1997) 83.
[17] L. Liping, L. Guangshe, R.L. Smith, H. Inomata, Chem. Mater. 12 (2000) 3705. [18] R.D. Sa´ncheza, J. Rivas, P. Vaqueiro, M.A. Lo´pez-Quintela, D. Caeiro, J. Magn. Magn. Mater. 247 (2002) 92. [20] C. Chaneac, E. Tronc, J.P. Jolivet, J. Mater. Chem. 6 (1996) 1905. [21] H.K. Jun, J.H. Koo, T.J. Lee, Energy Fuels 18 (2004) 41. [22] J.M.D. Coey, Phys. Rev. Lett. 27 (1971) 1140. [23] S. Ponce-Castaneda, J.R. Marttnez, S.A. Palomares-Sa´nchez, J. Sol–Gel Sci. Technol. 25 (2002) 37. [24] G.V.S. Rao, C.N.R. Rao, J.R. Ferraro, Appl. Spectrosc. 24 (1970) 436. [25] T. Sato, T. Iijima, M. Seki, N.J. Inagaki, J. Magn. Magn. Mater. 65 (1987) 252. [26] M. Rajendran, R.C. Pullar, A.K. Bhattacharya, D. Das, S.N. Chintalapudi, C.K. Majumdar, J. Magn. Magn. Mater. 232 (2001) 71. [27] R.H. Kodama, A.E. Berkowitz, E.J. McNiff, S. Foner, Phys. Rev. Lett. 77 (1996) 394. [28] C. Caizer, M. Stefanescu, J. Phys. D: Appl. Phys. 35 (2002) 3035. [29] G. Bate, in: E.P. Wohlfarth (Ed.), Ferromagnetic Materials vol. 2, North-Holland, Amsterdam, 1980, p. 431.
Sol–gel preparation and characterization of CoFe2O4–SiO2 nanocomposites Xiang-Hui Huang*, Zhen-Hua Chen College of Materials Science and Engineering, Hunan University, Changsha 410082, China Received 18 June 2004; accepted 20 September 2004 by T.T.M. Palstra Available online 20 October 2004
Abstract Magnetic nanocomposites formed by cobalt ferrite particles dispersed in a silica matrix were prepared by a sol–gel process. The effects of the thermal treatment temperature and the salt concentration on the structural and magnetic properties of the composites were investigated. By controlling these parameters, CoFe2O4/SiO2 nanocomposites with different crystallite size and magnetic properties were obtained. By increasing the annealing temperature and salt concentration, composites with a progressive increase in the coercive field and of the density of magnetization were produced. In particular, a nanocomposite, with a Fe/Si molar concentration of 21%, obtained by drying the gel at 150 8C and further annealing at 800 8C, has a coercivity of 2000 Oe, which is more than twice higher than the coercivity of bulk cobalt ferrite. q 2004 Elsevier Ltd. All rights reserved. PACS: 71.55.Jv; 72.80.Tm; 74.25.Ha; 75.50.Dd Keywords: B. Chemical synthesis; A. Silica; B. Magnetic properties; A. Ferrites
1. Introduction There is an increasing interest in magnetic ferrite nanoparticles, because of their broad applications in several technological fields including permanent magnets, magnetic fluids, magnetic drug delivery, microwave devices and highdensity information storage. Though g-Fe2O3 and Co-doped magnetites are already being used in recording media, renewed interest is being shown to cobalt ferrite (CoFe2O4), which is considered as a potential candidate for high-density recording. This is because of its magnetic properties such as strong anisotropy and hence, high coercivity at room temperature and moderate saturation magnetization, along with good mechanical hardness and chemical stability. These characteristics and also the fact that the magnetic properties of the ferrite particles are strongly dependent on * Corresponding author. Tel./fax: C86 731 882 1648. E-mail address: [email protected] (X.-H. Huang). 0038-1098/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2004.09.060
their size, justify any effort to produce size-tuned cobalt ferrite particles with diameters ranging from the superparamagnetic threshold of 10 nm to the critical singledomain size of 70 nm [1,2]. A lot of synthetic strategies for preparing nanosized cobalt ferrite have been presented [3–10]. While the nanoparticles obtained usually have a strong tendency to aggregate, which makes it very difficult to exploit their unique physical properties. Dispersion of the nanoparticles in a matrix is one method for reducing particle agglomeration and this technique allows one to stabilize the particles and to study their formation reactions. Sol–gel process has some advantages in making inorganic composite materials containing highly dispersed magnetic fine particles; the process facilitates a good and homogeneous dispersion of the particles into the inorganic matrix. The sol–gel derived amorphous silica matrix is an excellent host for supporting different types of guest nanoparticles, the porous nature of the amorphous silica matrix provides nucleation sites for nanomagnetic particles and minimizes the aggregation
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phenomena imposing an upper limit to the size of the particles. The large variety of parameters affecting the process opens up the possibility of versatile control of the structure and chemical properties of these products. Various magnetic systems like Fe2O3/SiO2 [11–15], Ni/ SiO2 [16] and NiFe2O4/SiO2 [17] dispersed in silica matrix with applications in areas such as catalysis and electronics have been studied. The properties of these materials depend strongly on the particle size, the particle–matrix interactions and the degree of dispersion of the nanoparticles in the matrix. The ability to control these parameters by chemical modifications of the method of preparation is of crucial importance now-a-days from both a fundamental and an industrial point of view [11]. We deal in this paper with the structure and magnetic study by infrared spectroscopy, X-ray diffraction, transmission electron micrographs and differential scanning calorimetry of xerogels mixed with iron, cobalt salts added as precursors to form cobalt ferrite embedded in a silica matrix. Special attention is given to the correlation between the structure and magnetic properties of cobalt ferrite embedded in a silica matrix, for different salt concentration and after-heat treatment at elevated temperatures.
2. Experimental Nanocomposites of cobalt ferrite dispersed in a silica matrix were prepared by sol–gel process using tetraethylorthosilicate (TEOS) as a precursor of silica and metallic nitrates as precursors of the ferrite. The TEOS/EtOH/H2O and Fe/Co molar ratios were controlled at 1:4:8 and 2:1, respectively. To determine the effect of the salt concentration, samples with Fe/Si molar concentration between 3 and 21% were prepared. The sols were prepared by
Fig. 1. X-ray diffraction patterns of the sample with Fe/Si molar concentration of 21% dried at 150 8C and then calcined at different temperatures.
dissolving Fe and Co nitrates in deionized water, adding the alcoholic solution of Si (OC2H5)4 and drops of HCl. After vigorous stirring for 1 h, the sols were allowed to gel at room temperature for 5 days in partially closed glass vessels. The obtained gels were put into ovens for further drying at 150 8C for 24 h to obtain xerogels. The xerogels were thermally treated at temperatures between 400 and 800 8C for 2 h in air to form CoFe2O4/SiO2 nanocomposites. The CoFe2O4/SiO2 nanocomposites were characterized for phases using a powder X-ray diffractometer (Cu Ka, Siemens D500 diffractometer). The mid-infrared (IR) spectra, from 4000 to 400 cmK1, were recorded using a Nicolet-510 spectrophotometer on pellets obtained dispersing the samples in KBr. The average crystallite diameters (DXRD) were calculated from the X-ray peak broadening of the (311) diffraction peak using Scherrer’s formula. A transmission electron microscope (TEM, Hitachi-800 microscopy) was employed for studies of particle and crystallite characteristics of cobalt ferrite in silica. Thermal behaviors of the samples were carried out in air at a heating rate of 10 8C/min, using a Netzsch STA 449C thermal analysis system. The magnetic properties of the nanocomposites were measured using a vibrating sample magnetometer. The hysteresis loops of the nanocomposites were collected using a vibrating sample magnetometer with a maximum applied magnetic field of 20 KOe. Ms values at room temperature were obtained by extrapolation to infinite field in M versus 1/H2 plot [18].
3. Results and discussion X-ray diffraction patterns of the sample with Fe/Si molar concentration of 21% fired at various temperatures are shown in Fig. 1. The very broad peak at 2q of around 27 in these XRD traces for all the four samples is attributed to the amorphous nature of the SiO2 matrix. The results demonstrate that the xerogel is amorphous. Weak peaks assigned to CoFe2O4 appear at 400 8C, suggesting that CoFe2O4 have nucleated in silica matrix. An increase in the intensity and a narrowing in the diffraction band are observed with increasing temperature. The changes in the matrix microstructure and the pore environment before and after heat treatment at various temperatures were followed by IR spectroscopy on the CoFe2O4/SiO2 nanocomposites with Fe/Si molar concentration of 21% (Fig. 2). For the xerogel, the broad bands centered at 1638.85 cmK1 are assigned to the H–O–H bending vibration of the absorbed water. Obviously, there are certain amounts of micropores that exist in the present xerogel, which must contain physical absorbed water molecules. The band at 1449.92 cmK1 is associated with the anti-symmetric NOK stretching vibration directly 3 arising from the residual nitrate groups in the xerogel. Strong absorptions at 1083.16, 802.03 and 464.01 cmK1 indicate the formation of silica network [13]. The band at
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860.09 cmK1 is assigned to Si–O–Fe. The presence of Si– O–Fe vibrations reflects some interaction between the highly isolated Fe3C ions and the nearest silica matrix. The Si–O–Fe bond is also evident by the presence of another band at 586.57 cmK1, which is associated with the Fe–O stretching in Si–O–Fe bonds [20]. The faint absorption band at 667.05 cmK1 is assigned to the stretching vibrating mode of Co–O band [21]. The presence of Si–O–Fe and Co–O bonds sufficiently reflects the chemical nature of the transition metals involved in the xerogel. That is, these transition metal ions do not participate directly in the sol– gel chemistry even though they were introduced into the starting solutions in the form of soluble inorganic salts. For samples obtained at treatment temperature of 400 8C, the intensities for the broad bands associated with the absorbed water are drastically weakened. The absorption at 1449.92 cmK1 disappears, which is a consequence of complete decomposition of the nitrate species as confirmed
Fig. 2. IR spectra of the nanocomposite samples treated at different temperatures.
847
by DSC analysis. The characteristic absorptions for the silica network remain nearly the same as those of the xerogel, while the bands at 568.14 and 861.72 cmK1 slightly increase in intensity, which can be ascribed to the enhanced interactions between the CoFe2O4 clusters and silica matrix. For the samples obtained at a treatment temperature of 600 8C, the absorption band at 1079.71 cmK1 for bSi–O– Sib of the SiO4 tetrahedron is further broadened, while that for Si–O–Si or O–Si–O bending mode at 465.70 cmK1 is much weaker, which corresponds to a rearrangement process of silica network [22]. It should be noted that a new band appears at 847.53 cmK1. Correspondingly, the absorption of the Fe–O stretching band in Fe–O–Si bonds increases in intensity. These facts reflect the formation of CoFe2O4 clusters that is accompanied with the rearrangement of silica network and with the enhancement of the Si– O–Fe bonds between the CoFe2O4 clusters and the surrounding silica network. For samples heat treated at 800 8C, the IR spectrum changes greatly compared with that for samples heat treated at 600 8C. The absorption at 1080.18 cmK1 for bSi–O–Sib of the SiO4 tetrahedron grows narrower and stronger, while the band at 860.21 cmK1 becomes very weak with the absence of the Si–O–Fe bonds. These results reflect the broken Si–O–Fe bond, which coincides with the disappearance of Fe–O stretching band at 574.79 cmK1 for Si–O–Fe bonds. The band intensity of samples heat treated at 800 8C at 800.01 cmK1 for the Si–O– Si symmetric stretch increases and the stronger absorption band at 457.94 cmK1 for Si–O–Si or O–Si–O bending mode reappears. The absorption band at 678.59 cmK1 becomes very strong, which can be associated with the characteristic Co–O stretching modes in CoFe2O4 phase [23]. The breakage of the Fe–O–Si bonds in the interface between the clusters and matrix and the formation of CoFe2O4 clusters are probably results of the transformation from FeO6 octahedron to FeO4 tetrahedron [24]. Hysteresis loops at 77 K (Fig. 3) show a drastic change in shape from the initial gel, which is paramagnetic to that of the sample heated at 400 8C, which is clearly superparamagnetic at this temperature. An increase in the initial susceptibility (slope M/H) with the temperature takes place up to 800 8C, indicating a gradual increase in the particle size. From 600 to 800 8C, the sample exhibits hysteresis at 77 K and both coercivity and remanent magnetization increase with the annealing temperature. This type of behavior is entirely consistent with a model of particle growth in the system in such a way that the differences in the magnetic parameters are associated with changes in particle size [24]. X-ray diffraction patterns for CoFe2O4/SiO2 nanocomposites prepared at different Fe/Si molar concentrations are presented in Fig. 4. Samples with less than an 8% Fe/Si molar concentration give only broad peaks, characteristic of amorphous silica (data not shown). The (311) reflection band narrows as the Fe/Si molar concentration increases from 8 to 21%, indicating an increase in crystallite size. The
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Fig. 3. Hysteresis loops at 77 K of the evolution with temperature from xerogel to the final CoFe2O4/SiO2 nanocomposite.
Fig. 4. X-ray diffraction patterns for CoFe2O4/SiO2 nanocomposites with different Fe/Si molar concentrations.
average diameters of the crystallite determined through the (311) reflection of the spinel structure are listed in Table 1. All of the nanocomposites consist of well-dispersed CoFe2O4 particles, nearly spherical, embedded in the silica network as shown in Fig. 5 for the sample with Fe/Si molar concentration of 21%.
Fig. 5. TEM micrograph of the sample with Fe/Si molar concentration of 21% calcined at 800 8C for 2 h following drying 150 8C.
The hysteresis loops of the nanocomposites were collected at RT using a vibrating sample magnetometer with a maximum applied magnetic field of 20 KOe (Fig. 6). Ms values at room temperature were obtained by extrapolation to infinite field in M versus 1/H2 plot (Table 1). The increase in the density of magnetization with the increase in the Fe/Si molar concentration can be observed very clearly. The changes in the magnetic properties of the nanocomposites can be accounted for by the modification of the average size of nanocrystallites with the Fe/Si molar concentration (Table 1). For nanocomposites with Fe/Si molar concentration of 8%, the Ms value is measured to be 1.21 emu gK1 at 20 KOe. The magnetization does not show saturation even at a field of 20 KOe for this sample. The sample is found to be superparamagnetic from the complete reversibility of the M–H curve recorded at room temperature. The superparamagnetic behavior of this sample is caused by its small crystallite size (6.2 nm) as revealed by XRD characterization. As previously reported [25], when the crystallite size is less than 14 nm, CoFe2O4 will present a predominantly superparamagnetic behavior at room temperature, therefore, sample with an average crystallite size of about 6 nm display superparamagnetic property. Ms increases with the increasing of the diameters of CoFe2O4 nanocrystallites and its maximum value is 16.6 emu gK1. Considering only the ferrite mass and assuming that the final
Table 1 Crystalline size and magnetic parameters of NiFe2O4/SiO2 nanocomposites Samples
Fe/Si molar concentration (%)
Crystalline size DXRD (311) (nm)
Ms (emu gK1) RT
Hc (Oe) RT
A B C D
8 13 18 21
6.2 14.5 17.4 21.8
1.21 8.08 12.4 16.6
0 900 1300 2000
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Fig. 6. Hysteresis loops at RT for the CoFe2O4/SiO2 nanocomposites with different Fe/Si molar concentrations.
composition was equal to that of the as-prepared gel, the maximum value of density of magnetization is about 55 emu gK1. The density of magnetization is lower than the reported values for bulk cobalt ferrite (80 emu gK1) [2], but it is in fairly good agreement with the values measured in CoFe2O4 particles of similar size [26]. The decrease in the density of magnetization with the decrease in the average diameter of the nanocrystallites can be attributed to surface effects [22,27] and core-shell morphology. The surface effects are the result of finite-size scaling of nanocrystallites, which in turn leads to a non-collinearity of magnetic moments on their surface. These effects are more intense in ferromagnetic system, where the exchange interaction occurs through the oxygen ion O2K (superexchange) [28]. The absence of the oxygen ion at the surface or the presence of another atom (ion) in the form of an impurity leads to a break of the superexchange bonds between the magnetic cations which induce surface spin disorder. Due to the above-mentioned effects, the density of magnetization in the nanocrystallites is lower than that of bulk cobalt ferrite. The decrease is more pronounced with the increase in the surface–volume ratio of the nanocrystallites and the decrease in the average diameter of nanocrystallites, respectively. The coercivity field (Hc) (Table 1) decreases to zero with the decreasing of the Fe/Si molar concentration. The observed behavior is conditioned by the magnetic structure that corresponds to a single-domain configuration
of the crystallites. The maximum value of Hc is as high as 2000 Oe, which is much higher than the coercivity of bulk cobalt ferrite (980 Oe) [29].
4. Conclusion Superparamagnetic and ferromagnetic CoFe2O4/SiO2 nanocomposites were synthesized by sol–gel processing and subsequently thermal treatment. A wide range of cobalt ferrite particle sizes within the nanometer scale is obtained by this method. By changing the concentration of nitrate salts, particles with an average diameter from 6.2 to 21.8 nm can be prepared. The magnetic properties exhibit a strong dependence on the crystalline size; Ms values increase with the increase in crystalline sizes ranging from 1.21 to 16.6 emu gK1. The maximum value of Hc is as high as 2000 Oe, which is much higher than the coercivity of bulk cobalt ferrite.
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