- Email: [email protected]
SiO2 nanocomposites by sol–gel method
ARTICLE IN PRESS
Journal of Crystal Growth 271 (2004) 287–293 www.elsevier.com/locate/jcrysgro
Preparation of CoFe2O4/SiO2 nanocomposites by sol–gel method Xianghui Huang, Zhenhua Chen College of Materials Science and Engineering, Hunan University, Changsha, 410082, China Received 8 May 2004; accepted 22 July 2004 Communicated by D.T.J. Hurle Available online 3 September 2004
Abstract Nanocomposites with cobalt ferrite nanoparticles uniformly dispersed in silica have been successfully synthesized. The effects of the thermal treatment temperature and the initial drying temperature on the structural and magnetic properties of the nanocomposites were examined. The crystalline phase, the particle size, and the homogeneity of the resulting nanocomposites were studied by X-ray diffraction, IR spectroscopy, differential scanning calorimetry, transmission electron spectroscopy and vibrating sample magnetometer. The xerogels were amorphous. Heat treatment at 400 1C resulted in CoFe2O4 clusters being partially formed and when the heat treatment temperature was increased to 600 1C, CoFe2O4 clusters were formed in large quantities. The formation reaction of CoFe2O4 clusters was accompanied by a rearrangement of the silica matrix network. On further increasing the heat treatment temperature to 800 1C, materials with CoFe2O4 nanocrystals, well crystalline dispersed in the silica matrix, could be obtained. By increasing the annealing temperature, composites with a progressive increase of the coercivity and of the remanent magnetization were produced. The remarkable effect of initial drying temperature on the particle size of cobalt ferrite suggested that a well-established silica network provided an effective confinement to the coarsening and aggregation of CoFe2O4 nanoparticles. r 2004 Elsevier B.V. All rights reserved. PACS: 75.01A; 89.02 Keywords: A1. Nanostructures; A1. X-ray diffraction; B2. Magnetic materials; B3. Infrared devices
1. Introduction
Corresponding author. Tel./fax: +86-7318-821648
E-mail address: [email protected] (Z.H. Chen).
Spinel ferrite nanoparticles have been intensively investigated in recent years because of their remarkable electrical and magnetic properties and wide practical applications in information storage
0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.07.064
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system, ferrofluid technology, magnetocaloric refrigeration and medical diagnostics. Among spinel ferrites, cobalt ferrite, CoFe2O4 is especially interesting because of its high cubic magnetocrystalline anisotropy, high coercivity and moderate saturation magnetization. Recently, cobalt ferrite nanoparticles was also known to be a photomagnetic material that shows an interesting lightinduced coercivity change [1,2]. A lot of synthetic strategies for preparing nanosized cobalt ferrite have been presented [3–10]. 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 to reduce particle agglomeration and this technique allows one to stabilize the particles and to study their formation reactions. In recent years, systems such as Fe2O3/SiO2 have been intensively studied, revealing a behavior different from that of bulk systems and serving as a model for the study of small particles [11–15]. The properties of these materials depend strongly on the particle size, the particle–matrix interactions, and the degree of dispersion of nanoparticles in the matrix. The ability to control these parameters by chemical modifications of the method of preparation is of crucial importance nowadays from both a fundamental and an industrial point of view [11]. In this paper, we report on the dispersion of CoFe2O4 nanoparticles in a silica matrix prepared using a sol–gel procedure and follow the structural and magnetic evolutions by IR spectroscopy, Xray diffraction, transmission electron micrographs, differential scanning calorimetry and vibrating sample magnetometer. Experiments were conducted to study the influence of the preparation variables, such as the initial drying temperature and thermal treatment of the gels on the structural and magnetic properties of cobalt ferrite embedded in a silica matrix.
2. Experimental procedure Nanocomposites of cobalt ferrite dispersed in a silica matrix were prepared by sol–gel process. The starting material tetraethylorthosilicate (TEOS)
was used as a precursor of silica, while ferric nitrate and cobalt nitrate were employed as the reactant iron and cobalt species. The TEOS: ethanol: H2O and Fe: Co molar ratios were controlled at 1:4:8 and 2:1, respectively. The Fe/Si molar ratio was initially adjusted to 16%. The sols were prepared by dissolving Fe and Co nitrates in deionized water, adding the alcoholic solution of Si(OC2H5)4 and a few 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 25, 110 and 150 1C, respectively, to obtain xerogels. The xerogels were placed in a furnace and thermally treated between 400 and 800 1C in air, with a heating rate of 10 1C/min, and kept for 2 h at each temperature. Thermal behavior of the samples was examined by differential scanning calorimetry (DSC), using a Netzsch-404PC thermal analysis system, in an air flux and with a heating rate of 10 1C/min. Mid-infrared (IR) spectra, from 4000 to 400 cm 1, were recorded using a Nicolet-510 spectrophotometer on pellets obtained dispersing the samples in KBr. The samples were characterized by powder X-ray diffraction (XRD) using a y 2y Siemens D500 diffractometer working with Cu Ka radiation, a graphite monochromator on the diffracted beam and a scintillation counter with pulse-height discriminator. Transmission electron microscopy (TEM) observations were carried out in bright-field (BF) mode, using a Hitachi-800 microscopy operating at 100 kV and with a JEOL CX200 microscope operating at 200 kV to study the morphological properties of the samples. TEM samples were prepared by dispersing the powders in acetone in an ultrasonic bath and dropping on a conventional carbon-coated copper grid. Mean particle size (m) and standard deviation (s) were calculated from TEM micrographs, at 100 000 magnification, counting around 100 particles. From these data, the degree of polydispersity, defined as s/m; was calculated. The hysteresis loops of the nanocomposites were collected at RT using a vibrating sample
ARTICLE IN PRESS X.H. Huang, Z.H. Chen / Journal of Crystal Growth 271 (2004) 287–293
magnetometer with a maximum applied magnetic field of 10 KOe.
3. Results and discussion It has been found that if nonreactive species are inserted into the initial solution (called ‘‘sol’’), the solid oxide network gradually groups around these species during the sol–gel reaction and finally gives a doped gel with dispersed species engaged in the pores of the gel [16]. This theory has been proved in different systems such as Fe/SiO2 [17], Fe2O3/ SiO2 [11–15,18], Cu/SiO2 [19], etc. The evolution of the xerogels with temperature was studied in detail in an attempt to understand the formation of cobalt ferrite nanocrystals in gels. Fig. 1 shows the X-ray diffraction pattern for the xerogel and the samples obtained after a 2 h heat treatment of the xerogel at various temperatures. The results demonstrate that the xerogel obtained after drying at 150 1C was amorphous and no nitrate crystallites were detectable, while the corresponding IR spectra (Fig. 3) of the xerogel showed a band at 1449.92 cm 1, which is associated with the anti-symmetric NO3 1 stretching vibration directly arising from the residual nitrate groups in the xerogel. These indicate that nitrate
Fig. 1. X-ray diffraction patterns of the samples dried at 150 1C and the samples obtained after calcination of the xerogel at various temperatures.
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salts were uniformly dispersed in the gel. In fact, when the nanoparticles’ precursors were not well dispersed, peaks due to the nitrate phase were visible in the XRD spectra. The decomposition temperature of iron nitrate is 125 1C, while according to the DSC, nitrate salts in the gel were decomposed at about 400 1C, which indicated the entry of Co and Fe ions into the silica network [20]. The XRD spectra of samples reveal that weak peaks assigned to CoFe2O4 appeared at 400 1C, suggesting that the particles of CoFe2O4 had been nucleated in the silica matrix. An increase in the intensity and a narrowing in the diffraction band were observed with increasing temperature. During the formation of CoFe2O4 nanocrystals, no traces of quartz, cristobalite or intermediate products, e.g., Fe2SiO4 or Co2SiO4 were formed even at temperatures as high as 800 1C, which can be accounted for the homogeneity in composition. Hysteresis loops at 77 K (Fig. 2) showed a drastic change in shape from the xerogel, which was paramagnetic to that of the sample heated at 400 1C, which was clearly superparamagnetic at this temperature. An increase in the initial susceptibility (slope M/H) with the temperature took place up to 800 1C indicating a gradual increase in the particle size. From 600 1 to 800 1C, the sample exhibited hysteresis at 77 K and both coercivity and remanent magnetization increased 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 [21]. The changes in the matrix microstructure and the pore environment before and after the heat treatment at various temperatures were followed by IR spectroscopy (Fig. 3). For the xerogel, the broad band centered at 1638.85 cm 1 is assigned to the H–O–H bending vibration of the absorbed water. Obviously, there were certain amounts of micropores that existed in the present xerogel, which must contain physically absorbed water molecules. The band at 1449.92 cm 1 is associated with the anti-symmetric NO3 stretching vibration directly arising from the residual nitrate groups in the xerogel. Strong absorptions at 1083.16, 802.03 and 464.01 cm 1 indicate the formation of a silica
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Fig. 2. Hysteresis loops at 77 K of the samples dried at 150 1C and the samples obtained after a 2 h heat treatment of the xerogel at various temperatures.
network [13]. The band at 860.09 cm 1 is assigned to Si–O–Fe. The presence of Si–O–Fe vibrations reflected some interaction between the highly isolated Fe3+ ions and the nearest silica matrix. The Si–O–Fe bond was also evident by the presence of another band at 586.57 cm 1, which is associated with the Fe–O stretching in Si–O–Fe bonds [22]. Another faint absorption band at 667.05 cm 1 is assigned to the stretching vibrating mode of the Co–O band [23]. The presence of Si–O–Fe and Co–O bonds sufficiently reflected the chemical nature of the transition metals involved in the xerogel. That is, these transition metal ions did 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 a treatment temperature of 400 1C, the intensities for the broad bands associated with the absorbed water were drastically weakened. The absorption at 1449.92 cm 1 disappeared, which was a consequence of complete decomposition of the nitrate species as confirmed by DSC analysis. The characteristic absorptions for the silica network remained nearly the same as those of the xerogel, while the bands at 568.14 and
861.72 cm 1 slightly increased in intensity, which could be ascribed to the enhanced interactions between the CoFe2O4 clusters and silica matrix. For the samples obtained at a treatment temperature of 600 1C, the absorption band at 1079.71 cm 1 for Si–O–Si of the SiO4 tetrahedron was further broadened, while that for the Si–O–Si or O–Si–O bending mode at 465.70 cm 1 was much weaker, which corresponded to a rearrangement process of the silica network [24]. It should be noted that a new band appeared at 847.53 cm 1. Correspondingly, the absorption of the Fe–O stretching band in Fe–O–Si bonds increased in intensity. These facts reflect the formation of CoFe2O4 clusters that was accompanied by the rearrangement of the 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 1C, the IR spectrum changed greatly compared with that for samples heat treated at 600 1C. The absorption at 1080.18 cm 1 for Si–O–Si of the SiO4 tetrahedron grew narrower and stronger, while the band at 860.21 cm 1 became very weak with the absence of the Si–O–Fe bonds. These
ARTICLE IN PRESS X.H. Huang, Z.H. Chen / Journal of Crystal Growth 271 (2004) 287–293
Fig. 3. IR spectra of the nanocomposite samples treated at different conditions.
results reflect the broken Si–O–Fe bonds, which coincided with the disappearance of Fe–O stretching band at 574.79 cm 1 for Si–O–Fe bonds. The
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band intensity of samples heat treated at 800 1C at 800.01 cm 1 for the Si–O–Si symmetric stretch increased, and the stronger absorption band at 457.94 cm 1 for the Si–O–Si or O–Si–O bending mode reappeared. The absorptions that appeared at 678.59 cm 1 became very strong, which can be associated with the characteristic Co–O stretching modes in the CoFe2O4 phase [25]. The breakage of the Fe–O–Si bonds in the interface between the clusters and matrix and the formation of CoFe2O4 clusters were probably a result of the transformation from FeO6 octahedron to FeO4 tetrahedron [26]. To further study the confinement of silica matrix on the growth of CoFe2O4 particles at the calcinations temperature, wet gels were dried at different temperatures, namely 50, 110 and 150 1C, respectively, before being calcined at 800 1C for 2 h. The choice of the three drying temperature was based on the observation in DSC study. According to the study, the formation of amorphous silica began at about 65 1C and was completed at temperatures at about 156 1C, above which cracking tended to occur due to the volume shrinkage during drying of the gel. The average sizes of the cobalt ferrite particles were evaluated from TEM micrographs. It is apparent that the average particles size of CoFe2O4 decreased with increasing the drying temperature from 50 to 150 1C (Fig. 4). It should be emphasized that the drying temperature not only affected the particle size but also the particle size distribution, as can be observed by the values of the polydispersity degree (s/m) obtained from TEM micrographs (Fig. 4). The s/mvalue of 70% obtained for the samples dried at 50 1C was drastically reduced to 14% for the samples dried at 150 1C. The electron micrographs of the samples calcined at 800 1C for 2 h following drying at 150 and 50 1C (Fig. 5) also evidently supported the above behavior. Such a strong influence of the drying temperature on the average particle size of CoFe2O4 can be explained as follows: When the gel precursor was dried at a temperature much lower than 156 1C, the formation of a silica network structure was not completed. The subsequent rapid increase in temperature (10 1C/min) during calcinations did
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ment and improved the uniformity of CoFe2O4 particles.
4. Conclusion
Fig. 4. Average diameters (m) and polydispersity degree (s/m) of the ferrite particles in the samples calcined at 800 1C for 2 h following drying at various temperatures.
The microstructural evolution of CoFe2O4 nanocrystals dispersed in amorphous silica was followed with IR, XRD, and VSM. Magnetic properties could be correlated with the microstructural evolution brought about by heat treatment of the nanocrystals dispersed in silica matrix at successively higher treatment temperatures. The remarkable effect of initial drying temperature on the particle size of nanophase cobalt ferrite suggested that a well-established silica network provided an effective confinement to the coarsening and aggregation of CoFe2O4 nanoparticles. References
Fig. 5. TEM micrographs of the samples calcined at 800 1C for 2 h following drying at (a) 150 1C and (b) 50 1C.
not favor the establishment of a silica network structure. In fact, a too rapid heating gave rise to the cracking of a partially completed network structure. Furthermore, the cobalt ferrite particles formed at a high temperature could also obstruct the further establishment of a silica network. Therefore, the lower the drying temperature, the less complete would the silica structure be. On the other hand, a well-established silica network provided a more effective confinement to the growth of CoFe2O4 particles [19]. As a result, the CoFe2O4 particles in the sample dried at 150 1C coarsened at a much slower rate than that in the sample dried at 50 1C. Therefore, increasing the drying temperature before heat treatment favored the formation of a complete silica network, which in turn hindered the growth of CoFe2O4 nanoparticles and resulted in an effective refine-
[1] A.K. Giri, K. Pongsaksawad, M. Sorescu, S. Majetich, IEEE Trans. Magn. 36 (2000) 3029. [2] A.K. Giri, E.M. Kirkpatrick, P. Moongkhamklang, S.A. Majetich, Appl. Phys. Lett. 80 (2002) 2341. [3] N. Moumen, P. Veillet, M.P. Pileni, J. Magn. Magn. Mater. 149 (1995) 67. [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] Jae-Gwang Lee, Jae Yun Park, Chul Sung Kim, J. Mater. Sci. 33 (1998) 3965. [9] YeongIl Kima, Don Kima, ChoongSub Leeb, 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 (3) (2002) 590. [13] F. del 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.
ARTICLE IN PRESS X.H. Huang, Z.H. Chen / Journal of Crystal Growth 271 (2004) 287–293 [16] F. Bentivegna, J. Ferre, M. Nyolt, J.P. Jamet, D. Imhoff, J. Appl. Phys. 83 (1998) 7776. [17] R.D. Shull, J.J. Ritter, A.J. Shapiro, L.J. Swartzendruber, J. Appl. Phys. 67 (9) (1990) 4490. [18] B.J. Clapsaddle, A.E. Gash, J.H. Satcher Jr., R.L. Simpson, J. Non-Cryst. Solids 31 (2003) 190. [19] Z.H. Zhou, J.M. Xue, H.S.O. Chan, J. Wang, Mater. Chem. Phys. 75 (2002) 181. [20] A.S. Albuquerque, J.D. Ardisson, W.A.A. Macedo, J. Magn. Magn. Mater. 192 (1999) 277.
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[21] H. Izutsu, P.K. Nair, K. Maeda, Y. Kiyozumi, F. Mizukami, Mater. Res. Bull. 32 (1997) 1303. [22] C. Chaneac, E. Tronc, J.P. Jolivet, J. Mater. Chem. 6 (12) (1996) 1905. [23] H.K. Jun, J.H. Koo, T.J. Lee, Energ. Fuel. 18 (2004) 41. [24] J.M.D. Coey, Phys. Rev. Lett. 27 (1971) 1140. [25] S. Ponce-Castaneda, J.R. Marttnez, S.A. Palomares Sanchez, J. Sol–Gel. Sci. Technol. 25 (2002) 37. [26] G.V.S. Rao, C.N.R. Rao, J.R. Ferraro, Appl. Spectrosc. 24 (1970) 436.
Journal of Crystal Growth 271 (2004) 287–293 www.elsevier.com/locate/jcrysgro
Preparation of CoFe2O4/SiO2 nanocomposites by sol–gel method Xianghui Huang, Zhenhua Chen College of Materials Science and Engineering, Hunan University, Changsha, 410082, China Received 8 May 2004; accepted 22 July 2004 Communicated by D.T.J. Hurle Available online 3 September 2004
Abstract Nanocomposites with cobalt ferrite nanoparticles uniformly dispersed in silica have been successfully synthesized. The effects of the thermal treatment temperature and the initial drying temperature on the structural and magnetic properties of the nanocomposites were examined. The crystalline phase, the particle size, and the homogeneity of the resulting nanocomposites were studied by X-ray diffraction, IR spectroscopy, differential scanning calorimetry, transmission electron spectroscopy and vibrating sample magnetometer. The xerogels were amorphous. Heat treatment at 400 1C resulted in CoFe2O4 clusters being partially formed and when the heat treatment temperature was increased to 600 1C, CoFe2O4 clusters were formed in large quantities. The formation reaction of CoFe2O4 clusters was accompanied by a rearrangement of the silica matrix network. On further increasing the heat treatment temperature to 800 1C, materials with CoFe2O4 nanocrystals, well crystalline dispersed in the silica matrix, could be obtained. By increasing the annealing temperature, composites with a progressive increase of the coercivity and of the remanent magnetization were produced. The remarkable effect of initial drying temperature on the particle size of cobalt ferrite suggested that a well-established silica network provided an effective confinement to the coarsening and aggregation of CoFe2O4 nanoparticles. r 2004 Elsevier B.V. All rights reserved. PACS: 75.01A; 89.02 Keywords: A1. Nanostructures; A1. X-ray diffraction; B2. Magnetic materials; B3. Infrared devices
1. Introduction
Corresponding author. Tel./fax: +86-7318-821648
E-mail address: [email protected] (Z.H. Chen).
Spinel ferrite nanoparticles have been intensively investigated in recent years because of their remarkable electrical and magnetic properties and wide practical applications in information storage
0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.07.064
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system, ferrofluid technology, magnetocaloric refrigeration and medical diagnostics. Among spinel ferrites, cobalt ferrite, CoFe2O4 is especially interesting because of its high cubic magnetocrystalline anisotropy, high coercivity and moderate saturation magnetization. Recently, cobalt ferrite nanoparticles was also known to be a photomagnetic material that shows an interesting lightinduced coercivity change [1,2]. A lot of synthetic strategies for preparing nanosized cobalt ferrite have been presented [3–10]. 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 to reduce particle agglomeration and this technique allows one to stabilize the particles and to study their formation reactions. In recent years, systems such as Fe2O3/SiO2 have been intensively studied, revealing a behavior different from that of bulk systems and serving as a model for the study of small particles [11–15]. The properties of these materials depend strongly on the particle size, the particle–matrix interactions, and the degree of dispersion of nanoparticles in the matrix. The ability to control these parameters by chemical modifications of the method of preparation is of crucial importance nowadays from both a fundamental and an industrial point of view [11]. In this paper, we report on the dispersion of CoFe2O4 nanoparticles in a silica matrix prepared using a sol–gel procedure and follow the structural and magnetic evolutions by IR spectroscopy, Xray diffraction, transmission electron micrographs, differential scanning calorimetry and vibrating sample magnetometer. Experiments were conducted to study the influence of the preparation variables, such as the initial drying temperature and thermal treatment of the gels on the structural and magnetic properties of cobalt ferrite embedded in a silica matrix.
2. Experimental procedure Nanocomposites of cobalt ferrite dispersed in a silica matrix were prepared by sol–gel process. The starting material tetraethylorthosilicate (TEOS)
was used as a precursor of silica, while ferric nitrate and cobalt nitrate were employed as the reactant iron and cobalt species. The TEOS: ethanol: H2O and Fe: Co molar ratios were controlled at 1:4:8 and 2:1, respectively. The Fe/Si molar ratio was initially adjusted to 16%. The sols were prepared by dissolving Fe and Co nitrates in deionized water, adding the alcoholic solution of Si(OC2H5)4 and a few 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 25, 110 and 150 1C, respectively, to obtain xerogels. The xerogels were placed in a furnace and thermally treated between 400 and 800 1C in air, with a heating rate of 10 1C/min, and kept for 2 h at each temperature. Thermal behavior of the samples was examined by differential scanning calorimetry (DSC), using a Netzsch-404PC thermal analysis system, in an air flux and with a heating rate of 10 1C/min. Mid-infrared (IR) spectra, from 4000 to 400 cm 1, were recorded using a Nicolet-510 spectrophotometer on pellets obtained dispersing the samples in KBr. The samples were characterized by powder X-ray diffraction (XRD) using a y 2y Siemens D500 diffractometer working with Cu Ka radiation, a graphite monochromator on the diffracted beam and a scintillation counter with pulse-height discriminator. Transmission electron microscopy (TEM) observations were carried out in bright-field (BF) mode, using a Hitachi-800 microscopy operating at 100 kV and with a JEOL CX200 microscope operating at 200 kV to study the morphological properties of the samples. TEM samples were prepared by dispersing the powders in acetone in an ultrasonic bath and dropping on a conventional carbon-coated copper grid. Mean particle size (m) and standard deviation (s) were calculated from TEM micrographs, at 100 000 magnification, counting around 100 particles. From these data, the degree of polydispersity, defined as s/m; was calculated. The hysteresis loops of the nanocomposites were collected at RT using a vibrating sample
ARTICLE IN PRESS X.H. Huang, Z.H. Chen / Journal of Crystal Growth 271 (2004) 287–293
magnetometer with a maximum applied magnetic field of 10 KOe.
3. Results and discussion It has been found that if nonreactive species are inserted into the initial solution (called ‘‘sol’’), the solid oxide network gradually groups around these species during the sol–gel reaction and finally gives a doped gel with dispersed species engaged in the pores of the gel [16]. This theory has been proved in different systems such as Fe/SiO2 [17], Fe2O3/ SiO2 [11–15,18], Cu/SiO2 [19], etc. The evolution of the xerogels with temperature was studied in detail in an attempt to understand the formation of cobalt ferrite nanocrystals in gels. Fig. 1 shows the X-ray diffraction pattern for the xerogel and the samples obtained after a 2 h heat treatment of the xerogel at various temperatures. The results demonstrate that the xerogel obtained after drying at 150 1C was amorphous and no nitrate crystallites were detectable, while the corresponding IR spectra (Fig. 3) of the xerogel showed a band at 1449.92 cm 1, which is associated with the anti-symmetric NO3 1 stretching vibration directly arising from the residual nitrate groups in the xerogel. These indicate that nitrate
Fig. 1. X-ray diffraction patterns of the samples dried at 150 1C and the samples obtained after calcination of the xerogel at various temperatures.
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salts were uniformly dispersed in the gel. In fact, when the nanoparticles’ precursors were not well dispersed, peaks due to the nitrate phase were visible in the XRD spectra. The decomposition temperature of iron nitrate is 125 1C, while according to the DSC, nitrate salts in the gel were decomposed at about 400 1C, which indicated the entry of Co and Fe ions into the silica network [20]. The XRD spectra of samples reveal that weak peaks assigned to CoFe2O4 appeared at 400 1C, suggesting that the particles of CoFe2O4 had been nucleated in the silica matrix. An increase in the intensity and a narrowing in the diffraction band were observed with increasing temperature. During the formation of CoFe2O4 nanocrystals, no traces of quartz, cristobalite or intermediate products, e.g., Fe2SiO4 or Co2SiO4 were formed even at temperatures as high as 800 1C, which can be accounted for the homogeneity in composition. Hysteresis loops at 77 K (Fig. 2) showed a drastic change in shape from the xerogel, which was paramagnetic to that of the sample heated at 400 1C, which was clearly superparamagnetic at this temperature. An increase in the initial susceptibility (slope M/H) with the temperature took place up to 800 1C indicating a gradual increase in the particle size. From 600 1 to 800 1C, the sample exhibited hysteresis at 77 K and both coercivity and remanent magnetization increased 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 [21]. The changes in the matrix microstructure and the pore environment before and after the heat treatment at various temperatures were followed by IR spectroscopy (Fig. 3). For the xerogel, the broad band centered at 1638.85 cm 1 is assigned to the H–O–H bending vibration of the absorbed water. Obviously, there were certain amounts of micropores that existed in the present xerogel, which must contain physically absorbed water molecules. The band at 1449.92 cm 1 is associated with the anti-symmetric NO3 stretching vibration directly arising from the residual nitrate groups in the xerogel. Strong absorptions at 1083.16, 802.03 and 464.01 cm 1 indicate the formation of a silica
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Fig. 2. Hysteresis loops at 77 K of the samples dried at 150 1C and the samples obtained after a 2 h heat treatment of the xerogel at various temperatures.
network [13]. The band at 860.09 cm 1 is assigned to Si–O–Fe. The presence of Si–O–Fe vibrations reflected some interaction between the highly isolated Fe3+ ions and the nearest silica matrix. The Si–O–Fe bond was also evident by the presence of another band at 586.57 cm 1, which is associated with the Fe–O stretching in Si–O–Fe bonds [22]. Another faint absorption band at 667.05 cm 1 is assigned to the stretching vibrating mode of the Co–O band [23]. The presence of Si–O–Fe and Co–O bonds sufficiently reflected the chemical nature of the transition metals involved in the xerogel. That is, these transition metal ions did 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 a treatment temperature of 400 1C, the intensities for the broad bands associated with the absorbed water were drastically weakened. The absorption at 1449.92 cm 1 disappeared, which was a consequence of complete decomposition of the nitrate species as confirmed by DSC analysis. The characteristic absorptions for the silica network remained nearly the same as those of the xerogel, while the bands at 568.14 and
861.72 cm 1 slightly increased in intensity, which could be ascribed to the enhanced interactions between the CoFe2O4 clusters and silica matrix. For the samples obtained at a treatment temperature of 600 1C, the absorption band at 1079.71 cm 1 for Si–O–Si of the SiO4 tetrahedron was further broadened, while that for the Si–O–Si or O–Si–O bending mode at 465.70 cm 1 was much weaker, which corresponded to a rearrangement process of the silica network [24]. It should be noted that a new band appeared at 847.53 cm 1. Correspondingly, the absorption of the Fe–O stretching band in Fe–O–Si bonds increased in intensity. These facts reflect the formation of CoFe2O4 clusters that was accompanied by the rearrangement of the 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 1C, the IR spectrum changed greatly compared with that for samples heat treated at 600 1C. The absorption at 1080.18 cm 1 for Si–O–Si of the SiO4 tetrahedron grew narrower and stronger, while the band at 860.21 cm 1 became very weak with the absence of the Si–O–Fe bonds. These
ARTICLE IN PRESS X.H. Huang, Z.H. Chen / Journal of Crystal Growth 271 (2004) 287–293
Fig. 3. IR spectra of the nanocomposite samples treated at different conditions.
results reflect the broken Si–O–Fe bonds, which coincided with the disappearance of Fe–O stretching band at 574.79 cm 1 for Si–O–Fe bonds. The
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band intensity of samples heat treated at 800 1C at 800.01 cm 1 for the Si–O–Si symmetric stretch increased, and the stronger absorption band at 457.94 cm 1 for the Si–O–Si or O–Si–O bending mode reappeared. The absorptions that appeared at 678.59 cm 1 became very strong, which can be associated with the characteristic Co–O stretching modes in the CoFe2O4 phase [25]. The breakage of the Fe–O–Si bonds in the interface between the clusters and matrix and the formation of CoFe2O4 clusters were probably a result of the transformation from FeO6 octahedron to FeO4 tetrahedron [26]. To further study the confinement of silica matrix on the growth of CoFe2O4 particles at the calcinations temperature, wet gels were dried at different temperatures, namely 50, 110 and 150 1C, respectively, before being calcined at 800 1C for 2 h. The choice of the three drying temperature was based on the observation in DSC study. According to the study, the formation of amorphous silica began at about 65 1C and was completed at temperatures at about 156 1C, above which cracking tended to occur due to the volume shrinkage during drying of the gel. The average sizes of the cobalt ferrite particles were evaluated from TEM micrographs. It is apparent that the average particles size of CoFe2O4 decreased with increasing the drying temperature from 50 to 150 1C (Fig. 4). It should be emphasized that the drying temperature not only affected the particle size but also the particle size distribution, as can be observed by the values of the polydispersity degree (s/m) obtained from TEM micrographs (Fig. 4). The s/mvalue of 70% obtained for the samples dried at 50 1C was drastically reduced to 14% for the samples dried at 150 1C. The electron micrographs of the samples calcined at 800 1C for 2 h following drying at 150 and 50 1C (Fig. 5) also evidently supported the above behavior. Such a strong influence of the drying temperature on the average particle size of CoFe2O4 can be explained as follows: When the gel precursor was dried at a temperature much lower than 156 1C, the formation of a silica network structure was not completed. The subsequent rapid increase in temperature (10 1C/min) during calcinations did
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ment and improved the uniformity of CoFe2O4 particles.
4. Conclusion
Fig. 4. Average diameters (m) and polydispersity degree (s/m) of the ferrite particles in the samples calcined at 800 1C for 2 h following drying at various temperatures.
The microstructural evolution of CoFe2O4 nanocrystals dispersed in amorphous silica was followed with IR, XRD, and VSM. Magnetic properties could be correlated with the microstructural evolution brought about by heat treatment of the nanocrystals dispersed in silica matrix at successively higher treatment temperatures. The remarkable effect of initial drying temperature on the particle size of nanophase cobalt ferrite suggested that a well-established silica network provided an effective confinement to the coarsening and aggregation of CoFe2O4 nanoparticles. References
Fig. 5. TEM micrographs of the samples calcined at 800 1C for 2 h following drying at (a) 150 1C and (b) 50 1C.
not favor the establishment of a silica network structure. In fact, a too rapid heating gave rise to the cracking of a partially completed network structure. Furthermore, the cobalt ferrite particles formed at a high temperature could also obstruct the further establishment of a silica network. Therefore, the lower the drying temperature, the less complete would the silica structure be. On the other hand, a well-established silica network provided a more effective confinement to the growth of CoFe2O4 particles [19]. As a result, the CoFe2O4 particles in the sample dried at 150 1C coarsened at a much slower rate than that in the sample dried at 50 1C. Therefore, increasing the drying temperature before heat treatment favored the formation of a complete silica network, which in turn hindered the growth of CoFe2O4 nanoparticles and resulted in an effective refine-
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