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Simplified synthesis of single-crystalline magnetic CoFe2O4 nanorods by a surfactant-assisted hydrothermal process
ARTICLE IN PRESS
Journal of Crystal Growth 270 (2004) 156–161
Simplified synthesis of single-crystalline magnetic CoFe2O4 nanorods by a surfactant-assisted hydrothermal process G.B. Ji, S.L. Tang*, S.K. Ren, F.M. Zhang, B.X. Gu, Y.W. Du National Laboratory of Solid State Microstructure and Department of Physics, Nanjing University, Nanjing 210093, People’s Republic of China Received 28 February 2004; accepted 4 June 2004 Available online 28 July 2004 Communicated by M. Kawasaki
Abstract A simple hydrothermal route, using cetyltrimethylammonium bromide (CTAB) as the surfactant, has been proposed for synthesizing single-crystalline spinal cobalt ferrite (CoFe2O4) nanorods. Transmission electron microscope (TEM) and scanning electron microscopy (SEM) images indicated that the final product consists of nanorods with a diameter of about 25 nm and length up to about 120 nm.Furthermore, high-resolution TEM (HRTEM) revealed a clear lattice structure of the [1 0 0] planes in the nanorods. The magnetic properties of the CoFe2O4 nanorods were characterized using vibrating sample magnetometer (VSM). The possible formation mechanism for the CTAB-assisted hydrothermal synthesis of CoFe2O4 nanorods has been discussed. r 2004 Elsevier B.V. All rights reserved. PACS: 75.50.Ss; 81.10.Dn; 81.16.Be; 81.20.Ka Keywords: A2. Hydrothermal crystal growth; B1. Nanomaterials; B2. Magnetic materials
1. Introduction Cobalt ferrite (CoFe2O4), with a partially inverse spinel structure, is one of the most important and most abundant magnetic materials. As a conventional magnetic material, with a Curie temperature (TC) around 793 K, CoFe2O4 is well *Corresponding author. Tel.: +86-25-3593817; fax: +86-2583595535. E-mail addresses: [email protected] (G.B. Ji), [email protected] (S.L. Tang).
known to have large magnetic anisotropy, moderate saturation magnetization, remarkable chemical stability and a mechanical hardness, which make it a good candidate for the recording media [1,2]. CoFe2O4 ultrafine powders [3–5] and films [6–8] have attracted considerable attention for their wide range of technological applications such as transformer cores, recording heads, antenna rods, memory, ferrofluids, biomedical application and sensors, etc. [9–11]. Over recent years, the chemical solution routes were successively emerging as effective, convenient,
0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.06.025
ARTICLE IN PRESS G.B. Ji et al. / Journal of Crystal Growth 270 (2004) 156–161
2. Experimental procedure The purities of all the reagents used are of analytical grade without further purification. In a typical procedure for the preparation of CoFe2O4 nanorods, 1 g of cationic surfactant cetyltrimethylammonium bromide (CTAB) was dissolved in 35 ml deionized water to form a transparent solution. Then ferric chloride hexahydrate (FeCl3.6H2O) of 1 g was added to the solution. After 10 min stirring, stoichiometric amount of CoCl2 was introduced into the mixed solution under vigorous stirring. Deionized water was added to make the solution for a total volume of 40 ml, and pH of the solution was adjusted to 11.0. Before being transferred to a Teflon-lined autoclave of 50.0 ml capacity, the solution mixture was pretreated under an ultrasonic water bath for 30– 40 min.Hydrothermal synthesis was carried out at 130 C for 15 h in an electric oven without shaking or stirring. Afterwards, the autoclave was allowed to cool to room temperature gradually. The black precipitate collected was washed with distilled water three times in an ultrasonic bath to remove any possible impurities. The solid was then heated at 80 C and dried under vacuum for 2 h. The crystallographic information of the asprepared samples was analyzed by power X-ray diffraction (XRD) with Cu Ka radiation
( at a scanning rate of 1 min 1 and (l=1.5406 A) the detective range from 15 to 70 . Morphologies and structures of the samples were investigated using a JSM-6700F scanning electron microscopy (SEM) and a HITACHI-2000 transmission electron microscopy (TEM). The X-ray photoelectron spectroscopy (XPS) data were obtained by a photoelectron spectrometer with Mg Ka source, a concentric hemispherical analyzer operating in a fixed analyzer transmission mode and a multichannel detector. The magnetic properties were measured using vibrating sample magnetometer (VSM, Lakeshore, Model 7300 series).
3. Results and discussion All the samples were obtained under hydrothermal conditions at 130 C. Fig. 1a shows the XRD pattern of the samples obtained after hydrothermal treatment in the absence of CTAB. No impurity phase such as FeCl3 and CoCl2 was detected from this pattern. The broadened nature of these diffraction peaks indicates that the grain sizes of the samples are on nanometer scale. The XRD patterns of the samples obtained in the presence of CTAB are given in Fig. 1b. All XRD peaks can be indexed to the spinal structure with lattice constant of ( which exhibits that the prepared CoFe2O4 a=8.39 A, nanorods are single phase. In addition, all peaks have
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Intensity (a.u.)
less energy demanding and less materials consuming synthetic techniques for material preparation [12–13]. However, the hydrothermal route is one of the most used ones, owing to its economics and the high degree of compositional control [14]. In addition, the hydrothermal synthesis requires neither extremely high processing temperature nor sophisticated processing. For example, ferrites can be prepared via the hydrothermal method at a temperature of about 150 C, whereas the solidstate method requires a temperature of 800 C [15]. Hydrothermal synthesis of several ferrites has been reported. However, there is little report on the synthesis of single-crystalline CoFe2O4 nanorods. We present here a simple hydrothermal route without a preformed template for the preparation of CoFe2O4 nanorods.
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Fig. 1. X-ray diffraction patterns of the prepared CoFe2O4 particles and nanorods via hydrothermal at 130 C treated for 15 h without CTAB (a) and with CTAB (b).
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Fig. 2. TEM images of the samples obtained in the absence of CTAB (a), in the presence of CTAB (b), magnified section of a bundle of the CoFe2O4 nanorods and the corresponding diffraction pattern of these nanorods (inset) (c) and HRTEM image of an individual nanorod (spacing=0.48 nm) (d).
Fig. 3. SEM images of the synthesized nanorods taken at different magnifications.
much higher intensities than those measured for the sample synthesized without the addition of CTAB, indicating higher crystallinity. Fig. 2 presents the representative TEM images of the samples prepared using the hydrothermal process. Fig. 2a is for the sample obtained in the absence of CTAB, CoFe2O4 particles with an irregular-shape revealed. The typical morphology of the samples obtained in the presence of CTAB after 15 h of hydrothermal treatment is shown in Fig. 2b, which clearly reveal the effect of CTAB on the nucleation and growth process of CoFe2O4. Interestingly, the morphology changed drastically
when the surfactant was applied.It has also been demonstrated that a large number of CoFe2O4 nanorods were formed, accompanied by the disappearance of the particles. The mean size is about 120 nm in length and 25 nm in diameter. The results clearly show that the surfactant CTAB is responsible for the formation of nanorods in large quantities. The electron diffraction (ED) pattern (shown in the inset of Fig. 2c) obtained along a typical individual nanorod confirms the single crystalline structure of the CoFe2O4 nanorods, and can be indexed as the [1 1 1], [3 1 1], and [2 2 0] zone axis of CoFe2O4. High-resolution TEM
ARTICLE IN PRESS G.B. Ji et al. / Journal of Crystal Growth 270 (2004) 156–161 16000
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(HRTEM) image shown in Fig. 2d further supports the single-crystalline nature of these nanorods. It is clear that the nanorod is structurally uniform with a lattice spacing of about 0.48 nm, which corresponds to the (1 1 1) lattice plane. Combined with the ED results it is confirmed that in the presence of the surfactant CTAB the CoFe2O4 nanorods grow along [1 0 0], one of the easy magnetic axes of CoFe2O4. The SEM images of the as-prepared samples shown in Fig. 3 have revealed the presence of a large quantity of nanorods with typical length ranging from 100 to 130 nm and diameter of about 25 nm. The overall chemical composition for the asprepared samples was determined by X-ray photoelectron spectroscopy (XPS). In the XPS analysis, the samples were exposed to the monochromic X-radiation and the properties of the inner-shell electrons were probed. Fig. 4(a–c) display the XPS spectra of the prepared samples. The XPS analysis indicates that the sample is composed of Fe, Co and oxygen, with corresponding binding energies of 780.35 eV (Co 2p3/2), 796.04 eV (Co 2p1/2), 710.55 eV (Fe 2p3/2), 724.29 eV (Fe 2p1/2), 529.75 eV (O 1 s), respectively, and the value states were proved to be Co2+, Fe3+ and O2 . The analysis of the Co 2p, Fe 2p and O 1 s peaks gave Co:Fe and Fe:O atomic ratios closely to 1:2 and 1:2, respectively, as expected for the stoichiometric composition of cobalt ferrite. XPS measurement results outlined above again support the result of XRD, shows that the cobalt ferrite has been synthesized. These observations suggest that the surfactant CTAB play a key role in controlling the nucleation and growth of the CoFe2O4 nanorods. For the reaction system in the presence of CTAB, the surface tension of solution is reduced due to the existence of surfactant, which lower the energy needed for the formation of a new phase. CTAB is an ionic compound, which ionizes completely in water [16]. The resultant CTA+ is a positively charged tetrahedron with a long hydrophobic tail, while the growth unit for CoFe2O4 crystal is considered to be Co–Fe–OH . Therefore, ionpairs between CTA+ and Co–Fe–OH could be formed due to electrostatic interaction, the Co– Fe–OH particles are negatively charged, and so
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Fig. 4. XPS spectra of Co 2p (a), Fe 2p (b) and O 1 s (c) core levels for the prepared cobalt ferrite nanorods.
CTA+ ions were adsorbed on the particle surface to form a film. The film is assembled and floatable. When the surfactant molecules leave, Co–Fe–OH will be carried away in the form of ion pairs, in the
ARTICLE IN PRESS G.B. Ji et al. / Journal of Crystal Growth 270 (2004) 156–161
160 0.10
a b
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of low magnetization is not yet clear. Further study is in progress to clarify the possible origin.
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4. Conclusions
0.02 0.00 -0.02 -0.04 -0.06 -0.08 -0.10 -10000 -8000 -6000 -4000 -2000
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H (Oe)
Fig. 5. Typical magnetic hysteresis loops measured at room temperature for the prepared particles (a) and nanorods (b), respectively.
crystallization process, surfactant molecules may serve as a growth controller, as well as an agglomeration inhibitor, by forming a covering film on the newly formed CoFe2O4 crystal. As reported previously [17], the adsorption of growth units on crystal surfaces strongly affects the growth rate and orientation of the crystals. The molecules in the film tend to be perpendicular to the absorbed surface, and the growth units would tend to face-land onto the growing interface. Since this kind of landing and dehydration will result in Co–O–Fe bonds, which make this landing mode predominant in competition with other ones such as vertex- and edge-landing. CoFe2O4 crystal should grow preferentially and the dehydration steps are repeated in the following procedures. Fig. 5 shows the typical hysteresis loops for the as-prepared particles and nanorods measured at room temperature, respectively. It shows that the CoFe2O4 particles and CoFe2O4 nanorods have magnetization at 10 kOe of 83 and 66 m g 1, respectively. It is of interest to note that the magnetization of CoFe2O4 nanoparticles is larger than that of the nanorods. The similar result was also obtained for Fe3O4 nanowires [18] and was explained in terms of the pinning of antiparallel magnetic domains due to the spatial confinement effect and the high shape anisotropy of the nanorods, preventing them from magnetizing in directions other than along their axes. The reason
In summary, a simple synthesis route, the CTAB-assisted hydrothermal method, has been used to fabricate single-crystalline phase CoFe2O4 nanorods. The prepared CoFe2O4 nanorods have diameters of 25 nm and lengths up to 120 nm.ED and HRTEM results indicate that the single-crystalline nanorods grow along [1 0 0]. The magnetization of the CoFe2O4 nanorods is lower than that of the CoFe2O4 nanoparticles, attributed to the spatial confinement effects and high shape anisotropy. Furthermore, the formation mechanism of the CoFe2O4 nanorods has been explained as the adsorption of CTA+, which affected the growth rate and orientation of crystals. It is expected that this method could also be a promising technique to fabricate onedimensional nanostructures of other magnetic materials.
Acknowledgements This work was supported by the Project No. 50171033 of National Natural Science Foundation of China and a national key project of fundamental research (973, No. G 1999064508)
References [1] E.S. Murdock, et al., IEEE Trans. Magn. 28 (1992) 3078. [2] S.N. Okuno, et al., Appl. Phys. 71 (1992) 5926. [3] V. Blaskov, V. Petkov, V. Rusanov, L.M. Martinez, B. Martinez, J.S. Munoz, M. Mikhov, J. Magn. Magn. Mater. 162 (1996) 331. [4] V. Pillai, D.O. Shah, J. Magn. Magn. Mater. 163 (1996) 243. [5] T. Meaz, S. Morup, J. Magn. Magn. Mater. 149 (1995) 14. [6] J.W.D. Martens, W.L. Peeters, H.M. Van Noort, M. Errnan, J. Phys. Chem. Solids 46 (1985) 411. [7] K. Suzuki, T. Namikawa, T. Yamazaki, Jpn. J. Appl. Phys. 27 (1988) 361.
ARTICLE IN PRESS G.B. Ji et al. / Journal of Crystal Growth 270 (2004) 156–161 [8] S.N. Okuno, S. Hashimoto, K. Inomata, J. Appl. Phys. 71 (1992) 5926. [9] F. Bodker, S. Morup, S. Linderoth, Phys. Rev. Lett. 72 (1994) 282. [10] M. Abe, T. Itoh, Y. Tamaura, Thin Solid Films 216 (1992) 155. [11] C.V. Gopal Reddy, S.V. Manorama, V.J. Rao, J. Mater. Sci. Lett. 19 (2000) 775. [12] F.F. Lange, Science 273 (1996) 903. [13] W.S. Sheldrich, M. Wachhold, Angew. Chem. Int. Edit. 36 (1997) 206.
161
[14] M. Rozman, M. Drofenik, J. Am. Chem. Soc. 78 (1995) 2449. [15] M.C. D’arrigo, C. Leonellic, G.C. Pellacani, J. Am. Ceram. Soc. 81 (1998) 3041. [16] L. Yan, Y.D. Li, Z.X. Deng, J. Zhuang, X. Sun, Int. J. Inorg. Mater. 3 (2001) 633. [17] T. Yoshida, M. Tochimoto, D. Schlettwein, D. Wohrle, T. Sugiura, H. Minoura, Chem. Mater. 11 (1999) 2657. [18] J. Wang, Q.W. Chen, C. Zeng, B.Y. Hou, Adv. Mater. 16 (2004) 137.
Journal of Crystal Growth 270 (2004) 156–161
Simplified synthesis of single-crystalline magnetic CoFe2O4 nanorods by a surfactant-assisted hydrothermal process G.B. Ji, S.L. Tang*, S.K. Ren, F.M. Zhang, B.X. Gu, Y.W. Du National Laboratory of Solid State Microstructure and Department of Physics, Nanjing University, Nanjing 210093, People’s Republic of China Received 28 February 2004; accepted 4 June 2004 Available online 28 July 2004 Communicated by M. Kawasaki
Abstract A simple hydrothermal route, using cetyltrimethylammonium bromide (CTAB) as the surfactant, has been proposed for synthesizing single-crystalline spinal cobalt ferrite (CoFe2O4) nanorods. Transmission electron microscope (TEM) and scanning electron microscopy (SEM) images indicated that the final product consists of nanorods with a diameter of about 25 nm and length up to about 120 nm.Furthermore, high-resolution TEM (HRTEM) revealed a clear lattice structure of the [1 0 0] planes in the nanorods. The magnetic properties of the CoFe2O4 nanorods were characterized using vibrating sample magnetometer (VSM). The possible formation mechanism for the CTAB-assisted hydrothermal synthesis of CoFe2O4 nanorods has been discussed. r 2004 Elsevier B.V. All rights reserved. PACS: 75.50.Ss; 81.10.Dn; 81.16.Be; 81.20.Ka Keywords: A2. Hydrothermal crystal growth; B1. Nanomaterials; B2. Magnetic materials
1. Introduction Cobalt ferrite (CoFe2O4), with a partially inverse spinel structure, is one of the most important and most abundant magnetic materials. As a conventional magnetic material, with a Curie temperature (TC) around 793 K, CoFe2O4 is well *Corresponding author. Tel.: +86-25-3593817; fax: +86-2583595535. E-mail addresses: [email protected] (G.B. Ji), [email protected] (S.L. Tang).
known to have large magnetic anisotropy, moderate saturation magnetization, remarkable chemical stability and a mechanical hardness, which make it a good candidate for the recording media [1,2]. CoFe2O4 ultrafine powders [3–5] and films [6–8] have attracted considerable attention for their wide range of technological applications such as transformer cores, recording heads, antenna rods, memory, ferrofluids, biomedical application and sensors, etc. [9–11]. Over recent years, the chemical solution routes were successively emerging as effective, convenient,
0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.06.025
ARTICLE IN PRESS G.B. Ji et al. / Journal of Crystal Growth 270 (2004) 156–161
2. Experimental procedure The purities of all the reagents used are of analytical grade without further purification. In a typical procedure for the preparation of CoFe2O4 nanorods, 1 g of cationic surfactant cetyltrimethylammonium bromide (CTAB) was dissolved in 35 ml deionized water to form a transparent solution. Then ferric chloride hexahydrate (FeCl3.6H2O) of 1 g was added to the solution. After 10 min stirring, stoichiometric amount of CoCl2 was introduced into the mixed solution under vigorous stirring. Deionized water was added to make the solution for a total volume of 40 ml, and pH of the solution was adjusted to 11.0. Before being transferred to a Teflon-lined autoclave of 50.0 ml capacity, the solution mixture was pretreated under an ultrasonic water bath for 30– 40 min.Hydrothermal synthesis was carried out at 130 C for 15 h in an electric oven without shaking or stirring. Afterwards, the autoclave was allowed to cool to room temperature gradually. The black precipitate collected was washed with distilled water three times in an ultrasonic bath to remove any possible impurities. The solid was then heated at 80 C and dried under vacuum for 2 h. The crystallographic information of the asprepared samples was analyzed by power X-ray diffraction (XRD) with Cu Ka radiation
( at a scanning rate of 1 min 1 and (l=1.5406 A) the detective range from 15 to 70 . Morphologies and structures of the samples were investigated using a JSM-6700F scanning electron microscopy (SEM) and a HITACHI-2000 transmission electron microscopy (TEM). The X-ray photoelectron spectroscopy (XPS) data were obtained by a photoelectron spectrometer with Mg Ka source, a concentric hemispherical analyzer operating in a fixed analyzer transmission mode and a multichannel detector. The magnetic properties were measured using vibrating sample magnetometer (VSM, Lakeshore, Model 7300 series).
3. Results and discussion All the samples were obtained under hydrothermal conditions at 130 C. Fig. 1a shows the XRD pattern of the samples obtained after hydrothermal treatment in the absence of CTAB. No impurity phase such as FeCl3 and CoCl2 was detected from this pattern. The broadened nature of these diffraction peaks indicates that the grain sizes of the samples are on nanometer scale. The XRD patterns of the samples obtained in the presence of CTAB are given in Fig. 1b. All XRD peaks can be indexed to the spinal structure with lattice constant of ( which exhibits that the prepared CoFe2O4 a=8.39 A, nanorods are single phase. In addition, all peaks have
(311)
11000 10000 9000 8000
Intensity (a.u.)
less energy demanding and less materials consuming synthetic techniques for material preparation [12–13]. However, the hydrothermal route is one of the most used ones, owing to its economics and the high degree of compositional control [14]. In addition, the hydrothermal synthesis requires neither extremely high processing temperature nor sophisticated processing. For example, ferrites can be prepared via the hydrothermal method at a temperature of about 150 C, whereas the solidstate method requires a temperature of 800 C [15]. Hydrothermal synthesis of several ferrites has been reported. However, there is little report on the synthesis of single-crystalline CoFe2O4 nanorods. We present here a simple hydrothermal route without a preformed template for the preparation of CoFe2O4 nanorods.
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(422)
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Fig. 1. X-ray diffraction patterns of the prepared CoFe2O4 particles and nanorods via hydrothermal at 130 C treated for 15 h without CTAB (a) and with CTAB (b).
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Fig. 2. TEM images of the samples obtained in the absence of CTAB (a), in the presence of CTAB (b), magnified section of a bundle of the CoFe2O4 nanorods and the corresponding diffraction pattern of these nanorods (inset) (c) and HRTEM image of an individual nanorod (spacing=0.48 nm) (d).
Fig. 3. SEM images of the synthesized nanorods taken at different magnifications.
much higher intensities than those measured for the sample synthesized without the addition of CTAB, indicating higher crystallinity. Fig. 2 presents the representative TEM images of the samples prepared using the hydrothermal process. Fig. 2a is for the sample obtained in the absence of CTAB, CoFe2O4 particles with an irregular-shape revealed. The typical morphology of the samples obtained in the presence of CTAB after 15 h of hydrothermal treatment is shown in Fig. 2b, which clearly reveal the effect of CTAB on the nucleation and growth process of CoFe2O4. Interestingly, the morphology changed drastically
when the surfactant was applied.It has also been demonstrated that a large number of CoFe2O4 nanorods were formed, accompanied by the disappearance of the particles. The mean size is about 120 nm in length and 25 nm in diameter. The results clearly show that the surfactant CTAB is responsible for the formation of nanorods in large quantities. The electron diffraction (ED) pattern (shown in the inset of Fig. 2c) obtained along a typical individual nanorod confirms the single crystalline structure of the CoFe2O4 nanorods, and can be indexed as the [1 1 1], [3 1 1], and [2 2 0] zone axis of CoFe2O4. High-resolution TEM
ARTICLE IN PRESS G.B. Ji et al. / Journal of Crystal Growth 270 (2004) 156–161 16000
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(HRTEM) image shown in Fig. 2d further supports the single-crystalline nature of these nanorods. It is clear that the nanorod is structurally uniform with a lattice spacing of about 0.48 nm, which corresponds to the (1 1 1) lattice plane. Combined with the ED results it is confirmed that in the presence of the surfactant CTAB the CoFe2O4 nanorods grow along [1 0 0], one of the easy magnetic axes of CoFe2O4. The SEM images of the as-prepared samples shown in Fig. 3 have revealed the presence of a large quantity of nanorods with typical length ranging from 100 to 130 nm and diameter of about 25 nm. The overall chemical composition for the asprepared samples was determined by X-ray photoelectron spectroscopy (XPS). In the XPS analysis, the samples were exposed to the monochromic X-radiation and the properties of the inner-shell electrons were probed. Fig. 4(a–c) display the XPS spectra of the prepared samples. The XPS analysis indicates that the sample is composed of Fe, Co and oxygen, with corresponding binding energies of 780.35 eV (Co 2p3/2), 796.04 eV (Co 2p1/2), 710.55 eV (Fe 2p3/2), 724.29 eV (Fe 2p1/2), 529.75 eV (O 1 s), respectively, and the value states were proved to be Co2+, Fe3+ and O2 . The analysis of the Co 2p, Fe 2p and O 1 s peaks gave Co:Fe and Fe:O atomic ratios closely to 1:2 and 1:2, respectively, as expected for the stoichiometric composition of cobalt ferrite. XPS measurement results outlined above again support the result of XRD, shows that the cobalt ferrite has been synthesized. These observations suggest that the surfactant CTAB play a key role in controlling the nucleation and growth of the CoFe2O4 nanorods. For the reaction system in the presence of CTAB, the surface tension of solution is reduced due to the existence of surfactant, which lower the energy needed for the formation of a new phase. CTAB is an ionic compound, which ionizes completely in water [16]. The resultant CTA+ is a positively charged tetrahedron with a long hydrophobic tail, while the growth unit for CoFe2O4 crystal is considered to be Co–Fe–OH . Therefore, ionpairs between CTA+ and Co–Fe–OH could be formed due to electrostatic interaction, the Co– Fe–OH particles are negatively charged, and so
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(c)
534
532
530
528
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Fig. 4. XPS spectra of Co 2p (a), Fe 2p (b) and O 1 s (c) core levels for the prepared cobalt ferrite nanorods.
CTA+ ions were adsorbed on the particle surface to form a film. The film is assembled and floatable. When the surfactant molecules leave, Co–Fe–OH will be carried away in the form of ion pairs, in the
ARTICLE IN PRESS G.B. Ji et al. / Journal of Crystal Growth 270 (2004) 156–161
160 0.10
a b
0.08
of low magnetization is not yet clear. Further study is in progress to clarify the possible origin.
0.06
M (emu/mg)
0.04
4. Conclusions
0.02 0.00 -0.02 -0.04 -0.06 -0.08 -0.10 -10000 -8000 -6000 -4000 -2000
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H (Oe)
Fig. 5. Typical magnetic hysteresis loops measured at room temperature for the prepared particles (a) and nanorods (b), respectively.
crystallization process, surfactant molecules may serve as a growth controller, as well as an agglomeration inhibitor, by forming a covering film on the newly formed CoFe2O4 crystal. As reported previously [17], the adsorption of growth units on crystal surfaces strongly affects the growth rate and orientation of the crystals. The molecules in the film tend to be perpendicular to the absorbed surface, and the growth units would tend to face-land onto the growing interface. Since this kind of landing and dehydration will result in Co–O–Fe bonds, which make this landing mode predominant in competition with other ones such as vertex- and edge-landing. CoFe2O4 crystal should grow preferentially and the dehydration steps are repeated in the following procedures. Fig. 5 shows the typical hysteresis loops for the as-prepared particles and nanorods measured at room temperature, respectively. It shows that the CoFe2O4 particles and CoFe2O4 nanorods have magnetization at 10 kOe of 83 and 66 m g 1, respectively. It is of interest to note that the magnetization of CoFe2O4 nanoparticles is larger than that of the nanorods. The similar result was also obtained for Fe3O4 nanowires [18] and was explained in terms of the pinning of antiparallel magnetic domains due to the spatial confinement effect and the high shape anisotropy of the nanorods, preventing them from magnetizing in directions other than along their axes. The reason
In summary, a simple synthesis route, the CTAB-assisted hydrothermal method, has been used to fabricate single-crystalline phase CoFe2O4 nanorods. The prepared CoFe2O4 nanorods have diameters of 25 nm and lengths up to 120 nm.ED and HRTEM results indicate that the single-crystalline nanorods grow along [1 0 0]. The magnetization of the CoFe2O4 nanorods is lower than that of the CoFe2O4 nanoparticles, attributed to the spatial confinement effects and high shape anisotropy. Furthermore, the formation mechanism of the CoFe2O4 nanorods has been explained as the adsorption of CTA+, which affected the growth rate and orientation of crystals. It is expected that this method could also be a promising technique to fabricate onedimensional nanostructures of other magnetic materials.
Acknowledgements This work was supported by the Project No. 50171033 of National Natural Science Foundation of China and a national key project of fundamental research (973, No. G 1999064508)
References [1] E.S. Murdock, et al., IEEE Trans. Magn. 28 (1992) 3078. [2] S.N. Okuno, et al., Appl. Phys. 71 (1992) 5926. [3] V. Blaskov, V. Petkov, V. Rusanov, L.M. Martinez, B. Martinez, J.S. Munoz, M. Mikhov, J. Magn. Magn. Mater. 162 (1996) 331. [4] V. Pillai, D.O. Shah, J. Magn. Magn. Mater. 163 (1996) 243. [5] T. Meaz, S. Morup, J. Magn. Magn. Mater. 149 (1995) 14. [6] J.W.D. Martens, W.L. Peeters, H.M. Van Noort, M. Errnan, J. Phys. Chem. Solids 46 (1985) 411. [7] K. Suzuki, T. Namikawa, T. Yamazaki, Jpn. J. Appl. Phys. 27 (1988) 361.
ARTICLE IN PRESS G.B. Ji et al. / Journal of Crystal Growth 270 (2004) 156–161 [8] S.N. Okuno, S. Hashimoto, K. Inomata, J. Appl. Phys. 71 (1992) 5926. [9] F. Bodker, S. Morup, S. Linderoth, Phys. Rev. Lett. 72 (1994) 282. [10] M. Abe, T. Itoh, Y. Tamaura, Thin Solid Films 216 (1992) 155. [11] C.V. Gopal Reddy, S.V. Manorama, V.J. Rao, J. Mater. Sci. Lett. 19 (2000) 775. [12] F.F. Lange, Science 273 (1996) 903. [13] W.S. Sheldrich, M. Wachhold, Angew. Chem. Int. Edit. 36 (1997) 206.
161
[14] M. Rozman, M. Drofenik, J. Am. Chem. Soc. 78 (1995) 2449. [15] M.C. D’arrigo, C. Leonellic, G.C. Pellacani, J. Am. Ceram. Soc. 81 (1998) 3041. [16] L. Yan, Y.D. Li, Z.X. Deng, J. Zhuang, X. Sun, Int. J. Inorg. Mater. 3 (2001) 633. [17] T. Yoshida, M. Tochimoto, D. Schlettwein, D. Wohrle, T. Sugiura, H. Minoura, Chem. Mater. 11 (1999) 2657. [18] J. Wang, Q.W. Chen, C. Zeng, B.Y. Hou, Adv. Mater. 16 (2004) 137.