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Author's personal copy Materials Chemistry and Physics 123 (2010) 254–259
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Magnetic behavior of cobalt ferrite nanowires prepared by template-assisted technique Said. M. El-Sheikh, Farid A. Harraz ∗ , Mamhoud M. Hessien Advanced Materials Technology Department, Central Metallurgical Research & Development Institute (CMRDI), Al-Felezat Street, Tebbin, P.O. Box 87, Helwan, Cairo 11421, Egypt
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Article history: Received 13 October 2009 Received in revised form 29 March 2010 Accepted 4 April 2010 Keywords: Cobalt ferrite Nanowires Template synthesis Thermal decomposition Magnetic properties
a b s t r a c t Spinel cobalt ferrite nanowires were successfully prepared in mesoporous silica SBA-15 as a host matrix followed by slow thermal decomposition of the precursors inside the silica-based template. The formation and phase control of as-synthesized nanostructured cobalt ferrites were confirmed by X-ray diffraction (XRD) measurements at different annealing temperatures ranging from 500 to 1000 ◦ C. The one-dimensional spinel nanostructures were identified by recording the transmission electron microscopy (TEM) images after a selective removal of the silica template in aqueous solution of NaOH. The final product was also characterized using infrared spectroscopy (FT-IR) and vibration sample magnetometer (VSM). The presence of SBA-15 lowers the formation temperature of cobalt ferrite nanowires compared to the corresponding bulk material. The nanowires annealed up to 700 ◦ C exhibited magnetic behavior characteristic for soft magnetic materials, whereas samples annealed at temperature higher than 700 ◦ C revealed magnetic behavior characteristic for hard magnetic materials with rectangular form and large coercive field. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Fabrication of one-dimensional (1D) nanostructured materials leads to a unique behavior which has never been observed within traditional bulk materials [1]. Recently, 1D materials are considered ideal systems for the investigation of magnetic, catalytic, optical, electrical and chemical properties based on the highest anisotropic properties [2]. The ferromagnetic nanowires are one of particular category of 1D materials with high coercivities and ratio of remanences to saturation magnetization close to one [2]. Therefore, many researchers have reported different techniques for preparation of nanowires such as chemical vapor deposition [3], laser ablation [4], thermal decomposition [5] and arc discharge [6]. However, magnetic behaviors of such particular morphologies still need to be developed for magnetic recording application such as high-density digital recording disks. For instant, preparation of nanocomposites based on spinel ferrites dispersed in silica is one of the research topics that recently received increasing attention [7]. This is actually because ferrite nanoparticles usually have a strong tendency to aggregate, which makes it difficult to exploit their unique physical properties. Nanocomposites can effectively retain the nature of the nanocrystals by dispersing them in inorganic matrices because of the expected
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limited-particle agglomeration and narrow grain size distribution. Recently, mesoporous silica such as SBA-15 is commonly used as a host matrix for fabricating nanowires, due to its homogenous and well-ordered channel structures of pores (5–7 nm) with high surface area (≈900 m2 g−1 ) and thermal stability [2]. It was reported for example that SBA-15 is a better support for the preparation of various nanowires such as Pt [8] Ag [9] Fe2 O3 [10] and (Zn, Mn)S [11]. Although much work has been devoted to the synthesis of metal nanowires in mesoporous templates, the preparation of ferrite nanowires has been less explored. Selective separation of the nanowires from the templates is another important topic in the fabrication of well-ordered nanostructures based on wires as building blocks. Cobalt ferrite (CoFe2 O4 ) is one of the promising hard magnetic materials that posses high coercivity, moderate saturation magnetization, high electromagnetic performance and excellent chemical stability and mechanical hardness [12]. Several studies have been performed to prepare cobalt ferrite nanostructures mostly in the form of nanoparticles. For example, a series of highly ordered mesoporous materials (cobalt ferrite-SBA-15) has been prepared by Du et al. [13] by impregnation of cobalt salt, iron salt, and citric acid with as-synthesized SBA-15. Their finding indicated that such materials have highly ordered hexagonal mesoporous structure with open pore systems. Also the magnetic properties they obtained showed that these materials are ferromagnetic with very low saturation magnetization (4.3 emu g−1 ). In addition, the obtained nanopar-
Author's personal copy Said.M. El-Sheikh et al. / Materials Chemistry and Physics 123 (2010) 254–259
ticles inside the host matrix exhibited low crystallinity and used mainly for catalytic application depending on the high surface area of SBA-15. Therefore, to date there are challenges to control the size and morphology, crystallinity and unidirectional growth of nanowires. 1D cobalt ferrite has been prepared previously using carbon nanotubes [14], however no study has been reported for the preparation of 1D cobalt ferrite nanowires using as-synthesized mesoporous silica SBA-15 template. In addition, the magnetic characteristics of such 1D nanowires are expected to be unique and completely different from the properties generated from the nanoparticles reported in previous works. So, it still needs to be investigated for their further possible use in magnetic storage media. The aim of the present work is to fabricate cobalt ferrite 1D nanowires with controlled size and shape by employing a templateassisted technique; mesoporous silica SBA-15 template is utilized in this study. Different annealing temperatures ranging from 500 to 1000 ◦ C were used to identify the best formation temperature of nanowires. The magnetic properties of as-synthesized nanostructures, after extraction from host matrix, were also correlated with the controlling reaction parameters and annealing temperatures. 2. Experimental Hexagonally ordered SBA-15 was synthesized, following the procedure described in Ref. [15], in acidic media using poly(ethylene glycol)–poly(propylene glycol)–poly(ethylene glycol) triblock copolymer, EO20 PO70 EO20 (Pluronic 123) as a templating agent. In a typical preparation method, 8 g of Pluronic 123 was dissolved in 60 ml of water and 240 ml of 2N HCl solution at 35 ◦ C. After addition of 18.2 ml of TEOS the reaction mixture was stirred at 35 ◦ C for 24 h and then at 80 ◦ C for 48 h. The final solid product was separated by filtration, washed with water and dried in air at room temperature. The resulting material was evacuated at 350 ◦ C for 2 h and left under vacuum for 24 h before using. The CoFe2 O4 nanostructures were synthesized as follows: the iron and cobalt precursors 13.80 g of Fe(NO3 )3 ·9H2 O and 5.03 g of Co(NO3 )2 ·6H2 O were firstly dissolved in a 10 ml volume of de-ionized water with 2:1 molar ratio of Fe/Co. The volume of solution was identical to the pore volume of SBA-15 (1.05 cm3 g−1 ) as determined by analysis of surface area and pore volume of the sample. 1.05 ml of previous solution was added dropwise to a flask containing stirred solution of 1 g SBA-15 dispersed in (20 ml) of methanol. Then, the flask was closed and the resulting mixture was left stirring overnight for a complete impregnation process [16]. The flask was then opened with continuous stirring the mixture until a complete removal of the solvent. The filled material was then dried in air in an oven at 100 ◦ C for 2 h. The calcination process of the as-prepared samples at 500–1000 ◦ C for 5 h were performed separately in a tube furnace equipped with a tubular quartz reactor, to investigate the formation of cobalt ferrite nanostructures inside SBA-15 pores. The calcination temperatures were raised from room temperature with a rate of 0.5 ◦ C min−1 . Cobalt ferrite nanowires were released by dissolving as-prepared nanocomposites in 2.0 M NaOH for 8 h and collected by repeating centrifugation and washing with water and ethanol followed by drying at 100 ◦ C for 2 h. The CoFe2 O4 bulk sample, 900 ◦ C bulk, was prepared by dissolving 1.38 g of Fe(NO3 )3 ·9H2 O and 0.50 g of Co(NO3 )2 ·6H2 O in 1 ml de-ionized water and drying the solution at 80 ◦ C for 24 h, then calcined the remainder portion at 900 ◦ C for 5 h. Phase identification of the produced nanopowder was preformed at room temperature by X-ray diffraction (XRD, Bruker axs D8, Germany) with Cu K␣ radiation ( = 1.5406 Å) in 2 range from 20 to 80◦ . Crystallite size is automatically calculated by Scherer’s equation from XRD data. Morphology of the samples was investigated using transmission electron microscopy (TEM, JEOL-JEM-1230, Japan). Infrared (IR) spectra were recorded by FT-IR spectrometer using JASCO 3600. The magnetic properties of as-synthesized materials were measured by vibration sample magnetometer (VSM; 9600-1 LDJ, USA).
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Fig. 1. XRD patterns of cobalt ferrite nanostructures obtained at different annealing temperatures of 500, 600, 700, 800, 900 ◦ C. The silica-based template was selectively removed using 2 M NaOH. Top pattern corresponds to the as-formed cobalt ferrite/SBA-15 nanocomposite annealed at 900 ◦ C.
A partial crystallization could be observed in the samples treated at temperature range of 600–800 ◦ C. Whereas the diffraction pattern of the sample treated at 900 ◦ C can be indexed to the spinel cobalt ferrite phase (JCPDS card # 79-1744). The top XRD pattern depicted in Fig. 1 corresponds to the as-formed cobalt ferrite/mesoporous silica nanocomposite annealed at 900 ◦ C without extraction of host silica matrix. This sample exhibits a diffraction hump at about 2 = 16–28◦ , which belongs to the amorphous silica walls of SBA-15 and the others peaks are completely similar to the sample prepared at 900 ◦ C with extraction of host silica matrix, which assigned previously to the spinel cobalt ferrite phase. The resulting XRD patterns of 1000◦ as well as the CoFe2 O4 bulk sample annealed at 900 ◦ C are shown in Fig. 2. As one can observe, a further increase in annealing temperature to 1000 ◦ C led to increasing the crystallization rate of spinel cobalt ferrite, simultaneously with the appearance of ␣-Fe2 O3 (hematite) phase (JCPDS# 89-0598). Identical diffraction pattern of bulk sample is obtained with relatively lower degree of crystallinity. The formation of such iron oxide phase in 1000◦ sample may be related to the different mobilities of iron and cobalt ions under the present experimental conditions. This may result in a faster diffusion of iron ions to the mesoporous silica outer surface, which in turn may lead to the formation of iron oxide particles outside the porous structure. However, in case of balk sample annealed at 900 ◦ C, one may attribute the formation of iron oxide phase to the incomplete phase transformation to spinel CoFe2 O4 at such annealing temperature of
3. Results and discussion 3.1. Characterization of as-formed nanostructures 3.1.1. X-ray diffraction analysis (XRD) The XRD patterns of cobalt ferrite nanostructures obtained after removal of the silica templates in 2 M NaOH and annealed at different temperatures (500–900 ◦ C) are shown in Fig. 1. As can be seen, low crystalline phase is formed at low temperature of 500 ◦ C.
Fig. 2. XRD patterns of released cobalt ferrite nanostructure annealed at 1000 ◦ C. The as-formed bulk sample annealed at 900 ◦ C is also shown.
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the stretching vibration of M–O (M = Fe, Co) bands in CoFe2 O4 compound [19] compared with other samples prepared at lower temperatures. This may be related to the effect of temperature at 1000 ◦ C, at which a grain growth of cobalt ferrite inside the host matrix may take place followed by getting outside from the host matrix. In addition, a weak absorption band found at 475 cm−1 is likely related to the formation of Fe2 O3 particles, in a good agreement with the XRD data shown in Fig. 2. Fig. 3(b) shows FT-IR spectra measured for the ferrite material after a selective removal of SBA-15 by NaOH. As expected, the intensive absorption peaks representing SBA-15 matrix approximately disappeared. A new series of absorption bands is detected. Strong absorption bands around 500–700 cm−1 and centered at 581–593 cm−1 are assigned to the stretching vibration of M–O (M = Fe, Co) bands in CoFe2 O4 compounds [19]. The high frequency band found at 581–593 cm−1 is attributed to the tetrahedral complexes (Fe3+ –O2− ) and variation in the band position is due to the difference in the (Fe3+ –O2− ) distance for octahedral and tetrahedral complex. The presence of absorption band 849 cm−1 is evidence to the formation of Fe–Co alloy in the cobalt ferrite [20]. An OH starching vibration due to physisorbed water near 3458 cm−1 is observed with an OH deformation vibration near 1690 cm−1 due to surface hydroxyls [21,22].
Fig. 3. FT-IR spectra of (a) CoFe2 O4 /SBA-15 nanocomposite synthesized at different annealing temperatures and (b) CoFe2 O4 nanowires after a selective removal of SBA-15 by NaOH.
900 ◦ C. This means that the presence of silica-based template not only confined the wire formation but also lowered the formation temperature of CoFe2 O4 phase. The XRD results indicate consequently that a complete phase transformation to spinel phase of CoFe2 O4 inside SBA-15 nanopores is achieved above 800 ◦ C with the observation of increasing the degree of crystallinity with annealing temperature. Furthermore, the crystallite size calculated from Scherer’s equation of as-formed products indicated an increase in the sizes with increasing the annealing temperature; sizes of 8, 13.5 and 78.1 nm were obtained at 800, 900, and 1000 ◦ C, respectively. The crystallite size of the nanocomposite is 12.7 nm while the value obtained for the bulk sample is 72 nm. 3.1.2. Fourier transform infrared spectroscopy (FT-IR) Fig. 3(a) shows FT-IR spectra recorded in 400–4000 cm−1 range for as-synthesized CoFe2 O4 /SBA-15 nanocomposite prepared with different temperatures from 500 to 1000 ◦ C. Four intensive absorption bands are observed at 1230–1220, 1094–1084, 808–803 and 466 cm−1 which can be assigned to asymmetric and symmetric stretching vibration of Si–O–Si framework of SAB-15 template. The broad absorption bands detected at 3441–3427, 1639–1630 cm−1 and the weak band appeared at 981–966 cm−1 are related to Si–OH stretching and bending vibration, respectively. The observed weak absorption band at 592–565 cm−1 can be attributed to the stretching vibration of the tetragonal groups (Fe3+ –O2− ) [17]. By increasing the temperature, one can observe a shift in band position to a higher wavenumber, which is likely owing to the onset of crystallization [18]. At 1000 ◦ C strong absorption bands observed around 500–700 cm−1 and centered at 581–593 cm−1 are assigned to
3.1.3. Transmission electron microscopy (TEM) The removal of silica template was performed by treating the as-formed nanocomposite in 2 M NaOH solution in order to release the cobalt ferrite initially formed inside silica nanotemplate. TEM images of the silica free samples calcined at 800 and 900 ◦ C were taken and shown in Fig. 4. The image of ferrite sample calcined at 900 ◦ C is shown in Fig. 4(a). The same sample but with a magnified image and ED pattern is also illustrated in Fig. 4(b). The diameter of the produced nanowires in the broadest region is ∼8–10 nm, which is wider than the pore diameter of SBA-15 template (5–7 nm). This is probably related to the presence of micropores on silica walls of SBA-15 channels, which increased in diameter with increasing the calcination temperature [23]. During the annealing process, decomposition of the precursors extended from the mesoporous channels to microporous and led to the formation of 1D cobalt ferrite. This means that the present SBA-15 acts effectively as a template during the formation of ordered nanowires for samples treated at 900 ◦ C or above. This observation actually points out that the impregnation of SBA-15 matrix by cobalt and iron precursors followed by decomposition procedure at high temperatures has no significant effect on the pore structure of the host silica template. TEM image of the sample calcined at 800 ◦ C is shown in Fig. 4(c). The image shows only nanoparticles of cobalt ferrite. With increasing calcination temperature to 900 ◦ C the length of spinel cobalt ferrite nanowires extended to reach about 2–3 m in length (images (a) and (b)). The 1D nanowires of spinel cobalt ferrite grow and fill the nanochannels uniformly and the nanowires are apparently continuous by further heating from 800 to 900 ◦ C. This may be attributed to nanoparticles and nanowires aggregation and the grain growing until reaching the compact state at 900 ◦ C to form long nanowires with a careful control of the reaction condition. As shown earlier, the diameter of the nanowires of assynthesized cobalt ferrite at 1000 ◦ C was larger than the pore diameter of the template which is probably due to the existence of micropores on the amorphous silica walls. These micropores led to the formation of channeled cobalt ferrite nanowires at very high annealing temperature. Cobalt and iron precursors are not only filled in the mesopores but also are accessible into the micropores. With increasing annealing temperature the diameter of these micropores increased and the distribution of micropore size become wider [1,2]. The decomposition of the precursor extended accordingly from the mesopores channel to the micropores during
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Fig. 5. Magnetic hysteresis behavior of cobalt ferrite nanowires produced by the template synthesis route and annealed at different temperatures between 500 and 1000 ◦ C. Table 1 Magnetic properties, measured at room temperature, for Co Ferrite nanostrutures grown at different temperatures by the assistance of mesoporous silica template. Temperature (◦ C)
500 600 700 900 1000 Bulk sample, 900 ◦ C
Magnetic parameters Ms (emu/g)
Hc (Oe)
Mr (emu/g)
23.85 39.25 45.63 51.81 61.60 45.09
14.42 74.99 85.74 251.2 593 1088
0.05641 1.729 2.295 5.996 18.62 18.06
3.2. Magnetic properties The magnetization of the produced cobalt ferrite nanowires was measured at room temperature under an applied field of 15 kOe and the corresponding hysteresis loops were consequently obtained. Plot of magnetization (M) as a function of applied field (H) at different annealing temperatures is shown in Fig. 5 and the corresponding magnetic parameters are listed in Table 1. Fig. 6, on
Fig. 4. TEM images for free CoFe2 O4 nanostructures synthesized using silica-based template and annealed at: (a) 900 ◦ C, (b) magnified image for image (a) with SAED pattern and (c) 800 ◦ C.
the calcination process. Hence, the highly crystalline cobalt ferrite formed at 1000 ◦ C led to a diameter of nanowires being larger than the pore diameter of the template.
Fig. 6. Saturation magnetization and coercivity of as-formed nanowires as a function of annealing temperature.
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the other hand, illustrates the saturation magnetization and the coercivity as a function of annealing temperatures from 500 to 1000 ◦ C. In general, the cobalt ferrite nanowires annealed up to 700 ◦ C exhibit magnetic behavior characteristic for soft magnetic materials due to the deviation from rectangular form and there was very low coercivity (
