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Synthesis and characterization of CoFe2O4 magnetic particles prepared by co-precipitation method: Effect of mixture procedures of initial solution
Journal of Alloys and Compounds 450 (2008) 532–539
Synthesis and characterization of CoFe2O4 magnetic particles prepared by co-precipitation method: Effect of mixture procedures of initial solution Jing Wang a,b,∗ , Tong Deng b , Yulong Lin a , Caiqin Yang a , Wenhong Zhan a a
School of Pharmaceutical Sciences, Hebei Medical University, Shijiazhuang 050017, PR China b Institute of Process Engineering, Chinese Academy of Sciences, Beijing, PR China
Received 21 November 2006; received in revised form 13 February 2007; accepted 20 February 2007 Available online 23 February 2007
Abstract CoFe2 O4 magnetic particles were prepared by co-precipitation method in 60 ◦ C homogeneous aqueous solution without any subsequent heat treatment. It was found that the mixing procedure and Fe2+ /Fe3+ ratio of initial solution were critical in the preparation of CoFe2 O4 particles in particle size, magnetization characters, uniformity in particle size and even cation distribution in spinel structure. Two different procedures were used to precipitate CoFe2 O4 magnetic particles. Evidenced by XRD, Mossbauer analyses and magnetization determination, particles in comparative uniformity average size were obtained in procedure A, denoted as normal pH regulation procedure, in which NaOH solution was dropped into the mixture solution of iron ions, and with the decreasing in Fe2+ /Fe3+ ratio of initial solution, the particle size decreased, which followed the same rule of diversification in saturation magnetization. Uniformity in particle size lowered when procedure B, referred to as reverse pH regulation procedure, where ferrous and cobalt ions were dropped into alkaline solution, was used to precipitate CoFe2 O4 . In both procedures, with the decreasing in Fe2+ /Fe3+ ratio of initial solution, the saturation magnetization decreased, while the magnetic coercivity decreased but increased sharply when Fe2+ /Fe3+ ratio of initial solution was 0. © 2007 Elsevier B.V. All rights reserved. Keywords: CoFe2 O4 ; Mossbauer; Average size; X-ray diffraction; Magnetic measurement
1. Introduction Ferrites are used in many technological applications that include permanent magnets, microwave absorbers, catalysts chemical, sensors and biomedical applications [1–6] because of their remarkable electrical and magnetic properties. Among spinel ferrites, cobalt ferrite, CoFe2 O4 is especially interesting because of their high cubic magnetocrystalline anisotropy, high coercivity and moderate saturation magnetization. Recently, cobalt ferrite nanoparticle was also known to be a photomagnetic material which shows an interesting light-induced coercivity change [7,8]. The size and magnetic properties of cobalt ferrite nanoparticles prepared by the co-precipitation method can be greatly varied depending on pH, salt concentration, temperature, stirring speed, counterion nature, etc. Kim et al. carried out systematic study for the effect of the precipitation temperature on the change
in XRD crystallinity and the average size of the nanoparticles in formation of CoFe2 O4 in homogeneous aqueous solution by the co-precipitation method [9]. As far as we know, there is no detailed study for the effect of mixing procedure and Fe2+ /Fe3+ ratio of initial solution on the size distribution, Mossbauer characteristic, saturation magnetization and magnetic coercivity in precipitation of CoFe2 O4 using co-precipitation method. In our previous study, the effect of mixing procedure of initial solution on the formation of Co-bearing and Ga-bearing magnetite were demonstrated and gave some interesting results [10]. So the aim of this work is to investigate mixing procedure and Fe2+ /Fe3+ ratio of initial solution effect on the size of CoFe2 O4 and characterize their magnetic properties, XRD and Mossbauer spectroscopy. 2. Experimental details 2.1. Chemicals and materials
∗
Corresponding author. Tel.: +86 311 86265627; fax: +86 311 86052053. E-mail address: [email protected] (J. Wang).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.02.099
All the chemicals used in this work were of reagent grade. They were commercially available and used as purchased without further purification. The stock
J. Wang et al. / Journal of Alloys and Compounds 450 (2008) 532–539
533
solutions were prepared by dissolving calculated amounts of the chemicals, including FeSO4 ·7H2 O, CoSO4 ·7H2 O and NaOH in distilled water. Fe3+ stock solution was prepared by dissolving Fe2 (SO4 )3 ·xH2 O in distilled water and determined the Fe3+ concentration by ICP.
2.2. Sample preparation Cobalt ferrite samples were prepared from the mixed Fe2+ , and Co2+ solutions, which were made from their stock solutions and had a constant initial total iron ions concentration and pre-determined amounts of Co2+ to total iron ions molar ratios 1:2. In both procedures, the ratio of Fe2+ /Fe3+ in initial solution were determined ranging from 100:0, 80:20, 50:50, 20:80 to 0:100, which were numbered as A1, B1; A2, B2; A3, B3; A4, B4 and A5, B5, respectively. 5 mol/L sodium hydroxide solution was used to regularize the pH of the mixed solutions while agitating, and in-line aerial oxidation was simultaneously provided. The pH values of the reaction systems were kept at between 11 and 11.5 by dropping 5 mol/L NaOH into the reaction suspension during the whole reaction period and the temperatures were controlled at 60 ◦ C. In procedure A, Sodium hydroxide solution was dropped to mixture of Fe2+ , Fe3+ and Co2+ until the pH values were up to 11, while in procedure B, the mixture of Fe2+ , Fe3+ and Co2+ were dropped into the sodium hydroxide solution whose quantity was equal to that used in the same stage in procedure A. The oxidation–reduction potentials (ORP) of the suspensions were in situ monitored using Ag/AgCl and Pt electrodes as the oxidation reaction progressed, and fine cobalt ferrite particles were precipitated. The oxidation reactions were supposed to be completed when ORP of the suspensions rose abruptly as Fe2+ was used to precipitate the CoFe2 O4 . The precipitates were filtrated, washed and dried.
Fig. 1. XRD patterns of precipitates with different starting Fe2+ /Fe3+ ratios synthesized by procedure A. A1 (100:0); A2 (80:20); A3 (50:50); A4 (20:80); A5 (0:100).
2.3. Characterization A Rigaku D/max-RB diffractometer (XRD) equipped with a Cu anode was used to determine the structure of the ferrites. The particle morphology was observed by scanning electron microscopic observation (Hitachi S-250MK3); The magnetization was determined using a vibrating sample magnetometer (American Lakeshore-7300) with a maximum applied field of 10 kOe at room temperature. The room temperature Mossbauer spectra (American Austi S-600) were obtained with a conventional constant acceleration transmission setup and a Co57 /Rh source (the isomer shifts are reported with respect to ␣-Fe). Samples were diluted with sucrose so as to obtain about 10 mg cm−1 Fe.
3. Results and discussion 3.1. The analysis of samples precipitated by procedure A 3.1.1. The XRD spectra and SEM micrograph The powder XRD patterns of samples precipitated with different Fe3+ /Fe2+ in initial solution by procedure A are shown in Fig. 1. Obviously, the sample, denotes as A1, shows the clear pattern corresponding to well known structure of CoFe2 O4 which has a cubic, spinel type lattice [11] and well resolved at (2 2 0), (3 1 1), (2 2 2), (4 0 0), (4 2 2), (5 1 1) and (4 4 0) reflections as in XRD patterns. With the increasing of Fe3+ concentration in starting solution, the intensity of XRD diffraction of the samples weaken and show broad, and the peaks of reflection of (2 2 0), (2 2 2), (4 2 2), (5 1 1) disappear in samples A4 and A5, but the (3 1 1), (4 0 0), (4 4 0) peaks are still clearly seen, especially (3 1 1). We have estimated the particle sizes of the samples using Debye–Scherrer equation according to the most intense peaks of reflection (3 1 1), and the results are shown in Fig. 2. As the Fe3+ in starting solution increases, the average particle size of samples decrease sharply from 130 nm of sample prepared with Fe2+ to 25 nm of sample prepared with Fe3+ . Otherwise, on the
Fig. 2. The crystalline size and magnetization of samples with different Fe3+ content in initial solution in procedure A.
morphological examination from SEM micrographs (Fig. 3) of the particles, it is suggested that the size of particles decreases with the increase of Fe3+ in the initial solution and it is observed that aggregation enhanced owing to the decreasing size of particles. The√unit cell parameters are estimated, respectively, using a = d h2 + k2 + l2 from the peak of the reflection (3 1 1) and listed in Table 1. The values of samples prepared with excessive Fe2+ in starting solution are close to the known of bulk CoFe2 O4 Table 1 The unit cell parameters of samples synthesized by procedure A Cell parameter
˚ a (A)
A1
A2
A3
A4
A5
8.414
8.386
8.406
8.381
8.316
534
J. Wang et al. / Journal of Alloys and Compounds 450 (2008) 532–539
Fig. 3. SEM micrographs of precipitates with different starting Fe2+ /Fe3+ ratios synthesized by procedure A. A1 (100:0); A2 (80:20); A3 (50:50); A4 (20:80); A5 (0:100).
J. Wang et al. / Journal of Alloys and Compounds 450 (2008) 532–539
Fig. 4. Magnetization loop of precipitates with different starting Fe2+ /Fe3+ ratios synthesized by procedure A. A1 (100:0); A2 (80:20); A3 (50:50); A4 (20:80); A5 (0:100).
535
Fig. 5. XRD patterns of precipitates with different starting Fe2+ /Fe3+ ratios synthesized by procedure B. B1 (100:0); B2 (80:20); B3 (50:50); B4 (20:80); B5 (0:100).
(8.395 ± 0.005). With the increasing of Fe3+ in starting solution, the intensity of peak of the reflection weakened. However, the ˚ of samples lattice parameter a, estimated as 8.381 and 8.316 A A4, A5, are small than those of samples A1, A2 and A3. It is indicated that there are more defect sites in the samples A4 and A5, which were prepared from excessive Fe3+ in starting solution. 3.1.2. Magnetization characterization The room temperature magnetizations for applied magnetic field are shown in Fig. 4. The value of magnetization of the sample A1 is much higher and tend to be saturated at a high field. The magnetization values and magnetic coercivity at 10 kOe are 65.693 emu/g and 986.66 Oe, which are close to the saturation magnetization and magnetic coercivity of bulk cobalt ferrite, known as 65 emu/g and 980 Oe [12]. As the Fe3+ in starting solution increased, the values of magnetization decrease sharply from 65.693 emu/g of sample A1 prepared with Fe2+ to 18.97 emu/g of sample A5 prepared with Fe3+ , and meanwhile, the values of magnetic coercivity decrease from 986.66 Oe of sample A1 to 126.89 Oe of sample A4, however, sharply increases to 594.12 Oe in sample A5 prepared from Fe3+ starting solution. 3.2. The analysis of samples precipitated by procedure B 3.2.1. The XRD spectra and SEM micrograph Fig. 5 shows the powder XRD patterns of the samples precipitated by procedure B with different Fe3+ concentration in initial solutions. Contrast to the samples in procedure A, the samples in procedure B keep to the same rules of diversification in intensity of XRD reflections (Fig. 5) with the different Fe3+ concentration. As can be seen from the average particle size (Fig. 6) and the unit cell parameters (Table 2) of samples, with the increasing of Fe3+ concentration in starting solution, the average particle size decrease sharply (B1 → B2 → B3 samples), but the average particle sizes don’t decrease further (B3 → B4 → B5 samples)
Fig. 6. The crystalline size and magnetization of samples with different Fe3+ content in initial solution in procedure B.
with the Fe3+ concentration increasing further. However, with the increasing of the Fe3+ concentration in starting solution and decreasing of average particle size of samples B1 → B2 → B3, the unit cell parameters have no distinct difference. The unit cell parameters decrease in the samples B4 and B5, which were prepared from excessive Fe3+ in starting solution, especially B5, while, it is no instinct difference in the average particle size of samples B3 → B4 → B5 (Fig. 6). It is concluded that increasing of Fe3+ concentration in starting solution within the certain range (such as A1 → A2 → A3 and B1 → B2 → B3 samples) makes no distinct difference in Table 2 The unit cell parameters of samples synthesized by procedure B Cell parameter
˚ a (A)
B1
B2
B3
B4
B5
8.410
8.420
8.423
8.380
8.320
536
J. Wang et al. / Journal of Alloys and Compounds 450 (2008) 532–539
Fig. 7. SEM micrographs of precipitates with different starting Fe2+ /Fe3+ ratios synthesized by procedure B. B1 (100:0); B2 (80:20); B3 (50:50); B4 (20:80); B5 (0:100).
J. Wang et al. / Journal of Alloys and Compounds 450 (2008) 532–539
unit cell parameters, but result in decreasing in the average particle size. The unit cell parameters decrease sharply when Fe3+ is used to precipitate the CoFe2 O4 . On the morphological examination from SEM micrographs (Fig. 7) of the particles prepared using procedure B, the decreasing of particle size and aggregation increasing of the particles are also observed with the increasing of Fe3+ in starting solution. 3.2.2. Magnetization characterization The room temperature magnetizations of samples prepared using procedure B for applied magnetic field are shown in Fig. 8. The magnetization values of sample B1 at 10 kOe is 62.583 emu/g, which is close to the saturation magnetization of bulk cobalt ferrite, known as 65 emu/g, but the magnetic coercivity, valued as 1162.5 Oe, is higher than that of bulk cobalt ferrite, known as 980 Oe. With the increasing of Fe3+ in starting solution, the average particle size decreases, but the saturation magnetizations have no distinct difference (Fig. 6), especially in samples B2, B3 and B4, whose saturation magnetizations are 51.217, 52.887, and 50.603 emu/g, respectively. Of sample B5, although the calculated average particle size is close to that of sample B4, saturation magnetizations decreases sharply, and the magnetic coercivity increases again. It is inferred that the particles size is not the only factor which determine the value of saturation magnetization. As for the abrupt increasing coercivities of samples A5 and B5, we gave the possible explanation as follows: In those two procedures, Since Fe(OH)2 , Fe(OH)3 and Co(OH)2 co-exist in the initial solutions of the sample Nos. 1–4, the mechanism of CoFe2 O4 formation must be different from that of samples No. 5 produced from the initial solution containing Fe(OH)3 and Co(OH)2 phase. So, the products were in different magnetic phase, the samples No. 5 in two procedures were in the hard magnetic phase due to their large coercivities. In conclusion, there are differences in the dependence of magnetization values on particle sizes between procedure A and
Fig. 8. Magnetization loop of precipitates with different starting Fe2+ /Fe3+ ratios synthesized by method B. B1 (100:0); B2 (80:20); B3 (50:50); B4 (20:80); B5 (0:100).
537
B. This difference might be explained by the difference of size distribution in two samples. If the samples B have a wider size distribution than the samples A, even though the average particle size is not much different from that of the sample A, the total magnetization will be dominated by the larger particles because the magnetization of the larger particles is much greater than the smaller particles [9]. The wider sizes distribution in samples B have different effect on the magnetization and average particle size. So the magnetic properties of the samples B are not reasonable seemingly. However, when all Fe was Fe3+ in starting solution, magnetization value of sample B5 prepared by procedure B was very small, which cannot give the reason now. 3.3. Mossbauer spectra analyses of A and B samples We have measured 57 Fe Mossbauer spectra of all samples prepared using different procedures in order to study the superparamagnetic nature, the size distribution and cation distribution. Fig. 9 shows the spectra of samples prepared using procedure A measured at room temperature. As shown in Fig. 9, the Mossbauer spectrum of ferrite CoFe2 O4 at room temperature show a spectrum of absorption with doublet of six peaks, reflecting normal Zeeman splittings of 57 Fe nuclei. In spectrums of sample A1 and A2, the room temperature 57 Fe Mossbauer spectras show magnetic hyperfine splitting structure of two overlapping six-line hyperfine patterns, corresponding to 57 Fe in B- and A-sites of the spinel lattice. Besides six strong peaks (sextet), reflecting normal Zeeman splits of 57 Fe nuclei, a superparamagnetic (SPM) quadrupole doublet is presented between the third and the fourth peaks, which would result from two factors. On one hand, this SPM behavior would depend on the non-magnetic phase including Fe content exist in the
Fig. 9. The Mossbauer spectra of samples precipitated by A procedure with different starting Fe2+ /Fe3+ ratios synthesized by procedure A1 (100:0); A2 (80:20); A4 (20:80); A5 (0:100).
538
J. Wang et al. / Journal of Alloys and Compounds 450 (2008) 532–539
Table 3 Values of hyperfine magnetic field (Hhf) and bond area (%)
Table 4 Values of hyperfine magnetic field (Hhf) and bond area (%).
Samples no.
Type
Hhf (kOe)
Area (%)
Samples no.
Type
Hhf (kOe)
Area (%)
A1
6(A) 6(B) 2
477.06 416.22
51.473 40.8461 7.681
B1
6(A) 6(B) 2
469.63 407.12
46.316 47.1952 6.489
A2
6(A) 6(B) 2
475.16 434.87
43.982 45.420 10.599
B2
6(A) 6(B) 2
469.90 405.19
46.976 45.8941 7.130
A4
6 2
449.22
37.752 62.648
B4
466.07 422.26
439.22
34.392 29.788 35.8202
A5
6 2
6(A) 6(B) 2
B5
6 2
447.71
45.558 54.442
28.607 71.393
“6” denotes sextet peaks and “2” denotes SPM doublet peaks.
“6” denotes sextet peaks and “2” denotes SPM doublet peaks.
samples, which indicates that a small quantity of non-magnetic ferric oxide were precipitated in the process of precipitation of CoFe2 O4 . On the other hand, this SPM behavior would result from the particle size effect of the nanosized crystallites, which infers that a small quantity of nanosized crystallites were precipitated. Furthermore, with the increasing of Fe3+ concentration in initial solution and the decreasing of particle size, as a result, the superparamagnetic (SPM) quadrupole doublet peaks were observed in the spectrums of sample A4 and A5, which resulted from the particle size effect of the nanosized crystallites, and the fraction of iron ions resulting from SPM increased (listed in Table 3). The cobalt ferrite spectrum was fitted using two sextets, one representing the A sites and the other the B sites. The fraction of iron ions in A- and B-sites were directly determined by the relative area ratios of the sub-spectra of corresponding A and B,
and the results were listed in Table 3. Theoretically, the cations would show inverse distribution in CoFe2 O4 spinel structure, and the cation distribution is [Fe3+ ]A [Fe3+ , Co2+ ]B , the fraction of iron ions in A- and B-sites were 50%, respectively. As can be seen from Table 3, the fraction of iron ions in A-sites of sample A1 approximated 51%, and B-sites 40%, which indicated that cations in sample A1 are in inverse distribution. With increasing of Fe3+ concentration in starting solution, the fraction of iron ions in A- sites decreased in sample A2, which inferred that the fraction of Co2+ occupied the A-site and induced the decreasing of hyperfine magnetic field (Hhf) of A-sites. Fig. 10 and Table 4 show the spectra and Mossbauer parameters of samples prepared using procedure B measured at room temperature, respectively. The spectra of B4 was a transition spectra from normal Zeeman splittings of 57 Fe necler to superparamagnetic (SPM) quadrupole doublet. Moreover, in procedure B, increasing of Fe3+ concentration in starting solution within the certain range (B1 → B2 → B3 samples) made no distinct difference in cation distribution in spinel structure deduced from the fraction of iron ions in A- and B-sites and hyperfine magnetic field (Hhf) of A-sites as a result (Table 4). 4. Conclusion
Fig. 10. The Mossbauer spectra of samples precipitated by B procedure with different starting Fe2+ /Fe3+ ratios synthesized by procedureB. B1 (100:0); B2 (80:20); B4 (20:80); B5 (0:100).
The feasibility of cobalt ferrites generation at 60 ◦ C by a controlled aerial oxidation or co-precipitation procedure of starting ferrous, ferric and cobalt ions solution was demonstrated. In two procedures, which were denoted as normal pH regulation procedure and reverse pH regulation procedure, with increasing of Fe3+ concentration in starting solution, all of particle size, saturation magnetization values and magnetic coercivity decreased, but when Fe3+ was used to precipitate the CoFe2 O4 , magnetic coercivity increased sharply. On samples prepared by normal pH regulation procedure, the dependence of saturation magnetization values on calculated average particle size showed linearity character, which inferred that the value of calculated average particle size approximated that of actual size. However, the same rule did not found on the samples prepared by reverse pH regulation procedure, which indicated that the value of calculated average particle size did
J. Wang et al. / Journal of Alloys and Compounds 450 (2008) 532–539
not represent the actual size of particles. In conclusion, the samples B have a wider size distribution than that of the samples A. Otherwise, the cation distribution in spinel structure were changed or modified by the different procedures and the ratio of Fe3+ /Fe2+ in starting solution. In normal pH regulation procedure, the concentration of Co2+ enter into A-sites increased with Fe3+ concentration in starting solution, but in reverse pH regulation procedure, the increasing of Fe3+ concentration in starting solution couldn’t affect the cation distribution of samples markedly. Acknowledgement The financial support for this work by the National Natural Science Fund of China (Grant No. 20076048) is greatly acknowledged.
539
References [1] J.D. Adam, S.V. Krishnaswamy, S.H. Talisa, K.C. Yoo, J. Magn. Mater. 83 (1990) 419. [2] J.G. Lee, J.Y. Park, Y.J. Oh, C.S. Kim, J. Appl. Phys. 84 (5) (1998) 2801. [3] S.C. Petrosius, R.S. Dradgo, V. Young, G.C. Grunewald, J. Am. Chem. Soc. 115 (1993) 6131. [4] A.K. Gupta, M. Gupta, Biomaterials 26 (2005) 3995. [5] X.H. Huang, Z.H. Chen, Scripta Mater. 54 (2006) 169. [6] C.C. Berry, A.S.G. Curtis, J. Phys. D Appl. Phys. 36 (2003) R198. [7] A.K. Giri, K. Pellerin, W. Pongsaksawad, M. Sorescu, S. Majetich, IEEE Trans. Magn. 36 (2000) 3029. [8] A.K. Giri, E.M. Kirkpatrick, P. Moongkhamklang, S.A. Majetich, Appl. Phys. Lett. 80 (2002) 2341. [9] Y.Il. Kim, D. Kim, C.S. Lee, Physica B 337 (2003) 42–51. [10] J. Wang, T. Deng, Y.J. Dai, J. Alloys Compd. 419 (2006) 155. [11] JCPDS database 22-1086. [12] G. Bate, in: E.P. Wohlfarth (Ed.), Ferromagnetic Materials, vol. 2, NorthHolland, Amsterdam, 1980, p. 431.
Synthesis and characterization of CoFe2O4 magnetic particles prepared by co-precipitation method: Effect of mixture procedures of initial solution Jing Wang a,b,∗ , Tong Deng b , Yulong Lin a , Caiqin Yang a , Wenhong Zhan a a
School of Pharmaceutical Sciences, Hebei Medical University, Shijiazhuang 050017, PR China b Institute of Process Engineering, Chinese Academy of Sciences, Beijing, PR China
Received 21 November 2006; received in revised form 13 February 2007; accepted 20 February 2007 Available online 23 February 2007
Abstract CoFe2 O4 magnetic particles were prepared by co-precipitation method in 60 ◦ C homogeneous aqueous solution without any subsequent heat treatment. It was found that the mixing procedure and Fe2+ /Fe3+ ratio of initial solution were critical in the preparation of CoFe2 O4 particles in particle size, magnetization characters, uniformity in particle size and even cation distribution in spinel structure. Two different procedures were used to precipitate CoFe2 O4 magnetic particles. Evidenced by XRD, Mossbauer analyses and magnetization determination, particles in comparative uniformity average size were obtained in procedure A, denoted as normal pH regulation procedure, in which NaOH solution was dropped into the mixture solution of iron ions, and with the decreasing in Fe2+ /Fe3+ ratio of initial solution, the particle size decreased, which followed the same rule of diversification in saturation magnetization. Uniformity in particle size lowered when procedure B, referred to as reverse pH regulation procedure, where ferrous and cobalt ions were dropped into alkaline solution, was used to precipitate CoFe2 O4 . In both procedures, with the decreasing in Fe2+ /Fe3+ ratio of initial solution, the saturation magnetization decreased, while the magnetic coercivity decreased but increased sharply when Fe2+ /Fe3+ ratio of initial solution was 0. © 2007 Elsevier B.V. All rights reserved. Keywords: CoFe2 O4 ; Mossbauer; Average size; X-ray diffraction; Magnetic measurement
1. Introduction Ferrites are used in many technological applications that include permanent magnets, microwave absorbers, catalysts chemical, sensors and biomedical applications [1–6] because of their remarkable electrical and magnetic properties. Among spinel ferrites, cobalt ferrite, CoFe2 O4 is especially interesting because of their high cubic magnetocrystalline anisotropy, high coercivity and moderate saturation magnetization. Recently, cobalt ferrite nanoparticle was also known to be a photomagnetic material which shows an interesting light-induced coercivity change [7,8]. The size and magnetic properties of cobalt ferrite nanoparticles prepared by the co-precipitation method can be greatly varied depending on pH, salt concentration, temperature, stirring speed, counterion nature, etc. Kim et al. carried out systematic study for the effect of the precipitation temperature on the change
in XRD crystallinity and the average size of the nanoparticles in formation of CoFe2 O4 in homogeneous aqueous solution by the co-precipitation method [9]. As far as we know, there is no detailed study for the effect of mixing procedure and Fe2+ /Fe3+ ratio of initial solution on the size distribution, Mossbauer characteristic, saturation magnetization and magnetic coercivity in precipitation of CoFe2 O4 using co-precipitation method. In our previous study, the effect of mixing procedure of initial solution on the formation of Co-bearing and Ga-bearing magnetite were demonstrated and gave some interesting results [10]. So the aim of this work is to investigate mixing procedure and Fe2+ /Fe3+ ratio of initial solution effect on the size of CoFe2 O4 and characterize their magnetic properties, XRD and Mossbauer spectroscopy. 2. Experimental details 2.1. Chemicals and materials
∗
Corresponding author. Tel.: +86 311 86265627; fax: +86 311 86052053. E-mail address: [email protected] (J. Wang).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.02.099
All the chemicals used in this work were of reagent grade. They were commercially available and used as purchased without further purification. The stock
J. Wang et al. / Journal of Alloys and Compounds 450 (2008) 532–539
533
solutions were prepared by dissolving calculated amounts of the chemicals, including FeSO4 ·7H2 O, CoSO4 ·7H2 O and NaOH in distilled water. Fe3+ stock solution was prepared by dissolving Fe2 (SO4 )3 ·xH2 O in distilled water and determined the Fe3+ concentration by ICP.
2.2. Sample preparation Cobalt ferrite samples were prepared from the mixed Fe2+ , and Co2+ solutions, which were made from their stock solutions and had a constant initial total iron ions concentration and pre-determined amounts of Co2+ to total iron ions molar ratios 1:2. In both procedures, the ratio of Fe2+ /Fe3+ in initial solution were determined ranging from 100:0, 80:20, 50:50, 20:80 to 0:100, which were numbered as A1, B1; A2, B2; A3, B3; A4, B4 and A5, B5, respectively. 5 mol/L sodium hydroxide solution was used to regularize the pH of the mixed solutions while agitating, and in-line aerial oxidation was simultaneously provided. The pH values of the reaction systems were kept at between 11 and 11.5 by dropping 5 mol/L NaOH into the reaction suspension during the whole reaction period and the temperatures were controlled at 60 ◦ C. In procedure A, Sodium hydroxide solution was dropped to mixture of Fe2+ , Fe3+ and Co2+ until the pH values were up to 11, while in procedure B, the mixture of Fe2+ , Fe3+ and Co2+ were dropped into the sodium hydroxide solution whose quantity was equal to that used in the same stage in procedure A. The oxidation–reduction potentials (ORP) of the suspensions were in situ monitored using Ag/AgCl and Pt electrodes as the oxidation reaction progressed, and fine cobalt ferrite particles were precipitated. The oxidation reactions were supposed to be completed when ORP of the suspensions rose abruptly as Fe2+ was used to precipitate the CoFe2 O4 . The precipitates were filtrated, washed and dried.
Fig. 1. XRD patterns of precipitates with different starting Fe2+ /Fe3+ ratios synthesized by procedure A. A1 (100:0); A2 (80:20); A3 (50:50); A4 (20:80); A5 (0:100).
2.3. Characterization A Rigaku D/max-RB diffractometer (XRD) equipped with a Cu anode was used to determine the structure of the ferrites. The particle morphology was observed by scanning electron microscopic observation (Hitachi S-250MK3); The magnetization was determined using a vibrating sample magnetometer (American Lakeshore-7300) with a maximum applied field of 10 kOe at room temperature. The room temperature Mossbauer spectra (American Austi S-600) were obtained with a conventional constant acceleration transmission setup and a Co57 /Rh source (the isomer shifts are reported with respect to ␣-Fe). Samples were diluted with sucrose so as to obtain about 10 mg cm−1 Fe.
3. Results and discussion 3.1. The analysis of samples precipitated by procedure A 3.1.1. The XRD spectra and SEM micrograph The powder XRD patterns of samples precipitated with different Fe3+ /Fe2+ in initial solution by procedure A are shown in Fig. 1. Obviously, the sample, denotes as A1, shows the clear pattern corresponding to well known structure of CoFe2 O4 which has a cubic, spinel type lattice [11] and well resolved at (2 2 0), (3 1 1), (2 2 2), (4 0 0), (4 2 2), (5 1 1) and (4 4 0) reflections as in XRD patterns. With the increasing of Fe3+ concentration in starting solution, the intensity of XRD diffraction of the samples weaken and show broad, and the peaks of reflection of (2 2 0), (2 2 2), (4 2 2), (5 1 1) disappear in samples A4 and A5, but the (3 1 1), (4 0 0), (4 4 0) peaks are still clearly seen, especially (3 1 1). We have estimated the particle sizes of the samples using Debye–Scherrer equation according to the most intense peaks of reflection (3 1 1), and the results are shown in Fig. 2. As the Fe3+ in starting solution increases, the average particle size of samples decrease sharply from 130 nm of sample prepared with Fe2+ to 25 nm of sample prepared with Fe3+ . Otherwise, on the
Fig. 2. The crystalline size and magnetization of samples with different Fe3+ content in initial solution in procedure A.
morphological examination from SEM micrographs (Fig. 3) of the particles, it is suggested that the size of particles decreases with the increase of Fe3+ in the initial solution and it is observed that aggregation enhanced owing to the decreasing size of particles. The√unit cell parameters are estimated, respectively, using a = d h2 + k2 + l2 from the peak of the reflection (3 1 1) and listed in Table 1. The values of samples prepared with excessive Fe2+ in starting solution are close to the known of bulk CoFe2 O4 Table 1 The unit cell parameters of samples synthesized by procedure A Cell parameter
˚ a (A)
A1
A2
A3
A4
A5
8.414
8.386
8.406
8.381
8.316
534
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Fig. 3. SEM micrographs of precipitates with different starting Fe2+ /Fe3+ ratios synthesized by procedure A. A1 (100:0); A2 (80:20); A3 (50:50); A4 (20:80); A5 (0:100).
J. Wang et al. / Journal of Alloys and Compounds 450 (2008) 532–539
Fig. 4. Magnetization loop of precipitates with different starting Fe2+ /Fe3+ ratios synthesized by procedure A. A1 (100:0); A2 (80:20); A3 (50:50); A4 (20:80); A5 (0:100).
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Fig. 5. XRD patterns of precipitates with different starting Fe2+ /Fe3+ ratios synthesized by procedure B. B1 (100:0); B2 (80:20); B3 (50:50); B4 (20:80); B5 (0:100).
(8.395 ± 0.005). With the increasing of Fe3+ in starting solution, the intensity of peak of the reflection weakened. However, the ˚ of samples lattice parameter a, estimated as 8.381 and 8.316 A A4, A5, are small than those of samples A1, A2 and A3. It is indicated that there are more defect sites in the samples A4 and A5, which were prepared from excessive Fe3+ in starting solution. 3.1.2. Magnetization characterization The room temperature magnetizations for applied magnetic field are shown in Fig. 4. The value of magnetization of the sample A1 is much higher and tend to be saturated at a high field. The magnetization values and magnetic coercivity at 10 kOe are 65.693 emu/g and 986.66 Oe, which are close to the saturation magnetization and magnetic coercivity of bulk cobalt ferrite, known as 65 emu/g and 980 Oe [12]. As the Fe3+ in starting solution increased, the values of magnetization decrease sharply from 65.693 emu/g of sample A1 prepared with Fe2+ to 18.97 emu/g of sample A5 prepared with Fe3+ , and meanwhile, the values of magnetic coercivity decrease from 986.66 Oe of sample A1 to 126.89 Oe of sample A4, however, sharply increases to 594.12 Oe in sample A5 prepared from Fe3+ starting solution. 3.2. The analysis of samples precipitated by procedure B 3.2.1. The XRD spectra and SEM micrograph Fig. 5 shows the powder XRD patterns of the samples precipitated by procedure B with different Fe3+ concentration in initial solutions. Contrast to the samples in procedure A, the samples in procedure B keep to the same rules of diversification in intensity of XRD reflections (Fig. 5) with the different Fe3+ concentration. As can be seen from the average particle size (Fig. 6) and the unit cell parameters (Table 2) of samples, with the increasing of Fe3+ concentration in starting solution, the average particle size decrease sharply (B1 → B2 → B3 samples), but the average particle sizes don’t decrease further (B3 → B4 → B5 samples)
Fig. 6. The crystalline size and magnetization of samples with different Fe3+ content in initial solution in procedure B.
with the Fe3+ concentration increasing further. However, with the increasing of the Fe3+ concentration in starting solution and decreasing of average particle size of samples B1 → B2 → B3, the unit cell parameters have no distinct difference. The unit cell parameters decrease in the samples B4 and B5, which were prepared from excessive Fe3+ in starting solution, especially B5, while, it is no instinct difference in the average particle size of samples B3 → B4 → B5 (Fig. 6). It is concluded that increasing of Fe3+ concentration in starting solution within the certain range (such as A1 → A2 → A3 and B1 → B2 → B3 samples) makes no distinct difference in Table 2 The unit cell parameters of samples synthesized by procedure B Cell parameter
˚ a (A)
B1
B2
B3
B4
B5
8.410
8.420
8.423
8.380
8.320
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Fig. 7. SEM micrographs of precipitates with different starting Fe2+ /Fe3+ ratios synthesized by procedure B. B1 (100:0); B2 (80:20); B3 (50:50); B4 (20:80); B5 (0:100).
J. Wang et al. / Journal of Alloys and Compounds 450 (2008) 532–539
unit cell parameters, but result in decreasing in the average particle size. The unit cell parameters decrease sharply when Fe3+ is used to precipitate the CoFe2 O4 . On the morphological examination from SEM micrographs (Fig. 7) of the particles prepared using procedure B, the decreasing of particle size and aggregation increasing of the particles are also observed with the increasing of Fe3+ in starting solution. 3.2.2. Magnetization characterization The room temperature magnetizations of samples prepared using procedure B for applied magnetic field are shown in Fig. 8. The magnetization values of sample B1 at 10 kOe is 62.583 emu/g, which is close to the saturation magnetization of bulk cobalt ferrite, known as 65 emu/g, but the magnetic coercivity, valued as 1162.5 Oe, is higher than that of bulk cobalt ferrite, known as 980 Oe. With the increasing of Fe3+ in starting solution, the average particle size decreases, but the saturation magnetizations have no distinct difference (Fig. 6), especially in samples B2, B3 and B4, whose saturation magnetizations are 51.217, 52.887, and 50.603 emu/g, respectively. Of sample B5, although the calculated average particle size is close to that of sample B4, saturation magnetizations decreases sharply, and the magnetic coercivity increases again. It is inferred that the particles size is not the only factor which determine the value of saturation magnetization. As for the abrupt increasing coercivities of samples A5 and B5, we gave the possible explanation as follows: In those two procedures, Since Fe(OH)2 , Fe(OH)3 and Co(OH)2 co-exist in the initial solutions of the sample Nos. 1–4, the mechanism of CoFe2 O4 formation must be different from that of samples No. 5 produced from the initial solution containing Fe(OH)3 and Co(OH)2 phase. So, the products were in different magnetic phase, the samples No. 5 in two procedures were in the hard magnetic phase due to their large coercivities. In conclusion, there are differences in the dependence of magnetization values on particle sizes between procedure A and
Fig. 8. Magnetization loop of precipitates with different starting Fe2+ /Fe3+ ratios synthesized by method B. B1 (100:0); B2 (80:20); B3 (50:50); B4 (20:80); B5 (0:100).
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B. This difference might be explained by the difference of size distribution in two samples. If the samples B have a wider size distribution than the samples A, even though the average particle size is not much different from that of the sample A, the total magnetization will be dominated by the larger particles because the magnetization of the larger particles is much greater than the smaller particles [9]. The wider sizes distribution in samples B have different effect on the magnetization and average particle size. So the magnetic properties of the samples B are not reasonable seemingly. However, when all Fe was Fe3+ in starting solution, magnetization value of sample B5 prepared by procedure B was very small, which cannot give the reason now. 3.3. Mossbauer spectra analyses of A and B samples We have measured 57 Fe Mossbauer spectra of all samples prepared using different procedures in order to study the superparamagnetic nature, the size distribution and cation distribution. Fig. 9 shows the spectra of samples prepared using procedure A measured at room temperature. As shown in Fig. 9, the Mossbauer spectrum of ferrite CoFe2 O4 at room temperature show a spectrum of absorption with doublet of six peaks, reflecting normal Zeeman splittings of 57 Fe nuclei. In spectrums of sample A1 and A2, the room temperature 57 Fe Mossbauer spectras show magnetic hyperfine splitting structure of two overlapping six-line hyperfine patterns, corresponding to 57 Fe in B- and A-sites of the spinel lattice. Besides six strong peaks (sextet), reflecting normal Zeeman splits of 57 Fe nuclei, a superparamagnetic (SPM) quadrupole doublet is presented between the third and the fourth peaks, which would result from two factors. On one hand, this SPM behavior would depend on the non-magnetic phase including Fe content exist in the
Fig. 9. The Mossbauer spectra of samples precipitated by A procedure with different starting Fe2+ /Fe3+ ratios synthesized by procedure A1 (100:0); A2 (80:20); A4 (20:80); A5 (0:100).
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Table 3 Values of hyperfine magnetic field (Hhf) and bond area (%)
Table 4 Values of hyperfine magnetic field (Hhf) and bond area (%).
Samples no.
Type
Hhf (kOe)
Area (%)
Samples no.
Type
Hhf (kOe)
Area (%)
A1
6(A) 6(B) 2
477.06 416.22
51.473 40.8461 7.681
B1
6(A) 6(B) 2
469.63 407.12
46.316 47.1952 6.489
A2
6(A) 6(B) 2
475.16 434.87
43.982 45.420 10.599
B2
6(A) 6(B) 2
469.90 405.19
46.976 45.8941 7.130
A4
6 2
449.22
37.752 62.648
B4
466.07 422.26
439.22
34.392 29.788 35.8202
A5
6 2
6(A) 6(B) 2
B5
6 2
447.71
45.558 54.442
28.607 71.393
“6” denotes sextet peaks and “2” denotes SPM doublet peaks.
“6” denotes sextet peaks and “2” denotes SPM doublet peaks.
samples, which indicates that a small quantity of non-magnetic ferric oxide were precipitated in the process of precipitation of CoFe2 O4 . On the other hand, this SPM behavior would result from the particle size effect of the nanosized crystallites, which infers that a small quantity of nanosized crystallites were precipitated. Furthermore, with the increasing of Fe3+ concentration in initial solution and the decreasing of particle size, as a result, the superparamagnetic (SPM) quadrupole doublet peaks were observed in the spectrums of sample A4 and A5, which resulted from the particle size effect of the nanosized crystallites, and the fraction of iron ions resulting from SPM increased (listed in Table 3). The cobalt ferrite spectrum was fitted using two sextets, one representing the A sites and the other the B sites. The fraction of iron ions in A- and B-sites were directly determined by the relative area ratios of the sub-spectra of corresponding A and B,
and the results were listed in Table 3. Theoretically, the cations would show inverse distribution in CoFe2 O4 spinel structure, and the cation distribution is [Fe3+ ]A [Fe3+ , Co2+ ]B , the fraction of iron ions in A- and B-sites were 50%, respectively. As can be seen from Table 3, the fraction of iron ions in A-sites of sample A1 approximated 51%, and B-sites 40%, which indicated that cations in sample A1 are in inverse distribution. With increasing of Fe3+ concentration in starting solution, the fraction of iron ions in A- sites decreased in sample A2, which inferred that the fraction of Co2+ occupied the A-site and induced the decreasing of hyperfine magnetic field (Hhf) of A-sites. Fig. 10 and Table 4 show the spectra and Mossbauer parameters of samples prepared using procedure B measured at room temperature, respectively. The spectra of B4 was a transition spectra from normal Zeeman splittings of 57 Fe necler to superparamagnetic (SPM) quadrupole doublet. Moreover, in procedure B, increasing of Fe3+ concentration in starting solution within the certain range (B1 → B2 → B3 samples) made no distinct difference in cation distribution in spinel structure deduced from the fraction of iron ions in A- and B-sites and hyperfine magnetic field (Hhf) of A-sites as a result (Table 4). 4. Conclusion
Fig. 10. The Mossbauer spectra of samples precipitated by B procedure with different starting Fe2+ /Fe3+ ratios synthesized by procedureB. B1 (100:0); B2 (80:20); B4 (20:80); B5 (0:100).
The feasibility of cobalt ferrites generation at 60 ◦ C by a controlled aerial oxidation or co-precipitation procedure of starting ferrous, ferric and cobalt ions solution was demonstrated. In two procedures, which were denoted as normal pH regulation procedure and reverse pH regulation procedure, with increasing of Fe3+ concentration in starting solution, all of particle size, saturation magnetization values and magnetic coercivity decreased, but when Fe3+ was used to precipitate the CoFe2 O4 , magnetic coercivity increased sharply. On samples prepared by normal pH regulation procedure, the dependence of saturation magnetization values on calculated average particle size showed linearity character, which inferred that the value of calculated average particle size approximated that of actual size. However, the same rule did not found on the samples prepared by reverse pH regulation procedure, which indicated that the value of calculated average particle size did
J. Wang et al. / Journal of Alloys and Compounds 450 (2008) 532–539
not represent the actual size of particles. In conclusion, the samples B have a wider size distribution than that of the samples A. Otherwise, the cation distribution in spinel structure were changed or modified by the different procedures and the ratio of Fe3+ /Fe2+ in starting solution. In normal pH regulation procedure, the concentration of Co2+ enter into A-sites increased with Fe3+ concentration in starting solution, but in reverse pH regulation procedure, the increasing of Fe3+ concentration in starting solution couldn’t affect the cation distribution of samples markedly. Acknowledgement The financial support for this work by the National Natural Science Fund of China (Grant No. 20076048) is greatly acknowledged.
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