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Structure and properties of magnetite formed in the presence of nickel(II) ions
Materials Research Bulletin, Vol. 33, No. 11, pp. 1609 –1619, 1998 Copyright © 1998 Elsevier Science Ltd Printed in the USA. All rights reserved 0025-5408/98 $19.00 1 .00
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STRUCTURE AND PROPERTIES OF MAGNETITE FORMED IN THE PRESENCE OF NICKEL(II) IONS
Tatsuo Ishikawa1*, Hiroshi Nakazaki1, Akemi Yasukawa1, Kazuhiko Kandori1, and Makoto Seto2 1 School of Chemistry, Osaka University of Education, 4-698-1 Asahigaoka, Kashiwara-shi, Osaka 582, Japan 2 Research Reactor Institute, Kyoto University, Kumatori, Osaka 590-04, Japan (Refereed) (Received December 29, 1997; Accepted April 10, 1998)
ABSTRACT Fe3O4 particles prepared by oxidation of Fe(OH)2 precipitated from FeCl2 solutions containing different amounts of Ni21 from 0 to 30 mol% were examined by various means. The thermogravimetry (TG) results show that the addition of Ni21 raised the air-oxidation temperature of Fe3O4 particles from ca. 200 to ca. 400°C, which means that the Fe3O4 particles were stabilized to air-oxidation by the doped Ni21. Fe21 content of the materials obtained at Ni/(Ni 1 Fe) $ 0.1 mol% was significantly greater than the theoretical Fe21 content of Fe3O4. Fe57 Mo¨ssbauer spectra indicated that Fe31 ions in B sites of the Ni-doped Fe3O4 are exchanged in part by Fe21. The dc electrical conductivity was increased from 1024 to 1022 S cm21 by Ni21 loading. © 1998 Elsevier Science Ltd
KEYWORDS: A. oxides, B. crystal growth, C. thermogravimetric analysis (TGA), C. Mo¨ssbauer spectroscopy, D. electrical properties INTRODUCTION Particles and thin films of magnetite (Fe3O4), one of the iron oxides, are raw materials extensively used for production of magnetic fluids, magnetic printer inks, microwave absorbers, catalysts, chemical sensors, and so on. This is because among the various spinel-type
*To whom correspondence should be addressed. Fax: 81-729-78-3394. 1609
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ferrites, Fe3O4 shows a relatively large coercive force and electrical conductivity [1– 4]. However, fine Fe3O4 particles are unstable to oxidation and easily oxidized in air even at room temperature, which brings troublesome problems for various uses; thus, stabilization of fine Fe3O4 particles to air-oxidation is desired [5]. Fe3O4 particles can be prepared by oxidation of an aqueous Fe(OH)2 suspension. Since the composition of oxidation products of Fe(OH)2 depends on the pH of the Fe(OH)2 suspensions and the oxidation rate of Fe21 [6], Fe3O4 particles free from impurities may be synthesized by controlling the oxidation rate of Fe(OH)2. It has been reported that the oxidation rate of Fe21 is proportional to concentrations of OH2 and Fe21 ions and O2 [7,8] and that divalent metal ions in trace quantities such as Co21 and Cu21 function as catalysts for the oxidation of Fe21 [9]. Therefore, these divalent metal ions can be expected to influence the constitution of oxidation products of Fe(OH)2 and to produce pure Fe3O4 particles. The aim of the present study was to prepare novel Fe3O4 particles with a high stability to air-oxidation. We paid particular attention to the influence of Ni21 on the formation, structure, and properties of Fe3O4 particles. The structure and properties of the Fe3O4 particles formed by oxidizing Fe(OH)2 particles with O2 in the presence of Ni21 were investigated by various means. EXPERIMENTAL Materials. 100 cm3 of the solutions containing various quantities of FeCl2 and NiCl2 from 0 to 30 mol% in Ni/(Fe 1 Ni) were prepared using O2-free deionized distilled water in an N2 atmosphere The total amount of these chlorides in the solutions were adjusted to 0.15 mol. To the solutions, 100 cm3 of 15 mol dm23 NaOH solution were added. The resulting suspensions were aged in a Teflon beaker placed in an autoclave filled with N2 gas at 100°C for 24 h. After this aging, the pH of the supernatant solution was reduced below 9 by repeating replacement of the supernatant with O2-free water. The precipitates were oxidized by bubbling pure O2 gas into the suspensions at a flow rate of 1.4 dm3 min21 at 70°C for 3 h. The resulting precipitates were filtered, washed with deionized distilled water, and finally dried in vacuo at room temperature for 16 h. A commercial Fe3O4 supplied by Wako Pure Chemical Industries, of which the Fe21 content and the specific surface area were 24.3 mol% of the total Fe and 21 m2 g21, was used as a reference sample. Characterization. The materials thus prepared were examined by the following conventional techniques. Powder X-ray diffraction (XRD) patterns were recorded on a Rigaku diffractometer using Ni-filtered Cu Ka radiation (30 kV and 15 mA). Fe and Ni contents were determined using a Seiko induced coupled plasma (ICP) spectrophotometer by first dissolving the materials in a 12 mol dm23 HCl solution by heating and then diluting the solution with water to a desired concentration. Fe21 ions in the particles were assayed by titrating with a K2Cr2O7 solution using sodium diphenylamine-4-sulphonate as an indicator. The dissolution of the samples and the titration were performed in an N2 stream. This titration was confirmed to be not influenced by the coexisting Ni21 by a preliminary experiment. Infrared (IR) spectra in KBr were taken using a Digilab Fourier transform infrared (FTIR) spectrometer. X-ray photoelectron spectroscopy (XPS) was carried out using a Shimadzu XPS spectrometer. Morphology of the particles was observed with a Jeol transmission electron microscope. Thermogravimetry (TG) curves were measured by a Seiko thermoanalyzer at a heating rate of 5°C min21 in an air stream and an N2 stream. N2 adsorption isotherms were
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FIG. 1 XRD patterns of the products at different X. X: a, 0; b, 0.1; c, 1; d, 10; e, 20; f, 30. E: Fe3O4; F: Ni2O3; 3: unknown peak. measured by an automatic volumetric adsorption apparatus at the boiling point of N2. Prior to adsorption, the samples were outgassed at 100°C for 2 h. Saturation magnetism and coercive force were determined using a Toei vibration magnetometer (VSM) at room temperature. The dc electrical conductivity of samples (200 mg) pressed into disks with 1 cm diameter under 1.2 t cm22 was measured in air at room temperature. Fe57 Mo¨ssbauer spectra were taken by an Elscint spectrometer using a Co57 source in an Rh matrix. RESULTS AND DISCUSSION XRD. Figure 1 shows XRD patterns of the materials prepared by adding different quantities of Ni21 to the starting solutions. Hereafter, the amount of Ni21 added to the starting solutions
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FIG. 2 IR spectra of the products at different X. X: a, 0; b, 0.01; c, 0.07; d, 0.1; e, 1; f, 10; g, 30.
is designated as X in mol%. As seen in this figure, the peaks due to (111) and (222) planes of Fe3O4 (JCPDS 19-629) at 2u 5 18.3° and 37.1°, respectively, are intensified by adding Ni21 at X $ 10. The d values and peak intensity ratios documented for Fe3O4 are close to those for NiFe2O3 (JPCDS 10-325), thus it is difficult to distinguish between the materials from the present XRD patterns. The product with X 5 10 shows the peak of Ni2O3 at 2u 5 44.2° (JPCDS 14-481) in addition to the Fe3O4 peaks and the Ni2O3 peaks grow with an increase in X. The product with X 5 30 shows two peaks at 2u 5 19.2° and 33.7° in addition to the peaks of Fe3O4 and Ni2O3. These peaks were not due to Ni(OH)2, NiOOH, or NiO, but could not be assigned for the present. Ni contents of the products determined by ICP were identical with those of the starting solutions, i.e., all of the Ni21 ions added to the solutions were involved in the formed particles. This may be attributed to the similar ionic radii of Ni21 (0.069 nm), Fe21 (0.074 nm), and Fe31 (0.064 nm). It is inferred from the XRD results in Figure 1 that some of added Ni21 ions are oxidized to Ni31 existing as Ni2O3 and the others substitute for Fe21 and/or Fe31 of Fe3O4.
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FIG. 3 TEM photographs of the particles formed at different X. FTIR. Figure 2 depicts IR spectra of the products at different X. As seen from this figure, the products at X # 0.07 give rise to the spectra having weak bands of a-FeOOH at 890 and 795 cm21 [10]. The a-FeOOH bands of the products at X $ 0.1 become very weak and the 585 cm21 band of Fe3O4 [11] markedly diminishes, as seen in curves d and e, although the XRD patterns of these products show the Fe3O4 peaks (Fig. 1). It is of interest that the materials formed in the range X 5 0.1–1 are more transparent to infrared rays compared to the other materials. Such drastic depression of the 585 cm21 band due to skeleton vibrations of Fe3O4 would reflect an extreme change of the bond strength of O–Fe–O. Transmission Scanning Microscopy (TEM) and Surface Area. TEM pictures of the particles synthesized at different X are shown in Figure 3. The particle sizes are almost unchanged at X # 5 but reduced at X $ 10. The specific surface area of the products at different X is decreased from 46 to 12 m2 g21 by increasing X from 0 to 20 in spite of the depression of particle size. The t-plots [12] and the pore size distribution curves [13] obtained from the N2 adsorption isotherms, not shown here, proved that all the samples are mesoporous but not microporous and that their porosity is reduced with increasing X. Consequently, the addition of Ni21 makes the particles more fine but less porous. Thermogravimetry. For estimation of the stability of the Ni-doped materials to air-oxidation, TG curves of the samples formed at different X were measured in an air stream and an N2 stream. They are illustrated in Figure 4. Curve a of the product in the absence of Ni21 exhibits a maximum at ca. 250°C. On increasing X, this maximum disappears and only a weight gain appears between ca. 400 and ca. 600°C (curves c and d). Such large weight gain does not appear in curve d9 traced in an N2 stream (dotted line). A monotonical weight gain
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FIG. 4 TG curves measured in an air stream (solid curves a– e) and an N2 stream (dotted curves a9 and d9) on the products at different X. X: a, a9, 0; b, 0.07; c, 0.1; d, d9, 10; e, 20. Curve f was collected on the commercial Fe3O4. in curves a9 and d9 would be due to trace amounts of O2 in the used N2 gas with a purity of 99.99%. Therefore, the weight gain can be assigned to oxidation of Fe3O4 by O2 in air. Note that the products at X $ 0.1 (curves c– e) exhibit a much higher oxidation temperature than that of the products with X # 0.1 (curves a and b) or the commercial Fe3O4 (curve f). The temperature of the weight gain in curves c– e is lowered by increasing X because of the reduction of particle size. If the weight gain is caused by the oxidation reaction of Fe3O4 with O2, 4Fe3O4 1 O2 3 6Fe2O3, the theoretical weight gain amounts 3.5 wt%. The weight gain of the products at X $ 10 in curves d and e is larger than the theoretical value. Fe21 Contents. As suggested by the TG results, the Ni-doped samples showed extraordinarily large weight gain. This may occur because the products with Ni21 contain a larger quantity of Fe21 than Fe3O4. To corroborate this, we determined Fe21 contents of the products by titrating with K2Cr2O7. Figure 5 plots the Fe21 content expressed in mol% of the
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FIG. 5 Fe21 contents of the products at different X determined by titration.
whole metals against X. Interestingly, the Fe21 content abruptly increases up to 68 mol% at X 5 20 with increasing X. If Fe21 ions of Fe3O4 were exchanged with Ni21 ions, the Fe21 content should not be greater than the Fe21 content of pure Fe3O4 (33.3 mol%). However, as seen from Figure 5, the products with X $ 0.1 contain larger than 33.3 mol% Fe21, which means that the Fe31 ions of Fe3O4 are replaced by not only the added Ni21 but also Fe21. The XRD patterns of the products at X # 1 showed only the peaks of Fe3O4 (Fig. 1). This appears to negate the possibility that the products contain FeO, Fe(OH)2, and green rust I with 43– 80 mol% Fe21 [14]. To our knowledge, the spinel structure including such a large amount of divalent cations has not been reported so far. It can be, therefore, inferred that the substitution of Fe31 by Ni21 and/or Fe21 in the Fe3O4 crystals generates the formation of Ni31 as shown below by XPS and that of oxygen defects or OH2 in the O22 sites to maintain the charge balance; however there is no experimental evidence for the latter case at the moment and such a large excess of Fe21 cannot be interpreted only by the oxidation of Ni21. A possible reason for the excess Fe21 is that Fe21 ions in the particles are protected against oxidation with O2 by formation of a stable surface layer. Mo¨ssbauer Spectroscopy. To gain information on the sites of Fe21 ions in the Ni-doped materials, their Mo¨ssbauer spectra were taken and are displayed in Figure 6. These spectra show that all the samples contains Fe21 and Fe31 in A and B sites of the Fe3O4 lattice, however the peak area ratio of Fe31 (A and B sites) to Fe21 and Fe31 (B sites) deviates from unity of the theoretical ratio for Fe3O4. The addition of Ni21 intensifies the peaks of Fe21 and Fe31 (B sites) and weakens those of Fe31 (A and B sites). This agrees well with the aforementioned results that Fe21 content is increased by doping Ni21 and that Fe31 ions in B sites are substituted by Fe21 and/or Ni21.
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FIG. 6 Mo¨ssbauer spectra of the products at different X. X: a, 0; b, 0.07; c, 0.1; d, 20. XPS. To clarify the origin of the high stability of the Ni-doped materials to air-oxidation, the surface characterization was done by XPS. Figure 7 depicts the XPS spectra of the products at different X. As seen from spectrum b the peak of Ni2p3/2 appears at 855.8 eV, which is close to the literature value (855.9 eV) of Ni(OH)2 [15]. With increasing X, this band shifts up to 859.8 eV and grows (spectrum e), indicating that Ni21 is oxidized to Ni31. Spectrum a of the product in the absence of Ni21 shows a peak of Fe2p3/2 at 711.6 eV, which is shifted to 715.2 eV and diminished by increasing X. The reported binding energies of FeO, Fe3O4, and a-Fe2O3 are 709.2, 710.7, and 711.0 eV, respectively [16]. The binding energy of more than 711.6 eV of the Fe2p3/2 peak obtained in this study is closer to that of a-Fe2O3 than that of Fe3O4. Therefore, the surface layer of the Ni-doped materials seems to contain less Fe21 than the bulk phase. It is well known that fine Fe3O4 particles are chemically unstable and easily oxidized in air into a solid solution with g-Fe2O3 [17–20]. However, g-Fe2O3 was difficult to detect because its XRD peaks and IR bands are close to those of Fe3O4 and the g-Fe2O3 content would be slight in this case. Figure 8 plots the Ni contents of the surface layer estimated from the Ni2p3/2 peak area and those of the whole particle determined by ICP vs. X. The Ni content of the whole particle (solid circles) increases linearly with X. Whereas the Ni content of the surface layer (open circles) rises steeply up to 60 mol% with increasing X and then remains constant. It is noteworthy that the surface Ni content is much larger than that of the whole particle at a small X, implying that the doped Ni is concentrated in the
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FIG. 7 XPS spectra of the products at different X. X: a, 0; b, 1; c, 2.5; d, 5; e, 30. surface layer. As seen from the XRD and XPS results, the surface layer is supposed to contain Ni2O3 and less Fe21, compared to the bulk phase. This would be a reason for the high stability to air-oxidation of Fe3O4 formed in the presence of Ni21. Electrical Conductivity, Saturation Magnetization, and Coercive Force. Figure 9 shows dc electrical conductivity of the materials formed at different X. The conductivity represented by the solid circles steeply rises with increasing X and then decreases. The conductivity of the commercial Fe3O4 was 8.3 3 1026 S cm21 much less than the values given in Figure 9. Since the high conductivity of Fe3O4 is generally considered to be due to electron hopping between Fe21 and Fe31, the steep increase in conductivity at small X can be ascribed to the increase of Fe21 content by loading Ni21. However, the number of the Fe21–Fe31 pairs in the Fe3O4 crystals is eventually reduced at large Fe21 content, so that a maximum conductivity appears at X 5 2.5. Figure 9 plots the saturation magnetization (ss) and coercive force (Hc) measured at room temperature against X. The ss shown by the triangles is 70 –90 Am2kg21, slightly less than the theoretical value of 92 Am2kg21, due to substitution of Fe31 by Ni21 and Fe21 with a smaller magnetic moment. The Hc shown by the squares is minimized at X 5 0.1 and near to 92–137 Oe of the values reported for the cubic Fe3O4 particles with the size of 0.10 – 0.22 mm [21].
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FIG. 8 Plots of Ni contents of the surface layer (E) and the whole particle (F) vs. X.
FIG. 9 Electrical conductivity (F), saturation magnetization (ss, ‚), and coercive force (Hc, h) of the products at different X.
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ACKNOWLEDGMENTS The authors thank Mr. Masao Fukusumi of Osaka Municipal Technical Research Institute for his help with TEM observation and Mr. Yasuhiro Fujii of Nissan Chemical Industries for his helping with VSM measurements. This study was supported in part by Nippon Sheet Glass Foundation for Materials Science and Technology, the Cosmetology Research Foundation and the Grants-in-Aid for Scientific Research (B) and (C) of the Ministry of Education, Science, Sports and Culture, Japan. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21.
Y.K. Kim and M. Oliveria, IEEE Trans. Mag. 30, 1316 (1994). N.F. Borrelli, S.L. Chen, and J.A. Murphy, IEEE Trans. Mag. Mag-8, 648 (1972). C.F. Jefferson and C. K. Baker, IEEE Trans. Mag. Mag-4, 460 (1968). S. Okamoto, IEEE Trans. Mag. Mag-10, 923 (1974). M.P. Sharrock and R.E. Bodnar, J. Appl. Phys. 57, 3919 (1985). T. Misawa, K. Hashimoto, and S. Shimodaira, Corros. Sci. 14, 131 (1974). P.C. Singer and W. Stumm, Science 167, 3291 (1970). W. Stumm and J.J. Morgan, Aquatic Chemistry, Wiley and Son, New York (1981). W. Stumm and G.F. Lee, Ind. Eng. Chem. 53, 143 (1961). A. Simon, H.H. Emons, and W. Schneiter, Z. Anorg. Allg. Chem. 312, 18 (1961). R.D. Waldron, Phys. Rev. 99, 1727 (1955). B.C. Lippens and J.H. de Boer, J. Catal. 4, 319 (1965). D. Dollimore and G.R. Heal, J. Appl. Chem. 14, 109 (1964). W. Feitknecht and G. Keller, Z. Anorg. Chem. 262, 61 (1950). T. Ishikawa, A. Nagashima, and K. Kandori, J. Mater. Sci. 26, 6231 (1991). H.S. Mcintyre and D.G. Zetaruk, Anal. Chem. 49, 1521 (1977). G. Ertl and K. Wandelt, Surf. Sci. 50, 479 (1975). J.P. Jolivet, E. Tronc, E. Barbe, and J. Livage, J. Colloid Interface Sci. 138, 465 (1990). P.S. Sidhu, R.J. Gilkes, and A.M. Posner, J. Inorg. Nucl. Chem. 39, 1953 (1977). H. Kojima and K. Handa, IEEE Trans. Mag. Mag-16, 11 (1980). D.J. Dunlop, IEEE Trans. Mag. Mag-8, 211 (1972).
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STRUCTURE AND PROPERTIES OF MAGNETITE FORMED IN THE PRESENCE OF NICKEL(II) IONS
Tatsuo Ishikawa1*, Hiroshi Nakazaki1, Akemi Yasukawa1, Kazuhiko Kandori1, and Makoto Seto2 1 School of Chemistry, Osaka University of Education, 4-698-1 Asahigaoka, Kashiwara-shi, Osaka 582, Japan 2 Research Reactor Institute, Kyoto University, Kumatori, Osaka 590-04, Japan (Refereed) (Received December 29, 1997; Accepted April 10, 1998)
ABSTRACT Fe3O4 particles prepared by oxidation of Fe(OH)2 precipitated from FeCl2 solutions containing different amounts of Ni21 from 0 to 30 mol% were examined by various means. The thermogravimetry (TG) results show that the addition of Ni21 raised the air-oxidation temperature of Fe3O4 particles from ca. 200 to ca. 400°C, which means that the Fe3O4 particles were stabilized to air-oxidation by the doped Ni21. Fe21 content of the materials obtained at Ni/(Ni 1 Fe) $ 0.1 mol% was significantly greater than the theoretical Fe21 content of Fe3O4. Fe57 Mo¨ssbauer spectra indicated that Fe31 ions in B sites of the Ni-doped Fe3O4 are exchanged in part by Fe21. The dc electrical conductivity was increased from 1024 to 1022 S cm21 by Ni21 loading. © 1998 Elsevier Science Ltd
KEYWORDS: A. oxides, B. crystal growth, C. thermogravimetric analysis (TGA), C. Mo¨ssbauer spectroscopy, D. electrical properties INTRODUCTION Particles and thin films of magnetite (Fe3O4), one of the iron oxides, are raw materials extensively used for production of magnetic fluids, magnetic printer inks, microwave absorbers, catalysts, chemical sensors, and so on. This is because among the various spinel-type
*To whom correspondence should be addressed. Fax: 81-729-78-3394. 1609
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ferrites, Fe3O4 shows a relatively large coercive force and electrical conductivity [1– 4]. However, fine Fe3O4 particles are unstable to oxidation and easily oxidized in air even at room temperature, which brings troublesome problems for various uses; thus, stabilization of fine Fe3O4 particles to air-oxidation is desired [5]. Fe3O4 particles can be prepared by oxidation of an aqueous Fe(OH)2 suspension. Since the composition of oxidation products of Fe(OH)2 depends on the pH of the Fe(OH)2 suspensions and the oxidation rate of Fe21 [6], Fe3O4 particles free from impurities may be synthesized by controlling the oxidation rate of Fe(OH)2. It has been reported that the oxidation rate of Fe21 is proportional to concentrations of OH2 and Fe21 ions and O2 [7,8] and that divalent metal ions in trace quantities such as Co21 and Cu21 function as catalysts for the oxidation of Fe21 [9]. Therefore, these divalent metal ions can be expected to influence the constitution of oxidation products of Fe(OH)2 and to produce pure Fe3O4 particles. The aim of the present study was to prepare novel Fe3O4 particles with a high stability to air-oxidation. We paid particular attention to the influence of Ni21 on the formation, structure, and properties of Fe3O4 particles. The structure and properties of the Fe3O4 particles formed by oxidizing Fe(OH)2 particles with O2 in the presence of Ni21 were investigated by various means. EXPERIMENTAL Materials. 100 cm3 of the solutions containing various quantities of FeCl2 and NiCl2 from 0 to 30 mol% in Ni/(Fe 1 Ni) were prepared using O2-free deionized distilled water in an N2 atmosphere The total amount of these chlorides in the solutions were adjusted to 0.15 mol. To the solutions, 100 cm3 of 15 mol dm23 NaOH solution were added. The resulting suspensions were aged in a Teflon beaker placed in an autoclave filled with N2 gas at 100°C for 24 h. After this aging, the pH of the supernatant solution was reduced below 9 by repeating replacement of the supernatant with O2-free water. The precipitates were oxidized by bubbling pure O2 gas into the suspensions at a flow rate of 1.4 dm3 min21 at 70°C for 3 h. The resulting precipitates were filtered, washed with deionized distilled water, and finally dried in vacuo at room temperature for 16 h. A commercial Fe3O4 supplied by Wako Pure Chemical Industries, of which the Fe21 content and the specific surface area were 24.3 mol% of the total Fe and 21 m2 g21, was used as a reference sample. Characterization. The materials thus prepared were examined by the following conventional techniques. Powder X-ray diffraction (XRD) patterns were recorded on a Rigaku diffractometer using Ni-filtered Cu Ka radiation (30 kV and 15 mA). Fe and Ni contents were determined using a Seiko induced coupled plasma (ICP) spectrophotometer by first dissolving the materials in a 12 mol dm23 HCl solution by heating and then diluting the solution with water to a desired concentration. Fe21 ions in the particles were assayed by titrating with a K2Cr2O7 solution using sodium diphenylamine-4-sulphonate as an indicator. The dissolution of the samples and the titration were performed in an N2 stream. This titration was confirmed to be not influenced by the coexisting Ni21 by a preliminary experiment. Infrared (IR) spectra in KBr were taken using a Digilab Fourier transform infrared (FTIR) spectrometer. X-ray photoelectron spectroscopy (XPS) was carried out using a Shimadzu XPS spectrometer. Morphology of the particles was observed with a Jeol transmission electron microscope. Thermogravimetry (TG) curves were measured by a Seiko thermoanalyzer at a heating rate of 5°C min21 in an air stream and an N2 stream. N2 adsorption isotherms were
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FIG. 1 XRD patterns of the products at different X. X: a, 0; b, 0.1; c, 1; d, 10; e, 20; f, 30. E: Fe3O4; F: Ni2O3; 3: unknown peak. measured by an automatic volumetric adsorption apparatus at the boiling point of N2. Prior to adsorption, the samples were outgassed at 100°C for 2 h. Saturation magnetism and coercive force were determined using a Toei vibration magnetometer (VSM) at room temperature. The dc electrical conductivity of samples (200 mg) pressed into disks with 1 cm diameter under 1.2 t cm22 was measured in air at room temperature. Fe57 Mo¨ssbauer spectra were taken by an Elscint spectrometer using a Co57 source in an Rh matrix. RESULTS AND DISCUSSION XRD. Figure 1 shows XRD patterns of the materials prepared by adding different quantities of Ni21 to the starting solutions. Hereafter, the amount of Ni21 added to the starting solutions
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FIG. 2 IR spectra of the products at different X. X: a, 0; b, 0.01; c, 0.07; d, 0.1; e, 1; f, 10; g, 30.
is designated as X in mol%. As seen in this figure, the peaks due to (111) and (222) planes of Fe3O4 (JCPDS 19-629) at 2u 5 18.3° and 37.1°, respectively, are intensified by adding Ni21 at X $ 10. The d values and peak intensity ratios documented for Fe3O4 are close to those for NiFe2O3 (JPCDS 10-325), thus it is difficult to distinguish between the materials from the present XRD patterns. The product with X 5 10 shows the peak of Ni2O3 at 2u 5 44.2° (JPCDS 14-481) in addition to the Fe3O4 peaks and the Ni2O3 peaks grow with an increase in X. The product with X 5 30 shows two peaks at 2u 5 19.2° and 33.7° in addition to the peaks of Fe3O4 and Ni2O3. These peaks were not due to Ni(OH)2, NiOOH, or NiO, but could not be assigned for the present. Ni contents of the products determined by ICP were identical with those of the starting solutions, i.e., all of the Ni21 ions added to the solutions were involved in the formed particles. This may be attributed to the similar ionic radii of Ni21 (0.069 nm), Fe21 (0.074 nm), and Fe31 (0.064 nm). It is inferred from the XRD results in Figure 1 that some of added Ni21 ions are oxidized to Ni31 existing as Ni2O3 and the others substitute for Fe21 and/or Fe31 of Fe3O4.
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FIG. 3 TEM photographs of the particles formed at different X. FTIR. Figure 2 depicts IR spectra of the products at different X. As seen from this figure, the products at X # 0.07 give rise to the spectra having weak bands of a-FeOOH at 890 and 795 cm21 [10]. The a-FeOOH bands of the products at X $ 0.1 become very weak and the 585 cm21 band of Fe3O4 [11] markedly diminishes, as seen in curves d and e, although the XRD patterns of these products show the Fe3O4 peaks (Fig. 1). It is of interest that the materials formed in the range X 5 0.1–1 are more transparent to infrared rays compared to the other materials. Such drastic depression of the 585 cm21 band due to skeleton vibrations of Fe3O4 would reflect an extreme change of the bond strength of O–Fe–O. Transmission Scanning Microscopy (TEM) and Surface Area. TEM pictures of the particles synthesized at different X are shown in Figure 3. The particle sizes are almost unchanged at X # 5 but reduced at X $ 10. The specific surface area of the products at different X is decreased from 46 to 12 m2 g21 by increasing X from 0 to 20 in spite of the depression of particle size. The t-plots [12] and the pore size distribution curves [13] obtained from the N2 adsorption isotherms, not shown here, proved that all the samples are mesoporous but not microporous and that their porosity is reduced with increasing X. Consequently, the addition of Ni21 makes the particles more fine but less porous. Thermogravimetry. For estimation of the stability of the Ni-doped materials to air-oxidation, TG curves of the samples formed at different X were measured in an air stream and an N2 stream. They are illustrated in Figure 4. Curve a of the product in the absence of Ni21 exhibits a maximum at ca. 250°C. On increasing X, this maximum disappears and only a weight gain appears between ca. 400 and ca. 600°C (curves c and d). Such large weight gain does not appear in curve d9 traced in an N2 stream (dotted line). A monotonical weight gain
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FIG. 4 TG curves measured in an air stream (solid curves a– e) and an N2 stream (dotted curves a9 and d9) on the products at different X. X: a, a9, 0; b, 0.07; c, 0.1; d, d9, 10; e, 20. Curve f was collected on the commercial Fe3O4. in curves a9 and d9 would be due to trace amounts of O2 in the used N2 gas with a purity of 99.99%. Therefore, the weight gain can be assigned to oxidation of Fe3O4 by O2 in air. Note that the products at X $ 0.1 (curves c– e) exhibit a much higher oxidation temperature than that of the products with X # 0.1 (curves a and b) or the commercial Fe3O4 (curve f). The temperature of the weight gain in curves c– e is lowered by increasing X because of the reduction of particle size. If the weight gain is caused by the oxidation reaction of Fe3O4 with O2, 4Fe3O4 1 O2 3 6Fe2O3, the theoretical weight gain amounts 3.5 wt%. The weight gain of the products at X $ 10 in curves d and e is larger than the theoretical value. Fe21 Contents. As suggested by the TG results, the Ni-doped samples showed extraordinarily large weight gain. This may occur because the products with Ni21 contain a larger quantity of Fe21 than Fe3O4. To corroborate this, we determined Fe21 contents of the products by titrating with K2Cr2O7. Figure 5 plots the Fe21 content expressed in mol% of the
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FIG. 5 Fe21 contents of the products at different X determined by titration.
whole metals against X. Interestingly, the Fe21 content abruptly increases up to 68 mol% at X 5 20 with increasing X. If Fe21 ions of Fe3O4 were exchanged with Ni21 ions, the Fe21 content should not be greater than the Fe21 content of pure Fe3O4 (33.3 mol%). However, as seen from Figure 5, the products with X $ 0.1 contain larger than 33.3 mol% Fe21, which means that the Fe31 ions of Fe3O4 are replaced by not only the added Ni21 but also Fe21. The XRD patterns of the products at X # 1 showed only the peaks of Fe3O4 (Fig. 1). This appears to negate the possibility that the products contain FeO, Fe(OH)2, and green rust I with 43– 80 mol% Fe21 [14]. To our knowledge, the spinel structure including such a large amount of divalent cations has not been reported so far. It can be, therefore, inferred that the substitution of Fe31 by Ni21 and/or Fe21 in the Fe3O4 crystals generates the formation of Ni31 as shown below by XPS and that of oxygen defects or OH2 in the O22 sites to maintain the charge balance; however there is no experimental evidence for the latter case at the moment and such a large excess of Fe21 cannot be interpreted only by the oxidation of Ni21. A possible reason for the excess Fe21 is that Fe21 ions in the particles are protected against oxidation with O2 by formation of a stable surface layer. Mo¨ssbauer Spectroscopy. To gain information on the sites of Fe21 ions in the Ni-doped materials, their Mo¨ssbauer spectra were taken and are displayed in Figure 6. These spectra show that all the samples contains Fe21 and Fe31 in A and B sites of the Fe3O4 lattice, however the peak area ratio of Fe31 (A and B sites) to Fe21 and Fe31 (B sites) deviates from unity of the theoretical ratio for Fe3O4. The addition of Ni21 intensifies the peaks of Fe21 and Fe31 (B sites) and weakens those of Fe31 (A and B sites). This agrees well with the aforementioned results that Fe21 content is increased by doping Ni21 and that Fe31 ions in B sites are substituted by Fe21 and/or Ni21.
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FIG. 6 Mo¨ssbauer spectra of the products at different X. X: a, 0; b, 0.07; c, 0.1; d, 20. XPS. To clarify the origin of the high stability of the Ni-doped materials to air-oxidation, the surface characterization was done by XPS. Figure 7 depicts the XPS spectra of the products at different X. As seen from spectrum b the peak of Ni2p3/2 appears at 855.8 eV, which is close to the literature value (855.9 eV) of Ni(OH)2 [15]. With increasing X, this band shifts up to 859.8 eV and grows (spectrum e), indicating that Ni21 is oxidized to Ni31. Spectrum a of the product in the absence of Ni21 shows a peak of Fe2p3/2 at 711.6 eV, which is shifted to 715.2 eV and diminished by increasing X. The reported binding energies of FeO, Fe3O4, and a-Fe2O3 are 709.2, 710.7, and 711.0 eV, respectively [16]. The binding energy of more than 711.6 eV of the Fe2p3/2 peak obtained in this study is closer to that of a-Fe2O3 than that of Fe3O4. Therefore, the surface layer of the Ni-doped materials seems to contain less Fe21 than the bulk phase. It is well known that fine Fe3O4 particles are chemically unstable and easily oxidized in air into a solid solution with g-Fe2O3 [17–20]. However, g-Fe2O3 was difficult to detect because its XRD peaks and IR bands are close to those of Fe3O4 and the g-Fe2O3 content would be slight in this case. Figure 8 plots the Ni contents of the surface layer estimated from the Ni2p3/2 peak area and those of the whole particle determined by ICP vs. X. The Ni content of the whole particle (solid circles) increases linearly with X. Whereas the Ni content of the surface layer (open circles) rises steeply up to 60 mol% with increasing X and then remains constant. It is noteworthy that the surface Ni content is much larger than that of the whole particle at a small X, implying that the doped Ni is concentrated in the
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FIG. 7 XPS spectra of the products at different X. X: a, 0; b, 1; c, 2.5; d, 5; e, 30. surface layer. As seen from the XRD and XPS results, the surface layer is supposed to contain Ni2O3 and less Fe21, compared to the bulk phase. This would be a reason for the high stability to air-oxidation of Fe3O4 formed in the presence of Ni21. Electrical Conductivity, Saturation Magnetization, and Coercive Force. Figure 9 shows dc electrical conductivity of the materials formed at different X. The conductivity represented by the solid circles steeply rises with increasing X and then decreases. The conductivity of the commercial Fe3O4 was 8.3 3 1026 S cm21 much less than the values given in Figure 9. Since the high conductivity of Fe3O4 is generally considered to be due to electron hopping between Fe21 and Fe31, the steep increase in conductivity at small X can be ascribed to the increase of Fe21 content by loading Ni21. However, the number of the Fe21–Fe31 pairs in the Fe3O4 crystals is eventually reduced at large Fe21 content, so that a maximum conductivity appears at X 5 2.5. Figure 9 plots the saturation magnetization (ss) and coercive force (Hc) measured at room temperature against X. The ss shown by the triangles is 70 –90 Am2kg21, slightly less than the theoretical value of 92 Am2kg21, due to substitution of Fe31 by Ni21 and Fe21 with a smaller magnetic moment. The Hc shown by the squares is minimized at X 5 0.1 and near to 92–137 Oe of the values reported for the cubic Fe3O4 particles with the size of 0.10 – 0.22 mm [21].
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FIG. 8 Plots of Ni contents of the surface layer (E) and the whole particle (F) vs. X.
FIG. 9 Electrical conductivity (F), saturation magnetization (ss, ‚), and coercive force (Hc, h) of the products at different X.
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ACKNOWLEDGMENTS The authors thank Mr. Masao Fukusumi of Osaka Municipal Technical Research Institute for his help with TEM observation and Mr. Yasuhiro Fujii of Nissan Chemical Industries for his helping with VSM measurements. This study was supported in part by Nippon Sheet Glass Foundation for Materials Science and Technology, the Cosmetology Research Foundation and the Grants-in-Aid for Scientific Research (B) and (C) of the Ministry of Education, Science, Sports and Culture, Japan. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21.
Y.K. Kim and M. Oliveria, IEEE Trans. Mag. 30, 1316 (1994). N.F. Borrelli, S.L. Chen, and J.A. Murphy, IEEE Trans. Mag. Mag-8, 648 (1972). C.F. Jefferson and C. K. Baker, IEEE Trans. Mag. Mag-4, 460 (1968). S. Okamoto, IEEE Trans. Mag. Mag-10, 923 (1974). M.P. Sharrock and R.E. Bodnar, J. Appl. Phys. 57, 3919 (1985). T. Misawa, K. Hashimoto, and S. Shimodaira, Corros. Sci. 14, 131 (1974). P.C. Singer and W. Stumm, Science 167, 3291 (1970). W. Stumm and J.J. Morgan, Aquatic Chemistry, Wiley and Son, New York (1981). W. Stumm and G.F. Lee, Ind. Eng. Chem. 53, 143 (1961). A. Simon, H.H. Emons, and W. Schneiter, Z. Anorg. Allg. Chem. 312, 18 (1961). R.D. Waldron, Phys. Rev. 99, 1727 (1955). B.C. Lippens and J.H. de Boer, J. Catal. 4, 319 (1965). D. Dollimore and G.R. Heal, J. Appl. Chem. 14, 109 (1964). W. Feitknecht and G. Keller, Z. Anorg. Chem. 262, 61 (1950). T. Ishikawa, A. Nagashima, and K. Kandori, J. Mater. Sci. 26, 6231 (1991). H.S. Mcintyre and D.G. Zetaruk, Anal. Chem. 49, 1521 (1977). G. Ertl and K. Wandelt, Surf. Sci. 50, 479 (1975). J.P. Jolivet, E. Tronc, E. Barbe, and J. Livage, J. Colloid Interface Sci. 138, 465 (1990). P.S. Sidhu, R.J. Gilkes, and A.M. Posner, J. Inorg. Nucl. Chem. 39, 1953 (1977). H. Kojima and K. Handa, IEEE Trans. Mag. Mag-16, 11 (1980). D.J. Dunlop, IEEE Trans. Mag. Mag-8, 211 (1972).