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The synthesis of iron–nickel alloy nanoparticles using a reverse micelle technique
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 307 (2006) 250–256 www.elsevier.com/locate/jmmm
The synthesis of iron–nickel alloy nanoparticles using a reverse micelle technique Irena Bana,, Miha Drofenika,b, Darko Makovecb a
Faculty of Chemistry and Chemical Engineering, University of Maribor, Smetanova 17, 2000 Maribor, Slovenia b Jozˇef Stefan Institute, Jamova 39, 1111 Ljubljana, Slovenia Received 16 February 2006 Available online 11 May 2006
Abstract Nanosized Fe0.2Ni0.8 particles were prepared by reducing their salts with sodium borohydride (NaBH4) in cationic water-in-oil (w/o) microemulsions of water/cetyl-trimethyl-amonium bromide (CTAB) and n-butanol/isooctane at 25 1C. According to the TEM and X-ray diffraction analyses, the synthesized particles were around 4–12 nm in size. Due to their nanodimensions, the particles had a primitive cubic (pc) structure rather than the body-centered cubic (BCC) structure of the bulk material. An examination of the synthesis from the reverse micelle reveals that the morphology of the iron–nickel alloy nanoparticles depends mainly on the microemulsion’s composition. The magnetization of the nanoparticles was much lower than that of the bulk material, reflecting the influence of the nanodimensions on the particles’ magnetizations. r 2006 Elsevier B.V. All rights reserved. PACS: 81.05 Bx Keywords: Nickel–iron alloys; Nanoparticles; Reverse micelles; Microemulsion; Magnetic properties
1. Introduction Magnetic nanoparticles usually show novel magnetic, electronic, optical and chemical properties, which are different to those of the bulk materials because of the extreme small sizes and the large specific surface areas. They have many potential applications, such as materials for catalysis, in electronic and mechanical devices, and in the medicine [1]. For this reason, research on the syntheses and characterization of magnetic nanoparticles has received increasing attention in recent years. Magnetic nanoparticles show interesting particle-sizedependent structural properties. Nanosized iron–nickel alloys are of particular interest because of their magnetic and mechanical properties, and because of their martensitic transformation [2]. An important aspect of the Fe–Ni alloy system is its structural evolution, with a change from bodycentered cubic (BCC) for the Fe-rich alloy to face-centered Corresponding author. Tel.: +386 2 2294 417; fax: +386 2 2527 774.
E-mail address: [email protected] (I. Ban). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.04.010
cubic (FCC) for the alloy with a larger Ni content [3]. Submicron Fe–Ni alloy particles have been prepared by various methods [3–9]. It has also been reported that the structure of iron–nickel alloys depends to a great extent on the composition and size of the iron–nickel alloy particles [10]. Bulk iron–nickel alloys usually exhibit a FCC crystal structure, which is often retained at low temperatures in nanosized particles. Many factors influence the stability of the FCC phase at low temperatures, e.g., the composition, the stress or strain associated with the synthesis, and the final size of the particles. Furthermore, the iron–nickel alloy containing 65–90 wt% nickel is well known as permalloy, and is important because of its magnetic permeability and low hysteresis losses. This paper describes the synthesis of Fe–Ni alloy nanoparticles using surfactant-based templates and examines their structural and magnetic properties. The selfassembly of a surfactant to form hydrophobic-structured domains led to an opportunity to apply these systems as the primary substance for the synthesis of materials with unique properties.
ARTICLE IN PRESS I. Ban et al. / Journal of Magnetism and Magnetic Materials 307 (2006) 250–256
2. Experimental methods The iron (II) chloride, nickel (II) chloride and sodium borohydride (NaBH4) were from E. Merck (Darmstadt); the isooctane and n-butanol were supplied by Flucka and the cationic water/cetyl-trimethyl-amonium-bromide (CTAB) surfactant was obtained from ALFA. The aqueous solution containing cations was obtained by dissolving appropriate amounts of iron and nickel chlorides. Two types of microemulsions were prepared by solubilizing aqueous Ni2+ (0.4 M) and Fe2+ (0.1 M) ions or NaBH4 (0.8 M) into a mixture of CTAB, n-butanol and isooctane. CTAB and n-butanol were used as the surfactant and co-surfactant, respectively. The stable, one-phase, microemulsion compositions were selected according to the pseudo-ternary phase diagram taken from Ref. [11]. The composition of the microemulsion used is shown in the composition diagram in Fig. 1. The synthesis of iron–nickel alloy nanoparticles was performed by mixing equal volumes of both microemulsion solutions. The solution turned black after mixing and was allowed to react for 1 h at 25 1C under nitrogen to minimize the oxidation. The micelle solution was disrupted by adding an excess amount of ethanol. The surfactant was removed with repeated methanol washing. The resulting powder containing the aggregates of Fe0.2Ni0.8 was black. The structural characterization was performed using electron microscopy. The as-synthesized Fe–Ni alloy powder was heat treated in an evacuated silica glass tube at 700 1C for 5 h. The particle morphology was inspected by high-resolution TEM (HREM) using a fieldemission electron-source scanning-transmission electron microscope (STEM) (JEOL 2010 F), operated at 200 kV. For the TEM investigations, the nanoparticles were deposited on a copper-grid-supported transparent carbon foil. The XRD measurements were performed on a AXSBruker/Siemens/D5005 diffractometer using Cu-Ka radiation (l ¼ 0.1542 nm). The XRD analysis data were
Fig. 1. Compositions selected for the synthesis of nanoparticles.
251
compared with the stored Powder Diffraction Files (PDFs) [12]. The magnetizations of the samples were measured using a Manics DSM-10 suscepto-magnetometer and a superconducting quantum interference device (SQUID) at temperatures from room temperature to 5 K. The specific surfaces of the synthesized nanopowders were determined by measuring the volume of adsorbed nitrogen at low temperatures using the BET method. The thermal properties of the synthesized nanopowders were determined with differential scanning calorimetry and thermo-gravimetric analysis (TA-4000 System Mettler). 3. Results and discussion The iron–nickel alloy nanoparticles were prepared from the selected microemulsion compositions within the CTAB and n-butanol/isooctane system at 25 1C(Fig. 1). Fig. 2 shows a typical XRD spectrum for the synthesized iron–nickel alloy nanoparticles. The spectrum shows three characteristic broad peaks at 2y ¼ 341, 481 and 611. The pattern does not match with the face-centered lattice reported for the iron–nickel alloy [12]. It could, however, be indexed according to the primitive cubic (pc) lattice with a ¼ 0.263 nm. According to the pc lattice, the three peaks observed belong to the (1 0 0), (1 1 0) and (1 1 1) reflections, as noted in Fig. 2. The impurities in the heat-treated sample at 700 1C are a result of the reaction of the nickel–iron alloy nanopowder with silica glass. The diffraction pattern, which were ascribed to impurities, can be assigned to the nickel and iron silicates according to PDF files. No oxides could be detected in the mixture after the heat treatment. Fig. 3 shows the TEM images and the corresponding electron diffraction patterns of two characteristic samples. Sample A (Fig. 3(a)) showed spherical nanoparticles, approximately 2.5 nm in size. HREM imaging (Fig. 3(b)) suggested that the nanoparticles had relatively poor crystallinity. The TEM image of sample C (Fig. 3(c)) showed particles with a rod-like shape, 3–5-nm wide and about 100-nm long. However, the HREM imaging (Fig. 3(d)) revealed that these particles were actually elongated agglomerates, which consisted of ‘‘chains’’ of much smaller spherical nanoparticles, approximately 4 nm in size. The electron diffraction patterns taken for these two samples (insets of Figs. 3(a) and (c)) show three broad, diffuse rings, which could be indexed with the pc cell of a 0.26 nm, observed using XRD. The (1 0 0) ring showing an inter-planar distance d of approximately 0.26 nm is the brightest, and merged with the (1 1 0) ring around a d-value of 0.19 nm, whereas the (1 1 1) ring around a d-value of 0.14 nm is well separated. The rings are diffuse and highly broadened due to their poor crystallinity and the small size of the nanoparticles. The EDX microanalysis of both samples was consistent with the starting composition of Fe0.2Ni0.8. The XRD as well as the electron diffraction in the TEM suggested that the iron–nickel alloy nanoparticles have a pc
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252
111(bcc)
6000
Lin (Counts)
200 (bcc)
4000 220 (bcc) (b)
2000 100 (pc) 110 (pc)
111 (pc)
(a)
0 30
40
50
60
70
80
2-Theta - Scale Fig. 2. (a) Typical X-ray powder diffraction patterns of as-synthesized nickel–iron alloy sample A and (b) sample A heat treated at 700 1C in vacuum.
Fig. 3. Morphology of synthesized iron–nickel alloy nanoparticles: (a) TEM image of sample A with spherical nanoparticles (inset: a corresponding electron diffraction pattern). (b) TEM image of sample C with rod-like agglomerates of nanoparticles, (c) HREM image of sample A, (d) HREM image of sample C.
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Table 1 Composition of microemulsions, specific surface area (As), average grain size obtained from specific surface area (dA), average particle size obtained from XRD (dx) and specific magnetization (Ms) Sample
[H2O]/[CTAB] W
[1-butanol]/[CTAB] P
As (m2/g)
dA (nm)
dx (nm)
Ms (emu/g)
A B C
16.17 20.2 27
4.9 4.9 4.9
160.21 60.54 82.93
4.2 11.3 8.2
4.0 3.7 3.2
12 12 6
lattice with 0.263 nm. The lattice parameter of 0.263 nm is close to the sum of the atomic radii of nickel (rNi ¼ 0.124 nm) and iron (rFe ¼ 0.126 nm), i.e., 0.250 nm. It is supposed that the lattice sites of iron and nickel are randomly distributed over the lattice sites with two iron atoms per eight primitive lattice unit cells. If the iron and nickel positions in the lattice were ordered we would expect the unit-cell parameter to be at least doubled. To the best of our knowledge, the primitive lattice structure of the nickel–iron alloy has not been documented in the literature yet. Only the FCC and the BCC structures have been reported [3,7]. In all other reported cases of iron–nickel alloy synthesis, whether in bulk form or as nanoparticles, the preparation conditions were associated with higher temperatures. In our case, the synthesis proceeded at room temperature. One argument that can be put forward is the decrease of the coordination of the atoms in the alloy when changing the crystal structure to pc. Namely, when the surface of a system strongly increases the number of unsaturated bonds on the particles’ surfaces increases as well. This means that the system can mitigate the large change in the unsaturated bond increase during the formation of nanoparticles to change the coordination in the structure. The coordination is 12 in FCC, 8 in BCC and 6 in pc. When the structure changes to pc, the system can mitigate the increase in the number of unsaturated bonds when increasing the surface area of the system-assembly of nanoparticles. When the samples were heat treated in a vacuum at 700 1C for 6 h, the usual BCC structure was obtained. In Fig. 2(b), the X-ray diffraction pattern of the heat-treated sample A shows the presence of the reflection of the BCC structure in addition to the reflections of the retained, untransformed, pristine pc phase. Besides, some reflections of impurities formed during the heat treatment of Fe–Ni alloy nanoparticles inside the silica glass tube, can be noticed. The as-synthesized phase transforms to the BCC phase, characteristic for Ni–Fe alloys [3,7]. As shown in Fig. 3, the morphologies of the synthesized nanoparticles change with the composition of the microemulsion used. Two samples, A and C, exhibited the same structure and composition; therefore, we can suppose that the difference in the habit of the synthesized particles must lie in the structure of the reverse micelles, out of which the particles were grown. The formation of elliptical or very long, rod-like reverse micelles, which are known to exist in
microemulsion systems that comprise CTAB as the surfactant [13–15] can be the reason for the formation of rod-like ‘‘particles’’, which are in fact one-dimensional and/or uniaxial agglomerates adapted to the reversemicelle geometry. For the nanoparticles that were formed in the ‘‘water pools’’ of reverse micelles and were observed to be elongated and/or acicular, there were no further processes taking place during the particle syntheses. Therefore, it is clear that the reason for the formation of the elongated agglomerates must be the micellar microstructure. It has also been reported that an alkali substance [16], salts or cosurfactant [17,18], such as 1-hexanol [19,20], might induce a spherical-to-worm-like transition. The BET-specific surface areas per unit mass of samples are given in Table 1. The average grain size obtained from the specific surface area in relation to the particle dimension estimated from the Sherrer equation are similar for sample A, but differ for samples B and C, which were agglomerated. The real size of the particles can be estimated from the HREM images, and they are fairly close to the dx values. The relatively small average grain size associated with the low saturation magnetization matches the powder morphology and the corresponding magnetic properties. The saturation magnetization of the nanoparticles is much lower than that of the bulk (80 emu/g). There are many factors that can cause a decrease in the magnetization with the grain size, e.g., crystal lattice defects, the mass effects of absorbed water, chemical and physical changes on the surface, magnetic degradation on the surface, redistribution of cation sites in the lattice. Since the grain size of the synthesized permalloy nanoparticles, dx ¼ 3–4 nm, is very small it is the particles’ ‘‘volume’’ of nonmagnetic regions on the grain surfaces, which is relatively large, that cannot contribute to the saturation magnetization during measurements [20–22]. Though the magnetization measurements at room temperature up to 10 kOe did not fully saturate the sample, which indicates a significant superparamagnetic contribution to the magnetization, the shape does indicate considerable magnetic ordering at room temperature (Fig. 4). The temperature dependence of the magnetization investigated under zero-field cooling, MZFC and field cooling, MFC revealed the superparamagnetic behavior of
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Fig. 4. The hysteresis of nickel–iron alloy nanoparticles of samples A and C at room temperature.
0.008 0.008
0.007
FC 0.006 m [emu]
0.005 m [emu]
FC
0.006
0.004
0.004 0.002 ZFC
0.003 0.000
0.002 0
2
4
6
0.001 0.000
8 10 12 14 16 18 T [K]
ZFC 0
50
100
150
200
250
T [K]
Fig. 5. Temperature dependence of the magnetization curves under ZFC and FC of nickel–iron nanoparticles of sample A.
the magnetic nanoparticles (Fig. 5). The sharp maximum of the MZFC at Tmax ¼ 7 K and the deviation of the MZFC and MFC curves close to Tmax indicate a narrow particle size distribution. It is well known that the superparamagnetic blocking temperature decreases with decreasing particle size. The blocking temperature should roughly satisfy the relationship TB ¼ KV/30 kB, where K is the anisotropy constant, kB the Boltzman constant, and V the average volume of the particles. Taking the measured blocking temperature of 7 K and the particle diameter of 4.0 nm, one can estimate the anisotropy constant of the magnetic particles to be K ¼ 8.6 104 J/m3. This value is about one order of magnitude larger than that of permalloy, which is 0.7 104 J/m3 [23]. The TG analyses in Fig. 6 show that the weight loss is continuous and is most probably a result of
water evaporation in the temperature interval up to 100 1C; in the temperature interval 100–250 1C the desorption of the surfactant from the alloy surface is to be expected, and after that the disintegration of the retained material on the surface occurs. The total weight loss is about 20 wt%. On the DSC diagram, Fig. 7, two endothermic peaks were identified, i.e., around 96 and 368 1C, respectively. The endothermic peak at 96 1C, which is irreversible, characterizes a process of water evaporation, while the endothermic peak at 368 1C was attributed to the Curie transition temperature. The Curie transition temperature of nanoparticles is lower than that of the bulk, which occurs at temperatures close to 580 1C. In the case when we are dealing with nanosized particles the transition temperature is certainly much lower. The finite-size scaling theory predicts that the shift in the transition temperature from that of the bulk should depend on the size of the system [24]. Namely, the increase in the surface-to-volume ratio leads to a decrease in the overall ordering of the magnetic phase due to the fact that a large fraction of the atoms on a disordered surface, about 60% of them, are less tightly subjected to a super-exchange interaction, and also the crystal structure is less ordered in comparison to the grain interior. A less intense and more broad endothermic peak of the second-order transition occurs as a result of a series of various temperatures, dependent on the crystallite sizes, where the transition takes place. We did not observe an exothermic peak contribution due to crystallization. This is in agreement with the TEM observation of the samples after heat treatment, which reveals that in this temperature region no increase in the grain size was noticed. Such a grain-size increase would usually accompany a crystallization process. On the other hand, the remarkable exothermic peak at 595 1C can be assigned to the transition from the pc to the BCC structure of the Ni–Fe alloy nanoparticles.
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Fig. 6. TG analyses of nickel–iron alloy nanoparticles.
Fig. 7. The DSC diagram of as-synthesized nickel–iron alloy nanoparticles.
4. Conclusion In this paper, we report a methodology for the synthesis of Fe–Ni alloy nanoparticles. The morphology of the iron–nickel alloy nanoparticles depends mainly on the microemulsion composition. According to nanodimension, the particles exhibit a primitive cubic (pc) structure in difference to body-centered cubic (BCC) structure of the bulk material. The temperature dependence of the magnetization investigated reveal the superparamagnetic behavior. The sharp maximum of the MZFC indicates a narrow particle size distribution. The magnetization of the nanoparticles is much lower than that of the bulk material, reflecting the influence of the nanodimensions on the particles’ magnetizations.
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I. Sˇafarik, M. Sˇafarikova, Monatsh. Chem. 133 (2002) 737. S. Kajiwara, S. Ohno, K. Honma, Philos. Mag. A 63 (1991) 625. C. Kuhrt, L. Schultz, J. Appl. Phys. 73 (4) (1993) 1975. S.D. Kaloshkin, V.V. Tcherdyntsev, Yu.V. Baldokhin, I.A. Tomilin, E.V. Shelekhov, J. Non-Cryst. Solids 287 (2001) 329. Y.H. Zhou, M. Harmelin, J. Bogot, Mater. Sci. Eng. A 124 (1990) 124. G. Dumpich, E.F. Wassermann, V. Manns, W. Keune, S. Marayama, Y. Miyoko, J. Magn. Magn. Mater. 67 (1987) 55. K.V.P.M. Shafi, A. Gedanken, R.B. Goldfarb, I. Felner, J. Appl. Phys. 81 (10) (1997) 6901. B.L. Cushing, V. Golub, C.J. O’Connor, J. Phys. Chem. Solids 65 (2004) 825. X.G. Li, A. Chiba, S. Takahashi, J. Magn. Magn. Mater. 170 (1997) 330.
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[10] H. Wang, Q. Liu, J. Zhang, T.Y. Hsu, Nanotechnology 14 (2003) 696. [11] C.C. Wang, D.H. Chen, T. Huang, Physicochem. Eng. Aspects 189 (2001) 145. [12] DIFFRACplus, Seach-match, Version 3.0, PDF1, PDF2, 1997. [13] Y. Liu, W. Wang, Y. Zhan, C. Zheng, G. Wang, Mater. Lett. 56 (2002) 496. [14] S. Xu, H. Zhou, J. Xu, Y. Li, Langmuir 18 (2002) 10503. [15] M.P. Pileni, A. Hammounda, I. Lisiecki, L. Motte, N. Moumen, J. Tanori, Control of the size and shape of nanoparticles, in: E. Pelizzetti (Ed.), Fine Particles Science and Technology, Kluwer Academic Publishers, Dordrecht, 1996, pp. 413–429.
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F.G. Sanchez, C.C. Ruiz, J. Lumin. 69 (1996) 179. X. Fang, C. Yang, J. Colloid Interf. Sci. 212 (1999) 242. J. Yang, Curr. Opin. Colloid Interf. Sci. 7 (2002) 276. B. Lindman, H. Wennerstrom, Topics Curr. Chem. 87 (1980) 1. Z. Lin, J.J. Cai, L.E. Scriven, H.T. Davis, J. Phys. Chem. 98 (1994) 5984. R.H. Kodama, J. Magn. Magn. Mater. 200 (1999) 359. T. Sato, T. Iijima, M. Seki, N. Inagaki, J. Magn. Magn. Mater. 65 (1987) 252. S. Chikazumi, Physics of Magnetism, Wiley, New York, 1964, p. 363. J.P. Chen, C.M. Sorenson, K.J. Klabunde, G.C. Hadjipanayis, E. Devlin, A. Kostikas, Phys. Rev. B 54 (13) (1996) 9288.
Journal of Magnetism and Magnetic Materials 307 (2006) 250–256 www.elsevier.com/locate/jmmm
The synthesis of iron–nickel alloy nanoparticles using a reverse micelle technique Irena Bana,, Miha Drofenika,b, Darko Makovecb a
Faculty of Chemistry and Chemical Engineering, University of Maribor, Smetanova 17, 2000 Maribor, Slovenia b Jozˇef Stefan Institute, Jamova 39, 1111 Ljubljana, Slovenia Received 16 February 2006 Available online 11 May 2006
Abstract Nanosized Fe0.2Ni0.8 particles were prepared by reducing their salts with sodium borohydride (NaBH4) in cationic water-in-oil (w/o) microemulsions of water/cetyl-trimethyl-amonium bromide (CTAB) and n-butanol/isooctane at 25 1C. According to the TEM and X-ray diffraction analyses, the synthesized particles were around 4–12 nm in size. Due to their nanodimensions, the particles had a primitive cubic (pc) structure rather than the body-centered cubic (BCC) structure of the bulk material. An examination of the synthesis from the reverse micelle reveals that the morphology of the iron–nickel alloy nanoparticles depends mainly on the microemulsion’s composition. The magnetization of the nanoparticles was much lower than that of the bulk material, reflecting the influence of the nanodimensions on the particles’ magnetizations. r 2006 Elsevier B.V. All rights reserved. PACS: 81.05 Bx Keywords: Nickel–iron alloys; Nanoparticles; Reverse micelles; Microemulsion; Magnetic properties
1. Introduction Magnetic nanoparticles usually show novel magnetic, electronic, optical and chemical properties, which are different to those of the bulk materials because of the extreme small sizes and the large specific surface areas. They have many potential applications, such as materials for catalysis, in electronic and mechanical devices, and in the medicine [1]. For this reason, research on the syntheses and characterization of magnetic nanoparticles has received increasing attention in recent years. Magnetic nanoparticles show interesting particle-sizedependent structural properties. Nanosized iron–nickel alloys are of particular interest because of their magnetic and mechanical properties, and because of their martensitic transformation [2]. An important aspect of the Fe–Ni alloy system is its structural evolution, with a change from bodycentered cubic (BCC) for the Fe-rich alloy to face-centered Corresponding author. Tel.: +386 2 2294 417; fax: +386 2 2527 774.
E-mail address: [email protected] (I. Ban). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.04.010
cubic (FCC) for the alloy with a larger Ni content [3]. Submicron Fe–Ni alloy particles have been prepared by various methods [3–9]. It has also been reported that the structure of iron–nickel alloys depends to a great extent on the composition and size of the iron–nickel alloy particles [10]. Bulk iron–nickel alloys usually exhibit a FCC crystal structure, which is often retained at low temperatures in nanosized particles. Many factors influence the stability of the FCC phase at low temperatures, e.g., the composition, the stress or strain associated with the synthesis, and the final size of the particles. Furthermore, the iron–nickel alloy containing 65–90 wt% nickel is well known as permalloy, and is important because of its magnetic permeability and low hysteresis losses. This paper describes the synthesis of Fe–Ni alloy nanoparticles using surfactant-based templates and examines their structural and magnetic properties. The selfassembly of a surfactant to form hydrophobic-structured domains led to an opportunity to apply these systems as the primary substance for the synthesis of materials with unique properties.
ARTICLE IN PRESS I. Ban et al. / Journal of Magnetism and Magnetic Materials 307 (2006) 250–256
2. Experimental methods The iron (II) chloride, nickel (II) chloride and sodium borohydride (NaBH4) were from E. Merck (Darmstadt); the isooctane and n-butanol were supplied by Flucka and the cationic water/cetyl-trimethyl-amonium-bromide (CTAB) surfactant was obtained from ALFA. The aqueous solution containing cations was obtained by dissolving appropriate amounts of iron and nickel chlorides. Two types of microemulsions were prepared by solubilizing aqueous Ni2+ (0.4 M) and Fe2+ (0.1 M) ions or NaBH4 (0.8 M) into a mixture of CTAB, n-butanol and isooctane. CTAB and n-butanol were used as the surfactant and co-surfactant, respectively. The stable, one-phase, microemulsion compositions were selected according to the pseudo-ternary phase diagram taken from Ref. [11]. The composition of the microemulsion used is shown in the composition diagram in Fig. 1. The synthesis of iron–nickel alloy nanoparticles was performed by mixing equal volumes of both microemulsion solutions. The solution turned black after mixing and was allowed to react for 1 h at 25 1C under nitrogen to minimize the oxidation. The micelle solution was disrupted by adding an excess amount of ethanol. The surfactant was removed with repeated methanol washing. The resulting powder containing the aggregates of Fe0.2Ni0.8 was black. The structural characterization was performed using electron microscopy. The as-synthesized Fe–Ni alloy powder was heat treated in an evacuated silica glass tube at 700 1C for 5 h. The particle morphology was inspected by high-resolution TEM (HREM) using a fieldemission electron-source scanning-transmission electron microscope (STEM) (JEOL 2010 F), operated at 200 kV. For the TEM investigations, the nanoparticles were deposited on a copper-grid-supported transparent carbon foil. The XRD measurements were performed on a AXSBruker/Siemens/D5005 diffractometer using Cu-Ka radiation (l ¼ 0.1542 nm). The XRD analysis data were
Fig. 1. Compositions selected for the synthesis of nanoparticles.
251
compared with the stored Powder Diffraction Files (PDFs) [12]. The magnetizations of the samples were measured using a Manics DSM-10 suscepto-magnetometer and a superconducting quantum interference device (SQUID) at temperatures from room temperature to 5 K. The specific surfaces of the synthesized nanopowders were determined by measuring the volume of adsorbed nitrogen at low temperatures using the BET method. The thermal properties of the synthesized nanopowders were determined with differential scanning calorimetry and thermo-gravimetric analysis (TA-4000 System Mettler). 3. Results and discussion The iron–nickel alloy nanoparticles were prepared from the selected microemulsion compositions within the CTAB and n-butanol/isooctane system at 25 1C(Fig. 1). Fig. 2 shows a typical XRD spectrum for the synthesized iron–nickel alloy nanoparticles. The spectrum shows three characteristic broad peaks at 2y ¼ 341, 481 and 611. The pattern does not match with the face-centered lattice reported for the iron–nickel alloy [12]. It could, however, be indexed according to the primitive cubic (pc) lattice with a ¼ 0.263 nm. According to the pc lattice, the three peaks observed belong to the (1 0 0), (1 1 0) and (1 1 1) reflections, as noted in Fig. 2. The impurities in the heat-treated sample at 700 1C are a result of the reaction of the nickel–iron alloy nanopowder with silica glass. The diffraction pattern, which were ascribed to impurities, can be assigned to the nickel and iron silicates according to PDF files. No oxides could be detected in the mixture after the heat treatment. Fig. 3 shows the TEM images and the corresponding electron diffraction patterns of two characteristic samples. Sample A (Fig. 3(a)) showed spherical nanoparticles, approximately 2.5 nm in size. HREM imaging (Fig. 3(b)) suggested that the nanoparticles had relatively poor crystallinity. The TEM image of sample C (Fig. 3(c)) showed particles with a rod-like shape, 3–5-nm wide and about 100-nm long. However, the HREM imaging (Fig. 3(d)) revealed that these particles were actually elongated agglomerates, which consisted of ‘‘chains’’ of much smaller spherical nanoparticles, approximately 4 nm in size. The electron diffraction patterns taken for these two samples (insets of Figs. 3(a) and (c)) show three broad, diffuse rings, which could be indexed with the pc cell of a 0.26 nm, observed using XRD. The (1 0 0) ring showing an inter-planar distance d of approximately 0.26 nm is the brightest, and merged with the (1 1 0) ring around a d-value of 0.19 nm, whereas the (1 1 1) ring around a d-value of 0.14 nm is well separated. The rings are diffuse and highly broadened due to their poor crystallinity and the small size of the nanoparticles. The EDX microanalysis of both samples was consistent with the starting composition of Fe0.2Ni0.8. The XRD as well as the electron diffraction in the TEM suggested that the iron–nickel alloy nanoparticles have a pc
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252
111(bcc)
6000
Lin (Counts)
200 (bcc)
4000 220 (bcc) (b)
2000 100 (pc) 110 (pc)
111 (pc)
(a)
0 30
40
50
60
70
80
2-Theta - Scale Fig. 2. (a) Typical X-ray powder diffraction patterns of as-synthesized nickel–iron alloy sample A and (b) sample A heat treated at 700 1C in vacuum.
Fig. 3. Morphology of synthesized iron–nickel alloy nanoparticles: (a) TEM image of sample A with spherical nanoparticles (inset: a corresponding electron diffraction pattern). (b) TEM image of sample C with rod-like agglomerates of nanoparticles, (c) HREM image of sample A, (d) HREM image of sample C.
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Table 1 Composition of microemulsions, specific surface area (As), average grain size obtained from specific surface area (dA), average particle size obtained from XRD (dx) and specific magnetization (Ms) Sample
[H2O]/[CTAB] W
[1-butanol]/[CTAB] P
As (m2/g)
dA (nm)
dx (nm)
Ms (emu/g)
A B C
16.17 20.2 27
4.9 4.9 4.9
160.21 60.54 82.93
4.2 11.3 8.2
4.0 3.7 3.2
12 12 6
lattice with 0.263 nm. The lattice parameter of 0.263 nm is close to the sum of the atomic radii of nickel (rNi ¼ 0.124 nm) and iron (rFe ¼ 0.126 nm), i.e., 0.250 nm. It is supposed that the lattice sites of iron and nickel are randomly distributed over the lattice sites with two iron atoms per eight primitive lattice unit cells. If the iron and nickel positions in the lattice were ordered we would expect the unit-cell parameter to be at least doubled. To the best of our knowledge, the primitive lattice structure of the nickel–iron alloy has not been documented in the literature yet. Only the FCC and the BCC structures have been reported [3,7]. In all other reported cases of iron–nickel alloy synthesis, whether in bulk form or as nanoparticles, the preparation conditions were associated with higher temperatures. In our case, the synthesis proceeded at room temperature. One argument that can be put forward is the decrease of the coordination of the atoms in the alloy when changing the crystal structure to pc. Namely, when the surface of a system strongly increases the number of unsaturated bonds on the particles’ surfaces increases as well. This means that the system can mitigate the large change in the unsaturated bond increase during the formation of nanoparticles to change the coordination in the structure. The coordination is 12 in FCC, 8 in BCC and 6 in pc. When the structure changes to pc, the system can mitigate the increase in the number of unsaturated bonds when increasing the surface area of the system-assembly of nanoparticles. When the samples were heat treated in a vacuum at 700 1C for 6 h, the usual BCC structure was obtained. In Fig. 2(b), the X-ray diffraction pattern of the heat-treated sample A shows the presence of the reflection of the BCC structure in addition to the reflections of the retained, untransformed, pristine pc phase. Besides, some reflections of impurities formed during the heat treatment of Fe–Ni alloy nanoparticles inside the silica glass tube, can be noticed. The as-synthesized phase transforms to the BCC phase, characteristic for Ni–Fe alloys [3,7]. As shown in Fig. 3, the morphologies of the synthesized nanoparticles change with the composition of the microemulsion used. Two samples, A and C, exhibited the same structure and composition; therefore, we can suppose that the difference in the habit of the synthesized particles must lie in the structure of the reverse micelles, out of which the particles were grown. The formation of elliptical or very long, rod-like reverse micelles, which are known to exist in
microemulsion systems that comprise CTAB as the surfactant [13–15] can be the reason for the formation of rod-like ‘‘particles’’, which are in fact one-dimensional and/or uniaxial agglomerates adapted to the reversemicelle geometry. For the nanoparticles that were formed in the ‘‘water pools’’ of reverse micelles and were observed to be elongated and/or acicular, there were no further processes taking place during the particle syntheses. Therefore, it is clear that the reason for the formation of the elongated agglomerates must be the micellar microstructure. It has also been reported that an alkali substance [16], salts or cosurfactant [17,18], such as 1-hexanol [19,20], might induce a spherical-to-worm-like transition. The BET-specific surface areas per unit mass of samples are given in Table 1. The average grain size obtained from the specific surface area in relation to the particle dimension estimated from the Sherrer equation are similar for sample A, but differ for samples B and C, which were agglomerated. The real size of the particles can be estimated from the HREM images, and they are fairly close to the dx values. The relatively small average grain size associated with the low saturation magnetization matches the powder morphology and the corresponding magnetic properties. The saturation magnetization of the nanoparticles is much lower than that of the bulk (80 emu/g). There are many factors that can cause a decrease in the magnetization with the grain size, e.g., crystal lattice defects, the mass effects of absorbed water, chemical and physical changes on the surface, magnetic degradation on the surface, redistribution of cation sites in the lattice. Since the grain size of the synthesized permalloy nanoparticles, dx ¼ 3–4 nm, is very small it is the particles’ ‘‘volume’’ of nonmagnetic regions on the grain surfaces, which is relatively large, that cannot contribute to the saturation magnetization during measurements [20–22]. Though the magnetization measurements at room temperature up to 10 kOe did not fully saturate the sample, which indicates a significant superparamagnetic contribution to the magnetization, the shape does indicate considerable magnetic ordering at room temperature (Fig. 4). The temperature dependence of the magnetization investigated under zero-field cooling, MZFC and field cooling, MFC revealed the superparamagnetic behavior of
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Fig. 4. The hysteresis of nickel–iron alloy nanoparticles of samples A and C at room temperature.
0.008 0.008
0.007
FC 0.006 m [emu]
0.005 m [emu]
FC
0.006
0.004
0.004 0.002 ZFC
0.003 0.000
0.002 0
2
4
6
0.001 0.000
8 10 12 14 16 18 T [K]
ZFC 0
50
100
150
200
250
T [K]
Fig. 5. Temperature dependence of the magnetization curves under ZFC and FC of nickel–iron nanoparticles of sample A.
the magnetic nanoparticles (Fig. 5). The sharp maximum of the MZFC at Tmax ¼ 7 K and the deviation of the MZFC and MFC curves close to Tmax indicate a narrow particle size distribution. It is well known that the superparamagnetic blocking temperature decreases with decreasing particle size. The blocking temperature should roughly satisfy the relationship TB ¼ KV/30 kB, where K is the anisotropy constant, kB the Boltzman constant, and V the average volume of the particles. Taking the measured blocking temperature of 7 K and the particle diameter of 4.0 nm, one can estimate the anisotropy constant of the magnetic particles to be K ¼ 8.6 104 J/m3. This value is about one order of magnitude larger than that of permalloy, which is 0.7 104 J/m3 [23]. The TG analyses in Fig. 6 show that the weight loss is continuous and is most probably a result of
water evaporation in the temperature interval up to 100 1C; in the temperature interval 100–250 1C the desorption of the surfactant from the alloy surface is to be expected, and after that the disintegration of the retained material on the surface occurs. The total weight loss is about 20 wt%. On the DSC diagram, Fig. 7, two endothermic peaks were identified, i.e., around 96 and 368 1C, respectively. The endothermic peak at 96 1C, which is irreversible, characterizes a process of water evaporation, while the endothermic peak at 368 1C was attributed to the Curie transition temperature. The Curie transition temperature of nanoparticles is lower than that of the bulk, which occurs at temperatures close to 580 1C. In the case when we are dealing with nanosized particles the transition temperature is certainly much lower. The finite-size scaling theory predicts that the shift in the transition temperature from that of the bulk should depend on the size of the system [24]. Namely, the increase in the surface-to-volume ratio leads to a decrease in the overall ordering of the magnetic phase due to the fact that a large fraction of the atoms on a disordered surface, about 60% of them, are less tightly subjected to a super-exchange interaction, and also the crystal structure is less ordered in comparison to the grain interior. A less intense and more broad endothermic peak of the second-order transition occurs as a result of a series of various temperatures, dependent on the crystallite sizes, where the transition takes place. We did not observe an exothermic peak contribution due to crystallization. This is in agreement with the TEM observation of the samples after heat treatment, which reveals that in this temperature region no increase in the grain size was noticed. Such a grain-size increase would usually accompany a crystallization process. On the other hand, the remarkable exothermic peak at 595 1C can be assigned to the transition from the pc to the BCC structure of the Ni–Fe alloy nanoparticles.
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Fig. 6. TG analyses of nickel–iron alloy nanoparticles.
Fig. 7. The DSC diagram of as-synthesized nickel–iron alloy nanoparticles.
4. Conclusion In this paper, we report a methodology for the synthesis of Fe–Ni alloy nanoparticles. The morphology of the iron–nickel alloy nanoparticles depends mainly on the microemulsion composition. According to nanodimension, the particles exhibit a primitive cubic (pc) structure in difference to body-centered cubic (BCC) structure of the bulk material. The temperature dependence of the magnetization investigated reveal the superparamagnetic behavior. The sharp maximum of the MZFC indicates a narrow particle size distribution. The magnetization of the nanoparticles is much lower than that of the bulk material, reflecting the influence of the nanodimensions on the particles’ magnetizations.
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