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water association colloids
Journal of Magnetism and Magnetic Materials 155 (1996) 67-69
•ii• ELSEVIER
Journal of amnalnatlsm magnetic materials
Preparation of oL-Feparticles by reduction of ferrous ion in lecithin/cyclohexane/water association colloids Jason L. Cain, Scott
R.
Harrison, Jacqueline A. Nikles, David E. Nikles
*
CenterJbr Materials for Information Technology. The Unicersio' of Alabama, Tuscaloosa. AL 35487-0336, USA
Abstract Ferromagnetic a-Fe particles were prepared by reduction of Fe 2+ by sodium borohydride in the presence of lecithin/cyclohexane/water association colloids. The particles consisted of a chains of iron spheres, 50-150 nm in diameter, held together by worm-like lecithin assemblies, about 100 nm in diameter and micrometers in length. The coercivity depended on the reaction medium and ranged from 4 to 419 Oe, with the highest coercivity particles obtained in the cyclohexane-rich region of the ternary phase diagram.
1. Introduction There have been many reports in the literature of the preparation of 5-100 nm size magnetic particles in reverse micelles. Magnetite particles were prepared by precipitation of mixtures of FeC12 and FeCI 3 in aerosol OT (AOT)/water/isooctane [1] or A O T / w a t e r / c y c l o h e x a n e [2] microemulsions. Barium ferrite nanoparticles were prepared by the coprecipitation of precursor carbonates in cetyltrimethylammonium bromide ( C T A B ) / w a t e r / butanol/octane microemulsions, followed by calcining [3]. Cobalt [4] or iron [5] nanoparticles were prepared by sodium borohydride reduction of Co 2+ or Fe z+ solubilized in A O T / w a t e r / h y d r o c a r b o n microemulsions. Our objective is to prepare acicular iron particles with particle size 50 nm and coercivities greater than 2000 Oe. The approach is to use association colloids as templates to control the size and shape of iron particles formed in their presence. Reverse micelles consist of small droplets of water dispersed in a hydrocarbon solvent by a surfactant. When AOT is used as the surfactant, the droplets are spherical and monodisperse over a wide range of temperature, pressure and composition [6]. The Stokes radii for the droplets range from 1.5 to 10 nm with the size depending on w, the molar ratio of water to surfactant. The requirement for high coercivity means that the iron particles must be acicular and has led to the consideration of ternary surfactant/water/hydrocarbon systems that give rod-like
Corresponding author. dnikles @ualvm.ua.edu.
Fax:
+ 1-205-348-9104;
email:
assemblies. Replacing the sodium counter ion in AOT with a bivalent counter ion, e.g. Cd 2+ Cu 2+, or Co 2+ gave rod-like water-in-oil assemblies at low values of w [7,8]. We have chosen to work with the l e c i t h i n / cyclohexane/water system. When water is added to hydrocarbon solutions of soybean lecithin until the value of w exceeds l, there is a tremendous increase in the solution viscosity [9]. Light scattering and neutron diffraction experiments have shown that the lecithin forms long, flexible tubes with an aspect (length to diameter) ratio approaching 100 [10]. Here we examine the use of lecithin tubes as templates for the formulation of acicular iron particles.
2. Preparation of oL-Fe particles Soybean lecithin (Fisher Scientific), 8.00 g, was dissolved in 92.00 g cyclohexane by stirring for 12 h. The next day a solution of 0.22 g (1.1 mmol) FeCI 2 • 4H,_O in 11.00 g water was added to the lecithin solution. The mixture was stirred vigorously for 15 min to mix, gently stirred for 3 h to ensure complete mixing, and then allowed to equilibrate for three days without stirring. After three days the mixture phase-separated into an optically clear, yellow-colored upper phase, and a viscous, turbid lower phase. The upper phase was discarded and the lower phase was used for iron formation. Under nitrogen, sodium borohydride, 0.0847 g (2.24 mmol), was added and the mixture was stirred for 5 min, whereupon a black precipitate formed. The supernatant liquid was decanted away from the Fe particles, the particles were rinsed five times with absolute ethanol, and then allowed to dry under vacuum at room temperature overnight. The particles were stable to oxidation for a number of days; long enough for structural and magnetic characterization.
0304-8853/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0 3 0 4 - 8 8 5 3 ( 9 5 ) 0 0 6 5 6 - 7
68
J.L. Cain et al. // Journal of Magnetism and Magnetic Materials 155 (1996) 67-69
3. Results and discussion Reduction of Fe 2÷ in l e c i t h i n / c y c l o h e x a n e / w a t e r association colloids gave iron particles that were either superparamagnetic or ferromagnetic, depending on the reaction conditions. X-ray diffraction showed only the Fe(110) reflection at d = 202.7 pm. In our initial experiments with values of w near 1, an equal volume of ethanol was added as a cosolvent to avoid phase separation. The data in Table 1 show the effects of the reaction conditions on the magnetic properties of the Fe particles. A low Fe 2÷ concentration, 12.3 m M (trial l) gave superparamagnetic particles, while higher concentrations, above 15 mM, gave ferromagnetic particles. Further increasing the Fe 2+ concentration, from 24 mM (trial 3) to 34 mM (trial 4), increased H c from 4 to 150 Oe. Doubling w from 4.3 to 8.8 (trial 5), and eliminating the added ethanol, increased Hc to 277 Oe. Further increases in w gave two phases, even with addition of ethanol, and ethanol was not used in the remainder of this study.
Table 1 ct-Fe particles prepared by borohydride reduction of Fe 2+ in lecithin/cyclohexane/water association colloids stabilized by ethanol cosolvent. Trial 5 contained no ethanol Trial I 2 3 4 5 a
Lecithin
Cyclohexane
(~)
(~)
6.9 6.9 9.0 9.0 5.9
92.2 92.2 90.1 90.1 92.2
w 5.7 5.7 4.3 4.3 8.8
[Fe 2+ ]
Hc
mM
(Oe)
12.3 17.3 23.7 33.6 21.0
a 12 4 150 277
Fig. 1. Transmission electron micrograph of the iron particles prepared in trial 17.
The data in Table 2 show the effects of colloid composition on the coercivity of the Fe particles. For these compositions the mixture formed two phases; a yellow, transparent low density phase and a turbid, viscous, higher density phase. The low density phase was separated and the addition of sodium borohydride to this phase did not give iron particles. This phase was therefore discarded in all subsequent experiments. Because of this phase separation, the composition of the lower phase was not certain
Superparamagnetic.
Table 2 ct-Fe particles prepared by sodium borohydride reduction of Fe 2÷ in lecithin/cyclohexane/water association colloids Trial
Lecithin (%)
Cyclohexane (%)
w
[Fe 2+ ] mM
He (Oe)
6 7 8 9 10 11 12 13 14 15 16 17 18 19
22.7 22.7 22.7 14.8 10.0 13.3 16.7 9.7 10.2 8.5 8.9 7.2 4.6 5.9
37.9 42.2 42.2 45.8 47.1 52.2 59.1 71.0 74.6 76.3 80.4 82.9 91.7 92.9
48 42 42 73 18 108 40 55 41 50 33 38 22 5.5
44.8 31.1 15.4 40.1 28.4 27.1 24.2 17.5 13.8 13.4 9.5 9.5 8.0 20.9
97 124 141 250 319 216 285 390 380 382 337 419 68 101
Fig. 2. Transmission electron micrograph of the iron particles prepared in trial 17 (close up).
J.L. Cain et al. / Journal of Magnetism and Magnetic Materials 155 (1996) 67-69
and will be characterized in future experiments. Addition of sodium borohydride to the lower phase immediately gave a black precipitate, which in all cases was ferromagnetic a-Fe. Until the composition of the lower phase is determined, no conclusions can be drawn about the correlation between phase composition and coercivity. Transmission electron micrographs for the highest coercivity iron particles from trial 17 (Fig. 1) show worm-like assemblies about 100 nm in diameter and micrometers in length, each containing many iron particles. This structure was observed for a number of samples having high coercivity, Fig. 2 shows that the iron particles were roughly spherical with diameters in the range 50-150 nm. The particles were held together in a chain of spheres by the cylindrical lecithin assembly. Clearly, lecithin forms large acicular assemblies in the cyclohexane-rich region of the phase diagram and the assemblies persisted when Fe 2+ was dissolved in the tubes. However, the iron particles did not assume the acicular shape of the tubes. Instead, many iron particles were nucleated and grew into spheres in each tubular assembly. Future work will search for reaction conditions where few iron particles nucleate in the tubes, and then grow into acicular particles, templated by the lecithin tubes.
69
Acknowledgements This work was supported primarily by the MRSEC Program of the National Science Foundation under Award Number DMR-9400399. Scott Harrison was supported by the National Science Foundation's Research Experiences for Undergraduates Program during the summer of 1994.
References [1] M. Gobe, K. Kon-No, K. Kandori and A. Kitahara, J Colloid Interface Sci 93 (1983) 293. [2] K.M. Lee, C.M. Sorensen, K.J. Klabunde and G.C. Hadjipanayis, IEEE Trans. Magn. 28 (1992) 3180. [3] V. Pillai, P, Kumar, M.S. Multani and D.O. Shah, Colloids Surfaces A: Physicochem. Eng. Aspects 80 (1993) 69. [4] J.P. Chen, K.M. Lee, C.M. Sorensen, K.J. Klabunde and G.C. Hadjipanayis, J. Appl. Phys. 75 (1994) 5876. [5] J. Rivas, M.A. Lopez-Quintela, J.A. Lope×-Perez, L. Liz and R.J. Duro, IEEE Trans. Magn. 29 (1993) 2655. [6] M. Zulauf and H.-F. Eicke, J. Phys. Chem. 83 (1979) 480. [7] C. Petit, P. Li×on and M.P. Pileni, Langmuir 7 (1991) 2620. [8] J. Eastoe, G. Fragneto, B.H. Robinson, T.F. Towey, R.K. Heenan and F.J. Leng, J. Chem. Soc. Faraday Trans. 88 (1992) 461. [9] R. Scartazzini and P.L. Luisi, J. Phys. Chem. 92 (1988) 829. [10] P. Schurtenberger, L.J. Magid, S.M, King and P. Lindner, J. Phys. Chem. 95 (1991) 4173.
•ii• ELSEVIER
Journal of amnalnatlsm magnetic materials
Preparation of oL-Feparticles by reduction of ferrous ion in lecithin/cyclohexane/water association colloids Jason L. Cain, Scott
R.
Harrison, Jacqueline A. Nikles, David E. Nikles
*
CenterJbr Materials for Information Technology. The Unicersio' of Alabama, Tuscaloosa. AL 35487-0336, USA
Abstract Ferromagnetic a-Fe particles were prepared by reduction of Fe 2+ by sodium borohydride in the presence of lecithin/cyclohexane/water association colloids. The particles consisted of a chains of iron spheres, 50-150 nm in diameter, held together by worm-like lecithin assemblies, about 100 nm in diameter and micrometers in length. The coercivity depended on the reaction medium and ranged from 4 to 419 Oe, with the highest coercivity particles obtained in the cyclohexane-rich region of the ternary phase diagram.
1. Introduction There have been many reports in the literature of the preparation of 5-100 nm size magnetic particles in reverse micelles. Magnetite particles were prepared by precipitation of mixtures of FeC12 and FeCI 3 in aerosol OT (AOT)/water/isooctane [1] or A O T / w a t e r / c y c l o h e x a n e [2] microemulsions. Barium ferrite nanoparticles were prepared by the coprecipitation of precursor carbonates in cetyltrimethylammonium bromide ( C T A B ) / w a t e r / butanol/octane microemulsions, followed by calcining [3]. Cobalt [4] or iron [5] nanoparticles were prepared by sodium borohydride reduction of Co 2+ or Fe z+ solubilized in A O T / w a t e r / h y d r o c a r b o n microemulsions. Our objective is to prepare acicular iron particles with particle size 50 nm and coercivities greater than 2000 Oe. The approach is to use association colloids as templates to control the size and shape of iron particles formed in their presence. Reverse micelles consist of small droplets of water dispersed in a hydrocarbon solvent by a surfactant. When AOT is used as the surfactant, the droplets are spherical and monodisperse over a wide range of temperature, pressure and composition [6]. The Stokes radii for the droplets range from 1.5 to 10 nm with the size depending on w, the molar ratio of water to surfactant. The requirement for high coercivity means that the iron particles must be acicular and has led to the consideration of ternary surfactant/water/hydrocarbon systems that give rod-like
Corresponding author. dnikles @ualvm.ua.edu.
Fax:
+ 1-205-348-9104;
email:
assemblies. Replacing the sodium counter ion in AOT with a bivalent counter ion, e.g. Cd 2+ Cu 2+, or Co 2+ gave rod-like water-in-oil assemblies at low values of w [7,8]. We have chosen to work with the l e c i t h i n / cyclohexane/water system. When water is added to hydrocarbon solutions of soybean lecithin until the value of w exceeds l, there is a tremendous increase in the solution viscosity [9]. Light scattering and neutron diffraction experiments have shown that the lecithin forms long, flexible tubes with an aspect (length to diameter) ratio approaching 100 [10]. Here we examine the use of lecithin tubes as templates for the formulation of acicular iron particles.
2. Preparation of oL-Fe particles Soybean lecithin (Fisher Scientific), 8.00 g, was dissolved in 92.00 g cyclohexane by stirring for 12 h. The next day a solution of 0.22 g (1.1 mmol) FeCI 2 • 4H,_O in 11.00 g water was added to the lecithin solution. The mixture was stirred vigorously for 15 min to mix, gently stirred for 3 h to ensure complete mixing, and then allowed to equilibrate for three days without stirring. After three days the mixture phase-separated into an optically clear, yellow-colored upper phase, and a viscous, turbid lower phase. The upper phase was discarded and the lower phase was used for iron formation. Under nitrogen, sodium borohydride, 0.0847 g (2.24 mmol), was added and the mixture was stirred for 5 min, whereupon a black precipitate formed. The supernatant liquid was decanted away from the Fe particles, the particles were rinsed five times with absolute ethanol, and then allowed to dry under vacuum at room temperature overnight. The particles were stable to oxidation for a number of days; long enough for structural and magnetic characterization.
0304-8853/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0 3 0 4 - 8 8 5 3 ( 9 5 ) 0 0 6 5 6 - 7
68
J.L. Cain et al. // Journal of Magnetism and Magnetic Materials 155 (1996) 67-69
3. Results and discussion Reduction of Fe 2÷ in l e c i t h i n / c y c l o h e x a n e / w a t e r association colloids gave iron particles that were either superparamagnetic or ferromagnetic, depending on the reaction conditions. X-ray diffraction showed only the Fe(110) reflection at d = 202.7 pm. In our initial experiments with values of w near 1, an equal volume of ethanol was added as a cosolvent to avoid phase separation. The data in Table 1 show the effects of the reaction conditions on the magnetic properties of the Fe particles. A low Fe 2÷ concentration, 12.3 m M (trial l) gave superparamagnetic particles, while higher concentrations, above 15 mM, gave ferromagnetic particles. Further increasing the Fe 2+ concentration, from 24 mM (trial 3) to 34 mM (trial 4), increased H c from 4 to 150 Oe. Doubling w from 4.3 to 8.8 (trial 5), and eliminating the added ethanol, increased Hc to 277 Oe. Further increases in w gave two phases, even with addition of ethanol, and ethanol was not used in the remainder of this study.
Table 1 ct-Fe particles prepared by borohydride reduction of Fe 2+ in lecithin/cyclohexane/water association colloids stabilized by ethanol cosolvent. Trial 5 contained no ethanol Trial I 2 3 4 5 a
Lecithin
Cyclohexane
(~)
(~)
6.9 6.9 9.0 9.0 5.9
92.2 92.2 90.1 90.1 92.2
w 5.7 5.7 4.3 4.3 8.8
[Fe 2+ ]
Hc
mM
(Oe)
12.3 17.3 23.7 33.6 21.0
a 12 4 150 277
Fig. 1. Transmission electron micrograph of the iron particles prepared in trial 17.
The data in Table 2 show the effects of colloid composition on the coercivity of the Fe particles. For these compositions the mixture formed two phases; a yellow, transparent low density phase and a turbid, viscous, higher density phase. The low density phase was separated and the addition of sodium borohydride to this phase did not give iron particles. This phase was therefore discarded in all subsequent experiments. Because of this phase separation, the composition of the lower phase was not certain
Superparamagnetic.
Table 2 ct-Fe particles prepared by sodium borohydride reduction of Fe 2÷ in lecithin/cyclohexane/water association colloids Trial
Lecithin (%)
Cyclohexane (%)
w
[Fe 2+ ] mM
He (Oe)
6 7 8 9 10 11 12 13 14 15 16 17 18 19
22.7 22.7 22.7 14.8 10.0 13.3 16.7 9.7 10.2 8.5 8.9 7.2 4.6 5.9
37.9 42.2 42.2 45.8 47.1 52.2 59.1 71.0 74.6 76.3 80.4 82.9 91.7 92.9
48 42 42 73 18 108 40 55 41 50 33 38 22 5.5
44.8 31.1 15.4 40.1 28.4 27.1 24.2 17.5 13.8 13.4 9.5 9.5 8.0 20.9
97 124 141 250 319 216 285 390 380 382 337 419 68 101
Fig. 2. Transmission electron micrograph of the iron particles prepared in trial 17 (close up).
J.L. Cain et al. / Journal of Magnetism and Magnetic Materials 155 (1996) 67-69
and will be characterized in future experiments. Addition of sodium borohydride to the lower phase immediately gave a black precipitate, which in all cases was ferromagnetic a-Fe. Until the composition of the lower phase is determined, no conclusions can be drawn about the correlation between phase composition and coercivity. Transmission electron micrographs for the highest coercivity iron particles from trial 17 (Fig. 1) show worm-like assemblies about 100 nm in diameter and micrometers in length, each containing many iron particles. This structure was observed for a number of samples having high coercivity, Fig. 2 shows that the iron particles were roughly spherical with diameters in the range 50-150 nm. The particles were held together in a chain of spheres by the cylindrical lecithin assembly. Clearly, lecithin forms large acicular assemblies in the cyclohexane-rich region of the phase diagram and the assemblies persisted when Fe 2+ was dissolved in the tubes. However, the iron particles did not assume the acicular shape of the tubes. Instead, many iron particles were nucleated and grew into spheres in each tubular assembly. Future work will search for reaction conditions where few iron particles nucleate in the tubes, and then grow into acicular particles, templated by the lecithin tubes.
69
Acknowledgements This work was supported primarily by the MRSEC Program of the National Science Foundation under Award Number DMR-9400399. Scott Harrison was supported by the National Science Foundation's Research Experiences for Undergraduates Program during the summer of 1994.
References [1] M. Gobe, K. Kon-No, K. Kandori and A. Kitahara, J Colloid Interface Sci 93 (1983) 293. [2] K.M. Lee, C.M. Sorensen, K.J. Klabunde and G.C. Hadjipanayis, IEEE Trans. Magn. 28 (1992) 3180. [3] V. Pillai, P, Kumar, M.S. Multani and D.O. Shah, Colloids Surfaces A: Physicochem. Eng. Aspects 80 (1993) 69. [4] J.P. Chen, K.M. Lee, C.M. Sorensen, K.J. Klabunde and G.C. Hadjipanayis, J. Appl. Phys. 75 (1994) 5876. [5] J. Rivas, M.A. Lopez-Quintela, J.A. Lope×-Perez, L. Liz and R.J. Duro, IEEE Trans. Magn. 29 (1993) 2655. [6] M. Zulauf and H.-F. Eicke, J. Phys. Chem. 83 (1979) 480. [7] C. Petit, P. Li×on and M.P. Pileni, Langmuir 7 (1991) 2620. [8] J. Eastoe, G. Fragneto, B.H. Robinson, T.F. Towey, R.K. Heenan and F.J. Leng, J. Chem. Soc. Faraday Trans. 88 (1992) 461. [9] R. Scartazzini and P.L. Luisi, J. Phys. Chem. 92 (1988) 829. [10] P. Schurtenberger, L.J. Magid, S.M, King and P. Lindner, J. Phys. Chem. 95 (1991) 4173.