Monodisperse MFe2O4 (M ) Fe, Co, Mn) Nanoparticles

Monodisperse MFe2O4 (M ) Fe, Co, Mn) Nanoparticles Shouheng Sun,*,† Hao Zeng,† David B. Robinson,† Simone Raoux,‡ Philip M. Rice,‡ Shan X. Wang,§ and Guanxiong Li§ Contribution from the IBM T. J. Watson Research Center, Yorktown Heights, New York 10598, IBM Almaden Research Center, 650 Harry Road, San Jose, California 95120, and Department of Materials Science and Engineering, Stanford UniVersity, Stanford, California 94305 Received August 22, 2003; E-mail: ssun@us.ibm.com Abstract: High-temperature solution phase reaction of iron(III) acetylacetonate, Fe(acac)3, with 1,2- hexadecanediol in the presence of oleic acid and oleylamine leads to monodisperse magnetite (Fe3O4) nanoparticles. Similarly, reaction of Fe(acac)3 and Co(acac)2 or Mn(acac)2 with the same diol results in monodisperse CoFe2O4 or MnFe2O4 nanoparticles. Particle diameter can be tuned from 3 to 20 nm by varying reaction conditions or by seed-mediated growth. The as-synthesized iron oxide nanoparticles have a cubic spinel structure as characterized by HRTEM, SAED, and XRD. Further, Fe3O4 can be oxidized to Fe2O3, as evidenced by XRD, NEXAFS spectroscopy, and SQUID magnetometry. The hydrophobic nanoparticles can be transformed into hydrophilic ones by adding bipolar surfactants, and aqueous nanoparticle dispersion is readily made. These iron oxide nanoparticles and their dispersions in various media have great potential in magnetic nanodevice and biomagnetic applications. Introduction Magnetic iron oxide nanoparticles and their dispersions in various media have long been of scientific and technological interest. The cubic spinel structured MFe2O4, or MOâFe2O3, represents a well-known and important class of iron oxide materials where oxygen forms an fcc close packing, and M2+ and Fe3+ occupy either tetrahedral or octahedral interstitial sites.1 By adjusting the chemical identity of M2+, the magnetic configurations of MFe2O4 can be molecularly engineered to provide a wide range of magnetic properties. Due in part to this versatility, nanometer-scale MFe2O4 materials have been among the most frequently chosen systems for studies of nanomagnetism and have shown great potential for many important technological applications, ranging from information storage and electronic devices to medical diagnostics and drug delivery. Dispersions of magnetic MFe2O4 nanoparticles, es- pecially magnetite (Fe3O4) nanoparticles, have been used widely not only as ferrofluids in sealing, oscillation damping, and position sensing2 but also as promising candidates for biomol- ecule tagging, imaging, sensing, and separation.3 Depending on the chemical identity of M2+, the densely packed solid state form of nanocrystalline MFe2O4-based materials, on the other hand, can have either high magnetic permeability and electrical resistivity (for M representing one or the mixed components from Co, Li, Ni, Zn, etc.) or half-metallicity (for M ) Fe), and may be a potential candidate for future high-performance electromagnetic4 and spintronic devices.5 To use MFe2O4 nanoparticles for future highly sensitive magnetic nanodevice and biomedical applications, a practical route to monodisperse MFe2O4 nanoparticles with diameters smaller than 20 nm and a tight size distribution (less than 10% standard deviation) is needed. A commonly used solution phase procedure for making such particles has been the coprecipitation of M2+ and Fe3+ ions by a base, usually NaOH or NH3âH2O in an aqueous solution6 or in a reverse micelle template.7 Although this precipitation method is suitable for mass production of † IBM T. J. Watson Research Center. ‡ IBM Almaden Research Center. § Stanford University. (1) (a) West, A. R. Basic Solid State Chemistry; John Wiley & Sons: New York, 1988; pp 356-359. (b) O’Handley, R. C. Modern Magnetic Materials-Principles and Applications; John Wiley & Sons: New York, 2000; pp 126-132. (2) Raj, K.; Moskowitz, R. J. Magn. Magn. Mater. 1990, 85, 233. (3) (a) Ha¨feli, U.; Schu¨tt, W.; Teller, J.; Zborowski, M. Scientific and Clinical Applications of Magnetic Carriers; Plenum Press: New York, 1997. (b) Oswald, P.; Clement, O.; Chambon, C.; Schouman-Claeys, E.; Frija, G. Magn. Reson. Imaging 1997, 15, 1025. (c) Hergt, R.; Andra, W.; d’Ambly, C. G.; Hilger, I.; Kaiser, W. 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Phys. ReV. B 1991, 44, 13319. (c) Anisimov, V. I.; Elfimov, I. S.; Hamada, N.; Terakura, K. Phys. ReV. B 1996, 54, 4387. (d) Gong, G. Q.; Gupta, A.; Xiao, G.; Qian, W.; Dravid, D. P. Phys. ReV. B 1997, 56, 5096. (e) Coey, J. M. D.; Berkowitz, A. E.; Balcells, L. I.; Putris, F. F.; Parker, F. T. Appl. Phys. Lett. 1998, 72, 734. (f) Li, X. W.; Gupta, A.; Xiao, G.; Gong, G. Q. J. Appl. Phys. 1998, 83, 7049. (g) Kiyomura, T.; Maruo, Y.; Gomi M. J. Appl. Phys. 2000, 88, 4768. (h) Moore, R. G. C.; Evans, S. D.; Shen, T.; Hodson, C. E. C. Physica E 2001, 9, 253. (i) Versluijs, J. J.; Bari, M. A.; Coey, J. M. D. Phys. ReV. Lett. 2001, 87, 26601. (j) Soeya, S.; Hayakawa, J.; Takahashi, H.; Ito, K.; Yamamoto, C.; Kida, A.; Asano, H.; Matsui, M. Appl. Phys. Lett. 2002, 80, 823. (k) Sorenson, T. A.; Morton, S. A.; Dan Waddill, G.; Switzer, J. A. J. Am. Chem. Soc. 2002, 124, 7604. Published on Web 12/10/2003 10.1021/ja0380852 CCC: $27.50 © 2004 American Chemical Society J. AM. CHEM. SOC. 2004, 126, 273-279 9 273 magnetic MFe2O4 ferrofluids, it does require careful adjustment of the pH value of the solution for particle formation and stabilization, and it is difficult to control sizes and size distributions, particularly for particles smaller than 20 nm. An alternative approach to monodisperse iron oxide nanoparticles is via high-temperature organic phase decomposition of an iron precursor, for example, decomposition of FeCup3 (Cup: N- nitrosophenylhydroxylamine, C6H5N(NO)O-)8 or decomposition of Fe(CO)5 followed by oxidation to Fe2O3.9 The latter process has recently been extended to the synthesis of monodisperse cobalt ferrite (CoFe2O4) nanoparticles.10 Although significant progress in making monodisperse Fe2O3 and CoFe2O4 nano- particles has been made in organic phase reactions, there is still no general process for producing MFe2O4, especially Fe3O4 nanoparticles with the desired size and acceptable size distribu- tion. Recently, we reported a convenient organic phase process for making monodisperse Fe3O4 nanoparticles through the reaction of Fe(acac)3 and a long-chain alcohol.11 Our further experiments indicated that this reaction could be readily extended to the synthesis of MFe2O4 nanoparticles (with M ) Co, Ni, Mn, Mg, etc.) by simply adding a different metal acetylacetonate precursor to the mixture of Fe(acac)3 and 1,2- hexadecanediol. Here we present detailed syntheses and char- acterization of Fe3O4 and related MFe2O4 nanoparticles (with M ) Co and Mn as two examples) with sizes tunable from 3 to 20 nm in diameter. The process involves high-temperature (up to 305 °C) reaction of metal acetylacetonate with 1,2-hexade- canediol, oleic acid, and oleylamine. The size of the oxide nanoparticles can be controlled by varying the reaction tem- perature or changing metal precursors. Alternatively, with the smaller nanoparticles as seeds, larger monodisperse nanopar- ticles up to 20 nm in diameter can be synthesized by seed- mediated growth. The process does not require a low-yield fractionation procedure to achieve the desired size distribution and is readily scaled up for mass production. The nanoparticles can be dispersed into nonpolar or weakly polar hydrocarbon solvent, such as hexane or toluene. The hydrophobic nanopar- ticles can be transformed into hydrophilic ones by mixing with a bipolar surfactant, tetramethylammonium 11-aminounde- canoate, allowing preparation of aqueous nanoparticle disper- sions. These iron oxide nanoparticles and their dispersions in various media have great potential in magnetic nanodevice and biomagnetic applications. Experimental Section The synthesis was carried out using standard airless procedures and commercially available reagents. Absolute ethanol, hexane, and dichlo- romethane (99%) were used as received. Phenyl ether (99%), benzyl ether (99%), 1,2-hexadecanediol (97%), oleic acid (90%), oleylamine (>70%), cobalt(II) acetylacetonate, Mn(II) acetylacetonate, and poly- ethylenimine (water-free, average Mw ca. 25 000) were purchased from Aldrich Chemical Co. Iron(III) acetylacetonate was from Strem Chemicals, Inc. Tetramethylammonium 11-aminoundecanoate was prepared by titrating a methanolic suspension of 11-aminoundecanoic acid with methanolic tetramethylammonium hydroxide (both from Aldrich), evaporating the solvent under reduced pressure, and recrystal- lizing in tetrahydrofuran. Synthesis of 4 nm Fe3O4 Nanoparticle Seeds. Fe(acac)3 (2 mmol), 1,2-hexadecanediol (10 mmol), oleic acid (6 mmol), oleylamine (6 mmol), and phenyl ether (20 mL) were mixed and magnetically stirred under a flow of nitrogen. The mixture was heated to 200 °C for 30 min and then, under a blanket of nitrogen, heated to reflux (265 °C) for another 30 min. The black-brown mixture was cooled to room temperature by removing the heat source. Under ambient conditions, ethanol (40 mL) was added to the mixture, and a black material was precipitated and separated via centrifugation. The black product was dissolved in hexane in the presence of oleic acid (�0.05 mL) and oleylamine (�0.05 mL). Centrifugation (6000 rpm, 10 min) was applied to remove any undispersed residue. The product, 4 nm Fe3O4 nano- particles, was then precipitated with ethanol, centrifuged (6000 rpm, 10 min) to remove the solvent, and redispersed into hexane. Under identical conditions, reaction of Co(acac)2 (1 mmol) with Fe- (acac)3 led to 3 nm CoFe2O4 nanoparticles that could be readily dispersed into hexane, giving a dark red-brown hexane dispersion. Synthesis of 6 nm Fe3O4 Nanoparticle Seeds. Fe(acac)3 (2 mmol), 1,2-hexadecanediol (10 mmol), oleic acid (6 mmol), oleylamine (6 mmol), and benzyl ether (20 mL) were mixed and magnetically stirred under a flow of nitrogen. The mixture was heated to 200 °C for 2 h and then, under a blanket of nitrogen, heated to reflux (�300 °C) for 1 h. The black-colored mixture was cooled to room temperature by removing the heat source. Following the workup procedures described in the synthesis of 4 nm particles, a black-brown hexane dispersion of 6 nm Fe3O4 nanoparticles was produced. Similarly, by adding Co(acac)2 or Mn(acac)2, 10 nm CoFe2O4 or 7 nm MnFe2O4 nanoparticle seeds can be made. Synthesis of 8 nm Fe3O4 Nanoparticles via 6 nm Fe3O4 Seeds. Fe(acac)3 (2 mmol), 1,2-hexadecanediol (10 mmol), benzyl ether (20 mL), oleic acid (2 mmol), and oleylamine (2 mmol) were mixed and magnetically stirred under a flow of N2. A 84 mg sample of 6 nm Fe3O4 nanoparticles dispersed in hexane (4 mL) was added. The mixture was first heated to 100 °C for 30 min to remove hexane, then to 200 °C for 1 h. Under a blanket of nitrogen, the mixture was further heated to reflux (�300 °C) for 30 min. The black-colored mixture was cooled to room temperature by removing the heat source. Following the workup procedures described in the synthesis of 4 nm particles, a black-brown hexane dispersion of 8 nm Fe3O4 nanoparticles was produced. Similarly, 80 mg of 8 nm Fe3O4 seeds reacted with Fe(acac)3 (2 mmol) and the diol (10 mmol) led to 10 nm Fe3O4 nanoparticles. Using this seed-mediated growth, bigger nanoparticles of Fe3O4 up to 20 nm, CoFe2O4 up to 20 nm, or MnFe2O4 up to 18 nm have been made. Synthesis of Hydrophilic Fe3O4 Nanoparticles. Under ambient conditions, a hexane dispersion of hydrophobic Fe3O4 nanoparticles (about 20 mg in 0.2 mL) was added to a suspension of tetramethy- lammonium 11-aminoundecanoate (about 20 mg in 2 mL) in dichlo- romethane. The mixture was shaken for about 20 min, during which time the particles precipitated and separated using a magnet. The solvent and nonmagnetic suspension were decanted, and the precipitate was washed once with dichloromethane and separated again using a magnet (6) See for example: (a) Kang, Y. S.; Risbud, S.; Rabolt, J. F.; Stroeve, P. Chem. Mater. 1996, 8, 2209. (b) Hong, C.-Y.; Jang, I. J.; Horng, H. E.; Hsu, C. J.; Yao, Y. D.; Yang, H. C. J. Appl. Phys. 1997, 81, 4275. (c) Fried, T.; Shemer, G.; Markovich, G. AdV. Mater. 2001, 13, 1158. (d) Tang, Z. X.; Sorensen, C. M.; Klabunde, K. J.; Hadjipanayis, G. C. J. Colloid Interface Sci. 1991, 146, 38. (e) Zhang, Z. J.; Wang, Z. L.; Chakoumakos, B. C.; Yin, J. S. J. Am. Chem. Soc. 1998, 120, 1800. (f) Neveu, S.; Bec, A.; Robineau, M.; Talbol, D. J. Colloid Interface Sci. 2002, 255, 293. (7) See for example: (a) Pileni, M. P.; Moumen, N. J. Phys. Chem. B 1996, 100, 1867. (b) Liu, C.; Zou, B.; Rondinone, A. J.; Zhang, Z. J. J. Phys. Chem. B 2000, 104, 1141. (8) Rockenberger, J.; Scher, E. C.; Alivisatos, P. A. J. Am. Chem. Soc. 1999, 121, 11595. (9) (a) Bentzon, M. D.; van Wonterghem, J.; Mørup, S.; Tho¨le´n, A.; Koch, C. J. Philos. Mag. B 1989, 60, 169. (b) Hyeon, T.; Lee, S. S.; Park, J.; Chung, Y.; Na, H. B. J. Am. Chem. Soc. 2001, 123, 12798. (c) Guo, Q.; Teng, X.; Rahman, S.; Yang, H. J. Am. Chem. Soc. 2003, 125, 630. (d) Redl, F. X.; Cho, K.-S.; Murray, C. B.; O’Brien, S. Nature 2003, 423, 968. (10) Hyeon, T.; Chung, Y.; Park, J.; Lee, S. S.; Kim, Y.-W.; Park, B. H. J. Phys. Chem. B 2002, 106, 6831. (11) Sun, S.; Zeng, H. J. Am. Chem. Soc. 2002, 124, 8204. A R T I C L E S Sun et al. 274 J. AM. CHEM. SOC. 9 VOL. 126, NO. 1, 2004 Qingyu Highlight Qingyu Highlight Qingyu Highlight Qingyu Highlight MSE Highlight MSE Highlight MSE Highlight to remove excess surfactants before drying under N2. The product was then dispersed in deionized water (18 M¿) or 1 mM phosphate buffer at neutral pH. Nanoparticle Characterization. Fe, Co, Mn, and S elemental analyses of the as-synthesized nanoparticle powders were performed on inductively coupled plasma-optic emission spectrometry (ICP-OES) at Galbraith Laboratories (Knoxville, TN). To prepare samples for elemental analysis, the particles were precipitated from their hexane dispersion by ethanol, centrifuged, washed with ethanol, and dried. Samples for transmission electron microscopy (TEM) analysis were prepared by drying a dispersion of the particles on amorphous carbon- coated copper grids. Particles were imaged using a Philips CM 12 TEM (120 kV). The structure of the particles was characterized using HRTEM and selected area electron diffraction (SAED) on a JEOL TEM (400 kV). X-ray powder diffraction patterns of the particle assemblies were collected on a Siemens D-500 diffractometer under Co KR radiation (ì ) 1.788965 Å). Near-edge X-ray absorption fine structure (NEXAFS) spectroscopy was performed at the Advanced Light Source at beamline 7.3.1.1, which was equipped with a spherical grating monochromator and had an energy resolution of E/¢E ) 1800. Magnetic studies were carried out using a MPMS2 Quantum Design SQUID magnetometer with fields up to 7 T and temperatures from 5 to 350 K. Infrared spectra of dried particles pressed into KBr pellets were obtained on a Nicolet Nexus 670 FTIR spectrometer. A homemade spin valve sensor12 was used to detect a single layer of 16 nm Fe3O4 nanoparticles. Results and Discussion Fe3O4 Synthesis. As illustrated in Scheme 1, reaction of Fe- (acac)3 with surfactants at high temperature leads to monodis- perse Fe3O4 nanoparticles, which can be easily isolated from reaction byproducts and the high boiling point ether solvent. If phenyl ether was used as solvent, 4 nm Fe3O4 nanoparticles were separated, while the use of benzyl ether led to 6 nm Fe3O4. As the boiling point of benzyl ether (298 °C) is higher than that of phenyl ether (259 °C), the larger sized Fe3O4 particle obtained from benzyl ether solution seems to indicate that high reaction temperature will yield larger particles. However, regardless of the size of the particles, the key to the success of making monodisperse nanoparticles is to heat the mixture to 200 °C first and remain at that temperature for some time before it is heated to reflux at 265 °C in phenyl ether or at �300 °C in benzyl ether. Directly heating the mixture to reflux from room temperature would result in Fe3O4 nanoparticles with wide size distribution from 4 to 15 nm, indicating that the nucleation of Fe3O4 and the growth of the nuclei under these reaction conditions is not a fast process. The low cost of Fe(acac)3 and the high yields it produces makes it an ideal precursor for Fe3O4 nanoparticle synthesis. The more expensive Fe(acac)2 or Fe(II) acetate can also be used but yields no better result than Fe(acac)3. Fe(II) (D-gluconate) is another good precursor for Fe3O4 synthesis. In benzyl ether, the reaction of Fe(II) (D-gluconate) with a 3-fold excess of each of oleic acid and oleylamine and a 5-fold excess of 1,2- hexadecanediol led to nearly monodisperse 8 nm Fe3O4 nano- particles. Several different alcohols and polyalcohols have been tested for their reactions with Fe(acac)3. It was found that 1,2- hydrocarbon diols, including 1,2-hexadecanediol and 1,2- dodecanediol, react well with Fe(acac)3 to yield Fe3O4 nano- particles. Long-chain monoalcohols, such as stearyl alcohol and oleyl alcohol, can also be used, but particle quality is worse and product yield is poorer than those with diols in the synthesis of Fe3O4 nanoparticle seeds. However, in the seed-mediated growth process, these monoalcohols can be used to form larger Fe3O4 nanoparticles.11 Oleic acid and oleylamine are necessary for the formation of particles. Sole use of oleic acid during the reaction resulted in a viscous red-brown product that was difficult to purify and characterize. On the other hand, the use of oleylamine alone produced iron oxide nanoparticles in a much lower yield than the reaction in the presence of both oleic acid and oleylamine. When the 4 nm particles were oxidized by bubbling oxygen through the dispersion at room temperature, they precipitated from hexane as a red-brown powder (the characterization of a similar product is discussed below). Adding more oleic acid did not cause re-dispersion of this powder into hexane. However, adding oleylamine did, leading to an orange-brown hexane dispersion. This is consistent with the previous observation that ç-Fe2O3 nanoparticles can be stabilized by alkylamine surfac- tants,13 suggesting that -NH2 coordinates with Fe(III) on the surface of the particles. The larger Fe3O4 nanoparticles can also be made by seed- mediated growth. This method has been recently applied to larger metallic nanoparticle and nanocomposite synthesis14 and is believed to be an alternative way of making monodisperse nanoparticles along with LaMer’s method through fast super- saturated-burst nucleation15 and Finke’s method via slow, continuous nucleation and fast, autocatalytic surface growth.16 In our synthesis, the small Fe3O4 nanoparticles, the seeds, are mixed with more materials as shown in Scheme 1 and heated,