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Synthesis of Nanomagnets Dispersed in Colloidal Silica Cages with Applications in Chemical Separation Pedro Tartaj,* Teresita Gonza´lez-Carren˜o, and Carlos J. Serna Instituto de Ciencia de Materiales de Madrid, CSIC, Cantoblanco, 28049, Madrid, Spain Received January 22, 2002. In Final Form: April 19, 2002 This Letter describes a simple, continuous, and generalized method to prepare nanomagnets dispersed in dense submicrospherical silica cages. The method exploits the subtle interplay between the parameters controlling particle formation in aerosol pyrolysis processes. We find that the magnetic properties of the composites can be easily tuned so as to produce suspensions that display reversible sedimentation behavior in an applied magnetic field gradient. Our results indicate that this material can be used for chemical separation in medicine, biology, and environmental protection.
Nanoclusters are ultrafine particles of nanometer dimensions located in the transition region between atoms and bulk solids. Below a critical size, magnetic clusters become monodomain and exhibit a number of outstanding physical properties such as giant magnetoresistance, superparamagnetism, and quantum tunneling of the magnetization. Magnetic nanoclusters are normally prepared as composites in diamagnetic matrixes to avoid their natural tendency to form aggregates. Among the different composites, those containing γ-Fe2O3 nanocrystals have potential applications in magnetic-tape media, magnetooptical devices, magnetic refrigerators, and catalysis.1,2 In recent years special attention has been focused on magnetically assisted chemical separation (MACS) of nuclear waste, biochemical products, and cells, which are of interest in biology, medicine, and environmental protection.3 The thermal decomposition of liquid aerosols (aerosol pyrolysis) represents a convenient procedure for obtaining finely dispersed particles of predictable shape, size, and variable composition. The resulting powders generally consist of spherical particles, the final diameter of which can be predetermined from that of the original droplets. The method offers certain advantages over other more commonly used techniques (such as precipitation from homogeneous solution) as it is simple, rapid, and continuous. As an example, the preparation of nonaggregated phosphorescence nanoparticles has been reported very recently.4 Herein we report the synthesis by pyrolysis of aerosols of γ-Fe2O3 nanocrystals dispersed in dense microspherical silica cages. We have previously been able to prepare hollow magnetic spheres coated by a thin layer of SiO2 by aerosol pyrolysis.5 However, this microstructure is not appropriate for MACS, since irreversible aggregation of particles is observed after magnetic sedimentation (i.e., * To whom correspondence may be addressed. E-mail: ptartaj@ icmm.csic.es. (1) Ziolo, R. F.; Giannelis, E. P.; Weinstein, B. A.; O’Horo, M. P.; Ganguli, B. N.; Mehrotra, V.; Russel, M. W.; Huffman, D. R. Science 1992, 257, 219. (2) (a) Onodera, S.; Kondo, H.; Kawana, T. MRS Bull. 1996, 21, 28. (b) Guerrero, H.; Rosa, G.; Morales, M. P.; Del Monte, F.; Moreno, E. M.; Levy, D.; Pe´rez del Real, R.; Belenguer, T.; Serna, C. J. Appl. Phys. Lett. 1997, 71, 2698. (c) McMichael, R. D.; Shull, R. D.; Swartzendruber, L. J.; Bennet, L. H.; Watson, R. E. J. Magn. Magn. Mater. 1992, 111, 29. (d) Ida, T.; Tsuiki, H.; Ueno, A.; Tohji, K.; Udagawa, Y.; Iwai, K.; Sano H. J. Catal. 1987, 106, 428. (3) (a) Bergemann, C.; Mu¨ller-Schulte, D.; Oster, J.; Brassard, L.; Lu¨bbe, A. S. J. Magn. Magn. Mater. 1999, 194, 45. (b) Nun˜ez, L.; Kaminski, M. D. J. Magn. Magn. Mater. 1999, 194, 102. (4) Xia, B.; Lenggoro, I. W.; Okuyama, K. Adv. Mater. 2001, 13, 1579.
after magnetic sedimentation, the initial state is not reestablished by simple shaking). In this case, the magnetic layer is formed by a large number of nanoparticles, and therefore collective phenomena are expected to take place. For MACS a homogeneous distribution of the magnetic material in a diamagnetic matrix is essential because it helps to screen the anisotropic magnetic dipolar attraction force, which renders the suspension unstable even in the absence of an external magnetic field.6 Moreover, the silica cage helps to prevent interparticle sintering and stabilizes the γ-Fe2O3 phase,7 allowing finetuning of the magnetic properties with temperature. In contrast, methods that use organic matrixes do not allow the control of properties with temperature.8 In addition a high loading of magnetic material is essential to generate the response of the material to a magnetic field gradient precluding the use of some methods described earlier.6 The aerosol device used for the preparation of nanomagnets dispersed in microspherical silica cages is described elsewhere.5 Tetraethoxysilane (TEOS) and iron nitrate in the right proportions were dissolved in methanol at a total salt concentration of 1 M. The solution (atomized at 1.6 mL min-1 with an air pressure of 1.7 kg cm-2) was directed to a first furnace kept at 250 °C to favor the evaporation of the solvent and therefore the precipitation of solute. The solid aerosol was then decomposed in a second furnace, which was held at 500 °C. Finally, the particles obtained were collected with an electrostatic filter (voltage supply 8 kV). Essential to the production of the magnetic nanoparticles dispersed in colloidal silica cages was the selection of experimental parameters such as the nature and concentration of precursors and the working temperature. Thus, the total salt concentration was set to a value of 1 M to favor the formation of dense spherical particles instead hollow ones.5 Meanwhile, the temperature of the furnaces was optimized to avoid high solvent pressure inside the particles that could cause their fragmentation. Three samples with different iron oxide content were prepared: samples F20 (20 mol % Fe2O3 content), F40 (40 mol % Fe2O3 content), and F80 (80 mol % Fe2O3 content). The iron oxide content was varied to (5) Tartaj, P.; Gonza´lez-Carren˜o, T.; Serna, C. J. Adv. Mater. 2001, 13, 1620. (6) Philipse, A. P.; van Bruggen, M. P. B.; Pathmamanoharan, C. Langmuir 1994, 10, 92. (7) Ennas, G.; Musinu, A.; Piccaluga, G.; Zadda, D.; Gatteschi, D.; Sangregorio, C.; Stanger, J. L.; Concas, G.; Spano, G. Chem. Mater. 1998, 10, 495. (8) Shisho, H.; Kawahashi, N J. Colloid Interface Sci. 2000, 226, 91.
10.1021/la025566a CCC: $22.00 © 2002 American Chemical Society Published on Web 05/14/2002
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Figure 1. TEM micrographs of sample F20 after the aerosol pyrolysis process (A) and further heated in a conventional furnace for 2 h at 900 (B), 1050 (C), and 1200 °C (D). Note in the sample heated at 1050 °C the presence of γ-Fe2O3 (dark regions) nanoparticles smaller than 20 nm dispersed in a microspherical silica particle (lighter regions). At this temperature, the enrichment of silica on particle outerlayers is clearly observed. It is important to note that similar microstructures to that displayed in micrographs B and C were observed for smaller and bigger particles.
determine its effect on the magnetic characteristic of samples, which are essential for MACS. Samples of the as-synthesized products consisted of spherical particles of an average size of 250 ( 200 nm (Figure 1) independently of the iron oxide content. Surface analyses (see ref 9 for details) carried out on the powders indicated that the outer surface was mainly silica, which can be associated with differences in solubility of the components.5 In particular, the component with the lowest solubility (in our case iron nitrate) is expected to precipitate first producing an enrichment of the component with the highest solubility (TEOS is fully soluble on methanol) on the surface. This silica enrichment at the surface is beneficial because of the presence of surface silanol groups that can easily react with alcohols and silane coupling agents10 to provide the ideal anchorage for covalent bounding of specific ligands. Moreover, the silica outerlayer helps to screen the anisotropic magnetic dipolar attraction force between the cages, and confers stability at high volume fractions, large variations in pH, and electrolyte concentration.11 From X-ray diffraction (XRD), the as prepared samples showed no traces of crystalline phases, and thus further annealing in a conventional furnace for 2 h was necessary to develop the crystallinity. At high temperatures, samples F80 and F40 contained both γ- and R-Fe2O3 phases (Figure 2). Meanwhile, sample F20 heated at temperatures from 800 to 1100 °C only contained the γ-phase,12 the crystallinity of which increased with temperature (Figure 2, Table 1). At 1200 °C the onset of the γ- to R-transformation took place. Thus, the proportion of iron oxide and the temperature determined the relative content of R-Fe2O3/ γ-Fe2O3. Given that an increase of the crystallite size favors (9) The isoelectric point (iep) of heated samples gave a value between 2 and 3, which is in the range of SiO2 (2-3) and different to that of γ-Fe2O3 (6-7). (10) Ulman, A. Chem. Rev. 1996, 96, 1533. (11) Mulvaney, P.; Liz-Marza´n, L. M.; Giersig, M.; Ung, T. J. Mater. Chem. 2000, 10, 1259.
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Figure 2. XRD patterns of samples F20, F40, and F80 heated at different temperatures: γ, γ-Fe2O3; H, R-Fe2O3. Table 1. Crystallite Size (DXray) Determined from the (311) X-ray Reflection of γ-Fe2O3 Using the Scherrer Equation, Saturation Magnetization (MS), and Coercivity Field (HC) Values Determined at RT for Sample F20 Heated at Different Temperaturesa DXray (nm) MS (emu/g) HC (Oe) a
800 °C
900 °C
1000 °C
1050 °C
1100 °C
4.5 (0.5) 10.1 0
5.5 (0.5) 11.5 0
9 (1) 13.5 0
12 (1) 15.6 0
17 (1) 16.9 30
The values inside parentheses are the standard deviation.
the γ- to R-phase transformation,7 it was not surprising that the samples with the highest iron oxide content (shorter diffusion distances between Fe2O3 nuclei) were enriched with the R-form. The thermal evolution of sample F20 was also followed by TEM (Figure 1). A progressive increase in crystallite size was also observed for the γ-phase. At 1200 °C, the particles lost their spherical shape as a consequence of the interparticle sintering coming fundamentally from the silica phase (Figure 1). The magnetic characterization (see ref 13 for details) was carried out for sample F20, where only γ-Fe2O3 is present, at temperatures from 800 to 1100 °C. All samples except the one heated at 1100 °C exhibit superparamagnetic behavior (i.e., zero coercivity field), which is consistent with the very small crystallite size (Table 1). Also the values of the saturation magnetization (MS) increase with the increase of the crystallite size. To discern which of the F20 samples heated at different temperatures are adequate for magnetic separation, their sedimentation in aqueous suspensions was followed by optical absorbance measurements. All samples were stable in the absence of magnetic field (no significant sedimentation was observed after 10 min). A progressive decrease in sedimentation times under a magnetic field was (12) The γ-formation always takes place through the oxidation of Fe3O4 (Cornell, R. M.; Schwertmann, U. The iron oxides. Structure, properties, reactions, occurrence and uses; VCH: Weinheim, 1996). In our case, the formation of the γ-phase is probably associated with the presence of carbonaceous species coming from the methanol solvent and the silica precursor (TEOS). These species on heating promote the reduction of part of the Fe(III) to Fe(II), favoring the formation of Fe3O4 nanoparticles, which are immediately oxidized to γ-Fe2O3 because the pyrolysis process is carried out on air. (13) Coercivity fields and saturation magnetization values were obtained from hysteresis loops registered in a vibrating sample magnetometer (model VTI, Oxford Instruments) up to a field of 7 T.
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Figure 3. (a) Variation with time of the absorbance (expressed as a % of initial value) under a magnetic field (square magnet of 1 cm × 1 cm × 0.3 cm with a Br of 1 T fixed at the bottom of the cell) for suspensions of sample F20 heated at temperatures in which only γ-Fe2O3 was detected by XRD. (b) Reversibility sedimentation behavior (represented as the relative absorbance difference) of sample F20 heated at 1050 and 1100 °C. For each experiment the curve displays the difference in sedimentation velocity (variation of absorbance with time) under no magnetic field of a dispersion containing the initial sample and that left to sediment under a magnetic field and then dispersed by shaking at a speed of 1500 rpm (MS2 Minishaker, IKA).
observed with the increase of the processing temperature (Figure 3a), which is consistent with the increase of crystallite size and the MS values (Table 1). Reversibility studies were also carried out by optical absorbance measurements (Figure 3b) for the samples that have fast sedimentation times (F20 heated at 1050 and 1100 °C). Only the sample heated to 1050 °C has
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highly reversible behavior (only a 2% difference in absorption between suspensions containing the initial sample and that previously left to sediment). Unlike in the sample heated to 1100 °C, the difference is greater and increases with time (Figure 3b), which clearly indicates that simple shaking of the sample was not energetic enough to break all the aggregates formed during magnetic sedimentation (irreversibility). The reason for this difference must lie in the different magnetic behavior exhibited by the samples. The sample heated at 1050 °C is superparamagnetic, while the one heated at 1100 °C has a small but nonzero coercivity field at room temperature (30 Oe, Table 1), which could be responsible for the formation of aggregates during the sedimentation induced by the magnetic field. In conclusion, we have shown that the aerosol-assisted pyrolysis of solutions containing iron nitrate and TEOS in different proportions is able to produce magnetic nanoparticles dispersed in microspherical silica cages. The magnetic properties of the composites can be tuned to produce suspensions that display reversible sedimentation behavior. This rapid, simple, and generalizable synthetic route is especially convenient for the production of composites adequate for magnetically assisted chemical separation in medicine, biology, and environmental protection. Acknowledgment. Financial support from CICYT (PB98-0525) and Pacti (C001999-AX011) is gratefully acknowledged. The help of Professor O’Grady and Dr. Creffield is also gratefully acknowledged. LA025566A
