olloidal Synthesis of Magnetic Nanoparticles V. Salgueirino Maceira and M. Farle 1 Institut für Physik, Universität Duisburg-Essen, Lotharstr. 1, 47048 Duisburg 1. Introduction 1 The synthesis of monodisperse magnetic nanoparticles with sizes ranging from 2 to 20 nm can be performed using colloidal chemical approaches. This approach involves rapid injection of reagents into a hot surfactant solution followed by aging at high temperature, or the mixing of reagents at low temperature under controlled conditions. The size of the nanoparticles can be controlled by systematically adjusting the reaction parameters, such as time, temperature, and the concentrations of reagents and stabilizing surfactants. In general, particle size increases with increasing reaction time, because more monomeric species are generated, and with increasing reaction temperature, because the rate of reaction is increased. Also, on the synthesis of colloidal magnetic nanoparticles, it is of primary importance to control the surface properties of the nanoparticles, since the surface will be the main factor determining colloidal stability. 2. Silica coated cobalt nanoparticles The main difficulty for the use of pure magnetic metals arises from their instability toward oxidation in air, which increases as the size gets smaller. The deposition of a silica shell around magnetic nanoparticles offers some advantages, like exceptional stability in aqueous dispersions, easy surface modification that allows the preparation of nonaqueous colloids, considerable reduction of the oxidation process rate and easy control of interparticle interactions, both in solution and within structures, through shell thickness. There is a procedure, described by Kobayashi, 2 which permits the preparation of o nanoparticles in aqueous solution and their coating with well-defined silica shells. 2.1 Synthesis The synthesis of amorphous cobalt nanoparticles coated with silica in aqueous/ethanolic solution is possible with the following method. The synthesis of cobalt nanoparticles in water involves the reduction of cobalt ions by sodium borohydride (NaBH 4 ) and the stabilization by citrate ions (see list of compounds) present in solution as soon as the cobalt chloride (ol 2.6 ) solution is injected into the flask. The process can take place at low temperature, around 2-4 but also at Room temperature and bubbling Nitrogen in the solution during the experiment. Subsequently, adding an ethanolic solution with APS ((3- aminopropyl)trimethoxysilane) and TES (tetraethoxysilane) produces the deposition of silicon oxide on the surface of cobalt nanoparticles, in a way to protect them against aggregation and oxidation (Figure 1a). This step involves the hydrolysis/condensation of both compounds, APS and TES in water/ethanol, and the silica shell deposited onto the nanoparticles can range from few nanometers to more than 1 hundred (Figures 1b and c). 1 Hyeon, T. hem ommun. 2003, (8), 927. 2 Kobayashi, Y. et al. J. Phys. hem B, 2003, 107, 7420. AG Farle Seite -1-
a) o TES /EtH o Figure 1. (a) Scheme of the surface reactions involved in the formation of a silica shell on cobalt particles. (b), (c) TEM images of silica coated cobalt nanoparticles with a shell thickness ranging from few nanometers to more than 100 nm. 2.2 Results and Discussion Following the method described in Reference led us to; a matrix of cobalt and silicon oxide (Figure 2a), but no nanoparticles, very nice and very monodisperse 4 nm cobalt particles (Figure 2b), in water! polydisperse cobalt nanoparticles trapped in a silica matrix (Figure 2c), and some silica coated cobalt nanoparticles (Figure 2d). Bubbling Nitrogen in the solutions before and during the experiment minimizes the presence of xigen in the system which helps to get obalt nanoparticles instead of obalt AG Farle Seite -2-
xide nanoparticles. However, in the absence of xygen the nanoparticles are not stabilized by the citrate ions and we get a matrix of cobalt and silicon oxide (Figure 2a). Figure 2. TEM images showing; (a) amorphous matrix of cobalt and silicon oxide, (b) monodisperse 4 nm cobalt particles, (c) polydisperse cobalt trapped in a silica matrix, (d) 250 nm silica coated cobalt nanoparticles and some smaller silica particles without metallic core. 0 Based on these preliminary results we interpret that the growth of core shell particles proceeds as follows: Immediately after the injection of the cobalt chloride solution the nucleation of particles takes place and these nuclei are surrounded by the citrate ions (Figure 2b). During the growth of the particles, because of the chemical inability of these ions to protect the cobalt nanoparticles, polydisperse nanoparticles will be formed. Those 4 nanometers coparticles are an intermediate state, and there is still some material in the solution that could be added (probably those black dots around the particles are some clusters of cobalt (Figure 2b)), forming very poly disperse nanoparticles. A fast deposition of silica on the surface of the cobalt nanoparticles is required in order to avoid the aggregation, and the hydrolysis/condensation of APS and TES will take place immediately in an ethanol/water volume ratio of 4:1. Since Trisodium citrate is not a very AG Farle Seite -3-
efficient stabilizer of cobalt sols, it is readily displaced by the silane coupling agent (APS) used to activate the surface toward silica deposition. Unfortunately, this very fast deposition traps more than one particle, and usually the result is the kind of structures that are shown in Figure 2c. By chance some silica coated cobalt nanoparticles were prepared (Figure 2d), which shows that the optimal conditions for the production of these core-shell colloids still need to be found. 2.3 onclusion In summary, this recipe will have to be modified in order to obtain a reproducible synthesis of silica coated cobalt nanoparticles. It is necessary to have stable cobalt nanoparticles so that the silicon oxide can be slowly deposited on the surface, producing amorphous o nanoparticles surrounded by homogeneous shells of silica. The stabilizer has the key role for that, since it has to be able to avoid the aggregation of particles. With citrate ions the solution of o nanoparticles stays black during only 1 or 2 hours and after that, it became transparent with some aggregates at the bottom of the flask. So, there is not enough time to form a silica shell around the nanoparticles, which indeed, has to be done very slowly, in order to surround every nanoparticle separately. 3. FePt Nanoparticles The iron-platinum (FePt) alloys have been investigated because of their important applications in magnetism. Depending on the Fe to Pt elemental ratio, these alloys can display different structures, having dramatic effects on the magnetic properties. But to fully understand the magnetics of these FePt nanomaterials, it is essential to synthesize monodisperse FePt nanoparticles with controlled size and composition. Promising applications for these systems are ultra-high density magnetic recording media, being necessary the obtaining of ordered arrays of magnetic nanoparticles and biological applications such as magnetic resonance imaging contrast enhancement and drug delivery. In the case of drug delivery, magnetic fields can be utilized to direct the particles (and thus the drug) to specific locations within the body. Therefore, the transfer to an aqueous solution with no loss of structural or magnetic properties is required. 3.1 Experimental Section Monodisperse FePt nanocrystals with good control on particle size and composition can be produced by a solution-phase chemical procedure. This procedure consists in the reduction of platinum acetylacetonate (Pt(acac) 2 ) by a diol (1,2-hexadecanediol) and decomposition of iron pentacarbonyl (Fe() 5 ) in the presence of oleic acid and oleylamine stabilizers in hightemperature solutions. 3 Their composition can be adjusted by controlling the molar ratio of iron pentacarbonyl to platinum salt. Fe() 5 is volatile and thermally unstable, gradually releasing and Fe at ambient temperature. So, the high-temperature condition of the synthesis needs an excess of Fe() 5. A correlation between the molar ratio of Fe() 5 /Pt(acac) 2 and FePt composition has to be established in order to determine the final composition of the particles. These particles are then isolated and purified by centrifugation after the addition of a flocculent (for example, ethanol) and can be redispersed in nonpolar solvents in variety of concentrations. For transferring the FePt nanoparticles to water, the surface properties have to be modified through the exchange of surfactants. So, after centrifugation of the sample by adding 3 Sun, S. et al. Science, 2000, 287, 1989. AG Farle Seite -4-
ethanol, the precipitate can be dried in order to eliminate all of the solvent. The nanoparticles can be redispersed in TMAH (tetramethylammonium hydroxide)/water solution. 4 Sonification was necessary to separate the nanoparticles and obtain a clear solution of FePt nanoparticles in water. 3.2 Results and Discussion The binary FePt nanoparticles are stabilized by oleic acid and oleylamine. If the final product is Fe-rich, a combination of oleic acid/oleylamine in a ratio of >1 is needed for particle stabilization during the purification process. If the final product is Pt-rich, a combination of oleic acid/oleylamine in a ratio of <1 is required for the stabilization. This corresponds to the fact that Fe tends to bind to the carboxylic group of the oleic acid while Pt tends to bind to the amino group of the oleylamine (see list of compounds). Several samples of FePt with different proportion of Fe and Pt have been prepared, all of them stabilized in hexane with a combination of oleic acid/oleylamine. The TEM images (Figure3) show different samples of FePt prepared following S. Sun s recipe. Fig. 3a shows a HRTEM image of highly crystalline Fe 32 Pt 68 nanoparticles with an average diameter of 3,7 nm. Figures 3b and c show TEM images of hexagonally close-packed 2D arrays of 2,7 nm large Fe 60 Pt 40 nanoparticles on a TEM grid. Figure 4 shows the size distribution of both samples. a 5 nm b 25 nm c 40 nm Figure 3. TEM images showing; (a) crystalline 3,7 nm FePt nanoparticles, (b), (c) hexagonally close-packed arrays of 2,7 nm FePt nanoparticles on a TEM grid. The deposition of a colloidal dispersion of FePt particles from hexane solution resulted in the nucleation of colloidal crystals of FePt nanocrystals. Figure 5 shows TEM images of hexagonal arrangements of FePt nanoparticles. Lattice planes of different orientation are observed within a single piece of colloidal crystal. 4 Euliss, L. E. et al. Nanoletters, 2003, 3, 1489. AG Farle Seite -5-
40 Fe 32 Pt 68 d=3,77nm (SD=8%) 60 Fe 60 Pt 40 d=2,72 nm (SD=12%) 50 30 40 n 20 n 30 20 10 10 0 2,0 2,5 3,0 3,5 4,0 4,5 5,0 d (nm) 0 2,0 2,5 3,0 3,5 4,0 4,5 5,0 d (nm) Figure 4. Histograms computed with the software Scion Image for Fe 32 Pt 68 and Fe 60 Pt 40 nanoparticles 100 nm Figure 5. TEM images of an hexagonal arrangement of FePt nanoparticles Having in mind biological applications these FePt nanoparticles have to be stable in water. Therefore it is necessary to change the surfactants that stabilize the nanoparticles in non polar solvents. The first step consists on the removal of the organic shell (oleic acid/oleylamine), which can be done by means of the purification process. So, part of this organic shell is removed and this fact helps to transfer the nanoparticles to water. The stabilization of particles in water takes place due to the formation of a double layer of ions around every nanoparticle. The surfactant TMAH (tetramethylammonium hydroxide) releases in water the cations TMA + and the anions H - which should form this double layer attached to the surface of the nanoparticles. Following the method mentioned above, the solutions of FePt nanoparticles appear very stable and very well redispersed, both in water and hexane. Figure 6a shows FePt solutions in water (A W ) and in hexane (A H ), both very clear and very stable. Figure 6b shows FePt nanoparticles, in hexane (top of the two-phase mixture, left) and in water (bottom of the twophase mixture, right) which demonstrates that once the nanoparticles are in water they do not transfer back to non-polar solvents like hexane. The transfer of FePt nanoparticles to water was also studied by TEM. Figure 7 shows FePt nanoparticles on TEM grids from a hexane suspension (a) and from aqueous suspension AG Farle Seite -6-
(b). These images indicate that no degradation or aggregation takes place upon transfer from organic to aqueous solution. Figure 6. Photographs of (a) FePt solutions in water (A W ) and in hexane (A H ), (b) FePt nanoparticles either in hexane (left), either in water (right) in a two-phase mixture of hexane (top) and water (bottom). Figure 7. TEM images of FePt nanoparticles on TEM grids from a hexane solution (a) and from aqueous solution (b). AG Farle Seite -7-
3.3 onclusion The recipe designed by S. Sun allows us to prepared very monodisperse and crystalline FePt nanoparticles with different proportions of the two metals. Due to their monodispersity it is possible to prepare hexagonally close-packed 2D arrays and nanoparticle-based selfassembly, i.e. colloid crystals. Furthermore, in exchanging the surfactants that protect them against aggregation and oxidation it has been possible to transfer them to water, obtaining very stable aqueous suspensions of FePt nanoparticles. LIST F MPUNDS 1. itrate ions Na H Na Na - H - - + 3Na + itric Acid (Trisodium Salt) itrate Ion 2. APS ((3-aminopropyl)trimethoxysilane) N Si(Me) 3 3. TES (tetraethoxysilane) H 3 H 3 Si H 3 H 3 AG Farle Seite -8-
4. PVP (poly(vinylpyrrolidone)) N ( ) n 5. Pt(acac) 2 (Platinum acetylacetonate) H 3 H 3 H Pt H H 3 H 3 6. Fe() 5 (Ironpentacarbonyl) Fe 7. 1,2-hexadecanediol H H H 3 H AG Farle Seite -9-
8. leic Acid H 3 H H 9. leylamine H 3 H H H 10. TMAH (Tetramethylammonium hydroxide) N H H 3 H 3 N H 3 H 3 Attended onferences Research Training Network: orrelation of Structure and Magnetism in Novel Nanoscale Magnetic Particles olloidal Synthesis of Magnetic Nanoparticles ctober 17-18, 2003, Prague (zech Republic). AG Farle Seite -10-