One-dimensional Assemblies of silica-coated cobalt nanoparticles; Magnetic Pearl-Necklaces

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1 One-dimensional Assemblies of silica-coated cobalt nanoparticles; Magnetic Pearl-Necklaces Verónica Salgueiriño-Maceira, 1,2 * Miguel A. Correa-Duarte, 3 Fred Hucht 4, Michael Farle 4 1 Department of Electrical Engineering, Arizona State University, Tempe, AZ, USA, 2 Instituto de Investigacións Tecnolóxicas, Dpto. de Química-Física, Universidade de Santiago de Compostela, 15782, Spain 3 Department of Mechanical and Aerospace Engineering, Arizona State University, Tempe, AZ, USA, (CEJDGTGKEJ2J[UKM7PKXGTUKV¼VWKUDWTI'UUGPWKUDWTI)GTOCP[ * Corresponding Author vero.salgueirinomaceira@asu.edu Abstract Silica-coated cobalt nanoparticles with diameters of 20 to 30 nm were found to selforganize into chains and loops during synthesis when driven by a weak external magnetic field. The magnetic dipole-dipole interaction is shown to be the driving force for this self-organization. A lower critical diameter of approximately 12 nm of the particles is estimated for the chain formation to become energetically stable at the synthesis temperature. The method, although simple, produces structures resembling pearl-necklaces, comparable to 1D systems obtained in more laborious processes. Molecular dynamic simulations taking magnetic dipolar forces into account reproduce the observed self-assembled structures very well. 1

2 One-dimensional nanocolloids, such as nanorods and nanowires have received considerable attention over the last years owing to their high potential for specialized optical and magnetic applications. 1,2 The high sensitivity of nanorods and nanowires to surface conditions is an advantage for instance, for sensor devices, but the same property can cause their instability and loss of desired properties. These considerations lead to the necessity of insulated one-dimensional nanoscale colloids with environmentally stable polymerized coatings. It was demonstrated that encapsulation of semiconductor, metal and/or magnetic nanoparticles 3 and rods 4 with silica in the form of amorphous inorganic polymers prevented their aggregation in a liquid and improved their chemical stability. In a wide range of applications 5,6, it is of primary importance to control the surface properties of the nanoparticles. Moreover, magnetic nano-assemblies require a high degree of understanding of the interactions between neighboring nanoparticles and the magnetism of the individual entities. Control of the distance between particles, their size and magnetic properties allows to tailor the magnetic properties of the whole structure. Herein, we report the first synthesis of unique silica-coated chains of 30 nm cobalt nanoparticles resembling nanoscale pearl necklaces in colloidal suspension, which may be used as a first step in the processing of novel macroscopic magnetic materials. Additionally, a quantitative model based on molecular dynamics simulation is presented confirming the importance of magnetic dipolar interactions for the formation process, Kobayashi et al 3g reported the synthesis of silica-coated cobalt nanoparticles which permits the preparation of amorphous Co cores of various sizes in aqueous solution and their coating with well-defined silica shells. Varying the process led us to produce silicacoated chains of 30 nm cobalt nanoparticles (Figure 1). The synthesis of these silicacoated chains of cobalt nanoparticles was performed as follow; 0.1 ml (0.4 M) of Cobalt Chloride Hexahydrate (Fluka) in H 2 O were added to an aqueous solution of NaBH 4 (Riedel de Haen) and Citric Acid Monohydrate (Riedel de Haen) (16.63 mg (4.4 mm) of NaBH 4 and 8.4 mg (0.4 mm) of Citric Acid Monohydrate were added to 100 ml of previous deaerated Milli-Q water (18 MΩcm)) under magnetic stirring. Nitrogen was bubbled during the whole process. Immediately after cobalt reduction, 400 ml of an ethanolic solution containing 15 µl of APS (3-aminopropyl trimethoxisilane) (Aldrich) 2

3 and 15 µl of TEOS (tetraethoxysilane) (Aldrich) was added. 15 minutes later the solution was centrifuged and the precipitate redispersed in ethanol (40 ml). 50 nm 100 nm Figure 1. TEM images of silica-coated cobalt nanoparticles self-assembled into 1D-structures. Figure 1 shows transmission electron microscopy (TEM) images of pearl necklace-like structures, formed by silica-coated cobalt nanoparticles with an average diameter of 32± 5 nm and a silica shell thickness of 3-5 nm. The images were taken after evaporating a drop of the suspension directly after synthesis. Our experimental process, which presents slight differences compared to the previous one, leads to a bigger average size of cobalt nanoparticles ( > 20 nm) and offers the deposition of thinner shells of silica (2-3 nm) since the hydrolysis and condensation of a smaller amount of TEOS are allowed to take place in a shorter period of time. The formation of chains of cobalt nanoparticles is attributed to the magnetic dipole-dipole interaction between neighboring particles as discussed in more detail below. The reduction of Co 2+ ions by NaBH 4 produces clusters of metallic cobalt, poorly stabilized by the citrate ions present in the solution, which become aggregated and form nanoparticles. When the volume of the nanoparticles further increases, one has to consider two different size regimes which are separated by a critical volume, i.e. critical diameter D C which characterizes a blocking temperature above which the magnetization is fixed to the geometry of the particles and below which the magnetization can freely 3

4 rotate independent of the motion or rotation of the particle itself. One could loosely term this a transition from a superparamagnetic to ferromagnetic blocked (SP-FM) state. In the first case, the magnetization is oriented along the easy magnetic anisotropy axis, and in the later case the time average of the magnetizationt of the particle is zero. This transition depends in a correlated way on the volume, magnetic anisotropy and the temperature of the particles. During the synthesis the temperature is fixed and the magnetic volume of the Co particles increases. In the experiment we find that at approximately a critical diameter of nm the formation of chains sets in. This process as outlined schematically in Figure 2 is possible only, when the magnetization of the particle does not fluctuate during the time of magnetic interaction in the suspension. The north and south poles of the dipolar nanomagnets will be mutually attracted while particles oriented such that their magnetization are parallel to each other and the interaction axis is perpendicular to the magnetization direction will repel each other thus favoring the formation of pearl necklaces 7 Figure 2. Scheme of formation of chains of silica-coated cobalt nanoparticles. For small enough spheres like the ones observed in this work the formation of magnetic domains within the particle is energetically unfavorable. And even if magnetic domains would appear for somewhat larger particles, one should note that previous results on micron sized core-shell colloids have shown that magnetic domain effects can be reduced 4

5 in external fields 8,9,10 During the synthesis the suspension is stirred by a magnetic rod a small magnetic field is present favoring the formation of chains. As a result of the dipolar interactions the larger cobalt clusters form chains and sometimes closed loops in order to minimize the magnetostatic energy. The observed spontaneous self-assembly of the nanoparticles yielding unique structures such as chains resembling pearl necklaces and loops (Figure 3) proofs the importance of dipolar interactions even in wet-chemical synthesis. 200 nm 100 nm Figure 3. Loops and chains of cobalt nanoparticles with different thickness of the outer silica shell. Figure 3 shows TEM images of closed loops and chains of silica-coated cobalt nanoparticles suggesting that the SP-FM transition took place at room temperature for 32±5 nm average diameter cobalt particles. The deposition of the silica shells avoids agglomeration and protects them (but not fully) against oxidation.3i Nevertheless; a further deposition of silica tends to form not only loops of nanoparticles but more fractallike structures (figure 3, left). Bigger aggregates of cobalt nanoparticles were also found in the samples, which can easily be related to the poor citrate-stabilization during the synthesis and therefore, a further optimization of the experimental method is needed toward a completed magnetic characterization of the silica-coated cobalt nanoparticles and their magnetic interactions leading to 1D-structures. 5

6 Figure 4: Molecular dynamics simulation for the formation of Co pearl necklaces findingg the same chain and loop like structures as observed in the experiment. To obtain a quantitative understanding of the chain formation process due to magnetic dipolar forces quantitative molecular dynamic simulations taking realistic magnetic parameters into account were performed. The simulations were performed for a model of magnetic hard spheres with dipolar interaction ( ( ˆ ( HS ) µ µ 3 µ r )( r ) U µ U ij = µ j + 4π 0 ˆ 3 i j i ij ij rij ij between two particles i and j at distance r ij, wheres particle i, and a hard sphere potential µ i denotes the magnetic moment of (HS) U ij. A detailed description of this work will be published elsewhere 11. The dipolar binding energy E (0) of two spherical ferromagnetic particles with magnetic moment density µ, volume V and non-magnetic shell to fully- 2 (0) µ 0µ V magnetized core radius ratio a is given by E =. Assuming a fully magnetically 3 12a saturated core detailed MD simulations show [1] that the chain building transition takes kb Tc place at 0.056, from which we get the critical diameter D ( 0) c /a = 8.6 nm at T c =300 E K for spheres with the magnetic moment (1.7 µ B ) of Co Using a 1.1 which corresponds 6

7 to the experimentally observed average ratio of the diamagnetic silica shell to the ferromagnetic Co core we calculate D c =11.4 nm. Above this theoretical diameter the particles will form chains at room temperature which are stable against thermal excitations. It is not surprising that the calculated critical diameter is smaller than the one experimentally observed, since turbulence in the suspension and the Co silicides and Co oxides present at the Co/SiOx interface which reduce the magnetization of the core are not accounted for in the simulation. In the case of the experimentally found reduced magnetization a larger critical volume is required to obtain the necessary magnetic interaction strength E (0) for stable chains at room temperature. The configurations found in our simulations 11 (Figure 4) are very similar to the experimental findings. In summary, the synthesis of one-dimensional nano chains and loops through dipoledipole interactions between silica-coated cobalt nanoparticles is reported. The method which allows the stabilization of cobalt nanoparticles in water solution by a surrounding thin layer of silica was modified, producing chains of nanoparticles. Acknowledgement This work was supported in part by the Deutsche Forschungsgemeinschaft SFB 445 and the European Community s human potential programme under Contract No. HPRN-CT [Magnetic Nanoscale Particles]. References 1. a) Duan, X., Huang, Y., Agarwal, R., Lieber, C. M., Nature, 2003, 421, 241, b) Cui, Y., Wei, Q., Park, H., Lieber, C. M., Science, 2001, 293, 1289, c) Hu, J., Li, L., S., Yang, W., Manna, L., Wang, L.W., Alivisatos, A. P., Science, 2001, 292, 2060, d) Puntes, V. F., Krishnan, K. M., Alivistos, A. P., Science, 2001, 291, a) Dumestre, F., Chaudret, B., Amiens, C., Fromen, M.-C., Casanove, M.-J., Renaud, P., Zurcher, P., Angew. Chem. Int. Ed., 2002, 41, 4286, b) Dumestre, F., Chaudret, B., Amiens, C., Respaud, M., Fejes, P., Renaud, P., Zurcher, P., Angew. Chem. Int. Ed., 2003, 42, a) Liz-Marzan, L. M., Giersig, M., Mulvaney, P., Langmuir, 1996, 12, 4329, b) Bruchez, M. Jr., Moronne, M., Gin, P., Weiss, S., Alivisatos, A. P., Science, 1998, 7

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