Self-Organization of Magnetic Nanosized Cobalt Particles

Abstract

The synthesis of nanoparticles, characterized by a low size distribution, is a new challenge in solid-state chemistry. Due to their small size, nanoparticles exhibit novel materials properties that differ considerably from those of the bulk solid state. In this emerging field, finely divided magnetic nanoparticles are desirable owing to their broad range of applications, especially in data storage devices and sensors. A great deal of work on large magnetic nanoparticles has been carried out, but, although the magnetic properties of isolated atoms are well understood, there are still questions about the development of magnetic order on a macroscopic scale. The creation of perfect nanometer-scale magnetic crystallites identically replicated with long-range order in a state that can be manipulated and understood in terms of a pure macromolecular substance is an ultimate challenge in present materials research and could help us to understand the formation of ferromagnetism. To develop this application, it is crucial to be able to control the spatial arrangement of these nanoparticles in 2D or 3D arrays. Recently, in our laboratory spontaneous arrangement of particles either into monolayer organized hexagonal networks or into 3D face-centered cubic (fcc) arrangements was observed with silver sulfide and silver nanosized clusters. Similar arrangements with metal particles such as gold or silver and CdSe semiconductors have been reported elsewhere. Here, we report, for the first time, self-organization of magnetic cobalt nanoparticles into 2D superlattices. The magnetic properties of isolated and organized particles are compared. Reverse micelles are water in oil droplets stabilized by a monolayer of surfactant (e.g., sodium bis(2-ethylhexyl)sulfosuccinate, usually called Na(AOT)). The diameter of the droplets is controlled by the volume of solubilized water and varies from 0.5 to 18 nm. In this liquid solution, as a result of Brownian motion, collision between droplets induces exchange between water pools. In a previous paper we demonstrated that nanosized cobalt particles can be produced by using reverse micelles as a microreactor. Cobalt particles are obtained by mixing two micellar solutions having the same diameter ([AOT] = 0.25 M): one contains 10 M Co(AOT)2 (cobalt bis(2-ethylhexyl)sulfosuccinate) and the other one 2 10 M sodium tetrahydroborate (NaBH4, sodium borohydride). After mixing, the micellar solution remains optically clear and its color immediately turns from pink to black, indicating the formation of colloidal particles. The synthesis is performed in 3 nm diameter reverse micelles. The average diameter of cobalt nanoparticles, determined by transmission electron microscopy (TEM), is 6.4 nm with a polydispersity of 21 % (Fig. 1a). The particles are very well dispersed and no aggregation occurs. However the particles are very readily oxidized. High-resolution TEM of the particles after synthesis shows a well-developed crystalline phase. The spacing distance of the lattice fringes is 2.15 Š which is consistent with the bulk value of fcc cobalt. No trace of oxide shell is observed on the surface. To form superlattices, the cobalt nanocrystallites are extracted from the reverse micelles. Trioctylphosphine (4 mL/ mL) is added to the micellar solution containing the cobalt nanoparticles. The solvent is then evaporated at 40 C under vacuum and a solid mixture of trioctylphosphine-coated nanoparticles and surfactant is obtained. The surfactant is removed by ethanol addition and a black solid remains, which is easily redispersed in pyridine. This surface treatment is carried out under nitrogen in a glove box to avoid oxidation. At the end of the process, the coating is thick enough to prevent the particles from being oxidized. The coated particles can be stored without taking any precautions for at least one week without any aggregation or oxidation. The size of the coated particles redispersed in pyridine can be determined by TEM (Fig. 1b). A size selection is made by extraction of the nanoparticles from reverse micelles (Fig. 1). The average size decreases from 6.4 nm to