Reversible Magnetic Agglomeration: A Mechanism for Thermodynamic Control over Nanoparticle Size.

We present a method for the synthesis and precise size control of magnetic nanoparticles in a reversible magnetic agglomeration mechanism. In this approach, nanoparticles nucleate and grow until a critical susceptibility is reached, in which magnetic attraction overcomes dispersive forces, leading to agglomeration and precipitation. This phase change in the system arrests nanoparticle growth and gives true thermodynamic control over the size of nanoparticles. We then show that increasing the alkyl chain length of the surfactant, and hence increasing steric stabilization, allows nanoparticles to grow to larger sizes before agglomeration occurs. Therefore, simply by choosing the correct surfactant, the size and magnetic properties of iron nanoparticles can be tailored for a particular application. With the continuous addition of the precursor solution, we can repeat the steps of nucleation, growth, and magnetic agglomeration indefinitely, making the approach suitable for large scale syntheses.

Non-volatile iron carbonyls as versatile precursors for the synthesis of iron-containing nanoparticles.

The most commonly used method for the formation of well-defined iron and iron-containing heterometallic nanoparticles is the thermal decomposition of iron pentacarbonyl (Fe(CO)5). However, iron pentacarbonyl is highly toxic and volatile, which introduces safety concerns and drastically diminishes control over the reaction stoichiometry. Here we alleviate these issues by beginning with an easy-to-handle solid, triiron dodecacarbonyl (Fe3(CO)12). The issue of poor solubility of this cluster is addressed by its reaction with amine, which renders the cluster fully soluble in common high boiling point solvents. This reaction generates non-volatile anionic iron carbonyl species in solution which are subsequently used as the nanoparticle precursor. We demonstrate that the thermolysis of this novel precursor solution yields well-defined Fe, Fe1-xCox, and Fe1-xPtx nanoparticles. In addition, the same approach overcomes the solubility issue of another poorly soluble iron carbonyl compound, diiron nonacarbonyl (Fe2(CO)9). By using these precursors in an array of nanoparticle-forming reactions, we demonstrate a convenient replacement for the commonly used Fe(CO)5, producing particles of similar quality, but without the drawbacks of the precursor volatility and high toxicity.