Control of Phase in Phosphide Nanoparticles Produced by Metal Nanoparticle Transformation: Fe2P and FeP

The transformation of Fe nanoparticles by trioctylphosphine (TOP) to phase-pure samples of either Fe2P or FeP is reported. Fe nanoparticles were synthesized by the decomposition of Fe(CO)5 in a mixture of octadecene and oleylamine at 200 °C and were subsequently reacted with TOP at temperatures in the region of 350−385 °C to yield iron phosphide nanoparticles. Shorter reaction times favored an iron-rich product (Fe2P), and longer reaction times favored a phosphorus-rich product (FeP). The reaction temperature was also a crucial factor in determining the phase of the final product, with higher temperatures favoring FeP and lower temperatures Fe2P. We also observe the formation of hollow structures in both FeP spherical nanoparticles and Fe2P nanorods, which can be attributed to the nanoscale Kirkendall effect. Magnetic measurements conducted on phase-pure samples suggest that ∼8 × 70 nm Fe2P rods are ferromagnetic with a Curie temperature between 215 and 220 K and exhibit a blocking temperature of 179 K, whereas FeP is metamagnetic with a Néel temperature of ∼120 K. These data agree with the inherent properties of bulk-phase samples and attest to the phase purity that can be achieved by this method.