Magnetic Core−Shell Nanoparticles from Nanoscale-Induced Phase Segregation

Nanoscaling of the nickel phosphides profoundly alters their domains of stability. An original mechanism of “nanoscale-induced phase segregation” was uncovered in the present study: the appearance of two crystallized phases inside each single nanoparticle (here, Ni 2 P and Ni fcc), where the single-phase product (Ni 3 P) would have been preferred at the bulk scale. This behavior was obtained by reacting at low temperature (<220 °C) substoichiometric amounts of white phosphorus (P 4 ) on well-defined monodisperse Ni nanoparticles in solution. Phosphorus insertion inside the Ni fcc nanoparticles triggers the crystallization of a Ni 2 P core surrounded by a Ni shell. The crystallization process was monitored by HRTEM, EFTEM, XRD, and SQUID analyses and revealed a direct transformation of Ni fcc to a core−shell structure without any other Ni x P y crystallized intermediate. This core−shell Ni−P system was tuned by adjusting the amount of P 4 added, providing tunable magnetic shells supported on monodisperse nanoparticles.