Tailoring Co2P Shell of Co@Co2P Nanorods through the P-Source and Influence on Their Stability in Water

Cobalt nanorods (NRs) with diameters below 20 nm exhibiting both high magnetization and high magnetic coercivity are interesting building blocks for different applications ranging from permanent magnets to biosensors. One of the challenges is to develop simple methods of protecting such NRs to make them resistant to oxidation without degrading their magnetic properties, thus opening the way to a wider field of uses. This study presents the reactivity of Co NRs prepared by the polyol process with two different P-sources, tris(diethylamino)phosphine (DEAP) and the tris(trimethylsilyl)phosphine (PTMS). We show that depending on P-source, temperature, and P/Co molar ratio, Co NRs can be progressively transformed into core–shell Co@Co2P NRs of increasing shell thickness and then to Co2P@CoP NRs. All of these transformations retain the anisotropic shape of the particles, showing that the phosphidation proceeds at least partially through a topotactic reaction. Complementary to the electron microscope and X-ray diffraction analyses, the magnetic properties of the NRs at different stages of the phosphidation allow the precise following of the transformation of the NRs. When the phosphide shell is very thin, the disappearance of the exchange bias shows that the native cobalt oxide shell of the raw Co NRs is transformed first. Thanks to the optimal diameter of the bare Co nanorods, the formation of a thick phosphide shell is possible, without degradation of the ferromagnetic properties of the Co core for thicker Co2P shells, as the high coercive field of the Co@Co2P NRs shows that both the shape and the magneto-crystalline anisotropy of the cobalt core are preserved. It is only when the transformation of metallic Co to Co2P@CoP is complete that the NRs are no more magnetic. The core–shell Co@Co2P NRs present a very good resistance toward oxidation in water, which is not so common with ferromagnetic nanoparticles of such size.