Formation mechanisms of Ni2P nanocrystals using XANES and EXAFS spectroscopy

Ni2P nanocrystals have been synthesized by a ligand stabilization method with adopting nickel acetylacetonate (Ni(acac)2) as a nickel precursor and trioctylphosphine (TOP) as a phosphorous precursor in the presence of trioctylphosphine oxide (TOPO) as a coordinating solvent. In order to better understand the formation of Ni2P nanocrystals, various synthetic routes were employed with varying the mixing order of TOP or TOPO with Ni(acac)2 in the complexation and nucleation steps, and the phosphidation temperatures of Ni–TOP solution in TOPO. In all cases the formation of Ni2P nanocrystals having an average size in range of 5–11 nm was achieved, but with slightly larger particle distribution of ca. 11 nm being observed for the case where the TOP was injected after a nucleation of Ni–TOPO was formed. In particular, XAFS techniques of EXAXS and XANES which are effective to characterize either amorphous or crystalline materials were applied to examine the structural and electrical properties of the precursor, intermediate and final materials. This led to elucidate the role and the effect of TOP and TOPO on the formation of Ni2P nanocrystals. It is concluded that the formation of Ni2P nanocrystals proceeds through three consecutive steps of complexation, nucleation and phosphidation. In phosphidation, TOP works as a reducing agent on the Ni precursor as well as a phosphorous source through the cleavage of P–C bonds.