Synthesis\nof Amorphous and Various Phase-Pure Nanoparticles\nof Nickel Phosphide with Uniform Sizes via a Trioctylphosphine-Mediated\nPathway

Nickel phosphides are of particular interest because\nthey are highly\nactive and stable catalysts for petroleum/biorefinery and hydrogen\nproduction. Despite their significant catalytic potential, synthesizing\nvarious phase-pure nickel phosphide nanoparticles of uniform size\nremains a challenge. In this work, we develop a robust trioctylphosphine\n(TOP)-mediated route to make highly uniform phase-pure Ni<sub>12</sub>P<sub>5</sub>, Ni<sub>2</sub>P, and Ni<sub>5</sub>P<sub>4</sub> nanoparticles.\nThe synthetic route forms amorphous Ni<sub>70</sub>P<sub>30</sub> nanoparticle\nintermediates. The reactions can be stopped at the amorphous stage\nwhen amorphous particles are desired. The amount of P incorporation\ncan be controlled by varying the ratio of TOP to Ni­(II). The mechanism\nfor composition control involves the competition of the kinetics of\ntwo processes: the addition of the reduced Ni and the incorporation\nof P into Ni. Uniform Ni<sub>70</sub>P<sub>30</sub> amorphous nanoparticles\ncan be generated at a high TOP-to-Ni­(II) ratio, where the P incorporation\nkinetics is made to dominate. Ni<sub>70</sub>P<sub>30</sub> can later\nbe transformed into phase-pure Ni<sub>12</sub>P<sub>5</sub>, Ni<sub>2</sub>P, and Ni<sub>5</sub>P<sub>4</sub> nanocrystals of uniform\nsize. The transformation can be controlled precisely by modulating\nthe temperature. A UV–vis study coupled with theoretical modeling\nreveals Ni(0)-TOP<sub><i>x</i></sub> complexes along the\nsynthetic path. This approach may be expanded to create other metal\ncompounds, potentially enabling the synthesis of uniform nanoparticles\nof a greater variety.