Nickel Phosphide Nanoparticles with Hollow, Solid, and Amorphous Structures

Conversion of unary metal nanoparticles (NPs) upon exposure to oxygen, sulfur, selenium, and phophorus precursors usually produces hollow metal oxide, sulfide, selenide, or phosphide NPs through the Kirkendall effect. Here, nanostructural control of mixed-phase Ni 2 P/Ni 12 P 5 (represented as Ni x P y ) NPs prepared through the thermolysis of nickel acetylacetonate using trioctylphosphine (TOP) as a ligand and phosphorus precursor is reported. The P:Ni molar ratio controls the NP size and is the key factor in determining the nanostructure. For P:Ni molar ratios of 1−3, nickel NPs form below 240 °C and subsequently convert to crystalline-hollow Ni x P y NPs at 300 °C. For higher P:Ni ratios, a Ni-TOP complex forms that requires higher temperatures for NP growth, thus favoring direct formation of Ni x P y rather than nickel. Consequently, for P:Ni molar ratios of >9, amorphous-solid Ni x P y NPs form at 240 °C and become crystalline-solid Ni x P y NPs at 300 °C. For intermediate P:Ni molar ratios of ∼6, both growth mechanisms result in a mixture of hollow and solid Ni x P y NPs. Similar results have been obtained using tributylphosphine or triphenylphosphine as the phosphorus source, but trioctylphosphine oxide cannot serve as a phosphorus source.