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 Ni2P/Ni12P5 (represented as NixPy) 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 degrees C and subsequently convert to crystalline-hollow NixPy NPs at 300 degrees C. For higher P:Ni ratios, a Ni-TOP complex forms that requires higher temperatures for NP growth, thus favoring direct formation of NixPy rather than nickel. Consequently, for P:Ni molar ratios of > 9, amorphous-solid NixPy NPs form at 240 degrees C and become crystalline-solid NixPy NPs at 300 degrees C. For intermediate P:Ni molar ratios of similar to 6, both growth mechanisms result in a mixture of hollow and solid NixPy NPs. Similar results have been obtained using tributylphosphine or triphenylphosphine as the phosphorus source, but trioctylphosphine oxide cannot serve as a phosphorus source.