Synthetic Levers Enabling Independent Control of Phase, Size, and Morphology in Nickel Phosphide Nanoparticles

Abstract

Simultaneous control of phase, size, and morphology in nanoscale nickel phosphides is reported. Phase-pure samples of discrete nanoparticles of Ni12P5 and Ni2P in hollow and solid morphologies can be prepared in a range of sizes (10-32 nm) by tuning key interdependent synthetic levers (P:Ni precursor ratio, temperature, time, oleylamine quantity). Size and morphology are controlled by the P:Ni ratio in the synthesis of the precursor particles, with large, hollow particles formed at low P:Ni and small, solid particles formed at high P:Ni. The P:Ni ratio also impacts the phase at the crystallization temperature (300-350 °C), with metal-rich Ni12P5 generated at low P:Ni and Ni2P at high P:Ni. Moreover, the product phase formed can be decoupled from the initial precursor ratio by the addition of more "P" at the crystallization temperature. This enables formation of hollow particles (favored by low P:Ni) of Ni2P (favored by high P:Ni). Increasing temperature and time also favor formation of Ni2P, by generating more reactive P and providing sufficient time for conversion to the thermodynamic product. Finally, increasing oleylamine concentration allows Ni12P5 to be obtained under high P:Ni precursor ratios that favor solid particle formation. Oleylamine concentration also acts to "tune" the size of the voids in particles formed at low P:Ni ratios, enabling access to Ni12P5 particles with different void sizes. This approach enables an unprecedented level of control over phase and morphology of nickel phosphide nanoparticles, paving the way for systematic investigation of the impact of these parameters on hydrodesulfurization activities of nickel phosphides.