<i>In Situ</i> Atomic-Scale Observation of Phase Evolution in Nickel Phosphide Nanoparticles

Nickel phosphides are promising earth-abundant, low-cost catalysts for hydrogen/oxygen evolution reactions and CO₂ reduction. However, their formation mechanisms remain poorly understood and difficult to control. This particularly applies to mechanisms determining phase evolution, crystallinity, and morphology under reactive conditions, factors that critically influence catalytic activity and stability. Here, we employ environmental transmission electron microscopy to directly observe the conversion of nickel nanoparticles into nickel phosphide phases under controlled phosphine atmosphere and temperatures. A three-stage Ni-to-Ni₂P conversion sequence is observed: (i) surface nucleation, (ii) rapid particle-size expansion, and (iii) crystallographic restructuring and faceting. Phase selectivity depends on the phosphine pressure and temperature: Ni₂P forms at both low and high pressures, Ni₂P and Ni₅P₄ coexist at intermediate pressure, and Ni₁₂P₅ emerges under no phosphine supply (residual phosphine may have remained) at elevated temperatures. We capture the temperature-driven Ni₂P-to-Ni₁₂P₅ transition. These insights offer strategies to control the phase and morphology for improved catalytic performance.