In Situ Atomic-Scale Observation of Phase Evolution in Nickel Phosphide Nanoparticles

Area of Science:

• Materials Science
• Catalysis
• Nanotechnology

Background:

• Nickel phosphides are earth-abundant, cost-effective catalysts for crucial reactions like hydrogen evolution, oxygen evolution, and CO2 reduction.
• Understanding nickel phosphide formation mechanisms is vital for controlling phase, crystallinity, and morphology, which directly impact catalytic activity and stability.
• Current knowledge gaps hinder the precise control over these factors under reactive conditions.

Purpose of the Study:

• To directly observe and elucidate the in-situ conversion mechanisms of nickel nanoparticles into various nickel phosphide phases.
• To investigate the influence of phosphine pressure and temperature on nickel phosphide phase evolution, crystallinity, and morphology.
• To provide fundamental insights for developing strategies to control nickel phosphide catalysts for enhanced performance.

Main Methods:

• Environmental transmission electron microscopy (ETEM) was utilized to directly visualize the transformation of nickel nanoparticles.
• Controlled phosphine atmospheres and temperatures were employed during the in-situ observations.
• Analysis focused on identifying nucleation, particle expansion, crystallographic restructuring, and faceting during the conversion process.

Main Results:

• A distinct three-stage conversion sequence from nickel to nickel phosphide (Ni₂P) was observed: surface nucleation, particle expansion, and crystallographic restructuring.
• Phase selectivity was found to be dependent on phosphine pressure and temperature, with different nickel phosphide phases (Ni₂P, Ni₅P₄, Ni₁₂P₅) forming under varied conditions.
• The temperature-driven transition between Ni₂P and Ni₁₂P₅ phases was successfully captured, highlighting the dynamic nature of these materials.

Conclusions:

• The study provides direct, real-time insights into the formation mechanisms of nickel phosphides.
• Understanding these mechanisms enables precise control over the phase and morphology of nickel phosphide catalysts.
• These findings pave the way for designing and optimizing nickel phosphide-based catalysts for improved efficiency and stability in various catalytic applications.