Room Temperature Ion Beam Synthesis of Ultra-Fine Molybdenum Carbide Nanoparticles: Toward a Scalable Fabrication Route for Earth-Abundant Electrodes

Abstract

Molybdenum carbides are promising low-cost electrocatalysts for electrolyzers, fuel cells, and batteries. However, synthesis of ultrafine, phase-pure carbide nanoparticles (diameter < 5 nm) with large surface areas remains challenging due to uncontrollable agglomeration that occurs during traditional high temperature syntheses. This work presents a scalable, physical approach to synthesize molybdenum carbide nanoparticles at room temperature by ion implantation. By tuning the implantation conditions, various molybdenum carbide phases, stoichiometries, and nanoparticle sizes can be accessed. For instance, molybdenum ion implantation into glassy carbon at 30 keV energy and to a fluence of 9 × 10¹⁶ at cm^-2 yields a surface η-Mo₃ C₂ with a particle diameter of (10 ± 1) nm. Molybdenum implantation into glassy carbon at 60 keV to a fluence of 6 × 10¹⁶ at cm^-2 yields a buried layer of ultrafine γ'-MoC/η-MoC nanoparticles. Carbon ion implantation at 20 keV into a molybdenum thin film produces a 40 nm thick layer primarily composed of β-Mo₂ C. The formation of nanoparticles in each molybdenum carbide phase is explained based on the Mo-C phase diagram and Monte-Carlo simulations of ion-solid interactions invoking the thermal spike model. The approaches presented are widely applicable for synthesis of other transition metal carbide nanoparticles as well.

Keywords: ion beam synthesis; molybdenum carbide; nanoparticles; phase selection; thermal spikes.