Controllable synthesis of ultrasmall Cu nanoparticles through spark ablation pathway regulation for efficient hydrogen evolution

Green hydrogen production via water electrolysis requires efficient, low-cost catalysts for the hydrogen evolution reaction (HER) to reduce energy consumption and system costs. Copper nanoparticles offer a promising low-cost alternative because they combine high conductivity, abundant surface sites, and earth abundance. However, conventional synthetic methods either require large amounts of ligands, which compromises environmental sustainability, or lack precise control over nanoparticle size. Here, we develop a ligand-free spark ablation in liquid (SAL) strategy for the size-controlled synthesis of Cu nanoparticles. By elucidating the formation and growth mechanisms, we show that spark discharge produces Cu nanoparticles through two competing mechanisms: plasma-mediated mechanism and molten-droplet mechanism. The thermal state of the discharge system controls the dominant ablation pathway and therefore determines the final particle-size distribution. The smallest and most uniform Cu nanoparticles deliver excellent hydrogen evolution performance, exhibiting a lower overpotential than commercial Pt/C at current densities above 400 mA cm −2 and stable operation for over 50 h. Mechanistic analysis indicates that their superior activity arises from an altered rate-determining step during HER. These findings establish SAL as a clean and controllable platform for producing metal nanoparticle catalysts, and provide mechanistic guidance for designing high-performance Cu-based electrocatalysts.