Crystalline and amorphous structure selectivity of ignoble high-entropy alloy nanoparticles during laser ablation in organic liquids is set by pulse duration.

Abstract

High-entropy alloy nanoparticles (HEA NPs) represent a promising material class with significant potential in various applications, such as heterogeneous catalysis or magnetic devices. This is due to their exceptional compositional tunability arising from the synergistic interplay of multiple elements within a single particle. While laser-synthesized, surfactant-free colloidal HEA NPs have already been reported, the underlying formation mechanism remains unknown, particularly the underexplored preference of amorphous over crystalline structures warrants further investigation. Herein, we present a systematic study of laser-generated equimolar CrMnFeCoNi nanoparticles, focusing on structural differences, arising from varying pulse durations during synthesis in organic solvents (acetone, ethanol, acetonitrile). In a systematic experimental series using high-resolution transmission electron microscopy, scanning transmission electron microscopy with energy-dispersive X-ray spectroscopy, selected-area electron diffraction, X-ray diffraction, electron energy loss spectroscopy, in situ heating, post-irradiation experiments, and differential scanning calorimetry we demonstrate that a pulse-duration-driven structural difference occurs during laser ablation in liquid is observable to the three utilized solvents. While picosecond-pulsed laser ablation in liquid produces polycrystalline HEA NPs, nanosecond-pulsed laser ablation favors a metastable amorphous structure. Particle cores in all cases exhibit a homogeneous distribution of the metals Cr, Mn, Fe, Co, and Ni, while particle shells were found to vary between manganese-enriched oxide layers and thin graphitic carbon coatings. The discovery of the structure-directing mechanism allows one to select between crystalline or amorphous HEA NP products, simply by choice of the laser pulse duration in the same, well-scalable setup, giving access to colloidal particles that can be further downstream processed to heterogeneous catalysts or magnets. In that context, the outstanding temperature stability up to 375 °C (differential scanning calorimetry) or 500 °C (transmission electron microscopy) may motivate future application-relevant work.