Fabrication of high and medium entropy transition metal alloy submicrometer particles by subnanosecond laser ablation in water
image: Submicrometer particle fabrication was achieved through subnanosecond laser pulses in water.
Credit: Osaka Metropolitan University
High-entropy alloy (HEA) nanoparticles have emerged as promising materials for catalysis and photonics due to the synergistic effects of multiple constituent elements at the nanoscale. However, synthesizing these multi-element systems remains challenging, particularly at the nano- and submicron scales, as differing physical and chemical properties of the elements lead to compositional heterogeneity and phase separation.
In this study, Osaka Metropolitan University researchers fabricated FeCoNi, CrFeCoNi, and CrMnFeCoNi submicrometer particles (SMPs) via laser ablation in liquid using subnanosecond laser pulses at 355, 532, and 1064 nm in water. STEM-EDX analysis of individual SMPs revealed that they could be categorized into four structural types based on the spatial elemental distribution: core-shell, crescent-shaped segregation, uniform distribution, and phase separation. In addition, a critical size-dependent transition in the elemental composition was observed at a diameter of approximately 200 nm, especially when using 355 nm laser pulses, where medium-sized SMPs showed significant elemental deviation, specifically enrichment of Cr, Mn, or Fe depending on the alloy, whereas large-sized SMPs maintained a stoichiometry nearly consistent with the bulk targets, but exhibited segregation.
Furthermore, the results indicate that preferential oxidation at the particle surface serves as the primary driving force for the segregation of elements like Cr, Mn, and Fe toward the shell. This work provides critical insights into the complex formation mechanisms of multi-elemental alloy particles in subnanosecond laser ablation in water and offers a wavelength-dependent strategy for controlling the surface properties and stoichiometry of HEA particles.
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