Femtosecond Laser Ablation of High-Entropy Ceramics RE₂(Ce_0.2Zr_0.2Hf_0.2Sn_0.2Ti_0.2)₂O₇ (RE = Sm, Eu, Er) in Liquid for Colloid Synthesis

Femtosecond laser synthesis and processing of colloids offer a platform for novel nanomaterial synthesis and applications, advancing side by side with high-end material innovations. In this work, RE2(Ce0.2Zr0.2Hf0.2Sn0.2Ti0.2)2O7 (RE = Sm, Eu, Er), one kind of A2B2O7-type (A = rare earth cation, B = tetravalent metallic cation) high-entropy oxide ceramic is chosen as the ablation target for colloid synthesis by femtosecond laser ablation in ethanol. Both surface structural and colloidal morphologies are analyzed to unveil the effects of photothermal (melting) and photomechanical (cracking) phenomena on colloid synthesis. High-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) jointly show that the synthesized nanomaterials in different sizes and shapes are mainly in the amorphous state, irrespective of the pristine samples’ composition. Energy-dispersive X-ray spectroscopy (EDS) mapping of as-prepared colloids indicates the emergence of elemental segregation, mainly occurring to small particles (mainly <10 nm); whereas for large particles, multiple elements are homogeneous, reflecting the high entropy characteristic of the samples. The discovery of a tailed nanostructure with elongated void embedment indicates special spallation dynamics containing void freezing or bubble capturing. Our findings may help to better understand the ablation dynamics relative to ceramics’ colloid formation. RE2(Ce0.2Zr0.2Hf0.2Sn0.2Ti0.2)2O7 (RE = Sm, Eu, Er) are most frequently considered to be good candidates for next-generation thermal barrier coatings in the film form; downsizing the ceramic dimensions into nanoscale via laser synthesis in liquids may expand its widespread applications in various fields such as optics, photonics, environment, catalysis, and biomedicine.