Nanosecond UV-Laser Fragmentation of LaB₆ for Scalable Production and Size Control of Nano-Sized Particles

Nanosecond laser fragmentation in liquids has recently emerged as a highly efficient alternative to picosecond laser fragmentation and ablation for nanoparticle production, but its applicability to mechanically robust, refractory ceramics in purely photothermal regime remains unclear. Here, LaB₆ is chosen as a model material due to its relevance as an electron emitter, NIR shield, and radiosensitizer. Optical-thermal considerations show that, despite the shallow optical penetration depth of UV light in LaB₆, nanosecond-scale thermal diffusion enables near-volumetric heating of particles up to ~1.3 µm, thereby defining feedstock size requirements for efficient fragmentation. A distinct fragmentation threshold of ~1.8 J cm⁻² is identified, at which nanoparticle formation sets in. In pure water, the nanoparticle mass yield saturates at ~45%, indicating that shielding and related processes limit effective photothermal energy deposition rather than laser fluence. To overcome these limitations, the influence of chemical additives was systematically investigated. Stabilizing agents and radical scavengers suppress fragmentation efficiency, whereas UV-absorbing FeCl₃ markedly enhances optical coupling and enables near-quantitative microparticle-to-nanoparticle conversion. Hofmeister-active salts further provide a handle to tune the particle size distribution: chaotropic anions (Br⁻, I⁻) suppress the persistence of large particles and kinetically quench growth, enabling size control down to the 2 sub-20 nm regime without altering the crystalline LaB₆ phase. Overall, this work establishes nanosecond UV laser fragmentation of LaB₆ microparticles as an efficient, scalable, and tunable route for processing refractory ceramic materials and highlights the central role of photothermal energy deposition and particle-liquid interfacial chemistry in controlling fragmentation efficiency and nanoparticle size.