Mechanochemical synthesis of hexagonal NaGdF₄ nanoparticles: From the influence of stabilizing agents on hexagonal phase to the formation of core-shell structures

Abstract

Hexagonal-phase NaLnF₄ upconversion nanoparticles (UCNPs) are conventionally synthesized via thermal decomposition methods that require elevated temperatures, prolonged reaction times, and organic solvents, thereby limiting process sustainability. In this context, mechanochemistry emerges as an environmentally responsible alternative for UCNP synthesis, as it minimizes the use of toxic solvents and circumvents the need for high-temperature conditions. In this study, NaGd0.80F₄: Yb0.18Er0.02 upconversion nanoparticles were prepared through high-energy ball milling using lanthanide nitrates as precursors and sodium oleate or sodium citrate as surface ligands. The mechanochemical strategy enabled effective control over crystalline phase formation and particle morphology. Experimental parameters, including milling time and the molar ratio between fluoride and lanthanide sources, were systematically optimized to promote the formation of the hexagonal crystalline phase. Transmission electron microscopy revealed spheroidal nanoparticles with an average diameter of 15.6 nm. Surface functionalization played a critical role in directing crystalline structure and stabilizing particle morphology. The synthesized nanoparticles exhibited characteristic Er³⁺ upconversion emissions at 520, 545, and 654 nm under 980 nm excitation, confirming the preservation of their optical functionality. Furthermore, the proposed methodology can be extended to the fabrication of core–shell architectures, thereby broadening the technological applicability of these nanomaterials. Collectively, these findings demonstrate that mechanochemical synthesis constitutes an efficient and sustainable approach for producing UCNPs with controlled structural and optical properties.