SiO2-Er3+:NaGdF4 upconversion coatings prepared by combining green hydrothermal and sol-gel methods

This work reports the successful preparation of SiO 2 -Er 3+ :NaGdF 4 upconversion (UC) coatings using a pre-crystallized nanoparticles sol-gel route. The synthesis combines the use of a green hydrothermal method for the controlled crystallization of β-NaGdF 4 nanocrystals with a low-temperature sol-gel process to obtain homogeneous coatings. Dopamine was employed as a surface functionalizing agent, which provided excellent colloidal stability and prevented nanoparticle agglomeration during the incorporation into the SiO 2 sol. Different amounts of Er 3+ dopant were explored and the best UC performance was observed for 6 mol% Er 3+ :NaGdF 4 . The resulting coatings also exhibited remarkable UC luminescence under near-infrared (NIR) excitation, attributed to the preservation of the crystalline phase and the uniform distribution of nanocrystals within the silica matrix. Structural and optical characterizations (XRD, FTIR, TEM, and photoluminescence spectroscopy) confirmed the successful integration of Er 3+ :NaGdF 4 within the silica matrix. This study presents a promising approach for the fabrication of rare-earth-based UC coatings for applications in photonics and optoelectronics.