Coating and Transforming the Y(OH)CO₃ Shell on Upconversion Nanoparticles

Abstract

Along with the promising applications of lanthanide doped upconversion nanomaterials in diverse fields such as biology, anticounterfeiting, and lasering, the demand for multifunctional upconversion nanomaterials is increasing. One effective means of obtaining these nanomaterials is to fabricate upconversion nanomaterial-based heterostructures, which may provide superior properties as compared to the sum of the parts. However, obtaining heterostructured upconversion nanomaterials remains challenging mainly because of the crystal lattice mismatch between upconversion nanomaterials and other materials. Typically used strategies for synthesizing upconversion nanomaterial-based heterostructures are applicable only to limited types of materials. Alternatively, transformation of the intermediate layer is a promising strategy used to obtain these heterostructures. Nevertheless, this method remains in its infancy and, to date, only a few intermediate layers have been developed. New types of intermediate layers are therefore highly desirable. In this study, we show that amorphous Y(OH)CO3 can be a promising candidate as an intermediate layer for fabricating upconversion nanoparticle-based heterostructures. As a proof-of-concept experiment, ligand-free NaGdF4:Yb/Tm upconversion nanoparticles were first prepared as core nanoparticles. The Y(OH)CO3 shell was then directly coated on the NaGdF4:Yb/Tm upconversion nanoparticles in an aqueous solution using urea and Y(NO3)3, by a homogeneous precipitation approach. The thickness of the resulting Y(OH)CO3 shell could be tuned by adjusting the amounts of either urea or Y(NO3)3. The as-coated Y(OH)CO3 shell could be easily converted to YOF by heating at 300 °C, yielding NaGdF4:Yb/Tm@YOF core-shell heterostructured nanoparticles. In addition, we found that the NaGdF4 core could be transformed to lanthanide oxide fluoride if the NaGdF4:Yb/Tm@Y(OH)CO3 core-shell nanoparticles were heated at 350 °C. We also observed that treating the NaGdF4:Yb/Tm@Y(OH)CO3 core-shell nanoparticles at even higher temperatures (e.g., 400 °C) produced aggregations of nanoparticles without regular morphologies. The transformation of the shell can be attributed to the decomposition of Y(OH)CO3 and reactions between the Y(OH)CO3 shell and NaGdF4 core. Meanwhile, the transformation of the NaGdF4 core at relatively high temperatures could be primarily due to the reactions between Y(OH)CO3 and NaGdF4. Notably, in this study, the core-shell structured nanoparticles, with either a Y(OH)CO3 or YOF shell, maintained the photon upconversion properties of NaGdF4:Yb/Tm upconversion nanoparticles. In addition, the method used here could be extended to the coating of other shells such as Tb(OH)CO3 and Yb(OH)CO3 on upconversion nanoparticles. Moreover, the NaGdF4:Yb/Tm@Y(OH)CO3 core-shell nanoparticles could be transformed to other nanoparticles with novel structures such as yolk-shell nanoparticles. These results can pave the way for preparing upconversion nanoparticle-based heterostructures and multifunctional composites...

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