Optimizing Upconversion Luminescence in BaYF5:Er³⁺/Yb³⁺ Nanocrystals Through Controlled Solvothermal Synthesis

Abstract

This study reports a facile solvothermal route to synthesize Er³⁺/Yb³⁺‑codoped BaYF₅ nanocrystals with tunable size and morphology. By systematically varying the fluoride source, pH, solvent, surfactants, Yb³⁺ concentration, temperature, and reaction time, we identified conditions that markedly enhance upconversion (UC) luminescence. NaBF₄ emerged as the superior fluoride source, delivering up to several‑fold stronger green (520–540 nm) and red (654 nm) emission compared with NH₄F or NaF. The addition of 5 % polyetherimide (PEI) as a surfactant further amplified UC output, whereas citric acid (CIT) increased particle size and reduced brightness.

Background

Upconversion nanophosphors (UCNPs) are pivotal in diverse applications—from solid‑state lasers and bioimaging to anti‑counterfeiting and quantum sensing—due to their unique ability to convert low‑energy photons into visible light. Fluoride hosts, especially NaYF₄, are widely used because of their low phonon energies and chemical stability, yet recent research has expanded to BaYF₅ and related BREF₅ (B = Mg, Ba, Ca, Sr) systems, which exhibit superior UC performance. Er³⁺/Yb³⁺ codoping is particularly effective: Yb³⁺ acts as an efficient sensitizer that transfers energy to Er³⁺, which then emits in the green and red spectral regions. BaYF₅ is an ideal host owing to its lattice parameters and ionic radius compatibility with Er³⁺ and Yb³⁺.

Key factors influencing UC efficiency include particle size, morphology, crystalline phase, and dopant distribution. Spherical particles with uniform size distribution promote light scattering and surface‐to‐volume ratio, enhancing UC emission. This work explores how synthetic parameters shape these properties and ultimately the luminescence output.

Experimental

All reagents were analytical grade (Ba(OH)₂·xH₂O, Y(NO₃)₃·6H₂O, Yb₂O₃, (CH₃COO)₃Er, NaBF₄, NH₄F, NaF, oleic acid, ethanol). Yb₂O₃ was first dissolved in dilute HNO₃ to yield Yb(NO₃)₃. In a typical synthesis, stoichiometric amounts of Ba(OH)₂·xH₂O, Y(NO₃)₃·6H₂O, (CH₃COO)₃Er, Yb(NO₃)₃, and NaBF₄ were dissolved in deionized water. Oleic acid and ethanol were added in predetermined ratios; the pH was adjusted to 9 using NH₃·H₂O. After 30 min of stirring, the mixture was sealed in a Teflon‑lined autoclave and heated at 200 °C for 16 h (or varied as needed). The product was collected by centrifugation, washed with ethanol and water, and dried at 60 °C for 12 h.

Characterization

Phase purity was confirmed by X‑ray diffraction (XRD) on a Bruker D8 Advance (Cu Kα, 10–70° 2θ). Photoluminescence (PL) was recorded on an Edinburgh Instruments FLS‑920 with 980‑nm excitation. Morphology and composition were examined by SEM (S‑3400N‑II) and EDS.

Results and Discussion

Phase and Crystallinity

All samples crystallized in the tetragonal BaYF₅ phase (JCPDS 46‑0039). No secondary phases appeared when NaBF₄ was used; a minor BaF₂ impurity was observed only at pH 4, indicating that higher pH suppresses unwanted by‑products. Peaks shifted to higher 2θ values, consistent with lattice contraction due to the smaller ionic radii of Er³⁺ and Yb³⁺ compared with Y³⁺. Crystallite size increased with higher temperature and longer reaction time, as quantified by the Scherrer equation.

Particle Morphology

SEM images revealed spherical particles ranging from ~30 nm (optimal conditions: 220 °C, 24 h) to ~180 nm (high temperature, 220 °C, 16 h). PEI addition yielded well‑dispersed, uniform spheres, whereas CIT caused aggregation and growth to micrometer scales, especially at CIT/Y = 4:1.

Upconversion Luminescence

UC spectra exhibited characteristic green (520, 540 nm) and red (654 nm) bands from Er³⁺ transitions. Emission intensity rose with crystallinity and particle uniformity. The optimal Yb³⁺ concentration was 20 %—higher levels led to concentration quenching. NaBF₄ as fluoride source produced the brightest emission; NaF and NH₄F yielded weaker signals due to rapid, uncontrolled F⁻ release that hampers crystal growth. PEI (5 %) further enhanced UC output by improving crystallinity and limiting particle growth, whereas CIT reduced brightness by enlarging particle size and diluting dopant concentration.

Mechanistic Insight

The UC process follows the established Yb³⁺ → Er³⁺ energy transfer pathway: Yb³⁺ absorbs 980 nm photons, transfers energy to Er³⁺, populating the ⁴I₁₁/₂ and ⁴F₇/₂ states. Subsequent non‑radiative relaxations lead to green (⁴S₃/₂, ²H₁₁/₂ → ⁴I₁₅/₂) and red (⁴F₉/₂ → ⁴I₁₅/₂) emissions. Structural factors such as particle size and crystallinity modulate cross‑relaxation pathways and surface quenching, thereby influencing the observed spectra.

Conclusion

BaYF₅:20 %Yb³⁺/2 %Er³⁺ nanocrystals with controlled morphology and high crystallinity were successfully prepared via solvothermal synthesis. NaBF₄ as fluoride source and 5 % PEI surfactant were critical for achieving uniform spherical particles (~30 nm) and maximal UC emission. The study provides a clear roadmap for tailoring synthesis parameters to optimize UC performance in BaYF₅ hosts.