Hydrothermal synthesis combined with calcination of NaGd(SO₄)₂ nanoparticles and their luminescent properties from single doped Eu³⁺ and co-doped Yb³⁺,Er³⁺

The NaGd(SO₄)₂: Eu³⁺/Yb³⁺,Er³⁺ nanoparticles were successfully synthesized by a facile hydrothermal method combined with calcination. DTA-TG-DTG, FT-IR, and XRD results reveal that the precursor is composed of gadolinium hydrate with sulfate groups and can convert into pure NaGd(SO₄)₂ nanoparticles with a dispersed fishing net shape after calcined at 800 °C for 2 h. Upon excitation at 273 nm UV light, the emission spectra of Eu³⁺-doped phosphors consist of ⁵D₀ → ⁷F_j (j = 1, 2, 3, 4) transitions and the target NaGd(SO₄)₂: Eu³⁺ shows a highest red luminescent intensity than NaGd(SO₄)₂·H₂O: Eu³⁺ and Gd₂O₃: Eu³⁺ phosphors, corresponding to the ⁵D₀ → ⁷F₂ transition of Eu³⁺. The calculated lifetime of NaGd(SO₄)₂ is determined to be 2.345 ms. The up-conversion luminescence (UCL) mechanism of Yb³⁺ and Er³⁺ co-doped NaGd(SO₄)₂ and Gd₂O₃ phosphors were also discussed in the typical energy level diagram for their possible UCL processes.