Proving Monodispersed Core@Shell Model NaGdF4:Tb@NaGdF4:Ce by Redox Reaction

Abstract

A series of Ce³⁺- and Tb³⁺-doped NaGdF₄ core and core@shell monodispersed nanoparticles (NPs), with different doping concentrations of Ce³⁺ and a fixed concentration of Tb³⁺ in a NaGdF₄ core (Ce (x = 1–20 at. %)–Tb (y = 5 at. %)) (C-CT-x) and core@shell NaGdF₄:20Ce@NaGdF₄:5Tb (CS-CT-20), NaGdF₄:5Tb@NaGdF₄:20Ce (CS-TC-20), and NaGdF₄:5Tb@NaGdF₄:50Ce (CS-TC-50) NPs have been prepared using the thermolysis method. Ce³⁺ shows an absorption peak at 250–260 nm. Gd³⁺ shows absorption peaks at 278 and 310 nm, and Tb³⁺ shows absorption peaks at 350 and 378 nm (main). The emission peak of Ce³⁺ is broad in the range of 320–400 nm. In the Ce–Tb system, there is an energy transfer from Ce³⁺ to Tb³⁺, and thus, a significant enhancement in luminescence intensity (488 and 545 nm) is observed. This is due to overlapping of the absorption peak of Tb³⁺ with the emission peak of Ce³⁺. The energy transfer from Ce³⁺ to Tb³⁺ is significantly improved in core@shell NPs, compared to core NPs. In order to prove the core@shell model, these NPs are further surface functionalized with SiO₂. All of the silica-coated samples are treated with KMnO₄/ascorbic acid. It is found that Ce³⁺ in the surface/shell is more exposed to KMnO₄/ascorbic acid, which proves the monodispersed core–shell model by the redox reaction.