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

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.