Proving Monodispersed\nCore@Shell Model NaGdF<sub>4</sub>:Tb@NaGdF<sub>4</sub>:Ce by Redox\nReaction

A series of Ce<sup>3+</sup>- and Tb<sup>3+</sup>-doped\nNaGdF<sub>4</sub> core and core@shell monodispersed nanoparticles\n(NPs), with\ndifferent doping concentrations of Ce<sup>3+</sup> and a fixed concentration\nof Tb<sup>3+</sup> in a NaGdF<sub>4</sub> core (Ce (x = 1–20\nat. %)–Tb (y = 5 at. %)) (C-CT-x) and core@shell NaGdF<sub>4</sub>:20Ce@NaGdF<sub>4</sub>:5Tb (CS-CT-20), NaGdF<sub>4</sub>:5Tb@NaGdF<sub>4</sub>:20Ce (CS-TC-20), and NaGdF<sub>4</sub>:5Tb@NaGdF<sub>4</sub>:50Ce (CS-TC-50) NPs have been prepared using the thermolysis method.\nCe<sup>3+</sup> shows an absorption peak at 250–260 nm. Gd<sup>3+</sup> shows absorption peaks at 278 and 310 nm, and Tb<sup>3+</sup> shows absorption peaks at 350 and 378 nm (main). The emission peak\nof Ce<sup>3+</sup> is broad in the range of 320–400 nm. In\nthe Ce–Tb system, there is an energy transfer from Ce<sup>3+</sup> to Tb<sup>3+</sup>, and thus, a significant enhancement in luminescence\nintensity (488 and 545 nm) is observed. This is due to overlapping\nof the absorption peak of Tb<sup>3+</sup> with the emission peak of\nCe<sup>3+</sup>. The energy transfer from Ce<sup>3+</sup> to Tb<sup>3+</sup> is significantly improved in core@shell NPs, compared to\ncore NPs. In order to prove the core@shell model, these NPs are further\nsurface functionalized with SiO<sub>2</sub>. All of the silica-coated\nsamples are treated with KMnO<sub>4</sub>/ascorbic acid. It is found\nthat Ce<sup>3+</sup> in the surface/shell is more exposed to KMnO<sub>4</sub>/ascorbic acid, which proves the monodispersed core–shell\nmodel by the redox reaction.