Controlled synthesis, formation mechanism and upconversion luminescence of NaYF4: Yb, Er nano-/submicrocrystals via ionothermal approach

In order to deepen the fundamental understanding of IL-mediated synthesis of nano-/submicrostructure, hydrophilic ILs ([Emim][BF4], [Bmim][BF4] and [Omim][BF4]), which act as solvents, templates, as well as fluorine source, have been employed to synthesize rare earth doped NaYF4 upconversion nano-/submicrocrystals (UC-NMCs). The imidazolium cations provide the capping reagent to prevent the nucleation centers from aggregation and growing, while the tetrafluoroborate anions introduce a new fluorine source according to partial hydrolysis. It is demonstrated that the properties of IL, such as viscosity, polarity, solvency and interfacial tension, extremely affect the dissolution, diffusion and nucleation process of lanthanide ions in IL. Morphology and size of the final products can thus be tailored by synthetical parameters, like imidazolium cations, cosolvents, Ln3+ and fluoride concentrations, as well as ionothermal time. Based on the experimental results, the possible mechanism of the nucleation and growth of UC-NMCs in IL is discussed.