*2.6. Photoluminescent Properties of the LaPO4:Ce<sup>3</sup>*+*, Tb3*<sup>+</sup> *Phosphors*

Figure 8 shows the excitation spectra of all the calcined LaPO4:Ce<sup>3</sup><sup>+</sup>, Tb3<sup>+</sup> phosphors by monitoring the 5D4→7F5 emission of Tb3<sup>+</sup> (λem <sup>=</sup> 543 nm) at room temperature. As clearly shown in Figure 8, the obtained fluorescent materials absorb excitation energy in the range of 240–310 nm with a maximum excitation wavelength at 277 nm, which may be related to the f–d transitions of Ce3+. In addition, several small peaks can be detected in the range of 310–400 nm, which could be caused by the f–f transitions of Tb3<sup>+</sup> [37,38]. Because of the forbidden nature of these transitions, their oscillator strength is much weaker than that of the spin-allowed 4f1–4f05d1 Ce3<sup>+</sup> transitions [22]. The excitation spectra consist of the strong excitation band of Ce3<sup>+</sup> and the weak excitation bands of Tb3+, revealing that Tb3<sup>+</sup> are essentially excited by Ce3<sup>+</sup>. In fact, several of the weak f–f excitation bands of Tb3<sup>+</sup> are only present in the region of the Ce3<sup>+</sup> emission. Thus, energy transfer from Ce3<sup>+</sup> to Tb3<sup>+</sup> occurs [21]. The emission spectra of all the calcined LaPO4:Ce3<sup>+</sup>, Tb3<sup>+</sup> phosphors at an excitation of 277 nm are shown in Figure 9. All the calcined LaPO4:Ce<sup>3</sup>+, Tb3<sup>+</sup> phosphors show obvious photoluminescence in the spectral range of 450–650 nm, and the four emission peaks at 487, 543, 584, and 621 nm can be assigned to the 5D4–7F6, 5D4–7F5, 5D4–7F4, and 5D4–7F3 transitions, respectively, of Tb3<sup>+</sup> [39,40]. Among these peaks, the green emission at 543 nm, which corresponds to the 5D4–7F5 transition of Tb3+, is the predominant peak. The spectral properties of the phosphors prepared by an ionic-liquid-driven supported liquid membrane system are essentially the same as those prepared by other synthetic methods, in which the improved ionic-liquid-driven supported liquid membrane system is a new and effective method to prepare LaPO4:Ce3<sup>+</sup>, Tb3<sup>+</sup> phosphors.

**Figure 8.** The excitation spectra of the calcined LaPO4:Ce3<sup>+</sup>, Tb3<sup>+</sup> phosphors prepared under different conditions.

**Figure 9.** The emission spectra of the calcined LaPO4: Ce3<sup>+</sup>, Tb3<sup>+</sup> phosphors prepared under different conditions.

#### **3. Materials and Methods**

#### *3.1. Materials*

The lanthanum sulfate hydrate, cerium sulfate hydrate, terbium sulfate hydrate, lanthanum nitrate hydrate, cerium nitrate hydrate, and terbium nitrate hydrate were provided by the Baotou Research Institute of Rare Earths, and the phosphoric acid solution was purchased from Aladdin (Shanghai, China). The chemicals used in the experiments were of analytical grade. The HVHP-04700 (pore size 0.45 μm and thickness 125 μm, ø: 5.5 cm, DUPAPORE®), a hydrophobic porous polyvinylidene fluoride film, was obtained from Millipore Corp. The ionic liquids (ILs) were selected from [C4mim][BF4]

and [C4mim][Tf2N] produced by the Center for Green Chemistry and Catalysis, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences. Figure 10 shows the molecular structures of the ionic liquids used in this study.

**Figure 10.** The molecular structures of the different ionic liquids used.

*3.2. Preparation of the Ionic-Liquid-Driven Supported Liquid Membrane, the La, Ce, and Tb Supply Phase, and the PO4 <sup>3</sup>*<sup>−</sup> *Supply Phase*

To prepare the ionic-liquid-driven supported liquid membrane, the hydrophobic porous polyvinylidene fluoride film (HVHP-04700) was immersed in an ionic liquid (≥200 μL of either [C4mim][BF4] or [C4mim][Tf2N]) for more than 2 h. For the La, Ce, and Tb supply phase, a lanthanum sulfate, cerium sulfate, and terbium sulfate mixed solution (or a lanthanum nitrate, cerium nitrate, and terbium nitrate mixed solution) was prepared in a calibrated volumetric flask by dissolving each compound in ultrapure water at a suitable La/Ce/Tb molar ratio. Each compound was placed in an ultrasound cleaner for 20 min to ensure complete dissolution. The PO4 <sup>3</sup><sup>−</sup> supply phase (1 M) was prepared in a calibrated volumetric flask by the dilution of concentrated phosphoric acid.
