*3.3. LaPO4:Ce<sup>3</sup>*+*, Tb3*<sup>+</sup> *Precursor Synthetic Process for the Membrane Reaction*

The experiment was carried out in a glass cell system with self-adjusting diffusion which consisted of two glass units sandwiching a functional membrane ([C4mim][BF4] or [C4mim][Tf2N]). The glass units were filled with 50 mL of the rare earth mixture (rare earth sulfates or rare earth nitrates) at a certain ratio and 50 mL of the phosphoric acid solution (1 M). To ensure a homogeneous system, both solutions were stirred with a magnetic stirrer at 1000 rpm (Mini MR, IKA). After the complete reaction had occurred at room temperature, the white product was collected, centrifuged, and washed with ethanol more than 5 times, and then dried at 60 ◦C in a drying oven for 12 h to obtain the precursor. Under a set temperature (1000 ◦C), the LaPO4:Ce, Tb precursor powder was calcined for 1 h under a reducing atmosphere to finally produced the green-emitting phosphors.

#### *3.4. Characterization Methods*

The concentration of P in the ultrapure water phase and the rare earth ion concentration of the LaPO4:Ce<sup>3</sup><sup>+</sup>, Tb3<sup>+</sup> samples were measured using a HORIBA-Jobin Yvon ULTIMA 2 series by inductively coupled plasma-atomic emission spectroscopy (ICP-AES). The Raman spectra were measured using the Horiba Jobin Yvon S.A.S. LabRAM Aramis. The structures and phase purities of the as-prepared LaPO4:Ce<sup>3</sup><sup>+</sup>, Tb3<sup>+</sup> samples were identified using X-ray diffraction analysis with a Bruker AXS D8 Advance Powder X-ray diffractometer (Cu Kα radiation, λ = 1.5418 Å). The morphologies, energy spectrum of membranes, and the as-prepared products were observed under a ZEISS SIGMA 500 field emission scanning electron microscope. The excitation and emission spectra were taken on an Edinburgh FLS980 spectrometer equipped with a 450 W ozone-free xenon arc lamp as the excitation source.

#### **4. Conclusions**

In summary, LaPO4:Ce, Tb phosphors with monoclinic structures and good photoluminescence were successfully synthesized using a novel, controllable, and efficient ionic-liquid-driven supported liquid membrane system. The release rate of PO4 <sup>3</sup><sup>−</sup> from the liquid membrane with different ionic liquids was different. The phosphors prepared by this method exhibited micro-spherical (when using rare earth sulfates as the raw material) and nanoscale stone-shape (when using rare earth nitrates as the raw material) morphologies due to the influence of the different anions. These studies indicate that ionic-liquid-driven supported liquid membrane systems are a promising method for preparing LaPO4:Ce3<sup>+</sup>, Tb3<sup>+</sup> phosphors.

**Author Contributions:** Formal analysis, J.L.; funding acquisition, F.Y.; investigation, J.L., Z.Z., R.B., and H.Z.; methodology, J.L.; project administration, F.Y.; resources, F.Y.; supervision, H.D., F.Y., and L.S.; writing—original draft, J.L.; writing—review and editing, H.D., F.Y., and L.S.

**Funding:** The project was sponsored by the Clean Nuclear Energy System Fuels and Materials Joint Innovation Key Laboratory of Fujian Province, The National Natural Science Foundation of China (51865044), the Key Laboratory of Baiyunobo Rare Earth Resource Researches and Comprehensive Utilization (2018Z2004), the Science and Technology Service Network Initiative (2017T3002), the Science and Technology Key R&D Programs of Jiangxi Province (20171ACH80013), and the Science and Technology Projects of Xiamen (3502Z20172031).

**Acknowledgments:** The authors are grateful to characterizations provided by the Public Technology Service Center, Xiamen Institute of Rare Earth Materials.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


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International Journal of *Molecular Sciences*
