*2.5. Structure and Morphologies of the LaPO4:Ce3*<sup>+</sup>*, Tb3*<sup>+</sup> *Phosphors*

In this study, we used an ionic-liquid-driven supported liquid membrane system to prepare phosphors. The whole system 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) and 50 mL of the phosphoric acid solution (1 M). The PO4 3− crossed the functional membrane to react with the rare earth ions in this system. Finally, the phosphors were prepared by calcining the precursors. The powder samples prepared from different rare earth ion sources (rare earth nitrates and rare earth sulfates) in the [C4mim][BF4] functional membrane were labelled BN and BS, respectively, and powder samples prepared from different rare earth ion sources (rare earth nitrates and rare earth sulfates) in the [C4mim][Tf2N] functional membrane were labelled NN and NS, respectively.

X-ray diffraction patterns were employed to determine the phase purities and crystal structures of the phosphor products. Figure 5a shows the XRD patterns of the precursors prepared under different conditions (different rare earth solutions and different ionic liquids). The vertical bars show the standard hexagonal LaPO4 peak positions (JCPDS No. 04-0635). Figure 5a shows that the diffraction peaks of all the precursors can be readily indexed to the hexagonal structure of LaPO4 in the P6222 space group (JCPDS No 04-0635). Figure 5b shows the XRD patterns of the as-prepared LaPO4:Ce, Tb phosphor samples prepared under different conditions (different rare earth solutions and different ionic liquids). The vertical bars show the standard monoclinic LaPO4 peak positions (JCPDS No. 32-0493). From Figure 5b, it is obvious that peaks at 2θ = 19.04◦, 21.74◦, 27.08◦, 28.88◦, and 42.48◦ are present after annealing at 1000 ◦C, which may be attributed to the (011), (101), (200), (120), and (221) reflections of the monazite crystalline structure of lanthanum phosphate. A monoclinic phase (space group: P21/n) of pure LaPO4 (JCPDS No. 32-0943) was obtained. By comparing the XRD pattern of the as-prepared precursors and LaPO4:Ce, Tb phosphor samples, we found that after annealing at 1000 ◦C, the fluorescent powder XRD peaks were sharper, the crystallinity was better and the structure of LaPO4:Ce, Tb had changed from a hexagonal to a monoclinic crystal phase. In addition, after the sample was calcined at 1000 ◦C, all the diffraction peaks shifted to the right compared with the standard diffraction peaks. This is because the radii of Ce3<sup>+</sup> (~0.1034 nm) and Tb3<sup>+</sup> (~0.0923 nm) are smaller than the radius of La3<sup>+</sup> (~0.1061 nm) in the LaPO4:Ce3<sup>+</sup>, Tb3<sup>+</sup> crystals, which leads to lattice contraction and a reduction of interplanar distance. Thus, based on the Bragg diffraction principle 2dsinθ = λ, where the decrease of the d value increases the diffraction angle, the diffraction peak positions of the XRD patterns move towards larger angles [23,36].

The morphology, size, and microstructural details were investigated by scanning electron microscopy. Figure 6 shows the SEM micrographs of the precursors prepared under different conditions (different rare earth solutions and different ionic liquids). Notably, the morphologies of the precursors are similar in the different ionic liquid functional membranes but show different morphologies for the different rare earth sources. When the anion of the rare earth mixed solution was sulfate, the samples exhibited a spherical morphology with particle sizes in the range of 600–800 nm, and a rough surface which consisted of aggregates of smaller particles. When the anion of the rare

earth mixed solution was nitrate, the samples exhibited a flower-like structure with a diameter of approximately 30 nm and a length of approximately 200 nm. According to our previous research, we believe the reason for the formation of this globular structure is due to the template effect of SO4 <sup>2</sup><sup>−</sup> [34]. Figure 7 shows SEM micrographs of the as-prepared LaPO4:Ce, Tb phosphor samples prepared under different conditions (different rare earth solutions and different ionic liquids). Similarly to the precursors, the annealed samples had similar morphologies when prepared with different ionic liquid functional membranes but different morphologies when prepared with different rare earth sources. After sintering, the samples prepared with rare earth sulfates as the raw material maintained their spherical structure, but the aggregation between spheres was more severe than that in the precursor samples. The small particles on the spherical surface of the sintered samples were larger than those on the surface of the respective precursor. However, after sintering, the samples prepared with rare earth nitrates as the raw material changed from a nanowire flower-like structure to a stone-like structure. The particle sizes were in the range 30–300 nm. The results show that the crystal size of all the samples increased after calcining, and due to the templating effect of SO4 <sup>2</sup>−, the samples with rare earth sulfates as the raw material continued to maintain their large micro-sized spherical morphology, while the shape of the samples prepared using rare earth nitrates as the raw material grew from a flower-like structure into a stone-like morphology.

**Figure 5.** XRD patterns of the precursors (**a**) and calcined LaPO4:Ce3<sup>+</sup>, Tb3<sup>+</sup> phosphors (**b**) prepared under different conditions.

**Figure 6.** *Cont*.

**Figure 6.** SEM images of the precursors prepared under different conditions: (**a**) BS, (**b**) NS, (**c**) BN, and (**d**) NN. (EHT for extra high tension, WD for working distance)

**Figure 7.** SEM images of calcined LaPO4:Ce3<sup>+</sup>, Tb3<sup>+</sup> phosphors prepared under different conditions: (**a**) BS, (**b**) NS, (**c**) BN, and (**d**) NN. (EHT for extra high tension, WD for working distance)
