**1. Introduction**

Recently, rare-earth-ion-doped multicomponent compounds have attracted considerable attention due to their potential applications in the fields of electroluminescent devices, high-resolution displays, biological labels, and integrated optics [1–4]. Among these rare-earth-doped oxide phosphors, trivalent-cerium- and terbium-coactivated LaPO4 is significant because of its low solubility in water, its high thermal stability, and its high-efficiency energy transfer between Ce3<sup>+</sup> and Tb3<sup>+</sup> [5–8]. Due to the 4f orbital properties of La3+, lanthanide phosphate is transparent in the visible region and has been proven to be an ideal host structure for other lanthanide ions, resulting in luminescent materials in the UV-visible region [9–11]. In Ce3<sup>+</sup> and Tb3<sup>+</sup> co-doped LaPO4, Ce<sup>3</sup><sup>+</sup> with optically allowed d–f transitions is an effective activator for Tb3<sup>+</sup> emission [12,13]. LaPO4:Ce<sup>3</sup>+, Tb3<sup>+</sup> powders have been widely used as the green component of three band emission type fluorescent lamps [14,15]. In addition, LaPO4:Ce<sup>3</sup>+, Tb3<sup>+</sup> phosphors have drawn continuous research attention in several other applications, including transparent fillers/markers, biomedical purposes, and plasma display panels [13,16–18].

Phosphor particles should be spherical in shape with no aggregation and their particle size should be in the micron range (<3 μm) with a narrow size distribution [16,19]. Spherical phosphor particles are more advantageous for the optical and geometric structure of the phosphor layer. The size of the phosphor affects the number of phosphor particles needed to produce the best coating for a particular application [20]. The shape, size distribution, and other microstructural characteristics of phosphors can be well controlled by different synthetic methods and reaction conditions. To date, several methods have been reported for the synthesis of phosphate phosphor materials, such as coprecipitation [21–23], solvothermal methods [24,25], electrospinning methods [26], solid-state methods [27], sol-gel processes [10,12], and spray pyrolysis [14]. Of these, coprecipitation is a common industrial synthetic method used to produce rare earth oxide powders and has the advantages of being feasible, low-cost, and environmentally friendly. Beyond that, fluorescent powders prepared by the coprecipitation method have uniform particle sizes, low agglomeration, and low phase impurities [21]. However, many factors such as reaction temperature, aging time, pH value, and solution concentration need to be controlled, which limits the development of coprecipitation methods. It is still challenging to simply prepare phosphors with favorable morphologies and excellent luminescent performance.

Ionic liquids are a kind of green solvent which includes a wide range of liquids, excellent thermal stability, a wide electrochemical window, and a low vapor pressure [28–32]. Recently, much attention has been paid to the study of ionic liquids in supported liquid membrane systems [33,34]. Ionic-liquid-driven supported liquid membrane systems have shown the advantages of high flux, high efficiency, strong durability, and environmental friendliness, and have made great progress in gas separation, organic separation, metal ion separation, and chemical reactions. Our team first committed to the use of an ionic-liquid-driven supported liquid membrane system to prepare CePO4 inorganic nanomaterials. In doing so, we could easily control the morphologies (rod or sphere) of rare earth luminescent materials by adjusting the pH and the concentration of SO4 <sup>2</sup><sup>−</sup> [34]. Here, we used a facile ionic-liquid-driven supported liquid membrane method to prepare rare earth ion (Ce3+, Tb3+) co-doped LaPO4 phosphors with different morphologies (spherical and stone-like shapes). The preparation procedure, the role of the ionic liquid supported liquid membrane, characterization of the crystal structure, and photoluminescent properties of the synthesized LaPO4:Ce3<sup>+</sup>, Tb3<sup>+</sup> phosphors are reported in the following sections. This method has been proven to be easily controlled, simple, and mild, and the phosphors prepared by this method show good morphological and photoluminescent properties.

### **2. Results and Discussion**

#### *2.1. Ionic-Liquid-Driven HVHP Membrane Characterization*

In this experiment, we use a microporous ionic-liquid-driven HVHP membrane as selective ion channels that can selectively transfer PO4 <sup>3</sup><sup>−</sup> from the PO4 <sup>3</sup><sup>−</sup> supply phase into the mixed rare earth ion supply phase to prepare phosphors. We referred to the methods of Krzysztof A. et al. [35] to characterize the ionic-liquid-driven HVHP membrane. A Raman study was performed to investigate the ionic liquid presence on the surface and inside the HVHP membrane. The spectra of the inner part of the ionic-liquid-driven HVHP membrane was recorded up to 40 μm below the surface. As shown in Figure 1, the Raman vibration modes of ionic liquids can be observed on the surface of and in the interior of their corresponding functional membranes, which proves ionic liquids' presence on the surface and inside the HVHP membranes. Figure 2 shows SEM micrographs of the cross-section of the untreated HVHP membrane, as well as SEM micrographs and the corresponding map microanalysis (B or S) of the cross-section of the resulting [C4mim][BF4]- and [C4mim][Tf2N]-driven HVHP membranes. The micrographs and map microanalysis show that the ionic liquid infiltrates the reticular surface of the membrane.

**Figure 1.** Comparison of Raman spectra of (a) untreated HVHP membrane, (b) [C4mim][BF4], (c) the surface of the [C4mim][BF4]-driven HVHP, (d) the internal part (40 μm below the surface) of the [C4mim][BF4]-driven HVHP, (e) [C4mim][Tf2N], (f) the surface of the [C4mim][Tf2N]-driven HVHP membrane and (g) the internal part (40 μm below the surface) of the [C4mim][Tf2N]-driven HVHP membrane.

**Figure 2.** Plot of (**a**) micrographs of the cross-section of the untreated HVHP membrane, (**b**) micrographs of the cross-section of the [C4mim][BF4]-driven HVHP, (**c**) map microanalysis (B) of [C4mim][BF4]-driven HVHP, (**d**) micrographs of the cross-section of [C4mim][Tf2N] and (**e**) map microanalysis (S) of the [C4mim][Tf2N]-driven HVHP membrane.

#### *2.2. Membrane Reaction Mechanism*

We referred to the mechanism study of PanPan Zhao et al. [34] to propose a possible membrane reaction mechanism. The entire process can be divided into two parts: the liquid membrane transport stage and the precipitation reaction stage. The porous HVHP membrane is a good hydrophobic barrier which can effectively separate the two aqueous phases. After being immersed in an ionic liquid, the function of the HVHP film changes significantly. The microporous HVHP membrane containing an ionic liquid consists of selective ion channels which can selectively transfer PO4 <sup>3</sup><sup>−</sup> from the PO4 <sup>3</sup><sup>−</sup> supply phase into the mixed rare earth ion supply phase; the microporous HVHP membrane without ionic liquid cannot do this (Figure 3a). A precipitation reaction occurs upon PO4 <sup>3</sup><sup>−</sup> contacting the mixed rare earth ion supply phase on the other side of the HVHP membrane. Throughout the process, the cation (imidazolium) of the ionic liquid is responsible for the selective transfer of PO4 3− from the PO4 <sup>3</sup><sup>−</sup> supply phase to the mixed rare earth ion supply phase. The anion is responsible for controlling the mixed rare earth ion supply phase and the release rate and ionic liquid hydrophobicity are correlated [34]. The hydrophilicity follows the order [N(SO2CF3)2] <sup>−</sup> < [BF4] −, so the release rate of PO4 <sup>3</sup><sup>−</sup> of the ionic-liquid-driven HVHP membrane is in this order (Figure 3a). The reaction appears to be a liquid-liquid extraction and occurs in the ionic liquid-film phase at the membrane interface. In addition, due to the thinness and high porosity of the porous HVHP membrane, the numerous ion transport channels are very short, meaning the precipitation reaction occurs quickly and efficiently. The experimental device and a schematic diagram of the reaction mechanisms are shown in Figure 4.

**Figure 3.** Plot of (**a**) the concentration of PO4 <sup>3</sup><sup>−</sup> that has crossed the liquid membrane within 60 min using different ionic liquids in the liquid membrane phase and (**b**) picture of the phosphor reaction process and images of the precursor solution color within 60 min when using different ionic liquids in the liquid membrane phase.

**Figure 4.** Ionic-liquid-driven supported liquid membrane system and schematic diagram of reaction mechanism.

#### *2.3. Transmittance of PO4 <sup>3</sup>*<sup>−</sup> *under the Action of the Two Functional Membranes*

To investigate the transfer efficiency of PO4 <sup>3</sup><sup>−</sup> by the different functional membranes, we performed the following experiment devices: two glass units sandwiching blank-, [C4mim][BF4]- or [C4mim][Tf2N]-infiltrated membranes. The glass units were filled with 50 mL deionized water 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. Samples of 10 μL were taken from the deionized water phase every 10 min. Then, the samples were diluted and the transfer of PO4 <sup>3</sup><sup>−</sup> under the action of three kinds of membranes was measured with ICP. Figure 3a shows the changes of PO4 <sup>3</sup><sup>−</sup> concentration in 50 mL of a deionized water phase under the action of three different membranes within 60 min. The PO4 <sup>3</sup><sup>−</sup> concentration remained at 0 under the action of the blank-infiltrated membrane while the PO4 3− concentration increased under the action of the ionic liquid functional membranes. This suggests that PO4 <sup>3</sup><sup>−</sup> cannot cross the blank-infiltrated membrane but can cross ionic liquid functional membranes. There is a clear difference between the [C4mim][BF4] functional membrane and the [C4mim][Tf2N] functional membrane, which indicates that the transfer efficiency of the [C4mim][BF4] functional membrane toward PO4 <sup>3</sup><sup>−</sup> is much greater than that of the [C4mim][Tf2N] functional membrane. Figure 3b shows a picture of the phosphor reaction process and images of the precursor solution color when using rare earth sulfates as the rare earth supply phase for the different ionic liquid systems at different times under 254 nm irradiation. As shown in Figure 3b, precipitation occurs only in the rare earth phases, which indicates that the rare earth ions cannot pass through the membrane channels but phosphate can. The sample solutions all emitted green fluorescence when under a 254 nm light source and the fluorescence brightness increased with increasing reaction time. This proves that the membrane transfer rate of PO4 <sup>3</sup><sup>−</sup> for the [C4mim][BF4] functional membrane is markedly faster than that for the [C4mim][Tf2N] functional membrane, which is consistent with the results of previous research. In the actual production application, we can choose an appropriate functional membrane with different ionic liquids to achieve the effect of controlling the rate of production.
