ABSTRACT

A LiCaPO₄:Eu²⁺ phosphor with high photoluminescence was synthesized using a polymerizable complex (PC) method employing a water-soluble polyethylene glycol-conjugated phosphate ester (PEG-P). PEG-P could be obtained from a reaction among polyethylene glycol 300, phosphorus pentoxide, and pyrophosphoric acid. The PEG-P prepared was stable in an aqueous condition. A transparent solution and gel were obtained when the PEG-P was used as a source of P during the PC method, whereas the use of H₃PO₄ caused an undesirable precipitate. The LiCaPO₄:Eu²⁺ obtained via the PC method employing the PEG-P showed higher emission intensity than those synthesized by a solid state reaction method and the PC method employing H₃PO₄. The high luminescence properties of the sample synthesized using the PEG-P may be attributed to high homogeneity of constituents in the sample.

1. Introduction

Phosphate phosphors have been well-known as one of the most important luminescence materials because of their excellent thermal stability and high emission intensity as well as lower temperature synthesis [1,2]. Among them, Eu²⁺-activated ABPO₄ phosphors (A and B are monoand divalent cations, respectively) have been reported as blue-emitting phosphors excited by near UVLEDs. For instance, KSrPO₄:Eu²⁺, KBaPO₄:Eu²⁺, and LiCaPO₄:Eu²⁺ have excellent luminescence properties including quantum efficiency and thermal quenching behavior. Therefore, they are considered to be potential application as phosphors for the white light emitting diodes [3-5].

Several kinds of methods were applied to synthesis of inorganic powders, such as solid state reaction (SSR), solution-based method, and combustion process. In the synthesis of phosphors, the SSR method is the most extensively used. However, the SSR method includes some drawbacks such as low homogeneity, and non-uniform particles morphology and size. Generally, solution-based synthesis is considered to be a desirable approach because it can produce highly homogeneous compounds in the atomic level [6,7]. Homogeneity of constituents and control of morphology and size of particles are requisites for highly efficient phosphors. One of limitations in a synthesis of phosphate phosphors using solution methods is requirement of appropriate P source, which is soluble and stable in an aqueous condition. The use of conventional phosphate reagents such as phosphoric acid or ammonium phosphate produce precipitates with metal ions in an aqueous condition [8]. Precipitates would lead to samples with inhomogeneous composition accompanying by secondary phases. In ion-activated type phosphors, distribution of an activator is closely connected to luminescence intensity in the view of concentration quenching. It indicates that inhomogeneity results in low performance of phosphor. Additionally, in a highly homogeneous sample, high amount of rare-earth can be doped, and as a result, high luminescence intensity would be achieved.

Solution-based synthesis highly requires the use of appropriate raw materials, which do not produce precipitate with any metals present in a given aqueous solution. Therefore, development of a new water-soluble P source that does not form precipitates with other metal ions is indispensable to realization of synthesis of extremely homogeneous phosphate compounds with high performance by aqueous solution-based methods. It is known that precipitates between phosphorus and metals could be prevented by introduction of condensed chain-structured phosphates [9,10]. Recently, we have succeeded in a synthesis of a phosphate phosphor with high luminescence by a solution method using such a phosphate oligomer [11]. The prepared oligomer had good solubility and stability in an aqueous condition, though approximately 50% H₃PO₄ unreacted remained. Therefore, it can be expected that if a phosphorus source with further less H₃PO₄ can be prepared, the range of synthesis of phosphate phosphors with even better photoluminescence properties can be greatly expanded. The condensed chain-structured phosphates are commercially available and are synthesized from a rather simple reaction among alcohol, phosphorus pentoxide, and polyphosphoric acid [12]. Especially, polyethylene glycol (PEG) is considered to be a good candidate for its condensation with the phosphate moiety because PEG is widely used as a cross-linking agent for promoting formation of “gel” in a variety of “sol-gel”-based solution methods. Polyethylene glycol-conjugated phosphate ester (PEG-P) is considered to be soluble and stable in an aqueous solution, and also PEG-P may have a role as a cross-linking agent. In this study, we report the synthesis of PEG-P whose chemical structures are deduced from ¹H and ³¹P{¹H} NMR measurements, and then we stress the great advantage of the use of PEG-P as a P source for the synthesis of Eu²⁺-doped LiCaPO₄, chosen as a model among phosphate-based phosphors, by demonstrating the superior photoluminescence properties of the target phosphor synthesized by the polymerizable complex (PC) method employing the PEG-P instead of the conventional P source, that is H₃PO₄.

2. Experimental Section

2.1. Synthesis of PEG-P

Pyrophosphoric acid (10 mmol phosphorus, Kanto Chemical) and polyethylene glycol 300 (PEG300, 10 mmol Kanto Chemical) were mixed at 323 K and kept for 2 h. Phosphorus pentoxide P₄O₁₀ (10 mmol phosphorus, Wako Chemical) was weighed in an inert atmosphere to prevent its hydrolysis, and then slowly added into the mixture. After P₄O₁₀ was dispersed homogeneously, temperature of the mixture increased to 358 K and kept for 5 h. Finally, 10 mmol of PEG300 was added, and the reaction was continued for 12 h at 358 K. The entire reaction was conducted in N₂ atmosphere. Gel with high viscosity was formed and it was dissolved in distilled water. The gel was dissolved in D₂O, and pH was adjusted to 13 using 10 M NaOH to prepare a solution for NMR analysis. ¹H and ³¹P{¹H} NMR spectra of the prepared PEG-P were recorded on a Bruker AVANCE 400 spectrometer (400 MHz for ¹H resonance) to analyze the chemical structure of PEG-P. The phosphorus concentration of diluted PEG-P was determined using an inductively-coupled plasma (ICP) method (Perkin Elmer; Optima 3300XL) prior to its use for the synthesis of Eu²⁺-doped LiCaPO₄ by the PC method described in the following section.

2.2. Preparation of Eu²⁺-Doped LiCaPO₄

A Eu²⁺-doped LiCaPO₄ phosphor was synthesized via the PC method employing the PEG-P as a P source. LiNO₃ (99%, Kanto Chemical), Ca(NO₃)₂∙4H₂O (99.5%, Kanto Chemical), and Eu(NO₃)₃, which was prepared by dissolution of Eu₂O₃ in HNO₃, were dissolved in a citric acid (CA) solution at a ratio of Li:Ca:Eu:CA = 1:0.97:0.03:8. The mixture was firstly heated at 353 K to allow chelation for 2 h, and then, the PEG-P and propylene glycol (PG) were added into the solution at a molar ratio of 1:8. The temperature was subsequently increased to 423 K to promote gel formation. The formed gel was heated at 1123 K in air to remove the organic content, and then it was reduced at 1373 K under a flow of Ar containing 4% H₂ for 3 h. To enhance the phase purity, post-heat treatment was conducted at 1073 K for 18 h in an Ar/4%H₂ atmosphere. A Eu²⁺-doped LiCaPO₄ phosphor was also synthesized by the PC method using H₃PO₄ as the P source. In addition, synthesis using the SSR method was carried out for comparison by stoichiometric mixing raw materials including Eu₂O₃ (Furuuchi Chemical), and Li₂CO₃ (Wako Chemical), CaCO₃, and (NH₄)H₂PO₄ (both from Kanto Chemical). X-ray diffraction analysis (XRD, Bruker AXS; D2 Phaser) was conducted. Excitation and emission spectra of phosphors were recorded using a fluorescence spectrometer (Hitachi; F-4500) at room temperature. Quantum efficiencies of the samples were evaluated using fluorescence spectrometer (Jasco; FP-6500).

3. Results and Discussion

Scheme 1 shows estimated chemical structures of the synthesized PEG-P; (a) monoand (b) di-esters, and Figure 1 shows ¹H (a) and ³¹P{¹H} (b) NMR spectra of the obtained PEG-P. In the ¹H NMR spectra (Figure 1(a)), each peak was assigned to each hydrogen marked with “1 - 6” in Scheme 1(a). No peaks of “4” and “5” were observed in ¹H NMR spectra of PEG and H₃PO₄ (not shown here), which in turn indicates that the observation of these peaks evidences formation of a conjugation between PEG300 and phosphate. The ³¹P{¹H} NMR spectrum (Figure 1(b)) indicates that only one side OH group of PEG was conjugated with phosphate. The integration

area corresponding to the peak “2” was greatly decreased after the reaction, and finally, it could be estimated that more than 90% of PEG 300 was bonded with phosphate. From the ³¹P{¹H} NMR spectrum, it could be confirmed that the PEG-P contained H₃PO₄, monoester, and diester with ratio of 17.4:72.5:10.1 (Figure 1(b)). Previously, we reported preparation of a phosphate oligomer starting from ethylene glycol and phosphoric acid by promoting their esterification reaction under reflux [11]. The yield of the phosphate oligomer was about 55.4% with 44.6% of unreacted H₃PO₄. This indicates that the present method can produce the condensed chain-structured phosphate with less H₃PO₄ than the reported method. The use of such a P source having a large proportion of esters containing a small amount of H₃PO₄ is considered to be suitable for solution-based synthesis of phosphate-based phosphors because of less opportunity that precipitates form resulting from interaction between H₃PO₄ and metal ions present in a given solution. Another important characteristic of PEG-P is its stability in H₂O, which was confirmed by the fact that the initial small proportion of H₃PO₄ remained unchanged over 2 months.

To demonstrate the advantages of the use of PEG-P in a solution-based method, a LiCaPO₄ doped with Eu²⁺ phosphor was synthesized by the PC method employing the PEG-P. Figure 2 shows XRD patterns of LiCaPO₄: Eu_0.03 synthesized by the PC method employing the PEG-P under various conditions; (a) 1123 K for 3 h in air, (b) 1373 K for 3 h in Ar/4%H₂ atmosphere, and (c) postheating of (b) at 1073 K for 18 h. No precipitate was formed when the PEG-P was added into an aqueous solution containing LiNO₃, Ca(NO₃)₂·4H₂O, and Eu(NO₃)₃. When the sample obtained after the heat-treatment at 1123 K in air, a single phase LiCaPO₄ was formed without any impurity phases as shown in Figure 2(a). Synthesis of a single phase LiCaPO₄:Eu²⁺ phosphor was rarely achieved in previous studies, and they suffered from significant contamination by impurities, such as Li₃PO₄ and Ca₃(PO₄)₂ [13-15]. It should therefore be stressed here that it was possible to obtain a single phase of LiCaPO₄ resulting from the achievement of highly homogeneous distribution of constituents. Reduction at 1373 K for 3 h gave the sample exhibiting the highest emission intensity among various reduction temperatures (1073 - 1473 K). However, at this relatively higher temperature, strong reflections due to impurity phases such as Li₃PO₄ and Ca₃(PO₄)₂ showed up (Figure 2(b)).

This is due to decomposition of LiCaPO₄ at such a high temperature. Reaction between Li₃PO₄ and Ca₃(PO₄)₂ appears to be possible when the corresponding phase diagram for these two compounds is taken into account [15,16]. The post-heat treatment of the above-mentioned sample at 1073 K for 18 h (Figure 2(c)) resulted in almost complete elimination of Li₃PO₄ and Ca₃(PO₄)₂, which accompanied formation of LiCaPO₄ resulting from a back reaction between the two impurities, and consequently an almost single phase of LiCaPO₄ was formed with a very tiny amount of Ca₃(PO₄)₂.

Figure 3 shows XRD patterns of LiCaPO₄: synthesized by the PC method employing PEG-P or H₃PO₄ and the SSR method after post-heat treatment at 1073 K for 18 h. All the samples mainly consisted of LiCaPO₄ as assigned to the JCPDS Card (LiCaPO₄, No. 79-1396). The sample synthesized by the PC method employing PEG-P was the almost single phase LiCaPO₄ with negligible extent of impurity phase, whereas the samples synthesized by the PC method using phosphoric acid and SSR method contained a large amount of impurities, which were Li₃PO₄ and Ca₃(PO₄)₂. During the PC method employing H₃PO₄, precipitate was observed in

the mixture solution while PEG-P didn’t make any precipitate. It can be expected that low homogeneity led to formation of a large amount of Li₃PO₄ and Ca₃(PO₄)₂ and these phases remained even after post-heat treatment. As mentioned above, phase purity is closely related to photoluminescence properties. The use of a novel PEG-P as a P source in the PC method could improve phase purity of LiCaPO₄, and it is one of advantages of the PEG-P in the PC method.

Figure 4 shows the SEM and the corresponding EDS maps of Eu in LiCaPO₄: phosphors prepared by the SSR method (a, b) and the PC method employing PEG-P (c, d) after the post-heat treatment. The state of Eu distribution is closely related to the photoluminescence properties of given phosphors [7]. 7 mol% Eudoped samples were analyzed to obtain clearer images of Eu distribution. As to the sample synthesized using the SSR method, inhomogeneity, especially, deficient Eu area as marked with the white arrow in Figure 4(b) corresponding to the black arrow in Figure 4(a) could be observed. On the other hand, as to the sample prepared using the PC method, the EDS mapping image showed uniform distribution of Eu without observation of deficient and localization of Eu ions. These results suggest that the PC method allowed highly homogeneous distribution of each element throughout the Eu²⁺-doped LiCaPO₄ phosphor, which may result in improvement of the corresponding photoluminescence properties.

Figure 5 shows excitation and emission spectra of the LiCaPO₄: phosphors synthesized using the SSR method and the PC method employing the PEG-P or H₃PO₄ after post-heat treatment at 1073 K for 18 h. The sample synthesized by the PC method using H₃PO₄ showed the lowest emission intensity. As stated above, the emission intensity of materials is closely related to the homogeneity of each element. The formation of pre-

cipitate between H₃PO₄ and metals during the mixing step in the PC method implies poor homogeneity despite of a solution-based method. The SSR method could produce better condition of sample compared to that of the PC method employing H₃PO₄, because it exhibited higher emission intensity. Formation of undesirable precipitate in the PC method is thought to be a critical factor for homogeneity. The strongest emission intensity was observed from the sample synthesized via the PC method using the PEG-P as a P source. The internal quantum efficiencies of LiCaPO₄: prepared using the SSR method and the PC method employing PEG-P under excitation at 375 nm were 53.7% and 67.6%, respectively, although the corresponding absorption rate (81.3%) of the sample prepared by the PC method using PEG-P was slightly smaller than that of the one prepared by the SSR method (82.7%). The PEG-P was stable in the aqueous condition and the use of PEG-P didn’t produce any precipitates with coexisting metal ions, and it allowed us to obtaining a highly homogeneous phosphor sample. As a result, despite of a lower absorption rate, the sample obtained from the PC method using PEG-P showed the high emission intensity and enhanced quantum efficiency.

4. Conclusion

A stable and water soluble phosphate ester was prepared by the reaction using pyrophosphoric acid, phosphorus pentoxide, and PEG300. The method could produce P source with less H₃PO₄. It was confirmed that the synthesized PEG-P didn’t make any precipitate in a mixture of metal salts while the use of H₃PO₄ resulted in formation of a precipitate. An almost single phase of LiCaPO₄ was obtained using the PEG-P, while SSR and PC method using H₃PO₄ led to formation of impurity phases, which were Li₃PO₄ and Ca₃(PO₄)₂. As a result, the phosphor prepared by the PC method using the synthesized

PEG-P exhibited enhanced photoluminescence properties, such as the highest emission intensity and quantum efficiency compared to other methods. Enhanced photoluminescence properties seemed to be attributed to homogeneous distribution of constituents in the atomic level. Therefore, the PEG-P is expected to be applicable in the solution-based synthesis of various kinds of phosphate-based ceramic compositions with enhanced material properties.

5. Acknowledgements

This work was partially supported by a Grant-in-Aid for Scientific Research on Innovative Areas of “Fusion Materials: Creative Development of Materials and Exploration of Their Function through Molecular Control” (no. 2206) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan and a Grantin-Aid for JSPS Fellows (24∙9285) from Japan Society for the Promotion of Science (JSPS).

REFERENCES

NOTES

^*Corresponding author.