1. Introduction

White light-emitting diodes (WLEDs) have attracted much attention owing to their unique advantages such as a high luminescence, high power efficiency, long lifespan, and well-established fabrication process^[1]. Most commercial WLEDs consist of blue light-emitting diode chips with a down- conversion phosphor made of yttrium aluminum garnet (YAG) doped with cerium (Ce)^[2]. Ce-doped YAG phosphors with a broad emission band from 500 to 650nm exhibit excellent yellow luminescent properties and have attracted immense interest for practical applications. Among the three stable phases in the Y₂O₃-Al₂O₃ binary system, including YAG, the perovskite YAlO_-, and monoclinic yttrium aluminum (YAM), the YAG phase is the most favorable for generating efficient luminescence in phosphors^[3]. It has been investigated using various methods, such as hydrothermal, precipitation, sol-gel, spray pyrolysis, laser ablation, and solid- state reactions, for the synthesis of YAG phosphors ^[4-9]. However, the traditional solid-state reaction of Y₂O₃- Al₂O₃ requires high-temperature processing near 1600°C to synthesize a pure YAG phase^[8]. Furthermore, many synthesis methods suffer from complex, time- consuming, and expensive manufacturing processes, together with inhomogeneous and irregular crystal properties^{[8, 10]}.

Green synthesis methods, which are low-cost and environmentally friendly processes for the preparation of various nano/microparticles, have attracted considerable^[11]. To obtain pure YAG crystals using a simple and low-temperature process, a green precipitation method can be utilized. In general, precipitating agents such as HCO₃^-, CO₃^2-, or OH^- ions are used for the direct precipitation of YAG particles^[12-14]. Sodium hydroxide (NaOH) and sodium bicarbonate (NaHCO₃) are widely used as precipitating agents for the precipitation of YAG phosphors. NaOH is a strong base and is widely used in the industry as a precipitating agent for metals, while the weak base NaHCO₃ can precipitate rare-earth elements. It is necessary to optimize the precipitation to produce high-quality YAG:Ce phosphors with excellent crystallinity and generate high luminance.

In this study, YAM:Ce and YAG:Ce nanophosphors were prepared by a simple and feasible green precipitation process using NaOH and NaHCO₃ as precipitating agents, respectively. Furthermore, the YAG:Ce nanoparticles were carefully optimized with thermal and pressing treatments, which improved their light-emitting performance and the crystallinity of the nanophosphors. The structural, energetic, and emission characteristics of the fabricated YAG:Ce nanophosphors were systematically investigated. Post-pressing treatment of the YAG:Ce nanoparticles under a high pressure (60.7 MPa) significantly enhanced the photoluminescence (PL) emission intensity by a factor of 2.9 compared to that for nanoparticles not subjected to pressing treatment. This simple, low-cost, and environmentally friendly precipitation and post-treatment method provides new opportunities for the development of high-performance YAG:Ce nanophosphors.

2. Results and Discussion

Ce-doped yttrium aluminum oxide (YAO) nanoparticles were synthesized using a simple precipitation process. A schematic of the precipitation process is shown in Fig. 1(a). NaOH and NaHCO₃ were used as the precipitating agents to generate Ce-doped YAO nanoparticles. The Y, Al, and Ce precursor droplets were mixed with a precipitating agent. The Y and Al precursors were introduced in a ratio of 3:5. The particles were then annealed at 1100°C. The obtained YAG:Ce nanoparticles were pressed at a high pressure of 60.7 MPa to improve the PL emission of the nanoparticles. The resulting nanoparticles had sizes ranging from several tens to hundreds of nanometers. Fig. 1(b) shows photographs of the YAG:Ce pellets and powders prepared with NaHCO₃. A transmission electron microscopy (TEM) image of the YAG:Ce nanoparticles is shown in Fig. 1(c). The atomic composition of the as-prepared and post-annealed YAG:Ce nanoparticles, analyzed by energy-dispersive X-ray analysis, is listed in Table 1.

Table 1. Atomic composition of as-prepared and post-annealed YAG:Ce nanoparticles analyzed by energy-dispersive X-ray analysis

Fig. 1. (a) Schematic of precipitation process for Ce-doped yttrium aluminum oxide (YAO) nanoparticles. (b) Photograph of YAG:Ce pellets and powders. (c) TEM image of YAG:Ce nanoparticles (scale bar: 100 nm)

Fig. 2 shows the X-ray diffraction (XRD) patterns of the Ce-doped YAO nanoparticles prepared using various precipitating agents. The pattern for the nanoparticles produced using NaOH features peaks corresponding to the (120), (130), (201), and (320) planes, with (201) and (320) as the major peaks, indicating a monoclinic yttrium aluminum (Y₄Al₂O₉, YAM) crystal structure. In contrast, the use of NaHCO₃ results in a cubic yttrium aluminum garnet (Y₃Al₅O₁₂, YAG) crystal structure, with the pattern showing peaks corresponding to the (211), (400), (420), and (422) planes, and a major peak for the (420) plane. Meanwhile, the YAO nanoparticles (prepared without using NaOH) not subject to thermal annealing exhibit low crystallinity without any notable peaks.

Fig. 2. XRD pattern of Ce-doped YAO nanoparticles precipitated using NaOH and NaHCO₃

Fig. 3 shows the Fourier-transform infrared (FTIR) spectra of the Ce-doped YAO nanoparticles prepared using NaOH and NaHCO₃ as precipitating agents without thermal annealing. The absorption bands at approximately 3000-3600cm^-1 and 1540- 1870cm^-1 correspond to the O-H stretching vibration of water and the C=O stretching vibrations of HCO₃^- and CO₃^2-, respectively. The C-H bending of HCO₃^- is observed at approximately 1600-1300cm^-1. The absorption peak at 830cm^-1 corresponds to Al-O vibration. The Ce-doped YAO nanoparticles precipitated using NaOH and NaHCO₃ show similar absorption behavior.

Fig. 3. FTIR spectra of as-prepared Ce-doped YAO nanoparticles precipitated using NaOH and NaHCO₃

The PL emission and excitation spectra of YAG:Ce nanoparticles are shown in Fig. 4(a). The Ce³⁺ excitation spectra yield broad bands located at approximately 353 and 470nm, which are attributed to the electron transition from the 4f state to the 5d state of Ce³⁺. The emission band at approximately 550nm is attributed to an electron transition of Ce³⁺, where the excited electrons of Ce³⁺ at the lowest crystal-splitting component of the 5d1 level fall down to the ground states of the ²F_5/2 and ²F_7/2 levels. Fig. 4(b) shows the PL intensity of the YAG:Ce nanoparticles as a function of Ce doping concentration. The highest PL intensity is observed for 1.0mol.% Ce-doped YAG nanoparticles. Above 1.0mol.%, the PL intensity decreases with increasing Ce concentration, indicating the concentrating quenching effect. The PL quenching at high Ce concentrations is due to the energy transfer process:

²D_3/2 → ²F_5/2 + ²F_7/2.

where I is the emission intensity and C is the dopant concentration. K and β are constants for the host lattice system and Q is the interaction type between rare-earth ions, where Q is 3,6,8, or 10, depending on whether the interactions are an exchange, dipole-dipole, dipole-quadrupole, or quadrupole- quadrupole interaction, respectively. As shown in the inset of Fig. 4(b), the calculated Q value was 6.43, which is close to 6. This result suggests that a dipole-dipole interaction between Ce³⁺ ions (5D3/2) is the major concentration quenching source for YAG:Ce nanoparticles at high Ce concentrations.

Fig. 4(c) shows the PL emission and excitation spectra of the YAG:Ce nanoparticles (1.0mol.%) with and without pressing treatment. The post- pressed YAG:Ce nanoparticles with a high pressure of 60.7 MPa exhibit remarkably higher PL emission (∼550nm) and excitation (∼470nm) intensities than the YAG:Ce nanoparticles without pressing treatment. The pressing treatment leads to enhancement of the PL emission intensity of YAG:Ce nanoparticles by a factor of 2.9 compared to that of nanoparticles without pressing treatment, which could be attributable to enhancement of the crystallinity of the nanoparticles.

Fig. 4. (a) PL emission and excitation spectra and (b) PL intensity of YAG:Ce nanoparticles as a function of Ce doping concentration. (c) PL emission and excitation spectra of YAG:Ce nanoparticles (1.0 mol.%) with and without pressing treatment

3. Conclusion

In summary, we successfully prepared YAM:Ce and YAG:Ce nanoparticles by a simple precipitation process using NaOH and NaHCO₃ precipitating agents, respectively. This approach offers fast and facile manufacturing of high-performance YAG:Ce nanophosphors. The structural, energetic, and PL characteristics of the nanoparticles were systematically investigated. We observed the concentration quenching effect of YAG:Ce nanophosphors at high Ce doping concentrations. YAG:Ce nanoparticles optimized with thermal and pressing treatments led to highly improved PL performance and crystallinity. The YAG:Ce nanoparticles pressed at a high pressure of 60.7 MPa significantly enhanced the PL emission intensity by a factor of 2.9 compared to that of nanoparticles without pressing treatment. It is expected that this simple, low-cost, environmentally friendly precipitation approach together with the post-treatments investigated in our work can be an effective way to fabricate high-performance YAG:Ce nanophosphors. In future studies, alternative synthesis methods, such as sol-gel and hydrothermal techniques, will be explored for comparison, along with investigations into thermal and humidity stability, and further exploration of applications such as high-brightness LEDs.

4. Experimental Details

YAM:Ce and YAG:Ce nanoparticles were prepared using a precipitation process. Yttrium nitrate hexahydrate [Y(NO₃)₃∙6H₂O] (Sigma-Aldrich), aluminum chloride hexahydrate [AlCl₃∙6H₂O] (Sigma-Aldrich), and cerium nitrate hexahydrate [Ce(NO₃)₃.6H₂O] (Sigma- Aldrich) were mixed with water to prepare the precursor solution. The concentration of precursor solution was fixed at 0.4M. Sodium hydroxide (NaOH) and sodium bicarbonate (NaHCO₃) diluted in water were used as precipitating agents. The precipitating agent concentration was fixed at 0.714 M., and the precipitation process was performed by adding the precipitating agent solution to the precursor solution drop by drop. The nanoparticles generated by precipitation were rinsed with water until a pH of 7 was obtained and subsequently dried for 24h at 80°C. The dried nanoparticles were denoted as “as-prepared.” The as-prepared nanoparticles were annealed in a furnace at 1100°C for 2 h to produce the YAG:Ce particles. Some YAG:Ce nanoparticles were compacted into pellets using a manual hydraulic press. The nanoparticles were placed in a steel die and subjected to a pressure of 60.7 MPa to form YAG pellets. The crystal structure of the nanoparticles was characterized by XRD (X’pert MPD System) using a Cu Kα X-ray tube, and the results were analyzed using analytical software (HighScore Plus 3). TEM (JEOL JEM-2100F) was performed at 200 kV. FTIR spectra over the range 4000-400cm^-1 were recorded using an Agilent Cary640 spectrometer. The composition of the nanoparticles was examined by energy-dispersive XRD analysis (Horiba). PL and excitation spectra were examined using a fluorescence spectrophotometer (Hitachi, F-4500).