1. Introduction

Europium-doped yttrium oxide (Y₂O₃:Eu³⁺) is a well-known red-emitting phosphor widely used in various areas such as fluorescent lamps, plasma display panels, flat-panel and field emission displays, and cathode-ray tubes [1–9]. Y₂O₃:Eu³⁺ materials involve abundant 4f electronic configurations and rich energy levels due to the Eu³⁺ luminescence activator [4,5,10]. Within the host Y₂O₃ lattice, Eu³⁺ exhibits efficient luminescence emissions with excellent color quality. In addition, Y₂O₃:Eu³⁺ materials have high chemical durability and thermal stability, simple chemical composition, low toxicity, and excellent thermal conductivity [1–9]. Recently, Y₂O₃:Eu³⁺ materials have been suggested as a red-emitting photon down converter to generate white light emission with near ultraviolet (UV)- or UV-light-emitting diodes (LEDs) due to the strong light absorption in the UV region of the Eu³⁺ activator [11–14]. With high emission efficiency group-III nitride-based LEDs (GaN, GaAlN, InGaN, and InGaAlN), which extend the emission spectrum to the green, blue, and ultraviolet regions, development of photon down-conversion materials (LED phosphors), which will combine with blue/UV LEDs to generate high quality white light, has recently been suggested as one of the most important research and development plans for LED-based solid-state lighting technology [15–17]. Development of such phosphor-converted white LEDs would provide a solid-state light source for next-generation lighting industry and display systems, and advance solid-state lighting technology by increasing energy efficiency, which would lead to cheaper lighting and power costs, reduce fossil fuel dependency, and help decrease carbon emissions [15–17]. The synthesis and characterization of high-efficiency Y₂O₃:Eu³⁺ LED-phosphors are therefore significant.

While bulk and nano-sized Y₂O₃:Eu³⁺ materials with various morphologies have been synthesized [3,6,7,18–45], the most promising Y₂O₃:Eu³⁺ LED-phosphor could be Y₂O₃:Eu³⁺ nanospheres. With sizes of a few hundred nanometers, it is expected that the application of such nanospheres (a photon down converter) on the top of LEDs may lead to the enhancement of the light extraction efficiency. It is known that, for high-quality luminescence, the preferred morphology of phosphor particles is a perfectly spherical shape as spherical phosphors have high packing densities and low scattering of light [29,40,46–48]. Moreover, recent studies have shown that the deposition of TiO₂ arrays with spheres of 500-900 nm in diameter on the top of GaN-based LEDs could enhance the light extraction efficiency by 4-5 times of that of planar LEDs [49,50]. While similar studies with Y₂O₃:Eu³⁺ nanospheres and arrays with smaller sizes of nanospheres have not been reported, the mechanism for increasing the light extraction efficiency suggests that Y₂O₃:Eu³⁺ nanospheres with a few hundred nanometers in diameter may serve as an efficient LED-phosphor.

Y₂O₃:Eu³⁺ nanoparticles with spherical or sphere-like shapes have been synthesized with various approaches [3,6,11,20,22–26,29,32,35,36,40,43]. However, most of the previous syntheses focused on nanoparticles with sizes smaller than 100 nm [3,6,11,22–26,35,36,40,43]. Reports of syntheses of Y₂O₃:Eu³⁺ nanospheres with sizes of 100-500 nm are relatively rare [20,29,32]. Li et al. employed a homogeneous precipitation method to synthesize Y₂O₃:Eu³⁺ spheres with ~200 nm in diameter [32]. Using a similar approach, Yamagata et al. also obtained nanospheres with sizes of 200-300 nm for potential applications in orthodontic adhesive fluorescent [20]. On the other hand, Yan et al. used a low-temperature reflux method to synthesize Y₂O₃:Eu³⁺ nanospheres with sizes of 80-140 nm [29].

Here, we report the synthesis of Y₂O₃:Eu³⁺ nanospheres using a modified co-precipitation method. The nanospheres with a wide range of sizes from 40 nm to 334 nm were obtained by tuning the annealing temperature and synthesis time (reaction time). The optimal concentration of Eu³⁺ was determined. We also report investigations of the structural and luminescent properties of the Y₂O₃:Eu³⁺ nanospheres with X-ray diffraction, SEM imaging, and photoluminescence measurements. Strong red emission and UV excitation of the Y₂O₃:Eu³⁺ nanospheres synthesized after a phase transformation at high annealing temperatures were identified, indicating a potential of applying Eu³⁺-doped Y₂O₃ nanospheres in high-efficiency white LEDs.

2. Experimental details

We employed a modified urea co-precipitation method similar to the one used by Fukushima et al. [51]. According to the desired concentration (5, 8, 10, and 15 mol% of Eu³⁺), a total of 0.0080 mole of Y₂O₃ and Eu₂O₃ were placed with 80 mL of nitric acid in a flask under a hot plate set to 300 °C while stirring magnetically. Once the solution had fully reacted into Y(NO₃)₃ and Eu(NO₃)₃, as indicated by a transparent solution, the hot plate was heated to 540 °C until the solution evaporated, forming a light yellow translucent solid. The flask was transferred to an unheated plate with a magnetic stirrer, and 25 g of urea was added. Deionized water (DIW) was added to make a solution of 180 mL. The solution was magnetically stirred until the solid on the bottom had dissolved and the solution was completely transparent. The flask was then placed in an 80 °C water bath for 1, 2, 3, 4, or 5 hours (reaction time) while magnetically stirring. The solution turned translucent milky white as it was heated at 80 °C, eventually reaching a thick white color indicating that the reaction had completed. The resulting solution was centrifuged repeatedly at 15000 rpm for 15 minutes, each time being rinsed (once each with DIW and ethanol) and placed in an ultrasonic cleaner for 10 minutes to remove impurities. This left a white solid that was either left overnight or heated in an oven for around an hour at 80 °C to dry. Finally, the dried powder was annealed at various temperatures ranging from 300 to 900 °C (annealing temperatures) for 1 hour.

Scanning electron microscopy (SEM) images of the obtained samples were taken on the Inspect S50 (FEI Company) SEM. X-ray diffraction (XRD) measurements were performed to study the structure of the samples, using a Rigaku SmartLab diffractometer with Cu Kα1 radiation, λ = 1.54 Å operating at 40 kV and 44 mA. The XRD patterns were obtained at room temperature at an angular range (2θ) of 3°–80° with a step of 0.01°. Photoluminescence spectra were measured using a spectrofluorophotometer (Shimadzu, RF-6000) with a xenon lamp as the excitation source.

3. Results and discussions

Figure 1 shows the SEM images of five samples with the Eu³⁺ concentration of 5 mol% synthesized at 80 °C for 4 hours and thermally annealed at various temperatures, or not annealed. All five samples are composed of nanospheres, which have a size range of approximately 58-212 nm in diameter. Before annealing, the nanospheres have an average size of ~100 nm and a size range of 85-112 nm [Fig. 1(a)]. After annealing at a relatively low temperature (300 °C), the nanoparticles are still spherical in shape, but the size range becomes broader (58-141 nm) and the average size is slightly smaller [Fig. 1(b)]. However, both larger and smaller nanospheres are observed. Annealing at a temperature of 500 °C resulted in nanospheres with a size range of 87-212 nm and a greater average size of ~130 nm [Fig. 1(c)].

 

Fig. 1 SEM images of nanospheres with the Eu concentration of 5 mol% synthesized at 80 °C for 4 hours, with or without a follow-up annealing process: (a) without annealing, (b)-(f) after annealing for 1 hour at an annealing temperature of 300 °C (b), 500 °C (c), 700 °C (d), and 900 °C (e, f).

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With a higher annealing temperature (700 °C), the nanospheres have a size range of 87-143 nm, and the average size of ~120 nm is slightly reduced [Fig. 1(d)]. Finally, with a very high annealing temperature (900 °C), the nanospheres have an average size of ~145 nm in diameter, significantly greater than those of the nanospheres obtained with lower annealing temperatures, as shown in Figs. 1(e) and 1(f).

The size variation with the annealing temperature can be understood with information about the structure of the nanospheres. Figure 2 shows the XRD patterns of the five samples. The XRD spectra clearly show that the nanospheres obtained before annealing and with annealing at the temperatures of 300 °C and 500 °C do not exhibit any crystalline features, indicating that the nanospheres are amorphous in structure. The amorphous nanospheres obtained prior to annealing will experience thermal decomposition and recombination during the annealing process. At 300 °C, decomposition results in the formation of small nanospheres [Fig. 1(b)]. Meanwhile, larger nanospheres may be formed by the recombination of small particles. It appears that decomposition is slightly more preferred than recombination at 300 °C, resulting in the formation of nanospheres with a slightly smaller mean size. At 500 °C, recombination dominates, making the nanospheres greater in size [Fig. 1(c)].

 

Fig. 2 XRD spectra of the samples with the Eu³⁺ concentration of 5 mol% synthesized at 80 °C for 4 hours, both with and without a follow-up annealing process. The annealed samples were obtained after annealing for 1 hour at an annealing temperature of 300 °C, 500 °C, 700 °C, and 900 °C, respectively.

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On the other hand, the nanospheres undergo a phase transformation from an amorphous structure to a cubic structure when annealing at temperatures above 500 °C (at 700 °C and 900 °C). A similar phase transformation of bulk Y₂O₃ at an annealing temperature of 600 °C was previously reported [44].

As shown in Fig. 2, the nanospheres that were annealed at 700 °C and 900 °C have a crystal structure. The locations and relative intensities of the XRD intensity peaks shown in Fig. 2 agree with those of the cubic phase of Y₂O₃ [52,53]. The four major peaks at approximately 29° (2θ), 34°, 48°, and 57° (Fig. 2) correspond to the (222), (400), (440), and (622) diffraction directions of cubic Y₂O₃, respectively. The other four peaks in the range of ~20-46°, ~20°, 35°, 39°, 42°, and 46°, correspond to the (211), (411), (322), (134), and (125) directions, respectively. In addition, stronger XRD intensities of the Y₂O₃:Eu³⁺ nanospheres obtained after annealing at 900 °C suggest that higher annealing temperature may yield higher crystallinity. With an annealing temperature of 700 °C, the phase transformation to the crystalline structure results in a slight decrease in the size of the nanospheres. With a higher annealing temperature of 900 °C, the sintering of small nanoparticles generates larger nanospheres, which exhibit well-ordered cubic structure. The general trend is that higher annealing temperatures yield larger nanospheres, particularly when the nanospheres are with a crystal structure.

Figure 3(a) shows the photoluminescence spectra of the five samples with nanospheres under the excitation of 250-nm UV light. Strong emission intensities are found in the two samples annealed at 700 °C and 900 °C, which are composed of Y₂O₃:Eu³⁺ nanospheres with a cubic structure. On the other hand, the three samples with an amorphous structure (unannealed and annealed at 300 °C and 500 °C) all show very weak emission intensities.

 

Fig. 3 (a) Photoluminescence emission spectra of the nanospheres with the Eu³⁺ concentration of 5 mol% synthesized at 80 °C for 4 hours, with or without a follow-up annealing process (1-hour annealing at four different annealing temperatures). (b) The corresponding excitation spectra of the nanospheres.

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The strongest emission of each of the two samples with a cubic structure is located at 611.6 nm. This orange-red line is due to the transition of ⁵𝐷₀ → ⁷𝐹₂ of the Eu³⁺ ions that occupy the lattice of cubic Y₂O₃ [23,45]. It is known that the ⁵𝐷₀ → ⁷𝐹₂ transitions of Eu³⁺ occupying the C₂ site in cubic Y₂O₃ are allowed by the forced electric dipole mechanism [45]. The 611.6-nm line of the Y₂O₃:Eu³⁺ nanospheres is slightly longer in wavelength than the corresponding line of bulk Y₂O₃:Eu³⁺ nanospheres (611.3 nm) [45]. The other two significant peaks of the 900 °C sample are located at 630 nm (red) and 709.8 nm (deep red). The corresponding peaks of the 700 °C sample are almost the same in location (629.1 nm and 709.7 nm). The former peak is due to a transition from ⁵𝐷₀ to one Stark component of ⁷𝐹₂ while the latter is due to a ⁵𝐷₀ → ⁷𝐹₄ transition. Both transitions are electric dipole allowed when the Eu³⁺ ions occupy the C₂ sites in cubic Y₂O₃ [4,18,45]. The intensity ratio of the 611.6-nm line to the second strongest emission line (the 630-nm line or the 629.1-nm line) is as high as ~7 (the 900 °C sample) or ~5 (700 °C sample).

As shown in Fig. 3(a), there are more weak peaks for each sample. The weak peaks for the 900 °C sample are located at 592.8 nm (orange), 652 nm (red), 662.4 nm (red), and 689.2 nm (red). The corresponding locations of the peaks for the 700 °C sample are 592.8 nm, 653 nm, 661.5 nm, and 689.2 nm, respectively. These peaks also belong to the ⁵𝐷₀ → ⁷𝐹_𝐽 (𝐽 = 0, 1, 2, 3, 4) transitions of the Eu³⁺ ions in the Y₂O₃:Eu³⁺ nanospheres [18,45]. The 592.8-nm peak is due to a ⁵𝐷₀ → ⁷𝐹₁ transition while the 652-nm and 662.4-nm peaks correspond to the ⁵𝐷₀ → ⁷𝐹₃ transitions. The ⁵𝐷₀ → ⁷𝐹₁ transitions are weak because they are electric dipole forbidden and magnetic dipole allowed (magnetic dipole transitions usually have much weaker intensities than electric dipole transitions). The ⁵𝐷₀ → ⁷𝐹₃ transitions are electric dipole forbidden per the Judd-Ofelt theory and could occur only via J-mixing (the weak interaction between the 4f orbitals and the chemical environment); therefore, the transitions have weak intensities [54,55]. Similar to the deep red line (the 709.7-nm peak), the 689.2-nm peak belongs to the ⁵𝐷₀ → ⁷𝐹₄ transitions.

In addition, the emission intensities of the 900 °C sample are observed to be stronger than those of the 700 °C sample. This is due to the fact that the 900 °C sample is composed of larger nanospheres (as shown in the SEM images) and higher crystallinity (as shown in the XRD patterns).

The excitation spectra of the nanospheres were also measured by monitoring the 611-nm emission, and the results are shown in Fig. 3(b). Similar to the emission spectra, the excitation spectra of the nanospheres with a crystalline structure (the 700 °C and 900 °C samples) are significantly stronger in intensity than those of the other three samples that are composed of nanospheres with an amorphous structure. For both the 700 °C and 900 °C samples, the strong excitation bands in the wavelength region below 270 nm are due to the charge transfer transitions [18,39]. The two bands with the peaks at 223 nm and 255 nm for the 900 °C sample are attributed to the O^2‒ → Y³⁺ and O^2‒ → Eu³⁺ transitions, respectively. The corresponding peaks for the 700 °C sample are located at ~220 nm and 256 nm.

The weaker excitation lines in the wavelength range of 350-550 nm are due to the f → f transitions of Eu³⁺ in cubic Y₂O₃, among them, the 364-nm, 382-nm, 396-nm, 417-nm, 467-nm, and 535-nm lines for the 900 °C sample are due to the ⁷𝐹₀ → ⁵𝐷₄, ⁷𝐹₀ → ⁵L₇, ⁷𝐹₀ → ⁵L₆, ⁷𝐹₀ → ⁵𝐷₃, ⁷𝐹₀ → ⁵𝐷₂, and ⁷𝐹₀ → ⁵𝐷₁ transitions, respectively [4,18]. The corresponding excitation lines for the 700 °C sample are at 363 nm, 382 nm, 395 nm, 414 nm, 467 nm, and 534 nm, respectively. While the corresponding peaks of the two samples are almost at the same locations, the intensities of the transitions for the 900 °C sample are considerably stronger than those for the 700 °C sample. This is similar to the case of emission spectra.

To investigate the role of the synthesis time (the reaction time ‒ the amount of time the urea and the Y/Eu-compounds are in the heated water bath) in forming nanospheres, a set of 5 experiments were carried out with the reaction times of 1, 2, 3, 4, and 5 hours. Figure 4 shows the SEM images of the Y₂O₃:Eu³⁺ nanospheres with the Eu³⁺ concentration of 10 mol% synthesized at 80 °C for 2-5 hours (the SEM images for the 1-hour sample were not taken). Each sample was annealed for 1 hour at 700 °C. The SEM images clearly show that the reaction time plays a significant role in the size of the nanospheres formed. With a reaction time of 2 hours, the size range is ~40-70 nm [Fig. 4(a)]. When the reaction time was increased to 3 hours, the size range is ~40-95 nm [Fig. 4(b)]. Both samples are composed of nanospheres with sizes below 100 nm in diameter. However, when the reaction time reached 4 hours, the nanospheres obtained have a size range from ~100 nm to ~210 nm [Fig. 4(c)]. The largest nanospheres, with a size range of ~164-334 nm, were obtained when the reaction time was 5 hours, as shown in Fig. 4(d). These results suggest that longer reaction time generates large nanospheres.

 

Fig. 4 SEM images of the Y₂O₃:Eu³⁺ nanospheres with the Eu³⁺ concentration of 10 mol% synthesized at 80 °C for (a) 2 hours, (b) 3 hours, (c) 4 hours, and (d) 5 hours. Each sample was annealed for 1 hour at 700 °C.

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Finally, another set of experiments, with 4 different concentrations of Eu³⁺, were performed in order to determine the optimal concentration of Eu³⁺ for photoluminescence emission, and the results for the emission spectra of the samples obtained are shown in Fig. 5. The 8% sample has the strongest emission intensity, followed by the 10% and 5% samples, while the 15% sample shows the weakest intensity. In terms of photoluminescence emission, therefore, the optimal Eu³⁺ concentration is determined to be ~8%. While it is greater than the 5 mol% optimal concentration reported for the Eu³⁺-doped Y₂O₃ nanospheres with small sizes (~5 nm in diameter) [26], the 8 mol% optimal Eu³⁺ concentration determined in this study is consistent with a recent study [20], in which 8 mol% of Eu³⁺ was found to be optimal for the nanospheres with larger sizes (200-300 nm).

 

Fig. 5 Photoluminescence emission spectra of the Y₂O₃:Eu³⁺ nanospheres with different Eu concentrations (5-15 mol%) synthesized at 80 °C for 4 hours, followed by thermal annealing at 700 °C for 1 hour.

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The emission spectrum of the 8% sample contains seven obvious peaks at 612 nm (the strongest line), 629.2 nm (the 2nd strongest), 593.3 nm (the 3rd), 709.9 nm (the 4th), 654.1 nm, 660.8 nm, and 691.4 nm. These emissions can be compared with the corresponding lines of the 5% sample discussed earlier: 611.6 nm (strongest), 629.1 nm (the 2nd strongest), 709.7 nm (the 3rd), 592.8 nm (the 4th), 653 nm, 661.5 nm, and 689.2 nm. Overall, compared to the 5% sample, the peaks of the 8% sample shift slightly to the red side.

4. Conclusions

The Eu³⁺-doped Y₂O₃ nanospheres with sizes in the range of 40-334 nm in diameter were synthesized by a low-cost urea-assisted co-precipitation method and an annealing process. The sizes of the nanospheres can be controlled by the reaction time and annealing temperature. The general trend is that longer reaction times and higher annealing temperatures result in the formation of larger nanospheres. The nanospheres obtained with an annealing temperature of 500 °C or below exhibit an amorphous structure. However, the nanospheres were found to undergo a phase transformation at an annealing temperature higher than 500 °C, and the obtained nanospheres have a crystalline phase (cubic Y₂O₃). The nanospheres with a cubic crystal structure exhibit strong red photoluminescent emission, strong UV excitation, and large sizes (100-334 nm) suitable for enhancement of the light extraction efficiency when coated on LEDs, suggesting an application as a potentially efficient red phosphor to generate white light. In addition, the optimal concentration of Eu³⁺ for the photoluminescent emission and excitation was determined to be ~8 mol%.