ABSTRACT

The Pr³⁺ ion-doped BaY₂ZnO₅ phosphor with the orthorhombic structure was synthesized successfully using a sol-gel method in this study. The SEM images show that the BaY₂ZnO₅:Pr³⁺ phosphor particles are aggregational but have an isotropic distribution for 2 mol% Pr³⁺ ions doped. Under an excitation wavelength of 311 nm, the emission bands that appear in the emission spectra are due to the ³P₀→³H_4,5,6, ¹D₂→³H₄ and ³P₀→³F₂ electron transition of Pr³⁺ ion, and it is the same as that for solid state reaction preparation. Comparing to the solid state reaction preparation, the intensities of the ³P₀→³H₄ transition were increased by about 6.5 times for sol-gel method. The enhancement in emission intensity is because the activators have more homogeneous contribution in host for the sol-gel method preparation. In addition, the color tone did not change very obviously, which located around the green light region for Pr³⁺ ion concentrations increasing. The color stability is better for sol-gel method than that for the solid state reaction preparation.

Keywords:

Phosphor, Rare Earth Oxides, Optical Properties

1. Introduction

Oxide phosphors have recently gained much attention for applications such as screens infield-emission displays (FEDs) [1] , plasma display panels (PDPs) [2] [3] and for white color light-emitting diodes (LEDs) [4] because the intrinsic problems such as their higher chemical stability and resistance to moisture relative to that of traditional phosphors are rare earth or transition metal activated sulphides such as ZnS, SrS, and CaS. A lot of efforts have been done to discover the novel host materials as well as activators with high photoluminescence properties including emission intensity, high quantum yield and so on for phosphor applications [5] [6] .

The emission of lanthanide ion, Pr³⁺ ion, in the visible region occurs because of the transition of the ³P₀ level, which contains two dominant transitions from the fluorescent ³P₀ level to the lower ³H₆ and ³H₄ states [7] . The emission spectra of Pr³⁺ are significantly different in different hosts and can take the form of red, green, or even blue emissions [8] [9] .

BaY₂ZnO₅ has the orthorhombic structure with space of Pbnm [10] , and it consists of YO₇, BaO₁₁, and ZnO₅ polyhedra. BaY₂ZnO₅ is an excellent host for various activator ions-doped phosphors. Some previous studies investigated the effect of the size, morphology of particles, and activator concentrations on the photoluminescence properties of BaY₂ZnO₅:RE³⁺ phosphors [11] [12] [13] [14] . According to our previous study [14] , the emission spectra show that the ³P₀→³H_4,5,6 transitions are dominant for the Pr³⁺-doped BaY₂ZnO₅ phosphor. The color tone changes from green to greenish and finally yellow, as the Pr³⁺ ion concentration increases.

Many efforts have been done to enhance the emission intensities of phosphors and the improvement of particle morphologies via fluxes addition, chemical methods utilized to mix activators well or by doping with ions of different radii. Various chemical methods have been investigated for synthesizing fine-sized grains, because these have been reported to enhance the emission efficiency and intensity of phosphors [15] [16] . These methods include sol-gel [17] [18] [19] , hydrothermal [20] [21] [22] , and precipitate techniques [23] . Among these methods, the sol-gel process is an attractive route that starts from molecular precursors and forms an oxide network via inorganic polymerization reactions, and offers both product and processing advantages, such as high purity, ultrahomogeneity, and reduction of the calcining temperature, which could decrease the grain size and enhance the emission efficiency and intensity of phosphors.

In this investigation, the Pr³⁺ ions-doped BaY₂ZnO₅ phosphor was prepared using a sol-gel method to reduce the preparation parameters, improve the surface morphology, and hope to increase to the luminescence efficiency of phosphor. The influences of Pr³⁺ ion concentrations on the resulting structure and the photoluminescence (PL) properties of BaY₂ZnO₅:Pr³⁺ phosphor were also studied. The results indicated that the calcination conditions are lower than that of the solid state reaction, and the intensities of emission peak increased by about 6.5 times for the sol-gel method.

2. Experimental Procedure

In this study, BaY₂ZnO₅ compounds doped with various concentrations of Pr³⁺ ions were synthesized using a sol-gel method. The raw materials of barium nitride [Ba(NO₃)₂], yttrium acetate [Y(OOCCH₃)₃・4H₂O], zinc acetate [Zn(OOCCH₃)₂] and praseodymium acetate [Pr(OOCCH₃)₃・xH₂O] with a purity of 99.99% were supplied by Alfa Aesar. At the first, barium nitride (0.01 mole), zinc acetate (0.01 mole), yttrium acetate (0.02 - 2x mole) and praseodymium acetate (2x mole, x = 0 - 0.005) were separately dissolved in 100 ml of deionized water, respectively. Secondly, barium nitride, zinc acetate, yttrium acetate and praseodymium acetate solutions were mixed in a round bottom flask. When the precursor was completely dissolved in the solution, predetermined amounts of citric acid and ethylene glycol (equal molar ratio) were added to the solution mentioned above as a chelating agent and stabilizing agent, respectively. The amounts of citric acid and ethylene glycol were determined by the ratio of citric acid to metal cations. At last, the powders obtained were calcined in air at 1200˚C for 6 h.

Characterization

Powders were analyzed for crystal structure by X-ray diffractometry (XRD; Rigaku Dmax-33 x-ray diffractometer, Tokyo, Japan) using Cu-Karadiation with a source power of 30 kV and a current of 20 mA to identify the possible phases formed after heat treatment. The surface morphologies of phosphors were examined using high resolution scanning electron microscopy (HR-SEM, S4200, Hitachi). Optical absorption spectra were measured at room temperature using a Hitachi U-3010 UV-vis spectrophotometer. Both the excitation and emission spectra of the phosphors were measured using a Hitachi F-7000 fluorescence spectrophotometer with a 150 W xenon arc lamp as the excitation source at room temperature.

3. Results and Discussion

3.1. Structures

Figure 1 shows the X-ray diffraction patterns of the Ba(Y_1-xPr_x)₂ZnO₅(x = 0 - 0.05) powders calcined in air at 1200˚C for 6 h. The XRD results show that all of the diffraction peaks of the BaY₂ZnO₅:Pr³⁺ phosphors can be attributed to the orthorhombic structure (JCPDS 89-5856) for Pr³⁺ ion concentration from 0 to 5 mol%, and there is no second phase appears in the spectra, which demonstrates that the Pr³⁺ ion substitutes the Y³⁺ ion. The calcination conditions are lower than that of 1250˚C/12h for conventional ceramics processing reported by the solid state reaction [14] . When the Pr³⁺ content is further increased, the intensities of the diffraction peaks seem to be decrease, and the diffraction peak of the (131) shift to a lower diffraction angle. The shift of the peak position is ∆θ = 0.68˚ for the Pr³⁺ ion concentration is 0 and 5 mol%. It is due to the Pr³⁺ ion (0.99 Å, 6-coordinated) has a larger radius than that of Y³⁺ ion (0.90 Å, 6-coordinated), the lattice distorts and intra-stress occurs, nonuniform strain in the vicinity of the Pr³⁺ ions is induced when a trivalent praseodymium ion is introduced to replace trivalent yttrium ion in the BaY₂ZnO₅:Pr³⁺ system.

Table 1 is the grain size calculated using the Scherrer’s equation for different concentrations of Pr³⁺ ion-doped BaY₂ZnO₅ phosphor. The grain sizes for BaY₂ZnO₅:Pr³⁺ phosphors are all in the nanoscale range, and increase initially then decrease for Pr³⁺ ion concentration increase further. The largest grain size occurs when Pr³⁺ ion concentration is 2 mol%. The results are in good accordance with the analysis of XRD patterns for the intensity of (131) diffraction peak is increase then decrease as the Pr³⁺ ion concentration increases.

3.2. Microstructures

The size of phosphor particles should be as homogeneous as possible without any aggregates or agglomerates. Moreover, the surface of the phosphor particles should also be as smooth as possible and have a high degree of crystallization to improve efficiency. Figure 2 shows the FE-SEM surface morphology of BaY₂ZnO₅ doped with 0, 1, 2, and 5 mol% Pr³⁺ ions. By the results, most of these phosphor particles have polyhedral shapes and irregular. These particles are agglomerates of smaller particles during the calcination process due to that they are on the nano-scale with large surface energies when the particles produced using this sol-gel method. When the Pr³⁺ ion concentration doping is 2 mol% (Figure 2(c)), the particles seem to be more uniform distribution and regular sizes. These phosphor powders with better surface morphologies will display a preferable photoluminescence property, and it will be good accordance with the optical properties measurement.

Figure 3 shows the absorption spectra of the BaY₂ZnO₅ doped with different Pr³⁺ ion concentrations phosphor calcined at 1200˚C in air for 6 h. For the host material, it shows two absorption broads in the UV region. The strong absorption band in the region from 200 to 270 nm is attributed to the band-to-band transitions, whereas the weaker broad band from 270 to 400 nm can be attributed to the tightly bound Frankel excitons, which are usually observed close to the bandgap in large-bandgap crystals [24] [25] . The absorption edge of the BaY₂ZnO₅ host is located at ~378 nm. When Pr³⁺ ion doped, a stronger broad band in the range of 250 - 350 nm and a series of small peaks between 400 and 500 nm appear in the spectrum, and that are associated with the f-f transition of the Pr³⁺ ion. The stronger broad band centered at 311 nm can be attributed to the 4f - 5d characteristics transition absorption of Pr³⁺ ion. According to the studies [26] [27] [28] , the typical Pr³⁺-activated oxide phosphors always demonstrate strong 4f - 5d transition band absorption at approximately 200 - 330 nm, and it is in accordance with the results for our study. In addition, there also appears a series of absorption peaks from 440 to 500 nm and 580 to 620 nm, this can be attributed to the 4f orbital characteristics transition for the ³H₄→³P_J (J = 0, 1, 2) transition of the Pr³⁺ ion.

Figure 4(a) shows the excitation spectra for BaY₂ZnO₅ doped with various Pr³⁺ ion concentrations calcined in air at 1200˚C for 6 h using the sol-gel method, the singles were obtained by monitoring the emission wavelength of 513 nm for the ³P₀→³H₄ transition. A strong broad band from 250 - 350 nm and some sharp peaks in the longer wavelength region can be observed in the excitation spectra. It is similar to the results observed in the absorption spectra, the 4f - 5d transition of Pr³⁺ ion centered at 311 nm was located in the broad band region. These sharp peaks centered at 458 nm, 485 nm and 495 nm in the longer wavelength region from 450 to 500 nm can be assigned to the f-f transitions of Pr³⁺ ion for the ³H₄→³P₂, ³H₄→³P₁ and ³H₄→³P₀, transition, respectively [29] [30] . The excitation intensity has a maximum value for Pr³⁺ ion 2 mol% doped, and then decreasing as the Pr³⁺ ion concentration increases which is due to concentration quenching effect.

Figure 4(b) is the comparison for excitation spectra of BaY₂ZnO₅:2mol%Pr³⁺ phosphor synthesized using the solid state reaction and sol-gel method. As can be seen, different synthesization did not change the shape of excitation curve but did change the intensities of the excitation peak. The excitation intensity of 4f - 5d transition for BaY₂ZnO₅:2mol%Pr³⁺ phosphor synthesized by the sol-gel method is 1.5 times higher than that for the solid state reaction. In Addition, The peak of the 4f - 5d transition shifts from 315 (solid state reaction) to 311 nm (sol-gel method). This shift was associated with the size of the BaY₂ZnO₅:2mol%Pr³⁺ phosphors. It is caused by BaY₂ZnO₅:2mol%Pr³⁺ phosphor prepared using a sol-gel method (calcination at 1200˚C) with a smaller particle sizes then that for solid state reaction (calcination at 1250˚C). According to the studies [31] , for a smaller particle size, lattice parameters were usually smaller than those of the bigger one because of the huge surface stress, which leads a stronger ligands field and hence the blue shift was often observed.

Figure 5(a) is the emission spectra under an excitation of 311 nm for BaY₂ZnO₅ doped with various Pr³⁺ ion concentrations prepared using the sol-gel method, and Figure 4(b) is the comparison of the emission spectra for BaY₂ZnO₅ doped with 2 mol% Pr³⁺ ions prepared by the solid state reaction and sol-gel method. As can be seen in Figure 5(a), there are several light emitting peaks in the visible light ranges for 490 - 520 nm, and 530 - 560 nm, respectively. The emission peaks at 496, 499 and 515 nm are assigned to the ³P₀→³H₄ transitions, those at 530, 539 and 555 nm are the ³P₀→³H₅ transitions. It reached a maximum when the Pr³⁺ concentration was 2 mol%, and decreased with the increasing Pr³⁺ concentration, which indicated that the concentration quenching is active when x > 0.02. Comparison to the solid state reaction (Figure 5(b)), the intensities of the ³P₀→³H₄ transition for BaY₂ZnO₅:2mol%Pr phosphor increased by about 6.5 times for sol-gel method. The enhancement in emission intensity is because the activators have more homogeneous contribution in host for the sol-gel method preparation.

For BaY₂ZnO₅:Pr³⁺ phosphor prepared using the sol-gel method, different concentrations of Pr³⁺ ion-doping has no effect on the wave shape, but did change the emission peaks intensities. The Commission Internationale de I’Eclairage (CIE) color coordinates of the different color tones for Ba(Y_1-xPr_x)ZnO₅ phosphors prepared using the sol-gel method that are excited at 311 nm are shown in Figure 6. As the Pr³⁺ ion concentration increasing, the color tone did not change very obviously, and which located around the green light region. It possesses a better color stability for sol-gel method synthesization, because the color tone changes from green to greenish and finally to yellow light region as the Pr³⁺ ion concentrations increasing for solid state reaction preparation.

4. Conclusion

In this study, the orthorhombic structure for Pr³⁺ ion-doped BaY₂ZnO₅ was synthesized successfully using a sol-gel method, and the calcination conditions are lower than that for the conventional ceramics processing. The SEM images show that the BaY₂ZnO₅:Pr³⁺ phosphor particles are aggregational but have an isotropic distribution for 2 mol% Pr³⁺ ions doped. Under an excitation wavelength of 311 nm, the emission bands that appear in the emission spectra are due to the ³P₀→³H_4,5,6, ¹D₂→³H₄ and ³P₀→³F₂ electron transition of Pr³⁺ ion, and it is the same as that for solid state reaction preparation. The optimum condition for Pr³⁺ ion concentration doped is 2 mol%, and the intensities of the ³P₀→³H₄ transition are increased by about 6.5 times for sol-gel method. The enhancement in emission intensity is because the activators have more homogeneous contribution in host for the sol-gel method preparation. In addition, the color tone did not change very obviously, which located around the green light region for Pr³⁺ion concentrations increasing. The color stability is better for sol-gel method than that for solid state reaction preparation.

Acknowledgements

The authors would like to thank the Ministry of Science and Technology of the Republic of China for financially supporting this project under grant MOST 107-2622-E-150-005-CC3.

Conflicts of Interest

The authors declare no conflicts of interest regarding the publication of this paper.

Cite this paper

Shih, H.-R., Tsai, M.-T., Teoh, L.-G. and Chang, Y.-S. (2019) Structure and Photoluminescence Properties of Pr³⁺ Ion-Doped BaY₂ZnO₅ Phosphor Prepared Using a Sol-Gel Method. Journal of Modern Physics, 10, 91-101. https://doi.org/10.4236/jmp.2019.102008

References