Effect of Particle Sizes of Raw Materials on the Luminescence Properties of Eu2O3-Doped BaZrO3 Phosphors

In this study, the raw materials of BaCO3 and ZrO2 powders with different average diameters were used to prepare BaZrO3:Eu0.025 phosphors by the solid-state reaction method. The raw materials with different average diameters were prepared by mixing them by the ball milling method and/or grinding them in a nanoscale grinding machine. As different grinding processes were used, the BaCO3 and ZrO2 powders had the average diameters of 2124, 707, 645, and 496 nm. When raw materials with the average diameter of 2124 nm were used and the calcination temperature was changed from 1100 to 1400 °C, the BaZrO3:Eu0.025 phosphors showed two strong orange emission bands with peaks at the wavelengths of 574 and 596 nm and two weak emission bands with peaks at 620 and 650 nm. The maximum intensity of the emission spectrum of the BaZrO3:Eu0.025 phosphors increased with increasing calcination temperature. When the raw materials with the average particle sizes of 707, 645, and 496 nm were used and the calcination temperature was changed from 950 to 1150 °C, the BaZrO3:Eu0.025 phosphors showed one broad blue emission band with central wavelengths located at 464–466 nm. In this study, we prove that when the sizes of the source materials are different, the luminescence properties of the BaZrO3:Eu0.025 phosphors will have large variations after they are calcined at different temperatures.

Eu 3+ -doped BaZrO 3 phosphors also show the merits of brightness and flexible industrial processing ability, and they are suitable for lighting and display devices. However, little work has been performed on the perovskite-type BaZrO 3 prepared by different fabrication processes. Previously, the solid-state reaction method was explored to synthesize BaZrO 3 :Eu 3+ phosphors. (5,9,10) The luminescence properties and the effects of dopant concentration on the PL characteristics of BaZrO 3 :Eu 3+ phosphors were investigated on the basis of excitation and emission spectra. Kanie et al. synthesized Eu-doped BaZrO 3 fine particles with high crystallinity by the hydrothermal reaction method and they obtained spherical, rhombic dodecahedral, and flower-shaped BaZrO 3 fine particles. (11) They also found the effects of the size and shape of the prepared BaZrO 3 -based powder grains on the fluorescence properties of the Eu-doped BaZrO 3 phosphors. (11) Two strong bands at 596 and 618 nm corresponding to the 5 D 0 -7 F 1 (596 nm) and 5 D 0 -7 F 2 (618 nm) transitions of Eu 3+ ions were actually observed, respectively. The two important emission peaks at 574 and 650 nm corresponding to the 5 D 0 -7 F 0 (574 nm) and 5 D 0 -7 F 3 (650 nm) transitions of Eu 3+ ions, respectively, were not observed in their prepared Eu-doped BaZrO 3 phosphors. This result suggests that the particle sizes have a large effect on the properties of Eu-doped BaZrO 3 phosphors. Nevertheless, no research studies have focused on the effect of the particle size of raw materials on the luminescent properties of BaZrO 3 -based phosphors.
Previously, we found that different calcining processes would affect the luminescent properties of BaZrO 3 :Eu 0.025 powders. (5) Therefore, in this study, we used raw materials with different particle sizes to synthesize the BaZrO 3 :Eu 0.025 phosphors by the traditional solid-state reaction method and investigated the effect of particle sizes on the luminescent properties. First, we used the ball milling method to mix and grind the raw materials and the solid-state reaction method to directly calcine the BaZrO 3 :Eu 0.025 powders at the temperature of 1100-1400 °C. Second, the raw materials were mixed and ground by a nano-powder grinding machine to semimicron scale. After that process, the BaZrO 3 :Eu 0.025 powders were calcined at temperatures of 1000-1150 °C. We found that different sizes of the raw materials used as precursors resulted in the BaZrO 3 phosphors having different optical properties.
In this study, the physical and optical properties of BaZrO 3 powders with different particle sizes as a function of calcining temperature are presented. The raw materials with the average diameter of 2124 nm were calcined at 1100-1400 °C to form BaZrO 3 :Eu 0.025 phosphors. They showed two strong orange emission bands with peaks at the wavelengths of 574 and 596 nm and two weak emission bands with peaks at around 620 and 650 nm under the excitation of the optimum optical wavelength of 271 nm. When the average diameters of the raw materials were 707, 645, and 496 nm, the two strong orange emission bands at the wavelengths of 574 and 596 nm and the two weak emission bands at around 620 and 650 nm were not observed in the BaZrO 3 :Eu 0.025 phosphors. We found that the particle sizes of the source materials have considerably large effects on the properties of the emission spectrum of BaZrO 3 :Eu 0.025 phosphors and we will investigate the luminescent properties of BaZrO 3 :Eu 0.025 phosphors with different average diameters of raw materials in detail.

Experimental Procedures
BaZrO 3 :Eu 0.025 powder was synthesized by the solid-state reaction method. BaCO 3 , ZrO 2 , and Eu 2 O 3 powders were weighed according to the composition formula BaCO 3 + ZrO 2 + 0.0125 Eu 2 O 3 (BaZrO 3 :Eu 0.025 ). Depending on the stoichiometric ratio, the reactants were ground by two different processes. For the first process, the raw materials of BaCO 3 and ZrO 2 powders were mixed and ground using agate balls of 10-20 mm diameter, and after being mixed in acetone, dried, and ground, the mixing powders were calcined at 1100-1400 °C for 2 h. For the second process, the raw materials of BaCO 3 and ZrO 2 powders were also mixed and ground using the agate balls. After being mixed in acetone, dried, and ground, the mixing powders were put in a nanoscale grinding machine and then ground using agate balls with different average diameters: 1.2-2.0, 0.8-1.2, and 0.6-0.8 mm, respectively. After those processes, the mixing powders were calcined at 950-1150 °C for 2 h.
For the raw materials ground by different processes, a laser particle size analyzer (Brookhaven Instruments Corporation: 90PLUS) was used to determine the average particle sizes of the ground raw materials. Morphology variations of the ground BaZrO 3 :Eu 0.025 -based raw materials were measured by field emission scanning electron microscopy (FESEM). The crystalline structures of the calcined BaZrO 3 :Eu 0.025 powders were measured on the basis of X-ray diffraction (XRD) patterns with Cu Kα radiation (λ = 1.5418 Å). The PL properties of the BaZrO 3 :Eu 0.025 phosphors were recorded at room temperature in the wavelength range of 300-800 nm on a Hitachi F-4500 fluorescence spectrophotometer. Liu and Wang excited the BaZr 1-x Eu x O 3 powders (BaZrO 3 doped with Eu 3+ ) with λ em = 597 and 258 nm, and they found that 258 nm had a better excitation effect on the BaZr 1−x Eu x O 3 powders. (12) This result suggests that we would need to identify the optimum optical wavelength for exciting the BaZrO 3 :Eu 0.025 powders. In this study, the 3D-scanning process was used to find the optimum optical wavelength, which was dependent on the average diameters and calcining temperature of the BaZr 1-x Eu x O 3 phosphors. We found that for the BaZrO 3 :Eu 0.025 phosphors, the optimum value was around 269-288 nm, and the BaZrO 3 :Eu 0.025 powders excited by other wavelengths had weaker PL intensities.

Results and Discussion
FESEM was used to examine the morphology of the ground BaZrO 3 :Eu 0.025 powders, and Fig.  1 shows the FESEM images of the BaZrO 3 :Eu 0.025 powders as a function of the grinding process. The observation results indicated that as the grinding process was changed, the particle sizes of raw materials apparently changed as well. When the agate balls of 10-20 mm diameter were used, as Fig. 1(a) shows, the particle sizes of the raw materials were of micrometer order and the average diameter was 2124 nm. After the grinding process, the powders with the average diameter of 2124 nm were further ground using agate balls of a smaller diameter in a nanoscale grinding machine. As the agate balls had the diameter of 1.2-2.0 mm, as Fig. 1(b) shows, the particle sizes of the raw materials actually decreased, and the average diameter was 707 nm. When smaller agate balls were used, for example, the diameters of the agate balls were 0.8-1.2 and 0.6-0.8 mm, as Figs. 1(c) and 1(d) show, the particle sizes of the raw materials actually decreased and the average diameters were 645 and 496 nm, respectively. These results prove that the nanoscale grinding machine can be used to grind the raw materials into smaller sizes and the diameter of the agate balls used is an important factor affecting the particle sizes of the raw materials.
In order to achieve good photoluminescence properties, the preparation of the BaZrO 3 :Eu 0.025 powders forming the cubic perovskite phase is very important because the crystallization of the BaZrO 3 :Eu 0.025 powders will affect their photoluminescence properties. Because the prepared raw materials of BaZrO 3 :Eu 0.025 powders have different diameters, they affect the crystallization temperature, as confirmed by the XRD patterns in Fig. 2. The results in Fig. 2 will show that not all calcined BaZrO 3 :Eu 0.025 powders have a stable cubic perovskite feature as the average diameter of the raw materials and the calcining temperature are changed. As Fig. 2(a) shows, when the average diameter of the raw materials of BaZrO 3 :Eu 0.025 powders was 2124 nm, the 2θ value of the (110) diffraction peak was constant at 30.18 as the calcining temperature increased from 1100 to 1400 °C. As the calcining temperature was increased, the diffraction intensity of the (110) peak of the BaZrO 3 :Eu 0.025 powders apparently increased. When the calcining temperature was 1100, 1200, 1300, and 1400 °C, the full widths at half maximum (FWHMs) for the (110) peak of the BaZrO 3 :Eu 0.025 powders were 0.26, 0.25, 0.19, and 0.17, respectively. These results suggest that the crystallization [judged from the diffraction intensity and FWHM of the (110) peak] of BaZrO 3 :Eu 0.025 powders calcined at higher temperatures is better than that of BaZrO 3 :Eu 0.025 powders calcined at lower temperatures.
When the average diameter of the raw materials of BaZrO 3 :Eu 0.025 powders was 707 nm and the calcining temperature was 1000 °C, as Fig. 2(b) shows, the calcined powders showed an amorphous phase. The 2θ value of the (110) diffraction peak remained at 30.16 as the calcining temperature increased from 1050 to 1150 °C. The results in Fig. 2(b) show that as the calcining temperature was increased, the diffraction intensity of the (110) peak (FWHM) of the BaZrO 3 :Eu 0.025 powders first increased (decreased) as the calcining temperature increased from 1000 to 1050 °C, reached a maximum (minimum) at 1100 °C, and then decreased (increased) at 1150 °C. The FWHMs for the (110) peak of the BaZrO 3 :Eu 0.025 powders were 0.45, 0.32, and 0.35, when the calcining temperatures were 1050, 1100, and 1150 °C, respectively. The diffraction intensity of the (110) peak shows a similar trend to the FWHM of the (110) peak. These results suggest that when the raw materials When the average diameters of the raw materials of BaZrO 3 :Eu 0.025 powders were 645 and 496 nm, as Figs. 2(c) and 2(d) show, the calcined powders showed an amorphous phase when the calcining temperatures were 1000 and 950 °C, respectively. If the calcining temperature was increased, the diffraction intensity (FWHM) of the (110) peak of the calcined BaZrO 3 :Eu 0.025 powders first increased (decreased) as the calcining temperature increased from 1000 to 1050 °C for 645 nm and from 950 to 1000 °C for 496 nm, reached a maximum (minimum) at 1050 °C for 645 nm and at 1000 °C for 496 nm, and then decreased (increased) at 1100 °C for 645 nm and at 1050 °C for 496 nm. Therefore, the average diameter of the raw materials of BaZrO 3 :Eu 0.025 powders decreases, and the calcining temperature required to form the stable cubic perovskite feature is decreased. The results in Fig. 2 also suggest that for different average diameters of the raw materials of BaZrO 3 :Eu 0.025 powders, the calcining temperature will have a large effect on the variations of the crystalline intensity and the FWHM for the (110) peak of BaZrO 3 :Eu 0.025 powders.
The reason for the decrease in the required calcining temperature is that if the average diameter of the raw materials decreases, the surface areas of BaCO (5) The shift of 2θ of the (110) diffraction peak to a higher value will cause a decrease in the lattice constant of the BaZrO 3 perovskite cubic structure. Owing to the difference in the sizes of Eu 3+ and Zr 4+ ions, we believe that the reason to cause this result is that the more Eu 3+ ions will occupy the Zr 4+ sites. Those results in Fig. 2 show that 2θ value of (110) diffraction peak has been shifted to higher value. This result proves that as Eu 3+ ions are substituted for the Zr 4+ sites in the BaZrO 3 :Eu 0.025 phosphors, the lattice constant increases and 2θ value of (110) diffraction peak will be shifted to smaller value. The PL emission spectra of the   Yang et al. found that β-SiAlON:Eu phosphors exhibited a typical rodlike morphology. (13) The low Eu 2+ concentration would cause the β-SiAlON:Eu phosphors to have a blue emission and the high Eu 2+ doping concentration would cause the β-SiAlON:Eu phosphors to have a green emission. (13) This result suggests that when Eu 2 O 3 is used as the dopant, the 5 D 0 -7 F J (J = 0, 1, 2, 3, and 4) transitions are not the only transitions of Eu 3+ ions; some other transitions of Eu 3+ ions exist, which will generate different color emissions. Thus, the results in Figs. 3 . Previously, we found that as the Mn +2 dopant concentration and the calcining temperature of Zn 2 SiO 4 phosphors increase, the chance for MnO (or MnO 2 ) to substitute for ZnO increases and hence the concentration of Mn +2 ions increases. Energy transfer between Mn +2 and Mn +2 ions is expected to occur, which will take the excitation energy very far from the absorption location. (14) This result suggests that with a smaller average diameter of the raw materials of BaZrO 3 :Eu 0.025 powders, the concentration of Eu 3+ ions and their probability of substituting for Zr 4+ increase. The opportunity for the energy transfer between Eu 3+ and Eu 3+ ions to occur is expected to increase; then, the PLmax value of BaZrO 3 :Eu 0.025 phosphors increases. The second important trend is that with the average diameters of the raw materials of 707, 645, and 496 nm, the PLmax value of the BaZrO 3 :Eu 0.025 phosphors first increases, reaches a maximum, and then decreases as the calcining temperature is increased. Figure 2 shows that at the calcining temperatures of 1150, 1100, and 1050 °C for the powders with the average diameters of 707, 645, and 496 nm, respectively, the degeneration of the crystallinity of BaZrO 3 :Eu 0.025 powders is apparently observed, which is believed to be the cause of this trend.

Conclusions
When the average diameter of the raw materials of BaZrO 3 :Eu 0.025 powders was 2124 nm and the calcining temperature was increased from 1100 to 1400 °C, the diffraction intensity (FWHM) of the (110) peak of the BaZrO 3 :Eu 0.025 powders apparently increased (decreased). The BaZrO 3 :Eu 0.025 phosphors had two strong bands corresponding to the 5 D 0 -7 F 0 (574 nm) and 5 D 0 -7 F 1 (596 nm) transitions of Eu 3+ ions and two weak bands corresponding to the 5 D 0 -7 F 2 (620 nm) and 5 D 0 -7 F 3 (650 nm) transitions of Eu 3+ ions. The temperature needed for BaZrO 3 :Eu 0.025 powders to form the perovskite cubic structure decreased as the average diameters of the raw materials decreased. When the average diameters of the raw materials of BaZrO 3 :Eu 0.025 powders were 707, 645, and 496 nm, the calcining temperatures that resulted in the maximum diffraction intensity (minimum FWHM) of the (110) peak of the BaZrO 3 :Eu 0.025 powders were 1100, 1050, and 1000 °C, respectively. BaZrO 3 :Eu 0.025 phosphors made from raw materials with average diameters of 707, 645, and 496 nm all showed one broad blue emission band with central wavelengths located at 464-466 nm. Also, the calcined BaZrO 3 :Eu 0.025 powders with the largest diffraction intensity and the smallest FWHM of the (110) peak had the largest PLmax value. These results prove that the crystallization of BaZrO 3 :Eu 0.025 powders is the most important factor affecting their photoluminescence properties.