Synthesis of Nanosized Ce3+, Eu3+-Codoped YAG Phosphor in a

Jul 22, 2008 - Supercritical Water System. Jae-Wook Lee,†,‡ Jae-Hyuk Lee,† Eun-Ji Woo,† Hyungwoong Ahn,§ Joon-Soo Kim,‡ and. Chang-Ha Lee*,...
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5994

Ind. Eng. Chem. Res. 2008, 47, 5994–6000

Synthesis of Nanosized Ce3+,Eu3+-Codoped YAG Phosphor in a Continuous Supercritical Water System Jae-Wook Lee,†,‡ Jae-Hyuk Lee,† Eun-Ji Woo,† Hyungwoong Ahn,§ Joon-Soo Kim,‡ and Chang-Ha Lee*,† Department of Chemical Engineering, Yonsei UniVersity, Seoul 120-749, Korea, Policy DiVision, Korea Institute of Geoscience & Mineral Resources, Daejeon 305-350, Korea, and Refining Process Technologies, SK Energy Institute of Technology, SK Energy, 140-1 Wonchon-dong, Yuseong-gu, Daejeon 305-712, Korea

Luminescent yttrium aluminum garnet (Y3Al5O12) nanoparticles codoped with Ce3+ and Eu3+ (YAG: Ce3+,Eu3+) were continuously synthesized by directly feeding potassium hydroxide solution and a metal salt solution to supercritical water (SCW). The effects of the Ce3+-to-Eu3+ concentration ratio on the photoluminescence of the synthesized nanoparticles were studied using a continuous SCW tubular reactor. At 20-s reaction time and a pH of 9.10 in the SCW reactor, the average size of the prepared phosphor nanoparticles was 60-150 nm and cubic or hexagonal particles coexisted with the spherical particles. The phosphor nanoparticles presented a broad emission band in the green-yellow spectral region due to Ce3+, as well as a sharp emission peak at around 610 nm in the red spectral region due to Eu3+. Without further thermal treatment, the YAG:Ce3+,Eu3+ phosphor synthesized in the continuous reactor under SCW conditions showed strong luminescence properties. Simultaneously, enhancement of the red spectral emission intensity in the YAG:Ce3+,Eu3+ phosphor could be controlled by increasing the Eu3+ concentration. Introduction Inorganic phosphors have been extensively investigated for use in display panel applications of various types, such as plasma display panels (PDPs), vacuum fluorescent displays (VFDs), and field-emission displays (FEDs).1 Since high-brightness GaN-based LEDs were developed in 1993,2 increasing interest has focused on white light-emitting diode (LED) phosphors for illumination based on ultraviolet (UV)/blue LEDs. This is because LED lamps have many advantages such as long lifetimes (∼100000 h), high rendering indexes, high luminous efficiencies, and a concurrent reduction in environmental pollution.3 A yellow phosphor excited by a blue LEDs results in white light emission. In the fabrication of white LEDs, the most common method is to combine a GaNbased blue LED and a yellow phosphor because of their low cost in comparison with white LEDs, which are composed of red, green, and blue LEDs. Also, the luminous flux is high compared to the combination of near-ultraviolet (near-UV) LEDs with blue, green, and red phosphors.4 Yttrium aluminum garnet- (YAG-) based phosphors have been used widely in the field of luminescence because of their stability under conditions of high irradiance with an electron beam.5,6 Therefore, YAG is often used as the host material for doping materials in full-color phosphors.7 Moreover, Y3Al5O12: Ce3+ (YAG:Ce3+), which has a garnet structure, has drawn great attention as a yellow phosphor. YAG:Ce3+ is an exceptional Ce3+ phosphor with an emission at relatively long wavelengths and a high efficiency under excitation because of its complete crystal-field splitting of the 2D level of Ce3+.8 Consequently, YAG:Ce3+ is the most suitable phosphor that can be utilized in white LEDs in combination with blue LEDs.9 * To whom correspondence should be addressed. E-mail: leech@ yonsei.ac.kr. † Yonsei University. ‡ Korea Institute of Geoscience & Mineral Resources. § SK Energy.

A white LED with a blue LED in combination with a YAG: Ce3+ phosphor as an illumination source is commercially available. However, such a combination has a poor color-rendering index (100. Photoluminescence Properties of YAG:Ce3+,Eu3+ Phosphors. As stated in the Introduction, red emission needs to be added to yellow emission to improve color-rendering index of YAG:Ce3+ phosphors by enhancing the emission intensity in the red spectral region, and there are two known efficient redemitting activator ions: Eu3+ and Pr3+.4,11,12 In this study, Eu3+ was used as a coactivator in YAG:Ce3+, and Eu was added to the YAG:Ce phosphors as a codoping element. Figures 8–10 show the photoluminescence (PL) spectra of the synthesized YAG:Ce3+,Eu3+ phosphors. Because the excitation band of Ce3+ in the YAG host material is at around 460 nm, a 460-nm blue light excitation source was applied to the synthesized phosphors, and the PL spectra were then measured in the range of 500-700 nm.13 As shown in these figures, both a broad emission band and a sharp emission peak were observed. The broad emission band in the green-yellow spectral region stems from Ce3+, and the sharp emission peak in the red spectral region comes from Eu3+. Figure 11, which shows the energytransfer route from Ce3+ to Eu3+, explains these phenomena well. When the YAG:Ce3+ phosphor is illuminated by blue light, it strongly absorbs the blue light, and yellow light is emitted by the transition from the lowest 2D(5d) band to the 2F7/2 and 2 F5/2 states of the Ce3+ ion.4,34 Furthermore, it is clear that the sharp peak at around 610 nm can be ascribed to the 5D0 f 7F2

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Figure 7. SEM images of Ce3+,Eu3+-codoped YAG phosphor at different Ce3+ and Eu3+ concentrations; (a) Ce3+ 5 at. %: Eu3+ 15 at. %, (b) Ce3+ 10 at. %: Eu3+ 10 at. %, (c) Ce3+ 15 at. %: Eu3+ 5 at. %, (d) and (e) TEM images of (b).

transition of Eu3+ because the YAG:Ce3+ phosphor does not have any sharp emission peak at this wavelength. Figure 8 presents the emission spectra of YAG:Ce3+,Eu3+ phosphors containing 5 at. % Ce3+ activator and 0, 5, 10, and 15 at. % Eu3+ coactivator. At the fixed 5 at. % concentration of Ce3+ activator, the emission intensity of Eu3+ increases with increasing amount of the Eu3+ coactivator. Therefore, the case with 5 at. % Ce3+ and 15 at. % Eu3+ yields the highest enhancement effect of the red spectral region. Figures 9 and 10 show the emission spectra of YAG: Ce3+,Eu3+ phosphors containing 0, 5, 10, and 15 at. % Eu3+ coactivator withh Ce3+ activator concentrations of 10 and 15

at. %, respectively. As in the previous case, the emission intensity of Eu3+ increases with increasing amount of Eu3+ coactivator. However, Figures 8–10 show that the intensity of the Eu3+ light emission and its dependence on the amount of Eu3+ coactivator decreases as the amount of Ce3+ is increased, resulting in a decreasing enhancement in the red spectral region.35 In addition, in the case of concentration quenching stemming from the increased amount of Eu3+ coactivator, it is possible to observe a decrease in the intensity of the Eu3+ light emission. Thus, the emission intensity of Eu3+ can be increased by increasing the amount of Eu3+ and/or decreasing the amount

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Figure 8. PL emission spectra of Ce3+,Eu3+-codoped YAG phosphor with varying amounts of Eu3+ at 5 at. % Ce3+: (a) 0, (b) 5, (c) 10, (d) 15 at. % Eu3+ (run 1).

Figure 11. Schematic energy level diagrams for Ce3+ and Eu3+ in YAG.

of Ce3+ activator and Eu3+ coactivator to obtain the optimum color-rendering index and color reproducibility. Conclusions

Figure 9. PL emission spectra of Ce3+,Eu3+-codoped YAG phosphor with varying amounts of Eu3+ at 10 at. % Ce3+: (a) 0, (b) 5, (c) 10, (d) 15 at. % Eu3+ (run 2).

Figure 10. PL emission spectra of Ce3+,Eu3+-codoped YAG phosphor with varying amounts of Eu3+ at 15 at. % Ce3+: (a) 0, (b) 5, (c) 10, (d) 15 at. % Eu3+ (run 3).

of Ce3+. The opposite changes decrease the emission intensity of Eu3+. Therefore, the degree of enhancement in the red spectral region can be suitably adjusted by varying the amounts

Ce3+-doped YAG phosphor nanoparticles were synthesized under SCW conditions (400 °C and 280 bar) using a continuous tubular reactor. The phosphor products synthesized with potassium hydroxide solution were nearly spherical in shape and much smaller than those produced with ammonium bicarbonate solution as a precipitator. Then, Ce3+,Eu3+-codoped YAG phosphor nanoparticles were synthesized under the same conditions with potassium hydroxide solution. The average size of the prepared phosphor nanoparticles at 20-s residence time and pH 9.10 was 60-150 nm, and cubic or hexagonal particles coexisted with the spherical particles. Without further thermal treatment, the SCW method can produce YAG:Ce3+,Eu3+ phosphor nanoparticles in a short reaction time. Through the use of Eu3+ as a coactivator, the emission intensities of YAG:Ce3+-based phosphors were enhanced in the red spectral region. Furthermore, adjusting the Ce3+ and Eu3+ concentrations in the SCW continuous reactor could control the degree of enhancement in the red spectral region in order to obtain the optimum color-rendering index and color reproducibility. Hence, nanosized Ce3+,Eu3+-codoped YAG phosphors synthesized under SCW conditions can be good candidate materials for the application of a white light source using blueemitting LEDs. Acknowledgment The authors thank the Korean Ministry of Environment for supporting this work as “The Eco-technopia 21 Project”.

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ReceiVed for reView March 14, 2008 ReVised manuscript receiVed June 14, 2008 Accepted June 20, 2008 IE800421W