Microemulsion-Solvothermal Synthesis and Tunable Emission of

Nov 5, 2012 - The products with donut-like morphology comprised of numerous nanorods exhibit a blue-light emission of Eu2+ and an improved red-light e...
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Microemulsion-Solvothermal Synthesis and Tunable Emission of YBO3:Eu for White-Light-Emitting Diodes Wanping Chen* and Ahong Zhou Department of Chemistry and Chemical Engineering, Huaihua University, HuaiHua, 418008, P. R. China ABSTRACT: Well-dispersed YBO3:Eu phosphors were synthesized by a reverse microemulsion method with solvothermal treatment. Xray diffraction (XRD), scanning electron microscopy (SEM), and photoluminescence spectroscopy (PL) were used to characterize the products. The products with donut-like morphology comprised of numerous nanorods exhibit a blue-light emission of Eu2+ and an improved red-light emission of Eu3+, and their relative intensity can be tuned by controlling the Eu concentration to generate white light. The absorption band locates in the spectral range 250−450 nm with a maximum at about 370 nm, which just overlaps the emission band of near-ultraviolet light-emitting diodes (LEDs). This indicates that the YBO3:Eu have potential application in white LEDs. The luminescence mechanism is explained simply, and the YBO3:Eu were also synthesized by a chemical precipitation to compare their morphology and luminescence properties. equal contributions of 5D0−7F1 and 5D0−7F2 emission, which gives rise to an orange-red emission instead of red emission and thus hampers its application. Therefore, some investigators try to enhance the 5D0−7F2 emission intensity through all kinds of chemical means such as controlling the morphology and size of YBO3:Eu3+.8−11 In this manuscript, a reverse microemulsion with solvothermal treatment was employed to synthesize YBO3:Eu3+. X-ray diffraction (XRD), scanning electron microscopy (SEM), and photoluminescence spectroscopy (PL) were used to characterize the products. The products comprised of numerous nanorods show donut-like morphology. More importantly, the presence of a blue-light emission of Eu2+ ion and an improved red-light emission of Eu3+ ion is observed, and their relative intensity can be tuned to generate white light under UV light excitation. The absorption band locates in the spectral range 250−450 nm with a maximum at about 370 nm, which just partially overlaps the emission band of near-UV LEDs (∼370−410 nm). The results indicate that the as-synthesized YBO3:Eu have potential application in white LEDs.

1. INTRODUCTION Light-emitting diodes (LEDs) have played an increasingly important role in many applications such as lighting and display due to their advantages of high brightness, reliability, low power consumption, high efficiency, and long working time.1 Among these applications, the white LEDs have been considered to be the most important solid-state light sources to substitute for the widely used incandescent lamps and fluorescent lamps.2 Currently, the most general approach to generate white light from LEDs is the combination of blue LEDs (∼460 nm) or near-ultraviolet (UV) LEDs (∼370−410 nm) with phosphors.3 For example, the yellow phosphor (Y,Gd)3Al5O12:Ce3+ is combined with blue LEDs to achieve white-light emission, which has been widely used in the field of lighting. Nevertheless, a disadvantage of the combination of the single phosphor with blue LEDs is a low color rendering index (CRI) due to the absence of red light. Therefore, an additional red phosphor is necessary for achieving white LEDs with a high CRI. However, the commercial red phosphors activated by Eu2+ ion such as SrS:Eu2+ and Sr2Si5N8:Eu2+ are insufficient for current white LED application, and SrS:Eu2+ is sensitive to water and Sr2Si5N8:Eu2+ requires rigorous synthesis conditions.4 Besides the phosphors doped with Eu2+ ion, the phosphors doped with Eu3+ ion are proposed as potential red phosphors for white LEDs by some investigators due to their high quantum efficiency, color rendering, and photostability.4−7 Generally, two problems need to be solved for LED phosphors activated by Eu3+ ion: first, the weak absorption of Eu3+ ion in blue and near-UV spectral range and, second, low CRI due to orange-red light emission derived from the 5D0−7F1 transition of Eu3+ ion. YBO3:Eu3+ has been one of the best red phosphors to be widely used and extensively researched. However, the characteristic emission of Eu3+ in YBO3 is composed of almost © 2012 American Chemical Society

2. EXPERIMENTAL SECTION The starting materials including Y2O3 (99.99%), Eu2O3 (99.99%), H3BO3 (AR), NH3·H2O (AR, 5 mol/L), Triton X100 (AR), n-hexanol (AR), cyclohexane (AR), and n-hexanol (AR) were used without further purification. Stoichiometric amounts of Y2O3, Eu2O3, and H3BO3 (excess 3 mol %) were dissolved in diluted nitric acid to form a mixture solution for Received: August 17, 2012 Revised: October 10, 2012 Published: November 5, 2012 24748

dx.doi.org/10.1021/jp308189q | J. Phys. Chem. C 2012, 116, 24748−24751

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synthesizing YBO3:xEu3+ phosphor. A typical doping concentration of Eu (x value) is 0.01. A reverse microemulsion with solvothermal technology (microemulsion-solvothermal, MS) was employed to synthesize YBO3:xEu. Two kinds of Triton X-100 reverse microemulsions of the mixture solution above and NH3·H2O solutions were prepared separately, and then mixed under continuous stirring. In the reverse microemulsion, the volume ratio of cyclohexane, Triton X-100, n-hexanol, and aqueous solution was kept a constant at 22:5:5:1.5. The mixed microemulsion systems were transferred into 50 mL Teflon-lined stainless-steel autoclaves. The autoclaves were heated at 150 °C for 24 h, and then cooled to room temperature naturally. Ultimately, the resulting precipitates were separated and washed several times by centrifugation with distilled water, and dried at 80 °C for several hours, and then the MS products were obtained. For comparison, YBO3:0.01Eu were also synthesized by chemical precipitation (CP). Appropriate amounts of NH3·H2O solutions were added into part of the mixture solution above to form a white precipitate. The precipitates were separated and washed several times by centrifugation with distilled water. The precipitates were dried at 80 °C for several hours and calcinated at 900 °C for 5 h, and then the CP products were obtained. The emission and excitation spectra of all products were recorded by an Edinburgh FLS 920 combined fluorescence lifetime and steady-state spectrometer. The phase compositions of the products were measured with a powder X-ray diffractometer (D/max 2200vpc, Rigaku, Japan) using Cu Kα radiation (λ = 1.5046 Å). Micrographs were recorded using a scanning electron microscope (JSM-6330F, JEOL, Japan) operating at 15 kV. All measures were carried out at room temperature.

of CP products are linear and those of MS products are obviously broadening, which implied that the size of MS products is much smaller than that of CP products.12 3.2. SEM Images of YBO3:Eu. Figure 2 presents the typical scanning electron microscope (SEM) images of CP products

Figure 2. SEM images of YBO3:0.01Eu: (a) CP products; (b) MS products. The inset c is a magnification of part b (MS products).

and MS products. It can be seen that the products are welldispersed and have different morphologies and sizes. The CP products have an ellipse-like morphology and a large size of about 500 nm (Figure 2a). The MS products show a uniform donut-like morphology with a diameter of about 5 μm (Figure 2b). A magnified image of the donut-like products is presented in inset C, which exhibits that they are actually based on the oriented assembly of numerous nanorods with a length of about 200 nm. This can be used to explain the broadening of the XRD patterns, as shown in Figure 1b. Although the size of MS products is much larger, the XRD pattern is the result of X-ray diffraction from the nanorods. 3.3. Luminescence Spectra of YBO3:Eu. Figure 3 presents the excitation and emission spectra of the CP products

3. RESULTS AND DISCUSSION 3.1. XRD Patterns of YBO3:Eu. The phase purity of all products was characterized by powder X-ray diffraction (XRD) at room temperature. All XRD patterns are in good agreement with the JCPD Standard Card No. 74-1929 of YBO3. No second phase was found. Typical XRD patterns are displayed in Figure 1, which indicates that all products are single phase and the crystal structure of YBO3:Eu is identical to that of YBO3. However, the diffraction peaks are obviously different for the product synthesized by the two methods. The diffraction peaks

Figure 3. Excitation and emission spectra of CP products YBO3:0.01Eu.

of YBO3:0.01Eu. By monitoring the emission of 590 nm, the products exhibit a strong charge transfer band (CTB) in the spectral range 220−270 nm with a maximum at about 250 nm, and the absorption of the 7F0 −5L6 transition is present (Figure 3a). Under 394 and 254 nm excitation, typical emission lines derived from the 5D0−7FJ (J = 0, 1, 2, 3, 4) transitions of Eu3+ are observed (Figure 3b). The emission of 5D0−7F1 is dominant, showing that the Eu3+ ion occupies an inversion

Figure 1. XRD patterns of YBO3:0.01Eu: (a) CP products; (b) MS products. 24749

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symmetry site in the phosphor.10 Therefore, The CP products show an orange-red emission and will result in poor red chromatic coordination. Figure 4 presents the normalized excitation and emission spectra of MS products. Under given UV light excitation, the

Eu2+ ions are considered to exist mainly on the surface of the MS products. The existence of Eu2+ ions may be further confirmed from the excitation spectra. By monitoring the emission of 609 and 591 nm, the as-synthesized products show a weak 4f−4f transition absorption of Eu3+ and a strong Eu−O charge transfer band (CTB) centered at ∼245 nm (curves d and e). However, the CTB shows obviously broadening toward the long-wavelength side, as denoted by a black arrow. Besides the linear absorption corresponding to the 7F0 → 5D4 (365 nm) and 7F0 → 5L6 (394 nm) transitions of Eu3+, the products also exhibit a weak absorption band in the spectral range 300−400 nm (denoted by an asterisk). By monitoring the emission of 460 nm, the absorption band becomes very obvious in the spectral range 250−450 nm, as displayed by curve f. The absorption spectra with a maximum at about 370 nm are comprised of band I (∼370 nm) and band II (∼280 nm). The two absorption bands are assigned to the absorption of the 4f− 5d transition of Eu2+. Figure 5 shows the normalized emission spectra of the assynthesized YBO3:xEu (x = 0.01, 0.05, 0.10, and 0.20) under

Figure 4. Normalized excitation and emission spectra of MS products YBO3:0.01Eu.

characteristic emission of Eu3+ is observed in the spectral range 580−730 nm. As can be seen, compared with the CP products, the change in the relative intensity of 5D0−7F1 and 5D0−7F2 emission is clear. Under 245 and 394 nm excitation, the relative intensity of 5D0−7F2 emission is enhanced, as displayed by curves a and b. Especially, under 267 nm excitation, the 5 D0−7F2 emission becomes dominant (see curve c). These show that the site symmetry of Eu3+ ion is lowered.10 Though the products with the donut-like morphology have a large size, the products are comprised of numerous nanorods with many Eu3+ centers at the surface. Therefore, the local symmetry of the Eu3+ ion on the surface is lowered and the emission intensity of 5D0−7F2 is enhanced. In addition, the results also show that there are several different Eu3+ lattice sites existing in the as-synthesized YBO3:Eu. As can be seen from Figure 4, besides the red-light emission, the as-synthesized YBO3:0.01Eu also show a blue-light emission under 267 and 394 nm excitation. Their maxima locate at about 460 and 400 nm, respectively, which are attributed to the 5d−4f transition emission of Eu2+ located in different lattice sites. The mixed valence Eu has been observed in some specific inorganic compounds also, which has been considered as an approach for white emission with some effectively controllable means.12−14 Here, it is hypothesized that the solvothermal synthesis system provides a reducing condition for the Eu2+ presence, because the partial reduction of Eu3+ ions into Eu2+ ions can be achieved in the solvothermal synthesis system containing several organic reagents; note that many similar reports can be found.15,16 Simultaneously, the partial substitution of Eu3+ ions by Eu2+ ions in the surface of the nanorods may reduce the surface free energy and eliminate the instability resulting from the large surface and numerous defects of the nanorods. Therefore, the

Figure 5. Normalized emission spectra of MS products YBO3:xEu with x = 0.01, 0.05, 0.10, and 0.20 under 394 nm excitation.

394 nm excitation. When the x value is 0.01, the intensity of blue-light emission is the strongest. When the x value is 0.2, the blue-light emission becomes very weak. The relative intensity of the blue-light emission gradually reduces with the increasing Eu concentration, which indicates the relative number of Eu2+ gradually reduces. The Eu2+ amount may be supposed to keep constant and not increase with the increasing Eu concentration, because the Eu2+ ions locate mainly in the surface of the products, and the products are uniform and their surface is not increased. However, the relative number of Eu3+ will gradually increase with the increasing Eu concentration. Therefore, the relative intensity of blue-light emission of Eu2+ gradually reduces corresponding to the red-emission of Eu3+. It implies the emission color can be effectively tuned by controlling the Eu concentration. As displayed in Figure 6, points a, b, c, and d denote the change trend in the chromatic coordination. With the x value increase from 0.01 to 0.20, the chromatic coordination shifts from the blue area to the red area through the white area. 24750

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(6) Ju, Z.; Wei, R.; Gao, X.; Liu, W.; Pang, C. Opt. Mater. 2011, 33, 909−913. (7) Wang, L.; Wang, Y. J. Lumin. 2011, 131, 1479−1481. (8) Guo, X.; Wang, Y.; Zhang, J. J. Cryst. Growth 2009, 311, 2409− 2417. (9) Jiang, X.; Yan, C.; Sun, L.; Wei, Z.; Liao, C. J. Solid State Chem. 2003, 175, 245−251. (10) Li, Z.; Zeng, J.; Li, Y. Small 2007, 3, 438−443. (11) Boyer, D.; Bertrand-Chadeyron, G.; Mahiou, R.; Capera, C.; Cousseins, J. J. Mater. Chem. 1999, 9, 211−214. (12) Mao, Z.; Wang, D.; Lu, Q.; Yu, W.; Yuan, Z. Chem. Commun. 2009, 346−348. (13) Zeuner, M.; Pagano, S.; Matthes, P.; Bichler, D.; Johrendt, D.; Harmening, T.; Pöttgen, R.; chnick, W. J. Am. Chem. Soc. 2009, 131, 11242−11248. (14) Mao, Z.; Wang, D. Inorg. Chem. 2010, 49, 4922−4927. (15) Lu, Z.; weng, L; Song, S; Zhang, P; Luo, X; Ren, X. Ceram. Int. 2012, 38, 5305−5310. (16) Hua, R; Sun, H; Jiang, H; Shi, C. Chem. Res. Chin. Univ. 2006, 22, 423−426. Figure 6. Chromatic coordination of MS products YBO3:xEu with x = 0.01, 0.05, 0.10, and 0.20 under 394 nm excitation.

4. CONCLUSIONS Well-dispersed YBO3:Eu phosphors were synthesized by a reverse microemulsion method with solvothermal treatment (MS) and chemical precipitation (CP), respectively. Compared with the CP products, the MS products have a donut-like morphology, are comprised of numerous nanorods, and show strong absorption in the spectral range 250−450 nm with a maximum at ∼370 nm, which partly overlaps the emission band of near-UV LED (∼370−410 nm). Furthermore, a blue-light emission (∼460 nm) of Eu2+ and an improved red-light emission of Eu3+ with the dominant emission of 5D0−7F2 (∼610 nm) were obtained simultaneously, and their relative intensity can be tuned to generate white light by controlling the Eu concentration. This indicated that the MS YBO3:Eu phosphors have potential application in white LEDs.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +86-0745-2851014. Fax: +86-0745-2851014. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work is financially supported by the National Natural Science Foundation of China (51102106) and by the Science Research Project of the Education Department of Hunan Province (11C0986).

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REFERENCES

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