Comparative investigation on synthesis and photoluminescence of YAG:Ce phosphor

Materials Science and Engineering B 106 (2004) 251–256

Comparative investigation on synthesis and photoluminescence of YAG:Ce phosphor Yuexiao Pan, Mingmei Wu, Qiang Su∗ Department of Chemistry, State Key Laboratory of Optoelectronic Materials and Technologies, School of Chemistry and Chemical Engineering, Sun Yat-Sen (Zhongshan) University, Guangzhou 510275, PR China Received 7 April 2003; accepted 15 September 2003

Abstract Phosphor yttrium aluminum garnet Y3 Al5 O12 (YAG), activated with trivalent cerium (Ce3+ ) was synthesized by different methods: solid-state reaction (SS), co-precipitation (CP) with ammonium bicarbonate, sol–gel (SG) with citric acid and combustion (CB) with urea. The crystallization, morphologies, particle size and luminescent character of the phosphors obtained under different experimental conditions were studied. The influences of various factors on the luminescence intensity of the phosphor were investigated, such as sintering time, repeated calcinations, reaction temperature, and quantities of urea used in CB. © 2003 Elsevier B.V. All rights reserved. Keywords: Ceramics; Light emitting diodes; Optical properties; Oxides

1. Introduction Recently, yttrium aluminum garnet (YAG), activated with trivalent cerium Y3−x Al5 O12 :xCe3+ (YAG:Ce), has been found to be an efficient phosphor for converting the blue LED radiation into a very broad yellow emission band [1–3]. The yellow emission from YAG:Ce is intense enough to complement the residual blue light which escapes through the phosphor in order to produce a white light. By far, YAG:Ce has been the most excellent phosphor satisfactorily applied in white phosphor-based LED commercial market. The property of the phosphor would be crucial to the ultimate applications. It is well known that Y2 O3 –Al2 O3 system exists in three different crystal phases: YAlO3 (YAP with a perovskite structure), Y4 Al2 O9 (YAM with a monoclinic structure) and Y3 Al5 O12 (YAG with a cubic garnet structure) [4–6]. YAG is much more stable than the other two intermediate phases in conventional solid-state reactions [7,8]. Therefore, high temperature sintering with repeated grinding and milling is required for preparing pure YAG by solid-state reaction (SS) [7–11]. A number of investigations proved that the ∗ Corresponding author. Tel.: +86-20-84111038; fax: +86-20-84111038. E-mail address: [email protected] (Q. Su).

0921-5107/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2003.09.031

wet chemical methods could decrease the sintering temperature efficiently [12–17]. To obtain ultrafine YAG powders at relatively low temperature, wet chemical methods, such as co-precipitation (CP) [18–22], sol–gel (SG) [16,23–26], combustion (CB) [17,27–29], and hydrothermal methods [30,31] are extensively applied, because through these methods, the starting materials can be mixed at molecular level. In our work, we have synthesized pure phase of YAG with a number of different methods and a comparative study has been made. The work is extremely significant for practical large-scale production of phosphor assisted white LEDs.

2. Experiment Y2 O3 (99.99%) and CeO2 (99.99%) were used as rare earth sources of YAG:Ce in following synthesis methods. Al(OH)3 (CP) and 99.9% Al(NO3 )3 ·9H2 O were used as the sources of Al, respectively, in SS and in wet chemical methods (CP, SG, CB). 2.1. Synthesis of YAG:Ce by solid-state reaction method The starting materials Y2 O3 , CeO2 , Al(OH)3 were mixed with a final molar ratio of 2.88Y:5Al:0.12Ce. The mixture was milled thoroughly and transferred into a furnace for

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crystallization firstly at 1300 ◦ C for 10 h under CO atmosphere, and then at 1500 ◦ C for 10 h to enhance both the degree of crystallization and the luminescence intensity of the products. Repeated milling was needed between the two treatments for higher homogeneity. For the wet chemical preparation, Y(NO3 )3 solution was formed by dissolving Y2 O3 in HNO3 and the excess HNO3 was removed by evaporation in fume cupboard. Likewise, Ce(NO3 )3 solution was obtained from CeO2 and HNO3 . Al(NO3 )3 solution was prepared by dissolving Al(NO3 )3 ·9H2 O in distilled water and the concentration of salt solution was measured by EDTA. 2.2. Synthesis of YAG:Ce by co-precipitation method with ammonium bicarbonate NH4 HCO3 With a total volume of 50 ml, the solution was prepared with a final molar ratio of 2.94Y:5Al:0.06Ce or 2.88Y:5Al:0.12Ce, where 0.018 mol of Y3+ was used. The solution was added dropwise to 100 ml of ammonium bicarbonate NH4 HCO3 (0.16 mol) precipitant solution with magnetic stirring at room temperature. After aging for 15 min, the resultant suspension was filtered and washed with distilled water. Dried at 100 ◦ C for 2 h, the powder was then transferred into a furnace and calcined at high temperature above 1000 ◦ C under CO atmosphere produced by incomplete oxidation of activated carbon. 2.3. Synthesis of YAG:Ce by sol–gel method with citric acid (C6 H8 O7 ·H2 O) With a total volume of 20 ml, the solution was prepared with a final molar ratio of 2.88Y:5Al:0.12Ce, where 0.006 mol of Y3+ was used. The solution was added dropwise to a mixture containing 1.5 g of citric acid in 5 ml of ethylene glycol (EG) with constant magnetic stirring until the mixture became transparent. After heating at 200 ◦ C for 5 h, the color of the solution changed from pale yellow to bright yellow, and then to brown. When the excess solvent was removed, the solution became more viscous without any precipitation or turbidity. At last, a dark brown glassy resin-like substance was observed. After further firing at 400 ◦ C for 2 h, a dry dark solid mass was formed. When the solid was cooled to room temperature, it was ground into a powder with agate mortar. Nanoscale yellow powder was obtained after calcinations were carried out at 1000 ◦ C in air. 2.4. Synthesis of YAG:Ce by combustion method with urea (CO(NH2 )2 ) The well mixed salt solution with 2.94Y:5Al:0.06Ce or 2.88Y:5Al:0.12Ce was heated till boiling. Solid urea with three times of the molecular of Y3+ was added to the boiling solution. With continuous heating and evaporation, the solution became opaque and then swelled into a white foam.

After the foam was dried, it was subsequently milled gently and then sintered at 1000 ◦ C in air for 5 h. 2.5. Characterization The products were characterized by powder X-ray diffraction (XRD) collected on a Philips model PW1830 diffractometer with graphite monochromator and Cu K␣ radiation (λ = 0.1541 nm). Scanning electron microscopy (SEM) images were taken on a JEOL JSM-6330F field emission scanning electron microscope. Samples were gold coated prior to SEM analysis. Particle sizes and shapes were observed by transmission electron microscopy (TEM) on a JEM-1010 electron microscope under an accelerated voltage of 100 kV. A sample for TEM examination was prepared by depositing an ultrasonically dispersed suspension of powder from a mixed solution of alcohol and water on a carbon-coated copper grid. The luminescence properties of all the phosphors were studied on an Aminco Bowman Series 2 fluorescence spectrophotometer at room temperature.

3. Results and discussion 3.1. X-ray powder diffraction The forming of YAG synthesized by SS, SG, CB, and CP was investigated by XRD. The results were shown in Fig. 1. A mixture of YAG, YAP, and other phases was formed by SS at 1300 ◦ C under CO for 10 h (Fig. 1a), which indicates this temperature was too low to obtain pure YAG phase by SS. Pure YAG phase appeared after repeated sintering at 1500 ◦ C for 10 h (Fig. 1b), which is in good agreement with JCPDS Card (no. 33-40). Compared with SS, wet chemical methods lowered the crystallization temperature evidently because metal centers were homogeneously cross-linked and distributed in the precursor. Only as low as 1000 ◦ C in air was necessary to obtain pure YAG phase by SG (Fig. 1c), CB (Fig. 1d), and CP (Fig. 1e), which is substantially lower than that employed in SS. 3.2. Morphology and size of YAG:Ce phosphors A series of SEM and TEM micrographs were made to characterize and compare the microstructure of Ce doped YAG phosphor prepared by different methods. Uniform and spherical nanoparticles of YAG:Ce with homogeneous microstructure were obtained by CP and SG at 1000 ◦ C for 5 h (Fig. 2a and b). From TEM (insets) of the product obtained by CP method with the use of ammonium bicarbonate NH4 HCO3 , we can observe that the diameter of uniform spherical particles is not greater than 50 nm. This is because NH4 HCO3 produces a carbonate precursor with an approximate composition of NH4 AlY0.6 (CO3 )1.9 (OH)2 ·0.8H2 O [32]. In SG, citric acid acts as chelate ligands to metal ions to yield a rigid polyester network in which metal atoms are

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aggregations were obtained (Fig. 2d). It is noticeable that in these wet chemical methods, the Ce ions are spontaneously reduced by sintering their precursors because the combustion of organic components provide a reducing atmosphere. In contrast, a reducing atmosphere must be provided in SS for the reduction of Ce ions. For large-scale application of the phosphor, besides of the particle size and morphology, the luminescence intensity should also be seriously considered.

(e)

(631) (444) (640) (721) (642)

(532)

G

G G G GPP G P G G G PP M

(651) (800)

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3.3. Photoluminescence property of YAG:Ce

(422) (431)

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2 Theta (Degree) Fig. 1. XRD patterns of YAG:Ce prepared under different experimental conditions: by solid-state reaction, first sintered at 1300 ◦ C under CO flow for 10 h (a) and then repeated sintering at 1500 ◦ C under CO flow for 10 h (b); by sol–gel method (c) and combustion method (d), sintering the precursors, respectively, at 1000 ◦ C in air for 5 h; by co-precipitation method (e), sintering the precursor at 1000 ◦ C under CO for 5 h. Note: “G” means YAG, “P” means YAP, “M” means YAM.

distributed homogeneously. Thus, the hydrolysis of metal atoms can be inhibited and chemical processing can be controlled [33–36]. However, aggregation of the uniform particles is appeared to be more serious in SG (Fig. 2b). This phenomenon could be due to the bridging of adjacent particles through the hydrogen bonding of water and the significant capillary action generated during the drying process in the precursors [37]. The aggregated and sticky sample will have trouble in practical application in LEDs because the particles would clog the injector during fabrication. Agglomeration is less serious possibly due to weaker hydrogen bonding in CP through the sintering process. The YAG particles with large irregular shapes were obtained by CB with urea (CO(NH2 )2 ) after the precursor was sintered at 1000 ◦ C for 5 h prior to grounding or milling (Fig. 2c). In CB, a large quantity of gases, such as N2 , CO2 , and oxides of nitrogen were produced by the combustion of urea, causing the products composing many pores (Fig. 2c, inset). Beside, it also causes the products to become foam-like and loose. The brittle products were easily grounded into a powder form. After repeated sintering at high temperature in SS, large and irregular particles of YAG with extensive

The occurrence of more than one Ce3+ absorption bands in the excitation spectrum (Fig. 3, left) in the region between 200 and 500 nm, i.e., three broad bands with maxima at 233, 340, and 463 nm, respectively, are attributed to the crystal-field splitting of the 5d state. Obviously, the broad band covers from 400 to 500 nm is most intense, which provides basic possibilities to the applications of the phosphor with the use of blue GaN LEDs. In the emission spectrum (Fig. 3, right), the broad emission band is located in the range from 500 to 650 nm, which is a kind of ideal yellow light that complements the blue light emitted by GaN single crystal to generate white light. Generally, sintering temperature and reaction time are two important factors that affect the crystallization and luminescence intensity. For co-precipitation method, with the increase of temperature, the luminescence intensity is improved significantly as shown in Fig. 4. The emission intensity also increases with the reaction time, as indicated in Fig. 5. However, the intensity remains constant when the reaction time exceeded 20 h regardless of the sintering temperature. Repeated sintering and grounding is a tool frequently used in solid-state reaction method for better crystallization and a more complete reaction. The intensity increases considerably after upon repeated sintering, but the process is inefficient beyond the third sintering process for the co-precipitation method (Fig. 4d and e). Among the factors, increasing the sintering temperature is a more efficient means for two reasons: first, it improves the crystallization of YAG particles; second, high temperature favors the doping Ce ions into YAG lattice. Both the shape and the position of the spectra show little change with the increase of luminescence intensity, which indicates sintering temperature, reaction time, and repeated calcinations are not significant factors for the shifting of Ce emission, this indicates that the product obtained by the co-precipitation method is more homogeneous due to the mixing of starting materials at molecular level. The shifting will be discussed later. In CB, as mentioned earlier, inert or reducing gases were produced in the combustion of urea. Note that the luminescent intensity was dependent on the quantity of urea used (Fig. 6). If the urea was not sufficient, Ce ions could not be reduced fully into Ce3+ . However, the luminescence decreased steadily when too much urea was added. According to the experimental results, the optimum concentration of

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Fig. 2. SEM images of YAG:Ce prepared at 1000 ◦ C for 5 h under CO flow by co-precipitation method (a; inset: TEM), in air by sol–gel method (b; inset: TEM), by combustion method (c; inset: a part of enlarged SEM), and first sintered at 1300 ◦ C for 10 h and then at 1500 ◦ C for 10 h under CO flow by solid-state reaction (d).

urea used in CB for preparing bright YAG:Ce phosphor is approximately 250 mol% of Y3+ . Calcination was carried at a temperature of 1000 ◦ C for wet chemical methods CP, SG, CP, and at 1300 ◦ C for SS methods. Luminescence intensity of the products obtained through CP, SG, and CB methods calcined at 1000 ◦ C was much higher than that of the product through SS method calcined at 1300 ◦ C (Fig. 7). This is due to a molecular-level mixing of raw materials and the decrease of crystallization temperature through the combustion of organic reagents in

wet chemical methods. The lower emission intensity of the phosphor obtained by SS at 1300 ◦ C is mainly attributed to an incomplete crystallization of YAG according to XRD (Fig. 1a). Furthermore, since the gases produced by the decomposition of organic molecules in SG and CB methods reduced the Ce ions and protects Ce3+ from oxidizing, phosphors can be directly prepared from their precursors just in air, while CO should be offered for reducing Ce ions in SS. However, after the second time of sintering the phosphors

(f)

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(f) 1500 C 5 h 1 time o (e) 1200 C 5 h 4 times o (d) 1200 C 5 h 3 times o (c) 1200 C 5 h 2 times o (b) 1200 C 5 h 1 time o (a) 1000 C 5 h 1 time

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Wavelength (nm) Fig. 3. Photoluminescence spectra of 2 mol% Ce doped YAG prepared by co-precipitation method. Excitation spectrum (λem = 532 nm), and emission spectrum (λex = 470 nm).

Fig. 4. Emission spectra of 4 mol% Ce doped YAG prepared by co-precipitation method at different temperature and for different sintering times: at 1000 ◦ C for 5 h for once (a), at 1200 ◦ C for 5 h once (b), for twice (c), for three times (d), for four times (e) with repeated milling, and at 1500 ◦ C for 5 h for once (f) (λex = 470 nm).

Y. Pan et al. / Materials Science and Engineering B 106 (2004) 251–256

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(a) 24 h (b) 20 h (c) 10 h (d) 5 h (e) 3 h

(a) SS (b) CB (c) SG (d) CP

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550

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Wavelength (nm) Fig. 6. Emission spectra of 2 mol% Ce doped YAG prepared by combustion method with different quantity of urea. The molecular ratio of urea to Y3+ is denoted as x (λex = 470 nm).

Intensity (a.u.)

(b) (c)

700

Fig. 8. Emission spectra of 4 mol% Ce doped YAG prepared at 1000 ◦ C for 5 h under CO flow by co-precipitation method (d), in air by combustion method (b) and sol–gel method (c), and sintered at 1300 ◦ C for 10 h under CO flow by solid-state reaction (a). Then all samples are sintered at 1500 ◦ C for 10 h under CO flow, respectively (λex = 470 nm).

at 1500 ◦ C under CO flow, the luminescence intensity of the phosphor obtained by SS was the strongest, and that by CB was the second and those by SG and CP were relatively weak (Fig. 8). The emission intensities of the products obtained by the wet chemistry synthesis methods are lower than that by solid-state reaction method, this may be due to the different morphology of the products (as shown in Fig. 2) and different environment around the Ce3+ ion, which induces the different splitting of the d-level and spectral band shift (as shown in Fig. 8). It is well known that the 5d–4f emission of Ce3+ depends strongly on the crystal field. In Figs. 7 and 8, red shifting happens in the Ce emission from the products derived from CB and SS. Taking both the morphology and particle size into consideration, the significantly smaller particles obtained by SG and CP have a higher surface tension than that of the bulk and hence the 5d level would have a stronger crystal-field splitting. In addition, the phosphors with large-size particles from SS and CB show a more intense luminescence than those with small-size particles from CP and SG.

4. Conclusion

(d)

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(a) CB (b) SG (c) CP (d) SS

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Fig. 5. Emission spectra of 4 mol% Ce doped YAG prepared by co-precipitation method first at 1000 ◦ C under CO flow for different reaction time (λex = 470 nm).

(a) (b)

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Wavelength (nm) Fig. 7. Emission spectra of 4 mol% Ce doped YAG prepared at 1000 ◦ C for 5 h under CO flow by combustion method (a), in air by sol–gel method (b) and co-precipitation method (c), and sintered at 1300 ◦ C for 10 h under CO flow by solid-state reaction (d) (λex = 470 nm).

We have synthesized YAG:Ce by solid-state reaction, co-precipitation, sol–gel, and combustion methods, respectively. The characters of these methods are discussed and compared. At high temperature, such as at 1500 ◦ C, solid-state reaction and combustion method are preferred for high luminescence intensity though large irregular particles have been obtained. Those from co-precipitation and sol–gel methods give rise to spherical particles with small size but have severe aggregations and relatively low luminescence intensity although the crystallization temperature is much lower. Red shifting happens in the Ce emission from the products derived from combustion method and

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solid-state reaction compared with those from sol–gel and co-precipitation methods because the smaller particles obtained by sol–gel and co-precipitation methods have a higher surface tension than that of the bulk derived by combustion method and solid-state reaction. Increasing the sintering temperature could promote the luminescence intensity because it improves the crystallization of YAG particles and favors the doping Ce ions into YAG lattice.

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Acknowledgements [19]

This work is financially supported by Provincial Natural Science foundation of Guangdong province and NSFC. It is a pleasure to acknowledge Guangdong Yangjiang Rare Earth Factory for providing us rare earth oxides. Careful proofreading and fruitful discussion by Dr. Wing-Cheong Wong at Hong Kong Government Laboratory, is greatly appreciated. The authors are greatly indebted to Prof. Donald S. McClure at Princeton University for his much useful guidance and suggestion. Thanks are due to Dr. Benli Chu in our group for many helpful discussions. References

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