Combustion synthesis of Eu3+ activated Y3Al5O12 phosphor nanoparticles

Journal of Alloys and Compounds 327 (2001) 82–86

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Combustion synthesis of Eu 31 activated Y 3 Al 5 O 12 phosphor nanoparticles Shi Shikao*, Wang Jiye Department of Chemistry, Hebei Normal University, Shijiazhuang 050091, China Received 12 January 2001; received in revised form 28 March 2001; accepted 28 March 2001

Abstract The precursor powders of Eu 31 activated by a combustion process with urea as a fuel, Y 3 Al 5 O 12 (YAG:Eu), are prepared. Sintering the precursor powders at 12008C for 2 h, YAG:Eu phosphor is synthesized. The diffraction profile of as-prepared YAG:Eu phosphor can be indexed as a garnet structure, as revealed by X-ray diffraction (XRD) data. Grains of YAG:Eu phosphor appear to be elliptical and their sizes range from 60 to 90 nm, as indicated by morphological studies from scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The luminescent intensities of the YAG:Eu phosphor strongly depend on the additional quantities of urea and the concentration of Eu 31 .  2001 Elsevier Science B.V. All rights reserved. Keywords: Y 3 A 5 O 12 ; Combustion synthesis; Phosphor; Eu 31 ion; Nanoparticle

1. Introduction Y 3 Al 5 O 12 or YAG garnet phases are excellent materials with stable physical and chemical properties, which have been extensively used as hosts for lasers and phosphors [1,2]. Single crystals of Cr 31 doped YAG are applied in solid state lasers. Ce 31 activated YAG powder is a fastresponse flying-spot scanner phosphor, and Tb 31 doped YAG gives a characteristic narrow-band phosphor suitable for contrast-enhanced display applications [3,4]. The grain size of phosphor powders prepared through conventional high-temperature solid-state reactions is of the order of 5–20 mm. Phosphors of small particles must be obtained by grinding and milling the larger phosphor particles [5]. The processing steps readily introduce additional defects and greatly reduce luminescence efficiency. For phosphor applications, it is desirable to have a fine particle size for high resolution and chemical purity for optimum chromaticity and brightness [6]. With the development of scientific technologies on materials, several chemical synthesis techniques, such as sol–gel [5,7], coprecipitation [8,9] and combustion methods, have received great attention recently. They have been utilized to produce amorphous YAG precursor powders, which are then crystallized to form YAG by sintering at

much lower temperatures. All of these methods start with liquid phases so that each required component can be accurately controlled. For sol–gel, or coprecipitation techniques, processing steps to prepare the precursor powders are complicated and the duration time is long. However, the combustion process to prepare the precursor powders is quite simple and the combustion reaction time only takes a few seconds. The preparation of nanostructured phosphors for potential commercial applications has rarely been investigated. Our research interests in synthesizing YAG:Eu nanophases are motivated by the attempt to improve the luminescence efficiency of phosphors used for cathode ray tube screens and high-definition projection television. In this paper, combustion synthesis is applied to prepare the precursor powders of YAG:Eu. Sintering at appropriate temperatures, YAG:Eu phosphor nanoparticles are synthesized. The microstructure and photoluminescence (PL) spectra of the as-prepared YAG:Eu powders are reported on X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and PL spectroscopy.

2. Experimental *Corresponding author. E-mail address: [email protected] (S. Shikao).

Aqueous nitrate solutions of Y 31 and Eu 31 are prepared by dissolving high-purity Y 2 O 3 and Eu 2 O 3 with HNO 3

0925-8388 / 01 / $ – see front matter  2001 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 01 )01399-8

S. Shikao, W. Jiye / Journal of Alloys and Compounds 327 (2001) 82 – 86 Table 1 Effects of the quantities of urea and the concentration of Eu 31 on the emission intensities of (Y 12x Eu x ) 3 Al 5 O 12 phosphor nanoparticles a Quantities of urea

Concentration of Eu 31 (X)

I( 5 D 0 → 7 F 1 )

I( 5 D 0 → 7 F 2 )

1W

0.06

2W 2.5W 3W 2.5W 2.5W

0.06 0.06 0.06 0.04 0.10

No combustion, only producing brown gas (NO 2 ) 0.68 0.55 1 0.71 0.61 0.48 0.51 0.47 0.32 0.33

a W means the quantities of urea as the molar ratio of urea to M 31 is 3 / 2. The relative emission intensity is 1, as the quantities of urea are 2.5W and the concentration of Eu 31 (X) is 0.06.

and some deionized water. Al(NO 3 ) 3 solution is gained by Al(NO 3 ) 3 ?9H 2 O dissolving in deionized water. The respective nitrate solutions with a cationic molar ratio for Y:Eu:Al of (12x):x:5 / 3 (x,1) are mixed in a cylindrical container (90 mm diameter3200 mm height) and undergo boiling to dehydration. Then CO(NH 2 ) 2 (urea) is added to the boiling solution (see Table 1), which immediately decomposes, generating traces of NO 2 and other combustibles such as NH 3 . The mixture then ignites, leading to smooth deflagration with enormous swelling, producing a white foam. In all, the combustion process lasts only a few seconds. The chemical equations of the reactions may be as follows: 4M(NO 3 ) 3 5 2M 2 O 3 1 12NO 2 1 3O 2 (M 5 Y, Eu, Al) CO(NH 2 ) 2 1 H 2 O 1 2NO 2 5 2NH 3 1 CO 2 1 2O 2 1 N 2 The voluminous and foamy product can be easily milled, becoming the precursor powder of YAG:Eu phosphor. The

Fig. 1. Synthesis of Y 3 Al 5 O 12 :Eu phosphor nanoparticles by combustion methods with urea.

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well-milled precursor powder is subsequently fired at 12008C for 2 h, producing fine YAG:Eu phosphor. X-ray diffraction (XRD) patterns of the YAG:Eu phosphor are recorded using a D/ max-rA X-ray diffractometer. The morphological studies are carried out on a S-570 scanning electron microscope (SEM) and a H-600 transmission electron microscope (TEM). The PL spectra of as-prepared Eu-activated YAG particles are measured using a Hitachi F-4500 fluorescence spectrophotometer. The synthesis of YAG:Eu nanoparticles is summarized in a flow diagram and represented in Fig. 1.

3. Results and discussions

3.1. Crystal structure The structure of the precursor powder of YAG:Eu prepared by the combustion process is determined using XRD. Since no obvious diffraction peaks are observed, it can be concluded that the powders are amorphous. With the addition of heat to the precursor powders of YAG:Eu at 12008C for 2 h, typical cubic garnet diffraction peaks are predominant in their XRD patterns (Fig. 2). These results are in agreement with JCPDS file 33-40. YAG has the garnet structure and is an intermediate phase of the Y 2 O 3 –Al 2 O 3 system. As can be seen from the phase diagram [10], YAG and the other two intermediate phases, YAlO 3 (YAP with a perovskite structure) and Y 4 Al 2 O 9 (YAM with a monoclinic structure) are line compounds. In conventional solid-state reactions [11], the YAG phase appears at 14008C, coexisting with other phases Al 2 O 3 , Y 2 O 3 , YAM and YAP. Only when 20% BaF 2 (as a flux) is added and the reaction temperature is over 15008C, does the product yield pure YAG phase. Although no flux is added, YAG phase with high purity can be obtained at 12008C through the combustion process to the starting materials, whereas it is impossible to happen for conventional high-temperature solid-state reactions.

Fig. 2. XRD diagram of YAG:Eu phosphor nanoparticles sintered at 12008C.

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Fig. 3. SEM micrographs of precursor powders (a) of YAG:Eu and phosphor nanoparticles sintered at 12008C (b). TEM image of phosphor nanoparticles (c).

S. Shikao, W. Jiye / Journal of Alloys and Compounds 327 (2001) 82 – 86

3.2. Morphology and grain size The microstructure of the precursor powders and YAG:Eu phosphor sintered at 12008C are studied on their SEM micrographs. The morphology of the precursor powders are seriously inordinate and unconsolidated (Fig. 3a). The particles of YAG:Eu phosphor sintered at 12008C are well-distributed and most appear to be irregularly spherical or elliptical (Fig. 3b). In order to achieve accurate data of the grain size of YAG:Eu phosphor, its TEM image is recorded in Fig. 3c. The grain size of YAG:Eu phosphor sintered at 12008C is in the range of 60–90 nm, as can be estimated from Fig. 3c. Therefore, the as-prepared YAG:Eu phosphor is not necessary to be reground before it is applied.

3.3. Luminescent characteristics The emission spectra of YAG:0.06Eu phosphor nanoparticles obtained after firing at 12008C for 2 h are shown in two different regions, peaking at 590 and 613 nm (Fig. 4a). The emission lines correspond to 5 D 0 → 7 F 1 (590

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nm) and 5 D 0 → 7 F 2 (613 nm) transitions of Eu 31 , which are well addressed in the literature [12]. Since the line at 590 nm is more prominent, the magnetic dipole transition is predominant, indicating the Eu 31 ions lie in centrosymmetrical sites. The forbidden transition of 5 D 0 → 7 F 2 (613 nm) is secondary. The absence of the 5 D 0 → 7 F 0 31 transition that occurs for a linear crystal field at the Eu site, can also be noticed. The excitation spectrum of the 5 D 0 → 7 F 1 (590 nm) emission for YAG:0.06Eu is given in Fig. 4b. It is the charge transfer (CT) band lying between 200 and 270 nm, with the maximum is near 240 nm. The spectral energy distribution of Eu 31 emission is strongly dependent on the quantities of urea (fuel). As urea is added in calculated stoichiometric quantities that molar ratio of urea to M 31 is 3 / 2 (supposed to be W ), no combustion process is observed, the only result being the production of large amounts of brown gas (NO 2 ). The reason for this result is that urea can be easily decomposed or directly reacted with O 2 at high temperatures. Therefore, urea must be added in excess amounts. The optimum quantities of urea to be added are confirmed to be 2.5W (see Table 1). The spectral energy distribution is also strongly dependent on the europium concentration [13], as can be seen in Table 1. The emission wavelength does not vary with the europium concentration. But the emission intensity ratio between the transitions of 5 D 0 → 7 F 1 and 5 D 0 → 7 F 2 changes a lot with the europium concentration. As the concentration (X) of Eu 31 is 0.06, the optimum luminescence intensities of the phosphor nanoparticles can be achieved.

4. Conclusions The present investigation gives an overall picture of the synthesis of Eu 31 activated YAG by employing a combustion with urea as a fuel process at a final sintering temperature of 12008C for 2 h. All combustion processes are safe and instantaneous. Single-phase, cubic YAG garnet is gained although no flux is mixed. The phosphor particles are well-distributed and the grain size is in the range of 60–90 nm. The emission spectra of as-prepared YAG:Eu correspond to 5 D 0 → 7 F 1 (590 nm) and 5 D 0 → 7 F 2 (613 nm) transitions of Eu 31 . The spectral energy distribution of Eu 31 emission strongly depend on the quantities of urea (fuel) and the concentration of Eu 31 . The optimum preparative conditions are urea quantities 52.5W and X (the concentration of Eu 31 )50.06.

Acknowledgements Fig. 4. Emission spectrum (a) ( lex 5254 nm) and excitation spectrum (b) ( lem 5590 nm) of (Y 0.94 Eu 0.06 ) 3 Al 5 O 12 phosphor nanoparticles sintered at 12008C.

This work is supported by Natural Science Foundation of Hebei Province (No. 299193), People’s Republic of

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China. Thanks are due to Professor Liu Xingren, Changchun Institute of Physics, Chinese Academy of Sciences, for his help.

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