Synthesis and luminescence properties of microemulsion-derived Y3Al5O12: Eu3+ Phosphors

(1)

ARTICLE IN PRESS

G Model

JALCOM-17979; No. of Pages 6

Journal of Alloys and Compounds xxx (2008) xxx–xxx

Contents lists available atScienceDirect

Journal of Alloys and Compounds

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / j a l l c o m

Synthesis and luminescence properties of microemulsion-derived

Y

3 Al

5 O

12 : Eu

3+

Phosphors

Chung-Hsin Lu

a,∗

_{, Chien-Hao Huang}

a

_{, Bing-Ming Cheng}

b

a_{Electronic and Electro-optical Ceramics Lab, Department of Chemical Engineering, National Taiwan University, Taipei, Taiwan, ROC} b_{National Synchrotron Radiation Research Center, Hsinchu, Taiwan, ROC}

a r t i c l e i n f o

Article history:

Received 11 December 2007 Received in revised form 21 May 2008 Accepted 25 May 2008 Available online xxx Keywords: Microemulsion Phosphors Synthesis Garnet

a b s t r a c t

Europium-ion doped yttrium aluminum garnet (YAG: Eu3+_{) phosphors were prepared via both the} solid-state-reaction method and the microemulsion process. The solid-solid-state-reaction-derived powders required repeated calcination at 1600◦C to produce phase-pure YAG: Eu3+_{, and the formed phosphors had} irreg-ular morphology and a broad particle-size distribution. The microemulsion route was demonstrated to signiﬁcantly lower the synthesis temperature of pure garnet phase to 1400◦_{C, and substantially reduce} the particle size to submicron order. With a rise in the calcination temperature, the peak intensities in the excitation and emission spectra also increased as a result of increased amount of YAG phase. Synchrotron radiation-excited luminescence studies indicate that YAG: Eu3+_{exhibited broad excitation peaks in} vac-uum ultraviolet (VUV) range and strong red emission peaks at 590 nm. The VUV-excited emission spectra demonstrate that the microemulsion-derived YAG: Eu3+_{phosphors generated more intense} lumines-cence than the solid-state-derived specimens. The enhanced lumineslumines-cence characteristics are considered to be attributed to the reduced particle size and improved morphology of the microemulsion-derived phosphors.

1. Introduction

Ever since the introduction of tricolor fluorescent lamps using rare-earth ion-doped phosphors, research on luminescence appli-cations is focused on the development of new types of phosphors. Newly launched devices such as plasma display panels (PDP) and field emission displays (FED) have attracted much attention because of their thinness, high brightness and superior picture quality. For improving the performance of displays, developing high-quality phosphors is a must. Besides the luminescence yield, the particle size, density, and morphology of phosphors also have to be well controlled[1–6]. The requirement that efficient phos-phors utilize discharged inert gas as an excitation source has motivated vacuum ultraviolet (VUV) excited luminescence stud-ies. When excited at 147 nm by discharged Xe gas, few phosphors have shown the potential for plasma display panels applications [1]. The phosphors used in PDP include BaMgAl10O17: Eu2+ _and

∗ Corresponding author at: Electronic and Electro-optical Ceramics Lab, Depart-ment of Chemical Engineering, National Taiwan University, No. 1, Sec. 4, Roosevelt Road, Taipei 106, Taiwan, ROC.

E-mail address:[email protected](C.-H. Lu).

Y2SiO5: Ce3+for blue emission, BaAl12O19: Mn2+and Zn2SiO4: Mn2+ for green emission, and YBO3: Eu3+ for red emission [7–12]. In most of the earlier reports, UV-light sources have been used to analyze the luminescence properties of the phosphors; however, rare studies have involved observations on VUV-excited emis-sion.

Yttrium aluminum garnet (Y3Al5O12, YAG) is an important mate-rial used for different kinds of electronic devices. Among the lanthanide-doped YAG materials, europium-ion-doped phosphors are regarded as potential red phosphors for applications in var-ious luminescent devices. The luminescence properties of YAG: Eu3+ _{have been reported in crystals and ceramics prepared via} the solid-state route[13,14]. However, phosphors prepared via the solid-state route usually have irregular morphology, large particles and wide particle size distributions. In order to reduce the parti-cle size and improve the morphology of the prepared YAG: Eu3+ phosphors, a microemulsion process was adopted in this study. The reaction routes of microemulsion-derived YAG: Eu3+ _were investigated and compared with those of the solid-state-reaction-derived phosphors. The prepared phosphors were investigated under high-energetic VUV excitation using synchrotron radia-tion as a light source. The effects of the prepararadia-tion condiradia-tions on the VUV excited luminescence of the phosphors were also studied.

(2)

ARTICLE IN PRESS

G Model

2 C.-H. Lu et al. / Journal of Alloys and Compounds xxx (2008) xxx–xxx

Fig. 1. Flowchart of the microemulsion process for synthesizing YAG: Eu3+_phosphors.

2. Experimental

2.1. Synthesis of YAG: Eu3+_phosphors

Yttrium oxide, aluminum oxide and europium oxide were mixed stoichiomet-rically according to the chemical formula (Y0.95Eu0.05)3Al5O12. The mixtures were ball-milled using zirconia (ZrO2) balls with ethyl alcohol as the dispersing agent for 96 h. The slurry was subsequently dried in a vacuum-rotator dryer. The dried precursors were calcined at 800–1500◦_{C for 4 h, and then at 1600}◦_{C for 4 h, before} being ground again. The calcination and grinding processes were repeated for three times. The samples calcined at 1600◦C for three times is denoted SS-16.

Fig. 1depicts the microemulsion process employed in this study for preparing YAG: Eu3+_{phosphors. The microemulsion system was prepared in a mixture of water} and oil with a water-to-oil (W/O) volume ratio of 3/7. Cyclohexane was chosen as the oil phase, and octylphenyl ether (OP-10) and 1-hexanol were used as the surfactant and co-surfactant, respectively. The amounts of cyclohexane, OP-10 and 1-hexanol were ﬁxed at 50, 20, and 30 vol% of the total volume of the oil phase. The aqueous phase was prepared by dissolving appropriate amounts of yttrium nitrate, aluminum nitrate, and europium oxide in de-ionized water. The concentrations of yttrium ions, europium ions and aluminum ions in the solution were set to 0.285, 0.015, and 0.5 M, respectively. The cationic solutions were mixed according to the composition of (Y0.95Eu0.05)3Al5O12. After the prepared aqueous phase was mixed with the oil phase, the mixed solution was agitated for 1 h using a homogenizer to yield the well-emulsiﬁed solution, which was slowly dropped into 180◦_{C hot kerosene to} evaporate water in the microemulsion. The obtained gels were dried at 500◦C for 2 h in air to remove the residual oil and obtain the precursor powders. The precursor powders were then ground and further calcined at various elevated temperatures ranging from 900 to 1400◦_{C for 4 h. The samples calcined at 1000, 1200, and 1400}◦_C are denoted EM-10, EM-12, and EM-14, respectively.

2.2. Characterization of YAG: Eu3+_phosphors

The phase purity and crystallinity of the powders were analyzed using an X-ray diffractometer (XRD, MAC science MXP3) with Cu K␣ radiation. The morphology and particle sizes of the powders were examined using a scanning electron microscope (SEM, Hitachi S-800). The photoluminescence performance of the phosphor was recorded using a ﬂuorescence spectrophotometer (Hitachi, F-4500) at room

tem-perature. The luminescence characteristics of YAG: Eu3+_{phosphors in the vacuum} ultraviolet radiation range were investigated using synchrotron radiation at beam line BL03A at the National Synchrotron Radiation Research Center, Hsinchu, Taiwan. The VUV light produced in the center was dispersed with a high-flux cylindrical grating monochromator. The intensity of this VUV light was monitored by means of light reflected from a LiF beam splitter. A small fraction of the reflected beam passed through one additional LiF plate and was incident on a glass window coated with sodium salicylate. The fluorescence signal was detected by a photomultiplier tube in a photon-counting mode.

3. Results and discussion

3.1. Reaction routes of solid-state-reaction-derived YAG: Eu3+

phosphors

Fig. 2 shows the X-ray diffraction patterns of the solid-state-reaction-derived YAG: Eu3+_{powders calcined under various} conditions. After calcination at 800◦C, all detected peaks corre-sponded to Y2O3 and Al2O3 (Fig. 2(I)). After heating at 1000◦C, the reaction between Y2O3 and Al2O3began to occur, and mon-oclinic Y4Al2O9(YAM)[15]formed along with a small amount of orthorhombic YAlO3[16]and hexagonal YAlO3(YAP)[17](Fig. 2(II)). When the precursor powders were heated at 1200◦C, the YAM phase disappeared, along with an increase in the intensity of orthorhombic YAP peaks and a decrease in the intensity of hexag-onal YAP peaks (Fig. 2(III)). When the heating temperature was elevated to 1400, 1500 and 1600◦C, Y3Al5O12appeared as the dom-inant phase; however, several peaks of orthorhombic YAP were still present (Fig. 2(IV)–(VI)). It is revealed that pure YAG: Eu3+_is difﬁcult to obtain via the solid-state reaction. In order to obtain single-phased YAG: Eu3+_{, the grinding and calcination processes} at 1600◦C for 4 h were performed for three times. After the

(3)

calci-ARTICLE IN PRESS

G Model

C.-H. Lu et al. / Journal of Alloys and Compounds xxx (2008) xxx–xxx 3

Fig. 2. X-ray diffraction patterns of YAG: Eu3+_{phosphors prepared via the solid-state} reaction method followed by (a) calcination at (I) 800◦_{C, (II) 1000}◦_{C, (III) 1200}◦_C, (IV) 1400◦_{C and (V) 1500}◦_{C for 4 h, and (b) repeated grinding and heating at 1600}◦_C for (VI) one time, (VII) two times, and (VIII) three times for 4 h.

nation process was performed at 1600◦C for the second time, the amount of orthorhombic YAP signiﬁcantly decreased (Fig. 2(VII)). As the calcination process was carried out at 1600◦C for three times, orthorhombic YAP completely disappeared and pure YAG: Eu3+_{phosphors were obtained as shown in}_{Fig. 2}_{(VIII), with the} evidence that the recorded XRD pattern of pure YAG: Eu3+_powders matched well with the standard powder diffraction ﬁle of YAG in the ICDD database (No. 33-0040)[18].

3.2. Reaction routes and morphology of the microemulsion-derived YAG: Eu3+_phosphors

Fig. 3 illustrates the X-ray diffraction patterns of the microemulsion-derived YAG: Eu3+_{precursors calcined at various} temperatures. For the sample calcined at 500◦C as shown in Fig. 3(I), no characteristic XRD peak was observed implying the absence of crystallized phase. After calcination at 1000◦C, YAG began to form with the coexistence of monoclinic YAM and hexago-nal YAP (Fig. 3(II)). For the 1100◦C-calcined sample, hexagonal YAP disappeared, and YAG coexisted with YAM. With a further increase in the heating temperature, the amount of YAM decreased along with an increase in the amount of YAG. When the heating temper-ature reached 1400◦C, pure YAG phase was successfully obtained.

It was found that the main intermediate compounds formed in the microemulsion-derived precursors and solid-state-reaction-derived precursors were YAM and YAP. In addition, the microemulsion-derived precursors yielded phase-pure YAG after calcination at 1400◦C, which is 200◦C lower than that required in the solid-state-reaction process. The lowering of the forma-tion temperature of pure YAG is considered to be attributed to the enhanced homogeneity of constituent species and increased

Fig. 3. X-ray diffraction patterns of YAG: Eu3+ _{phosphors prepared via the} microemulsion process followed by calcination at (I) 500◦_{C, (II) 1000}◦_{C, (III) 1100}◦_C, (IV) 1200◦_{C, (V) 1300}◦_{C and (VI) 1400}◦_{C in air for 4 h.}

reactivity in the microemulsion-derived precursors. In compari-son with the sol–gel process[19], the microemulsion process needs to increase the heating temperatures for obtaining the pure com-pound. Heating at elevated temperatures results in improvement in crystallinity.

Fig. 4(a)–(c) shows the morphology of YAG: Eu3+ _precursors prepared via the microemulsion process followed by calcination at various temperatures. The formed powders mainly comprised spherical and uniformly submicron-sized particles. As the calci-nation temperature was raised from 1000 to 1400◦C, the average particle size enlarged from 100 to around 300 nm (seeFig. 4(a) and (c)). On the other hand, the powders obtained via the solid-state reaction exhibited irregular morphology and large particles size (around 1–3␮m) as shown inFig. 4(d). It is revealed that the microemulsion process effectively led to reduced particle size and improved morphology of YAG: Eu3+_{powders. The coarsening and} agglomeration of grains occurring in the solid-state reaction was a result of high-temperature calcination and prolonged heating.

3.3. UV-excited luminescence characteristics of microemulsion-derived YAG: Eu3+_phosphors

Fig. 5illustrates the photoluminescence excitation spectra of the microemulsion-derived YAG: Eu3+_{precursors calcined at various} temperatures. Monitoring at 590 nm yielded a band with a max-imum at 238 nm for the sample calcined at 1000◦C (EM-10) as depicted inFig. 5(a). This band was attributed to the presence of the YAG phase and the appearance of the excitation spectra con-ﬁrmed the entrance of Eu3+ _{ions into the crystallographic sites} of YAG. When the calcination temperatures were raised to 1200 and 1400◦C, similar excitation spectra were observed for samples EM-12 and EM-14 as illustrated inFig. 5(b) and (c). However, the intensities of the peaks in the excitation spectra increased with heating temperature due to the increase in the amount of YAG phase

(4)

ARTICLE IN PRESS

G Model

Fig. 4. SEM photographs of the microemulsion-derived YAG: Eu3+_{phosphors with calcination at (a) 1000}◦_{C, (b) 1200}◦_{C and (c) 1400}◦_{C for 4 h, and (d) the solid-state-derived} YAG: Eu3+_{phosphors with calcination at 1600}◦_{C for three times.}

as shown inFig. 3. The 395-nm peak in the excitation spectra is asso-ciated with the intra-conﬁgurational 4f6 _{transitions of Eu}3+_ions [20]. In addition, the band observed in the region of 200–275 nm corresponds to the charge transfer band (CTB) of Eu3+_–O2−_[21]_.

Fig. 6illustrates the emission spectra of microemulsion-derived YAG: Eu3+_{calcined at various temperatures. For the sample calcined} at 1000◦C (EM-10) as shown inFig. 6(a), the 238-nm excited spec-trum reveals the coexistence of YAM: Eu3+_{and YAG: Eu}3+_{. The main} peaks at 590 and 613 nm correspond to the5_D

0→7F1transition of Eu3+_{in YAG and the}5_D

0→7F2transition of Eu3+in YAM, respec-tively. For the sample calcined at 1200◦C (EM-12) as depicted in Fig. 6(b), the similar emission spectrum was observed; however, an increase in the intensity of the 590-nm peak and a decrease in the intensity of 613 nm peak were also found. This indicates that the amount of YAG was increased and the amount of YAM was cor-respondingly reduced. As the temperature was raised to 1400◦C (EM-14), only the spectrum belonging to YAG was observed and the intensities of the peaks were signiﬁcantly increased. The dom-inant peak at 590 nm corresponds to the5_D

0→7F1transition and the other peak at 608 nm is related to the5_D0_→7_F2_transition_[22]_. The intensity of the transitions between different J levels depends on the symmetry of the local environment of the Eu3+ activators[23]. According to the transition selection rulesJ = 0±1 and S = 0, magnetic dipole transition (5_D0_→7_F1_{) is allowed,} and electric dipole transition (5_D

0→7F2) is forbidden. However, in some cases in which Eu3+ _{activators occupy sites without} inversion symmetry, the parity forbiddance is not strictly main-tained and the spectra resulting from the electric dipole transition

(5)

ARTICLE IN PRESS

G Model

C.-H. Lu et al. / Journal of Alloys and Compounds xxx (2008) xxx–xxx 5

Fig. 6. Photoluminescence emission spectra upon excitation at 238 nm for the microemulsion-derived YAG: Eu3+_{phosphors calcined at various temperatures.}

3.4. VUV-excited luminescence characteristics of the solid-state-reaction-derived and microemulsion-derived YAG: Eu3+_phosphors

The excitation spectra in the VUV region monitored at 590 nm were recorded for YAG: Eu3+_{phosphors prepared via the} solid-state reaction (SS-16) and microemulsion route (EM-14). Similar

Fig. 7. Photoluminescence excitation spectra of YAG: Eu3+_{phosphors (a) EM-14 and} (b) SS-16, monitored atemission= 590 nm and recorded via a synchrotron radiation facility.

Fig. 8. Photoluminescence emission spectra of YAG: Eu3+_{phosphors (a) EM-14 and} (b) SS-16, upon VUV-light excitation and recorded via a synchrotron radiation facil-ity.

excitation characteristics are observed inFig. 7. The excitation spec-tra result from the 4fn−1_{–5d transition of Eu}3+_{, host absorption,} excitonic transition and charge transfer. The band gap of YAG is reported to be 6.5 eV (190 nm)[23]. Taking into consideration of the energy of the onset of host absorption, the excitation spectra can be divided into two regions: (i) charge transfer absorption above 190 nm, and (ii) fundamental absorption below 190 nm. However, the relative intensities of these two regions reveal a considerably large difference between the samples prepared via these two meth-ods. The excitation spectrum of the microemulsion-derived YAG: Eu3+_{reveals a strong absorption in the fundamental absorption} region. During the solid-state reactions, grinding processes were repeated for several times. Defects were formed on the surface of powders during grinding. It is considered that formed defects in the solid-state-derived powders cause to reduce the fundamental absorption.

Fig. 8 shows the emission spectra of YAG: Eu3+ _phosphors synthesized via the two processes, under excitation at 147 and 173 nm with VUV light. Similar emission spectra in the 580 to 700 nm region are observed for powders prepared via both pro-cesses. The major emission peak near 590 nm is attributed to the 5_D0_→7_F1_{magnetic dipole transition of Eu}3+_{ions, while the minor} peaks at 608 nm is assigned to5_D

0→7F2 electric dipole transi-tions[21]. The intensity ratio (R = I608/I590) between5D0→7F2(at 608 nm) and 5_D0_→7_F1 _{(at 590 nm) transitions can be used to} evaluate the site symmetry for Eu3+_._[23] _{It was found that the} microemulsion-derived powders had a larger R value (R = 3.9) than the solid-state-derived powders (R = 3.7), suggesting greater distor-tion of the local environment of Eu3+_{ions in microemulsion-derived} YAG. The probability of5_D

0→7F2transition is increased as a result of this distortion. The emission intensity of the microemulsion-derived phosphors was found to be higher than that of the solid-state-derived phosphors. It is attributed to the decreased particle size and improved morphology of the microemulsion-derived powders. In the solid-state reaction, the prolonged heating promotes agglomeration and coarsening of grains, and repeated

(6)

ARTICLE IN PRESS

G Model

grinding produces defects on the surface of the phosphors, thereby increasing non-radiative relaxations and causing a decrease in the emission efﬁciency. Conclusively, the microemulsion process was conﬁrmed to be a potential method to prepare YAG: Eu3+_phosphors with improved photoluminescence properties.

4. Conclusions

Y3Al5O12: Eu3+(YAG: Eu3+) phosphors were prepared via both the solid-state reaction and microemulsion process. The solid-state reaction required high-temperature heating to prepare pure garnet phase, and the formed particles exhibited irregular morphology with a broad particle-size distribution. On the other hand, the microemulsion route effectively lowered the synthesis tempera-ture of pure garnet phase to 1400◦C and reduced the particle size to submicron order. The intensities of the peaks in the excitation and emission spectra markedly increased with the calcination tempera-ture owing to an increase in the amount of YAG phase. According to the results of the synchrotron radiation-excited luminescence stud-ies, it is conﬁrmed that YAG: Eu3+_{exhibited broad excitation peaks} in VUV range and strong red emission peaks at 590 nm. In addition, the microemulsion-derived YAG: Eu3+_{phosphors were found to} generate more intense luminescence than the solid-state-derived powders. The reduced particle size and improved morphology in the microemulsion-derived phosphors are regarded as the factors for enhancing the luminescence characteristics.

Acknowledgement

The authors would like to thank Mr. Hong-Kai Chen and Miss Hsiao-Chi Lu at the National Synchrotron Radiation Research Center, Taiwan for their help in measuring the VUV excitation spectra.

References

[1] K. Nonomura, H. Higashino, R. Murai, Mater. Res. Bull. 11 (2002) 898. [2] X. Zhang, Q. Li, Y. Tu, Y. Tang, J. Xia, Y. Zheng, B. Wang, H. Yin, L. Tong, SID 02

Digest 33 (2002) 748.

[3] Y.C. Kang, I.W. Lenggoro, S.B. Park, K. Okuyama, Mater. Res. Bull. 35 (2000) 789.

[4] K. Betsui, F. Namiki, Y. Kanazawa, H. Inoue, Fujitsu Sci. Tech. J. 35 (1999) 229.

[5] K. Yoshikawa, Y. Kanazawa, M. Wakitani, T. Shinoda, A. Otsuka, Tech. Rep. Inst. TV Eng. Jpn. 16 (1992) 17.

[6] J. Koike, T. Kojima, R. Toyonaga, A. Kagami, T. Hase, S. Inaho, J. Electrochem. Soc. 126 (1979) 1008.

[7] K.C. Mishra, M. Raukas, G. Marking, P. Chen, P. Boolchand, J. Electrochem. Soc. 152 (2005) H183.

[8] D.W. Cooke, J.K. Lee, B.L. Bennett, J.R. Groves, L.G. Jacobsohn, E.A. McKigney, Appl. Phys. Lett. 88 (2006) 103108.

[9] D.Y. Lee, Y.C. Kang, H.D. Park, S.K. Ryu, J. Alloy. Compd. 353 (2003) 252. [10] P. Thiyagarajan, M. Kottaisamy, M.S. Ramachandra Rao, J. Electrochem. Soc. 154

(2007) H297.

[11] E.J. Popovici, L. Muresan, H. Amalia, E. Indrea, M. Vasilescu, J. Alloy. Compd. 434–435 (2007) 809.

[12] K.S. Sohn, I.W. Zeon, H. Chang, S.K. Lee, H.D. Park, Chem. Mater. 14 (2002) 2140.

[13] M. Sekita, H. Haneda, S. Shirasaki, T. Yanagitani, J. Appl. Phys. 69 (1991) 3709. [14] J.A. Koningstein, Phys. Rev. 136 (1964) A717.

[15] Powder Diffraction File, Card No. 33-0041, Joint committee on powder diffrac-tion standards, Swarthmore, PA.

[19] C.H. Lu, W.T. Hsu, C.H. Hsu, H.C. Lu, B.M. Cheng, J. Alloys Compd. 456 (2008) 57.

[20] B.G. Wybourne, Spectroscopic properties of rare-earths, Interscience, New York, 1965.

[21] Y.H. Zhou, J. Lin, S.B. Wang, H.J. Zhang, Opt. Mater. 20 (2002) 13.

[22] D. Ravichandran, R. Roy, A.G. Chakhovskoi, C.E. Hunt, W.B. White, S. Erdei, J. Lumin. 71 (1997) 291.

[23] C.W. Thiel, H. Cruguel, Y. Sun, G.J. Lapeyre, R.M. Macfarlane, R.W. Equall, R.L. Cone, J. Lumin. 94 (2001) 1.