Preparation and characterization of garnet phosphor nanoparticles derived from oxalate coprecipitation

Article ID jssc.1999.8202, available online at http://www.idealibrary.com on

Preparation and Characterization of Garnet Phosphor Nanoparticles

Derived from Oxalate Coprecipitation

Teng-Ming Chen,

 S. C. Chen, and Chao-Jung Yu

Received October 7, 1998; in revised form February 2, 1999; accepted February 5, 1999

Terbium-activated Y3Al5O12 (YAG : Tb) phosphor

nanopar-ticles with homogeneous grain size and crystallinity have been prepared at 100033C by heat treatment of a metal oxalate precur-sor derived from an alkaline coprecipitation route. The di4rac-tion pro5le of as-prepared YAG : Tb nanoparticles can be indexed as a garnet structure and exhibits peak broadening phenonmenon, as revealed by X-ray di4raction (XRD) data. Grains of YAG : Tb nanoparticles appear to be irregularly spherical or elliptical and their sizes range from 60 to 70 nm, as indicated by morphological studies from bright-5eld transmis-sion electron microscopy (TEM) imaging. Furthermore, the photoluminescence (PL) spectra of the (Y2.9Tb0.1)Al5O12 phase

were investigated to determine the energy level of electron transition related to luminescence processes and the possible blue shift in the excitation or emission PL spectra.  1999 Academic Press

Key Words: (Y, Tb)3Al5O12; phosphor nanoparticles; oxalate

coprecipitation; photoluminescence.

INTRODUCTION

Rare-earth activated YAlO (YAG: R) garnet phases have shown considerable potential as rugged phosphors and scintillation materials because they exhibit a thermally stable lattice, a well-determined crystal structure and, most importantly, they resist saturation at high current excitation (1}3). At high electron excitation power densities (P), the light emitted from phosphors generally levels o!, mainly due to material heating and the associated luminescence quenching problems (1). For instance, the cathodolumines-cence (CL) intensity of D emission (j"544 nm) of epi-taxial YAG "lms doped with Tb> was observed to be nonlinear as a function of P under electron bombardment (2). The CL intensity of the YAG : Tb specimens exhibit deviation from linearity and saturate at P exceeding 10 watt/m, corresponding to an excitation current density

of 3;10\ amp/cm (2, 3). On the other hand, YAG : R

To whom correspondence should be addressed.

phosphors are typically synthesized by solid-state reactions between component oxides with or without #uxes, which generally require prolonged heat treatment at elevated temperature and repeated grinding or milling to assure

product purity and composition homogeneity (4, 5).

Other routes leading to the formation of various R-doped YAG's have also been investigated extensively, namely, cop-recipitation (6, 7), sol}gel (8, 9), chemical vapor deposition (10), and thermal pyrolysis of metallorganic precursors (11). On the other hand, nanoparticles of yttrium iron garnet (YIG) were recently synthesized by thermal pyrolysis of ethylene glycol/citrate gel precursors (12), but the preparation of nanostructured phosphors for potential commercial applications has rarely been investigated and related synthetic conditions were not well established. Bhargava et al. reported nanocrystals of Mn-doped ZnS with sizes varying from 3.5 to 7.5 nm that can yield both high luminescent e$ciencies and exhibit signi-"cant lifetime shortening (13). On the other hand, Zhang

et al. prepared nanocrystalline YSiO: Eu and discovered

that both luminescent intensity and quenching

concentra-tions of Eu> were enhanced in YSiO:Eu

nano-particles (14). Our research interests in synthesizing YAG : Tb nanophases are motivated by the attempt to improve the luminescence e$ciency and study the variation of lifetime of phosphors used for cathode ray tube (CRT) screens and high-de"nition projection television (HDTV).

Among the large number of YAG : R phosphors reported, Tb> has long been selected as an activator mainly because of its simple structure of ground and excited energy levels (15). Concentration quenching (16) and cross-relax-ation e!ects (17) in the YAG : Tb system have also been thoroughly investigated. In this paper we demonstrate a methodology for the synthesis of a series of YAG : Tb phases and report the microstructure and photolumines-cence (PL) spectra of the as-prepared YAG : Tb nanopar-ticles on the basis of investigations on X-ray di!raction (XRD), transmission electron microscopy (TEM), and PL spectroscopy.

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FIG. 2. The XRD pattern of products derived from calcination and sintering of metal oxalate coprecipitates for (a) 12 h, (b) 16 h, (c) 20 h, and (d) 24 h, respectively.

FIG. 1. _{Synthesis of (Y\VTbV)AlO phosphor nanoparticles by} al-kaline oxalic acid (OA)-triethylamine (TEA) coprecipitation.

SYNTHESIS OF YAG : Tb PHOSPHORS

High-purity _{Tb(NO)) 6HO, Y(NO) )5HO and}

Al(NO)) 6HO with a cationic molar ratio for Tb : Y :Al

of x : (3!x) : 5 (0.05(x(3) and 2.5 mmol of Y> were dissolved in 25 ml deionized water. An aqueous solution of coprecipitant was prepared by dissolving 0.025 mol of oxalic acid (OA) in 10 ml of deionized water. The pH of OA solution was then adjusted to an optimal value of 10.2 by adding 25 ml of triethylamine (TEA). The solution of the metal nitrates was then added dropwise into the OA}TEA solution with vigorous stirring. During mixing a white pre-cipitate formed and the solution was then cooled in an ice-water bath. After separation of the precipitate and "l-trate, the white gel-like precipitate was then dried and the obtained precipitate was subsequently calcined at 3003C followed by another thermal treatment at 5003C for 1 h, respectively, and then sintered at 10003C for 24 h in air. The preparation of YAG : Tb nanoparticles is summarized in a #ow diagram and represented in Fig. 1. The resulting "ne powder used for XRD, TEM imaging, and PL spectra measurements was obtained by milling the calcined prod-ucts with a Retsch MM 2000 mixer mill.

The X-ray di!raction (XRD) patterns of Tb-doped YAG phases were measured on a MAC Science MXP-3 auto-matic di!ractometer using graphite-monochromatized and Ni-"ltered CuKa radiation. The specimens for bright-"eld transmission electron microscopy (TEM) were obtained by ultrasonically dispersing the nanoparticles in ethanol, and dispersed drops of the specimens were then transferred onto a Cu grid covered with a carbon "lm. Subsequently, the morphological studies were carried out on a Hitachi H-600 electron microscope operating at 100 kV and with a magni-"cation of ca. 100,000X. The PL spectra of as-prepared

FIG. 3. Bright-"eld TEM images showing the morphology and the size distribution of as-prepared (Y Tb )AlO nanoparticles.

Tb-doped YAG nanoparticles were measured by using a

Shimadzu RF-5301PC spectro#uorophotometer. The

chemical compositions of cations in both precipitates and "ltrates were checked with a Perkin Elmer SCIEX ELAN 500 inductively coupled plasma atomic emission spectr-ometer (ICP-AES).

A series of (Y\VTbV)AlO phases with 0.05(x(3.0 were prepared from an oxalate precursor derived from an alkaline coprecipitation route with an optimal pH of 10.2 and at "nal sintering temperature of 10003C. However, for simplicity, only the results of our investigation on the phase with x"0.1 will be reported in this paper.

The evolution of XRD pro"les for phases derived from the metal oxalate precipitates with nominal composition (Y Tb )AlO as a function of calcination and sintering time is illustrated in Fig. 2. The products obtained from oxalate precursors after sintering at 10003C for 12 and 16 h were found to be poorly crystalline yttrium aluminates such as YAlO (JCPDS "le 34-368) and YAlO (JCPDS "le 16-219). Our observations are consistent with the observed sequence of formation of yttrium aluminates in the YO} AlO system as a function of temperature regardless of the YO: AlO molar ratio described by Kinsman et al. (18) However, with longer heat treatment at the same temper-ature for 20 h the di!raction pro"le of as-prepared sample can clearly be identi"ed as the YAG type structure (space group Ia3d, No. 230) and the (Y Tb )AlO phase exhi-biting strong XRD intensity was obtained only after sinter-ing at 10003C for 24 h. The broadensinter-ing of di!raction peaks reported in Fig. 2d is also a good indication of the intrinsi-cally nano-sized nature of as-prepared YAG : Tb phases. The preparation conditions described here are apparently much milder than those generally adopted by solid-state method (7). The indexed XRD pro"le for (Y Tb )AlO is then shown in Fig. 2d from which the cell parameters

a derived is 12.028(2) A> compared to 12.026(1) A> for pristine

YAlO. The results of our investigations on the forma-tion of (Y\VTb)AlO solid soluforma-tion with 0.054x (3.0 and the quenching e!ect of Tb> concentration on luminescence intensity have been reported elsewhere (19).

To investigate the conservation of cation stoichiometry in the process of coprecipitation, we have also carried

out ICP}AES analyses on the compositions of

(Y Tb )AlO samples. The typical loss of cations in the as-prepared (Y Tb )AlO sample has been determined

to be (5.54;10\, (1.70;10\, and (9.60;10\

mol% for Tb, Y, and Al cations (19), respectively, which is considered to be not detrimental to the performance of YAG : Tb phosphors.

The bright-"eld TEM images showing the morphology and the size distribution of (Y Tb )AlO nanoparticles are shown in Fig. 3. In all samples of as-prepared YAG : Tb phosphor grains are aggregated and the morphology ap-pears to be irregularly spherical or elliptical (without faceted

borders) and their sizes range from 60 to 70 nm, as estimated from TEM imaging micrographs. The attempt to further reduce the grain size of YAG : Tb nanoparticles by modify-ing the synthetic conditions (e.g., coprecipitation pH, rela-tive ratio of TEA/OA) is currently under investigation.

The ambient temperature photoluminescence (PL) spec-trum of (Y Tb )AlO phosphor nanoparticles sintered at 10003C for 24 h is illustrated in Fig. 4. The YAG : Tb phases are e$cient green phosphors emitting a typical Tb> line spectrum under UV (j"273 nm) excitation. The broad excitation peak centered at 273 nm is attributed to a 4f}5d

FIG. 4. _{Ambient temperature PL spectra of (Y Tb )AlO phosphor nanoparticles excited by UV radiation (j"273 nm).}

charge transfer (CT) transition (20) and is followed by typi-cal Tb> green emission. As a result of absorbing UV radiation, Tb> ion is excited to a 4f 5d state; it then decays stepwise from this state to the D or the D state which then returns to FH ( j"3,4,5,6) states by emitting visible light (20). No blue shift was observed in the excitation or emission PL spectra of the (Y Tb )AlO nanoparticles with respect to those of the (Y Tb )AlO phase syn-thesized from the solid-state route. The absence of a blue shift in the PL spectra could probably be attributed to size and agglomeration e!ects of the nanoparticles. The emis-sion peaks with wavelength below 490 nm originate from theD state to various F\ ground state levels (15). At greater wavelengths the major emission peaks appearing at 545 (strongest), 586, and 622 nm are attributed to the D state to F, F, and F, states, respectively.

CONCLUSION

Nanoparticles of YAG : Tb phosphor have been syn-thesized by employing an alkaline oxalate coprecipitation route at a "nal sintering temperature of 10003C, which is apparently much lower than that adopted by conventional

solid-state method. The grain size of as-prepared

(Y Tb )AlO nanoparticles was estimated to be 60 to 70 nm, as indicated by bright-"eld TEM morphological studies. We found that the characteristics of ambient tem-perature PL spectra for (Y Tb )AlO nanoparticles is comparable to those of YAG : Tb phases synthesized via

solid-state method. However, no blue shift in the excitation or emission PL spectra of the (Y Tb )AlO nano-particles was observed.

ACKNOWLEDGMENT

We are grateful for long-term "nancial support from National Science Council of Taiwan, R.O.C. under Contracts NSC86-2113-M-009-006 and NSC88-2113-M-009-013. The bright-"eld TEM micrographs were meas-ured at the Department of Materials Science and Engineering of NCTU.

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