Materials Science and Engineering A334 (2002) 246–249

Synthesis of nanometer Y2O3:Eu phosphor and its luminescenceproperty

Junying Zhang *, Zilong Tang, Zhongtai Zhang, Wangyang Fu, Jin Wang,Yuanhua Lin

Department of Materials Science and Engineering, State Key Laboratory of New Ceramics and Fine Processing, Tsinghua Uni�ersity,Beijing 100084, People’s Republic of China

Received 29 May 2001; received in revised form 10 September 2001

Abstract

Metal nitrate aqueous solution was mixed with citric acid to synthesize the red emission nanometer Y2O3:Eu phosphor bysimple sol–gel processing. Structure characterization of the phosphor was carried out by XRD and TEM. The samples are mostlyamorphous when the dry gel is calcined below 500 °C and crystallize completely at 600 °C, and the crystallity increases withincreasing calcining temperature. The mean particle size increases from about 10 to 50 nm when the calcining temperatureincreases from 600 to 900 °C, while that calcined at 1250 °C is about 100 nm. The luminescence property of the phosphor wasanalyzed by measuring the excitation and emission spectra. The luminescence intensity increases when the calcining temperatureincreases and luminescence intensity is higher than that prepared by solid-state reaction method when the concentration of Eu3+

and the calcining temperature are the same. © 2002 Elsevier Science B.V. All rights reserved.

Keywords: Nanometer materials; Phosphors; Sol–gel; Luminescence

www.elsevier.com/locate/msea

1. Introduction

Y2O3 is an excellent substrate for phosphors becauseof its chemical stability. The Y2O3:Eu as an efficientred-emission phosphor has been used in fluorescentlights (FL). Recently, it has been applied widely in highresolution (HR) and projection TVs (PTVs), projectiondevices and low voltage display such as cathode raytube (CRT), plasma display panels (PDP) and fieldemission displays (FED) [1–5]. Traditionally, Y2O3:Euphosphor are prepared by high-temperature solid-statereaction at 1400–1500 °C for several hours [6,7]. Thephosphor particle synthesized by this method is massiveand the grain size is very big and must be ground ormilled to get finer powder. The luminescence efficiencyof the phosphors greatly decreases in this process andthe morphology of the grain is changed. Through thesol–gel process, it is possible to synthesize fine-grain

phosphors. It is easy to control the composition and ahigh degree of uniformity is available. The active pre-cursors result in low calcining temperatures, minimizingthe potential for cross contamination [8–12].

In this paper nanometer Y2O3:Eu was synthesized bya simple sol–gel process. Y2O3 and Eu2O3 were dis-solved in diluted nitric acid and citric acid was em-ployed as a chelating agent. The phosphor wascharacterized by XRD, TEM and spectrofluorometer.

2. Experimental

The starting materials are Y2O3, Eu2O3 (99.99%),citric acid and nitric acid (AR). Fig. 1 is the flowscheme for the process of preparing Y1.9O3:Eu0.1 phos-phor. Initially, 30 g amount of Y2O3 and Eu2O3 (10at.%) were reacted with diluted HNO3 to get aqueoussolution and the amount of HNO3 is just enough todissolve Y2O3 and Eu2O3. The citric acid was thenadded in this solution with the 2:1 molar ratio of themetal ions and citric acid. The concentration of metal

* Corresponding author. Tel.: +86-10-6277-2623; fax: +86-10-6277-1130.

E-mail address: zzt@tshinghua.edu.cn (J. Zhang).

0921-5093/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved.PII: S0921 -5093 (01 )01812 -3

J. Zhang et al. / Materials Science and Engineering A334 (2002) 246–249 247

ions in the so-obtained solution is 1 mol/l, and pH isabout 0.5. The solution was heated at 95 °C for about2 h while stirring to get transparent gel. The gel wasdried at 95 °C for about 1 h to get a gray fluffy

powder. The dry gel was calcined at different tempera-ture for 3 h to get the phosphor powder.

For comparison, Y1.9O3:Eu0.1 was also prepared bytraditional solid-state reaction method. Y2O3 andEu2O3 were mixed in stoichiometric amount and H3BO3

(2 wt.%) was added in the mixture in order to decreasethe synthesis temperature. The powder was milled for 4h in water using agate-grinding media in a nalgenecontainer. The milled slurry was dried and then granu-lated through a 200 mesh screen. Later the powder wascalcined at 1300 °C for 3 h to get phosphor powder.

X-ray diffraction patterns were obtained with aRigakuD/MaxIIIB X-ray diffractometer (XRD) usingCuK� radiation. A Hitachi H-80 transmitting electronmicroscopy (TEM) was used to observe the morphol-ogy of the particles. The excitation and emission spec-tra of the phosphor powders were recorded using aHitachi M850 spectrofluorometer. Each excitation andemission spectra were corrected using sodium salicylate.For comparison of the phosphor powder, both excita-tion and emission spectra were measured at a fixedband pass of 0.5 nm with the same instrument parame-ters. The emission spectra were excited at the wave-length of highest peak of excitation spectra.

3. Results and discussion

XRD patterns for the powders obtained by calciningthe dry gel at different temperatures for 3 h are shownin Fig. 2. The powders calcined at 500 °C are mostlyamorphous with only weak evidence of crystallinity.When the gel was calcined at 600 °C, there are nopeaks from Eu2O3, so single phase of Y2O3 is obtainedwhile Eu2O3 enters Y2O3 crystal lattice. The crys-tallinity of the phosphor increases with increasing cal-cining temperature up to 1250 °C. The line broadeningis obvious resulting from very fine crystal size andcrystallite size can be calculated using the Scherrerequation shown in Eq. (1) [13].

D=0.9�/� cos(�) (1)

Where D is the crystal size of the powder, � is thewavelength of the light, � is the full-width in radiationat half-maximum (FWHM) of the peak, and � is theBragg angle of the X-ray diffraction peak. A well-crys-tallized Y2O3 is employed as standard for instrumentpeak calibration.

The FWHM and crystal size of the phosphor cal-cined at different temperatures are shown in Table 1.The average crystal size of the phosphor powder is 14nm for the powders calcined at 600 °C, and that cal-cined at 900 °C is about 80 nm. As the calciningtemperature increases, the crystal size increases and thatcalculated for the powders calcined at 1100 °C is about100 nm, while that at 1250 °C is about 250 nm.

Fig. 1. Synthesis procedure of Y2O3:Eu phosphor.

Fig. 2. XRD patterns of the powder calcined at different tempera-tures.

Table 1FWHM of the XRD patterns and calculated crystal size of thephosphor

Calcining temperature (°C)1250600 1100900700

FWHM (°) 0.380.76 0.190.230.25253.0113.680.235.4Crystal size (nm) 14.1

J. Zhang et al. / Materials Science and Engineering A334 (2002) 246–249248

Fig. 3. TEM micrograph of the phosphor calcined at different temperatures: (a) 600 (b) 700 (c) 900 and (d) 1250 °C.

Fig. 3 is the TEM micrograph of the phosphorobtained by calcining the dry gel directly at differenttemperatures. When the gel was calcined at 600 °C, thepoor crystallity results in very small crystal size and theaverage crystal size is about 10 nm. When the calciningtemperature increases, the crystal size increases and thephosphor particles have regular shape. The averagecrystal size increases to about 50 nm at 900 °C which isin accordance with the results obtained by XRD dif-

fraction patterns. When the calcining temperature is upto 1250 °C, the average crystal size increases to about100 nm which is smaller than that calculated by XRDdiffraction patterns. The reason for the difference maybe due to the fact that the crystal size of the standardsamples for the X-ray diffraction measurement is notlarge enough.

Fig. 4 shows the excitation and emission spectra ofY2O3:Eu phosphor. The excitation spectrum of the red

J. Zhang et al. / Materials Science and Engineering A334 (2002) 246–249 249

Fig. 4. Excitation and emission spectra of the phosphor.

makes the Eu3+ enter the Y2O3 crystal lattice moreeasily, so the activator concentration is higher althoughthe same amount of Eu2O3 is doped in the Y2O3

substrate. Fig. 5 shows the luminescence intensity as afunction of calcining temperature. The luminescenceintensity increase as the calcining temperature increases,this may be due to the fact that good activation andhigh crystallity are obtained at high temperatures.

4. Conclusions

This study has analyzed the synthesis process ofY2O3:Eu phosphor by a sol–gel process. Based on theresults presented herein, the following can be con-cluded.1. Nanometer Y2O3:Eu phosphor can be synthesized

by a simple sol–gel process.2. The phosphor crystallizes completely at 600 °C and

the average size is about 100 nm even when thecalcining temperature is up to 1250 °C. The lu-minescence intensity increases when the calciningtemperature increases.

3. Emission intensity of the phosphor obtained bysol–gel processing is higher than that prepared bysolid-state reaction method. The phosphor obtainedby sol–gel process is fine enough for applicationwithout grinding or milling.

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Fig. 5. Emission intensity as a function of calcining temperature.

fluorescence (�=612 nm) shows a wide band with thepeak at about 233.5 nm which is attributed to transitiontoward the charger transfer state (CTS) due to Eu�Ointeraction. The emission spectrum excited by 233.5 nmUV consists of lines in the red spectral area. These linescorrespond to transition from the excited 5D0 level to7FJ (J=0, 1, 2, 3, 4) level of the 4F6 configuration ofthe Eu3+ ion [14]. The most intense line at 612 nmcorresponds to the hypersensitive transition between the5D0 and 7F2 level of the Eu3+ ion. The spectra of thenanometer Y2O3:Eu phosphor are similar to that ob-tained by traditional solid-state reaction method withthe same calcining temperature and the same concen-tration of Eu3+, but the luminescence intensity of theformer is higher than that of the latter. The phosphorparticle synthesized by sol–gel processing has regularshape, and scattering of light evolved from the phos-phor decreases, so the luminescence intensity is higher.Furthermore, the high surface of the nanometer powder