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Journal of Luminescence 126 (2007) 503–507

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Hydrothermal synthesis and luminescent properties of LnPO4:Tb,Bi(Ln ¼ La,Gd) phosphors under UV/VUV excitation

Yuhua Wanga,�, Chunfang Wua, Jie Weib

aDepartment of Materials Science, School of Physical Science and Technology, Lanzhou University, Lanzhou, Gansu Province, 730000, PR ChinabKey Lab for Magnetism and Magnetic Materials of MOE, School of Physical Science and Technology, Lanzhou University, Lanzhou, Gansu Province,

730000, PR China

Received 21 March 2006; received in revised form 22 September 2006; accepted 25 September 2006

Available online 13 November 2006

Abstract

Monoclinic LnPO4:Tb,Bi (Ln ¼ La,Gd) phosphors were prepared by hydrothermal reaction and their luminescent properties under

ultraviolet (UV) and vacuum ultraviolet (VUV) excitation were investigated. LaPO4:Tb,Bi phosphor and GdPO4:Tb phosphor showed

the strongest emission intensity under 254 and 147 nm excitation, respectively, because of the different energy transfer models. In UV

region, Bi3+ absorbed most energy then transferred to Tb3+, but in VUV region it was the host which absorbed most energy and

transferred to Tb3+.

r 2006 Published by Elsevier B.V.

PACS: 78.55.�m

Keywords: VUV; Phosphor; Luminescence; Hydrothermal synthesis; Phosphate

1. Introduction

Plasma display panels (PDPs) have gained considerableattention for display in television sets and computers due totheir small bulk and for being completely flat. But PDPsstill have several problems that must be solved before theycan be used to replace the traditional cathode ray tube(CRT), such as the luminescent properties of the phos-phors. Their luminescent properties have a great effect onthe display quality. Three-primary-color phosphors appliedin PDPs are excited mainly by a 147 nm ray generated bythe discharge of mixed xenon/neon gas. Among the threephosphors, green phosphors are widely investigated, as thehuman eye perceives more toward the green in the visiblespectrum.

The commercial applied green phosphor is Zn2SiO4:Mn,which has a long decay time. Some researchers [1,2] havetried to decreased the decay time of Zn2SiO4:Mn by

e front matter r 2006 Published by Elsevier B.V.

min.2006.09.006

ing author. Tel.: +86931 8912772; fax: +86 931 8913554.

ess: wyh@lzu.edu.cn (Y. Wang).

increasing the Mn concentration, but got a decreasedluminescent intensity. Tb-activated green emitting lantha-num phosphate phosphors have been well studied and arewidely used in compact fluorescent lamps owing to theirquantum efficiency and stability at high temperatures [3,4].Also, compared to silicate, borate and oxide-basedstandard PDP phosphors the lanthanum phosphate-basedphosphors exhibit high absorption in VUV region. Therewere several researchers who have reported luminescentproperties of Eu3+ in GdPO4. Chen et al. [5] discussed theenergy transfer between Gd3+ and Eu3+ in GdPO4:Eu andLi and Hong [6] observed the high absorption of PO4

3� inVUV region. Also, Justel et al. [7] points out that rare earth(RE) phosphate is an efficient host under VUV excitation.As far as the photoluminescence of Tb-doped REphosphate phosphor is concerned, Lee et al. [8,9] investi-gated the influence of the precursor types of phosphorus onthe morphology and VUV characteristics of GdPO4:Tbphosphor particles prepared by spray pyrolysis. Wu et al.[10] expressively discussed the effect of Gd3+ on the energytransfer from the PO4

3� to the activators. In this paper, we

ARTICLE IN PRESSY. Wang et al. / Journal of Luminescence 126 (2007) 503–507504

prepared the Tb-doped LaPO4 and GdPO4 phosphors by alow temperature synthesis method-hydrothermal reaction,and in order to improve the luminescence intensity of theTb in phosphate, we also co-doped Bi3+ in the host. Theexcitation and emission properties of the phosphors in UVand VUV region were investigated.

2. Experimental procedure

The starting materials were Gd2O3 (99.9%), La2O3

(99.9%), Tb4O7 (99.9%), Bi2O3 (A.R.) and (NH4)2HPO4

(A.R.). Stoichiometric amount of each oxides weredissolved in diluted nitric by heating. Stoichiometric(NH4)2HPO4 was dissolved in distilled water and the(NH4)2HPO4 solution was then added to the RE ionsolution. The pH value of the clear mixed solution wascontrolled at 2 by using ammonia. Finally, the mixture wastransferred to a Teflon-lined stainless steel autoclave with afilling capacity of 40%. The hydrothermal reaction at240 1C for 6 h and then cooled to room temperature. Theobtained suspension was filtered, washed repeatedly withdistilled water and dried in an oven at 60 1C. Forcomparison, GdPO4:Tb was also prepared by the pre-cipitation method and treated at 900 1C for 2 h.

The crystal structure of samples was characterized by X-ray powder diffraction operating at 40 kV/60mA, usingmonochromatized CuKa radiation. The morphology of theproducts was characterized with a Hitachi S-4800 scanningelectron microscope (SEM). The excitation and emissionspectra were measured by a FLS920T spectrophotometerequipped with VM504 vacuum monochromator, and the

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Fig. 1. XRD pattern

VUV excitation spectra were corrected by the sodiumsalicylate at the same measurement conditions.

3. Results and discussion

The XRD pattern of Gd0.9Tb0.1PO4 is shown in Fig. 1.The XRD patterns of other samples were also similar.Gd0.9Tb0.1PO4 had a monazite structure with a monoclinicsystem and its cell parameters were a ¼ 0.6658 nm,b ¼ 0.6850 nm, c ¼ 0.6358 nm and a ¼ 90.01, b ¼ 104.21,g ¼ 90.01. According to Ref. [11], Bi3+(r ¼ 0.103 nm) hasan ionic radius similar to Gd3+(r ¼ 0.094 nm) and La3+

(0.103 nm), which provide a chance for Bi3+ to substituteGd3+ or La3+ ion. Also, Van Zon et al. [12] found thatBi3+ ion in LaPO4-Bi occupies the La3+ site by analyzingthe EXAFS data. In order to show the final concentrationof Bi3+ in our product, the metal content in the sampleLa0.85Tb0.10Bi0.05PO4 was determined by the inducedcoupled plasma (ICP) spectroscopy (IRIS Advantage,Jarrell-Ash, USA). The final concentration of La, Tb andBi are 88.0, 7.9 and 4.1mol%, respectively. In the XRDpattern of the Bi doped phosphate in our work, no otherphase was observed except for the monoclinic phase. So Biwas successfully doped into the host.Fig. 2 shows the UV excitation spectra of Gd0.9Tb0.1PO4

(GT), Gd0.85Tb0.1Bi0.05PO4 (GTB), La0.9Tb0.1PO4 (LT)and La0.85Tb0.1Bi0.05PO4 (LTB). There were two strongexcitation bands peaking at 246–243 nm in the phosphorsLTB and GTB. According to the results of Refs. [13,14],they can be ascribed to the 1S0-

3P1 transition of Bi3+. It isindicated that an energy transfer occurred from Bi3+ toTb3+ in LnPO4 (Ln ¼ La,Gd), and the transfer efficiency

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s of GdPO4:Tb.

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0.0

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271

246

GTB

LTB

GT

LT

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nsity

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Fig. 2. UV excitation spectra of GT, GTB, LT and LTB monitored at

543 nm.

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LTBIn

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a.u.

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Fig. 3. Emission spectra of GT, GTB, LT and LTB under 254 nm

excitation.

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Fig. 4. VUV excitation spectra of GT, GTB, LT and LTB monitored at

543 nm.

Y. Wang et al. / Journal of Luminescence 126 (2007) 503–507 505

in LTB was higher than in GTB. The 1S0-3P1 transition of

Bi3+ in LTB had a red shift compared to GTB. The reasonwas that La3+ has a rather bigger electronegativity (1.09)than Gd3+ (0.91) [15], and the 1S0-

3P1 transition of Bi3+

needed less energy in LTB. The 4f 8-4f75d transition ofTb3+ ions between 220 and 260 nm was covered up by the1S0-

3P1 transition of Bi3+. So the 4f 8-4f75d transitionof Tb3+ could not be made out in LTB and GTB. But itcould be seen distinctly in LT and GT. The features of thecrystal-field-split five 5d-levels of Tb3+ in LnPO4

(Ln ¼ La,Gd) are same as Nakazawa [16] reported. Usingthe formulation proposed in this reference:

W fdðnÞ ¼ V fd1 þ ufdðn1Þ þ DUðnÞ,

and the data listed in it, we got Wfd(8) ¼35.0+0.89� 8+2.5 ¼ 44.62(1000 cm�1) (The value isequal to 224 nm) in LaPO4. (Where Wfd(n) is the energyof the lowest f–d transition of f n-series ions, the sum of thefirst two terms on the right-hand side of equation,Vfd1+ufd?(n�1), is the one-electron-scheme transitionenergy of the ions, from the 4f–5d orbits. The value of thesecond term, DU(n), is the correction for the many-electroneffects due to exchange and spin–orbit interactions between4f-electrons.) In Fig. 2, the f–d transition of Tb3+ began at228 nm, which was close to the calculated value mentionedabove. A few of the small peaks from 280 to 380 nm wereascribed to the 4f-4f transition of Tb3+ ions. Theexcitation spectroscopic characteristics of the trivalentRE ions are due to the transitions between the levels ofthe 4f n electronic configuration of the trivalent RE ion andthe levels of the 4f n�15d electronic configuration, where a4f electron is promoted to a 5d localized level [17]. The4f n-4f n�15d electronic transitions are characterized bystrong Frank-Condon factors with broadband absorptionspectra in the VUV and UV. On the contrary, theintraconfigurational 4f n-4f n transitions are parity for-bidden. They are forced by the crystal field configurationmixing and they appear to be weak and sharp. So the f–dtransition or f–f transition of RE ion can be judged by the

width of the excitation spectra, and the assignment of theabove mentioned are in accordance with the rule. A peak at271nm in GT was prominent, which was due to the 8S7/2-

6I7/2 transition of Gd3+ ions. It was an evidence to showthe occurrence of the energy transfer from Gd3+ to Tb3+.Fig. 3 gives the emission spectra of GT, GTB, LT, LTB

under 254 nm excitation. There were four peaks at 487,543, 583 and 621 nm, respectively, which were 5D4-

7FJ

(J ¼ 3, 4, 5, 6) transition emission of Tb3+ ions. The5D3-

7FJ (J ¼ 3, 4, 5, 6) transition emission of Tb3+ ionswere not observed in these samples for the occurrence ofthe radiationless relaxation from 5D3 state to lower 5D4

level that is called cross-relaxation [18]. The emissionintensity was in the order LTB4GTB4LT4GT, whichwas in accordance with the excitation spectra.Fig. 4 shows the VUV excitation spectra of GT, GTB,

LT and LTB monitored at 543 nm. The main band peaking

ARTICLE IN PRESSY. Wang et al. / Journal of Luminescence 126 (2007) 503–507506

at about 142 nm in GT and LT really corresponded to thehost-related absorption band because its position did notchange with the activators. Theoretical calculation [19]showed that the lowest intra-molecular 2t2-2a, 3t2transition energy of the tetrahedral PO4

3� molecule wasaround 7–10 eV (124–177 nm). These results coincided withthe energy we observed. The intensity of host-relatedexcitation band peaking at 142 nm in the GT was higherthan that in the LT, which suggested that the energytransfer from host to Tb3+ was higher in the former than inthe latter for the presence of the Gd3+ [20]. Similarphenomenon was observed in LnCa4O (BO3)3:Tb

3+(Ln ¼La,Gd), and the samples of GdCa4O(BO3)3:Tb

3+ showhigher host excitation efficiency than LaCa4O(BO3)3:Tb

3+

[21]. The author interpreted that it was because of theexistence of Gd3+, which had an effect on the hostabsorption and can transfer the excitation energy to theluminescent center such as Tb3+. The intensity of hostexcitation band decreased much with doping Bi3+, asshown in Fig. 4. It could be explained that a hostsensitization competition took place between Bi3+ andTb3+ ions as Ref. [14], and it suggested that the addition ofBi3+ was bad to the 543 nm emission when excited by147 nm. Several small peaks in the 160–220 nm area wereascribed to the 4f 8-4f75d transition of Tb3+ ions, and thepeak at 235 nm to the 1S0-

3P1 transition of Bi3+ ions. Onesmall peak at 273 nm was observed that was due to the 8S7/2-

6I7/2 transition of Gd3+. The excitation intensity ofLTB was rather low compared to GT, GTB, and LT. Thereason was further investigated.

Fig. 5 shows the emission spectra of GT, GTB, LT andLTB under 147 nm excitation. Four peaks at 487, 543, 583and 620 nm could be seen, which were ascribed to5D4-

7FJ (J ¼ 3, 4, 5, 6) transition emission of Tb3+ ions.The emission intensity was ordered according toGT4GTB4LT4LTB. It showed that GT would be a

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583620

Fig. 5. Emission spectra of GT, GTB, LT and LTB under 147 nm

excitation.

promising green phosphor for PDPs. The Tb3+ emissionintensity order under 147 nm excitation was not the same asthe Tb3+ emission intensity order under 254 nm excitation,because the former used the Bi3+ but the latter used thehost transfer the excitation energy to Tb3+.The 147 nm excitation emission spectra of GT phosphor

prepared by hydrothermal reaction 240 1C/6 h (abbrevia-tion as HR) and precipitation method with heat treatmentat 900/2 h (PM) were also compared in Fig. 6. The twosamples showed almost the same emission intensity. Thissuggested that the hydrothermal reaction was an effectiveand cost-saving method because its reaction temperaturewas 660 1C lower than the precipitation method. Mean-while, the morphology of the phosphors prepared by HRand PM were shown in Fig. 7. As could be seen, theparticles of phosphor prepared by PM are irregular withsmall particles of about 0.3 mm and long particles in lengthof about 1.2 mm. Phosphor prepared by HR had approxi-mately a spherical shape and a narrow size distribution.The mean size was 0.8 mm. The use of spherical particlesmay increase packing densities and brightness thanirregular shaped particles. It could be concluded that HRis a better method to prepare phosphor for PDPs.

4. Conclusion

In order to find out an efficient green phosphor forPDPs, Tb3+ doped phosphate phosphors were prepared bymild hydrothermal reaction and their luminescent proper-ties under UV and VUV excitation were investigated.LaPO4:Tb,Bi phosphor had a strong excitation bandpeaking at about 246 nm and had an energy transfer fromBi3+ to Tb3+ with high efficiency that led to the stronggreen emission of Tb3+ in the UV region and under 147 nmexcitation GdPO4:Tb phosphors showed the strongest

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Fig. 6. Emission spectra of GdPO4:Tb phosphor prepared by HR and PM

under 147 nm excitation.

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Fig. 7. SEM photographs of GdPO4:Tb phosphor prepared by PM

(a) and HR (b).

Y. Wang et al. / Journal of Luminescence 126 (2007) 503–507 507

emission intensity because the host absorbed VUV energyand transferred to Tb3+. It was concluded that theefficiency of energy transfer from host to Tb3+ is high inthe presence of Gd3+ excited at 147 nm, but the content ofGd3+ should be carefully controlled. The phosphorprepared by HR had approximately spherical shape, whichwas good for the application in PDPs compared to thephosphor prepared by PM. So, Tb-doped gadolinium

phosphate phosphor, as prepared by hydrothermal synth-esis, was a promising phosphor for PDPs. Furtherinvestigations are being planned in our laboratory.

Acknowledgments

This work was supported by Program for New CenturyExcellent Talents in University of China (NCET, 04-0978),the Key Science Research Project of Ministry of Educationof China (105170) and Specialized Research Fund for theDoctoral Program of Higher Education of China (SRFDP,20040730019).

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