Journal of Luminescence 143 (2013) 93–95

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Journal of Luminescence

0022-23http://d

n Tel.:E-m

journal homepage: www.elsevier.com/locate/jlumin

A novel UV-emitting phosphor: LiSr4(BO3)3: Pb2+

İlhan Pekgözlü n

Bartin University, Faculty of Engineering, Department of Environmental Engineering, Bartin 74100, Turkey

a r t i c l e i n f o

Article history:Received 24 December 2012Received in revised form19 March 2013Accepted 29 March 2013Available online 29 April 2013

Keywords:Inorganic borateLuminescenceXRD

13/$ - see front matter & 2013 Elsevier B.V. Ax.doi.org/10.1016/j.jlumin.2013.03.057

+90 378 223 5459; fax: +90 378 223 5258.ail address: pekgozluilhan@yahoo.com

a b s t r a c t

Pure and Pb2+ doped LiSr4(BO3)3 materials were prepared by a solution combustion synthesis method.The phase analysis of all synthesized materials were determined using the powder XRD. The synthesizedmaterials were investigated using spectrofluorometer at room temperature. The excitation and emissionbands of LiSr4(BO3)3: Pb2+ were observed at 284 and 328 nm, respectively. The dependence of theemission intensity on the Pb2+ concentration for the LiSr4(BO3)3 were studied in detail. It was observedthat the concentration quenching of Pb2+ in LiSr4(BO3)3 is 0.005 mol. The Stokes shifts of LiSr4(BO3)3: Pb2+

phosphor was calculated to be 4723 cm–1.& 2013 Elsevier B.V. All rights reserved.

1. Introduction

Divalent lead cation (Pb2+), as a well-known dopant for manydifferent host lattices, is of great scientific, medical and industrialinterest. Because of the diversity of the photoluminescent proper-ties, it provides the possibilities of fabricating novel phosphormaterials [1]. Inorganic luminescent materials containing metalions with s2 (Pb2+ etc.) configuration can be used in X-ray imagingdevices, low pressure lamps, and high-energy physics. For example,BaSi2O5: Pb2+ emits a broad band around 350 nm under UVexcitation, which is one of the earliest known phosphors forphotocopying lamps [2]. Inorganic borate phosphors have attractedmuch attention due to their high stability, easy synthesis, and highUV transparency [3]. The compound of LiSr4(BO3)3, an example ofalkaline-earth borates, is characterized by having an association ofBO3 triangle, (Sr1O6 octahedra/Sr2O8 polyhedra), and LiO6 octahedra.The crystal structure of LiSr4(BO3)3 has been studied in detail byChen and coworkers [4]. Recently, although the luminescent proper-ties of Ce3+, Tb3+, Eu3+, Dy3+, Eu2+, LiSr4(BO3)3 have been reported[5–9], photoluminescence properties of Pb2+ doped LiSr4(BO3)3 hasnot yet been studied.

In this study, pure LiSr4(BO3)3 and LiSr4(BO3)3 materials withdifferent mol ratios of Pb2+ were prepared by a solution combus-tion synthesis method. The synthesized materials were character-ized by using the powder X Ray Diffraction. After synthesis andcharacterization of all synthesized LiSr4(BO3)3 materials, thephotoluminescence properties of these phosphors were studiedin detail using a spectrofluorometer.

ll rights reserved.

2. Experimental

Pure and Pb2+ doped LiSr4(BO3)3 materials were prepared by asolution combustion synthesis method followed by heating of theprecursor combustion ash at 800 1C in air. LiNO3 (Riedel-de-Haen≥99%), Sr(NO3)2 (Merck ≥99%), H3BO3(Merck ≥99.8%), Pb(NO3)2(Riedel-de-Haen ≥99.5%), and CO(NH2)2 (Fluka ≥99.5%) were usedas starting materials. The precursor solutions were introduced intoa muffle furnace and maintained at 500 1C for 10 min. The precursorpowders were removed from furnace. The voluminous and foamycombustion ashes were easily milled to obtain a precursor powderof LiSr4−xPbx(BO3)3 (x¼0, 0.0005, 0.001, 0.0025, 0.005, 0.01, 0.015,0.02, 0.03, 0.04 and 0.05). The well-mixed precursor powders werethoroughly mixed and then heated secondly up to 600 1C for 1 h inair. After milling, the samples were slowly heated at 800 1C for 8 hin air.

The XRD structural analysis of pure and Pb2+ doped LiSr4(BO3)3materials were performed on an X-ray Phillips X'Pert Pro equippedwith CuKα (30 kV, 15 mA, λ¼1.54051 Å) radiation at room tempera-ture. Scanning was generally performed between 101 and 901 2θ.Measurement was made with 0,03301 step size at 25 1C tempera-ture. The photoluminescence spectra were measured at roomtemperature with a Thermo Scientific Lumina fluorescence spectro-meter equipped with a 150W Xenon lamp.

3. Results and discussion

3.1. X- ray powder diffraction analysis

The XRD pattern of pure and Pb2+ doped LiSr4(BO3)3 is presentedin Fig. 1, which is in agreement with the JCPDS (17-0861). This

10 20 30 40 50 60 70 80 90

2θ

pure (500 °C)

ICSD 17-0861

pure (800 °C)

%0,5Pb (800 °C)

%2Pb (800 °C)400

323431

521

404

532444

800

804844

808 848 1204 1244

Fig. 1. XRD pattern obtained for LiSr4−xPbx(BO3)3 (x¼0, 0.005, and 0.02).

0

5000

10000

15000

20000

25000

200 250 300 350 400 450 500Wavelength (nm)

Rel

ativ

e In

tens

ity

1- 0,005 mol Pb2- 0,015 mol Pb3- 0,03 mol Pb

11

22

33

Fig. 2. The excitation (a) and emission (b) spectra of LiSr4−xPbx(BO3)3 (x¼0.005,0.015, and 0.03) at room temperature (λexc¼284 and λem¼328 nm).

Table 1The spectroscopic properties of some Pb2+ doped inorganic hosts, at roomtemperature.

Host λexc λems Stokes shift(cm−1) Ref.

SrB2O4 270 363 9489 [14]SrB4O7 254 303 6367 [15]SrAl2B2O7 277 420 12,292 [16]SrLaBO4 254 470 [17]Sr6YAl(BO3)6 277 371 9147 [18]La2Sr5Mg(BO3)6 254 361 9162 [18]SrHfO3 269 340 [19]Sr2Mg(BO3)2 260 330 8159 [20]LiSr4(BO3)3 284 328 4723 This study

Fig. 3. Emission spectra of pure and Pb2+ (0.05, 0.1, 0.25, 0.5, 1, 1.5, 2, 3, 4, and 5 mol%):LiSr4(BO3)3 phosphors (λexc¼284 nm).

İ. Pekgözlü / Journal of Luminescence 143 (2013) 93–9594

indicated that Pb2+ could be doped into LiSr4(BO3)3 compoundinstead of Sr up to mol fraction of 0.05 without formation of anysecond phases. The two possible sites available for the incorporatingPb2+ in LiSr4(BO3)3 lattice are either the Li+ sites or the Sr2+ sites.The Pb2+ (1.19 Å for C.N¼6) ion has a much larger ionic radius,compared with that of Li+ (0.76 Å for C.N¼6) ion. The Sr2+ ions havetwo different Sr1O6 octahedra and Sr2O8 polyhedra environments.The ionic radii of Sr2+ for (C.N¼6) and (C.N¼8) are 1.18 and 1.26 Å,respectively [8]. However, the ionic radii of Pb2+ for (C.N¼6) and(C.N¼8) are 1.19 and 1.29 Å, respectively. So it would be expectedthat Pb2+ would replace Sr2+ in LiSr4(BO3)3 lattice.

3.2. Photoluminescence properties

The luminescence properties of Pb2+ in host materials is diverse.It can be described by the 1S0-3P0,1 transition, which originatesfrom the 6 s2-6 s16p2 interconfigurational transition. Typically atroom temperature, emission is observed from the 3P1-1S0 transi-tion [10], although at low temperatures the highly forbidden3P0-1S0 emission is also observed [11]. As seen in Fig. 2a, theexcitation band of the synthesized phosphor LiSr4(BO3)3: Pb2+ wasobserved at 284 nm, which is assigned to the 1S0-3P1 transition.The emission band was observed at 328 nm from the 3P1 excitedstate level to the 1S0 ground state upon excitation with 284 nm(Fig. 2b). The emission band of LiSr4(BO3)3: Pb2+ lies between300 nm and 400 nm and is in the UV region. Due to no splittingor multiple peaks in the photoluminescence spectra, it is believedthat the Pb2+ ions are incorporated into only one site in LiSr4(BO3)3.So, activator ion (Pb2+) here is supposed to occupy the Sr2+ sites andnot Li+ sites according to the ionic size considerations. In manyinorganic hosts, although the absorption band of Pb2+ is in the nearUV spectral region, the emission band of Pb2+ is usually observed in300–450 nm range (Table 1). This diversity is depending strongly onthe site occupied by Pb2+ ions, electronegativity of the ligand,crystal structure of the host lattice and temperature [12–14].Up till now, the photoluminescence properties of Pb2+ was inves-tigated in various inorganic hosts with different structure, suchas SrB2O4 [14], SrB4O7 [15], SrAl2B2O7 [16], SrLaBO4 [17], Sr6YAl(BO3)6 [18], La2Sr5Mg(BO3)6 [18], SrHfO3 [19], and Sr2Mg(BO3)2 [20]with the intention of studying the effect of crystal structure on thephotoluminescence of Pb2+. As seen in Table 1, Pb2+ ions occupy theSr2+ sites in these hosts, which have different crystal structures, andthe emission of Pb2+ in these hosts is located at characteristicallydifferent positions due to the electronegativity of the ligands [14].So, the Pb2+ ion in different inorganic hosts emit varies from UV togreen region, depending on crystal structure of the host lattice [21].

It is known that the luminescence intensities of Pb2+ dopedphosphors always depend on the doped Pb2+ ions concentration

[12,16,22–24]. Hence, it has also investigated that the photolumi-nescence spectra of LiSr4(BO3)3 with different Pb2+ doping con-centrations. As seen in Fig. 2, with different Pb2+ dopingconcentration, the positions and shapes of the photoluminescencebands have exhibited no obvious changes. The dependence of theemission intensity on the Pb2+ doping concentration for the LiSr4−xPbx(BO3)3 (0.0005≤x≤0.05) is shown in Fig. 3. With increasing Pb2+

doping concentration, the emission intensity of LiSr4(BO3)3: Pb2+

increases up and reaches a maximum at 0.005 mol. When thedoping concentration of Pb2+ ion in LiSr4(BO3)3 exceeds 0.005 mol,the emission intensity of the synthesized phosphor decreases.Recently, the concentration quenching of Pb2+ ion in Li6CaB3O8.5

[12], SrAl2B2O7 [16], SrZnO2 [22], ZnTiO3 [23], and Sr5(PO4)3Cl [24],was studied in detail by scientists. Based on observation, it has

İ. Pekgözlü / Journal of Luminescence 143 (2013) 93–95 95

been attributed to the migration of excitation energy to thequenching centers (traps) or to the cross-relaxation mechanisms[16,22,25,26]. So, it can be expressed that the concentrationquenching of Pb2+ in LiSr4(BO3)3 phosphor is 0.005 mol.

Finally, the Stokes shift of the synthesized phosphorLiSr4(BO3)3: Pb2+ was calculated to be 4723 cm–1 using the excita-tion band at 284 nm and the emission band at 328 nm. If it iscompared the Stokes shift of Pb2+ in LiSr4(BO3)3 with that of Pb2+

substituted for coordinated Sr2+ in the other hosts (Table 1), it isobserved that the present value (4723 cm–1) is so small. As a resultof this small Stokes shift in LiSr4(BO3)3: Pb2+, it can be expressedthat there is a small relaxation in the excited state.

4. Conclusion

Pure and Pb2+ doped LiSr4(BO3)3 materials were prepared by asolution combustion synthesis method followed by heating of theprecursor combustion ash at 800 1C in air. The synthesized materialswere characterized using powder XRD. The XRD pattern of allsynthesized phosphor is in agreement with the JCPDS (17-0861).The photoluminescent properties of LiSr4(BO3)3: Pb2+ phosphor withdifferent mol ratios of Pb2+ were investigated using a spectro-fluorometer at room temperature. The emission band of LiSr4(BO3)3:Pb2+ was observed at 328 nm from the 3P1 excited state level to the1S0 ground state upon excitation with 284 nm. The dependence ofthe emission intensity on the Pb2+ concentration for the LiSr4(BO3)3were studied in detail. It was observed that the concentrationquenching of Pb2+ in LiSr4(BO3)3 is 0.005 mol. Finally, the Stokes

shift of LiSr4(BO3)3: Pb2+ was calculated to be 4723 cm–1. As a result,LiSr4(BO3)3: Pb2+ make it as the good candidates for the broadbandUV application.

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