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

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

Journal of Luminescence 143 (2013) 93–95 Contents lists available at SciVerse ScienceDirect Journal of Luminescence journal homepage: www.elsevier.c...

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Journal of Luminescence 143 (2013) 93–95

Contents lists available at SciVerse ScienceDirect

Journal of Luminescence 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

art ic l e i nf o

a b s t r a c t

Article history: Received 24 December 2012 Received in revised form 19 March 2013 Accepted 29 March 2013 Available online 29 April 2013

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 synthesized materials were investigated using spectrofluorometer at room temperature. The excitation and emission bands of LiSr4(BO3)3: Pb2+ were observed at 284 and 328 nm, respectively. The dependence of the emission intensity on the Pb2+ concentration for the LiSr4(BO3)3 were studied in detail. It was observed that 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.

Keywords: Inorganic borate Luminescence XRD

1. Introduction

2. Experimental

Divalent lead cation (Pb2+), as a well-known dopant for many different host lattices, is of great scientific, medical and industrial interest. Because of the diversity of the photoluminescent properties, it provides the possibilities of fabricating novel phosphor materials [1]. Inorganic luminescent materials containing metal ions with s2 (Pb2+ etc.) configuration can be used in X-ray imaging devices, low pressure lamps, and high-energy physics. For example, BaSi2O5: Pb2+ emits a broad band around 350 nm under UV excitation, which is one of the earliest known phosphors for photocopying lamps [2]. Inorganic borate phosphors have attracted much attention due to their high stability, easy synthesis, and high UV transparency [3]. The compound of LiSr4(BO3)3, an example of alkaline-earth borates, is characterized by having an association of BO3 triangle, (Sr1O6 octahedra/Sr2O8 polyhedra), and LiO6 octahedra. The crystal structure of LiSr4(BO3)3 has been studied in detail by Chen and coworkers [4]. Recently, although the luminescent properties of Ce3+, Tb3+, Eu3+, Dy3+, Eu2+, LiSr4(BO3)3 have been reported [5–9], photoluminescence properties of Pb2+ doped LiSr4(BO3)3 has not yet been studied. In this study, pure LiSr4(BO3)3 and LiSr4(BO3)3 materials with different mol ratios of Pb2+ were prepared by a solution combustion synthesis method. The synthesized materials were characterized by using the powder X Ray Diffraction. After synthesis and characterization of all synthesized LiSr4(BO3)3 materials, the photoluminescence properties of these phosphors were studied in detail using a spectrofluorometer.

Pure and Pb2+ doped LiSr4(BO3)3 materials were prepared by a solution combustion synthesis method followed by heating of the precursor 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 used as starting materials. The precursor solutions were introduced into a muffle furnace and maintained at 500 1C for 10 min. The precursor powders were removed from furnace. The voluminous and foamy combustion ashes were easily milled to obtain a precursor powder of 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 were thoroughly mixed and then heated secondly up to 600 1C for 1 h in air. After milling, the samples were slowly heated at 800 1C for 8 h in air. The XRD structural analysis of pure and Pb2+ doped LiSr4(BO3)3 materials were performed on an X-ray Phillips X'Pert Pro equipped with CuKα (30 kV, 15 mA, λ¼1.54051 Å) radiation at room temperature. Scanning was generally performed between 101 and 901 2θ. Measurement was made with 0,03301 step size at 25 1C temperature. The photoluminescence spectra were measured at room temperature with a Thermo Scientific Lumina fluorescence spectrometer equipped with a 150 W Xenon lamp.

3. Results and discussion 3.1. X- ray powder diffraction analysis

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

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

94

25000

404 323 521 431

444 532

804 800

%2Pb (800 °C)

844 808

848

1204

1244

%0,5Pb (800 °C)

pure (800 °C)

pure (500 °C)

15000

20

30

40

50

60

70

80

1

2

2

3

3

1- 0,005 mol Pb 2- 0,015 mol Pb 3- 0,03 mol Pb

10000

5000

ICSD 17-0861

10

1 20000

Relative Intensity

400

0 200

250

90

300

350

400

450

500

Wavelength (nm)

2θ Fig. 1. XRD pattern obtained for LiSr4−xPbx(BO3)3 (x ¼0, 0.005, and 0.02).

indicated that Pb2+ could be doped into LiSr4(BO3)3 compound instead of Sr up to mol fraction of 0.05 without formation of any second phases. The two possible sites available for the incorporating Pb2+ 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 have two different Sr1O6 octahedra and Sr2O8 polyhedra environments. The ionic radii of Sr2+ for (C.N¼ 6) and (C.N¼ 8) are 1.18 and 1.26 Å, respectively [8]. However, the ionic radii of Pb2+ for (C.N¼6) and (C.N¼ 8) are 1.19 and 1.29 Å, respectively. So it would be expected that 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 originates from the 6 s2-6 s16p2 interconfigurational transition. Typically at room temperature, emission is observed from the 3P1-1S0 transition [10], although at low temperatures the highly forbidden 3 P0-1S0 emission is also observed [11]. As seen in Fig. 2a, the excitation band of the synthesized phosphor LiSr4(BO3)3: Pb2+ was observed at 284 nm, which is assigned to the 1S0-3P1 transition. The emission band was observed at 328 nm from the 3P1 excited state level to the 1S0 ground state upon excitation with 284 nm (Fig. 2b). The emission band of LiSr4(BO3)3: Pb2+ lies between 300 nm and 400 nm and is in the UV region. Due to no splitting or multiple peaks in the photoluminescence spectra, it is believed that 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 and not Li+ sites according to the ionic size considerations. In many inorganic hosts, although the absorption band of Pb2+ is in the near UV spectral region, the emission band of Pb2+ is usually observed in 300–450 nm range (Table 1). This diversity is depending strongly on the 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 investigated in various inorganic hosts with different structure, such as 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 the photoluminescence of Pb2+. As seen in Table 1, Pb2+ ions occupy the Sr2+ sites in these hosts, which have different crystal structures, and the emission of Pb2+ in these hosts is located at characteristically different positions due to the electronegativity of the ligands [14]. So, the Pb2+ ion in different inorganic hosts emit varies from UV to green region, depending on crystal structure of the host lattice [21]. It is known that the luminescence intensities of Pb2+ doped phosphors always depend on the doped Pb2+ ions concentration

Fig. 2. The excitation (a) and emission (b) spectra of LiSr4−xPbx(BO3)3 (x¼ 0.005, 0.015, and 0.03) at room temperature (λexc ¼ 284 and λem ¼ 328 nm).

Table 1 The spectroscopic properties of some Pb2+ doped inorganic hosts, at room temperature. Host

λexc

λems

Stokes shift(cm−1)

Ref.

SrB2O4 SrB4O7 SrAl2B2O7 SrLaBO4 Sr6YAl(BO3)6 La2Sr5Mg(BO3)6 SrHfO3 Sr2Mg(BO3)2 LiSr4(BO3)3

270 254 277 254 277 254 269 260 284

363 303 420 470 371 361 340 330 328

9489 6367 12,292

[14] [15] [16] [17] [18] [18] [19] [20] This study

9147 9162 8159 4723

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 (λexc ¼ 284 nm).

[12,16,22–24]. Hence, it has also investigated that the photoluminescence spectra of LiSr4(BO3)3 with different Pb2+ doping concentrations. As seen in Fig. 2, with different Pb2+ doping concentration, the positions and shapes of the photoluminescence bands have exhibited no obvious changes. The dependence of the emission intensity on the Pb2+ doping concentration for the LiSr4−x Pbx(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 the doping 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

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been attributed to the migration of excitation energy to the quenching centers (traps) or to the cross-relaxation mechanisms [16,22,25,26]. So, it can be expressed that the concentration quenching of Pb2+ in LiSr4(BO3)3 phosphor is 0.005 mol. Finally, the Stokes shift of the synthesized phosphor LiSr4(BO3)3: Pb2+ was calculated to be 4723 cm–1 using the excitation band at 284 nm and the emission band at 328 nm. If it is compared the Stokes shift of Pb2+ in LiSr4(BO3)3 with that of Pb2+ substituted for coordinated Sr2+ in the other hosts (Table 1), it is observed that the present value (4723 cm–1) is so small. As a result of this small Stokes shift in LiSr4(BO3)3: Pb2+, it can be expressed that there is a small relaxation in the excited state. 4. Conclusion Pure and Pb2+ doped LiSr4(BO3)3 materials were prepared by a solution combustion synthesis method followed by heating of the precursor combustion ash at 800 1C in air. The synthesized materials were characterized using powder XRD. The XRD pattern of all synthesized phosphor is in agreement with the JCPDS (17-0861). The photoluminescent properties of LiSr4(BO3)3: Pb2+ phosphor with different mol ratios of Pb2+ were investigated using a spectrofluorometer at room temperature. The emission band of LiSr4(BO3)3: Pb2+ was observed at 328 nm from the 3P1 excited state level to the 1 S0 ground state upon excitation with 284 nm. The dependence of the emission intensity on the Pb2+ concentration for the LiSr4(BO3)3 were studied in detail. It was observed that the concentration quenching of Pb2+ in LiSr4(BO3)3 is 0.005 mol. Finally, the Stokes

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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 broadband UV application.

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