Photoluminescence properties of a novel red emitting Sr7Zr(PO4)6:Eu3+ phosphor

Optical Materials 37 (2014) 866–869

Contents lists available at ScienceDirect

Optical Materials journal homepage: www.elsevier.com/locate/optmat

Short Communication

Photoluminescence properties of a novel red emitting Sr7Zr(PO4)6:Eu3+ phosphor Zhi-wei Zhang ⇑, Peng-xin Shen, Ya-nan Wu, Xian-fu Zhang, Jian-ping Zhang, Wei-guo Zhang, Dong-jun Wang Chemical Engineering College, Hebei Normal University of Science and Technology, Qinhuangdao 066600, China

a r t i c l e

i n f o

Article history: Received 24 January 2014 Received in revised form 30 March 2014 Accepted 10 May 2014 Available online 20 June 2014 Keywords: Phosphors Sr7Zr(PO4)6:Eu3+ Red-emitting phosphor Luminescence

a b s t r a c t A novel red-emitting phosphor Sr7Zr(PO4)6:Eu3+ has been synthesised by a high-temperature solid-state reaction. X-ray powder diffraction (XRD) analysis and FT-IR spectra confirmed the phase formation of Sr7Zr(PO4)6:Eu3+ materials. The photoluminescence excitation and emission spectra, the concentration dependence of the emission intensity, decay curves, and ultraviolet–visible absorption spectroscopy of the phosphor were investigated. The results showed that the phosphor could be efficiently excited by the near ultraviolet (NUV) light region from 350 to 450 nm, and it exhibited red light emission. The decay time was also determined for various concentrations of Eu3+ in Sr7Zr(PO4)6. The calculated color coordinates lies in the red region. Therefore, these obtained results suggest that the prepared phosphors exhibit great potential for use as red emitting phosphor for near ultraviolet white light emitting diodes (NUV WLEDs). Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction White light-emitting diodes (wLEDs) has excellent performance, such as high brightness, low power consumption, environmentally friendly, long service lifetime, and good reliability, therefore, they has extensively been applied in solid-state lightings [1,2]. The commonly-used wLEDs is combination of a blue light emitting InGaN chip and a yellow phosphor YAG:Ce pumped by the chip [3]. However, this combination method exhibits a poor color rendering index (<80) for the lack of a red light component [4–6]. Although many attempts have been made to develop redemitting phosphors that can be pumped by near-UV LEDs, the current red phosphors have some drawbacks, for example, sulfides and oxysulfides (CaS:Eu2+ [7], Y2O2S:Eu3+ [8]) are not stable, another example, nitrides and oxynitrides (Sr2Si5N8:Eu2+ [9], SrAlSi4N7:Eu2+ [10], b-SiAlON:Pr3+ [11]) are difficult to synthesis. Therefore, there needs to be much more focus on the red-emitting phosphors. Recently, potential and effective technological applications, mainly related to optical and optoelectronic domains (Scintillators, lasers, etc.), have significantly revived interest in new materials crystallizing with eulytite structure. Sr7Zr(PO4)6, as a member of a big family of eulytite compounds, has a cubic structure with space ⇑ Corresponding author. Tel.: +86 335 2039067. E-mail addresses: [email protected], [email protected] (Z.-w. Zhang). http://dx.doi.org/10.1016/j.optmat.2014.05.029 0925-3467/Ó 2014 Elsevier B.V. All rights reserved.

group of I43d (Z = 2) [12]. The structure has a three-dimensional network formed by mixed metal (Sr/Zr)-oxygen octahedra connected by edge sharing and forming corrugated chains. The octahedra are additionally linked by independent PO4 tetrahedra groups by sharing corners. Sr and Zr atoms randomly occupy two positions in the octahedra [13]. Such a structure is favorable to being a luminescent material. For example, Qin has reported M7Zr(PO4)6 (M = Ca, Sr, Ba) as novel phosphors with unusual self-activated luminescence due to the Zr4+ to O2 charge transfer transitions [13]. However, to the best of our knowledge, there is no report on the research of Eu3+-doped Sr7Zr(PO4)6 phosphors. Thus, in this paper we reported the preparation and investigation on luminescence properties of a series of novel red-emitting Sr7Zr(PO4)6:Eu3+ phosphors. 2. Experimental section 2.1. Synthesis of samples Sr7xZr(PO4)6:xEu3+ (x = 0.10, 0.40, 0.60, 0.70, 0.90, 1.10, and 1.30) phosphors were synthesised using a solid state reaction method. Sr(NO3)24H2O (99.9%, 50 mol%), (NH4)2HPO4 (99.9%, 42 mol%), ZrO2 (99.9%, 8 mol%), and Eu2O3 (99.99%) were obtained from Sinopharm Chemical Reagent, Co. Ltd., Shanghai China, and used as received without further purification. Stoichiometric amounts of all the above reagents were mixed thoroughly in

867

Z.-w. Zhang et al. / Optical Materials 37 (2014) 866–869

alcohol by ball milling using an agate mortar and pestle. They were pre-sinter at 750 °C for 3 h in air, and re-sintered at 1200 °C for 4 h. Finally, the samples were ground into powder for characterization.

3. Results and discussion 3.1. Phase identification Fig. 1 illustrates the X-ray diffraction (XRD) patterns of Sr7xZr(PO4)6:xEu3+ (x = 0.10, 0.40, 0.60, 0.70, 0.90, 1.10, and 1.30) phosphors with different doping Eu3+ contents (x). All the observed diffraction peaks can be indexed to the standard data of Sr7Zr(PO4)6 with JCPDS card No. 34-0065 [13]. No obvious impurity phase was detected when Eu3+ ions were doped into the host lattice, indicating that all samples are of single phase. The FT-IR spectrum of Sr7Zr(PO4)6 has been examined and the bands that can be associated with –OH, P–O, and ZrO2, respectively (Fig. 2). The peak at 3483 and 2362 cm1 is attributed to the stretching and bending modes of physical adsorbed OH for the samples. The presence of OH groups is harmful to effectiveness of luminescence, and they will be removed from the samples in the following work. According to the Refs. [14,15], the bands can be specified as follows: the mode at 1047 and 909 cm1 were ascribed to m3 P–O and m1 P–O–P, respectively. The mode at 547 cm1 correspond to vibrations of ZrO2.

Transmittance (%)

The phase purities were analyzed using a D/MAX2500TC X-ray powder diffractometer. The photoluminescence excitation (PLE), emission spectra (PL) and the luminescence decay times were measured using a steady- and transient-state fluorescence spectrometer (FLS920), emission monochromator is R928 and the slit width of excitation is 2 nm. The excitation and fluorescence spectra have been reasonably corrected. The Fourier transform infrared (FT-IR) spectrometer 8900 was used to measure the FT-IR spectra in the 400–4000 cm1 range using the KBr pellets. Ultraviolet–visible spectroscopy was measured using U-4100 UV–VIS-NIR spectrophotometer. All measurements were carried out at room temperature of 22 °C.

60 45

2362

30

3483 15 0

909 547

1047

4000

3200

2400

1600

800

Wavenumber (cm-1 ) Fig. 2. Infrared spectra of Sr7Zr(PO4)6.

maximum at about 270 nm) and several excitation bands located at 319 nm (7F0,1 ? 5H3,6), 362 nm (7F0,1 ? 5D4), 382 nm (7F0,1 ? 5L7), 394 nm (7F0 ? 5L6), 414 nm (7F1 ? 5D3), 465 nm (7F0 ? 5D2), respectively. The broad absorption from 220 to 310 nm can be attributed to charge-transfer transition from negative oxygen ion (2p6) to the empty state of 4f7 of Eu3+ ion [ligand-to-metal charge-transfer band (CTB)]. The spectroscopic

(a)

x =0.10 x =0.40 x =0.60 x =0.70 x =0.90 x =1.10 x =1.30

Relative intensity (a.u.)

2.2. Characterization of samples

75

3.2. Luminescence properties 200

The fluorescence excitation spectra of typical sample Sr7xZr(PO4)6:xEu3+ monitoring at the 5D0 ?7F2 emission (612 nm) were shown in Fig. 3(a). The excitation spectra of Eu3+ doped Sr7Zr(PO4)6 clearly indicate a broad absorption from 220 to 310 nm (with a

300

x= 0.40 x= 0.60 x= 0.70

350

400

450

(b) Intensity (a.u.)

5 D → 7F 0 2

0.0

0.2

0.4

0.6

0.8

x= 1.30

20

30

40

50

60

70

80

2 theta (deg) Fig. 1. XRD patterns for Sr7xZr(PO4)6:xEu3+ (x = 0.10, 0.40, 0.60, 0.70, 0.90, 1.10 and 1.30) powders.

1.2

1.4

x=0.10 x=0.40 x=0.60 x=0.70 x=0.90 x=1.10 x=1.30

x= 1.10

570

1.0

x

x= 0.90

PDF#34-0065

500

Wavelength (nm)

Intensity (a.u.)

x= 0.10

250

600

630

660

690

720

750

780

Wavelength (nm) Fig. 3. (a) PLE spectra; (b) PL spectra of Sr7xZr(PO4)6:xEu3+ (x = 0.10, 0.40, 0.60, 0.70, 0.90, 1.10, and 1.30) phosphors with different amounts of Eu3+. The inset of (b): relation between the relative intensity of 5D0 ? 7F2 transition and concentration of Eu3+.

Z.-w. Zhang et al. / Optical Materials 37 (2014) 866–869

properties of Eu3+ doped phosphates were well analyzed in Refs. [16–19], and our results agree with those papers. The strongest absorption band of 7F0 ? 5L6 transition at 394 nm matches well with the output wavelength of near-UV chips in phosphor-converted wLEDs. Another strong excitation peak in the blue region is located at 465 nm, whose intensity is inferior to that of 394 nm. Fig. 3(b) illustrates the PL spectra of Sr7xZr(PO4)6:xEu3+ phosphors excited at 394 nm. The bands with the maxima at 579, 591, 612, 652 and 700 nm of Sr7xZr(PO4)6:xEu3+ phosphor are assigned to the 5D0 ? 7FJ (J = 0, 1, 2, 3, 4) transition of Eu3+, respectively [11]. In addition, from the emission spectra, the red emission (5D0 ? 7F2) is stronger than the orange emission (5D0 ? 7F1), indicating that Eu3+ is located in a noncentrosymmetric position in the Sr7Zr(PO4)6 matrix [20]. The inset of Fig. 3(b) shows relation between the integral intensity of 5D0 ? 7F2 transition and concentration of Eu3+. As shown in the inset of Fig. 3(b), as the concentration increases, the emission intensity increases, and it reaches to maximum at x = 1.10, which was taken as the optimum concentration. The intensity of the luminescence increases with Eu3+ concentration but with a clear tendency to saturation, which in fact suggests that at higher Eu contents some quenching of the Eu3+ luminescence occurs. According to M. Puchalska, the cause may be that the material is highly defected (for divalent strontium is trivalent europium) and a similar effect has already been observed for CaAl4O7:Eu3+ and CaGa4O7:Eu3+ [21,22].

100000

x =0.10 x =0.40 x =0.60 Intensity (a.u.)

868

x =0.90 x =1.10 x =1.30

10000

1000

0

2

4

6

8

10

Times (ms) Fig. 5. The luminescence decay curves of Sr7xZr(PO4)6:xEu3+ (x = 0.10, 0.40, 0.60, 0.90, 1.10, and 1.30) phosphors.

3.4. Luminescence decay times The PL decay curves of Sr7xZr(PO4)6:xEu3+ (x = 0.10, 0.40, 0.60, 0.70, 0.90, 1.10, and 1.30) phosphors with various activation concentrations were measured, as shown in Fig. 5. The PL decay curves of Eu3+ emissions obey a single-exponential law and fit using equation as follows [23]:

3.3. Ultraviolet–visible reflection spectroscopy

I ¼ A expðt=sÞ

To investigate the influence of Eu3+ doping on the ultraviolet– visible reflection spectroscopy of Sr7Zr(PO4)6 host, the diffuse reflection spectra of Sr7Zr(PO4)6 and Sr5.90Zr(PO4)6:1.10Eu3+ were measured and shown in Fig. 4. The Sr7Zr(PO4)6 host shows energy absorption in the 250–300 nm range. As shown in Fig. 4, Sr7Zr(PO4)6 and Sr5.90Zr(PO4)6:1.10Eu3+ exhibit the steep absorption edges in the range of 300–400 nm and 260–320 nm, respectively, which is due to the intrinsic transition of host lattice. It is indicated that with Eu3+-doping (x = 1.10), the absorption band tends to shift toward the shorter wavelength. For Sr5.90Zr(PO4)6:1.10Eu3+, the absorption peaks at 320 and 500 nm are ascribed to the f–f electron transition of Eu3+ ions, which is consistent with the conclusion of the excitation spectrum analysis. The similar analysis can by referred to the paper [23].

where I is the luminescence intensity; A, t, and s are constant, time, and decay time for the exponential components, respectively. The decay times (s) of Eu3+ ion are calculated to be 2.260, 2.232, 2.372, 2.375, 2.385, and 2.360 ms, corresponding to x = 0.10, 0.40, 0.60, 0.90, 1.10, and 1.30, respectively.

ð6Þ

3.5. CIE analysis Commission International de I’Eclairage (CIE) 1931 x–y chromaticity diagram of Sr7xZr(PO4)6:xEu3+ (x = 0.10, 0.40, 0.60, 0.70, 0.90, 1.10, and 1.30) phosphors are presented in Fig. 6. The CIE chromaticity coordinates for these phosphors were located in the red region at A (x = 0.648, y = 0.352), B (x = 0.648, y = 0.352), C (x = 0.648, y = 0.352), D (x = 0.648, y = 0.352), E (x = 0.648,

105

Reflectance (%)

90 x =0.00

75

x =1.10

60

45

30 200

300

400

500

600

700

Wavelength (nm) Fig. 4. UV–vis spectra of Sr7Zr(PO4)6 and Sr5.90Zr(PO4)6:1.10Eu3+ phosphors.

Fig. 6. CIE chromaticity coordinates of Sr7xZr(PO4)6:xEu3+ (x = 0.10, 0.40, 0.60, 0.70, 0.90, 1.10, and 1.30) phosphors.

Z.-w. Zhang et al. / Optical Materials 37 (2014) 866–869

y = 0.352), F (x = 0.648, y = 0.352), and G (x = 0.648, y = 0.352), hardly change with increasing Eu3+ contents. The inset of Fig. 6 shows luminescence photographs of Sr6.90Zr(PO4)6:0.10Eu3+ (A) and Sr5.90Zr(PO4)6:1.10Eu3+ (F) phosphors excited at 365 nm. Sr6.90Zr(PO4)6:1.10Eu3+ phosphor can emit more brighter red light than that of Sr5.90Zr(PO4)6:0.10Eu3+. Under NUV excitation, Sr5.90Zr(PO4)6:1.10Eu3+ phosphor emitted a bright red light. 4. Conclusions In summary, a series of novel red-emitting phosphors, Sr7xZr(PO4)6:xEu3+ (x = 0.10, 0.40, 0.60, 0.70, 0.90, 1.10, and 1.30) phosphors were synthesised by a high temperature solid state method. The phosphor has four intense emission bands centering at 591, 612, 652 and 700 nm upon 394 nm excitation. The optimum concentration of Eu3+ for Sr7xZr(PO4)6:xEu3+ is determined as 1.10. The decay times (s) of Eu3+ ion are calculated to be 2.260, 2.232, 2.372, 2.375, 2.385, and 2.360 ms, corresponding to x = 0.10, 0.40, 0.60, 0.90, 1.10, and 1.30, respectively. The above obtained results indicated the Sr7Zr(PO4)6:Eu3+ phosphor could act as a kind of potential candidate for wLEDs. Acknowledgments We acknowledge the Major Program of the Hebei Higher Education Institutions of China (Grant No.: ZD20131015) and innovation team fund of Hebei Normal University of Science and Technology (No. CXTD2012-05). We also gratefully acknowledge instrumental analysis center of Hebei Normal University of Science and Technology. References [1] S. Nakamura, G. Fasol, The Blue Laser Diode: GaN Based Light Emitters and Lasers, Springer, Berlin, 1997. [2] R.-J. Xie, N. Hirosaki, Silicon-based oxynitride and nitride phosphors for white LEDs: a review, Sci. Technol. Adv. Mater. 8 (2007) 588–600. [3] A. Birkel, K.A. Denault, N.C. George, C.E. Doll, B. Hery, A.A. Mikhailovsky, C.S. Birkel, B.-C. Hong, R. Seshadri, Rapid microwave preparation of highly efficient Ce3+-substituted garnet phosphors for solid state white lighting, Chem. Mater. 24 (2012) 1198–1204. [4] C.H. Huang, T.M. Chen, W.R. Liu, Y.C. Chiu, Y.T. Yeh, S.M. Jang, A single-phased emission-tunable phosphor Ca9Y(PO4)7:Eu2+, Mn2+ with efficient energy transfer for white-light-emitting diodes, ACS Appl. Mater. Interfaces 2 (2010) 259–264.

869

[5] S. Choi, Y. Young, H. Jung, Eu2+ and Mn2+ activated single phase white emitting phosphor Na(Sr, Ba)PO4 for phosphor converted-LEDs, Mater. Lett. 75 (2012) 186–188. [6] P. Li, Z. Wang, Z. Yang, Q. Guo, Sr2B2P2O10:Eu2+, Mn2+, Ba2+: a potential singlephase white light-emitting phosphor for UV light emitting diodes, J. Electrochem. Soc. 157 (2010) H504–H509. [7] X.H. He, Y. Zhu, Improvement of morphology and luminescence of CaS:Eu2+ red-emitting phosphor particles via carbon-containing additive strategy, J. Mater. Sci. 43 (2008) 1515–1519. [8] X.Y. Wu, Y.J. Liang, R. Chen, M.Y. Liu, Y.Z. Li, Preparation and photoluminescence properties of Y2O3:Eu, Bi phosphors by molten salt synthesis for white light-emitting diodes, J. Mater. Sci. 46 (2011) 5581–5586. [9] C.W. Yeh, W.T. Chen, R.S. Liu, S.F. Hu, H.S. Sheu, J.M. Chen, H.T. Hintzen, Origin of thermal degradation of Sr2xSi5N8:Eux phosphors in air for light-emitting diodes, J. Am. Chem. Soc. 134 (2012) 14108–14117. [10] J. Ruan, R.J. Xie, N. Hirosaki, T. Takeda, Nitrogen gas pressure synthesis and photoluminescent properties of orange-red SrAlSi4N7:Eu2+ phosphors for white light-emitting diodes, J. Am. Ceram. Soc. 94 (2011) 536–542. [11] T.C. Liu, B.M. Cheng, S.F. Hu, R.S. Liu, Highly stable red oxynitride b-SiAlON:Pr3+ phosphor for light-emitting diodes, Chem. Mater. 23 (2011) 3698–3705. [12] E.H. Arbib, B. Elouadi, J.P. Chaminade, J. Darriet, The crystal structure of the phosphate eulytite Ba3Bi(PO4)3, Mater. Res. Bull. 35 (2000) 761–773. [13] L. Qin, D.L. Wei, Y.L. Huang, S.I. Kim, Y.M. Yu, H.J. Seo, New self-activated eulytite-type compounds of M7Zr(PO4)6 (M = Ca, Sr, Ba), J. Alloy. Compd. 574 (2013) 305–309. [14] C.C. Silva, A.S.B. Sombra, Structural studies of calcium phosphate doped with titanium and zirconium obtained by high-energy mechanical alloying, Phys. Scr. 80 (2009) 065801–065805. [15] Y. Zhang, G.F. Yin, S.F. Zhu, D.L. Zhou, Y.H. Wang, Y. Li, L. Luo, Preparation of bCa3(PO4)2 bioceramic powder from calcium carbonate and phosphoric acid, Curr. Appl. Phys. 5 (2005) 531–534. [16] R.Y. Yang, Y.M. Peng, Y.K. Su, Novel red-emitting microwave-assisted-sintered LiSrPO4:Eu3+ phosphors for application in near-UV white light-emitting diodes, J. Electron. Mater. 42 (2013) 2910–2914. [17] W. Szuszkiewicz, B. Keller, M. Guzik, J. Legedziewicz, Application of lanthanide (Eu, Nd) spectroscopy as a structural probe of diluted double phosphates, J. Alloy. Compd. 341 (2002) 297–306. [18] S. Szulia, M. Kosmowska, H.A. Kołodziej, M. Sobczyk, G. Czupin´ska, Dielectric relaxation and electronic spectroscopy of double potassium yttrium tetraoxophosphate(V) K3Y(PO4)2 doped by neodymium and europium ions, J. Mol. Struct. 1006 (2011) 409–418. [19] B.Z. Yang, Z.P. Yang, Y.F. Liu, F.C. Lu, P.L. Li, Y.M. Yang, X. Li, Synthesis and photoluminescence properties of the high-brightness Eu3+-doped Sr3Y(PO4)3 red phosphors, Ceram. Int. 38 (2012) 4895–4900. [20] W. Li, J. Lee, Microwave-assisted sol–gel synthesis and photoluminescence characterization of LaPO4:Eu3+, Li+ nanophosphors, J. Phys. Chem. C 112 (2008) 11679–11684. [21] M. Puchalska, Y. Gerasymchuk, E. Zych, Optical properties of Eu3+-doped CaAl4O7 synthesized by the Pechini method, Opt. Mater. 32 (2010) 1117–1122. [22] M. Puchalska, E. Zych, Effect of Na+ co-dopant and activator concentration on luminescent properties of CaGa4O7:Eu3+, J. Lumin. 132 (2012) 2879–2884. [23] G.R. Dillip, B. Deva, Prasad Raju, A study of the luminescence in near UVpumped red-emitting novel Eu3+-doped Ba3Ca3(PO4)4 phosphors for white light emitting diodes, J. Alloy. Compd. 540 (2012) 67–74.