Synthesis and luminescent properties of Ba2V2O7:Sm3+

JOURNAL OF RARE EARTHS, Vol. 35, No. 2, Feb. 2017, P. 135

Synthesis and luminescent properties of Ba2V2O7:Sm3+ LI Fei (李 飞), FANG Hongwei (方宏威) , CHEN Yonghu (陈永虎)* (Department of Physics, University of Science and Technology of China, Hefei 230026, China) Received 17 May 2016; revised 15 August 2016

Abstract: Samarium doped pyrovanadate Ba2V2O7:Sm3+ phosphors were synthesized by traditional high-temperature solid-state reaction method. The phase and the structure of the samples were characterized by powder X-ray diffraction (XRD), and the luminescent properties and the energy transfer mechanism of the material were investigated using quantitative photoluminescence (PL) spectroscopy. The excitation spectrum of the sample exhibited a broad ultraviolet (UV) band with maximum at around 341 nm due to V–O charge transfer transition of the host. The emission spectrum displayed a yellow-greenish broadband (peaking at around 498 nm) coming from the host Ba2V2O7 and three narrow peaks (at 561, 599 and 646 nm) attributed to the dopant Sm3+ ions. The PL spectra revealed the energy transfer from the host to the Sm3+ ions. In addition, the color coordinates and the color temperature of the phosphor Ba1.95V2O7:5%Sm3+ were (0.314, 0.365) and 6135 K, respectively, under 365 nm excitation, suggesting it to be a candidate of single-phase converting phosphors for white-light-emitting diodes (WLEDs) with near-UV chips. Keywords: phosphor; Ba2V2O7:Sm3+; luminescence; WLED; rare earths

In recent years, white-light-emitting diodes (WLEDs) have attracted more and more attention for their advantages such as high efficiency, compactness, long operational lifetime, and environmental friendliness[1,2]. Among different kinds of WLED lamps, phosphors converted WLEDs working with a combination of phosphors and LED chips to achieve white light emission, are the most promising to replace nowadays incandescent and fluorescent lamps as major lighting sources[3]. There are two primary approaches in commercial use to obtain phosphors converted WLEDs. One is the combination of phosphors and a blue LED chip[4,5], for example, YAG: Ce3+ yellow phosphor excited with a blue InGaN chip was the most typical and successful in this branch due to its high luminescence efficiency[5,6]. However, because of the lack of red-light components in this device, YAG: Ce3+ phosphor has disadvantages such as poor color rendering index (CRI, smaller than 80) and high correlated color temperature (CCT, higher than 7000 K)[4,7,8]. The second approach is a combination of two or three different color phosphors with a UV (or near-UV) LED chips[9–11]. However, as different hosts involved usually have different aging conditions, the combination may suffer color shift with hosts aging. In order to enhance the color stability, single-phase white phosphors excited by UV LED is proposed to be a potential solution, since different activators in only one host would experience similar aging conditions. As a result, the single-phased WLEDs have the advantages such as small color aberra-

tion, high color rendering, and high efficiency. As common inorganic materials, vanadates have excellent electronic, optical, and chemical properties mainly due to the 3d electrons of vanadium[12–14]. Since energy would transfer from V-O charge-transfer bands to the luminescence center, the absorption bands in many vanadates is usually quite broad and intense[15]. Besides, alkaline-earth metal pyrovanadate M2V2O7 (M=Ba, Sr) have a strong self-activated broadband emission[16,17], which is induced by one-electron charge-transfer transition from the oxygen 2p orbital to the vacant 3d orbital of V5+ in the tetrahedral VO4 with Td symmetry[18]. On the other hand, rare earth ions have played an important role in modern lighting and display fields due to the abundant emission colors based on their 4f-4f or 5d-4f transitions. Among rare earth ions, Sm3+ (4f5) ion is one of the most interesting rare earth ions due to its fluorescence properties. The emission of Sm3+ is situated in the orange spectral region and consists of three narrow characteristic peaks attributed to transitions from the excited 4G5/2 level to the state 6H5/2 (around 561 nm), 6H7/2 (around 590 nm) and 6H9/2 (around 646 nm), respectively[19,20]. Nowadays, many Sm3+-doped materials have been applied in various fluorescent devices, color displays, and temperature sensors, etc.[21] In this paper, Sm3+ ions were doped into self-activated pyrovanadate host Ba2V2O7 with a yellow-green emission broadband to investigate their potential as single-phase white phosphors used for WLED.

Foundation item: Project supported by the National Natural Science Foundation of China (11574298, 11374291, 11274299) * Corresponding author: CHEN Yonghu (E-mail: [email protected]; Tel.: +86-551-63606024) DOI: 10.1016/S1002-0721(17)60891-3

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1 Experimental A series of Ba2V2O7:Sm3+ phosphors with different Sm3+ concentration were synthesized by high-temperature solid-state reactions. BaCO3 (A.R.), V2O5 (A.R.) and Sm2O3 (99.99%) were used as starting materials. Stoichiometric amounts of the raw materials have been mixed and thoroughly ground in an agate mortar, and then preheated at 500 ºC for 4 h in air. Subsequently, the obtained samples were ground again and then re-calcined at 1000 ºC for 6 h in air. To study the influence of Sm3+ ions concentration on fluorescence properties of Ba2V2O7: xSm3+, Ba2–xSmxV2O7 samples with x=0, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07 and 0.08 (molar ratio), respectively, were prepared in the experiment. The crystal structures were analyzed by a MXPAHF rotating anode X-ray diffractometer (Cu Kα radiation). The XRD profiles were collected in the range 10º<2θ< 70º. Photoluminescence excitation (PLE) and emission (PL) spectra were characterized on a HITACHI 850 fluorescence spectrometer with a 150 W Xe lamp as an excitation source.

2 Results and discussion 2.1 Phase and crystal structure analysis Fig. 1 shows the XRD patterns of Ba2V2O7:xSm3+ samples and the standard data of Ba2V2O7 (PDF#76-612). Fig. 1(2), (3), (4) and (5) represent the XRD patterns of Ba2V2O7:xSm3+ samples with x=0, 0.01, 0.02 and 0.03, respectively. The XRD patterns of Fig. 1(2), (3), (4) and (5) match well with those of standard cards of Ba2V2O7 (PDF#76-612), indicating that single-phase phosphors Ba2V2O7:Sm3+ were obtained and the dopant of Sm3+ ions had negligible effect on the lattice structure of Ba2V2O7 host. Besides, in Ba2V2O7 host, Ba2+ had four sites in which two sites had 7 coordination numbers, one site had 8 coordination numbers and the last site had 9

Fig. 1 Powder XRD patterns for structural analysis of samples Ba2V2O7:xSm3+ (x=0, 0.01, 0.02, 0.03 for (2), (3), (4), (5), respectively) and the standard cards of Ba2V2O7 (PDF#76-612) (1)

coordination numbers. The ion radii with different coordination numbers in Ba2V2O7 are shown in Table 1[22]. Since the revised effective ionic radii of Sm3+ were close to those of Ba2+, it was very easy for Sm3+ ions to substitute the Ba2+ of the host lattice. Fig. 2 shows the structure of Ba2V2O7 and V2O74– dimer group. In Ba2V2O7 lattice, vanadium atom is tetrahedrally coordinated and two VO4 tetrahedra form dimer (V2O7)4– group by sharing one oxygen atom[23]. Because the emission originated from the charge-transfer (CT) of VO43– group is strongly dependent on the structure of VO43– group in the self-activated Ba2V2O7 host[16,17], the structure of V2O74– group in Ba2V2O7 is shown in Fig. 2(b). It can be seen from Fig. 2(b) that the Table 1 Revised effective radii (10–1 nm) of Ba2+ and Sm3+ ions with different coordination number (CN) in Ba2V2O7 Ions 2+

Ba

Sm3+

CN=7

CN=8

CN=9

1.38

1.42

1.47

1.02

1.079

1.32

Fig. 2 Schematic crystal structure diagrams of Ba2V2O7 (a) The structure of Ba2V2O7 unit cell (consisting of four different VO4 dimers) (V5+ ions outside the unit cell are not plotted); (b) The structure of one VO4 dimer (V2O74– group)

LI Fei et al., Synthesis and luminescent properties of Ba2V2O7:Sm3+

two VO4 tetrahedra are distorted when combined together to form a dimer. 2.2 Reflection spectrum The reflection spectrum of the undoped host and the Sm3+-doped host were also measured. As shown in Fig. 3, both the pure host and the doped host are subject to the same reflection trough at around 300 nm, which was due to the strong absorption of V2O74– group from 1A1 to 1T2. The trough structure around 472 nm was ascribed to the absorption of V2O74– group from 1A1 to 3T2, which was more prominent in the reflection spectrum of the pure host. The band at 1083 nm found only at the reflection spectrum of the Sm3+-doped sample could be attributed to the intra-configurational 4f-4f transition 3H5/2→6F11/2 of Sm3+. 2.3 Photoluminescence analysis The excitation spectrum (monitoring 646 nm emission) and emission spectrum (excited at 341 nm) of the sample Ba1.97V2O7:0.03Sm3+ are presented in Fig. 4. The emission spectrum (excited at 341 nm) contains two groups of emission coming from two distinctly different origins.

Fig. 3 Reflection spectrum of the undoped host Ba2V2O7 and the doped host Ba2V2O7:Sm3+

Fig. 4 PLE (monitoring 646 nm emission) and PL (excited at 341 nm) spectra of Ba1.97V2O7:0.03Sm3+ (Each peak is marked with their corresponding transitions. CT stands for charge transfer of V-O in V2O74– group)

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One is a blue-greenish broadband ranging from 400 to 590 nm with a maximum at about 490 nm. It can be attributed to the direct band gap transition from conduction band (CB) to valence band (VB) of V2O74– group[17,18,23]. The second group of emissions are three narrow lines located at 561 nm (green), 599 nm (orange) and 646 nm (red), respectively, which can be ascribed to the 4 G5/2→6H5/2, 4G5/2→6H7/2 and 4G5/2→6H9/2 transitions of Sm3+ ions[19–21]. Among the three peaks, the orange emission peak at 599 nm corresponding to the electric dipole 4 G5/2→6H7/2 transition obviously dominated. The excitation spectrum (monitoring at 646 nm) had a broadband ranging from 260 to 395 nm with the maximum occurring at about 341 nm, which corresponded to the charge transfer state (CTS) transition from valence band (VB) to conduction band (CB) of V2O74– group[17,18,23]. The excitation peaks attributed to the characteristic intrinsic 4f–4f transitions of Sm3+ can also be found at 405 nm, but much weaker compared with that of CTS transition, indicating that the energy transfer from O-V charge transfer (CT) bands of the V2O74– group to Sm3+ ions was very efficient. To investigate the Sm3+ concentration dependence of the emission properties, the emission spectra of Ba2V2O7: xSm3+ (x=0, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07 and 0.08, respectively) phosphors under excitation at 341nm are recorded in Fig. 5. It is clear from Fig. 5 that all the samples have similar emission spectral shape. Besides, the integral intensity of emission origin from V2O74– group and Sm3+ ions are both shown in Fig. 6. As the doping concentration of Sm3+ increases, the integral intensity of broadband around 490 nm is reduced monotonically due to the V2O74– group, and meanwhile the integral intensity of Sm3+ ions emission is enhanced slowly compared with V2O74– group. This conspicuous phenomenon further confirmed the energy transfer from the vanadate host to the Sm3+ activator. In our case, the energy transfer was enhanced as the doping concentration of Sm3+ increased at least until 8%. However, the total quantum efficiency would decrease with the increase of

Fig. 5 PL spectra of Ba2V2O7:xSm3+ (x=0, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07 and 0.08, respectively) under excitation at 341 nm

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Fig. 6 Dependence of PL intensity of V2O74– emission (498 nm) and Sm3+ emission (599 nm) on the concentration of Sm3+ in Ba2V2O7:xSm3+ phosphor

the Sm3+ concentration, since the additional nonradiative quenching ways were introduced with the doping of Sm3+ ions. The decay curves of host with different Sm3+ concentration are shown in Fig. 7. The samples Ba2V2O7:xSm3+ (x=0, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06 and 0.07, respectively) were excited by a pulsed laser of 355 nm with a rate of 10 Hz, and the intensity of host was recorded at 490 nm. It is found that the decay time decreases slowly as the Sm3+ concentration increases from 0 to 0.06, and then almost unchanged to 0.07. The results further confirmed the energy transfer from host to Sm3+ ions, and the transfer efficiency would enhance with the increase of Sm3+ concentration at least until x=0.06. The energy transfer in the system can be described based on the energy diagram as shown in Fig. 8. As shown in Fig. 2(b), two VO43– group formed a dimer V2O74– group by sharing one oxygen atom[17,23], thus the spin triplet (3T1,2) and two spin singlet states (1T1,2 and 1 A1,2) of VO43– group[23] were respectively diffused. When excited, a non-radiative transition occurred from

Fig. 7 The decay curves of host with different Sm3+ concentration (x=0, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06 and 0.07, respectively). The samples were excited by a pulsed Laser of 355 nm with a rate of 10 Hz, and the intensity of host was recorded at 490 nm

JOURNAL OF RARE EARTHS, Vol. 35, No. 2, Feb. 2017

Fig. 8 Energy diagram of V2O74– group and Sm3+ ion and energy transfer (ET) process from dimer V2O74– group to Sm3+ ions in Ba2V2O7:Sm3+ phosphors

the spin singles (1T) level to the spin triplet (3T) in the excited states of the vanadate group V2O74– [17,23]. When Sm3+ ions were doped, new energy levels occurred and part of energy available in V2O74– group was absorbed by Sm3+ ions and the electrons of Sm3+ ion was raised from the ground state 6H5/2 to the high level excited states. Then these excited electrons would relax to the lower level by a non-radiative process, finally, the 4G5/2 level was populated. As de-excitation occurred in a radiative way from 4G5/2 to 6HJ (J=5/2, 7/2, and 9/2), the photoluminescence emission of Sm3+ could be obtained. In this way, energy transfer (ET) process from dimer V2O74– group to Sm3+ ions in Ba2V2O7:Sm3+ phosphors happened. The above phenomenon was also reported in other phosphors such as Sr3(VO4)2:Sm3+ [20]. The color coordinates and correlated color temperature (CCT) of Ba2V2O7:Sm3+ phosphor samples with different dopant Sm3+ ion concentrations excited at 341 and 365 nm[20,24,25] are listed in Table 2. The corresponding color coordinates under excitation at 341 and 365 nm are also shown in the Commission Internationale de L'Eclairage (CIE) diagram in Fig. 9. When excited at 341 nm, the color tone of the samples shifts gradually from blue-greenish (0.233, 0.354) to white (0.316, 0.365) and eventually to orange (0.393, 0.375) as doping concentration of Sm3+ ions increased, implying that the CIE chromaticity coordinates would be tunable via adjusting the concentration of Sm3+ ions and the phosphor with different color compositions can be obtained via changing the Sm3+ concentration and white light can be achieved at an appropriate doping concentration. Therefore, the white light can be obtained through simple alteration of the doping Sm3+ concentration to meet the needs of different illumination applications. To confirm the potential applications for WLEDs with near-UV chips, the samples were also excited at 365 nm which was the ordinary UV-LED emission wavelength[24,25] and white

LI Fei et al., Synthesis and luminescent properties of Ba2V2O7:Sm3+

139

Fig. 9 CIE chromaticity diagram for sample Ba2V2O7:xSm3+ (x=0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07 and 0.08, respectively) excited at 341 nm (a) and 365 nm (b) (The inset is the photograph of the Ba1.95V2O7:0.05Sm3+ powder excited at 365 nm) Table 2 Color coordinates (X, Y), color temperature (CCT), dominant wavelength (DW) and color purity (C.P.) of Ba2V2O7:xSm3+ (x=0, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07 and 0.08, respectively) under excitation at 341 nm (a) and 365 nm (b) (a) Sample

X

Y

CCT/K

DW/nm

C.P./%

Ba2V2O7

0.233

0.354

10882

494

32.9849

Ba2V2O7:0.01Sm3+

0.242

0.355

10241

495

29.3431

Ba2V2O7:0.02Sm3+

0.248

0.356

9822

495

27.6001

Ba2V2O7:0.03Sm3+

0.256

0.357

9300

496

24.5106

Ba2V2O7:0.04Sm3+

0.27

0.358

8465

497

19.897

3+

Ba2V2O7:0.05Sm

0.289

0.361

7417

500

13.5922

Ba2V2O7:0.06Sm3+

0.316

0.365

6156

518

6.41958

Ba2V2O7:0.07Sm3+

0.352

0.369

4814

571

16.4703

Ba2V2O7:0.08Sm3+

0.393

0.375

3673

583

30.3991

Sample

X

Y

CCT/K

DW/nm

C.P./%

(b)

Ba2V2O7

0.245

0.36

10882

494

32.9849

Ba2V2O7:0.01Sm3+

0.245

0.36

9886

496

27.9656

Ba2V2O7:0.02Sm3+

0.248

0.362

9643

498

27.2836

Ba2V2O7:0.03Sm3+

0.261

0.368

8754

498

22.618

3+

Ba2V2O7:0.04Sm

0.281

0.378

7592

503

16.0842

Ba2V2O7:0.05Sm3+

0.314

0.394

6135

535

13.5586

Ba2V2O7:0.06Sm3+

0.358

0.414

4785

565

31.5897

Ba2V2O7:0.07Sm3+

0.406

0.419

3697

576

47.6427

3+

0.461

0.402

2623

586

58.9147

Ba2V2O7:0.08Sm

temperature dependence of the luminescence, the emission spectrum of the sample Ba1.96V2O7:0.04Sm3+ excited at 355 nm at different temperature from 25 to 210 ºC were recorded as presented in Fig. 10. When the temperature is risen, the intensity of emission due to the V2O74– group is quenched rapidly, and meanwhile the intensity of Sm3+ ions emission only decreases slowly comparing with V2O74– group. Besides, the broadband around 490 nm due to the V2O74– group is split into two broadband in which one is due to the transition from 3T2 to 1A1 around at 472 nm, the another is due to the transition from 3T1 to 1A1 around at 533 nm. The corresponding color coordinates of sample Ba1.96V2O7: 0.04Sm3+ under an excitation of 355 nm at different temperatures are shown in the CIE diagram in Fig. 11. When excited at 341 nm, the color tone of the samples shifts gradually from bluish white (0.283, 0.360) at 25 ºC and eventually to orange (0.585, 0.436) at 210 ºC as the temperature raised. This color shifting caused by the temperature quenching of the luminescence is disadvantageous for the practical WLED applications. The further efforts should be put into the optimization of the sample prepa-

light can be obtained at the doping concentration of 0.05 mol.% with chromaticity coordinates of (0.314, 0.365) and CCT of 6135 K. Therefore the Ba1.95V2O7:0.05Sm3+ recommends itself to be a promising single-phased converting phosphor working for WLEDs with near-UV chips. 2.4 Thermal stabilities For applications of high power LEDs, thermal stability of phosphor should be considered. To characterize the

Fig. 10 Emission spectrum of the sample Ba1.96V2O7:0.04Sm3+ excited at 355 nm at different temperatures from 25 to 210 °C

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Fig. 11 CIE chromaticity diagram for sample Ba1.96V2O7:0.04 Sm3+ excited at 355 nm at different temperatures from 25 to 210 °C

ration to eliminate the various quenching centers so as to improve its thermal stability.

3 Conclusions A novel single-phased white-light phosphor Ba2V2O7: Sm3+ was synthesized by traditional high-temperature solid-state method. X-ray powder analysis confirmed the formation of single phase of Ba2V2O7. The excitation spectrum of Ba2V2O7:Sm3+ showed a broadband ranging from 260 to 395 nm, which could be attributed to the absorption of (VO4)3– group while the excitation peaks originated from Sm3+ were weak compared with VO43– group. Under the UV excitation (341 nm), the Ba2V2O7:Sm3+ generates both blue-greenish emission attributed to (VO4)3– and orange emission attributed to Sm3+. The CIE coordinates and color temperature of Ba1.95V2O7:0.05Sm3+ phosphor were (0.314, 0.365) and 6135 K under excitation at 365 nm, suggesting itself to be a potential candidate as the single-phased converting phosphor working for WLEDs with near-UV chips.

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