Enhanced red emission of NaSrVO4:Eu3+ phosphor via Bi3+co-doping for the application to white LEDs

Enhanced red emission of NaSrVO4:Eu3+ phosphor via Bi3+co-doping for the application to white LEDs

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Ceramics International xx (xxxx) xxxx–xxxx

Contents lists available at ScienceDirect

Ceramics International journal homepage: www.elsevier.com/locate/ceramint

Enhanced red emission of NaSrVO4:Eu3+ phosphor via Bi3+co-doping for the application to white LEDs ⁎

Yu Zenga, Kehui Qiub, , Ziqi Yanga, Yunlei Bua, Wentao Zhanga, Junfeng Lia a b

College of Materials and Chemistry & Chemical Engineering, Chengdu University of Technology, Chengdu 610059, China Institute of Materials Science and Technology, Chengdu University of Technology, Chengdu 610059, Sichuan, China

A R T I C L E I N F O

A BS T RAC T

Keywords: White LEDs NaSrVO4:Eu3+, Bi3+ Red emitting phosphors Luminescence properties

A series of NaSrVO4:Eu3+ and NaSrVO4:Eu3+, Bi3+ phosphors were synthesized by the combustion method fueled by citric acid. The structure and photoluminescence properties of the phosphors were examined by X-ray powder diffraction (XRD) scanning electron microscope (SEM) and photoluminescence spectroscopy (PL). The samples are well crystallized in the monoclinic phase with the P21/n space group. According to the photoluminescence spectra, the phosphors are efficiently excited by near-UV light and exhibited a bright red emission centered at ~625 nm. The photoluminescence intensities of the NaSrVO4:Eu3+, Bi3+ phosphors are significantly enhanced by the efficient energy transfer between Bi3+ and Eu3+. The emission intensity of NaSrVO4 is maximized by co-doping with 15 mol% Eu3+ and 3 mol% Bi3+. The sensitization mechanism was also deduced from energy level diagrams of the Eu3+ and Bi3+ ions.

1. Introduction The serious global energy crisis and challenges of climate change have driven the development of white light-emitting diodes (w-LEDs) for solid-state lighting applications. Besides being environmentally friendly, w-LEDs deliver high quantum efficiency, long lifetime, and good color reducibility [1–3]. Commercial w-LEDs are currently fabricated by combining a blue LED chip with yellow phosphor (YAG:Ce3+) [4]. However, the deficiency of red emission degrades the color rendering index of these w-LEDs (CRI < 80) and limits their use to high color temperatures [5,6]. The above drawbacks could be resolved by constructing a UV chip that pumps red, green, and blue phosphors [7], or by combining a blue chip with red and blue phosphors [8]. Both of these methods require efficient red-emitting phosphors that strongly absorb from the UV to the blue region, and exhibit high thermal and chemical stability. Moreover, the color purity, efficiency and stability of the existing red phosphors are inferior to those of blue and green phosphors. Thus, novel red-emitting phosphors are urgently sought. In the past few decades, rare earth ions doped with vanadate phosphors have become widely applied in solid-state lighting. The utility of these phosphors is attributed to the excellent properties of the vanadates; namely, high chemical stability, high thermal stability, good crystallinity and high visible light transparency [9–13]. They also exhibit broad and intense O2-–V5+ charge transfer bands (CTB) in



the UV wavelength region [14–18]. Therefore, energy can be efficiently transferred between VO43- and the rare earth ions, enhancing the emission intensity of the luminescent center. Among the rare-earth ions, Eu3+ is especially promising for its narrow and intense red emission at approximately 620 nm, which originates from 5D0→7F2 transitions. The typical absorptions at approximately 393 nm and 463 nm match well with those of conventional n-UV (380–410 nm) and blue (450–470 nm) LED chips, respectively. Therefore, Eu3+-attracted vanadate phosphors have been extensively researched. Recently, rare-earth ions doped with alkali–alkaline orthovanadate (A+B2+VO43-) have attracted much attention owing to their potential application in solid-state lighting such as NaSrVO4 and NaCaVO4 [19]. As one of the vanadates, they possess the excellent properties of vanadates (high chemical stability, high thermal stability, good crystallinity and high visible light transparency); also, the alkali metal and alkaline earth metal oxide is more abundant and less expensive than yttrium oxide for commercially used red phosphors (YVO4:Eu3+). In this work, Eu3+ doped NaSrVO4 are synthesized by a combustion method fueled by citric acid and the photoluminescence properties are also investigated. To enhance the red emission of NaSrVO4:Eu3+ phosphor, the Bi3+ ions are doped as a sensitizer. We show that doping with small amounts of Bi3+ significantly improves the photoluminescence intensity of the NaSrVO4:Eu3+ phosphors. We also systematically investigate the influences of the activator (Eu3+) and sensitizer (Bi3+)

Corresponding author. E-mail addresses: [email protected], [email protected] (K. Qiu).

http://dx.doi.org/10.1016/j.ceramint.2016.10.016 Received 5 September 2016; Received in revised form 30 September 2016; Accepted 1 October 2016 Available online xxxx 0272-8842/ © 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Please cite this article as: Zeng, Y., Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.10.016

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Fig. 3. Excitation spectra of NaSr1-xVO4:xEu3+(0≤x≤20 mol%) phosphors monitored at 625 nm. Fig. 1. X-ray diffraction patterns of (a) NaSrVO4, (b) NaSr0.85VO4:0.15Eu3+, (c) NaSr0.82VO4:0.15Eu3+, 0.03Bi3+ phosphors annealed at 900 ◦C for 1 h.

concentrations on the photoluminescence intensity. 2. Experimental 2.1. Preparation A series of NaSr1-x-yVO4:xEu3+, yBi3+ were prepared by the combustion method. The raw materials were high-purity europium oxide (Eu2O3), analytical reagent strontium nitrate (Sr(NO3)2), bismuth nitrate pentahydrate (Bi(NO3)3·5H2O) ammonium metavanadate (NH4VO3), citric acid (C6H8O7·H2O) and 2 mol/L nitric acid (HNO3). The proper amount of Eu2O3 was dissolved in HNO3 and mixed with a stoichiometric ratio of Sr(NO3)2, Bi(NO3)3·5H2O, NH4VO3, and citric acid in distilled water. The mixed solution was heated at 70– 80 °C with stirring for 30 min, forming a blue–black viscous sol precursor. Finally, the precursor was placed in a furnace, which had been preheated to 900 °C, and heated for 1 h in air. Passive cooling yielded the red emission phosphors.

Fig. 4. Emission spectra of NaSr1-xVO4:xEu3+(0≤x ≤20 mol%) phosphors under excitation at 393 nm. Inset plots the relation between the emission intensity of the 5D0→7F2 transition of NaSr1-xVO4:xEu3+ phosphors and the doping concentration of Eu3+ ions. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

2.2. Characterization

slit width was 1 nm.

The crystal structures of the red emitting phosphors were evaluated by X-ray diffraction (XRD, X′pert PRO PANalytical diffractometer) with Cu–Kα radiation (operated at 40 kV and 30 mA). The diffraction angle 2θ was ranged from 10° to 60° using a step size of 0.05°. The morphologies were characterized by a scanning electron microscope (SEM, Inspect, F50). The emission and excitation spectra of these synthesized phosphors were recorded on a Hitachi F-4600 spectrometer equipped with a 150 W Xe lamp at a scan speed of 10 nm/s. The

3. Results and discussion Fig. 1 shows XRD patterns of NaSrVO4, NaSr0.85VO4:0.15Eu3+, and NaSr0.82VO4:0.15Eu3+, 0.03Bi3+. All diffraction peaks can be indexed to the monoclinic phase of NaSrVO4 (JCPDS:32–1160) with the space group P21/n (a=7.22 Å, b=9.83 Å, c=5.73 Å, V=405.38 Å3, β=94.57°).

Fig. 2. SEM images of (a) NaSrVO4, (b) NaSr0.85VO4:0.15Eu3+, and (c) NaSr0.82VO4:0.15Eu3+, 0.03Bi3+.

2

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Fig. 5. (a) Excitation spectra of NaSr0.85-yVO4:0.15Eu3+, yBi3+(0≤y ≤4 mol%) phosphors and (b) Gaussian fittings to the excitation spectrum of NaSr0.82VO4:0.15Eu3+, 0.03Bi3+.

Fig. 6. Emission spectra of NaSr0.85-yVO4:0.15Eu3+, yBi3+(0≤y ≤4 mol%) phosphors.

Fig. 8. Energy level diagrams of Bi3+ and Eu3+ ions, and the energy transfer process from Bi3+ to Eu3+ in NaSrVO4:Eu3+, Bi3+ phosphor.

(V=405.38 Å3), which is caused by the slightly smaller ionic radii of Eu3+(0.095 nm) and Bi3+(0.096 nm) than that of Sr2+(0.112 nm). Similar results were reported in Sm3+ doped NaSrVO4 [20]. Fig. 2 shows the SEM images of (a) NaSrVO4, (b) NaSr0.85VO4:0.15Eu3+, and (c) NaSr0.82VO4:0.15Eu3+, 0.03Bi3+, respectively. The morphology of all the samples is a uniform rod bunches with an agglomeration. Moreover, the particles are all in the micrometer range, which is appropriate for w-LEDs applications. The photoluminescence excitation (PLE) spectra of the NaSr13+ xVO4:xEu (0≤x≤20 mol%) phosphors monitored by 625 nm emission is shown in Fig. 3. The broad band in the 200–350 nm range can be ascribed to O2-→V5+ and O2-→Eu3+ CTB transitions [21,22]. The sharp lines in the 360–500 nm range correspond to f–f transitions of the Eu3+ ions. The peaks centered at approximately 360, 380, 393, 415, and 464 nm correspond to 7F0→5D4, 7F0→5L7, 7F0→5L6, 7F0→5D3, and 7 F0→5D2 transitions, respectively [23]. The emission spectra of NaSr1-xVO4:xEu3+(0≤x≤20 mol%) under excitation with 393 nm near-UV light are shown in Fig. 4. The dominant red emission bands at ~618 and 625 nm are attributed to the electric dipole transition 5D0→7F2, indicating that the Eu3+ ions locate at the sites of non-inversion symmetry. The other weak peaks centered around 538, 592, 659 and 705 nm correspond to 5D1→7F1, 5 D0→7F1, 5D0→7F3, and 5D0→7F4 transitions, respectively [24,25]. The inset in the top right hand corner plots the intensity of 5D0→7F2 emission by the NaSr1-xVO4:xEu3+ phosphors versus the Eu3+ ion

Fig. 7. Intensities of 5D0→7F2 emission of NaSr0.85-yVO4:0.15Eu3+, yBi3+(0≤y ≤4 mol %) samples under different excitation wavelengths.

Impurity peaks related to Bi and Eu are not observed, indicating that the samples were single-phase and their crystal structure was unaffected by small doping of Eu3+ and Bi3+ ions. The corresponding unit cell volumes of NaSr0.85VO4:0.15Eu3+ and NaSr0.82VO4:0.15Eu3+, 0.03Bi3+ in Fig. 1 were V=402.77 Å3 (a=7.213 Å, b=9.812 Å, c=5.708 Å, β=94.49°), and V=401.39 Å3 (a=7.209 Å, b=9.797 Å, c=5.699 Å, β=94.29°), respectively. It should be noted that doping with Eu3+ and Bi3+ decreases the volume of pristine NaSrVO4 3

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Fig. 9. (a) Integrated areas of the emissions caused by the 5D0→7F2 and 5D0→7F1 transitions and (b) the ratio of 5D0→7F2 to 5D0→7F1 transitions for the NaSr0.85-yVO4:0.15Eu3+, yBi3+(0≤y≤4 mol%) as a function of Bi3+ concentration. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

yBi3+, (0≤y ≤4 mol%) samples monitored at 625 nm emission. The intensity of the CT band of O2−→V5+ and the 7F0→5L6, 5D2 transitions of Eu3+ were largely improved at 3 mol% Bi3+. In addition, as the doping concentration of Bi3+ increased from 0 to 4 mol%, the CT band remarkably shifted from 277 nm to 294 nm, possibly because the absorption band of Bi3+ overlapped the CT band of the O2−→V5+ transition. To confirm this speculation, we conducted a peak differentiation and imitation analysis. As shown in Fig. 5(b), the broad band within the 200–350 nm range separated into two peaks at ~272 nm and ~305 nm, corresponding to the VO43- group transition and the 1 S0→3P1 transition of Bi3+ ions, respectively [22]. These results favorably agree with our speculation. The 1S0→3P1 transition of Bi3+ ions strongly suggests a sensitizer role for the Bi3+ ions, with energy transfer between the Bi3+ and Eu3+ ions [27,28]. Fig. 6 shows the emission spectra of NaSr0.85-yVO4:0.15Eu3+, 3+ yBi (0≤y ≤4 mol%) phosphors under excitation at 393 nm. All profiles present the same peaks at different relative intensities. As before, the dominant emissions are the 5D0→7F2 transitions of Eu3+. The intensity of these emissions increases with increasing Bi3+ concentration up to 3 mol%. This result is attributable to energy transfer from Bi3+ to Eu3+ and the increasing distortion of the crystal structure of NaSrVO4. However, when the doping concentration of Bi3+ exceeds the optimum level of 3 mol%, the excess Bi3+ can aggregate, forming trapping centers that dissipate the absorbed energy instead of transferring it to activator ions [29,30]. This energy dissipation reduces the red emissions originating from the 5D0→7F2 transition of Eu3+. Fig. 7 plots the intensities of the 5D0→7F2 transition in the NaSr0.853+ 3+ phosphors as functions of Bi3+ concentration yVO4:0.15Eu , yBi under different excitation wavelengths. To better understand the energy transfer mechanism in the NaSr13+ 3+ phosphors, we constructed energy level diagrams x-yVO4:xEu , yBi of the Bi3+ and Eu3+ ions, and determined the energy transfer process from Bi3+ to Eu3+ in the NaSrVO4:Eu3+, Bi3+ phosphor. The diagrams are presented in Fig. 8. As is well known, Bi3+ ion has an outer 6S2 electronic configuration. Its ground state is a spin-orbit singlet (1S0), and its excited state has a 6s6p configuration. The lower spin-parallel state (6s6p) yields a low-energy 3P state, which splits into 3P0, 3P1, and 3 P2 (in order of increasing energy) due to the spin-orbit interaction [31]. Alternatively, the Bi3+ ions in the Bi3+ doped NaSrVO4:Eu3+ phosphors are pumped into excited states, and energy is transferred from the higher energy state to the 5D4 level of Eu3+. Thus, Bi3+ ions behave as sensitizers that significantly improve the PL intensity of the NaSrVO4:Eu3+ phosphors. The integrated areas of the emission intensities at 625 nm (5D0→7F2) and 592 nm (5D0→7F1) as a function of the Bi3+ concentration are shown in Fig. 9(a). As is known, the 5D0→7F2 and 5D0→7F1

Fig. 10. CIE 1931 chromaticity diagram for (a) commercial Y2O3:Eu3+, (b) NaSr0.85VO4:0.15Eu3+, (c) NaSr0.82VO4:0.15Eu3+, 0.03Bi3+, and (d) standard red light under 393 nm excitation. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

doping concentration. The intensity of this emission strongly depends on the Eu3+ concentration. More specifically, the emission intensity is maximized at an Eu3+ concentration of 15 mol% (x=15 mol%), and reduces at higher and lower concentrations. This trend can be explained by the concentration quenching effect. As the concentration of Eu3+ ions increases, the distance between the ions reduces, enabling energy migration among the activators. According to Blasse, the critical transfer distance (Rc) of concentration quenching is given by [26]:

⎛ 3V ⎞1/3 R C=2 × ⎜ ⎟ ⎝ 4πxc N ⎠

(1)

where V is the volume of the unit cell, xc is the critical concentration of activator (Eu3+), and N is the number of cations in the NaSrVO4 unit cell. Setting xc, V, and N to 0.15, 402.77 Å3 and 4, respectively, the critical energy transfer distance is approximated as 10.86 Å, indicating that concentration quenching of Eu3+ in NaSrVO4:Eu3+ is dominated by multipolar interaction. Fig. 5(a) presents the excitation spectra of NaSr0.85-yVO4:0.15Eu3+, 4

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transitions are responsible for the red and orange light emissions, respectively. Thus, the stronger intensities of 5D0→7F2 than 5D0→7F1 transition indicate high red color purity of Bi3+co-doped NaSrVO4:Eu3+ phosphors [32–34]. Fig. 9(b) exhibits the ratio of 5D0→7F2 to 5D0→7F1 transition, which is a direct embodiment of red color purity. Obviously, the photoluminescence properties of the NaSrVO4:Eu3+, Bi3+ are enhanced by controlling the concentration of Bi3+ and the optimum doping concentration of Bi3+ is 3 mol%. The performance of the phosphors was evaluated by determining their color coordinates using the Commission International de l′Eclairage (CIE) 1931 calculator program. Under excitation at 393 nm, the CIE chromaticity coordinates of NaSr0.85VO4:0.15Eu3+ and NaSr0.82VO4:0.15Eu3+, 0.03Bi3+ were (x=0.644, y=0.356) and (x=0.658, y=0.342), respectively (see Fig. 10). These color coordinates more closely approximate those of the NTSC standard (x=0.67, y=0.33) than commercial red emitting phosphor Y2O3:Eu3+(x=0.64, y=0.36). That is, the Bi3+ sensitized NaSrVO4:Eu3+ phosphors are eminently suitable for white LEDs. 4. Conclusion In conclusion, we synthesized a series of NaSrVO4:Eu3+ and NaSrVO4:Eu3+, Bi3+ phosphors by the combustion method. Under excitation with near-UV light (393 nm), all samples emitted intense red light peaking at ~625 nm. The emission originated from the 5D0→7F2 electric dipole transition of Eu3+ ions. We also systematically evaluated the influences of the activator (Eu3+) and sensitizer (Bi3+) concentrations on the emission intensity. The optimum concentrations of Eu3+ and Bi3+ in NaSr1-x-yVO4:xEu3+, yBi3+ were x=0.15 and y=0.03. As a sensitizer, Bi3+ absorbs the excitation energy and transfers it to the Eu3+ ions, enhancing the red emission of Eu3+ ions. The CIE chromaticity coordinates (x=0.658, y=0.342) of the NaSr0.82VO4:0.15Eu3+, 0.03Bi3+ phosphor approximated the NTSC standard values. All of these results indicate the suitability of this novel red phosphor in white LEDs. Acknowledgement We would like to acknowledge the financial supports from the Key Scientific and Technological Research and Development Program (Grant no. 2014GZ0090) in Sichuan Province, PR China. References [1] F. Hong, L. Zhou, L. Li, Q. Xia, X. Luo, Combustion synthesis and luminescent properties of red-emitting Ca4-xAl6WO16:xeu3+ phosphors and photoluminescence enhancement by Bi3+ co-doping, Opt. Commun. 316 (2014) 206–210. [2] J. Zhao, C. Guo, J. Yu, R. Yu, Spectroscopy properties of Eu3+ doped Ca9R(VO4)7 (R=Bi, La, Gd and Y) phosphors by sol-gel method, Opt. Laser Technol. 45 (2013) 62–68. [3] S.S. Pitale, M. Gohain, I.M. Nagpure, O.M. Ntwaeaborwa, B.C.B. Bezuidenhoudt, H.C. Swart, A comparative study on structural, morphological and luminescence characteristics of Zn3(VO4)2 phosphor prepared via hydrothermal and citrate-gel combustion routes, Physica B: Condens. Matter 407 (2012) 1485–1488. [4] S.NakamuraG.Fasol, The blue laser diose, Berlin Heidelberg, 1997. [5] X. He, K. Qiu, X. Lu, K. Zhao, Z. Jiang, Enhanced luminescence properties in (Sr12+ oxynitride phosphor, J. Solid State Chem. 220 (2014) xBax)2.97SiO3N4/3:0.03Eu 172–176. [6] W. Zhang, J. Li, Y. Wang, J. Long, K. Qiu, Synthesis and luminescence properties of NaLa(MoO4)2-xAGx:Eu3+(AG=SO42-, BO33-) red phosphors for white light emitting

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