Co-precipitation synthesis, photoluminescence properties and theoretical calculations of MgWO4:Eu3+ phosphors

Xu Fenga, Wenlin Feng*ab, Meng Xiaa, Ke Wanga, Honggang Liuc, Dashen Denga, Xiang Qina, Weikun Yaoa and Wentao Zhua
aDepartment of Applied Physics, Chongqing University of Technology, Chongqing 400054, China. E-mail: wenlinfeng@126.com; Tel: +86 23 6256 3272
bChongqing Key Laboratory of Modern Photoelectric Detection Technology and Instrument, Chongqing 400054, China
cDepartment of Material Science, Sichuan University, Chengdu 610064, China

Received 28th October 2015 , Accepted 23rd January 2016

First published on 27th January 2016


Abstract

A novel red light emitting phosphorescent material MgWO4:Eu3+ is successfully synthesized by the co-precipitation method. The structure, luminescent properties and fluorescence lifetime of the prepared phosphors are characterized using X-ray diffraction (XRD), photoluminescence and luminescence decay spectra. The results indicate that an effective electric dipole transition 5D07F2 of Eu3+ red emission with maxima at 615 nm can be achieved in the MgWO4 host while being excited by near-UV light (393 nm) or blue light (464 nm). The CIE chromaticity coordinates of MgWO4:Eu3+ phosphors exhibit emission in the red-light region. Moreover, to explain the fluorescence spectra of MgWO4:Eu3+ phosphors, a complete energy matrix is successfully built by an effective operator Hamiltonian including free ion and crystal field interactions. The fluorescence spectra for the Eu3+ ion at the monoclinic (C2v) Mg2+ site of the MgWO4 crystal are calculated from this matrix. The fitting values are in agreement with the observed results.


1. Introduction

Luminescent materials containing rare earth ions are able to absorb energy in the ultraviolet or blue regions and emit visible light. Recently, these materials have drawn increasing interest due to their potential applications in display devices, fluorescent labels, white light emitting diodes, etc.1–3 Rare earth ions have been playing an important role in phosphors due to the abundant emission color based on their 5d–4f or 4f–4f transitions, such as the trivalent europium ion, which is a red emitting activator due to its 5D07FJ (J = 0–6) transitions. The Eu3+ ion is expected to be one of the most promising species that provides optical devices in the red color region and many investigations have been carried out in various hosts. For example, Eu3+ activated tungstate or molybdate phosphors have excellent luminescent efficiency, color purity and stability.4–6 They can be well excited by ultraviolet (393 nm) or visible light (464 nm) which is nicely in agreement with the widely applied near-UV or blue LED chips. Therefore, they have attracted much attention in recent years. Many investigations, such as those on Ba1−xMoO4:xEu3+,7 CaMoO4:Eu,8 (Ba1−xEux)WO4,9 SiO2@CaMoO4:Eu3+,10 (Ca1−xyLuy)MoO4:xEu3+,11 and (Ca1−xyLny)MoO4:xEu3+ (Ln = Y, Gd),12 etc., have been developed to compensate LED for the red deficiency of the output light. Comparison of two methods of preparation of the traditional solid-state and co-precipitation method, the solid-state method for preparing phosphors has a few disadvantages, such as high calcination temperature and large grain size of the samples (generally more than 1100 °C and 2 μm).8,13 Compared with the conventional solid-state method, the co-precipitation method is a low-cost, controllable particle size and low energy consumption synthesis technique for preparing phosphors.14,15 Thus, in order to improve the crystallite size and its distribution, Eu3+ doped MgWO4 phosphors are fabricated by using co-precipitation method instead of the common solid-state reaction. Their structure, luminescent properties and fluorescence decay time are studied in detail. At the same time, taking into account that a fundamental theoretical understanding of the fluorescent mechanism for Eu3+ ion at the monoclinic (C2v) Mg2+ site of MgWO4 crystal has not been set up, the explanation for energy levels of MgWO4:Eu3+ by crystal-field theory is necessary. Thus, in the present study, the fluorescent spectra for Eu3+ ion at the monoclinic (C2v) Mg2+ site of MgWO4 crystal are calculated by the complete diagonalization (of energy matrix) method. The calculated data are compared with the experimental results.

2. Experimental section

Firstly, Eu2O3 (99.99%), MgCl2 (A.R.), Na2WO4 (A.R.), HCl (A.R.) aqueous solutions were prepared, respectively. Then according to the appropriate stoichiometric ratio, MgCl2 and/or Eu2O3, HCl solutions were added sequentially into Na2WO4 solution under stirring to form a clear and homogeneous mixture. Thirdly, this mixture solution was neutralized by the precipitating aid agent into deposit precursor at 80 °C, wherein NH3·H2O and NH4HCO3 were taken as precipitating aid agent respectively. At last, the precursor was pre-fired at 400 °C for 2 h, and fired at 800 °C for 2 h to improve the crystallization and luminescent intensity of MgWO4:Eu3+.

The XRD measure of phosphor samples was taken on a X-ray diffractometer (Shimadzu, XRD-7000S/L) with Cu Kα radiation (λ = 0.15406 nm) operating at 40 kV and 30 mA. The morphology and energy dispersive X-ray microanalysis (EDX) spectrum of the product were characterized using a scanning electron microscopy (SEM, MIRA3, Tescan). A fluorescence spectrometer (Hitachi, F-4600 type) was used to record the photoluminescence spectra of the samples. The decay time was characterized by using FLS-980 fluorescence spectrometer (Edinburgh Instruments). All measures were recorded at room temperature.

3. Results and discussions

3.1 Crystal structure

The results of XRD patterns for all the samples present similar profiles which are shown in Fig. 1(a). XRD examination of Mg1−xWO4:xEu3+ samples with x = 0, 0.01, 0.02, 0.03, 0.04 and 0.05 reveals a monoclinic sanmartinite-type of structure. It is found that the peaks from XRD are the phase of MgWO4 for low doping (below about x = 0.05). In Fig. 1(b) we present the EDX spectrum of a little rectangular region, which shows that the elements of the region are Mg, W, Eu and O. It is thus safe to conclude that the phosphor observed here comes from the phase Mg0.96Eu0.04WO4. The nanocrystallite size can be estimated from the Scherrer formula,16 D = /β[thin space (1/6-em)]cos[thin space (1/6-em)]θ, where k (0.89) is a constant and D is the average crystallite size, λ is the X-ray wavelength (0.15406 nm), β is the full width at half maximum (FWHM) of an observed peak and θ is the diffraction angle. By using the Scherrer formula to the FWHM of the ([1 with combining macron]11), (111), (001) and (110) diffraction peaks, the average crystallite sizes of MgWO4:Eu3+ can be calculated as 49.1 nm.
image file: c5ra22631g-f1.tif
Fig. 1 (a) XRD patterns of Mg1−xWO4:xEu3+ (x = 0, 0.01, 0.02, 0.03, 0.04, 0.05). All peaks can be indexed by a monoclinic sanmartinite-type of structure, indicating that the phase here is Mg1−xEuxWO4. (b) The EDX spectrum of Mg0.96Eu0.04WO4. The inset in (b) shows a SEM picture. The little rectangle marks the position where we took the EDX spectrum.

3.2 SEM image of MgWO4:Eu3+

The typical SEM image is shown in Fig. 2. From Fig. 2, it reveals that the surface morphology of the sample (4 mol% of Eu3+ doping case) is irregular. It is made of numerous tiny particles. These particles sizes of MgWO4:Eu3+ are mainly in the range of 0.2–1 μm. It is worth noting that the particle size of the result of SEM is generally different from the crystallite size from the observation in XRD. One particle may be one or more crystallite grains of the aggregates. Thus, the sizes are different in SEM and XRD results.17
image file: c5ra22631g-f2.tif
Fig. 2 SEM image of Mg0.96Eu0.04WO4 phosphor.

3.3 Luminescence of the samples

The excitation (EX) spectra of MgWO4:Eu3+ phosphor are shown in Fig. 3(a). It can be seen that the EX spectrum monitored under 615 nm compose of a wide band centered at 320 nm and several intense sharp lines. The broad band from 290–350 nm is attributed to the charge transfer (CT) transition of O2− → W6+ and O2− → Eu3+.20 Several intense sharp lines in the range of 350–500 nm originate from the intra-configurational 4f → 4f transitions of Eu3+ ions in the host. The peaks centered at 393 nm and 464 nm are corresponding to the 7F05L6 and 7F05D2 transitions respectively. The absorption of 7F05L6 at 393 nm and 7F05D2 at 464 nm are strong, which is coupled well with the characteristic emission from near-UV and blue GaN-based LED chips and their intensities are much stronger than that of CT band. Other peaks locate at 361, 375, 381, 413, 455, 470 and 490 nm, corresponding to 7F05D4, 7F05L7, 7F05L7, 7F05D3, 7F05D2, 7F05D2 and 7F05D1 transitions of Eu3+.
image file: c5ra22631g-f3.tif
Fig. 3 Photoluminescence spectra of Mg1−xWO4:xEu3+ (x = 0, 0.01, 0.02, 0.03, 0.04, 0.05): (a) the emission at λem = 615 nm; (b) the excitation at λex = 393 nm, and (c) λex = 464 nm.

The EM spectra of MgWO4:Eu3+ phosphor under the excitation of 393 and 464 nm respectively are also shown in Fig. 3(b) and (c). All emission (EM) spectra consist of sharp lines with wavelengths ranging from 500 to 750 nm, which are associated with the 5D07FJ (J = 1, 2, 3, and 4) transitions from the excited levels of Eu3+ to the ground state.18,19 It can be observed from Fig. 3 that the intensities of samples increase with the increasing of the dopant concentrations and reach a maximum at 4 mol% Eu, which may be due to the improvement of the lattice sites of the activated centres of the phosphors. Then the EM intensities decrease significantly as the concentrations continuously increase from 4 mol% Eu, which is attributed to the concentration quenching. It indicates that the concentration of activators has an important influence on the lattice of phosphors.

There are no observed differences for the EM band shape and position under the excitation of 393 or 464 nm except for luminescent intensity. Under the excitation of near-UV light (∼393 nm) and blue-light irradiation (∼464 nm), the EM spectra is composed of groups of sharp lines which belong to the characteristic emission of Eu3+ ion. The main emission is at 615 nm, originating from the electric dipole transitions (5D07F2) of Eu3+, which reveals that Eu3+ ions occupy the lattice sites without inversion symmetry. Other f–f transitions of Eu3+ ion, such as 579, 594, 657 and 706 nm, which are associated with the 5D07FJ (J = 0, 1, 3, 4) transitions from the excited levels of Eu3+ to the ground state, are relatively weak. This indicates that it is advantageous to obtain a phosphor with good Commission Internationale de l'Eclairage (CIE) chromaticity coordinates. From the EM spectral data of Fig. 3, the average color coordinates (x ≈ 0.439, y ≈ 0.303) of MgWO4:Eu3+ are obtained as you can see in Fig. 4 and they are in the red region. Therefore, the phosphors MgWO4:Eu3+ can be effectively excited by near-UV and blue LED, and emit red light.


image file: c5ra22631g-f4.tif
Fig. 4 Commission Internationale de l'Eclairage (CIE) average chromaticity diagram for MgWO4:Eu3+ phosphors.

3.4 The decay curves

The typical fluorescence decay pattern of Mg0.96WO4:Eu0.043+ is shown in Fig. 5. The decay curve is nonexponential and well fitted into a two-exponential function21,22
 
I = A1[thin space (1/6-em)]exp(−t/τ1) + A2[thin space (1/6-em)]exp(−t/τ2) (1)
in which τ1 and τ2 are time constants and A1 and A2 are coefficients. The average lifetime can be calculated by using the formula
 
image file: c5ra22631g-t1.tif(2)

image file: c5ra22631g-f5.tif
Fig. 5 Fluorescence decay time of 615 nm emission of Mg0.96WO4:Eu0.043+ under excitation into the 5D0 at 464 nm.

As can be seen, the average lifetime in Mg0.96WO4:Eu0.043+ is 718.16 μs, which is caused by the nonradiative relaxation channels and reduction of the 5D0 lifetime in the Mg0.96WO4:Eu0.043+ phosphor.

3.5 Crystal-field calculations

The effective operator Hamiltonian for rare-earth ions in crystals with C2v site symmetry can be written as23–27
 
H = Hfree ion + HCF(C2v) (3)
in which the free ion Hamiltonian can be expressed as25–28
 
image file: c5ra22631g-t2.tif(4)

The physical meaning of these free ion parameters has been described by Crosswhite and Carnall et al.26,27 The crystal field Hamiltonian with C2v site symmetry can be written, in the Wybourne notation, as28,29

 
HCF(C2v) = B20C20 + B20(C2,−2 + C2,2) + B40C40 + B42(C4,−2 + C4,2) + B44(C4,−4 + C4,4) + B60C60 + B62(C6,−2 + C6,2) + B64(C6,−4 + C6,4) + B66(C6,−6 + C6,6) (5)
in which Bkq are the crystal-field parameters, and Ckq are the Racah spheric tensor operators. A complete energy matrix is constructed from the effective operator Hamiltonian and then the energy-level calculations of Eu3+ ion in MgWO4 crystal are carried out by means of this matrix. In the calculations, the free-ion parameters in the energy matrix are taken as follows,30 Eav ≈ 63[thin space (1/6-em)]786 cm−1, the Coulomb repulsions F2 ≈ 82[thin space (1/6-em)]786 cm−1, F4 ≈ 59[thin space (1/6-em)]401 cm−1, F6 ≈ 42[thin space (1/6-em)]644 cm−1, the spin–orbit coupling parameter ζ4f ≈ 1332 cm−1, the two-body and three-body interaction parameters α ≈ 19.8 cm−1, β ≈ −617 cm−1, γ ≈ 1460 m−1, T2 ≈ 370 cm−1, T3 ≈ 40 cm−1, T4 ≈ 40 cm−1, T6 ≈ −330 cm−1, T7 ≈ 380 cm−1, T8 ≈ 370 cm−1, the Marvin integrals M0 ≈ 2.38 cm−1, M2 ≈ 1.33 cm−1, M4 ≈ 0.90 cm−1, the parameters related to the electrostatic correlated magnetic interaction P2 ≈ 303 cm−1, P4 ≈ 227 cm−1, P6 ≈ 152 cm−1. Thus, in the above complete energy matrix, only the crystal-field parameters are unknown. These parameters are often taken as adjustable data because they depend upon the central metal ion, the ligands and the nature of metal–ligand bonds. By fitting the calculated optical absorption spectra to the observed values of the Eu3+ center in MgWO4 crystal, we get
B20 ≈ 315.20 cm−1, B22 ≈ 780.02 cm−1, B40 ≈ 1109.01 cm−1, B42 ≈ 2072.65 cm−1,
 
B44 ≈ −1443.31 cm−1, B60 ≈ −605.44 cm−1, B62 ≈ −468.13 cm−1, B64 ≈ −1824.14 cm−1, B66 ≈ 189.16 cm−1 (6)

The comparisons between the calculated optical spectral data and the experimental values are listed in Table 1. Note that only a few low-lying multiplets are given in Table 1.

Table 1 Experimental and calculated optical spectra (cm−1) as well as the error (%) of Eu3+ at the Mg2+ site of MgWO4 phosphors
Mult. Calc. Expt Errora
a Error = |Ecalc.Eexpt|/Eexpt.
7F0 0 0 0
7F1 216    
  423    
  430 436 1.38
7F2 770    
  830    
  1045 1011 3.36
  1094    
  1604    
7F3 1752    
  1830    
  2024    
  2041 2050 0.44
  2078    
  2096    
  2129    
7F4 2505    
  2714    
  2933 2944 0.37
  2969    
  3056    
  3116 3107 0.29
  3151    
  3184    
  3508    
7F5 3695    
  3710    
  3922    
  3945    
  3948    
  3996    
  4023    
  4168    
  4539    
  4615    
  4657    
7F6 4892    
  4903    
  5110    
  5141    
  5272    
  5376    
  5376    
  5384    
  5518    
  5545    
  5604    
  5860    
  5860    
5D0 17[thin space (1/6-em)]287    
5D1 19[thin space (1/6-em)]008    
  19[thin space (1/6-em)]063    
  19[thin space (1/6-em)]072 20[thin space (1/6-em)]408 6.54
5D2 21[thin space (1/6-em)]425 21[thin space (1/6-em)]277 0.70
  21[thin space (1/6-em)]478    
  21[thin space (1/6-em)]523 21[thin space (1/6-em)]552 0.34
  21[thin space (1/6-em)]549    
  21[thin space (1/6-em)]584 21[thin space (1/6-em)]978 1.79
5D3 24[thin space (1/6-em)]243 24[thin space (1/6-em)]213 0.12
  24[thin space (1/6-em)]295    
  24[thin space (1/6-em)]343    
  24[thin space (1/6-em)]345    
  24[thin space (1/6-em)]361    
  24[thin space (1/6-em)]371    
  24[thin space (1/6-em)]388    
5L6 24[thin space (1/6-em)]539    
  24[thin space (1/6-em)]597    
  24[thin space (1/6-em)]780    
  24[thin space (1/6-em)]803    
  24[thin space (1/6-em)]921    
  25[thin space (1/6-em)]023    
  25[thin space (1/6-em)]099    
  25[thin space (1/6-em)]146    
  25[thin space (1/6-em)]350    
  25[thin space (1/6-em)]383    
  25[thin space (1/6-em)]497 25[thin space (1/6-em)]445 0.20
  25[thin space (1/6-em)]550    
  25[thin space (1/6-em)]552    
5L7 26[thin space (1/6-em)]142    
  26[thin space (1/6-em)]157    
  26[thin space (1/6-em)]188    
  26[thin space (1/6-em)]245 26[thin space (1/6-em)]248 0.01
  26[thin space (1/6-em)]252    
  26[thin space (1/6-em)]262    
  26[thin space (1/6-em)]282    
  26[thin space (1/6-em)]302    
  26[thin space (1/6-em)]359    
  26[thin space (1/6-em)]360    
  26[thin space (1/6-em)]405    
  26[thin space (1/6-em)]506    
  26[thin space (1/6-em)]510    
  26[thin space (1/6-em)]536    
  26[thin space (1/6-em)]689 26[thin space (1/6-em)]667 0.08
5D4 27[thin space (1/6-em)]671    
  27[thin space (1/6-em)]673    
  27[thin space (1/6-em)]695 27[thin space (1/6-em)]701 0.02


4. Conclusions

A series of Eu3+-activated MgWO4 phosphors are synthesized with the co-precipitation method. The synthesized phosphors can be efficiently excited by 393 and 464 nm which are well matched with the emission of near-UV LED and blue LED chips. The EM intensity increases with the increasing of Eu3+ doping concentration but concentration quenching occurs when the Eu concentration in MgWO4:Eu3+ is higher than 4 mol%.

In terms of those suitable crystal-field parameters, the calculated optical spectral data show excellent agreement with the experimental values (see Table 1). The disparities (error in Table 1) between calculated and experimented spectral values are so small, which suggests that the optical spectra can be satisfactorily explained by the diagonalization (of energy matrix) method for Eu3+ ion at C2v point symmetry. In addition, the good agreement between theory and experiment has also proved that the Eu3+ replacing Mg2+ site with C2v symmetry in MgWO4 crystal is reasonable and this is also consistent with the present experimental results.

Acknowledgements

This work was supported by the National Science Foundation of China (Grant no. 51574054), the Scientific Research Foundation for Returned Scholars, MHRSS, China (Grant no. 2015-157), and the Chongqing Research Program of Basic Research and Frontier Technology (Grant no. cstc2015jcyjA50034).

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