Comparative study on photoluminescence efficiencies of Sm3+-doped MeWO4 (Me = Ba, Sr, Ca, and Mg) phosphors H.-Y. He & Y. Wang
Journal of Materials Science: Materials in Electronics ISSN 0957-4522 Volume 24 Number 12 J Mater Sci: Mater Electron (2013) 24:4847-4852 DOI 10.1007/s10854-013-1486-6
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Author's personal copy J Mater Sci: Mater Electron (2013) 24:4847–4852 DOI 10.1007/s10854-013-1486-6
Comparative study on photoluminescence efficiencies of Sm3+-doped MeWO4 (Me 5 Ba, Sr, Ca, and Mg) phosphors H.-Y. He • Y. Wang
Received: 9 August 2013 / Accepted: 31 August 2013 / Published online: 13 September 2013 Ó Springer Science+Business Media New York 2013
Abstract The Sm-doped MeWO4 (Me = Ba, Sr, Ca, and Mg) phosphors were synthesized with a sol–gel method and studied for their microstructures and photoluminescence efficiencies. The hosts of the phosphors show very weak blue emissions except for the MgWO4. The Sm3? cation in the all hosts shows red emissions. Significantly, the photoluminescence efficiency of Sm3? in CaWO4 and BaWO4 was largest as the calcination temperature equals 600 and 700 °C, respectively. The BaWO4 may be a new potential host of rare earth-doped phosphors.
1 Introduction It is well known that rare earth ions in many hosts can emit various luminescences under ultraviolet and/or infrared excitations and so have been widely studied for luminescence application. The Sm3? (4f5) ion is one of the most interesting trivalence rare earth ions because the fluorescence properties emitting 4G5/2 level exhibits relative high quantum efficiency. A number of earlier studies on spectroscopic properties of Sm3? ions in different hosts have revealed that the fluorescence yield of this rare earth ion is strongly dependent on its environment inside the structure network [1–5]. Alkaline-earth metal tungstate have the general formula MeWO4 (Me = Mg, Ca, Sr, Ba) with the monoclinic or tetraganol structure. These materials have been studied for more than 50 years because of their great technological
H.-Y. He (&) Y. Wang College of Materials Science and Engineering, Shaanxi University of Science and Technology, Xi’an 710021, Shaanxi, China e-mail:
[email protected]
importance [6]. Because of their ability in visible luminescence, these materials appear the basis of their wide use as phosphors, laser materials, and scintillation detectors [7– 9]. Recently, the photoluminescences of some rare earth cation in CaWO4 host have been widely studied [10–16]. The photoluminescences of some rare earth cations in SrWO4 [17] and Ca(Sr,Ba)WO4 [18] hosts have also been reported. However, the comparison of photoluminescence efficiencies of rare earth cations in different alkaline-earth metal tungstates has rarely been reported in previous literature. In addition, the photoluminescence of host can affects the application properties of the doped phosphors. It has been reported that some MeWO4 powders show blue-green luminescences under ultraviolet excitation [19–22]. However, their intensities are rarely compared with the photoluminescence intensities of the Sm3? cation in these hosts in previous literature. In this work, we reported the photoluminescences of the Sm3?-doped MeWO4 (Me = Mg, Ca, Sr, Ba) phosphors synthesized by a sol–gel method followed by calcination at different temperatures and compare their efficiencies.
2 Experimental procedure The starting materials used are all analytical agents without further processing. The Sm3?-doped MeWO4 phosphors were synthesized by a sol–gel method. First, 0.0006 mol Sm2O3 powder was dissolved into 10 ml HCl aqueous solution and expanded to 100 ml with deionic water (noted as S). 0.01 mol Ba(CH3COOH)2, 0.01 mol Sr(NO3)2, 0.01 mol Ca(NO3)24H2O, and 0.01 mol MgCl26H2O were seperately dissolved into 50 ml deionic water (noted as Ms). 0.01/3 mol (NH4)10W12O414H2O was dissolved
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into 100 ml deionic water with magnetic stirring and heating at 80–100 °C (noted as W). Subsequently, the solution S was separated into four equal parts by volume and then dropped into each one of the solutions Ms with constant magnetic stirring, and then the solution W was added into the above mixed solutions with same method. Such four mixed solutions have concentrations of 0.0097 mol/l for various M2? cation, 0.0003 mol/l for Sm3? cation and 0.01 mol/l for the WO42- anion. Finally, the mixed solutions were dried at 100 °C for 12 h and then calcined at 600 and 700 °C for 2 h, respectively. The cooling procedure was naturally performed in the furnace. The structural phase of the synthesized phosphors was identified at room temperature using a X-ray diffractometer (XRD, CuKa1, k = 0.15406 nm, Model No. D/Max2200PC, Rigaku, Japan). The microstructures of the phosphors were analyzed by a field emission scanning electron microscope (SEM, Model No: JXM-6700F, Japan). The photoluminescences of the phosphors were measured on a photoluminescence spectrophotometer (Model No: F-4500, Japan).
cards. As increasing calcination temperature to 700 °C, the lattice parameters of the doped MgWO4 phosphor decrease, while the parameter a and c of the other phosphors decrease and exceed values reported in JCPDS cards, respectively. The SEM micrographs of the Sm3?-doped MeWO4 phosphors are shown in Figs. 2 and 3. The all phosphors
3 Results and discussion The XRD patterns of the Sm3?-doped MeWO4 (Me = Ba, Sr, Ca, and Mg) phosphors are shown in Fig. 1. The phosphors are all identified as MeWO4 (Me = Ba, Sr, Ca, and Mg) with tetragonal structure except for the MgWO4 having monoclinic structure according to the reported cards (JSPDS: 43-0646, 85-0587, 72-1624, and 27-0789). No any impurities are detected out, which could indicate that the Sm3? cation is incorporated into the MeWO4 host lattices. The intensities of the XRD peaks increase with increase in the calcination temperature, generally indicating increases in crystllinities. The average particle sizes and the lattice parameters are determined with XRD data analysis and listed in Table 1. The lattice parameters of the phosphors calcined at 600 °C are smaller than that reported in JCPDS
Table 1 Lattice parameter of the phosphor determined with XRD data analysis
Powders
BaWO4:Sm SrWO4:Sm CaWO4:Sm MgWO4:Sm
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Fig. 1 XRD patterns of the Sm-doped MEWO4 powders calcined at a 600 °C and b 700 °C
Calcination temperature (°C)
˚) Lattice parameter (A
Average particle size (nm)
a
b
c
600
5.5748
5.5748
12.6998
700
5.5525
5.5525
12.7112
43.1
600
5.4195
5.4195
11.9434
40.0
700
5.3963
5.3963
11.9566
40.4
600
5.2452
5.2452
11.3559
26.3
700
5.2134
5.2134
11.3902
32.2
600
4.9234
5.6705
4.6831
48.3
700
4.6841
5.6633
4.6794
49.2
41.8
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Fig. 2 SEM micrographs of the Sm-doped a MgWO4, b CaWO4, c SrWO4 and d BaWO4 powders calcined 600 °C
Fig. 3 SEM micrographs of the Sm-doped a MgWO4, b CaWO4, c SrWO4 and d BaWO4 powders calcined 700 °C
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Fig. 5 a Excitation and b emission spectra of the Sm3? in the Smdoped MeWO4 calcined at 600 °C
Fig. 4 a Excitation and emission spectra of the host for the Smdoped MeWO4 calcined at b 600 °C and c 700 °C
consist of the large particles and some small particles with granular morphology. Average particle sizes increase as increase in calcination temperature. The sizes seem to be larger than that determined with XRD analysis, which could shows that the nanoparticles are polycrystalline.
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To investigate photoluminescence of the Sm-doped MeWO4 phosphors, the photoluminescence spectra of the phosphors were measured by selecting appropriate wavelengths of monitoring and excitation in the steady-state type of measurements. Figure 4 shows the photoluminescence spectra of host of Sm3?-doped MeWO4 phosphors under monitoring emission at 310 nm and excitation at 470 nm. The excitation peak is centered at *310 nm. The emission peaks are centered at *470 nm showing blue color. Excepting the Sm3?-doped MgWO4 phosphors, other phosphors show very weak photoluminescence. Figures 5 and 6 shows photoluminescence spectra of Sm3? in the phosphors under monitoring emission at 651 nm and excitation at 406 nm. The excitation spectra show eight peaks centered at *365, *379, *406, *421.0, *463, *471, *482, and *493 nm. They can be respectively assigned to the Sm3? electronic transitions of 6 H5/2 ? 4L17/2, 6H5/2 ? 6P7/2, 6H5/2 ? 4F7/2, 6H5/2 ? 4 P5/2, 6H5/2 ? 4I13/2, 6H5/2 ? 4I11/2, 6H5/2 ? 4I9/2, and 6 H5/2 ? 4G7/2. The peaks position irregularly varies in small range of *5 nm as varying the host from MgWO4 to
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Fig. 7 Energy level scheme for all the observed excitation and emission bands of Sm3? in the powders
Fig. 6 a Excitation and b emission spectra of the Sm3? in the Smdoped MEWO4 calcined at 700 °C
CaWO4, SrWO4 and BaWO4. The prominent excitation at *406 nm was selected for the measurement of emission spectra of Sm3? in the phosphors. Three emission bands are observed in the emission spectra as shown in Fig. 5. The emission band centered at *567 nm is assigned to 4 G5/2 ? 6H5/2 transition, while the bands centered at *605 and *651 nm are assigned to 4G5/2 ? 6H7/2 and 4 G5/2 ? 6H9/2 transitions, respectively. The emission peak shifts to short wavelength as varying the host from MgWO4 to CaWO4, SrWO4 and BaWO4. The total shift is about 9 nm. There is not observable shift due to different calcination temperatures. Among the three emission bands, the transition 4G5/2 ? 6H9/2 (*651 nm) are intensest, being different from the intensest transition 4G5/2 ? 6H7/2 (*605 nm) as usually reported in some literature [1–5, 10, 17]. This implies that the Sm3? in the phosphors show red emission although almost the same intensity of the emission lines at 651 and 605 nm in natural CaWO4 crystals (scheelite) was reported in previous literature [23]. Importantly, the emission peaks of Sm3? cation in the CaWO4 and the BaWO4 hosts are strongest as calcinations temperature equals 600 and
700 °C, respectively. The energy level scheme for the observed excitations and emissions is proposed in Fig. 7.
4 Conclusion The Sm3?-doped MeWO4 (Me = Ba, Sr, Ca, and Mg) phosphors have been successfully synthesized by a sol–gel method. The hosts of the phosphors show very weak blue emissions except for the MgWO4. The Sm3? cation in all hosts shows red emission. These emission are red than usually reported orange-red since the transition 4 G5/2 ? 6H9/2 (*651 nm) is intensest among three emission peaks. Significantly, the photoluminescence efficiency of the Sm3? cation in the CaWO4 and the BaWO4 hosts is largest when calcination temperature is 600 and 700 °C, respectively. The BaWO4 may be new potential host for rare-earth-doped phosphors.
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