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Effects of the Concentration of Eu3+ Ions and Synthesizing Temperature on the Luminescence Properties of Sr2−xEuxZnMoO6 Phosphors

Effects of the Concentration of Eu3+ Ions and Synthesizing Temperature on the Luminescence... applied sciences Article 3+ Effects of the Concentration of Eu Ions and Synthesizing Temperature on the Luminescence Properties of Sr Eu ZnMoO Phosphors 2x x 6 1 2 3 4 , Chi-Yu Lin , Su-Hua Yang , Jih-Lung Lin and Cheng-Fu Yang * Department of Aero-Electronic Engineering, Air Force Institute of Technology, Kaohsiung 820, Taiwan, R.O.C.; takiincku@yahoo.com.tw Department of Electronic Engineering, National Kaohsiung University of Applied Sciences, Kaohsiung 807, Taiwan, R.O.C.; shya@cc.kuas.edu.tw Department of Aeronautics and Astronautics, Air Force Academy, Kaohsiung 820, Taiwan, R.O.C.; lin2737.cafa@msa.hinet.net Department of Chemical and Material Engineering, National University of Kaohsiung, Kaohsiung 811, Taiwan, R.O.C. * Correspondence: cfyang@nuk.edu.tw; Tel.: +886-7-591-9283 Academic Editors: Teen-Hang Meen, Antonio Facchetti and Giorgio Biasiol Received: 13 September 2016; Accepted: 22 December 2016; Published: 27 December 2016 Abstract: The effect of Eu O concentration on the luminescence properties of double perovskite 2 3 (cubic) Sr Eu ZnMoO phosphors was thoroughly investigated using different synthesizing 2x x 6 temperatures. Phosphors with the composition Sr Eu ZnMoO , where Eu O was substituted for 2x 6 2 3 SrO and x was changed from 0 to 0.12, were synthesized by the solid-state method at temperatures of 900–1200 C, respectively. Analysis of the X-ray diffraction (XRD) patterns showed that even when the synthesizing temperature was 1100 C, secondary or unknown phases were observed in 3+ Sr Eu ZnMoO ceramic powders. The effect of the concentration of Eu ions on the luminescence 2x 6 properties of the Sr Eu ZnMoO phosphors was readily observable because no characteristic 2x x 6 emission peak was observed in the Sr ZnMoO phosphor. Two characteristic emission peaks 2 6 5 7 5 7 at 597 and 616 nm were observed, which correspond to the D – F and D – F transitions of 0 1 0 2 3+ Eu ions, respectively. The two characteristic emission peaks of the Sr Eu ZnMoO phosphors 2x x 6 3+ were apparently influenced by the synthesizing temperature and the concentration of Eu ions. 3+ When x was larger than 0.08, a concentration quenching effect of Eu ions in the Sr Eu ZnMoO 2x x 6 phosphors could be observed. The lifetime of the Sr Eu ZnMoO phosphors decreased as the 2x x 6 synthesizing temperature increased. A linear relation between temperature and lifetime was obtained by using a fitting curve of t = 0.0016  T + 3.543, where t was lifetime and T was synthesizing temperature. Keywords: Sr Eu ZnMoO phosphors; double perovskite oxides; synthesizing temperature; 2x x 6 lifetime 1. Introduction UV light can be generated as a consequence of electronic transitions of light sources through an Hg discharge. In low-pressure Hg discharge, the main emission line is located at a wavelength of 254 nm. This light is invisible and harmful to human bodies, so it has to be converted into visible light, which can be done with a combination of luminescent materials. These luminescent materials can strongly absorb light of that wavelength and efficiently convert it into visible light. Recently, white light emitting diodes (LEDs) have become popular because they have several advantages, including high efficiency, long lifetime, and low power consumption. Red phosphors are also helpful for generating Appl. Sci. 2017, 7, 30; doi:10.3390/app7010030 www.mdpi.com/journal/applsci Appl. Sci. 2017, 7, 30 2 of 11 white light when they are excited by blue or near-UV lights. To obtain phosphors with highly efficient emissions, it is important to choose the right compound materials and ensure they have outstanding physical and chemical stability. Numerous studies have explored different luminescent materials to enable the development of suitable phosphors. When lanthanide contraction ions are introduced into host materials, unfilled 4f electron orbitals result, and these have attracted considerable attention. The resulting phosphors emit very luminescent emissions with specific light wavelengths because of variations in the energy level of some free electrons [1–4]. These phosphors have been the most promising candidates for applications in fluorescent lamps and flat panel display devices, such as electroluminescence panels, plasma display panels, and field emission displays. A large number of isotropic compounds with perovskite structures, such as double perovskite oxides, have the general formula A BB’O , in which BO and B’O octahedra are corner-shared, 2 6 6 6 alternately. The great flexibility of A and B(B’) sites in A BB’O allows very rich substitutions, 2 6 and this framework forms cube-octahedral cavities filled by A-site cations [5,6]. Double perovskites with the formula A BB’O , where A uses an alkaline earth, B and B’ are metal transition magnetic 2 6 and nonmagnetic ions, and O is oxygen, have been investigated as magnetic materials for many years. For example, Sr CrMoO has been studied as a half-metallic system [7]. Kobayashi et al. 2 6 recently reported room-temperature low-field magnetic resistance in the ordered double perovskite Sr FeMoO [8]. Complete ordering of Fe and Mo on the B and B’ sites of this metallic A BB’O double 2 6 2 6 perovskite is predicted to give half-metallic ferromagnetism with localized majority-spin electrons on the Fe atoms [9,10]. Recently, the study of A BB’O -based materials has increased due to various 2 6 technological applications, such as inorganic oxide luminescent materials. The emitting materials are usually composed of activators and a host lattice. Some host lattice materials can produce light themselves, and some can produce light when doped with rare-earth activators (ions). Rare-earth ions are known to exist in various valence states, although the trivalent state is the most prevalent. Rare-earth ions can be applied in lighting devices and display panels due to their abundant energy levels across a wide spectrum range, from ultraviolet to near infrared. Sm- and Eu-based ions are the most commonly used dopants because they are stable 3+ 3+ 2+ in trivalent (Sm and Eu ), as well as divalent (Eu ), states. The luminescence of rare-earth ions doped in perovskite-type ceramics was actively investigated in the 1960s and 1970s because of interest in their ferroelectricity, phase transitions, and semiconducting properties [11]. Recently, many studies have shown that the double perovskite structure with a composition of A BMO (A = Ba, 2 6 3+ Sr; B = Ca, Zn; M = Mo, W) is activated by trivalent europium ions (Eu ) [12–15]. Phosphors 3+ activated by Eu are considered ideal red sources because of their sharp emission lines in the red 3+ region [12–16]. Eu -doped double-perovskite materials have a broad excitation band ranging from 3+ UV to visible light, and they also show highly efficient red luminescence. For that, Eu -doped double molybdenum (Mo)-based double perovskite oxides have attracted significant attention for their 3+ possible application as luminescent materials, such as Sr MgMo W O : Eu [17], Sr Ca(Mo/W)O : 2 1x 6 2 6 3+ 3+ 3+ 3+ 3+ Eu [18], Sr CaMoO : Eu [19], (Ba,Sr) CaMoO : Eu , Yb [20], Ca LaMO : Eu (M = Sb, Nb, 2 6 2 6 2 6 Ta) [21], and A CaMoO (A = Sr, Ba) [6], respectively. 2 6 3+ To the best of our knowledge, the luminescent characteristics of concentrated Eu ions in Sr Eu ZnMoO phosphors, which are molybdenum-based double-perovskite oxides, have not 2x 6 3+ been reported. In this study, the red-emitting phosphors of Eu -doped Mo-based double-perovskite Sr Eu ZnMoO oxides were synthesized by the conventional high-temperature solid state reaction 2x 6 3+ method. We found that Eu concentration and the synthesizing temperature of the Sr Eu ZnMoO 2x x 6 phosphors had a large effect on their luminescent characteristics. The concentration quenching effect of 3+ Eu ions in Sr Eu ZnMoO phosphors was found and would be well discussed [22]. In this study, 2x x 6 the first important novelty is that we have not found any similar studies about Sr Eu ZnMoO 2x 6 phosphors. The second important novelty is that we found a linear relationship between the synthesizing temperature and lifetime of Sr Eu ZnMoO phosphors. We also investigated the 2x 6 relationship between the synthesized temperature and lifetime of Sr Eu ZnMoO phosphors. 2x x 6 Appl. Sci. 2017, 7, 30 3 of 11 2. Materials and Methods Sr Eu ZnMoO powders were synthesized through the solid-state reaction method. 2x 6 Stoichiometric amounts of SrCO , ZnO, MoO , and Eu O were weighed according to the composition 3 3 2 3 formula of (2 x) SrCO + ZnO + MoO + (0.5 x) Eu O , where x = 0, 0.04, 0.06, 0.08, 0.10, and 3 3 2 3 0.12, respectively. After being mixed in acetone, dried, and ground, the solid-state reaction method was used to heat the Sr Eu ZnMoO compositions in an air atmosphere. The Sr Eu ZnMoO x x 2x 6 2x 6 powders were heated to 900 C, 1000 C, 1100 C, and 1200 C for 4 h. When the synthesizing temperature was equal to 1300 C, the Sr Eu ZnMoO was melted and gathered together to not 2x 6 be used as part of the phosphors. The crystalline structures of the synthesized Sr Eu ZnMoO 2x x 6 powders were measured using X-ray diffraction (XRD) (Bruker, Boston, MA, USA ) patterns with Cu K radiation ( = 1.5418 Å) and with a scanning speed of 2 per minute. Photoluminescence (PL) properties were recorded at room temperature in the wavelength range of 450–800 nm on a Hitachi F-4500 fluorescence spectrophotometer (Hitachi, Tokyo, Japan). In the past, we had found that 271 nm had a better excitation effect on BaZr Eu O powders [16]. This result suggested that we would also 1x x 3 need to find the optimum optical wavelength for exciting the Sr Eu ZnMoO powders. In this study, 2x x 6 the three-dimensional (3D) scanning process using a spectrophotometer (Hitach, Tokyo, Japan) was used to find the optimum optical wavelength, and this value was dependent on the compositions of the Sr Eu ZnMoO phosphors and the synthesizing temperature. We found that the optimum exciting 2x x 6 optical wavelength for all of the Sr Eu ZnMoO powders was 350 nm, and the Sr Eu ZnMoO 2x x 6 2x x 6 powders excited by other wavelengths had the weaker PL intensities. 3. Results and Discussion To achieve high PL properties, the preparation of Sr Eu ZnMoO powders forming 2x 6 the double-perovskite phase is very important, because crystallization of Sr Eu ZnMoO 2x x 6 powders influences their photoluminescent properties. Figure 1 shows the XRD patterns of our Sr Eu ZnMoO powders as a function of the synthesizing temperature. The strong peaks occurred 2x x 6 at around 31.9 for the (220) diffraction peak of the six host lattices. Those results suggest that the XRD patterns showed stable double-perovskite features regardless of the synthesizing temperature and 3+ Eu concentration. As Figure 1a shows, when the synthesizing temperature of the Sr Eu ZnMoO powders was 2x x 6 900 C and as the x value increased from 0 to 0.12, the 2 value of the (220) diffraction peak shifted from 31.85 to 31.87 and the full width at half maximum (FWHM) values for the (220) diffraction peak were in the range of 2 = 0.25 –0.27 . When the synthesizing temperature was 1200 C, the 2 value of the (220) diffraction peak shifted from 31.88 to 31.90, and the FWHM values for the (220) diffraction peak of the Sr Eu ZnMoO powders were in the range of 2 = 0.19–0.20 , as the x value of the 2x 6 Sr Eu ZnMoO powders increased from 0 to 0.12. The results in Figure 1a–d show that the 2 of the 2x x 6 (220) diffraction peak shifted to a higher value and the FWHM values for the (220) diffraction peaks of the Sr Eu ZnMoO powders decreased as the synthesizing temperature increased. 2x x 6 The ideal cubic double-perovskite structure (with the same space group Pm3m (221)) can be described by a faced-centered cubic (fcc) lattice with lattice constant 2a [23]. The B(B’) ion is coordinated by the B’(B) ion using an O ion as an intermediate in the middle, and the lengths of B–O and B’–O are considered to be equal. After relaxation, both lattice constants and atomic positions reduce the ideal cubic structure (space group Fm3m) to a tetragonal structure (space group I4/mmm). There are two O atoms located on the z-axis with B and B’ atoms sitting between, and the four O atoms are 1 2 located on the xy-plane; the same as the B and B’ atoms. The angle of the B–O–B’ remains at 180 during structural optimization, whereas the lattice constant and bond length change. The lattice a can be calculated by using (1) the reflection peaks (011), (111), (200), and (220) from the XRD patterns in Figure 1; and (2) the closeness of the c/a ratio to the ideal value of 2. As the synthesizing temperature was 900 C, the Sr Eu ZnMoO powders exhibited a cubic crystal structure with the cell parameters 2x x 6 p p 3+ changing from a = b = c/ 2 = 0.3968 nm to a = b = c/ 2 = 0.3971 nm as the concentration of Eu ions Appl. Sci. 2017, 7, 30 4 of 11 increased from 0 to 0.12. The cell parameters for Sr Eu ZnMoO powders synthesized at 1200 C 2x x 6 Appl. Sci. 2017, 7, 30 4 of 11 were also calculated from the XRD patterns shown in Figure 1, and the cell parameters changed from p p 3+ a = b = c/ 2 = 0.3971 nm to a = b = c/ 2 = 0.3974 nm as the concentration of Eu ions increased 3+ changed from a = b = c/√2 = 0.3971 nm to a = b = c/√2= 0.3974 nm as the concentration of Eu ions from 0 to 0.12. These results were in good agreement with those from the Joint Committee on Powder increased from 0 to 0.12. These results were in good agreement with those from the Joint Committee Diffraction Standards (JCPDS) file number 742474. on Powder Diffraction Standards (JCPDS) file number 742474. (a) (b) o: unknown phase x:Eu O x=0.06 x=0.06 o: unknown phase x:Eu O 2 3 2 3 x=0.05 x=0.05 x x x x x=0.04 x=0.04 x=0.03 x=0.03 x=0.02 x=0.02 x=0 x=0 o o o o oo o o 20 30 40 50 60 70 80 20 30 40 50 60 70 80 2θ (Degree) 2θ (Degree) (d) (c) o: unknown phase x:Eu O x=0.06 x=0.06 2 3 x=0.05 x=0.05 x=0.04 x=0.04 x=0.03 x=0.03 x=0.02 x=0.02 x=0 x=0 20 30 40 50 60 70 80 20 30 40 50 60 70 80 2θ (Degree) 2θ (Degree) Figure 1. XRD patterns of Sr2−xEuxZnMoO6 phosphors synthesized at (a) 900 °C; (b) 1000 °C; (c) 1100 Figure 1. XRD patterns of Sr Eu ZnMoO phosphors synthesized at (a) 900 C; (b) 1000 C; 2x 6 °C; and (  d) 1200 °C for 4  h, respectively. (c) 1100 C; and (d) 1200 C for 4 h, respectively. As the results of the X-ray diffraction (XRD) patterns showed in Figure 1 were compared, these As the results of the X-ray diffraction (XRD) patterns showed in Figure 1 were compared, results indicated that the diffraction intensities of unknown phases for peaks located at around 2θ = 3+ these28 results .44° and indicated 33.09° incre thata the sed dif asfraction the concent intensities ration o of f E unknown u ions increased an phases for peaks d decrlocated eased as atthe around 3+ synthesizing temperature increased. Those peaks are in good agreement with the (222) and (400) 2 = 28.44 and 33.09 increased as the concentration of Eu ions increased and decreased as the peaks of JCPDS file number 120393 for cubic Eu2O3. The decrease in the diffraction intensities of peaks synthesizing temperature increased. Those peaks are in good agreement with the (222) and (400) peaks 3+ 2+ located at around 2θ = 28.44° and 33.09° prove that more Eu ions will substitute the sites of Sr ions of JCPDS file number 120393 for cubic Eu O . The decrease in the diffraction intensities of peaks 2 3 as the synthesizing temperature is raised. Even when the synthesizing temperature was 1100 °C, 3+ 2+ located at around 2 = 28.44 and 33.09 prove that more Eu ions will substitute the sites of Sr secondary or unknown phases were observed in the Sr2−xEuxZnMoO6 ceramic powders. These Eu2O3 ions as the synthesizing temperature is raised. Even when the synthesizing temperature was 1100 C, and secondary or unknown phases were not observed when the synthesizing temperature was 1200 secondary or unknown phases were observed in the Sr Eu ZnMoO ceramic powders. These Eu O 2x x 6 2 3 °C. These results suggest that when the same synthesizing temperature is used, the concentration of and secondary or unknown phases were not observed when the synthesizing temperature was 1200 C. 3+ Eu ions has no apparent effect on the crystallization of Sr2−xEuxZnMoO6 powders; hence, the 3+ These results suggest that when the same synthesizing temperature is used, the concentration of Eu synthesizing temperature is an important factor in determining the crystalline properties of 3+ ions has no apparent effect on the crystallization of Sr Eu ZnMoO powders; hence, the synthesizing Sr2−xEuxZnMoO6 powders. Additionally, the concentration of Eu ions and the synthesizing 2x x 6 temperature affect the photoluminescent properties of Sr2−xEuxZnMoO6 phosphors. temperature is an important factor in determining the crystalline properties of Sr Eu ZnMoO 2x x 6 3+ XRD patterns for Sr2−xEuxZnMoO6 phosphors synthesized at 1200 °C for 4 h in the narrow range powders. Additionally, the concentration of Eu ions and the synthesizing temperature affect the of 29–35° are shown in Figure 2. These results are significant. Initially, the splitting of the (220) photoluminescent properties of Sr Eu ZnMoO phosphors. 2x x 6 diffraction peak was observed in the Sr2ZnMoO6 and Sr1.98Eu0.02ZnMoO6 phosphors, but it was not XRD patterns for Sr Eu ZnMoO phosphors synthesized at 1200 C for 4 h in the narrow 2x x 6 observed in other Sr2−xEuxZnMoO6 phosphors. The two split peaks of the Sr2ZnMoO6 phosphors were range of 29–35 are shown in Figure 2. These results are significant. Initially, the splitting of the located at 2θ = 31.83° and 31.88°, and the two split peaks of the Sr1.98Eu0.02ZnMoO6 phosphors were (220) diffraction peak was observed in the Sr ZnMoO and Sr Eu ZnMoO phosphors, but it 2 6 1.98 0.02 6 located at 2θ = 31.81° and 31.89°. These results suggest that the Sr2ZnMoO6 and Sr1.98Eu0.02ZnMoO6 was not observed in other Sr Eu ZnMoO phosphors. The two split peaks of the Sr ZnMoO x 3+ 2x 6 2 6 phosphors revealed a perovskite structure with the tetragonal phase. As the concentration of Eu phosphors were located at 2 = 31.83 and 31.88 , and the two split peaks of the Sr Eu ZnMoO 1.98 0.02 6 Intensity (a.u.) Intensity (a.u .) (011) (011) (110) (110) (111) (220) (220) (101) (101) (222) (222) (200) (200) (400) (422) (422) (220) (220) (310) (310) Intensi ty (a.u.) Intensit y (a.u.) (011) (011) (110) (110) (220) (220) (101) (101) (222) (222) (200) (200) (422) (422) (220) (220) (310) (310) Appl. Sci. 2017, 7, 30 5 of 11 phosphors were located at 2 = 31.81 and 31.89 . These results suggest that the Sr ZnMoO and 2 6 Appl. Sci. 2017, 7, 30 5 of 11 Sr Eu ZnMoO phosphors revealed a perovskite structure with the tetragonal phase. As the 1.98 0.02 6 3+ concentration of Eu was more than 0.02, the Sr Eu ZnMoO phosphors would have transformed 2x x 6 was more than 0.02, the Sr2−xEuxZnMoO6 phosphors would have transformed from the tetragonal from the tetragonal phase to the (pseudo-)cubic phase, since the splitting of the (220) diffraction peak phase to the (pseudo-)cubic phase, since the splitting of the (220) diffraction peak was not observed. was not observed. Since the Sr Eu ZnMoO phosphors were tetragonal phase or (pseudo-)cubic 2x x 6 Since the Sr2−xEuxZnMoO6 phosphors were tetragonal phase or (pseudo-)cubic phase, even when the 3+ phase, even when the concentration of Eu ions was increased to 0.12, this result also proves that 3+ 3+ concentration of Eu ions was increased to 0.12, this result also proves that the Eu ions would have 3+ 2+ 3+ the Eu ions would have substituted into the sites of Ba ions. If the Eu ions had substituted into 2+ 3+ 2+ substituted into the sites of Ba ions. If the Eu ions had substituted into the sites of Zn (or Mo) ions, 2+ the sites of Zn (or Mo) ions, the Sr Eu ZnMoO phosphors would have revealed other crystalline 2x x 6 the Sr2−xEuxZnMoO6 phosphors would have revealed other crystalline phases, or more secondary phases, or more secondary phases would have been revealed in the Sr Eu ZnMoO phosphors 2x x 6 phases would have been revealed in the Sr2−xEuxZnMoO6 phosphors rather than in the double- rather than in the double-perovskite features. perovskite features. 1200 C x=0.12 x=0.10 x=0.08 x=0.06 x=0.04 x=0 29 31 33 35 2θ (Degree) Figure 2. XRD patterns of Sr2−xEuxZnMoO6 phosphors synthesized at 1200 °C for 4 h. Figure 2. XRD patterns of Sr Eu ZnMoO phosphors synthesized at 1200 C for 4 h. 2x 6 3+ 3+ Previously, Lin et al. successfully synthesized novel near-UV and blue-excited Eu , Tb -co- 3+ Previously, Lin et al. successfully synthesized novel near-UV and blue-excited Eu , doped one-dimensional strontium germanate full-color nanophosphors by a simple sol- 3+ Tb -co-doped one-dimensional strontium germanate full-color nanophosphors by a simple 3+ 3+ hydrothermal method. They found that incorporation of the Eu and Tb ions into strontium 3+ 3+ sol-hydrothermal method. They found that incorporation of the Eu and Tb ions into strontium 3+ germanate resulted in a slight shrinkage of the lattice constants and the unit cell volume because Eu 3+ germanate resulted in a slight shrinkage of the lattice constants and the unit cell volume because Eu 3+ 2+ 3+ 3+ and Tb had smaller radii than Sr , indicating the Eu and Tb ions had been incorporated into the 3+ 2+ 3+ 3+ and Tb had smaller radii than Sr , indicating the Eu and Tb ions had been incorporated into host lattice of SrGe4O9 and did not change the crystal structure [24]. Those results also suggested that the host lattice 3+ of SrGe O and did not change 2+ the crystal structure [24]. Those results also suggested 4 9 the Eu ions substituted into the sites of Sr ions. The 2θ value of the (220) diffraction peak was 3+ 2+ that the Eu ions substituted into the sites of Sr ions. 3+ The 2 value of the (220) diffraction peak shifted to a higher value, as the concentration of Eu was equal to, or greater than, 0.02. The ion 3+ 2+ 3+ 2+ was shifted to a higher value, as the concentration of Eu was equal to, or greater than, 0.02. The ion radius of Sr is 0.130 nm and the ion radius of Eu is 0.1087 nm. Hence, as more Sr would have been 2+ 3+ 2+ 3+ radius of Sr is 0.130 nm and the ion radius of Eu is 0.1087 nm. Hence, as more Sr would have substituted by Eu , the ionic radius of the Sr2−xEuxZnMoO6 phosphors would have decreased, 3+ 2+ been incre substituted asing the 2 byθ va Eulue o , the f th ionic e (22radius 0) diffract of the ion pe Sr ak. The Eu ZnMoO ion radiphosphors us of Eu is would 0.131 nm have , w decr hich eased, is 2x 6 2+ 2+ 2+ incr thought to be the sa easing the 2 value me as t of th he i e (220) on radius diffraction of Sr . If Eu peak. 2O3The exists ion as E radius u ions, the cel of Eu isl pa 0.131 rameters of the nm, which is 2+ 2+ Sr2−xEuxZnMoO6 phosphors would not have been changed. The shift of the (220) diffraction peak to a thought to be the same as the ion radius of Sr . If Eu O exists as Eu ions, the cell parameters 2 3 higher 2θ value or the decrease in the cell parameters can prove that Eu2O3 existed in the of the Sr Eu ZnMoO phosphors would not have been changed. The shift of the (220) diffraction 2x x 6 3+ Sr2−xEuxZnMoO6 phosphors and in the Eu state. peak to a higher 2 value or the decrease in the cell parameters can prove that Eu O existed in the 2 3 3+ In order to find the best doping concentra 3+ tion of Eu , we synthesized a series of Sr2−xEuxZnMoO6 Sr Eu ZnMoO phosphors and in the Eu state. 2x x 6 phosphors (x = 0 to 0.12) and measured their emission 3+ spectra. The PL emission spectra of In order to find the best doping concentration of Eu , we synthesized a series of Sr Eu ZnMoO 2x 6 Sr2−xEuxZnMoO6 powders excited at a wavelength of 350 nm are shown in Figure 3 for the light phosphors (x = 0 to 0.12) and measured their emission spectra. The PL emission spectra of wavelength range of 400–650 nm. The spectra in Figure 3 show that that all the excitation spectra of Sr Eu ZnMoO powders excited at a wavelength of 350 nm are shown in Figure 3 for the light 2x 6 Sr2−xEuxZnMoO6 phosphors (except the Sr2ZnMoO6 powder) consisted of two parts: one broad band wavelength range of 400–650 nm. The spectra in Figure 3 show that that all the excitation spectra in the 400–575 nm region and two sharp peaks located at 597 nm and 616 nm. However, for the of Sr Eu ZnMoO phosphors (except the Sr ZnMoO powder) consisted of two parts: one broad 2x 6 2 6 Sr2ZnMoO6 powder, even when the synthesizing temperature was 1200 °C, the two sharp peaks of band in the 400–575 nm region and two sharp peaks located at 597 nm and 616 nm. However, Sr2−xEuxZnMoO6 phosphors located at 597 nm and 616 nm were not found in the emission spectra. for the Sr ZnMoO powder, even when the synthesizing temperature was 1200 C, the two sharp 2 6 This means that no characteristic peaks were observed in Sr2ZnMoO6 powder even when the peaks of Sr Eu ZnMoO phosphors located at 597 nm and 616 nm were not found in the emission 2x x 6 synthesizing temperature was 1200 °C. Obviously, the broad band in the 400–575 nm region is spectra. This means that no characteristic peaks were observed in Sr ZnMoO powder even when 2 6+ 2 6 assignable to the well-known O –Mo charge transfer band (CTB) [25]. As Eu2O3 was substituted for the synthesizing temperature was 1200 C. Obviously, the broad band in the 400–575 nm region is SrO, the Sr2−xEuxZnMoO6 phosphors had the same spectral profile, but with a different concentration 2 6+ 3+ assignable to the well-known O –Mo charge transfer band (CTB) [25]. As Eu O was substituted for of Eu ions, the characteristic peaks were observed. 2 3 Intensity (a.u.) (220) Appl. Sci. 2017, 7, 30 6 of 11 SrO, the Sr Eu ZnMoO phosphors had the same spectral profile, but with a different concentration 2x x 6 3+ of Eu Appl.ions, Sci. 2017 the , 7, 30 characteristic peaks were observed. 6 of 11 1200 1200 x=0 x=0 (a) (b) x=0.04 x=0.04 x=0.06 x=0.06 5 7 D - F 900 900 0 1 x=0.08 x=0.08 x=0.10 x=0.10 x=0.12 x=0.12 5 7 D - F 5 7 600 0 1 600 D - F 0 2 5 7 D - F 0 2 300 300 400 450 500 550 600 650 400 450 500 550 600 650 Wavelength(nm) Wavelength(nm) 1200 1200 x=0 x=0 (d) (c) x=0.04 x=0.04 5 7 5 7 D - F x=0.06 x=.0.06 D - F 0 2 0 1 5 7 5 7 900 900 x=0.08 x=0.08 D - F D - F 0 2 0 1 x=0.10 x=0.10 x=0.12 x=0.12 600 600 0 0 400 450 500 550 600 650 400 450 500 550 600 650 Wavelength(nm) Wavelength(nm) Figure 3. Emission spectra of Sr2−xEuxZnMoO6 phosphors synthesized at (a) 900 °C; (b) 1000 °C; (c) Figure 3. Emission spectra of Sr Eu ZnMoO phosphors synthesized at (a) 900 C; (b) 1000 C; 2x x 6 1100 °C; and (d) 1200 °C, respectively. (c) 1100 C; and (d) 1200 C, respectively. When the synthesizing temperature was changed from 900 to 1200 °C, the emission spectra of When the synthesizing temperature was changed from 900 to 1200 C, the emission spectra of the Sr2−xEuxZnMoO6 phosphors consisted of sharp peaks in two strong bands at 597 and 616 nm, 5 7 5 7 3+ the Sr correspondi Eu ZnMoO ng to the phosphors D0– F1 (597 consisted nm) and of D0sharp – F2 (616 nm peaks) t in rans two itions of Eu strong bands ions. These r at 597 and esults 616 nm, 2x x 6 5 7 5 7 5 7 3+ 5 7 prove that the two emission peaks of the Sr2−xEuxZnMoO6 phosphors, D0– F1 at 597 nm and D0– F2 corresponding to the D – F (597 nm) and D – F (616 nm) transitions of Eu ions. These results 0 1 0 2 3+ 5 7 5 7 at 616 nm, were excited by the addition of Eu ions. When the synthesizing temperature was raised prove that the two emission peaks of the Sr Eu ZnMoO phosphors, D – F at 597 nm and D – F 2x x 6 0 1 0 2 to 1100 and 1200 °C, the intensity of the broa3+ dened emission went from 400 nm to 575 nm, increasing at 616 nm, were excited by the addition of Eu ions. When the synthesizing temperature was raised to 3+ with the increase in the Eu ion concentration. However, the emission intensities of the 1100 and 1200 C, the intensity of the broadened emission went from 400 nm to 575 nm, increasing with Sr2−xEuxZnMoO6 phosphors were influenced by the synthesizing temperature and the concentration 3+ the increase in the Eu ion concentration. However, the emission intensities of the Sr Eu ZnMoO 2x x 6 3+ of the Eu ions. 3+ phosphors were influenced by the synthesizing temperature and the concentration of the Eu ions. 3+ In a previous study, the spectra of BaZrO3 doped with Eu powders consisted of a series of 3+ In a previous study, the spectra of BaZrO doped with Eu powders consisted of a series of resolved emission peaks located at 576 nm, 597 nm, 616 nm, 623 nm, 651 nm, 673 nm, 696 nm, and 5 7 3+ 5 7 resolved emission peaks located at 576 nm, 597 nm, 616 nm, 623 nm, 651 nm, 673 nm, 696 nm, 704 nm, which are assignable to the D0– FJ (J = 0, 1, 2, 3, 4) transitions of Eu ions, namely, D0– F0 5 7 3+ 5 7 5 7 5 7 5 7 and (704 576 nm), nm, which D0– F1 (5 ar 97 nm) e assignable , D0– F2 ( to 616 the nm, D 623 – nm F ()J , = D00, – F 1, 3 (6 2,51 nm) 3, 4) ,transitions and D0– F4 (67 of Eu 3 nm, 69 ions, 6 nm namely , , 0 J 5 7 5 7 5 7 5 7 5 7 5 7 and 704 nm) [16]. Liu and Wang’s research also showed that the emission intensities of D0– F0 (576 D – F (576 nm), D – F (597 nm), D – F (616 nm, 623 nm), D – F (651 nm), and D – F 0 0 0 1 0 2 0 3 0 4 5 7 5 7 nm), D0– F1 (597 nm), and D0– F2 (616, 623 nm) had almost the same values, even if the Eu (673 nm, 696 nm, and 704 nm) [16]. Liu and Wang’s research also showed that the emission intensities concentration in the BaZr1−xEuxO3 phosphor powders was different [26]. These previous results [6,16], 5 7 5 7 5 7 of D – F (576 nm), D – F (597 nm), and D – F (616, 623 nm) had almost the same values, even if 0 0 0 1 0 2 3+ and the results this study, suggest that the transitions of Eu ions between different energy bands are the Eu concentration in the BaZr Eu O phosphor powders was different [26]. These previous 1x 3 affected by the host materials of the prepared phosphors. 3+ results [6,16], and the results this study, suggest that the transitions of Eu ions between different As the synthesizing temperature was changed from 900 °C to 1100 °C, the intensities of the two energy bands are affected by the host materials of the prepared phosphors. 3+ emission peaks of the Sr2−xEuxZnMoO6 phosphors was enhanced by increasing the Eu doping As the synthesizing temperature was changed from 900 C to 1100 C, the intensities of the two concentration and reached a maximum value at x = 0.08. In contrast, the intensities of the two 3+ 3+ emission peaks of the Sr Eu ZnMoO phosphors was enhanced by increasing the Eu doping emission peaks of the Sr2−xEuxxZnMoO6 phosphors decreased when the Eu doping ratio was more 2x 6 3+ concentration than 0.08. Thi andsr proves that the co eached a maximum ncentration-quenc value at x = 0.08. hing effect o In contrast, f Eu the dop intensities ing happened of the two in the emission 3+ Sr2−xEuxZnMoO6 phosphors. When 1200 °C was used as the sintering temperature, the intensities of peaks of the Sr Eu ZnMoO phosphors decreased when the Eu doping ratio was more than 0.08. 2x 6 3+ This proves that the concentration-quenching effect of Eu doping happened in the Sr Eu ZnMoO 2x x 6 Intensity Intensity Intensity Intensity Appl. Sci. 2017, 7, 30 7 of 11 3+ the two emission peaks of the Sr2−xEuxZnMoO6 phosphors increased as the concentration of Eu ions rose. The results in Figure 3 also show that when the temperature changed from 1100 °C to 1200 °C, the emission peak varied significantly. For the Sr2−xEuxZnMoO6 phosphors, when the synthesizing temperature was changed from 1100 °C to 1300 °C, the emission peak with the maximum intensity 5 7 5 7 shifted from D0– F1 (597 nm) to D0– F2 (616 nm). Nevertheless, the strong band at 576 nm 5 7 corresponding to the D0– F0 transition was not observed in the emission spectra. Appl. Sci. 2017, 7, 30 7 of 11 The maximum emission intensities (PLmax values) of the Sr2−xEuxZnMoO6 phosphors in the 5 7 5 7 transitions of the D0– F1 (597 nm) and D0– F2 (616 nm) peaks are presented in Figure 4 as a function 3+ of the synthesizing temperature and Eu ion concentration. Those results suggest again that the PL phosphors. When 1200 C was used as the sintering temperature, the intensities of the two emission 3+ characteristics of the Sr2−xEuxZnMoO6 phosphors were strongly affected by the synthesizing peaks of the Sr Eu ZnMoO phosphors increased as the concentration of Eu ions rose. The results 2x 6 3+ temperature and the concentration of Eu ions. As the x value of the Sr2−xEuxZnMoO6 phosphors was in Figure 3 also show that when the temperature changed from 1100 C to 1200 C, the emission peak 5 7 5 7 smaller than 0.10, the emission intensities of the D0– F1 (597 nm) and D0– F2 (616 nm) peaks first varied significantly. For the Sr Eu ZnMoO phosphors, when the synthesizing temperature was 2x 6 increased, reached a maximum, and then decreased as the synthesizing temperature increased. When 5 7 changed from 1100 C to 1300 C, the emission peak with the maximum intensity shifted from D – F 0 1 5 7 the x values of Sr2−xEuxZnMoO6 phosphors were 0.10 and 0.12, the emission intensity of the D0– F1 5 7 5 7 (597 nm) to D – F (616 nm). Nevertheless, the strong band at 576 nm corresponding to the D – F 0 2 0 0 (597 nm) first increased, then decreased at 1100 °C, and then increased at 1200 °C. The emission transition was not 5observed 7 in the emission spectra. intensity of the D0– F2 (616 nm) peaks increased with the increase in synthesizing temperature. These The maximum emission intensities (PL values) of the Sr Eu ZnMoO phosphors in the results suggest that 1100 °C is an important synthesizing temperature for Sr2−xEuxZnMoO6 phosphors max 2x x 6 5 7 5 7 because the transition of PL properties happens at this temperature, but the reasons for this are not transitions of the D – F (597 nm) and D – F (616 nm) peaks are presented in Figure 4 as a function 0 1 0 2 3+ known. of the synthesizing temperature and Eu ion concentration. Those results suggest again that the The results in Figures 3 and 4 present an important result regarding Sr2−xEuxZnMoO6 phosphors: PL characteristics of the Sr Eu ZnMoO phosphors were strongly affected by the synthesizing 2x x 6 5 7 the PLmax values of the transition of the 3+ D0– F1 (597 nm) critically increased as the synthesizing temperature and the concentration of Eu ions. As the x value of the Sr Eu ZnMoO phosphors 2x x 6 temperature increased from 900 °C to 1000 °C, then they did not apparently increase as the 5 7 5 7 was smaller than 0.10, the emission intensities of the D – F (597 nm) and D – F (616 nm) peaks 0 1 0 2 synthesizing temperature increased from 1000 °C to 1100 °C, as Figure 4a shows. However, for first increased, reached a maximum, and then decreased as the synthesizing temperature increased. Sr2−xEuxZnMoO6 phosphors with x = 0.04, 0.06, and 0.08, the PLmax values of the transition of the D0– When the x values of Sr Eu ZnMoO phosphors were 0.10 and 0.12, the emission intensity of the 7 2x x 6 F2 (616 nm) linearly increased as the synthesizing temperature increased from 900 °C to 1100 °C. For 5 7 D – F (597 nm) first increased, then decreased at 1100 C, and then increased at 1200 5C. The 7 emission 0 Sr21−xEuxZnMoO6 phosphors with x = 0.10 and 0.12, the PLmax values of the transition of the D0– F2 (616 5 7 intensity of the D – F (616 nm) peaks increased with the increase in synthesizing temperature. nm) linearly increa 0 sed 2 as the synthesizing temperature increased from 900 °C to 1200 °C, as Figure These 4b shows. results suggest When the synthesizin that 1100 C g temperature w is an important as 12 synthesizing 00 °C, the emi temperatur ssion intensi ety of for the transi Sr Eu ti ZnMoO on 2x x 6 5 7 of D0– F2 (616 nm) for Sr1.9Eu0.1ZnMoO6 and Sr1.88Eu0.12ZnMoO6 phosphors was higher than that of phosphors because the transition of PL properties happens at this temperature, but the reasons for this 5 7 D0– F1 (597 nm), as Figure 4 shows. are not known. (b) (a) 900 900 700 700 500 500 x=0.04 x=0.04 x=0.06 x=0.06 300 300 x=0.08 x=0.08 x=0.10 x=0.10 x=0.12 x=0.12 100 100 900 1000 1100 1200 900 1000 1100 1200 o o Calcining temperature ( C) Calcining temperature ( C) Figure 4. Emission intensity of Sr2−xEuxZnMoO6 phosphors as a function of synthesizing temperature. Figure 4. Emission intensity of Sr Eu ZnMoO phosphors as a function of synthesizing temperature. 2x x 6 (a) Emission peak of 597 nm; and (b) emission peak of 616 nm. (a) Emission peak of 597 nm; and (b) emission peak of 616 nm. 5 7 5 7 5 7 5 7 The transitions of D0– F1 (588 nm), D0– F2 (612 nm), D0– F3 (649 nm), and D0– F4 (695 nm) for The results in Figures 3 and 4 present an important result regarding Sr Eu ZnMoO phosphors: 2x x 6 3+ 3+ 3+ the Eu ions can be simultaneously observed in the emission spectrum of the Eu , Tb -co-doped 5 7 the PL values of the transition of the D – F (597 nm) critically increased as the synthesizing max 0 1 one-dimensional strontium germinate, full-color nano-phosphors [24]. However, introduction of the temperature increased from 900 C to 1000 C, then they did not apparently increase as the 3+ Eu ions in the Sr2−xEuxZnMoO6 lattice results in disorder. The disorder in the structure will cause 5 7 synthesizing temperature increased from 1000 C to 1100 C, as Figure 4a shows. However, transitions in D0– F2 and point defects in the lattice because of differences in the chemical valence 3+ 2+ 3+ for Sr and in Eu the ZnMoO ionic radiu phosphors s between Ewith u ionx s = and 0.04, Sr ion 0.06, s. Th and e Eu 0.08, ions coul the PL d occupy ei values ther of the si of the transition tes 2x x 6 max 5 7 of the D – F (616 nm) linearly increased as the synthesizing temperature increased from 900 C to 0 2 1100 C. For Sr Eu ZnMoO phosphors with x = 0.10 and 0.12, the PL values of the transition 2x x 6 max 5 7 of the D – F (616 nm) linearly increased as the synthesizing temperature increased from 900 C to 0 2 1200 C, as Figure 4b shows. When the synthesizing temperature was 1200 C, the emission intensity 5 7 of the transition of D – F (616 nm) for Sr Eu ZnMoO and Sr Eu ZnMoO phosphors was 0 2 1.9 0.1 6 1.88 0.12 6 5 7 higher than that of D – F (597 nm), as Figure 4 shows. 0 1 5 7 5 7 5 7 5 7 The transitions of D – F (588 nm), D – F (612 nm), D – F (649 nm), and D – F (695 nm) 0 1 0 2 0 3 0 4 3+ 3+ 3+ for the Eu ions can be simultaneously observed in the emission spectrum of the Eu , Tb -co-doped one-dimensional strontium germinate, full-color nano-phosphors [24]. However, introduction of the 3+ Eu ions in the Sr Eu ZnMoO lattice results in disorder. The disorder in the structure will cause 2x x 6 5 7 transitions in D – F and point defects in the lattice because of differences in the chemical valence 0 2 Intensity Intensity Appl. Sci. 2017, 7, 30 8 of 11 3+ 2+ 3+ and in the ionic radius between Eu ions and Sr ions. The Eu ions could occupy either of the sites 2+ 2+ 2+ of Sr or the sites of Zn (Mo ) in the Sr Eu ZnMoO phosphor ’s double-perovskite structure. 2x 6 3+ If Eu ions occupy a lattice site with a strict center of symmetry, the odd terms of the static crystal Appl. Sci. 2017, 7, 30 8 of 11 field vanish. This will lead to electric dipole transitions being strictly forbidden for purely electric 5 7 transitions and the transitions of the D – F (597 nm) will predominantly produce the emission. 0 1 2+ 2+ 2+ of Sr or the sites of Zn (Mo ) in the Sr2−xEuxZnMoO6 phosphor’s double-perovskite structure. If 3+ 2+ 2+ The Eu ions could occupy either the tetrahedral sites of Sr or the octahedral sites of Zn or 3+ Eu ions occupy a lattice site with a strict center of symmetry, the odd terms of the static crystal field 6+ Mo in the Sr Eu ZnMoO double-perovskite crystal structure. Consequently, as both of these sites 2x x 6 vanish. This will lead to electric dipole transitions being strictly forbidden for purely electric 5 7 5 7 are centrosymmetric, electric dipole transitions 5 7 ( D – F , only the D – F transition is observed transitions and the transitions of the D0– F1 (597 nm 0) will predom j =0,2 inantly pro 0 duce the emission. 2 3+ 2+ 2+ in this study) The Eu are ion forbidden s could ocand, cupy either the t as such, should etrahedr not al sit appear es of Srin or t the he octahedral sites o spectra. As the synthesizing f Zn or 5 7 6+ temperatur Mo in t e h is e 900 Sr2−xEu C, xZnMoO the fact 6 double that the -pero Dvskit – Fe cry electric stal struct dipole ure.transition Consequently at 615 , as both of these sites nm prevails over the 0 2 5 7 5 7 5 7 are centrosymmetric, electric dipole transitions ( D0– Fj=0,2, only the D0– F2 transition is observed in D – F magnetic dipole transition at 595 nm in the case of Sr Eu ZnMoO phosphors leads us to 0 1 2x x 6 3+ this study) are forbidden and, as such, should not appear in the spectra. As the synthesizing the conclusion that the Eu ions are generally located in a disordered manner in the Sr Eu ZnMoO 2x x 6 5 7 temperature is 900 °C, the fact that the D0– F2 electric dipole transition at 615 nm prevails over the 3+ powders [27]. The charge-compensating oxygen vacancies surrounding the Eu ions will lead to the 5 7 D0– F1 magnetic dipole transition at 595 nm in the case of Sr2−xEuxZnMoO6 phosphors leads us to the deviation from the point symmetry and relaxation of electric dipole transitions selection rules, with the 3+ conclusion that the Eu ions are generally located in a disordered manner in the Sr2−xEuxZnMoO6 5 7 3+ appearance of the D – F transition lines in the spectra [27]. Additionally, the Eu luminescent 0 2 3+ powders [27]. The charge-compensating oxygen vacancies surrounding the Eu ions will lead to the 5 7 centers contributing to the D – F transition line are probably located at the Sr Eu ZnMoO 0 2 2x x 6 deviation from the point symmetry and relaxation of electric dipole transitions selection rules, with 5 7 3+ particle surface or subsurface and will dominate the emission of Sr Eu ZnMoO phosphors when 2x x 6 the appearance of the D0– F2 transition lines in the spectra [27]. Additionally, the Eu luminescent 3+ 5 7 the synthesizing temperature is 1200 C [27]. If Eu ions occupy a non-centrosymmetric lattice site, centers contributing to the D0– F2 transition line are probably located at the Sr2−xEuxZnMoO6 particle surface or subsurface and will dominate the emission of Sr2−xEuxZnMoO6 phosphors when the both electric and magnetic transitions are possible, and the non-symmetric site occupancy will be a 3+ 5 7 synthesizing temperature is 1200 °C [27]. If Eu ions occupy a non-centrosymmetric lattice site, both dominant reason for causing the emission of D – F (612 nm) transition [28]. 0 2 electric and magnetic transitions are possible, and the non-symmetric site occupancy will be a The definition of fluorescence (or phosphorescence) lifetime is the time for the intensity of a single 5 7 dominant reason for causing the emission of D0– F2 (612 nm) transition [28]. emission peak to decrease to 1/e (approximately 37%) of its original intensity. The decay curves of The definition of fluorescence (or phosphorescence) lifetime is the time for the intensity of a 3+ Sr Eu ZnMoO phosphors were obtained at different concentrations of Eu (x = 0.04, 0.06, 0.08, 2x x 6 single emission peak to decrease to 1/e (approximately 37%) of its original intensity. The decay curves 0.010, and 0.12), and Figure 5 shows the intensity decay of the luminescence of Sr Eu ZnMoO 2x x 6 3+ of Sr2−xEuxZnMoO6 phosphors were obtained at different concentrations of Eu (x = 0.04, 0.06, 0.08, phosphors as a function of the synthesizing temperature, which can be used to calculate the lifetime. 0.010, and 0.12), and Figure 5 shows the intensity decay of the luminescence of Sr2−xEuxZnMoO6 The wavelength of the exciting light was 271 nm, and the measured wavelength for the intensity phosphors as a function of the synthesizing temperature, which can be used to calculate the lifetime. 5 7 decay The wa was 597 velenm ngth of ( D the exci – F ), ti because ng light wa it had s 271 the nm maximum , and the mea emission sured wa intensity velength f , as or the the synthesizing intensity 0 1 5 7 decay was 597 nm ( D0– F1), because it had the maximum emission intensity, as the synthesizing temperature was 900–1100 C. Typically, the standard form of the single exponential decay function is: temperature was 900–1100 °C. Typically, the standard form of the single exponential decay function is: A(t) = A exp[-(t/)] (1) A(t) = A0 exp[-(t/τ)] (1) where A is the initial population,  is constant for the decay time , and t is time. Figure 5 shows that where A0 is the initial population, τ is constant for the decay time , and t is time. Figure 5 shows that the all curves of decay time can be fitted well by a single-exponential function. Figure 5 also shows the all curves of decay time can be fitted well by a single-exponential function. Figure 5 also shows that if the same synthesizing temperature is used, the observed decay time is almost unchanged even that if the same synthesizing temperature is used, the observed decay time is almost unchanged even the x value in Sr Eu ZnMoO phosphor increases from 0.04 to 0.12. Moreover, the decay time is 2x x 6 the x value in Sr2−xEuxZnMoO6 phosphor increases from 0.04 to 0.12. Moreover, the decay time is a 3+ a single exponential function for Sr Eu ZnMoO phosphors with different concentrations of Eu x 3+ 2x 6 single exponential function for Sr2−xEuxZnMoO6 phosphors with different concentrations of Eu ions ions and synthesized at different temperatures, because the activator lies in the same coordination and synthesized at different temperatures, because the activator lies in the same coordination environment [29]. environment [29]. (b) (a) x=0.04 x=0.04 x=0.06 x=0.06 x=0.08 x=0.08 x=0.10 x=0.10 x=0.12 x=0.12 0.1 0.1 0.0 0.5 1.0 1.5 2.0 2.5 3.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Time (ms) Time (ms) Figure 5. Cont. Intensity Intensity Appl. Sci. 2017, 7, 30 9 of 11 Appl. Sci. 2017, 7, 30 9 of 11 (c) (d) 1 1 Appl. Sci. 2017, 7, 30 9 of 11 (c) (d) 1 1 x=0.04 x=0.04 x=0.06 x=0.06 x=0.08 x=0.08 x=0.10 x=0.10 x=0.12 x=0.12 0.1 0.1 0.0 0.5 1.0 1.5 2.0 2.5 3.0 0.00.5 1.01.5 2.02.5 3.0 x=0.04 x=0.04 x=0.06 x=0.06 Time (ms) Time (ms) x=0.08 x=0.08 x=0.10 x=0.10 Figure 5. Luminescence lifetime of Sr2−xEuxZnMoO6 phosphors synthesized at (a) 900 °C; (b) 1000 °C; x=0.12 x=0.12 Figure 5. Luminescence lifetime of Sr Eu ZnMoO phosphors synthesized at (a) 900 C; (b) 1000 C; 2x 6 0.1 0.1 (c) 1100 °C; and (d) 1200 °C, respectively. 0.0 0.5 1.0 1.5 2.0 2.5 3.0 0.00.5 1.01.5 2.02.5 3.0 (c) 1100 C; and (d) 1200 C, respectively. Time (ms) Time (ms) From the results shown in Figure 5, the lifetimes of Sr2−xEuxZnMoO6 phosphors, which are From the results shown in Figure 5, the lifetimes of Sr Eu ZnMoO phosphors, which are 2x x 6 Figure 5. Luminescence lifetime of Sr2−xEuxZnMoO6 phosphors synthesized at (a) 900 °C; (b) 1000 °C; compared in Table 1, were measured at 2.04–2.10 ms, 1.93–2.03 ms, 1.78–1.82 ms, and 1.65–1.67 ms, compared in Table 1, were measured at 2.04–2.10 ms, 1.93–2.03 ms, 1.78–1.82 ms, and 1.65–1.67 ms, (c) 1100 °C; and (d) 1200 °C, respectively. when the synthesizing temperature was 900 °C, 1000 °C, 1100 °C, and 1200 °C, respectively. The when the synthesizing temperature was 900 C, 1000 C, 1100 C, and 1200 C, respectively. lifetimes of Sr2−xEuxZnMoO6 phosphors apparently decreased with the increase in the synthesizing From the results shown in Figure 5, the lifetimes of Sr2−xEuxZnMoO6 phosphors, which are temperature. When the synthesizing temperature was 900 °C and 1000 °C, the lifetimes of the The lifetimes of Sr Eu ZnMoO phosphors apparently decreased with the increase in the 2x 6 compared in Table 1, were measured at 2.04–2.10 ms, 1.93–2.03 ms, 1.78–1.82 ms, and 1.65–1.67 ms, Sr2−xEuxZnMoO6 phosphors varied more. When the synthesizing temperature was 1100 °C and 1200 synthesizing temperature. When the synthesizing temperature was 900 C and 1000 C, the lifetimes when the synthesizing temperature was 900 °C, 1000 °C, 1100 °C, and 1200 °C, respectively. The °C, the Sr2−xEuxZnMoO6 phosphors had less variation in their lifetimes. In Figure 6 we use a fitting of the Sr Eu ZnMoO phosphors varied more. When the synthesizing temperature was 1100 C 2x 6 lifetimes of Sr2−xEuxZnMoO6 phosphors apparently decreased with the increase in the synthesizing curve to find the relationship between lifetime and synthesizing temperature; the equation is: and 1200 C, the Sr Eu ZnMoO phosphors had less variation in their lifetimes. In Figure 6 we use 2x x 6 temperature. When the synthesizing temperature was 900 °C and 1000 °C, the lifetimes of the t = −0.0016 × T + 3.543 (2) a fitting curve to find the relationship between lifetime and synthesizing temperature; the equation is: Sr2−xEuxZnMoO6 phosphors varied more. When the synthesizing temperature was 1100 °C and 1200 °C, the Sr2−xEuxZnMoO6 phosphors had less variation in their lifetimes. In Figure 6 we use a fitting where t represents the emitting lifetime of the luminescent material and T represents the synthesizing t = 0.0016  T + 3.543 (2) curve to find the relationship between lifetime and synthesizing temperature; the equation is: temperature (°C). In other words, the luminescence lifetime of Sr2−xEuxZnMoO6 phosphors can be obtained by using Equation (2), as the synthesizing temperature is changed from 900 °C to 1200 °C. t = −0.0016 × T + 3.543 (2) where t represents the emitting lifetime of the luminescent material and T represents the synthesizing These results suggest that the luminescence lifetime of Sr2−xEuxZnMoO6 phosphors can be a function temperatur where t e represents the emi ( C). In other wor tting l ds,ithe fetime of luminescence the luminescent ma lifetime3+ teri of al Sr and Eu T represen ZnMoO ts the synthesizing phosphors can be of synthesizing temperature, and that the concentration of Eu ions not on 2x ly can affect 6 their emitting temperature (°C). In other words, the luminescence lifetime of Sr2−xEuxZnMoO6 phosphors can be obtained by using Equation (2), as the synthesizing temperature is changed from 900 C to 1200 C. intensity but also can affect their luminescence lifetime. obtained by using Equation (2), as the synthesizing temperature is changed from 900 °C to 1200 °C. These results suggest that the luminescence lifetime of Sr Eu ZnMoO phosphors can be a function 2x 6 These results suggest that the luminescence lifetime of Sr2−xEuxZnMoO6 phosphors can be a function Table 1. Luminescence lifetimes of Sr2−xEuxZnMoO6 phosphors as a function of the sy 3+ nthesizing of synthesizing temperature, and that the concentration of Eu ions not only can affect their emitting 3+ 3+ of synthesizing temperature, and that the concentration of Eu ions not only can affect their emitting temperature and concentration of Eu ions. intensity but also can affect their luminescence lifetime. intensity but also can affect their luminescence lifetime. Synthesizing Temperature X =0.04 X =0.06 X =0.08 X =0.10 X = 0.12 Table 1. Luminescence lifetimes of Sr Eu ZnMoO phosphors as a function of the synthesizing 900 °C 2x 2. x05 2. 61 2.04 2.04 2.05 Table 1. Luminescence lifetimes of Sr2−xEuxZnMoO6 phosphors as a function of the synthesizing 3+ temperature and concentration of Eu ions. 1000 °C 2.03 2.03 1.93 1.97 1.99 3+ temperature and concentration of Eu ions. 1100 °C 1.81 1.83 1.78 1.82 1.82 Synthesizing Temperature X =0.04 X =0.06 X =0.08 X =0.10 X = 0.12 Synthesizing Temperature X = 0.04 X = 0.06 X = 0.08 X = 0.10 X = 0.12 1200 °C 1.65 1.65 1.66 1.66 1.67 900 °C 2.05 2.1 2.04 2.04 2.05 900 C 2.05 2.1 2.04 2.04 2.05 1000 °C 2.03 2.03 1.93 1.97 1.99 1000 C 2.03 2.03 1.93 1.97 1.99 x=0.04 2.4 x=0.06 1100 C 1.81 1.83 1.78 1.82 1.82 1100 °C 1.81 1.83 1.78 1.82 1.82 x=0.08 1200 C 1.65 1.65 1.66 1.66 1.67 x=0.10 1200 °C 1.65 1.65 1.66 1.66 1.67 2.1 x=0.12 fiting curve x=0.04 1. 2.8 4 x=0.06 x=0.08 x=0.10 2.1 1.5 x=0.12 fiting curve 1.8 1.2 900 1000 1100 1200 Calcining temperature ( C) 1.5 Figure 6. Fitting curve of lifetime for Sr2−xEuxZnMoO6 phosphors as a function of the synthesizing 3+ temperature and concentration of Eu ions. 1.2 900 1000 1100 1200 Calcining temperature ( C) Figure 6. Fitting curve of lifetime for Sr2−xEuxZnMoO6 phosphors as a function of the synthesizing Figure 6. Fitting curve of lifetime for Sr Eu ZnMoO phosphors as a function of the synthesizing 2x 6 3+ temperature and concentration of Eu ions. 3+ temperature and concentration of Eu ions. Intensity Intensity Time (ms) Time (ms) Intensity Intensity Appl. Sci. 2017, 7, 30 10 of 11 4. Conclusions For Sr Eu ZnMoO powders, the 2 value of the (220) diffraction peak shifted to a higher value 2x 6 and the FWHM value for the (220) diffraction peak decreased as the synthesizing temperature increased. 3+ When the x value (the concentration of Eu ions) increased from 0 to 0.12 and the synthesizing temperatures rose from 900 C to 1200 C, the cell parameters of the Sr Eu ZnMoO powders were 2x x 6 p p p changed from a = b = c/ 2 = 0.3968 nm to a = b = c/ 2 = 0.3971 nm and from a = b = c/ 2= 0.3971 nm to a = b = c/ 2 = 0.3974 nm (cubic crystal structure). When the synthesizing temperature was 900–1100 C, the emission intensities of the Sr Eu ZnMoO phosphors peaked at x = 0.08 and decreased as the 2x 6 +3 3+ concentration of Eu ions increased. There was an observable concentration-quenching effect of Eu ions in the Sr Eu ZnMoO phosphors. The emission spectrum of Sr ZnMoO powder consisted of 2x x 6 2 6 one broad band in the 400–575 nm region, and the emission spectra of the Sr Eu ZnMoO phosphors 2x x 6 (except for the Sr ZnMoO powder) consisted of two parts: one broad band in the 400–575 nm region 2 6 and two sharp peaks located at 597 nm and 616 nm. The lifetimes of the Sr Eu ZnMoO phosphors, 2x x 6 3+ which depended on synthesizing temperature and were independent of the concentration of Eu ions, were 2.04–2.10 ms, 1.93–2.03 ms, 1.78–1.82 ms, and 1.65–1.67 ms when the synthesizing temperature was 900 C, 1000 C, 1100 C, and 1200 C, respectively. We found that when the synthesizing temperature was in the range of 900–1200 C, the relationship between the lifetime and the synthesizing temperature could be determined by the following equation: t = 0.0016  T + 3.543, where t and T represent the emitting lifetime of the luminescent material and the synthesizing temperature ( C). Acknowledgments: The authors would like to acknowledge the financial supports of MOST 104-2221-E-390- 013-MY2, MOST 105-2622-E-390-003-CC3, and MOST 105-2221-E-344-002. Author Contributions: S.-H.Y. and J.-L.L. helped proceeding the experimental processes, measurements, and data analysis; C.-Y.L. and C.-F.Y. organized the paper and encouraged in paper writing; Also, C.-Y.L. and C.-F.Y. helped proceeding the experimental processes and measurements. Conflicts of Interest: The authors declare no conflict of interest. References 1. Binnemans, K.; Görller-Walrand, C. On the color of the trivalent lanthanide ions. Chem. Phys. Lett. 1995, 235, 163–174. [CrossRef] 2. Dorenbos, P. The 5d level of positions of the trivalent lanthanides in inorganic compounds. J. Lumin. 2000, 91, 155–176. [CrossRef] N N1 3. Reid, M.F.; Pieterson, L.V.; Meijerink, A. Trends in parameters for 4f $ 4f 5d spectra of lanthanide ions in crystals. J. Alloys Compd. 2002, 344, 240–245. [CrossRef] 7 6 2+ 4. Dorenbos, P. 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Effects of the Concentration of Eu3+ Ions and Synthesizing Temperature on the Luminescence Properties of Sr2−xEuxZnMoO6 Phosphors

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applied sciences Article 3+ Effects of the Concentration of Eu Ions and Synthesizing Temperature on the Luminescence Properties of Sr Eu ZnMoO Phosphors 2x x 6 1 2 3 4 , Chi-Yu Lin , Su-Hua Yang , Jih-Lung Lin and Cheng-Fu Yang * Department of Aero-Electronic Engineering, Air Force Institute of Technology, Kaohsiung 820, Taiwan, R.O.C.; takiincku@yahoo.com.tw Department of Electronic Engineering, National Kaohsiung University of Applied Sciences, Kaohsiung 807, Taiwan, R.O.C.; shya@cc.kuas.edu.tw Department of Aeronautics and Astronautics, Air Force Academy, Kaohsiung 820, Taiwan, R.O.C.; lin2737.cafa@msa.hinet.net Department of Chemical and Material Engineering, National University of Kaohsiung, Kaohsiung 811, Taiwan, R.O.C. * Correspondence: cfyang@nuk.edu.tw; Tel.: +886-7-591-9283 Academic Editors: Teen-Hang Meen, Antonio Facchetti and Giorgio Biasiol Received: 13 September 2016; Accepted: 22 December 2016; Published: 27 December 2016 Abstract: The effect of Eu O concentration on the luminescence properties of double perovskite 2 3 (cubic) Sr Eu ZnMoO phosphors was thoroughly investigated using different synthesizing 2x x 6 temperatures. Phosphors with the composition Sr Eu ZnMoO , where Eu O was substituted for 2x 6 2 3 SrO and x was changed from 0 to 0.12, were synthesized by the solid-state method at temperatures of 900–1200 C, respectively. Analysis of the X-ray diffraction (XRD) patterns showed that even when the synthesizing temperature was 1100 C, secondary or unknown phases were observed in 3+ Sr Eu ZnMoO ceramic powders. The effect of the concentration of Eu ions on the luminescence 2x 6 properties of the Sr Eu ZnMoO phosphors was readily observable because no characteristic 2x x 6 emission peak was observed in the Sr ZnMoO phosphor. Two characteristic emission peaks 2 6 5 7 5 7 at 597 and 616 nm were observed, which correspond to the D – F and D – F transitions of 0 1 0 2 3+ Eu ions, respectively. The two characteristic emission peaks of the Sr Eu ZnMoO phosphors 2x x 6 3+ were apparently influenced by the synthesizing temperature and the concentration of Eu ions. 3+ When x was larger than 0.08, a concentration quenching effect of Eu ions in the Sr Eu ZnMoO 2x x 6 phosphors could be observed. The lifetime of the Sr Eu ZnMoO phosphors decreased as the 2x x 6 synthesizing temperature increased. A linear relation between temperature and lifetime was obtained by using a fitting curve of t = 0.0016  T + 3.543, where t was lifetime and T was synthesizing temperature. Keywords: Sr Eu ZnMoO phosphors; double perovskite oxides; synthesizing temperature; 2x x 6 lifetime 1. Introduction UV light can be generated as a consequence of electronic transitions of light sources through an Hg discharge. In low-pressure Hg discharge, the main emission line is located at a wavelength of 254 nm. This light is invisible and harmful to human bodies, so it has to be converted into visible light, which can be done with a combination of luminescent materials. These luminescent materials can strongly absorb light of that wavelength and efficiently convert it into visible light. Recently, white light emitting diodes (LEDs) have become popular because they have several advantages, including high efficiency, long lifetime, and low power consumption. Red phosphors are also helpful for generating Appl. Sci. 2017, 7, 30; doi:10.3390/app7010030 www.mdpi.com/journal/applsci Appl. Sci. 2017, 7, 30 2 of 11 white light when they are excited by blue or near-UV lights. To obtain phosphors with highly efficient emissions, it is important to choose the right compound materials and ensure they have outstanding physical and chemical stability. Numerous studies have explored different luminescent materials to enable the development of suitable phosphors. When lanthanide contraction ions are introduced into host materials, unfilled 4f electron orbitals result, and these have attracted considerable attention. The resulting phosphors emit very luminescent emissions with specific light wavelengths because of variations in the energy level of some free electrons [1–4]. These phosphors have been the most promising candidates for applications in fluorescent lamps and flat panel display devices, such as electroluminescence panels, plasma display panels, and field emission displays. A large number of isotropic compounds with perovskite structures, such as double perovskite oxides, have the general formula A BB’O , in which BO and B’O octahedra are corner-shared, 2 6 6 6 alternately. The great flexibility of A and B(B’) sites in A BB’O allows very rich substitutions, 2 6 and this framework forms cube-octahedral cavities filled by A-site cations [5,6]. Double perovskites with the formula A BB’O , where A uses an alkaline earth, B and B’ are metal transition magnetic 2 6 and nonmagnetic ions, and O is oxygen, have been investigated as magnetic materials for many years. For example, Sr CrMoO has been studied as a half-metallic system [7]. Kobayashi et al. 2 6 recently reported room-temperature low-field magnetic resistance in the ordered double perovskite Sr FeMoO [8]. Complete ordering of Fe and Mo on the B and B’ sites of this metallic A BB’O double 2 6 2 6 perovskite is predicted to give half-metallic ferromagnetism with localized majority-spin electrons on the Fe atoms [9,10]. Recently, the study of A BB’O -based materials has increased due to various 2 6 technological applications, such as inorganic oxide luminescent materials. The emitting materials are usually composed of activators and a host lattice. Some host lattice materials can produce light themselves, and some can produce light when doped with rare-earth activators (ions). Rare-earth ions are known to exist in various valence states, although the trivalent state is the most prevalent. Rare-earth ions can be applied in lighting devices and display panels due to their abundant energy levels across a wide spectrum range, from ultraviolet to near infrared. Sm- and Eu-based ions are the most commonly used dopants because they are stable 3+ 3+ 2+ in trivalent (Sm and Eu ), as well as divalent (Eu ), states. The luminescence of rare-earth ions doped in perovskite-type ceramics was actively investigated in the 1960s and 1970s because of interest in their ferroelectricity, phase transitions, and semiconducting properties [11]. Recently, many studies have shown that the double perovskite structure with a composition of A BMO (A = Ba, 2 6 3+ Sr; B = Ca, Zn; M = Mo, W) is activated by trivalent europium ions (Eu ) [12–15]. Phosphors 3+ activated by Eu are considered ideal red sources because of their sharp emission lines in the red 3+ region [12–16]. Eu -doped double-perovskite materials have a broad excitation band ranging from 3+ UV to visible light, and they also show highly efficient red luminescence. For that, Eu -doped double molybdenum (Mo)-based double perovskite oxides have attracted significant attention for their 3+ possible application as luminescent materials, such as Sr MgMo W O : Eu [17], Sr Ca(Mo/W)O : 2 1x 6 2 6 3+ 3+ 3+ 3+ 3+ Eu [18], Sr CaMoO : Eu [19], (Ba,Sr) CaMoO : Eu , Yb [20], Ca LaMO : Eu (M = Sb, Nb, 2 6 2 6 2 6 Ta) [21], and A CaMoO (A = Sr, Ba) [6], respectively. 2 6 3+ To the best of our knowledge, the luminescent characteristics of concentrated Eu ions in Sr Eu ZnMoO phosphors, which are molybdenum-based double-perovskite oxides, have not 2x 6 3+ been reported. In this study, the red-emitting phosphors of Eu -doped Mo-based double-perovskite Sr Eu ZnMoO oxides were synthesized by the conventional high-temperature solid state reaction 2x 6 3+ method. We found that Eu concentration and the synthesizing temperature of the Sr Eu ZnMoO 2x x 6 phosphors had a large effect on their luminescent characteristics. The concentration quenching effect of 3+ Eu ions in Sr Eu ZnMoO phosphors was found and would be well discussed [22]. In this study, 2x x 6 the first important novelty is that we have not found any similar studies about Sr Eu ZnMoO 2x 6 phosphors. The second important novelty is that we found a linear relationship between the synthesizing temperature and lifetime of Sr Eu ZnMoO phosphors. We also investigated the 2x 6 relationship between the synthesized temperature and lifetime of Sr Eu ZnMoO phosphors. 2x x 6 Appl. Sci. 2017, 7, 30 3 of 11 2. Materials and Methods Sr Eu ZnMoO powders were synthesized through the solid-state reaction method. 2x 6 Stoichiometric amounts of SrCO , ZnO, MoO , and Eu O were weighed according to the composition 3 3 2 3 formula of (2 x) SrCO + ZnO + MoO + (0.5 x) Eu O , where x = 0, 0.04, 0.06, 0.08, 0.10, and 3 3 2 3 0.12, respectively. After being mixed in acetone, dried, and ground, the solid-state reaction method was used to heat the Sr Eu ZnMoO compositions in an air atmosphere. The Sr Eu ZnMoO x x 2x 6 2x 6 powders were heated to 900 C, 1000 C, 1100 C, and 1200 C for 4 h. When the synthesizing temperature was equal to 1300 C, the Sr Eu ZnMoO was melted and gathered together to not 2x 6 be used as part of the phosphors. The crystalline structures of the synthesized Sr Eu ZnMoO 2x x 6 powders were measured using X-ray diffraction (XRD) (Bruker, Boston, MA, USA ) patterns with Cu K radiation ( = 1.5418 Å) and with a scanning speed of 2 per minute. Photoluminescence (PL) properties were recorded at room temperature in the wavelength range of 450–800 nm on a Hitachi F-4500 fluorescence spectrophotometer (Hitachi, Tokyo, Japan). In the past, we had found that 271 nm had a better excitation effect on BaZr Eu O powders [16]. This result suggested that we would also 1x x 3 need to find the optimum optical wavelength for exciting the Sr Eu ZnMoO powders. In this study, 2x x 6 the three-dimensional (3D) scanning process using a spectrophotometer (Hitach, Tokyo, Japan) was used to find the optimum optical wavelength, and this value was dependent on the compositions of the Sr Eu ZnMoO phosphors and the synthesizing temperature. We found that the optimum exciting 2x x 6 optical wavelength for all of the Sr Eu ZnMoO powders was 350 nm, and the Sr Eu ZnMoO 2x x 6 2x x 6 powders excited by other wavelengths had the weaker PL intensities. 3. Results and Discussion To achieve high PL properties, the preparation of Sr Eu ZnMoO powders forming 2x 6 the double-perovskite phase is very important, because crystallization of Sr Eu ZnMoO 2x x 6 powders influences their photoluminescent properties. Figure 1 shows the XRD patterns of our Sr Eu ZnMoO powders as a function of the synthesizing temperature. The strong peaks occurred 2x x 6 at around 31.9 for the (220) diffraction peak of the six host lattices. Those results suggest that the XRD patterns showed stable double-perovskite features regardless of the synthesizing temperature and 3+ Eu concentration. As Figure 1a shows, when the synthesizing temperature of the Sr Eu ZnMoO powders was 2x x 6 900 C and as the x value increased from 0 to 0.12, the 2 value of the (220) diffraction peak shifted from 31.85 to 31.87 and the full width at half maximum (FWHM) values for the (220) diffraction peak were in the range of 2 = 0.25 –0.27 . When the synthesizing temperature was 1200 C, the 2 value of the (220) diffraction peak shifted from 31.88 to 31.90, and the FWHM values for the (220) diffraction peak of the Sr Eu ZnMoO powders were in the range of 2 = 0.19–0.20 , as the x value of the 2x 6 Sr Eu ZnMoO powders increased from 0 to 0.12. The results in Figure 1a–d show that the 2 of the 2x x 6 (220) diffraction peak shifted to a higher value and the FWHM values for the (220) diffraction peaks of the Sr Eu ZnMoO powders decreased as the synthesizing temperature increased. 2x x 6 The ideal cubic double-perovskite structure (with the same space group Pm3m (221)) can be described by a faced-centered cubic (fcc) lattice with lattice constant 2a [23]. The B(B’) ion is coordinated by the B’(B) ion using an O ion as an intermediate in the middle, and the lengths of B–O and B’–O are considered to be equal. After relaxation, both lattice constants and atomic positions reduce the ideal cubic structure (space group Fm3m) to a tetragonal structure (space group I4/mmm). There are two O atoms located on the z-axis with B and B’ atoms sitting between, and the four O atoms are 1 2 located on the xy-plane; the same as the B and B’ atoms. The angle of the B–O–B’ remains at 180 during structural optimization, whereas the lattice constant and bond length change. The lattice a can be calculated by using (1) the reflection peaks (011), (111), (200), and (220) from the XRD patterns in Figure 1; and (2) the closeness of the c/a ratio to the ideal value of 2. As the synthesizing temperature was 900 C, the Sr Eu ZnMoO powders exhibited a cubic crystal structure with the cell parameters 2x x 6 p p 3+ changing from a = b = c/ 2 = 0.3968 nm to a = b = c/ 2 = 0.3971 nm as the concentration of Eu ions Appl. Sci. 2017, 7, 30 4 of 11 increased from 0 to 0.12. The cell parameters for Sr Eu ZnMoO powders synthesized at 1200 C 2x x 6 Appl. Sci. 2017, 7, 30 4 of 11 were also calculated from the XRD patterns shown in Figure 1, and the cell parameters changed from p p 3+ a = b = c/ 2 = 0.3971 nm to a = b = c/ 2 = 0.3974 nm as the concentration of Eu ions increased 3+ changed from a = b = c/√2 = 0.3971 nm to a = b = c/√2= 0.3974 nm as the concentration of Eu ions from 0 to 0.12. These results were in good agreement with those from the Joint Committee on Powder increased from 0 to 0.12. These results were in good agreement with those from the Joint Committee Diffraction Standards (JCPDS) file number 742474. on Powder Diffraction Standards (JCPDS) file number 742474. (a) (b) o: unknown phase x:Eu O x=0.06 x=0.06 o: unknown phase x:Eu O 2 3 2 3 x=0.05 x=0.05 x x x x x=0.04 x=0.04 x=0.03 x=0.03 x=0.02 x=0.02 x=0 x=0 o o o o oo o o 20 30 40 50 60 70 80 20 30 40 50 60 70 80 2θ (Degree) 2θ (Degree) (d) (c) o: unknown phase x:Eu O x=0.06 x=0.06 2 3 x=0.05 x=0.05 x=0.04 x=0.04 x=0.03 x=0.03 x=0.02 x=0.02 x=0 x=0 20 30 40 50 60 70 80 20 30 40 50 60 70 80 2θ (Degree) 2θ (Degree) Figure 1. XRD patterns of Sr2−xEuxZnMoO6 phosphors synthesized at (a) 900 °C; (b) 1000 °C; (c) 1100 Figure 1. XRD patterns of Sr Eu ZnMoO phosphors synthesized at (a) 900 C; (b) 1000 C; 2x 6 °C; and (  d) 1200 °C for 4  h, respectively. (c) 1100 C; and (d) 1200 C for 4 h, respectively. As the results of the X-ray diffraction (XRD) patterns showed in Figure 1 were compared, these As the results of the X-ray diffraction (XRD) patterns showed in Figure 1 were compared, results indicated that the diffraction intensities of unknown phases for peaks located at around 2θ = 3+ these28 results .44° and indicated 33.09° incre thata the sed dif asfraction the concent intensities ration o of f E unknown u ions increased an phases for peaks d decrlocated eased as atthe around 3+ synthesizing temperature increased. Those peaks are in good agreement with the (222) and (400) 2 = 28.44 and 33.09 increased as the concentration of Eu ions increased and decreased as the peaks of JCPDS file number 120393 for cubic Eu2O3. The decrease in the diffraction intensities of peaks synthesizing temperature increased. Those peaks are in good agreement with the (222) and (400) peaks 3+ 2+ located at around 2θ = 28.44° and 33.09° prove that more Eu ions will substitute the sites of Sr ions of JCPDS file number 120393 for cubic Eu O . The decrease in the diffraction intensities of peaks 2 3 as the synthesizing temperature is raised. Even when the synthesizing temperature was 1100 °C, 3+ 2+ located at around 2 = 28.44 and 33.09 prove that more Eu ions will substitute the sites of Sr secondary or unknown phases were observed in the Sr2−xEuxZnMoO6 ceramic powders. These Eu2O3 ions as the synthesizing temperature is raised. Even when the synthesizing temperature was 1100 C, and secondary or unknown phases were not observed when the synthesizing temperature was 1200 secondary or unknown phases were observed in the Sr Eu ZnMoO ceramic powders. These Eu O 2x x 6 2 3 °C. These results suggest that when the same synthesizing temperature is used, the concentration of and secondary or unknown phases were not observed when the synthesizing temperature was 1200 C. 3+ Eu ions has no apparent effect on the crystallization of Sr2−xEuxZnMoO6 powders; hence, the 3+ These results suggest that when the same synthesizing temperature is used, the concentration of Eu synthesizing temperature is an important factor in determining the crystalline properties of 3+ ions has no apparent effect on the crystallization of Sr Eu ZnMoO powders; hence, the synthesizing Sr2−xEuxZnMoO6 powders. Additionally, the concentration of Eu ions and the synthesizing 2x x 6 temperature affect the photoluminescent properties of Sr2−xEuxZnMoO6 phosphors. temperature is an important factor in determining the crystalline properties of Sr Eu ZnMoO 2x x 6 3+ XRD patterns for Sr2−xEuxZnMoO6 phosphors synthesized at 1200 °C for 4 h in the narrow range powders. Additionally, the concentration of Eu ions and the synthesizing temperature affect the of 29–35° are shown in Figure 2. These results are significant. Initially, the splitting of the (220) photoluminescent properties of Sr Eu ZnMoO phosphors. 2x x 6 diffraction peak was observed in the Sr2ZnMoO6 and Sr1.98Eu0.02ZnMoO6 phosphors, but it was not XRD patterns for Sr Eu ZnMoO phosphors synthesized at 1200 C for 4 h in the narrow 2x x 6 observed in other Sr2−xEuxZnMoO6 phosphors. The two split peaks of the Sr2ZnMoO6 phosphors were range of 29–35 are shown in Figure 2. These results are significant. Initially, the splitting of the located at 2θ = 31.83° and 31.88°, and the two split peaks of the Sr1.98Eu0.02ZnMoO6 phosphors were (220) diffraction peak was observed in the Sr ZnMoO and Sr Eu ZnMoO phosphors, but it 2 6 1.98 0.02 6 located at 2θ = 31.81° and 31.89°. These results suggest that the Sr2ZnMoO6 and Sr1.98Eu0.02ZnMoO6 was not observed in other Sr Eu ZnMoO phosphors. The two split peaks of the Sr ZnMoO x 3+ 2x 6 2 6 phosphors revealed a perovskite structure with the tetragonal phase. As the concentration of Eu phosphors were located at 2 = 31.83 and 31.88 , and the two split peaks of the Sr Eu ZnMoO 1.98 0.02 6 Intensity (a.u.) Intensity (a.u .) (011) (011) (110) (110) (111) (220) (220) (101) (101) (222) (222) (200) (200) (400) (422) (422) (220) (220) (310) (310) Intensi ty (a.u.) Intensit y (a.u.) (011) (011) (110) (110) (220) (220) (101) (101) (222) (222) (200) (200) (422) (422) (220) (220) (310) (310) Appl. Sci. 2017, 7, 30 5 of 11 phosphors were located at 2 = 31.81 and 31.89 . These results suggest that the Sr ZnMoO and 2 6 Appl. Sci. 2017, 7, 30 5 of 11 Sr Eu ZnMoO phosphors revealed a perovskite structure with the tetragonal phase. As the 1.98 0.02 6 3+ concentration of Eu was more than 0.02, the Sr Eu ZnMoO phosphors would have transformed 2x x 6 was more than 0.02, the Sr2−xEuxZnMoO6 phosphors would have transformed from the tetragonal from the tetragonal phase to the (pseudo-)cubic phase, since the splitting of the (220) diffraction peak phase to the (pseudo-)cubic phase, since the splitting of the (220) diffraction peak was not observed. was not observed. Since the Sr Eu ZnMoO phosphors were tetragonal phase or (pseudo-)cubic 2x x 6 Since the Sr2−xEuxZnMoO6 phosphors were tetragonal phase or (pseudo-)cubic phase, even when the 3+ phase, even when the concentration of Eu ions was increased to 0.12, this result also proves that 3+ 3+ concentration of Eu ions was increased to 0.12, this result also proves that the Eu ions would have 3+ 2+ 3+ the Eu ions would have substituted into the sites of Ba ions. If the Eu ions had substituted into 2+ 3+ 2+ substituted into the sites of Ba ions. If the Eu ions had substituted into the sites of Zn (or Mo) ions, 2+ the sites of Zn (or Mo) ions, the Sr Eu ZnMoO phosphors would have revealed other crystalline 2x x 6 the Sr2−xEuxZnMoO6 phosphors would have revealed other crystalline phases, or more secondary phases, or more secondary phases would have been revealed in the Sr Eu ZnMoO phosphors 2x x 6 phases would have been revealed in the Sr2−xEuxZnMoO6 phosphors rather than in the double- rather than in the double-perovskite features. perovskite features. 1200 C x=0.12 x=0.10 x=0.08 x=0.06 x=0.04 x=0 29 31 33 35 2θ (Degree) Figure 2. XRD patterns of Sr2−xEuxZnMoO6 phosphors synthesized at 1200 °C for 4 h. Figure 2. XRD patterns of Sr Eu ZnMoO phosphors synthesized at 1200 C for 4 h. 2x 6 3+ 3+ Previously, Lin et al. successfully synthesized novel near-UV and blue-excited Eu , Tb -co- 3+ Previously, Lin et al. successfully synthesized novel near-UV and blue-excited Eu , doped one-dimensional strontium germanate full-color nanophosphors by a simple sol- 3+ Tb -co-doped one-dimensional strontium germanate full-color nanophosphors by a simple 3+ 3+ hydrothermal method. They found that incorporation of the Eu and Tb ions into strontium 3+ 3+ sol-hydrothermal method. They found that incorporation of the Eu and Tb ions into strontium 3+ germanate resulted in a slight shrinkage of the lattice constants and the unit cell volume because Eu 3+ germanate resulted in a slight shrinkage of the lattice constants and the unit cell volume because Eu 3+ 2+ 3+ 3+ and Tb had smaller radii than Sr , indicating the Eu and Tb ions had been incorporated into the 3+ 2+ 3+ 3+ and Tb had smaller radii than Sr , indicating the Eu and Tb ions had been incorporated into host lattice of SrGe4O9 and did not change the crystal structure [24]. Those results also suggested that the host lattice 3+ of SrGe O and did not change 2+ the crystal structure [24]. Those results also suggested 4 9 the Eu ions substituted into the sites of Sr ions. The 2θ value of the (220) diffraction peak was 3+ 2+ that the Eu ions substituted into the sites of Sr ions. 3+ The 2 value of the (220) diffraction peak shifted to a higher value, as the concentration of Eu was equal to, or greater than, 0.02. The ion 3+ 2+ 3+ 2+ was shifted to a higher value, as the concentration of Eu was equal to, or greater than, 0.02. The ion radius of Sr is 0.130 nm and the ion radius of Eu is 0.1087 nm. Hence, as more Sr would have been 2+ 3+ 2+ 3+ radius of Sr is 0.130 nm and the ion radius of Eu is 0.1087 nm. Hence, as more Sr would have substituted by Eu , the ionic radius of the Sr2−xEuxZnMoO6 phosphors would have decreased, 3+ 2+ been incre substituted asing the 2 byθ va Eulue o , the f th ionic e (22radius 0) diffract of the ion pe Sr ak. The Eu ZnMoO ion radiphosphors us of Eu is would 0.131 nm have , w decr hich eased, is 2x 6 2+ 2+ 2+ incr thought to be the sa easing the 2 value me as t of th he i e (220) on radius diffraction of Sr . If Eu peak. 2O3The exists ion as E radius u ions, the cel of Eu isl pa 0.131 rameters of the nm, which is 2+ 2+ Sr2−xEuxZnMoO6 phosphors would not have been changed. The shift of the (220) diffraction peak to a thought to be the same as the ion radius of Sr . If Eu O exists as Eu ions, the cell parameters 2 3 higher 2θ value or the decrease in the cell parameters can prove that Eu2O3 existed in the of the Sr Eu ZnMoO phosphors would not have been changed. The shift of the (220) diffraction 2x x 6 3+ Sr2−xEuxZnMoO6 phosphors and in the Eu state. peak to a higher 2 value or the decrease in the cell parameters can prove that Eu O existed in the 2 3 3+ In order to find the best doping concentra 3+ tion of Eu , we synthesized a series of Sr2−xEuxZnMoO6 Sr Eu ZnMoO phosphors and in the Eu state. 2x x 6 phosphors (x = 0 to 0.12) and measured their emission 3+ spectra. The PL emission spectra of In order to find the best doping concentration of Eu , we synthesized a series of Sr Eu ZnMoO 2x 6 Sr2−xEuxZnMoO6 powders excited at a wavelength of 350 nm are shown in Figure 3 for the light phosphors (x = 0 to 0.12) and measured their emission spectra. The PL emission spectra of wavelength range of 400–650 nm. The spectra in Figure 3 show that that all the excitation spectra of Sr Eu ZnMoO powders excited at a wavelength of 350 nm are shown in Figure 3 for the light 2x 6 Sr2−xEuxZnMoO6 phosphors (except the Sr2ZnMoO6 powder) consisted of two parts: one broad band wavelength range of 400–650 nm. The spectra in Figure 3 show that that all the excitation spectra in the 400–575 nm region and two sharp peaks located at 597 nm and 616 nm. However, for the of Sr Eu ZnMoO phosphors (except the Sr ZnMoO powder) consisted of two parts: one broad 2x 6 2 6 Sr2ZnMoO6 powder, even when the synthesizing temperature was 1200 °C, the two sharp peaks of band in the 400–575 nm region and two sharp peaks located at 597 nm and 616 nm. However, Sr2−xEuxZnMoO6 phosphors located at 597 nm and 616 nm were not found in the emission spectra. for the Sr ZnMoO powder, even when the synthesizing temperature was 1200 C, the two sharp 2 6 This means that no characteristic peaks were observed in Sr2ZnMoO6 powder even when the peaks of Sr Eu ZnMoO phosphors located at 597 nm and 616 nm were not found in the emission 2x x 6 synthesizing temperature was 1200 °C. Obviously, the broad band in the 400–575 nm region is spectra. This means that no characteristic peaks were observed in Sr ZnMoO powder even when 2 6+ 2 6 assignable to the well-known O –Mo charge transfer band (CTB) [25]. As Eu2O3 was substituted for the synthesizing temperature was 1200 C. Obviously, the broad band in the 400–575 nm region is SrO, the Sr2−xEuxZnMoO6 phosphors had the same spectral profile, but with a different concentration 2 6+ 3+ assignable to the well-known O –Mo charge transfer band (CTB) [25]. As Eu O was substituted for of Eu ions, the characteristic peaks were observed. 2 3 Intensity (a.u.) (220) Appl. Sci. 2017, 7, 30 6 of 11 SrO, the Sr Eu ZnMoO phosphors had the same spectral profile, but with a different concentration 2x x 6 3+ of Eu Appl.ions, Sci. 2017 the , 7, 30 characteristic peaks were observed. 6 of 11 1200 1200 x=0 x=0 (a) (b) x=0.04 x=0.04 x=0.06 x=0.06 5 7 D - F 900 900 0 1 x=0.08 x=0.08 x=0.10 x=0.10 x=0.12 x=0.12 5 7 D - F 5 7 600 0 1 600 D - F 0 2 5 7 D - F 0 2 300 300 400 450 500 550 600 650 400 450 500 550 600 650 Wavelength(nm) Wavelength(nm) 1200 1200 x=0 x=0 (d) (c) x=0.04 x=0.04 5 7 5 7 D - F x=0.06 x=.0.06 D - F 0 2 0 1 5 7 5 7 900 900 x=0.08 x=0.08 D - F D - F 0 2 0 1 x=0.10 x=0.10 x=0.12 x=0.12 600 600 0 0 400 450 500 550 600 650 400 450 500 550 600 650 Wavelength(nm) Wavelength(nm) Figure 3. Emission spectra of Sr2−xEuxZnMoO6 phosphors synthesized at (a) 900 °C; (b) 1000 °C; (c) Figure 3. Emission spectra of Sr Eu ZnMoO phosphors synthesized at (a) 900 C; (b) 1000 C; 2x x 6 1100 °C; and (d) 1200 °C, respectively. (c) 1100 C; and (d) 1200 C, respectively. When the synthesizing temperature was changed from 900 to 1200 °C, the emission spectra of When the synthesizing temperature was changed from 900 to 1200 C, the emission spectra of the Sr2−xEuxZnMoO6 phosphors consisted of sharp peaks in two strong bands at 597 and 616 nm, 5 7 5 7 3+ the Sr correspondi Eu ZnMoO ng to the phosphors D0– F1 (597 consisted nm) and of D0sharp – F2 (616 nm peaks) t in rans two itions of Eu strong bands ions. These r at 597 and esults 616 nm, 2x x 6 5 7 5 7 5 7 3+ 5 7 prove that the two emission peaks of the Sr2−xEuxZnMoO6 phosphors, D0– F1 at 597 nm and D0– F2 corresponding to the D – F (597 nm) and D – F (616 nm) transitions of Eu ions. These results 0 1 0 2 3+ 5 7 5 7 at 616 nm, were excited by the addition of Eu ions. When the synthesizing temperature was raised prove that the two emission peaks of the Sr Eu ZnMoO phosphors, D – F at 597 nm and D – F 2x x 6 0 1 0 2 to 1100 and 1200 °C, the intensity of the broa3+ dened emission went from 400 nm to 575 nm, increasing at 616 nm, were excited by the addition of Eu ions. When the synthesizing temperature was raised to 3+ with the increase in the Eu ion concentration. However, the emission intensities of the 1100 and 1200 C, the intensity of the broadened emission went from 400 nm to 575 nm, increasing with Sr2−xEuxZnMoO6 phosphors were influenced by the synthesizing temperature and the concentration 3+ the increase in the Eu ion concentration. However, the emission intensities of the Sr Eu ZnMoO 2x x 6 3+ of the Eu ions. 3+ phosphors were influenced by the synthesizing temperature and the concentration of the Eu ions. 3+ In a previous study, the spectra of BaZrO3 doped with Eu powders consisted of a series of 3+ In a previous study, the spectra of BaZrO doped with Eu powders consisted of a series of resolved emission peaks located at 576 nm, 597 nm, 616 nm, 623 nm, 651 nm, 673 nm, 696 nm, and 5 7 3+ 5 7 resolved emission peaks located at 576 nm, 597 nm, 616 nm, 623 nm, 651 nm, 673 nm, 696 nm, 704 nm, which are assignable to the D0– FJ (J = 0, 1, 2, 3, 4) transitions of Eu ions, namely, D0– F0 5 7 3+ 5 7 5 7 5 7 5 7 and (704 576 nm), nm, which D0– F1 (5 ar 97 nm) e assignable , D0– F2 ( to 616 the nm, D 623 – nm F ()J , = D00, – F 1, 3 (6 2,51 nm) 3, 4) ,transitions and D0– F4 (67 of Eu 3 nm, 69 ions, 6 nm namely , , 0 J 5 7 5 7 5 7 5 7 5 7 5 7 and 704 nm) [16]. Liu and Wang’s research also showed that the emission intensities of D0– F0 (576 D – F (576 nm), D – F (597 nm), D – F (616 nm, 623 nm), D – F (651 nm), and D – F 0 0 0 1 0 2 0 3 0 4 5 7 5 7 nm), D0– F1 (597 nm), and D0– F2 (616, 623 nm) had almost the same values, even if the Eu (673 nm, 696 nm, and 704 nm) [16]. Liu and Wang’s research also showed that the emission intensities concentration in the BaZr1−xEuxO3 phosphor powders was different [26]. These previous results [6,16], 5 7 5 7 5 7 of D – F (576 nm), D – F (597 nm), and D – F (616, 623 nm) had almost the same values, even if 0 0 0 1 0 2 3+ and the results this study, suggest that the transitions of Eu ions between different energy bands are the Eu concentration in the BaZr Eu O phosphor powders was different [26]. These previous 1x 3 affected by the host materials of the prepared phosphors. 3+ results [6,16], and the results this study, suggest that the transitions of Eu ions between different As the synthesizing temperature was changed from 900 °C to 1100 °C, the intensities of the two energy bands are affected by the host materials of the prepared phosphors. 3+ emission peaks of the Sr2−xEuxZnMoO6 phosphors was enhanced by increasing the Eu doping As the synthesizing temperature was changed from 900 C to 1100 C, the intensities of the two concentration and reached a maximum value at x = 0.08. In contrast, the intensities of the two 3+ 3+ emission peaks of the Sr Eu ZnMoO phosphors was enhanced by increasing the Eu doping emission peaks of the Sr2−xEuxxZnMoO6 phosphors decreased when the Eu doping ratio was more 2x 6 3+ concentration than 0.08. Thi andsr proves that the co eached a maximum ncentration-quenc value at x = 0.08. hing effect o In contrast, f Eu the dop intensities ing happened of the two in the emission 3+ Sr2−xEuxZnMoO6 phosphors. When 1200 °C was used as the sintering temperature, the intensities of peaks of the Sr Eu ZnMoO phosphors decreased when the Eu doping ratio was more than 0.08. 2x 6 3+ This proves that the concentration-quenching effect of Eu doping happened in the Sr Eu ZnMoO 2x x 6 Intensity Intensity Intensity Intensity Appl. Sci. 2017, 7, 30 7 of 11 3+ the two emission peaks of the Sr2−xEuxZnMoO6 phosphors increased as the concentration of Eu ions rose. The results in Figure 3 also show that when the temperature changed from 1100 °C to 1200 °C, the emission peak varied significantly. For the Sr2−xEuxZnMoO6 phosphors, when the synthesizing temperature was changed from 1100 °C to 1300 °C, the emission peak with the maximum intensity 5 7 5 7 shifted from D0– F1 (597 nm) to D0– F2 (616 nm). Nevertheless, the strong band at 576 nm 5 7 corresponding to the D0– F0 transition was not observed in the emission spectra. Appl. Sci. 2017, 7, 30 7 of 11 The maximum emission intensities (PLmax values) of the Sr2−xEuxZnMoO6 phosphors in the 5 7 5 7 transitions of the D0– F1 (597 nm) and D0– F2 (616 nm) peaks are presented in Figure 4 as a function 3+ of the synthesizing temperature and Eu ion concentration. Those results suggest again that the PL phosphors. When 1200 C was used as the sintering temperature, the intensities of the two emission 3+ characteristics of the Sr2−xEuxZnMoO6 phosphors were strongly affected by the synthesizing peaks of the Sr Eu ZnMoO phosphors increased as the concentration of Eu ions rose. The results 2x 6 3+ temperature and the concentration of Eu ions. As the x value of the Sr2−xEuxZnMoO6 phosphors was in Figure 3 also show that when the temperature changed from 1100 C to 1200 C, the emission peak 5 7 5 7 smaller than 0.10, the emission intensities of the D0– F1 (597 nm) and D0– F2 (616 nm) peaks first varied significantly. For the Sr Eu ZnMoO phosphors, when the synthesizing temperature was 2x 6 increased, reached a maximum, and then decreased as the synthesizing temperature increased. When 5 7 changed from 1100 C to 1300 C, the emission peak with the maximum intensity shifted from D – F 0 1 5 7 the x values of Sr2−xEuxZnMoO6 phosphors were 0.10 and 0.12, the emission intensity of the D0– F1 5 7 5 7 (597 nm) to D – F (616 nm). Nevertheless, the strong band at 576 nm corresponding to the D – F 0 2 0 0 (597 nm) first increased, then decreased at 1100 °C, and then increased at 1200 °C. The emission transition was not 5observed 7 in the emission spectra. intensity of the D0– F2 (616 nm) peaks increased with the increase in synthesizing temperature. These The maximum emission intensities (PL values) of the Sr Eu ZnMoO phosphors in the results suggest that 1100 °C is an important synthesizing temperature for Sr2−xEuxZnMoO6 phosphors max 2x x 6 5 7 5 7 because the transition of PL properties happens at this temperature, but the reasons for this are not transitions of the D – F (597 nm) and D – F (616 nm) peaks are presented in Figure 4 as a function 0 1 0 2 3+ known. of the synthesizing temperature and Eu ion concentration. Those results suggest again that the The results in Figures 3 and 4 present an important result regarding Sr2−xEuxZnMoO6 phosphors: PL characteristics of the Sr Eu ZnMoO phosphors were strongly affected by the synthesizing 2x x 6 5 7 the PLmax values of the transition of the 3+ D0– F1 (597 nm) critically increased as the synthesizing temperature and the concentration of Eu ions. As the x value of the Sr Eu ZnMoO phosphors 2x x 6 temperature increased from 900 °C to 1000 °C, then they did not apparently increase as the 5 7 5 7 was smaller than 0.10, the emission intensities of the D – F (597 nm) and D – F (616 nm) peaks 0 1 0 2 synthesizing temperature increased from 1000 °C to 1100 °C, as Figure 4a shows. However, for first increased, reached a maximum, and then decreased as the synthesizing temperature increased. Sr2−xEuxZnMoO6 phosphors with x = 0.04, 0.06, and 0.08, the PLmax values of the transition of the D0– When the x values of Sr Eu ZnMoO phosphors were 0.10 and 0.12, the emission intensity of the 7 2x x 6 F2 (616 nm) linearly increased as the synthesizing temperature increased from 900 °C to 1100 °C. For 5 7 D – F (597 nm) first increased, then decreased at 1100 C, and then increased at 1200 5C. The 7 emission 0 Sr21−xEuxZnMoO6 phosphors with x = 0.10 and 0.12, the PLmax values of the transition of the D0– F2 (616 5 7 intensity of the D – F (616 nm) peaks increased with the increase in synthesizing temperature. nm) linearly increa 0 sed 2 as the synthesizing temperature increased from 900 °C to 1200 °C, as Figure These 4b shows. results suggest When the synthesizin that 1100 C g temperature w is an important as 12 synthesizing 00 °C, the emi temperatur ssion intensi ety of for the transi Sr Eu ti ZnMoO on 2x x 6 5 7 of D0– F2 (616 nm) for Sr1.9Eu0.1ZnMoO6 and Sr1.88Eu0.12ZnMoO6 phosphors was higher than that of phosphors because the transition of PL properties happens at this temperature, but the reasons for this 5 7 D0– F1 (597 nm), as Figure 4 shows. are not known. (b) (a) 900 900 700 700 500 500 x=0.04 x=0.04 x=0.06 x=0.06 300 300 x=0.08 x=0.08 x=0.10 x=0.10 x=0.12 x=0.12 100 100 900 1000 1100 1200 900 1000 1100 1200 o o Calcining temperature ( C) Calcining temperature ( C) Figure 4. Emission intensity of Sr2−xEuxZnMoO6 phosphors as a function of synthesizing temperature. Figure 4. Emission intensity of Sr Eu ZnMoO phosphors as a function of synthesizing temperature. 2x x 6 (a) Emission peak of 597 nm; and (b) emission peak of 616 nm. (a) Emission peak of 597 nm; and (b) emission peak of 616 nm. 5 7 5 7 5 7 5 7 The transitions of D0– F1 (588 nm), D0– F2 (612 nm), D0– F3 (649 nm), and D0– F4 (695 nm) for The results in Figures 3 and 4 present an important result regarding Sr Eu ZnMoO phosphors: 2x x 6 3+ 3+ 3+ the Eu ions can be simultaneously observed in the emission spectrum of the Eu , Tb -co-doped 5 7 the PL values of the transition of the D – F (597 nm) critically increased as the synthesizing max 0 1 one-dimensional strontium germinate, full-color nano-phosphors [24]. However, introduction of the temperature increased from 900 C to 1000 C, then they did not apparently increase as the 3+ Eu ions in the Sr2−xEuxZnMoO6 lattice results in disorder. The disorder in the structure will cause 5 7 synthesizing temperature increased from 1000 C to 1100 C, as Figure 4a shows. However, transitions in D0– F2 and point defects in the lattice because of differences in the chemical valence 3+ 2+ 3+ for Sr and in Eu the ZnMoO ionic radiu phosphors s between Ewith u ionx s = and 0.04, Sr ion 0.06, s. Th and e Eu 0.08, ions coul the PL d occupy ei values ther of the si of the transition tes 2x x 6 max 5 7 of the D – F (616 nm) linearly increased as the synthesizing temperature increased from 900 C to 0 2 1100 C. For Sr Eu ZnMoO phosphors with x = 0.10 and 0.12, the PL values of the transition 2x x 6 max 5 7 of the D – F (616 nm) linearly increased as the synthesizing temperature increased from 900 C to 0 2 1200 C, as Figure 4b shows. When the synthesizing temperature was 1200 C, the emission intensity 5 7 of the transition of D – F (616 nm) for Sr Eu ZnMoO and Sr Eu ZnMoO phosphors was 0 2 1.9 0.1 6 1.88 0.12 6 5 7 higher than that of D – F (597 nm), as Figure 4 shows. 0 1 5 7 5 7 5 7 5 7 The transitions of D – F (588 nm), D – F (612 nm), D – F (649 nm), and D – F (695 nm) 0 1 0 2 0 3 0 4 3+ 3+ 3+ for the Eu ions can be simultaneously observed in the emission spectrum of the Eu , Tb -co-doped one-dimensional strontium germinate, full-color nano-phosphors [24]. However, introduction of the 3+ Eu ions in the Sr Eu ZnMoO lattice results in disorder. The disorder in the structure will cause 2x x 6 5 7 transitions in D – F and point defects in the lattice because of differences in the chemical valence 0 2 Intensity Intensity Appl. Sci. 2017, 7, 30 8 of 11 3+ 2+ 3+ and in the ionic radius between Eu ions and Sr ions. The Eu ions could occupy either of the sites 2+ 2+ 2+ of Sr or the sites of Zn (Mo ) in the Sr Eu ZnMoO phosphor ’s double-perovskite structure. 2x 6 3+ If Eu ions occupy a lattice site with a strict center of symmetry, the odd terms of the static crystal Appl. Sci. 2017, 7, 30 8 of 11 field vanish. This will lead to electric dipole transitions being strictly forbidden for purely electric 5 7 transitions and the transitions of the D – F (597 nm) will predominantly produce the emission. 0 1 2+ 2+ 2+ of Sr or the sites of Zn (Mo ) in the Sr2−xEuxZnMoO6 phosphor’s double-perovskite structure. If 3+ 2+ 2+ The Eu ions could occupy either the tetrahedral sites of Sr or the octahedral sites of Zn or 3+ Eu ions occupy a lattice site with a strict center of symmetry, the odd terms of the static crystal field 6+ Mo in the Sr Eu ZnMoO double-perovskite crystal structure. Consequently, as both of these sites 2x x 6 vanish. This will lead to electric dipole transitions being strictly forbidden for purely electric 5 7 5 7 are centrosymmetric, electric dipole transitions 5 7 ( D – F , only the D – F transition is observed transitions and the transitions of the D0– F1 (597 nm 0) will predom j =0,2 inantly pro 0 duce the emission. 2 3+ 2+ 2+ in this study) The Eu are ion forbidden s could ocand, cupy either the t as such, should etrahedr not al sit appear es of Srin or t the he octahedral sites o spectra. As the synthesizing f Zn or 5 7 6+ temperatur Mo in t e h is e 900 Sr2−xEu C, xZnMoO the fact 6 double that the -pero Dvskit – Fe cry electric stal struct dipole ure.transition Consequently at 615 , as both of these sites nm prevails over the 0 2 5 7 5 7 5 7 are centrosymmetric, electric dipole transitions ( D0– Fj=0,2, only the D0– F2 transition is observed in D – F magnetic dipole transition at 595 nm in the case of Sr Eu ZnMoO phosphors leads us to 0 1 2x x 6 3+ this study) are forbidden and, as such, should not appear in the spectra. As the synthesizing the conclusion that the Eu ions are generally located in a disordered manner in the Sr Eu ZnMoO 2x x 6 5 7 temperature is 900 °C, the fact that the D0– F2 electric dipole transition at 615 nm prevails over the 3+ powders [27]. The charge-compensating oxygen vacancies surrounding the Eu ions will lead to the 5 7 D0– F1 magnetic dipole transition at 595 nm in the case of Sr2−xEuxZnMoO6 phosphors leads us to the deviation from the point symmetry and relaxation of electric dipole transitions selection rules, with the 3+ conclusion that the Eu ions are generally located in a disordered manner in the Sr2−xEuxZnMoO6 5 7 3+ appearance of the D – F transition lines in the spectra [27]. Additionally, the Eu luminescent 0 2 3+ powders [27]. The charge-compensating oxygen vacancies surrounding the Eu ions will lead to the 5 7 centers contributing to the D – F transition line are probably located at the Sr Eu ZnMoO 0 2 2x x 6 deviation from the point symmetry and relaxation of electric dipole transitions selection rules, with 5 7 3+ particle surface or subsurface and will dominate the emission of Sr Eu ZnMoO phosphors when 2x x 6 the appearance of the D0– F2 transition lines in the spectra [27]. Additionally, the Eu luminescent 3+ 5 7 the synthesizing temperature is 1200 C [27]. If Eu ions occupy a non-centrosymmetric lattice site, centers contributing to the D0– F2 transition line are probably located at the Sr2−xEuxZnMoO6 particle surface or subsurface and will dominate the emission of Sr2−xEuxZnMoO6 phosphors when the both electric and magnetic transitions are possible, and the non-symmetric site occupancy will be a 3+ 5 7 synthesizing temperature is 1200 °C [27]. If Eu ions occupy a non-centrosymmetric lattice site, both dominant reason for causing the emission of D – F (612 nm) transition [28]. 0 2 electric and magnetic transitions are possible, and the non-symmetric site occupancy will be a The definition of fluorescence (or phosphorescence) lifetime is the time for the intensity of a single 5 7 dominant reason for causing the emission of D0– F2 (612 nm) transition [28]. emission peak to decrease to 1/e (approximately 37%) of its original intensity. The decay curves of The definition of fluorescence (or phosphorescence) lifetime is the time for the intensity of a 3+ Sr Eu ZnMoO phosphors were obtained at different concentrations of Eu (x = 0.04, 0.06, 0.08, 2x x 6 single emission peak to decrease to 1/e (approximately 37%) of its original intensity. The decay curves 0.010, and 0.12), and Figure 5 shows the intensity decay of the luminescence of Sr Eu ZnMoO 2x x 6 3+ of Sr2−xEuxZnMoO6 phosphors were obtained at different concentrations of Eu (x = 0.04, 0.06, 0.08, phosphors as a function of the synthesizing temperature, which can be used to calculate the lifetime. 0.010, and 0.12), and Figure 5 shows the intensity decay of the luminescence of Sr2−xEuxZnMoO6 The wavelength of the exciting light was 271 nm, and the measured wavelength for the intensity phosphors as a function of the synthesizing temperature, which can be used to calculate the lifetime. 5 7 decay The wa was 597 velenm ngth of ( D the exci – F ), ti because ng light wa it had s 271 the nm maximum , and the mea emission sured wa intensity velength f , as or the the synthesizing intensity 0 1 5 7 decay was 597 nm ( D0– F1), because it had the maximum emission intensity, as the synthesizing temperature was 900–1100 C. Typically, the standard form of the single exponential decay function is: temperature was 900–1100 °C. Typically, the standard form of the single exponential decay function is: A(t) = A exp[-(t/)] (1) A(t) = A0 exp[-(t/τ)] (1) where A is the initial population,  is constant for the decay time , and t is time. Figure 5 shows that where A0 is the initial population, τ is constant for the decay time , and t is time. Figure 5 shows that the all curves of decay time can be fitted well by a single-exponential function. Figure 5 also shows the all curves of decay time can be fitted well by a single-exponential function. Figure 5 also shows that if the same synthesizing temperature is used, the observed decay time is almost unchanged even that if the same synthesizing temperature is used, the observed decay time is almost unchanged even the x value in Sr Eu ZnMoO phosphor increases from 0.04 to 0.12. Moreover, the decay time is 2x x 6 the x value in Sr2−xEuxZnMoO6 phosphor increases from 0.04 to 0.12. Moreover, the decay time is a 3+ a single exponential function for Sr Eu ZnMoO phosphors with different concentrations of Eu x 3+ 2x 6 single exponential function for Sr2−xEuxZnMoO6 phosphors with different concentrations of Eu ions ions and synthesized at different temperatures, because the activator lies in the same coordination and synthesized at different temperatures, because the activator lies in the same coordination environment [29]. environment [29]. (b) (a) x=0.04 x=0.04 x=0.06 x=0.06 x=0.08 x=0.08 x=0.10 x=0.10 x=0.12 x=0.12 0.1 0.1 0.0 0.5 1.0 1.5 2.0 2.5 3.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Time (ms) Time (ms) Figure 5. Cont. Intensity Intensity Appl. Sci. 2017, 7, 30 9 of 11 Appl. Sci. 2017, 7, 30 9 of 11 (c) (d) 1 1 Appl. Sci. 2017, 7, 30 9 of 11 (c) (d) 1 1 x=0.04 x=0.04 x=0.06 x=0.06 x=0.08 x=0.08 x=0.10 x=0.10 x=0.12 x=0.12 0.1 0.1 0.0 0.5 1.0 1.5 2.0 2.5 3.0 0.00.5 1.01.5 2.02.5 3.0 x=0.04 x=0.04 x=0.06 x=0.06 Time (ms) Time (ms) x=0.08 x=0.08 x=0.10 x=0.10 Figure 5. Luminescence lifetime of Sr2−xEuxZnMoO6 phosphors synthesized at (a) 900 °C; (b) 1000 °C; x=0.12 x=0.12 Figure 5. Luminescence lifetime of Sr Eu ZnMoO phosphors synthesized at (a) 900 C; (b) 1000 C; 2x 6 0.1 0.1 (c) 1100 °C; and (d) 1200 °C, respectively. 0.0 0.5 1.0 1.5 2.0 2.5 3.0 0.00.5 1.01.5 2.02.5 3.0 (c) 1100 C; and (d) 1200 C, respectively. Time (ms) Time (ms) From the results shown in Figure 5, the lifetimes of Sr2−xEuxZnMoO6 phosphors, which are From the results shown in Figure 5, the lifetimes of Sr Eu ZnMoO phosphors, which are 2x x 6 Figure 5. Luminescence lifetime of Sr2−xEuxZnMoO6 phosphors synthesized at (a) 900 °C; (b) 1000 °C; compared in Table 1, were measured at 2.04–2.10 ms, 1.93–2.03 ms, 1.78–1.82 ms, and 1.65–1.67 ms, compared in Table 1, were measured at 2.04–2.10 ms, 1.93–2.03 ms, 1.78–1.82 ms, and 1.65–1.67 ms, (c) 1100 °C; and (d) 1200 °C, respectively. when the synthesizing temperature was 900 °C, 1000 °C, 1100 °C, and 1200 °C, respectively. The when the synthesizing temperature was 900 C, 1000 C, 1100 C, and 1200 C, respectively. lifetimes of Sr2−xEuxZnMoO6 phosphors apparently decreased with the increase in the synthesizing From the results shown in Figure 5, the lifetimes of Sr2−xEuxZnMoO6 phosphors, which are temperature. When the synthesizing temperature was 900 °C and 1000 °C, the lifetimes of the The lifetimes of Sr Eu ZnMoO phosphors apparently decreased with the increase in the 2x 6 compared in Table 1, were measured at 2.04–2.10 ms, 1.93–2.03 ms, 1.78–1.82 ms, and 1.65–1.67 ms, Sr2−xEuxZnMoO6 phosphors varied more. When the synthesizing temperature was 1100 °C and 1200 synthesizing temperature. When the synthesizing temperature was 900 C and 1000 C, the lifetimes when the synthesizing temperature was 900 °C, 1000 °C, 1100 °C, and 1200 °C, respectively. The °C, the Sr2−xEuxZnMoO6 phosphors had less variation in their lifetimes. In Figure 6 we use a fitting of the Sr Eu ZnMoO phosphors varied more. When the synthesizing temperature was 1100 C 2x 6 lifetimes of Sr2−xEuxZnMoO6 phosphors apparently decreased with the increase in the synthesizing curve to find the relationship between lifetime and synthesizing temperature; the equation is: and 1200 C, the Sr Eu ZnMoO phosphors had less variation in their lifetimes. In Figure 6 we use 2x x 6 temperature. When the synthesizing temperature was 900 °C and 1000 °C, the lifetimes of the t = −0.0016 × T + 3.543 (2) a fitting curve to find the relationship between lifetime and synthesizing temperature; the equation is: Sr2−xEuxZnMoO6 phosphors varied more. When the synthesizing temperature was 1100 °C and 1200 °C, the Sr2−xEuxZnMoO6 phosphors had less variation in their lifetimes. In Figure 6 we use a fitting where t represents the emitting lifetime of the luminescent material and T represents the synthesizing t = 0.0016  T + 3.543 (2) curve to find the relationship between lifetime and synthesizing temperature; the equation is: temperature (°C). In other words, the luminescence lifetime of Sr2−xEuxZnMoO6 phosphors can be obtained by using Equation (2), as the synthesizing temperature is changed from 900 °C to 1200 °C. t = −0.0016 × T + 3.543 (2) where t represents the emitting lifetime of the luminescent material and T represents the synthesizing These results suggest that the luminescence lifetime of Sr2−xEuxZnMoO6 phosphors can be a function temperatur where t e represents the emi ( C). In other wor tting l ds,ithe fetime of luminescence the luminescent ma lifetime3+ teri of al Sr and Eu T represen ZnMoO ts the synthesizing phosphors can be of synthesizing temperature, and that the concentration of Eu ions not on 2x ly can affect 6 their emitting temperature (°C). In other words, the luminescence lifetime of Sr2−xEuxZnMoO6 phosphors can be obtained by using Equation (2), as the synthesizing temperature is changed from 900 C to 1200 C. intensity but also can affect their luminescence lifetime. obtained by using Equation (2), as the synthesizing temperature is changed from 900 °C to 1200 °C. These results suggest that the luminescence lifetime of Sr Eu ZnMoO phosphors can be a function 2x 6 These results suggest that the luminescence lifetime of Sr2−xEuxZnMoO6 phosphors can be a function Table 1. Luminescence lifetimes of Sr2−xEuxZnMoO6 phosphors as a function of the sy 3+ nthesizing of synthesizing temperature, and that the concentration of Eu ions not only can affect their emitting 3+ 3+ of synthesizing temperature, and that the concentration of Eu ions not only can affect their emitting temperature and concentration of Eu ions. intensity but also can affect their luminescence lifetime. intensity but also can affect their luminescence lifetime. Synthesizing Temperature X =0.04 X =0.06 X =0.08 X =0.10 X = 0.12 Table 1. Luminescence lifetimes of Sr Eu ZnMoO phosphors as a function of the synthesizing 900 °C 2x 2. x05 2. 61 2.04 2.04 2.05 Table 1. Luminescence lifetimes of Sr2−xEuxZnMoO6 phosphors as a function of the synthesizing 3+ temperature and concentration of Eu ions. 1000 °C 2.03 2.03 1.93 1.97 1.99 3+ temperature and concentration of Eu ions. 1100 °C 1.81 1.83 1.78 1.82 1.82 Synthesizing Temperature X =0.04 X =0.06 X =0.08 X =0.10 X = 0.12 Synthesizing Temperature X = 0.04 X = 0.06 X = 0.08 X = 0.10 X = 0.12 1200 °C 1.65 1.65 1.66 1.66 1.67 900 °C 2.05 2.1 2.04 2.04 2.05 900 C 2.05 2.1 2.04 2.04 2.05 1000 °C 2.03 2.03 1.93 1.97 1.99 1000 C 2.03 2.03 1.93 1.97 1.99 x=0.04 2.4 x=0.06 1100 C 1.81 1.83 1.78 1.82 1.82 1100 °C 1.81 1.83 1.78 1.82 1.82 x=0.08 1200 C 1.65 1.65 1.66 1.66 1.67 x=0.10 1200 °C 1.65 1.65 1.66 1.66 1.67 2.1 x=0.12 fiting curve x=0.04 1. 2.8 4 x=0.06 x=0.08 x=0.10 2.1 1.5 x=0.12 fiting curve 1.8 1.2 900 1000 1100 1200 Calcining temperature ( C) 1.5 Figure 6. Fitting curve of lifetime for Sr2−xEuxZnMoO6 phosphors as a function of the synthesizing 3+ temperature and concentration of Eu ions. 1.2 900 1000 1100 1200 Calcining temperature ( C) Figure 6. Fitting curve of lifetime for Sr2−xEuxZnMoO6 phosphors as a function of the synthesizing Figure 6. Fitting curve of lifetime for Sr Eu ZnMoO phosphors as a function of the synthesizing 2x 6 3+ temperature and concentration of Eu ions. 3+ temperature and concentration of Eu ions. Intensity Intensity Time (ms) Time (ms) Intensity Intensity Appl. Sci. 2017, 7, 30 10 of 11 4. Conclusions For Sr Eu ZnMoO powders, the 2 value of the (220) diffraction peak shifted to a higher value 2x 6 and the FWHM value for the (220) diffraction peak decreased as the synthesizing temperature increased. 3+ When the x value (the concentration of Eu ions) increased from 0 to 0.12 and the synthesizing temperatures rose from 900 C to 1200 C, the cell parameters of the Sr Eu ZnMoO powders were 2x x 6 p p p changed from a = b = c/ 2 = 0.3968 nm to a = b = c/ 2 = 0.3971 nm and from a = b = c/ 2= 0.3971 nm to a = b = c/ 2 = 0.3974 nm (cubic crystal structure). When the synthesizing temperature was 900–1100 C, the emission intensities of the Sr Eu ZnMoO phosphors peaked at x = 0.08 and decreased as the 2x 6 +3 3+ concentration of Eu ions increased. There was an observable concentration-quenching effect of Eu ions in the Sr Eu ZnMoO phosphors. The emission spectrum of Sr ZnMoO powder consisted of 2x x 6 2 6 one broad band in the 400–575 nm region, and the emission spectra of the Sr Eu ZnMoO phosphors 2x x 6 (except for the Sr ZnMoO powder) consisted of two parts: one broad band in the 400–575 nm region 2 6 and two sharp peaks located at 597 nm and 616 nm. The lifetimes of the Sr Eu ZnMoO phosphors, 2x x 6 3+ which depended on synthesizing temperature and were independent of the concentration of Eu ions, were 2.04–2.10 ms, 1.93–2.03 ms, 1.78–1.82 ms, and 1.65–1.67 ms when the synthesizing temperature was 900 C, 1000 C, 1100 C, and 1200 C, respectively. We found that when the synthesizing temperature was in the range of 900–1200 C, the relationship between the lifetime and the synthesizing temperature could be determined by the following equation: t = 0.0016  T + 3.543, where t and T represent the emitting lifetime of the luminescent material and the synthesizing temperature ( C). 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Published: Dec 27, 2016

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