Novel multi-wavelength effectively excited ZnWO4-WO3:Eu3+ multiphase red phosphor for white light-emitting diodes

Journal of Alloys and Compounds 807 (2019) 151668

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Novel multi-wavelength effectively excited ZnWO4-WO3:Eu3þ multiphase red phosphor for white light-emitting diodes Shaojie Bai a, Yun Liu b, *, Guoqiang Tan a, Huijun Ren c, Dinghan Liu b, Kai Wang a, Rong Wang b, Yi Zhu b, Sen Ye a a

School of Materials Science and Engineering, Shaanxi Key Laboratory of Green Preparation and Functionalization for Inorganic Materials, Xi'an, 710021, China College of Electrical and Information Engineering, Xi'an, 710021, China c School of Arts and Sciences, Shaanxi University of Science and Technology, Xi'an, 710021, China b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 4 June 2019 Received in revised form 30 July 2019 Accepted 31 July 2019 Available online 1 August 2019

The discovery of phosphor with efficient red emission is a crucial step in the development of the white light-emitting diodes (wLEDs). Herein, based on the regulation of the W/Zn ratio, the Eu3þ-activated ZnWO4-WO3∙0.5H2O-WO3 multiphase phosphor was designed successfully via the one-step hydrothermal treatment route. The effects of the W/Zn ratio on the phase structure, composition, morphology, luminescence property and lifetime of the obtained samples were systematically and comparatively investigated. The obtained product not only maintains the multi-wavelength excitation feature of ZnWO4:Eu3þ phosphor, but also sharply improves the characteristic emission intensity of Eu3þ ions. It is found that the WO3∙0.5H2O crystal has a special 3D interconnected channel structure, which is a relatively much access to the chemical behavior and permits the ready insertion of part of Eu3þ ions, and this coordination environment can obtain strong pure red light emission. Further, the ZnWO4-WO3:Eu3þ phosphor obtained after dehydration treatment can be excited more efficiently in the deep-ultraviolet (DUV), near-ultraviolet (NUV) and blue light regions, and its quantum efficiencies reach 18%, 22% and 21.2%, respectively. Besides, it also exhibits better CIE chromaticity coordinate, higher color purity than commercial Y2O2S:Eu3þ phosphor. Therefore, the ZnWO4-WO3:Eu3þ is promising for the application in red phosphor with different wavelengths for wLEDs. © 2019 Elsevier B.V. All rights reserved.

Keywords: Multiphase phosphor White light-emitting diodes WO3∙0.5H2O 3D interconnected channel structure

1. Introduction Nowadays, considerable interest is in the field of phosphorconversion white light-emitting diodes (pc-wLEDs) has been gained owing to their superior advantages, including long lifetime, short respond time, high efficiency, environmental friendliness, low energy consuming, and so on [1e5]. Generally, the ultimate outstanding performances of pc-wLEDs devices are inseparable from high-quality phosphors [6]. Currently, the commercial pcwLEDs are to combine a blue-emitting InGaN LED chip with yellow-emitting YAG:Ce3þ phosphor, but they have a low color rendering index (CRI<80) and a high correlated color temperature (CCT>4500K) due to the shortage of red light component in the spectra [7,8]. To date, one of the most valuable strategies is to

* Corresponding author. E-mail address: [email protected] (Y. Liu). https://doi.org/10.1016/j.jallcom.2019.151668 0925-8388/© 2019 Elsevier B.V. All rights reserved.

stimulate the red, blue and green phosphors through combining the deep-ultraviolet (DUV), near-ultraviolet (NUV) or blue light LED chip, which can produce more balanced white emission spectrum and much higher-rendering index [9,10]. Unfortunately, most commercial red phosphors are suffering from low efficiency, color purity and unstability, which greatly limits the development of wLEDs [11]. Besides, it should also be noted that if the phosphor can be efficiently excited by multi-wavelength bands, its application potential will inevitably be increased. As a result, a novel redemitting phosphor with satisfactory high brightness, efficient and strong absorption in the multi-wavelength region should be urgently developed. In general, the host lattice sites and the surrounding coordination environment of rare earth ions are regarded as important driving factors of luminescence behavior [12]. Because of this, it is necessary to explore an excellent matrix that can provide an appropriate coordination field environment for rare earth ions and is propitious to dominate the luminescent properties of the

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phosphor. Recently, a variety of potential matrices have been widely studied, such as vanadates, tungstates, molybdates, phosphates, nitrides, fluorides and silicates [13e19]. As for tungstates, especially the ZnWO4 with the wolframite-type structure has become one of the most promising candidates due to its stable chemical property, high average refractive, short decay time and relatively low phonon energy [20,21]. For Eu3þ-activated ZnWO4 host phosphor, it can produce the pure red light emission through narrow 5D0/7F2 characteristic transition of Eu3þ ions because Eu3þ ions are located at the non-center symmetrical lattices, as well as it can be well pumped by NUV or blue light LED chips [22]. Meanwhile, under DUV region excitation, the ZnWO4 host can also effectively transfer energy to Eu3þ ions so as to obtain red light emission due to its self-activating ability [23]. These characteristics have become the potential for it to be excited by multi-wavelength bands. But until now, there is still some restrictions on further improve the luminescence efficiency, which contributes to the serious obstacles for its development and commercial application. The luminescence efficiency is closely related to fluorescence emission intensity. In the past few years, the approaches to enhance fluorescence intensity are focused on sensitizer/activator ions codoping systems, which has been demonstrated as an efficient pathway to modify the excitation wavelength and enhance the luminescence intensity [24]. For instance, not only the ZnWO4:Eu3þ, Bi3þ and BaY(BO)Cl:Bi3þ, Eu3þ phosphors can be effectively excited by multi-wavelength bands, but also the sensitizer Bi3þ can transfer the absorbed energy to Eu3þ, resulting in the improved emission intensity [9,25]. Besides this, the controlling size, morphology, structure, doping concentration and defects of materials by adding organic solvents are also considered to be the dominant factors affecting luminescence properties [24,26,27]. However, these paths are insignificant for greatly enhancing the luminescence intensity, and some organic solvents may cause environmental pollution. It is known that multiphase materials are able to achieve a unique performance by combining the advantages of each single phase, which provides a new idea for the design of phosphors [28,29]. In this work, the ZnWO4-WO3:Eu3þ multiphase phosphor were actively synthesized by designing W/Zn stoichiometric ratios of 1:1, 3:1, 5:1, 7:1, 9:1 and 11:1. These samples were abbreviated as ZWx:Eu (x ¼ 1, 3, 5, 7, 9 and 11. for example, ZW1:Eu). In addition to maintaining the original feature excited by multi-wavelength bands of ZnWO4:Eu3þ phosphor, the design of material could also be expected to modify the host to obtain a phosphor with excellent performance under multi-wavelength excitation. Therefore, a novel red-emitting ZnWO4-WO3:Eu3þ multiphase phosphor with high color purity, efficient and strong absorption in the DUV, NUV and blue regions was successfully obtained by material design. Moreover, the effects of the W/Zn ratio on the phase structure, morphology, luminescence property and fluorescence lifetime of the products were studied in detail. And the thermal behavior of the obtained representative sample was also investigated.

Eu(NO3)3 solution. In the typical synthesis process of ZnW7:Eu sample, 2.85 mmol of Zn(NO3)2$6H2O and 0.15 mmol of Eu(NO3)3 were dissolved into 15 mL of deionized water. The 21 mmol of NaWO4$2H2O was dissolved in 20 mL of deionized water to obtain aqueous solution. Then the NaWO4 solution was slowly dropwise into mixed solution of Zn(NO3)2 and Eu(NO3)3 under magnetic stirring to get a white precipitate. Next, the white precipitate solution was continuously stirred for 40 min at room temperature and then the pH was adjusted to 6 by using nitric acid until solution was clear. Subsequently, the mixtures were added into a 60 mL Teflon-lined stainless-steel autoclave with a filling factor of 80%, which was heated to 180  C and maintained at the temperature for 12 h until reduced to room temperature naturally. Finally, the product was then collected by centrifugation and washed three times with deionized water and absolute ethanol respectively, and then dried at 80  C for 12 h. Similarly, for other samples of ZnWx:Eu (x ¼ 1, 3, 5, 9 and 11), the amount of NaWO4$2H2O only needs to be varied to 3 mmol, 9 mmol, 15 mmol, 27 mmol and 33 mmol, respectively. Finally, the ZnW7:Eu sample was only calcined in the muffle furnace at 500  C for 2 h. In addition, for the purpose of comparison, the sample I was prepared by using a unique raw material NaWO4$2H2O. The sample II was prepared by removing the raw material Zn(NO3)2$6H2O used in the preparation of ZW7:Eu sample, while the amount of other raw materials and the preparation procedure remained unchanged.

2. Experiment

3. Results and discussion

2.1. Synthesis

3.1. XRD, TG/DSC and crystal structures

The ZWx:Eu (x ¼ 1, 3, 5, 7, 9 and 11) samples with different stoichiometric ratios were synthesized through a mild hydrothermal synthesis route, where the doping concentration of Eu3þ was set as the optimum doping concentration of 5 mol% when Eu3þ doped the pure phase ZnWO4 host [30]. The Zn(NO3)2$6H2O (99%), NaWO4$2H2O (99.5%), and Eu2O3 (99%) were used as raw materials. The Eu2O3 was dissolved in dilute nitric acid solution, and then the excess acid was removed by heating and stirring to obtain 0.1 mol/L

The XRD patterns of ZWx:Eu (x ¼ 1, 3, 5, 7, 9 and 11) samples are displayed in Fig. 1. When the W/Zn ratio is 1:1, it is apparent that all the diffraction peaks can be well correlated with the monoclinic ZnWO4 phase (JCPDS File No. 15-0774), and no obvious secondary phase can be detected. However, the additional diffraction peaks of the cubic WO3∙0.5H2O phase (JCPDS File No. 84-1851) and monoclinic WO3 phase (JCPDS File No. 43-1035) are found when the W/ Zn ratios are varied to 3:1, 5:1 and 7:1 respectively, where the peaks

2.2. Characterization The phase structures were characterized through using powder X-ray diffractometer (XRD, D8 Advance, Bruker) with Cu Ka radiation in the range of 10 < 2q < 80 at a scanning rate of 5 /min. The sizes and morphologies were inspected by a Hitachi S-4800 scanning electron microscope (SEM) at an acceleration voltage of 5 kV. The transmission electron microscope (TEM) and high-resolution (HRTEM) were measured by an FEI Tecnai-G2-F20 microscope with an acceleration voltage of 200 kV. The photoluminescence (PL) excitation, emission spectra and lifetimes were recorded by an F4600 fluorescence spectrophotometer with a 150-W Xenon lamp made in Hitachi, Japan as an excitation source. The measurement of quantum efficiency was performed on Edinburgh FS5 spectrofluorometer equipped with an integrating sphere coated with barium sulfate. The surface analyses were taken on an X-ray photoelectron spectroscopy (XPS) using an AXIS SUPRA electron energy spectrometer with monochromatic Al Ka radiation as an X-ray excitation source. The thermogravimetric (TG) analysis and differential scanning calorimetry (DSC) curves were determined by a Germany STA449F3 simultaneous thermal analyzer in air with a heating rate of 5  C/min from room temperature to 800  C. The Fourier transform infrared spectroscopy (FT-IR) was carried out by a Bruker instrument with a Vertex70 model. All the measurements were performed at room temperature.

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Fig. 1. XRD patterns of the ZWx:Eu (x ¼ 1, 3, 5, 7, 9 and 11) and C-ZW7:Eu samples.

at 2q ¼ 14.9 , 28.7 and 30.0 are indexed to the (111), (311) and (222) crystal planes of WO3∙0.5H2O phase and the peaks at 2q ¼ 23.5 and 2q ¼ 34.2 are associated with the (020) and (202) crystal planes of WO3 phase, respectively. Moreover, it can also be observed that the intensity of diffraction peaks of the WO3∙0.5H2O and WO3 phases is increased gradually as the W/Zn ratio is increased, which suggests that the component content may change. To gain more insight to understand the effects of the W/Zn ratio on the component content, the relative component content of ZnWx:Eu (x ¼ 1, 3, 5, 7, 9 and 11) samples by pseudo-quantitative analysis of a specific phase are summarized in Table 1. It is evaluated from the measured XRD patterns by using the Eq. (1), [12]:

C¼

I ðsingle phaseÞ I ðtotal phaseÞ

(1)

of WO3∙0.5H2O phase become more and stronger, whereas the diffraction peaks of ZnWO4 phase are gradually becoming weaker and the WO3 phase disappears. Combined with Table 1, this is mainly because the generation of more WO3∙0.5H2O phase makes the diffraction peaks of WO3∙0.5H2O phase much stronger, so that the relative intensities of ZnWO4 and WO3 phases are too low to be clearly observed. In a nutshell, it can be determined that the obtained non-stoichiometric ratios of ZWx:Eu (x ¼ 3, 5, 7, 9 and 11) samples are a multiphase mixture of ZnWO4-WO3∙0.5H2O-WO3, and the matrix with different component content can be obtained by tailoring W/Zn ratios. Because the coordination environment around the luminescence center is considered to be an important factor affecting the luminescence behavior, exploring the doping positions of rare earth ions can greatly help to understand the change of luminescence properties. Thus far, several studies have reported that Eu3þ ions can replace Zn2þ sites in ZnWO4:Eu3þ phosphor because the hexacoordinated Eu3þ ions (1.087 Å) and Zn2þ ions (0.88 Å) have a closer ionic radius than that of the hexacoordinated W6þ ions (0.74 Å) [31,32], but the appearance of the mixed phase makes this all uncertain. Therefore, in order to investigate Eu3þ ions occupancy possibility in the ZnWO4-WO3∙0.5H2O-WO3 mixture, the phase structures of sample I-II are also studied (sample I-II are described in the previous preparation section). As shown in Fig. 2, all the diffraction peaks could be well classified as the cubic WO3∙0.5H2O phase, and the narrow half width and sharp peaks reveal that the products are well crystallized. The results indicate that the cubic WO3∙0.5H2O phase in the ZnWO4-WO3∙0.5H2O-WO3 mixture is formed because the unreacted sodium tungstate is crystallized after acid precipitation. According to previous literature reports, in the common full oxide pyrochlore of the general formula A2B2O6O0 , while the cubic WO3∙0.5H2O phase is a representative of the defect pyrochlore, its A positions are unoccupied and the water molecules are located at the O0 positions [33]. Some present studies have noted the particularity of the WO3∙0.5H2O crystal structure (in Fig. 3). It consists of the distorted octahedral [WO6] clusters connected by sharing oxygen atom to form the layered structures, and then the layered structures are linked along [111] direction, resulting in the formation of the three-dimensional (3D) interconnected tunnels structure. The water molecules sit at the windows between the large cavities, which forms a hydrogen bond with an oxygen ligand of the next layer [34e36]. This structure is relatively more open on chemical behavior and permits the ready insertion of cations [37]. Considering that the ionic radius of

where C corresponds to the relative component content; I (single phase) is the intensity of the diffraction peaks corresponding to only one phase in the mixed phase; I (total phase) is the summation of the intensity of all diffraction peaks in the mixed phase. From Table 1, as the W/Zn ratio is increased from 3:1 to 7:1, the WO3∙0.5H2O relative content is enhanced sharply, and the WO3 relative content also has a little increase. Furthermore, when the W/Zn ratio is above 7:1, it can be observed that the diffraction peaks

Table 1 Relative component content of ZWx:Eu (x ¼ 1, 3, 5, 7, 9 and 11) as a function the W/ Zn ratio. W/Zn ratio

Monoclinic ZnWO4

Cubic WO3$0.5H2O

Monoclinic WO3

1:1 3:1 5:1 7:1 9:1 11:1

100% ~94% ~82% ~79% ~3% ~2%

e ~5% ~15% ~18% ~97% ~98%

e ~1% ~3% ~3% e e

3

Fig. 2. XRD patterns of the sample I-II and sample II calcined at 500  C for 2 h.

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Fig. 3. Crystal structure of ZnWO4, WO3∙0.5H2O and WO3.

hexacoordinated Eu3þ ions is somewhat lower than that of the size of the tunnel pore (1.26 Å) and water molecule (1.41 Å) [38], but larger than the ionic radius of hexacoordinated W6þ ions, it implies that Eu3þ ions are more likely to be located at the water molecule positions even if there is a problem of charge imbalance. According to the Bragg equation 2dsinq ¼ nl, when the smaller Eu3þ ions substitute the larger water molecules, it will cause the d value of the interplanar spacing to decrease. To keep the formula constant, the diffraction peaks will turn to the higher-angle direction. Indeed, it can be found from the magnified picture in Fig. 2 that the diffraction peaks of the Eu3þ ions doped WO3∙0.5H2O sample are significantly shifted to a large angle compared with un-doped WO3∙0.5H2O sample, which indicates that Eu3þ ions can be successfully doped into the WO3∙0.5H2O crystal. For the monoclinic WO3 phase, since its crystal structure is closely arranged into a layered structure by the octahedral [WO6] clusters and then formed by sharing oxygen atoms (in Fig. 3) [39], it is very difficult to provide the replaceable or interstitial sites for the Eu3þ ions. Meanwhile, the monoclinic WO3 phase component content is extremely low, so it can be ignored. Therefore, it can be judged that the Zn2þ sites of the ZnWO4 phase and channel locations of WO3∙0.5H2O phase can be the potential locations of the Eu3þ ions replacement in the ZnWO4WO3∙0.5H2O-WO3 mixture. In order to facilitate to understand the impacts of crystal water of the ZnWO4-WO3∙0.5H2O-WO3 mixture on its luminescence properties, the thermal behaviors of the representative ZW7:Eu sample are investigated by TG-DSC to gain the optimum calcination temperature for dehydration treatment. As shown in Fig. 4, it can be seen that the TG curve can be separated into two parts. The first step ranges from room temperature to 450  C, which contains the evaporation of surface-absorbed water and the release of crystalline water [39]. The process of weight loss in the second step occurs between 550  C and 600  C. Correspondingly, as depicted in the DSC curve, there is an exothermic peak about 600  C, which may be due to the decomposition of the original phase. Since the dehydration treatment of the ZW7:Eu sample is carried out at 500  C

Fig. 4. TG-DSC curves of the ZW7:Eu sample.

with 2 h for the subsequent investigation (abbreviated as CZW7:Eu). As a comparison, the dehydration treatment of WO3∙0.5H2O:Eu sample is also performed (abbreviated as CWO3∙0.5H2O:Eu). Combined with Fig. 1, the C-ZW7:Eu sample is similar to that of the ZW7:Eu sample, and there are no significant differences to be found. Fig. 2 shows that the diffraction peaks shift significantly to a larger angle, suggesting that water molecules can be removed under this condition. Therefore, the dehydration treatment will make the ZW7:Eu sample become a mixture of ZnWO4-WO3, which has the significant effects on its luminescence performance eventually. 3.2. SEM, TEM and HRTEM analyses The effects of the W/Zn ratio on the morphologies and sizes of the products are studied by SEM, TEM and HRTEM, respectively. From the SEM micrographs (seen in Fig. 5a), the ZW1:Eu sample

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Fig. 5. (aef) SEM images of ZWx:Eu (x ¼ 1, 3, 5, 7, 9 and 11) samples; (geh) SEM images of C-ZW7:Eu sample.

consists of agglomerated nanoparticles, but no other morphologies are observed. From the TEM image (in Fig. 6a), it can be observed more clearly that the ZW1:Eu sample is composed of uniform nanorods with an average length of 40 nm and the diameter of 10 nm. From the HRTEM image (in Fig. 6b), the lattice fringes of 0.471 nm and 0.246 nm can correspond well to the (110) and (002) planes of ZnWO4, respectively. However, as the W/Zn ratio is set to 3:1, 5:1 and 7:1, respectively, the products are composed of the large-scale nanorods and a spot of two-dimensional (2D) nanosheets with an average thickness of 15e20 nm, where the 2D nanosheets are intercalated in the nanorods to form a rose-like structure (seen in Fig. 5b, c and d, respectively). To confirm the

composition of the 2D nanosheets, the TEM and HRTEM images of the ZW7:Eu sample are displayed. As shown in Fig. 6c, the ZW7:Eu sample contains two structures of the nanorods and 2D nanosheets, which is consistent with the results observed in the SEM image. The HRTEM image (in Fig. 6d) of 2D nanosheets area shows the clear lattice fringes, which reveals the high crystallinity. The distances between adjacent fringes are measured to be 0.297 nm and 0.590 nm, which are matched with the (222) and (111) planes of the cubic WO3$0.5H2O phase, and a spacing of 0.264 nm between two fringes corresponds to the (202) plane of the monoclinic WO3 phase. Considering the similar morphology of crystalline tungsten oxide mixtures, WO3-WO3$0.5H2O synthesized by microwaveassisted hydrothermal method has been reported [40]. Therefore, an implication of this is the possibility that the generated 2D nanosheets is a two-phase mixture of WO3$0.5H2O-WO3. When the W/Zn ratio is further adjusted to 9:1 and 11:1, the SEM images show that the shapes of samples are irregular block formed by agglomerated nanoparticles (in Fig. 5e and f), which testifies the importance of the content of WO2 4 in the formation of WO3-WO3$0.5H2O nanosheets. The morphology of C-ZW7:Eu sample (in Fig. 5g) is composed of a large number of block structures, where the block structures are composed of nanoparticles embedded in a frame. More interestingly, many isolated octahedral pyramids can also be observed (in Fig. 5h), which is mainly due to the shrinkage of the structure caused by the release of crystal water in the WO3$0.5H2O phase [39]. 3.3. FTIR, XPS, PLE and PL analyses

Fig. 6. (a) and (c) TEM images of the ZW1:Eu and ZW7:Eu samples; (b) and (d) HRTEM images of the ZW1:Eu and ZW7:Eu samples.

Fig. 7 presents the FTIR spectra of the ZnWx:Eu (x ¼ 1, 3, 5, 7, 9 and 11), C-ZW7:Eu and WO3$0.5H2O samples. It can be observed that the ZWx:Eu (x ¼ 1, 3, 5, 7, 9 and 11) and WO3$0.5H2O samples have two clear absorption bands in the range of 3200e3600 cm1 and 1500e1700 cm1, which can be ascribed to the stretching vibration of OH groups and the bending vibration of H-O-H groups, respectively [41]. These absorption bands are mainly derived from the crystalline water of the WO3$0.5H2O phase and the adsorbed water of samples. Moreover, it can be noticed that the absorption band disappears in the C-ZW7:Eu sample, confirming that the water can be completely removed at elevated temperature calcination.

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Fig. 7. FTIR spectra of the ZWx:Eu (x ¼ 1, 3, 5, 7, 9 and 11), C-ZW7:Eu and WO3$0.5H2O samples.

Furthermore, for the ZW1:Eu sample, the sharp shoulder absorption peak at 865 cm1 corresponds to the stretching vibration of WO, and the peaks at 670 cm1 and 600 cm1 are caused by the stretching and bending modes of the Zn-O-W groups. The characteristic signals of the symmetric and asymmetric deformation modes of W-O bonds and Zn-O bonds in [WO6] and [ZnO6] groups are located at 465 cm1 and 420 cm1, respectively [20,42]. Subsequently, these sharp shoulder absorption peaks are also found in the ZWx:Eu (x ¼ 3, 5, 7, 9 and 11) and C-ZW7:Eu samples, which indicates that these samples contain the monoclinic ZnWO4 phase. In particular, the absorption peaks are noticed at 963 cm1 in the WO3$0.5H2O and ZWx:Eu (x ¼ 3, 5, 7, 9 and 11) samples, which is associated with W-O stretching modes of [WO6] groups in the cubic WO3$0.5H2O phase. At the same time, the weak vibration peaks at 795 cm1 can also be detected in the ZW5:Eu and ZW7:Eu samples, which could be attributed to the O-W-O stretching vibrations of [WO6] groups in the monoclinic WO3 phase [43]. However, the OW-O stretching modes of the monoclinic WO3 phase are not observed in the ZWx:Eu (x ¼ 3, 9 and 11) samples because of the low relative phase contents of WO3. These results further support the fact that the obtained non-stoichiometric ratios of the ZWx:Eu (x ¼ 3, 5, 7, 9 and 11) samples are a multiphase mixture of ZnWO4WO3∙0.5H2O-WO3. To further ascertain the surface elemental composition, the oxidation state and doping position of Eu3þ ions, both representative ZW1:Eu and ZW7:Eu samples are analyzed by XPS and shown in Fig. 8. The carbon C1s peak (284.6 eV) is used as a reference to calibrate binding energies. Fig. 8a exhibits the wide range spectra of both samples, which testifies the presence of Zn, W, O, C and Eu elements in both samples [31]. Fig. 8bee reveal the high-resolution spectra of Zn2p, W4f, O1s and Eu3d of both samples. In Fig. 8b, the two main peaks corresponding to Zn2p1/2 and Zn2p3/2 are detected at the binding energies of 1044.67 eV and 1021.52 eV in the ZW1:Eu sample, while the two peaks are situated at the binding energies of 1044.80 eV and 1021.67 eV in the ZW7:Eu sample [44]. These are ascribed to Zn2þ ions in ZnWO4. The W4f spectrum of the ZW1:Eu sample consists of two spineorbit doublets at the binding energies of 37.69 eV and 35.55 eV, which is attributed to W4f5/2 and W4f7/2,

respectively [45]. However, for the ZW7:Eu sample, two peaks of W4f5/2 and W4f7/2 are located at the binding energies of 37.57 eV and 35.43 eV, respectively. These results indicate that both samples contain only W6þ oxidation state. The O1s oxidation state peak is shown in Fig. 8d. For the ZW1:Eu sample, the asymmetric O1s peak is deconvoluted into three peaks at 530.38, 531.50 and 532.81 eV, matching the oxygen coordination in Zn-O, W-O and surface chemisorbed oxygen, respectively [44]. For the ZW7:Eu sample, it can be split to three peaks at the binding energies of 530.25, 530.60 and 533.01 eV, respectively. Obviously, the above results show that there is just very weak shift of the binding energies of Zn2p, W4f and O1s in both samples, which can be assigned to the chemical environment changes when the additional WO3 and WO3$0.5H2O phases are generated [46,47]. It is worthwhile to note that the Eu3d3/2 and Eu3d5/2 peaks of the ZW1:Eu sample are found at the binding energies of 1134.49 eV and 1163.39 eV, while the two peaks of the ZW7:Eu sample are obviously shifted to the lower binding energies located at 1133.62 eV and 1162.52 eV, such a behavior reveals that the Eu3þ ions coordination environment has obviously been changed (Fig. 8e). Consequently, the findings of this investigation suggest that the whole Eu3þ ions may be doped into WO3∙0.5H2O host lattice or simultaneously located at ZnWO4 and WO3∙0.5H2O host lattice in the ZnWO4-WO3∙0.5H2O-WO3 mixture. Fig. 9 shows the excitation spectra monitoring 616 nm of representative the ZW1:Eu, ZW7:Eu, WO3∙0.5H2O:Eu and CZW7:Eu samples. For the ZW1:Eu sample, the excitation spectrum consists of a series of sharp linear peaks and a broadband peak. The linear peaks are assigned to the characteristic intra-4f transitions of Eu3þ ions, which is 7F0/5D4 (361 nm), 7F0/5G2 (376 nm), 7 F0/5G3 (382 nm), 7F0/5L6 (394 nm), 7F0/5D3 (416 nm), 7 F0/5D2 (465 nm) and 7F0/5D1(537 nm), respectively [48]. The broadband peak in the range of 200e350 nm is derived from the charge transfer (CT), which corresponds to the overlap of both the O2/W6þ and O2/Eu3þ transitions in the ZnWO4 host. For all of them, the stronger transition is the charge transfer band (CTB). However, for the ZW7:Eu sample, the intensity of characteristic peaks of Eu3þ ions has risen drastically, but the CTB has declined. Another important finding is that the WO3∙0.5H2O:Eu sample has not the obvious CT. That is to say that the CT occurs only when Eu3þ ions are doped into the ZnWO4 host lattice. Combined with XPS analysis results, it can be determined that Eu3þ ions are simultaneously located in the ZnWO4 and WO3∙0.5H2O host in the mixture of ZnWO4-WO3∙0.5H2O-WO3, so that less Eu3þ ions enter the ZnWO4 host, resulting in the CTB decreased. Moreover, one unanticipated finding is that the 7F0/5L6 (394 nm) and 7F0/5D2 (465 nm) characteristic transitions in the C-ZW7:Eu sample not only exhibit almost the same excitation intensity as the ZW7:Eu sample, but also its CTB shows the significant increase due to the increase of crystallinity of ZnWO4 crystal after calcination has a significant effect on the luminescence properties. It means that the C-ZW7:Eu sample can be matched well with the DUV, NUV and blue light LED chips. Fig. 10aec show the emission spectra of the ZWx:Eu (x ¼ 1, 3, 5, 7, 9 and 11), C-ZW7:Eu and WO3∙0.5H2O:Eu samples under the excitation of 295 nm, 394 nm and 465 nm respectively. There are a series of characteristic emission peaks of Eu3þ ions in all the samples clearly observed in the range of 550e750 nm, which corresponds to 5D0/7F1(595 nm), 5D0/7F2(616 nm), 5 D0/7F3(655 nm), 5D0/7F4(705 nm) typical transitions, respectively [48]. Especially, the characteristic emission peaks at 616 nm and 595 nm are ascribed to the electric dipole transition (ED) and the magnetic dipole transition (MD), respectively. It is generally accepted that the hypersensitive ED transition 5D0/7F2 usually dominates in the emission spectrum when Eu3þ ions are localized

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7

Fig. 8. XPS survey spectra (a) full, (b) Zn2p, (c) W4f, (d) O1s, (e) Eu3d of the ZW1:Eu and ZW7:Eu samples.

at the sites with non-inversion symmetry, whereas the insensitive MD transition 5D0/7F1 is predominant when Eu3þ ions occupy the inversion symmetry sites [49]. It can be seen the 5D0/7F2 typical transition is the strongest in all of the emission peaks, which certifies that Eu3þ ions are situated at the sites with non-inversion symmetry in the ZnWO4-WO3∙0.5H2O-WO3 mixture host. Moreover, it can also be seen that, at the 295 nm excitation, the samples exhibit a broad band emission ranging from 350 nm to 550 nm, which is ascribed to the electron transition between the O 2p orbitals and the empty d orbitals of the central W6þ ions in WO2 4 groups. This indicates an effective energy transfer from WO2 4 groups to Eu3þ ions [31]. Fig. 10d provides the emission intensity contrast diagram at

617 nm of the ZWx:Eu (x ¼ 1, 3, 5, 7, 9 and 11), C-ZW7:Eu and WO3∙0.5H2O:Eu samples under the excitation of 295 nm, 394 nm and 465 nm respectively. Obviously, with successive enhance of the W/Zn ratio, the emission intensity of the obtained samples is increased sharply under 394 nm and 465 nm excitation, reaches a maximum intensity at the W/Zn ratio of 7:1, and then is decreased gradually as the W/Zn ratio is beyond 7:1. Consistent with our original intention, the emission intensity of the ZW7:Eu sample is about 5 and 8 times stronger than that of the ZnWO4:Eu3þ sample, respectively. What is surprising is that the emission intensity of the C-ZW7:Eu sample is not only still excellent under 394 nm excitation, but the intensity is slightly improved under 465 nm excitation. More importantly, it also has strong luminescence intensity under

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will get more powerful red light emission when Eu3þ ions are located at the lattice coordination environment of WO3∙0.5H2O crystal. This result also indirectly explains that the reason why the performance of the ZW7:Eu sample is greatly enhanced while part of Eu3þ ions are doped into the WO3∙0.5H2O host lattice. However, as far as we know, the rare earth ions doped pyrochlore WO3∙0.5H2O luminescent materials have not been reported. Undoubtedly, this finding is a critical issue for the future research of luminescent materials, which provides a new alternative matrix. 3.4. Decay curves and CIE coordinates

Fig. 9. Excitation spectra monitoring 616 nm of the ZW1:Eu, ZW7:Eu, WO3∙0.5H2O:Eu and C-ZW7:Eu samples.

295 nm excitation. In other words, the C-ZW7:Eu sample is capable of achieving the best luminescence intensity under multiwavelength excitation including DUV, NUV and blue light regions. This study can be well used to develop the target aimed at a highly efficient phosphor excited by commercial chips of different wavelengths. In addition, an equally noteworthy fact is that the emission intensity of the WO3∙0.5H2O:Eu sample is higher than that of the ZW7:Eu sample under 394 nm excitation, which further indicates that Eu3þ ions can be introduced into the WO3∙0.5H2O host and it

The decay behaviors of Eu3þ ions in the ZWx:Eu (x ¼ 1, 3, 5, 7, 9 and 11) and WO3∙0.5H2O:Eu samples are also performed with 394 nm excitation by monitoring the emission at 616 nm and the decay curves are expressed in Fig. 11. The decay curves of all the samples can be well fitted by a single exponential function (2) [49]:

It ¼ I0 expð  t=tÞ þ A

(2)

where It and I0 correspond to the PL intensities at time t and 0, t is the lifetime of an excited state energy level and A is a constant. Here, the average lifetimes of the ZWx:Eu (x ¼ 1, 3, 5, 7, 9 and 11) and WO3∙0.5H2O:Eu samples are calculated to be 0.819 ms, 0.631 ms, 0.612 ms, 0.588 ms, 0.589 ms, 0.633 ms and 0.551 ms, respectively. Clearly, as the W/Zn ratio is increased from 1:1 to 7:1, the decay lifetimes exhibit a downward tendency and get closer to the lifetime of the WO3∙0.5H2O:Eu sample. The results further indicate that when the W/Zn ratio is increased gradually, an increasing number of WO3∙0.5H2O crystals are generated, which

Fig. 10. (aec) Emission spectra of the ZWx:Eu (x ¼ 1, 3, 5, 7, 9 and 11), C-ZW7:Eu and WO3∙0.5H2O:Eu samples under the excitation of 295 nm, 394 nm and 465 nm respectively; (d) Intensity contrast diagram at 617 nm.

S. Bai et al. / Journal of Alloys and Compounds 807 (2019) 151668

9

Fig. 11. Decay curves of the ZWx:Eu (x ¼ 1, 3, 5, 7, 9 and 11) and WO3∙0.5H2O:Eu samples with 394 nm excitation by monitoring at 616 nm.

will cause more Eu3þ ions to enter its ontology. Consequently, following the successive increases from 7:1 to 11:1, the lifetimes are increased slightly, which may be due to the formation of excessive WO3∙0.5H2O crystals corresponding to the diluting of Eu3þ ions doping concentration. The quantum efficiency (QE) is often adopted as an important parameter in evaluating its potential application for solid state lighting. The QE can be calculated by the following Eq. (3), [50]:

ð

h¼ð

LS ð ER  ES

(3)

where LS is the emission spectra of the sample, ES and ER represent the excitation light with and without the sample in the intergrating sphere. Table 2 compares the QEs of the C-ZW7:Eu sample and commercial Y2O2S:Eu3þ phosphor. The QEs of C-ZW7:Eu sample reach 22% and 21.2% under the excitation of 394 nm and 465 nm respectively, which are much higher than Y2O2S:Eu3þ phosphor. However, the QE of C-ZW7:Eu sample is lower than that of Y2O2S:Eu3þ phosphor under the excitation of deep UV region. Considering the optimization of doping concentration and

Table 2 The quantum efficiencies of the C-ZW7:Eu sample and commercial Y2O2S:Eu3þ phosphor. samples Y2O2S:Eu3þ

C-ZW7:Eu

Excitation Wavelength(nm)

QE(%)

Reference

lex ¼ 317 nm lex ¼ 394 nm lex ¼ 465 nm lex ¼ 295 nm lex ¼ 394 nm lex ¼ 465 nm

35% 9.6% 11.6% 18% 22% 21.2%

[51] [51] [50] Present work Present work Present work

synthetic procedure, the quantum efficiency has the potential to be further optimized. Fig. 12 shows the emission spectra of the C-ZW7:Eu sample and Y2O2S:Eu3þ phosphor at different excitation wavelengths. It can be observed that the red emission intensity of C-ZW7:Eu sample are about 1.7 times stronger than that of Y2O2S:Eu3þ phosphor. Furthermore, the CIE (Commission International de L0 Eclairage) chromaticity coordinates and the color purity of the ZWx:Eu (x ¼ 1, 3, 5, 7, 9 and 11) and C-ZW7:Eu samples are listed in Table 3. The chromaticity diagrams are presented in Fig. 13. As Table 3 shows, the CIE coordinates of C-ZW7:Eu sample are determined to be (0.624, 0.323), (0.666, 0.333) and (0.666, 0.333) under the

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excitation of 295 nm, 394 nm and 465 nm, respectively, which is very close to the standard red light (0.67, 0.33) and much better than the commercial red-emitting phosphor of Y2O2S:Eu3þ (0.622, 0.351) (in Fig. 13a). Moreover, it can be seen that the luminescence color of the ZWx:Eu (x ¼ 1, 3, 5, 7, 9 and 11) samples can be adjusted from orange to white region through changing the W/Zn ratio (in Fig. 13b). And the color coordinate of ZW11:Eu sample is close to the standard white chromaticity (0.33, 0.33), which further embodies research potential of these mixed-phase phosphors. Besides, the color purity is also an important reference for measuring the quality of the light source. To better characterize the quality of the obtained red light, the color purity of the obtained samples can be calculated by using the Eq. (4), [52]:

Fig. 12. Emission spectra of the C-ZW7:Eu and Y2O2S:Eu3þ sample at different excitation wavelengths.

qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðx  xi Þ2 þ ðy  yi Þ2 Color purity ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi  100% ðxd  xi Þ2 þ ðyd  yi Þ2

(4)

where (x, y), (xi, yi) and (xd, yd) refer to the CIE coordinates of the samples, the equal energy white illuminant point and the CIE

Table 3 The CIE chromaticity coordinates and the color purity of the ZnW7:Eu and C-ZW7:Eu samples. Samples

CIE(x,y) Color purity (l ¼ 295 nm)

CIE(x,y) Color purity (l ¼ 394 nm)

CIE(x,y) Color purity (l ¼ 465 nm)

ZW1:Eu

(0.494, 0.320)

ZW3:Eu

(0.444, 0.319)

ZW5:Eu

(0.429, 0.321)

ZW7:Eu

(0.406, 0.321)

ZW9:Eu

(0.345, 0.316)

ZW11:Eu

(0.324, 0.312)

C-ZW7:Eu

(0.624, 0.323) ~82.7%

(0.652, ~90.6% (0.659, ~92.5% (0.662, ~93.4% (0.666, ~94.5% (0.653, ~90.9% (0.651, ~90.3% (0.666, ~94.5%

(0.629, ~84.7% (0.654, ~91.1% (0.659, ~92.6% (0.664, ~93.9% (0.645, ~88.7% (0.644, ~88.5% (0.666, ~94.5%

0.347) 0.341) 0.338) 0.333) 0.346) 0.349) 0.333)

0.371) 0.345) 0.314) 0.335) 0.355) 0.356) 0.333)

Fig. 13. (a) The CIE chromaticity diagram of the C-ZW7:Eu and Y2O2S:Eu3þ sample at different excitation wavelengths; (b) The CIE chromaticity diagram of the ZWx:Eu (x ¼ 1, 3, 5, 7, 9 and 11) samples under the excitation of 295 nm.

S. Bai et al. / Journal of Alloys and Compounds 807 (2019) 151668

coordinates of the dominant emission wavelength, respectively. Here, (xi, yi) and (xd, yd) equal to (0.333, 0.333) and (0.685, 0.315), respectively. Hence, under the excitation of 295 nm, 394 nm and 465 nm respectively, the color purity of the C-ZW7:Eu sample is determined to be approximately 82.7%, 94.5% and 94.5%. The good CIE coordinate and the high color purity indicate that the C-ZW7:Eu sample is an ultimate candidate of red-emitting phosphor in pcWLEDs. 3.5. Conclusion In summary, a series of ZnWO4-WO3∙0.5H2O-WO3 multiphase phosphors with different component contents are fabricated via designing the W/Zn stoichmetr. When the W/Zn ratio is set to 7:1, the emission intensity of product is about 5 and 8 times as much as that of the pure phase ZnWO4:Eu3þ phosphor under 394 nm and 465 nm excitation respectively. The fluorescence enhancement effect is also verified because the WO3∙0.5H2O crystal has a 3D interconnected channel structure in which the water molecular positions can be replaced by part of Eu3þ ions, and then this coordination surrounding is more favorable for Eu3þ ions red light emission. It is more notable that, under the excitation of 295 nm, 394 nm and 465 nm respectively, the ZnWO4-WO3:Eu3þ phosphor obtained after dehydration treatment exhibits meritorious CIE coordinates (0.624, 0.323; 0.666, 0.333; 0.666, 0.333), high color purity (82.7%; 94.5%; 94.5%) and quantum efficiencies (18%; 22%; 21.2%) As a consequence, the ZnWO4-WO3:Eu3þ phosphor is expected to become an excellent and comprehensive candidate of red phosphor for wLEDs.

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Acknowledgements

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This research was financially supported through the Project of the National Natural Science Foundation of China (Grants No. 51272148 and No. 51772180).

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