The influence of Eu³⁺ doping on the studies of luminescent properties and quantum efficiency of ZnWO₄ phosphor

Abstract

ZnWO₄:Eu³⁺ red nanophosphors have been synthesized by a simple environmentally friendly and low-cost molten salt method. The phase structure, morphology, SAED, photo-luminescent (PL), lifetime, color rendering, quantum efficiency and stability properties of the phosphor were investigated in detail. The XRD results indicated that all Eu³⁺ ions doping ZnWO₄ samples crystallize in a single crystal wolframite structure and exhibit superior crystallinity. Under the excitation of 394 and 465 nm, ZnWO₄:Eu³⁺ phosphors can be exhibited red light emission. The photoluminescence excitation spectra of ZnWO₄:Eu³⁺ phosphors show several emission peaks, on account of the f–f transitions of Eu³⁺ ions. The quantum efficiency of ZnWO₄:0.07Eu³⁺ phosphors were calculated to be 40.5%, the CIE chromaticity coordinate of ZnWO₄:0.07Eu³⁺ phosphors were (0.631, 0.364) closed to the red phosphors standard value (0.670, 0.330), based on the Judd–Ofelt analysis, Judd–Ofelt intensity parameters (Ω₂/Ω₄) includes symmetry of Eu³⁺ ions crystal field environment, it was suggested that ZnWO₄:0.07Eu³⁺ phosphors were a potential phosphor in LEDs. This research sheds new light on improving the quantum efficiency, stability properties and CIE chromaticity coordinate.

1 Introduction

With the rapid development of society, people pay great attention to the luminescent material that to strengthen the luminescent properties, a lot of literatures have reported [1] that rare earth ions doping have important applications in luminescent properties. In the tungstate materials [2], Zinc tungstate is special, it is different from other materials, which belongs to a kind of self-activating phosphor, ZnWO₄-based materials are divided into single crystal and nanoparticles and so on [3, 4]. Tungstate materials have attracted intensive attention originating from typical merits, such as high light yield, high average refractive index, physical, chemical, and structural properties, which has been widely used in lighting and display devices, including plasma display panels, white light-emitting diodes (WLEDs) [5]. Up to now, one of the main obstacles is improving the stability properties, which can be extremely difficult for its development and commercial in WLEDS.

There are several methods were prepared for the rare earth ions doping in improving the luminescent properties of the phosphor. The literature has reported that Dang Yuan et al. A novel electrochemical sensor for the selective determination of hydroquinone and catechol using synergic effect of electropolymerized nicotinic acid film and Cd-doped ZnWO₄ nanoneedle [6], Minzhu Zhao et al. Investigation of energy transfer mechanism and luminescent properties in Eu³⁺ and Sm³⁺ co-doped ZnWO₄ phosphors [7], Chen Guiqiang et al. Improving red emission by co-doping Li⁺ in ZnWO₄:Eu³⁺ phosphors [8], Chai Xiaona et al. Upconversion luminescent and temperature-sensing properties of Ho³⁺/Yb³⁺-codoping ZnWO₄ phosphors based on luminescent intensity ratio [9], Chunyang Li et al. Color changing from white to red emission for ZnWO₄:Eu³⁺ nanophosphors at a different temperature [10], therefore, in this work, the nanocrystals of ZnWO₄:Eu³⁺ phosphors have been prepared by the molten salt method.

As it can be observed in the literature [11], Eu³⁺ ions doping frequently play important roles in morphologies as well as in luminescent properties of phosphors. At the same time, it was used for an activator by the rare earth ions doping the phosphors. The purpose of this work are used to investigate the luminescent properties of phosphors, Judd–Ofelt [12] the theoretical analysis, quantum efficiency and stability properties. So far, to the best of our knowledge, there has not yet been any report similar to our work. For the first time, the goal of this work is discussed in detail for the luminescent properties of phosphors and Judd–Ofelt analysis. The contributions of this paper can be summarized as follows better understand luminescent properties, quantum efficiency and stability properties of the sample.

2 Experimental methods

2.1 Preparation of ZnWO₄ and ZnWO₄:Eu³⁺ specimen

All chemical reagents are analytical reagent (AR) and from Sinopharm Chemical Reagent Co., Ltd. China. All chemicals were utilized without further purification. In brief, all specimens operated at a fixed doping concentration of the Eu³⁺ ions was 7 mol%, Eu³⁺ ions doping ZnWO₄ nanophosphors were prepared via the molten salt method. Using analytically pure (AR) Na₂WO₄·2H₂O (99.5%), Eu₂O₃ (99.99%) and Zn(CH₃COO)₂·2H₂O (99%) as raw materials and LiNO₃ (99%)–NaNO₃ (99%) as molten salt media. The raw materials Na₂WO₄·2H₂O and Zn(CH₃COO)₂·2H₂O were weighed out according to the composition of ZnWO₄. The molten salt with the weigh ratio about LiNO₃:NaNO₃ to 1:3 and dissolved. Then, the raw materials were added to the molten salt solution. The mixture of raw materials and molten salt (molar ratio of 1:4), and then placed in a mortar to grind for 30 min, subsequently, the mixture was heated to the target temperature (180 °C) at a speed rate of 5 °C/min and kept for 8 h in the room temperature. After cooling, the solidified melt was washed with distilled water to remove the sodium salt and residual lithium salt. The silver nitrate solution was not tested until a white precipitate was produced, which demonstrated that there was no excess Cl⁻ in the solution. Ultimately, the obtained specimens were dried at 60 °C for analysis.

2.2 Material characterization

The structure of obtained powders were characterized by X-ray diffraction patterns (XRD, D/max, 2200 PC) at a 2θ range from 10° to 70° with 0.02°/step size with Cu-Kα radiation (λ = 1.5405 Å). Rietveld refinement of the sample was presented by the General Structure Analysis System software (GASA). The morphologies of samples were observed by field emission scanning electron microscopy (Q45, FEI, America). UV–Vis–NIR spectrophotometer (Cary5000, Agilent, America) was used to record the absorption spectra in the range of 200–800 nm. The Raman spectrum was tested by a microscopic confocal laser spectrometer (Renishaw-invia, Renishaw, Britain). The light source was the laser of an argon ion laser with a spectral resolution of 2 cm⁻¹. XPS (X-ray photoelectron spectroscopy, Axis Supra, Britain) was measured the composition of the elements and oxygen vacancy of the sample. The excitation spectra and emission spectra were detected by Photoluminescence spectrometer (F-4600, Hitachi, Japan) and the decay time was obtained by using the xenon lamp as an excitation source. The quantum efficiency of the phosphors were conducted with a Quantum-QY Plus UV–NIR absolute PL quantum yield spectrometer (C9920-03, Hamamatsu photonics K.K., Japan) with samples in powder form and the temperature-dependent PL spectra were evaluated on Hitachi FLS-980 Fluorescence Spectrophotometer. All the above measurements were performed out at room temperature.

3 Results and discussion

3.1 XRD characterization

The XRD spectra of ZnWO₄ phosphor and ZnWO₄:0.07Eu³⁺ crystal particles were obtained by the LTMS method in LiNO₃–NaNO₃ at 8 h. As shown in Fig. 1, it can be observed all the diffraction peaks can be perfectly indexed to monoclinic crystal structured ZnWO₄ (JCPDS card No. 15-0774), which demonstrates that the substitution of Eu³⁺ ions have a little modest effect on the crystal structured of ZnWO₄. Furthermore, under the same synthesis condition, all the observed diffraction peaks of the ZnWO₄ and ZnWO₄:0.07Eu³⁺ phosphors are strong and sharp, which implies good crystallinity and observes from the diffraction pattern, this paper found no other secondary and mixed phase in the figure. Therefore, all prepared samples are the monoclinic wolframite structure, as can be observed from the partial enlarge from 28º to 33º of the figure. Obviously, it can be noticed that the diffraction peaks of the diffraction pattern shift toward a lower angle deviation due to the Eu³⁺ ions doping which was described to expand the lattice. According to the similar ionic radius and stability of the crystal system, the ion radius of the Eu³⁺ (0.95 Ǻ) is close to the ionic radius of Zn²⁺ (0.74 Ǻ), however, the larger than the W⁶⁺ (0.60 Ǻ) ions radius. Therefore, the Eu³⁺ ions will preferentially replace the Zn²⁺ position in the ZnWO₄ matrix. Based on the Bragg equation:

$$ 2d\sin \theta = n\lambda $$

(1)

Fig. 1

XRD patterns of the ZnWO₄ and the ZnWO₄:0.07Eu³⁺

where d is the interplanar distance of corresponding crystal plane, θ is the diffraction angle, λ is the wavelength of the X-ray, where n is the number of reflection orders (generally speaking the value of the n is 1), it is interestingly found that with the reduction of the diffraction angle, interplanar distance of corresponding crystal plane becomes larger. According to the above analysis, this may be because the Eu³⁺ ions preferentially replace the Zn²⁺ position in the ZnWO₄ lattice, which lead to an increase in the spacing between the crystal faces. The analysis of these results suggests that the sharp and high diffraction peaks with the higher crystallinity of the phosphor. From the crystal structure diagram of the ZnWO₄ of the view, ZnO₆ and WO₆ octahedrons are linked to each other by shared oxygen, it can easily conclude that the introduction of Eu³⁺ entered into the ZnWO₄ which could bring defects and increase the oxygen vacancies of the surface [13].

3.2 TEM analysis and EDS spectrum analysis

Figure 2 exhibits the SEM images of the (a), HRTEM images of the (b), SAED (c) of the ZnWO₄:0.07Eu³⁺ phosphors. The sample has a homogeneous morphology with a diameter of the 7–20 nm which content with the pattern of the XRD. The ZnWO₄:0.07Eu³⁺ phosphors with rod-like morphology due to the higher aspect ratio, to clearly understand the crystallinity and purity of the sample by observed the HRTEM. As can be observed in Fig. 2b, the average gap is the lattice fringe of 0.48 and 0.545 nm corresponding to the lattice plane of the (100) and (010) respectively, which further confirmed the formation of the monoclinic structure of wolframite ZnWO₄. The SAED figures were shown in Fig. 2c. The picture is consists of a few laps, it is generally accepted that the samples were polycrystalline, it is composed of a lattice, therefore it was concluded that the specimen of a single crystal.

Fig. 2

a TEM image, b HRTEM image and c SAED pattern of ZnWO₄:0.07Eu³⁺ phosphors

3.3 XPS analysis

To obtain further information on the analysis chemical composition of elements and surface state of the ZnWO₄:0.07Eu³⁺ nanorods, the XPS measurements were displayed in Fig. 3. The survey spectrum as illustrated in Fig. 3a, indicating the presence of Zn 2p, W 4f, O 1s, Eu 3d in the as-obtained sample without other elemental signals being observed. With carbon C 1s peak Tag the binding energies (BE) [14]. The XPS peak of C 1s is due to accidental hydrocarbons from the XPS measurement. As can be observed in Fig. 3b, the BE values of C 1s is 284.8 eV, it suggests that the doped Eu³⁺ is trivalent. In the XPS spectrum of Zn 2p, two characteristic peaks located at 1021.88 and 1044.98 eV (Fig. 3c) can be assigned to Zn 2p_3/2 and Zn 2p_1/2, respectively. However, for the pure ZnWO₄, Zn 2p_3/2 and Zn 2p_1/2 are centered at 1021.72 and 1044.94 eV. The XPS spectrum of W 4f exhibits two peaks that occurred in 35.38 and 37.48 eV (Fig. 3d), originating from the W 4f_7/2 and W 4f_5/2 of W⁶⁺, respectively. Compared with pure ZnWO₄, it can easily conclude that there is a shift of 0.63 eV and 0.67 eV to the high binding energy. As displayed in Fig. 3e, crystal lattice oxygen was observed in the peak at 530.88 eV in the Eu³⁺ doping ZnWO₄ nanocrystal. In comparison with the pure ZnWO₄ phosphor, the BE value of O 1s is 530.71 eV which in accordance with the holding peak of the O²⁻ ion. For the XPS spectrum of Eu 3d, the main two peaks at the binding energies of 1126 and 1155 eV can be ascribed to the Eu 3d_5/2 and Eu 3d_3/2 of Eu³⁺ (Fig. 3f) [15]. The energy of the peaks is 17,145 eV which originated from the Eu 3d_3/2 of the ZnWO₄ which in good agreement with the previous work [16]. Based on the above-mentioned analysis results, it can be concluded that the valence C, Zn, W, O and Eu elements are 0, + 2, + 6, − 2, + 3, respectively.

Fig. 3

XPS spectra of ZnWO₄:0.07Eu³⁺ for the chemical states of a broden spectra, b C, c Zn d W, e O and f Eu

3.4 The ultraviolet–visible reflectance absorption spectra analysis and optical band gap energy analysis

The band structure was conducted by ultraviolet–visible reflectance absorption spectra to investigate the intrinsic electronic of ZnWO₄:0.07Eu³⁺ nano-rods (Fig. 4a). In comparison with the band structure of the pure ZnWO₄ with the same synthesis, the condition was depicted in Fig. 4a. As can be seen in the picture, Eu³⁺ ions doping the ZnWO₄ can strengthen the optical response of the ZnWO₄ phosphors, the band gap energy of the phosphors, which the equation is expressed as follows [17]:

$$ \alpha hv = B\left( {hv - E_{{\text{g}}} } \right)^{\frac{1}{t}} $$

(2)

Fig. 4

a Ultraviolet–Visible reflectance absorption spectra b (αhν)²–hν curves of the ZnWO₄ and ZnWO₄:0.07Eu³⁺

where α is absorption coefficient, B is the characteristic constant of relating to the material, where hν is the photon energy, h is Planck's constant (h = 4.14 × 10^–15 eVs). E_g is the optical band gap energy, t is the exponent associated with electron transitions, where t = 1/2 is for directly allowed transition, t = 2/3 means directly prohibit transitions, t = 2 represents indirect transitions are allowed, t = 3 stands for indirect forbidden transition, from the literature [18], the optical transition of ZnWO₄ is directly allowed, therefore where the value of t is 0.5, as presented in Fig. 4b. On the basis of the above Eq. (2), the optical band gap of pure ZnWO₄ and ZnWO₄:0.07Eu³⁺ phosphors were estimated to be 3.18 and 3.16 eV, respectively. As expected, the band gap values of ZnWO₄:0.07Eu³⁺ phosphors were lower than of the pure ZnWO₄ phosphor, indicating that the as-prepared ZnWO₄:0.07Eu³⁺ phosphors have a higher intrinsic electronic conductivity, and may be demonstrated that the superior electrochemical performances. In general, owing to rare earth ions doping, resulted in the impurity levels are formed, the result suggests that Eu³⁺ ions enter into the ZnWO₄ host. Confirming the ZnWO₄:0.07Eu³⁺ phosphors have a higher absorptive capacity than the ZnWO₄ host. In this figure, strong absorption band was observed at 350–490 nm, it was obviously observed that have one broad absorption band locating at 350–490 nm, which shows strong absorption ability located in 465 nm, at the same time, there is an obvious absorption peak, which point on the absorption spectrum corresponds to the 465 nm, deriving from the characteristic transition (⁷F₀ → ⁵D₂) of the Eu³⁺ [19].

3.5 Raman spectrum analysis

The structure of ZnWO₄ phosphor was further analyzed using the Raman spectrum, which is an effective way of studying the vibrational modes of lattices and molecules, there are a series of literatures [20] have reported that vibrational modes of ZnWO₄ contain 36 patterns. The Raman spectrum is classified into external vibration and internal vibration mode, Raman activity parameters is A_g and B_g, and the optical. This can be formulated as follows:

$$ \Gamma_{{({\text{Raman}})}} = 8A_{{\text{g}}} + 10B_{{\text{g}}} $$

(3)

where g is Raman activity parameters, the Raman spectra of the ZnWO₄ was connected with other reports [21], and the pure peak can be observed prominent absorptions in the spectral range of 100–1000 cm⁻¹ which correspond to six characteristic vibration patterns in Fig. 5. The bands at ~ 588 and 719 cm⁻¹ are linking to symmetrical stretching vibrations connecting O atom in Zn–O–W [22], the two bands nearby 424 and 468 cm⁻¹ are depicting into asymmetric deformation of Zn–O and W–O in ZnO₆ and WO₆ octahedral ligand [23]. The two bands located at ~ 875 and 826 cm⁻¹ were originating from the stretching pattern of WO₆ octahedron [24]. Raman spectrum was also completed to comprehend the influence of the doping. As displayed in Fig. 5, the relevant reports show the Raman shifts of the ZnWO₄ at 123, 141, 162, 190, 276, 320, 340, 405, 515, 542, 677, 709, 785 and 903 cm⁻¹, which explains this task. The three Raman bands located at ~ 123, 190 and 542 cm⁻¹ can be described into the symmetric stretching of the ZnO₆ octahedron clusters, at the same time, the rest of the bands are a feature of the vibration pattern of WO₆ octahedron group. Vibration patterns are divided into internal and external modes. The internal extension patterns of WO₆ octahedron group were expressly solved as six Raman bands for the A_1g symmetric extension (~ 903 cm⁻¹). The E_g (709 and 785 cm⁻¹) and T_2g (2A_g + B_g, ~ 405, 340 and 190 cm⁻¹) were corresponding to asymmetric stretching and bending deformation of W–O bonds. The Raman band located at ~ 162 cm⁻¹ was ascribed from the inter-chain torsion and distortion, however, that at ~ 276 cm⁻¹ might be named as distortion vibration (A_g) of cationic sub-lattice of WO₆ octahedron group [25]. In addition, bands in the 500–600 cm⁻¹ range are connected with W–O–W symmetric extension and the other located at ~ 141, 320 and 677 cm⁻¹ are corresponding to the external vibration pattern of WO₆ octahedron cluster movement against Zn²⁺ [26]. The pure rod-like ZnWO₄ shows the strongest Raman, due to larger aspect ratio, hence led to the stronger Raman peaks [27]. Furthermore, the flexural vibration of the WO₆ octahedron group (190 cm⁻¹), locating at ~ 190 cm⁻¹ originating from the symmetric extension vibration of ZnO₆ octahedron group, which verified that pure ZnWO₄ host with higher lattice symmetry, because no other defects were created ZnWO₄ the crystal structure. As shown in figure, the samples have to turn into the high band, when the Eu³⁺ occupy the Zn²⁺ of the ZnWO₄ host lattice, resulting in the distortion of the lattice. That could be caused by the small lattice and the offset of the Raman. In the internal vibration modes, Zn–O bonds are lower than the W–O bonds of the WO₆ octahedron group.

Fig. 5

Raman spectra of ZnWO₄ specimen

3.6 Photoluminescence (PL) analysis

The excitation spectrum of ZnWO₄:0.07Eu³⁺ phosphors were obtained by monitoring the emission wavelength at 616 nm, as displayed in Fig. 6a. The emission spectra can be observed the characteristic emission peaks of the Eu³⁺ [28], located in 594, 616, 654 and 705 nm, which originated from the transition of Eu³⁺, corresponding to the transition of ⁵D₀ → ⁷F₁, ⁵D₀ → ⁷F₂, ⁵D₀ → ⁷F₃, ⁵D₀ → ⁷F₄, respectively [29]. Electron transition ⁵D₀ → ⁷F₂ is predominant, when the transition intensity of the ⁵D₀ → ⁷F₂ higher than the transition intensity of the ⁵D₀ → ⁷F₁, suggesting the Eu³⁺ ions occupy the asymmetric site of the lattice and no inversion center. Under the 616 nm emission spectra, in the wavelength wide range of 340–500 nm, it can be found that the suitable amount of purity in the products may produce some defects to become the new luminescent center, which is favorable for the luminescence of ZnWO₄:0.07Eu³⁺ phosphors.

Fig. 6

a Excitation spectrum of ZnWO₄:0.07Eu³⁺ phosphors and b emission spectrum of ZnWO₄:0.07Eu³⁺ phosphors under diverse excitation

Figure 6b displays the emission spectra of ZnWO₄:0.07Eu³⁺ phosphors under the excitation of 394 and 465 nm. It is interestingly to find the strongest intensity that was observed under 465 nm excitation, as shown in figure, all samples consist of the characteristic peak of the Eu³⁺, the peaks center at 594 and 616 nm, originating from the ⁵D₀ state, and can be assigned to the magnetic dipole transition of Eu³⁺ (⁵D₀ → ⁷F₁) [30] and electric dipole transition of Eu³⁺ (⁵D₀ → ⁷F₂) [31], respectively. The weaker peaks at 654 nm, caused by the characteristic transition of the Eu³⁺ ions (⁵D₀ → ⁷F₃) [16], the peak located at 705 nm which was assigned as the characteristic transition of the Eu³⁺ (⁵D₀ → ⁷F₄).

3.7 Lifetime analysis

The single exponential fitting decay curve of ZnWO₄:0.07Eu³⁺ phosphors were illustrated in Fig. 7. All decay life curves could be well fitted into first order exponential decay model function, and the single exponential fitting decay can be calculated by following formula [32]:

$$ I(t) = I_{0} + A\exp \left( {{{ - t} \mathord{\left/ {\vphantom {{ - t} \tau }} \right. \kern-\nulldelimiterspace} \tau }} \right) $$

(4)

Fig. 7

The fluorescence decay of ZnWO₄:0.07Eu³⁺ phosphors

where I(t) is fluorescent intensity at time t, I(0) is the background intensity, A is the constant, τ is the lifetime, the single exponential decay model of the phosphor was mainly due to Eu³⁺-doped phosphors have same coordination environment into the host lattice and only exist in one deactivation process, the decay lifetime of the phosphor is 0.64 ms.

3.8 Electronic energy level scheme analysis

Figure 8 shows schematic diagram of energy level transitions of Eu³⁺ under excitation of 394 nm, excited by ultraviolet light 394 nm, Eu³⁺ ions were excited from ⁷F₁ to ⁵D₂ level and released to ⁵D₀ state were called the way of the non-radiative process. Eu³⁺ absorbed the photon energy from the ground state return to ⁵L₆, the energy was absorbed by the phosphors which emit non-radiative process will be dispersed to the crystal lattice. When ⁵D₀ has dwelled, Eu³⁺ ions display efficient visible emissions because of the multi-channel transitions. The sensitivity or the effect of ligand ion strongly on the emission intensity of the ⁵D₀ → ⁷F₂ transition is a target to study the local site symmetry of Eu³⁺ ions in the crystal lattice. Based on the above analysis, the symmetry site was occupied by the Eu³⁺ ions, therefore the electric dipole transition suggests that Eu³⁺ ions were located at an asymmetric site without an inversion center. There are peaks located at 594, 616, 655 and 702 nm that corresponding to the radiative transition of the ⁵D₀ → ⁷F₁, ⁵D₀ → ⁷F₂, ⁵D₀ → ⁷F₃ and ⁵D₀ → ⁷F₄ respectively. The corresponding photon energy can be obtained by calculating the photon energy equation [33]:

$$ E = h\nu = \frac{hc}{\lambda } $$

(5)

Fig. 8

Electronic energy level scheme of Eu³⁺ excited under 394 nm excitation

where E is the photon energy, h is the Planck's constant, ν is the frequency of light, c is the vacuum speed of light, the value of c is 3.0 × 10⁸ m/s, λ is excitation and emission wavelength. The energy of the non-radiative transition was released up to 1.01 eV.

3.9 The quantum efficiency (QE) and thermal stabilities of the phosphor analysis

The quantum efficiency is one of a key parameter used to be estimated the practical application, under the 394 nm excitation, the quantum efficiency ZnWO₄:0.07Eu³⁺ phosphors were measured with a Quantum-QY Plus UV–NIR absolute PL quantum yield spectrometer (C9920-03, Hamamatsu photonics K.K., Japan) with samples in powder form, and reference sample is BaWO₄ was illustrated in Fig. 9. The instrument is composed of photonic multi-channel analyzer PMA-12 (the detector) and PLQY measurement software-PMA as well as the photonic multi-channel analyzer (C10028) with the addition of supplementary units, which contains an excitation laser unit and a filter unit (A10094). The test measurements were measured at room temperature. The quantum efficiency of the phosphors can be obtained as will see in the following the formula [34]:

$$ \Phi_{{{\text{PL}}}} = \frac{{N\left( {{\text{Em}}} \right)}}{{N\left( {{\text{Abs}}} \right)}} = \frac{{\int {\frac{\lambda }{hc}\left[ {I_{{{\text{em}}}}^{{{\text{sample}}}} \left( \lambda \right) - I_{{{\text{em}}}}^{{{\text{reference}}}} \left( \lambda \right)} \right]} \,\,d\lambda }}{{\int {\frac{\lambda }{hc}\left[ {I_{{{\text{ex}}}}^{{{\text{reference}}}} \left( \lambda \right) - I_{{{\text{ex}}}}^{{{\text{sample}}}} \left( \lambda \right)} \right]} \,d\lambda }} $$

(6)

Fig. 9

Quantitative excitation and emission spectra of the ZnWO₄:0.07Eu³⁺ and BaWO₄ sample

N(Abs) respects the number of photons absorbed of the sample, where N(Em) is the number of photons emitted from a sample, λ is the wavelength, h is Planck’s constant and the value of h is 6.63 × 10^–34 J·s, c is the velocity of light and the value is equal to 3 × 10¹⁷ nm/s, $I_{{{\text{ex}}}}^{{{\text{sample}}}}$ represents the integrated intensities of the excitation light with sample, however, $I_{{{\text{ex}}}}^{{{\text{reference}}}}$ means integrated intensities of the excitation light without a sample, $I_{{{\text{em}}}}^{{{\text{sample}}}}$ and $I_{{{\text{em}}}}^{{{\text{reference}}}}$ are the photoluminescence intensities with and without a sample, respectively. Based on the above test, the Quantum efficiency value of ZnWO₄:0.07Eu³⁺ phosphors were calculated to be 40.5%. In recent years, for most of the phosphors, Quantum yield of phosphors were further improved in practical applications via adjusting the reaction conditions and the amount Eu³⁺ ions doping.

Figure 10 depicts thermal stabilities of the ZnWO₄:0.07Eu³⁺ phosphors, the thermal stability is an important parameter for phosphors in WLEDs. Generally speaking, the temperature of the phosphors have an influence on thermal stability, attaching to its great influence on the efficiency of the phosphors. Integrated intensity of Eu³⁺ ions as a function of increasing temperature were depicted in Fig. 10. The integrated intensity of ZnWO₄:0.07Eu³⁺ phosphors gradually declines, when the temperature changes from 180 to 300 °C, and dramatically decreases above 220 °C, it was mainly due to the thermal quenching, then it remains 90% of its initial intensity at 220 °C and reserves 85% at 240 °C. To further analysis the phosphor's thermal stability, the activation energy can be calculated by the Arrhenius equation as follows [35]:

$$ I_{T} = \frac{{I_{0} }}{{1{\text{ } + \text{ D}}\exp \left( { - \frac{{E_{{\text{a}}} }}{KT}} \right)}} $$

(7)

Fig. 10

Integrated intensity of Eu³⁺ ions as a function of increasing temperature

where I₀ and I_T represents the initial luminescence intensity and intensity at testing temperature and at different room temperature, respectively. D denotes a constant for a certain matrix, k is the Boltzmann constant and the value of k is 8.629 × 10^–5 eVK⁻¹, E_a is activation energy of the thermal quenching, based on the above Eq. (7), the activation energy E_a of ZnWO₄:0.07Eu³⁺ phosphors were calculated to be 0.578 eV. In comparison with CaGd₄F₁₄:Ce³⁺ phosphor and Y₂MoSiO₈:Eu³⁺ phosphor. According to the literature [36, 37], the activation energy (E_a) of CaGd₄F₁₄:Ce³⁺ phosphor and Y₂MoSiO₈:Eu³⁺ phosphor are 0.503 and 0.29 eV, respectively. ZnWO₄:0.07Eu³⁺ phosphors have higher thermal stability, suggesting that it has better potential for indoor application of WLED [38].

3.10 Color coordinates analysis

Under the control of annealing temperature, upon the excitation by near ultraviolet light CIE chromaticity coordinates of the Eu³⁺-doped ZnWO₄ phosphors were studied in Fig. 11. It is generally recognized that the color purity was calculated by the coordinate weighted average value relative to the light source point and the main control wavelength point, the formula is as follows [39]:

$$ {\text{Color}}\,\,{\text{purity}} = \sqrt {\frac{{\left( {x - x_{i} } \right)^{2} + \left( {y - y_{i} } \right)^{2} }}{{\left( {x_{d} - x_{i} } \right)^{2} + \left( {y_{d} - y_{i} } \right)^{2} }}} $$

(8)

Fig. 11

CIE chromaticity diagram for the ZnWO₄:0.07Eu³⁺ phosphors a ZnWO₄ and b ZnWO₄:0.07Eu³⁺

where (x, y) is color coordinate of phosphor light source, (x_i, y_i) is standard white light source color coordinates, (x_d, y_d) is the main emission coordinate of the emission spectrum, correlation color temperature is also an important parameter to measure the quality of light source, correlation color temperature was described as the following equation [40]:

$$ {\text{CCT}} = - \,449n^{3} + 3525n^{2} - 6823.3n + 5520.33 $$

(9)

$$ n = \frac{{x - x_{e} }}{{y - y_{e} }} $$

(10)

where the value of (x_e, y_e) is (0.3320, 0.1858), (x, y) represents color coordinates of phosphors. The parameters of the above were represented, the relevant result was summarized as follows in Table 1. In this work, it is well known that it has important applications in the white light field of solid-state lighting. CIE standard white light source is the (0.33, 0.33). In this paper, color coordinate of ZnWO₄:0.07Eu³⁺ phosphors were calculated is (0.631, 0.364) which is close to that of the NTSC standard CIE chromaticity coordinate value for (0.67, 0.33) and better than that of the commercial red Y₂O₂S:Eu³⁺, the color coordinates of ZnWO₄:0.07Eu³⁺ phosphors are closer to the standard red light region. Therefore, in comparing with doping or un-doping, it is interestingly found that the color purity of the phosphor was improver than un-doping the ZnWO₄ host. It was suggested that ZnWO₄:0.07Eu³⁺ phosphors were applied in red-emitting phosphor as the next candidate for WLED.

Table 1 Fluorescence parameters of ZnWO₄ and ZnWO₄:0.07Eu³⁺ phosphors under 394 nm excitation

3.11 Judd–Ofelt theory analysis

The theory can be used to explain the relationship between transition and relative intensity, which has become a powerful tool to study the luminescent properties of ions doping in solid-state lighting, it also provides information about the structural environment of metal ions and the chemical bonds of ions. According to the literature [41], the electric dipole transition intensity can be calculated the value of strength parameters Ω_λ (λ = 2, 4, 6), there are two transition modes for Eu³⁺-doped the phosphors, one pattern is magnetic dipole transition of the ⁵D₀ → ⁷F₁, another mode is electric dipole transition, the magnetic dipole transition probability was expressed as follows [12]:

$$ A_{{{\text{R}}\left( {{\text{mo}}} \right)}} = \frac{{64\pi^{4} \xi^{3} n^{3} S_{{{\text{md}}}} }}{{3h\left( {2J + 1} \right)}} $$

(11)

The S_md mean magnetic dipole line oscillator strength, the value of S_md is 7.83 × 10^–42 esu² cm², it is a constant and isn't change with the substrate, where n is index of refraction of the host, h is Planck constant, ξ is average number of transitions, ⁵D₀ → ⁷F_J corresponding to the electric dipole transition (J = 2, 4, 6), the radiative transition probability was calculated by following formula [42]:

$$ A_{{\left( {{\text{JO}}} \right)}} = \frac{{64\pi^{4} e^{2} \xi^{3} }}{{3h\left( {2J{ + }1} \right)}}\varepsilon \sum\limits_{\lambda = 2,4,6} {\Omega_{\lambda } \left\langle {\psi J\left\| {U^{\lambda } } \right\|\psi^{\prime}J^{\prime}} \right\rangle }^{2} $$

(12)

where e is the unit charge, ɛ is the refraction factor and the value of ɛ is ε = n(n² + 2)²/9, < ψJ||U^λ||ψ′J′ > ² represents reduce the square of the matrix elements. The data of reduced matrix element was listed in Table 2. On the basis of the uniqueness of Eu³⁺, strength parameters Ω_λ originated from ⁵D₀ → ⁷F_λ (λ = 2, 4, 6), therefore A_(JO) shown by the following:

$$ A_{{\left( {{\text{JO}}} \right)}} = \frac{{64\pi^{4} e^{2} \xi^{3} }}{{3{\text{h}}(2J + 1)}}\frac{{n\left( {n^{2} + 2} \right)}}{9}^{2} \Omega_{\lambda } \left\langle {\psi J\left\| {U\lambda } \right\|\psi^{\prime}J^{\prime}} \right\rangle^{2} $$

(13)

Table 2 The data of reducing matrix element Eu^{3+ 5}D₀ → ⁷F_{2, 4, 6} transitions

The ratio of the emission peak area corresponding to the transition of the ⁵D₀ → ⁷F_J (J = 2, 4, 6) to the emission peak area corresponding to the transition from ⁵D₀ → ⁷F₁, which is the intensity ratio of electric and magnetic dipole transitions as following equation [43]:

$$ \frac{{A_{{\left( {{\text{JO}}} \right)}} }}{{A_{{{\text{R}}({\text{mo}})}} }} = \frac{{\int {I_{{\left( {{\text{JO}}} \right)}} {\text{d}}\lambda } }}{{\int {I_{{\text{(mo)}}} {\text{d}}\lambda } }} $$

(14)

As mentioned in figure, the transition of the ⁵D₀ → ⁷F₆ was not observed in the emission spectrum, so the value of the ⁵D₀ → ⁷F₆ transition is equal to zero, radiation lifetime τ, lifetime contains radiative transition probability and non-radiative transition probability [44], the equation is shown as follows:

$$ \frac{1}{\tau } = A_{{\text{R}}} + A_{{\text{N}}} $$

(15)

where A_R is the radiative transition, A_R = ∑A_J (J = 0, 1, 2, 3, 4), the quantum efficiency was calculated as follows [45]:

$$ \eta = \frac{1}{{A_{{\text{R}}} + \frac{{A_{{\text{N}}} }}{{A_{{\text{R}}} }}}} $$

(16)

The ratio of η is radiative transition probability to the total radiative transition probability. The stimulated cross-sectional area (σ_e) of the sample was studied, it's a measure which used to judge the laser performance of materials, the formula is expressed as follows [46]:

$$ \sigma_{{\text{e}}} \left( {\lambda_{{\text{p}}} } \right) = \left( {\frac{{\lambda^{4}_{{\text{p}}} }}{{8\pi cn^{2} \Delta \lambda_{{{\text{eff}}}} }}} \right)A_{{\text{R}}} $$

(17)

λ_p means the wavelength of emission peak, C is for the speed of light, Δλ_eff is the effective bandwidth, the effective bandwidth is shown the equation [47]:

$$ \Delta \lambda_{{{\text{eff}}}} = \int {\frac{{I\left( \lambda \right)\,{\text{d}}\lambda }}{{I_{\max } }}} $$

(18)

Table 3 shows some J–O parameters of Eu³⁺ ions in the diverse host, in comparison with the context, it can also be observed that the quantum efficiency of our powders were higher than other matrices, which indicated the radiative transition probability of Eu³⁺ ions in ZnWO₄ phosphors were larger. Therefore ZnWO₄:Eu³⁺ phosphors may manifest the higher luminescent efficiency.

Table 3 Comparison of the intensity parameters (Ω₂, Ω₄), radiative transition probability (A_R), radiative lifetime (τ) and quantum efficiency (η) of Eu³⁺-doping diverse host

Based on the theoretical calculation, when the temperature is 180 °C, the intensity parameter Ω₂ value of ZnWO₄:Eu³⁺ phosphors are 3.509 × 10^–20 cm², it can be observed the intensity parameters of ZnWO₄:0.07Eu³⁺ crystal continues to increase. At the same time, the intensity parameter Ω₄ of ZnWO₄:Eu³⁺ phosphors are 1.080 × 10^–20 cm², respectively, the transition of ⁵D₀ → ⁷F₄ is very weak in the spectra. It is concluded that Ω₄ is insensitive to the symmetry of its surroundings. All parameters were calculated in Table 3. The higher quantum efficiency indicates that the phosphors have better luminescent properties.

All relevant parameters were collected in Table 4, it can be seen that the excitation cross-sectional area corresponding to the electric dipole transition from ⁵D₀ → ⁷F₂ of phosphors had a larger value. The larger stimulated cross-section area is more favorable for the laser with high gain, the luminescent performance of the phosphors are longer and better. Indicating that the transition of ⁵D₀ → ⁷F₂ provides a good laser effect, more widely have potential applications in the optical display devices.

Table 4 Effective bandwidth of the emission transition (∆λ_eff), stimulating emission cross-section (σ_e) and gain bandwidth for ZnWO₄:0.07Eu³⁺ phosphors

4 Conclusion

In this paper, we propose molten salt method-based hydrothermal reaction methods which makes principled use of Eu³⁺ doping. The Eu³⁺-doped ZnWO₄:0.07Eu³⁺ phosphors have been successfully fabricated by the molten salt method. Under the excitation of 394 and 465 nm, the phosphors can be effectively excited. By introducing the Eu³⁺ ions into the ZnWO₄ matrix, quantum efficiency can be obtained enhance. It is usually accepted that the intrinsic characteristics have influence on the fluorescent properties. Based on the above calculated results, the chromaticity coordinates of ZnWO₄:0.07Eu³⁺ phosphors are (0.631, 0.364), the chromaticity coordinates of standard red phosphor is (0.67, 0.33), which suggests that ZnWO₄:0.07Eu³⁺ phosphors locate in the red region, on the basis of calculating the J–O parameters (Ω₂ > Ω₄), denoting that Eu³⁺ ions occupy the lowest symmetric of the phosphor. To the best of our knowledge, we are the first to use the molten salt method for studying quantum efficiency and stabilities properties in ZnWO₄:0.07Eu³⁺ phosphors. Therefore ZnWO₄:0.07Eu³⁺ phosphors can become a potential phosphor in WLEDs.