1 Introduction

The light emitting diodes (LEDs) have enticed great consideration as an illuminator for solid state lighting since 1993. Correlated with the conventional fluorescent lamps, solid state lighting technology has the benefits as low energy consumption, environmental stability and potent output and energy efficient [1–3]. The commercially gettable tricolor phosphors for White LEDs are Y2O2S:Eu3+ for red [4], BaMgAl10O17:Eu2+ for blue [5] and ZnS (Cu+, Al3+) for green [6]. While comparing with blue and green phosphors, the thermal and chemical stability of the sulphide based red phosphors are found to be inadequate. They exhibit poor absorbance in near UV region and their efficiency is also approximately eight times lesser than blue and green phosphors for white LED applications [7]. In order to get around this shortcoming, a well- efficient red-emitting phosphor to be synthesised is a vital part. Therefore, at the moment, significant attention is sharpened on finding an alternative novel low cost red phosphor which is thermally and chemically more stable and also exhibits more excellent luminous efficiency. Recently, a special class of Re3+ doped NaLa(WO4)2 tungstates are attractive and have been investigated intensely. The host (NLW) exhibits high chemical stability, very low phonon frequencies, optical transparency over a wide wavelength range and site-selective doping capability.Thus, It exhibits a wide range of applications in vivo imaging of tissues and cells, solid state light emitting applications, scintillators and thermally stimulated luminescence (TSL) dosimetry [8]. Among different rare earth elements trivalent europium (Eu3+) ion is widely recognized as an activator for red emission (around 612 nm), which has been used in most commercial red phosphors. The intra-4f-shell down-conversion transitions (5D0–7Fj (j = 1, 2, 3, 4) of Eu3+ ions are strongly dependent on crystal structure of the host and sensitive to the local environment where the rare-earth has been situated [9]. Among all transition of Eu3+, the transition due to 5D0–7F2 is the most intense and hypersensitive transition and emits red color. The transition probabilities are strongly influenced by host lattice, specially the covalent nature of host and site symmetry of the occupation [10]. Also, most of the synthesis reports on these materials remain in the stage of solid phase synthesis or high temperature liquid techniques. These methods usually need high temperatures, time consuming heating processes and frequent grinding. The grinding process damages the phosphor surfaces, resulting in the loss of emission intensity. In addition, the aggregation and inhomogeneous shapes are also unavoidable, which inhibit the absorption of the excitation energy and therefore reduce the emission intensity [11, 12].

To the best of our knowledge, the reports focused on the synthesis and optical properties of rare-earth-doped NaLa(WO4)2 are still limited. Based on these investigations, in this paper, NaLa(WO4)2:Eu3+ (x = 0.01, 0.05, 0.07, 0.09, 0.11) phosphors were at low temperature in ethylene glycol as reaction medium. The structural and optical properties of the materials were studied in detail.

2 Experimental

2.1 Synthesis

The phosphors of NaLa(WO4)2:Eu3+ (x = 0.01, 0.05, 0.07, 0.09, 0.11) were synthesized at low temperature (140 °C reflexed for 2h) in ethylene glycol (EG) medium. Lanthanum(III) nitrate hexahydrate (La(NO3)3·6H2O, AR) Europium(III) oxide (Eu2O3, 99.99%, Aldrich), Sodium tungstate dehydrate (Na2WO4·2H2O, AR) were used as sources of La3+, Eu3+ and WO4 2− respectively. In a typical synthesis procedure, stoichiometric ratio of La(NO3)3·6H2O and Eu2O3 were dissolved together in Conc. HNO3 and excess acid was removed by evaporation with water. To this solution, stoichiometric ratio of Na2WO4·2H2O was added followed by 50 ml of EG. The PH of the solution was balanced to have 9–9.5 using NaOH solution. The resulting solution was refluxed for 2 h at 140 °C. The resulting white precipitate was collected by centrifugation at 10,000 r min−1 for 15 min. The phosphor material is washed many times with acetone and finally dried under an ambient atmosphere. The dried sample was used for characterization.

2.2 Characterization

X-ray powder diffraction (XRD) analysis was out using Pan Analytical X’pert pro X-ray diffractometer with Cu K-alpha radiation (λ = 1.5406 Å) at a scanning rate of 0.020 per second. The XRD patterns were obtained in the range of 0° ≤ 2θ ≤ 70° and were compared with the JCPDS data. Fourier Transform infrared spectroscopy (FTIR) measurements were carried out in the wavelength range of 400–4000 cm−1 with a Nicolet 6700 FTIR equipped with a deuterated triglycine sulfate detector. Raman spectra were recorded on a RFS/100/S Bruker FT-Raman spectrometer with a Nd doped yttrium aluminium garnet laser providing excitation light at 1064 nm in the frequency range of 150–1000 cm−1. The size, shape and structure of the samples were characterized by emission scanning electron microscopy (FE-SEM, Hitachi S-4800). Transmission electron microscopy (TEM) was recorded on a JEOL JEM-2100 at an accelerating voltage of 200 kV. The measurements of PL and excitation (PLE) spectra were performed by a Jobin Yuvon Flurolog-3-11 Spectroflurometer at room temperature with 450 W xenon lamp was used as the excitation source (200–700 nm). The excitation and emission slit width were set to 2 nm. All spectroscopic measurements of the phosphors were carried out at room temperature.

3 Results and discussion

3.1 Structural and morphology characterization

Figure 1 shows the XRD patterns of Eu3+ doped NaLa(WO4)2 phosphors with different doping concentration of Eu3+. The diffraction patterns of NaLa(WO4)2:xEu3+ fit well with the tetragonal scheelite crystal structure of NaLa(WO4)2 (JCPDS No. 79-1118). The strong and sharp diffraction peaks indicate a good crystallinity of the prepared samples. In addition, the diffraction intensities of all of the XRD patterns are sharp, revealing that the highly crystalline products can be synthesized by this method. The diffraction peaks of NaLa(WO4)2:Eu3+ are shifted to higher angles when compared to that of standard NaLa(WO4)2 due to small ionic size of Eu3+ (106.6 pm) than La3+ (116.0 pm) and Na+ (116.0 pm) in the host lattice. Lattice parameters were calculated for all NLW phosphors as shown in Table 1. The gradual decrease of the cell parameters due to the La substitution by Eu ions can be observed in Fig. 2. This result is justified since ionic radii of Eu3+ is smaller than of La3+ and Na+ for octahedral coordination [13] as in this case. The calculated values are comparable with those observed in bulk crystals [8]. Figure 3a shows the typical FE-SEM image of the as-prepared NaLa(WO4)2:0.09 Eu3+ phosphor. It can be found that the size and morphology of the product is not relatively uniform and regular. Meanwhile, some particles are agglomerated into bulk, which leads to the broad size range of particles. Figure 3b shows the TEM image of the as-prepared NaLa(WO4)2:0.09 Eu3+ phosphor. The products composed of a number of round nano-sized particles. The average grain size was estimated by counting 100 particles. Figure 3c show the statistic result of particle sizes. The average particle diameter of the as-prepared sample was estimated to be around 35 nm. The EDX spectrum of the sample prepared by this method confirms the presence of sodium (Na), Lanthanum (La), Tungsten (W) and oxygen (O) elements (Fig. 3d). No impurity peaks of other elements can be detected, which can effectively support the XRD result.

Fig. 1
figure 1

XRD pattern of Na La(WO4)2:xEu3+ (x = 0.01, 0.05, 0.07, 0.09 and 0.11)

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Table 1 Lattice parameters of pure and Eu-doped NLW powders
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Fig. 2
figure 2

Lattice parameters variations as a function of Eu concentration

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Fig. 3
figure 3

a FE-SEM image of the as-prepared NaLa(WO4)2:0.09 Eu3+ phosphor. b TEM image of the as-prepared NaLa(WO4)2:0.09 Eu3+ phosphor. c Particle size analysis of the NaLa(WO4)2:0.09 Eu3+ phosphor. d EDX spectrum of NaLa(WO4)2:0.09 Eu3+ phosphor

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3.2 FTIR analysis

The FT-IR spectra of the of NaLa(WO4)2:xEu3+ (x = 0.01, 0.05, 0.07, 0.09, 0.11) phosphors are shown in Fig. 4.The FTIR spectra show vibration bands due to the characteristic vibrations of W–O at around 1,000 cm−1. The peaks at around 2960 cm−1 are due to stretching vibrations of CH2 groups of EG molecule. A strong large absorption band around 657–966 cm−1 for NaLa(WO4)2:xEu3+ nano particles are related to anti-symmetric stretching vibration of O–W–O groups in WO4 2− tetrahedron clusters. The bands at around 3419 and 1631 cm−1 can be referred to stretching vibration of O–H and H–O–H bending mode vibration of H2O, respectively [14]. They are the usual vibrations of water molecules physically consumed on the sample surface from air, which is entirely different from coordinated water in compounds [15].

Fig. 4
figure 4

Raman spectra of NaLa(WO4)2:xEu3+ (x = 0.01, 0.05, 0.07, 0.09, 0.11) phosphors

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3.3 Raman analysis

The Raman spectra of NaLa(WO4)2:xEu3+ (x = 0.01, 0.05, 0.07, 0.09, 0.11) phosphors in the range from 200 to 1000 cm−1 is shown in Fig. 5. Raman spectra of the as prepared phosphor materials illustrates many peaks attributing to the Raman-active internal mode vibrations in the tetrahedral [WO4]2− units. Those at around 916.23, 796.23, 372.43 and 334.25 cm−1 which correspond to ν1(Ag),ν3(Eg), ν4(Bg) and ν2(Ag) vibrations for NaLa(WO4)2 [17, 18]. The very small variations observed on the positions of Raman modes could be emerging from different influences, such as synthesizing methods, average crystal size and interaction forces between the ions [16]. An analysis of the spectra greatly exhibited that all Raman-active modes of NaLa(WO4)2:xEu3+ prepared in this work is component of a tetragonal structure and in agreement with those formerly reported in the literature [17, 18].

Fig. 5
figure 5

FTIR spectra of NaLa(WO4)2:xEu3+ (x = 0.01, 0.05, 0.07, 0.09, 0.11) phosphors

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3.4 Photoluminescence properties, CIE chromaticity coordinates and thermal properties

Figure 6 shows the PLE spectra of the NaLa(WO4)2:xEu3+ (x = 0.01, 0.05, 0.07, 0.09 and 0.11) phosphor monitored at an emission wavelength of 615 nm. The PLE spectra of Na La(WO4)2:Eu3+ revealed a broad band between 220 and 300 nm with a band maximum at 249 nm, which is due to the charge transfer from the completely filled 2p orbital of O2− ions to the partially filled 4f orbitals of Eu3+ ions [19, 20]. Thus, this band is also called as charge transfer band (CTB). The other excitation bands from 300 to 550 nm correspond to the intra configurational f–f transition of Eu3+ ions. The transitions were indexed as (7F0 → 5D4) at 361 nm, (7F0 → 5G2) at 381 nm, (7F0 → 5L6) at 394 nm, (7F0 → 5D3) at 412 nm, (7F0 → 5D2) at 462 nm, and (7F0 → 5D1) at 530 nm. Figure 7 shows the PL spectra of the NaLa(WO4)2:xEu3+ samples with different Eu3+ ion concentrations monitored at an excitation wavelength of 394 nm. The PL spectra exhibited typical Eu3+ emission bands due to the presence of well known magnetic-dipole (MD) and electric dipole (ED) transitions. The photoluminescence emission spectra consists of sharp lines with wavelength ranging from 530 nm to 710 nm, which are associated with the 5D0 → 7FJ (J = 1, 2, 3, 4) transitions from the excited Eu3+ to the ground state. The main emission peak is the 5D0 → 7F2 transitions of Eu3+ at 615 nm; other transitions from 5D0 → 7F1, 5D0 → 7F3, and 5D0 → 7F4 located in the range of 530–710 nm are weak [8]. The 5D0 → 7F2 electric dipole transition of Eu3+ is highly sensitive to its surroundings and occurs dominantly only when the Eu3+ ion is in a noncentrosymmetric site. The emission intensity of the phosphor increases up to 9 mol% and then starts decreasing due to concentration quenching phenomena. As the Eu3+ ion concentration increased, the distance between Eu–Eu decreased and the energy was transferred non-radiatively between the Eu3+ ions, thus leads to the concentration quenching effect [21–25]. The lifetime of the Eu3+ (5D0–7F2) emission was measured and the normalized decay curves monitored for Na La(WO4)2:0.09 Eu3+ phosphors under excitation at 394 nm are shown in Fig. 8a. They fit well with the first-order exponential decay formula:

$$I\left( t \right)\text{ }=\text{ }I{{o}^{+}}A\text{ }\exp \text{ }\left( t/\tau \right)$$
(1)
Fig. 6
figure 6

PLE spectra of the NaLa(WO4)2:0.09Eu3+ monitored at an emission wavelength of 615 nm

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Fig. 7
figure 7

PL spectra of the NaLa(WO4)2:xEu3+ (x = 0.01, 0.05, 0.07, 0.09 and 0.11) phosphors under the excitation wavelength of 396 nm

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Fig. 8
figure 8

a Representative emission decay curve for 615 nm (5D0–7F2 of Eu3+) emission of 9 mol% Eu3+-doped Na La(WO4)2 phosphor (λexc = 394 nm). b The PL spectra (λex = 394 nm) of the NaLa(WO4)2:0.09 Eu3+ phosphor at different temperatures in the range of 25–250 °C. c CIE chromaticity diagram for NaLa(WO4)2:0.09 Eu3+ phosphor (λex = 394 nm)

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where I and I0 are the luminescence intensity at time t and 0, A is a constant, and τ represents the characteristic decay time for exponential component and the value of lifetime is 0.62 ms. The result shows that the lifetime is short enough for potential applications in displays and lights. The decay curve also indicates that all Eu3+ ions occupy the same atom environment, which is consistent with the description of the crystal structure above.

The thermal stability of the phosphor is an important area to be studied since the material is to be used in high-powered LEDs. The emission spectra measured for Na La(WO4)2:0.09 Eu3+ under 394 nm excitation at different temperatures between 25 and 250 °C are shown in Fig. 8b. The figure shows the temperature dependence of the luminescence quenching. The emission intensity of the phosphor decreases as the temperature increases up to 250 °C. Obviously, when the temperature increased to 150 °C, the corresponding PL intensity dropped to 81.3% of its initial value. This result indicates that the NaLa(WO4)2:0.09 Eu3+ phosphor has a relatively good thermal quenching effect. The thermal quenching phenomenon arises due to electron–phonon interaction and that can be very well explained using the configurational coordinate diagram. Initially, the Eu3+ luminescence center in the excited state is activated by phonon interactions which are released through a crossover between the 5D0 excited state and 7Fj (j = 1, 2, 3, 4) ground state [25, 26]. Thus, the emission intensity decreases as the temperature increases. CIE diagram (Fig. 8c) illustrates that the obtained phosphor nanoparticles show red emissions (x = 0.664, y  = 0.336)when excited by a single wavelength (λex = 394 nm) and thus the obtained results confirm that the SSM prepared Na La(WO4)2:0.09 Eu3+ phosphor is a promising red emitting components for near-UV InGaN-based white LED.

4 Conclusion

Red emitting NaLa(WO4)2:xEu3+ phosphors have been successfully prepared at low temperature via ethyelene glycol route. Their structural and optical properties were discussed in detail. The XRD study reveals tetragonal structure for Eu3+ doped NaLa(WO4)2. Upon 394 nm near UV excitation, the NaLa(WO4)2:0.09Eu3+ phosphor showed strong red emission lines at 615 nm corresponding to forced electric dipole transitions. The optimum doping concentration of Eu3+ content in NaLa(WO4)2 for the red emission is found to be 9 mol%. All these favourable properties indicate that SSM prepared NaLa(WO4)2:0.09Eu3+ red phosphor is a promising candidate for the phosphor converted white LEDs.