Hydrothermal synthesis of NaLa(WO₄)₂:Eu³⁺ octahedrons and tunable luminescence by changing Eu³⁺ concentration and excitation wavelength

Abstract

NaLa(WO₄)₂:Eu³⁺ phosphors with different Eu³⁺ concentrations have been synthesized by a hydrothermal method. The phase is confirmed by XRD analysis, which shows a pure-phase NaLa(WO₄)₂ XRD pattern for all of NaLa(WO₄)₂:Eu³⁺ phosphors. The SEM and TEM images indicate that all of NaLa(WO₄)₂:Eu³⁺ phosphors have a octahedral morphology. These suggest that the Eu³⁺ doping has no influence on the structure and growth of NaLa(WO₄)₄ particles. By monitoring the emission of Eu³⁺ at 615 nm, NaLa(WO₄)₂:Eu³⁺ phosphors show excitation bands originating from both host and Eu³⁺ ions. Under the excitation at 271 nm corresponding to WO₄ ²⁻ groups, emission bands coming from the ¹A₁ → ³T₁ transition with the WO₄ ²⁻ groups and the ⁵D₀ → ⁷F_j (j = 0, 1, 2, 3 and 4) transitions of Eu³⁺ are observed. The emission intensity relating to WO₄ ²⁻ groups decreases with increasing Eu³⁺ concentration. But emission intensities of Eu³⁺ increase firstly and then decreases because of concentration quenching effect. Under the excitation at 395 nm corresponding to ⁷F₀ → ⁵L₆ transition of Eu³⁺, only characteristic Eu³⁺ emission bands can be observed. The results of this work suggest that tunable luminescence can be obtained for Eu³⁺ doped NaLa(WO₄)₂ phosphors by changing Eu³⁺ concentration and excitation wavelength.

1 Introduction

More and more attentions are paid to phosphor materials in recent years due to their applications in fields of solid state lighting [1], solar cells [2], sensors [3], medical and biological labels [4]. To meet the requirements of these applications, phosphors materials are expected to show high-efficiency and color controllable emissions, high physical and chemical stabilities, and so on. The luminescent properties of phosphor materials are affected by some factors, such as host, excitation wavelength, temperature, size and morphology. Rare earth ions play an important role in phosphor materials owing to their abundant emission colors originating from the 4f–4f or 5d–4f transitions [5]. For example, as one of widely used activators of red emission, Eu³⁺ often shows emission bands originating from ⁵D₀ → ⁷F_j (j = 0, 1, 2, 3 and 4) transitions, whereas the relative intensity of these emission bands are influenced highly by the site of Eu³⁺ in the host. There will be a relative strong ⁵D₀ → ⁷F₁ transition and a relative weak ⁵D₀ → ⁷F₂ transition when Eu³⁺ locates a site without an inversion [6].

Double tungstates with the formula of MRE(WO₄)₂ (M = alkali-metal ions, RE = rare earth ions) are considered as potent candidates for the excellent physical and chemical stability and luminous efficiency [7]. The efficient energy transfer from the host matrix to the localized states of lanthanide ions also benefits the luminescent property. These materials have the scheelitr-type structure and contain the WO₄ ²⁻ tetrahedral anions (W⁶⁺ is coordinated by four oxygen atoms in a tetrahedral site, and the RE³⁺/M⁺ site is eight coordinated). They exhibit excellent luminescence-property and can meet the various demands for exploring high-quality phosphor materials when they are doped with rare earth ions. A large number of rare earth ions doped double tungstates, such as NaGd(WO₄)₂:Tb³⁺ [8], NaLu(WO₄)₂:Eu³⁺ [2], NaLa(WO₄)₂:Er³⁺/Yb³⁺ [9], NaLa(WO₄)₂:Eu³⁺/Tb³⁺/Tm³⁺ [10], NaGd(WO₄)₂:Ho³⁺/Yb³⁺ [11] and NaGd(WO₄)₂:Tm³⁺/Dy³⁺/Eu³⁺ [12], have been synthesized successfully by different methods.

In this wok, we report on the hydrothermal synthesis of NaLa(WO₄)₂:Eu³⁺ phosphors. Hydrothermal method, as a typical solution-based approach, has been proved to be an effective and convenient synthesis technique for preparing uniform and well-dispersed phosphor particles. The hydrothermal process can be controlled by an appropriate choice of reaction parameters, such as temperature, time, pH value and surfactant. Moreover, low reaction temperature, mild reaction conditions, size-selective growth, controllable morphology, phase purity, smaller particle size, and narrow particle size distribution are the advantages of the hydrothermal synthesis method. We find that the obtained NaLa(WO₄)₂:Eu³⁺ phosphors have the octahedral morphology and the luminescence can be tuned by changing Eu³⁺ concentration and excitation wavelength.

2 Materials and methods

A series of NaLa(WO₄)₂:x mol%Eu³⁺ (x = 0, 3, 6, 9, 12 and 15) phosphors were synthesized by a hydrothermal process. La₂O₃ (99.99 %), Eu₂O₃ (99.99 %), (NH₄)₆W₇O₂₄·6H₂O (AR) and NaNO₃ (AR) were used as raw materials. NaOH (AR) was used to adjust the pH values. All raw materials were used directly without further purification. La(NO₃)₃ and Eu(NO₃)₃ solutions were obtained firstly by dissolving La₂O₃ and Eu₂O₃ into HNO₃ with constant stirring and heating at about 50 °C until all materials were dissolved completely. The mole ratios of Eu₂O₃:La₂O₃ were 0, 3:97, 6:94, 9:91, 12:88 and 15:85, respectively. Then, (NH₄)₆W₇O₂₄·6H₂O and NaNO₃ were added to the above solution with stirring for about 30 min. The pH value of solution was adjusted to be 7 by adding appropriate NaOH, which induces the formation of suspension solution. The suspension solution was subsequently transferred into a 100 ml Teflon vessel and sealed in a stainless steel autoclave. The autoclave with its contents was treated under hydrothermal conditions for 10 h at 180 °C. When the reaction was completed, it was naturally cooled to room temperature and a white precipitate was obtained. Afterwards, the precipitates were filtered by centrifugation and washed several times using deionized water and ethanol. The final products were dried at 70 °C for 24 h in air.

The X-ray powder diffraction (XRD) measurements were carried out on a Rigaku-Dmax 2500 diffractometer using Cu Kα radiation (λ = 0.15405 nm). The morphologies and microstructures were studied by scanning electron microscopy (SEM, Hitachi S-4800) and transmission electron microscopy (TEM, JEOL-2100F). Selected-area electron diffraction (SAED) and high-resolution TEM (HRTEM) were examined on a JEM-2010F high-resolution transmission electron microscope. The luminescence was performed on a HitachiF-7000 spectrophotometer equipped with a 150 W xenon lamp as the excitation source.

3 Results and discussion

The phase of obtained NaLa(WO₄)₂:Eu³⁺ phosphors has been confirmed by XRD analysis. It is clear from Fig. 1 that all diffractions are well according with the pure-phase NaLa(WO₄)₂ (JCPDS No. 79-1118) and no secondary phase is detected. This indicates that the phosphors doped with different Eu³⁺ concentrations remain a single phase. According to the effective ionic radii of cations, Eu³⁺ (1.066 Å for CN = 8) ions are proposed to occupy the sites of La³⁺ (1.160 Å for CN = 8) or Na⁺ (1.180 Å for CN = 8). However, on the basis of the valence state analysis, the Eu³⁺ ions are much more probably occupying the La³⁺ sites. The diffraction peaks shift slightly to higher 2θ angles with the increasing Eu³⁺ concentrations because of the substitution of La³⁺ by Eu³⁺ and the smaller ionic radius of Eu³⁺ comparing with La³⁺. The lattice parameters were calculated using least square refinement program CHECKCELL. The calculated lattice parameter as a function of Ca concentration is shown in Fig. 2. The above results suggest that the Eu³⁺ ions have been effectively incorporated into the host lattices by replacing La³⁺ sites without altering the host structure and the well-crystallized NaLa(WO₄)₂:Eu³⁺ samples can be obtained under the present hydrothermal conditions.

Fig. 1

XRD patterns of pure NaLa(WO₄)₂ and Eu³⁺ doped NaLa(WO₄)₂ phosphors

Fig. 2

Lattice parameters of NaLa(WO₄)₂:x mol%Eu³⁺ (x = 0, 3, 6, 9, 12 and 15) phosphors

Figure 3 gives the SEM (Fig. 3a) and TEM (Fig. 3b) images of NaLa(WO₄)₂:12 mol%Eu³⁺ phosphors. From images, one can see that monodispersed particles with a well-defined octahedral morphology are obtained. Generally, the Ostwald ripening process is a well accepted mechanism for the crystal growth. In the Ostwald ripening process, the initial formation of tiny crystalline nuclei in a supersaturated medium is followed by crystal growth. In this process the larger particles grow at the cost of the small ones due to the energy difference between large particles and small particles. Initially, the mixture solution containing La(NO₃)₃, Eu(NO₃)₃, NaNO₃ and (NH₄)₆W₇O₂₄ with a pH value of 7 yields the formation of abundant amorphous NaLa(WO₄)₂:Eu³⁺ particles, which serves as precursors of the crystal. Next, the hydrothermal process induces the crystallization, in which amorphous particles nucleate and hydroxyl anions are adsorbed selectively. Finally, the crystal grows with certain faces and octahedral crystals are formed. All of NaLa(WO₄)₂:Eu³⁺ phosphors with different Eu³⁺ concentrations have similar morphology, indicating that the Eu³⁺ concentration has no influence on morphology. The bright spots in the SAED pattern (Fig. 3c) demonstrate the single crystalline nature of NaLa(WO₄)₂:12 mol%Eu³⁺ phosphors, which is in accordance with the clear lattice fringes in the HRTEM image in Fig. 3d. Additionally, the distance between two adjacent lattice fringes was calculated to be 0.294 nm, which corresponds with the (004) plane of tetragonal NaLa(WO₄)₂.

Fig. 3

SEM (a), TEM (b), SAED pattern (c) and HRTEM (d) images of NaLa(WO₄)₂:12 mol%Eu³⁺ phosphor

Figure 4 shows the excitation spectra of NaLa(WO₄)₂ and NaLa(WO₄)₂:12 mol%Eu³⁺ phosphors by monitoring 615 nm emission of Eu³⁺. Both NaLa(WO₄)₂ and NaLa(WO₄)₂:12 mol%Eu³⁺ phosphors have a strong and broad band ranging from 200 to 350 nm with a maximum at about 271 nm, which is the charge transfer band (CTB) of WO₄ ²⁻ groups and O²⁻ → Eu³⁺ transition from an oxygen 2p state excited to an Eu³⁺ 4f state. Mostly, the contribution of these two transitions cannot be distinguished obviously because of the spectral overlap. For NaLa(WO₄)₂:12 mol%Eu³⁺, a series of sharp excitation lines in the range of 350–500 nm also can be observed, which is the typical intra-4f transitions of Eu³⁺. The excitation lines in this region come from ⁷F₀ → ⁵D₄, ⁷F₀ → ⁵G_J, ⁷F₀ → ⁵L₇, ⁷F₀ → ⁵L₆, ⁷F₀ → ⁵D₃ and ⁷F₀ → ⁵D₂ transitions of Eu³⁺, respectively. Herein, the presence of excitation band of WO₄ ²⁻ groups by monitoring Eu³⁺ emission suggests an energy transfer from WO₄ ²⁻ groups to Eu³⁺.

Fig. 4

Excitation spectra of NaLa(WO₄)₂ and NaLa(WO₄)₂:12 mol%Eu³⁺ phosphor

Figure 5 shows the emission spectra of NaLa(WO₄)₂:x mol%Eu³⁺ (x = 0, 3, 6, 9, 12 and 15) phosphors under an excitation at 271 nm. All of Eu³⁺ doped NaLa(WO₄)₂ phosphors have similar profiles and each one consists of two parts: one is the broad band in the range of 350–575 nm with a maximum at about 468 nm and the other consists of several emission lines in the range of 575–725 nm. The broad band peaking at about 468 nm is due to the ¹A₁ → ³T₁ transition with the WO₄ ²⁻ groups. The pure NaLa(WO₄)₂ only has this emission band under excitation at 271 nm. The sharp emission bands with maximum at 590, 599, 615, 623 and 706 nm result from the ⁵D₀ → ⁷F_j (j = 0, 1, 2, 3 and 4) transitions of Eu³⁺, respectively. The emission band peaking at 615 nm (originating from the ⁵D₀ → ⁷F₂ electric dipole transition of Eu³⁺) is strongest, suggesting that Eu³⁺ locates a non-symmetry site in NaLa(WO₄)₂ host. For NaLa(WO₄)₂:Eu³⁺ phosphors, the emission intensity of the host decreases gradually with increasing Eu³⁺ concentrations. The emission intensity corresponding to Eu³⁺ increases with increasing Eu³⁺ concentrations and reaches a maximum at x = 12. The concentration of Eu³⁺ more than 12 mol% in NaLa(WO₄)₂ host induces the decrease of Eu³⁺ emission due to the concentration quenching effect. The changes of emission intensity relating to transition of host and Eu³⁺ confirm the existence of energy transfer from the host to Eu³⁺ in NaLa(WO₄)₂:Eu³⁺ phosphors. But WO₄ ²⁻ groups do not transfer all of the absorbed energy to Eu³⁺ ions so that the emission of host can be observed. Also, such energy transfer phenomenon demonstrates that the as-synthesized NaLa(WO₄)₂:Eu³⁺ is a solid solution, in which Eu³⁺ has successfully incorporated into NaLa(WO₄)₂ lattice, agreeing well with the XRD results [13].

Fig. 5

Emission of NaLa(WO₄)₂:x mol%Eu³⁺ (x = 0, 3, 6, 9, 12 and 15) phosphors under excitation at 271 nm

Figure 6 shows the emission spectra of NaLa(WO₄)₂:x mol%Eu³⁺ (x = 0, 3, 6, 9, 12 and 15) phosphors under excitation at 395 nm. The results show that the excitation wavelength has obvious influence on the luminescence of NaLa(WO₄)₂:Eu³⁺ phosphors. There are only characteristic emission bands of Eu³⁺ ions, indicating that the energies absorbed by the ⁷F₀ → ⁵L₆ transition cannot be transferred to WO₄ ²⁻ groups. This advises that excitation induced emissions are possible from the single emitting component by dissolving the suitable amount of Eu³⁺ ions in the calcium site due to WO₄ polarization in the luminescence. This result is similar to that of CaMoO₄:Eu³⁺ phosphors reported by Raju et al. [14]. But the dependence of Eu³⁺ emission on the Eu³⁺ concentration is similar to those of results obtained by 271 nm excitation.

Fig. 6

Emission of NaLa(WO₄)₂:x mol%Eu³⁺ (x = 0, 3, 6, 9, 12 and 15) phosphors under excitation at 395 nm

In order to determine the energy transfer mechanism from host to activators in NaLa(WO₄)₂:Eu³⁺ phosphors, it is necessary to know the critical distance (R_c) between the neighbouring Eu³⁺ ions. The critical transfer distance (R_c) can be calculated by the formula of ${\text{R}}_{\text{C}} \approx 2\left( {3{\text{V}}/4\uppi{\text{XN}}} \right)^{{\frac{1}{3}}}$, where ${\text{X }}$ is the critical concentration of Eu³⁺, ${\text{V}}$ is the volume of the unit cell, and ${\text{N}}$ is the number of available sites of the dopant in the unit cell [15]. For NaLa(WO₄)₂ host, the values of V, X and N are 332.698 Å³, 0.12 and 2, respectively. The critical distance is calculated to be about 13.86 Å. With the increase of Eu³⁺ concentration, the energy transfer from the host to Eu³⁺ becomes more efficient, and the probability of energy migration between activators increases simultaneously. When the distance becomes small enough, the concentration quenching phenomenon takes place and the energy migration is hindered. In general, there are two main mechanisms for a non-radiative energy transfer process: one is exchange interaction, and the other is electric multipolar interactions [16]. The value of R_c acquired above indicates the little possibility of exchange interaction since it can be predominant only for about 5 Å. Hence, we can conclude that the energy transfer mechanism from the host to Eu³⁺ is dominated by the electric multipolar interactions.

It is well known that the intensities of the transitions between different J-number levels are dependent on the symmetry of the local environment of Eu³⁺ activators in terms of the Judd–Ofelt theory [6, 17]. The magnetic dipole transition (⁵D₀ → ⁷F₁, orange emission) will dominate if Eu³⁺ is in a site with inversion symmetry, whereas electric dipole transition (⁵D₀ → ⁷F₂, red emission) dominates when Eu³⁺ locates a site without inversion symmetry. Therefore, the luminescent intensity ratio of ⁵D₀ → ⁷F₂ to ⁵D₀ → ⁷F₁ (R/O) is used to measure the degree of distortion from inversion symmetry of the local chemical environment surrounding Eu³⁺ in the host. As shown in Figs. 4 and 5, the emission intensity of ⁵D₀ → ⁷F₂ is much higher than that of ⁵D₀ → ⁷F₁ for NaLa(WO₄)₂:Eu³⁺ phosphors. The R/O values of NaLa(WO₄)₂:Eu³⁺ phosphors with different Eu³⁺ concentrations are shown in Fig. 7. It reveals that the R/O values increase with the increasing Eu³⁺ concentrations, indicating Eu³⁺ occupies more distorted local environment. The color purity of Eu³⁺ doping phosphors can be characterized as the R/O value [18].

Fig. 7

Dependence of the luminescent intensity ratio of ⁵D₀ → ⁷F₂ to ⁵D₀ → ⁷F₁ on Eu³⁺ concentration

4 Conclusion

We have synthesized a series of NaLa(WO₄)₂:Eu³⁺ phosphors by a hydrothermal method. The Eu³⁺ doping has no influence on the phase structure and morphology of obtained NaLa(WO₄)₂:Eu³⁺ samples. The Eu³⁺ concentration in NaLa(WO₄)₂ host and the excitation wavelength have obvious influence on luminescence of NaLa(WO₄)₂:Eu³⁺ phosphors. Under the excitation at 271 nm corresponding to WO₄ ²⁻ groups, emission bands coming from the ¹A₁ → ³T₁ transition with the WO₄ ²⁻ groups and the ⁵D₀ → ⁷F_j (j = 0, 1, 2, 3 and 4) transitions of Eu³⁺ are observed. The emission intensity relating to WO₄ ²⁻ groups decreases with increasing Eu³⁺ concentration. But emission intensities of Eu³⁺ increase firstly and then decreases because of concentration quenching effect. Under the excitation at 395 nm corresponding to ⁷F₀ → ⁵L₆ transition of Eu³⁺, only characteristic Eu³⁺ emission bands can be observed. The results of this work suggest that tunable luminescence can be obtained for Eu³⁺ doped NaLa(WO₄)₂ phosphors by changing Eu³⁺ concentration and excitation wavelength.