1. Introduction

Rare earth ions (RE) activated compounds have broad absorption and present emission from UV-visible to IR wavelength range duo to transitions of 4f electrons [1]. It has been widely investigated for luminescence or display. Among RE activators, Eu³⁺ is one of the most popular ions for its efficient ⁵D₀ to ⁷F_J (J = 0-6) transitions in red wavelength region [2]. Eu³⁺-doped materials are attractive as red-emitting phosphors for lighting and display such as white light-emitting diodes (W-LEDs). Of various hosts, molybdate oxides are attractive hosts owing to energy transfer from Mo-O complex to RE ions [3]. Molybdate phosphors have been intensively investigated for solid-state lighting [4], for example, CaMoO₄:Eu³⁺ [5], Gd₂MoO₆:Eu³⁺ [6], La₂Mo₂O₉:Eu³⁺ [7], Gd₂(MoO₄)₃:Eu³⁺ [8], NaYEu(MoO₄)₂:Eu³⁺ [3].

Molybdates with a formula of Ln₂Mo₄O₁₅ (Ln = lanthanides and yttrium), two times of M^IIIMo₂O_7.5, form a large family of compounds with interesting physical properties depending on crystal structures [9]. The structure of Ln₂Mo₄O₁₅ can be divided into three types, type I: La₂Mo₄O₁₅ (monoclinic space group P2₁/n) [10]; type II: Ln₂Mo₄O₁₅ (Ln = Ce-Tb) (triclinic space group P-1) [9]; and type III: Ln₂Mo₄O₁₅ (Ln = Dy–Lu and Y) (monoclinic space group P2₁/c) [11]. The well-known and intensively investigated properties of Ln₂Mo₄O₁₅ are anomalous negative thermal expansion (NTE) over a temperature range in 30-500 °C [12], for example, Y₂Mo₄O₁₅ shows NTE property (25-100 °C) with a TEC of −1.3 × 10⁻⁵ K⁻¹ [11]. Laufer et al [13] reported the structural and luminescence properties of Eu³⁺-doped Y₂[MoO₄]₂[Mo₂O₇] with the dominated ⁵D₀→⁷F₂ transitions.

In this work, a red-emitting nano-phosphor of Eu³⁺-doped Y₂Mo₄O₁₅ was synthesized by the Pechini method. The structure was investigated by powder X-ray diffraction (XRD) measurement and structure refinements, SEM and TEM measurements. The photoluminescence excitation and emission together with the decay curves were measured; The luminescence properties related to application such as the absolute internal quantum efficiency (QE) and thermal stability were reported.

2. Experimental

Y₂Mo₄O₁₅:Eu³⁺ was prepared by the Pechini method. The raw materials are (NH₄)₆Mo₇O₂₄·4H₂O, Eu₂O₃ and Y₂O₃ (99.9%). Firstly, Eu₂O₃ and Y₂O₃ were dissolved with nitric acid; the solution was complexed by citric acid with two times molar weight of Ln³⁺. The solution was neutralized by ammonium hydroxide (28% wt.). Secondly, (NH₄)₆Mo₇O₂₄·4H₂O was dissolved (60-70 °C) by adjusting the pH to 7.0 and complexing them with citric acid in two times molar weight of molybdate. Metal complexation in both cases was promoted by heat treatment at 100 °C for 2 h. Thirdly, the as-prepared two solutions were mixed together with adding aqueous polyvinylalcohol (PVA) to adjust the viscoelastic, which was stirred for 3 h to obtain a homogeneous viscous solution for the spin-coating on clean glasses. The precursor film can be obtained by natural withering of the coated glasses. Finally, the film was annealed to the desired temperature (800 °C) producing Y₂Mo₄O₁₅:Eu³⁺ with the heating rate of 5 °C /min. Note that, we cannot obtain the pure crystal phase when the doping level is above x = 0.4.

XRD was carried on a Rigaku D/Max diffractometer at 40 kV, 30 mA with Bragg-Brentano geometry using CuKα radiation (λ = 1.5405 Å). The photoluminescence (PL) spectra were recorded on a Perkin-Elmer LS-50B with Monk-Gillieson type monochromators. For the measurements of luminescence decay, the samples were excited by a pulsed Nd:YAG laser at 355 nm (Spectron Laser System SL802G). The luminescence was dispersed by the 75 cm monochromator (ActonResearch Corp. Pro-750) and multiplied by the PMT (Hamamatsu R928). The data were displayed and recorded with the LeCloy 9301 digital storage oscilloscope. PL quantum efficiency (QE), i.e., internal QE, was measured by a standard Edinburgh Instruments FLS-920 spectrometer equipped with an Edinburgh instruments integrating sphere. The excitation wavelength was selected from the output of the xenon lamp by a monochromator, which is connected with CCD sensor and a computer by light guides. QE value was calculated by the quantum yield measurement software.

3. Results

3.1 Phase formation and SEM

The structural refinement of Y₂Mo₄O₁₅:Eu³⁺ was finished by the GSAS program of Rietveld method shown in Fig. 1(a).The sample grows in pure monoclinic phase of P2₁/c (14). The structure sketch map of Y₂Mo₄O₁₅ is shown in Fig. 1(b), which was modeled using the Diamond Crystal and Molecular Structure Visualization software on the results of the refinements. The framework is constructed by the MoO₄ groups. O8 atom lying at a center of symmetry bridges two Mo(2)O₄ tetrahedra to form a pyromolybdate Mo(2)₂O₇ groups. Each Y(Eu)³⁺ is coordinated with seven oxygen to constitute singly capped trigonal prism with the bond distances of 2.2514(3) to 2.3224(2) Å. Mo(1) and Mo(2) centers are almost tetrahedron with Mo-O bond lengths from 1.689(3) to 1.878(2) Å. Mo(2)₂O₇ polyhedron and two Mo(1)O₄ tetrahedra joined together by sharing O14 atoms to form an entire Mo₄O₁₅ group.

 

Fig. 1 (a) a representative experimental (crossed) and calculated (red solid line) X-ray diffraction profiles of Y_2-2xEu_2xMo₄O₁₅ (x = 0.4), R_p = 0.1402, R_wp = 0.0705, R(F²) = 0.170, and χ² = 1.615; (b):the sketch maps of monoclinic structure of Y₂Mo₄O₁₅ viewed along [100].

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The typical scanning electron microscopy (SEM) image of Y_2−2xEu_2xMo₄O₁₅ (x = 0.4) are shown in Fig. 2(a).The particles are obviously agglomerated. Figure 2(c) is a representative transmission electron microscopy (TEM) image. The nano-powders consist of roughly spherical. The synthesized particles are smaller than 100 nm. The TEM image confirms that the Y_2−2xEu_2xMo₄O₁₅ nanoparticles are well crystallized with a single-phase structure. The minimum and maximum diameters are 9 nm and 90 nm, respectively. The statistical average size of the particles by the micrograph is about 35 nm.

 

Fig. 2 SEM micrographs (a) and the TEM image (b) of Y_2-2xEu_2xMo₄O₁₅ (x = 0.4) nanoparticles.

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3.2 The luminescence and excitation spectra

Figure 3(a) presents the photoluminescence of Y_2-2xEu_2xMo₄O₁₅ (x = 0.05, 0.2, 0.3, 0.4). The emission lines are the characteristic transitions from ⁵D₀→⁷F_J (J = 0, 1, 2, 3, 4) of Eu³⁺ ions. The ⁵D₀→⁷F₂ transitions show the dominated intensities. This is in agreement with the Laufer et al’s reports [13]. The value of (⁵D₀→⁷F₂)/(⁵D₀→⁷F₁) for Y_2-2xEu_2xMo₄O₁₅ (x = 0.4) is 3.51, which indicates low ligand symmetry and high bond covalency. This larger ratio is favorable to improve the color purity of the red phosphor [2].

 

Fig. 3 (a): Luminescence of Y_2-2xEu_2xMo₄O₁₅ (x = 0.05, 0.2, 0.3, 0.4) excited at 395 nm, inset is the absolute QEs efficiencies on Eu³⁺-doping; (b): typical excitation spectrum of Y_2-2xEu_2xMo₄O₁₅ (x = 0.05, 0.2, 0.4) (λ_em = 616 nm).

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Figure 3(b) shows the excitation spectra obtained by monitoring the ⁵D₀→⁷F₂ emission (616 nm). It consists of a broad band at 310 nm and some lines in 370-550 nm. The broad excitation bands with an asymmetry shape can be decomposed by two Gaussian components. A band at 275 nm is identified as the charge transfer (CT) transition from O²⁻ to Eu³⁺. The band centered at 330 nm is attributed to CT from O-orbitals to Mo-4d orbitals. The sharp lines are characteristic 4f–4f transitions indicating that Y₂Mo₄O₁₅:Eu³⁺ can be efficiently excited by UV-LED (350–400 nm) and blue-LED chips (450 nm). The presence of the excitation bands due to MoO₄ groups in Y₂Mo₄O₁₅ (Fig. 3(b)) gives the fact that there is an energy transfer from the host to Eu³⁺. The Eu³⁺ could be activated through two channels, i.e., to sensitize MoO₄ groups in the lattices and then transferring the energy to Eu³⁺ ions, or to sensitize the Eu³⁺ ions directly via the f-f transitions. Usually, the excitations due to MoO₄ groups are spin and parity allowed, and therefore they usually are more efficient for the luminescence of Eu³⁺ ions.

3.3 Absolute PL quantum efficiency

Absolute PL QE is one of important parameters for a phosphor’s application. Inset Fig. 3(a) shows the relationship between the QEs and the Eu³⁺-doping (x) of Y_2-2xEu_2xMo₄O₁₅ (x = 0.05-0.4). The efficiency increases with the enhancement of Eu³⁺ doping and the reach at a maximum at x = 0.4. Consequently the optimal QE in Y_2-2xEu_2xMo₄O₁₅ (x = 0.4) nano-powders is decided to be 63% (λ_ex = 395 nm). This value is higher than that of commercial red-emitting Y₂O₂S:Eu³⁺ (QE = 35%, λ_ex = 317 nm, λ_em = 611 nm) [14].

3.4 Thermal stability of Eu³⁺-luminescence

The thermal stability on temperature is one of important technological parameters for applications in W-LEDs. Usually W-LED packages could tolerate high temperatures (>100 °C) and light fluxes (>10 W/cm²) in comparison with traditional fluorescent lighting [15]. The thermal quenching was also investigated by the temperature-dependent emission as shown in Fig. 4(a).The integrated intensities decrease with increasing the temperature to 150 °C with the intensity 80% of the initial value (20°C). Generally, the quenching at high temperature is due to energy migration and transfer to nonradiative traps within the lattice [16]. To confirm the influence of temperature on the energy transfer, the decay curves were measured shown Fig. 4(b). It is obvious that the decay of the ⁵D₀ level at 15 and 350 K have a similar exponential profile. The lifetime keeps nearly the same value of 0.69 ms at 15-350 K indicating that the temperatures have no obvious influence on Eu³⁺. The lifetime starts to drop at 350 K by a decrease of the emission intensity. The luminescence lifetimes of the Eu³⁺ ions are calculated as a function of temperature and displayed in Fig. 4(b). The thermal activation energy (∆E) for the thermal quenching was determined by the equation as follows:

 

Fig. 4 (a): the spectra of Y_2-2xEu_2xMo₄O₁₅ (x = 0.4) at the selected temperatures; inset shows the temperature dependence of the integrated intensity normalized to the value at 20 °C; (b): the decay curves of ⁵D₀→⁷F₂ transition (616 nm) under excitation of 355 nm at different temperature; inset is the dependence of the lifetimes on temperature.

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4. Discussions

One of the motivations in this work is based on the fact that MoO₄ groups exhibit intensive absorption of near-UV light due to CT transition from O^2- to Mo⁶⁺ and therewith efficient energy transfer from MoO₄ to Eu³⁺ could happen in the hosts [4]. In this process, there are energy migration and quenching processes among MoO₄, which compete with the energy transfer process and consequently reduce the luminescent efficiency. Anyway, it is well-known that a vital influence on the luminescence efficiency is structure in Eu³⁺-activated hosts, for example, the microstructures of Eu³⁺ has great influences on the luminescence.

The maximum QE 63% of Y_2-2xEu_2xMo₄O₁₅ (x = 0.4) indicates that this is an efficient red-emitting phosphor. It has been confirmed that in highly charged hosts MO_x such as M = V, Mo, W, etc. the excitation energy can be transferred from MO_x groups to Eu³⁺ ions via either an exchange or super-exchange mechanism. The efficiency of the energy transfer depends on the M-O-Eu bond angle [15]. Usually, a 180° angle would maximize wave-function overlap and enhances the CT efficiency from MO_x to Eu³⁺. For example, Blasse et al. concluded that energy transfer VO₄^3-→ Eu³⁺ is not efficient in Eu³⁺-doped vanadate garnets due to the V–O–Eu bond angle is 90 or 120°, which reduce the wave-function overlap and the exchange energy transfer efficiency. As shown in Fig. 1 (b), the Mo–O–Eu bond angle is 165°, which would enhance the CT efficiency MoO₄→Eu³⁺ in Y₂Mo₄O₁₅.

It is a common phenomenon that a heavy Eu³⁺-doping leads to a “concentration quenching” of the red luminescence due to a possible energy migration among Eu³⁺ ions. In un-doped Y₂Mo₄O₁₅ structure, the minimum distance between the Y³⁺ ions is 5.5670 Å. By an approximation, the shortest distances between En³⁺ ions in 40% mol Eu³⁺-doped Y₂Mo₄O₁₅ is at least 5.5670 × 2 = 11.134 Å. It is far longer that the reported values in some Eu³⁺-concentrated phosphors such as Eu₃BWO₉ (4.2117 Å) [16]. This indicates that Y₂Mo₄O₁₅ host lattices can provide a long distance between Eu³⁺ ions even at high concentration. This can limit the luminescence quenching by energy transfer of Eu³⁺ ions hampering the energy migration to the killer centers.

Figure 5 presents the excitation spectra of Y_2-2xEu_2xMo₄O₁₅ (x = 0.4) detected by monitoring ⁵D₀→⁷F₂ luminescence of 616 nm. The sharp emission line with ⁷F₀→⁵D₀ transition at 579.1 nm (17,268 cm⁻¹). One F₀→⁵D₀ transition together with the three-fold splitting of ⁷F₁ level and five-fold splitting of ⁷F₂ level indicate that there is only one crystallographic Eu³⁺ site with symmetry C_s in this lattice. The Eu³⁺ ions have low-ðeld sites with small ΔE (⁷F₁) of 148 cm⁻¹ in Y₂Mo₄O₁₅ indicating that Eu-O bonding is more covalent. Usually in Eu³⁺-doped compound disordered distribution of Eu³⁺ ions is responsible for the thermal quenching. In Y₂Mo₄O₁₅ the Eu³⁺ ions are distributed in the host with a high “ordered state” in only one crystallographic. The excitation energy can have a quick diffusion and give an emission from Eu³⁺ ions. This could be one reason for the high thermal stability for the Eu³⁺ in Y₂Mo₄O₁₅.

 

Fig. 5 The excitation spectra of Y_2-2xEu_2xMo₄O₁₅ (x = 0.4) detected in the wavelength region of ⁷F₀→⁵D₀ transitions at 300 and 15 K. The spectra were obtained with a 580 nm filter by monitoring ⁵D₀→⁷F₂ luminescence of 616 nm of Eu³⁺ from the sample.

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5. Conclusions

Nano-sized red-emitting phosphor of Eu³⁺-doped Y₂Mo₄O₁₅ has been synthesized by pechini method with the average size of 35 nm. The crystal phase and structure were performed by XRD structural refinements. The sample keeps monoclinic P2₁/c crystalline phase below the doping level 40 mol% of the Eu³⁺ substitution for Y³⁺ ions in the lattices. Eu³⁺-doped Y₂Mo₄O₁₅ show effective excitation in near-UV and blue light regions. The phosphor shows the strong emission from the ⁵D₀→⁷F₂ transitions of Eu³⁺ ions (616 nm) with CIE coordinates (x = 0.662, y = 0.334). Under the excitation of near UV light, Y₂Mo₄O₁₅:Eu³⁺ presents maximum absolute internal QE value of 67% and a high thermal activation energy ∆E of 0.395 eV. The luminescence properties are ascribed to the special structure characteristic such as big Mo–O–Eu bond angle, the long distances between Eu³⁺ ions, only one crystallographic site with a high “ordered state” distributed in the lattices.