Luminescence properties and energy transfer of Tb³⁺, Eu³⁺ co-doped YTaO₄ phosphors obtained via sol–gel combustion process

Abstract

Tantalate is considered as a valuable and efficient luminescence host because of its intense absorption in the ultraviolet area and excellent chemical properties. In this work, a series of pure YTaO₄:Eu³⁺ and/or Tb³⁺ crystals were prepared via a sol–gel combustion method. The morphology, structure, and optical properties of the samples were discussed in detail. The Eu³⁺, Tb³⁺ co-doped YTaO₄ samples are consisted of small spherical particles of around 18 nm. The prepared YTaO₄:Tb³⁺ and/or Eu³⁺ samples exhibit the characteristic wide excitation band around 210–300 nm, the characteristic narrow red emission of Eu³⁺ (⁵D₀ → ⁷F₂) transitions and green emission of the Tb³⁺ (⁵D₄ → ⁷F₅) transitions when excited by UV light. It is focused on the energy transfer processes from the YTaO₄ to Tb³⁺ as well as Eu³⁺ ions and from Tb³⁺ to Eu³⁺ ions of YTaO₄:Eu³⁺/Tb³⁺ phosphors. Color-tunable emissions are realized through adjusting the types of rare earth ion (Eu³⁺ and Tb³⁺) and relative doping concentrations excited by a single wavelength. That is to say, the obtained Tb³⁺ and Eu³⁺ co-doped YTaO₄ phosphors have a promising prospect in lasers, white light diodes (WLED), fluorescent lamp, and field emission display devices, etc.

Introduction

At present, owing to the advantages of friendly environment, nice color rendering index and excellent luminescent performance, rare earth active oxide phosphors have been got quite attention in imaging, lighting, display, and other fields. In general, high temperature solid-state method is a common way to synthesize rare earth activated oxide phosphors. Unfortunately, the traditional solid reaction route is not beneficial to control the particle size and separation of components, and may lead to other problems such as long reaction times and stoichiometric losses caused by evaporation at high temperature in general [1]. Therefore, it is still necessary to develop some timesaving and low temperature methods to synthesize the target product such as the hydrothermal method, supersonic microwave co-assistance method, sol–gel method[2–6]. Among these routes, sol–gel combustion method is a good choice because it can obtain stable precursors and nanometer phosphors with homogeneous composition and high purity. Compared with bulk materials, nanophosphors have a large number of surface atoms. Therefore, the interaction between nanophosphor and the incident UV light is much higher than that of its bulk counterpart. Additionally, nanophosphors have the advantages of better absorption, high conversion efficiency, and high color rendering [7–11]. All of these advantages of reduced dimension phosphors make them easier to use in modern applications [12].

Compared with the common LED used for lighting, the deep UV LED with the wavelength of 240–280 nm has the outstanding advantages of safety, environmental protection, cold light source, pure light quality, high efficiency, rich wavelength type, easy to compound spectral energy, convenient modulation, etc., which is much suitable for civil use. In addition to being a common germicidal lamp, deep UV LED has been widely used in medical and health fields [13, 14]. At the same time, with the outbreak of the coronavirus, the research on UV-C (280–200 nm) LED with bactericidal effect is more and more urgent. Recently, rare earth ions doped tantalate phosphors have received considerable concern for their important applications in lighting fields [15–19]. YTaO₄ is known as an excellent chemical and photoactive host material in virtue of its luminescence performance, high density, high irradiation hardness, strong X-ray absorption, and stable physical and chemical properties [20]. YTaO₄ has two crystal phases, one is tetragonal phase (T), namely scheelite structure, with space group I41/a [21], the other is monoclinic phase, including the pyrite phase (M) with I2/a space group and the prime phase (Mʹ) with P2/a space group [22, 23]. The difference of the two structures is essentially on account of the different coordination number of atoms. With M structure, Ta atoms occupy tetrahedral lattice, while in Mʹ structure, Ta atoms exist in twisted octahedral structure [24, 25]. As reported, Rare earth-doped YTaO₄ crystal with Mʹ structure can present the charge transfer process more effectively and provide splendid luminescent emission [26–28].

Eu³⁺ along with Tb³⁺ ions doped phosphors have been extensively studied on account of their ideal luminescent properties [29, 30]. Besides, based on energy transfer mechanism, the phosphors doped with Eu³⁺/Tb³⁺ ions can realize tunable multicolor emission. Particularly, thanks to the ⁵D₀ → ⁷F_{1, 2} transitions, the Eu³⁺-doped phosphors can emit red light [31–37]. Tb³⁺ can be used as both activator and sensitizer to achieve green light emission due to its ⁵D₄ → ⁷F₅ transitions at approximately 547 nm [38, 39] and improve the emission intensity of activator ions (Eu³⁺). In addition, Tb³⁺ ions also broaden the UV–visible absorption region when Tb³⁺ along with Eu³⁺ ions co-doping in some hosts owing to the existence of more level transitions [40–43]. It means that owing to the effective energy transfer, the characteristic emission light of Tb³⁺ and Eu³⁺ ions can be adjusted to obtain color-tunable phosphors excited by ultraviolet light. Moreover, the strong excitation band in the ultraviolet region of YTaO₄ host can effectively transfer absorbed energy to the doped activators. Thereby, it allows the energy to be more fully transmitted to rare earth ions to improve the fluorescence efficiency of the phosphors. Hence, we focus on synthesizing color adjustable phosphors by doping Tb³⁺ and Eu³⁺ in YTaO₄ host.

In this work, we prepared the phosphors with the composition of YTaO₄:Eu³⁺/Tb³⁺ by sol–gel combustion method. Photoluminescence properties, energy transfer, and adjustable color were studied closely. Due to the excellent luminescence properties and colorimetric properties of YTaO₄:Ln³⁺ (Ln³⁺  = Eu³⁺ and/or Tb³⁺) samples, there will be potential phosphors used in the fields of display and lighting.

Experimental section

Materials

Yttrium oxide (Y₂O₃) (99.99%), Terbium oxide (Tb₄O₇) (99.99%), Europium oxide (Eu₂O₃) (99.99%), Tantalum oxide (Ta₂O₅) (99.99%), Citric acid (CA) (Analytical reagent, AR), Nitric acid (HNO₃), Ammonia, Ammonium nitrate (NH₄NO₃) (AR) were used as starting materials. The reagents we used were analytical grade and utilized as purchased without further purification.

Preparation

The corresponding quantities of Y₂O₃, Eu₂O₃, and Tb₄O₇ were put into dilute HNO₃ solution, stirred, and heated in ambient atmosphere, and then after evaporation, crystallization, and cooling, the obtained rare earth nitrate crystals were finally configured into the corresponding aqueous solutions to obtain 0.1 mol L⁻¹ Y(NO₃)₃, 0.05 mol L⁻¹ Eu(NO₃)₃ and 0.05 mol L⁻¹ Tb(NO₃)₃ solutions. Eu³⁺-, Tb³⁺-doped YTaO₄ nanocrystals were obtained by sol–gel combustion method. The typical steps for preparing YTaO₄:0.07Tb³⁺, 0.02Eu³⁺ are as follows: First, 0.5 mmol of Ta₂O₅ was completely dissolved in HF acid (40%), and then ammonia solution (25%) was added until the pH equals to 9. The obtained precipitate was centrifuged and washed with deionized water to get rid of the redundant F⁻, and then dissolved in citric acid aqueous solution at 50 ℃. Subsequently, 0.97 mmol Y(NO₃)₃, 0.01 mmol Eu(NO₃)₃, 0.02 mmol Tb(NO₃)₃ and 1.20 g of NH₄NO₃ were added into the above solution. Under vigorous stirring at 80 ℃, a high viscosity gel with pale-yellow was formed. Further, the gel was heated at 180 ℃ for 3 h to get a highly condensed porous network dry gel. After that, it was transferred to the muffle furnace and heated to 500 ℃ for 3 h. Finally, grind the solid taupe mass obtained in the previous process into a fine powder and put it into the alumina crucible then for heat treatment at 1000 ℃ for 5 h. The target phosphors were obtained and used for further characterization. The synthesis steps of other samples are the same as that of YTaO₄:0.07Tb³⁺, 0.02 Eu³⁺ samples except for doping different concentrations of rare earth ions.

Characterization

X-ray diffraction (XRD) measurements were carried out on a Rigaku-Dmax-RA diffractometer using Cu Kα radiation (λ = 0.15406 nm), operating at a scanning speed of 10° min^–1 in the 2θ range from 10° to 90°. The morphologies of the samples were analyzed using a FEI-30 field emission scanning electron microscope (FE-SEM) equipped with an energy dispersive X-ray spectrometer (EDX). The photoluminescence measurements were carried out on a HITACHIF-7000 spectrophotometer equipped with a 150-W Xenon lamp as the excitation source. All the measurements were carried out at room temperature.

Results and discussion

Phase identification, structure, and morphology analysis

The XRD patterns of YTaO₄:Eu³⁺/Tb³⁺ samples synthesized via sol–gel combustion method are shown in Fig. 1a. We can see that all measured peak positions match with YTaO₄ standard card (PDF# 24-1425) properly. The obtained YTaO₄ powder is a monoclinic structure (M' structure) with P2/a space group and the lattice parameters are a = 5.29, b = 5.45, c = 5.11 Å, and Z = 2. No any other impure phase is detected, suggesting that the replacement of Y³⁺ ions by Eu³⁺ and Tb³⁺ions does not affect the purity of the YTaO₄. This is because the radii of Y³⁺ (r = 0.908 Å), Tb³⁺ (r = 0.923 Å) and Eu³⁺ (r = 0.947 Å) ions are similar, and all three ions have the same coordination number (CN = 8). Figure 1b shows the lattice structure as well as atomic coordination of YTaO₄. We can see that each tantalum ion and yttrium ion are encircled by six and eight oxygen ions in the YTaO₄Mʹ-type structure, respectively, to form a distorted octahedron (TaO₆) and a twisted cube (YO₈).

Fig. 1.

a XRD patterns of the as-prepared YTaO₄:Tb³⁺, YTaO₄:Eu³⁺ and YTaO₄:0.07Tb³⁺, 0.02 Eu³⁺ samples, and the corresponding standard data of YTaO₄ (PDF No. 24-1425) are given as reference, b the crystal structure of YTaO₄ host

The morphology and elemental component of YTaO₄:0.07Tb³⁺, 0.02Eu³⁺ sample are characterized by FE-SEM and EDX, respectively. Figure 2a illuminates that the phosphors are uniform nanoparticles with a small number of agglomerates and the particle size is about 18 nm, seen from the size distribution histogram of the samples (insert of Fig. 2a). Figure 2b exhibits the EDX spectrum of YTaO₄:0.07Tb³⁺, 0.02Eu³⁺ phosphor, and we can see the existence of O, Ta, Y, Tb, Eu, and Si elements. The Si element comes from silicon wafer.

Fig. 2.

a FE-SEM image and b EDX spectrum of YTaO₄:0.07Tb³⁺, 0.02 Eu³⁺ sample. Inset of a is the size distribution histogram

Luminescence properties

The photoluminescence excitation (PLE) and photoluminescence (PL) spectra of Tb³⁺ singly activated YTaO₄ samples are presented in Fig. 3a. When monitored at 547 nm (⁵D₄ → ⁷F₅ transition of Tb³⁺), there is a strong broadband around 210–300 nm with a maximum at 270 nm in PLE spectrum, corresponding to the charge transfer band of Tb³⁺–O²⁻ and O²⁻–Ta⁵⁺ in TaO₄³⁻ group [13], which lead to better luminescent properties of Tb³⁺ ions in YTaO₄ host for UV excitation [44]. It also indicates that the existence of energy transfer between the host lattice (TaO₄³⁻ group) and the active centers (Tb³⁺ ions) [45]. In addition, the f-f transitions of Tb³⁺ ions from the ground state (⁷F₆) to the ⁵D₂ (354 nm), ⁵L₁₀ (369 nm) and ⁵G₆ (378 nm) states result in several sharp PLE peaks ranged from 320 to 380 nm [46, 47]. Upon 270 nm excitation, the representative emission peaks at 492 nm, 547 nm, 585 nm, and 621 nm in the PL spectrum are corresponded to ⁵D₄ → ⁷F_{6, 5, 4, 3} transitions of Tb³⁺ ions, respectively [48]. The strongest emission peak of YTaO₄:0.07Tb³⁺ phosphor situates at 547 nm in the green area. The PL spectra of the YTaO₄:xTb³⁺ phosphors under 270 nm excitation illuminate that the luminescence intensity of Tb³⁺ reaches maximum at x = 0.07. When Tb³⁺ concentration exceeds 0.07, the emission intensity decreases owing to the concentration quenching effect [49, 50].

Fig. 3.

PL excitation spectrum (left) for YTaO₄: 0.07Tb³⁺ and emission spectra (right) for YTaO₄:xTb³⁺ (x = 0.01–0.08) samples. Inset is enlarged excitation spectrum of YTaO₄: 0.07Tb³⁺ at the range of 320 to 380 nm

The PLE and PL spectra of the Eu³⁺ singly activated YTaO₄ phosphor are illustrated in Fig. 4. When 613 nm (⁵D₀ → ⁷F₂ transition) is used as monitoring wavelength, the PLE spectrum consists of a broad band of 210—300 nm centered at 258 nm and a few of weak sharp peaks ranging from 300 to 450 nm, which are corresponded to the charge transfer band transitions of Eu³⁺–O²⁻ and O²⁻–Ta⁵⁺ [51–53] and the f-f transitions of Eu³⁺ from the ground state (⁷F₀) to the ⁵H₆ (321 nm), ⁵D₄ (362 nm), ⁵G₄ (384 nm), ⁵L₆ (395 nm), ⁵D₃ (418 nm), ⁵D₂ (466 nm) and ⁵D₁ (537 nm) states [54, 55]. At 258 nm excitation, in PL spectrum, we can see the main peaks at around 578, 593, 613 and 656 nm, which correspond to the ⁵D₀ → ⁷F₀, ⁵D₀ → ⁷F₁, ⁵D₀ → ⁷F₂ and ⁵D₀ → ⁷F₃ transitions of Eu³⁺ [56]. In addition, the symmetry of the doping position of Eu³⁺ directly affects the emission intensity of ⁵D₀ → ⁷F₁ and ⁵D₀ → ⁷F₂. If the Eu³⁺ ion has a site with inversion symmetry, the ⁵D₀ → ⁷F₁ (593 nm) transition dominates, while if the Eu³⁺ ion holds a site with non-inversion symmetry, the ⁵D₀ → ⁷F₂ transition predominates. In our study, the peak at 613 nm (⁵D₀ → ⁷F₂) is the strongest, which shows that Eu³⁺ ions locate at non-inversion symmetry center in YTaO₄ [57]. It can be concluded that the luminescence intensity reaches maximum at y = 0.02 from the PL spectra of the YTaO₄:yEu³⁺ (y = 0.005–0.04) phosphors under 258 nm excitation. When Eu³⁺ concentration exceeds to 0.02, the emission intensity reduces owing to the concentration quenching effect [38].

Fig. 4.

PL excitation spectrum (left) for YTaO₄: 0.02 Eu³⁺ and emission spectra (right) for YTaO₄:yEu³⁺ (y = 0.005–0.04) samples

To obtain the multicolor tunable emission and further prove the possibility of energy transfer process of the YTaO₄:Tb³⁺, Eu³⁺ phosphors, a series of YTaO₄:xTb³⁺, 0.02Eu³⁺ (x = 0.01, 0.03, 0.05, 0.07, 0.09) phosphors were synthesized. The emission spectra of YTaO₄:xTb³⁺, 0.02Eu³⁺ (x = 0.01, 0.03, 0.05, 0.07 and 0.09) phosphors excited by 270 nm are illustrated in Fig. 5, which shows the variation of luminescence intensity and the intense characteristic emission peaks of Tb³⁺ and Eu³⁺ ions. Because of the concentration quenching, the emission intensity of Tb³⁺ increases first and then decreases with the increase of Tb³⁺content. Although the concentration of Eu³⁺ is constant, the emission of Eu³⁺ (613 nm) increases gradually in a certain range with the increase of the Tb³⁺ content, which due to the energy transfer from Tb³⁺ to Eu³⁺ ions, but then the emission of Eu³⁺ (613 nm) decreases when Tb³⁺ content is from 0.05 to 0.09, which due to the concentration quenching of Eu³⁺, the results prove that Tb³⁺ can transfer energy to Eu³⁺ [30].

Fig. 5.

Emission spectra of YTaO₄:xTb³⁺, 0.02Eu³⁺ (x = 0.01, 0.03, 0.05, 0.07, 0.09) phosphors under 270 nm excitation, and inset is variation of the normalized intensities at 547 and 613 nm emission corresponding to the Tb³⁺ content (x)

To further understand the energy transfer process between Tb³⁺ and Eu³⁺, the emission spectra of YTaO₄:0.07Tb³⁺, yEu³⁺ (y = 0.01, 0.02, 0.03, 0.04, 0.05) samples under 270 nm excitation are shown in Fig. 6. In the emission spectra of YTaO₄:0.07Tb³⁺, yEu³⁺ phosphors, we can observe the characteristic sharp peaks of Eu³⁺ and Tb³⁺ ions. As there is increase in doping concentration of Eu³⁺ (y), the emission intensity of Eu³⁺ continues to rise until y = 0.03, afterward the characteristic emission intensity of Eu³⁺ begins to decrease. In addition, when the doping concentration of Eu³⁺ goes up, the emission intensity of Tb³⁺ declines as a result of the energy transfer from Tb³⁺ to Eu³⁺ in YTaO₄ host [42].

Fig. 6.

Emission spectra of YTaO₄:0.07Tb³⁺, yEu³⁺ (y = 0.01, 0.02, 0.03, 0.04, 0.05) phosphors under 270 nm excitation, and inset is variation of the normalized intensities at 547 and 613 nm emission corresponding to the Eu³⁺ content (y)

Figure 7 shows the fluorescence lifetime of YTaO₄:0.07Tb³⁺, yEu³⁺ (y = 0, 0.01, 0.02, 0.03, 0.04, 0.05). Test data can be fitted and processed according to the following formula:

$$I=I_0+\text{A}{e}^{-t/\tau }$$

Fig. 7.

Decay curves of YTaO₄:0.07Tb³⁺, yEu³⁺ (y = 0, 0.01, 0.02, 0.03, 0.04, 0.05) phosphors under the excitation of 270 nm and monitoring at 547 nm

Here I corresponds to the fluorescence intensity at t, I₀ corresponds to the fluorescence intensity of the t = 0, and τ is on behalf of the fluorescence lifetime. The fluorescence lifetime of Tb³⁺ are 2.42, 1.73, 1.72, 1.54, 1.36 and 1.23 ms for y = 0, 0.01, 0.02, 0.03, 0.04, 0.05, respectively. With the increase of Eu³⁺ doping concentration, the fluorescence lifetime of Tb³⁺ decreases gradually as we can obviously see, which further proves that Tb³⁺ transfers energy to Eu³⁺ ions.

The energy transfer efficiency could be roughly calculated by the related luminescence intensities as the following equation:

$${\eta }_{\text{ET}}=1-\frac{I_{\text{S}}}{I_{\text{SO}}}$$

Here I_s and I_s0 correspond to the luminescence intensity of Tb³⁺ ions with and without Eu³⁺ ions, respectively. Figure 8 shows that the energy transfer efficiency increases gradually till about 86% of phosphors with the increase of Eu³⁺ content, indicating that the effective energy transfer between Tb³⁺ and Eu³⁺ is occurred in YTaO₄ host.

Fig. 8.

Energy transfer efficiency (ƞ_ET) from Tb³⁺ to Eu³⁺ in YTaO₄:0.07Tb³⁺, yEu³⁺ (y = 0–0.05) samples under 270 nm excitation

According to the energy transfer theory of Dexter and Reisfeld, the possible mechanism can be analyzed by the following formula [58]:

$$I_{\text{so}}/I_{\text{s}}\propto C^{n/3}$$

where I_so and I_s are the luminescence intensity of Tb³⁺ with or without Eu³⁺, respectively. C is the sum concentration of Tb³⁺ and Eu³⁺, and n = 6, 8, and 10 are corresponding to the dipole–dipole, dipole-quadrupole, and quadrupole–quadrupole interactions, respectively. Plots of the value of I_so/I_s and C^n/3 are depicted in Fig. 9. It is found that when n = 8, the fitting result is the best, indicating that the energy transfer mechanism is dipole-quadrupole interaction.

Fig. 9.

The dependence I_so/I_s of Tb³⁺ on the $C_{Tb+Eu}^{n/3}$ (n = 6, 8, 10) in YTaO₄:0.07Tb³⁺, yEu³⁺ (y = 0–0.05) phosphors

Energy transfer mechanism

Figure 10 describes the energy levels and energy transfer mechanism from Tb³⁺ to Eu³⁺ in YTaO₄ host which explain the polychromatic luminescence mechanism of YTaO₄:Tb³⁺, Eu³⁺ clearly. For YTaO₄:Tb³⁺, Eu³⁺ phosphors, both Tb³⁺/Eu³⁺–O²⁻ charge transfer band and TaO₄³⁻ group make contributions for the emissions of Tb³⁺ and Eu³⁺ [13]. In the whole process of excitation, electrons are transferred from O²⁻ valence to the Tb³⁺ and Eu³⁺ conduction band upon the excitation of ultraviolet light. Subsequently, the electrons transfer to ⁵D₃ state of Tb³⁺ ions and ⁵L₆ of Eu³⁺ ions, respectively. As is known to all, electrons in excited state are unstable. Therefore, the electrons of Tb³⁺ ions relax from ⁵D₃ to ⁵D₄ level through non-radiative transition, then transmit to ⁷F₆, ⁷F₅, ⁷F₄ and ⁷F₃ energy level, leading to the emission of 492 nm, 547 nm, 585 nm, and 621 nm, respectively. Similarly, the electrons of Eu³⁺ relax from ⁵L₆ to ⁵D₀ state, then go back to ⁷F₁ and ⁷F₂, corresponding to the emission of 593 nm and 613 nm, respectively. In addition, the transitions from ground state ¹A₁ of TaO₄³⁻ group to excited levels take place and TaO₄³⁻ group transfers energy to Tb³⁺ and Eu³⁺ ions leading to the characteristic emission of Tb³⁺ and Eu³⁺ ions. Meanwhile, the characteristic emission of Eu³⁺ ions can be checked, owing to that some electrons of Tb³⁺ ions make a non-radiative transition from ⁵D₄ to the ⁵D₀ of Eu³⁺ ions. Due to the manageable energy transfer efficiency, the YTaO₄:Tb³⁺, Eu³⁺ samples upon excitation of ultraviolet light can show the Tb³⁺ and Eu³⁺ emissions with different intensities at the same time. Consequently, the emission color of phosphors is adjustable.

Fig. 10.

Schematic energy-level diagram of Tb³⁺ and Eu³⁺ in YTaO₄ and the energy transfer process

Adjustable color

The luminescence properties of YTaO₄:0.07Tb³⁺ (1), YTaO₄:0.07Tb³⁺, yEu³⁺ (y = 0.01(2), 0.02 (3), 0.03 (4), 0.04 (5) and 0.05(6)) and YTaO₄:0.02Eu³⁺ (7) samples upon 270 nm excitation have been investigated in detail, and the corresponding CIE chromaticity diagrams are shown in Fig. 11. It is worth mentioning that the CIE chromaticity coordinate of YTaO₄:0.07Tb³⁺ locates in the green emission zone with 1 point (0.3119, 0.5294). The emissive color of YTaO₄:0.02Eu³⁺ is in the red emission zone and the CIE chromaticity coordinate is calculated to be (0.4165, 0.2909) (7 point). With the change of doping amount of Eu³⁺ ion, the YTaO₄:0.07Tb³⁺, yEu³⁺ (y = 0.01–0.05) phosphors can achieve color adjustable emission from green to yellow and the corresponding CIE chromaticity coordinates are calculated to be (2, 0.368, 0.5176), (3, 0.3864, 0.5142), (4, 0.3854, 0.4838), (5, 0.2634, 0.5935), (6, 0.3754, 0.4225). The results show that multi color luminescence can be achieved through adjusting the Eu³⁺/Tb³⁺ ratio.

Fig. 11.

CIE chromaticity diagram of YTaO₄:0.07Tb³⁺ (1), YTaO₄:0.07Tb³⁺, yEu³⁺ (y = 0.01(2), 0.02 (3), 0.03 (4), 0.04 (5) and 0.05(6)) and YTaO₄:0.02Eu³⁺ (7) samples upon 270 nm excitation

Conclusions

In summary, Eu³⁺-and/or Tb³⁺-doped Mʹ-YTaO₄ phosphors were prepared by a sol–gel combustion method, which needs lower temperature than before. Under excitation at 270 nm, due to the typical ⁵D₄–⁷F₅ transition of Tb³⁺, the YTaO₄:Tb³⁺ phosphors show characteristic green emission. Meanwhile, the PL intensity is strongly dependent on the Tb³⁺ doping concentration and the optimal doping concentration is determined to be 0.07. YTaO₄:Eu³⁺ phosphors show excellent red emission on account of the ⁵D₀–⁷F_{1, 2} transitions. The emission color of the samples is transformed from green to red by adjusting the doping ratio of Tb³⁺ and Eu³⁺. The energy transfers from YTaO₄ host to Tb³⁺ and Eu³⁺ and Tb³⁺–Eu³⁺ are systematically discussed. The energy transfer efficiency of Tb³⁺–Eu³⁺ in the YTaO₄:0.07Tb³⁺,0.05Eu³⁺ phosphor can reach up to about 86%. From the above experimental results, it can be concluded that the YTaO₄:Tb³⁺, Eu³⁺ phosphors have a broad application prospect in phosphor conversion LEDs.

Compliance with ethical standards

Conflict of interest

There are no conflicts of interest to declare.