Controlled Synthesis of Tb³⁺/Eu³⁺ Co-Doped Gd₂O₃ Phosphors with Enhanced Red Emission

Abstract

(Gd_0.93−xTb_0.07Eu_x)₂O₃ (x = 0–0.10) phosphors shows great potential for applications in the lighting and display areas. (Gd_0.93−xTb_0.07Eu_x)₂O₃ phosphors with controlled morphology were prepared by a hydrothermal method, followed by calcination at 1100 °C. XRD, FE-SEM, PL/PLE, luminescent decay analysis and thermal stability have been performed to investigate the Eu³⁺ content and the effects of hydrothermal conditions on the phase variation, microstructure, luminescent properties and energy transfer. Optimum excitation wavelength at ~308 nm nanometer ascribed to the 4f⁸-4f⁷5d¹ transition of Tb³⁺, the (Gd_0.93−xTb_0.07Eu_x)₂O₃ phosphors display both Tb³⁺and Eu³⁺ emission with the strongest emission band at ~611 nm. For increasing Eu³⁺ content, the Eu³⁺ emission intensity increased as well while the Tb³⁺ emission intensity decreased owing to Tb³⁺→Eu³⁺ energy transfer. The energy transfer efficiencies were calculated and the energy transfer mechanism was discussed in detail. The lifetime for both the Eu³⁺ and Tb³⁺ emission decreases with the Eu³⁺ addition, the former is due to the formation of resonant energy transfer net, and the latter is because of contribution by Tb³⁺→Eu³⁺ energy transfer. The phosphor morphology can be controlled by adjusting the hydrothermal condition (reaction pH), and the morphological influence to the luminescent properties (PL/PLE, decay lifetime, etc.) has been studied in detail.

1. Introduction

The stable physical and chemical properties of Gd₂O₃ with cubic structure make it an important inorganic compound in luminescence applications. The Gd³⁺ in Gd₂O₃ could be easily substituted by an alternative rare earth activator ion (Eu³⁺, Tb³⁺, etc.) due to their similar ion radius (Gd³⁺, Eu³⁺, and Tb³⁺ have ion radii of 1.053 Å, 1.066 Å and 1.040 Å for coordination number 8) [1]. The Eu³⁺, Tb³⁺and Dy³⁺ doped Gd₂O₃ matrix can emit vivid red, green and yellow colors, which in turn supports their use in the field of lighting and display [2,3,4].

The (Gd_0.93−xTb_0.07Eu_x)₂O₃ system was chosen in light of: (1) the luminescent properties of phosphor are greatly affected by the particle morphology and size, which relied on the synthesis route used. [5,6,7]. The hydrothermal method is usually selected to control the particle morphology and size [8,9,10], which is also applied in the preparation of Gd_0.93−xTb_0.07Eu_xO₃ systems in this work. Based on this, luminescent properties due to particle morphology and size were studied in detail; (2) due to higher ⁶I_J excited state of Gd³⁺ compared to ⁵D_3,4 and ⁵D_0,1 emission states of Tb³⁺ and Eu³⁺, the Gd³⁺ can sensitize the luminescence of Tb³⁺ and Eu³⁺ through Gd³⁺→Tb³⁺, Gd³⁺→Eu³⁺ energy transfer [11,12]. Meanwhile, Tb³⁺→Eu³⁺ energy transfer reported in numerous works can also boost Eu³⁺ red emission [13], and the energy transfer of Gd³⁺→Tb³⁺→Eu³⁺ may also occur; (3) the lower electronegativity (1.20) of Gd³⁺ compared to Y³⁺ (1.22) and Lu³⁺ (1.27) may result in easier inter- configurational transition, which can induce new properties and further improve the red emission intensity. Better luminescence features of Eu³⁺ and Tb³⁺ in Gd₂O₃ than Y₂O₃ and Lu₂O₃ lattices may then be obtained, which is further validated by experiments in this work.

In this paper, a series of (Gd_0.93−xTb_0.07Eu_x)₂O₃ (x = 0–0.10) phosphors were prepared through hydrothermal method, and the particle size and morphology were tuned by varying the reaction pH values. The phase structure, microstructure, luminescent properties, energy transfer efficiency and mechanism were analyzed by the combination of XRD, FE-SEM, PLE/PL and luminescent decay analysis. Moreover, morphology and size effect of the particle on the luminescent properties were investigated. In the sections that follow, we report in detail the synthesis, morphology/size controlled, luminescent traits, energy transfer and thermal stability of the phosphors.

2. Results and Discussion

The XRD patterns of precursors with different Eu³⁺ content are shown in Figure 1a. The diffraction peaks can be indexed as pure Gd(OH)₃ (JCPDS NO.38-1042). All the samples show the same diffraction behavior, indicating that the Eu³⁺ addition does not significantly affect the crystal structure of the precursor. Figure 1b displays the XRD patterns of (Gd_0.93−xTb_0.07Eu_x)₂O₃ (x = 0–0.10) sintered at 1100 °C as a function of Eu³⁺ content (reaction pH = 9.0, hydrothermal temperature: 140 °C). The diffraction peaks of the calcined products can be indexed as pure Gd₂O₃ phase (JCPDS NO. 43-1014) and no other phases are observed. All the samples show the same diffraction behavior indicating that the Eu³⁺ addition does not affect the crystal structure.

Figure 2 illustrates the FE-SEM images of the (Gd_0.93−xTb_0.07Eu_x)₂O₃ precursor sintered at 1100 °C with x = 0.04 (a) and x = 0.1 (b), respectively (reaction pH = 9.0, hydrothermal temperature: 140 °C). All the precursors display rod-resemble structures with diameters of ~100 nm and lengths of ~500 nm. Comparison of the FE-SEM images in Figure 2a (x = 0.04) and Figure 2b (x = 0.1) shows that the Eu³⁺ incorporation does not alter the particle morphology. The particles (Gd_0.93−xTb_0.07Eu_x)₂O₃ calcined at 1100 °C possess good dispersion and uniform morphology (Figure 2c,d), and the rod-like morphology of the precursor persists. The main variation was that the particles grew and the overall outline was clearer and more easily distinguished.

Figure 3 shows FE-SEM micrographs of (Gd_0.89Tb_0.07Eu_0.04)₂O₃ precursor synthesized at various pH values (pH 8–12, hydrothermal temperature: 140 °C). As we can see the particle morphology and size can be controlled by varying the pH value during synthesis. For the pH value of 8.0, the particles exhibit a tubular morphology (Figure 3a) with diameter of ~200 nm and length of ~800 nm. In contrast, a pH value of 9.0 results in a rod-like particle morphology (Figure 3b). The formation of tubular and rod-shaped phosphors strongly depends on the mass transfer rate. At a low pH value of 8.0, the mass transfer speed of inner part is lower than the outer region, which leads to tube formation. As the pH increased to 9.0, the mass transfer speed between inner and outer region is comparable which leads to the formation of the rod morphology. While the pH value is further adjusted from 9.0 to 12.0, the precursor size with rod-like shape gradually decreased from diameter of ~120 nm and length of ~500 nm to ~80 nm and ~100 nm, respectively. The reduction of the size is principally attributed to large nucleation density resulting from large pH value [14].

Figure 4 shows the excitation spectrum of the (Gd_0.93−xTb_0.07Eu_x)₂O₃ (x = 0.02–0.1) samples (reaction pH = 9.0, hydrothermal temperature: 140 °C, calcined temperature: 1100 °C) as a function of Eu³⁺ content at an emission wavelength of 542 nm (Tb³⁺ emission, Figure 4a) and 611 nm (Eu³⁺ emission, Figure 4b), respectively. With monitoring at 542 nm, the PLE spectra of the (Gd_0.93−xTb_0.07Eu_x)₂O₃ (x = 0.02–0.1) system displays one strong and broad peak centered at ~308 nm which is ascribed to the 4f⁸-4f⁷5d¹ transition of Tb³⁺ [15], whereas by monitoring at 611 nm (Figure 4b), the PLE spectra of (Gd_0.93−xTb_0.07Eu_x)₂O₃ phosphors contain two excitation bands at ~248 nm and ~308 nm which is ascribed to the charge transfer band (CTB) of Eu³⁺ [16] and the 4f⁸-4f⁷5d¹ transition of Tb³⁺, respectively. In addition, as we can see from the inline graph of b, the CTB excitation peak of Eu³⁺ at ~258 nm overlapped the characteristic transition ⁸S_7/2-⁶I_J of Gd³⁺ implying the Gd³⁺→Eu³⁺ energy transfer. The occurrence of Gd³⁺ and Tb³⁺ on the PLE spectra monitoring the Eu³⁺ emission provide clear information for energy transfer of the Gd³⁺→Eu³⁺ and Tb³⁺→Eu³⁺ [17,18]. Therefore, not only the Tb³⁺ but also Eu³⁺ ions can be energized at ~308 nm. The PL spectra with 308 nm excitation are analyzed and presented in Figure 4c.

The PL spectra show the strongest emission band at ~611 nm (⁵D₀-⁷F₂ transition of Eu³⁺) accompanied by other relatively weak emission bands at ~542 nm, ~580 nm, ~593 nm, ~654 nm and ~687 nm contributed to the ⁵D₄-⁷F₅ transition of Tb³⁺, ⁵D₀-⁷F₀ transition of Eu³⁺, ⁵D₀-⁷F₁ transition of Eu³⁺, ⁵D₀-⁷F₃ transition of Eu³⁺, and ⁵D₀-⁷F₄ transition of Eu³⁺, respectively [19,20,21,22]. Both the appearance of the ⁵D₀-⁷F₀ transition of Eu³⁺ and the higher emission intensity of ⁵D₀-⁷F₂ transition of Eu³⁺ (~611 nm) compared with ⁵D₀-⁷F₁ transition of Eu³⁺ (~593 nm) imply that more Eu³⁺ occupies the relatively low symmetric lattice (C₂) [23,24]. The intensity of the emission at 611 nm increases with an increasing Eu³⁺ content (up to x = 0.04), and then decreases because of the concentration quenching. Furthermore, the emission intensity of Tb³⁺ at ~542 nm (the inset in Figure 4c) decreases resulting from the energy transfer of Tb³⁺→Eu³⁺. Comparing the PL spectra of (Gd_0.89Tb_0.07Eu_0.04)₂O₃, (Gd_0.96Eu_0.04)₂O₃ and (Y_0.96Eu_0.04)₂O₃ (Figure 4c), the emission intensity is found in the order (Gd_0.89Tb_0.07Eu_0.04)₂O₃ > (Gd_0.96Eu_0.04)₂O₃ > (Y_0.96Eu_0.04)₂O₃ due to the efficient Gd³⁺→Eu³⁺ and Tb³⁺→Eu³⁺ energy transfer.

The luminescence quenching type of Eu³⁺ in solid phosphors can be obtained through evaluating the parameter s as indicated in Equation (1) [25,26,27,28]:

$$\log (\frac{I}{c})=(-\frac{s}{\mathrm{d}})\log (c)+\log f$$

(1)

where I represents the Eu³⁺ emission intensity, c is the Eu³⁺ concentration, d = 3 for a regular sample, f is a constant, and s is the electric multipole index. When values of 3, 6, 8 and 10 are assigned to s, different exchange interaction, dipole-dipole, dipole-quadrupole, and quadrupole-quadrupole electric interactions are obtained, respectively. The log(I/c)-log(c) plot that corresponds to emission at 611 nm is shown in Figure 4d. The fitted slope (−s/3) was calculated to be −1.13, thus s = 3.42 (~3) for the (Gd_0.93−xTb_0.07Eu_x)₂O₃ systems, indicating that concentration quenching is mostly caused by the energy transfer between Eu³⁺ ions [26,29].

The energy level diagram and energy transfer between Gd³⁺, Tb³⁺ and Eu³⁺ are shown in Figure 5. At 275 nm excitation, the electrons of Gd³⁺ are excited from the ⁸S_7/2 to the ⁶I_J state, then relaxed to ⁶P_7/2 state. On the other hand, UV excitation makes the electrons of Tb³⁺ and Eu³⁺ shift from the ⁷F_J (J = 3, 4, 5, 6 for Tb³⁺) and ⁷F_J (J = 0, 1, 2, 3, 4 for Eu³⁺) to the ⁵D₃ (Tb³⁺) and ⁵D₁ (Eu³⁺) states followed by relaxation to ⁵D₄ (Tb³⁺) and ⁵D₀ (Eu³⁺), respectively. Because the energy level of the ⁶P_7/2 state lies higher than the ⁵D₄ levels of Tb³⁺ and the ⁵D₀ level of Eu³⁺, the part energy of Gd³⁺ can be transferred to Tb³⁺ and Eu³⁺ [30], respectively. Meanwhile, energy transfer from Tb³⁺ to Eu³⁺ due to the higher energy level of ⁵D₄ (Tb³⁺) compared to ⁵D₀ (Eu³⁺) can happen. The electrons of ⁵D₄ (Tb³⁺) and ⁵D₀ (Eu³⁺) states jump back to the ground state ⁷F_J, thereby producing green (Tb³⁺) and red (Eu³⁺) emissions [31].

In order to calculate the energy transfer efficiency between Tb³⁺ and Eu³⁺, the luminescence decay behavior of Tb³⁺ at 542 nm was investigated using the x = 0.04 and the results are shown in Figure 6a. As we can see that the kinetics of decay follow a single exponential decay behavior:

$$I=A\exp (-\frac{t}{{\tau }_R})+B$$

(2)

where I refers to luminescence intensity, t represents the decay time τ_R denotes the lifetime and A and B are the constants [32]. The fitted result yields A = 8424.79 ± 821.77 (au), B = 100.24 ± 24.60 (au) and τ_R = 0.16 ± 0.01 ms. The lifetime values for Tb³⁺ shown in the inset of Figure 6a decrease gradually with increasing Eu³⁺ content because of energy transfer Tb³⁺→Eu³⁺. with transfer efficiency (η_ET) being obtained by evaluating the lifetime of Tb³⁺ with (τ_S) and without (τ_S0) Eu³⁺ doping through Equation (3) [33]:

$${\eta }_{ET}=1-\frac{{\tau }_S}{{\tau }_{S0}}$$

(3)

The results of energy transfer efficiency calculation are shown in Figure 6b. As can be seen, η_ET has a positive correlation with Eu³⁺ concentration where increased Eu³⁺ content, from x = 0.02 to x = 0.10, leads to gradually enhanced efficiency of energy transfer, from 89.7% to 98.7%. By consequence, the sensitizer of Tb³⁺ plays a critical part in the luminescence emission of Eu³⁺ with large η_ET value predominantly generating from substantial overlapping of spectra between the ⁵D₄→⁷F_J emissions of Tb³⁺ and the ⁷F_0,1→⁵D_0,1 absorption of Eu³⁺ [34]. Figure 6c shows the lifetime value of Eu³⁺ for 611 nm emission relative to Eu³⁺ content, through where we can see that the lifetime of Eu³⁺ decreases from 2.24 to 1.19 ms with Eu³⁺ addition from x = 0.02 to x = 0.10, resulting from the formation of a resonant energy transfer net among the activators. Figure 6d depicts the CIE chromaticity coordinates for (Gd_0.89Tb_0.07Eu_0.04)₂O₃ phosphors with 308 nm excitation. The CIE chromaticity coordinate and color temperature are determined to be (~0.64, ~0.35) and ~2439 K, respectively, as a result the phosphors gives a vivid red color.

The energy transfer mechanism between Tb³⁺→Eu³⁺ can be analyzed according to Dexter’ and Reisfeld’s theory [35,36], and the explanation is given as in the equations below:

$$\ln \frac{I_{S0}}{I_S}\propto C$$

(4)

$$\frac{I_{S0}}{I_S}\propto C^{\frac{n}{3}}$$

(5)

where C is the summed concentration of doped ions Tb³⁺ and Eu³⁺; I_S0 and I_S are the emission intensities of Tb³⁺ for 542 nm emission with and without Eu³⁺; lnI_s0/I_s-C corresponds to exchange interactions, and lnI_s0/I_s-C^n/3 for n = 6, 8, 10 represent the dipole-dipole, dipole-quadrupole and quadrupole-quadrupole electric interactions, respectively. The plots of lnI_s0/I_s-C and lnI_s0/I_s-C^n/3 are illustrated in Figure 7. By comparing the fitted factor values (R), the best linear relationship was found for n = 10, which clearly shows energy transfer from Tb³⁺→Eu³⁺ in the (Gd_1−xTb_0.07Eu_x)₂O₃ phosphor is dominated by quadrupole-quadrupole electric interactions [27].

Considering that the change of hydrothermal pH values can alter the particle morphology (Figure 3), and the shape/size has a significant effect on the luminescent properties, we investigated the PL spectra of the (Gd_0.89Tb_0.07Eu_0.04)₂O₃ sample as a function of pH value (pH = 8–12, Figure 8a, hydrothermal temperature: 140 °C, calcined temperature: 1100 °C). From Figure 8, we can conclude that the pH value variation has no influence to the shape of the emission peak, however it affects the emission intensity of Eu³⁺ dramatically. The emission intensity first decreases with the increasing pH till pH = 9.0. Thereafter it increases as the pH further increases up to 12.0. When the pH varies from 8.0 to 9.0, and the particle morphology changes from tubular to rods, with the latter presenting directional growth as described in Figure 3b–e. The phosphors with rod-like morphology could decrease the electric dipole transition probabilities of Eu³⁺, therefore decreasing the luminescence intensity [28]. For pH changing from 9.0 to 12.0, the particle dimension progressively decreases while the surface area gradually increases. As a result, the luminescent center number on the particle surface increases leading to an improved intensity of emission.

Figure 8b displays the lifetime values of the 611 nm emission with different synthesis pH values. The lifetime increases from 1.42 to 2.02 ms with the pH increasing from 8.0 to 12.0. The extended lifetime can be expressed via Equation (6) [25,37]:

$${\tau }_R~\frac{1}{f(ED)}\frac{{\lambda }_0^2}{{[\frac{1}{3}(n_{eff}{}^2+2)]}^2{n}_{eff}}$$

(6)

where f(ED) and λ₀ are represent the dipole transition oscillator strength and the wavelength in vacuum, respectively. n_eff is the effective refractive index which is influenced by the particle size and decreases for smaller particles when applied to intermediately-sized particles as in this work. Thus, the n_eff decreased at a larger given pH value, and a longer lifetime was obtained. The influences of the defects of lattice on luminescent lifetime, nevertheless, can in no way be totally excluded. Deep traps are believed to be capable of arresting electrons temporarily, thus leading to a longer lifetime.

The thermal stability for phosphor materials is an important parameter for its potential application. The influences of temperature variation to the intensity of emission was investigated in the range of 298–523 K using (Gd_0.89Tb_0.07Eu_0.04)₂O₃ as an example (reaction pH = 8.0, hydrothermal temperature: 140 °C, calcination temperature: 1100 °C), and the activation energy was also calculated in this work. Owing to the thermal quenching, the emission intensity of (Gd_0.89Tb_0.07Eu_0.04)₂O₃ phosphor decreased with increasing temperature (Figure 9a). The temperature resulted thermal quenching can be explained using Arrhenius equation [27,38]:

$$\ln (\frac{I_0}{I}-1)=\ln A-\frac{E_a}{kT}$$

(7)

where E_a is the activation energy, T denotes temperature, A is a constant and k refers to the Boltzmann constant. I₀ is the emission intensity at room temperature while I corresponds to the emission intensity at the related operating temperature. The variation of ln[(I₀−I)/I] in terms of 1/kT for the thermal quenching is shown in Figure 9b. The slope of the fitting curve is −0.211, which corresponds to the E_a value of 0.211 eV being almost the same as the 0.212 eV value for the Gd₂O₃:Dy³⁺/Eu³⁺ system [39,40]. The larger activation energy means that the synthesized phosphor has a more stable thermal stability compared to other reported phosphors and can be potentially used in lighting and display areas [41].

3. Summary

Pure-phase (Gd_0.93−xTb_0.07Eu_x)₂O₃ (x = 0.02–0.1) phosphors with controlled morphology were synthesized by hydrothermal method, followed by calcination. The combined technologies of XRD, FE-SEM, PLE/PL, decay behavior and thermal stability have been applied to analyze the products. The analysis results can be summarized as follows:

(1)

Increasing Eu³⁺ content does not change the particle morphology, but both the particle shape and size can be controlled by tuning the pH value used in the hydrothermal synthesis. The particle morphology varies from tubular to rod-like when the pH value increases from 8.0 to 9.0. The rod-like particle size decreases with the pH value when increased from 9.0 to 12.0;

(2)

(Gd_0.93−xTb_0.07Eu_x)₂O₃ phosphors exhibit a vivid red emission with a CIE chromaticity coordinate and color temperature of (~0.64, ~0.35) and ~2439 K, respectively. The quenching concentration was x = 0.04, and determined to be due to energy transfer between Eu³⁺. Comparing to the (Gd_0.96Eu_0.04)₂O₃ and (Y_0.96Eu_0.04)₂O₃ oxides, the (Gd_0.89Tb_0.07Eu_0.04)₂O₃ possesses better luminescent properties due to Tb³⁺→Eu³⁺, Gd³⁺→Eu³⁺ energy transfer;

(3)

The influence of particle shape or size on the luminescence features, e.g. PLE/PL, lifetime, of resultant phosphors was investigated. The related energy transfer efficiency, mechanism, process and thermal stability were also analyzed in detail.

4. Experimental Procedures

The chemical reagents used in the synthesis include rare earth oxides (Gd₂O₃, Tb₄O₇, and Eu₂O₃, 99.99% pure, Jining Zhongkai New Type Material Science Co. Ltd, Jining, China), ammonia (NH₃∙H₂O, analytical grade 25 wt%) and nitric acid (HNO₃, analytical grade 68 wt%). Both acids were purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). All reagents were utilized as starting material with no additional purification.

The whole synthesis process is shown in Figure 10. The rare earth nitrates RE(NO₃)₃ (RE = Gd, Tb, Eu) were provided via dissolving the corresponding oxides, Gd₂O₃, Tb₄O₇ and Eu₂O₃, in hot nitric acid. RE(NO₃)₃ was mixed as mother salt and stirred for 30 minutes according to the stoichiometric ratio (Gd_0.93−xTb_0.07Eu_x)₂O₃. Ammonia was used to adjust the pH of the mother salt, and the resulting turbid liquid was aged for 30 min. The turbid liquids were transferred to an autoclave and heated in an oven for 24 h. Upon completion of the reaction, the suspension was cooled to room temperature, followed by centrifugation and repeated washing using distilled water and alcohol to give a precipitate. The wet precipitate was dried at 180 °C for 24 h in air. The precursors were firstly decomposed at 600 °C for 4 h in the air, and then calcined at 1100 °C for 4 h in Ar/H₂ (5 vol.% H₂) gas mixture to obtain the resultant oxides. The Eu³⁺ content (x = 0–0.10) and reaction pH (pH = 8.0–12.0) were varied to study their effects on the particle morphology and size.

Phosphor phases were identified by X-ray diffractometry (XRD, Model PW3040/60, PANALYTICAL B.V, Almelo, The Netherlands) with nickel-filtered CuKα radiation and a 4° 2θ/min scanning speed. Particle morphological distribution was studied by field-emission scanning electron microscopy (FE-SEM, Model JSM-7001F, JEOL, Tokyo, Japan). Photoluminescence excitation (PLE) and photoluminescence (PL) spectra of the phosphors were collected by a FP-6500 fluorospectrophotometer (JASCO, Tokyo, Japan) at room temperature, which has an integrating sphere (Model ISF-513, JASCO) of diameter of 60 mm and an excitation source, Xe lamp, 150 W. The decay kinetic of Eu³⁺ and Tb³⁺ emission was acquired at room temperature. By exciting the phosphor powder at a chosen wavelength, the emission intensity was detected as to the elapsed time immediately after the excitation light was blocked by a shutter.