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

(Gd0.93−xTb0.07Eux)2O3 (x = 0–0.10) phosphors shows great potential for applications in the lighting and display areas. (Gd0.93−xTb0.07Eux)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 (Gd0.93−xTb0.07Eux)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.


Introduction
The stable physical and chemical properties of Gd 2 O 3 with cubic structure make it an important inorganic compound in luminescence applications. The Gd 3+ in Gd 2 O 3 could be easily substituted by an alternative rare earth activator ion (Eu 3+ , Tb 3+ , etc.) due to their similar ion radius (Gd 3+ , Eu 3+ , and Tb 3+ have ion radii of 1.053 Å, 1.066 Å and 1.040 Å for coordination number 8) [1]. The Eu 3+ , Tb 3+ and Dy 3+ doped Gd 2 O 3 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−x Tb 0.07 Eu x ) 2 O 3 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−x Tb 0.07 Eu x O 3 systems in this work. Based on this, luminescent properties due to particle morphology and size were studied in detail; (2) due to higher 6 I J excited state of Gd 3+ compared to 5 D 3,4 and 5 D 0,1 emission states of Tb 3+ and

Results and Discussion
The XRD patterns of precursors with different Eu 3+ content are shown in Figure 1a. The diffraction peaks can be indexed as pure Gd(OH) 3 (JCPDS NO. . All the samples show the same diffraction behavior, indicating that the Eu 3+ addition does not significantly affect the crystal structure of the precursor. Figure 1b displays the XRD patterns of (Gd 0.93−x Tb 0.07 Eu x ) 2 O 3 (x = 0-0.10) sintered at 1100 • C as a function of Eu 3+ content (reaction pH = 9.0, hydrothermal temperature: 140 • C). The diffraction peaks of the calcined products can be indexed as pure Gd 2 O 3 phase (JCPDS NO. 43-1014) and no other phases are observed. All the samples show the same diffraction behavior indicating that the Eu 3+ addition does not affect the crystal structure.
Molecules 2019, 24, x FOR PEER REVIEW 2 of 12 [11,12]. Meanwhile, Tb 3+ →Eu 3+ energy transfer reported in numerous works can also boost Eu 3+ red emission [13], and the energy transfer of Gd 3+ → Tb 3+ → Eu 3+ may also occur; (3) the lower electronegativity (1.20) of Gd 3+ compared to Y 3+ (1.22) and Lu 3+ (1.27) may result in easier interconfigurational transition, which can induce new properties and further improve the red emission intensity. Better luminescence features of Eu 3+ and Tb 3+ 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 (Gd0.93 −xTb0.07Eux)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.

Results and Discussion
The XRD patterns of precursors with different Eu 3+ content are shown in Figure 1a. The diffraction peaks can be indexed as pure Gd(OH)3 (JCPDS NO. . All the samples show the same diffraction behavior, indicating that the Eu 3+ addition does not significantly affect the crystal structure of the precursor. Figure 1b displays the XRD patterns of (Gd0.93−xTb0.07Eux)2O3 (x = 0-0.10) sintered at 1100 °C as a function of Eu 3+ 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 Eu 3+ addition does not affect the crystal structure.  Figure 2 illustrates the FE-SEM images of the (Gd0.93−xTb0.07Eux)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 Eu 3+ incorporation does not alter the particle morphology. The particles (Gd0.93−xTb0.07Eux)2O3 calcined at 1100 °C possess good dispersion and uniform morphology (Figure 2c and 2d), 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 (Gd0.89Tb0.07Eu0.04)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 3 shows FE-SEM micrographs of (Gd 0.89 Tb 0.07 Eu 0.04 ) 2 O 3 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 3 shows FE-SEM micrographs of (Gd0.89Tb0.07Eu0.04)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−x Tb 0.07 Eu x ) 2 O 3 (x = 0.02-0.1) samples (reaction pH = 9.0, hydrothermal temperature: 140 • C, calcined temperature: 1100 • C) as a function of Eu 3+ content at an emission wavelength of 542 nm (Tb 3+ emission, Figure 4a) and 611 nm (Eu 3+ emission, Figure 4b), respectively. With monitoring at 542 nm, the PLE spectra of the (Gd 0.93−x Tb 0.07 Eu x ) 2 O 3 (x = 0.02-0.1) system displays one strong and broad peak centered at~308 nm which is ascribed to the 4f 8 -4f 7 5d 1 transition of Tb 3+ [15], whereas by monitoring at 611 nm (Figure 4b), the PLE spectra of (Gd 0.93−x Tb 0.07 Eu x ) 2 O 3 phosphors contain two excitation bands at~248 nm and 308 nm which is ascribed to the charge transfer band (CTB) of Eu 3+ [16] and the 4f 8 -4f 7 5d 1 transition of Tb 3+ , respectively. In addition, as we can see from the inline graph of b, the CTB excitation peak of Eu 3+ at~258 nm overlapped the characteristic transition 8 S 7/2 -6 I J of Gd 3+ implying the Gd 3+ →Eu 3+ energy transfer. The occurrence of Gd 3+ and Tb 3+ on the PLE spectra monitoring the Eu 3+ emission provide clear information for energy transfer of the Gd 3+ →Eu 3+ and Tb 3+ →Eu 3+ [17,18]. Therefore, not only the Tb 3+ but also Eu 3+ ions can be energized at~308 nm. The PL spectra with 308 nm excitation are analyzed and presented in Figure 4c.  Figure 4 shows the excitation spectrum of the (Gd0.93−xTb0.07Eux)2O3 (x = 0.02-0.1) samples (reaction pH = 9.0, hydrothermal temperature: 140 °C, calcined temperature: 1100 °C) as a function of Eu 3+ content at an emission wavelength of 542 nm (Tb 3+ emission, Figure 4a) and 611 nm (Eu 3+ emission, Figure 4b), respectively. With monitoring at 542 nm, the PLE spectra of the (Gd0.93−xTb0.07Eux)2O3 (x = 0.02-0.1) system displays one strong and broad peak centered at ~308 nm which is ascribed to the 4f 8 -4f 7 5d 1 transition of Tb 3+ [15], whereas by monitoring at 611 nm (Figure 4b), the PLE spectra of (Gd0.93−xTb0.07Eux)2O3 phosphors contain two excitation bands at ~248 nm and ~308 nm which is ascribed to the charge transfer band (CTB) of Eu 3+ [16] and the 4f 8 -4f 7 5d 1 transition of Tb 3+ , respectively. In addition, as we can see from the inline graph of b, the CTB excitation peak of Eu 3+ at ~258 nm overlapped the characteristic transition 8 S7/2-6 IJ of Gd 3+ implying the Gd 3+ →Eu 3+ energy transfer. The occurrence of Gd 3+ and Tb 3+ on the PLE spectra monitoring the Eu 3+ emission provide clear information for energy transfer of the Gd 3+ →Eu 3+ and Tb 3+ →Eu 3+ [17,18]. Therefore, not only the Tb 3+ but also Eu 3+ ions can be energized at ~308 nm. The PL spectra with 308 nm excitation are analyzed and presented in Figure 4c.  Figure (c) shows the emission spectra (λex = 308 nm) of (Gd0.93−xTb0.07Eux)2O3 (x = 0.02-0.1), the (Gd0.96Eu0.04)2O3 (λex = 258 nm) and (Y0.96Eu0.04)2O3 (λex = 258 nm) were included for comparison. Inset is excitation spectra corresponding to Gd1.78Tb0.14Eu0.08O3, Gd1.92Eu0.08O3 and Y1.92Eu0.08O3 with emission peak of 611 nm. The inset in (c) is the enlarged graph of the Tb 3+ emission peak. Figure ( The PL spectra show the strongest emission band at ~611 nm ( 5 D0-7 F2 transition of Eu 3+ ) accompanied by other relatively weak emission bands at ~542 nm, ~580 nm, ~593 nm, ~654 nm and ~687 nm contributed to the 5 D4-7 F5 transition of Tb 3+ , 5 D0-7 F0 transition of Eu 3+ , 5 D0-7 F1 transition of Eu 3+ , 5 D0-7 F3 transition of Eu 3+ , and 5 D0-7 F4 transition of Eu 3+ , respectively [19][20][21][22]. Both the appearance of the 5 D0-7 F0 transition of Eu 3+ and the higher emission intensity of 5 D0-7 F2 transition of Eu 3+ (~611 nm) The PL spectra show the strongest emission band at~611 nm ( 5 D 0 -7 F 2 transition of Eu 3+ ) accompanied by other relatively weak emission bands at~542 nm,~580 nm,~593 nm,~654 nm and~687 nm contributed to the 5 D 4 -7 F 5 transition of Tb 3+ , 5 D 0 -7 F 0 transition of Eu 3+ , 5 D 0 -7 F 1 transition of Eu 3+ , 5 D 0 -7 F 3 transition of Eu 3+ , and 5 D 0 -7 F 4 transition of Eu 3+ , respectively [19][20][21][22]. Both the appearance of the 5 D 0 -7 F 0 transition of Eu 3+ and the higher emission intensity of 5 D 0 -7 F 2 transition of Eu 3+ (~611 nm) compared with 5 D 0 -7 F 1 transition of Eu 3+ (~593 nm) imply that more Eu 3+ occupies the relatively low symmetric lattice (C 2 ) [23,24]. The intensity of the emission at 611 nm increases with an increasing Eu 3+ content (up to x = 0.04), and then decreases because of the concentration quenching. Furthermore, the emission intensity of Tb 3+ at~542 nm (the inset in Figure 4c) decreases resulting from the energy transfer of Tb 3+ →Eu 3+ . Comparing the PL spectra of (Gd 0.89 Tb 0.07 Eu 0.04 ) 2  The luminescence quenching type of Eu 3+ in solid phosphors can be obtained through evaluating the parameter s as indicated in Equation (1) [25][26][27][28]: where I represents the Eu 3+ emission intensity, c is the Eu 3+ 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.  [26,29]. The energy level diagram and energy transfer between Gd 3+ , Tb 3+ and Eu 3+ are shown in Figure 5. At 275 nm excitation, the electrons of Gd 3+ are excited from the 8 S 7/2 to the 6 I J state, then relaxed to 6 P 7/2 state. On the other hand, UV excitation makes the electrons of Tb 3+ and Eu 3+ shift from the 7 F J (J = 3, 4, 5, 6 for Tb 3+ ) and 7 F J (J = 0, 1, 2, 3, 4 for Eu 3+ ) to the 5 D 3 (Tb 3+ ) and 5 D 1 (Eu 3+ ) states followed by relaxation to 5 D 4 (Tb 3+ ) and 5 D 0 (Eu 3+ ), respectively. Because the energy level of the 6 P 7/2 state lies higher than the 5 D 4 levels of Tb 3+ and the 5 D 0 level of Eu 3+ , the part energy of Gd 3+ can be transferred to Tb 3+ and Eu 3+ [30], respectively. Meanwhile, energy transfer from Tb 3+ to Eu 3+ due to the higher energy level of 5 D 4 (Tb 3+ ) compared to 5 D 0 (Eu 3+ ) can happen. The electrons of 5 D 4 (Tb 3+ ) and 5 D 0 (Eu 3+ ) states jump back to the ground state 7 F J , thereby producing green (Tb 3+ ) and red (Eu 3+ ) emissions [31].
The luminescence quenching type of Eu 3+ in solid phosphors can be obtained through evaluating the parameter s as indicated in Equation (1) where I represents the Eu 3+ emission intensity, c is the Eu 3+ 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 (Gd0.93−xTb0.07Eux)2O3 systems, indicating that concentration quenching is mostly caused by the energy transfer between Eu 3+ ions [26,29]. The energy level diagram and energy transfer between Gd 3+ , Tb 3+ and Eu 3+ are shown in Figure  5. At 275 nm excitation, the electrons of Gd 3+ are excited from the 8 S7/2 to the 6 IJ state, then relaxed to 6 P7/2 state. On the other hand, UV excitation makes the electrons of Tb 3+ and Eu 3+ shift from the 7 FJ (J = 3, 4 ,5, 6 for Tb 3+ ) and 7 FJ (J = 0, 1, 2, 3, 4 for Eu 3+ ) to the 5 D3 (Tb 3+ ) and 5 D1 (Eu 3+ ) states followed by relaxation to 5 D4 (Tb 3+ ) and 5 D0 (Eu 3+ ), respectively. Because the energy level of the 6 P7/2 state lies higher than the 5 D4 levels of Tb 3+ and the 5 D0 level of Eu 3+ , the part energy of Gd 3+ can be transferred to Tb 3+ and Eu 3+ [30], respectively. Meanwhile, energy transfer from Tb 3+ to Eu 3+ due to the higher energy level of 5 D4 (Tb 3+ ) compared to 5 D0 (Eu 3+ ) can happen. The electrons of 5 D4 (Tb 3+ ) and 5 D0 (Eu 3+ ) states jump back to the ground state 7 FJ, thereby producing green (Tb 3+ ) and red (Eu 3+ ) emissions [31].  In order to calculate the energy transfer efficiency between Tb 3+ and Eu 3+ , the luminescence decay behavior of Tb 3+ 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: 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 Tb 3+ shown in the inset of Figure 6a decrease gradually with increasing Eu 3+ content because of energy transfer Tb 3+ →Eu 3+ . with transfer efficiency (η ET ) being obtained by evaluating the lifetime of Tb 3+ with (τ S ) and without (τ S0 ) Eu 3+ doping through Equation (3) [33]: Molecules 2019, 24, x FOR PEER REVIEW 6 of 12 In order to calculate the energy transfer efficiency between Tb 3+ and Eu 3+ , the luminescence decay behavior of Tb 3+ 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: where I refers to luminescence intensity, t represents the decay time τR 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 τR = 0.16 ± 0.01 ms. The lifetime values for Tb 3+ shown in the inset of Figure 6a decrease gradually with increasing Eu 3+ content because of energy transfer Tb 3+ →Eu 3+ . with transfer efficiency (ηET) being obtained by evaluating the lifetime of Tb 3+ with (τS) and without (τS0) Eu 3+ doping through Equation  The results of energy transfer efficiency calculation are shown in Figure 6b. As can be seen, ηET has a positive correlation with Eu 3+ concentration where increased Eu 3+ 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 Tb 3+ plays a critical part in the luminescence emission of Eu 3+ with large ηET value The results of energy transfer efficiency calculation are shown in Figure 6b. As can be seen, η ET has a positive correlation with Eu 3+ concentration where increased Eu 3+ 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 Tb 3+ plays a critical part in the luminescence emission of Eu 3+ with large η ET value predominantly generating from substantial overlapping of spectra between the 5 D 4 → 7 F J emissions of Tb 3+ and the 7 F 0,1 → 5 D 0,1 absorption of Eu 3+ [34]. Figure 6c shows the lifetime value of Eu 3+ for 611 nm emission relative to Eu 3+ content, through where we can see that the lifetime of Eu 3+ decreases from 2.24 to 1.19 ms with Eu 3+ 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 0.89 Tb 0.07 Eu 0.04 ) 2 O 3 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 Tb 3+ →Eu 3+ can be analyzed according to Dexter' and Reisfeld's theory [35,36], and the explanation is given as in the equations below: where C is the summed concentration of doped ions Tb 3+ and Eu 3+ ; I S0 and I S are the emission intensities of Tb 3+ for 542 nm emission with and without Eu 3+ ; 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 Tb 3+ →Eu 3+ in the (Gd 1−x Tb 0.07 Eu x ) 2 O 3 phosphor is dominated by quadrupole-quadrupole electric interactions [27].
Molecules 2019, 24, x FOR PEER REVIEW 7 of 12 predominantly generating from substantial overlapping of spectra between the 5 D4→ 7 FJ emissions of Tb 3+ and the 7 F0,1→ 5 D0,1 absorption of Eu 3+ [34]. Figure 6c shows the lifetime value of Eu 3+ for 611 nm emission relative to Eu 3+ content, through where we can see that the lifetime of Eu 3+ decreases from 2.24 to 1.19 ms with Eu 3+ 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 (Gd0.89Tb0.07Eu0.04)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 Tb 3+ →Eu 3+ can be analyzed according to Dexter' and Reisfeld's theory [35,36], and the explanation is given as in the equations below: where C is the summed concentration of doped ions Tb 3+ and Eu 3+ ; IS0 and IS are the emission intensities of Tb 3+ for 542 nm emission with and without Eu 3+ ; lnIs0/Is-C corresponds to exchange interactions, and Is0/Is-C n/3 for n = 6, 8, 10 represent the dipole-dipole, dipole-quadrupole and quadrupole-quadrupole electric interactions, respectively. The plots of lnIs0/Is-C and Is0/Is-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 Tb 3+ →Eu 3+ in the (Gd1−xTb0.07Eux)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 (Gd0.89Tb0.07Eu0.04)2O3 sample as a function of pH value (pH = 8-12, Figure 8a, 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 0.89 Tb 0.07 Eu 0.04 ) 2 O 3 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 Eu 3+ 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 Eu 3+ , 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. 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 Eu 3+ 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 Eu 3+ , 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.  where f(ED) and λ0 are represent the dipole transition oscillator strength and the wavelength in vacuum, respectively. neff 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 neff 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 (Gd0.89Tb0.07Eu0.04)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 (Gd0.89Tb0.07Eu0.04)2O3 phosphor decreased with increasing temperature (Figure 9a). The temperature resulted thermal quenching can be explained using Arrhenius equation [27,38]: where f (ED) and λ 0 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 0.89 Tb 0.07 Eu 0.04 ) 2 O 3 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 0.89 Tb 0.07 Eu 0.04 ) 2 O 3 phosphor decreased with increasing temperature (Figure 9a). The temperature resulted thermal quenching can be explained using Arrhenius equation [27,38]: where E a is the activation energy, T denotes temperature, A is a constant and k refers to the Boltzmann constant. I 0 is the emission intensity at room temperature while I corresponds to the emission intensity at the related operating temperature. The variation of ln[(I 0 −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 Gd 2 O 3 :Dy 3+ /Eu 3+ 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].
where Ea 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 Ea value of 0.211 eV being almost the same as the 0.212 eV value for the Gd2O3:Dy 3+ /Eu 3+ 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].

Summary
Pure-phase (Gd0.93−xTb0.07Eux)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 Eu 3+ 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) (Gd0.93−xTb0.07Eux)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 Eu 3+ . Comparing to the (Gd0.96Eu0.04)2O3 and (Y0.96Eu0.04)2O3 oxides, the (Gd0.89Tb0.07Eu0.04)2O3 possesses better luminescent properties due to Tb 3+ →Eu 3+ , Gd 3+ →Eu 3+ 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.

Summary
Pure-phase (Gd 0.93−x Tb 0.07 Eu x ) 2 O 3 (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 Eu 3+ 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; (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.

Experimental Procedures
The chemical reagents used in the synthesis include rare earth oxides ( The whole synthesis process is shown in Figure 10. The rare earth nitrates RE(NO 3 ) 3 (RE = Gd, Tb, Eu) were provided via dissolving the corresponding oxides, Gd 2 O 3 , Tb 4 O 7 and Eu 2 O 3 , in hot nitric acid. RE(NO 3 ) 3 was mixed as mother salt and stirred for 30 minutes according to the stoichiometric ratio (Gd 0.93−x Tb 0.07 Eu x ) 2 O 3 . 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/H 2 (5 vol.% H 2 ) gas mixture to obtain the resultant oxides. The Eu 3+ 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. 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 (Gd0.93−xTb0.07Eux)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 Eu 3+ 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 Eu 3+ and Tb 3+ 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.  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 Eu 3+ and Tb 3+ 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.

Conflicts of Interest:
The authors declare no conflict of interest.