Introduction

Rare-earth oxides (RE2O3, RE = rare-earth element) as the host materials have been studied widely by lots of researchers, including their synthesis and luminescent behaviors in the past few years. Owing to their unique physical and chemical properties [1,2,3], the rare-earth (RE) activated ions-doped RE2O3 oxide has been widely applied in the white LEDs, cathode-ray tubes (CRT), field-emission displays (FED), electroluminescent devices (ED), plasma display panels (PDP) and so on [4,5,6]. Among the rare-earth oxide RE2O3, the Gd2O3 might be the best known. The Gd2O3 with cubic-structured (space group: \( Ia\bar{3} \)) is the promising host material which is ascribed to its low photon energy and high refractive index [7, 8]. The Gd3+ in Gd2O3 systems can be easily replaced by the activated ions (such as Eu3+, Tb3+, Dy3+ and so on) and exhibit the colorful emission under the UV excitation. The Eu3+ and Tb3+ ions-activated Gd2O3 are the famous red and green-emitting phosphors, respectively, owing to their simple chemical composition, excellent luminescent efficiency, high color purity and so on [9,10,11].

The (Gd0.98−xTb0.02Eu x )2O3 systems have been chosen in this paper according to the following main reasons: (1) the luminescent properties of the phosphors heavily depend on the particle size and morphology [12,13,14] which in turn depend on the synthesis route used [15]. The phosphors with spheres, nanorods, nanowires, nanoplates, and flower plates morphologies have been widely prepared by various methods in recent years, such as the complexing-agent-assisted hydrothermal process [12], the homogeneous precipitation method [16], the carbonate precipitation [17] and so on. Until so far, the Gd2O3:Tb3+/Eu3+ phosphors which are prepared by the urea-based homogeneous precipitation combined with the polyol method have not been widely studied yet. In addition, the ratio of ethylene glycol/deionized water (EG/DI) plays an important role in the morphology and luminescent property of the phosphors which has been demonstrated in the preparation of (Y,Gd)2O3:Eu3+ microsheet solution using the continue microwave irradiation [18, 19]. Based on these considerations, this work proposed a facile urea-based homogeneous precipitation combined with the polyol method to prepare the Gd2O3:Tb3+/Eu3+ phosphors with different sizes as well as morphologies, and the morphology/size can be efficiently controlled by adjusting the ratio of EG/DI content; (2) the emission intensity can be improved via the energy transfer between the rare-earth ions. The Tb3+ ions have been frequently used as the sensitizer to enhance the Eu3+ emission in the Gd3Al5O12:Tb/Eu [20], Y2O3:Tb/Eu [21], Lu2O3:Tb/Eu [22] phosphor systems and so on. Furthermore, the green emission of Tb3+ (5D4 → 7F1,2 transition of Tb3+) and red emission of Eu3+ (5D0 → 7F1,2 transition of Eu3+) can be sensitized by the efficient Gd3+ → Tb3+ and Gd3+ → Eu3+ energy transfers, respectively [23, 24]. It can be expected that the luminescent properties of Eu3+ and Tb3+ in Gd2O3 systems will be better than in the Y2O3 and Lu2O3 host matrices, which has been verified in the following experiments in this paper. Based on these, it can predict that the efficient Gd3+ → Tb3+, Gd3+ → Eu3+ and Tb3+ → Eu3+ energy transfers exist in (Gd0.98−xTb0.02Eu x )2O3 systems. It is reported that the part or whole energy of Tb3+ could be transferred to the Eu3+, further enhances the red emission of Eu3+ [20], which can allow the emission color to be readily tuned through adjusting the Eu3+ concentration [25,26,27,28]; (3) the Gd3+ has more smaller electro-negativity (1.20) than that of the Y3+ (1.22) and the Lu3+ (1.27), which may result in an easier charge transfer (CT) and enhance the emission intensity [29, 30]; (4) the luminescent mechanism especially the thermal stability, activation energy and energy transfer mechanism rarely reported in previous works has been investigated in detail in the present work.

The (Gd0.98Tb0.02Eu0.02)2O3 samples with different morphologies and particle sizes have been successfully prepared by the urea-based homogeneous precipitation combined with the polyol method. The particle size and morphology can be effectively controlled by adjusting the ratios of EG/DI content, and the emission color has been known to be tuned though varying the content of Eu3+ [25,26,27,28]. The properties of precursors and resultant products have been systematically investigated using the combined techniques of XRD, XPS, ICP-OES, FE-SEM, HR-TEM, PLE/PL spectroscopy and fluorescence decay analysis. In the following sections, we report the synthesis, morphology evolution, precipitation kinetic, and the luminescent properties (PLE/PL, decay behavior, thermal stable, activation energy, etc.) of the Gd2O3:Tb3+/Eu3+ phosphors.

Experimental section

Materials

The starting chemicals used in the present work mainly contain the Gd2O3, Tb4O7 and Eu2O3 (99.99% pure, Huizhou Ruier Rare Chemical Hi-Tech Co. Ltd., Huizhou, China), HOCH2CH2OH (AR, Tianjin Kermel Chemical Reagent Co. Ltd., Tianjin, China), CO(NH2)2·12H2O (AR, Sinopharm Chemical Reagent Co. Ltd., Shanghai, China), and HNO3 (AR, Sinopharm Chemical Reagent Co. Ltd., Shanghai, China). All the chemicals were used as received without further purification.

Preparation procedure

The rare-earth nitrates RE(NO3)3 (RE = Gd, Tb and Eu) were obtained via dissolving the Gd2O3, Tb4O7 and Eu2O3 in the hot HNO3. The mother salt solution was achieved though mixing the RE(NO3)3 solutions according to the formula of (Gd0.98−xTb0.02Eu x )2O3. Adding the CO(NH2)2·12H2O and different solvent (pure DI water or mixed solution of DI water and EG) to the mother solution and the total volume were kept at 500 mL. The mixed solutions were firstly homogenized under stirring for 60 min at 25 °C and then heated to 90 ± 1 °C within 60 min. Keeping the temperature at 90 ± 1 °C for 2 h, the precipitations were collected by centrifugation, washed repeatedly with DI water and alcohol, and dried in the air at 80 °C for 12 h. The dried precursors were firstly calcined in the air at 600 °C for 4 h to produce oxides and then calcined at 1000 °C for 2 h in N2/H2 (80 vol% N2) gas mixture. The total contents of Gd3+, Tb3+ and Eu3+ all were 0.015 mol/L in each case. The EG volume content, expressed as y = EG/(EG + DI) volume (y = 0, 0.1, 0.2, 0.3, 0.4, 0.5), was varied to reveal EG addition effect on the particle morphology and size.

Characterization

The phase evolution was measured by the powder X-ray diffraction (XRD) using nickel-filtered CuKα radiation in the 2θ range 10°–50° at a scan speed of 4.0° 2θ/min (Model D8 ADVANCE, BRUKER Co., Germany). The chemical states of the constituents were performed by using the X-ray photoelectron spectroscopy (XPS) with an MgK X-ray source, energy of 1220 eV, and the operating voltage of at 10 kV (ESCALAB 250 XI, Thermo Scientific, America). The cation contents of the samples were revealed by ICP-OES analysis (Model SPS3520DD-UV, SII Technologies, Chiba, Japan) with a detection limit of 0.01 wt%, following standard procedures, and the average of three measurements was used to denote the content for each element. The morphologies of the synthesized precursors and oxides were performed by a field-emission scanning electron microscope (FE-SEM) with an acceleration voltage of 10 kV (QUANTA FEG 250, FEI Co., America). The micromechanism of phosphors was performed by HR-Transmission Electron Microscope (HR-TEM) with the 200 kV acceleration voltage (JEM-2100F, JEOL, Japan). The PLE/PL spectra of the (Gd0.98−xTb0.02Eu x )2O3 samples were obtained by a Fluorescence Spectrophotometer (FP-6500, JASCO Co., Japan) at room temperature equipped with a Φ60-mm intergating sphere (ISF-513, JASCO, Tokyo, Japan), and the excitation source is the 150-W Xe-lamp.

Results and discussion

The morphologies of (Gd0.98−xTb0.02Eu x )2O3 (x = 0–0.05) precursors using pure water as the reaction solvent are shown in Fig. 1. It can be seen that all the precursors have the similar average particle size of ~ 300 nm, regardless of different Eu3+ content; the samples exhibit good dispersion and spherical morphology. The reason may be that the reaction has the similar nucleation rate of precipitation process, which is ascribed to the concentrations of the urea and Tb3+ ions were fixed, and the concentrations of Gd and Eu varied little.

Figure 1
figure 1

FE-SEM of (Gd0.98−xTb0.02Eu x )2O3 precursors with different Eu contents: a x = 0, b x = 0.001, c x = 0.005, d x = 0.01, e x = 0.02, f x = 0.03, g x = 0.05, respectively

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The formation of (Gd0.98−xTb0.02Eu x )2O3 solid solution depends on the nucleation/growth processes, and the precipitation occurs at a certain degree of supersaturation S, which is given by the following formula [31]:

$$ S = {{a_{A} a_{B} } \mathord{\left/ {\vphantom {{a_{A} a_{B} } {K_{\text{sp}} }}} \right. \kern-0pt} {K_{\text{sp}} }} $$
(1)

where the a A and the a B are the activities of partially hydrolyzed cation ([Ln(OH) x (H2O) y ]3−x, x + y = 6, Ln = Gd, Tb, Eu) and the CO32−, respectively. The Ksp is the solubility product constant. Only when the S achieves to the critical supersaturation S*, the nucleation/growth processes can start. As earlier reports [32], the composition of the precursor synthesized via urea-based homogeneous precipitation has been determined to be lanthanide basic carbonate Ln(OH)CO3·nH2O. The solubility of lanthanide basic carbonate in the water increases with the ionic radius of Ln3+ ions decreasing. According to the lanthanide contraction law, it can be known that the value of Ksp increases according to priority of the Tb(OH)CO3 > Gd(OH)CO3 > Eu(OH)CO3. Thus, the complexity of stable nuclei of Ln(OH)CO3 formation increases in the same order. The homogeneous nucleation of Eu(OH)CO3 starts in priority, and then the precipitations of the Tb(OH)CO3 and Gd(OH)CO3 occur on the already formed Eu(OH)CO3 nuclei through heterogeneous nucleation.

In order to better understand the precipitation mechanism, we have regularly sampled and analyzed the particles formed in the different reaction stages, using the (Gd0.96Tb0.02Eu0.02)2O3 sample as an example. Figure 2 shows the FE-SEM morphologies of the precipitation at the different reaction times. From which it can be seen that the colloidal particles grow quite uniformly with the reaction time, which indicates that the mixed systems can follow the LaMer model [33]. The growth rate of the nanoparticles can be consistent with the cubic-root law which is given the following formula:

$$ D\left( t \right) = \sqrt[3]{Kt} $$
(2)

where the D(t) is the average particle diameter at the time of t, in which the t is reaction time, and the K is the growth rate. Figure S1 (Supporting Information Figure S1) shows the relationship between particle size D(t) and the reaction time t, and the inset in Fig. S1 (in the Supporting Information) is the relationship particle size D3(t) and the reaction time t. From which it can be seen that the relationship between D(t)3 and t presents good relationship which confirms with formula (2), indicating that the particle growth is surface-diffusion controlled even for the complicated Gd/Tb/Eu ternary system.

Figure 2
figure 2

Time-course evolution of particle size for (Gd0.96Tb0.02Eu0.02)2O3 precursor, growth time: a 20 min, b 20 min, c 60 min, d 80 min, e 100 min, f 120 min, respectively

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Figure 3a shows the XRD patterns of the (Gd0.98−xTb0.02Eu x )2O3 samples calcined at the same temperature of 1000 °C for 4 h. As shown in Fig. 3a, the Tb3+ and Eu3+ ions additions do not alter the crystal structure, and all the (Gd0.98−xTb0.02Eu x )2O3 samples can be well indexed to the Gd2O3 with cubic structure (space group: \( Ia\bar{3} \)(206), JCPDS No. 43-1014) [2], which indicates that the pure phase (Gd0.98−xTb0.02Eu x )2O3 can be formed at 1000 °C. The main peak (222) is zoomed in Fig. 3b. It can be seen that the positions of main peak drift to the lower angle with the Eu3+ content increasing due to the lager ionic radius of Eu3+ (0.1066 nm) than that of Gd3+ (0.1053 nm) and Tb (0.1040 nm) [34]. Fixed at the same Tb3+ content of 2 at.%, Fig. 3c shows the calculated lattice constants of the (Gd0.98−xTb0.02Eu x )2O3 samples as a function of the Eu3+ content. Clearly, the cell parameter linearly increases with Eu3+ concentration increasing and follows Vegard’s law, indicating that the homogeneous solid solutions have already been formed.

Figure 3
figure 3

a XRD patterns of (Gd0.98−xTb0.02Eu x )2O3 precursors with different Eu contents (x value, x = 0–0.05). b The amplification diagram of main peak (222). c The lattice constants of the (Gd0.98−xTb0.02Eu x )2O3 oxides as a function of the Eu3+ content. d The XPS survey scan of (Gd0.96Tb0.02Eu0.02)2O3 oxides. e–g Are the high-resolution O1s, Gd 4d/Tb 4d and Eu 3d XPS spectra of (Gd0.96Tb0.02Eu0.02)2O3 oxides, respectively

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Figure 3d shows the typical XPS survey scan of the (Gd0.96Tb0.02Eu0.02)2O3 samples. As shown in Fig. 3d, the Gd, Tb, Eu, O and C exist in the XPS survey spectra, indicating that the Tb3+ and Eu3+ ions have effectively been introduced into the Gd2O3 matrix materials. The XPS band of C1s was due to the adsorbed impurity carbon [35]. As shown in Fig. 3e, the two O1s XPS bands of samples are found at the 532.7 and 530.3 eV BE positions, owing to the O2− of (Gd0.96Tb0.02Eu0.02)2O3 and the surface oxygen vacancies [35, 36]. The Gd 4d and Tb 4d were observed at the bands of 148.5 and 143.1 eV, respectively, which is shown in Fig. 3f. The positions of bands are consistent with the energy level for Gd and Tb in Gd2O3 and Tb2O3, respectively [37]. Thus, the valence states of the elements in the sample can be verified. Further observation is that only the Tb3+ ions is found at 148.5 eV in the 4d core-level spectrum of (Gd0.96Tb0.02Eu0.02)2O3 sample, which indicates that the Tb4+ was almost completely reduced to the Tb3+ [38]. Figure 3g shows the high XPS resolution Eu 3d spectra of the (Gd0.96Tb0.02Eu0.02)2O3 sample; the major bands of Eu 3d are found at 1123.9, 1133.9, 1154.6 and 1163.1 eV, respectively. The positions of energy bands are associated with the Eu3+ in Eu2O3 system [39].

The cation contents of the (Gd0.98−xTb0.02Eu x )2O3 phosphors obtained by the ICP-OES analysis are shown in Table 1. As shown in Table 1, the analyzed contents of samples are consistent with the values which were calculated from the prescribed chemical formula. The results indicate that the urea-based homogeneous precipitation method used in this work is very suitable to prepare rare-earth oxide phosphors with controllable chemical composition.

Table 1 Calculated and analyzed cation contents for nine typical phosphors
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Figures S2a–S2g (Supporting Information Figure S2) show the FE-SEM of (Gd0.98−xTb0.02Eu x )2O3 precursors calcined at 1000 °C using pure water. As can be seen that the resultant products still keep the good dispersions and the spherical morphologies of precursors even calcined at 1000 °C. The reason for this phenomenon is that the negligible aggregations of spherical precursors make the contact areas between particles very limited [30, 40]. Figures S2h and S2i (in the Supporting Information) show the selected area electron diffraction (SAED) pattern and high-resolution (HR) TEM image of (Gd0.96Tb0.02Eu0.02)2O3 phosphors, respectively. As shown in Fig. S2h (in the Supporting Information), the results indicate that the (Gd0.96Tb0.02Eu0.02)2O3 samples possess the highly single crystalline nature of the nanostructures. Further observation is that the interplanar distance (d) of the (Gd0.96Tb0.02Eu0.02)2O3 samples is ~ 0.274 nm (Supporting Information Figure S2i), which is consistent with the results of Gd2O3 in the database (d(400) = 0.270 nm, JCPDS No. 43-1014).

The PLE properties of the (Gd0.98−xTb0.02Eu x )2O3 phosphors using pure water as reaction solvent are displayed in Fig. 4. By monitoring the Tb3+ emission at 547 nm (the 5D4 → 7F5 transition of Tb3+, Fig. 4a), all the PLE spectra mainly contain three groups of excitation bands at ~ 223, ~ 276 and ~ 314 nm, respectively, which are arising from the 4f8 → 4f75d1 transition of Tb3+. Further observation is that the weak intensity peak (7F6 → 5L10–7 and 7F6 → 5D3 intra-4f8 transitions of Tb3+) appears at ~ 375 nm (Fig. 4a). It can be clearly seen that the excitation bands intensities vary remarkably and decrease steadily with the Eu3+ contents increasing. By monitoring the Eu3+ emission at 613 nm (the 5D0 → 7F2 transition of Eu3+, Fig. 4b), besides the charge transfer (CTB, ~ 243 nm), 7F0,1 → 5L7 (~ 380 nm) and 7F0,1 → 5L6 (~ 394 nm) transitions of Eu3+, the 4f8 → 4f75d1 transitions of Tb3+ also appear at ~ 275 and ~ 314 nm in the PLE spectra, confirming the highly efficient energy transfer from Tb3+ to Eu3+. It should be noted that the excitation peaks at ~ 275 and 314 nm overlap the characteristic transition 8S7/2 → 6I J of Gd3+, providing an evidence of the energy transfer from Gd3+ to Tb3+ and Eu3+, respectively.

Figure 4
figure 4

A comparison of the PLE behaviors of the (Y0.96Tb0.02Eu0.02)2O3, (Lu0.96Tb0.02Eu0.02)2O3 and (Gd0.98−xTb0.02Eu x )2O3 phosphors, the PLE spectra were obtained by monitoring the 547 nm (a) and 613 nm (b) emissions, respectively

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From above analysis, the exciting (Gd0.98−xTb0.02Eu x )2O3 phosphors at 314 nm can efficiently achieve the Tb3+ and Eu3+ emissions. Further observation is that the excitation peaks of (Y0.96Tb0.2Eu0.2)2O3 (at ~ 305 nm) and (Lu0.96Tb0.2Eu0.2)2O3 (at ~ 303 nm) show gradually blueshift, but the excitation peaks of (Y0.96Tb0.2Eu0.2)2O3 (at ~ 231 nm) and (Lu0.96Tb0.2Eu0.2)2O3 (at ~ 235 nm) show gradually redshift on the PLE spectra compared to the (Gd0.98−xTb0.02Eu x )2O3 phosphors. This is mainly due to the lower electro-negativity of Gd3+ (1.20) than that of Y3+ (1.22) and Lu3+ (1.27). The lower electro-negativity would enhance the crystal field splitting of the 5d energy level, which can shift the low energy excitation peaks to the long wavelengths, and high energy excitation peaks to the short wavelength [24].

Figure 5 shows the photoluminescence properties of the (Gd0.98−xTb0.02Eu x )2O3 phosphors under 314 nm wavelength excitation with Eu3+ content changing, and the PL spectra of (Y0.96Tb0.02Eu0.02)2O3 and (Lu0.96Tb0.02Eu0.02)2O3 samples are also included for the comparison. The (Gd0.98Tb0.02)2O3 (x = 0) phosphor exhibits four groups of typical Tb3+ emission bands at ~ 490 nm (blue), 547 nm (green, the strongest), 595 nm (orange-red) and 623 nm (red) which are associated with the 5D4 → 7F J (J = 3, 4, 5, 6) transitions of Tb3+ which are marked in Fig. 5a, respectively. However, the Tb3+/Eu3+ co-doped samples not only contain the typical Tb3+ emission bands, but also the typical Eu3+ emission bands at ~ 595, 613 nm (the strongest), 655 and 708 nm attributing to the 5D0 → 7F J (J = 1, 2, 3, 4) transitions of Eu3+, respectively. There are two aspects which should be noted: (1) the intensities of blue emissions (< 480 nm) derived from the 5D3 → 7F J transitions of Tb3+ (J = 3, 4, 5, 6) are too weak to be detectable. The phenomena can be explained as follows: the spectral energy distribution of Tb3+ emission is strongly dependent on the Tb3+ concentrations. As the Tb incorporation increasing, the intensity of 5D3 emission steadily decreased while the 5D4 emission increased due to the cross-relaxation from 5D3 to 5D4 level. Owing to the high Tb3+ concentrations used in the present work, the 5D3 emission is almost completely quenched; (2) the strongest emission of Eu3+ appears at ~ 613 nm for the 5D0 → 7F2 transition rather than ~ 595 nm for the 5D0 → 7F1 transition in the Tb3+/Eu3+ co-doped samples. The mainly reason is that the occupancy of Eu3+ at C2 sites without inversion symmetry (75%) is higher than the S6 sites (25%) with inversion symmetry [41, 42].

Figure 5
figure 5

a Emission spectra of the (Y0.96Tb0.02Eu0.02)2O3, (Lu0.96Tb0.02Eu0.02)2O3 and (Gd0.98−xTb0.02Eu x )2O3 phosphors under 314 nm excitation and b presents relative intensities of the 547 nm emission of Tb3+ and the 613 nm emission of Eu3+ as well as the I613/I547 intensity ratio, as a function of the Eu content (the x value). c Is the relationship between log(I/c) and log(c) of the (Gd0.98−xTb0.02Eu x )2O3 phosphors for Eu3+ emission

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Figure 5b shows the relative intensities of the 547 and 613 nm emissions as well as the I613/I547 intensity ratios under 314 nm excitation. The emission intensities of Eu3+ increased with the Eu3+ content increasing up to 2 at.% (x = 0.02), and then decreased due to the concentration quenching. At the maximum Eu3+ emission intensity, the total activator concentration CTb+Eu of 4 at.% is close to the 5 at.% which had been widely reported in Gd2O3 system doped with either Eu3+ or Tb3+. The interaction type of luminescence quenching for solid phosphors can be concluded via analyzing the constant s according to the following formula [34, 43]:

$$ \log \left( {{I \mathord{\left/ {\vphantom {I c}} \right. \kern-0pt} c}} \right) = \left( {{{ - s} \mathord{\left/ {\vphantom {{ - s} d}} \right. \kern-0pt} d}} \right)\log c + \log f $$
(3)

where the I is the emission intensity, the c is the activator concentration, the d is the sample dimension (d = 3 for regular sample), the f is a constant, and the s is the index of electric multipole. The s values of 6, 8, and 10 represent the dipole–dipole, dipole–quadrupole, and quadrupole–quadrupole electric interactions, respectively, whereas s = 3 corresponds to the exchange interaction. The relationship between log(I/c) and log(c) for 613 nm emission is shown in Fig. 5c. The slope (−s/3) is determined to be − 0.71 yielding the s value of the (Gd0.98−xTb0.02Eu x )2O3 samples is ~ 2.13, indicating that the concentration quenching is mainly due to the energy transfer between Eu3+ and Eu3+ [44, 45]. The Tb3+ emission intensity gradually decreased with the Eu3+ contention increasing while the I613/I547 intensity ratio increased, which can provide the strong evidence of the highly efficient energy transfer from Tb3+ to Eu3+. The luminescent intensity (regardless of Tb3+ or Eu3+ emission) of the (Gd0.96Tb0.02Eu0.02)2O3 was higher than that of the (Y0.96Tb0.02Eu0.02)2O3 and (Lu0.96Tb0.2Eu0.02)2O3 samples, which can provide an directly evidence of the Gd3+ → Tb3+ and the Gd3+ → Eu3+ energy transfers, respectively.

To further explain the energy transfer from Tb3+ to Eu3+, the decay kinetics of Tb3+ (5D4 → 7F5 transition) and Eu3+ (5D0 → 7F2 transition) have been investigated (λex = 314 nm), and then the lifetimes (τR) and the energy transfer efficiencies (η) of the phosphors can be calculated. The fluorescence decay curves of Tb3+ and Eu3+ emissions can be fitted with single exponential decay in the following formula:

$$ I = A\exp \left( {{{ - t} \mathord{\left/ {\vphantom {{ - t} {\tau_{\text{R}} }}} \right. \kern-0pt} {\tau_{\text{R}} }}} \right) + B $$
(4)

where the τR is fluorescence lifetime, the t is decay time, the I is the relative fluorescence intensity, and A and B are the constants. It can be seen that the lifetime values of the Tb3+ emission (the inset of Fig. 6a) decreased from ~ 2.98 to ~ 1.67 ms with the Eu3+ content (the x value) increasing from x = 0 to x = 0.05, attributing to the efficient Tb3+ → Eu3+ energy transfer. Meanwhile, the lifetime values of the Eu3+ emission slowly and continuously decrease at the higher concentration of Eu3+ (the inset of Fig. 6b). The higher Eu3+ concentration will lead to the formation of a resonant energy transfer network between the activators, which can act as an additional channel to the non-radiative centers of particle surface and therefore shorten the lifetime.

Figure 6
figure 6

Fluorescence decay curves for the 547 nm (a) and 613 nm (b) emission of (Gd0.96Tb0.02Eu0.02)2O3 phosphors. The embedded graphs a and b are the fluorescence lifetime of the 547 and 613 nm for the (Gd0.98−xTb0.02Eu x )2O3 phosphors as a function of the Eu3+ content, respectively. c Energy transfer efficiency and average separation distance (R) of the activators, as a function of the Eu3+ content (the x value). The insets in graph a and b are the lifetimes of the 547 and 613 nm emissions for the (Gd0.98−xTb0.02Eu x )2O3 phosphors as a function of the Eu3+ contents

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The energy transfer efficiency (η) of Tb3+ → Eu3+ can be calculated by the following formula:

$$ \eta = 1 - \frac{\tau }{{\tau_{0} }} $$
(5)

where the τ and the τ0 are the lifetimes of Tb3+ in the presence and absence of Eu3+, respectively. The results of calculation are shown in Fig. 6c. It can be seen that the Tb3+ → Eu3+ energy transfer efficiency increases from 7.62 to 43.74% with the Eu3+ content (the x value) increasing from x = 0.001 to x = 0.05, respectively.

In order to further elaborate the Tb3+ → Eu3+ energy transfer mechanism, the mode and type of the energy transfer have been calculated theoretically. The mode of energy transfer (exchange interaction and multipolar interaction) depends on the average separation distance R between the donor and acceptor. The average separation distances for Tb3+ and Eu3+ in (Gd0.98−xTb0.02Eu x )2O3 phosphors can be calculated via the Blasse formula [46]:

$$ R = 2\left( {\frac{3V}{{4\pi C_{{{\text{Tb}} + {\text{Eu}}}} N}}} \right)^{{{1 \mathord{\left/ {\vphantom {1 3}} \right. \kern-0pt} 3}}} $$
(6)

where the CTb+Eu is the total concentrations of Tb3+ and Eu3+, the V is the volume of unit cell, and the N is the number of available sites for the dopant per unit cell. The per unit cell of Gd2O3 sample has 80 atoms, among which 32 are the Ln3+ and thus the N = 32. The value of the V can be obtained from the lattice constant which is shown in Fig. S2c (in the Supporting Information). The R values of calculation are shown in Fig. 6c, as a function of Eu3+ content (the x value), showing that all the R values are significantly larger than the 0.3–0.4 nm. It indicates that the mode of Tb3+ → Eu3+ energy transfer is the electric multipolar interactions in the present work.

According to the early reported energy transfer process theory which is suggested by Dexter and Reisfeld [46,47,48,49], the types of interaction of electric multipole interactions can be analyzed by the following formula:

$$ \frac{{\eta_{0} }}{\eta }\infty C^{{{n \mathord{\left/ {\vphantom {n 3}} \right. \kern-0pt} 3}}} $$
(7)

where the C is the total concentrations of Tb3+ and Eu3+ which could be replaced by the Eu3+ content owing to the fixed Tb3+ content, the η0 and η are the luminescence quantum efficiencies of the Tb3+ in the absence and presence of Eu3+, respectively. Meanwhile, the η0/η ratio can be replaced by the PL intensity ratio IS0/I S . The (IS0/I S )∞Cn/3 with n = 6, 8 and 10 corresponds to dipole–dipole, dipole–quadrupole, and quadrupole–quadrupole interactions, respectively. The relationships between (IS0/I S ) and Cn/3 are shown in Fig. S3 (Supporting Information Figure S3), from which it can be seen that the linear relationship with n = 6 (Supporting Information Figure S3a) was the best through the comparison of fitting factor values (R2). The results indicate that the Tb3+ → Eu3+ energy transfer mechanism of the (Gd0.98−xTb0.02Eu x )2O3 samples is dominantly electric dipole–dipole interactions, as well as the critical distance (Rc) of energy transfer can be also proved.

The above analysis indicates that the multi-channel energy transfers may exist in the (Gd0.98−xTb0.02Eu x )2O3 phosphors which is shown in Fig. 7, such as Gd3+ → Eu3+, Gd3+ → Tb3+, Tb3+ → Eu3+ and Gd3+ → Tb3+ → Eu3+. By monitoring the 314 nm excitation wavelength, the electrons of the 8S7/2 ground state though absorbing the energy can transmit to the 6P J excited state of Gd3+, meanwhile the 4f8 electrons of Tb3+ transmit to the 5d1 state. The energy transfer of the Gd3+ → Eu3+ and Gd3+ → Tb3+ may happen owing to that the 5D3,4 (Tb3+) and the 5D0,1 (Eu3+) states in the energy diagram lies lower than the 6P J state of Gd3+ [50,51,52]. On the other hand, the electrons of the 7F J ground state of Tb3+ via absorbing the energy can transmit to the 5D3 excited state. Owing to the 5D1 emission state of Eu3+ is lower than the 5D3 state of Tb3+ [15]; thus the energy transfer of Gd3+ → Tb3+ → Eu3+ may also occur in the (Gd0.98−xTb0.02Eu x )2O3 phosphors. Then the electrons of Tb3+ and Eu3+ relaxed from 5D3 and 5D1 to the 5D4 and 5D0, respectively. Back-jumping electrons of Tb3+ and Eu3+ from 5D4 and 5D0 excited state to the 7F5 (5D4 → 7F5 transition of Tb3+) and 7F2 (5D0 → 7F2 transition of Eu3+) levels, respectively, and finally emit the green (547 nm) and red (613 nm) lights.

Figure 7
figure 7

Illustration of the energy transfer processes for the (Gd0.98−xTb0.02Eu x )2O3 phosphors

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The PLE/PL thermal properties of phosphors are the important technological factors for applying in LEDs. Therefore, the PLE/PL emission spectra with the change of temperatures at the range of 298–523 K have been obtained to investigate the influence of temperature on the luminescence properties, and the activation energy of thermal quenching can be determined, which are shown in Fig. 8. The temperature variation does not bring about any appreciable change to the peak shape and positions of PLE (Fig. 8a, b) and PL (Fig. 8c) bands, but the emission intensities of Tb3+ and Eu3+ in (Gd0.98−xTb0.02Eu x )2O3 descend monotonically with the temperature increasing. Owing to the temperature has influence on the energy transfer, thus the declined rates are different. The reason for the emission intensity decreasing is generally due to the thermal quenching which was caused by the thermal activation of the intersecting point between the ground and the excited states [53]. The emission intensities of Tb3+ and Eu3+ at 323 K can retain ~ 68.03 and ~ 51.93% of their corresponding original values at 273 K, respectively. To further investigate the temperature-dependent thermal quenching phenomenon, the Arrhenius equation below has been utilized to assess the activation energy [54]:

$$ \ln \left( {\frac{{I_{0} }}{I} - 1} \right) = \ln A - \frac{{E_{\text{a}} }}{kT} $$
(8)

where the Ea and T represent the objective activation energy and temperature (K), respectively. The A is a constant and the k is the Boltzmann constant (8.626 × 10−5 eV). The I 0 is the integrated emission intensity at room temperature, and the I is the integrated emission intensity at differently operated temperatures. Figure 8d shows the relationship between the ln[(I 0 /I) − 1] and 1/kT for the thermal quenching of the (Gd0.98−xTb0.02Eu x )2O3 phosphors. As can be seen that the slope of the best fitting line is − 0.2397, so that the activation energy E a is ~ 0.2397 eV. The relatively high activation energy achieved in this work indicates that it possesses good thermal stability and it is an excellent candidate for application in LEDs.

Figure 8
figure 8

Dependence of the PLE (a, b) and PL (c) spectra of (Gd0.96Tb0.02Eu0.02)2O3 phosphors on the temperature; d linear relationship of the ln[(I 0 /I) − 1] versus 1/kT activation energy graph for thermal quenching of the (Gd0.96Tb0.02Eu0.02)2O3 sample. The inset in c is the relative intensity of the 547 nm (Tb3+) and 613 nm (Eu3+) emissions as a function of the Eu3+ content

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As mentioned above, the relative I613/I545 intensity ratio changed with the Eu3+ addition, thus the emission color could be tuned via changing the relative Tb3+/Eu3+ content, and this can be verified by the CIE chromaticity coordinates analysis. The CIE chromaticity coordinates for the emission of (Gd0.98−xTb0.02Eu x )2O3 (x = 0-0.05) under 314 nm excitation are shown in Fig. 9a. The samples with different Eu3+ content (the x value) were calculated to have the color coordinates (x, y) of (0.33, 0.56), (0.36, 0.55), (0.41, 0.50), (0.46, 0.46), (0.51, 0.42), (0.54, 0.39), and (0.54, 0.37) for the x = 0, 0.001, 0.005, 0.01, 0.02, 0.03, and 0.05, respectively, roughly corresponding to the green, yellowish-green, yellow, deep yellow, yellowish-orange, orange-red, and red colors, respectively. The corresponding color temperature can be calculated using the following formulas [55]:

$$ T = - 437n^{3} + 3601n^{2} - 6861n + 5514.31 $$
(9)

and

$$ n = (x - 0.332)/(y - 0.1858) $$
(10)

The samples of the color temperature were ~ 5549 K (x = 0), ~ 5042 K (x = 0.001), ~ 4130 K (x = 0.005), ~ 3217 K (x = 0.01), ~ 2375 K (x = 0.02), ~ 1940 K (x = 0.03), and ~ 1834 K (x = 0.05). Figure 9b shows the vivid luminescence colors of the (Gd0.98−xTb0.02Eu x )2O3 phosphors with different Eu3+ contents under 254 nm UV excitation from a handheld UV lamp. It is clearly observed that the emission color moves from green to yellow and eventually red regions, indicating that the color-tunable photoluminescence can be achieved though varying the Eu content (the x value).

Figure 9
figure 9

CIE chromaticity diagram for the emission of (Gd0.98−xTb0.02Eu x )2O3 phosphors under 314 nm excitation (a). b is the appearances of luminescence for the x = 0, 0.001, 0.005, 0.01, 0.02, 0.03 and 0.05 samples under 254 nm excitation from a handheld UV lamp

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In order to study the particle size and morphology effects on the luminescent properties of phosphors, the (Gd0.96Tb0.02Eu0.02)2O3 phosphor with different morphologies and particle sizes as an example has been successfully obtained in present work. The effect of EG addition y (y = 0–0.5) on the particle sizes and morphologies of the (Gd0.96Tb0.02Eu0.02)2O3 precursors has been studied using FE-SEM analysis (Supporting Information Figure S4). The morphologies of (Gd0.96Tb0.02Eu0.02)2O3 precursors with EG content increasing from y = 0 (pure water) to y = 0.2 are spherical with good dispersion, and the average particle size of the precursors steadily decreases (Fig. S4a: φ a : ~ 300 nm, Fig. S4b: φ b : ~ 240 nm and Fig. S4c: φ c : ~ 200 nm). The particle sizes of the precursors decrease with EG content increasing which can be explained as follows:

The supersaturation S is inversely proportional to the solubility product Ksp in the process of chemical precipitation [31], and the Ksp is related to the solubility Cs. The Cs can be calculated though the given following formula [56, 57]:

$$ C_{\text{s}} \approx \exp \left[ { - \frac{{z^{ + } z^{{^{\_} }} e^{2} }}{{4\pi \varepsilon_{0} \varepsilon_{r} kT\left( {r^{z + } + r^{z - } } \right)}}} \right] $$
(11)

where the rz+ and rz− are the ion radius of take charge of z+, z−, respectively, the \( \varepsilon_{0} \) is the vacuum permittivity, the \( \varepsilon_{r} \) is the relative permittivity constant of solvent. The \( \varepsilon_{r} \) of the water (78.5) is larger than the EG (37.7) [40] leading to the smaller Cs, and thus the S increases. At the same time, the homogeneous nucleation rate (RN) can be calculated by the following formula [58]:

$$ R_{\text{N}} = A\exp \left( {\frac{{ - 16\pi \sigma_{SL} \upsilon^{{_{2} }} }}{{3k^{3} T^{3} \ln^{2} S}}} \right) $$
(12)

where the RN is the number of nuclei formed per unit time per unit volume, the A is a pre-exponential constant typically ranging from 1025 to 1056, the σSL is the surface tension at the liquid/solid interface, the υ is the atomic volume of the solute, the k is the Boltzmann constant, and the T is the temperature. The interface energy of EG (48.4 N/m) is smaller than water (72.8 N/m) [40], leading to the smaller surface tension at the liquid/solid interface σSL. Meanwhile, the increase in EG contents leads to the increase in supersaturation S and the decrease in interfacial tension σSL, which will increase the nucleation density leading to the smaller particle size. The results show that the spherical particle size will decrease with the EG content increasing. However, the spherical nanoparticles will be self-assembled when the particle size decreases to a certain degree, which are shown in Figs. S4d–S4f (Supporting Information Figure S4). As shown in Fig. S4d, the flower plate morphology begins to appear when the EG content achieves to 30 vol% (y = 0.3), and the spherical particles mix with flower plate particles existed in precursors. When the ratio of EG and DI water content increases up to y = 0.5 as shown in Fig. S4f (Supporting Information Figure S4), the precursors are completely changed into the flower shape structure by its self-assembled.

The FE-SEM images of the (Gd0.96Tb0.02Eu0.02)2O3 precursors calcined at 1000 °C with different EG content (y = 0–0.5) are shown in Fig. S5 (Supporting Information Figure S5). It can be seen that the resultant samples even calcined at 1000 °C still maintain the good dispersion and morphology of the precursors under the different EG contents.

The PLE (Fig. 10a under 547 nm emission and Fig. 10b under 613 nm emission) and PL (Fig. 10c) spectra of the (Gd0.96Tb0.02Eu0.02)2O3 phosphors with different particle sizes and morphologies have been performed. Figure 10 shows that using the EG as the reaction solvent does not alter the shapes and positions of the PLE and PL peaks, being the same to the PLE and PL bands marked in Figs. 4 and 5a. The intensities of the 613 and 547 nm emission for Eu3+ and Tb3+ with the change of EG content are shown in Fig. 10c inset. It can be seen that the emission intensities of Eu3+ and Tb3+ all steadily decrease with the EG content increasing from 0 to 20 vol% and then increase. The reason for that phenomenon is that the bigger specific surface of phosphor will result in the higher scattering of the light, and thus the deteriorated emission intensity was achieved [59,60,61].

Figure 10
figure 10

PLE (a, b) and PL (c) spectrum of (Gd0.96Tb0.02Eu0.02)2O3 phosphors with different EG content (0–50 vol%), the PLE spectrum was obtained by monitoring the 547 nm (a) and 613 nm (b) emission, while the PL spectrum was obtained under UV excitation at 314 nm. Inset in c is the relative intensity of the 613 nm (Eu3+) emission as a function of the EG content

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The substantial differences for the PL intensity may be caused by the particle morphology or the defects or the combined effects of two [45, 62]. For excited electrons, the higher defects content can improve its probability of non-radiative transitions, leading to the PL quenching [62]. On the other hand, the particle morphology can influence the PL intensity through the affecting the scattering degree of light produced and the packing density of phosphor crystals [45, 62]. In order to differentiate the two influencing factors, the decay curves of the 547 nm (5D4 → 7F5 transition of Tb3+) and 613 nm (5D0 → 7F2 transition of Eu3+) emission for the phosphors have been studied. Figures 11a and 11b show the luminescence decay curves of the (Gd0.96Tb0.02Eu0.02)2O3 (10 vol% EG) phosphor for the Tb3+ and Eu3+ emissions, respectively. The curves can be fitted with the single exponential decay by formula (4). The fitting results for the Tb3+ and Eu3+ emissions are shown in Fig. 11 with τR = 2.02 ± 0.01 (ms), A = 5089.37 ± 131.17 (a.u.), B = 8.54 ± 0.15 (a.u.) and τR = 2.03 ± 0.01 (ms), A = 2982.10 ± 158.43 (a.u.), B = 0.27 ± 0.22 (a.u.), respectively. Further observation is that the fluorescence lifetimes have little changes with the EG content (0–50 vol%) changing (the insets of Fig. 11a, b), and all the samples have the similar lifetimes (Tb3+ ~ 1.99 ± 0.05 ms and Eu3+ ~ 2.07 ± 0.05 ms). The results may show that there is no significant difference in the defect concentration between these samples, since the more defect states will lead to non-radiative relaxation rates increasing, which will shorten the lifetimes of Eu3+ emission [62]. Thus, it can be concluded that the significant difference in emission intensities of 613 and 547 nm heavily depends on the morphology of the particles. The average size of spherical particles gradually decreases with the EG content increasing from 0 to 20 vol%. Thus, the specific surface area of the particles is increased leading to the higher scattering of the light and the lower emission intensity. In addition, the agglomeration of small size particles may be the reason for the decrease in emission intensity. However, the average size of spherical particles gradually increases with the EG content continually increasing to the 50 vol%, resulting in the smaller scattering of the light, and thus improves the luminescence emission intensity [45, 62].

Figure 11
figure 11

Fluorescence decay curves for the 547 nm (a) and 613 nm (b) emission of the (Gd0.96Tb0.02Eu0.02)2O3 phosphors. The embedded graph is the fluorescence lifetime of the different EG content (y = 0–0.5)

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Conclusions

Color-tunable (Gd0.98−xTb0.02Eu x )2O3 (x = 0–.05) phosphors have been successfully obtained using the urea-based homogeneous precipitation method in the present work. Detailed characterizations by the combined techniques of XRD, XPS, ICP-OES, FE-SEM, HR-TEM, PLE/PL spectra and decay analysis have yielded the following main conclusions:

  1. 1.

    Growth of the particles is surface-diffusion related and follows the cubic-root law. Final sizes of the resultant particles are inversely proportional to nucleation density. Both the precursors and resultants of the (Gd0.98−xTb0.02Eu x )2O3 particles using the pure water as the solvent exhibit good dispersion and spherical morphology. Size-controlled and particle morphology changing from spheres to flower plate can be achieved via the EG addition owing to its lower permittivity constant and interface energy;

  2. 2.

    Under the UV excitation wavelength of 314 nm (4f8 → 4f75d1 transition of Tb3+), the phosphors display the typical Tb3+ and Eu3+ emissions together, with the green emission at 547 nm (5D4 → 7F5 transition of Tb3+) and red emission at 613 nm (5D0 → 7F2 transition of Eu3+) being dominant. The Tb3+ and Eu3+ emissions vary significantly with the Eu3+ incorporation, and the emission color can thus be readily tuned from approximately green to red via adjusting the Eu3+ content. The quenching concentration is determined to be 2.0 at.% (x = 0.02) which is ascribed to the exchange between Eu3+ ions;

  3. 3.

    Tb3+ → Eu3+ energy transfer can be demonstrated by the two aspects: (a) the presence of Tb3+ excitation bands on the PLE spectra monitoring the Eu3+ emission; (b) the lifetime values for Tb3+ emission decreased with Eu3+ addition. The Gd3+ → Eu3+ and Gd3+ → Tb3+ energy transfers are also existed in the (Gd0.98−xTb0.02Eu x )2O3 system. The efficiency of Tb3+ → Eu3+ energy transfer is calculated to be increased from 7.62 to 43.74% with the Eu3+ content (the x value) increasing from x = 0.001 to x = 0.05, respectively. The activation energy (Ea) is determined to be ~ 0.2397 eV through temperature-dependent analysis, indicating its good thermal stability;

  4. 4.

    The particle morphology/size does not alter the fluorescence lifetime, color coordinate, and color temperature, but brings about appreciable change to the emission intensity. Owing to the scattering of the light, the emission intensity firstly decreases and then improves with the EG addition which can be confirmed by the phosphor morphology of “spherical → spherical/flower shape → flower shape” variation.