Controlled processing of (Gd,Ln)2O3:Eu (Ln = Y, Lu) red phosphor particles and compositional effects on photoluminescence.

Synthesis of (Gd0.95-x Ln x Eu0.05)2O3 (Ln = Y and Lu, x = 0-0.95) powders via ammonium hydrogen carbonate (AHC) precipitation has been systematically studied. The best synthesis parameters are found to be an AHC/total cation molar ratio of 4.5 and an ageing time of 3 h. The effects of Y3+ and Lu3+ substitution for Gd3+, on the nucleation kinetics of the precursors and structural features and optical properties of the oxides, have been investigated. The results show that (i) different nucleation kinetics exist in the Gd-Y-Eu and Gd-Lu-Eu ternary systems, which lead to various morphologies and particle sizes of the precipitated precursors. The (Gd,Y)2O3:Eu precursors display spherical particle morphologies and the particle sizes increase along with more Y3+ addition. The (Gd,Lu)2O3:Eu precursors, on the other hand, are hollow spheres and the particle sizes increase with increasing Lu3+ incorporation, (ii) the resultant oxide powders are ultrafine, narrow in size distribution, well dispersed and rounded in particle shape, (iii) lattice parameters of the two kinds of oxide solid solutions linearly decrease at a higher Y3+ or Lu3+ content. Their theoretical densities linearly decrease with increasing Y3+ incorporation, but increase along with more Lu3+ addition and (iv) the two kinds of phosphors exhibit typical red emissions at ∼613 nm and their charge-transfer bands blue shift at a higher Y3+ or Lu3+ content. Photoluminescence/photoluminescence excitation intensities and external quantum efficiency are found to decrease with increasing value of x, and the fluorescence lifetime mainly depends on the specific surface areas of the powders.

Introduction

Eu3+-activated Gd2O3, Y2O3 and Lu2O3 as three kinds of well-known red phosphors are widely applied in various lighting and display systems. The three phosphors present typical red photoluminescence (PL) at ∼612 nm upon UV excitation into the charge-transfer (CT) band (electron transfer from O2− to Eu3+) [1-8]. Improved performance of displays requires high-quality phosphors for sufficient brightness, high resolution and long-term stability. Phosphor particles of spherical shape, ultrafine size, good dispersion and narrow size distribution are desired for three reasons: (i) spherical shape not only allows the formation of a denser phosphor layer by close packing, but also minimizes the light scattering on particle surface to increase the efficiency of luminescence and brightness [9], (ii) ultrafine powders provide small pixel sizes for improved resolution [10-13] and (iii) well-dispersed and narrow size-distributed particles produce uniform luminescence over the whole phosphor screen [14, 15].

The (Gd,Ln)2O3:Eu (Ln = Y, Lu) solid solution systems were studied based on the following three major considerations: The commercial Ln2O3:Eu phosphors are classically yielded by high-temperature solid-phase reaction and ball-milling treatment, which often gives rise to large particle size, considerable agglomeration and broad particle size distribution. Therefore, wet chemical methods are necessary in the synthesis of fluorescent powders. Based on previous reports [10–12, 14, 22–30], urea-based homogeneous precipitation (UBHP) technique is found to be a good tool for acquiring uniform and monodispersed particles. Compared with the UBHP, ammonium hydrogen carbonate (AHC) precipitation is relatively time-efficient, cost-effective, operation-simple and high-yield. In this work, (Gd,Ln)2O3:Eu (Ln = Y, Lu) red phosphors were studied for their synthesis, formation mechanism, compositional impacts and PL behaviors.

Compared with Y2O3:Eu and Lu2O3:Eu, Gd2O3:Eu phosphor displays relatively strong PL and CT bands due to the low electronegativity for Gd3+. In general, the PL intensity of Ln2O3:Eu can be significantly enhanced at elevated synthesis temperature. However, Gd2O3:Eu will undergo a transformation from a cubic to a monoclinic (or hexagonal) phase upon heating (> 1250 °C), by which its PL intensity would be sharply lowered.

The ionic radius of Y3+ (0.0900 nm) is close to that of Ho3+ (0.0901 nm) [16], leading to similar chemical properties, and thus Y is generally regarded as a member of the lanthanide family. Y2O3 is cost-effective, and hence Y3+-doped Gd2O3:Eu red phosphor not only owns low-cost advantage but also exhibits relatively high emission intensity. In addition, Y3+ doping can effectively suppress the phase transformation of Gd2O3 through solid-solution formation. (Gd,Y)2O3:Eu is a promising material to be used in the fields of fluorescent lamp, white LEDs, and can also be densified into transparent ceramic scintillator by high-temperature sintering [17, 18].

Lu3+ has smaller ionic radius than Y3+, and thus its doping is more effective in stabilizing the cubic structure of Gd2O3. Moreover, Lu3+ possesses high absorption of x-rays to reduce the multiple scattering of light because of its high density and high effective atomic number. It has been proved suitable as an x-ray-stopping powder in ceramic scintillators [19-21].

Experimental procedures

The starting materials were Gd(NO3)3·6H2O (> 99.95% pure, Kanto Chemical Co., Inc., Tokyo, Japan), Y(NO3)3·6H2O (> 99.99% pure, Kanto Chemical) and Eu(NO3)3·6H2O (> 99.95% pure, Kanto Chemical). Lu(NO3)3 was prepared by dissolving a Lu2O3 powder (> 99.95% pure, Kanto Chemical) in an excessive amount of nitric acid at ∼90 °C.

Precipitated precursors for (Gd0.95−LnEu0.05)2O3 were synthesized by dripping AHC (ultrahigh purity, Kanto Chemical) solution (1.5 M) into a stoichiometric amount of mother liquor (0.15 M for total cationic concentration) at selected AHC/total cation molar ratio (R) under magnetic stirring at room temperature. For typical synthesis, the x value was selected from 0 to 0.95 and the molar ratio of Eu3+ to total cations was fixed at 5 at.% because the concentration quenching of luminescence was found at 5 at.% for Y2O3:Eu [12, 31] and 5–7 at.% for Gd2O3:Eu [32-35]. The resultant suspension, after ageing for different times, was filtered and repeatedly washed with distilled water to remove the byproducts. Then it was rinsed with anhydrous alcohol to remove water and dried at 80 °C. The dried precursors were lightly crushed with an alumina mortar and a pestle, and finally calcined in a tube furnace under flowing oxygen gas (250 ml min−1) at selected temperatures for 4 h with a heating rate of 5 °C min−1.

Compositions of the precursors were determined by chemical analysis. Cation contents were detected by the inductively coupled plasma spectroscopic approach with an accuracy of 0.01 wt% (Model SPS3520, SII Nanotechnology Inc., Tokyo, Japan), carbon contents were assayed on a carbon/sulfur determinator with a detection limit of 0.01 wt% (Model CS-444LS, LECO Co., Michigan, USA), and NH4+ contents were determined by the standard distillation-titrimetric method with an experimental error of ±0.1 wt%. The precursors and their oxides were characterized using thermogravimetry (TG; Model 6200, SII Nanotechnology Inc.), x-ray diffractometry (XRD; Model RINT2200, Rigaku Corporation, Japan), Brunauer–Emmett–Teller analysis (BET; Model Autosorb-iQ, Quantachrome Instruments, Florida, USA), field-emission scanning electron microscopy (FE-SEM; Model S-5000, Hitachi, Tokyo), laser diffraction particle size analysis (LDPSA, Model Horiba LA-920, Kyoto, Japan) and fluorescence spectroscopy (Model FP-6500, JASCO, Tokyo).

For the cubic-structured C-type sesquioxide lattice, which has 80 atoms per unit cell, x-ray density was calculated from where M stands for the atomic weight of element i (i = Y/Lu, Gd, Eu and O), a is the lattice constant, NA is the Avogadro constant, while m and n denote the atomic percentages of Gd3+ and Eu3+, respectively.

The external quantum efficiency (εex, the total number of emitted photons divided by the total number of excitation photons) of the phosphors was deduced from where P(λ)/hv and E(λ)/hv are the number of photons in the emission and excitation spectra of the samples, respectively.

Results and discussion

AHC/total cation molar ratio (R) impact on (Gd0.75Y0.2Eu0.05)2O3 precursors and their oxides

R has significant effects on the nature of precipitated precursors, and the impact of R on the morphologies of the (Gd0.75Y0.2Eu0.05)2O3 precursors is shown in figure 1.

Figure 1.

FE-SEM micrographs showing morphologies of the (Gd0.75Y0.2Eu0.05)2O3 precursors obtained under typical synthetic conditions, with R = 1 (a), R = 2 (b), R = 3 (c), R = 4.5 (d), R = 7.5 (e) and R = 15 (f).

Figure 1 shows the morphologies of the (Gd0.75Y0.2Eu0.05)2O3 precipitated precursors obtained at R = 1–15 and an ageing time of 1 h. At R = 1–4.5, the colloidal precursors appear spherical and are composed of ultrafine primary particles (figures 1(a)–(d)), whereas those acquired at R = 7.5–15 display non-uniform mixtures and consist of very large and small platelike particles (figures 1(e) and (f)). In particular, at low R (1–2), the precursor powders strongly agglomerate, which is probably because the low pH is close to their isoelectric points.

Ln3+ ions are hydrated to [Ln(H2O)6]3+ and then undergo hydrolysis to form [Ln(OH)(HO)(6−]3− species in water solution (equations (3) and (4)). However, the AHC solution contains many kinds of ionic species, such as HCO3−, CO32−, OH−, H+ and NH4+. Compositions of the precursors formed from the reaction systems depend on the competition of the ionic species that combine with Ln3+ (equations (5)–(7)):

The XRD patterns of the (Gd0.75Y0.2Eu0.05)2O3 precursors obtained at R = 1–15 and an ageing time of 1 h are shown in figure S1 (available from stacks.iop.org/STAM/14/064202/mmedia) of the supporting information. The precursor powders produced at R = 1–4.5 exhibit the amorphous phases corresponding to their spherical particle shapes (figures 1(a)–(d)). As R increases above 7.5, the precursor powders improve their crystallinities and exhibit the characteristics of layered rare-earth carbonate structures (figures 1(e) and (f)), although the diffraction patterns cannot be identified by comparison with the data files of relevant rare-earth compounds of known chemical composition. Elemental analysis (table 1) reveals that the precursors prepared at R = 1–4.5 possess an approximate composition of hydrated carbonate, but low R (e.g. R = 1) would cause a cationic component deviation due to incomplete precipitation. At R ≽ 7.5, the platelike precursors are hydrated ammonium rare-earth carbonate. The platelike precursors may be considered as the layered compounds, which are composed of the cationic layers of Ln3+/NH4+ and interlayered anions of CO32−.

Table 1.

Elemental content (wt%) of precursor powders prepared under typical synthesis conditions.

R	Ageing (h)	Gd	Y	Eu	C	N	Approx. formula
1	1	47.2	4.9	3.4	6.4	–	(Gd0.67Y0.12Eu0.05)2(CO3)2.52·2.4H2O
4.5	1	44.8	6.7	2.8	7.3	–	(Gd0.75Y0.2Eu0.05)2(CO3)3·4H2O
4.5	10	45.8	7.2	2.9	7.3	0.92	(NH4)0.36(Gd0.75Y0.2Eu0.05)2(CO3)3.18·2.4H2O
7.5	1	37.8	5.6	2.4	7.4	3.1	(NH4)1.4(Gd0.75Y0.2Eu0.05)2(CO3)3.7·5.6H2O

The (Gd0.75Y0.2Eu0.05)2O3 precursors undergo a series of thermal decomposition processes to form oxides, including dehydration, decarbonization, crystallite growth, etc (figure S2 in the supporting information (available from stacks.iop.org/STAM/14/064202/mmedia)). Figure 2 reveals that the resultant oxide particles generally retain the original morphologies of their respective precursors. The inside grains of the oxides calcined from the precursors of high R (e.g. R = 7.5–15) are obviously sintered together and crystal boundaries appear (the insets in figures 2(e) and (f)) since the thick plates are difficult to collapse via simple calcination. At R = 4.5, the particles exhibit excellent morphology and the synthesis is of high yield, and hence ageing-time impact was studied in detail under fixed R = 4.5 in the next section.

Figure 2.

FE-SEM micrographs of the (Gd0.75Y0.2Eu0.05)2O3 oxides calcined at 1100 °C for 4 h, obtained from the precursors of R = 1 (a), R = 2 (b), R = 3 (c), R = 4.5 (d), R = 7.5 (e) and R = 15 (f).

Ageing-time effect on the (Gd0.75Y0.2Eu0.05)2O3 precursors and their oxides

Figure 3 displays the impact of ageing time on the properties of the (Gd0.75Y0.2Eu0.05)2O3 precursors and their oxides. At R = 4.5, the ageing time was selected from 1 to 10 h. With an ageing time of 3 h, the precursor also exhibits spherical shape (figure 3(a)) and its oxide is well dispersed (figure 3(d)). As the ageing time increases to 6 h, the precursor and its oxide transform into a platelike morphology (figures 3(b), (e)). When the ageing time is over 10 h, the precursor also keeps the layered structure, but may consist of two phases of the hydrated carbonate and the hydrated ammonium rare-earth carbonate, as shown by the combined results of the element analysis (table 1) and the XRD analysis (figure S3 in the supporting information).

Figure 3.

FE-SEM micrographs showing morphologies of the (Gd0.75Y0.2Eu0.05)2O3 precursors and their oxides: the precursors synthesized at an ageing time of 3 h (a), 6 h (b) and 10 h (c); and (d)–(f) for their oxides, respectively.

Synthesis and characterization of (Gd, Ln)2O3:Eu compounds

A series of (Gd0.95−LnEu0.05)2O3 (Ln = Y and Lu, x = 0–0.95) powders were synthesized under the optimized conditions of R = 4.5 and an ageing time of 3 h.

Figures 4(a)–(c) show the morphologies of (Gd0.95−YEu0.05)2O3 (x = 0, 0.6 and 0.95) precursors. It can be seen that all of the precursor powders exhibit spherical shapes corresponding to their amorphous phases (figure S4 in the supporting information) and the particle sizes increase with greater Y3+ addition (∼ 50, 68 and 78 nm for x = 0, 0.6 and 0.95, respectively, as measured from the FE-SEM micrographs). The composition-dependent particle size is closely related with the nucleation and particle growth processes, while the occurrence of precipitation depends on supersaturation given by the following equation [36]: where aA and aB are the activities of partially hydrolyzed cation [Ln(OH)(HO)] and anion (CO32−), and Ksp is the solubility product constant. Nucleation starts only when S reaches the critical supersaturation S∗>. The solubility of a lanthanide basic carbonate increases with decreased ionic radius of Ln3+ [12]. According to the lanthanide contraction law, the Ksp value increases in the order Eu(OH)CO3 < Gd(OH)CO3 < Y(OH)CO3. Thus, Eu (OH) CO3 and Gd(OH)CO3 are formed in priority to act as crystal nuclei (equation (8)). As the Eu3+ cation and AHC concentration are fixed, more Gd3+ leads to a higher nucleation density and thus smaller particle size. The deviating chemical composition of the precursor powder prepared at R = 1 for 1 h also supports our interpretations (table 1).

Figure 4.

FE-SEM micrographs of (Gd0.95−LnEu0.05)2O3 precursor powders with Ln = Y, x = 0 (a), x = 0.6 (b) and x = 0.95 (c); Ln = Lu, x = 0.1 (d), x = 0.6 (e) and x = 0.95 (f).

Figures 4(d)–(f) are FE-SEM micrographs showing the morphologies of (Gd0.95−LuEu0.05)2O3 (x = 0.1, 0.6, 0.95) precursors. It can be observed that a higher Lu3+ content leads to much larger particles (∼ 56, 78 and 101 nm for x = 0.1, 0.6 and 0.95, respectively, as measured from the FE-SEM micrographs) of frequently empty interiors. This phenomenon can be interpreted from the nucleation kinetics among Lu(OH)CO3, Eu(OH)CO3 and Gd(OH)CO3. Although the Ksp increases in the order Eu(OH)CO3 < Gd(OH)CO3 < Lu(OH)CO3, the relatively high degree of hydrolysis for Lu3+ may dominate here. The higher hydrolysis degree of Lu3+ induces a higher activity of [Lu(OH)(HO)]; therefore Lu(OH)CO3 is produced in priority in the Gd–Lu–Eu ternary system. Meanwhile, the high degree of hydrolysis also causes a low pH value of the nitrate solution (equation (4)). Thus, when the AHC is added, a fair amount of small bubbles would be generated via chemical decomposition of AHC (equation (9)), some of which provide crystal nuclei to form the hollow structures. According to our previous studies [37, 38], such a phenomenon is also observed for other rare-earth cations with high degrees of hydrolysis, such as Sc-containing compounds.

Figure 5 exhibits the morphologies of the (Gd0.95−YEu0.05)2O3 (x = 0, 0.6, 0.95) and (Gd0.95−LuEu0.05)2O3 (x = 0.1, 0.6, 0.95) powders calcined at 1100 °C for 4 h. The resultant oxide particles are ultrafine, uniform in size, well-dispersed and rounded in particle shape. The particle size distribution curve is given out in figure S5 of the supporting information (available from stacks.iop.org/STAM/14/064202/mmedia) by taking the (Gd0.95Eu0.05)2O3 powder as an example. This sample shows a narrow and unimodal particle size distribution.

Figure 5.

FE-SEM micrographs of (Gd0.95−LnEu0.05)2O3 oxide powders with Ln = Y, x = 0 (a), x = 0.6 (b) and x = 0.95 (c); Ln = Lu, x = 0.1 (d), x = 0.6 (e) and x = 0.95 (f).

Tables 2 and 3 show the SBET value, the grain size calculated by XRD, the statistical particle-size data from the FE-SEM micrographs obtained with the WinRoof image analysis software, and the particle size measured via LDPSA of the (Gd,Ln)2O3:Eu oxides.

Table 2.

SBET, crystallite size, SEM size and particle size of (Gd,Y)2O3:Eu oxides.

Y3+ content (%)	SBET (m2 g−1)	XRD (nm)	SEM (nm)	LDPSA (nm)
0	6.8	46.3	187	288
10	12.3	46.3	165	303
20	12.7	41.6	156	216
40	14.5	46.3	153	218
60	19.4	38.0	120	182
80	21.7	41.7	115	145
95	15.4	41.7	116	142

Table 3.

SBET, crystallite size, SEM size and particle size of (Gd,Lu)2O3:Eu oxides.

Lu3+ content (%)	SBET (m2 g−1)	XRD (nm)	SEM (nm)	LDPSA (nm)
10	15.4	46.3	126	199
20	18.3	38.0	106	173
40	19.2	38.0	105	216
60	19.7	34.8	87	154
80	16.8	34.8	78	224
95	15.4	34.8	80	328

The grain sizes of the oxide powders generally decrease with increasing Y3+ or Lu3+ incorporation, since the lattice constants of (Y0.95Eu0.05)2O3 (10.61 Å, figure 7) and (Lu0.95Eu0.05)2O3 (10.40 Å) are much smaller than that of (Gd0.95Eu0.05)2O3 (10.80 Å). Mass diffusion in (Y0.95Eu0.05)2O3 and (Lu0.95Eu0.05)2O3 may be slower than that in (Gd0.95Eu0.05)2O3 upon calcination and, therefore, smaller grain sizes result at a higher Y3+ or Lu3+ content. The SEM sizes of the oxides also generally decrease along with more Y3+ or Lu3+ addition. LDPSA is effective in detecting agglomerates [39]. Some relatively small particle sizes detected by LDPSA indicate less agglomeration and higher dispersion of the powders.

Figure 7.

Lattice constants and theoretical densities of the (Gd,Ln)2O3:Eu solid solutions with the composition (Gd0.95−YEu0.05)2O3 (a) and (Gd0.95−LuEu0.05)2O3 (b).

Figure 6 exhibits the XRD patterns of the (Gd,Ln)2O3:Eu oxide powders calcined at 1100 °C for 4 h. The XRD peaks of the two kinds of solid solutions shift toward the high-angle side along with more Y3+ or Lu3+ incorporation. Apparently, Y3+ and Lu3+ doping leads to a gradual contraction in the unit cell of the cubic crystal owing to the smaller ionic radius of Y3+ and Lu3+ than that of Gd3+ and Eu3+.

Figure 6.

XRD patterns of (Gd,Ln)2O3:Eu oxides with the composition (Gd0.95−YEu0.05)2O3 (a) and (Gd0.95−LuEu0.05)2O3 (b).

The calculated lattice constants and theoretical densities of the (Gd0.95−LnEu0.05)2O3 solid solutions are shown in figure 7. The cell parameters linearly decrease along with increasing x value, which observes Vegard's law, implying that homogeneous solid solutions have been formed. The theoretical densities calculated with the cell parameters linearly decrease with more Y3+ addition due to its atomic number which is lower than Gd3+, whereas the theoretical densities increase at a higher Lu3+ content since it is much heavier than Gd3+.

Figure 8 shows the PL spectra of (Gd,Ln)2O3:Eu phosphors. Both (Gd,Y)2O3:Eu and (Gd,Lu)2O3:Eu oxide powders calcined at 1100 °C exhibit the characteristic emissions of Eu3+ from 5D0 → 7F (J = 0, 1, 2, 3, 4) transitions. The sharpest emission peak at 613 nm and that at 630 nm are associated with the 5D0 → 7F2 transitions of Eu3+. The group of emission peaks at 587, 592 and 598 nm derives from 5D0 → 7F1 of Eu3+, while those at 582, 652 and 708 nm are assigned to the 5D0 → 7F0, 5D0 → 7F3 and 5D0 → 7F4 transitions, respectively.

Figure 8.

PL behaviors of the (Gd,Ln)2O3:Eu phosphors with the composition (Gd0.95−YEu0.05)2O3 (a) and (Gd0.95−LuEu0.05)2O3 (b). The PL spectra were obtained under excitations with the peak wavelengths of the CT bands shown in figure 9.

Figure 9.

PLE spectra of the (Gd,Ln)2O3:Eu phosphors with the composition (Gd0.95−YEu0.05)2O3 (a) and (Gd0.95−LuEu0.05)2O3 (b). The PLE spectra were obtained by monitoring the 613 nm emissions of Eu3+.

The excitation spectra of (Gd,Ln)2O3:Eu particles obtained with the emission at 613 nm are shown in figure 9. The main bands at 248–258 nm are due to the transition by CT, viz., electronic transition from the 2p orbital of O2− to the 4f orbital of Eu3+, the peaks at 217–235 nm as the left shoulder of the broad CT bands are ascribed to the absorption of rare-earth-oxide host lattice, and the right ones at 276 nm are owing to the 8S7/2 → 6I internal f–f transition of Gd3+ [6]. The overlapped bands from 300 to 335 nm arise from the 8S7/2 → 6P of Gd3+ and 7F0,1 → 5H3/5H6 transitions of Eu3+. The two types of bands can be distinguished by comparing with the non-Gd3+-containing samples, that is, the peaks at 308 and 314 nm are only for the 8S7/2 → 6P transition of Gd3+. The other weak peaks beyond 350 nm are assignable to the f–f Eu3+ transition.

The PL and photoluminescence excitation (PLE) intensities of (Gd,Ln)2O3:Eu powders mainly depend on the amount of Gd3+, since the differences in electronegativities among Gd3+, Y3+ and Lu3+ result in diverse abilities to attract electrons and even shifted-peak locations of CT bands [12, 22] (6 nm for (Gd,Y)2O3:Eu and 10 nm for (Gd,Lu)2O3:Eu). The low electronegativity for Gd3+ (1.20) leads to easy electron transfer, and thus PL intensity is enhanced, although the larger particle size of its oxide also contributes to the relatively strong emission. For (Gd,Y)2O3:Eu and (Gd,Lu)2O3:Eu powders of the same Gd3+ concentration, the PL and PLE intensities are in the order (Gd,Lu)2O3:Eu < (Gd,Y)2O3:Eu. This is also owing to the lower electronegativity of Y3+ (1.22) than that of Lu3+ (1.27). In this work, the (Gd0.95Eu0.05)2O3 phosphor exhibits PL intensity at 613 nm, about 1.7 times that of the (Y0.95Eu0.05)2O3 phosphor and 2.9 times that of the (Lu0.95Eu0.05)2O3 phosphor, which is similar to the relative CT band intensity ratios.

Calcination temperature also plays an important role in the luminescence of the resultant phosphors and the results are shown in figure 10 with the (Gd0.75Y0.2Eu0.05)2O3 sample as an example. The PL intensity increases with increasing calcination temperature owing to the more homogeneous component distribution among Gd3+, Y3+ and Eu3+, which alleviates localized concentration quenching of Eu3+. Improved crystallinity (figure S6 in the supporting information (available from stacks.iop.org/STAM/14/064202/mmedia)) and larger particle size are another two important reasons. The phosphor particles calcined at 500 °C exhibit low emission intensity probably because of the combined effects of disordered coordination between Ln3+ and O2− in the amorphous structure (figure S6 in the supporting information) and the presence of CO32− in the intermediate calcination product (figure S2 in the supporting information), which may raise the non-radiative relaxation rate. The PL intensity is significantly improved as the temperature increases to 600 °C, due to the phase transformation from an amorphous to a cubic phase. A linear increase in PL intensity is observed at a higher calcination temperature from 600 to 1300 °C (the inset in figure 10).

Figure 10.

The effects of calcination temperature on PL intensity of the (Gd0.75Y0.2Eu0.05)2O3 phosphors. The PL spectra were obtained via excitation at 258 nm.

Figure 11 shows the fluorescence lifetime of the (Gd,Ln)2O3:Eu phosphors, which was obtained from the fluorescence decay curves (figure S7 in the supporting information) by a single exponential equation where τ, t and I denote the fluorescence lifetime, delay time and relative intensity, respectively, and A and B are constants.

Figure 11.

Fluorescence lifetime of the (Gd,Ln)2O3:Eu phosphors.

The lifetimes of Eu3+-doped Y2O3, Gd2O3 and Lu2O3 phosphors vary in the range of 1.1–2.6 ms [6, 32, 40–44]. In this work, the fluorescence lifetime is observed at 1.48–2.23 ms for the (Gd,Y)2O3:Eu and 1.48–2.52 ms for the (Gd,Lu)2O3:Eu phosphors, which are close to each other and are in good agreement with the reported values. The fluorescence lifetime decreases at a higher calcination temperature largely due to the smaller specific surface area (larger particle size) and improved crystallinity (figure S8 in the supporting information) [45, 46].

The external quantum efficiency was reported to be in the wide range of 8.0–91.9% for the Y2O3:Eu [47-49], 4.6–23.6% for the Gd2O3:Eu [50, 51] and 30–90% for the Lu2O3:Eu phosphors [44, 52, 53]. Such significant deviation can be ascribed to varied particle sizes, particle morphologies and crystal defects, etc. As shown in figure 12, the εex is found in this study to be 63–100% for (Gd,Y)2O3:Eu and 46–100% for (Gd,Lu)2O3:Eu, and depends on the Gd3+ concentration, which may be attributed to the sensitization effect of Gd3+ [33]. In addition, the εex dramatically increases with increasing calcination temperature of the sample (figure S9 in the supporting information). A 700 °C increase from 600 to 1300 °C improves the εex by ∼83.3%, mainly owing to the crystal perfection.

Figure 12.

External quantum efficiency of the (Gd,Ln)2O3:Eu phosphors.

Conclusions

Well-dispersed (Gd,Ln)2O3:Eu powders have been synthesized via AHC precipitation under optimized conditions of R = 4.5 and an ageing time of 3 h. The nucleation kinetics between Gd–Y–Eu and Gd–Lu–Eu systems are different. For the Gd–Y–Eu ternary system, the nucleation order is found to be Y3+ < Gd3+ < Eu3+. On the other hand, the nucleation density increases in the order of Gd3+ < Eu3+ < Lu3+ in the Gd–Lu–Eu system. The grain sizes of the resultant oxides generally decrease with increasing Y3+ or Lu3+ addition. The lattice parameters of the two kinds of oxide solid solutions linearly shrink at a higher Y3+ or Lu3+ content. The theoretical densities of the (Gd,Ln)2O3:Eu phosphor linearly decrease with increasing Y3+ incorporation, but increase along with more Lu3+ addition. The (Gd,Ln)2O3:Eu phosphor particles exhibit typical red emissions at 613 nm upon UV excitation into the CT band at 248–258 nm. The center of the CT bands red shifts at a higher Gd3+ content. The PL/PLE intensities and εex of the (Gd,Ln)2O3:Eu phosphors improve at a higher Gd3+ concentration. Elevated calcination temperature enhances the PL intensity and the εex, however, lowers the lifetime of the (Gd,Ln)2O3:Eu phosphors. The two kinds of materials made in this work may find potential applications in luminescence and scintillation fields.