Monodispersed Colloidal Spheres for Uniform Y2O3:Eu3+ Red

J. Phys. Chem. C 2008, 112, 11707–11716

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Monodispersed Colloidal Spheres for Uniform Y2O3:Eu3+ Red-Phosphor Particles and Greatly Enhanced Luminescence by Simultaneous Gd3+ Doping Ji-Guang Li,*,†,‡ Xiaodong Li,† Xudong Sun,† and Takamasa Ishigaki‡ Key Laboratory for Anisotropy and Texture of Materials (Ministry of Education), School of Materials and Metallurgy, Northeastern UniVersity, Shenyang, 110004, China, and Nano Ceramics Center, National Institute for Materials Science, Namiki 1-1, Tsukuba, Ibaraki, 305-0044, Japan ReceiVed: March 16, 2008; ReVised Manuscript ReceiVed: June 3, 2008

Uniform red-phosphor spheres (∼60-300 nm in diameter) of Y2O3:Eu3+ binary and (Y,Gd)2O3:Eu3+ ternary systems exhibiting excellent emission at 610 nm have been converted from their colloidal precursor spheres synthesized via homogeneous precipitation. The precursor spheres (approximate composition: [(Y1-xGdx)1-yEuy](OH)CO3 · 1.3H2O, x ) 0-0.5 and y ) 0-0.11) are directly solid solutions, but arising from sequential nucleation each of the spheres has more Gd and especially Eu while having less Y going from the particle surface to the core. Eu3+ is more effective than Gd3+ in raising nucleation density, leading to rapidly decreased average size of the precursor particles at a higher Eu3+ addition. Diminishing the concentration gradients through adequate annealing is identified to be crucial to high luminous intensity of the oxide particles. At the optimal annealing temperature of 1000 °C, cation homogenization is achieved and the oxide particles largely retain their precursor morphologies, yielding dispersed uniform spheres of excellent luminescence. The (Y1-xEux)O1.5 phosphor particles exhibit typical red emissions at 610 nm upon UV excitation into the charge transfer band at ∼255 nm, and the quenching concentration of Eu3+ is found to be ∼5 at. %. Partially replacing Y3+ with Gd3+ (up to 50 at. %) while keeping Eu3+ at the optimal content of 5 at. % linearly improves the 610 nm emission, and the phosphor particles of [(Y0.5Gd0.5)0.95Eu0.05]O1.5 exhibit an luminous intensity ∼103% of that of a commercially available Y2O3:Eu red phosphor. The uniform phosphor spheres obtained in this work are expected to have wide applications in high-resolution display technologies of contemporary interest. 1. Introduction Eu3+ activated Y2O3 exhibits sharp red emissions at ∼610 nm arising from the 5D0 f 7F2 intra-4f electronic transitions of the Eu3+ ions, upon UV excitation into the charge transfer (CT) band at ∼254 nm. The material is currently the most widely used red phosphor finding frequent applications in areas such as fluorescent lamps, white LEDs, plasma display panels (PDP), flat-panel displays (FPD), field emission displays (FED), and cathode-ray tubes (CRT).1 For applications in tricolor fluorescent lamps, for example, the Y2O3:Eu3+ red phosphor constitutes ∼60 wt % of the total phosphor particles, with the other two being ∼30 wt % of CeMgAl11O19:Tb3+ green phosphor and ∼10 wt % of BaMgAl10O17:Eu2+ blue phosphor.2 Though new types of red phosphors, such as (oxy)nitrides, oxysulfides, and vanadates,3 are now under investigation and development, Y2O3: Eu3+ is still attractive in view of its excellent chemical and thermal stabilities, its simple chemical composition, and the relative ease of its synthesis. Luminescent behavior of a phosphor powder is dependent upon its particle size and size distribution. Smaller particles tend to have more surface luminous states arising from their increased surface/bulk volume ratio and may thus exhibit luminescent behaviors different from the bigger ones.4 Phosphor particles of a uniform size are thus beneficial to a highly efficient and uniform luminescence. For practical applications, current ad* To whom correspondence should be addressed at the National Institute for Materials Science. Telephone: +81-29-860-4394. Fax: +81-29-8604701. E-mail: [email protected]. † Key Laboratory for Anisotropy and Texture of Materials (Ministry of Education), School of Materials and Metallurgy, Northeastern University. ‡ Nano Ceramics Center, National Institute for Materials Science.

vances in high-resolution displays have placed new requirements on characteristics of the phosphor particles. To improve resolution by decreasing pixel size, phosphor particles of smaller size, sharp size distribution, and spherical shape are highly desired. A narrow size distribution helps to build a uniform phosphor layer, allowing a uniform luminescence on the whole phosphor screen. The spherical shape, on the other hand, may offer dual advantages over other morphologies in that (1) it allows a denser phosphor layer to be formed through close packing of the spheres5 and (2) it minimizes the light scattering on surfaces of the phosphor particles.6 With these two combined merits, the efficiency of luminescence and the brightness of the phosphor panel may be improved appreciably. Y2O3:Eu3+ phosphors are classically made via solid state reaction at temperatures well above 1000 °C. Repeated heating and extensive intermittent grinding are often necessary to homogenize the Eu3+ activators, finally yielding coarse particles of irregular shapes and considerable aggregation.2 Extensive post milling may alleviate aggregation and bring down the particle sizes to 1-3 µm. Such an operation, however, usually results in a dramatic decrease in luminous intensity and a significant broadening of the emission peaks, due to the defects introduced by crushing.7 As a consequence, the overall performance of the phosphor particles deteriorates considerably. Spray pyrolysis is a straightforward way to produce dispersed phosphor particles of spherical morphologies, but unfortunately the resultant particles mostly have hollow structures and show wide size distributions in the nanomicron regime.8 Other techniques have been able to produce Y2O3-based red phosphor particles of finer sizes and have their respective advantages, such as one-step synthesis, low processing temperature and so on,9d–g but

10.1021/jp802383a CCC: $40.75  2008 American Chemical Society Published on Web 07/15/2008

11708 J. Phys. Chem. C, Vol. 112, No. 31, 2008

Li et al.

TABLE 1: Elemental Contents of the Precursor Spheres Intended for Oxides [(Y1-xGdx)1-yEuy]2O3 sample id S1 S2 S3 S4 S5 S6 S7 S8 S9 S10

intended x, y values x x x x x x x x x x

) ) ) ) ) ) ) ) ) )

0, y ) 0.01 0, y ) 0.03 0, y ) 0.05 0, y ) 0.07 0, y ) 0.09 0, y ) 0.11 0.10, y ) 0.05 0.25, y ) 0.05 0.40, y ) 0.05 0.50, y ) 0.05

Y (wt %) 46.33 44.81 43.88 43.00 41.61 40.21 38.13 30.47 23.44 18.86

Gd (wt %)

Eu (wt %)

C (wt %)

approximate chemical formulas

7.50 17.93 27.67 33.34

0.80 2.37 3.94 5.55 7.03 8.51 3.83 3.65 3.53 3.40

6.32 6.24 6.24 6.24 6.19 6.13 6.12 5.80 5.58 5.38

(Y0.99Eu0.01) (OH)CO3 · 1.3H2O (Y0.97Eu0.03) (OH)CO3 · 1.37H2O (Y0.95Eu0.05) (OH)CO3 · 1.3H2O (Y0.93Eu0.07) (OH)CO3 · 1.22H2O (Y0.91Eu0.09) (OH)CO3 · 1.27H2O (Y0.89Eu0.11) (OH)CO3 · 1.33H2O [(Y0.9Gd0.1)0.95 Eu0.05] (OH)CO3 · 1.31H2O [(Y0.75Gd0.25)0.95 Eu0.05] (OH)CO3 · 1.26H2O [(Y0.6Gd0.4)0.95 Eu0.05] (OH)CO3 · 1.17H2O [(Y0.5Gd0.5)0.95 Eu0.05] (OH)CO3 · 1.25H2O

frequently find difficulties in effectively controlling particle morphology (size, size distribution, shape, aggregation) and sometimes even phase structure.9e–i The making of dispersed phosphor spheres of uniform sizes proves still rather challenging. Meanwhile, some lanthanides (Ln) are known to precipitate as basic carbonate colloidal spheres of uniform sizes by heating a mixed solution containing Ln3+ and a proper amount of urea at temperatures >83 °C.10 The in situ decomposition of urea releases precipitating ligands (mainly OH- and CO32-) slowly and homogeneously into the reaction system, avoiding localized distribution of the reactants and thus making it possible to exercise control over nucleation and particle growth. The methodology was accordingly termed as homogeneous precipitation (HP). It should be noted that the HP technique, though proved successful in making monodispersed colloidal spheres of some single lanthanides, has rarely been extended to multilanthanide systems. The lanthanides (including Y) are known to exhibit slowly yet successively changed physicochemical properties along with decreased ionic radius of the element (the lanthanide contraction phenomena).11 In practice, it was actually observed that light lanthanides tend to precipitate in a way different from heavier ones,10a,d in terms of chemical composition and morphology of the resultant particles. For mixed systems, concerns thus arise as to

whether the lanthanides are evenly distributed among the particles and within each particle. Such an issue was not well addressed in the works5b,10c,12 prior to our very recent study on the Y/Gd mixed system.13 The issue turns particularly important while applying the HP methodology to synthesize phosphor particles, since if not treated properly a nonuniform distribution of activators may lead to localized concentration quenching of luminescence and thus a poor luminescent performance. Monodispersed colloidal spheres of the Y/Eu binary system and even the Y/Gd/Eu ternary system have been successfully made in this work with the HP technique, and they are subsequently converted into luminescent oxide spheres of uniform sizes via thermal decomposition at proper temperatures. The simultaneous incorporation of Gd into the Y/Eu system was observed to greatly enhance luminescence of the finally resultant phosphor particles. Furthermore, new insights have been gained with regard to precipitation mechanisms of the mixed systems, which explain well the observed particle growth kinetics and compositional distributions of the lanthanides. The findings of this work are expected to have wide implications for other materials systems.

Figure 1. Typical FE-SEM micrographs showing morphologies of the colloidal precursor spheres for the Y/Eu binary systems. (a-d) samples S1, S3, S4, and S6 (Table1), respectively.

Monodispersed Colloidal Spheres

J. Phys. Chem. C, Vol. 112, No. 31, 2008 11709

Figure 2. Typical FE-SEM micrographs showing morphologies of the colloidal precursor spheres for the Y/Gd/Eu ternary systems. (a-d) samples S7, S8, S9, and S10 (Table 1), respectively.

Figure 3. Elemental mapping of Gd (a), Y (b), and Eu (c) distributions in the dried precursor spheres of sample S8 (Table 1). (d) SEM image of the particles.

2. Experimental Section 2.1. Synthesis of Monodispersed Colloidal Precursor Spheres. In a typical synthetic procedure, proper amounts of Y(NO3)3 · 6H2O (>99.99% pure, Kanto Chemical Co., Inc., Tokyo, Japan), Gd(NO3)3 · 6H2O (>99.95% pure, Kanto Chemical), Eu(NO3)3 · 6H2O (>99.95% pure, Kanto Chemical), and

urea (CO(NH2)2, >99% pure, Kanto Chemical) were dissolved in distilled water to make a total volume of 2000 mL. In each case, the total concentration of Eu3+, Gd3+, and Y3+ was kept constant at 0.015 mol/L while that of urea was kept at 0.5 mol/ L. The mixed solution, contained in a beaker wrapped with aluminum foil, was first homogenized under magnetic stirring


Li et al. the oxides were determined by fitting the observed reflections with a least-squares refinement program. X-ray densities of the solid-solution oxides were obtained from the following equation, assuming that Eu3+ and Gd3+ ions exclusively replace the Y3+ sites of the cubic-structured C-type Y2O3 sesquioxide lattice which has eighty atoms per unit cell:

dth )

32 × [(1 - m - n)MY + mMGd + nMEu + 1.5MO] a3NA (1)

Figure 4. Average diameters of the colloidal precursor spheres, as a function of the Eu/Gd content. Standard size deviations are given out as error bars, while the figure in brackets at each data point denotes the size deviation in percentage.

at room temperature for 2 h and was then heated on a hot plate to 90 ( 1 °C within 60 min. After reacting at 90 ( 1 °C for 2 h, the suspension was cooled naturally to ∼50 °C and the resultant colloidal particles were recovered via suction filtration through a 0.2-µm membrane. Byproducts of the reaction were removed by washing for 4 times the particles with distilled water via ultrasonication and suction filtration. After rinsing with anhydrous ethanol (except the part for elemental analysis), the particles were dried in an air oven at 100 °C for 24 h and were then calcined in a tube furnace under flowing O2 gas (100 mL/ min) at selected temperatures for 4 h to produce oxide particles. To understand size and composition evolution of the precursor particles, 30 mL of the colloidal suspension was sampled regularly since visible turbidity appeared in the originally clear solution. The sampled suspension was quenched with iced water and was centrifuged to achieve solid/liquid separation. After washing and drying as described above, the particles were used for further analysis. 2.2. Characterization Techniques. Compositions of the precursor particles were determined by elemental analysis. The Y3+, Gd3+, and Eu3+ contents were analyzed by the inductively coupled plasma (ICP) spectrophotometric method (Model IRIS Advantage, Nippon Jarrell-Ash Co. Ltd., Kyoto, Japan) with an accuracy of 0.01 wt %; carbon content was assayed on a simultaneous carbon/sulfur determinator with a detection limit of 0.01 wt % (Model CS-444LS, LECO, St. Joseph, MI); NH4+ content was determined by the standard distillation-titrimetric method with an experimental error of ( 0.1 wt %. Morphologies of the particles were observed via fieldemission scanning electron microscopy (FE-SEM, Model S-5000, Hitachi, Tokyo). Average diameter and standard size deviation of the particles were derived from 200 randomly selected particles with an image analysis software. Elemental mapping was performed on a field emission electron probe microanalyzer (FE-EPMA, Model JXA-8500F, JEOL, Tokyo) by analyzing the KR1 lines of Y, Gd, and Eu at 0.64487 nm, 0.2046 nm, and 0.21206 nm, respectively. Phase identification was performed via X-ray diffractometry (XRD) on a Philips PW1800 X-ray diffractometer (Philips Research Laboratories, Eindhoven, The Netherlands) operating at 40 kV/50 mA using nickel-filtered Cu-KR radiation with a scanning speed of 0.15 ° 2θ per minute. Lattice parameters of

where Mi stands for atomic weight of element i (i ) Y, Gd, Eu, and O), a the lattice constant, NA the Avogadro constant, while m and n denote the atomic percentages of Gd3+ and Eu3+, respectively. Specific surface areas (SBET) of the oxide powders calcined at 1000 ° were measured by the Brunauer-Emmett-Teller (BET) method on an automatic surface area analyzer (Model 4201, Beta Scientific Corporation, Albertson) via nitrogen adsorption at 77 K. Average particle diameter was estimated from the following equation, assuming that the particles are ideal solid spheres and have smooth surfaces:

DBET )

6000 dth × SBET

(2)

where dth is the theoretical density (g/cm3) calculated from eq 1, DBET is the average particle size (nm), and SBET is the specific surface area expressed in m2/g. Photoluminescence excitation (PLE) and photoluminescence (PL) properties of the [(Y1-xGdx)1-yEuy]O1.5 solid solutions were collected on a fluorescence spectrophotometer (Model F-4500, Hitachi, Tokyo) at room temperature with a 300 W Xe-lamp as the light source. Samples of the same weight (4 g) were separately packed in quartz cuvettes with cross-section sizes of 10 × 10 mm and a length of 45 mm. Measurements were conducted under identical conditions for all the samples with a scanning speed of 120 nm/min and slit sizes of 1.0 and 2.5 nm for the PLE and PL sides, respectively. 3. Results and Discussion 3.1. Characterizations of the Colloidal Precursor Spheres. Elemental contents of the dried precursor particles are summarized in Table 1, from which it can be seen that the particles contain considerable amounts of carbon (from CO32-, supplied by the hydrolysis of urea) besides the intended lanthanides. NH4+ was also analyzed, but its content is below the detection limit (0.1 wt %) in each case. From the results of elemental analysis and by considering molecular electrical neutrality, the precursor particles were all found to possess an approximate chemical composition of [(Y1-xGdx)1-yEuy](OH)CO3 · 1.3H2O (x ) 0-0.5, y ) 0-0.11). The formation of basic carbonate particles conforms to previous reports10,12a and our own observations for the Y/Gd binary system.13 In addition, the results of elemental analysis follow closely the aimed amounts of the lanthanides, suggesting quantitative precipitation for all the mixed systems. Chemical analysis of the supernatant after solid-particle removal, on the other hand, revealed negligible amounts of remnant cations, indicating an almost complete precipitation by reaction at 90 ( 1 °C for 2 h. Figure 1 shows morphologies of the precursor particles (redispersed from dried powders) for the Y/Eu binary systems. It can be seen that monodispersed colloidal spheres are obtainable with the HP technique and that a higher Eu content steadily leads to finer particles. The particles remain spherical,



Figure 5. Time-course evolution of particle size for sample S8 (Table 1). Growth time indicated in the FE-SEM micrographs. “0 min” denotes the starting point of analysis, where visible turbidity appears in the originally transparent solution.

Figure 6. Time-course evolution of particle size for sample S8 (Table 1). The inset is the fitting of the growth data with the cubic-root law.

Figure 7. Time-course evolution of particle composition for sample S8 (Table 1).

uniformly sized, and highly dispersed by partially replacing Y with Gd while keeping constant the Eu content at 5 at. % (y ) 0.05). For the [(Y1-xGdx)0.95Eu0.05](OH)CO3 (x ) 0-0.5) ternary systems, again, average diameter of the spheres turns successively smaller at a higher Gd addition (Figure 2). The observed uniform particle morphology and the composition-dependent systematic size variation implies that the colloidal spheres are solid solutions rather than mechanical mixtures of individual Y(OH)CO3, Gd(OH)CO3, and Eu(OH)CO3 phases. Elemental mapping of Y, Gd, and Eu (for sample S8, Table 1) indicates that each particle contains the three intended lanthanides and that they are quite evenly distributed among the particles (Figure 3).14 Thus it can be said that basic-carbonate solid solutions can be formed not only for the Y/Gd binary system13 but also for the more complicated Y/Gd/Eu ternary system, despite the even bigger size mismatch between Y3+ and Eu3+ (for 6-fold

coordination, rY3+ ) 0.0900 nm, rGd3+ ) 0.0938 nm, and rEu3+ ) 0.0947 nm).15 The substantial composition-dependent size variation of precursor particles was previously observed by us for the Y/Gd binary system and has been interpreted from the different nucleation probability of Y(OH)CO3 and Gd(OH)CO3.13 The basic-carbonate colloidal spheres are formed via nucleation/ growth processes, while the occurrence of precipitation needs supersaturation, S, given by16

S ) aAaB/Ksp

(3)

where aA and aB are in this work the activities of partially hydrolyzed cation ([Ln(OH)x(H2O)y]3-x, x+y ) 6, Ln)Y, Gd, Eu) and anion (CO32-), respectively, and Ksp is the solubility product constant. Nucleation starts only when S reaches the critical supersaturation S*. The solubility in water of a lanthanide


Figure 8. Phase evolution of sample S10 (Table 1), as a function of the calcination temperature.

Figure 9. Lattice constants of the oxides calcined at 1000 °C, as a function of the Eu/Gd content.

basic carbonate increases with decreased ionic radius of the Ln3+ ions,17 following the lanthanide contraction law. From the ionic radii of Y3+, Gd3+, and Eu3+, it can thus be said that the Ksp value increases in the order Y(OH)CO3 > Gd(OH)CO3> Eu(OH)CO3. It can then be inferred from eq 3 that stable nuclei of Y(OH)CO3 are the most difficult while those of Eu(OH)CO3 are the easiest to be formed. For the Y/Eu binary systems, homogeneous nucleation of Eu(OH)CO3 occurs in priority and then the precipitation of Y(OH)CO3 largely proceeds via heterogeneous nucleation on the already formed Eu(OH)CO3 nuclei. The higher the Eu content, the higher the nucleation density. As the cation and urea concentrations are fixed, an enhanced nucleation leaves behind fewer solutes for particle growth and thus average size of the final resultant particles steadily decreases at a higher Eu addition. Such a scenario similarly holds for the Y/Gd/Eu ternary systems when fixing the Eu addition at 5 at. %. Plotting the average particle diameter against the x value in the (Y1-xEux)(OH)CO3 binary system and in the [(Y1-xGdx)0.95Eu0.05](OH)CO3 ternary system both yields linear lines (Figure 4), implying that the mean particle size is inversely proportional

Li et al. to nucleation density. However, more pronounced effects were observed for Eu3+ than Gd3+ in raising nucleation density, as can be seen from the steeper slope of the linear line in the former case. This is also perceivable from the smaller Ksp of Eu(OH)CO3. To gain better understanding of the precipitation mechanism, regular sampling and analysis of the particles formed at different reaction stages were made with sample S8 as an example. The regularly sampled colloidal particles were observed to grow quite uniformly with reaction time (Figure 5), indicating that the LaMer model18 may apply in these mixed systems. The particle growth data can be well fitted with the cubic-root law (Figure 6) given byD(t) ) [3]{Kt}, where D(t) is the average particle diameter at time t, K is the growth rate and t is reaction time, indicating that particle growth is surface-diffusion controlled16 even for the complicated Y/Gd/Eu ternary system. Composition evolution of the colloidal spheres (sample S8, Table 1)) was investigated through elemental analysis via ICP of their instantaneous cation contents and the results are presented in Figure 7, where the contents of Y, Gd, and Eu have been normalized to their respective intended values to reveal the evolution more clearly. The normalized values would keep constant at 1.0 for the three elements during the whole process of reaction should stoichiometric precipitation takes place instantaneously. It was thus concluded from Figure 7 that hyper-stoichiometric precipitation of Eu and Gd (especially Eu) while hypo-stoichiometric precipitation of Y occurred up to 75 min of reaction. At the start of the analysis, where visible turbidity appeared in the originally transparent solution, the particles have an actual composition of (Y0.59Gd0.32Eu0.08)(OH)CO3, though the intended composition of [(Y0.75Gd0.25)0.95Eu0.05](OH)CO3 (that is, (Y0.7125Gd0.2375Eu0.05)(OH)CO3) was expected from a stoichiometric precipitation. Along with reaction and particle growth, more Y is built in and the particles turn overall stoichiometric after 75 min of reaction. Inferred from Figure 7, the colloidal particles have increased Eu and Gd (especially Eu) while decreased Y contents going from surfaces to the cores. The results of Figure 7 support well the sequential nucleation/precipitation hypnosis employed in explaining the observed compositional effects on average sizes of the final resultant colloidal particles. 3.2. Characterizations and Photoluminescence Behaviors of the Oxide Spheres. The basic carbonate precursors are readily converted into oxides via thermal decomposition. Figure 8 displays phase evolution of the basic carbonate spheres upon calcination, with the most heavily doped S10 sample (Table 1) as an example. The amorphous precursor directly transforms to oxide at 600 °C, and almost all the diffraction peaks corresponding to Y2O3-based solid solution of cubic structure (space group: Ia3j, JPCDS: 43-1036) have appeared at this temperature. The monoclinic-structured (Y1-xGdx)2O3:Eu3+ solidsolution phase, though reported to form in some cases,9e–h was not observed up to 1300 °C for all the samples in this study. A careful examination of the (222) diffraction, however, revealed that the position of this peak shifts with temperature up to 1000 °C, that is, it appears at 2θ ) 29.08, 28.84, 28.98, and 28.86° for the powders calcined at 600, 700, 850, and 1000 °C, respectively. This phenomenon suggests redistribution through interdiffusion of the differently sized Y3+, Gd3+, and Eu3+ ions to form a homogeneous solid solution of [(Y0.5Gd0.5)0.95Eu0.05]2O3 at higher temperatures. The redistribution of cations happens owing to the concentration gradients within each of the precursor sphere (Figure 7). Peak shifting is no longer observed at temperatures above 1000 °C, indicating the formation of



Figure 10. FE-SEM micrographs showing morphologies of the oxide particles calcined at 1000 °C, with (a) sample S3 [(Y0.95Eu0.05)O1.5], (b) sample S6 [(Y0.89Gd0.11)O1.5], (c) sample S8 [(Y0.75Gd0.25)0.95Eu0.05O1.5], and (d) sample S10 [(Y0.5Gd0.5)0.95Eu0.05O1.5]. Inset in Figure 10d is the observed luminescence of the phosphor powder under 254-nm excitation (taken with a digital camera).

TABLE 2: Some Properties of the [(Y1-xGdx)1-yEuy]2O3 Particles Obtained at 1000 °C sample id

SBET (m2/g)a

dth (g/cm3)b

DBET (nm)c

DSEM (nm)d

S1 S2 S3 S4 S5 S6 S7 S8 S9 S10

3.91 4.11 5.09 6.64 8.74 12.6 4.93 7.98 8.39 15.87

5.049 5.099 5.146 5.195 5.242 5.290 5.405 5.786 6.165 6.410

304 286 229 174 131 90 225 130 116 59

335 292 242 187 138 81 200 143 109 64

a Specific surface area via BET analysis. b X-ray density of the oxides. c Average particle size calculated from SBET. d Average particle size determined from SEM micrographs.

homogeneous solid solution at 1000 °C. Lattice parameter of the oxide powder pyrolyzed at 1000 °C increases with increasing Eu/Gd addition and follows the Vegard’s law (Figure 9), which provides another piece of evidence that homogeneous oxide solid-solutions have been produced at 1000 °C. Eu3+ is more effective than Gd3+ in enlarging the cell parameter, owing to its bigger ionic size. Figure 10 shows typical FE-SEM morphologies of the solidsolution oxides obtained at 1000 °C. Apparently, the spherical shape and the excellent dispersion of the precursor particles have largely been retained to the oxides. This is primarily due to two reasons: (1) negligible aggregation of the precursor spheres, which gives very limited contact areas among the particles, and (2) the highly refractory nature of the oxides (melting point: ∼2430 °C for Y2O3), which makes sintering difficult to occur. Nonetheless, partial sintering (neck formation among adjacent particles) was indeed observed occasionally (Figure 10b,d) when diameter of the precursor spheres goes down to ∼100 nm. Even though, they have a much better dispersion and a much narrower size distribution than those made via most of other techniques.9

Figure 11. Photoluminescence excitation (PLE) and photoluminescence (PL) spectra of the phosphor spheres calcined from sample S3 [(Y0.95Eu0.05)O1.5] at 1000 °C. The PLE spectrum was obtained by monitoring the 610 nm emission, while the PL spectrum was obtained under UV excitation at 255 nm. Inset is the relative intensity of the 610 nm emission as a function of the Eu3+ content, where the relative intensities were obtained by normalizing the observed 610-nm PL intensities of the samples to that of the Y0.99Eu0.01O1.5 (x ) 0.01) sample.

Such particle morphologies, combined with the excellent luminescent properties as shown in the following sections, would be very beneficial to various display applications. Some properties of the phosphor particles are summarized in Table 2, from which it can be seen that the average particle sizes determined via BET analysis and from SEM micrographs agree with each other and are within a deviation of 10%. Broadening analysis of the (222) diffraction peak with the Scherrer equation yielded average crystallite sizes of ∼53 nm for all the samples, irrespective of their compositions. It can thus be inferred that


Figure 12. Comparison of the PLE (a) and PL (b) behaviors of the phosphor spheres calcined at 1000 °C. Sample names are indicated in the figures. The inset in b is the relative intensity of the 610 nm emission, as a function of the Gd content, where the relative intensities were obtained by normalizing the observed 610-nm PL intensities of the samples to that of the Y0.95Eu0.05O1.5 (x ) 0) sample.

the nanosized particles (Figure 10b,d) are almost single crystalline while the submicron-sized ones are significantly multicrystalline. For a particle of ∼300 nm diameter (Table 2, sample S1), a simple estimation indicates that it may contain ∼200 primary crystallites. Owing to the mass loss upon thermal decomposition, the oxide particles generally show ∼25% reduction in the mean diameter when compared with their respective precursors. Figure 11 shows photoluminescence (PL) and photoluminescence excitation (PLE) spectra of the (Y0.95Eu0.05)2O3 spheres (sample S3, Figure 10a). The broad yet strong band with a maximum at ∼255 nm observed from the PLE spectrum is ascribed to the transition by charge transfer, that is, electronic transition from the 2p orbital of O2- to the 4f orbital of Eu3+ activators.19 The three groups of excitation peaks found in the ranges 290-350 nm, 350-500 nm, and 450-500 nm are assignable to the 7F0,1 f 5H3/5H6, 7F0,1 f 5L6, and 7F0,1 f 5D2 electronic transitions of the Eu3+ ions, respectively.20 Upon UV excitation at 255 nm, the phosphor spheres exhibit sharp lines ranging from 550 to 650 nm, which are associated with the transitions from the excited 5D0 state to the 7FJ (J ) 0, 1, 2) emission states of Eu3+ ions.21 It is the 5D0 f 7F2 electric dipole transition that gives a red emission at 610 nm, as typically observed. The peak has a full width at half-maximum (fwhm)

Li et al. of 4 nm, indicating that the emission is sharp and that the phosphor particles have excellent crystallinity and fewer defects. Intensity of the 610 nm emission improves steadily with increased content of the Eu3+ ions until reaching the optimal concentration of 5 at. % (Figure 11, the inset). A further increase in the Eu3+ content leads to decreased luminous intensities due to concentration quenching, a phenomenon arising from enhanced probability of energy transfer among the Eu3+ activators themselves due to the shortened distance among adjacent Eu3+ ions.22 A similar optimal Eu3+ content was also observed by other researchers.8b Keeping Eu3+ at the optimal content of 5 at. %, we studied the effects of Gd3+ addition on the PLE and PL properties of the red phosphors (Figure 12). The 8S7/2 f 6IJ intra f-f transition of Gd3+ ions is clearly observed on the PLE spectra at ∼276 nm20 for the Gd3+-containing samples (Figure 12a). The 8S7/2 f 6PJ transition of Gd3+, usually found at ∼311 nm,20 overlaps with the 7F0,1 f 5H3/5H6 transition of Eu3+ ions and is thus not clearly distinguishable. Partially replacing Y3+ with Gd3+ up to 50 at. % does not alter appreciably peak position of the charge transfer (CT) band but considerably red-shifts the band edge and significantly increases intensity of the CT band. These are primarily due to the increased covalency by Gd3+ addition, and might be understood by considering the bond structure of Eu3+-O2--Ln3+ (Ln ) Y3+ and Gd3+). Since the electronegativities of Y3+ and Gd3+ are 1.22 and 1.20, respectively,23 these ions tend to attract electrons in the order Y3+ > Gd3+, and as a result the excitation energy for the electron transfer from O2- to Eu3+ increases in the same order. Replacing Y3+ with Gd3+ allows an easier charge transfer, and thus leading to the enhanced intensity of the CT band and the red-shifted band edge. Figure 12b compares PL spectra of three typical samples. The incorporation of Gd3+ does not alter peak position of the 5D f 7F transition but greatly improves the emission intensity, 0 2 indicating that Gd3+ ions effectively sensitize the red emission through an efficient energy transfer from Gd3+ to Eu3+.20 The enhanced emission by Gd3+ addition can also be expected from the excitation spectra (Figure 12a). Intensity of the 610 nm emission increases linearly with increased Gd3+ addition, and the intensity of the [(Y0.5Gd0.5)0.95Eu0.05]O1.5 sample is roughly 1.4 times that of the (Y0.95Eu0.05)O1.5 one (Figure 12b, the inset). Nonetheless, Gd2O3 exhibits weaker stability than Y2O3 against atmospheric CO2 and H2O, owing to its higher basicity, and thus samples with even higher Gd3+ contents were not intended. PL intensity of the phosphor particles is also known to be sensitive to the detected surface. Though in this work the particle size is compositional dependent (the higher the Gd content, the smaller the particles), the observed differences in PL intensities are largely originated from Gd3+-doping effects rather than surface effects. This is evidenced by (1) the PL intensity improves linearly with the Gd3+ content but not with specific surface area of the sample (Table 2), and (2) no clear peak shifting of the excitation band21 was observed with decreased particle size (Figure 12a). The effects of calcination temperature on luminous intensity have been investigated with sample S10 [(Y0.5Gd0.5)0.95Eu0.05O1.5, Table 2] as an example, and the results are presented in Figure 13. It can be seen that decomposition temperature of the precursor affects significantly luminescence of the resultant phosphor particles, especially when it is below 1000 °C. A 300 °C increase in the temperature from 700 to 1000 °C yielded a 70% increase in the intensity of the 610 nm emission (Figure 13, inset). The drastically improved luminescence is largely


J. Phys. Chem. C, Vol. 112, No. 31, 2008 11715 tration gradients while largely retain the original particle morphologies, yielding homogeneous oxide solid-solutions of excellent luminescence. The uniform phosphor spheres of (Y1-xEux)O1.5 exhibit typical red emissions at 610 nm upon UV excitation into the charge transfer band at ∼255 nm. The quenching concentration of Eu3+ was found to be ∼5 at. % for the Y/Eu system. The emission intensity linearly improves by replacing Y3+ with Gd3+ up to 50 at. % while keeping the Eu3+ content at 5 at. %. The phosphor particles with a composition of [(Y0.5Gd0.5)0.95Eu0.05]O1.5 exhibits an emission intensity ∼103% of that of a commercially available Y2O3:Eu phosphor powder. The dispersed phosphor spheres made in this work may find wide applications in various high-resolution display technologies.

Figure 13. Intensity of the 610 nm emission for sample S10 [(Y0.5Gd0.5)0.95Eu0.05O1.5], as a function of the calcination temperature. The blue line corresponds to the commercial Y2O3:Eu red phosphor. Inset is the temperature-dependent relative intensity of the 610 nm emission, where the relative intensities were obtained by normalizing the observed 610-nm PL intensities of the samples to that of the sample calcined at 600 °C.

owing to the homogenized distribution of the component lanthanides within the phosphor particles, which alleviates localized concentration quenching, although improved crystallinity of the host lattice also contributes to the stronger emission. The phosphor particles obtained at 1000 °C exhibit a PL intensity ∼103% of that of a commercially available Y2O3:Eu phosphor powder of micron particle sizes (Kasei Optonix Ltd., Nara, Japan).24 Another 300 °C temperature increase from 1000 to 1300 °C only improves the emission intensity by ∼10%, mainly due to further improved crystallinity. Thus it can be said that in this work a homogeneous distribution of Eu3+ activators is more crucial than crystallinity to strong luminescence. Besides, the crystallites grow to ∼178 nm at 1300 °C, and the resultant particles lose their spherical morphology and become significantly aggregated. Y2O3:Eu phosphor powders of finer crystallite/particle sizes generally show decreased luminous intensities,25 though there have been recent reports showing that nanophosphors of good crystallinity and well-controlled morphologies exhibit enhanced luminescence.9d–f The excellent performance observed from our samples may largely be ascribed to the uniform particle morphology, the uniform distribution of Eu3+ among the phosphor particles, the good crystallinity, and low defect concentrations.26 4. Conclusions Monodispersed colloidal spheres with a general chemical composition of (Y,Gd,Eu)(OH)CO3 · 1.3H2O have been synthesized for the Y/Eu and Y/Gd/Eu mixed systems via urea-based homogeneous precipitation. The resultant particles were observed to be solid solutions rather than mechanical mixtures of individual Y(OH)CO3, Gd(OH)CO3, Eu(OH)CO3 phases. Growth of the particles is surface-diffusion related and follows the cubicroot law. Final sizes of the resultant particles are inversely proportional to nucleation density and are greatly affected by the Gd/Eu content. Differential nucleation/precipitation was identified with regard to Y, Gd, and Eu, and as a consequence the resultant particles have more Gd and Eu (especially Eu) while less Y going from particle surfaces to the cores. Calcining the basic-carbonate spheres at 1000 °C diminishes the concen-

Acknowledgment. Thanks are due to Mr. M. Ikeda of Nano Ceramics Center, National Institute for Materials Science (NIMS), for particle size analysis and to Mr. Y. Yajima and Mr. K. Kosuda of Materials Analysis Station, NIMS, for performing chemical analysis and elemental mapping of the precursors, respectively. This work was partially supported by Program for New Century Excellent Talents in University (Grant NCET-25-0290), the National Science Fund for Distinguished Young Scholars (Grant 50425413), and the National Natural Science Foundation of China (Grants 50672014 and 50772020). References and Notes (1) Blasse, G.; Grabmair, B. C. Luminescent Materials, Springer Verlag: Berlin, 1994. (2) Ekambaram, S.; Patil, K. C.; Maaza, M. J. Alloys Compd. 2005, 393, 81. (3) (a) Xie, R.-J.; Hirosaki, N.; Suehiro, T.; Xu, F.-F.; Mitomo, M. Chem. Mater. 2006, 18, 5578. (b) Kawahara, Y.; Petrykin, V.; Ichihara, T.; Kijima, N.; Kakihana, M. Chem. Mater. 2006, 18, 6303. (c) Riwotzki, K.; Hasse, M. J. Phys. Chem. B 1998, 102, 10129. (d) Meyssamy, H.; Riwotzki, K.; Kornowski, A.; Naused, S.; Hasse, M. AdV. Mater. 1999, 11, 840. (4) (a) Wu, C.; Qin, W.; Qin, G.; Zhao, D.; Zhang, J.; Huang, S.; Lu, S.; Liu, H.; Lin, H. Appl. Phys. Lett. 2003, 82, 520. (b) Wang, J.-W.; Chang, Y.-M.; Chang, H.-C.; Lin, S.-H.; Huang, L.-C. L.; Kong, X.-L.; Kang, M.W. Chem. Phys. Lett. 2005, 405, 314. (5) (a) Vechet, A.; Gibbons, C.; Davies, D.; Jing, X.; March, P.; Ireland, T.; Silver, J.; Newport, A. J. Vac. Sci. Technol. 1999, B17, 750. (b) Jing, X.; Ireland, T.; Gibbons, C.; Barber, D. J.; Silver, J.; Vecht, A.; Fern, G. J. Electrochem. Soc. 1999, 146, 4654. (c) Hirai, T.; Hiranoa, T.; Komasawa, I. J. Mater. Chem. 2000, 10, 2306. (6) (a) Yoo, J. S.; Lee, J. D. J. Appl. Phys. 1997, 81, 2810. (b) Vila, L. D.; Stucchi, E. B.; Davolos, M. R. Euro. J. Mater. Chem. 1997, 7, 2113. (7) Narita, K. Phosphor Handbook; CRC Press LLC: Boca Raton, FL, 1999. (8) (a) Kang, Y. C.; Roh, H. S.; Park, S. B. AdV. Mater. 2000, 12, 451. (b) Kim, E. J.; Kang, Y. C.; Park, H. D.; Ryu, S. K. Mater. Res. Bull. 2003, 38, 515. (c) Kang, Y. C.; Roh, H. S.; Park, S. B. J. Am. Ceram. Soc. 2001, 84, 447. (9) (a) Nelson, J. A.; Brant, E. L.; Wagner, M. J. Chem. Mater. 2003, 15, 688. (b) Chen, J.; Shi, Y.; Shi, J. J. Mater. Res. 2004, 19, 3586. (c) Byeon, S. H.; Ko, M. G.; Park, J. C.; Kim, D. K. Chem. Mater. 2002, 14, 603. (d) Lenggoro, I. W.; Itoh, Y.; Okuyama, K. J. Mater. Res. 2004, 19, 3534. (e) Camenzind, A.; Strobel, R.; Krumeich, F.; Pratsinis, S. E. AdV. Powder. Technol. 2007, 18, 5. (f) Camenzind, A.; Strobel, R.; Pratsinis, S. E. Chem. Phys. Lett. 2005, 415, 193. (g) Chen, J.-Y.; Shi, Y.; Shi, J.-L. J. Inorg. Mater. 2004, 19, 1260. (h) Tissue, B. M.; Yuan, H. B. J. Solid State Chem. 2003, 171, 12. (i) Sun, L.; Liao, C.; Yan, C. J. Solid State Chem. 2003, 171, 304. (10) (a) Matijevic´, E.; Hsu, W. P. J. Colloid Interface Sci. 1987, 118, 506. (b) Hsu, W. P.; Ro¨nnquist, L.; Matijevic´, E. Langmuir 1988, 4, 31. (c) Aiken, B.; Hsu, W. P.; Matijevic´, E. J. Am. Ceram. Soc. 1988, 71, 845. (d) Akinc, M.; Sordelet, D. AdV. Ceram. Mater. 1987, 2, 232. (e) Sordelet, D.; Akinc, M. J. Colloid Interface Sci. 1988, 122, 47. (11) Sidgwick, N. V. Chemical Elements and Their Compounds; Clarendon Press: Oxford, 1962; Vol. 1. (12) (a) Li, J.-G.; Ikegami, T.; Wang, Y.; Mori, T. J. Am. Ceram. Soc. 2003, 86, 915. (b) Yoon, H. S.; Jang, H. S.; Im, W. B.; Kang, J. H.; Jeon, D. Y. J. Mater. Res. 2007, 22, 2017.

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JP802383A

Monodispersed Colloidal Spheres for Uniform Y2O3:Eu3+ Red

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