(Y,Tb,Eu)₂O₃ monospheres for highly fluorescent films and transparent hybrid films with color tunable emission

Abstract

(Y,Tb,Eu)₂O₃ monospheres (∼180 nm in diameter), calcined from the precursors synthesized via urea-based homogeneous precipitation, were employed as the building blocks for highly fluorescent films and as the dispersion fillers for transparent polymer films. The oxide spheres, consisting of ∼40 nm grains, simultaneously displayed the green emission of Tb³⁺ at 543 nm and the red emission of Eu³⁺ at 614 nm under 270 nm UV excitation, with the emission color finely tunable from red to yellow and then to green by increasing the Tb content. The efficiency of Tb³⁺ → Eu³⁺ energy transfer was found to be ∼51.7% for (Y_0.95Tb_0.02Eu_0.03)₂O₃. Self-assembling the oxide monospheres produced close-packed phosphor films with transmittance ≥ 80% (600–850 nm region) and an emission intensity ∼6.8 times that of the powder form. Meanwhile, solution casting yielded transparent PVA films dispersed with the monospheres, which exhibit outstanding flexibility, high transmittance of ≥80% (450–850 nm region), and multi-color emission. The outcomes of this work may have wide implications for flexible luminescent devices and packaging and energy conversion materials.

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1. Introduction

Colloidal monospheres have been finding widespread applications in photonics, drug delivery, heterogeneous catalysis, biolabeling, solar cells, optical sensing, and combinatorial synthesis, and are ideal building blocks for functional devices.^1–6 The monospheres may self-assemble into arrays or domains, such as photonic crystals and close-packed films, that allow functionalities to be obtained not only from the spheres themselves but also from the long-range periodic structures.^1–6 Meanwhile, organic/inorganic composites are increasingly developed for wide applications because of their outstanding flexibility and optical clarity.^7–10 A small amount of suitable micro-/nano-fillers in polymer may lead to dramatically improved permeability and mechanical, thermal, chemical, electrical, and optical properties.^9,10 In this regard, monospheres as fillers may allow a homogeneous dispersion, which is particularly important to high performance hybrids.

Monospheres of lanthanide (including Y) oxides are classically produced by calcining the basic carbonate precursor spheres synthesized via urea-based homogeneous precipitation (UBHP),^11–15 and recently the synthesis has been successfully extended from single lanthanide to mixed lanthanide systems.^16–20 Due to the facile synthesis and the charming morphology and enhanced property of the monospheres, UBHP are being widely employed to produce solid spheres of different compositions,^21–24 hollow spheres,^25,26 and core–shell spheres^27–29 for special functionalities. It was, however, noticed that the fabrication of devices with monospheres as building blocks or fillers are yet limited. Y₂O₃:Eu³⁺ and Y₂O₃:Tb³⁺ are among the most important red and green emitting phosphors finding wide applications in fluorescent lamps, white light emitting diodes (white LEDs), plasma display panels (PDPs), flat panel displays (FDPs), field emission displays (FEDs), and cathode ray tubes (CRTs).^17–20 Color-tunable emission is an important issue in phosphor studies. Though this can be attained by mixing different proportions of Y₂O₃:Eu³⁺ and Y₂O₃:Tb³⁺ particles, the range and ability of color tuning would be limited since the Eu³⁺ and Tb³⁺ activators need different excitation wavelengths for their respective efficient emissions. On the other hand, energy transfer between two types of activators is widely utilized in the phosphor field to tune the emission color, to produce a specific color that cannot be attained with one single type of activator, and to enhance the desired emission. We thus synthesized in this work (Y,Tb,Eu)₂O₃ solid-solution submicron monospheres with the UBHP technique and fabricated highly fluorescent films via close-packing of the spheres and also transparent hybrid films with the spheres as fillers for polyvinyl alcohol (PVA), both for enhanced and color-tunable luminescence. In the following sections, we report the synthesis of monospheres and the fabrication and optical performance of the films.

2. Experimental section

2.1 Synthesis

The starting rare-earth sources are Y₂O₃, Tb₄O₇, and Eu₂O₃, all 99.99% pure products purchased from Huizhou Ruier Rare-Chem. Hi-Tech. Co. Ltd (Huizhou, China). The other reagents are of analytical grade and were purchased from Shenyang Chemical Reagent Factory (Shenyang, China). The nitrate solution of RE³⁺ (RE = Y, Tb and Eu) was prepared by dissolving the corresponding oxide with a slightly excessive amount of nitric acid, followed by evaporation at ∼90 °C to dryness to remove the superfluous acid. In a typical synthesis, a proper amount of urea (CO(NH₂)₂) was dissolved in the mixed nitrate solution to make a total volume of 1 L, which was then homogenized under magnetic stirring at room temperature for 1 h. In all the cases, the total concentration of RE³⁺ was kept constant at 0.015 M. The mixed solution was heated to 90 ± 1 °C within 60 min in a water bath, followed by natural cooling to ∼50 °C after reacting at 90 ± 1 °C for 2 h. The resultant colloidal particles were collected via centrifugation, washed with distilled water for three times to remove byproducts, rinsed with absolute ethanol, and then dried in air at 70 °C for 24 h. (Y,Tb,Eu)₂O₃ was obtained by calcining the precipitation products under flowing O₂ gas (200 mL min⁻¹) at selected temperatures for 4 h with a heating rate of 10 °C min at the ramp stage, followed by reduction in hydrogen (200 mL min⁻¹) at the same temperature for 4 h for the Tb³⁺ containing samples.

2.2 Fabrication of highly fluorescent films and transparent hybrid films

For assembly of fluorescent films, 60 mg of Y/Tb/Eu basic carbonate precursors was dispersed in 3 mL water via ultrasonication for 5 min to yield a suspension, to which 3 mL mixed solution of ethanol and normal butanol (3 : 2 in volume ratio) was added. After ultrasonic processing for 5 min, 100 μL polyethyenimine (PEI) was added, followed by ultrasonic treatment for 1 h. The resultant colloidal spheres were assembled into films on quartz substrates (10 mm in diameter) via dropping 100 μL of the colloidal suspension on the substrate followed by slow air drying. Prior to deposition, the quartz substrates were ultrasonically washed in sequence in acetone, ethanol, and distilled water, and were then immersed in a mixed solution of 30 vol% H₂SO₄ and H₂O₂ (3 : 1 in volume ratio) heated at 80 °C for 1 h. Subsequently, the substrates were kept in the mixed solution of H₂O, NH₄OH, and 30 vol% H₂O₂ (5 : 1 : 1 in volume ratio) to render surface hydrophilicity. (Y,Tb,Eu)₂O₃ fluorescent films were obtained by calcining the precursor films with the aforementioned procedure.

Hybrid films were prepared by solution casting. The colloidal spheres of (Y,Tb,Eu)₂O₃ (30 mg) in water was prepared by extensive ultrasonic treatment. Then 3 g polyvinyl alcohol (PVA) was dissolved in the colloidal suspension to make a total volume of 20 mL, which was performed under magnetic stirring at 80 °C for 1 h. Hybrid films were prepared by pouring the final suspension on a smooth plate (20 × 20 cm²), followed by water evaporation at room temperature. The dried film was readily removable from the plate.

2.3 Characterization techniques

Phase identification was performed by X-ray diffractometry (XRD, Model PW3040/60, Philips, Eindhoven, the Netherlands) operating at 40 kV/40 mA using nickel filtered Cu Kα radiation and a scanning speed of 4.0° 2θ/min. Lattice constants of the oxides were calculated from the XRD patterns using the software package X'Pert HighScore Plus version 2.0 (PANanalytical B.V., Almelo, the Netherlands). Morphologies of the products were observed via field emission scanning electron microscopy (FE-SEM, Model JSM-7001F, JEOL, Tokyo, Japan) and transmission electron microscopy (TEM, Model JEM-2000FX, JEOL, Tokyo). Energy dispersive X-ray (EDX) spectroscopy of the particles was also performed with the FE-SEM equipment mentioned above. Thermogravimetry (TG, Model SETSYS Evolution-16, SETARAM, France) of the dried precursor was made in flowing argon gas with a heating rate of 10 °C min⁻¹. Optical transmittance of the films was measured at room temperature with a UV-vis spectrophotometer (Lambda-750S, Perkin-Elmer, Shelton, Connecticut, USA). Photoluminescence and fluorescence decay were analyzed with an LS-55 fluorospectrophotometer (Perkin-Elmer).

3. Results and discussion

Fig. 1 shows FE-SEM morphologies of the resultant (Y_0.95Tb_xEu_0.05−x)(OH)CO₃·nH₂O (0 ≤ x ≤ 0.05) particles. At the fixed urea/RE³⁺ molar ratio (R) of 33.3, all the particles are monospheres with an average diameter of ∼250 nm (Fig. 1a–d). We previously reported that, for mixed lanthanide systems, the particle size can be remarkably affected by the relative contents of the component lanthanides,^16,17 which was not observed in this work due to the small total content (5 at%) and the relatively close sizes of Tb³⁺ and Eu³⁺ (for 6-fold coordination, r_Tb³⁺ = 0.0923 nm and r_Eu³⁺ = 0.0947 nm). Increasing R to 66.7 yielded significantly smaller spheres of ∼140 nm (Fig. 1e), owing to greatly increased nuclei density.^16–20 The monospheres made in this work are direct solid-solutions rather than a mechanical mixture of individual Y(OH)CO₃·nH₂O, Tb(OH)CO₃·nH₂O, and Eu(OH)CO₃·nH₂O particles, as previously confirmed by us via elemental mapping for the Gd/Y binary and Gd/Y/Eu ternary systems.^16,17 This is primarily due to the quite similar physicochemical properties of these rare-earth ions. EDX analysis of the monospheres also indicates that each individual particle simultaneously contains Y, Eu, and Tb elements (Fig. S1, ESI†).

Fig. 1 FE-SEM micrographs showing morphologies of the (Y_0.95Tb_xEu_0.05−x)(OH)CO₃·nH₂O monospheres obtained at R = 33.3, with x = 0 (a), 0.02 (b), 0.04 (c), and 0.05 (d). (e) is for the x = 0.04 sample obtained at R = 66.7.

The thermal decomposition of basic carbonate is dependent on the type of lanthanide. We found via TG a decomposition temperature of ∼800 °C for the Y/Tb/Eu ternary precursor (Fig. S2, ESI†), and by calcining at 800 °C for 4 h the resultant oxides have displayed the characteristic diffractions corresponding to the cubic structured Y₂O₃ (JCPDS file no. 89-5591, Fig. S3, ESI†). Fig. 2 shows FE-SEM morphologies of the (Y_0.95Tb_0.04Eu_0.01)₂O₃ powders calcined at various temperatures. It is seen that the spherical shape and excellent dispersion of the precursor particles can be well retained to the oxides up to 1000 °C (Fig. 2a and b). Shrinking of particle diameter from ∼250 to 180 nm was observed, owing to the removal of water via dehydration and dehydroxylation and the decomposition of carbonate ions. The particles lost their spherical shape at 1200 °C (Fig. 2c) and underwent significant sintering and aggregation at the even higher temperature of 1400 °C (Fig. 2d). Profile broadening analysis of the (222) diffraction by applying the Scherrer equation found average crystallite sizes of ∼36, 42, 47, and 65 nm for the oxides calcined at 800, 1000, 1200 and 1400 °C, respectively (Fig. S3, ESI†). Fitting the data with the equation dⁿ − d₀ⁿ = A exp(−Q/RT) yielded an activation energy of ∼92 kJ mol⁻¹ for crystal growth (n = 3, Fig. S4, ESI†). The crystallite size is much smaller than the observed particle size, indicating that the particles are significantly polycrystalline.

Fig. 2 FE-SEM micrographs showing morphologies of the (Y_0.95Tb_0.04Eu_0.01)₂O₃ powders calcined at (a) 800 °C, (b) 1000 °C, (c) 1200 °C, (d) 1400 °C. The precursor is exhibited in Fig. 1c.

Fig. 3 demonstrates XRD patterns of the (Y_0.95Tb_xEu_0.05−x)₂O₃ powders obtained at 1000 °C. No other phase was identified along with the cubic structured oxides. Calculation with the (222) diffraction yielded similar cell constants of ∼1.0615 nm for all the oxides, due to the small total content and the similar ionic sizes of Tb³⁺ and Eu³⁺. The value is larger than that of Y₂O₃ (a = 1.0596 nm, JCPDS no. 89-5591), since Tb³⁺ and Eu³⁺ are larger than Y³⁺ (for 6-fold coordination, r_Y³⁺ = 0.0900 nm). The oxide particles exhibit excellent dispersion, although the surface turned slightly rougher (Fig. 4). TEM observation also reveals the polycrystalline nature of the spheres (Fig. 4c), which is further confirmed by the ring-like selected area electron diffraction (SAED) pattern (the inset in Fig. 4c). The (222), (332), (400) and (431) planes are well resolved via SAED analysis, indicating high crystallinity of the oxide particles.

Fig. 3 XRD patterns of the (Y_0.95Tb_xEu_0.05−x)₂O₃ (0 ≤ x ≤ 0.05) powders calcined at 1000 °C.

Fig. 4 FE-SEM (a, b and d) and TEM (c) micrographs showing morphologies of the (Y_0.95Tb_xEu_0.05−x)₂O₃ particles calcined at 1000 °C, with (a) x = 0, (b) x = 0.02, (c) x = 0.04, (d) x = 0.05. The inset in (c) is the SAED pattern.

Fig. 5 shows PLE/PL spectra of the (Y_0.95Eu_0.05)₂O₃ (x = 0) and (Y_0.95Tb_0.05)₂O₃ (x = 0.05) powders. The x = 0 sample exhibits two intense excitation bands with maxima at 225 and 242 nm, which are assignable to the excitation of Y₂O₃ host and O²⁻ → Eu³⁺ charge transfer (CTB), respectively.^18–20,30 The oxide exhibits sharp emissions ranging from 450 to 750 nm upon UV excitation at 242 nm, which are associated with the transitions from the excited ⁵D₀ to the ⁷F_J (J = 0–3) ground states of Eu³⁺.^18–20,30 The dominant red emission at 614 nm arises from the hypersensitive ⁵D₀ → ⁷F₂ forced electric dipole transition of Eu³⁺ taking non-centrosymmetric C₂ sites. The x = 0.05 sample exhibits a broad and strong excitation band in the range of 250–330 nm, with a maximum at 270 nm, which is well-documented to be due to the 4f⁸ → 4f⁷5d¹ inter-configurational Tb³⁺ transition.³¹ When excited at 270 nm, the oxide displayed the typical ⁵D₄ → ⁷F_J (J = 5–2) transitions of Tb³⁺ at about 543, 587, 626, and 670 nm, respectively, with the green emission at 543 nm (⁵D₄ → ⁷F₅) being the strongest.

Fig. 5 Photoluminescence excitation and emission spectra of (Y_0.95Tb_xEu_0.05−x)₂O₃, with (a) x = 0 and (b) x = 0.05.

PLE/PL analysis of the (Y_0.95Tb_xEu_0.05−x)₂O₃ powders (0.01 ≤ x ≤ 0.05) was performed to define the excitation and luminescence properties. Monitoring the 543 nm green emission similarly yielded the 4f⁸ → 4f⁷5d¹ transition of Tb³⁺, whose intensity increases with increasing Tb incorporation (Fig. S5, ESI†). It is seen that, under 270 nm excitation, the characteristic emissions of both Tb³⁺ and Eu³⁺ simultaneously appeared on the PL spectra, with the green Tb³⁺ emission at 543 nm monotonically improves with increasing x (Fig. 6a), conforming to the tendency observed from the PLE spectra (Fig. S5, ESI†). We analyzed in Fig. 6b the relative intensity for these two emissions, together with the I(⁵D₄ → ⁷F₅)/I(⁵D₀ → ⁷F₂) intensity ratio, as a function of the Tb content (0.01 ≤ x ≤ 0.04). Clearly, the ratio increased from ∼0.31 at x = 0.01 to ∼1.45 at x = 0.04, suggesting color-tunable emissions. When excited at 270 nm, the emissions of these samples have CIE chromaticity coordinates of (0.62, 0.34) for x = 0, (0.56, 0.39) for x = 0.01, (0.53, 0.42) for x = 0.02, (0.47, 0.48) for x = 0.03, (0.43, 0.52) for x = 0.04, and (0.35, 0.60) for x = 0.05 (Fig. 7A), and thus the emission color spans a wide range from red to green, as also seen from the bright colors displayed in Fig. 7B.

Fig. 6 Photoluminescence spectra of (Y_0.95Tb_xEu_0.05−x)₂O₃ (a) and intensities of the 543 and 614 nm emissions normalized to the x = 0.01 sample (b). The I₆₁₃/I₅₄₃ intensity ratio versus Tb content (the x value) is included.

Fig. 7 CIE chromaticity diagram of the (Y_0.95Tb_xEu_0.05−x)₂O₃ (A) and multi-color emission of the oxide powders under 254 nm radiation from a hand-held UV lamp (B).

It was noticed from Fig. 6b that the intensity of the Eu³⁺ emission at ∼614 nm steadily decreases with increasing x to 0.03 and then increases for the x = 0.04 sample. The decrease is primarily owing to the decreased number of Eu³⁺ luminescent centers while the increase may suggest that the contribution from Tb³⁺ → Eu³⁺ energy transfer overwhelms the effects of activator concentration. In order to investigate the energy transfer, (Y_0.98−yTb_0.02Eu_y)₂O₃ (0.005 ≤ y ≤ 0.04) monospheres were similarly synthesized, but with the Tb content fixed at 2 at% (Fig. S6, S7 (ESI†) and 8). It is seen from the excitation spectra (λ_em = 543 nm) that the f–d transition of Tb exhibits steadily lower intensity with increasing Eu content, especially when y is over 0.01, suggesting efficient energy migration from Tb³⁺ to Eu³⁺ (Fig. S8a, ESI†). Accordingly, successively stronger Eu³⁺ emission was obtained for the y = 0.005–0.03 samples through exciting the Tb³⁺ ions at 270 nm (Fig. S8b, ESI†). The lowered Eu³⁺ emission at y = 0.04 suggests concentration quenching, and the optimal total activator concentration is thus 5 at%. This also indicates that no concentration quenching would take place for the (Y_0.95Tb_xEu_0.05−x)₂O₃ (0 ≤ x ≤ 0.05) phosphors. The I(⁵D₀ → ⁷F₂)/I(⁵D₄ → ⁷F₅) intensity ratio monotonically increases with increasing Eu incorporation (Fig. 9), similar to the finding of Fig. 6b. In the absence of concentration quenching, the efficiency (η_ET) of Tb³⁺ → Eu³⁺ energy transfer can be calculated from the luminescence intensity with the equation η_ET = 1 − I_S/I_S0,³¹ where I_S and I_S0 are the integrated intensities of the Tb³⁺ emission in the presence and in the absence of Eu³⁺ acceptor, respectively. The η_ET value, calculated from Fig. S8 (ESI†), steadily increased from ∼32.3% for y = 0.005 to 51.7% for y = 0.030 (Fig. 9). The high efficiency of energy transfer is primarily owing to the spectral overlap between the ⁵D₄ → ⁷F_J emission of Tb³⁺ and the ⁷F_0,1 → ⁵D_0,1,2 excitation absorption of Eu³⁺.³²

Fig. 8 FE-SEM micrographs showing morphologies of the (Y_0.98−yTb_0.02Eu_y)₂O₃ oxides calcined at 1000 °C for 4 h, with y = 0.005 (a), 0.01 (b), 0.02 (c), 0.03 (d), and 0.04 (e). Particle morphology of the precursors and XRD patterns of these oxides are shown in Fig. S6 and S7,† respectively.

Fig. 9 Correlation of I(⁵D₀ → ⁷F₂)/I(⁵D₄ → ⁷F₅) intensity ratio and η_ET with the Eu content for (Y_0.98−yTb_0.02Eu_y)₂O₃.

Fluorescence decay analysis (Fig. S9, ESI†) found that the 614 nm emission of Eu³⁺ has a lifetime almost constant at 1.4 ± 0.1 ms, though it tends to slightly increase with increasing Eu³⁺ incorporation, while the lifetime of the 545 nm Tb³⁺ emission significantly decreases from ∼1.22 ms for y = 0.005 to ∼0.66 ms for y = 0.03. The rapidly shortening lifetime of Tb³⁺ emission further confirms that the particles are codoped with Tb³⁺ and Eu³⁺ and there is efficient Tb³⁺ → Eu³⁺energy transfer.^31,33,34

The inset in Fig. 10a shows the microstructure of a phosphor film formed via self-assembly of the oxide monospheres on a quartz substrate. Owing to the excellent dispersion and the high morphological uniformity, the monospheres underwent close packing to form micron-sized single-crystalline domains, similar to the structural features of photonic crystals.¹⁸ Well organized spheres may form unique scattering layers at the air/sphere interface, and can thus exhibit more angular-dependent light patterns of the extraction modes. The enhanced extraction efficiency on the surface of a well ordered phosphor film may lead to significantly improved photoluminescence through the leaky and/or Bragg diffraction scattering modes.^33,34 Another distinct advantage is that the close packed phosphor film minimizes surface scattering of both the excitation and emission lights and can therefore substantially enhance luminescence. Indeed, the 614 nm Eu³⁺ emission of the (Y_0.95Eu_0.05)₂O₃ film shows an intensity ∼6.8 times that of the powder form (Fig. 10a). Enhanced emission was also observed by Do et al.^35,36 and Jeong et al.³⁷ for the photonic films of Y₂O₃/Gd₂O₃:Eu³⁺. Furthermore, the oxide films assembled in this work exhibit high transmittances of ≥80% at the wavelengths above 600 nm (Fig. 11) and show facilely tunable emission colors (Fig. 10b).

Fig. 10 Photoluminescence emission spectra of the (Y_0.95Eu_0.05)₂O₃ powder and film (a) and the appearance of multi-color emissions of the oxide films under 254 nm radiation from a hand-held UV lamp (b). The inset in (a) shows FE-SEM structure of the close-packed (Y_0.95Eu_0.05)₂O₃ film. The PL measurements are performed with the same weight of phosphor particles and are under identical instrument settings.

Fig. 11 Transmission spectra of the (Y_0.95Tb_0.04Eu_0.01)₂O₃ film on quartz and the (Y_0.95Tb_0.04Eu_0.01)₂O₃/PVA hybrid film.

Organic/inorganic hybrids are increasingly developed for wide applications because of their outstanding flexibility and optical clarity,^7–10 where uniform distribution of the inorganic component is crucial. Aggregated phosphor particles are hardly well dispersible, and thus the fabrication of polymer-based homogeneous hybrids is difficult. The submicron monospheres obtained in this work are, however, easily dispersible in solvents. The surfaces of these particles can also be readily functionalized by the O–H groups of water-soluble PVA, which further facilitates particle dispersion and the formation of stable colloidal suspension. Fig. 12 shows the (Y_0.95Tb_xEu_0.05−x)₂O₃/PVA transparent films successfully fabricated in this work via solution casting, which exhibit outstanding flexibility and at the same time high transmittances of ≥80% in the 450–850 nm region (Fig. 11). FE-SEM observation found that the spheres are dispersed in the PVA matrix as individuals rather than aggregates (Fig. S10, ESI†), and the films are smooth and dense without defects such as holes. The polymer films similarly exhibit bright emissions under UV radiation, and the emission color can be finely tuned from red to orange, yellow, and then green by varying the Tb³⁺/Eu³⁺ content.

Fig. 12 Multi-color emission of the (Y_0.95Tb_xEu_0.05−x)₂O₃ hybrid films under 254 nm radiation from a hand-held UV lamp.

4. Conclusions

Monodispersed (Y,Tb,Eu)₂O₃ spheres with diameters of ∼180 nm have been calcined from their precursors synthesized via urea-based homogeneous precipitation, which were then close-packed into highly fluorescent films and dispersed in polyvinyl alcohol (PVA) to form transparent hybrid films for color-tunable emission. Detailed characterizations of the products by the combined techniques of FE-SEM, TEM, XRD, TG and optical spectroscopy have yielded the following main conclusions:

(1) Calcining the (Y,Tb,Eu) (OH)CO₃·nH₂O precursor at the optimal temperature of 1000 °C yielded (Y,Tb,Eu)₂O₃ phosphor monospheres of excellent dispersion. The oxide particles are composed of ∼40 nm grains;

(2) The emission color of the oxide phosphors can be widely tuned from red via yellow to green with increasing Tb incorporation. The maximum efficiency of Tb³⁺ → Eu³⁺ energy transfer was found to be ∼51.7% for the (Y_0.95Tb_0.02Eu_0.03)₂O₃ composition;

(3) The close packed phosphor film self-assembled from (Y_0.95Eu_0.05)₂O₃ monospheres on quartz shows an emission intensity ∼6.8 times that of the powder form. The color tunable (Y_0.95Tb_xEu_0.05−x)₂O₃ (0 ≤ x ≤ 0.05) films have high transmittances of ≥80% in the 600–850 nm spectral region;

(4) Transparent (Y,Tb,Eu)₂O₃/PVA hybrid films have been successfully fabricated by solution casting with the oxide monospheres as fillers, which exhibits excellent flexibility and high transmittance of ≥80% in the wavelength region of 450–850 nm. Bright and color-tunable emission was achieved for the hybrids by incorporating phosphor spheres of different Tb³⁺/Eu³⁺ ratio.

Acknowledgements

This work was supported in part by the National Natural Science Foundation of China (Grants 51302032 and 51172038), the Fundamental Research Funds for the Central Universities (Grants N140204002 and N130810003), the Liaoning Province Doctor Startup Fund (Grant 20131035), and Grant-in-Aid for Scientific Research (KAKENHI, no. 26420686).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra04665c

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