Influence of synthesis conditions on the morphologies of ReBO₃ microstructures and white light emission of YBO₃:Eu³⁺ phosphors prepared by an oleic acid-assisted hydrothermal method

Abstract

Uniform ReBO₃ (Re = Y, Dy, Ho, Er and Yb) spherical microstructures were successfully prepared by an oleic acid (OA)-assisted hydrothermal method. In the preparation process of YBO₃, the solvent (water/ethanol) composition, OA amount, pH value, reaction temperature, the mole ratio of Y³⁺/BO₃³⁻ and reaction time would affect the shape, size, crystallinity and structure of the prepared sample. Spherical, porous spherical and 3D hexagonal flowers of YBO₃ crystals can be prepared by changing the synthesis factors, and their possible formation mechanisms were presented based on the experiment results. It is very interesting that YBO₃:3%Eu³⁺ and YBO₃:5%Eu³⁺ samples prepared by the OA-assisted method display a white light emission under excitation at 394 nm, and the quantum yield of the YBO₃:5%Eu³⁺ sample can reach 44.59%. However, when the excitation wavelength is 270 nm, YBO₃:5%Eu³⁺ shows orange-red emission and the quantum yield can reach 79.73%.These results may open up a new opportunity for YBO₃:Eu³⁺ phosphors in the application of the white light emitting diodes (WLEDs).

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1. Introduction

Lanthanide-doped luminescent materials have been utilized in lots of fields, due to the nature of the special 4f electron configuration of lanthanides, and their high stability, low toxicity, strong luminescence intensity and exceptional optical damage threshold.^1,2 Among rare earth inorganic luminescent materials, Eu³⁺ and Tb³⁺-doped rare earth orthoborates ReBO₃ (Re = Y and Gd) are considered to be attractive vacuum ultraviolet (VUV) phosphors, owing to their strong luminescence intensity and high efficiency under VUV excitation, hence they can be applied to Hg-free fluorescent lamps and plasma display panels.^3,4 Recently, it was reported that a Eu³⁺, Tb³⁺ co-doped YBO₃ phosphor can emit a white light under the excitation of a 365 nm UV light, which makes them have potential applications in LED devices.⁵ Therefore, the study on the preparation and luminescence properties of rare earth orthoborates has important academic and practical significance.

It is well known that, the properties of functional materials not only have a strong correlation with their intrinsic structures, but also external parameters, such as the size, morphology, and dimensionality.^6,7 Recently, three-dimensional (3D) hierarchical architectures, assembled by nanostructured building blocks such as nanoplates, nanoparticles, nanorods and so forth, have received more and more attentions by researchers. Emitting materials with 3D hierarchical structure not only possesses lots of well-known merits and have been applied to plasma display panel and LED,^8,9 but also can be used in the fields of wastewater treatment and catalysis.¹⁰ Up to now, most reports focus on preparing rare earth ions-doped LnBO₃ luminescent materials with 3D hierarchical structure by the hydro/solvothermal method. For example, uniform monodisperse YBO₃:Eu³⁺/Tb³⁺ microspheres have been obtained by the facile ethylene glycol-mediated solvothermal method.¹¹ Zhang et al. fabricated YBO₃:Eu³⁺, Tb³⁺ phosphors with the morphology of uniform flower-like assembled by a facile hydrothermal method based on the reaction of rare earth nitrate and K₂B₄O₇·4H₂O in water, ethanol and ethylene glycol, respectively.⁵ In recent years, the organic surfactants in the synthesis of micro/nanostructures are widely used to control the morphologies of samples.^12–15 Oleic acid (OA), as a very nice organic surfactant, is often used to modify the morphologies of compounds. For examples, Bu et al. used OA/oleylamine (OM) as a mixed surfactant to synthesize the monodisperse NaLa(MoO₄) bipyramid nanocrystals.¹⁶ Shang et al. studied the effect of molar ratio of OA/Ln³⁺ on the morphology of NaYF₄:10%Nd³⁺, 10%Yb³⁺, 2%Er³⁺ microcrystals.¹⁷ Klier and Kumke studied the upconversion luminescence properties of OA capped NaYF₄:Gd³⁺:Yb³⁺:Er³⁺ nanoparticles and found that it can be applied in biological systems.¹⁸ However, OA is used to modulate the shapes of rare earth orthoborates, which has not been reported.

The phosphors that can emit the white light are researched widely and prepared commercially in recent years, due to their application in solid state light emitting diode (LED) lamps. In most cases, codoping a sensitizer and an activator into the same host is an efficient way to generate the white emission by an effective energy transfer among activators. For instance, Gong et al. fabricated the Ce³⁺, Tb³⁺, Mn²⁺ co-doped NaCaBO₃ phosphor, and it can emit white light after the contents of Ce³⁺, Tb³⁺, Mn²⁺ were precisely controlled.¹⁹ Zhang et al. reported that YBO₃:Eu³⁺, Tb³⁺ phosphor emitted a white light, which originates from simultaneous blending of blue, green, and red emissions.⁵ Of course, doping single activator (as Ho³⁺, Dy³⁺, Eu³⁺) into inorganic compounds can also produce the white light. For instance, GdVO₄:Ho³⁺ emits the white light by means of the combination of the sharp Ho³⁺ transition lines with the broad lattice defect emission, and red LaPO₄:Eu³⁺ phosphor which is chemically modified by OA adsorbing on the surface, can generate white light.^20,21 Like LaPO₄:Eu³⁺, it was reported that YBO₃:Eu³⁺ phosphor only exhibits the orange-red emission, but YBO₃:Eu³⁺ that can emit the white light was not reported.

In this work, we focused on the controllable synthesis of ReBO₃ microsphere and YBO₃ with different morphologies by the OA-assisted hydrothermal method. Based on the effect of the experimental conditions on the morphology of prepared samples, the possible formation mechanism of samples with different morphologies has been discussed. Then doping Eu³⁺ into YBO₃ microsphere by the OA-assisted method makes it emit the white light, while the light color of YBO₃:Eu³⁺ can be varied by changing the Eu³⁺ dopant concentration or the excitation wavelength. Finally, the effect of shape on the luminescent properties of YBO₃:Eu³⁺ was discussed, which showed that YBO₃:Eu³⁺ with hexagonal microstructure presented the strongest emission intensity.

2. Experimental section

2.1. Reagents

Ln₂O₃ (Eu₂O₃, Tb₄O₇, Dy₂O₃, Ho₂O₃, Er₂O₃ and Yb₂O₃) was dissolved into concentrated nitric acid, the excessive nitric acid was evaporated by heating, and then deionized water was added to form the 0.05 M and 0.4 M Ln(NO₃)₃ solution, respectively. All the chemicals were of analytic grade and used them directly without further purification.

2.2. Sample preparation

The spherical ReBO₃ (Re = Y, Dy, Ho, Er and Yb) samples were prepared by the hydrothermal approach as follows: 5 mL 0.4 M Re(NO₃)₂ and 3 mmol H₃BO₃ were dissolved into 10 mL deionized water, and then 50 mL ethanol was added into the solution. Under vigorous stirring for 10 min, 0.2 mL oleic acid (OA; ρ = 0.889–0.895 g mL⁻¹) was added to the above mixture solution. The above solution was stirred until all the substances were fully dissolved and mixed, and then adjusted slowly to be pH = 7 by 1 mol L⁻¹ ammonia. After additional agitation for 90 min, the obtained white mixed solution was transferred to a 100 mL of stainless-steel autoclave with a Teflon liner, sealed, and maintained at 180 °C for 24 h. After cooling naturally to room temperature, the final product was collected by centrifugation, washed with deionized water and ethanol, and then dried in an oven at 100 °C for 12 h. YBO₃:Eu³⁺ and YBO₃:Tb³⁺ luminescent materials with the different dopant concentrations were prepared in the same way as that for YBO₃ except for adding a certain amount of Tb(NO₃)₃ or Eu(NO₃)₃ solution at the first step.

2.3. Characterization of sample

The X-ray diffraction (XRD) patterns of samples were performed on a PaNalytical X′ Pert PRO X-ray diffractometer with CuKα radiation (λ = 1.5405 nm; 40 kV, 40 mA). Scanning electron microscopy (SEM) images of samples were taken on a Hitachi S-3400 scanning electron microscope operated at 15 kV. The samples were coated with a thin layer of gold before scanning. The Fourier transform infrared (FT-IR) spectra were recorded on a Nicolet iN10 FT-IR spectrometer, and the sample to be measured was ground with KBr and pressed into thin wafer. The thermal gravimetric and different scanning calorimetry (TG-DSC) analyses of the dried samples prior to calcination were conducted on a thermal analyzer (SAT 449 F3, NETZSCH) at a heating rate of 10 °C min⁻¹ under a continuous-flow of air. All the measurements were tested at room temperature. Photoluminescence (PL) excitation and emission spectra were recorded on a FS5 spectrophotometer equipped with 150 W xenon lamp as the excitation source. The luminescence life times (τ) were examined by an Edinburgh FLS920 phosphorimeter. The quantum yield can be defined as the integrated intensity of the luminescence signal divided by the integrated intensity of the absorption signal. The absorption intensity was calculated by subtracting the integrated intensity of the light source with the sample in the integrating sphere from the integrated intensity of the light source with a blank sample in the integrating sphere. The X-ray photoelectron spectroscopy (XPS) spectra were obtained on a Thermo Scientific ESCALAB 250Xi, the number of scans was 10, and pass energy was 30 eV in high resolution spectra and 160 eV in survey spectra. All binding energies (BE) were determined with respect to the C1s line (284.6 eV) originating from adventitious carbon.

3. Results and discussion

3.1. Microstructure properties of ReBO₃ (Re = Y, Dy, Ho, Er and Yb)

The morphologies of ReBO₃ (Re = Y, Dy, Ho, Er and Yb) were investigated by SEM, which were shown in Fig. 1a–e. The results show that their samples present the similar spherical morphology, and the diameter of YbBO₃ sphere is about 15 μm and smaller than that (about 20 μm) of other ReBO₃ (Re = Y, Dy, Ho and Er) samples. And the surface of ReBO₃ (Re = Y, Dy, Ho and Er) is smoother than that of YbBO₃. We have found that the ionic radii of Re³⁺ have an important influence on the crystal sizes of ReBO₃ samples. Using the same preparation method, the pure hexagonal crystal of ReBO₃ (Re = La, Ce, Pr, Nd, Sm, Eu and Tb) could not be obtained. Fig. 1f gave the SEM images of the sample prepared by using Sm(NO₃)₃ and H₃BO₃, in which we observed lots of microwires and in its XRD pattern (Fig. 1g) the SmBO₃ phase cannot be observed. When we synthesized other ReBO₃ (Re = La, Ce, Pr, Eu, Gd, Tb) with the bigger lanthanide radius, the pure hexagonal phase of ReBO₃ also cannot be observed. These results indicated that for the lanthanide with larger radius, it is difficult to synthesize hexagonal phase ReBO₃.

Fig. 1 SEM images of (a) YBO₃, (b) DyBO₃, (c) HoBO₃, (d) ErBO₃, (e) YbBO₃, (f) SmBO₃ samples prepared with the solution (pH = 7) in the presence of 0.2 mL OA at 180 °C for 24 h and (g) their XRD patterns.

The X-ray diffraction patterns of the ReBO₃ (Re = Y, Dy, Ho, Er and Yb) samples prepared by the OA-assisted hydrothermal method are showed in the Fig. 1g. The results display that these samples belong to pure hexagonal phase with good crystal quality and no other phase can be observed. In addition, the positions of all diffraction peaks of samples shift toward smaller 2θ values with an increase in the lanthanide ionic radii.

3.2. Structure and morphology of YBO₃:Eu³⁺ and YBO₃:Tb³⁺ luminescent materials

Fig. 2 shows the SEM images and XRD patterns of the undoped and Eu³⁺/Tb³⁺-doped YBO₃ samples synthesized by OA-assisted hydrothermal route. All diffraction peaks of the undoped sample can be indexed to the hexagonal phase YBO₃ with lattice constants a = b = 0.3777 nm, c = 0.8810 nm, according to the JCPDS file no. 13-0531. YBO₃:3%Eu³⁺, and YBO₃:7%Tb³⁺ luminescent materials present the similar crystal structure and no discernible impurities and/or other phases can be detected, which indicates that all Eu³⁺ or Tb³⁺ ions occupied the sites of Y³⁺ ions in YBO₃ host and no individual EuBO₃ or TbBO₃ phase can be detected in their XRD patterns. The morphologies of YBO₃:3%Eu³⁺ and YBO₃:7%Tb³⁺ luminescent materials are similar to the sphere-like YBO₃ samples, indicating that the doped components have no influence on the morphological features of YBO₃. This kind of spherical morphology will be beneficial to the phosphors, because of the high packing density and reduced light scattering of these spheres.

Fig. 2 SEM images of (a) YBO₃, (b) YBO₃:3%Eu³⁺, (c) YBO₃:7%Tb³⁺ samples prepared with the solution (pH = 7) in the presence of 0.2 mL OA at 180 °C for 24 h, and (d) their XRD patterns.

3.3. Effect of synthesis conditions

The crystal growth process is not only determined by the internal structure but also affected by the external synthesis conditions. Herein, we investigated the solvent amount, reaction temperature, pH value in synthesis solution, OA amount and molar ratio of Y³⁺/BO₃³⁻ on the microstructure of the samples.

3.3.1 Effect of solvent amount. Fig. 3 shows the SEM images of samples prepared at 180 °C for 24 h with the different solvents, in which the ratio of deionized water/ethanol (mL/mL) was changed in the solvent of 60 mL and OA of 0.2 mL. As shown in Fig. 3, the appropriate solvent ratio of deionized water/ethanol is 20 mL/40 mL to 10 mL/50 mL (Fig. 3e and f). At the solvent ratios above, the obtained samples exhibited the similar and regular microspheres. When the mixture solvent of 10 mL water and 50 mL ethanol was used, the morphology of the prepared sample is dispersed, uniform microspheric structure with the diameter of 20 μm and smooth surface. And in the solvents with other ratios of water/ethanol, the spherical samples cannot be obtained (Fig. 3a–d and g). The results above show that ethanol in solvent plays an important of role in the formation of YBO₃ uniform microsphere, and when water/ethanol (mL/mL) is 2/4–1/5, the mixed solvents can provide a favorable environment for the crystal growth by the help of OA, resulting in the formation of regular YBO₃ microsphere.

Fig. 3 SEM images of samples prepared with different solvents: (a) deionized water/ethanol (mL/mL) = 60/0, (b) 50/10, (c) 40/20, (d) 30/30, (e) 20/40, (f) 10/50, and (g) 0/60 in the solution (pH = 7) containing 0.2 mL OA at 180 °C for 24 h.

3.3.2 Effect of OA amount. Fig. 4 shows the SEM images and XRD patterns of YBO₃ samples prepared at 180 °C for 24 h with different OA amounts in the mixture solvent of 10 mL water and 50 mL ethanol. As shown in Fig. 4a, when OA was not added in the solvent, the prepared sample presents the flower-like microsphere shape, which was composed of many nanoflakes linked closely by edge to edge and edge to surface conjunction. With the increasing in the OA amount, prepared YBO₃ spheres gradually became much more compact and holes in the sphere became smaller. For instance, when the OA was 0.1 mL, the obtained sample presents the compact microspheres (Fig. 4b), which was constructed densely by nanoflakes (see the inset); when OA was 0.2 mL, the sample looks like solid microspheres with smooth surface (Fig. 4c), but from the magnified SEM (see the inset), we can find that the surface is rough and the microspheres are also composed of many nanoflakes. When the amount of OA was added to 0.4 mL, there were too many flakes in the surface of YBO₃ sphere, so the optimum OA amount for preparing YBO₃ microsphere should be 0.1–0.2 mL.

Fig. 4 SEM images of YBO₃ prepared with OA (mL) of (a) 0, (b) 0.1, (c) 0.2, (d) 0.4 in the solution (pH = 7) at 180 °C for 24 h, and (e) their XRD patterns.

The results above show that OA plays an important role in controlling the morphology of the synthesized products. Since ammonia was added to adjust the pH value in the synthesis solution, OA can react with ammonia to form oleic acid ammonium,²² which can cap Y³⁺ ions through an ion exchange process in the mixed solvent. And then under the hydrothermal conditions, the chelating of Y³⁺–OA complexes was attacked by BO₃³⁻ and an anion-exchange reaction between BO₃³⁻ and OA occurred to form YBO₃ nuclei. When the OA amount was adjusted in the synthesis solution, the concentration of Y³⁺ in the solution can be controlled, resulting in controlling the crystal growth of YBO₃ basic monomers. These monomers interact and then aggregate together. Subsequently, the nanoparticle precursors grow and crystallize in this hydrothermal condition. While the OA molecules can also adsorb on the surface of the nanoparticle precursors to hold back the growth rate of crystals to form nanoflakes, and then guide the nanoflakes to link together and finally form microspheres.¹⁶

3.3.3 Effect of synthesis temperature. Fig. 5 shows the SEM images and XRD patterns of YBO₃ samples prepared at the different temperatures. Amorphous aggregates were formed in the sample prepared at 160 °C (Fig. 5a and d). When the sample was prepared at 180 °C, the uniform microspheres were obtained (Fig. 5b), and its diffraction peaks are quite sharp and agreement with the hexagonal phase of YBO₃. Continually increasing synthesis temperature to 200 °C, the uniform microspheres with diameter of ∼10 μm was obtained (Fig. 5c), which is smaller than that (∼15 μm) of the sample synthesized at 180 °C. Their X-ray diffraction peaks are indexed to the hexagonal phase YBO₃. This shows that the hydrothermal temperature has an obvious influence on the morphology of YBO₃ particles.

Fig. 5 SEM images of YBO₃ samples prepared at (a) 160 °C, (b) 180 °C, (c) 200 °C in the solution (pH = 7) containing of 0.2 mL OA for 24 h, and (d) their XRD patterns.

Generically, the crystallization process might be divided into two steps, nucleation stage and growth stage. If we could deeply understand the process of crystal nucleation and growth, its morphology would be more easily and smoothly controlled. The separation of nucleation and growth process is very necessary and key to generate uniform particles.²³ In the absence of Ostwald ripening, the final crystal size of sample is affected by the balance between the rates of nucleation and growth. Since the nucleation can be affected strongly by the synthesis temperature, herein, the balance of nucleation and growth rate can be adjusted by changing the synthesis temperature. As shown in Fig. 5, the sample prepared at 160 °C exhibited amorphous phase with irregular particle morphology. When the temperature was increased to 180 °C, the prepared product showed the hexagonal YBO₃ phase and the microsphere shape. This shows that increasing the synthesis temperature is in favor of accelerating growth rate of crystallites and the reaction of Y³⁺–OA with BO₃³⁻ to form YBO₃ crystals. When the synthesis temperature is further increased to 200 °C, the growth rate of crystallites and nucleation rate are simultaneously increased a lot, which needs to provide a higher seeds concentration by consuming the same amount of monomer, resulting in the higher particles concentration and smaller particles.^11,24

3.3.4 Effect of pH value. The effects of pH value in the synthesis solution (containing 0.2 mL OA) on the morphology and crystal phase of the YBO₃ products prepared at 180 °C for 24 h are shown in Fig. 6. The results show that the sample prepared in pH = 6 exhibited irregular particles with amorphous phase (Fig. 5a and e). When the pH value of synthesis solution was adjusted to 7, the monodispersed and hexagonal phase YBO₃ microspheres with the size of ∼20 μm were formed (Fig. 6b and e). Continually increasing the pH value to 8, the obtained microflower product still shows the hexagonal phase, and is composed of sheets (Fig. 6c and e). As the pH value reached to 9, the crystalline of hexagonal phase sample is lightly reduced and its microflowers exhibit hexagonal shape (Fig. 6d and e), comparing with the sample prepared at pH = 8. Continually increasing the pH value of synthesis solution, purified YBO₃ phase could not be synthesized.

Fig. 6 SEM images of samples at pH of (a) 6, (b) 7, (c) 8, (d) 9 in the solution containing 0.2 mL OA at 180 °C for 24 h, and (e) their XRD patterns.

These results show that the pH value in synthesis solution has a great influence on a formation of uniform YBO₃ microstructure, and the monodisperse YBO₃ microstructure could not be formed under the acidic or pH value of >9 conditions. In the acidic solution, H⁺ would restrict the dissociation of H⁺ from –COOH in OA molecules; hence Y³⁺ cannot be capped by OA, which makes that the concentration of Y³⁺ in solution cannot be controlled to affect the nucleation. So, the acidic condition is not suitable for the formation of YBO₃ crystals. In the strong alkaline conditions, the excess OH⁻ ions will surround OA molecules by means of the hydrogen bond. Hence, the electrostatic attraction between –COO⁻ and Y³⁺ will be decreased. Thus the excess of OH⁻ will make it difficult to form YBO₃. When the pH value of the solution was in the range of 7–9, the concentration of H⁺ and OH⁻ was in a suitable level and OA molecules could absorb on the surface of YBO₃ nanoparticles evenly through electrostatic absorption to modify the morphology of YBO₃ crystal.

3.3.5 Effect of B (or Y) amount. In the synthesis of the samples above, we controlled the molar ratio of Y³⁺/BO₃³⁻ being 2/3. Here, the effect of Y³⁺/BO₃³⁻ (or the BO₃³⁻ amount) on the shape YBO₃ sample was investigated. The results show that, when Y³⁺/BO₃³⁻ (mol) was 2/2, the sample prepared exhibited the cotton-like quasi-spherical microstructures (Fig. 7a). In Y³⁺/BO₃³⁻ = 2/3, the uniform spherical particles were obtained (Fig. 7b). Continually adjusting its molar ratio to 2/4, the morphology of sample became tyre-like particle and its size was decreased obviously (Fig. 7c). When Y³⁺/BO₃³⁻ was 2/6, the prepared samples presented the mixture morphology of tyre-like particles and spherical-like particle with rough surface. When the Y³⁺/BO₃³⁻ value was 2/8, the obtained sample exhibited non-uniform spherical morphology with highly porous structure. These results show that, with the increase in H₃BO₃ amount, more YBO₃ nuclei were formed and then the balance between the rates of nucleation and growth was destroyed, leading to the difference of their morphologies.

Fig. 7 SEM images of YBO₃ samples prepared at 180 °C for 24 h with the solution (pH = 7) containing 0.2 mL OA and Y³⁺/BO₃³⁻ (mol) of (a) 2/2, (b) 2/3, (c) 2/4, (d) 2/6, and (e) 2/8.

3.3.6 Effect of reaction time. To study the possible formation mechanism of YBO₃ sample, the effect of the preparation time on the shape and structure of the sample was studied, and the results are shown in Fig. 8 (pH = 7) and Fig. 9 (pH = 9). After 2 h of reaction, irregular aggregated particles with amorphous were formed (Fig. 8a and f). When the reaction time was prolonged to 8 h, a kind of cotton-shaped and flower-like sphere and many irregular particles were formed (Fig. 8b), and they are still amorphous (Fig. 8f). Like the sample (8 h), the sample prepared for 10 h are still irregular particles (Fig. 8c), and its X-ray diffraction peaks appeared (Fig. 8f) and are assigned to the hexagonal phase YBO₃ on the base of the main peak located at 2θ = 27.3°. After 14 h of reaction, the mixture of particles, flakes and irregular spheres with diameter of 5–30 μm was formed, and there are the characteristic diffraction peaks of hexagonal YBO₃ in its XRD pattern, in spite of their lower intensities. When the hydrothermal time was increased to 24 h, the amorphous particles were completely disappeared and uniform YBO₃ microspheres (Fig. 8e) formed, and its XRD pattern exhibits the better crystalline.

Fig. 8 SEM images of YBO₃ prepared with the solution (pH = 7) containing 0.2 mL OA at 180 °C for (a) 2 h, (b) 8 h, (c) 10 h, (d) 14 h, (e) 24 h, and (f) their XRD patterns.

Fig. 9 SEM images of YBO₃ prepared with the solution (pH = 9) containing 0.2 mL OA at 180 °C for (a) 10 h, (b) 15 h, (c) 16 h, (d) 24 h.

When the pH value in the synthesis solution was increased from 7 to 9, the effect of the synthesis time on the shape and structure of the sample has the similar variation rule as Fig. 8. However the shape of the sample prepared for 24 h is more regular hexagonal microflowers composed of sheets with hexagonal phase YBO₃, and its size of 3D microstructure is ∼15 μm and the space between nanosheets is ∼0.15 μm.

3.4. Possible formation mechanism

The results above show that the crystal evolution process included a nucleation, aggregation, oriented growth and ripening. (1) OA molecules strongly bonded with Y³⁺ ions to form complexes, resulting in a decrease in the amount of free Y³⁺ ions in the solution. Then the BO₃³⁻ ions attacked the complexes and replaced the OA molecules, while the nucleation process was taken place. (2) The amorphous YBO₃ particles were formed gradually, and they grew in a certain way to form nanoflakes. (3) The OA molecules absorbed on the surface of nanoflakes in all sides through the electrostatic interaction and formed a protective layer, thus restricting the growth rate of particles and promoting isotropic growth. Through molecule interaction between OA molecules absorbed on the nanoflakes, these primary building blocks would assemble into multilayered hierarchical structures. In terms of the theory of thermodynamics, the lower the Gibbs free energy, the greater the stability of materials is. Since spheroid has a minimal surface energy, multilayered hierarchical structures would transform into spherical conglomeration to reduce the total energy of the reaction system.²⁵ (4) In the induction of sufficient OA and further ripening process, the loose microspheres were gradually evolved into compact spheres. Thus, the uniform and monodispersed YBO₃ microspheres were obtained. A schematic illustration for the possible formation process of solid YBO₃ spheres is shown in Scheme 1. Comparing with the formation of YBO₃ microsphere, the YBO₃ 3D hexagonal microstructure was fabricated at pH = 9 (Scheme 2), in which the other reaction conditions remain unchanged. This is possibly that more OH⁻ existed in the solution (pH = 9) can be adsorbed on the surface of nanoflakes, resulting in the formation of 3D hexagonal microstructure.

Scheme 1 Schematic illustration for the possible formation process of YBO₃ samples in the solution (pH = 7).

Scheme 2 Schematic illustration for the possible formation process of YBO₃ samples in the solution (pH = 9).

3.5. PL properties of YBO₃:Eu³⁺/Tb³⁺

PL properties of the YBO₃:3%Eu³⁺ and YBO₃:7%Tb³⁺ microsphere phosphors were investigated at room temperature. As shown in Fig. 10A, the excitation spectrum of YBO₃:3%Eu³⁺ samples consists of a wide absorption band centered at 235 nm and some weak peaks, which can be assigned to the charge-transfer band between the O²⁻ and Eu³⁺ ions and the f–f transitions within 4f⁶ configuration of the Eu³⁺ ions, respectively.²⁶ Under excitation at 235 nm, the emission spectrum of YBO₃:3%Eu³⁺ shows a group of lines at about 580, 591, 611 (628), 652 (675), and 708 nm, corresponding the ⁵D₀–⁷F_J (J = 0, 1, 2, 3, 4) transition lines of the Eu³⁺ ions, in which the magnetic dipole transition of ⁵D₀–⁷F₁ at 591 nm is the most prominent, that is to say, the Y³⁺ sites occupied by Eu³⁺ ions have high symmetry in the YBO₃ host.²⁷ And the peaks located at 611 and 628 nm resulted from the typical forced electric dipole transition ⁵D₀–⁷F₂ of the Eu³⁺ ions. Since the orange transition peak of ⁵D₀–⁷F₁ is stronger than the red peak of ⁵D₀–⁷F₂, so the YBO₃:3%Eu³⁺ display orange-red emission, which can be confirmed by the luminescence photograph under excitation at 235 nm (inset of Fig. 10A).

Fig. 10 Excitation and emission spectra of (A) YBO₃:3%Eu³⁺ and (B) YBO₃:7%Tb³⁺ microspheres.

Fig. 10B shows the excitation and emission spectra of YBO₃:7%Tb³⁺ microsphere phosphors. In its excitation spectrum, the intense band at 236 nm can be assigned to the spin-allowed transition with high energy from 4f⁸ to 4f⁷5d¹ of the Tb³⁺ ions, and the band at 282 nm is attributed to the spin-forbidden transition with lower energy from 4f⁸ to 4f⁷5d¹ of the Tb³⁺ ions. The weak lines at 300–400 nm are due to the characteristic f–f transition from the 7f⁶ ground state to the different excited states of the Tb³⁺ ions in the YBO₃ lattice. In its emission spectrum of YBO₃:7%Tb³⁺ excited at 236 nm, the several peaks at 487, 544, 582 and 619 nm are corresponds to the ⁵D₄–⁷F_J (J = 6, 5, 4, 3) transition of the Tb³⁺ ions, respectively. The dominant peak located at 544 nm belongs to the green zone. Under ultraviolet excitation, the YBO₃:Tb³⁺ sample displays bright green emission, which can be confirmed by the luminescence photograph under excitation at 236 nm (inset of Fig. 10B).

Under excitation at 394 nm, the effect of the Eu³⁺ ions concentration on its luminescent intensity was investigated, and the results are shown in Fig. 11. Increasing concentration of Eu³⁺ ions has no influence on the peak position in the emission spectra, and the optimum concentration of Eu³⁺ in YBO₃ microsphere is 9%. We calculated the corresponding chromaticity coordinate of YBO₃:x%Eu³⁺ (x = 3, 5, 7, 9, 11), which are (0.33, 0.32), (0.34, 0.33), (0.40, 0.33), (0.42, 0.33) and (0.42, 0.32), respectively. As shown in the chromaticity coordinate values, the light color would be varied with the dopant concentration. When the concentration of Eu³⁺ is lower than 7%, the synergy of the broad green-blue light and the orange-red light of Eu³⁺ in the emission spectra of YBO₃:Eu³⁺ can give rise to the white light color. However, when the Eu³⁺ concentration is 7%, 9% and 11% respectively, its luminescent intensity is so strong that the light color has a little shift to the red-orange district. These results are close to the standard white light emission, which are very interesting. Generally, Eu³⁺-doped materials give red or orange/red emission. Here, YBO₃:3%Eu³⁺ and YBO₃:5%Eu³⁺ phosphors prepared by the OA-assisted hydrothermal method can generate the white color, which is similar to that reported by Li et al.²⁰ They made that white light could be emitted on LaPO₄:Eu³⁺ nanorods chemically modified by oleic acid. The reason is that, the covalent bonding of OA to the surface ions was modified by surrounding surface oxygen vacancies, resulting in a yield of the mid-gap states. And a synergy of the relevant interface mid-gap states and red emission of Eu³⁺ gives rise to the tunable colors from purplish pink through green-blue to white.²⁰ Hence, after OA adsorbed on the YBO₃:Eu³⁺ surface by chemical bonding, the PL properties of the sample was regulated to produce the white light color.

Fig. 11 The emission spectra of different concentrations Eu³⁺ doped YBO₃ under excitation at 394 nm.

To prove the role of OA on the luminescence of YBO₃:5%Eu³⁺, the luminescence properties of YBO₃:5%Eu³⁺ before and after calcinations at 700 °C were tested and the results are given in Fig. 12. Under excitation at 394 nm, the emission spectrum of sample without calcination consisted of several peaks at 591, 610, and 627 nm, corresponding to transition from ⁵D₀–⁷F_J of Eu³⁺. In the inserted emission spectra, a′ and b′ represent the magnified spectra of the bands located in the region of 430–560 nm for YBO₃:5%Eu³⁺ without and with calcination, respectively. As shown in the inserted emission spectra, a broad green-blue emission band at 430–560 nm was observed, ascribing to the surface of YBO₃:5%Eu³⁺ capped by OA (Fig. 12a).

Fig. 12 Emission spectra and light color of YBO₃:5%Eu³⁺ (a) before and (b) after calcination at 700 °C for 5 h under excitation at 394 nm.

After YBO₃:Eu³⁺ was calcined at 700 °C, the OA molecules adsorbed on the sample were removed, resulting in the higher luminescent intensity of Eu³⁺ ions and disappearance of the green-blue emission (Fig. 12b). Hence, the YBO₃:5%Eu³⁺ solid sphere without calcination presents the white light color, and YBO₃:5%Eu³⁺ calcined at 700 °C exhibits the purplish pink light color. Furthermore, we have found that the light color of as-prepared YBO₃:5%Eu³⁺ can vary with the excitation wavelength. Under being excited at 235 nm, the as-prepared sample showed purplish pink color (Fig. 10A), and under being excited at 394 nm, the sample exhibited an intense white light (Fig. 12a), indicating that the light color can be easily tuned by changing the excitation wavelength.

The FTIR spectra of YBO₃:5%Eu³⁺ phosphor and OA are shown in Fig. 13. The strong broad peaks at 800–1200 cm⁻¹ in Fig. 13a and b belong to the B–O vibrations of B₃O₉ (ref. 3 and 28) confirming the formation of YBO₃ phosphors. The small peak at 563 cm⁻¹ is assigned to an in-plane bending of the BO₄ group.²⁹ The broad peak at 3450 cm⁻¹ corresponds to the stretching vibrations of the hydroxide groups, due to the presence of moisture in the sample. For YBO₃:5%Eu³⁺ without calcination, the intensity of the peaks at ∼3450 cm⁻¹ is stronger than that for YBO₃:5%Eu³⁺ calcined, because of the presence of adsorbed water; the peak at 1650 cm⁻¹ can be ascribed to the H–O–H scissoring from free or absorbed water and the stretching vibration of –C=C– group; the weak peaks at 2935 and 2850 cm⁻¹ are attributed to the stretching of a –CH₂– and –CH₃ group; the strong peak at 1373 cm⁻¹ is ascribed to –CH₂– bending vibration mode. The above results indicate that, oleic acid and water have adsorbed on the surface of the YBO₃ microcrystals. The band at about 1550 cm⁻¹ can be detected and it is the characteristic peak of chemisorbed carboxylate.³⁰ Comparing to the IR spectra of free OA (Fig. 13c), it can be found that, the stretching vibration of the free acid C=O groups is very weak and hardly observed at 1700–1725 cm⁻¹ in the IR spectra of YBO₃:Eu³⁺ without calcination, which further confirms that oleic acid adsorbed on the surface of the sample by the chemical bonding of OA molecules to surface.²⁰

Fig. 13 FT-IR spectra of YBO₃:5%Eu³⁺ before (a) and after (b) calcination at 700 °C for 5 h and (c) OA.

As we known, in Eu ions – doped compounds, Eu³⁺, Eu²⁺ or both is mostly responsible for the tunable wavelength to white light. Therefore, the valence states of Eu ions in the as-prepared YBO₃:Eu³⁺ microsphere by oleic acid-assisted hydrothermal method should be determined by XPS measurements. As seen in XPS spectra in Fig. 14, two bands can be obviously detected, peaking at 1134.28 and 1162.88 eV, respectively. Based on the literature, these two bands are assigned to the Eu³⁺ 3d_3/2 and Eu³⁺ 3d_5/2, respectively. However, the peaks of the Eu²⁺ 3d binding energies (1127 and 1158 eV) almost were not detected in the XPS spectra,³¹ which indicating that the broad band in the emission spectra was not originating from the transition between 4f⁷ (⁸S_7/2) ground state to 4f⁶5d excited state of Eu²⁺. Herein, we can surmise that OA adsorbed in the surface of YBO₃:Eu³⁺ should be responsible for the generation of the white light color.

Fig. 14 XPS of Eu 3d spectra of YBO₃:5%Eu³⁺ sample before heat-treatment.

TG curve of YBO₃:5%Eu³⁺ is shown in Fig. 15. A slight weight loss was detected at <200 °C because of the presence of the free or adsorbed water in the sample, and an obvious weight loss existed at >360 °C, which is related to the decomposition of oleic acid.

Fig. 15 TG curve of YBO₃:5%Eu³⁺ microspheres.

The effect of morphology (solid microsphere, Fig. 2b), porous sphere (Fig. 4b) and hexagonal microstructure (Fig. 6d) of the YBO₃:5%Eu³⁺ sample on its PL property is shown in Fig. 16. In the excitation spectra of the YBO₃:5%Eu³⁺ microsphere and hexagonal microstructure at 591 nm (Fig. 16A), there are two broad bands at ∼215 nm and 230–250 nm, and the former is originated from host absorption band (HAB) and the latter is owing to O²⁻ to Eu³⁺ charge-transfer band (CTB) from the 2p orbital of O²⁻ to the 4f orbital of Eu³⁺. And the weak lines at 300–400 nm result from the f–f transition lines of Eu³⁺. The YBO₃:5%Eu³⁺ porous spheres exhibit a broad band at 214 nm owing to HAB and a very weak broad band at ∼230 nm ascribing to CTB (O²⁻–Eu³⁺), which indicates the lowest luminescent intensity of Eu³⁺ in YBO₃ porous sphere among three phosphors.

Fig. 16 (A) Excitation and (B) emission spectra of YBO₃:5%Eu³⁺ with different morphologies: (a) porous sphere, (b) solid sphere and (c) hexagonal microstructure.

As shown the emission spectra of YBO₃:5%Eu³⁺ (Fig. 16B), among three phosphors, the luminescence intensity of YBO₃:5%Eu³⁺ 3D hexagonal microstructure is the strongest and that of Eu³⁺ in YBO₃ porous sphere is the lowest, which are agreement with the results of the excitation spectra. The reason for these results is probably that, the YBO₃:5%Eu³⁺ porous sphere has more amount of the surfactant adsorbed on the surface and the surfactant on the surface would quench the luminescence of Eu³⁺, resulting in the lowest intensity of Eu³⁺ in YBO₃ porous sphere. The YBO₃:5%Eu³⁺ 3D hexagonal microstructures possess better crystalline and more Eu³⁺ ions located in the surface of hierarchical microstructure assembled by the nanoflakes, which results in the strongest luminescent intensity.

We have characterized the decay lifetime and the quantum efficiency of as-prepared YBO₃:Eu³⁺ by oleic acid-assisted hydrothermal method. Fig. 17 shows the luminescent decay curves of the YBO₃:5%Eu³⁺ sample before heat-treatment excited at 235 and 394 nm, respectively, which are fitted into a double-exponential function as I = A₁ exp(−τ/τ₁) + A₂ exp(−τ/τ₂). τ₁ and τ₂ are the fast and slow components of the luminescence lifetimes, and A₁ and A₂ are the fitting parameters, respectively. When the YBO₃:5%Eu³⁺ samples were excited at 235 nm, two lifetimes, the faster one τ₁ = 1.88 ms, and the slow one τ₂ = 5.20 ms, were obtained for the emission of Eu³⁺ at 591 nm. The average lifetime was defined as [τ] = (A₁τ₁² + A₂τ₂²)/(A₁τ₁ + A₂τ₂), which was 3.56 ms. When under the excitation wavelength of 270 nm, the quantum yield of as-prepared YBO₃:5%Eu³⁺ are 79.73%, which is higher than that of the reported YBO₃:Eu³⁺ phosphor.²⁷ When excited at 394 nm, two lifetimes, the faster one τ₁ = 0.73 ms, and the slow one τ₂ = 3.73 ms, were obtained for the emission of Eu³⁺ at 591 nm. The average lifetime for Eu³⁺ was 3.37 ms and its quantum yield can reach to 44.59%, which is higher than that of the white light emission from the reported YP_0.8V_0.2O₄:Dy³⁺ (1 at%), Sm³⁺ (0.75 at%) which is 25%.³² Based on the results, we concluded that as-prepared YBO₃:Eu³⁺ with uniform microstructures can emit the white light color and its quantum yield is 44.59%.These results may open up a new opportunity for YBO₃:Eu³⁺ phosphors in the application of the white light emitting diodes (WLEDs).

Fig. 17 Decay curves of the YBO₃:5%Eu³⁺ sample before heat-treatment were excited at (A) 235 and (B) 394 nm.

4. Conclusions

In summary, a series of uniform ReBO₃ (Re = Y, Dy, Ho, Er and Yb) microparticles were hydrothermally prepared by a facile and effective oleic acid (OA)-assisted method. It was found that the prepared samples showed a same morphology but different particle sizes, probably due to different Re³⁺ ions radii. In the preparation process of YBO₃ sample, the solvent (water/ethanol) composition, OA amount, pH value, reaction temperature, the mole ratio of Y³⁺/BO₃³⁻ and reaction time would affect the shape, size, crystallinity and structure of prepared sample. On the basis of these results, the possible formation mechanism of YBO₃ microspheres was proposed as following four steps: nucleation, aggregation, oriented growth and ripening.

When under the excitation wavelength of 270 nm, YBO₃:5% Eu³⁺ microspheres show the orange-red emission and the quantum yield of as-prepared YBO₃:5%Eu³⁺ can reach to 79.73%. It is very interesting that YBO₃:3%Eu³⁺ and YBO₃:5%Eu³⁺ samples prepared by the OA-assisted method display the white color under excitation at 394 nm, due to OA adsorbed on the surface by chemical bonding. And the quantum yield of the YBO₃:5%Eu³⁺ sample can reach to 44.59%. The combination of green-blue emission from OA-related mid-gap states to Eu³⁺ and orange-red emission from Eu³⁺ in YBO₃ realize white light emission, indicating that YBO₃:5%Eu³⁺ phosphors has a potential application in white light emitting diodes (WLEDs).

Acknowledgements

We would like to acknowledge the financial support from the National Basic Research Program of China (2010CB732300).

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