Large-scale fabrication of porous YBO3 hollow microspheres with tunable photoluminescence

Hollow lanthanide-doped compounds are some of the most popular materials for high-performance luminescent devices. However, it is challenging to find an approach that can fabricate large-scale and well-crystallized lanthanide-doped hollow structures and that is facile, efficient and of low cost. In this study, YBO3: Eu3+/Tb3+ hollow microspheres were fabricated by using a novel multi-step transformation synthetic route for the first time with polystyrene spheres as the template, followed by the combination of a facile homogeneous precipitation method, an ion-exchange process and a calcination process. The results show that the as-obtained YBO3: Eu3+/Tb3+ hollow spheres have a uniform morphology with an average diameter of 1.65 µm and shell thickness of about 160 nm. When used as luminescent materials, the emission colours of YBO3: Eu3+/Tb3+ samples can be tuned from red, through orange, yellow and green-yellow, to green by simply adjusting the relative doping concentrations of the activator ions under the excitation of ultraviolet light, which might have potential applications in fields such as light display systems and optoelectronic devices.


Introduction
Luminescent materials, especially lanthanide-doped materials, have attracted extensive synthetic interest due to their remarkable luminescence properties, such as various emission colours, high photochemical stability, low toxicity, narrow emission 2. Experimental

Materials
The rare earth oxides Y 2 O 3 (99.99%), Eu 2 O 3 (99.99%), Tb 4 O 7 (99.99%) and other chemicals were purchased from Aladdin Reagent Co. Ltd. Rare earth chloride stock solutions were prepared by dissolving the corresponding metal oxide in hydrochloric acid at an elevated temperature. All chemicals were analytical-grade reagents and used as purchased without further purification.

Preparation of monodispersed polystyrene microspheres
In a typical synthesis, the poly(N-vinylpyrrolidone) K30 stabilizer (1.0 g) was dissolved in ethanol (38.2 ml) in a three-necked round bottom flask fitted with a condenser and a magnetic stirrer. The reaction vessel was then heated to 70°C under a nitrogen blanket and purged with nitrogen for 2 h. Then, a solution of azoisobutyronitrile (0.15 g) pre-dissolved in styrene monomer (15 g) was added to the reaction vessel with vigorous stirring. The styrene polymerization was allowed to proceed for 12 h before cooling to room temperature. The product was purified by repeated centrifugation and washed with ethanol. A white fine powder (PS) was finally obtained after being dried in a vacuum oven at 50°C.

Preparation of the monodisperse YBO 3 hollow microspheres
First, 1 mmol of YCl 3 (0.2 M) aqueous solution and the as-prepared PS microspheres (100 mg) were added to 50 ml deionized water and well dispersed with the assistance of ultrasonication for 30 min. Then, 2.0 g of urea was dissolved in the solution under vigorous stirring. Finally, the mixture was transferred into a 100 ml flask and heated at 90°C for 2 h with vigorous stirring before the product was collected by centrifugation. The product was washed with deionized water and ethanol three times. Second, the asobtained sample was dispersed in deionized water by ultrasonication for 30 min. Then, 0.2 g of H 3 BO 3 dissolved in an appropriate amount of deionized water was dripped into the dispersion followed by further stirring. After additional agitation for 60 min, the as-obtained mixing solution was transferred into a Teflon bottle held in a stainless steel autoclave, sealed and maintained at 180°C for 24 h. As the autoclave was cooled to room temperature naturally, the precipitate was separated by centrifugation, washed with deionized water and ethanol in sequence, and then dried in air at 80°C for 12 h. Finally, the final YBO 3 hollow microspheres were obtained through a heat treatment at 800°C in air for 4 h with a heating rate of 1°C min −1 . The YBO 3 : Eu 3+ /Tb 3+ hollow microspheres were prepared in a similar procedure except that by adding corresponding EuCl 3 and TbCl 3 together with YCl 3 as the starting materials as described above.

Characterization
Powder X-ray diffraction (XRD) measurement was performed with a Rigaku-Dmax 2500 diffractometer with Cu Kα radiation (λ = 0.15405 nm). Raman spectra were obtained by a Lab RAM HR system of Horiba JobinYvon at room temperature using a 532 nm solid-state laser as excitation source. Thermogravimetric analysis (TGA) data were recorded with a thermal analysis instrument (SDT 2960, TA Instruments, New Castle, DE, USA) with a heating rate of 10°C min −1 in an air flow of 100 ml min −1 . The morphologies and composition of the as-prepared samples were inspected with a field emission scanning electron microscope (SEM, SU8010, Hitachi). Low-to high-resolution transmission electron microscopy (TEM) was performed using an FEI Tecnai G 2 S-Twin with a field emission gun operating at 200 kV. Images were acquired digitally with a Gatan multiople CCD camera. The PL excitation and emission spectra were recorded with a Hitachi F-4500 spectrophotometer equipped with a 150 W xenon lamp as the excitation source. All measurements were performed at room temperature.

Results and discussion
The strategy of preparing the YBO 3 hollow microspheres is highly repeatable and revealed in figure 1. The whole process can be mainly divided into four steps. (i) Synthesis of the well-monodisperse PS colloidal microspheres by dispersion polymerization [42]. (ii) Synthesis of the core-shell PS@Y(OH)CO 3 microspheres by the homogeneous precipitation method using urea as the precipitating agent. The PS microspheres have a lot of hydroxyl groups, which are beneficial to the adsorption of Y 3+ , OH − and CO 2− 3 (released from precipitator agent urea) (equations (3.1)-(3.3)). (iii) Formation of the core-shell PS@YBO 3 microspheres by an ion-exchange process under hydrothermal condition. Under hydrothermal process, the H 3 BO 3 is able to react with Y(OH)CO 3 to form some YBO 3 nanoparticles (equation (3.4)). Subsequently, the interface chemical transformation gradually continued to occur with the inner layer in the hydrothermal condition, resulting in the pure YBO 3 layer. (iv) Calcination of the core-shell PS@YBO 3 microspheres in air to remove the PS microsphere template to get the YBO 3 hollow spheres and increase of crystallinity of the final product. The main chemical reactions for the formation of the YBO 3 hollow microspheres could be represented as follows: and The phase purity and crystal structure of the obtained samples were examined by XRD ( figure 2). After the homogeneous precipitation reaction, no obvious diffraction peak appears in the pattern of the sample (PS@Y(OH)CO 3 ), indicating that the as-formed core-shell PS@Y(OH)CO 3 sample is amorphous. After the hydrothermal reaction, the diffraction pattern of the sample can be indexed to the hexagonal-vaterite phase of YBO 3 (JCPDS no. 16   After annealing at 800°C for 4 h, all of the diffraction peaks can also be well indexed to the tetragonal phase of YBO 3 , and no other impurity peaks can be detected, indicating the formation of a purely YBO 3 phase. It can also be seen that the diffraction peaks of the YBO 3 sample are very sharp and strong, revealing that the YBO 3 product with high crystallinity can be synthesized using this method. This is important for phosphors because high crystallinity generally means fewer traps and stronger luminescence. In order to further confirm the structure, a typical Raman spectrum of YBO 3 sample recorded at room temperature is shown in figure 3.   groups, and the vibrational modes of the B 3 O 9 groups. The other bands in the 250-1200 cm −1 region are related to the internal modes of the B 3 O 9 groups. Furthermore, some splits of the internal vibrational bands should be attributed to the crystal field effect which may reduce the site symmetry of the B 3 O 9 groups [43]. The size and morphology of the products were further examined by SEM and TEM measurements. Figure 4a,b shows that the PS microspheres consist of well-dispersed microspheres with an average size of 1.85 µm and their surfaces are smooth. After the homogeneous precipitation reaction, the Y(OH)CO 3 layers were coated around the PS microspheres (denoted as PS@Y(OH)CO 3 ). From the SEM image (figure 4c), it can be seen that the sample inherits the spherical morphology, and the surfaces are much rougher than those of the PS microsphere template because of the precipitation of a large amount of nanoparticles. The size of the PS@Y(OH)CO 3 is about 2.20 µm. Furthermore, detailed morphological identification was performed using TEM image analysis. Figure 4d presents a typical representative TEM image of the PS@Y(OH)CO 3 sample, which consists of rough surface microspheres and the core-shell structures can be easily found via different colours of core and shell. The average size of the asprepared sample is 2.20 µm in diameter and the thickness of the shell is about 175 nm. So the size of the PS@Y(OH)CO 3 microspheres is larger than that of the pure PS microspheres, which further confirms the formation of the Y(OH)CO 3 layer. When the PS@Y(OH)CO 3 core-shell microspheres were treated with H 3 BO 3 under hydrothermal conditions at 180 o C for 24 h, the product (denoted as PS@YBO 3 ) largely inherits the shape and dimension of the PS@Y(OH)CO 3 core-shell microspheres (figure 4e). The size of the product is similar to the core-shell PS@Y(OH)CO 3 microspheres in the size range of 2.20 µm. From the TEM image (figure 4f ), it can be seen that the average size of the core-shell microspheres is about 2.20 µm and the shell thickness is about 175 nm, which conforms to the size calculated from the SEM image.   Thermal decomposition of the PS microsphere template is a simple and conventional route to form a hollow structure. After the synthesis of the core-shell PS@YBO 3 microspheres, we also investigated the effect of calcination on the morphology of the as-prepared product. Figure 5 shows the TGA curves of the PS microspheres and the core-shell PS@YBO 3 microspheres. For the PS microspheres (black line), there is one weight loss which is attributed to the splitting burning of PS microspheres. For the coreshell PS@YBO 3 microspheres, there are two stages of weight loss (red line): one is a slow weight loss because of the dehydration and densification of the PS microspheres. The other one is the burning of the PS microspheres. Finally, the residual weight percentage is about 54.1%, which accounts for the final YBO 3 hollow microspheres, suggesting the considerably high yield of the hollow phosphors prepared using this method. So it can be concluded that the calcination process has a dual function: elimination of the PS microsphere cores to form hollow microspheres and the increase of crystallinity of the final product. The morphology, microstructure and elemental composition of YBO 3 sample were revealed by SEM, TEM and high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) (figure 6). The YBO 3 sample exhibits sphere-like structure with a diameter of approximately 1.65 µm (figure 6a). In particular, the hollow microspheres can be clearly visualized from the rupture of one sphere with a typical wall thickness of around 160 nm (figure 6a). The sharp contrast between the edge and centre part of the hollow structure is clearly visible in the TEM image (figure 6b). The measured d spacing of 0.327 nm in the high-resolution TEM image (inset in figure 6b) can be indexed to the lattice spacing of the (100) plane of YBO 3 . In order to investigate the elemental distribution, HAADF-STEM image and elemental maps were acquired for an individual sphere (figure 6c-f ). Elements Y, B and O are evenly distributed throughout the entire sphere, revealing that the YBO 3 hollow sphere can be synthesized by the combination of a facile homogeneous precipitation method, an ion-exchange process and a calcination process.
It is well known that Eu 3+ or Tb 3+ ions-doped YBO 3 samples can emit strong red or green emission under UV excitation, respectively. The excitation and emission spectra of the YBO 3 : 5 mol% Eu 3+ sample are shown in figure 7a. By means of monitoring at 590 nm, it was found that the excitation spectrum is composed of a strong absorption band centred at 237 nm and some weak lines, which are due to the    Figure 7b shows the excitation and emission spectra of the YBO 3 : 5 mol% Tb 3+ sample. The excitation spectrum of the YBO 3 : 5 mol% Tb 3+ sample monitored with 541 nm consists of two intense bands and some weak lines. The intense bands centred at 240 and 285 nm are attributed to the spin-allowed transition ( S = 0) with higher energy and the spin-forbidden transition ( S = 1) with lower energy from the 4f to the 5d level of the Tb 3+ ions, respectively [16,44]. The other weak lines are due to the characteristic f-f transitions of the Tb 3+ ions. The emission spectrum consists of a group of lines centred at about 489, 541, 587 and 619 nm, which correspond to the 5 D 4 → 7 F J (J = 6, 5, 4, 3) transitions of the Tb 3+ ions, respectively. The strongest one is located at 541 nm, corresponding to the 5 D 4 → 7 F 5 transition of Tb 3+ .
In order to further illustrate the tunable PL property of the YBO 3 sample, we co-doped Eu 3+ and Tb 3+ ions with different relative concentrations into the YBO 3 host lattice (total concentration: 5 mol%). The emission spectra of the YBO 3 : x mol% Eu 3+ , (5 − x) mol% Tb 3+ excited at 237 nm are depicted in figure 8 to show the succession of changes. It can be seen that the as-obtained YBO 3 : 5 mol% Eu 3+ sample shows the characteristic emission peaks of Eu 3+ ions. When Tb 3+ ions were doped into the YBO 3 host lattice, the YBO 3 : Eu 3+ /Tb 3+ samples show not only the characteristic emission of Eu 3+ ions, such as 590 nm ( 5 D 0 → 7 F 1 ), 610 and 624 nm ( 5 D 0 → 7 F 1 ), but also the characteristic emission of Tb 3+ ions, such as 489 nm ( 5 D 4 → 7 F 6 ) and 541 nm ( 5 D 4 → 7 F 5 ). As one might expect, on increasing the relative concentration ratio of Eu 3+ /Tb 3+ , the luminescence of the Eu 3+ ions gradually decreased, while that of Tb 3+ increased. Finally, the pure YBO 3 : 5 mol% Tb 3+ sample shows a bright green emission. As a result, the PL colour can be tuned from red, through orange, yellow and green-yellow, to green by simply adjusting the relative doping concentrations of the Eu 3+ and Tb 3+ ions. The result can be confirmed by the corresponding CIE chromaticity diagram for the emission spectra of the Eu 3+ and Tb 3+ co-doped YBO 3 samples ( figure 9). This result indicates that the as-obtained phosphors have the merit of multicolour emissions in the visible region when excited by a single wavelength of light, which might find potential applications in fields such as display systems and optoelectronic devices.

Conclusion
To sum up, YBO 3 with a well-dispersed hollow microsphere shape has been successfully synthesized via the combination of a facile homogeneous precipitation approach, an ion-exchange process and a calcination process. The morphology, crystal structure and luminescence property of the as-obtained hollow microspheres were characterized by XRD, SEM, TEM and PL. Furthermore, the PL colour of the YBO 3 : Eu 3+ , Tb 3+ samples can be controlled from red to orange to yellow to green-yellow and then to green by adjusting the relative doping concentrations of the activator ions, which indicates that the asobtained phosphors could have the merit of multicolour emissions in the visible region when excited at a single wavelength. The material has a very important potential application in many fields, such as light display systems and optoelectronic devices, owing to its multicolour emission.
Data accessibility. This article does not contain any additional data. Authors' contributions. Z.X., F.D. and Y.G. conceived and designed the study; H.Y. and F.A. performed the experiments and collected data; G.Z., L.H. and Y.S. analysed the data. Y.G., Y.B. and Z.X. wrote the paper. All authors gave final approval for publication.