Synthesis and luminescent properties of uniform monodisperse LuPO4:Eu3+/Tb3+ hollow microspheres

Uniform monodisperse LuPO4:Eu3+/Tb3+ hollow microspheres with diameters of about 2.4 µm have been successfully synthesized by the combination of a facile homogeneous precipitation approach, an ion-exchange process and a calcination process. The possible formation mechanism for the hollow microspheres was presented. Furthermore, the luminescence properties revealed that the LuPO4:Eu3+ and LuPO4:Tb3+ phosphors show strong orange-red and green emissions under ultraviolet excitation, respectively, which endows this material with potential application in many fields, such as light display systems and optoelectronic devices. Since the synthetic process can be carried out at mild conditions, it should be straightforward to scale up the entire process for large-scale production of the LuPO4 hollow microspheres. Furthermore, this general and simple method may be of much significance in the synthesis of many other inorganic materials.


Introduction
Nowadays, rare earth luminescent micro/nanomaterials which have lots of excellent physical and chemical properties arising 2017 The Authors. Published by the Royal Society under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/4.0/, which permits unrestricted use, provided the original author and source are credited.

Experimental section 2.1. Materials
The rare earth oxides Ln 2 O 3 (99.99%) (Ln = Lu and Eu) and Tb 4 O 7 (99.99%) were purchased from GZSUNKO new material Co., Ltd. Other chemicals were purchased from Sinopharm Chemical Reagent Co., Ltd. All chemicals were analytical-grade reagents and used as purchased without further purification. Rare earth chloride stock solutions were prepared by dissolving the corresponding metal oxide in hydrochloric acid at an elevated temperature.

Preparation of monodispersed polystyrene microspheres
Monodisperse PS colloidal microspheres were prepared by dispersion polymerization [35]. In a typical synthesis, the poly(N-vinylpyrrolidone) stabilizer (1.0 g) was dissolved in ethanol ( 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 monodisperse PS@Lu(OH)CO 3 microspheres
In the preparation procedure, 1 mmol of LuCl 3 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 precursors were washed by deionized water and ethanol three times.

Preparation of monodisperse hollow LuPO 4 microspheres
In a typical synthesis, the as-obtained PS@Lu(OH)CO 3 sample was dispersed in deionized water by ultrasonication for 30 min. Then, 0.2 g of NH 4 H 2 PO 4 dissolved in 10 ml 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 12 h. As the autoclave was cooled to room temperature naturally, the precipitates were separated by centrifugation, washed with deionized water and ethanol in sequence, and then dried in air at 80°C for 12 h. The final hollow LuPO 4 microspheres were obtained through a heat treatment at 800°C in air for 4 h with a heating rate of 1°C min −1 . Hollow LuPO 4 :Ln 3+ (Ln 3+ = Eu 3+ , Tb 3+ ) spheres were prepared in a similar procedure except by adding corresponding Eu 2 O 3 , and Tb 4 O 7 together with Lu 2 O 3 as the starting materials as described above.

Characterization
The X-ray diffraction (XRD) patterns of the samples were recorded on a D8 Focus diffractometer (Bruker) with Cu-Kα1 radiation (λ = 0.15405 nm). Fourier transform infrared spectroscopy (FT-IR) spectra were measured with a Perkin-Elmer 580B infrared spectrophotometer with the KBr pellet technique. Thermogravimetric data were recorded with Thermal Analysis Instrument (SDT 2960, TA Instruments, New Castle, DE) with the 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 on a field emission scanning electron microscope (FESEM, SU8010, Hitachi). Low-to high-resolution transmission electron microscopy (TEM) was performed using FEI Tecnai G 2 S-Twin with a field emission gun operating at 200 kV. Images were acquired digitally on a Gatan multiple CCD camera. The PL excitation and emission spectra were recorded with a Hitachi F-7000 spectrophotometer equipped with a 150 W xenon lamp as the excitation source. All measurements were performed at room temperature.

Results and discussion
The synthesis protocol of the hollow LuPO 4 microspheres is shown in scheme 1. In accordance with the previous reported method, the well-monodispersed PS colloidal microspheres were prepared by dispersion polymerization, and coated with a Lu(OH)CO 3 layer using urea-based chemical precipitation to form a monodisperse core-shell PS@Lu(OH)CO 3 microspheres (equations (3.1) and (3.2)). Subsequently, the core-shell PS@Lu(OH)CO 3 microspheres were reacted with NH 4 H 2 PO 4 under hydrothermal process, involving the replacement of the anions between CO 3 2− and PO 4 3− based on the Kirkendall effect [2,14]. The LuPO 4 bumpy skin layer was constructed and assembled on the surface of the PS microspheres as the reaction progress (equation (3.3)). Finally, the PS microspheres were removed through calcination of the PS@LuPO 4 sample and LuPO 4 hollow microspheres were formed.
and  . It can be observed in the TEM image that the surfaces of monodisperse PS microspheres are very smooth (figure 1b). After deposition of Lu(OH)CO 3 layer, the surface of PS@Lu(OH)CO 3 coreshell microsphere is bumpier than PS microsphere. The size of the PS@Lu(OH)CO 3 core-shell-structured microsphere (ca 2.50 µm) is larger than that of the bare PS microspheres (ca 2.4 µm) due to the amorphous Lu(OH)CO 3 shell (figure 1c). From the TEM image (figure 1d), the core-shell structure can be easily found via the dark cores and the grey shell. Then, the core-shell-structured PS@LuPO 4 microspheres with spiky LuPO 4 shell were synthesized through an ion-exchange process under hydrothermal reaction conditions. The SEM image (figure 1e) shows that the core-shell PS@LuPO 4 sample has an average size of about 2.55 µm and very rough surfaces with spikes. The TEM image (figure 1f ) reveals that the sample with a diameter of 2.55 µm has a spiky surface and a solid structure, which agrees well with the SEM image.    Figure 3 shows the XRD results of the PS spheres, the core-shell PS@Lu(OH)CO 3 microspheres, the core-shell PS@LuPO 4 microspheres and the final LuPO 4 hollow microspheres. In figure 3a for the PS spheres, an obviously broadened diffraction peak at 19°is observed, which can be assigned to the typical XRD pattern of PS spheres [10,36]. For the core-shell PS@Lu(OH)CO 3 microspheres (figure 3b), two broad bands at 30°and 47°can be observed, which imply that the as-formed coreshell PS@Lu(OH)CO 3 microspheres are amorphous. Figure 3c shows the XRD pattern of the sample that the core-shell PS@Lu(OH)CO 3 microspheres were treated with NH 4 H 2 PO 4 in the hydrothermal process. All of the diffraction peaks can be indexed as pure tetragonal phase, and coincide well with the standard data of LuPO 4 (JCPDS No. 84-0337). It means the product is the core-shell PS@LuPO 4 . After annealing of the core-shell PS@LuPO 4 microspheres at 800°C for 4 h, the position of all peaks does not change (figure 3d). However, the diffraction peaks become stronger and sharper due to the increase of crystallinity. This is important for phosphors, because high crystallinity generally means less traps and stronger luminescence. Thus, it can be concluded that the calcination process has a dual function: elimination of the PS spheres cores to form hollow microspheres and increase of crystallinity.
To further examine the chemical compositions of the samples, FT-IR spectroscopy was conducted for all the products ( figure 4). For the PS microspheres (figure 4a), the characteristic adsorption peaks at about 3150-2800, 1650-1350 and 600-820 cm −1 can be attributed to the stretching vibrations of aromatic C-H in-plane, stretching vibrations of aromatic C-C and bending vibrations of aromatic C-C out-of-plane, respectively [37]. Compared with the PS microspheres, the core-shell PS@Lu(OH)CO 3 microspheres not only exhibit characteristic absorption bands of the PS microspheres, but also   show the bands at 1543, 1408, 838, 760, 698 and 664 cm −1 , corresponding to CO(v as ), CO (v s ), CO (δ), OH-(δ) and CO (δ) (v as = asymmetric stretch; v s = symmetric stretch; δ = deformation) (figure 4b) [10,36]. This result further indicates that the compositions of the core-shell PS@Lu(OH)CO 3 are the PS microspheres and Lu(OH)CO 3 . For the core-shell PS@LuPO 4 microspheres (figure 4c), the vibration bands at 648 and 1023 cm −1 represent the characteristic adsorption of the phosphate groups [14,38]. It means that the core-shell PS@Lu(OH)CO 3 microspheres can convert to the coreshell PS@LuPO 4 microspheres during the hydrothermal process. In the FT-IR spectrum of the LuPO 4 hollow microspheres, all of the functional groups of the PS microspheres nearly disappear, and the characteristic adsorptions of the phosphate groups do not change, which demonstrates that the PS template can thoroughly be removed by calcination. We also studied the thermal behaviour of the PS spheres and the core-shell PS@LuPO 4 microspheres by thermogravimetric analysis (TGA) technique ( figure 5). There is one stage of weight loss for the PS spheres (line a), which can be attributed to the splitting burning of the PS spheres. For the core-shell PS@LuPO 4 microspheres, there are two stages of weight loss (line b): 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 70.53%, which accounts for the final LuPO 4 hollow microspheres, suggesting the considerably high yield of the hollow phosphors prepared by this method. Figure 6a shows  assigned to the 5 D 0 → 7 F 2 (618 nm), 5 D 0 → 7 F 3 (649 nm) and 5 D 0 → 7 F 4 (694 nm) transitions of Eu 3+ ions, respectively. Figure 6b depicts the excitation and emission spectra of the LuPO 4 :Tb 3+ hollow microspheres. The excitation spectrum is composed of two bands with a maximum at 225 and 270 nm due to the f → d transitions of Tb 3+ in the LuPO 4 lattice, and some weak lines in the longer wavelength. The peaks with weaker intensity are assigned to the transitions from the 7 F 6 ground state to the different excited states of the Tb 3+ ions, that is, 5 G 2 (350 nm), 5 D 2 (354 nm), 5 G 6 (370 nm) and 5 D 3 (381 nm), respectively. Upon the excitation at 225 nm, the obtained emission spectrum exhibits four obvious lines centred at 490, 545, 588 and 623 nm, that can be attributed to the transitions from the 5 D 4 excited state to the 7 F J (J = 6, 5, 4, 3) ground states of the Tb 3+ ions, respectively. The 5 D 4 → 7 F 5 transition at 545 nm is the most prominent group.

Conclusion
In summary, well-dispersed and homogeneous LuPO 4 hollow microspheres have been successfully achieved by the combination of a facile homogeneous precipitation approach, an ion-exchange process and a calcination process. The possible mechanism for the overall formation process of the hollow microspheres has been discussed in detail. Under UV excitation, the LuPO 4 :Ln 3+ (Ln 3+ = Eu, Tb) hollow microspheres exhibit orange-red and green emissions, respectively. Furthermore, it is expected that our synthetic approach may open a new way for monodisperse hollow microspheres that exhibit promising physicochemical properties.
Data accessibility. This paper does not have any additional data. Authors' contributions. Y.B., F.D. and Z.X. conceived and designed the study; Y.G., C.S. and H.Y. performed the experiments and collected data; G.Z. and Y.S. analysed the data. Y.G. and Z.X. wrote the paper. All authors gave final approval for publication.