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 peaks and large anti-Stokes shifts [1-8]. These advantages lead to their excellent performance in versatile applications such as lighting, information display technologies, solar cells, biological labelling and biomedical imaging technology [9-13]. Among lanthanide-doped materials, yttrium orthoborate (YBO3) is one category of useful host lattices for luminescence [14]. Thanks to the high damage threshold, high-vacuum ultraviolet (VUV) transparency and nonlinear optical efficiency resulting from the B–O structure, it exhibits extraordinarily high luminescence efficiency under VUV excitation and it is considered to be attractive candidate as VUV luminescent material, which has found applications in Hg-free fluorescent lamps and plasma display panels [15-18]. Up to now, YBO3 micro/nanocrystals with abundant morphologies have been synthesized through a variety of techniques, such as hydro/solvothermal techniques [19], vapour–liquid–solid method, electrospinning method [20], sol–gel routes and solid-phase method. Through these techniques, various morphologies of YBO3 materials have been synthesized, such as well-dispersed nanocrystals [21], one-dimensional nanowires and nanotubes [20], drum-like microcrystals [22] and three-dimensional flower-like architectures [23].

As a unique family of functional materials, hollow structure materials possess a large fraction of empty space and high surface area which endow them with broad applications in gas sensors, drug delivery, biomaterials, water treatment, supercapacitors, dye-sensitized solar cells, heterogeneous catalysts, fuel cells, etc. [4,9,24-31]. Thus, much attention has been devoted to synthesizing hollow structures of various functional materials. Basically, there are four main methods for synthesizing hollow structures: (i) conventional hard templating method, (ii) sacrificial templating method, (iii) soft templating method and (iv) template-free method. Among all these commonly used strategies, hard templating method is an established, industrially relevant, simple and scalable protocol to produce hollow materials [32]. Basically, the preparation of hollow structures using hard templating method consists typically of three steps: (i) template preparation, (ii) coating of the template and (iii) removal of the template [33-35]. Although there are a lot of reports on hard templating synthesis of hollow structures, including C3N4 [36], Ta3N5 [37], TiO2 [38], ZnO [39], BiMoO6 [40] and LaTiO2N [41], reports on YBO3 hollow structure are very rare. Furthermore, as is well known, YBO3 has been proved to be a very efficient host lattice for the luminescence of Eu3+ and Tb3+ ions. However, based on our knowledge, up to now, there is no report available on the preparation of Eu3+- and Tb3+-co-doped YBO3 hollow structure which shows tunable luminescence properties. Generally speaking, developing facile, cost-effective, environmentally friendly and scalable strategies for the synthesis of YBO3 spherical hollow structure is still a key challenge.

Herein, we used polystyrene (PS) microspheres as the template to synthesize YBO3 hollow spheres via the combination of a homogeneous precipitation method, an ion-exchange process and a calcination process. Besides, we also systematically investigated the photoluminescence (PL) colours of the YBO3 hollow spheres co-doped with Eu3+ and Tb3+ ions, which could be tuned from red, through yellow and green-yellow, to green by simply adjusting the relative doping concentrations of the activator ions. The development of this method would offer a new platform for the fabrication of other hollow structure materials.

Experimental

Materials

The rare earth oxides Y2O3 (99.99%), Eu2O3 (99.99%), Tb4O7 (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 YBO3 hollow microspheres

First, 1 mmol of YCl3 (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 as-obtained sample was dispersed in deionized water by ultrasonication for 30 min. Then, 0.2 g of H3BO3 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 YBO3 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 YBO3: Eu3+/Tb3+ hollow microspheres were prepared in a similar procedure except that by adding corresponding EuCl3 and TbCl3 together with YCl3 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 G2 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 YBO3 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)CO3 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 Y3+, OH− and (released from precipitator agent urea) (equations (3.1)–(3.3)). (iii) Formation of the core-shell PS@YBO3 microspheres by an ion-exchange process under hydrothermal condition. Under hydrothermal process, the H3BO3 is able to react with Y(OH)CO3 to form some YBO3 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 YBO3 layer. (iv) Calcination of the core-shell PS@YBO3 microspheres in air to remove the PS microsphere template to get the YBO3 hollow spheres and increase of crystallinity of the final product. The main chemical reactions for the formation of the YBO3 hollow microspheres could be represented as follows: 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)CO3), indicating that the as-formed core-shell PS@Y(OH)CO3 sample is amorphous. After the hydrothermal reaction, the diffraction pattern of the sample can be indexed to the hexagonal-vaterite phase of YBO3 (JCPDS no. 16-0277, space group P63/m, z = 2 and cell parameter α = 3.778 Å, c = 8.806 Å). After annealing at 800°C for 4 h, all of the diffraction peaks can also be well indexed to the tetragonal phase of YBO3, and no other impurity peaks can be detected, indicating the formation of a purely YBO3 phase. It can also be seen that the diffraction peaks of the YBO3 sample are very sharp and strong, revealing that the YBO3 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 YBO3 sample recorded at room temperature is shown in figure 3. The Raman peaks in the low wavenumber region, such as 185, 196 and 205 cm−1, should be related to the translations of the Y3+ cations and the B3O9 groups, and the vibrational modes of the B3O9 groups. The other bands in the 250–1200 cm−1 region are related to the internal modes of the B3O9 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 B3O9 groups [43].

Figure 1.

Schematic of the synthesis route of the YBO3 hollow microspheres.

Figure 2.

XRD patterns of (a) the core-shell PS@Y(OH)CO3 microspheres, (b) the core-shell PS@YBO3 microspheres and (c) the YBO3 hollow microspheres.

Figure 3.

Raman spectrum of the YBO3 hollow microspheres at room temperature.

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)CO3 layers were coated around the PS microspheres (denoted as PS@Y(OH)CO3). 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)CO3 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)CO3 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 as-prepared 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)CO3 microspheres is larger than that of the pure PS microspheres, which further confirms the formation of the Y(OH)CO3 layer. When the PS@Y(OH)CO3 core-shell microspheres were treated with H3BO3 under hydrothermal conditions at 180oC for 24 h, the product (denoted as PS@YBO3) largely inherits the shape and dimension of the PS@Y(OH)CO3 core-shell microspheres (figure 4e). The size of the product is similar to the core-shell PS@Y(OH)CO3 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.

Figure 4.

SEM and TEM images of (a,b) the PS spheres, (c,d) the core-shell PS@Y(OH)CO3 microspheres and (e,f) the core-shell PS@YBO3 microspheres.

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@YBO3 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@YBO3 microspheres. For the PS microspheres (black line), there is one weight loss which is attributed to the splitting burning of PS microspheres. For the core-shell PS@YBO3 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 YBO3 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 YBO3 sample were revealed by SEM, TEM and high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) (figure 6). The YBO3 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 YBO3. 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 YBO3 hollow sphere can be synthesized by the combination of a facile homogeneous precipitation method, an ion-exchange process and a calcination process.

Figure 5.

TGA curves of the PS spheres and the core-shell PS@YBO3 microspheres.

Figure 6.

(a) SEM, (b) TEM and (b, inset) high-resolution TEM images of the YBO3 hollow microspheres. (c) HAADF-STEM image of the YBO3 hollow microspheres and the corresponding elemental maps for (d) Y, (e) B and (f) O.

It is well known that Eu3+ or Tb3+ ions-doped YBO3 samples can emit strong red or green emission under UV excitation, respectively. The excitation and emission spectra of the YBO3: 5 mol% Eu3+ 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 charge transfer band between the O2− and Eu3+ ions and f–f transition of the Eu3+ ions, respectively. Upon excitation at 237 nm, the emission spectrum of the YBO3: 5 mol% Eu3+ sample displays four palpable peaks, which are centred at 590, 610, 624 and 650 nm. The four lines correspond to the 5D0 → 7F1, 5D0 → 7F2, 5D0 → 7F2 and 5D0 → 7F3 transitions of Eu3+ ions in YBO3, respectively. The most prominent emission peak, attributed to the 5D0 → 7F1 transition of Eu3+, is located at 590 nm.

Figure 7.

Photoluminescence excitation and emission spectra of as-prepared (a) YBO3: 5 mol% Eu3+ and (b) YBO3: 5 mol% Tb3+.

Figure 7b shows the excitation and emission spectra of the YBO3: 5 mol% Tb3+ sample. The excitation spectrum of the YBO3: 5 mol% Tb3+ 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 (ΔS = 0) with higher energy and the spin-forbidden transition (ΔS = 1) with lower energy from the 4f to the 5d level of the Tb3+ ions, respectively [16,44]. The other weak lines are due to the characteristic f–f transitions of the Tb3+ ions. The emission spectrum consists of a group of lines centred at about 489, 541, 587 and 619 nm, which correspond to the 5D4 → 7F (J = 6, 5, 4, 3) transitions of the Tb3+ ions, respectively. The strongest one is located at 541 nm, corresponding to the 5D4 → 7F5 transition of Tb3+.

In order to further illustrate the tunable PL property of the YBO3 sample, we co-doped Eu3+ and Tb3+ ions with different relative concentrations into the YBO3 host lattice (total concentration: 5 mol%). The emission spectra of the YBO3: x mol% Eu3+, (5 − x) mol% Tb3+ excited at 237 nm are depicted in figure 8 to show the succession of changes. It can be seen that the as-obtained YBO3: 5 mol% Eu3+ sample shows the characteristic emission peaks of Eu3+ ions. When Tb3+ ions were doped into the YBO3 host lattice, the YBO3: Eu3+/Tb3+ samples show not only the characteristic emission of Eu3+ ions, such as 590 nm (5D0 → 7F1), 610 and 624 nm (5D0 → 7F1), but also the characteristic emission of Tb3+ ions, such as 489 nm (5D4 → 7F6) and 541 nm (5D4 → 7F5). As one might expect, on increasing the relative concentration ratio of Eu3+/Tb3+, the luminescence of the Eu3+ ions gradually decreased, while that of Tb3+ increased. Finally, the pure YBO3: 5 mol% Tb3+ 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 Eu3+ and Tb3+ ions. The result can be confirmed by the corresponding CIE chromaticity diagram for the emission spectra of the Eu3+ and Tb3+ co-doped YBO3 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.

Figure 8.

Photoluminescence emission spectra of the Eu3+ and Tb3+ co-doped YBO3 samples under excitation at 240 nm (total concentration: 5 mol%): (a) YBO3: 5 mol% Eu3+; (b) YBO3: 4 mol% Eu3+, 1 mol% Tb3+; (c) YBO3: 3 mol% Eu3+, 2 mol% Tb3+; (d) YBO3: 2 mol% Eu3+, 3 mol% Tb3+; (e) YBO3: 1 mol% Eu3+, 4 mol% Tb3+; (f) YBO3: 5 mol% Tb3+.

Figure 9.

CIE chromaticity diagram for the emission spectra of the as-obtained Eu3+ and Tb3+ co-doped YBO3 samples: (a) YBO3: 5 mol% Eu3+; (b) YBO3: 4 mol% Eu3+, 1 mol% Tb3+; (c) YBO3: 3 mol% Eu3+, 2 mol% Tb3+; (d) YBO3: 2 mol% Eu3+, 3 mol% Tb3+; (e) YBO3: 1 mol% Eu3+, 4 mol% Tb3+; (f) YBO3: 5 mol% Tb3+.

Conclusion

To sum up, YBO3 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 YBO3: Eu3+, Tb3+ 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 as-obtained 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.