1. Introduction

Over the past few decades, the preparation of rare earth-doped nano- and micro-sized materials has attracted increasing attention due to their unique properties and potential applications in many fields such as phosphors, lasers, optoelectronic devices, optical telecommunication, and biological labels [1,2]. In particular, rare earth orthophosphates (REPO₄) are a very interesting class of host lattices for activator ions because of their many favorable properties, such as very high thermal stability (2300 °C), good photochemical stability, high refractive index (n = 1.5), high quantum yield and low toxicity [3,4]. As such, rare earth phosphates (REPO₄) have become a very promising material and play an increasingly important role in luminous performance [5–7].

There are many factors that affect luminescence properties, such as the activator concentration, crystallinity, grain size, morphology, and aggregation. Of these, a spherical phosphor morphology is helpful to improve brightness and resolution. The ideal morphology of fluorescent particles includes perfect sphericity (< 2 μm), a narrow size distribution, and free of aggregation. Moreover, the spherical morphology contributes to achieving high packing densities and low light scattering and has an important effect in core-shell structure applications [7,8].

Many researchers have prepared a variety of core-shell-structured fluorescent powders [9,10]. For example, the Lin’s research group prepared a series of core-shell structures of spherical luminous materials and studied the influence of the annealing temperature and SiO₂ core particle size on the luminescence intensity [9]. Sofia Dembski’s team also studied the effect of pH on luminescent core-shell nanoparticles [10]. These studies showed that the annealing temperature, SiO₂ particle size, and pH value all had important impacts on the luminescent performance. Increasing the annealing temperature and SiO₂ particle size and maintaining the pH value within a suitable range can grant property-enhanced luminescent materials [9,10]. However, few studies have reported on the effects of annealing temperature on the synthesis of SiO₂@LaPO₄:Ce³⁺/Tb³⁺ green phosphors. Song’s research group prepared a series of luminescent nanoparticles with SiO₂ coated rare earth doped phosphate [11,12]. In their works the SiO₂ as the shell can suppress the surface quenching effect and protect the luminescent ions in order to achieve the enhanced effects of upconversion luminescence.

In our present work, silica spheres were prepared as the core, and a LaPO₄:Ce³⁺/Tb³⁺ phosphor layer was used as the shell. The SiO₂ as the core can decrease the cost, meanwhile, the core-shell structured phosphors’ luminescence intensity can reach about seventy percent of the LaPO₄:Ce³⁺/Tb³⁺ phosphors’ under an optimum calcination temperature. The core-shell structured SiO₂@LaPO₄:Ce³⁺/Tb³⁺ particles can be obtained by a deposition method. As far as we know, the synthesis of SiO₂@LaPO₄:Ce³⁺/Tb³⁺ phosphors has rarely been reported to date. Herein, the morphology, size, crystal structure, and photoluminescence properties were investigated in detail. As the method used is economical, environmentally friendly, and has high-yield mass production, this route may open new possibilities to synthesize microsphere phosphors and extend their applications besides in LED message board field.

2. Experimental

2.1. Materials

All chemicals were used without further purification. La(NO₃)₃·6H₂O (purity, 99.9%) was purchased from Alfa Aesar Co., Ltd. (Ward Hill, MA, USA). CeO₂ (99.9%) and Tb₄O₇ (99.9%) were purchased from Shanghai Yuelong Non-Ferrous Metals Limited (Shanghai, China). Ethanol, ammonia and NaOH were obtained from Yantai Sanhe Chemical Reagent Co., Ltd. NaH₂PO₄ was acquired from Tianjin Guangcheng Chemicals Co., Ltd, and tetraethyl orthosilicate (TEOS) was purchased from Sinopharm Chemical Reagent Co., Ltd.

2.2. Synthesis of silica cores

The highly monodisperse SiO₂ spheres were prepared following the well-known Stöber method [13–15], i.e., tetraethoxysilane (TEOS) was hydrolyzed in an ethanol solution containing water and ammonia. In this experiment, 70 mL of NH₃·H₂O, 75 mL of ethanol and 3 mL of H₂O were mixed in a beaker, and then, 7 mL of TEOS was added dropwise under vigorous stirring for five minutes. Then, the beaker was sealed with polyvinyl chloride film and left to stir at room temperature for 4 h, resulting in the formation of a white silica colloidal suspension. After the reaction completed, the monodisperse silica powder spheres could be obtained by centrifugation, washing and drying at 80 °C for 12 h.

2.3. Synthesis of core-shell structured SiO₂@LaPO₄:Ce³⁺/Tb³⁺ phosphors

SiO₂ core-LaPO₄:Ce³⁺/Tb³⁺ shell phosphors (denoted as SiO₂@LaPO₄:Ce³⁺/Tb³⁺) were prepared using homogeneous precipitation. NaH₂PO₄ (1 mmol) was dissolved in 40 mL of deionized water with stirring for 10 min, and the pH value of the solution was adjusted to between 8 and 11 using NaOH (2 M) solution. Then, the as-prepared SiO₂ microspheres (0.2 g) were added to the solution and ultrasonicated for 10 min. Subsequently, 0.57 mmol of La (NO₃)₃·6H₂O, 0.29 mmol of Ce(NO₃)₃·6H₂O and 0.14 mmol of Tb(NO₃)₃·6H₂O were added in 40 mL of deionized H₂O. The resulting mixture was added dropwise into the solution prepared earlier and heated to 70 °C for 4 h with vigorous stirring. After cooling to room temperature, the precursor was washed repeatedly by deionized H₂O and ethanol and dried at 80 °C for 12 h. Then, the dried samples were heated to the desired temperature (600-900 °C) at a heating rate of 5 °C min⁻¹ and held there for 2 h. The pure powder phosphor LaPO₄:Ce³⁺/Tb³⁺ was prepared as above, just without 0.2 g SiO₂ added.

2.4. Measurements and characterization

The crystal structures of the as-prepared samples were determined on a Rigaku/ Max-3A X-ray diffractometer (XRD) using Cu Kα1 radiation (λ = 1.5406 Å). The morphologies and sizes of the samples were investigated using a field emission scanning electron microscope (FE-SEM, JSM-6700F) equipped with an energy dispersive spectrometer (EDS). The transmission electron microscope (TEM) and high-resolution TEM (HRTEM) images were obtained on a JEOL JEM-2100F with an accelerating voltage of 200 kV (JEOL, Japan). X-ray photoelectron spectroscopy (XPS) measurements were performed in a VG Scientific ESCALAB-250 spectro-meter. Photoluminescent (PL) emission spectra were obtained using a Hitachi F-4600FL spectrophotometer equipped with a 150 W-xenon lamp as the excitation source. All measurements were performed at room temperature (RT).

3. Results and discussion

3.1. XRD analysis

Figure 1 shows the XRD patterns of as-obtained samples annealed at different temperatures. As shown in Fig. 1(a), there is no diffraction peak is observed, except for a broad band centered at 2θ = 21.98°, which is the characteristic peak for amorphous SiO₂ (PDF#29-0085). For the annealed SiO₂@LaPO₄:Ce³⁺/Tb³⁺ particles (Fig. 1(b)-1(e)), besides an amorphous peak of SiO₂ particles centered at 2θ = 21.98°, diffraction peaks at 2θ = 21.115°, 26.782°, 28.521°, 30.894°, 36.618°, 41.906°, 46.034°, 48.375°, and 51.979° are present, which are attributed to the ($\overline{1}11$), (200), (012), (012), ($\overline{2}12$), ($\overline{3}11$), ($\overline{3}12$), (320), and (132) reflections of the crystalline lanthanum phosphate (monazite structure), respectively. The diffraction peaks can be indexed as the monoclinic phase of LaPO₄ with lattice constants a = 0.6837 nm, b = 0.7077 nm, and c = 0.651 nm (JCPDS No. 32-0493 P21/n (14)) [16]. All of the diffraction peaks belonging to crystalline LaPO₄ are present; thus, the coating of LaPO₄:Ce³⁺/Tb³⁺ has crystallized well on the surfaces of the bare silica particles. No additional phases were detected in the SiO₂@La_0.57Tb_0.14 Ce_0.29PO₄ samples, even annealing at 900 °C. Note that the diffraction peaks become sharper and stronger with increasing the annealing temperature due to the good crystallinity after annealing.

 

Fig. 1 X-ray diffraction patterns of (a) bare SiO₂, and SiO₂@LaPO₄:Ce³⁺/Tb³⁺ particles annealed at 600 °C (b), 700 °C (c), 800 °C (d), and 900 °C (e), respectively.

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3.2. SEM analysis

Figure 2 shows the SEM images of SiO₂ and SiO₂@LaPO₄:Ce³⁺/Tb³⁺ as well as the EDS spectrum of SiO₂@LaPO₄:Ce³⁺/Tb³⁺. We can observe from Fig. 2a that the as-formed SiO₂ have a spherical shape with an extremely smooth surface (average size 370 nm), a lack of aggregation, and a narrow size distribution. After functionalizing the silica particles with the LaPO₄:Ce³⁺/Tb³⁺ coating, the resulting SiO₂@LaPO₄:Ce³⁺/Tb³⁺ particles retain the morphological properties of the silica, i.e., these particles are still spherical, un-aggregated, and well dispersed. In contrast, the surfaces of the core-shell structured particles are rougher than those of the bare SiO₂ cores. This indicates that the additional layers of LaPO₄:Ce³⁺/Tb³⁺ material are well-coated on the silica surfaces using the proposed method. However, the SEM micrographs can only provide basic information on the morphology of the SiO₂@ LaPO₄:Ce³⁺/Tb³⁺ particles on a large scale (namely, all of the SiO₂ particles remain spherical and un-aggregated, subject to their sol-gel coating of the LaPO₄:Ce³⁺/Tb³⁺ layer), and the core-shell structure of the SiO₂@LaPO₄:Ce³⁺/Tb³⁺ was not be resolved from the SEM micrographs due to their low magnification. The EDS spectrum (Fig. 2c) shows two strong peaks of Si and O. Other distinct peaks can be attributed to P, La, Ce, and Tb. This provides additional evidence for the formation of a crystalline LaPO₄:Ce³⁺/Tb³⁺ coating on the SiO₂ particles.

 

Fig. 2 SEM images of (a) SiO₂ and (b) SiO₂@LaPO₄:Ce³⁺/Tb³⁺ annealing at 800°C and (c) the EDS spectrum of SiO₂@LaPO₄:Ce³⁺/Tb³⁺.

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3.3. TEM analysis

The morphologies and structures of the samples were further examined by TEM. As shown in Fig. 3(a), the core-shell-structured SiO₂@LaPO₄:Ce³⁺/Tb³⁺ particles can be seen clearly due to the different electron penetrability of the cores and shells. We can see from Fig. 3(a) that the products have a rough surface coated on the SiO₂ core and exhibit a color difference between the cores and shells. The cores are black spheres with an average size of 370 nm, and the shells are gray color with an average thickness of 30 nm. In Fig. 3(b), we can see that the rod-shaped LaPO₄:Ce³⁺/Tb³⁺ layer was successfully deposited on the SiO₂ core, which are consistent with the XPS results in Fig. 4. The HRTEM was shown in Fig. 3(c). The distance between neighboring planes is measured to be 0.451 nm, which is close to the 0.4848 nm of the (110) plane of monoclinic LaPO₄ [17].

 

Fig. 3 (a, b) TEM images of SiO₂@LaPO₄:Ce³⁺/Tb³⁺ samples annealing at 800°C and (c) a typical HRTEM image.

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Fig. 4 XPS spectrum of the SiO₂@LaPO₄:Ce³⁺/Tb³⁺ samples: the left-upper insert is the Tb 3d region, and the right-upper insert is the Ce 3d region.

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3.4. XPS analysis

The XPS spectra of SiO₂@LaPO₄:Ce³⁺/Tb³⁺ samples are shown in Fig. 4. The left-upper insert is the Tb 3d region, and the right-upper insert is the Ce 3d region. The XPS spectrum analysis can only detect particles of a few atomic layer thicknesses. It can be seen from the XPS spectra that the peaks located at 1280.27 eV, 1245.57 eV, 905.02 eV, 885.47 eV, 853.12 eV, 837.87 eV, 533.02 eV and 132.87 eV can be attributed to the binding energy of Tb 3d_3/2, Tb 3d_5/2, Ce 3d_3/2, Ce 3d_5/2, La 3d_3/2, La 3d_5/2, O 1s and P 2p, respectively. These results indicated the synthetic product contained the elements of Tb, Ce, La, O and P. These results are consistent with the EDS and are further evidence that a LaPO₄:Ce³⁺/Tb³⁺ layer is successfully coated on the SiO₂ core surface.

3.5. Luminescence study

Figures 5(a) and 5(b) exhibit the excitation and emission spectra of SiO₂@LaPO₄:Ce³⁺ and SiO₂@LaPO₄:Ce³⁺/ Tb³⁺ core-shell phosphor. In Fig. 5(a), the excitation spectrum (left) exhibits a broad band from 250 nm to 300 nm with a maximum at 280 nm, which corresponds to the f-d transitions of Ce³⁺ ions from the ground state ²F_5/2 to excited 5d states. The emission spectrum consists of two peaks centered at 320 and 337 nm, originating from the 5d excited state to the ²F_5/2 and ²F_7/2 ground state. The energy between the two peaks is 1838 cm⁻¹, basically consistent with the ground-state splitting of Ce³⁺ (2000 cm⁻¹). In Fig. 5(b), the excitation spectrum was obtained by monitoring the emission of Tb^{3+ 5}D₄-⁷F₅ transition at 545 nm. The absorption of Ce³⁺ ion is a broad band under ultraviolet and the strong excitation peak at 286 nm belongs to 4f-5d transition absorption of Ce³⁺ ion. The absorption at 240 nm and 264 nm correspond to 4f(²F_5/2) →5d and 4f(²F_7/2) →5d, respectively. The emission of Ce³⁺ ion is a broad band transition, which distributes at long wave ultraviolet. The emission peaks at 320 nm and 337 nm correspond to 5d→²F_5/2 and 5d→²F_7/2 transition, respectively. As a result of the emission band of cerium ion having an overlap with the terbium ion’s absorption band, the energy can be transferred from Ce³⁺ to Tb³⁺, thus the sensitization effect can be achieved favorably [18]. The emission spectrum was excited at 286 nm, including four terbium ion f-f transition emission peaks (492, 545, 583, and 623 nm). In addition, two cerium ion d-f transition emission peaks (320 nm, 337 nm) are located in the ultraviolet region. The terbium ion emission peaks correspond to the transition from the excited state of ⁵D₄ to the ground state of ⁷F_J (J = 6~3), among which the strongest green emission peak corresponds to the terbium ion transition of ⁵D₄→⁷F₅, respectively.

 

Fig. 5 Excitation and emission spectra for the luminescence of Ce³⁺ (a), Tb³⁺(b) in SiO₂@LaPO₄:Ce³⁺/Tb³⁺ particles, the energy transfer process (c), and the emission intensity with a sample heated to 800°C as inset (d).

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Figure 5(c) shows the energy level diagram and energy transfer process of Ce³⁺ and Tb³⁺ in LaPO₄:Ce³⁺/Tb³⁺ with electronic transitions. In this process, Ce³⁺ ions are firstly excited by ultraviolet light excitation, then the energy transfer takes place from Ce³⁺ to Tb³⁺ which non-radiatively decay to the upper excited level (⁵D₄) of Tb³⁺ [18]. Finally, radiative decay occurs from this level to various lower levels of ⁷F_J (J = 0, 1, 2, 3, 4, 5 and 6) [9].The energy levels of Tb³⁺ are suitable for the energy transfer to take place from the allowed Ce³⁺ emission of (f - d) upon excitation with UV light [15].

On the basis of the energy transfer and luminescence performance research, our study mainly focuses on the influencing factor of the annealing temperature. Figure 5(d) shows the PL intensity of SiO₂@LaPO₄:Ce³⁺/Tb³⁺ core-shell phosphors varies with the annealing temperature. It can be seen that the PL intensity basically increases with increasing the annealing temperature before 800 °C. The degree of crystallinity may change with the increasing of sintering temperature, which may be influence the PL intensity of samples. When the temperature reaches 900 °C the PL intensity declines instead. This phenomenon was attributed to the changing of phosphors’ particle size and surface morphology. The PL intensity of SiO₂@LaPO₄:Ce³⁺/Tb³⁺ core–shell phosphor annealed at 800 °C can reach about 71.25% that of the pure LaPO₄:Ce³⁺/Tb³⁺ powder phosphor. The picture of the sample heated to 800°C is displayed as inset, which exhibits an excellent green fluorescent under 286 nm excited.

4. Conclusions

The SiO₂@LaPO₄:Ce³⁺/Tb³⁺ core-shell-structured phosphors are successfully synthesized using a homogeneous precipitation method. The obtained SiO₂@LaPO₄:Ce³⁺/Tb³⁺ particles have perfect spherical morphology, a narrow size distribution, and good dispersion. When the samples annealed at 800 °C the PL intensity can achieve a maximum and reach about 71.25% that of the pure LaPO₄:Ce³⁺/Tb³⁺ powder phosphor. Both the SEM and TEM images suggest that the LaPO₄:Ce³⁺/Tb³⁺ layers are successfully deposited on the SiO₂ particles, which is further confirmed by XRD, EDS and XPS analysis. The SiO₂@LaPO₄:Ce³⁺/Tb³⁺ phosphors show strong green luminescence. This type of core-shell structured phosphor has potential applications in photoluminescence areas, such as lighting, LED message board, information display and detection. This method can also be extended to prepare various other core-shell phosphors with homogeneous morphology and can also decrease the cost of phosphors to some degree.