Facile hydrothermal synthesis and luminescent properties of Eu-doped CaF₂–YF₃ alkaline-earth ternary fluoride microspheres

Abstract

Large-scale CaF₂–YF₃ alkaline-earth ternary fluoride microspheres with diameters of about 2.5 μm were prepared by a facile hydrothermal method in the presence of disodium ethylenediamine tetraacetate (Na₂H₂L). The influences of several experimental parameters, such as reaction time, amount of Na₂H₂L, pH values, and fluoride source on the final products were investigated. The formation mechanism of the as-obtained microspheres was proposed on the basis of all these studies. It is also found that the addition amount of the Y³⁺ ions had an effect on the morphology of CaF₂–YF₃. The luminescence spectrum of Eu³⁺-doped CaF₂–YF₃ microspheres showed the strong characteristic dominant emission of the Eu³⁺ ions at 590 nm, indicating that the Eu³⁺ ions occupy a site of inversion symmetry in the CaF₂–YF₃ matrix.

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1. Introduction

It is well known that micro/nanomaterials syntheses have attracted considerable interest during the past few years. Great effort has been devoted to the investigation of morphology, size and dimensions of the micro/nanomaterials.^1–6 Among them, fluoride materials have drawn much attention due to their unique optical and electronic properties, such as low-energy phonons, high ionicity, electron-acceptor behavior, high resistivity and anionic conductivity. For example, the fabrication of metal fluoride nanomaterials, such as CaF₂ nanocubes,⁷ SrF₂ nanospheres,⁸ BaF₂ nanocubes⁹ or nanorods,¹⁰ and hexagonal GdF₃ plates with holes in the center,¹¹ has been reported for their application in UV lithography, UV-transparent optical lenses, surface conditioning of glass, and upconversion luminescence. However, compared with above mentioned binary lanthanide fluorides (LnF₃, Ln = lanthanide elements), ternary ALnF₄ or Ln_1−yM_yF_3−y (A = alkali metals, Ln = lanthanide elements, M = Ca, Sr, Ba, Cd) fluorides are rarely studied. It is reported that barium yttrium fluoride crystals such as BaY₂F₈ and BaYF₅ are excellent host matrixes which can be doped with various lanthanide ions, exhibiting strong up/down-conversion luminescence.¹²

Up to now, the fabrication of these ternary fluorides mainly involved solid state reactions,^13–16 which has disadvantages of uncompleted production, irregular morphology, and larger particle size distribution. In comparison, solution-based syntheses possess some advantages in economical energy and facilities and controllable reaction conditions which result in refined morphology and size as well. Nevertheless, there are only a few documented literatures on the syntheses of alkaline-earth ternary fluorides via solution-based methods until now. Wang's group modified the uneven size and shape of alkaline-earth fluoride to monodisperse ultra small nanospheres through lanthanide doping in solution.^17,18 Huang in our group reported the hydrothermal synthesis and luminescence of Eu-doped Ba_0.92Y_2.15F_8.29 submicrospheres.¹⁹ To our knowledge, there has been no literature available on the hydrothermal synthesis of cubic-phase CaF₂–YF_3.

Herein, we report the preparation of CaF₂–YF₃ alkaline-earth ternary fluoride microspheres by the hydrothermal method with the disodium ethylenediamine tetraacetate (EDTA-2Na or Na₂H₂L) as chelator and NaBF₄ as fluoride source, respectively. The influence of reaction time, the amount of Na₂H₂L, pH values and fluoride source were also systemically investigated. Besides, the effects of initial amount of Y³⁺ on the Y³⁺/Ca²⁺cation ratio in the final product were also studied. In order to explore the luminescent property and the environment of Y³⁺ ions in CaF₂–YF₃ host lattice, the luminescence of Eu³⁺ doped CaF₂–YF₃ were researched as well.

2. Experiment section

Reagents

The rare-earth oxides RE₂O₃ (RE = Y, Eu) (99.99%) were purchased from Shanghai Yuelong Non-Ferrous Metals Limited. Rare-earth nitrates stock solutions were obtained by dissolving the corresponding metal oxide in HNO₃ solution under heating with agitation. The excess HNO₃ was evaporated until the pH value was 4. The other chemicals (ethylenediamine tetraacetic acid, calcium chloride, and sodium fluoborate, AR) were purchased from Beijing Chemical Co. and used as received without further purification.

Preparation

In a typical procedure of preparing CaF₂–YF₃ microspheres, 1 mmol Na₂H₂L was first added into deionized water (20 mL). Sodium hydroxide aqueous solution (2 mol L⁻¹) was then added dropwise with vigorous stirring until EDTA was dissolved absolutely to form a transparent solution with a pH value of 4. Subsequently, 1 mmol CaCl₂ and 0.5 mmol Y(NO₃)₃ were introduced into the above solution. After the solution was stirred for 30 min, 20 mL of aqueous solution containing 1 mmol NaBF₄ was added into it under thorough stirring. After additional agitation for 15 min, The resultant solution was transferred into a 50 mL Teflon autoclave, which was then tightly sealed and heated at 180 °C for 24 h followed by cooling down to room temperature naturally. The resulting precipitates were washed with deionized water and ethanol each three times. The final product was dried at 60 °C in air.

A similar process was employed for preparing Eu-doped CaF₂–YF₃. A stoichiometric amount of Eu(NO₃)₃ was added at an initial stage with Y(NO₃)₃ and other processes the same afterwards.

Characterization

Powder X-ray diffraction measurements were performed on a Bruker D8 focus X-ray powder diffractometer with Cu Kα radiation (λ = 0.15405 nm). The size and morphology of the products were characterized using an Hitachi S-4800 field emission scanning electron microscope equipped with an energy dispersive X-ray (EDX) spectrometer, operated at an acceleration voltage of 10 kV. The size and morphology of the samples were inspected using a field emission scanning electron microscope equipped with an energy-dispersive spectrometer (EDS) (FE-SEM, S-4800, Hitachi, Japan). The transmission electron microscope (TEM) images and selected area electron diffraction (SAED) patterns were obtained by a JEOL H-8100 transmission electron microscope at the accelerating voltage of 200 kV. The photoluminescence excitation and emission spectra were recorded with an Hitachi F-4500 fluorescence spectrophotometer equipped with a 150W xenon lamp as the excitation source. All the measurements were performed at room temperature.

3. Results and discussion

Phase identification and morphology of as-synthesized CaF₂–YF₃ microspheres

The crystal structure and phase purity of the product were examined by X-ray powder diffraction. Fig. 1a shows the XRD pattern of the as-synthesized product. All the diffraction peaks are coincident with the cubic-phase Ca_0.9Y_0.1F_2.1 (JCPDS 79-0094, space group Fm3m, z = 8, and cell parameter α = 5.489 Å) well, except for a small shift toward the lower 2θ side, indicating a slight expansion of the crystal lattice (the calculated cell parameter α = 5.5194 Å) compared with the standard pattern. In the lattice of CaF₂, when replacing Ca²⁺ by Y³⁺, more F⁻ came into the lattice to balance the electric charge. In this case, the lattice expansion caused by the repulsion of the interstitial F⁻ is dominated, while the lattice contraction ascribed to the replacement of smaller ionic radius is inconspicuous.¹⁷ This explained the small shift toward lower 2θ side in the XRD pattern. Furthermore, the EDX spectrum analysis (Fig. 1b) confirms the presence of elements Ca, Y, and F in the product, which can effectively support the XRD result. It is speculated that the emergence of the slight amount of Na showed that Na⁺ ions entered the lattice in the hydrothermal synthesis process. The average crystallite size of the product can be estimated from the Scherrer formula D₁₁₁ = Kλ/(β cos θ), where λ is the X-ray wavelength (0.15405 nm), β is the full-width at half-maximum, θ is the diffraction angle, K is a constant (0.89), and D₁₁₁ means the size along the (111) direction. The estimated average crystallite size is 21.5 nm.

Fig. 1 (a) XRD pattern of the product obtained in the typical procedure and the standard data of cubic-phase Ca_0.9Y_0.1F_2.1 (JCPDS 79-0094) as a reference and (b) the corresponding EDX spectrum.

The morphology and size of the as-prepared product were further examined by SEM and TEM. As shown in the overall view SEM image in Fig. 2a, the product displayed as uniform microspheres with an average size of approximately 2.5 μm. The magnified SEM image (Fig. 2b) indicates that the microspheres with rough surfaces are stacked up by small nanoparticles. The TEM image further demonstrates that the sample is composed of monodisperse microspheres with a uniform spherical morphology (Fig. 2c). A careful observation of a single microsphere can also find the rough contour of the microsphere (Fig. 2d), which further imply that the microspheres are assembled by small nanoparticles. The observation is in consistent with crystal size obtained by the Scherrer formula on the basis of the XRD pattern. The corresponding SAED pattern shows dot rings, revealing the polycrystalline nature (Fig. 2e).

Fig. 2 (a and b) SEM images, (c and d) TEM images and (e) SAED pattern corresponding to (d) of the product obtained in the typical procedure.

Effect of the reaction time

In order to investigate the formation mechanisms of the CaF₂–YF₃ microspheres, time-dependent experiments were systemically carried out. Fig. 4 shows SEM images of the products obtained at 0.5 h, 1 h, 3 h and 12 h, respectively. The spherical morphology of the product began to form after 0.5 h with the mean diameter of 500 nm. However, the spheres showed polydispersity in dimensions after reacting for 0.5 and 1 h (Fig. 3a and b). As the reaction time was increased to 3 h, the size distribution because narrow and the sphere diameters increased to 2.5 μm (Fig. 3c). Further prolonged the reaction time to 12 h, the microspheres became tighter in structure and their diameters grew to 2.5 μm. The XRD patterns of the products corresponding to Fig. 3a–d are shown in Fig. 3e. All the products can be readily indexed into the cubic-phase CaF₂–YF₃, and the crystallinity increased as time went by.

Fig. 3 SEM images of the products with different reaction time: (a) 0.5 h; (b) 1 h; (c) 3 h; (d) 12 h and (e) XRD patterns corresponding to (a–d), respectively.

Fig. 4 SEM images of product produced with different amount of Na₂H₂L: (a) 0 mmol; (b) 0.2 mmol; (c) 0.4 mmol and (d) corresponding XRD patterns.

Effect of the addition amount of Na₂H₂L chelator

Many microspheres or submicrospheres were prepared with the assistance of additives, such as CTAB and EDTA. During the hydrothermal process, organic surfactants or inorganic ions have often been employed as morphology controlling agents to manipulate the growth behavior by selective absorption on high-energy facets, eventually favoring the formation of different morphologies. The influence of the chelator Na₂H₂L on the product was also observed by SEM images and XRD patterns. Contrast experiments were carried out by varying the amount of Na₂H₂L chelator. As shown in Fig. 4a, when no Na₂H₂L was added in the reaction, the product showed up as irregular plates and loose rod structures. Combining with their corresponding XRD patterns shown in Fig. 4d, we conclude the coexistence of cubic CaF₂ and minor YF₃. When the amount was less than 0.5 mmol, such as 0.2 mmol and 0.4 mmol, the morphology of the product was irregular spherical aggregates (<0.5 μm) even after 24 h incubation as shown in Fig. 4b and c. It is thus suggested that the chelator Na₂H₂L played an important role in the synthesis of pure phase CaF₂–YF₃. The Na₂H₂L can chelate with Ca²⁺ and Y³⁺ to form stable complex CaL²⁻ and YL⁻ which can release Ca²⁺ and Y³⁺ gradually to react with F⁻.^20,21 Because of Na₂H₂L absorbing on the surface of the product, the sample exhibits good monodispersity.^22,23

Effect of pH value

The pH value of the solution has great effect on the size and the final morphology of the hydrothermal product. The pH-dependent morphological evolution of the CaF₂–YF₃ microspheres was observed by SEM. When the reaction was carried out at pH = 2, some loose and irregular aggregates formed as well as microspheres (Fig. 5a), which show great similarity as the product obtained when Na₂H₂L is not enough. At pH = 3, together with microspheres were some polyhedron bulks (Fig. 5b). As is known, there is ionization equilibrium in the solution:

Fig. 5 SEM images of product produced with different pH values: (a) pH = 2, (b) pH = 3, (c) pH = 5 and (d) pH = 6.

When the pH value is low, the equilibrium shifts to the left and Na₂H₂L turns into ethylenediaminetetraacetic acid (EDTA), which is only slightly soluble in water and thus plays weaker chelating ability. In fact, we failed to prepare CaF₂–YF₃ microspheres when EDTA was used instead of Na₂H₂L while the other conditions were unchanged. At pH = 4, large scale of uniform microspheres were obtained (Fig. 2). When the pH value was further increased to 5 or 6, the spherical morphology was maintained but the size distribution became relatively wider (Fig. 5c and d). The reason is that activity of Na₂H₂L chelator increased with the pH value increasing, making Ca²⁺ and Y³⁺ be released more slowly. So, the nucleation rate was decreased. In a word, the successful fabrication of CaF₂–YF₃ microspheres has been realized in a pH range above 4, and the low pH value affects the morphology by means of reducing the activity of Na₂H₂L chelator.

Effect of alkaline source

When ammonia is chosen as the alkaline source instead of NaOH to adjust the pH values, two kinds of spheres in different size scales were formed, namely microspheres and nanospheres (Fig. 6a and b). Refer to the corresponding XRD patterns, an unexpected obvious diffraction peak at low 2θ could be found, comparing with the Ca_0.9Y_0.1F_2.1 cubic phase standard pattern. It is suggested that NH₄⁺ itself might participate in the reaction to course the unwanted XRD peak. This result indicates that the alkali source also plays an important role.

Fig. 6 (a and b) SEM images of product produced when ammonia is chosen as the alkaline source. (c) corresponding XRD patterns and the standard data of cubic-phase Ca_0.9Y_0.1F_2.1 (JCPDS 79-0094) as a reference.

Effect of fluoride source

The influence of fluoride source was also investigated by replacing NaBF₄ with NaF and NH₄F. Smaller nanoparticles form as illustrated in Fig. 7. The XRD patterns show that the products were both indexed as CaF₂–YF₃ ternary fluoride (Fig. 7c). However, irregular nanoparticles in diameter of 10–20 nm were yielded instead of uniform microspheres (Fig. 7a and b). The difference was attributed to the weak electrolytic ability of NaBF₄ in comparison with the strong electrolyte NaF and NH₄F. In NaBF₄ solution, the following ionic reaction happens:

BF₄⁻ + 3H₂O → H₃BO₃ + 3HF + F⁻

Fig. 7 SEM images of product by using different fluoride source: (a) NaF, (b) NH₄F, and (c) corresponding XRD patterns.

That is to say, F⁻ ions provided by NaBF₄ were in a lower concentration than the ones provided by NaF and NH₄F. The smaller size was ascribed to the fast nucleation rate when F⁻ ions were in a high concentration. Thus, the NaBF₄ in our case was supposed to decrease the nucleation rate and result in a larger size by releasing F⁻ ions slowly.

Mechanism of the microsphere growth

On the basis of the experimental conditions discussed above, the growth mechanism of CaF₂–YF₃ could be concluded into three steps as follows: (1) at room temperature, H_xL^(4−x)− combined with Ca²⁺ and Y³⁺ to form stable complex CaL²⁻ and YL⁻, respectively; (2) in high temperature and pressure atmosphere, F⁻ ions hydrolyzed from NaBF₄ reacted with cations dissociated from CaL²⁻ and YL⁻ to form numerous CaF₂–YF₃ nuclei; (3) the nuclei grew into nanoparticles and tended to aggregate into microspheres to minimize their surface energy. The small submicrospheres possessing higher surface energy had a faster growth rate than the large ones, which make the size uniformity possible after a long ripening process.^24,25 During the process, H_xL^(4−x)− absorbed on the surface of the product to keep the microspheres from aggregation. That is to say, Na₂H₂L in this reaction play the role of both chelator and stabilizer.

Effect of the amount of Y³⁺ ions

As shown in Fig. 8, the product didn't show any obvious difference in the microsphere morphology and size when the addition of Y³⁺ ions is in the range of 0.2–0.5 mmol. Meanwhile, the product lost its spherical shape and became loose and irregular when the value increased to 0.6 mmol, which might be due to the relative insufficiency of Na₂H₂L caused by excess of Y³⁺ ions.

Fig. 8 SEM images of the product prepared with different Y³⁺ ions (a): 0.2 mmol; (b): 0.3 mmol; (c): 0.4 mmol and (d): 0.6 mmol.

Luminescence of CaF₂–YF₃:Eu

It is well known that Eu³⁺ ions are widely used as a structure probes to detect the chemical environments in a matrix. The obtained microspheres in our system are proved to be suitable matrix of Eu³⁺ ions and didn't show any change in morphology after Eu³⁺ ions are introduced. The XRD patterns show a slight shift to lower angle for the Eu-doped CaF₂–YF₃ because of the larger ionic radius of Eu³⁺ (Fig. S1†). The EDS further confirmed the presence of element Eu in the doped samples (Fig. S1†). Fig. 9 shows the excitation and emission spectra of 5% Eu³⁺ doped CaF₂–YF₃. The excitation peak at 397 nm is assigned to the ⁷F₀ → ⁵L₆ transitions within the Eu³⁺ (4f⁶) configuration. Upon excitation at this wavelength, series of emission bands appear in the range of 500–700 nm, which is ascribed to the ⁵D₁ and ⁵D₀ to ⁷F_J (J = 1, 2, 3 and 4) transitions of the Eu³⁺ ions, respectively. In the CaF₂–YF₃:Eu crystal structure, the Y³⁺ and Eu³⁺ ions occupy the sites of the Ca²⁺ ions, where the Ca²⁺ ion bonds to eight F⁻ ions, forming a CaF₈ cubic polyhedron, as illustrated in the left-up inset panel in Fig. 9. The cubic polyhedron can be visualized to give Ca²⁺ high inversion centrosymmetry. In general, if the Eu³⁺ ions locate at a site of inversion symmetry, then the ⁵D₀ → ⁷F₁ is the main transition, contrariwise, the ⁵D₀ → ⁷F₂ is the dominated transition.^26,27 As seen in Fig. 9, the high intensity ratio between 590 nm (⁵D₀ → ⁷F₁) and 615 nm(⁵D₀ → ⁷F₂) indicates that the Eu³⁺ ions occupy a site of inversion symmetry in the CaF₂–YF₃ matrix, which is consistent with the above analysis.

Fig. 9 Excitation (left) and emission (right) emission spectra of Eu³⁺ doped CaF₂–YF₃.

4. Conclusion

In summary, our work provided a facile method to synthesize cubic CaF₂–YF₃ ternary fluoride microspheres through introducing alkaline-earth ions as dopants in a hydrothermal system. The synthetic method was based on the chelating and capping ability of Na₂H₂L. Our investigation reveals that the reaction conditions play important role in determining the final morphology and size of the products. Furthermore, the possible formation mechanism for the microspheres was termed into three steps as a nanoparticle aggregation process. Finally, the luminescent investigation indicates that the earth ions occupy a site of inversion symmetry in the cubic-phase CaF₂–YF₃ matrix.

Acknowledgements

This work is financially supported by the National Natural Foundation of China (Grant no. 21271167) and the Fund for Creative Research Groups (Grant no. 21221061), and the National Basic Research Program of China (973 Program, Grant no. 2014CB643803).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra04185b

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