Hierarchical bundles structure of β-NaLuF₄: facile synthesis, shape evolution, and luminescent properties

Abstract

Uniform β-NaLuF₄ nanorod bundles have been prepared through a facile two-step chemical conversion route without using any surfactants or capping reagents. During the process, lutetium precursors with square morphology were first prepared by a urea-assisted precipitation process, which then grew into the β-NaLuF₄ crystals under a mild hydrothermal condition. Different experimental parameters were examined to investigate the growth mechanisms of the lutetium precursors and the NaLuF₄ crystals. It is found that uniform hexagonal NaLuF₄ nanorod bundles can be obtained via a mild hydrothermal process of 180 °C for 24 h when using 0.5 g NaF and the lutetium precursors prepared with 0.18 g urea. Moreover, the down-conversion (DC) and up-conversion (UC) luminescent properties of NaLuF₄ were also investigated through doping different rare earth ions (Eu³⁺, Tb³⁺, Ce³⁺/Tb³⁺, Yb³⁺/Er³⁺, and Yb³⁺/Ho³⁺). It has been shown that the existence of Ce³⁺ ion can dramatically enhance the emission intensity of Ln³⁺ ion. And under 980 nm laser excitation, the Yb³⁺/Er³⁺ and Yb³⁺/Ho³⁺ doped NaLuF₄ samples exhibit light green and yellowish green UC luminescence, respectively. This effective and low-cost chemical conversion method, combined with the novel structure and excellent optical luminescent properties of the as-prepared β-NaLuF₄ nanocrystals, may endow this luminescent structure with promising potential to serve as biomedical candidates, solid-state lasers and display devices.

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1 Introduction

In recent years, rare earth (RE) fluorides (NaREF₄ and REF₃), with high refractive indexes and low phonon energies, have attracted great attention due to their wide potential applications in optical telecommunication, lasers, biochemical probes, infrared quantum counters, and medical diagnostics.¹ In particular, the hexagonal β-NaYF₄ crystals, with various dimensions and morphologies, have been investigated extensively in the past few years.² With the same crystalline plane as β-NaYF₄, β-NaLuF₄ is also expected to be one kind of excellent host material, which however is less reported.³ Moreover, the Lu³⁺ ion may be more favorable than the Y³⁺ ion for trivalent lanthanide (Ln³⁺) dopants’ emission due to the intensity-borrowing mechanism mixing the 4f and 5d orbitals of the Ln³⁺ ions via the lattice valence band level.⁴ It was also found that the Lu³⁺ ion doping can remarkably tune the luminescent properties (e.g. brightness and long afterglow performance) through influence on the emitters’ electronic population life time.⁵ Therefore, it is expected that the β-NaLuF₄ can exhibit some interesting and unexpected optical properties when substituted by various lanthanide ions.

The assembling of nanoplates, nanorods, nanotubes, nanowires, and nanospheres into two- (2D) or three-dimensional (3D) hierarchical superstructures is an important subject to build functional nano-systems which has received great research attention.⁶ This is because from those well-defined architectures can emerge some unique optical and electronic properties in many cases, which endow them with great versatility for some specific applications.⁷ By now, a number of techniques have been developed to synthesize those special nano-structures, such as sol–gel route,⁸ solid-state combinatorial chemistry method,⁹ microwave assisted process,¹⁰ and chemical vapor synthesis.¹¹ Besides, the chemical conversion process is also a promising technique to develop some special nano-/micro-structures which might be difficult or impossible to directly synthesize.¹² During the reaction process, precursors are first prepared to serve as the further reactants, also to control the shapes of the final products as a template. Therefore, the morphology of final products is usually very similar to that of the initial precursors.¹³ However, in some cases, a notable change of the morphology may take place between the precursors and final products when the precursors undergo a transformation process from which new forms are produced.¹⁴

In the present work, we report for the first time the preparation of β-NaLuF₄ nanorod assemblies through a facile hydrothermal assisted chemical conversion process. During the synthesis process, β-NaLuF₄ nanorods are attached and aggregated together to form the bundles assembly structure. On the other hand, we investigate both the down- and up-conversion properties of the as-prepared β-NaLuF₄ nanocrystals by doping with different kinds of lanthanide ions (Eu³⁺, Tb³⁺, Ce³⁺/Tb³⁺, Yb³⁺/Er³⁺, Yb³⁺/Ho³⁺). These doped β-NaLuF₄ superstructures display strong DC and UC emissions, and as such they can serve as efficient hosts for both types of luminescent materials. The results indicate that this simple chemical conversion method is an effective way for the preparation of β-NaLuF₄ phosphors, which can also be applied to prepare many other types of nanocrystals. Moreover, the special structural geometry and excellent optical luminescent properties of the as-prepared β-NaLuF₄ nanocrystals, together with this easy and low-cost synthetic method, will endow this luminescent structure with promising potential to serve as biomedical candidates, solid-state lasers and display devices.

2 Experimental section

Chemicals and materials

The initial materials including analytical grade NaF, HNO₃, Ce(NO₃)₃·6H₂O, and urea were purchased from Sinopharm Chemical Reagent Co., Ltd. All the rare earth oxides used in this work including Lu₂O₃, Eu₂O₃, Tb₄O₇, Yb₂O₃, Er₂O₃, and Ho₂O₃ (99.99%) were purchased from Changchun Applied Chemistry Science and Technology Limited, China. All the chemicals were used as received without further purification.

Synthesis of lutetium precursor nanoparticles

In a typical procedure, 1 mL of 1 M LuCl₃ aqueous solution was first added to 50 mL deionized water. Then a designed amount of urea (0.06 g, 0.12 g, and 0.18 g) was introduced to the solution with magnetic stirring. After additional agitation for 1 h, the solution was moved to a water bath at 90 °C for 4 h. Then it was removed, cooled to room temperature naturally, washed with deionized water several times, and finally redispersed in 5 mL deionized water. Precursors with different reaction times were also prepared to investigate the formation process.

Synthesis of NaLuF₄ nanorod bundles

Typically, 0.5 g NaF was dissolved in 30 mL deionized water with magnetic stirring for 15 min. Then 5 mL as-prepared precursor suspension was added into the solution slowly with continuous stirring. After additional agitation for 30 min, the mixture was transferred into a 50 mL autoclave held in a stainless steel autoclave, sealed, and maintained at 180 °C for 24 h. After the autoclave cooled to room temperature naturally, the precipitate was separated by centrifugation, washed with deionized water and ethanol several times, and dried at 60 °C for 12 h to obtain the final products. The samples derived from precursors prepared with 0.06, 0.12, and 0.18 g urea are designated as NaLuF₄-1, NaLuF₄-2, and NaLuF₄-3, respectively.

In order to investigate the synthesis process, other samples were prepared through a similar procedure using different amounts of NaF or with different reaction times. In addition, NaLuF₄:5%Eu³⁺, NaLuF₄:5%Tb³⁺, NaLuF₄:x%Ce³⁺,5%Tb³⁺ (x = 2, 5, 8, 10, and 15), NaLuF₄:17%Yb³⁺,3%Er³⁺, and NaLuF₄:17%Yb³⁺,3%Ho³⁺ were also prepared through a similar process except for adding a stoichiometric amount of corresponding RECl₃ instead of LuCl₃ aqueous solution at the initial stage. All the doping ratios of rare-earth ions are molar ratios in our experiments.

Characterization

XRD patterns were obtained on a Rigaku TR III diffractometer at a scanning rate of 10° min⁻¹ in the 2θ range from 5° to 80°, with Cu-Kα radiation (λ = 0.15405 nm), accelerating voltage and emission current of 40 kV and 200 mA. SEM images were obtained using a scanning electron microscope (SEM, S4800, Hitachi) equipped with an energy-dispersive X-ray spectrometer (EDS, JEOL JXA-840). Transmission electron microscopy (TEM) was performed on a FEI Tecnai G² S-Twin instrument with a field emission gun operating at 200 kV. Images were acquired digitally on a Gatan multiple CCD camera. The X-ray photoelectron spectra (XPS) were taken on a VG ESCALAB MK II electron energy spectrometer using Mg Kα (1253.6 eV) as the X-ray excitation source. Fourier transform infrared spectroscopy (FT-IR) was carried out on an ABB Bomen FTLA2000-100 spectrometer using KBr pellets. The photoluminescence (PL) measurements were performed on a Hitachi F-4500 spectrophotometer equipped with a 150 W xenon lamp as the excitation source. The UV–vis adsorption spectral values were measured on a TU-1901 spectrophotometer. The UC emission spectra were obtained using a 980 nm laser from an OPO (optical parametric oscillator, Continuum Surelite, USA) as the excitation source and detected by R955 (HAMAMATSU) from 400 to 800 nm. All the measurements were performed at room temperature.

3 Results and discussion

Phase identification and morphology of lutetium precursor

Fig. 1 shows the XRD patterns of the as-prepared lutetium precursors using different amounts of urea. It can be seen that all the diffraction peaks of the three patterns are very sharp and strong, which may indicate a high crystallinity of the samples. However, on the basis of the Joint Committee on Powder Diffraction Standards (JCPDS) reference database, a compound showing such a diffraction pattern cannot be found. Nevertheless, the diffraction peaks of the three samples are quite similar to each other in both their peak positions and the relative intensities, which means the three samples are the same compound. In order to further investigate the phase purity of as-prepared lutetium precursor and define its molecular formula, a series of other measurements were carried out. Fig. 2 gives the XPS results of the lutetium precursor prepared with 0.18 g urea. As shown, the binding energies of Lu 4d_3/2, Lu 4d_5/2, O1s, and C1s were found to be 207, 197, 531 and 284 eV, respectively, suggesting the existence of these elements in the precursor. The FT-IR spectrum of lutetium precursor prepared with 0.18 g urea is shown in Fig. 3. The broad band centred at 3418 cm⁻¹ can be attributed to the stretching vibration of OH⁻ group in the precursor.¹⁵ And the two strong peaks at 1443 and 1653 cm⁻¹ can be assigned to the υ₃ mode of carbonate. The other bands at approximately 1079, 842, and 785 (682) cm⁻¹ are attributed to υ₁, υ₂, and υ₄ modes of carbonate, respectively. In addition, the bending (550 cm⁻¹) mode of Lu–OH can also be clearly found.¹⁶ It can be inferred from the above analysis that OH⁻, CO₃^2–, Lu³⁺ and H₂O groups exist in the precursor. Although we still cannot define the precise molecular formula of the precursor, it can be identified as a kind of lutetium subcarbonate (Lu(OH)CO₃·nH₂O) according to the above analysis and previous reports.¹⁷

Fig. 1 XRD patterns of lutetium precursors prepared with 0.06 g (A), 0.12 g (B) and 0.18 g (C) urea using a water bath at 90 °C for 4 h.

Fig. 2 XPS of lutetium precursor prepared with 0.18 g urea using a water bath at 90 °C for 4 h.

Fig. 3 FT-IR spectrum of lutetium precursor prepared with 0.18 g urea using a water bath at 90 °C for 4 h.

The typical SEM images of the lutetium precursors prepared with different amounts of urea (0.06, 0.12, and 0.18 g) are shown in Fig. 4A–C, respectively. As shown, the three samples exhibit a quite similar square morphology which is assembled by some small particles. What is more, when the urea amount is increased from 0.06 g to 0.18 g, the particle sizes of the precursors decrease and the shapes become more regular. In the EDS of the precursor prepared with 0.18 g urea (Fig. 4D), the element signals for Lu, O, and C (Si is due to the silicon slice used in the measurement process) can be detected, fitting well with the previous XPS and FT-IR results. The TEM image of the precursor prepared with 0.18 g urea is shown in Fig. 4E, from which we can clearly see the square morphology of the sample with the length of each side about 150–200 nm. A possible formation process of the lutetium precursor is proposed in Fig. 4F based on the time-dependent experiments (Fig. S1†). At the beginning, the urea breaks down at 90 °C to produce excessive OH⁻ and CO₃^2– in the solution, and small nuclei are formed (Fig. S1A†). With increasing reaction time, these as-formed nuclei slowly grow (Fig. S1B†) and self-assemble into the square shaped particles (Fig. S1B,C†). Further increasing the reaction time to 4 h, more larger and regular assembled particles are obtained (Fig. S1D†). Notably, the small urea amount (molar ratio of Lu/urea is 1–3) plays an important role in the self-assembly process. As the amount of urea is not highly excessive, the formation and growth of nuclei is difficult, and the formed nuclei tend to assemble together. Therefore, a slight increase of the urea amount may lead to a more regular growth of nuclei and a decrease of assembled particles. This can also explain why the precursor prepared here is very different to the previous reports of lutetium precursor nanowires prepared with ammonia and the spherical RE precursors prepared with 1.5 g urea.¹⁸

Fig. 4 SEM images of lutetium precursors prepared with 0.06 g (A), 0.12 g (B) and 0.18 g (C) urea; EDS of lutetium precursor prepared with 0.18 g urea (D); TEM image of lutetium precursor prepared with 0.18 g urea (E); and schematic illustration for the formation process of lutetium precursor (F).

Phase identification and morphology of NaLuF₄

When the precursors reacted with 0.5 g NaF under hydrothermal conditions, hexagonal NaLuF₄ crystals could be obtained. Fig. 5 gives the XRD and SEM images of the as-prepared NaLuF₄ products using precursors prepared with 0.06, 0.12, and 0.18 g urea. We can see from the XRD patterns (Fig. 5A) that all the three samples can be well indexed to the pure hexagonal NaLuF₄ phase (JCPDS No. 27–0726) and no impurity peaks can be observed. The calculated cell lattice parameters for the three samples are listed in Table 1, and the standard data of hexagonal NaLuF₄ (JCPDS No. 27–0726) are also given for comparison. It can be seen that the calculated cell lattice parameters of the three samples are consistent with the standard data, revealing the high purity of the samples. Fig. 5B–D exhibit the SEM images of NaLuF₄-1, NaLuF₄-2, and NaLuF₄-3, respectively. As shown, all the three samples exhibit similar nanorod bundles morphology. A more detailed investigation of NaLuF₄-3, which possesses more uniform shape and size than those of NaLuF₄-1 and NaLuF₄-2, has been carried out, as shown in Fig. 6. Fig. 6A,B exhibit the SEM images with different magnifications. As shown, the as-prepared NaLuF₄-3 sample is entirely comprised of nanorod bundles with uniform size. It can be seen from the TEM image (Fig. 6C) that the length of those nanorod bundles is about 2 μm, while the width is about 200 nm. The magnified TEM image (Fig. 6D) also reveals the as-prepared bundle-like morphology. And the width of the small nanorods composing the bundles is around 20–25 nm. The EDS pattern (Fig. 6E) of the NaLuF₄ bundle reveals the existence of Na, Lu, and F elements (Si is due to the silicon slice used in the measurement process). The molar ratio of Na/Lu/F is determined to be 1/1.043/4.106, agreeing well with the theoretical value of 1/1/4. The HRTEM image (Fig. 6F) displays obvious lattice fringes with an interplanar spacing of 0.293 nm, which matches well with the d value of the (110) plane of the hexagonal-phased NaLuF₄.

Fig. 5 XRD patterns (A) and SEM images of NaLuF₄ crystals synthesized at 180 °C for 24 h with different lutetium precursors: precursor prepared with 0.06 g urea (B), precursor prepared with 0.12 g urea (C), and precursor prepared with 0.18 g urea (D). All of the samples were obtained with 0.5 g NaF.

Fig. 6 Low (A) and high (B) magnification SEM images, TEM images (C,D), EDS spectrum (D), and HRTEM image (F) of NaLuF₄ crystals synthesized at 180 °C for 24 h using 0.5 g NaF and lutetium precursor prepared with 0.18 g urea.

Table 1 Unit cell lattice constants for the as-prepared NaLuF₄ crystals synthesized at 180 °C for 24 h with different lutetium precursors

Samples	a, b/Å	c/Å	Cell volume/Å^3
JCPDS 27–0726	5.901	3.453	104.13
NaLuF4-1	5.928	3.489	104.97
NaLuF4-2	5.902	3.490	104.52
NaLuF4-3	5.890	3.481	104.39

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It is noticeable that the morphology of the final NaLuF₄ product and the precursors are quite different in our case, meaning the hydrothermal process has a significant influence on the morphology of the NaLuF₄ crystals. Therefore, a series of time-dependent experiments were performed to gain an insight into the conversion process from the precursor to NaLuF₄ crystals. The intermediate products obtained at different hydrothermal times were investigated by XRD and SEM, as shown in Fig. 7.

Fig. 7 XRD patterns (A) and SEM images of NaLuF₄ crystals synthesized at 180 °C for different reaction times: 1 h (B), 2 h (C), and 12 h (D); schematic illustration for the formation process of NaLuF₄ crystals (E). All of the samples were obtained using 0.5 g NaF and lutetium precursor prepared with 0.18 g urea.

As can be seen in Fig. 7A, hexagonal phase NaLuF₄ was already formed when the reaction was carried out at 180 °C for 1 h, meaning the phase formation process is very fast. On the other hand, the morphology evolution process is gradual and slow, as evidenced by the SEM images. Samples collected after the hydrothermal reaction conducted for 1 h were of some irregular 1D nanorods, as shown in Fig. 7B. Herein, under the hydrothermal condition, the lutetium precursors quickly dissolved and reacted with NaF in the solution to form new NaLuF₄ crystals. As the reaction time increased to 2 h, some of the nanorods assembled together into a bundle structure (Fig. 7C). As the reaction time is further increased to 12 h (Fig. 7D), the nanorods continuously aggregated to form more 3D nanorod bundles structures. And finally, well-crystallized β-NaLuF₄ with uniform bundles superstructures was obtained (Fig. 6). Although the actual crystallization and growth process in solution is still complex and remains mysterious, we can simply conclude it as a so-called two-stage growth process, which involves a fast nucleation of primary particles and a slow aggregation and growth of primary particles.¹⁹ It is worthwhile to point out that the aggregation and growth of the as-prepared NaLuF₄ nanorods bundles were totally spontaneous, since no surfactants or capping reagents were used during the formation process. A schematic illustration of the possible conversion processes is demonstrated in Fig. 7E. Initially, hydrothermal treatment of the precursor and NaF mixture led to the formation of NaLuF₄ particles through a quick dissolution-recrystallization mechanism.²⁰ The growth rates of different crystal facets were unequal due to the inhomogeneous nucleation and growth of primary particles, and small nanorods appeared. In the following stage, these separate nanorods tend to aggregate together along the long-axis in order to minimize the surface energy by reducing the exposed area. This step may be driven by several factors except for this thermodynamic demand, such as, crystal-face attraction, hydrogen bond interaction, and van der Waals forces.²¹

The effect of NaF content on the phase and morphology of as-synthesized NaLuF₄ was also investigated. Fig. 8 shows the XRD patterns and SEM images of NaLuF₄ synthesized using 0.16 g, 0.32 g, and 0.5 g NaF (the corresponding molar ratios of NaF/Lu are 4, 8, 12, respectively). It can be seen that when the NaF/Lu molar ratio is 4, the XRD pattern exhibits pure hexagonal NaLuF₄ but with a very low crystallization (Fig. 8A). And its SEM image consists of rods of irregular width and length (Fig. 8B). When the molar ratio reaches 8, crystallization of the NaLuF₄ crystals is improved (Fig. 8A). The shape of the sample is more regular and some of the rods assemble together (Fig. 8C). When the molar ratio increases to 12, highly crystalline hexagonal NaLuF₄ nanorod bundles are obtained (Fig. 8D). It has been reported that excessive fluoride source in a hydrothermal process can promote the formation of hexagonal-phased NaREF₄ since F⁻ ions can decrease its crystallization temperature remarkably.²² The critical value of NaF/Ln for the phase transition from cubic to hexagonal is usually always 12. However, in the present work, when the ratio of NaF/Lu is 4, hexagonal NaLuF₄ can be formed. It may be caused by two reasons. Firstly, during the hydrothermal conversion process, the lutetium precursor dissolves and then reacts with NaF which can form an oversaturation of NaF. Secondly, during the synthesis process of lutetium precursor, some of the Lu³⁺ ions may not react to form the products due to the low amount of urea used. Thus the NaF amount is relatively excessive in the hydrothermal process.

Fig. 8 XRD patterns (A) and SEM images of NaLuF₄ crystals synthesized at 180 °C for 24 h with different amounts of NaF: 0.16 g (B), 0.32 g (C), and 0.5 g (D). All of the samples were obtained using lutetium precursor prepared with 0.18 g urea.

Luminescent properties

It is well known that β-NaREF₄ is an excellent host lattice for the luminescence of various lanthanide ions. To investigate the luminescent properties of the as-prepared β-NaLuF₄ nanorod bundles, we discussed the DC and UC luminescent properties of NaLuF₄ doped with different RE ions.

Fig. 9A exhibits the excitation and emission spectra of the NaLuF₄:5%Eu³⁺, NaLuF₄:5%Tb³⁺, NaLuF₄:5%Ce³⁺,5%Tb³⁺, and their corresponding photographs taken under 254 nm UV lamp irradiation (all of the samples were synthesized at 180 °C for 24 h using 0.5 g NaF and lutetium precursor prepared with 0.18 g urea). As shown, the excitation and emission spectra of NaLuF₄:5%Eu³⁺ and NaLuF₄:5%Tb³⁺ are very similar to the previous reports of Eu³⁺ and Tb³⁺ doped crystals.²³ The excitation spectrum of NaLuF₄:5%Eu³⁺ consists of the characteristic excitation lines of Eu³⁺ within its 4f⁶ configuration from 200 to 400 nm. The peaks located at 319, 362, 378, and 395 nm can be assigned to the Eu³⁺ transitions of ⁷F₀ → ⁵H₆, ⁷F₀ → ⁵D₄, ⁷F₀ → ⁵G₂, and ⁷F₀ → ⁵L₆, respectively. Some weak lines at 253, 269, 287 and 299 nm can also be observed which have a small contribution to the excitation of Eu³⁺. Excitation into the strongest ⁷F₀ → ⁵L₆ transition of Eu³⁺ at 395 nm yields the emission spectrum of the sample, which is composed of the emission lines ascribed to the Eu³⁺ characteristic transitions of ⁵D₁ → ⁷F₁ (537 nm), ⁵D₁ → ⁷F₂ (556 nm), ⁵D₀ → ⁷F₁ (593 nm), ⁵D₀ → ⁷F₂ (615 nm). The excitation spectrum of NaLuF₄:5%Tb³⁺ is composed of the characteristic transitions of Tb³⁺ within its 4f⁸ configuration, which can be assigned to the respective transitions of ⁷F₆ → ⁵I₆ (285 nm), ⁷F₆ → ⁵H₆ (306 nm), ⁷F₆ → ⁵D₀ (319 nm), ⁷F₆ → ⁵G₂ (342 nm), ⁷F₆ → ⁵D₂ (356 nm), ⁷F₆ → ⁵G₆ (370 nm) and ⁷F₆ → ⁵D₃ (380 nm). Upon excitation into the ⁷F₆ → ⁵D₂ transition at 356 nm, characteristic transitions of Tb³⁺ ion (⁵D₄ → ⁷F₆ at 491 nm, ⁵D₄ → ⁷F₅ at 544 nm, ⁵D₄ → ⁷F₄ at 585 nm, and ⁵D₄ → ⁷F₃ at 622 nm) can be observed. However, when 5% Ce³⁺ ion was co-doped with Tb³⁺ ion into the NaLuF₄ matrix, a great change happens in the excitation spectrum. Compared to the excitation spectrum of NaLuF₄:5%Tb³⁺, a broad band at about 250 nm can be observed in NaLuF₄:5%Ce³⁺,5%Tb³⁺ which can be ascribed to the Ce³⁺ 4f¹–5d¹ characteristic transition.²⁴ Moreover, the broad band replaces the Tb^{3+ 7}F₆ → ⁵D₂ transition at 356 nm as the strongest excitation peak, indicating the energy transfer from Ce³⁺ to Tb³⁺ is dominant in NaLuF₄:5%Ce³⁺,5%Tb³⁺.²⁵ As for the emission spectra of NaLuF₄:5%Ce³⁺,5%Tb³⁺, all of the emission lines are located at the same positions to those of NaLuF₄:5%Tb³⁺. Furthermore, NaLuF₄:x%Ce³⁺,5%Tb³⁺ with different Ce³⁺ concentrations (0%, 2%, 5%, 8%, 10%, 15%) were prepared. The co-doping of Ce³⁺ ion does not influence the phase nor the morphology of the NaLuF₄ crystals, as can be seen in the XRD and SEM images of NaLuF₄:x%Ce³⁺,5%Tb³⁺ (Fig. S2†). Fig. 9B shows their emission spectra excited under the same conditions. As can be seen, the emission intensities of the samples first enhance with the increasing of Ce³⁺ concentration (2% and 5%), and decrease on further increase of the Ce³⁺ doping concentration (8%, 10%, and 15%). The emission intensity of Tb³⁺ is enhanced by a maximum of about 11 times when co-doping of 5%Ce³⁺ ion with 5%Tb³⁺ ion occurs in the NaLuF₄ crystals. It is believed that the energy transfer process plays an important role in improving the emission efficiency of materials. In the Tb³⁺ ion doped NaLuF₄ crystals, the electronic transitions within the 4fⁿ configurations of Tb³⁺ are strongly forbidden, resulting in the relatively weak emission intensity.²⁶ However, in the 5%Ce³⁺ and 5%Tb³⁺ co-doped NaLuF₄ samples, the Ce³⁺ ion acts as sensitizer, first to absorb the excitation energy, and then transfers it to the Tb³⁺ ions, where the energy is discharged as fluorescent emissions,²⁷ and an effective improvement of the luminescence can be observed. The schematic energy level diagram of Ce³⁺ and Tb³⁺ is shown in Fig. S3.† It is worth noting that the enhancement of luminescence can be gained because the energy transfer process from Ce³⁺ to Tb³⁺ is efficient and no Ce³⁺–Ce³⁺ energy transfer occurs. Therefore, when further increasing the Ce³⁺ concentration, the more abundant Ce³⁺ ions may act as energy quenchers and cause the decrease of emission intensity.²⁸

Fig. 9 (A) Excitation (left) and emission spectra (right) of NaLuF₄:5%Eu³⁺, NaLuF₄:5%Tb³⁺, and NaLuF₄:5%Ce³⁺,5%Tb³⁺ (inserts are the corresponding photographs taken under 254 nm UV lamp irradiation); (B) emission spectra of NaLuF₄:x%Ce³⁺,5%Tb³⁺ (x = 0, 2, 5, 8, 10, 15) excited at 250 nm under the same condition; (C) the CIE chromaticities of NaLuF₄:5%Eu³⁺, NaLuF₄:5%Tb³⁺, and NaLuF₄:5%Ce³⁺,5%Tb³⁺.

Fig. 9C shows the CIE chromaticity diagram of NaLuF₄:5%Eu³⁺, NaLuF₄:5%Tb³⁺, and NaLuF₄:5%Ce³⁺,5%Tb³⁺. The black crosses indicate the CIE chromaticity coordinate positions. The CIE coordinates for NaLuF₄:5%Eu³⁺, NaLuF₄:5%Tb³⁺, NaLuF₄:5%Ce³⁺,5%Tb³⁺ are determined as x = 0.5174, y = 0.3567 (red region), x = 0.2855, y = 0.3925 (green region), and x = 0.2561, y = 0.4554 (green region), respectively. The CIE chromaticity coordinates of the samples agree well with the PL spectra and corresponding photographs in Fig. 9A.

The UC luminescent properties of the NaLuF₄ crystals co-doped with 17% Yb³⁺ and 3% Ln³⁺ (Ln = Er and Ho) under 980 nm laser diode excitation were also investigated. As shown in Fig. 10A, the UC spectrum of NaLuF₄:17%Yb³⁺,3%Er³⁺ consists of two green emission bands at 522 and 541, and one red emission band centred at 654 nm, which can be assigned to ²H_11/2 → ⁴I_15/2, ⁴S_3/2 → ⁴I_15/2, and ⁴F_9/2 → 4I_15/2 transitions of Er³⁺, respectively.²⁹ As for NaLuF₄:17%Yb³⁺,3%Ho³⁺ in Fig. 10B, two emission bands centered at 542 (green region) and 648 nm (red region) were observed, which correspond to the ⁵S₂ → ⁵I₈ and ⁵F₅ → ⁵I₈ transitions of Ho³⁺, respectively.³⁰ Moreover, the positions and splittings of these optical transitions are comparable to those reported for Yb/Er and Yb/Ho co-doped single crystals,³¹ indicating that these ions have been successfully doped into the host lattice. Yet it should also be noted that the peak broadening and relative intensity ratios show some difference in the corresponding spectra, which may be due to the possible changes in the local structure around Er³⁺/Ho³⁺ and different surface group status in the as-prepared nanorod bundles structures.³² In order to investigate thoroughly the involved upconversion mechanisms in the as-prepared NaLuF₄:Yb³⁺,Er³⁺ and NaLuF₄:Yb³⁺,Ho³⁺ crystals, the excitation power-dependent UC emissions of green, and red were calculated. It is well known that the output UC luminescent intensity (I_UC) is proportional to the infrared excitation power (I_IR): I_UC ∝ I_IRⁿ. Here the n represents the absorbed photon numbers per visible photon emitted, and its value can be obtained from the slope of the fitted line of the plot of log(I_UC) versus log(I_IR). As shown in Fig. 10C, the slopes of the linear fits of log(I_UC) versus log(I_IR) for the green, and red emissions in the as-prepared NaLuF₄:17%Yb³⁺,3%Er³⁺ sample are 1.87, and 1.93, respectively, while the slopes in NaLuF₄:17%Yb³⁺,3%Ho³⁺ for green, and red emissions are 1.84, and 1.88. The results indicate that a two-photon process is involved to produce the green and red UC emissions, which is consistent with the previous reports of the Yb³⁺,Er³⁺ and Yb³⁺,Ho³⁺ doped crystals.³³ Based on this, the mechanism for the energy transfer process of Yb³⁺/Er³⁺/Ho³⁺ ions in the as-prepared NaLuF₄:Yb³⁺,Er³⁺ and NaLuF₄:Yb³⁺,Ho³⁺ crystals under 980 nm LD excitation is depicted in Fig. S4.† It can be seen that the green and red UC emissions of Yb³⁺/Er³⁺ and Yb³⁺/Ho³⁺ doped NaLuF₄ all need a two-photon energy transfer process. The DC and UC luminescence results powerfully manifest that the as-synthesized hexagonal NaLuF₄ nanorod bundles are an excellent host lattice for the luminescence of those ions. Fig. 10D shows the CIE chromaticity diagram of NaLuF₄:17%Yb³⁺,3%Er³⁺, and NaLuF₄:17%Yb³⁺,3%Ho³⁺. The CIE coordinates for NaLuF₄:17%Yb³⁺,3%Er³⁺, and NaLuF₄:17%Yb³⁺,3%Ho³⁺ are determined as x = 0.2722, y = 0.6772 (green region), and x = 0.3346 , y = 0.5831 (yellow region), respectively, which agrees well with the corresponding photographs inserted in Fig. 10A (light-green) and Fig. 10B (yellowish-green).

Fig. 10 Upconversion luminescence spectra of (A) NaLuF₄:17%Yb³⁺,3%Er³⁺ and (B) NaLuF₄:17%Yb³⁺,3%Ho³⁺ crystals under 980 nm LD excitation (inserts are the corresponding photographs); (C) pump power dependence of the green, and red upconverted emissions in NaLuF₄:17%Yb³⁺,3%Er³⁺ and NaLuF₄:17%Yb³⁺,3%Ho³⁺; (D) the CIE chromaticities of NaLuF₄:17%Yb³⁺,3%Er³⁺ and NaLuF₄:17%Yb³⁺,3%Ho³⁺.

4 Conclusion

In summary, NaLuF₄ nanorod bundles have been controllably synthesized through a facile chemical conversion approach. The precursors and NaLuF₄ crystals are well characterized by XRD, SEM, EDS, TEM, HRTEM, XPS, FT-IR, and PL. It is found that uniform hexagonal NaLuF₄ nanorod bundles can be obtained using 0.5 g NaF and the lutetium precursors prepared with 0.18 g urea via a mild hydrothermal process of 180 °C for 24 h. The formation processes of the lutetium precursors and NaLuF₄ nanorod bundles were also discussed based on time-dependant experiments. In addition, the DC luminescent properties of NaLuF₄:5%Eu³⁺, NaLuF₄:5%Tb³⁺, and NaLuF₄:x%Ce³⁺,5%Tb³⁺, as well as the UC luminescent properties of NaLuF₄:17%Yb³⁺,3%Er³⁺/Ho³⁺ were also investigated. It is found that the as-prepared hexagonal NaLuF₄ crystals with a novel bundles structure present both excellent DC and UC properties, indicating they are an excellent host lattice, also proving the designed chemical conversion method is an effective way for the preparation of β-NaLuF₄ phosphors.

Acknowledgements

Financial supports from the National Natural Science Foundation of China (NSFC 21271053), Research Fund for the Doctoral Program of Higher Education of China (20112304110021), Harbin Sci.-Tech. Innovation Foundation (RC2012XK017012) and the Fundamental Research Funds for the Central Universities of China are greatly acknowledged.

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Footnote

† Electronic Supplementary Information (ESI) available: SEM images of precursors prepared with different times; XRD patterns and SEM images of NaLuF₄ doped with different Ce concentrations; and simplified energy level of Ce/Tb, Yb/Er/Ho. See DOI: 10.1039/c2ra20991h

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