Fabrication and luminescence properties of YF₃:Eu³⁺ hollow nanofibers via coaxial electrospinning combined with fluorination technique

Abstract

Polyvinyl pyrrolidone (PVP)–PVP/[Y(NO₃)₃ + Eu(NO₃)₃] core–sheath composite nanofibers were prepared by coaxial electrospinning, and then Y₂O₃:Eu³⁺ hollow nanofibers were synthesized by calcination of the as-prepared composite nanofibers. For the first time, YF₃:Eu³⁺ hollow nanofibers were successfully fabricated by fluorination of the Y₂O₃:Eu³⁺ hollow nanofibers via a double-crucible method using NH₄HF₂ as fluorinating agent. The morphology and properties of the products were investigated in detail by X-ray diffraction, scanning electron microscope (SEM), transmission electron microscope (TEM), and fluorescence spectrometer. YF₃:Eu³⁺ hollow nanofibers were pure orthorhombic phase with space group Pnma and were hollow-centered structure with the mean diameter of 211 ± 29 nm. Fluorescence emission peaks of Eu³⁺ in the YF₃:Eu³⁺ hollow nanofibers were observed and assigned to the energy levels transitions of ⁵D₀ → ⁷F₁ (587 and 593 nm), ⁵D₀ → ⁷F₂ (615 and 620 nm), and the ⁵D₀ → ⁷F₁ hypersensitive transition at 593 nm was the dominant emission peak. Moreover, the emitting colors of YF₃:Eu³⁺ hollow nanofibers were located in the red region in CIE chromaticity coordinates diagram. The luminescent intensity of YF₃:Eu³⁺ hollow nanofibers was increased remarkably with the increasing doping concentration of Eu³⁺ ions and reached a maximum at 7 mol% of Eu³⁺. This preparation technique could be applied to prepare other rare earth fluoride hollow nanofibers.

Introduction

In the past few years, nanosized materials have been a subject of extensive research due to their unique properties. The rare earth fluoride luminescent nanomaterials have recently attracted much attention owing to their peculiar physical and chemical properties, for example, low phonon energy, high ionicity, high resistivity, and high anionic conductivity [1]. YF₃ is one of the most important host crystals for lanthanide-doped phosphors, providing a wide band gap (>10 eV) and suitable Y³⁺ sites where Y³⁺ can be easily substituted by other trivalent rare earth ions without additional charge compensation [2]. YF₃:Ln³⁺ nanomaterials have wide and potential applications in optical devices, phosphors, down/up conversion luminescent materials [1, 3], etc.

By far, a variety of efforts have been devoted to developing several simple and efficient methods for the fabrication of YF₃ or YF₃:Ln³⁺ luminescent nanomaterials with different morphologies and sizes, such as nanocrystals, nanowires, nanorods, nanobundles, nanoflowers, nanoplates, etc. [4–12]. Typical synthetic methods include hydrothermal methods [4, 5, 13], coprecipitation [14], microemulsion [6–8], pyrolysis [15, 16], solid state reaction [17], ultrasonic-assisted route [9, 18], microwave synthesis [2, 10], solvent extraction route [19], liquid–solid–solution (LSS) phase transfer synthesis [20], etc. Nevertheless, no reports on the preparation of YF₃:Ln³⁺ hollow nanofibers are found in the literature. Hollow nanofibers double the surface area compared with common solid nanofibers, which is very useful in surface-related applications such as chemical sensors or photocatalysis [21].

Electrospinning is a simple, straightforward and versatile method to fabricate micro- and nanofibers of various materials, including rare earth oxyfluoride nanofibers [22], rare earth oxide nanofibers [23–25] and hollow nanofibers [26], core–shell structured coaxial nanofibers [27] and nanobelts [28, 29], etc. The electrospinning-based synthesis method of hollow nanofibers, which includes monoaxial [30, 31] and coaxial electrospinning [21] method, has also been reported. The key point of coaxial electrospinning is the dual nozzle spinneret which has a smaller (inner) capillary inside the bigger (outer) capillary at the coaxial position [32].

Recently, Yang et al. [22] prepared the pure phase YOF:Ln³⁺ nanofibers via sintering the electrospun RE(CF₃COO)₃/PVP composite fibers at high-temperature. Nevertheless, pure phase YF₃:Ln³⁺ cannot be easily prepared using this process, because YF₃ is easy to be oxidized at high calcination temperature. Hence, preparation of pure phase YF₃:Ln³⁺ hollow nanofibers remains an important and challenging subject of study. In this paper, Y₂O₃:x%Eu³⁺ hollow nanofibers were prepared through calcining the coaxial electrospun PVP@PVP/[Y(NO₃)₃ + Eu(NO₃)₃] core–sheath composite nanofibers at 700 °C. YF₃:x%Eu³⁺[x = 1,3,5,7,9, x stands for molar ratio of Eu³⁺ to (Eu³⁺+Y³⁺)] hollow nanofibers were fabricated by fluorination of Y₂O₃:x%Eu³⁺ hollow nanofibers via a double-crucible method we newly proposed for the first time. The samples were systematically characterized and some meaningful results were obtained.

Experimental sections

Chemicals

Polyvinyl pyrrolidone (PVP) (K90, Mr = 1300000, AR) was bought from Tiantai Chemical Co., Ltd. Yttrium oxide (Y₂O₃) (99.99 %) and europium oxide (Eu₂O₃) (99.99 %) were purchased from Kemiou Chemical Co., Ltd. N,N-dimethylformamide (DMF, AR) and ammonium hydrogen fluoride (NH₄HF₂, AR) were purchased from Sinopharm Chemical Reagent Co., Ltd. Nitric acid (HNO₃, AR) and absolute ethyl alcohol were bought from Beijing Chemical Factory. All chemicals were directly used as received without further purification.

Preparation of PVP@PVP/[Y(NO₃)₃ + Eu(NO₃)₃] core–sheath composite nanofibers via coaxial electrospinning

In the preparation of sheath precursor solution, 1.8 g of RE(NO₃)₃·6H₂O (RE = Y³⁺ and Eu³⁺) dissolved in 16.4 g of DMF, and then 1.8 g of PVP was added into the above solution under magnetic stirring for 4 h to form 20 g of homogeneous transparent sheath solution. In the sheath solution, the mass ratios of PVP, rare earth nitrates and DMF were equal to 9:9:82. To prepare the core precursor solution, 1.35 g of PVP was dissolved in 13.65 g of absolute ethyl alcohol under magnetic stirring for 4 h to form 15 g of homogeneous transparent core solution.

Subsequently, a homemade coaxial electrospinneret was used in this study. A coaxial nozzle was designed to make core–sheath composite nanofibers, as shown in Fig. 1, the core solution was injected into the inner plastic syringe while the sheath solution was loaded into the outer one. A flat iron net used as a fiber collector was put 16 cm away from the spinneret. The two solutions were electrospun at room temperature under a positive high voltage of 13 kV and relative humidity was 30–60 %, and PVP@PVP/[Y(NO₃)₃ + Eu(NO₃)₃] core–sheath composite nanofibers were acquired on the collector.

Fig. 1

Schematic diagram of the coaxial electrospinning spinneret and the electrospinning setup

Fabrication of Y₂O₃:Eu³⁺ hollow nanofibers

The above PVP@PVP/[Y(NO₃)₃ + Eu(NO₃)₃] core–sheath composite nanofibers were calcined at 700 °C for 8 h with a heating rate of 1 °C/min. Then, the calcination temperature was decreased to 200 °C at a rate of 1 °C/min. Finally, samples were naturally cooled down to room temperature and Y₂O₃:Eu³⁺ hollow nanofibers were obtained.

Fabrication of YF₃:Eu³⁺ hollow nanofibers

Y₂O₃:Eu³⁺ hollow nanofibers were loaded into a small crucible. A few carbon rods were put into a big crucible, and then the small crucible was placed into the big crucible. Next, some NH₄HF₂ powders were loaded into the space between the two crucibles, and then the big crucible was covered with its lid. We call this process a double-crucible method. Finally, the crucibles were annealed at 280 °C for 2 h, then heated to 450 °C and remained for 3 h with the heating rate of 2 °C/min, then, the temperature was decreased to 200 °C at a rate of 1 °C/min followed by natural cooling down to ambient temperature. Thus, YF₃:Eu³⁺ hollow nanofibers were acquired.

Characterization methods

The as-prepared YF₃:Eu³⁺ hollow nanofibers were identified by a Rigaku D/max-RA X-ray powder diffractometer (XRD) with Cu Kα radiation. The operation voltage and current were kept at 30 kV and 20 mA, respectively. The morphology and internal structure of YF₃:Eu³⁺ hollow nanofibers were observed by a field emission scanning electron microscope (FESEM, XL-30, FEI Company) and a transmission electron microscope (TEM, JEM-2010). The excitation and emission spectra of samples were recorded with a HITACHI F-7000 fluorescence spectrophotometer using a Xe lamp as the excitation source. The histograms of diameters were drawn by Image-Pro Plus 6.0 and origin 8.5 softwares.

Results and discussion

Characterizations of the structure and morphology

Figure 2a shows the XRD patterns of Y₂O₃:Eu³⁺ hollow nanofibers (a) and YF₃:Eu³⁺ hollow nanofibers (b). It can be seen that XRD pattern of the Y₂O₃:Eu³⁺ hollow nanofibers is conformed to the cubic structure of Y₂O₃ (PDF#86-1107), and the space group is $\text{Ia} \bar{3}$. No peaks of any other phases or impurities are detected, indicating crystalline Y₂O₃:Eu³⁺ were obtained. The lattice constants of Y₂O₃:Eu³⁺ crystals are a = b = c = 10.6102 Å, respectively. The XRD analysis result of YF₃:Eu³⁺ hollow nanofibers demonstrates that the characteristic diffraction peaks [2θ = 24.6°(101), 25.9°(020), 27.8°(111), 30.96°(210), 44.6°(221), 45.6°(112), 46.9°(131), 49.0°(230), 55.0°(321), 76.8°(223), etc.] of the sample can be easily indexed to those of the pure orthorhombic phase with primitive structure of YF₃ (PDF#70-1935), and the space group is Pnma. No characteristic peaks were observed for other impurities, the lattice constants of YF₃:Eu³⁺ crystals are a = 6.354 Å, b = 6.854 Å, c = 4.395 Å, respectively.

Fig. 2

XRD patterns of Y₂O₃:Eu³⁺ (a) and YF₃:Eu³⁺ (b) hollow nanofibers (a), and YF₃:x%Eu³⁺ (x = 1,3,5,7,9) hollow nanofibers (b)

Figure 2b manifests XRD patterns of the YF₃:x%Eu³⁺ (x = 1,3,5,7,9) hollow nanofibers. As seen from the Fig. 2b, the XRD patterns of all samples are consistent with the orthorhombic structure of YF₃ (PDF#70-1935), and the space group is Pnma. In addition, with the increase of Eu³⁺ concentration, no obvious shifting of peaks can be detected, indicating that Y³⁺ may be substituted by Eu³⁺ successfully to form the luminescence center because of the similar radius between Y³⁺ and Eu³⁺.

Figure 3a, b demonstrate FESEM images of Y₂O₃:Eu³⁺ hollow nanofibers and YF₃:Eu³⁺ hollow nanofibers. It can be clearly seen that the morphology and diameters of YF₃:Eu³⁺ hollow nanofibers are similar to those of Y₂O₃:Eu³⁺ hollow nanofibers and morphology of fibers is hollow-centered structure. Therefore, we can safely conclude that the fluorination technique, we proposed here, can retain the morphology of the precursor nanofibers. Histograms of diameters of these fibers are indicated in Fig. 4. The average diameters of Y₂O₃:Eu³⁺ hollow nanofibers and YF₃:Eu³⁺ hollow nanofibers are, respectively, 213 ± 42 and 211 ± 29 nm under the confidence level of 95 %.

Fig. 3

FESEM images of Y₂O₃:Eu³⁺ hollow nanofibers (a) and YF₃:Eu³⁺ hollow nanofibers (b)

Fig. 4

Histograms of diameters of Y₂O₃:Eu³⁺ hollow nanofibers (a) and YF₃:Eu³⁺ hollow nanofibers (b)

The TEM images of Y₂O₃:Eu³⁺ hollow nanofibers and YF₃:Eu³⁺ hollow nanofibers are presented in Fig. 5. It is found that Y₂O₃:Eu³⁺ hollow nanofibers and YF₃:Eu³⁺ hollow nanofibers are composed of nanoparticles, and the diameters of Y₂O₃:Eu³⁺ and YF₃:Eu³⁺ hollow nanofibers are ca. 200 nm. The results are consistent with the results of SEM analysis. In the inset of Fig. 5b, the corresponding selected area electron diffraction (SAED) patterns of YF₃:Eu³⁺ hollow nanofibers exhibit typical polycrystalline diffraction patterns, indicating that polycrystalline YF₃:Eu³⁺ was obtained.

Fig. 5

TEM images of Y₂O₃:Eu³⁺ hollow nanofibers (a) and YF₃:Eu³⁺ hollow nanofibers (b), the inset shows the SAED patterns of YF₃:Eu³⁺ hollow nanofibers

Energy dispersive spectrum (EDS) analysis

The EDS analysis shown in Fig. 6 manifests that the presence of Y, O, Eu corresponds to Y₂O₃:Eu³⁺, and the presence of Y, F, Eu corresponds to YF₃:Eu³⁺. C exists in YF₃:Eu³⁺ hollow nanofibers due to carbon rods loaded into the big crucible during the fluorination process. Si comes from Si carrier for bearing the sample and Au peak is from the conductive film of Au-plated on the sample for SEM observation.

Fig. 6

EDS spectra of Y₂O₃:Eu³⁺ hollow nanofibers (a) and YF₃:Eu³⁺ hollow nanofibers (b)

Photoluminescence properties

Figure 7 demonstrates the PL excitation (monitored by 593 nm) and emission (excited by 394 nm) spectra of the YF₃:7%Eu³⁺ hollow nanofibers. The excitation spectrum (Fig. 7a) is dominated by the ⁷F₀ → ⁵L₆ transition of Eu³⁺ ions centered at about 394 nm. The position of other peaks is practically identical to the characteristic absorption bands for f–f transitions in Eu³⁺ ions [33]. The emission spectrum (Fig. 7b) consists of seven main peaks at 555, 587, 593, 615, 620, 651, and 692 nm, which originate from the ⁵D₁ → ⁷F₂, ⁵D₀ → ⁷F₁, ⁵D₀ → ⁷F₁, ⁵D₀ → ⁷F₂, ⁵D₀ → ⁷F₂, ⁵D₀ → ⁷F₃, and ⁵D₀ → ⁷F₄ transitions of Eu³⁺ ions, respectively. Among these emission peaks, the red emission at 593 nm attributed to ⁵D₀ → ⁷F₁ transition of Eu³⁺ ions is the strongest one.

Fig. 7

Excitation spectrum (a) and emission spectrum (b) of YF₃:7%Eu³⁺ hollow nanofibers

Figure 8 shows the excitation spectra and emission spectra of YF₃:Eu³⁺ hollow nanofibers with different doping concentrations of Eu³⁺ ions. It is found that the peak positions and spectral shapes of excitation and emission spectra do not vary with Eu³⁺ concentration for YF₃:Eu³⁺ hollow nanofibers, but the intensity of excitation and emission peaks for YF₃:x%Eu³⁺ hollow nanofibers strongly depends on the doping concentration of Eu³⁺ ions and the strongest excitation and emission spectra can be obtained when the doping molar concentration of Eu³⁺ is 7 %. Obviously, the luminescence intensity of YF₃:Eu³⁺ hollow nanofibers increases with the increase of the concentration of Eu³⁺ from the beginning, reaching a maximum value with the Eu³⁺ concentration of 7 %, and then decrease with the further increase in Eu³⁺ concentration. Below this value (7 %), the emission intensity is weak because there are no sufficient luminescent centers. Higher than this value, the luminescent intensity decreases due to the concentration quenching effect based on the energy transfer between adjacent luminescent centers. Namely, the optimum concentration for red emission of Eu³⁺ is 7 % in YF₃:Eu³⁺ hollow nanofibers. Based on Dexter’s theory [34], the average distances(R) between Eu³⁺ ions can be estimated by the following equation:

$$ R = 2\left( { 3V/ 4\pi XN} \right)^{ 1/ 3} $$

(1)

Fig. 8

Excitation (a) and emission (b) spectra of YF₃:x%Eu³⁺ (x = 1,3,5,7,9) hollow nanofibers

where V is the volume of the unit cell, X is the critical concentration, N is the number of available crystallographic sites occupied by the activator ions in the unit cell. For YF₃:Eu³⁺, V = 0.1914 nm³, N = z × 2 = 8, and then the average distances(R) between Eu³⁺ ions is 0.8675 nm when the doping concentration is 7 %.

The fluorescent decay curves of YF₃:x%Eu³⁺ hollow nanofibers with different concentration of Eu³⁺ (1, 3, 5, 7, and 9 %), as presented in Fig. 9, are used to calculate the lifetime and to investigate the luminescence dynamics of these samples. The samples are excited by 394 nm and monitored by 593 nm, and the curves accord with the single exponential decay:

$$ I_{t} = I_{0} { \exp }( - t/\tau ) $$

(2)

Fig. 9

Decay curves of YF₃:x%Eu³⁺ (x = 1,3,5,7,9) hollow nanofibers

where I _t is the intensity at time t, I ₀ is the intensity at t = 0 and τ is the decay lifetime [35]. All the curves can be fitted by single exponential procedures, and the lifetime values of YF₃:Eu³⁺ are 8.174, 11.916, 6.8818, 7.7203, and 7.1565 ms corresponding to the Eu³⁺ concentration of 1, 3, 5, 7, and 9 %, respectively.

In general, color can be represented by the Commission International del’Eclairage (CIE) 1931 chromaticity coordinates. The color coordinates for the red emission in the present experiment are calculated based on the corresponding emission spectra and the results are shown in Fig. 10. The coordinates (x, y) of YF₃:Eu³⁺ hollow nanofibers are (0.238, 0.220), (0.393, 0.315), (0.466, 0.344), (0.510, 0.348), and (0.402, 0.333) which correspond to the Eu³⁺ concentration of 1, 3, 5, 7, and 9 %, respectively. Among these hollow nanofibers, YF₃:1%Eu³⁺ hollow nanofibers show nearly blue emission owing to the presence of stronger intensity arising from ⁵D₁ → ⁷F₂ transitions of Eu³⁺ ions. These results indicate that the emission colors of YF₃:Eu³⁺ hollow nanofibers are tunable by changing the concentration of doping Eu³⁺ ions, which is considered to be a promising candidate for application in LEDs [35].

Fig. 10

CIE chromaticity coordinates diagram of YF₃:x%Eu³⁺ (x = 1,3,5,7,9) hollow nanofibers

Formation mechanism for YF₃:Eu³⁺ hollow nanofibers

We propose the formation mechanism for YF₃:Eu³⁺ hollow nanofibers, as shown in Fig. 11. First, the precursor spinning solutions were prepared and PVP@PVP/[Y(NO₃)₃ + Eu(NO₃)₃] core–sheath composite nanofibers were fabricated by coaxial electrospinning, and then Y₂O₃:Eu³⁺ hollow nanofibers were prepared through calcining the as-obtained composite nanofibers. During calcination process, PVP chain was broken and volatilize. With the increase in calcination temperature, nitrate was decomposed and oxidized to NO₂, Y³⁺ and Eu³⁺ were oxidized to form Y₂O₃:Eu³⁺ crystallites, many crystallites were combined into nanoparticles, then some nanoparticles were mutually connected to generate hollow-centered Y₂O₃:Eu³⁺ nanofibers. Next, Y₂O₃:Eu³⁺ hollow nanofibers were fluorinated using NH₄HF₂ as fluorinating agent. In the fluorinated process, NH₄HF₂ decomposed and reacted with Y₂O₃:Eu³⁺ hollow nanofibers to produce YF₃:Eu³⁺ hollow nanofibers. During the process, NH₄HF₂ powders and Y₂O₃:Eu³⁺ hollow nanofibers were separated by the small crucible, which prevented Y₂O₃:Eu³⁺ hollow nanofibers from morphology damage. If Y₂O₃:Eu³⁺ hollow nanofibers directly mix with NH₄HF₂ powders, melted NH₄HF₂ will cut the Y₂O₃:Eu³⁺ hollow nanofibers into pieces, as a result, the morphology of Y₂O₃:Eu³⁺ hollow nanofibers cannot be retained, as presented in Fig. 12. Carbon rods played an important role in the reduction via combination of O₂ to produced CO, which react with oxygen species of Y₂O₃:Eu³⁺ to give CO₂ in the heating process. The double-crucible method we proposed here is actually a solid–gas reaction, which has been proved to be an important method, not only can retain the morphology of Y₂O₃:Eu³⁺ hollow nanofibers, but also can fabricate YF₃:Eu³⁺ hollow nanofibers with pure phase at relatively low temperature.

Fig. 11

Formation mechanism of YF₃:Eu³⁺ hollow nanofibers

Fig. 12

SEM image of YF₃:Eu³⁺ particles prepared via direct mixing of Y₂O₃:Eu³⁺ hollow nanofibers with NH₄HF₂ powders

Conclusions

In summary, orthorhombic structure YF₃:Eu³⁺ hollow nanofibers were successfully fabricated via fluorination of the relevant Y₂O₃:Eu³⁺ hollow nanofibers which were obtained by calcining the coaxial electrospun core–sheath composite nanofibers. The morphology of YF₃:Eu³⁺ hollow nanofibers can be inherited from Y₂O₃:Eu³⁺ hollow nanofibers under the fluorination circumstance via a double-crucible method we newly proposed. The mean diameter of YF₃:Eu³⁺ hollow nanofibers is 211 ± 29 nm. YF₃:Eu³⁺ hollow nanofibers emit red emissions of predominant peaks at 593 and 587 nm originating from ⁵D₀ → ⁷F₁ transition of Eu³⁺ ions under the excitation of 394 nm ultraviolet light and the quenching concentration is 7 %. The present work provides a new route to fabricate hollow nanofibers of rare earth trifluoride.