Tunable multicolor luminescence and white light emission realized in Eu³⁺ mono-activated GdF₃ nanofibers with paramagnetic performance

Abstract

Luminescent–magnetic bifunctional GdF₃:Eu³⁺ nanofibers have been successfully fabricated via the combination of electrospinning followed by calcination with fluorination technique. The structure, morphologies, luminescence, and magnetic properties of the synthesized nanomaterials have been characterized by a variety of techniques. X-ray diffraction (XRD) analysis indicates that the as-obtained GdF₃:Eu³⁺ nanofibers have a pure orthorhombic structure. Scanning electron microscope (SEM) observations show that the directly electrospinning-made PVP/[Gd(NO₃)₃ + Eu(NO₃)₃] composite nanofibers have smooth surfaces, uniform size and good dispersion, and the surfaces of GdF₃:Eu³⁺ nanofibers become rough after calcination and fluorination process. The diameters of composite nanofibers and GdF₃:Eu³⁺ nanofibers are respectively 333.26 ± 1.80 nm and 86.54 ± 0.55 nm under the confidence level of 95%. Under 274 nm ultraviolet light excitation, GdF₃:Eu³⁺ nanofibers exhibit characteristic ⁵D_3,2,1,0 → ⁷F_J emission of Eu³⁺ and the trend of their color tones changes from blue, cold white, warm white to red by adjusting the molar concentration of Eu³⁺. In addition, all of the samples exhibit paramagnetic features and the magnetic properties of GdF₃:Eu³⁺ nanofibers are tunable by modulating the doping concentration of Eu³⁺ ions. More importantly, the tunable multicolor luminescence, white light emission and paramagnetic properties are simultaneously realized in single-phase GdF₃:Eu³⁺ nanofibers, which are ideally suited to applications in many fields such as solid-state lasers, lighting and displays, magnetic resonance imaging. This design conception and construction strategy developed in this work may provide some new guidance for the synthesis of other rare earth fluoride nanostructures with various morphologies.

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Introduction

Rare earth (RE) nanomaterials have aroused extensive attention owing to their extraordinary luminescent, magneto-optical, magnetic and optoelectronic peculiarities.^1–4 Luminescent–magnetic nanomaterials have both luminescent and magnetic properties, and have been applied in medical diagnostics and optical imaging,⁵ etc. Conventional luminescent–magnetic bifunctional nanomaterials are composites composed of luminescent materials and magnetic materials, and usually, magnetic materials reduce the luminous intensity of luminescent materials, leading to the fact that the luminescence intensity of the composites is lower than that of the luminescent material without magnetic material. Thus, it is worth researching how to prepare a single-phase material with simultaneous luminescence and magnetic properties for exploiting luminescent–magnetic devices of high precision communications and magnetic field detection.⁶ Host material is an important factor to obtain highly efficient luminescence emission. Among various luminescence host materials, RE fluorides with the formulae of REF₃ have been recognized as wonderful luminescent matrixes due to their superior characters, such as low phonon frequency, temperature of crystallization, and high coefficient of refraction and chemical durability.^7–11 These characters make RE fluorides possess lower possibilities of nonradiative transition and higher quantum efficiency than those of oxide matrixes,¹² and have potential applications in luminescent devices, displays,¹³ lasers wave guide communication,^14–17 biological imaging and detection,^18–20 magnetic resonance imaging (MRI),^18,21 etc. GdF₃ nanomaterial, as an important member of the RE fluorides, not only is a very good luminescence host material, but also possesses attractive magnetic properties at ambient temperature due to the intrinsic magnetic moment of Gd³⁺ ions. Herein, GdF₃:RE³⁺ nanomaterials have been extensively researched.^22–24

At present, many synthetic strategies have already been adopted for preparing GdF₃:RE³⁺ nanostructures. Typical synthetic approaches include hydrothermal and solvothermal methods,^24–27 precipitation,^22,23,28 microwave.²⁹ Different morphological GdF₃:RE³⁺ nanomaterials have been prepared by the above methods, including, submicroplates,²⁵ elliptic structure,²⁵ spheres,²⁸ nanorods,^30,31 nanoflowers.³¹ To our knowledge, no reports on GdF₃:RE³⁺ nanofibers are found in the references. Nanofiber, a novel sort of one-dimensional (1D) nanomaterial with particular morphology, has got a lot of attention due to its large aspect ratio.^32–34 Hence, the fabrication of GdF₃:RE³⁺ nanofibers is an urgent subject of study. Electrospinning technique is a direct and convenient process making uniform and ultralong 1D nanomaterials, such as nanofibers,^35–37 nanobelts,^38,39 hollow nanofibers,^40–43 Janus nanofibers,⁴⁴ ribbon-shaped coaxial nanocables,⁴⁵ and Janus nanobelts.^5,46 However, as far as we know, the final products are often oxides after calcining the electrospun composite nanomaterials. In our previous report, NaGdF₄:Dy³⁺ nanofibers and nanobelts were prepared by combining electrospinning followed by calcination with fluorination technique, which achieved luminescence and enhanced paramagnetic properties.⁴⁷ As an extension of previous work, the fluorination technique is used for forming inorganic GdF₃ nanofibers in this work. Eu³⁺ is often used as a red emission activator owing to its ⁵D₀ → ⁷F_J transitions (J = 0, 1, 2, 3, 4).^48,49 More importantly, the higher energy levels ⁵D_3,2,1 (blue, green, orange) emissions can be obtained relying upon the matrix (phonon energy and crystal texture) and the doping concentration of Eu³⁺. In this case, both the phonon energy of the matrix and the doping concentration of Eu³⁺ ought to be low enough to eliminate the nonradiative transition of Eu³⁺, respectively.^50,51 Thus, it is of great importance and interest to note that the emitting color of samples can be tuned and white light can be realized in specific host by adjusting the doping concentration of Eu³⁺. Consequently, it is expected that GdF₃:Eu³⁺ nanofibers are obtained and exhibit excellent multicolor luminescence, white light emission and magnetism properties owing to the particularity of GdF₃ host. To our knowledge, there are no reports on the fabrication of GdF₃:Eu³⁺ nanofibers to date.

Herein, PVP/[Gd(NO₃)₃ + Eu(NO₃)₃] composite nanofibers were firstly obtained via electrospinning process. Thereafter, the composite nanofibers were calcined into Gd₂O₃:Eu³⁺ nanofibers in air. Finally, GdF₃:Eu³⁺ nanofibers were fabricated by fluorinating the as-prepared Gd₂O₃:Eu³⁺ nanofibers with NH₄HF₂ powder as the fluorinating agent. The structure, morphology, luminescent and magnetic properties were researched in detail, and some meaningful results were obtained.

Experimental sections

Chemicals

Gadolinium oxide (Gd₂O₃, 99.99%), europium oxide (Eu₂O₃, 99.99%), concentrated nitric acid (HNO₃, AR), polyvinyl pyrrolidone (PVP) (K90, M_w ≈ 90 000, AR), N,N-dimethylformamide (DMF, AR), and ammonium hydrogen fluoride (NH₄HF₂, AR) were used in the experiments. All the reagents were of analytical grade and were directly used as received without further purification.

Preparation and formation process of GdF₃:Eu³⁺ nanofibers

GdF₃:x% Eu³⁺ [x = 0, 0.1, 0.5, 1, 2, 3, 5, 10, 13, 15, x stands for the molar ratio of Eu³⁺/(Gd³⁺ + Eu³⁺)] nanofibers were prepared by calcining the electrospun PVP/[Gd(NO₃)₃ + Eu(NO₃)₃] composite nanofibers followed by fluorinating the calcined products. In the typical procedure of preparing representative GdF₃:13% Eu³⁺ nanofibers, 0.5598 g of Gd₂O₃, and 0.0812 g of Eu₂O₃ (molar ratio of Gd³⁺ to Eu³⁺ was settled as 0.87 : 0.13) were dissolved with nitric acid and heated at 80 °C until excess nitric acid and water were evaporated to form 1.6000 g of mixed nitrates. The obtained mixed nitrates together with 0.9000 g of PVP were dissolved in 7.5000 g of DMF and magnetically stirred to form 10 g of homogeneous transparent spinning solution. In this spinning solution, the mass ratio of mixed nitrates, DMF, and PVP was fixed as 16 : 75 : 9. Subsequently, spinning solution with viscoelastic behavior was electrified and pushed by air pressure through using a traditional single-spinneret electrospinning setup at room temperature under the direct current high-voltage of 13 kV. The distance between the spinneret and collector was fixed at 18 cm, and the angle between the spinneret and the horizon was fixed at 15°. Accordingly, PVP/[Gd(NO₃)₃ + Eu(NO₃)₃] composite nanofibers were obtained by the above process. Then, the as-fabricated composite nanofibers were calcinated at 700 °C with a heating rate of 1 °C min⁻¹ for 4 h in air to obtain Gd₂O₃:13% Eu³⁺ nanofibers. During calcination process, PVP, nitrates and residual solvent were decomposed and eventually volatilized from the composite nanofibers. With the increase in calcination temperature, Gd³⁺, and Eu³⁺ could combine with O₂, coming from air, to form Gd₂O₃:Eu³⁺ nanoparticles, and these nanoparticles were interconnected to form Gd₂O₃:Eu³⁺ nanofibers. PVP acted as template during the formation of Gd₂O₃:Eu³⁺ nanofibers.

GdF₃:13% Eu³⁺ nanofibers were synthesized by fluorination of the as-prepared Gd₂O₃:13% Eu³⁺ nanofibers with NH₄HF₂ powder using as fluorination source. The as-prepared Gd₂O₃:13% Eu³⁺ nanofibers were put into a small crucible, a few carbon rods were loaded into a big crucible, and then the small crucible was placed into the big crucible. Next, some NH₄HF₂ powder was added into the gap between the two crucibles, and then the big crucible was covered with its lid. Subsequently, the crucibles were annealed at 280 °C for 2 h, then heated to 500 °C with the heating rate of 2 °C min⁻¹ and remained for 3 h, then the temperature was decreased to 200 °C at a rate of 1 °C min⁻¹ followed by natural cooling down to ambient temperature to acquire GdF₃:13% Eu³⁺ nanofibers. Schematic diagram of formation process for GdF₃:Eu³⁺ nanofibers is presented in Fig. 1. In the fluorination process, NH₄HF₂ decomposed and reacted with Gd₂O₃:Eu³⁺ nanofibers to generate GdF₃:Eu³⁺ nanofibers. During the process, NH₄HF₂ powder and Gd₂O₃:Eu³⁺ nanofibers were isolated by the small crucible, which prevented Gd₂O₃:Eu³⁺ nanofibers from morphology damage. If Gd₂O₃:Eu³⁺ nanofibers directly mix with NH₄HF₂ powder, melted NH₄HF₂ would disrupt the Gd₂O₃:Eu³⁺ nanofibers, as a result, the morphology of Gd₂O₃:Eu³⁺ nanofibers could not be retained. Carbon rods played an important role in the reduction via combination of O₂ to produce CO, which reacted with oxygen species of Gd₂O₃:Eu³⁺ nanofibers to give CO₂ in the heating process. The fluorination method has been proved to be an important method, not only can retain the morphology of Gd₂O₃:Eu³⁺ nanofibers, but also can fabricate GdF₃:Eu³⁺ nanofibers with pure phase at relatively low temperature. Other series of GdF₃:x% Eu³⁺ (x = 0, 0.1, 0.5, 1, 2, 3, 5, 10, 15) nanofibers were prepared by the similar procedures except for different doping concentration of Eu³⁺ ions.

Fig. 1 Schematic diagram of formation process for GdF₃:Eu³⁺ nanofibers.

Characterization

X-ray diffraction (XRD) measurements were carried out using a Rigaku D/max-RA X-ray diffractometer with Cu Kα radiation of 0.15406 nm. The morphologies and sizes of the samples were investigated by an XL-30 field emission scanning electron microscope (SEM) made by FEI Company. The purity of the products was examined by an Oxford ISIS-300 energy dispersive spectrometer (EDS) attached to SEM. The excitation and emission spectra of samples were recorded with a HITACHI F-7000 fluorescence spectrophotometer using a 150 W Xe lamp as the excitation source, and scanning speed was fixed at 1200 nm min⁻¹. The excitation and emission slits were respectively set to 2.5 and 2.5 nm. Then, the magnetic performances of samples were measured by a vibrating sample magnetometer (VSM, MPMS SQUID XL). The histograms of diameters distribution were drawn by Image-Pro-Plus 6.0 and origin 9.0 softwares. All the determinations were performed at room temperature.

Results and discussion

Crystallization behavior

Fig. 2 indicates the XRD patterns of Gd₂O₃:Eu³⁺ nanofibers and GdF₃:Eu³⁺ nanofibers. Well-defined diffraction peaks are acquired. It can be seen from Fig. 2a that diffraction peaks of Gd₂O₃:Eu³⁺ nanofibers can be conformed to those of the pure cubic phase Gd₂O₃ according to the PDF standard card of Gd₂O₃ (PDF#65-3181), and the space group is Ia3̄. Obvious diffraction peaks are situated near 2θ = 20.09° (211), 28.56° (222), 33.10° (400), 35.17° (411), 39.02° (332), 42.58° (134), 47.51° (440), 52.07° (611), 54.96° (145), 56.37° (622), 57.75° (136), 59.12° (444), 76.74° (622), and 79.12° (048), etc. No characteristic peaks are observed for other impurities, indicating crystalline Gd₂O₃:Eu³⁺ are prepared. From Fig. 2b, the XRD patterns of GdF₃:Eu³⁺ nanofibers demonstrate that the characteristic diffraction peaks [2θ = 23.29° (011), 24.36° (101), 25.49° (020), 27.55° (111), 30.03° (210), 34.05° (201), 35.55° (121), 41.09° (002), 43.01° (221), 45.42° (112), 46.21° (131), 47.84° (230), 48.97° (022), 51.03° (122), 53.57° (321), 58.55° (141), 64.86° (232), 72.43° (341), and 76.20° (223), etc.] of the sample can be readily indexed to those of the pure orthorhombic phase with primitive structure of GdF₃ (PDF#49-1804), and the space group is Pnma. No peaks of any other phases or impurities are detected, indicating crystalline GdF₃:Eu³⁺ with pure phase were successfully obtained.

Fig. 2 XRD patterns of Gd₂O₃:Eu³⁺ (a) and GdF₃:Eu³⁺ (b) nanofibers with PDF standard cards of Gd₂O₃ and GdF₃.

Morphology analysis

Fig. 3 demonstrates the microstructures of PVP/[Gd(NO₃)₃ + Eu(NO₃)₃] composite nanofibers, Gd₂O₃:13% Eu³⁺ nanofibers and GdF₃:13% Eu³⁺ nanofibers. It can be clearly seen that the morphology of samples is fibrous structure. Fig. 3a shows the PVP/[Gd(NO₃)₃ + Eu(NO₃)₃] composite nanofibers have smooth surface, good dispersion and uniform diameter. One can see from Fig. 3b and c that the surfaces of nanofibers become rough and the diameters of nanofibers decrease due to the decomposition and volatilization of PVP in the calcination process and the decomposition of NH₄HF₂, and growth of GdF₃:13% Eu³⁺ crystal during fluorination process. Moreover, the morphology of GdF₃:Eu³⁺ nanofibers is similar to those of Gd₂O₃:Eu³⁺ nanofibers. The diameter of GdF₃:Eu³⁺ nanofibers is smaller than that of Gd₂O₃:Eu³⁺ nanofibers. This is because F⁻ ion has smaller radius than O²⁻ ion. From the above observation, we can safely infer that the fluorination technique can remain the morphology of the precursor nanofibers. In order to investigate the diameter distributions of the samples, Image-Pro-Plus 6.0 software was used to measure the diameters of 100 nanofibers, and the results analyzed by Shapiro–Wilk method are normal distribution. As indicated in Fig. 4, the diameters of PVP/[Gd(NO₃)₃ + Eu(NO₃)₃] composite nanofibers, Gd₂O₃:13% Eu³⁺ nanofibers and GdF₃:13% Eu³⁺ nanofibers are 333.26 ± 1.80 nm, 157.38 ± 1.01 nm, and 86.54 ± 0.55 nm, respectively, under the 95% confidence level.

Fig. 3 SEM images of PVP/[Gd(NO₃)₃ + Eu(NO₃)₃] composite nanofibers (a), Gd₂O₃:13% Eu³⁺ (b) and GdF₃:13% Eu³⁺ nanofibers (c).

Fig. 4 Histograms of diameters distribution of PVP/[Gd(NO₃)₃ + Eu(NO₃)₃] composite nanofibers (a), Gd₂O₃:13% Eu³⁺ (b) and GdF₃:13% Eu³⁺ nanofibers (c).

Fig. 5 shows the EDS spectra of PVP/[Gd(NO₃)₃ + Eu(NO₃)₃] composite nanofibers, Gd₂O₃:13% Eu³⁺ nanofibers and GdF₃:13% Eu³⁺ nanofibers. The presence of C, O, Gd, Eu elements corresponds to the PVP/[Gd(NO₃)₃ + Eu(NO₃)₃] nanofibers, the presence of O, Gd, Eu elements corresponds to the Gd₂O₃:13% Eu³⁺ nanofibers and the presence of F, Gd, Eu elements corresponds to the GdF₃:13% Eu³⁺ nanofibers. Carbon peak comes from residual carbon and the used carbon rods in the calcination and fluorination process. Si comes from Si carrier for bearing the sample and Pt peak is from the conductive film of Pt plated on the sample for SEM observation. From the above analysis, we can safely conclude that GdF₃:13% Eu³⁺ nanofibers have been successfully fabricated.

Fig. 5 EDS spectra of PVP/[Gd(NO₃)₃ + Eu(NO₃)₃] composite nanofibers (a), Gd₂O₃:13% Eu³⁺ (b) and GdF₃:13% Eu³⁺ nanofibers (c).

Photoluminescence performance

Fig. 6 illustrates the room temperature photoluminescence excitation (PLE) and photoluminescence emission (PL) spectra of GdF₃:0.1% Eu³⁺ and GdF₃:13% Eu³⁺ nanofibers. The PLE spectra monitored by 594 nm (⁵D₀ → ⁷F₁ energy levels transition of Eu³⁺ ions) exhibit a strong sharp excitation peak at 274 nm assigned to Gd³⁺ (⁸S_7/2 → ⁶I_7/2) and a weak excitation peak centering at 395 nm, corresponding to the ⁷F₀ → ⁵L₆ energy levels transitions of Eu³⁺ ions, suggesting the efficient energy transfer occurred from the Gd³⁺ ions to the Eu³⁺ ions in GdF₃:Eu³⁺ nanofibers.²⁵

Fig. 6 Excitation and emission spectra of GdF₃:0.1% Eu³⁺ (a) and GdF₃:13% Eu³⁺ (b) nanofibers.

Upon excitation at 274 nm, GdF₃:0.1% Eu³⁺ nanofibers exhibit characteristic emission spectrum, as shown in Fig. 6a. The emission peaks originate from ⁵D_3,2,1,0 → ⁷F_J energy levels transitions of Eu³⁺, i.e., ⁵D₃ → ⁷F₁ (418 nm), ⁵D₃ → ⁷F₂ (430 nm), ⁵D₃ → ⁷F₃ (445 nm), ⁵D₂ → ⁷F₀ (464 nm), ⁵D₂ → ⁷F₂ (489 nm), ⁵D₂ → ⁷F₃ (510 nm), ⁵D₁ → ⁷F₁ (538 nm), ⁵D₁ → ⁷F₂ (555 nm), ⁵D₁ → ⁷F₃ (587 nm), ⁵D₀ → ⁷F₁ (594 nm), and ⁵D₀ → ⁷F₂ (615 nm).^50–52 The strongest peak is located at 430 nm, which results in a blue light emission. Meanwhile, emission spectrum of GdF₃:13% Eu³⁺ nanofibers is demonstrated in Fig. 6b, the obvious emission peaks at 594 and 615 nm are corresponded to the magnetic dipole transition ⁵D₀ → ⁷F₁ (594 nm) and the electric dipole transition ⁵D₀ → ⁷F₂ (615 nm) of Eu³⁺ ions. Because the emission of GdF₃:13% Eu³⁺ nanofibers is dominated by the ⁵D₀ emission of Eu³⁺, the red emission color is produced. This is quite different from that of GdF₃:0.1% Eu³⁺.

In a specific host, as GdF₃ with lower phonon frequency, when the doping concentration of Eu³⁺ ions is low enough, Eu³⁺ ions may emit not only from lower energy level ⁵D₀ (red), but also from higher excited states ⁵D₁, (yellow), ⁵D₂ (green) and ⁵D₃ (blue).⁵³ The energy level ⁵D_3,2,1 emissions are predominant at low doping concentration of Eu³⁺ ions.⁵¹ With increasing in Eu³⁺ doping concentration, the higher-level ⁵D_3,2,1 emissions are quenched gradually owing to the cross relaxation occurring between two neighboring Eu³⁺ ions, and the lower energy level ⁵D₀ emission becomes dominating, which can be observed from the emission spectrum of the GdF₃:13% Eu³⁺ nanofibers (Fig. 6b).

A schematic diagram for energy levels transitions process in Eu³⁺-doped GdF₃ nanofibers is shown in Fig. 7. Under the excitation at 274 nm, the Gd³⁺ ions firstly absorb energies of photons from ultraviolet light and are pumped to ⁶I_J level (at 36 500 cm⁻¹), and then it relaxes nonradiatively until it reaches the ⁶P_J levels.⁵⁴ Subsequent energy transfers from ⁶P_J levels to Eu³⁺ ions result in the blue, green, yellow, orange, and red emissions of Eu³⁺ ions, respectively.

Fig. 7 Schematic diagram for energy levels transitions process in Eu³⁺-doped GdF₃ nanofibers.

Fig. 8a demonstrates the PL spectra of GdF₃:x% Eu³⁺ nanofibers under the excitation at 274 nm (the ⁸S_7/2 → ⁶I_7/2 of Gd³⁺ ions) with different Eu³⁺ concentration of x = 0.1, 0.5, 1, 2, 3, 5, 10, 13 and 15. As depicted above, the strongest emission peak is located at 430 nm (⁵D₃ → ⁷F₂) in GdF₃:0.1% Eu³⁺ nanofibers, which results in a blue light emission. The emission peaks of GdF₃:0.5% Eu³⁺ nanofibers cover the whole visible spectral region, and 510 nm (⁵D₂ → ⁷F₃) is the strongest one, which leads to a white light emission. With further increasing in the concentration of Eu³⁺, ⁵D_3,2,1 emissions are quenched and ⁵D₀ emission becomes predominant. The optimum doping concentration is 13% for ⁵D₀ emission in GdF₃:Eu³⁺ nanofibers. The above situation can be expediently observed through the emission intensities of Eu³⁺ ions at 430, 464, 510, 594, 615 nm as a function of Eu³⁺ concentration, as shown in Fig. 8b. Based on Dexter's theory,⁵⁵ the average distances (R) between Eu³⁺ ions can be estimated by the following equation:

R = 2(3V/4πXN)^1/3 (1)

where V is the volume of the unit cell, X is the concentration, N is the number of available crystallographic sites occupied by the activator ions in the unit cell. For GdF₃:Eu³⁺, V = 201.47 ų, N = 4, and then the corresponding calculated R (Eu³⁺–Eu³⁺) values are 45.82 Å, 26.80 Å, and 9.04 Å when the doping concentrations are 0.1%, 0.5%, and 13%, respectively. As the Eu³⁺ doping concentration increases, the R becomes small enough to allow a resonant energy transfer, and the higher energy level is quenched by the cross relaxation process, such as Eu³⁺ (⁵D₃) + Eu³⁺ (⁷F₀) → Eu³⁺ (⁵D₂) + Eu³⁺ (⁷F₄), Eu³⁺ (⁵D₁) + Eu³⁺ (⁷F₀) → Eu³⁺ (⁵D₀) + Eu³⁺ (⁷F₃), which will exist only above a certain Eu³⁺ concentration, because this process depends on the interaction between two Eu³⁺ emission centers.⁵¹

Fig. 8 Emission spectra of GdF₃:x% Eu³⁺ (x = 0.1, 0.5, 1, 2, 3, 5, 10, 13, 15) nanofibers (a), and dependence of the emission intensity at different wavelengths on Eu³⁺ concentration (b).

Based on the above analysis, it can be concluded that the emission color of GdF₃:x% Eu³⁺ nanofibers can be tuned by only changing the molar concentration of Eu³⁺ ions when excited at 274 nm. Thus, the CIE parameters such as the color coordinates (x, y) and the color correlated temperature (CCT) are calculated to characterize the color emission. The CIE chromaticity coordinates for GdF₃:x% Eu³⁺ (x = 0.1, 0.5, 1, 2, 3, 5, 10, 13, 15) nanofibers excited at 274 nm are calculated based on the corresponding emission spectra, which are represented in the CIE diagram in Fig. 9 and the data are given in Table 1. From the CIE chromaticity diagram, it can be seen that the emission colors of GdF₃:x% Eu³⁺ nanofibers are tunable and the trend of their color tones changes from blue, cold white, warm white to red by adjusting the doping concentration of Eu³⁺, and corresponding digital photographs are also shown in Fig. 9, which is considered to be promising applications in the fields of blue LEDs,⁵⁶ WLEDs,⁵⁷ miniature color displays and light-emitting devices in near future.⁵⁸

Fig. 9 CIE chromaticity diagram and corresponding luminescence photographs for GdF₃:x% Eu³⁺ nanofibers.

Table 1 The CIE chromaticity coordinates and color temperature for GdF₃:x% Eu³⁺ nanofibers

Samples	CIE (x, y)	CCT/K
GdF3:0.1% Eu^3+ (a)	(0.222, 0.232)	73 563 K
GdF3:0.5% Eu^3+ (b)	(0.235, 0.272)	19 609 K
GdF3:1% Eu^3+ (c)	(0.312, 0.333)	6519 K
GdF3:2% Eu^3+ (d)	(0.362, 0.344)	4313 K
GdF3:3% Eu^3+ (e)	(0.421, 0.344)	2598 K
GdF3:5% Eu^3+ (f)	(0.465, 0.349)	1975 K
GdF3:10% Eu^3+ (g)	(0.519, 0.348)	2032 K
GdF3:13% Eu^3+ (h)	(0.521, 0.344)	2131 K
GdF3:15% Eu^3+ (i)	(0.511, 0.339)	2069 K

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To further investigate the luminescence dynamics, the PL decay curves of the Eu³⁺ ions (from ⁵D_3,2,1,0 to the ground states of ⁷F_J) in GdF₃:0.5% Eu³⁺ nanofibers are represented in Fig. 10. The curves accord with the single-exponential decay:

I_t = I₀ exp(−t/τ) (2)

where I_t is the intensity at time t, I₀ is the intensity at t = 0 and τ is the decay lifetime.⁵⁹ On the basis of this equation and the decay curves, the lifetime values for ⁵D₃ → ⁷F₂, ⁵D₂ → ⁷F₃, ⁵D₁ → ⁷F₁, and ⁵D₀ → ⁷F₁ of Eu³⁺ were determined to be 2.26, 3.58, 3.88, and 12.80 ms, respectively, in GdF₃:0.5% Eu³⁺ nanofibers. From the decay times, it can be seen that the decay times of the higher energy level (⁵D₃, ⁵D₂, ⁵D₁) emission are shorter than that of the lower energy level (⁵D₀) emission. This is because the higher energy level is more metastable than the lower energy level, and the electrons at those levels would like to either transition to the ground state (⁷F_J, J = 0, 1, 2, 3, 4) or relax to the low-energy level (⁵D₀) nonradiatively by multiphonon relaxation.⁵¹

Fig. 10 Decay curves for the luminescence of ⁵D₃ → ⁷F₂ (a), ⁵D₂ → ⁷F₃ (b), ⁵D₁ → ⁷F₁ (c), and ⁵D₀ → ⁷F₁ (d) of Eu³⁺ in GdF₃:0.5% Eu³⁺ nanofibers.

Magnetic properties analysis

Besides the aforementioned luminescence properties, the paramagnetic behavior of GdF₃ and GdF₃:x% Eu³⁺ (x = 0.1, 0.5, 5, 10, 13, 15) nanofibers is also studied. Fig. 11 shows the magnetization (M) of GdF₃ and GdF₃:x% Eu³⁺ nanofibers as a function of magnetic field (−20 to 20 kOe), and the magnetization values of samples are listed in Table 2. One can see that all of GdF₃ and GdF₃:x% Eu³⁺ nanofibers show paramagnetic properties. The magnetization of GdF₃ nanofibers is 2.46 emu g⁻¹ and GdF₃:x% Eu³⁺ nanofibers are 2.42, 2.33, 2.20, 2.18, 2.16, and 2.11 emu g⁻¹ for x = 0.1, 0.5, 5, 10, 13, and 15, respectively, which can be used for common bioseparation.⁶⁰ The magnetization of samples is sensitive to the concentration of magnetic RE³⁺ ions. To the best of our knowledge, the magnetic moment (3.40–3.51 A m²) and magnetic mass susceptibility of Eu³⁺ ion are less than the magnetic moment (7.94 A m²) and magnetic mass susceptibility of Gd³⁺ ion.⁶¹ For this reason, the magnetization values of GdF₃:x% Eu³⁺ nanofibers are slightly smaller than that of GdF₃ nanofibers, and decreases with increasing in doping concentration of Eu³⁺. These results indicate that the magnetic properties of GdF₃:Eu³⁺ nanofibers are tunable by changing the doping concentration of Eu³⁺ ions.

Fig. 11 Magnetization as a function of magnetic field for GdF₃:x% Eu³⁺ (x = 0, 0.1, 0.5, 5, 10, 13, 15) nanofibers.

Table 2 Magnetization of GdF₃:x% Eu³⁺ (x = 0, 0.1, 0.5, 5, 10, 13, 15) nanofibers

Samples	Magnetization (M)/(emu g^−1)
GdF3 nanofibers (a)	2.46
GdF3:0.1% Eu^3+ nanofibers (b)	2.42
GdF3:0.5% Eu^3+ nanofibers (c)	2.33
GdF3:5% Eu^3+ nanofibers (d)	2.20
GdF3:10% Eu^3+ nanofibers (e)	2.18
GdF3:13% Eu^3+ nanofibers (f)	2.16
GdF3:15% Eu^3+ nanofibers (g)	2.11

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Conclusions

Bifunctional GdF₃:Eu³⁺ nanofibers with pure orthorhombic phase are successfully fabricated by electrospinning combined with calcination and fluorination method. The diameters of GdF₃:Eu³⁺ nanofibers analyzed by Shapiro–Wilk method are normal distribution and are 86.54 ± 0.55 nm. Under the excitation of 274 nm ultraviolet light, GdF₃:Eu³⁺ nanofibers emit the characteristic ⁵D_3,2,1,0 → ⁷F_J energy level emissions of Eu³⁺. It is amazing to find that multicolor luminescence from blue to red including white light emission can be realized in a single component host by properly tuning single-activator Eu³⁺ ions molar concentration, which would be the excellent candidates in color displays. In addition, the paramagnetic properties of GdF₃:Eu³⁺ make them promising materials in future biomedical engineering applications, such as biological labels, bioseparation and MRI. The present work provides a new route to fabricate luminescent–magnetic bifunctional nanofibers of other rare earth fluorides.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (51573023, 50972020, 51072026), the Science and Technology Development Planning Project of Jilin Province (20130101001JC, 20070402).

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