DOI:
10.1039/C6RA21039B
(Paper)
RSC Adv., 2016,
6, 113045-113052
Tunable multicolor luminescence and white light emission realized in Eu3+ mono-activated GdF3 nanofibers with paramagnetic performance
Received
21st August 2016
, Accepted 24th November 2016
First published on 25th November 2016
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 5D3,2,1,0 → 7FJ 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.
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 → 7FJ transitions (J = 0, 1, 2, 3, 4).48,49 More importantly, the higher energy levels 5D3,2,1 (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, Mw ≈ 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.
![image file: c6ra21039b-f1.tif](/image/article/2016/RA/c6ra21039b/c6ra21039b-f1.gif) |
| 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 Ia
. 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.
![image file: c6ra21039b-f2.tif](/image/article/2016/RA/c6ra21039b/c6ra21039b-f2.gif) |
| 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.
![image file: c6ra21039b-f3.tif](/image/article/2016/RA/c6ra21039b/c6ra21039b-f3.gif) |
| 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). | |
![image file: c6ra21039b-f4.tif](/image/article/2016/RA/c6ra21039b/c6ra21039b-f4.gif) |
| 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.
![image file: c6ra21039b-f5.tif](/image/article/2016/RA/c6ra21039b/c6ra21039b-f5.gif) |
| 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+ (8S7/2 → 6I7/2) 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
![image file: c6ra21039b-f6.tif](/image/article/2016/RA/c6ra21039b/c6ra21039b-f6.gif) |
| 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 5D3,2,1,0 → 7FJ 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 5D3,2,1 emissions are predominant at low doping concentration of Eu3+ ions.51 With increasing in Eu3+ doping concentration, the higher-level 5D3,2,1 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 6IJ level (at 36
500 cm−1), and then it relaxes nonradiatively until it reaches the 6PJ levels.54 Subsequent energy transfers from 6PJ levels to Eu3+ ions result in the blue, green, yellow, orange, and red emissions of Eu3+ ions, respectively.
![image file: c6ra21039b-f7.tif](/image/article/2016/RA/c6ra21039b/c6ra21039b-f7.gif) |
| 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 8S7/2 → 6I7/2 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+, 5D3,2,1 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:
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 GdF
3:Eu
3+,
V = 201.47 Å
3,
N = 4, and then the corresponding calculated
R (Eu
3+–Eu
3+) 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 Eu
3+ 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 Eu
3+ (
5D
3) + Eu
3+ (
7F
0) → Eu
3+ (
5D
2) + Eu
3+ (
7F
4), Eu
3+ (
5D
1) + Eu
3+ (
7F
0) → Eu
3+ (
5D
0) + Eu
3+ (
7F
3), which will exist only above a certain Eu
3+ concentration, because this process depends on the interaction between two Eu
3+ emission centers.
51
![image file: c6ra21039b-f8.tif](/image/article/2016/RA/c6ra21039b/c6ra21039b-f8.gif) |
| 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
![image file: c6ra21039b-f9.tif](/image/article/2016/RA/c6ra21039b/c6ra21039b-f9.gif) |
| 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% Eu3+ (a) |
(0.222, 0.232) |
73 563 K |
GdF3:0.5% Eu3+ (b) |
(0.235, 0.272) |
19 609 K |
GdF3:1% Eu3+ (c) |
(0.312, 0.333) |
6519 K |
GdF3:2% Eu3+ (d) |
(0.362, 0.344) |
4313 K |
GdF3:3% Eu3+ (e) |
(0.421, 0.344) |
2598 K |
GdF3:5% Eu3+ (f) |
(0.465, 0.349) |
1975 K |
GdF3:10% Eu3+ (g) |
(0.519, 0.348) |
2032 K |
GdF3:13% Eu3+ (h) |
(0.521, 0.344) |
2131 K |
GdF3:15% Eu3+ (i) |
(0.511, 0.339) |
2069 K |
To further investigate the luminescence dynamics, the PL decay curves of the Eu3+ ions (from 5D3,2,1,0 to the ground states of 7FJ) in GdF3:0.5% Eu3+ nanofibers are represented in Fig. 10. The curves accord with the single-exponential decay:
|
It = I0 exp(−t/τ)
| (2) |
where
It 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
5D
3 →
7F
2,
5D
2 →
7F
3,
5D
1 →
7F
1, and
5D
0 →
7F
1 of Eu
3+ were determined to be 2.26, 3.58, 3.88, and 12.80 ms, respectively, in GdF
3:0.5% Eu
3+ nanofibers. From the decay times, it can be seen that the decay times of the higher energy level (
5D
3,
5D
2,
5D
1) emission are shorter than that of the lower energy level (
5D
0) 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 (
5D
0) nonradiatively by multiphonon relaxation.
51
![image file: c6ra21039b-f10.tif](/image/article/2016/RA/c6ra21039b/c6ra21039b-f10.gif) |
| 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.
![image file: c6ra21039b-f11.tif](/image/article/2016/RA/c6ra21039b/c6ra21039b-f11.gif) |
| 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% Eu3+ nanofibers (b) |
2.42 |
GdF3:0.5% Eu3+ nanofibers (c) |
2.33 |
GdF3:5% Eu3+ nanofibers (d) |
2.20 |
GdF3:10% Eu3+ nanofibers (e) |
2.18 |
GdF3:13% Eu3+ nanofibers (f) |
2.16 |
GdF3:15% Eu3+ nanofibers (g) |
2.11 |
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 5D3,2,1,0 → 7FJ 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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