Coaxial electrospinning preparation and properties of magnetic–photoluminescent bifunctional CoFe₂O₄@Y₂O₃:Eu³⁺ coaxial nanofibers

Abstract

CoFe₂O₄@Y₂O₃:Eu³⁺ magnetic–fluorescent bifunctional coaxial nanofibers have been successfully obtained via calcination of the [CoFe₂O₄/PVP]@[(Y(NO₃)₃ + Eu(NO₃)₃)/PVP] composite coaxial nanofibers which were fabricated by coaxial electrospinning technique. The diameter of CoFe₂O₄@Y₂O₃:5 %Eu³⁺ magnetic–fluorescent bifunctional coaxial nanofibers was 133 ± 17 nm. Strong fluorescence emission peaks of Eu³⁺ in the CoFe₂O₄@Y₂O₃:Eu³⁺ coaxial nanofibers were observed and assigned to ⁵D₀ → ⁷F₁ (588 nm), ⁵D₀ → ⁷F₁ (593 nm), ⁵D₀ → ⁷F₁ (599 nm), ⁵D₀ → ⁷F₂ (612 nm) and ⁵D₀ → ⁷F₂ (630 nm) energy levels transitions of Eu³⁺ ions, and the predominant emission peak was located at 612 nm. Compared with CoFe₂O₄/Y₂O₃:Eu³⁺ composite nanofibers, CoFe₂O₄@Y₂O₃:Eu³⁺ magnetic–fluorescent bifunctional coaxial nanofibers simultaneously provided higher magnetism and fluorescent intensity. The color, photoluminescent intensity and magnetism of the coaxial nanofibers can be tuned via adjusting the diversity and content of fluorescent compounds and the content of magnetic compounds. Formation mechanism of CoFe₂O₄@Y₂O₃:Eu³⁺ coaxial nanofibers was also presented. The bifunctional magnetic–photoluminescent CoFe₂O₄@Y₂O₃:Eu³⁺ coaxial nanofibers have potential applications in many fields due to their excellent magnetism and fluorescence.

1 Introduction

Electrospinning is one of the most effective and convenient techniques to process viscous solutions or melts into continuous fibers with diameters ranging from micrometer to submicron or nanometer. Some inorganic and organic nanomaterials have been prepared by electrospinning technique [1–7]. Recently, coaxial electrospinning provides an alternative route to design and fabricate new functionalized composite nanofibers with core–shell structures [8, 9].

Nowadays, rare-earth-doped nanomaterials have been widely used in high performance luminescent devices, magnets and other functional materials [10, 11]. Y₂O₃ has received considerable attention when doped with rare earth ions owing to their interesting properties [12, 13]. Y₂O₃ doped with Eu³⁺ ions is an important phosphor with a variety of applications in many luminescent and optical devices. The interest in the synthesis of Y₂O₃:Eu³⁺ phosphor is motivated by the need to improve the luminescence efficiency of materials [14].

Recently, magnetic nanomaterials have attracted increasing interest of scientists owing to their practical and potential applications, such as biomacromolecules separation, catalyst separation and drug/gene delivery [15–18]. Among the magnetic materials, cobalt ferrite (CoFe₂O₄) has been studied intensively due to its significant properties, such as moderate saturation magnetization, remarkable chemical and thermal stabilities and good mechanical hardness. It has been broadly used as a traditional high-frequency low loss power converter, stress sensor and actuator [19–22].

Magnetic–photoluminescent bifunctional nanomaterials simultaneously possess excellent magnetism and luminescence, and have been applied in medical diagnostics and optical imaging. Presently, studies are mainly focused on the preparation, properties and applications of magnetic–photoluminescent bifunctional nanoparticles [23–25]. In order to obtain new morphologies of magnetic–fluorescent nanomaterials, the fabrication of one-dimensional magnetic–fluorescent nanomaterials is an urgent subject of study. Dong, et al. have prepared the magnetic–photoluminescent bifunctional composite nanofibers and nanoribbons via electrospinning [2, 5, 8]. It has been proved that the existence of magnetic nanomaterials will greatly decrease the luminescence of rare earth compounds if magnetic nanomaterials directly blend with the rare earth luminescent compounds [26–28]. Therefore, if the strong luminescence of the magnetic–fluorescent bifunctional nanomaterials is to be achieved, rare earth compounds must be effectively isolated from magnetic nanomaterials in order to avoid direct contacting. The peculiar nanostructure of coaxial nanofibers can help to realize this academic idea if magnetic nanomaterials are only dispersed in the core of the coaxial nanofibers and rare earth luminescent compounds are merely dispersed in the shell of the coaxial nanofibers, and thus it is expected that the coaxial nanofibers exhibit excellent magnetic and fluorescent properties.

To the best of our knowledge, there have been no reports on the preparation of CoFe₂O₄@Y₂O₃:Eu³⁺ magnetic–fluorescent bifunctional coaxial nanofibers. Herein, we employ the electrospinning technique to prepare CoFe₂O₄@Y₂O₃:Eu³⁺ coaxial nanofibers with magnetic–photoluminescent bifunction. The morphology, structure, luminescence, magnetism and formation mechanism of the CoFe₂O₄@Y₂O₃:Eu³⁺ coaxial nanofibers were investigated in detail, and some meaningful results were obtained.

2 Experimental sections

2.1 Chemicals

Polyvinyl pyrrolidone (PVP, M_w = 130 0000) and N,N-dimethylformamide (DMF) were purchased from Tianjin Tiantai Fine Chemical Reagents Co. Ltd., HNO₃ and NaOH were bought from Beijing Chemical Company. Y₂O₃ (99.99 %), Eu₂O₃ (99.99 %), Fe(NO₃)₃·9H₂O and Co(NO₃)₂·6H₂O were bought from Sinopharm Chemical Reagent Co. Ltd. All chemicals were of analytical grade and directly used as received without further purification. Y(NO₃)₃·6H₂O and Eu(NO₃)₃·6H₂O were prepared by dissolving Y₂O₃ and Eu₂O₃ in HNO₃, followed by crystallizing from the solution through evaporating the water from the solution by heating.

2.2 Preparation of CoFe₂O₄ nanofibers and CoFe₂O₄/Y₂O₃:Eu³⁺ composite nanofibers via electrospinning

In a typical procedure of preparing spinning solution of CoFe₂O₄ nanofibers, 1 mmol of Fe(NO₃)₃·9H₂O, 0.5 mmol of Co(NO₃)₂·6H₂O and 2.2 g of PVP were dissolved into 15.8 g of DMF.

In order to perform the comparative experiment, CoFe₂O₄/Y₂O₃:Eu³⁺ composite nanofibers were also prepared. The spinning solution for preparing CoFe₂O₄/Y₂O₃:Eu³⁺ composite nanofibers was acquired as follows: 0.95 mmol of Y(NO₃)₃·6H₂O, 0.05 mmol of Eu(NO₃)₃·6H₂O, 1.5 g of PVP, 10 g of DMF and 1.5 g of CoFe₂O₄ nanofibers were mixed together.

These two spinning solutions were stirred for 24 h to form homogeneous mixture solutions for further electrospinning. The spinning solutions were injected into the traditional single axial electrospinning setup. Subsequently, the spinning solutions were electrospun under the high-voltage of 15 kV, and the distance between the spinneret and collector was fixed at 18–20 cm. The room temperature was 20–24 °C and the relative humidity was 30–40 %. In this way, the electrospun [(Co(NO₃)₂ + Fe(NO₃)₃)]/PVP composite nanofibers and CoFe₂O₄/[(Y(NO₃)₃ + Eu(NO₃)₃)]/PVP were fabricated on the collector. CoFe₂O₄ nanofibers and CoFe₂O₄/Y₂O₃:Eu³⁺ composite nanofibers can be obtained when the relevant composite nanofibers were annealed in air at 900 °C for 4 h and 700 °C for 4 h with the heating rate of 1 °C min⁻¹, respectively.

2.3 Fabrication of magnetic–luminescent coaxial nanofibers via coaxial electrospinning

In the preparation of core spinning solution, 1.3 g of CoFe₂O₄ nanofibers was added into 8.7 g of DMF, and dispersed ultrasonically for 20 min, then 1.3 g of PVP was dissolved into the above solution under stirring for 12 h.

A mixed solution of 0.95 mmol of Y(NO₃)₃·6H₂O, 0.05 mmol of Eu(NO₃)₃·6H₂O, 1.5 g of PVP and 8.5 g of DMF was prepared as the shell spinning solution with Eu³⁺ molar percentage of 5 %. Other series of CoFe₂O₄@Y₂O₃:x%Eu³⁺ [x = 1, 3, 7, x stands for molar ratio of Eu³⁺ to (Y³⁺+Eu³⁺)] coaxial nanofibers were also prepared by the similar procedure except for different percentage of rare earth nitrates in the spinning solution.

A homemade coaxial electrospinneret was used in this study. As shown in Fig. 1, the core spinning solution was injected into the inner plastic syringe while the shell spinning solution was loaded into the outer one. The spinning processing parameters were the same as they were in the above electrospinning process. [CoFe₂O₄/PVP]@[(Y(NO₃)₃ + Eu(NO₃)₃)/PVP] composite coaxial nanofibers were successfully prepared via the coaxial electrospinning. CoFe₂O₄@Y₂O₃:Eu³⁺ coaxial nanofibers were obtained after annealing the relevant composite coaxial nanofibers in air at 700 °C for 4 h with the heating rate of 1 °C min⁻¹.

Fig. 1

Schematic diagram of electrospinning setup

2.4 Fabrication of Y₂O₃:5 %Eu³⁺ powders by hydrothermal method

In order to compare the fluorescent properties of CoFe₂O₄@Y₂O₃:Eu³⁺ coaxial nanofibers and conventional fluorescent powders, Y₂O₃:5 %Eu³⁺ powders were synthesized according to the traditional method as described in the Ref. [12].

In the typical procedure of preparing the Y₂O₃:5 %Eu³⁺ powders, 0.95 mmol of Y₂O₃ and 0.05 mmol of Eu₂O₃ were dissolved in dilute HNO₃. After heating, a clear solution was obtained. The residual HNO₃ was removed by evaporation of the water together with the acid for several times and the final pH value was controlled to be around 5. The precipitate was formed by adding 10 wt % of NaOH aqueous solution to the above mixture with stirring at room temperature. The pH value of the solution was controlled to 8. The resulting suspensions were further stirred for 20 min at room temperature. Finally, the milky solutions were transferred to Teflon-lined autoclaves for hydrothermal treatment at 120 °C for 24 h. The resulting precipitates were first ground into powders, heated to 600 °C at a rate of 5 °C min⁻¹, and then maintained at that temperature for 2 h. The diameter of Y₂O₃:5 %Eu³⁺ powders is ca. 1–2 μm.

3 Characterization

The samples were identified by an X-ray powder diffractometer (XRD, Bruker D8 FOCUS) with Cu Kα radiation. The operation voltage and current were kept at 40 kV and 20 mA, respectively. The morphology and internal structure of samples were observed by a field emission scanning electron microscope (FESEM, XL-30) and a transmission electron microscope (TEM, JEM-2010), respectively. The fluorescent properties of samples were investigated by a Hitachi fluorescence spectrophotometer F-7000. Then, the magnetic performance of samples was measured by a vibrating sample magnetometer (VSM, MPMS SQUID XL). All measurements were performed at room temperature.

4 Results and discussion

4.1 Characterizations of the structure and morphology

The phase compositions of CoFe₂O₄ nanofibers, [CoFe₂O₄/PVP]@[(Y(NO₃)₃ + Eu(NO₃)₃)/PVP] composite coaxial nanofibers and CoFe₂O₄@Y₂O₃:5 %Eu³⁺ coaxial nanofibers are characterized by means of XRD analysis, as shown in Fig. 2. It can be seen that XRD patterns of the as-prepared CoFe₂O₄ nanofibers are readily indexed to those of the pure cubic spinel of CoFe₂O₄ (PDF#22-1086), and no characteristic diffraction peaks are observed for other impurities, as indicated in Fig. 2a. Figure 2b reveals that [CoFe₂O₄/PVP]@[(Y(NO₃)₃ + Eu(NO₃)₃)/PVP] composite coaxial nanofibers contain CoFe₂O₄ and the diffraction peak of the amorphous PVP (2θ = 22.2°) can also be observed. It is seen from Fig. 2c that XRD patterns of CoFe₂O₄@Y₂O₃:5 %Eu³⁺ coaxial nanofibers are conformed to the cubic phase with primitive structure of Y₂O₃ (PDF#73-1334) and the cubic spinel structure of CoFe₂O₄ (PDF#22-1086), indicating that CoFe₂O₄@Y₂O₃:5 %Eu³⁺ coaxial nanofibers are composed of crystalline CoFe₂O₄ and Y₂O₃:5 %Eu³⁺.

Fig. 2

XRD patterns of CoFe₂O₄ nanofibers (a), [CoFe₂O₄/PVP]@[(Y(NO₃)₃ + Eu(NO₃)₃)/PVP] composite coaxial nanofibers (b) and CoFe₂O₄@Y₂O₃:5 %Eu³⁺ coaxial nanofibers (c)

Figure 3 shows FESEM images of CoFe₂O₄ nanofibers, [CoFe₂O₄/PVP]@[(Y(NO₃)₃ + Eu(NO₃)₃)/PVP] composite coaxial nanofibers and CoFe₂O₄@Y₂O₃:5 %Eu³⁺ coaxial nanofibers, respectively. CoFe₂O₄ nanofibers have coarse surface, as shown in Fig. 3a. It is found from Fig. 3b that [CoFe₂O₄/PVP]@[(Y(NO₃)₃ + Eu(NO₃)₃)/PVP] composite coaxial nanofibers are extremely smooth. As seen from Fig. 3c, the diameter of CoFe₂O₄@Y₂O₃:5 %Eu³⁺ coaxial nanofibers is decreased owing to decomposition and volatilization of PVP and decomposition of rare earth nitrates, and the CoFe₂O₄@Y₂O₃:5 %Eu³⁺ coaxial nanofibers have relatively coarse surface. The diameters of CoFe₂O₄ nanofibers, [CoFe₂O₄/PVP]@[(Y(NO₃)₃ + Eu(NO₃)₃)/PVP] composite coaxial nanofibers and CoFe₂O₄@Y₂O₃:5 %Eu³⁺ coaxial nanofibers are 81 ± 11, 333 ± 46 and 133 ± 17 nm under the confidence level of 95 %, respectively, as demonstrated in Fig. 4.

Fig. 3

FESEM images of CoFe₂O₄ nanofibers (a), [CoFe₂O₄/PVP]@[(Y(NO₃)₃ + Eu(NO₃)₃)/PVP] composite coaxial nanofibers (b) and CoFe₂O₄@Y₂O₃:5 %Eu³⁺ coaxial nanofibers (c)

Fig. 4

Histograms of diameters distribution of CoFe₂O₄ nanofibers (a), [CoFe₂O₄/PVP]@[(Y(NO₃)₃ + Eu(NO₃)₃)/PVP] composite coaxial nanofibers (b) and CoFe₂O₄@Y₂O₃:5 %Eu³⁺ coaxial nanofibers (c)

The TEM images of [CoFe₂O₄/PVP]@[(Y(NO₃)₃ + Eu(NO₃)₃)/PVP] composite coaxial nanofibers and CoFe₂O₄@Y₂O₃:5 %Eu³⁺ coaxial nanofibers are presented in Fig. 5. As revealed in Fig. 5, an obvious coaxial structure can be seen in the [CoFe₂O₄/PVP]@[(Y(NO₃)₃ + Eu(NO₃)₃)/PVP] composite coaxial nanofibers and CoFe₂O₄@Y₂O₃:5 %Eu³⁺ coaxial nanofibers, and the CoFe₂O₄ nanofibers are clearly observed in the core of the coaxial nanofibers. The diameters of the [CoFe₂O₄/PVP]@[(Y(NO₃)₃ + Eu(NO₃)₃)/PVP] composite coaxial nanofibers and CoFe₂O₄@Y₂O₃:5 %Eu³⁺ coaxial nanofibers are 310–320 nm and 120–130 nm, respectively, which are in good agreement with those of FESEM analyses.

Fig. 5

TEM images of [CoFe₂O₄/PVP]@[(Y(NO₃)₃ + Eu(NO₃)₃)/PVP] composite coaxial nanofibers (a) and CoFe₂O₄@Y₂O₃:5 %Eu³⁺ coaxial nanofibers (b)

4.2 Fluorescent properties analysis

In order to obtain the optimum ratio of Eu³⁺ to (Eu³⁺+Y³⁺), a series of CoFe₂O₄@Y₂O₃:x%Eu³⁺ [x = 1, 3, 5, 7] coaxial nanofibers are fabricated by coaxial electrospinning process. The excitation and emission spectra of CoFe₂O₄@Y₂O₃:x%Eu³⁺ [x = 1, 3, 5, 7] coaxial nanofibers and CoFe₂O₄/Y₂O₃:5 %Eu³⁺ composite nanofibers are revealed in Figs. 6 and 7. Broad excitation bands extending from 200 nm to 275 nm are observed when monitoring wavelength is 612 nm in the samples. The peak at 236 nm assigned to the charge transfer band of O²⁻–Eu³⁺ can be also identified. Characteristic emission peaks of the Eu³⁺ observed in the samples are ascribed to the energy levels transitions of ⁵D₀ → ⁷F₁ (588 nm), ⁵D₀ → ⁷F₁ (593 nm), ⁵D₀ → ⁷F₁ (599 nm), ⁵D₀ → ⁷F₂ (612 nm) and ⁵D₀ → ⁷F₂ (630 nm), and the emission peak at 612 nm is the predominant emission peak. Obviously, the luminescence intensity of CoFe₂O₄@Y₂O₃:x %Eu³⁺ [x = 1, 3, 5, 7] coaxial nanofibers increases with the increase of the concentration of Eu³⁺ ions from the beginning, and reaches a maximum value with the Eu³⁺ concentration of 5 %, and then decreases with further increasing in Eu³⁺ concentration, indicating that the optimum doping molar concentration of Eu³⁺ ions is 5 %.

Fig. 6

Excitation spectra of CoFe₂O₄@Y₂O₃:x%Eu³⁺ [x = 1 (a), 3 (b), 5 (c), 7 (d)] coaxial nanofibers and CoFe₂O₄/Y₂O₃:5 %Eu³⁺ composite nanofibers (e)

Fig. 7

Emission spectra of CoFe₂O₄@Y₂O₃:x%Eu³⁺ [x = 1 (a), 3 (b), 5 (c), 7 (d)] coaxial nanofibers and CoFe₂O₄/Y₂O₃:5 %Eu³⁺ composite nanofibers (e)

To illustrate the advantages of the coaxial nanostructure of the magnetic–fluorescent bifunctional nanofibers, the CoFe₂O₄/Y₂O₃:5 %Eu³⁺ composite nanofibers, as a contrast sample, are also fabricated by mixing core spinning solution and shell spinning solution together at the mass ratio of 1:1 followed by electrospinning via the traditional single-nozzle electrospinning method. From the comparison between CoFe₂O₄@Y₂O₃:5 %Eu³⁺ coaxial nanofibers and CoFe₂O₄/Y₂O₃:5 %Eu³⁺ composite nanofibers, it is found that both excitation peaks intensity and emission peaks intensity of the CoFe₂O₄/Y₂O₃:5 %Eu³⁺ composite nanofibers are much more weaker than those of the CoFe₂O₄@Y₂O₃:5 %Eu³⁺ coaxial nanofibers, and this weak fluorescent emission intensity makes the CoFe₂O₄/Y₂O₃:5 %Eu³⁺ composite nanofibers impractical in fluorescent performance. From the UV–vis diffuse reflectance spectrum of CoFe₂O₄ nanofibers illustrated in Fig. 8, it is observed that the CoFe₂O₄ nanofibers can absorb visible light (400–700 nm) and much more strongly absorb ultraviolet light (<400 nm), the result is in good agreement with the references [29, 30]. Therefore, both the exciting light (236 nm) and emitting light (588–630 nm) can be absorbed by dark-colored CoFe₂O₄. As illustrated in Fig. 9, because the CoFe₂O₄ nanofibers are distributed over the whole parts of the CoFe₂O₄/Y₂O₃:5 %Eu³⁺ composite nanofibers, the exciting light has to pass through many CoFe₂O₄ nanofibers to reach the Y₂O₃:Eu³⁺ fluorescence center. In this process, a large part of the exciting light has been absorbed by the CoFe₂O₄ nanofibers, and thus the exciting light is much weakened before it reaches the Y₂O₃:Eu³⁺. Similarly, the emitting light emitted by the Y₂O₃:Eu³⁺ also has to pass through the CoFe₂O₄ nanofibers, which is severely weakened by the absorption of the CoFe₂O₄ nanofibers. Thus, the heavy loss in fluorescent emission intensity of CoFe₂O₄/Y₂O₃:5 %Eu³⁺ composite nanofibers results from the strong light absorption of the dark-colored CoFe₂O₄ to the exciting light and emitting light when CoFe₂O₄ and Y₂O₃:Eu³⁺ are blended together. As to the CoFe₂O₄@Y₂O₃:5 %Eu³⁺ coaxial nanofibers, the CoFe₂O₄ nanofibers only disperse in the core of the coaxial nanofibers so that the exciting light and emitting light almost will not be affected by the CoFe₂O₄ nanofibers. The discussion about the effect of magnetic compounds on luminescence performance in coaxial nanofibers and composite nanofibers has been reported in the Ref. [33], and our above research result is similar to the result reported this literature.

Fig. 8

UV–Vis diffuse reflectance spectrum of CoFe₂O₄ nanofibers

Fig. 9

Schematic diagrams of the situation of the exciting light and emitting light in CoFe₂O₄/Y₂O₃:Eu³⁺ composite nanofibers and CoFe₂O₄@Y₂O₃:Eu³⁺ coaxial nanofibers

For comparison, the emission spectra of the CoFe₂O₄@Y₂O₃:5 %Eu³⁺ coaxial nanofibers and Y₂O₃:5 %Eu³⁺ powders under the same excitation conditions (in order to avoid the experimental errors) are shown in Fig. 10a. It is known that the defects as quenching centers have severe drawback in luminescence intensity for nanomaterials because they provide nonradiative recombination centers for electrons and holes [11]. As seen from Fig. 10a, the CoFe₂O₄@Y₂O₃:5 %Eu³⁺ coaxial nanofibers have stronger emission intensity than that of Y₂O₃:5 %Eu³⁺ powders due to the fact that coaxial nanofibers have less surface defects than Y₂O₃:5 %Eu³⁺ powders.

Fig. 10

Emission spectra (a) and luminescence decay curves (b) of CoFe₂O₄@Y₂O₃:5 %Eu³⁺ coaxial nanofibers and Y₂O₃:5 %Eu³⁺ powders

The luminescent decay curves of CoFe₂O₄@Y₂O₃:5 %Eu³⁺ coaxial nanofibers and Y₂O₃:5 %Eu³⁺ powders, as shown in Fig. 10b, are used to calculate the lifetime and to investigate the luminescence dynamics of these samples. The samples are excited by 236 nm and monitored by 612 nm, and the curves can be well-fitted into a double-exponential function as I = A₁exp(−t/s₁) + A₂exp(−t/s₂), in which s₁ and s₂ are the fast and slow components of the luminescence lifetimes, A₁ and A₂ are the fitting parameters. The average lifetime values of CoFe₂O₄@Y₂O₃:5 %Eu³⁺ coaxial nanofibers and Y₂O₃:5 %Eu³⁺ powders are 1.51 and 1.47 ms, respectively.

According to the above results, the CoFe₂O₄@Y₂O₃:5 %Eu³⁺ coaxial nanofibers have stronger fluorescent intensity and longer luminescence lifetimes compared with the Y₂O₃:5 %Eu³⁺ powders.

4.3 CIE analysis

Generally, color is represented by the Commission Internationale de L’Eclairage (CIE) chromaticity coordinates and color ratios. The chromaticity coordinates and color ratios have been calculated from the emission spectra. The chromaticity coordinates of CoFe₂O₄@Y₂O₃:Eu³⁺ coaxial nanofibers and CoFe₂O₄/Y₂O₃:5 %Eu³⁺ composite nanofibers are indicated in Fig. 11. The coordinates (X, Y) of samples are (0.515, 0.317), (0.539, 0.331), (0.545, 0.342), (0.556, 0.356) and (0.476, 0.301), which correspond to CoFe₂O₄@Y₂O₃:x%Eu³⁺ [x = 1, 3, 5, 7] coaxial nanofibers and CoFe₂O₄/Y₂O₃:5 %Eu³⁺ composite nanofibers, respectively. Remarkably, the emission colors of samples are red owing to the strong emission intensity originating from ⁵D₀ → ⁷F₂ (612 nm) energy levels transitions of Eu³⁺ ions. According to the above results, it can be found that the emission colors of CoFe₂O₄@Y₂O₃:Eu³⁺ coaxial nanofibers can be tuned by adjusting the concentrations of Eu³⁺ ions, which are considered to be promising candidates for application in LEDs.

Fig. 11

CIE chromaticity coordinates diagram of CoFe₂O₄@Y₂O₃:x %Eu³⁺ [x = 1, 3, 5, 7] coaxial nanofibers and CoFe₂O₄/Y₂O₃:5 %Eu³⁺ composite nanofibers

4.4 Magnetic performance

The typical hysteresis loops for CoFe₂O₄@Y₂O₃:5 %Eu³⁺ coaxial nanofibers, [CoFe₂O₄/PVP]@[(Y(NO₃)₃ + Eu(NO₃)₃)/PVP] composite coaxial nanofibers and CoFe₂O₄/Y₂O₃:5 %Eu³⁺ composite nanofibers are shown in Fig. 12, and the saturation magnetizations of them are listed in Table 1. As shown in Table 1, the saturation magnetization of the CoFe₂O₄@Y₂O₃:5 %Eu³⁺ coaxial nanofibers, [CoFe₂O₄/PVP]@[(Y(NO₃)₃ + Eu(NO₃)₃)/PVP] composite coaxial nanofibers and CoFe₂O₄/Y₂O₃:5 %Eu³⁺ composite nanofibers are 16.1 , 14.6 and 11.4 emu g⁻¹, respectively.It can be seen that the saturation magnetization of the CoFe₂O₄@Y₂O₃:5 %Eu³⁺ coaxial nanofibers is higher than that of [CoFe₂O₄/PVP]@[(Y(NO₃)₃ + Eu(NO₃)₃)/PVP] composite coaxial nanofibers and CoFe₂O₄/Y₂O₃:5 %Eu³⁺ composite nanofibers. It is well known that the saturation magnetization of a magnetic composite material depends on the mass percentage of the magnetic substance in the magnetic composite material [27, 28], thus, the saturation magnetization of the coaxial nanofibers can be tuned via adjusting the content of magnetic compounds.

Fig. 12

Hysteresis loops of CoFe₂O₄@Y₂O₃:5 %Eu³⁺ coaxial nanofibers (a), [CoFe₂O₄/PVP]@[(Y(NO₃)₃ + Eu(NO₃)₃)/PVP] composite coaxial nanofibers (b) and CoFe₂O₄/Y₂O₃:5 %Eu³⁺ composite nanofibers (c)

Table 1 Saturation magnetization of samples

Combined with the fluorescent performance analyses, the CoFe₂O₄@Y₂O₃:5 %Eu³⁺ coaxial nanofibers have better fluorescent intensity and magnetic property compared with the CoFe₂O₄/Y₂O₃:5 %Eu³⁺ composite nanofibers. Based on the above experimental results, we can safely conclude that our academic ideas described in the introduction are successfully realized via coaxial electrospinning.

Compared with the magnetic–fluorescent bifunctional core–shell nanoparticles reported in the references [31, 32], CoFe₂O₄@Y₂O₃:Eu³⁺ coaxial nanofibers is a new kind of magnetic–fluorescent bifunctional nanomaterials with special morphology. It has obvious advantages owing to its length-diameter ratio, unique optical and magnetic properties. These excellent properties may make the coaxial nanofibers to possess the potential applications in the fields of medical diagnostics, drug target delivery and bioimaging [8, 33].

4.5 Formation mechanism for CoFe₂O₄@Y₂O₃:Eu³⁺ coaxial nanofibers

Formation mechanism for CoFe₂O₄@Y₂O₃:Eu³⁺ coaxial nanofibers is proposed on the basis of the above experimental results, as shown in Fig. 13. CoFe₂O₄ nanofibers are ultrasonically dispersed into DMF, then PVP is dissolved into the above solution to form core spinning solution. Y(NO₃)₃·6H₂O, Eu(NO₃)₃·6H₂O and PVP are mixed with DMF to form shell spinning solution with certain viscosity. [CoFe₂O₄/PVP]@[(Y(NO₃)₃ + Eu(NO₃)₃)/PVP] composite coaxial nanofibers are fabricated via coaxial electrospinning. PVP acted as template during the formation of [CoFe₂O₄/PVP]@[(Y(NO₃)₃ + Eu(NO₃)₃)/PVP] composite coaxial nanofibers. The core of the coaxial nanofibers only remains CoFe₂O₄ nanofibers owing to decomposition and volatilization of PVP during the calcination process. Simultaneously, the nitrates are decomposed and oxidized to NO₂, Y³⁺ and Eu³⁺ are oxidized to form Y₂O₃:Eu³⁺ nanocrystallite, many nanocrystallites are combined into nanoparticles, and these nanoparticles are mutually connected to generate Y₂O₃:Eu³⁺ shell of the coaxial nanofibers.

Fig. 13

Formation mechanism for CoFe₂O₄@Y₂O₃:Eu³⁺ coaxial nanofibers

5 Conclusions

In summary, uniform bifunctional magnetic–photoluminescent CoFe₂O₄@Y₂O₃:Eu³⁺ coaxial nanofibers are successfully prepared by the coaxial electrospinning. The diameter of the coaxial nanofibers is 133 ± 17 nm. Compared with the CoFe₂O₄/Y₂O₃:Eu³⁺ composite nanofibers, CoFe₂O₄@Y₂O₃:Eu³⁺ coaxial nanofibers simultaneously possess both higher fluorescent intensity and saturation magnetization. The color, photoluminescent intensity and magnetism of the coaxial nanofibers can be tuned via adjusting the diversity and content of fluorescent compounds and the content of magnetic compounds. Besides, other multifunctional nanomaterials such as electrical–magnetic, photoluminescent–electrical and photoluminescent–electrical–magnetic twi- or trifunctional nanostructures can be designed and fabricated based on this novel design conception and construct technology.