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Eu3+ ions composite nanofibers and nanoribbons
Materials Research Bulletin 47 (2012) 321–327
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Electrospinning preparation and photoluminescence properties of poly (methyl methacrylate)/Eu3+ ions composite nanofibers and nanoribbons Maoying Li a, Zhenyi Zhang a, Tieping Cao b, Yangyang Sun a, Pingping Liang a, Changlu Shao a,*, Yichun Liu a a Center for Advanced Optoelectronic Functional Materials Research, and Key Laboratory of UV Light-Emitting Materials and Technology of Ministry of Education, Northeast Normal University, 5268 Renmin Street, Changchun 130024, People’s Republic of China b Department of Chemistry, Baicheng Normal College, Baicheng, Jilin 137000, People’s Republic of China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 1 July 2011 Received in revised form 12 September 2011 Accepted 9 November 2011 Available online 19 November 2011
Nanofibers and nanoribbons of poly (methyl methacrylate) (PMMA)/Eu3+ ions composites with different concentration of Eu3+ ions were successfully prepared by using a simple electrospinning technique. From the results of scanning electron microscopy and energy-dispersive X-ray spectroscopy, we found that the morphology of the as-electrospun PMMA/Eu3+ ions composites could be changed from fiber to ribbon structure by adjusting the concentration of Eu3+ ions in the electrospun precursor solution. The coordination between the Eu3+ ions and PMMA molecules were investigated by Fourier transform infrared spectroscopy and differential thermal analysis. The photoluminescence (PL) properties of the as-electrospun PMMA/Eu3+ ions composites were studied in comparison to those of the Eu(NO3)3 powder. It was showed that the 5D0–7FJ (J = 0, 1, 2, 3, 4) emission appeared in the PL spectra of the aselectrospun PMMA/Eu3+ ions composites, whereas the 5D0–7F0 emission was completely absent in the PL spectra of Eu(NO3)3 powder due to the different local environments surrounding Eu3+ ions. It was interesting to note that the intensity ratios of the electric–dipole and magnetic–dipole transitions for the PMMA/Eu3+ ions composites could be enhanced significantly by increasing electrospinning voltage. ß 2011 Elsevier Ltd. All rights reserved.
Keywords: A. Composites A. Polymers B. Sol–gel chemistry D. Optical properties
1. Introduction With the steady and fast growing field of nanoscience and nanotechnology, one-dimensional (1D) nanomaterials have been the subject of intensive research due to their unique properties and potential technology applications. They are expected to play an important role as both interconnects and functional components in the fabrication of nanoscale electronic and optoelectronic devices [1,2]. Among the 1D nanomaterials, 1D organic/inorganic composite nanomaterials have been of immense interest in recent years, because they could sustain the advantages of mixing the organic nanomaterials (flexibility, processability, and light weight) and the inorganic nanomaterials (hardness, heat, and chemical resistance, and optical function). To our best knowledge, electrospinning is the most simple and effective technique that allows the fabrication of continuous 1D organic polymer-inorganic composite fibers. By controlling electrospinning parameters, some interesting structures could be obtained, such as beaded fibers, porous fibers, helical structures, ribbon structures and others [3–5]. This method
* Corresponding author. Tel.: +86 43185098803. E-mail address: [email protected] (C. Shao). 0025-5408/$ – see front matter ß 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2011.11.029
provides a means to bridge the dimensional and property gap between nano- and macroscale engineering materials and structures. By using this facile method, many kinds of polymer/ inorganic luminescent materials composite nanofibers with optoelectronic properties have been prepared, such as polymer (poly (vinyl pyrrolidone) (PVA) [6], poly (ethylene oxide) (PEO) [7], poly (vinyl alcohol) (PVP)/ZnO quantum dots [8], PVP/Ln3+ doped NaYF4 [9], PMMA/Alq3 [10], and PVP/CdS composite nanofibers [11]. Among the inorganic luminescent materials, rare earth (RE) ions exhibit a high yield of luminescence in the visible light region. In recent years, research and development of the luminescent material using these RE ions have attracted extensive attention of optoelectronic engineers [12–21]. Among the RE ions, the trivalent europium (Eu3+) ion is a well-known dopant for many different compounds, producing red emission because of its excellent luminescent properties coming from 4f–4f transition of Eu3+ ions and the ‘‘antenna effect’’ of ligands [22]. Moreover, the Eu3+ ions as a hard acid have strong preference for negatively charged atom. The polymer coordinates with Eu3+ ions which can change the microenvironment of the coordination sphere [23]. Thus, nowadays the synthesis and luminescence properties of 1D polymer/ europium complexes composite materials and polymer/Eu3+ ions
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composite materials have attracted considerable interest [24–26]. This kind of 1D nanostructural composite materials might be very useful to fabrication of nanoscale electronic and optoelectronic devices. In our present work, we reported a successful attempt to fabricate the PMMA/Eu3+ ions composite nanofibers and nanoribbons by using a simple electrospinning technique. From the characterization on the as-prepared samples, it was found that the fiber structure could shift to the ribbon structure by enhancing the concentration of Eu3+ ions in the precursor solution. The photoluminescence (PL) properties of the as-electrospun PMMA/ Eu3+ ions composites were studied in comparison to those of the Eu(NO3)3 powder. Furthermore, the composite nanofibers exhibited the larger intensity ratios of the electric–dipole to magnetic– dipole transitions at higher electrospinning voltages. 2. Experimental 2.1. Preparation of the PMMA/Eu3+ ions electrospun composites Firstly, 1 g of poly (methyl methacrylate) (PMMA) (Mn = 350,000) powders were dissolved in 9 mL of N,N-dimethylformamide (DMF) solution. After stirring at about 40 8C for 6 h, the 3.0 wt.%, 5.0 wt.%, 8.0 wt.%, 10.0 wt.% Eu(NO3)3 powder relative to PMMA was added to the above solution. Then, the above composite solutions were stirred at room temperature for 12 h. Thus, the precursor solutions were obtained. Subsequently, the above composite precursor solution was drawn into a hypodermic syringe for electrospinning under the voltage of 12 kV. The obtained PMMA/Eu3+ ions composites with different concentration of Eu3+ ions were collected at a distance about 15 cm to the syringe tip for the following characterizations. To further investigate the PL properties of PMMA/Eu3+ ions composites, the 5.0 wt.% PMMA/ Eu3+ ions electrospun composites were prepared by electrospinning under different voltage (10, 12, 14 kV). In the following discussion, the 3.0 wt.%, 5.0 wt.%, 8.0 wt.%, and 10.0 wt.% PMMA/ Eu3+ ions electrospun composites were denoted as EuP1, EuP2, EuP3, and EuP4 composites, respectively. 2.2. Characterization Scanning electron microscopy (SEM; XL-30 ESEM FEG, Micro FEI Philips) and fluorescence microscopy (Nikon, TE2000-U, Japan) was used to characterize the morphologies of the products. Fourier transform infrared (FT-IR) spectra were obtained on Magna 560 FTIR spectrometer with a resolution of 1 cm1. Differential thermal analysis (DTA) analysis was carried out on a NETZSCH STA 449C thermoanalyzer in N2 atmosphere. Photoluminescence (PL) spectra were collected with a Jobin-Yvon HR800 micro-Raman spectrometer using the 488 nm line of a He–Cd laser as the excitation source. 3. Results and discussion Fig. 1A–H showed the SEM images of the as-electrospun products with the different concentration of Eu3+ ions. As observed in Fig. 1A, it was found that these ultra-long EuP1 composite nanofibers were oriented randomly and had a smooth and uniform surface due to the amorphous nature of the PMMA/Eu(NO3)3 composite nanofibers. Meanwhile, Fig. 1E displayed the corresponding SEM images of the EuP1 composite nanofibers with higher magnification. It was showed that the diameters of these composite nanofibers ranged from 350 to 400 nm. Fig. 1B and F exhibited the SEM images of the EuP2 composite nanofibers with low and high magnification, respectively. Comparing with the EuP1 composite nanofibers, the structure and morphology of the
EuP2 composite nanofibers had little changed. However, as the concentration of Eu3+ ions increased, the morphology of the aselectrospun products was changed from the fiber to the ribbon structure. In Fig. 1C and G, these long continuous EuP3 composite nanoribbons with the width of 1–2 mm and thickness of 150– 250 nm had a smooth surface. Moreover, Fig. 1D and H showed the SEM images of the EuP4 composite nanoribbons with the different magnification. It was observed that the width of the EuP4 composite nanoribbons was about 0.5 mm smaller than that of EuP3 composite nanoribbons, while the thickness of EuP4 composite nanoribbons was about 10–30 nm smaller than that of EuP3 composite nanoribbons. Fig. 1I–L was the energydispersive X-ray (EDX) spectrum from Fig. 1A–D, respectively, which further confirmed that the as-electrospun products were composed of C, O, and Eu element. In addition, EDX analysis indicated that the concentration of Eu3+ ions were increased from the as-electrospun EuP1 to EuP4 composites, which was in agree with our experiment. As the matter of fact, many factors, such as the solution viscosity, surface tension, flow rate, applied voltage, and so on, could affect the morphology of the electrospun nanofibers. Fridrikh et al. proposed the existence of a terminal diameter for the polymer electrospun fibers [27]. And, the equation for the terminal jet radius (ht) could be depicted as follow: " ht ¼ g e¯
Q2 I2
#1=3 2 pð2 ln x 3Þ
where g was the surface tension, e¯ the dielectric permittivity of air, Q the liquid jet flow rate, I the total current, and x (x R/h, where R was the radius of curvature and h was the jet diameter) was the dimensionless wavelength of the instability responsible for the normal displacements. Thus, the terminal jet diameter depended directly on the flow rate, electric current, and surface tension of the fluid. In our present work, the flow rate and electric current hold constant. The variable was only the concentration of Eu3+ ions in the precursor solution. In the images of the EuP1 and EuP2 composite nanofibers (Fig. 1A and B), the diameters of these nanofibers had little changed. It was implied that the low concentration of Eu3+ ions in the precursor solution could hardly affect the surface tension of the fluid in our experiment. Interestingly, as the concentration of Eu3+ ions further increased, the morphology of as-electrospun products changed from the fiber to ribbon structure. Previously, many reach reported ribbonshaped fibers when electrospinning polymer solutions. However, there was no special model and theoretical study to explaining the formation mechanism of electrospun nanoribbons. Kang et al. hypothesized that the solvent and inorganic salt might play an important role for preparing the electrospun ribbons [28]. The inorganic salt could increase the charge density, resulting in enhancing the Coulombic repulsion forces. Fong et al. predicted that the ribbon formation was attributed to collapsing of hollow nanofibers [29]. And the hollow nanofibers might have formed because of rapid solvent evaporation. In our experiment, the high concentration of Eu3+ ions in the precursor solution could increase the Coulombic repulsion forces, which might lead to an increase in tensile strength during the electrospinning. On the other hand, a skin might be formed on the surface of electrospun fibers during the electrospinning process due to the rapid evaporation of DMF and indissoluble PMMA in DMF at room temperature, which reduced subsequent solvent evaporation. This skin might cause the formation of hollow tubes, which subsequently collapse to form ribbons. Thus, we believed that the Coulombic repulsion forces and rapid evaporation of solvent on the surface of electrospun fibers during the electrospinning process played a leading role on the formation of PMMA/Eu3+ ions composite nanoribbons in our work. Furthermore, Yu et al. found that the width and thickness of TiO2
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Fig. 1. SEM images of PMMA/Eu3+ ions electrospun composites with the low (A–D) and high (E–H) magnification: (A and E) EuP1 composite nanofibers; (B and F) EuP2 composite nanofibers; (C and G) EuP3 composite nanoribbons; (D and H) EuP4 composite nanoribbons; and EDX spectrum of the as-electrospun composites: (I) EuP1 composite nanofibers; (J) EuP2 composite nanofibers; (K) EuP3 composite nanoribbons; (L) EuP4 composite nanoribbons.
ribbons decreased with increasing the electrospinning voltage [30]. Some researchers reported that a higher voltage led to enhanced stretching of the solution due to the greater columbic force in the jet as well as the stronger electricfield [31]. In our experiment, the high concentration of Eu3+ ions in the precursor solution could increase the Coulombic repulsion forces, resulting in decreasing the width and the thickness of nanoribbons. The FT-IR spectra of the Eu(NO3)3 powder, PMMA nanofibers, and PMMA/Eu3+ ions electrospun composites with different concentration of Eu3+ ions were shown in Fig. 2. From the Fig. 2A, it was observed that the PMMA nanofibers exhibited strong peaks at around 1726 cm1, which was attributed to the C5 5O
stretching vibration. And, the peak centered at around 3000 cm1 was assigned to C–H stretching vibrations. As in the FT-IR spectra of PMMA/Eu3+ ions composites with different concentration of Eu3+ ions, besides the characteristic vibration bands of PMMA, a new intense peak at around 1640 cm1 (from Eu(NO3)3) was appeared, indicating that the as-electrospun composites were composed of PMMA and Eu(NO3)3. It was reported that the type of interaction between the ions and the carbonyl group might cause a shift or broaden in FT-IR peaks, because the coordination between ions and carbonyl oxygen [32,33]. To further investigation of the interaction between PMMA and Eu3+ ions in our experiment, the full width at half-maximum (FWHM) of the peaks at around
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Fig. 2. FT-IR spectra of the Eu(NO3)3 powder, PMMA electrospun nanofibers, and PMMA/Eu3+ ions electrospun composites: (a) PMMA nanofibers; (b) EuP1 composite nanofibers; (c) EuP2 composite nanofibers; (d) EuP3 composite nanoribbons; (e) EuP4 composite nanoribbons; (f) Eu(NO3)3 powder.
1726 cm1 for the as-electrsopun composites was given in Fig. 2B. Compared with the PMMA nanofibers, the FWHM of carbonyl group stretching vibration for the PMMA/Eu3+ ions composites was broadened. Moreover, the FWHM was broadened with increasing the concentration of Eu3+ ions in the as-electrospun composites. It was suggested that some interaction between PMMA molecules and Eu3+ ions existed through the coordination of Eu3+ ions to the carbonyl group of PMMA (See Scheme 1(A)). Furthermore, Scheme 1(B) showed the fluorescence microscopy images of EuP2 composite nanofibers. It was demonstrated that PMMA/Eu3+ ions composites were luminous under a fluorescence excitation. The thermal behavior was another effective way to study the interaction between the polymer composites, including the polymer/polymer, polymer/organic materials, and polymer/inorganic materials composites. Thus, to further confirm the interaction between the PMMA molecule and Eu3+ ions, the differential thermal analysis (DTA) of PMMA nanofibers and PMMA/Eu3+ composites with different concentration of Eu3+ ions were
investigated in Fig. 3. As observed in Fig. 3a, the DTA curve of the PMMA nanofibers showed a obvious decalescence valley at around 360 8C However, compared with the PMMA nanofibers, the decalescence valley of PMMA/Eu3+ composites shifted slightly toward a lower temperature with increasing the concentration of Eu3+ ions. Furthermore, a new exothermic peak was observed at around 470–520 8C in the PMMA/Eu3+ composites, indicating that the Eu3+ ions were successfully doped into the PMMA electrospun composites. It was worthwhile to note that the exothermic peak of the PMMA/Eu3+ composites shifted toward a higher temperature gradually with increasing the contents of Eu3+ ions. The shift of the decalescence valley and exothermic peak in the curves of the PMMA/Eu3+ composites demonstrated the existence of interactions between the PMMA molecule and Eu3+ ions. These interactions could be responsible for especial luminescent properties of the composites. The photoluminescence (PL) peaks from 5D0 to 7FJ (J = 0–4) transitions of Eu3+ ions could be observed under 488 nm light
Scheme 1. (A) Schematic diagram of the coordination interaction between the Eu3+ ions and the carbonyl group of PMMA. (B) The fluorescence microscopy image of EuP2 composite nanofibers.
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Fig. 3. DTA curves of the PMMA nanofibers and PMMA/Eu3+ ions electrospun composites: (a) PMMA nanofibers; (b) EuP1 composite nanofibers; (c) EuP2 composite nanofibers; (d) EuP3 composite nanoribbons; (e) EuP4 composite nanoribbons.
excitation, because this photon energy resonantly excited the 7 F2–5D2 transition of Eu3+ ions. Thus, the PL properties of PMMA/ Eu3+ ions composites with different concentration of Eu3+ ions were studied and compared with those of the Eu(NO3)3 powder under an excitation wavelength of lexc = 488 nm at room temperature. As we known, under 488 nm light excitation, the electrons could transfer from 5D2 state to 5D0 state by the nonradiative process of excited Eu3+ ions, later the electrons population on 5D0 state transfered to 7FJ (J = 0–4) state of Eu3+ ions [34]. The five emission peaks at around 581 nm, 592 nm, 614 nm, 649 nm and 678 nm could be observed in the PL spectrum of Eu3+ ions, which were attributed to the 5D0–7F0, 5D0–7F1, 5D0–7F2, 5 D0–7F3 and 5D0–7F4 transitions, respectively [35]. The 5D0–7F0 transition was basically forbidden due to the same J = 0 value. The 5 D0–7F2 emission was from an electric–dipole transition, which resulted in a large transition probability in the crystal field with inversion antisymmetry [34]. The 5D0–7F1 lines originated from a magnetic–dipole transition. When the 5D0–7F1 transition band was stronger than the 5D0–7F2 transition band, it could be induced that the Eu3+ ions occupied a site with inversion symmetry. Fig. 4 showed the normalized PL spectra of the Eu(NO3)3 powder and the as-electrospun composites. As observed in Fig. 4, the emission peaks from 5D0 to 7F1, 5D0 to 7F2, 5D0 to 7F3 and 5D0 to 7F4 transitions appeared for the Eu(NO3)3 powder, while the emission peak from 5D0 to 7F0 transition was completely absent. However, for the PMMA/Eu3+ ions composites, the five emission peaks from Eu3+ ions could be found. It was implied that the 5D0–7F0 transition become partly allowed in the PMMA/Eu3+ ions composites due to the distorted crystal field [33]. The relative intensity of the 5D0–7F3 transition emission peaks increased slightly with increasing the content of Eu3+ ions in the composites. Furthermore, the 5D0–7F3 transition emission peak of PMMA/Eu3+ ions composites exhibited a little red shift in contrast to that of Eu(NO3)3 powder. Those results indicated that the PMMA matrix structure evidently influenced the spectrum form. To investigate the local environments surrounding Eu3+ ions, the high-resolution emission spectra of 5D0–7F2 transitions for the various products were presented in Fig. 5. As observed in Fig. 5, two crystal-field splitting peaks of the 5D0–7F2 transition for Eu(NO3)3
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Fig. 4. The normalized photoluminescence (PL) spectra of the Eu(NO3)3 powder and the as-electrospun composites: (a) Eu(NO3)3; (b) EuP1 composite nanofibers; (c) EuP2 composite nanofibers; (d) EuP3 composite nanoribbons; (e) EuP4 composite nanoribbons.
powder could be found at 613.9 and 617.3 nm. For the electrospun composites, because of more disordered local environments surrounding Eu3+ ions from the interaction between PMMA molecules and Eu3+ ions, two crystal-field splitting peaks of the
Fig. 5. The high-resolution emission spectra of 5D0–7F2 transitions for the Eu(NO3)3 powder and the as-electrospun composites: (a) Eu(NO3)3 powder; (b) EuP1 composite nanofibers; (c) EuP2 composite nanofibers; (d) EuP3 composite nanoribbons; (e) EuP4 composite nanoribbons.
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Fig. 6. The photoluminescence (PL) spectra (ranged from 585 to 635 nm) of aselectrospun composites: (a) EuP1 composite nanofibers; (b) EuP2 composite nanofibers; (c) EuP3 composite nanoribbons; (d) EuP4 composite nanoribbons. The inserts was the intensity of 5D0–7F2 transition and the intensity ratios of 5D0–7F2 to 5 D0–7F1 transition for above composites versus the concentration of Eu3+ ions. 5
D0–7F2 transition were formed a broad band. Meanwhile, the main bands of 5D0–7F2 transition for Eu3+ in the composites exhibited a little red shift in contrast to that in the Eu(NO3)3 powder. Moreover, compared with the Eu(NO3)3 powder, the
emission bands of the electrospun composites became more broader, because many different kinds of Eu3+ ions site occupation would resulted with respect to the crystalline phase like Eu(NO3)3. The spectral broadening and shift showed that the relative contribution of different Stark components of the 5D0–7F2 transition was different [33,36]. To further understanding the PL properties of the as-electrospun composites, the PL spectra (ranged from 585 to 635 nm) of PMMA/Eu3+ ions composites with different concentration of Eu3+ ions were investigated. In our work, average data were used according to the results of the repeating measurements to increase the reliability. As observed in Fig. 6, two emission bands at around 592 and 617 nm could be found obviously. The emission band at 592 nm was from 5D0 to 7F1 transition (the magnetic–dipole transitions), while the emission band at 617 nm was the 5D0–7F2 transition (the electric–dipole transition). By comparing the PL spectra of the various PMMA/Eu3+ ions composites, it was found that the emission intensity increased obviously with increasing the concentration of Eu3+ ions in the PMMA/Eu3+ ions composites at the beginning (From EuP1 to EuP3). However, when the concentration of Eu3+ ions reached 10.0 wt.% (EuP4), the emission intensity evidently decreased, which might be attributed to the quenching phenomenon from the high concentration of Eu3+ ions [37,38]. In the inset of Fig. 6, it could be clearly seen that the emission intensity of 5D0–7F2 transition increased from EuP1 to EuP3, but decreased in EuP4. Furthermore, it had been reported that the intensity ratios of the electric–dipole and magnetic–dipole
Fig. 7. The SEM images of the 5.0 wt.% PMMA/Eu3+ ions composite nanofibers with different electrospinning voltage: (A) 10 kV; (B) 12 kV; (C) 14 kV. (D) The photoluminescence (PL) spectra (ranged from 585 to 635 nm) of as-electrospun composites with the electrospinning voltage of 10–14 kV. (E) The intensity of 5D0–7F2 transition and the intensity ratios of 5D0–7F2 to 5D0–7F1 transition for above composites versus the electrospinning voltage.
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transitions were hypersensitive to the symmetry of local environment surrounding the Eu3+ ions. Thus, the intensity ratio as a function of the Eu3+ ions concentration was given in the inset of Fig. 6. It was showed that the intensity ratios of the 5D0–7F2 to the 5D0–7F1 transition in those PMMA/Eu3+ ions composites were determined to be 1.65 for EuP1 composite, 2.43 for EuP2 composite, 4.49 for EuP3 composite, and 4.09 for EuP4 composite. The result suggested that the symmetry of the Eu3+ ions in the PMMA matrix decreased clearly from the EuP1 to EuP3 composites, but increased slightly in the EuP4 composites. It could be deduced that the EuP3 composites with the lowest symmetry that might be attributed to the enhanced coordination between the Eu3+ ions and the carbonyl group of PMMA by increasing of the concentration of Eu3+ ions in the electrospun composite. All the results suggested that the coordination of Eu3+ ions to the carbonyl group of PMMA led to the Eu3+ ions in a highly polarizable chemical environment [23,26]. In previous work, many research found that the electrospinning voltage was an important role to determine the distribution of inorganic nanomaterials and the spread of polymer chain in the electrospun polymer nanofibers, resulting in affecting the PL properties of these composite nanofibers [39]. Fig. 7A–C showed the SEM images of 5.0 wt.% PMMA/Eu3+ ions composite nanofibers with different electrospinning voltage. It could be seen that the diameters of these nanofibers had little change and ranged from 350 to 400 nm. However, as observed in Fig. 7D, the PL intensity of these nanofibers was significantly increased by increasing the electrospinning voltage from 10 to 14 kV. The result indicated that, with the increase of the electrospinning voltages, the Eu3+ ions got more dispersive in the composite nanofibers, while the PMMA chains got more spread, resulting in the stronger coordination between the Eu3+ ions and the carbonyl group of PMMA (see Scheme 2). In Fig. 7E, the intensity ratios of 5D0–7F2 to 5D0–7F1 transition were also increased by increasing the electrospinning voltage. It was suggested that the symmetry of the Eu3+ ions in the PMMA matrix was decreased slightly by increasing the electrospinning voltage due to the enhanced interaction between the PMMA and Eu3+ ions. 4. Summary
a simple electrospinning technique. The morphology of the aselectrospun PMMA/Eu3+ ions composites could be changed from the fiber to ribbon structure by adjusting the concentration of Eu3+ ions in the electrospun precursor solution. The coordination between the Eu3+ ions and PMMA molecules were investigated by Fourier transform infrared spectroscopy (FT-IR) and differential thermal analysis (DTA). The photoluminescence properties of aselectrospun PMMA/Eu3+ ions composites were studied in comparison to those of the Eu(NO3)3 powder. It was interesting to note that the intensity ratios of electric–dipole and magnetic–dipole transitions for the PMMA/Eu3+ ions composites could be enhanced significantly by increasing the electrospinning voltage. Acknowledgments The present work is supported financially by the National Natural Science Foundation of China (Nos. 50572014, 50972027, and 10647108) and the Program for New Century Excellent Talents in University (NCET-05-0322). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
In our present work, the nanofibers and nanoribbons of poly (methyl methacrylate) (PMMA)/Eu3+ ions composites with different concentration of Eu3+ ions were successfully prepared by using
[18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37]
3+
3+
Scheme 2. The scheme of the distribution of Eu ions in the PMMA/Eu composites nanofibers under the electrospinning process.
ions
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[38] [39]
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Electrospinning preparation and photoluminescence properties of poly (methyl methacrylate)/Eu3+ ions composite nanofibers and nanoribbons Maoying Li a, Zhenyi Zhang a, Tieping Cao b, Yangyang Sun a, Pingping Liang a, Changlu Shao a,*, Yichun Liu a a Center for Advanced Optoelectronic Functional Materials Research, and Key Laboratory of UV Light-Emitting Materials and Technology of Ministry of Education, Northeast Normal University, 5268 Renmin Street, Changchun 130024, People’s Republic of China b Department of Chemistry, Baicheng Normal College, Baicheng, Jilin 137000, People’s Republic of China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 1 July 2011 Received in revised form 12 September 2011 Accepted 9 November 2011 Available online 19 November 2011
Nanofibers and nanoribbons of poly (methyl methacrylate) (PMMA)/Eu3+ ions composites with different concentration of Eu3+ ions were successfully prepared by using a simple electrospinning technique. From the results of scanning electron microscopy and energy-dispersive X-ray spectroscopy, we found that the morphology of the as-electrospun PMMA/Eu3+ ions composites could be changed from fiber to ribbon structure by adjusting the concentration of Eu3+ ions in the electrospun precursor solution. The coordination between the Eu3+ ions and PMMA molecules were investigated by Fourier transform infrared spectroscopy and differential thermal analysis. The photoluminescence (PL) properties of the as-electrospun PMMA/Eu3+ ions composites were studied in comparison to those of the Eu(NO3)3 powder. It was showed that the 5D0–7FJ (J = 0, 1, 2, 3, 4) emission appeared in the PL spectra of the aselectrospun PMMA/Eu3+ ions composites, whereas the 5D0–7F0 emission was completely absent in the PL spectra of Eu(NO3)3 powder due to the different local environments surrounding Eu3+ ions. It was interesting to note that the intensity ratios of the electric–dipole and magnetic–dipole transitions for the PMMA/Eu3+ ions composites could be enhanced significantly by increasing electrospinning voltage. ß 2011 Elsevier Ltd. All rights reserved.
Keywords: A. Composites A. Polymers B. Sol–gel chemistry D. Optical properties
1. Introduction With the steady and fast growing field of nanoscience and nanotechnology, one-dimensional (1D) nanomaterials have been the subject of intensive research due to their unique properties and potential technology applications. They are expected to play an important role as both interconnects and functional components in the fabrication of nanoscale electronic and optoelectronic devices [1,2]. Among the 1D nanomaterials, 1D organic/inorganic composite nanomaterials have been of immense interest in recent years, because they could sustain the advantages of mixing the organic nanomaterials (flexibility, processability, and light weight) and the inorganic nanomaterials (hardness, heat, and chemical resistance, and optical function). To our best knowledge, electrospinning is the most simple and effective technique that allows the fabrication of continuous 1D organic polymer-inorganic composite fibers. By controlling electrospinning parameters, some interesting structures could be obtained, such as beaded fibers, porous fibers, helical structures, ribbon structures and others [3–5]. This method
* Corresponding author. Tel.: +86 43185098803. E-mail address: [email protected] (C. Shao). 0025-5408/$ – see front matter ß 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2011.11.029
provides a means to bridge the dimensional and property gap between nano- and macroscale engineering materials and structures. By using this facile method, many kinds of polymer/ inorganic luminescent materials composite nanofibers with optoelectronic properties have been prepared, such as polymer (poly (vinyl pyrrolidone) (PVA) [6], poly (ethylene oxide) (PEO) [7], poly (vinyl alcohol) (PVP)/ZnO quantum dots [8], PVP/Ln3+ doped NaYF4 [9], PMMA/Alq3 [10], and PVP/CdS composite nanofibers [11]. Among the inorganic luminescent materials, rare earth (RE) ions exhibit a high yield of luminescence in the visible light region. In recent years, research and development of the luminescent material using these RE ions have attracted extensive attention of optoelectronic engineers [12–21]. Among the RE ions, the trivalent europium (Eu3+) ion is a well-known dopant for many different compounds, producing red emission because of its excellent luminescent properties coming from 4f–4f transition of Eu3+ ions and the ‘‘antenna effect’’ of ligands [22]. Moreover, the Eu3+ ions as a hard acid have strong preference for negatively charged atom. The polymer coordinates with Eu3+ ions which can change the microenvironment of the coordination sphere [23]. Thus, nowadays the synthesis and luminescence properties of 1D polymer/ europium complexes composite materials and polymer/Eu3+ ions
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composite materials have attracted considerable interest [24–26]. This kind of 1D nanostructural composite materials might be very useful to fabrication of nanoscale electronic and optoelectronic devices. In our present work, we reported a successful attempt to fabricate the PMMA/Eu3+ ions composite nanofibers and nanoribbons by using a simple electrospinning technique. From the characterization on the as-prepared samples, it was found that the fiber structure could shift to the ribbon structure by enhancing the concentration of Eu3+ ions in the precursor solution. The photoluminescence (PL) properties of the as-electrospun PMMA/ Eu3+ ions composites were studied in comparison to those of the Eu(NO3)3 powder. Furthermore, the composite nanofibers exhibited the larger intensity ratios of the electric–dipole to magnetic– dipole transitions at higher electrospinning voltages. 2. Experimental 2.1. Preparation of the PMMA/Eu3+ ions electrospun composites Firstly, 1 g of poly (methyl methacrylate) (PMMA) (Mn = 350,000) powders were dissolved in 9 mL of N,N-dimethylformamide (DMF) solution. After stirring at about 40 8C for 6 h, the 3.0 wt.%, 5.0 wt.%, 8.0 wt.%, 10.0 wt.% Eu(NO3)3 powder relative to PMMA was added to the above solution. Then, the above composite solutions were stirred at room temperature for 12 h. Thus, the precursor solutions were obtained. Subsequently, the above composite precursor solution was drawn into a hypodermic syringe for electrospinning under the voltage of 12 kV. The obtained PMMA/Eu3+ ions composites with different concentration of Eu3+ ions were collected at a distance about 15 cm to the syringe tip for the following characterizations. To further investigate the PL properties of PMMA/Eu3+ ions composites, the 5.0 wt.% PMMA/ Eu3+ ions electrospun composites were prepared by electrospinning under different voltage (10, 12, 14 kV). In the following discussion, the 3.0 wt.%, 5.0 wt.%, 8.0 wt.%, and 10.0 wt.% PMMA/ Eu3+ ions electrospun composites were denoted as EuP1, EuP2, EuP3, and EuP4 composites, respectively. 2.2. Characterization Scanning electron microscopy (SEM; XL-30 ESEM FEG, Micro FEI Philips) and fluorescence microscopy (Nikon, TE2000-U, Japan) was used to characterize the morphologies of the products. Fourier transform infrared (FT-IR) spectra were obtained on Magna 560 FTIR spectrometer with a resolution of 1 cm1. Differential thermal analysis (DTA) analysis was carried out on a NETZSCH STA 449C thermoanalyzer in N2 atmosphere. Photoluminescence (PL) spectra were collected with a Jobin-Yvon HR800 micro-Raman spectrometer using the 488 nm line of a He–Cd laser as the excitation source. 3. Results and discussion Fig. 1A–H showed the SEM images of the as-electrospun products with the different concentration of Eu3+ ions. As observed in Fig. 1A, it was found that these ultra-long EuP1 composite nanofibers were oriented randomly and had a smooth and uniform surface due to the amorphous nature of the PMMA/Eu(NO3)3 composite nanofibers. Meanwhile, Fig. 1E displayed the corresponding SEM images of the EuP1 composite nanofibers with higher magnification. It was showed that the diameters of these composite nanofibers ranged from 350 to 400 nm. Fig. 1B and F exhibited the SEM images of the EuP2 composite nanofibers with low and high magnification, respectively. Comparing with the EuP1 composite nanofibers, the structure and morphology of the
EuP2 composite nanofibers had little changed. However, as the concentration of Eu3+ ions increased, the morphology of the aselectrospun products was changed from the fiber to the ribbon structure. In Fig. 1C and G, these long continuous EuP3 composite nanoribbons with the width of 1–2 mm and thickness of 150– 250 nm had a smooth surface. Moreover, Fig. 1D and H showed the SEM images of the EuP4 composite nanoribbons with the different magnification. It was observed that the width of the EuP4 composite nanoribbons was about 0.5 mm smaller than that of EuP3 composite nanoribbons, while the thickness of EuP4 composite nanoribbons was about 10–30 nm smaller than that of EuP3 composite nanoribbons. Fig. 1I–L was the energydispersive X-ray (EDX) spectrum from Fig. 1A–D, respectively, which further confirmed that the as-electrospun products were composed of C, O, and Eu element. In addition, EDX analysis indicated that the concentration of Eu3+ ions were increased from the as-electrospun EuP1 to EuP4 composites, which was in agree with our experiment. As the matter of fact, many factors, such as the solution viscosity, surface tension, flow rate, applied voltage, and so on, could affect the morphology of the electrospun nanofibers. Fridrikh et al. proposed the existence of a terminal diameter for the polymer electrospun fibers [27]. And, the equation for the terminal jet radius (ht) could be depicted as follow: " ht ¼ g e¯
Q2 I2
#1=3 2 pð2 ln x 3Þ
where g was the surface tension, e¯ the dielectric permittivity of air, Q the liquid jet flow rate, I the total current, and x (x R/h, where R was the radius of curvature and h was the jet diameter) was the dimensionless wavelength of the instability responsible for the normal displacements. Thus, the terminal jet diameter depended directly on the flow rate, electric current, and surface tension of the fluid. In our present work, the flow rate and electric current hold constant. The variable was only the concentration of Eu3+ ions in the precursor solution. In the images of the EuP1 and EuP2 composite nanofibers (Fig. 1A and B), the diameters of these nanofibers had little changed. It was implied that the low concentration of Eu3+ ions in the precursor solution could hardly affect the surface tension of the fluid in our experiment. Interestingly, as the concentration of Eu3+ ions further increased, the morphology of as-electrospun products changed from the fiber to ribbon structure. Previously, many reach reported ribbonshaped fibers when electrospinning polymer solutions. However, there was no special model and theoretical study to explaining the formation mechanism of electrospun nanoribbons. Kang et al. hypothesized that the solvent and inorganic salt might play an important role for preparing the electrospun ribbons [28]. The inorganic salt could increase the charge density, resulting in enhancing the Coulombic repulsion forces. Fong et al. predicted that the ribbon formation was attributed to collapsing of hollow nanofibers [29]. And the hollow nanofibers might have formed because of rapid solvent evaporation. In our experiment, the high concentration of Eu3+ ions in the precursor solution could increase the Coulombic repulsion forces, which might lead to an increase in tensile strength during the electrospinning. On the other hand, a skin might be formed on the surface of electrospun fibers during the electrospinning process due to the rapid evaporation of DMF and indissoluble PMMA in DMF at room temperature, which reduced subsequent solvent evaporation. This skin might cause the formation of hollow tubes, which subsequently collapse to form ribbons. Thus, we believed that the Coulombic repulsion forces and rapid evaporation of solvent on the surface of electrospun fibers during the electrospinning process played a leading role on the formation of PMMA/Eu3+ ions composite nanoribbons in our work. Furthermore, Yu et al. found that the width and thickness of TiO2
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Fig. 1. SEM images of PMMA/Eu3+ ions electrospun composites with the low (A–D) and high (E–H) magnification: (A and E) EuP1 composite nanofibers; (B and F) EuP2 composite nanofibers; (C and G) EuP3 composite nanoribbons; (D and H) EuP4 composite nanoribbons; and EDX spectrum of the as-electrospun composites: (I) EuP1 composite nanofibers; (J) EuP2 composite nanofibers; (K) EuP3 composite nanoribbons; (L) EuP4 composite nanoribbons.
ribbons decreased with increasing the electrospinning voltage [30]. Some researchers reported that a higher voltage led to enhanced stretching of the solution due to the greater columbic force in the jet as well as the stronger electricfield [31]. In our experiment, the high concentration of Eu3+ ions in the precursor solution could increase the Coulombic repulsion forces, resulting in decreasing the width and the thickness of nanoribbons. The FT-IR spectra of the Eu(NO3)3 powder, PMMA nanofibers, and PMMA/Eu3+ ions electrospun composites with different concentration of Eu3+ ions were shown in Fig. 2. From the Fig. 2A, it was observed that the PMMA nanofibers exhibited strong peaks at around 1726 cm1, which was attributed to the C5 5O
stretching vibration. And, the peak centered at around 3000 cm1 was assigned to C–H stretching vibrations. As in the FT-IR spectra of PMMA/Eu3+ ions composites with different concentration of Eu3+ ions, besides the characteristic vibration bands of PMMA, a new intense peak at around 1640 cm1 (from Eu(NO3)3) was appeared, indicating that the as-electrospun composites were composed of PMMA and Eu(NO3)3. It was reported that the type of interaction between the ions and the carbonyl group might cause a shift or broaden in FT-IR peaks, because the coordination between ions and carbonyl oxygen [32,33]. To further investigation of the interaction between PMMA and Eu3+ ions in our experiment, the full width at half-maximum (FWHM) of the peaks at around
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Fig. 2. FT-IR spectra of the Eu(NO3)3 powder, PMMA electrospun nanofibers, and PMMA/Eu3+ ions electrospun composites: (a) PMMA nanofibers; (b) EuP1 composite nanofibers; (c) EuP2 composite nanofibers; (d) EuP3 composite nanoribbons; (e) EuP4 composite nanoribbons; (f) Eu(NO3)3 powder.
1726 cm1 for the as-electrsopun composites was given in Fig. 2B. Compared with the PMMA nanofibers, the FWHM of carbonyl group stretching vibration for the PMMA/Eu3+ ions composites was broadened. Moreover, the FWHM was broadened with increasing the concentration of Eu3+ ions in the as-electrospun composites. It was suggested that some interaction between PMMA molecules and Eu3+ ions existed through the coordination of Eu3+ ions to the carbonyl group of PMMA (See Scheme 1(A)). Furthermore, Scheme 1(B) showed the fluorescence microscopy images of EuP2 composite nanofibers. It was demonstrated that PMMA/Eu3+ ions composites were luminous under a fluorescence excitation. The thermal behavior was another effective way to study the interaction between the polymer composites, including the polymer/polymer, polymer/organic materials, and polymer/inorganic materials composites. Thus, to further confirm the interaction between the PMMA molecule and Eu3+ ions, the differential thermal analysis (DTA) of PMMA nanofibers and PMMA/Eu3+ composites with different concentration of Eu3+ ions were
investigated in Fig. 3. As observed in Fig. 3a, the DTA curve of the PMMA nanofibers showed a obvious decalescence valley at around 360 8C However, compared with the PMMA nanofibers, the decalescence valley of PMMA/Eu3+ composites shifted slightly toward a lower temperature with increasing the concentration of Eu3+ ions. Furthermore, a new exothermic peak was observed at around 470–520 8C in the PMMA/Eu3+ composites, indicating that the Eu3+ ions were successfully doped into the PMMA electrospun composites. It was worthwhile to note that the exothermic peak of the PMMA/Eu3+ composites shifted toward a higher temperature gradually with increasing the contents of Eu3+ ions. The shift of the decalescence valley and exothermic peak in the curves of the PMMA/Eu3+ composites demonstrated the existence of interactions between the PMMA molecule and Eu3+ ions. These interactions could be responsible for especial luminescent properties of the composites. The photoluminescence (PL) peaks from 5D0 to 7FJ (J = 0–4) transitions of Eu3+ ions could be observed under 488 nm light
Scheme 1. (A) Schematic diagram of the coordination interaction between the Eu3+ ions and the carbonyl group of PMMA. (B) The fluorescence microscopy image of EuP2 composite nanofibers.
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Fig. 3. DTA curves of the PMMA nanofibers and PMMA/Eu3+ ions electrospun composites: (a) PMMA nanofibers; (b) EuP1 composite nanofibers; (c) EuP2 composite nanofibers; (d) EuP3 composite nanoribbons; (e) EuP4 composite nanoribbons.
excitation, because this photon energy resonantly excited the 7 F2–5D2 transition of Eu3+ ions. Thus, the PL properties of PMMA/ Eu3+ ions composites with different concentration of Eu3+ ions were studied and compared with those of the Eu(NO3)3 powder under an excitation wavelength of lexc = 488 nm at room temperature. As we known, under 488 nm light excitation, the electrons could transfer from 5D2 state to 5D0 state by the nonradiative process of excited Eu3+ ions, later the electrons population on 5D0 state transfered to 7FJ (J = 0–4) state of Eu3+ ions [34]. The five emission peaks at around 581 nm, 592 nm, 614 nm, 649 nm and 678 nm could be observed in the PL spectrum of Eu3+ ions, which were attributed to the 5D0–7F0, 5D0–7F1, 5D0–7F2, 5 D0–7F3 and 5D0–7F4 transitions, respectively [35]. The 5D0–7F0 transition was basically forbidden due to the same J = 0 value. The 5 D0–7F2 emission was from an electric–dipole transition, which resulted in a large transition probability in the crystal field with inversion antisymmetry [34]. The 5D0–7F1 lines originated from a magnetic–dipole transition. When the 5D0–7F1 transition band was stronger than the 5D0–7F2 transition band, it could be induced that the Eu3+ ions occupied a site with inversion symmetry. Fig. 4 showed the normalized PL spectra of the Eu(NO3)3 powder and the as-electrospun composites. As observed in Fig. 4, the emission peaks from 5D0 to 7F1, 5D0 to 7F2, 5D0 to 7F3 and 5D0 to 7F4 transitions appeared for the Eu(NO3)3 powder, while the emission peak from 5D0 to 7F0 transition was completely absent. However, for the PMMA/Eu3+ ions composites, the five emission peaks from Eu3+ ions could be found. It was implied that the 5D0–7F0 transition become partly allowed in the PMMA/Eu3+ ions composites due to the distorted crystal field [33]. The relative intensity of the 5D0–7F3 transition emission peaks increased slightly with increasing the content of Eu3+ ions in the composites. Furthermore, the 5D0–7F3 transition emission peak of PMMA/Eu3+ ions composites exhibited a little red shift in contrast to that of Eu(NO3)3 powder. Those results indicated that the PMMA matrix structure evidently influenced the spectrum form. To investigate the local environments surrounding Eu3+ ions, the high-resolution emission spectra of 5D0–7F2 transitions for the various products were presented in Fig. 5. As observed in Fig. 5, two crystal-field splitting peaks of the 5D0–7F2 transition for Eu(NO3)3
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Fig. 4. The normalized photoluminescence (PL) spectra of the Eu(NO3)3 powder and the as-electrospun composites: (a) Eu(NO3)3; (b) EuP1 composite nanofibers; (c) EuP2 composite nanofibers; (d) EuP3 composite nanoribbons; (e) EuP4 composite nanoribbons.
powder could be found at 613.9 and 617.3 nm. For the electrospun composites, because of more disordered local environments surrounding Eu3+ ions from the interaction between PMMA molecules and Eu3+ ions, two crystal-field splitting peaks of the
Fig. 5. The high-resolution emission spectra of 5D0–7F2 transitions for the Eu(NO3)3 powder and the as-electrospun composites: (a) Eu(NO3)3 powder; (b) EuP1 composite nanofibers; (c) EuP2 composite nanofibers; (d) EuP3 composite nanoribbons; (e) EuP4 composite nanoribbons.
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Fig. 6. The photoluminescence (PL) spectra (ranged from 585 to 635 nm) of aselectrospun composites: (a) EuP1 composite nanofibers; (b) EuP2 composite nanofibers; (c) EuP3 composite nanoribbons; (d) EuP4 composite nanoribbons. The inserts was the intensity of 5D0–7F2 transition and the intensity ratios of 5D0–7F2 to 5 D0–7F1 transition for above composites versus the concentration of Eu3+ ions. 5
D0–7F2 transition were formed a broad band. Meanwhile, the main bands of 5D0–7F2 transition for Eu3+ in the composites exhibited a little red shift in contrast to that in the Eu(NO3)3 powder. Moreover, compared with the Eu(NO3)3 powder, the
emission bands of the electrospun composites became more broader, because many different kinds of Eu3+ ions site occupation would resulted with respect to the crystalline phase like Eu(NO3)3. The spectral broadening and shift showed that the relative contribution of different Stark components of the 5D0–7F2 transition was different [33,36]. To further understanding the PL properties of the as-electrospun composites, the PL spectra (ranged from 585 to 635 nm) of PMMA/Eu3+ ions composites with different concentration of Eu3+ ions were investigated. In our work, average data were used according to the results of the repeating measurements to increase the reliability. As observed in Fig. 6, two emission bands at around 592 and 617 nm could be found obviously. The emission band at 592 nm was from 5D0 to 7F1 transition (the magnetic–dipole transitions), while the emission band at 617 nm was the 5D0–7F2 transition (the electric–dipole transition). By comparing the PL spectra of the various PMMA/Eu3+ ions composites, it was found that the emission intensity increased obviously with increasing the concentration of Eu3+ ions in the PMMA/Eu3+ ions composites at the beginning (From EuP1 to EuP3). However, when the concentration of Eu3+ ions reached 10.0 wt.% (EuP4), the emission intensity evidently decreased, which might be attributed to the quenching phenomenon from the high concentration of Eu3+ ions [37,38]. In the inset of Fig. 6, it could be clearly seen that the emission intensity of 5D0–7F2 transition increased from EuP1 to EuP3, but decreased in EuP4. Furthermore, it had been reported that the intensity ratios of the electric–dipole and magnetic–dipole
Fig. 7. The SEM images of the 5.0 wt.% PMMA/Eu3+ ions composite nanofibers with different electrospinning voltage: (A) 10 kV; (B) 12 kV; (C) 14 kV. (D) The photoluminescence (PL) spectra (ranged from 585 to 635 nm) of as-electrospun composites with the electrospinning voltage of 10–14 kV. (E) The intensity of 5D0–7F2 transition and the intensity ratios of 5D0–7F2 to 5D0–7F1 transition for above composites versus the electrospinning voltage.
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transitions were hypersensitive to the symmetry of local environment surrounding the Eu3+ ions. Thus, the intensity ratio as a function of the Eu3+ ions concentration was given in the inset of Fig. 6. It was showed that the intensity ratios of the 5D0–7F2 to the 5D0–7F1 transition in those PMMA/Eu3+ ions composites were determined to be 1.65 for EuP1 composite, 2.43 for EuP2 composite, 4.49 for EuP3 composite, and 4.09 for EuP4 composite. The result suggested that the symmetry of the Eu3+ ions in the PMMA matrix decreased clearly from the EuP1 to EuP3 composites, but increased slightly in the EuP4 composites. It could be deduced that the EuP3 composites with the lowest symmetry that might be attributed to the enhanced coordination between the Eu3+ ions and the carbonyl group of PMMA by increasing of the concentration of Eu3+ ions in the electrospun composite. All the results suggested that the coordination of Eu3+ ions to the carbonyl group of PMMA led to the Eu3+ ions in a highly polarizable chemical environment [23,26]. In previous work, many research found that the electrospinning voltage was an important role to determine the distribution of inorganic nanomaterials and the spread of polymer chain in the electrospun polymer nanofibers, resulting in affecting the PL properties of these composite nanofibers [39]. Fig. 7A–C showed the SEM images of 5.0 wt.% PMMA/Eu3+ ions composite nanofibers with different electrospinning voltage. It could be seen that the diameters of these nanofibers had little change and ranged from 350 to 400 nm. However, as observed in Fig. 7D, the PL intensity of these nanofibers was significantly increased by increasing the electrospinning voltage from 10 to 14 kV. The result indicated that, with the increase of the electrospinning voltages, the Eu3+ ions got more dispersive in the composite nanofibers, while the PMMA chains got more spread, resulting in the stronger coordination between the Eu3+ ions and the carbonyl group of PMMA (see Scheme 2). In Fig. 7E, the intensity ratios of 5D0–7F2 to 5D0–7F1 transition were also increased by increasing the electrospinning voltage. It was suggested that the symmetry of the Eu3+ ions in the PMMA matrix was decreased slightly by increasing the electrospinning voltage due to the enhanced interaction between the PMMA and Eu3+ ions. 4. Summary
a simple electrospinning technique. The morphology of the aselectrospun PMMA/Eu3+ ions composites could be changed from the fiber to ribbon structure by adjusting the concentration of Eu3+ ions in the electrospun precursor solution. The coordination between the Eu3+ ions and PMMA molecules were investigated by Fourier transform infrared spectroscopy (FT-IR) and differential thermal analysis (DTA). The photoluminescence properties of aselectrospun PMMA/Eu3+ ions composites were studied in comparison to those of the Eu(NO3)3 powder. It was interesting to note that the intensity ratios of electric–dipole and magnetic–dipole transitions for the PMMA/Eu3+ ions composites could be enhanced significantly by increasing the electrospinning voltage. Acknowledgments The present work is supported financially by the National Natural Science Foundation of China (Nos. 50572014, 50972027, and 10647108) and the Program for New Century Excellent Talents in University (NCET-05-0322). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
In our present work, the nanofibers and nanoribbons of poly (methyl methacrylate) (PMMA)/Eu3+ ions composites with different concentration of Eu3+ ions were successfully prepared by using
[18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37]
3+
3+
Scheme 2. The scheme of the distribution of Eu ions in the PMMA/Eu composites nanofibers under the electrospinning process.
ions
327
[38] [39]
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