Flexible hollow nanofibers: Novel one-pot electrospinning construction, structure and tunable luminescence–electricity–magnetism trifunctionality

Chemical Engineering Journal 284 (2016) 831–840

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Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej

Flexible hollow nanofibers: Novel one-pot electrospinning construction, structure and tunable luminescence–electricity–magnetism trifunctionality Yawen Liu, Qianli Ma, Ming Yang, Xiangting Dong ⇑, Ying Yang, Jinxian Wang, Wensheng Yu ⇑, Guixia Liu Key Laboratory of Applied Chemistry and Nanotechnology at Universities of Jilin Province, Changchun University of Science and Technology, Changchun 130022, China

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 A novel strategy and technique were

used to directly construct hollow nanofibers.  Hollow nanofibers were fabricated via one-pot coaxial electrospinning technology.  Photoluminescence, electricity and magnetism were assembled into hollow nanofibers.  Photoluminescence, electricity and magnetism of the hollow nanofibers can be tuned.  Design conception and construction strategy are of universal significance.

a r t i c l e

i n f o

Article history: Received 1 July 2015 Received in revised form 1 September 2015 Accepted 9 September 2015 Available online 14 September 2015 Keywords: Hollow nanofibers Luminescence Electrical conduction Magnetism Electrospinning

a b s t r a c t Novel photoluminescent–electrical–magnetic trifunctional flexible Tb(BA)3phen/PANI/Fe3O4/PVP (BA = benzoic acid, phen = phenanthroline, PANI = polyaniline, PVP = polyvinylpyrrolidone) hollow nanofibers were constructed by one-pot coaxial electrospinning process using coaxial spinneret. Very different from the traditional preparation process of hollow fibers via coaxial electrospinning, which need to firstly fabricate the coaxial fibers and followed by removing the core through high-temperature calcination or solvent extraction, in our present study, no core spinning solution is used to directly fabricate hollow nanofibers. The morphology and properties of the hollow nanofibers were characterized in detail by X-ray diffractometry, scanning electron microscopy, transmission electron microscope, N2 adsorption–desorption measurement system, fluorescence spectroscopy, 4-point probes resistivity measurement system and vibrating sample magnetometry. The Tb(BA)3phen/PANI/Fe3O4/PVP hollow nanofibers, with outer diameters of ca. 238 nm and inner diameters of ca. 80 nm, exhibit excellent photoluminescent performance, electrical conductivity and magnetic properties. Fluorescence emission peaks of Tb3+ ions are observed in Tb(BA)3phen/PANI/Fe3O4/PVP hollow nanofibers and assigned to the 5 D4 ? 7F6 (490 nm), 5D4 ? 7F5 (545 nm), 5D4 ? 7F4 (585 nm) and 5D4 ? 7F3 (621 nm) energy level transitions of Tb3+, and the 5D4 ? 5F5 hypersensitive transition at 545 nm is the predominant emission peak. The electrical conductivity of the hollow nanofibers reaches up to the order of 10 3 S cm 1. The luminescent intensity, electrical conductivity and magnetic properties of the hollow nanofibers can be tuned by adding various amounts of Tb(BA)3phen, PANI and Fe3O4 nanoparticles. The new-typed photolu minescent–electrical–magnetic trifunctional flexible hollow nanofibers hold potential for a variety of applications, including electromagnetic interference shielding, microwave absorption, molecular

⇑ Corresponding authors. Tel.: +86 0431 85582574; fax: +86 0431 85383815. E-mail addresses: [email protected] (X. Dong), [email protected] (W. Yu). http://dx.doi.org/10.1016/j.cej.2015.09.030 1385-8947/Ó 2015 Elsevier B.V. All rights reserved.

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electronics and biomedicine. This design conception and synthetic strategy developed in this study are of universal significance to construct more advanced multifunctional hollow one-dimensional nanomaterials. Ó 2015 Elsevier B.V. All rights reserved.

1. Introduction More recently, with the rapid development of materials science, much emphasis has been put on hollow micro/nanomaterials owing to their promising applications, such as catalysis [1], drug release [2], sensoring [3], energy storage [4] and biomedical engineering [5]. One-dimensional (1D) hollow nanomaterials, including nanotubes and hollow nanofibers, have attracted increasing interests during past decades because of their unique structure, high surface area and novel properties. Compared with common solid nanofibers [6], hollow nanofibers double the surface area and exhibit potential in surface-related applications such as chemical sensors [7] or photocatalysis [8,9]. Nanocomposites are the hot spots of research in the field of functional materials. In the past few years, a great deal of work has been done to study nanocomposites with multifunctionality, which greatly expanded the application of nanocomposites in various fields [10–18]. For instance, rare earth (RE)-doped luminescent materials are usually integrated with magnetic nanoparticles (NPs) to prepare magnetic–photoluminescent bifunctional nanocomposites [19–25]. To date, except the abovementioned zero-dimensional nanocomposites, multifunctional nanocomposites with 1D structure have been extensively pursued by scientists [26,27]. Ma et al. [28–30] fabricated Fe3O4/RE com plexes/polyvinylpyrrolidone (PVP) magnetic–photoluminescent bifunctional core–shell nanocables and nanobelts via electrospinning process. Wang et al. [31] prepared Fe2O3/[Eu(DBM)3(Bath)] (DBM = dibenzoylmethanate, Bath = bathophenanthroline)/PVP m agnetic–photoluminescent bifunctional composite nanofibers. Polyaniline (PANI) is one of the most important conducting polymers due to its high conductivity, good processibility and environmental stability, as well as its potential for a variety of applications [32–41]. Huang et al. [42] synthesized Fe3O4/PANI composite nanofibers via micro-emulsion polymerization of aniline in the presence of Fe3O4 NPs. Lun et al. [43] fabricated [Tb(BA)3phen + Eu(BA)3phen]/PANI/PVP (BA = benzoic acid, phen = phenanthroline) tunable color–electricity bifunctional composite nanofibers through facile one-pot electrospinning process. Electrospinning has been proved to be an outstanding technique to process viscous solutions or melts into continuous fibers with 1D nanostructure [44–46]. This method not only attracts extensive academic investigations, but is also applied in many areas such as filtration [47], luminescent materials [48,49], biological scaffolds [50], electrode materials [51] and nanocables [52]. Recent efforts have been made to improve electrospinning method and setup, and nanofibers with novel structures, such as coresheath, Janus, porous and hollow morphology, can be prepared if appropriate processing parameters are adopted [53–58]. In many cases, nanofibers with hollow structure were prepared by coaxial electrospinning technique, in which coaxial spinnerets replace the single spinneret in the conventional setup for electrospinning, followed by appropriate post-treatment, such as core solvent extracting or calcination process. Lee et al. [59] developed highly porous polymeric hollow nanofibers using a method based on coaxial electrospinning with inner silicon oil and outer polymer solutions, and then the silicon oil was extracted by soaking the fibers in n-hexane overnight. Li et al. [60] successfully synthesized YF3:Eu3+ hollow nanofibers via fluorination of the relevant Y2O3:

Eu3+ hollow nanofibers which were obtained by calcining the coaxial electrospun core–sheath composite nanofibers. Nevertheless, the core-removing method usually affects not only the core but also the shell, and do not fit to all the situations. Therefore, it is necessary to explore more universal methods to prepare hollow structured nanofibers. Herein, we employ one-pot coaxial electrospinning technique by using specially designed spinnerets to directly assemble photo luminescence–electricity–magnetism trifunction into flexible hollow nanofibers. We discovered a peculiar phenomenon using coaxial electrospinning apparatus that the hollow structured nanofibers can be directly obtained without using core spinning solution. Based on this method, Tb(BA)3phen/PANI/Fe3O4/PVP hollow nanofibers have been successfully fabricated via the one-pot coaxial electrospinning. To the best of our knowledge, up to now there is no report on the synthesis of novel hollow structured nanofibers possessing trifunctionality of photoluminescence, electricity and magnetism. More importantly, the synthetic strategy developed in this work can be extended to fabricate other hollow nanofibrous polymer-matrix materials, making the strategy widely applicable for preparing advanced multifunctional materials. The hollow structure of this kind of nanofiber makes it a potential material for applications of drug targeting, bioseparation and other fields. For example, various drugs can be encapsulated into the inner of the hollow nanofibers and control the release of drug molecules, thus realizing drug target delivering and slow releasing. 2. Experimental sections 2.1. Materials PVP K90 (Mw  130,0000), Tb4O7, BA, phen, FeCl3
6H2O, FeSO4
7H2O, NH4NO3, polyethyleneglycol (PEG, Mw  20,000), ammonia, oleic acid (OA), aniline (ANI), (IS)-(+)-camphor-10 sulfonic acid (CSA), ammonium persulfate (APS), anhydrous ethanol, CHCl3, N, N-dimethylformamide (DMF), nitric acid and deionized water were used. All the reagents were of analytical grade and used directly as received without further purification. The purity of Tb4O7 was 99.99%. The deionized water was homemade. 2.2. Synthesis of terbium complexes Tb(BA)3phen powders were synthesized according to the traditional method as described in Ref. [61]. 1.8692 g of Tb4O7 was dissolved in 10 mL of concentrated nitric acid and then crystallized by evaporation of excess nitric acid and water at the temperature of 120 °C, and Tb(NO3)3
6H2O was acquired. Tb(NO3)3 ethanol solution was prepared by adding 10 mL of anhydrous ethanol into the above Tb(NO3)3
6H2O. 3.6636 g of BA and 1.9822 g of phen were dissolved in 100 mL of ethanol. The Tb(NO3)3 solution was then added into the mixture solution of BA and phen with magnetic agitation for 3 h at 60 °C. Finally, the precipitate was collected by filtration and dried at 60 °C for 12 h. 2.3. Preparation of OA modified Fe3O4 NPs Fe3O4 NPs with the particle size of 8–10 nm were obtained via a facile coprecipitation synthetic method [62]. To improve the

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2.4. Preparation of spinning solutions for fabricating hollow nanofibers

Table 1 Compositions of the spinning solutions. Spinning solutions

Compositions ANI/g

CSA/g

APS/g

Tb(BA)3 phen/g

Fe3O4/g

PVP/g

1 2 3 4 5 6 7 8 9 10

0.1800 0.1800 0.1800 0.1800 0.1200 0.2400 0.3000 0.1800 0.1800 0.1800

0.2245 0.2245 0.2245 0.2245 0.1497 0.2993 0.3742 0.2245 0.2245 0.2245

0.4411 0.4411 0.4411 0.4411 0.2940 0.5881 0.7351 0.4411 0.4411 0.4411

0.7200 0.9000 1.0800 1.2600 1.0800 1.0800 1.0800 1.0800 1.0800 1.0800

0.6000 0.6000 0.6000 0.6000 0.6000 0.6000 0.6000 0.1800 0.3000 1.2000

0.6000 0.6000 0.6000 0.6000 0.6000 0.6000 0.6000 0.6000 0.6000 0.6000

monodispersity, stability, and solubility of Fe3O4 NPs in the spinning solution, PEG was used as the protective agents to prevent the particles from aggregation, and the as-prepared Fe3O4 NPs were coated with OA. A typical synthetic procedure is as follows: 5.4060 g of FeCl3
6H2O, 2.7800 g of FeSO4
7H2O, 4.04 g of NH4NO3 and 1.9 g of PEG were added into 100 mL of deionized water to form a uniform solution under vigorous stirring at 50 °C. At the same time, the reactive mixture was kept under argon atmosphere to prevent the oxidation of Fe2+. After the mixture had been bubbled with argon for 30 min, 0.1 mol L 1 of NH3
H2O was dropwise added into the mixture to adjust pH value above 11. Then the system was continuously bubbled with argon for 20 min at 50 °C, and black precipitates were formed. Per 1.0000 g of the as-prepared Fe3O4 NPs were ultrasonically dispersed in 50 mL of deionized water for 20 min. The suspension was subsequently heated to 80 °C under argon atmosphere with vigorous mechanical stirring for 30 min and then 0.5 mL of OA was slowly added. The reaction was stopped after heating and stirring the mixture for 40 min. The precipitates were collected from the solution by magnetic separation, washed with anhydrous ethanol for three times, and then dried in an electric vacuum oven at 60 °C for 6 h. The dosages of deionized water and OA were adjusted proportionately based on the used amount of Fe3O4 NPs.

In the preparation of spinning solutions, CSA was dissolved in 2.5000 g of DMF with magnetic stirring at room temperature, then ANI and 1.0000 g of CHCl3 were slowly added into the above solution, and PVP was added into the mixture with stirring as a viscosity improver and template for electrospinning process. The pre-blended solution was then cooled down to 0 °C in an ice-bath. APS was acted as an oxidant and dispersed into 2.0000 g of DMF at 0 °C and then slowly added into the above solution with magnetic stirring. The final mixture was allowed to react for 24 h at 0 °C and PANI was obtained by the polymerization of aniline [63]. Finally, a certain amount of the as-prepared Fe3O4 NPs were ultrasonically dispersed into 2.0000 g of CHCl3 for 20 min and added into the PANI solution. Then Tb(BA)3phen powders were also dissolved into the above emulsion under magnetic stirring for 12 h at room temperature, thus the spinning solution was obtained. In order to find the optimum ratio of Tb(BA)3phen and PANI, different spinning solutions containing various amounts of Tb(BA)3phen and PANI were respectively prepared, followed by introducing various amounts of APS and CSA. Besides, to investigate the impact of Fe3O4 NPs on the magnetic and fluorescent properties of the hollow nanofibers, various amounts of Fe3O4 NPs were introduced into spinning solutions. The dosages of these materials were listed in Table 1, and the products produced by spinning solutions 1–10 were denoted as S1–S10, respectively.

2.5. Fabrication of Tb(BA)3phen/PANI/Fe3O4/PVP flexible hollow nanofibers via one-pot coaxial electrospinning A homemade setup for coaxial electrospinning was used in this study as depicted in Fig. 1. The as-prepared spinning solution was loaded into the outer plastic syringe, while the inner plastic syringe was empty but the air. The coaxial needle was composed of a truncated 8 # stainless steel needle as the inner one and a truncated 16 # stainless steel needle as the outer one, and the tip of the inner needle protruded 0.5 mm out of the outer needle. The flow rate of the spinning solution was measured to be 0.0167 mL min 1. A flat iron net was put about 14 cm away from the tip of the coaxial needle

Fig. 1. Schematic illustration of the setup for one-pot coaxial electrospinning process.

Y. Liu et al. / Chemical Engineering Journal 284 (2016) 831–840

as a fiber collector. A positive direct current (DC) voltage of 13 kV was applied between the needle and the collector to generate stable, continuous PVP-based photoluminescent–electrical–magnetic trifunctional flexible hollow nanofibers at the room temperature of 15–20 °C, and the relative humidity was 20–30%. As seen from the detail of Taylor cone, the air inside guarantees the formation of hollow structure with fast evaporation of solvent through the electrospinning process. 2.6. Characterization methods The phase compositions of samples were identified by an X-ray powder diffractometer (XRD, Bruker, D8 FOCUS) with Cu Ka radiation. The operation voltage and current were kept at 40 kV and 20 mA, respectively. The morphology and internal structure of the as-prepared hollow nanofibers were observed by a fieldemission scanning electron microscope (FESEM, XL-30) and a transmission electron microscope (TEM, JEM-2010), respectively. The elemental analysis was performed by an energy-dispersive X-ray spectrometer (EDX, Oxford Instruments) attached to the FESEM. The Brunauer–Emmett–Teller (BET) surface area of the sample was analyzed by using a Micromeritics ASAP 2020 nitrogen adsorption apparatus. Before nitrogen adsorption measurement, the sample was degassed in a vacuum at 200 °C for 2 h. Then, the fluorescent performances of Tb(BA)3phen/PANI/Fe3O4/PVP hollow nanofibers were investigated by Hitachi fluorescence spectrophotometer F-7000. The UV–Vis absorption spectra were measured by a UV–Vis spectrophotometer (SHIMADZU UV mini 1240). The electrical conductivities of samples were determined by a 4-point probes resistivity measurement system (RTS-9 type). The magnetic properties were measured by a vibrating sample magnetometer (VSM, MPMS SQUID XL). All the measurements were performed at room temperature. 3. Results and discussion 3.1. Phase analyses In order to investigate the phase compositions of the Fe3O4 NPs and Tb(BA)3phen/PANI/Fe3O4/PVP hollow nanofibers, XRD was employed to analyze the samples. As shown in Fig. 2, the positions of diffraction peaks for Fe3O4 NPs match well with the PDF 74-0748 standard card of Fe3O4, which indicates that the Fe3O4 NPs are single phase and belong to the cubic system. Moreover, no characteristic peaks are observed for other impurities such as Fe2O3 and FeO (OH). XRD pattern of the Tb(BA)3phen/PANI/Fe3O4/PVP hollow nanofibers demonstrates that the hollow nanofibers contain Fe3O4 NPs, but the diffraction intensities of Fe3O4 are reduced due to the existence of amorphous Tb(BA)3phen, PANI and PVP.

Tb(BA)3phen/PANI/Fe3O4/PVP hollow nanofibers

Intensity (a.u.)

834

Fe3O4nanoparticles

PDF#74-0748 (Fe3O4)

20

30

40

50

60

70

2-Theta (degree) Fig. 2. XRD patterns of Fe3O4 NPs and Tb(BA)3phen/PANI/Fe3O4/PVP hollow nanofibers (S3) with the PDF standard card of Fe3O4.

endpoints of the nanofibers, the successful fabrication of Tb(BA)3phen/PANI/Fe3O4/PVP hollow nanofibers can be ascribed to the fact that the air flowed down from the inner needle has the ability to support the hollow structure of the Taylor cone and the subsequent electrospun nanofibers. In our study, it is found that CHCl3 used as an integral part of solvent in the spinning solutions plays an important role in the formation of the hollow structure owing to its lower surface tension, lower boiling point and higher vapor pressure. Fig. 3d shows the EDX analysis of the as-prepared hollow nanofibers, which confirms the presence of C, N, O, S, Fe and Tb elements in the hollow nanofibers. Many nanofibers conglutinate together at the overlapping locations, which could be partially responsible for the high mechanical resilience of the nanofibrous membranes. As shown in Fig. 4, a sheet of composite nanofibrous membrane with a size of 1.5  2.0 cm2 can be bent by hand and recoverable, indicating that the nanofibrous membranes are freestanding and mechanically flexible for the good bending and recovering performance. The internal structure of the nanofibers was further investigated by using TEM technique, as presented in Fig. 5. A clear hollow structure can be observed with an inner diameter of ca. 80 nm, which is consistent with the result of SEM analysis. The Fe3O4 NPs are uniformly dispersed in the shell of the hollow nanofibers. From the above analyses, we can safely conclude that the Tb (BA)3phen/PANI/Fe3O4/PVP hollow nanofibers have been successfully fabricated.

3.3. Photoluminescent performance 3.2. Morphology and internal structure The morphology of Tb(BA)3phen/PANI/Fe3O4/PVP hollow nanofibers was characterized by the combination of SEM, BET, TEM, and EDX analyses. As illustrated in Fig. 3, FESEM observation (Fig. 3a) indicates that the electrospun hollow nanofibers orientate randomly and interweave to form a network structure. A magnified FESEM image (Fig. 3a, inset) clearly reveals that the nanofibers possess a typical hollow structure. The surface of the flexible hollow nanofibers is relatively smooth and the BET specific surface area of the nanofibers is 19.08 m2 g 1. One can see from Fig. 3b that the corresponding pore diameter of the hollow-structured nanofibers is ca. 94.90 nm. Moreover, the hollow nanofibers have almost uniform outer diameters of 238 ± 1.04 nm as shown in Fig. 3c and inner diameters of approximately 80 nm. As seen from the

The performance of luminescent materials is influenced by the doping emitter concentration, so it is of great significance to determine the optimum concentration for the as-prepared hollow nanofibers. To perform this study, a series of Tb(BA)3phen/PANI/Fe3O4/ PVP hollow nanofibers were fabricated. The mass percentage of PANI to PVP and the mass ratio of Fe3O4 to PVP were fixed at 30% and 1:1, respectively. Fig. 6 demonstrates the excitation and emission spectra of Tb(BA)3phen/PANI/Fe3O4/PVP hollow nanofibers with different doping concentrations of Tb(BA)3phen from 120% to 210% (samples S1–S4). When monitored at 545 nm, the excitation spectra (Fig. 6, left) exhibit broad bands extending from 200 to 350 nm with a maximum intensity at 306 nm, which is attributed to the p ? p⁄ electronic transition of the ligands. The emission spectra of the samples are recorded at an excitation

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0.012

Pore volume dV/dD (cm 3/g)

a

b

0.010 0.008 0.006 0.004 0.002 0.000

500 nm 0

20

40

60

80

100

120

140

160

Pore diameter (nm) 45 40

C

c

d

Intensity (a.u.)

Percentage (%)

35 30 25 20 15

N S O Tb Fe

10

Fe Tb

5 0 180

200

220

240

260

280

300

2

320

4

6

Tb 8

Binding Energy (KeV)

Diameter (nm)

Fig. 3. FESEM image (a), pore size distribution (b), histogram of diameter distribution (c), and EDX spectrum (d) of Tb(BA)3phen/PANI/Fe3O4/PVP hollow nanofibers.

4000

6000

Intensity (a.u.) Fig. 4. Photographs for real substance (a), bending (b) and recovery (c) of a sheet of nanofibrous membrane.

Intensity(a.u.)

5000

3000

b.Tb(BA)3phen:PVP=150 % (S 2)

2500 2000

c.Tb(BA)3phen:PVP=180 % (S 3)

490

1500 1000 500

d.Tb(BA)3phen:PVP=210 % (S 4)

585 621

0

4000

a.Tb(BA)3phen:PVP=120 % (S 1)

545

3500

120

150

180

210

545

Masspercentage(%)

306 3000

λem=545 nm

λex=306 nm 333 d

2000

d 490 a

a 1000

585 0 200

250

300

350

500

550

600

621 650

Wavelength (nm) Fig. 6. Excitation (left) and emission spectra (right) of Tb(BA)3phen/PANI/Fe3O4/ PVP hollow nanofibers containing different mass percentages of Tb(BA)3phen. Inset: variation of intensities of luminescent peaks at 545 nm, 490 nm, 585 nm and 621 nm with respective mass percentage of Tb(BA)3phen for the hollow nanofibers.

Fe3O4 NPs

100 nm

Fig. 5. TEM image of Tb(BA)3phen/PANI/Fe3O4/PVP hollow nanofibers.

wavelength of 306 nm, as shown in the right part of Fig. 6. A series of characteristic emission peaks between 450 nm and 650 nm can be observed due to the energy level transitions of 5D4 ? 7F6 (490 nm), 5D4 ? 7F5 (545 nm), 5D4 ? 7F4 (585 nm) and 5D4 ? 7F3 (621 nm) of Tb3+ ions, and the predominant emission peak at 545 nm corresponds to the 5D4 ? 7F5 hypersensitive transition.

Y. Liu et al. / Chemical Engineering Journal 284 (2016) 831–840

6000

a

5

D4

7

a. PANI:PVP=20% (S5) b. PANI:PVP=30% (S3) c. PANI:PVP=40% (S6 ) d. PANI:PVP=50% (S7)

F5

545

λex=306 nm

Intensity(a.u.)

Intensity (a.u.)

5000

4000 3000

5

D4

7

F6

a

490 2000

5000

3000 2000

49 0

1000

5 85

6 21 20

30

40

50

Mass percentage(% )

d 5

D4

7

F4

585 500

545

4000

0

1000

0 450

b

550

5

7

D4

F3

Absorbance (a.u.)

836

545

490

621

600

650

Wavelength (nm)

700 300 450 600 750 900

Wavelength (nm)

Fig. 7. Comparison of photoluminescence spectra of Tb(BA)3phen/PANI/Fe3O4/PVP hollow nanofibers containing different mass percentages of PANI (a) and UV–Vis absorbance spectrum of PANI/PVP nanofibers (b).

Fig. 8. Schematic diagram of the situation of the exciting light and emitting light in Tb(BA)3phen/PANI/Fe3O4/PVP hollow nanofibers containing different percentages of PANI.

Furthermore, when the mass percentages of Tb(BA)3phen are varied from 120% to 180%, both the excitation and emission intensity markedly increase, and then only slightly increase when the mass percentages of Tb(BA)3phen continue to increase from 180% to 210%, as more clearly seen from the inset of Fig. 6. Therefore, the mass percentage of Tb(BA)3phen of 180% was adopted to prepare Tb(BA)3phen/PANI/Fe3O4/PVP hollow nanofibers. Additionally, a comparative study is carried out to demonstrate the effect of adding different amounts of PANI and Fe3O4 NPs on the photoluminescence properties of the hollow nanofibers. Firstly, the influence of PANI on the photoluminescence property is researched. The mass percentage of Tb(BA)3phen to PVP and the mass ratio of Fe3O4 NPs to PVP were fixed at 180% and 1:1, respectively. Tb(BA)3phen/PANI/Fe3O4/PVP hollow nanofibers containing different mass percentages of PANI ranging from 20% to 50% were fabricated (samples S3, S5–S7). It can be seen from Fig. 7a that the photoluminescence intensities decrease with the increasing amounts of PANI, which results from the light absorption of PANI mixed in the hollow nanofibers. Especially when the mass percentage of PANI exceeds 40%, the photoluminescence peaks decline significantly. The UV–Vis absorbance spectrum of PANI/PVP nanofibers shown in Fig. 7b clearly explains that PANI/PVP can strongly absorb visible light (400–760 nm) and much more easily absorb the ultraviolet light (300–400 nm). Moreover, a broad

absorption peak around 620 nm is also observed, meaning that PANI absorbs red light much stronger. As illustrated in Fig. 8, the exciting light in the hollow nanofibers has to pass through PANI to reach and excite Tb(BA)3phen. In this process, a large part of the exciting light has been absorbed by PANI, and thus the exciting light is much weakened before it reaches the Tb(BA)3phen. Similarly, the emitting light emitted by Tb(BA)3phen also has to pass through PANI and is absorbed by it. Consequently, both the exciting light and emitting light in the hollow nanofibers are absorbed by PANI, resulting in that the intensities of exciting light and emitting light are severely weakened, and furthermore, the light absorption becomes stronger with introducing more PANI into the hollow nanofibers. Generally, fluorescence color can be represented by the Commission Internationale de L’Eclairage (CIE) 1931 chromaticity coordinates. Fig. 9 depicts the CIE coordinate diagram of Tb(BA)3phen/PANI/Fe3O4/PVP hollow nanofibers with different percentages of PANI under the excitation of 306-nm ultraviolet light. It demonstrates that the emitting color of Tb(BA)3phen/PANI/Fe3O4/PVP hollow nanofibers shifts to blue region with introducing more PANI, due to the stronger light absorption of red light by PANI. Moreover, the decreased degree of the photoluminescence intensity for respective peak at 545 nm, 490 nm, 585 nm and 621 nm are distinct with the increasing amounts of PANI introduced into the hollow nanofibers, which

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fact that the intensities of exciting light and emitting light are severely weakened, and moreover, the light absorption becomes stronger with introducing more Fe3O4 NPs into the hollow nanofibers. Fig. 12 is the CIE coordinate diagram of Tb(BA)3phen/PANI/ Fe3O4/PVP hollow nanofibers with different mass ratios of Fe3O4 NPs under the excitation of 306-nm ultraviolet light. It demonstrates that the emitting color of Tb(BA)3phen/PANI/Fe3O4/PVP hollow nanofibers varies with introducing more Fe3O4 NPs, which can be attributed to the fact that Fe3O4 NPs have stronger light absorption at ultraviolet range. Thus, it is concluded that the existence of PANI and Fe3O4 NPs leads to the decrease in photoluminescence intensities, and furthermore, photoluminescence intensities become much weaker with more PANI and Fe3O4 NPs introduced into the hollow nanofibers. 3.4. Electrical conductivity analyses Fig. 9. CIE chromaticity coordinates diagram of Tb(BA)3phen/PANI/Fe3O4/PVP hollow nanofibers containing different percentages of PANI.

Except for the above photoluminescence property, the electrical conductivity of the trifunctional flexible hollow nanofibers is also researched. PANI is known as a conducting polymer and is distributed consecutively in Tb(BA)3phen/PANI/Fe3O4/PVP hollow nanofibers. Hence the introduction of more PANI into the hollow nanofibers facilitates the formation of continuous conductive network with more efficient charge transport, leading to enhancement in electrical conductivity of the hollow nanofibers. As summarized in Table 2, the average electrical conductivity value of the hollow nanofibers increases greatly from 7.37  10 4 to 3.27  10 3 S cm 1 when the mass percentage of PANI varies from 20% to 30%. With further increase in amount of PANI, the conductivity of the hollow nanofibers changes slightly. Combined with the above results of photoluminescence spectra analyses, it is evidenced that the optimum mass percentage of PANI to PVP is 30%. Besides, when fixing other parameters, the samples doped with various amounts of Fe3O4 NPs (S8–S10) are tested to discuss the effect of Fe3O4 NPs on the electrical conductivity, and the results demonstrate that the presence of Fe3O4 NPs has little influence on the conductivities of the samples.

affects the relative proportion of peaks, thus leading to the change of CIE coordinates. Meanwhile, the influence of Fe3O4 NPs on the photoluminescence property is also studied. The mass percentage of Tb(BA)3phen to PVP and that of PANI to PVP were maintained at 180% and 30%, respectively. Tb(BA)3phen/PANI/Fe3O4/PVP hollow nanofibers containing different mass ratios of Fe3O4 NPs were fabricated (samples S3, S8–S10). It is observed by comparing the photoluminescence spectra in Fig. 10a that the photoluminescence intensities decrease with the increasing amounts of Fe3O4 NPs introduced into the hollow nanofibers. This phenomenon can be explained by the absorbance spectrum of Fe3O4 NPs illustrated in Fig. 10b, which shows that light absorption of the dark-colored Fe3O4 NPs at ultraviolet wavelengths (k < 400 nm) is much stronger than that in visible region (k = 400–760 nm). As revealed in Fig. 11, the exciting light in the hollow nanofibers has to pass through Fe3O4 NPs to reach and excite Tb(BA)3phen. In this process, a large part of the exciting light has been absorbed by Fe3O4 NPs, and thus the exciting light is much weakened before it reaches the Tb(BA)3phen. Likewise, the emitting light emitted by Tb(BA)3phen also has to pass through Fe3O4 NPs and is absorbed by them. As a result, both the exciting light and emitting light in the hollow nanofibers are absorbed by Fe3O4 NPs, leading to the

5

D4

7

a.Fe3O4 :PVP=0.3:1(S8) b.Fe3O4 :PVP=0.5:1(S9)

F5

545

λex=306 nm

d.Fe3O4 :PVP= 2:1 (S10 ) 5000

5

D4

7

F6

490

d

2000

4000

545

3000 2000

490

1000 0

585 S3

S8

S9

Samples 5

1000

D4

7

F4

585 0 450

b

c.Fe3O4:PVP= 1:1 (S3 )

a 3000

Hysteresis loops of Fe3O4@OA NPs and Tb(BA)3phen/PANI/ Fe3O4/PVP hollow nanofibers containing different mass ratios of Fe3O4 NPs are used to confirm the magnetic properties of the

500

550

5

D4

7

621 S10

Absorbance (a.u.)

4000

a

Intensity(a.u.)

Intensity (a.u.)

5000

3.5. Magnetic properties

490 545

F3

621

600

Wavelength (nm)

650

700 300 450 600 750 900

Wavelength (nm)

Fig. 10. Comparison of photoluminescence spectra of Tb(BA)3phen/PANI/Fe3O4/PVP hollow nanofibers containing different mass ratios of Fe3O4 NPs (a) and UV–Vis absorbance spectrum of Fe3O4 NPs (b).

838

Y. Liu et al. / Chemical Engineering Journal 284 (2016) 831–840

Fig. 11. Schematic diagram of the situation of the exciting light and emitting light in Tb(BA)3phen/PANI/Fe3O4/PVP hollow nanofibers containing different mass ratios of Fe3O4 NPs.

30

a. Fe3O4 : PVP = 0.3:1 (S 8) b. Fe3O4 : PVP = 0.5:1 (S 9)

M (emu⋅g-1)

20 10

c. Fe3O4 : PVP = 1:1

(S3)

d. Fe3O4 : PVP = 2:1

(S 10)

e

e. Fe3O4 @OA nanoparticles

a

0 -10 -20 -30 -10000

Fig. 12. CIE chromaticity coordinates diagram of Tb(BA)3phen/PANI/Fe3O4/PVP hollow nanofibers containing different mass ratios of Fe3O4 NPs.

Table 2 Electrical conductivity of the samples doped with various amounts of PANI and Fe3O4 NPs. Samples

Constant amounts

Various amounts

Electrical conductivity (S cm 1)

S5 S3 S6 S7

Fe3O4: PVP = 1:1

PANI: PVP = 20% PANI: PVP = 30% PANI: PVP = 40% PANI: PVP = 50%

7.37  10 3.27  10 4.93  10 8.22  10

4

S8

PANI: PVP = 30%

Fe3O4: PVP = 0.3:1 Fe3O4: PVP = 0.5:1 Fe3O4: PVP = 2:1

2.91  10

3

2.11  10

3

1.75  10

3

S9 S10

3 3 3

samples, as shown in Fig. 13, and their saturation magnetizations are listed in Table 3. Magnetic measurements illustrate that the saturation magnetization value of the as-prepared Fe3O4@OA NPs is 29.25 emu g 1 and neither remanence nor coercivity is detected. And the reversible hysteresis behavior also indicates the Fe3O4 NPs exhibit superparamagnetism and fast magnetic responsivity. It should be noted that the trifunctional flexible hollow nanofibers still show good paramagnetism. From Table 3, it is clear that the

-5000

0

5000

10000

H (Oe) Fig. 13. Hysteresis loops of Fe3O4@OA NPs and Tb(BA)3phen/PANI/Fe3O4/PVP hollow nanofibers containing different mass ratios of Fe3O4 NPs.

Table 3 Saturation magnetization of Fe3O4@OA NPs and Tb(BA)3phen/PANI/Fe3O4/PVP hollow nanofibers containing different mass ratios of Fe3O4 NPs. Samples

Saturation magnetization (M) (emu g

Fe3O4@OA NPs Fe3O4: PVP = 2:1 (S10) Fe3O4: PVP = 1:1 (S3) Fe3O4: PVP = 0.5:1 (S9) Fe3O4: PVP = 0.3:1 (S8)

29.25 8.15 6.06 4.15 2.36

1

)

saturation magnetizations of the hollow nanofibers increase from 2.36 emu g 1 to 8.15 emu g 1 with the increasing amounts of Fe3O4 NPs. Hence, the magnetic properties of the hollow nanofibers can be tuned by adding various contents of Fe3O4 NPs.

4. Conclusions In summary, we have developed a facile route for fabricating novel photoluminescent–electrical–magnetic trifunctional flexible Tb(BA)3phen/PANI/Fe3O4/PVP hollow nanofibers via one-pot coaxial electrospinning. The mean diameter and the inner diameter of

Y. Liu et al. / Chemical Engineering Journal 284 (2016) 831–840

the hollow nanofibers are ca. 238 nm and ca. 80 nm, respectively. It is very gratifying to see that the hollow nanofibers possess excellent photoluminescent performance, electrical conductivity and magnetic properties. Furthermore, the photoluminescent intensity, electrical conductivity and magnetic properties of the hollow nanofibers can be tuned by adding different concentration of the luminescent material, as well the contents of PANI and Fe3O4 NPs, respectively. Based on this design conception and synthetic strategy, other photoluminescent–electrical–magnetic trifunctional hollow nanofibers can be fabricated. For instance, the photoluminescent color of the hollow nanofibers is tunable by adjusting the diversity and contents of the luminescent materials. Owing to these versatile properties, the novel hollow fibrous structured trifunctional material is expected to apply in various areas of electromagnetic interference shielding, molecular electronics, multifunctional nanodevices and biomedical field, etc. [64–66].

Acknowledgements This work was financially supported by the National Natural Science Foundation of China (NSFC 51573023, 50972020, 51072026), Specialized Research Fund for the Doctoral Program of Higher Education (20102216110002, 20112216120003), the Science and Technology Development Planning Project of Jilin Province (Grant Nos. 20130101001JC, 20070402).

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