Fibers and Polymers 2017, Vol.18, No.1, 50-56 DOI 10.1007/s12221-017-6895-3

ISSN 1229-9197 (print version) ISSN 1875-0052 (electronic version)

Polyaniline-doped TiO2/PLLA Fibers with Enhanced Visible-light Photocatalytic Degradation Performance Xiaoqiang Li1,2, Chen Shi2, Jidong Wang2, Jian Wang2, Mengjuan Li2, Hua Qiu2, Hong Sun1, and Kenji Ogino1* 1

Department of Organic and Polymer Materials Chemistry, Tokyo University of Agriculture & Technology, Tokyo 184-8588, Japan 2 College of Textile & Clothing, Jiangnan University, Wuxi 214122, China (Received September 9, 2016; Revised November 21, 2016; Accepted November 26, 2016)

Abstract: A simple and practical strategy has been developed for preparing polyaniline(PANi)-doped TiO /poly(l-lactide) (P@TiP-C) fibers by a combination of coaxial-electrospinning and in-situ polymerization. The TiO /PLLA composite fibers with TiO located on the surface were fabricated by coaxial-electrospinning, with PLLA as the core phase and a dispersion of TiO particles, a well-known photocatalyst, in the sheath phase. The aniline monomers were also located in the core phase and in-situ polymerized by ammonium persulfate (APS) after electrospinning. SEM images show that TiO particles were located on the surface of PLLA fibers. Photocatalytic degradation tests show that the P@TiP-C fibers exhibit enhanced photocatalytic activity for degradation of methyl orange under visible light, likely due to the synergistic effect of PANi and TiO . 2

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Keywords: In-situ polymerization, Photocatalyst, TiO , Coaxial-electrospinning, Polyaniline 2

in practical applications are in the form of powder or particles, which have many disadvantages, such as low efficiency of light absorption, difficult separation or recovery, easy to condensate in the suspension system, and easy to flow away in gas-solid systems. These problems limit further development and applications of TiO2 photocatalysts in industry. In order to solve these problems, TiO2 has been immobilized on various substrates, such as films, fibers, tubes and porous structures. Among them, TiO2 combined with fibrous structure draws much attention due do their high aspect ratio and good dispersity in pollutant aqueous solutions during photocatalytic progresses. Electrospinning is a technique that utilizes electric force to drive the spinning process and to produce fibers from polymer solutions or melts [12-14]. Unlike conventional spinning techniques (solution- or melt-spinning), which are capable of producing fibers with diameters in the micrometer range, electrospinning produces fibers with diameters in the nanometer range. Electrospun fibers possess many extraordinary properties, such as small diameters and large specific surface areas. Additionally, the non-woven fibrous mats made of electrospun polymer fibers offer a unique capability of readily controlling the pore sizes. Furthermore, electrospun fibers are produced through a top-down process, which results in continuous and low-cost fibers that are also relatively easy to align, assemble and process. Till now, many synthetic and natural polymers including, but not limited to, polylactide, polycaprolactone, poly(glycolic acid), poly(L-lactide-co-caprolactone) (PLLACL), collagen, and chitosan have been electrospun into fibrous mats for various applications [15-19]. It is well known that only about 4 % of the solar energy can be utilized by TiO2 for organic degradation, because of the rather high intrinsic band gap of TiO2 (3.2 eV for

Introduction Synthetic dyes are produced and used worldwide every year for industrial purposes, such as textile, leather tanning, food processing, cosmetics, electroplating, paper, and pharmaceutical industries; and about 5-10 % of this quantity is released into the ecosystem along with waste water [1-3]. There were many available efficient and economical methods to remove dyes from waste water [4-6]. In our previous study, polyaniline/filter-paper composite had been successfully prepared for the removal of coomassie brilliant blue from aqueous solutions [7]. However, photocatalysis is considered to be a much greener and more sustainable technology. In Zhang et al.’s report, a novel Fe3O4@MnO2 ball-in-ball hollow spheres with hierarchical hollow structure were synthesized by a combined processes of in-situ growth of metallic oxides and an etching method; the as-prepared composite spheres were applied to degrade methylene blue (MB) and exhibit the merits of excellent catalytic performance easy separation, good stability and easy-to-recycle [8]. Among various photocatalysts, TiO2 is a typical hydrophilic photocatalysts with the capability of degradation and selfcleaning, and has a wide range of applications including toxic chemical decomposition, protective/self-cleaning clothing, self-cleaning glass, and self-cleaning membranes [9-11]. The photocatalyst is able to create excited electrons in the conduction band and holes in the valence band when incident photons are absorbed. Thereafter, these electrons and holes result in formation of hydroxyl and oxygen radicals, which react with chemicals at the surface of the photocatalyst. The most frequently used TiO2 photocatalysts *Corresponding author: [email protected] 50

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anatase, 3.02 eV for rutile). In addition, there exists a high rate of electron-hole recombination in TiO2. In order to reduce the high intrinsic band cap of TiO2, many strategies have been employed, including dye sensitization, polymer modification, non-metals doping, semiconductor coupling, transition metal doping, and spatial structuring [20-22]. Zhao et al., reported the capability of enhancing photocatalytic activity by doping TiO2 with graphene under the UV and visible-light spectrum regions; they believed that more •OH radicals could be produced by the graphene@TiO2 composite than by pure TiO2 under UV and visible-light irradiation. [23] The conductive polymers of polyaniline have received many attentions because of its easy-to-synthesize, light weight, and high stability. Moreover, PANi can generate π-π transition under visible light, and injects the excited electrons into the conduction band of TiO2, and then the electrons transfer to an adsorbed electron acceptor to yield oxygenous radicals [24]. In this work, TiO2/poly(L-lactide) (TiO2/PLLA) nanofibrous composites (P@TiP-C) with TiO2 particles wrapped on the surface of PPLA fibers were fabricated using the method of coaxial-electrospinning, with PLLA serving as the filament material. Thereafter, polyaniline was in-situ polymerized in the PLLA fibers. The morphologies of the nanofibers were observed by scanning electron microscopy (SEM). The loading content of TiO2 was examined by thermo-gravimetric (TG) analysis, and the photocatalysis property of P@TiP-C fibers was evaluated by photocatalytic degrading methyl orange (MO) tests.

Experimental Materials Poly(l-lactide) (PLLA, Mw=50,000) was purchased from Jinan Daigang Biomaterial Co., Ltd. (Jinan, China). Titania nanoparticle powder (Degussa Aeroxide P-25, mixed anatase/ rutile phase) and trifluoroethanol (TFE) were purchased

from J&K Scientific Ltd. Aniline was distilled under reduced pressure before use, and ammonium persulfate (APS) was used as received. All other chemicals were commercially obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China) and used without any further purification. Electrospinning The core phase solution for coaxial-electrospinning were made to a PLLA concentration of 0.12 g/ml in TFE/aniline (99.5/0.5 in volume). PLLA was added to the TFE and magnetically stirred until all the PLLA dissolved, followed by adding aniline into the above solution. The sheath solution of TiO2/PLLA was made at concentration of 0.06 g/ml TiO2 and 0.02 g/ml PLLA in TFE. TiO2/PLLA/ TFE solution with TiO2 concentration of 0.12 g/ml and PLLA concentration of 0.12 g/ml was also prepared for fabricate TiO2/PLLA composite fibers. A variable high-voltage power supply was used in the electrospinning apparatus. Mixture solution of TiO2/PLLA/ TFE was pumped through a blunt needle with its diameter of 0.9 mm for conventional electrospinning at a flow rate of 1.0 ml/h using a syringe pump (Cole-Parmer Instrument Company, USA). For coaxial electrospinning, the setup equipment mainly consists of a syringe-like apparatus with an inner needle co-axially placed inside an outer one, as shown in Figure 1. Two separated syringe pumps were used to push the core phase and sheath phase, respectively. Solutions were pumped at a core flow rate of 0.8 ml/h and a sheath flow rate of 0.3 ml/h. A copper electrode connected the coaxial-apparatus directly with high voltage of 15 kV. An aluminum foil was wrapped on a rotation drum and connected ground for the collection of electrospun fibers. In-situ Polymerization of Aniline After electrospinning, the PLLA/TiO2/aniline precursor fibers were immediately peeled up from the aluminum foil. The in-situ polymerization of aniline was carried out in an

Figure 1. The formation processes of PANi-doped PLLA/TiO fibers from coaxial-electrospinning and in-situ polymerization. 2

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ice-water bath. The precursor fibers were soaked in 200 ml of 1 M HCl solution, followed by adding 0.5 g APS into the solution. Then the PANi was formed after a few minutes incubation both in the PLLA/TiO2 microfibers and in the HCl solution. After the polymerization, the obtained fibers were taken out and washed with HCl solution to remove the residues. Finally, the membranes were washed with distilled water for several times and dried in an oven at 40 oC overnight. The preparation procedure of PANi-doped PLLA/ TiO2 (P@TiP-C) fibers is schematically shown in Figure 1. We also found that the aniline amount significantly influenced the morphology of PLLA or TiO2/PLLA fibers. When the amount of aniline monomer surpasses the threshold value, the excessive aniline molecules tended to dissolve PLLA and it was difficult to produce fibers by electrospinning. Therefore, the aniline loading amount should be strictly controlled. Characterization The surface morphology of the samples was observed using a JSM-5900 scanning electron microscope (Japan Electron Optics Laboratory Co. Ltd.). Prior to examination, samples were gold sputter-coated under argon to render them electrically conductive. Images were taken at an excitation voltage of 5 kV. The X-ray diffractograms were obtained using a D/Max-BR diffractometer (RigaKu, Japan) with Cu Kα radiation in the 2θ range of 10-80 o at 40 mV and 300 mA. TG analysis was performed on a thermogravimetric analyzer (Universal-V4.5A TA instrument, USA). The samples were heated from 30 to 800 oC at a heating rate of 10 oC/min under nitrogen flow rate of 20 ml/min. The fibrous mats were daubed on one KBr disk and covered by another KBr disk, and then tested on a Spectrum 100 Fourier transform infrared (FTIR) spectrometer (Perkin-Elmer, US). X-ray photoelectron spectroscopy (XPS) data were obtained with an ESCALAB 250Xi (Thermo Fisher Scientific, USA) electron spectrometer using 300 W Al Kα radiation. Photocatalytic Activities The overall photocatalytic activities of TiO2-based electrospun fibers were evaluated using MO as the model dye indicator. Membranes with a dimension of 2.0 cm×2.0 cm were put into a micro-reactor (as shown in Figure 2) with 25 ml of MO solution (0.5 mg/l) and stored in the dark for 2 h to achieve the adsorption equilibrium for MO. A 250 W xenon lamp with a 420 nm cut-off glass filter was used as a visible-light source. At selected time intervals, decreases in the concentrations of MO solutions were analyzed at 465 nm using a TU-1901 spectrophotometer. The calibration curve of MO was obtained by measuring the absorbance of different predetermined concentrations of the samples. The photocatalytic activities of MO on TiO2 powder, coaxialelectrospun TiO2/PLLA (TiP) fibers and PANi-doped TiO2/ PLLA fibers from conventional electrospinning (P@TiP-B)

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Figure 2. Micro-reactor for photocatalytic decomposition of MO.

fibers were also determined as controls for the purpose of comparison.

Results and Discussion A schematic illustration of the synthetic route for the fabrication of P@TiP-C fibers is depicted in Figure 1. Specially, a facile coaxial-electrospinning method was used to produce TiO2/PLLA fibers, with the TiO2 wrapped on the surface of PLLA fibers rather than encapsulated them in the fibers. Furthermore, the fresh obtained composite fibers were treated with ASP solution to in-situ polymerize the aniline in the PLLA fibers. Characterizations The SEM images of pure PLLA fibers, obtained from electrospinning of a 0.12 g/ml PLLA/TFE solution, are shown in Figure 3(a)-(c). The pure PLLA fibers have sizes ranging from 200 to 700 nm, with an average diameter of ~450 nm. TiO2/PLLA fibers from blending electrospinning using the conventional nozzle are shown in Figure 3(d)-(f). These fibers had a lager average diameter than that of pure PLLA fibers. The morphology of TiO2/PLLA fibers spun using a coaxial nozzle was also studied by SEM and the images are shown in Figure 3(g)-(i). Obviously, there were some morphological differences between the fibers obtained from the conventional and the coaxial nozzle configurations. TiO2/PLLA fibers from coaxial-electrospinning showed smaller diameter, attributable to the additional solvent of sheath solution during the coaxial-electrospinning that diluted the overall polymer concentration and reduced the amount available for fiber formation. TiO2 particles in clusters with varied sizes are shown in Figure 3(d)-(i). It is noted that TiO2 clusters adhered to or were wrapped on the surface of the thin PLLA fibers obtained from coaxialelectrospinning. However, TiO2 clusters were encapsulated in the PLLA fibers obtained from blend-electrospinning. TG analysis was performed in order to better understand

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Figure 3. SEM microphotographs of various PLLA fibers with different magnifications; (a-c) pure PLLA fibers, (d-f) TiO /PLLA fibers form blend electrospinning, and (g-i) TiO /PLLA fibers from coaxial-electrospinning. 2

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Figure 4. TG curves of pure PLLA, TiO /PLLA composite fibers from conventional and coaxial-electrospinning. 2

the photocatalytic properties of the TiO2/PLLA fibrous composites made by conventional or coaxial-electrospinning. It is reasonable to predict that the TiO2 on the surface of polymer fibers could have much higher photocatalytic capability than the case that they are encapsulated with the same loading amount. However, TiO2 on the surface is easily washed off from the PLLA fibers. The loading amounts of TiO2 from conventional electrospinning and coaxial-

electrospinning are different in this work. The loading amount can be calculated from the TG results, which are shown in Figure 4. The TG curve of the pure PLLA was different from those of the TiO2/PLLA composites. Nearly 100 % of PLLA was decomposed at the temperature of 600 oC, while the TiO2/PLLA composites had 60.3 % (from coaxial electrospinning) and 46.3 % (from blend electrospinning) of the total weight left, respectively. These results indicate that more TiO2 (about 14 %) was loaded in the blendelectrospun PLLACL fibers than in the coaxial-electrospun fibers. Figure 5(a) shows the XRD patterns of TiO2/PLLA fibers and PANi-doped TiO2/PLLA, respectively. In the XRD pattern of the TiO2/PLLA fibers, five typical peaks at 2θ of 25.3 o, 37.9 o, 48.1 o, 54.0 o and 62.8 o can be indexed as (101), (004), (200), (105) and (204) planes of anatase TiO2, respectively. The peaks at 14.9 o are doping diffraction peaks characteristic of PANi. The peaks at 2θ=23.7 and 26.4 o corresponding to the periodicity parallel and perpendicular to PANi chains are observed [25,26]. This result confirms the formation of PANi in TiO2/PLLA fibers. The composition of P@TiP-C nanofibrous membranes could be confirmed by their FTIR spectra, as shown in Figure 5(b). Both spectra on TiO2/PLLA (fabricated from

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Figure 6. XPS pattern of P@TiP-C fibers. Figure 5. XRD patterns (a) and FTIR spectrum (b) of the P@TiPC and TiO /PLLA fibers. 2

coaxial-electrospinning) and P@TiP-C membranes, have the characteristic peak at approximately 3400 cm-1 can be assigned to the -OH group and the H-O-H stretching and bending vibrations, which may be caused by the adsorbed water molecules in the nanofibrous membranes. Moreover, on Figure 2(a), the peak at 2970 cm-1 originates from the stretching vibration of the C-H band on PLLA; and the peaks from 1500 to 1375 cm-1 can be assigned to the -CH and -C-C- bands, and the peak at 1730 cm-1 is attributed to the carbanyl group (-COO-) of PLLA. The sample of P@TiP-C shows the peaks at 1512 cm-1 and around 850760 cm-1, which indicate the existence of -C=N- and benzene rings, respectively. The information regarding the chemical and bonding environment of the PANi-doped PLLA/TiO2 fibrous membrane were ascertained using XPS. The survey of spectra was shown in Figure 6(a). With respect to the XPS spectra of O1s in Figure 6(b), the peak at 531.3 ascribes to -C=O (or -COO-) specie. The C1s peak in Figure 6(c) is elemental carbon at 284.6 eV, attributed mainly to sp2 hybridized carbon. The N1s peak is also presents at 399.6 eV. However, it is not expected as other reports that the peaks of Ti miss on the XPS spectra. Under considering the polymerization of PANi in this study, it is thought that TiO2 particles were

covered by the PANi; therefore, there are only C, N, O, and S detected by XPS. Photocatalytic Activities In this work, the photocatalytic activities of PANi-doped TiO2/PLLA fibers from coaxial-electrospinning followed by in-situ polymerization of PANi (P@TiP-C) were evaluated by measuring the degradation of methyl orange (MO) at room temperature under visible-light irradiation. Pure TiO2 powder (the same TiO2 used for preparing electrospun fibers), P@TiP fibers from conventional electrospinning followed by in-situ polymerization of PANi (P-doped T/PB), and TiO2/PLLA (T/P) fibers without PANi doping were used for the comparison. As shown in Figure 7, MO degradation is negligible on the blank sample without any photocatalyst under visible irradiation. Similarly, the pure TiO2 powder and TiO2/PLLA fibers without PANi-doping could not degrade MO within 3 hours. However, both the samples of P@TiP-B and P@TiPC fibers exhibited much better visible-light photocatalytic performance than for MO. In this work, the sample of P@TiP-C showed the best visible-light photocatalysts activity, as the MO was degraded by 81.3 %. These results demonstrate that the PANi doping improved the overall photocatalytic activity of TiO2/PLLA fibers for the degradation of MO. The mechanism of the enhancement

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conducting network of PANi, and subsequently transfer to the surface to react with water and oxygen to yield hydroxyl radical, which would oxidize MO [24]. As a result, the fast photogenerated charge separation and relatively slow charge recombination enhanced the photocatalytic activity of composite fibers [26]. The routes of formation of •OH and the photodegradation of MO can be described as follows: − TiO 2 + hv → (eCB ) + (h +VB ) → PANi(e− ) + h +

(1)

+ − TiO 2 (h VB ) + OH ads → ⋅OH + TiO 2

(2)

− TiO 2 (eCB ) + O 2ads → ⋅O 2 − + TiO 2

H2O

Figure 7. Visible-light photocatalytic degradation curves of MO on various samples.

→ ⋅OH + TiO 2 (3)

PANi(e− ) + O 2ads → ⋅O 2 − + PANi

H2O

→ ⋅OH + PANi (4)

MOads + ⋅OH → Degraded product

(5)

Conclusion

Figure 8. The schematic illustration of mechanism of the activation of photocatalytic activity for MO by P@TiP-C.

of photocatalytic activity for TiO2 by PANi is similar with previous study that had been reported by Zhao et al. [23], and summarized in Figure 8. From the experimental results that mentioned above, the doped PANi can enhance the photocatalytic activity of TiO2. Firstly, the PANi served as a photosensitizer for TiO2 to transfer more light-electrons under visible light irradiation, because its band gap (2.81 eV) is narrower than that of TiO2. Under visible-light irradiation, electrons (e−) can be excited from the valence band (VB) to the conduction band (CB), and creating a change vacancy or hole (h+), in the CB at the same time. However, most of these charges quickly recombine without doing any chemistry; less than 1 % of electrons and holes are trapped and participate in photocatalytic reactions. As the condition of PANi-doping, TiO2 particles on the surface of PLLA/PANi fibers, are in intimate contact with PANi molecular chains. As the TiO2 absorbs visible light, the excited-state electrons can be shuttled freely along the

In this study, photocatalytic fibrous composites were fabricated from blend- and coaxial-electrospinning, respectively, with PLLA as the filament material and TiO2 as the photocatalytic agent. It was found that TiO2 particles were wrapped on the PLLACL fibers when coaxial-electrospinning was used, but encapsulated in the PLLACL fibers by blendelectrospinning. The P@TiP fibers were fabricated from coaxial-electrospinning, followed by PANi-doping using insitu polymerization method. The new fibrous photocatalysts exhibit greatly improved visible-light photocatalytic activity for the degradation of MO, ascribed to the synergistic effect of TiO2 and PANi.

Acknowledgment This work was supported by Jiangsu Provincial Natural Science Foundation (NO. BK20130146) and National Natural Science Foundation of China (No. 51503083).

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