Journal of Colloid and Interface Science 424 (2014) 49–55
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Flexible polyaniline-coated TiO2/SiO2 nanofiber membranes with enhanced visible-light photocatalytic degradation performance Zhenyan Liu a, Yue-E. Miao a, Mingkai Liu a, Qianwei Ding a, Weng Weei Tjiu b, Xiaoli Cui c, Tianxi Liu a,⇑ a
State Key Laboratory of Molecular Engineering of Polymers, Department of Macromolecular Science, Fudan University, Shanghai 200433, PR China Institute of Materials Research and Engineering, A⁄STAR (Agency for Science, Technology and Research), 3 Research Link, Singapore 117602, Singapore c Department of Materials Science, Fudan University, Shanghai 200433, PR China b
a r t i c l e
i n f o
Article history: Received 12 January 2014 Accepted 1 March 2014 Available online 12 March 2014 Keywords: Titanium dioxide Silica Polyaniline Composite materials Visible-light photocatalysis Electrospinning
a b s t r a c t A simple and practical strategy has been developed for preparing polyaniline (PANi) coated TiO2/SiO2 nanofiber membranes by a combination of electrospinning, calcination and in situ polymerization. TiO2/SiO2 (TS) nanofibers are fabricated by electrospinning, followed by calcination. Then they are used as template for in situ polymerization of aniline monomers. SEM images show that PANi nanoparticles thus formed can be densely and uniformly coated on the surface of TS nanofibers. Photocatalytic degradation tests show that the as-prepared nanofiber membranes exhibit enhanced photocatalytic activity for degradation of methyl orange under visible light, which may be due to the synergistic effect of PANi and TiO2. Furthermore, the effect of polymerization time on the morphology and photocatalytic activity of the membrane is investigated. The free-standing membrane is flexible and easy to handle, which is promising for potential applications in photocatalysis and water remediation fields. Ó 2014 Elsevier Inc. All rights reserved.
Introduction Textile industry wastewater containing pigments and/or dyes can cause severe environmental pollution and the problem has urged relevant research on water decontamination. During recent decades, photocatalysis has attracted much attention in environmental restoration as a green and sustainable technology. Among various photocatalysts, TiO2 has been widely employed due to its lack of toxicity, low cost, high photocatalytic activity and photostability [1]. Despite these advantages, the practical applications of TiO2 need to deal with three major disadvantages: (1) TiO2 nanoparticles can easily form aggregates to minimize their surface area because of their high surface energy, which is unfavorable for photocatalytic reaction; (2) it is very difficult to separate TiO2 nanoparticles from treated water by conventional methods (including centrifugation and filtration), which may lead to loss of the photocatalyst and bring about secondary pollution; (3) the widespread applications of TiO2 are hindered by its low utilization of solar energy in the visible region. Therefore, there is an urgent need to develop a flexible nanostructured TiO2 photocatalyst with efficient visible photocatalytic activity.
⇑ Corresponding author. E-mail address: [email protected] (T. Liu). http://dx.doi.org/10.1016/j.jcis.2014.03.009 0021-9797/Ó 2014 Elsevier Inc. All rights reserved.
Many researchers have endeavored to introduce TiO2 on/into high-surface-area substrate. Among them, TiO2–SiO2 composites have been widely used in industrial applications and most extensively studied [2–4]. SiO2 is an excellent catalyst support because of its chemical inertia, thermal stability and adsorption of reactants. Particularly, the nanofibrous SiO2 supports have both high surface-area-to-volume ratio and favorable recycling properties. Electrospinning in combination with sol–gel processes has been proved to be a simple and effective method to produce polymer/ metal oxide (or metal sulfide) composite nanofibers. In many cases, the metal oxide (or metal sulfide) nanofibers can be obtained by subsequent pyrolysis [5–7]. Ding et al. fabricated the flexible and amphiphobic SiO2 nanofibrous mats via electrospinning the blend solutions of poly(vinyl alcohol) (PVA) and SiO2 gel, followed by calcination to remove the organic component [8]. Wang et al. prepared anatase mesoporous titanium nanofibers from calcination of the electrospun tetrabutyl titanate/poly(vinylpyrrolidone) (PVP)/pluronic123 (P123) composite nanofibers [9]. According to previous studies, fibrous photocatalysts are attractive candidates for practical applications because they possess a higher surfacearea-to-volume ratio and a three-dimensional (3D) open structure which make their surface active sites accessible to reactants more effectively. Xia et al. developed a new catalytic system by embedding Pt nanoparticles in the inner surfaces of CeO2 hollow fibers and the turnover frequency of the catalyst for CO oxidation was
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2–3 orders of magnitude higher than other systems [10]. ZnO quantum dots on hollow SiO2 nanofibers were prepared via electrospinning and exhibited high photocatalytic activity over Rhodamine B under ultraviolet light irradiation, which could be easily recycled due to its 1D nanostructure [11]. However, the TiO2 fibers obtained by electrospinning are usually quite brittle and easy to collapse into tiny pieces. To overcome this limitation, a number of methods have been reported to improve the flexibility or toughness of TiO2 films, which also contributes to easy separation and recycling. Sigmund et al. synthesized a flexible fiber mat composed of TiO2 and SiO2, offering potential applications as HEPA filters [12]. Fujishima et al. developed a method to integrate TiO2 nanoparticles (Degussa P25) into amorphous SiO2 nanofibers, which showed improved mechanical strength as well as good photocatalytic activity for reduction of silver ions and decomposition of acetaldehyde [13]. It is known that, owing to rather high intrinsic band gap of TiO2 (3.2 eV for anatase, 3.02 eV for rutile), only about 4% of the solar energy can be utilized. In addition, there exists a high rate of electron–hole recombination in TiO2. Various strategies have been employed to make TiO2 photocatalysts highly efficient under visible light, including dye sensitization, polymer modification, non-metals doping, semiconductor coupling, transition metal doping, and spatial structuring [14–16]. Conjugated polymers, such as polyaniline (PANi), poly(fluorine-co-thiophene) (PFT), polythiophene, polypyrrole and their derivatives can function as sensitizers to extend photoresponse of TiO2 into the visible region effectively [17–21]. PANi is one of the most fascinating conductive polymers and has received much attention for easy polymerization, high yield, relative high conductivities, low cost, and high stability [22–27]. Moreover, polymers are stable sensitizers in water compared to dyes because of their low solubility in water. Remarkable improvements in photocatalytic activity under visible light have been made by combining PANi and TiO2. Under visible-light irradiation, PANi generates p–p transition, injecting the excited electrons into the conduction band of TiO2, and then the electrons transfer to an adsorbed electron acceptor to yield oxygenous radicals [28]. Meanwhile, coupling photocatalysts with PANi proves to be an ideal system to promote photoinduced charge separation and inhibit charge recombination. Li et al. prepared a catalytic system of PANi/TiO2 nanocomposites, which exhibited higher photocatalytic activity and stability for degradation of methyl orange than the bare TiO2 and TiO2xNx under both UV and visible light irradiation [29]. In this work, we present a simple and practical strategy for preparing PANi coated TiO2/SiO2 (P/TS) nanofiber membranes by a combination of electrospinning, calcination and in situ polymerization. The nanofiber membranes thus obtained exhibit good flexibility, enhanced visible-light photocatalytic activity and recycling ability. The prepared membranes have large surface area and an open structure. Methyl orange (MO) is then used as the model dye pollutant to evaluate the photocatalytic degradation performance of the membranes. The effect of polymerization time on the morphology and photocatalytic activity of the membranes is also investigated.
was distilled under reduced pressure before use. Ammonium persulfate (APS) was used as received. All other reagents of analytical grade were purchased from Sinopharm Chemical Reagent Co., Ltd., and used without further purification. Synthesis of electrospun TiO2/SiO2 (TS) nanofiber membranes An aqueous TiO2/SiO2 sol was prepared through hydrolysis and polycondensation. 0.02 mol of H2O with a pH of 2.3 (pH adjusted by HNO3) was added to 0.008 mol of KH-560. Then, 0.002 mol of TEOS and 0.01 mol of Ti(OBu)4 were added to the above solution and stirred to produce a homogeneous solution. Then, 4 wt% PVP ethanol solution (12 mL) was slowly dropped into the TiO2/SiO2 sol to improve the electrospinnability. The mixture was vigorously stirred at room temperature for 5 h to produce a clear yellow solution. After that, the solution was transferred into a plastic syringe for electrospinning at a fixed electrical voltage of 15 kV. The pump speed was 2.0 mL/h, and the distance between the needle tip and collector was kept at 20 cm. The fiber membrane was collected on aluminum foil and then peeled off. The membranes were further calcined at 600, 700 and 800 °C respectively in air for 2 h to remove PVP, thus obtaining pure TiO2/SiO2 fibrous membranes for further experiments. The samples were labeled TS-600 °C, TS700 °C and TS-800 °C, respectively. Surface coating of polyaniline on TiO2/SiO2 (P/TS) nanofiber membranes Before coating PANi on TiO2/SiO2 (TS) nanofiber membranes, the as-prepared TS-800 °C membranes were rinsed with acetone (15 mL), ethanol (15 mL) and deionized water (15 mL) successively to increase their hydrophilicity. PANi coating was carried out via aniline polymerization on the surface of TS nanofibers in an icewater bath. First, pretreated TS membranes were soaked in 15 mL of aniline solution (1 mL of distilled aniline dissolved in 50 mL of 1 M HCl solution), followed by addition of 1 M HCl solution of APS (15 mL). The molar ratio of aniline and APS was 2:1. PANi emeraldine salt was formed after a few minutes and coated on the surface of TS nanofibers. After coating of PANi, the obtained P/TS membranes were taken out and washed with HCl solution to remove oligomer. Finally, the membranes were washed with deionized water for several times and dried in an oven at 40 °C overnight. The whole preparation procedure of polyaniline-coated TiO2/SiO2 fiber membrane is schematically shown in Fig. 1. Characterization X-ray diffraction (XRD) experiments were conducted on a PANalytical (X’Pert PRO) X-ray diffractometer using Cu Ka radiation (k = 0.1542 nm) at an acceleration voltage of 40 kV and a current of 40 mA. Raman spectra were obtained using a Renishaw inVia Reflex spectrometer with 632.8 nm laser excitation. Thermogravimetric analysis (Pyris 1 TGA) was performed under oxygen flow from 100 to 750 °C at a heating rate of 20 °C min1. Scanning electron microscope (SEM, Tescan) performed at an acceleration voltage of 20 kV was used to observe the morphology of samples.
Experimental
Measurements of the photocatalytic activities
Materials
Methyl orange (MO) was selected as the model dye indicator to evaluate the photocatalytic properties of the catalysts. Membranes with a dimension of 1.5 cm 0.8 cm were immersed into 3 mL of MO solution (1.5 mg/L) and stored in the dark for 2 h to achieve the adsorption equilibrium for MO. A 500 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
Poly(vinylpyrrolidone) (PVP, Mw = 1,300,000 g/mol) was purchased from Sigma–Aldrich. Tetraethyl orthosilicate (TEOS) was supplied by Shanghai Lingfeng Chemical Co., Ltd. Titanium nbutoxide (Ti(OBu)4) and KH-560 (3-Glycidyloxypropyltrimethoxysilane) were commercially obtained from Aladdin. Aniline (Ani)
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Fig. 1. Schematic illustration of the preparation of polyaniline coated TiO2/SiO2 nanofiber membrane.
solutions were analyzed at 465 nm using a Perkin–Elmer Lambda 35 UV–vis absorption spectrophotometer. Similar procedures were conducted for the recycling ability tests. The calibration curve of MO was prepared by measuring the absorbance of different predetermined concentrations of the samples. The photocatalytic activities of MO on TiO2 powder, TiO2/SiO2 (TS-800 °C) and PANi-coated SiO2 (P/S) nanofiber membranes were also carried out as reference samples for the purpose of comparison. SiO2 nanofiber membrane was prepared according to the literature [8].
Results and discussion Morphology and structure of polyaniline-coated TiO2/SiO2 nanofiber membranes TEOS and KH-560 were mixed into TiO2 sol to improve the mechanical performance of the nanofiber membrane. In consideration of both the mechanical property and photocatalytic activity, the optimized Ti/Si molar ratio and reaction time were determined through a series of experiments. Flexible TS nanofiber membranes are obtained and can be easily bent, as shown in Fig. 2A. Fig. 2B shows the SEM image of the TS nanofibers with uniform diameter and smooth surface. The work of Ding et al. [8] revealed that the SiO2 nanofiber mats were very flexible and therefore SiO2 probably functions as a flexibilizer in the TiO2–SiO2 system. With the reduction in surface defects, the number of crack initiation sites on the fiber surface is also likely to decrease, explaining the improved flexibility [12]. TGA curve in Fig. S1 suggests that PVP is removed completely by heating to 600 °C. Fig. 3 is the corresponding XRD patterns of the TS membranes calcined at 600, 700 and 800 °C, respectively. Curves (a) and (b) exhibit a diffuse pattern, showing the characteristics of amorphous TiO2 and SiO2. However, pure TiO2 is known to crystallize into anatase at a lower temperature of 400 °C [30]. Kim et al. reported that TiO2 in the composite was highly dispersed and did
not exist as crystals with sufficient size to be detected by XRD [31]. In the XRD pattern of the TS-800 °C sample, five typical peaks at 2h values of 25.3°, 37.9°, 48.1°, 54.0° and 62.8° can be indexed to (1 0 1), (0 0 4), (2 0 0), (1 0 5) and (2 0 4) planes of anatase TiO2, respectively. The diffraction intensities increase by increasing the calcination temperature, which indicates an improvement in the crystallinity of anatase TiO2. The results suggest that the formation of anatase phase would be extended to higher temperature in TiO2/ SiO2 composite fiber systems [12,32]. As shown in Fig. S2, the peaks centered at 2h = 9.3° and 14.9° are the characteristic doping diffraction peaks of PANi. The peaks at 2h = 20.8° and 25.4° corresponding to the periodicity parallel and perpendicular to PANi chains are also observed, respectively [33]. After PANi layers have been coated on the surface of TS nanofibers, the composite fibers exhibit a diffraction peak at 2h = 20.8° of PANi, which indicates a successful polymerization of PANi on TS nanofibers, while the peak at 2h = 25.4° of PANi and the peak at 2h = 25.3° of TiO2 almost overlap with each other. Raman spectroscopy has been applied as a powerful tool for characterizing the microstructure of nanomaterials. Fig. 4 demonstrates the Raman spectra of neat PANi powder, P/TS membrane and TS-800 °C membrane, respectively. The typical characteristic bands of PANi powder are observed in both neat PANi powder (Fig. 4a) and P/TS membrane (Fig. 4b). For instance, the C–C stretching vibration of the benzenoid and the C–N stretching of the cation radical are situated at 1594 cm1 and 1336 cm1. The bands at 1468, 1218, 1161, and 520 cm1 assigned to semiquinone radical cation structure, in-plane ring deformation, the C–H bending vibration in quinoid/phenyl groups and C–N–C torsion are also observed respectively [34,35]. The appearance of a band at 1336 cm1 corresponding to C–N stretching of the cation radical species confirms that PANi is in a doping state [35]. Moreover, the bands centered at 411 cm1 and 810 cm1 are related to the C–H deformation [36]. The Raman spectrum of TS-800 °C membrane shows three characteristic peaks of TiO2 at 515, 398, and 146 cm1 (Fig. 4c) [37]. On the contrary, the Raman signals for
Fig. 2. (A) Digital photo and (B) SEM image of the self-standing TS membrane.
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Fig. 3. XRD patterns of the TS membranes calcined at (a) 600, (b) 700, and (c) 800 °C.
TiO2 cannot be clearly observed in the P/TS membrane because the TS nanofibers have been covered by the PANi layers. Fig. S2 shows the SEM image of the PANi powder synthesized without using TS nanofibers. It can be seen that PANi particles are prone to form aggregates during the polymerization. Nevertheless, the in situ polymerization of aniline on the TS nanofiber surface serving as nucleation centers or growth template can effectively prohibit the agglomeration of PANi particles formed. Hence PANi nanoparticles can be densely and uniformly distributed on the surface of TS nanofibers (Fig. 5A and B). Compared with the morphology of PANi particles, the P/TS nanofiber membrane excels in flexibility and porosity and may have wider applications. The effect of reaction time on the morphology and properties of P/ TS nanofiber membranes was also investigated. The SEM images of the as-prepared P/TS nanofiber membranes (labeled as P/TS-0.5, P/ TS-1, P/TS-2, and P/TS-4) with reaction time of 0.5, 1, 2 and 4 h respectively are shown in Fig. 5. After reaction for 0.5 h, TS nanofibers became rough and were coated by a thin layer of PANi (as shown in Fig. 5A). However, some areas of TS nanofibers could not be fully covered by PANi nanoparticles because of the short polymerization time. Further increase in reaction time could
ensure the complete coating of TS nanofibers by PANi. The inset image in Fig. 5B shows the as-prepared PANi nanoparticles coated on TS nanofibers at high magnification, demonstrating that PANi nanoparticles with the diameter less than 100 nm are homogeneously covered on the surface of TS nanofibers. With further increase in the polymerization time, agglomeration of PANi particles began to occur, especially after 4 h, even leading to formation of many nanowires (as shown in Fig. 5C and D). When further prolonging polymerization time, the oligomers may be overlaid on the surface of TS nanofibers and react with each other, thus causing severe agglomeration of PANi particles and increase in the fiber diameter. As can be seen from Fig. 6, two weight loss regions appear in the TGA curve of PANi powder. The first weight loss of about 13% occurring from 110 to 400 °C can be attributed to desorption/ decomposition of adsorbed anions inherited from the synthesis, and the second one of about 84% from 400 to 680 °C results from the thermal decomposition of the PANi. The loading amounts of PANi on the TS nanofiber membranes are about 1.0%, 2.3%, 2.6%, and 5.1% for P/TS-0.5, P/TS-1, P/TS-2 and P/TS-4 samples, respectively. It confirms that the PANi loading increases gradually with the increase in polymerization time. Photocatalytic activities of polyaniline-coated TiO2/SiO2 nanofiber membranes The photocatalytic activities of the P/TS nanofiber membranes were evaluated by measuring the degradation of methyl orange (MO) at room temperature under visible-light irradiation. TiO2 powder, TS and P/S membranes were used as reference in comparison with the P/TS nanofibers. Before the photocatalytic degradation of MO, the photocatalyst was immersed in the solution and kept in the dark for 2 h in each condition to ensure the adsorption–desorption equilibrium. The absorbance values at 465 nm were chosen to illustrate the catalytic performance quantitatively. Fig. 7 shows the time dependent UV–vis spectra of the MO solution in the presence of P/TS-1. The obvious decrease in the absorption peaks of the UV absorption spectra of MO indicates an efficient degradation of MO on the membrane. The digital photos in the inset of Fig. 7 were taken to visually depict the whole degradation process. As can be seen from Fig. 8A, the degradation is negligible in the blank test of MO without any photocatalyst under visible illumination. The samples of TS nanofiber membrane and TiO2 powder have no notable effect on degradation of MO. By comparing the photocatalytic properties of P/S nanofiber membrane with the P/TS samples, we find that the latter ones exhibit better visible-light photocatalytic performance. The photocatalytic degradation efficiency of MO under visible light follows the order: P/TS-1 > P/TS0.5 > P/TS-2 > P/TS-4 > P/S > TS > TiO2 > blank. Among all the photocatalysts investigated here, the P/TS-1 nanofiber membrane possesses the best visible-light photocatalytic activity, as the MO is degraded by 87% in 90 min. The photodegradation of MO can fit pseudo-first-order kinetics in the range of 0–60 min (Fig. 8B):
lnðC 0 =CÞ ¼ kt
Fig. 4. Raman spectra of (a) neat PANi powder, (b) P/TS membrane, and (c) TS800 °C membrane.
where C0 is the initial concentration, C is the concentration of MO at time t, and the degradation rate constant k are 0.018 min1, 0.021 min1, 0.014 min1 and 0.010 min1 for P/TS-0.5, P/TS-1, P/ TS-2 and P/TS-4, respectively. When the polymerization time exceeds 1 h, the degradation rate of MO on P/TS samples decreases gradually under visible light though it is still superior to those on P/S membrane and TiO2. The above results reveal that the polymerization time of PANi has a significant influence on the photocatalytic activity of the as-prepared photocatalysts. We consider that the 3D open structure of the fibrous photocatalyst has a beneficial effect on the photocatalytic activity. By rough
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Fig. 5. SEM images of TS nanofiber membranes coated with PANi nanoparticles with different reaction time: (A) 0.5, (B) 1, (C) 2, and (D) 4 h. The insets in (A) and (B) are the SEM images at high magnifications.
Fig. 6. TGA curves of TS membrane, P/TS nanofiber membranes with different polymerization time and PANi powder. The inset shows low weight loss region of the curves.
Fig. 7. Time dependent UV–vis absorption spectra of the MO solution with P/TS-1. The insets are the corresponding digital photos of MO solutions at the same time when the UV–vis spectra are taken.
estimation, the fibrous structure is over 90% in porosity, which makes the diffusion of reactants easier. In addition, the surface of TS nanofibers contains a large number of hydroxyl groups, thus forming hydrogen bonding interactions with aniline monomers. Such a strong interaction ensures that the aniline monomers are uniformly adsorbed on the surface of TS nanofibers during the formation of the composites [38]. For P/TS photocatalysts, the positively charged backbone of PANi emeraldine salt on the surface undergoes chemical
interactions with the anionic dye (MO). Such an adsorption is favorable in promoting the photodegradation of the dyes [35]. Although TS nanofibers are nearly inactive under visible light, the combination of TS nanofibers with PANi leads to high photocatalytic activity for the degradation of MO. PANi has a band gap of 2.81 eV, narrower than that of TiO2, which shows absorption in the visible-light region. Thus PANi may function as a photosensitizer to TiO2. The excited-state electrons in PANi can readily migrate to the conduction band (CB) of TiO2 and
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Fig. 8. (A) Visible-light photocatalytic degradation curves and (B) kinetic linear fitting curves of MO on TiO2 powder, TS, P/S and P/TS nanofiber membranes with different polymerization time.
Fig. 9. (A) Recycling photocatalytic tests on P/TS-1 nanofiber membranes under visible-light irradiation, and (B) SEM image of the P/TS-1 membrane after five cycles. The inset in (B) is the SEM image at high magnification.
subsequently transfer to the surface to react with water and oxygen, yielding hydroxyl and superoxide radicals with strong oxidation capability [28]. Herein, fast photogenerated charge separation and relatively slow charge recombination occur, which significantly enhances the photocatalytic activity of the as-prepared P/ TS photocatalysts. The polymerization time of PANi (namely PANi loading amount) significantly influences the morphology of P/TS membrane and the photodegradation of MO (as shown in Fig. 8). The photocatalytic activity of P/TS membrane initially increases and then decreases with the increase in polymerization time. The optimum polymerization time for PANi on the TS nanofiber template is 1 h, which can be ascribed to the balance between the increase in PANi loading and the decrease in light absorption due to coating or coverage of PANi on the TS nanofibers. High amount of PANi loading prevents TiO2 from absorbing visible light, thus causing a rapid decrease in irradiation passing through the reaction system [21]. When the amount of PANi surpasses the threshold value, the excessive PANi nanoparticles tend to form a relatively thick layer and even aggregate on the surface of TS nanofibers, thus hindering the migration of excited electrons from the outer PANi layer to the inner TS nanofibers. Consequently, the number of radicals decreases and the photodegradation of the dye pollutant is thus affected [28]. Therefore, the amount of PANi loading should be adequately controlled.
The P/TS nanofiber membranes could be easily taken out of solutions after use owing to the excellent flexibility of the selfstanding TS nanofiber membranes. Furthermore, we studied the stability of the P/TS-1 nanofiber membrane with the best photocatalytic activity. Fig. 9A shows the recycling test results for the P/TS1 sample under visible-light irradiation. The degradation of MO slightly declines from 87% to 70% after five cycles, still providing a good performance. The most likely explanation for such a decrease is that the presence of residual organic dye in the nanofibers blocks part of active sites on the photocatalyst [39]. Fig. 9B is the SEM image of the membrane after five cycles of photocatalytic tests. It can be seen that the PANi nanoparticles are still densely immobilized on the surface of TS nanofibers after immersion in water for about 10 h (shown at high magnification in the inset image), and there is no apparent change compared with Fig. 5B. Obviously, the P/TS nanofiber membranes exhibit good photocatalytic stability for the photodegradation of dye pollutant. Conclusions In summary, PANi coated TiO2/SiO2 nanofiber membranes have been successfully prepared through an effective route. The new photocatalysts exhibit highly improved visible-light photocatalytic activity for the degradation of dye pollutant. The large specific surface area and porous 3D nanostructure of the composite
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membranes make their surface active sites fully accessible to reactants. The outstanding photoactivity of the P/TS may result from the synergistic effect of TiO2 and PANi. The optimum polymerization time for PANi proves to be 1 h, which can be attributed to the balance between the maximum increase in PANi loading and the minimum decrease in light absorption. The recycling tests reveal that the P/TS nanofiber membrane is stable and effective for the degradation of dye pollutant. The as-prepared nanofiber membranes prove to be flexible and easy to separate from the treated water, thus having potential applications in photocatalysis and water remediation fields. Acknowledgment The authors are grateful for financial support from the National Natural Science Foundation of China (51125011, 51373037). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis. 2014.03.009. References [1] X.Y. Zhang, Y.J. Sun, X.L. Cui, Z.Y. Jiang, Int. J. Hydrogen Energy 37 (2012) 811– 815. [2] H.J. Kim, Y.G. Shul, H.S. Han, Top. Catal. 35 (2005) 287–293. [3] W.Y. Dong, C.W. Lee, X.C. Lu, Y.J. Sun, W.M. Hua, G.S. Zhuang, S.C. Zhang, J.M. Chen, H.Q. Hou, D.Y. Zhao, Appl. Catal. B: Environ. 95 (2010) 197–207. [4] W.Y. Dong, Y.J. Sun, Q.W. Ma, L. Zhu, W.M. Hua, X.C. Lu, G.S. Zhuang, S.C. Zhang, Z.G. Guo, D.Y. Zhao, J. Hazard. Mater. 229 (2012) 307–320. [5] A. Greiner, J.H. Wendorff, Angew. Chem. Int. Ed. 46 (2007) 5670–5703. [6] X. Xia, S.L. Li, X. Wang, J.X. Liu, Q.F. Wei, X.W. Zhang, J. Mater. Sci. 48 (2013) 3378–3385. [7] P.S. Kumar, S. Nizar, J. Sundaramurthy, P. Ragupathy, V. Thavasi, S.G. Mhaisalkar, S. Ramakrishna, J. Mater. Chem. 21 (2011) 9784–9790. [8] M. Guo, B. Ding, X.H. Li, X.L. Wang, J.Y. Yu, M.R. Wang, J. Phys. Chem. C 114 (2010) 916–921. [9] D. Vu, Z.Y. Li, H.N. Zhang, W. Wang, Z.J. Wang, X.R. Xu, B. Dong, C. Wang, J. Colloid Interface Sci. 367 (2012) 429–435. [10] K. Yoon, Y. Yang, P. Lu, D.H. Wan, H.C. Peng, K.S. Masias, P.T. Fanson, C.T. Campbell, Y.N. Xia, Angew. Chem. Int. Ed. 51 (2012) 9543–9546.
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Contents lists available at ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Flexible polyaniline-coated TiO2/SiO2 nanofiber membranes with enhanced visible-light photocatalytic degradation performance Zhenyan Liu a, Yue-E. Miao a, Mingkai Liu a, Qianwei Ding a, Weng Weei Tjiu b, Xiaoli Cui c, Tianxi Liu a,⇑ a
State Key Laboratory of Molecular Engineering of Polymers, Department of Macromolecular Science, Fudan University, Shanghai 200433, PR China Institute of Materials Research and Engineering, A⁄STAR (Agency for Science, Technology and Research), 3 Research Link, Singapore 117602, Singapore c Department of Materials Science, Fudan University, Shanghai 200433, PR China b
a r t i c l e
i n f o
Article history: Received 12 January 2014 Accepted 1 March 2014 Available online 12 March 2014 Keywords: Titanium dioxide Silica Polyaniline Composite materials Visible-light photocatalysis Electrospinning
a b s t r a c t A simple and practical strategy has been developed for preparing polyaniline (PANi) coated TiO2/SiO2 nanofiber membranes by a combination of electrospinning, calcination and in situ polymerization. TiO2/SiO2 (TS) nanofibers are fabricated by electrospinning, followed by calcination. Then they are used as template for in situ polymerization of aniline monomers. SEM images show that PANi nanoparticles thus formed can be densely and uniformly coated on the surface of TS nanofibers. Photocatalytic degradation tests show that the as-prepared nanofiber membranes exhibit enhanced photocatalytic activity for degradation of methyl orange under visible light, which may be due to the synergistic effect of PANi and TiO2. Furthermore, the effect of polymerization time on the morphology and photocatalytic activity of the membrane is investigated. The free-standing membrane is flexible and easy to handle, which is promising for potential applications in photocatalysis and water remediation fields. Ó 2014 Elsevier Inc. All rights reserved.
Introduction Textile industry wastewater containing pigments and/or dyes can cause severe environmental pollution and the problem has urged relevant research on water decontamination. During recent decades, photocatalysis has attracted much attention in environmental restoration as a green and sustainable technology. Among various photocatalysts, TiO2 has been widely employed due to its lack of toxicity, low cost, high photocatalytic activity and photostability [1]. Despite these advantages, the practical applications of TiO2 need to deal with three major disadvantages: (1) TiO2 nanoparticles can easily form aggregates to minimize their surface area because of their high surface energy, which is unfavorable for photocatalytic reaction; (2) it is very difficult to separate TiO2 nanoparticles from treated water by conventional methods (including centrifugation and filtration), which may lead to loss of the photocatalyst and bring about secondary pollution; (3) the widespread applications of TiO2 are hindered by its low utilization of solar energy in the visible region. Therefore, there is an urgent need to develop a flexible nanostructured TiO2 photocatalyst with efficient visible photocatalytic activity.
⇑ Corresponding author. E-mail address: [email protected] (T. Liu). http://dx.doi.org/10.1016/j.jcis.2014.03.009 0021-9797/Ó 2014 Elsevier Inc. All rights reserved.
Many researchers have endeavored to introduce TiO2 on/into high-surface-area substrate. Among them, TiO2–SiO2 composites have been widely used in industrial applications and most extensively studied [2–4]. SiO2 is an excellent catalyst support because of its chemical inertia, thermal stability and adsorption of reactants. Particularly, the nanofibrous SiO2 supports have both high surface-area-to-volume ratio and favorable recycling properties. Electrospinning in combination with sol–gel processes has been proved to be a simple and effective method to produce polymer/ metal oxide (or metal sulfide) composite nanofibers. In many cases, the metal oxide (or metal sulfide) nanofibers can be obtained by subsequent pyrolysis [5–7]. Ding et al. fabricated the flexible and amphiphobic SiO2 nanofibrous mats via electrospinning the blend solutions of poly(vinyl alcohol) (PVA) and SiO2 gel, followed by calcination to remove the organic component [8]. Wang et al. prepared anatase mesoporous titanium nanofibers from calcination of the electrospun tetrabutyl titanate/poly(vinylpyrrolidone) (PVP)/pluronic123 (P123) composite nanofibers [9]. According to previous studies, fibrous photocatalysts are attractive candidates for practical applications because they possess a higher surfacearea-to-volume ratio and a three-dimensional (3D) open structure which make their surface active sites accessible to reactants more effectively. Xia et al. developed a new catalytic system by embedding Pt nanoparticles in the inner surfaces of CeO2 hollow fibers and the turnover frequency of the catalyst for CO oxidation was
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2–3 orders of magnitude higher than other systems [10]. ZnO quantum dots on hollow SiO2 nanofibers were prepared via electrospinning and exhibited high photocatalytic activity over Rhodamine B under ultraviolet light irradiation, which could be easily recycled due to its 1D nanostructure [11]. However, the TiO2 fibers obtained by electrospinning are usually quite brittle and easy to collapse into tiny pieces. To overcome this limitation, a number of methods have been reported to improve the flexibility or toughness of TiO2 films, which also contributes to easy separation and recycling. Sigmund et al. synthesized a flexible fiber mat composed of TiO2 and SiO2, offering potential applications as HEPA filters [12]. Fujishima et al. developed a method to integrate TiO2 nanoparticles (Degussa P25) into amorphous SiO2 nanofibers, which showed improved mechanical strength as well as good photocatalytic activity for reduction of silver ions and decomposition of acetaldehyde [13]. It is known that, owing to rather high intrinsic band gap of TiO2 (3.2 eV for anatase, 3.02 eV for rutile), only about 4% of the solar energy can be utilized. In addition, there exists a high rate of electron–hole recombination in TiO2. Various strategies have been employed to make TiO2 photocatalysts highly efficient under visible light, including dye sensitization, polymer modification, non-metals doping, semiconductor coupling, transition metal doping, and spatial structuring [14–16]. Conjugated polymers, such as polyaniline (PANi), poly(fluorine-co-thiophene) (PFT), polythiophene, polypyrrole and their derivatives can function as sensitizers to extend photoresponse of TiO2 into the visible region effectively [17–21]. PANi is one of the most fascinating conductive polymers and has received much attention for easy polymerization, high yield, relative high conductivities, low cost, and high stability [22–27]. Moreover, polymers are stable sensitizers in water compared to dyes because of their low solubility in water. Remarkable improvements in photocatalytic activity under visible light have been made by combining PANi and TiO2. Under visible-light irradiation, PANi generates p–p transition, injecting the excited electrons into the conduction band of TiO2, and then the electrons transfer to an adsorbed electron acceptor to yield oxygenous radicals [28]. Meanwhile, coupling photocatalysts with PANi proves to be an ideal system to promote photoinduced charge separation and inhibit charge recombination. Li et al. prepared a catalytic system of PANi/TiO2 nanocomposites, which exhibited higher photocatalytic activity and stability for degradation of methyl orange than the bare TiO2 and TiO2xNx under both UV and visible light irradiation [29]. In this work, we present a simple and practical strategy for preparing PANi coated TiO2/SiO2 (P/TS) nanofiber membranes by a combination of electrospinning, calcination and in situ polymerization. The nanofiber membranes thus obtained exhibit good flexibility, enhanced visible-light photocatalytic activity and recycling ability. The prepared membranes have large surface area and an open structure. Methyl orange (MO) is then used as the model dye pollutant to evaluate the photocatalytic degradation performance of the membranes. The effect of polymerization time on the morphology and photocatalytic activity of the membranes is also investigated.
was distilled under reduced pressure before use. Ammonium persulfate (APS) was used as received. All other reagents of analytical grade were purchased from Sinopharm Chemical Reagent Co., Ltd., and used without further purification. Synthesis of electrospun TiO2/SiO2 (TS) nanofiber membranes An aqueous TiO2/SiO2 sol was prepared through hydrolysis and polycondensation. 0.02 mol of H2O with a pH of 2.3 (pH adjusted by HNO3) was added to 0.008 mol of KH-560. Then, 0.002 mol of TEOS and 0.01 mol of Ti(OBu)4 were added to the above solution and stirred to produce a homogeneous solution. Then, 4 wt% PVP ethanol solution (12 mL) was slowly dropped into the TiO2/SiO2 sol to improve the electrospinnability. The mixture was vigorously stirred at room temperature for 5 h to produce a clear yellow solution. After that, the solution was transferred into a plastic syringe for electrospinning at a fixed electrical voltage of 15 kV. The pump speed was 2.0 mL/h, and the distance between the needle tip and collector was kept at 20 cm. The fiber membrane was collected on aluminum foil and then peeled off. The membranes were further calcined at 600, 700 and 800 °C respectively in air for 2 h to remove PVP, thus obtaining pure TiO2/SiO2 fibrous membranes for further experiments. The samples were labeled TS-600 °C, TS700 °C and TS-800 °C, respectively. Surface coating of polyaniline on TiO2/SiO2 (P/TS) nanofiber membranes Before coating PANi on TiO2/SiO2 (TS) nanofiber membranes, the as-prepared TS-800 °C membranes were rinsed with acetone (15 mL), ethanol (15 mL) and deionized water (15 mL) successively to increase their hydrophilicity. PANi coating was carried out via aniline polymerization on the surface of TS nanofibers in an icewater bath. First, pretreated TS membranes were soaked in 15 mL of aniline solution (1 mL of distilled aniline dissolved in 50 mL of 1 M HCl solution), followed by addition of 1 M HCl solution of APS (15 mL). The molar ratio of aniline and APS was 2:1. PANi emeraldine salt was formed after a few minutes and coated on the surface of TS nanofibers. After coating of PANi, the obtained P/TS membranes were taken out and washed with HCl solution to remove oligomer. Finally, the membranes were washed with deionized water for several times and dried in an oven at 40 °C overnight. The whole preparation procedure of polyaniline-coated TiO2/SiO2 fiber membrane is schematically shown in Fig. 1. Characterization X-ray diffraction (XRD) experiments were conducted on a PANalytical (X’Pert PRO) X-ray diffractometer using Cu Ka radiation (k = 0.1542 nm) at an acceleration voltage of 40 kV and a current of 40 mA. Raman spectra were obtained using a Renishaw inVia Reflex spectrometer with 632.8 nm laser excitation. Thermogravimetric analysis (Pyris 1 TGA) was performed under oxygen flow from 100 to 750 °C at a heating rate of 20 °C min1. Scanning electron microscope (SEM, Tescan) performed at an acceleration voltage of 20 kV was used to observe the morphology of samples.
Experimental
Measurements of the photocatalytic activities
Materials
Methyl orange (MO) was selected as the model dye indicator to evaluate the photocatalytic properties of the catalysts. Membranes with a dimension of 1.5 cm 0.8 cm were immersed into 3 mL of MO solution (1.5 mg/L) and stored in the dark for 2 h to achieve the adsorption equilibrium for MO. A 500 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
Poly(vinylpyrrolidone) (PVP, Mw = 1,300,000 g/mol) was purchased from Sigma–Aldrich. Tetraethyl orthosilicate (TEOS) was supplied by Shanghai Lingfeng Chemical Co., Ltd. Titanium nbutoxide (Ti(OBu)4) and KH-560 (3-Glycidyloxypropyltrimethoxysilane) were commercially obtained from Aladdin. Aniline (Ani)
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Fig. 1. Schematic illustration of the preparation of polyaniline coated TiO2/SiO2 nanofiber membrane.
solutions were analyzed at 465 nm using a Perkin–Elmer Lambda 35 UV–vis absorption spectrophotometer. Similar procedures were conducted for the recycling ability tests. The calibration curve of MO was prepared by measuring the absorbance of different predetermined concentrations of the samples. The photocatalytic activities of MO on TiO2 powder, TiO2/SiO2 (TS-800 °C) and PANi-coated SiO2 (P/S) nanofiber membranes were also carried out as reference samples for the purpose of comparison. SiO2 nanofiber membrane was prepared according to the literature [8].
Results and discussion Morphology and structure of polyaniline-coated TiO2/SiO2 nanofiber membranes TEOS and KH-560 were mixed into TiO2 sol to improve the mechanical performance of the nanofiber membrane. In consideration of both the mechanical property and photocatalytic activity, the optimized Ti/Si molar ratio and reaction time were determined through a series of experiments. Flexible TS nanofiber membranes are obtained and can be easily bent, as shown in Fig. 2A. Fig. 2B shows the SEM image of the TS nanofibers with uniform diameter and smooth surface. The work of Ding et al. [8] revealed that the SiO2 nanofiber mats were very flexible and therefore SiO2 probably functions as a flexibilizer in the TiO2–SiO2 system. With the reduction in surface defects, the number of crack initiation sites on the fiber surface is also likely to decrease, explaining the improved flexibility [12]. TGA curve in Fig. S1 suggests that PVP is removed completely by heating to 600 °C. Fig. 3 is the corresponding XRD patterns of the TS membranes calcined at 600, 700 and 800 °C, respectively. Curves (a) and (b) exhibit a diffuse pattern, showing the characteristics of amorphous TiO2 and SiO2. However, pure TiO2 is known to crystallize into anatase at a lower temperature of 400 °C [30]. Kim et al. reported that TiO2 in the composite was highly dispersed and did
not exist as crystals with sufficient size to be detected by XRD [31]. In the XRD pattern of the TS-800 °C sample, five typical peaks at 2h values of 25.3°, 37.9°, 48.1°, 54.0° and 62.8° can be indexed to (1 0 1), (0 0 4), (2 0 0), (1 0 5) and (2 0 4) planes of anatase TiO2, respectively. The diffraction intensities increase by increasing the calcination temperature, which indicates an improvement in the crystallinity of anatase TiO2. The results suggest that the formation of anatase phase would be extended to higher temperature in TiO2/ SiO2 composite fiber systems [12,32]. As shown in Fig. S2, the peaks centered at 2h = 9.3° and 14.9° are the characteristic doping diffraction peaks of PANi. The peaks at 2h = 20.8° and 25.4° corresponding to the periodicity parallel and perpendicular to PANi chains are also observed, respectively [33]. After PANi layers have been coated on the surface of TS nanofibers, the composite fibers exhibit a diffraction peak at 2h = 20.8° of PANi, which indicates a successful polymerization of PANi on TS nanofibers, while the peak at 2h = 25.4° of PANi and the peak at 2h = 25.3° of TiO2 almost overlap with each other. Raman spectroscopy has been applied as a powerful tool for characterizing the microstructure of nanomaterials. Fig. 4 demonstrates the Raman spectra of neat PANi powder, P/TS membrane and TS-800 °C membrane, respectively. The typical characteristic bands of PANi powder are observed in both neat PANi powder (Fig. 4a) and P/TS membrane (Fig. 4b). For instance, the C–C stretching vibration of the benzenoid and the C–N stretching of the cation radical are situated at 1594 cm1 and 1336 cm1. The bands at 1468, 1218, 1161, and 520 cm1 assigned to semiquinone radical cation structure, in-plane ring deformation, the C–H bending vibration in quinoid/phenyl groups and C–N–C torsion are also observed respectively [34,35]. The appearance of a band at 1336 cm1 corresponding to C–N stretching of the cation radical species confirms that PANi is in a doping state [35]. Moreover, the bands centered at 411 cm1 and 810 cm1 are related to the C–H deformation [36]. The Raman spectrum of TS-800 °C membrane shows three characteristic peaks of TiO2 at 515, 398, and 146 cm1 (Fig. 4c) [37]. On the contrary, the Raman signals for
Fig. 2. (A) Digital photo and (B) SEM image of the self-standing TS membrane.
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Fig. 3. XRD patterns of the TS membranes calcined at (a) 600, (b) 700, and (c) 800 °C.
TiO2 cannot be clearly observed in the P/TS membrane because the TS nanofibers have been covered by the PANi layers. Fig. S2 shows the SEM image of the PANi powder synthesized without using TS nanofibers. It can be seen that PANi particles are prone to form aggregates during the polymerization. Nevertheless, the in situ polymerization of aniline on the TS nanofiber surface serving as nucleation centers or growth template can effectively prohibit the agglomeration of PANi particles formed. Hence PANi nanoparticles can be densely and uniformly distributed on the surface of TS nanofibers (Fig. 5A and B). Compared with the morphology of PANi particles, the P/TS nanofiber membrane excels in flexibility and porosity and may have wider applications. The effect of reaction time on the morphology and properties of P/ TS nanofiber membranes was also investigated. The SEM images of the as-prepared P/TS nanofiber membranes (labeled as P/TS-0.5, P/ TS-1, P/TS-2, and P/TS-4) with reaction time of 0.5, 1, 2 and 4 h respectively are shown in Fig. 5. After reaction for 0.5 h, TS nanofibers became rough and were coated by a thin layer of PANi (as shown in Fig. 5A). However, some areas of TS nanofibers could not be fully covered by PANi nanoparticles because of the short polymerization time. Further increase in reaction time could
ensure the complete coating of TS nanofibers by PANi. The inset image in Fig. 5B shows the as-prepared PANi nanoparticles coated on TS nanofibers at high magnification, demonstrating that PANi nanoparticles with the diameter less than 100 nm are homogeneously covered on the surface of TS nanofibers. With further increase in the polymerization time, agglomeration of PANi particles began to occur, especially after 4 h, even leading to formation of many nanowires (as shown in Fig. 5C and D). When further prolonging polymerization time, the oligomers may be overlaid on the surface of TS nanofibers and react with each other, thus causing severe agglomeration of PANi particles and increase in the fiber diameter. As can be seen from Fig. 6, two weight loss regions appear in the TGA curve of PANi powder. The first weight loss of about 13% occurring from 110 to 400 °C can be attributed to desorption/ decomposition of adsorbed anions inherited from the synthesis, and the second one of about 84% from 400 to 680 °C results from the thermal decomposition of the PANi. The loading amounts of PANi on the TS nanofiber membranes are about 1.0%, 2.3%, 2.6%, and 5.1% for P/TS-0.5, P/TS-1, P/TS-2 and P/TS-4 samples, respectively. It confirms that the PANi loading increases gradually with the increase in polymerization time. Photocatalytic activities of polyaniline-coated TiO2/SiO2 nanofiber membranes The photocatalytic activities of the P/TS nanofiber membranes were evaluated by measuring the degradation of methyl orange (MO) at room temperature under visible-light irradiation. TiO2 powder, TS and P/S membranes were used as reference in comparison with the P/TS nanofibers. Before the photocatalytic degradation of MO, the photocatalyst was immersed in the solution and kept in the dark for 2 h in each condition to ensure the adsorption–desorption equilibrium. The absorbance values at 465 nm were chosen to illustrate the catalytic performance quantitatively. Fig. 7 shows the time dependent UV–vis spectra of the MO solution in the presence of P/TS-1. The obvious decrease in the absorption peaks of the UV absorption spectra of MO indicates an efficient degradation of MO on the membrane. The digital photos in the inset of Fig. 7 were taken to visually depict the whole degradation process. As can be seen from Fig. 8A, the degradation is negligible in the blank test of MO without any photocatalyst under visible illumination. The samples of TS nanofiber membrane and TiO2 powder have no notable effect on degradation of MO. By comparing the photocatalytic properties of P/S nanofiber membrane with the P/TS samples, we find that the latter ones exhibit better visible-light photocatalytic performance. The photocatalytic degradation efficiency of MO under visible light follows the order: P/TS-1 > P/TS0.5 > P/TS-2 > P/TS-4 > P/S > TS > TiO2 > blank. Among all the photocatalysts investigated here, the P/TS-1 nanofiber membrane possesses the best visible-light photocatalytic activity, as the MO is degraded by 87% in 90 min. The photodegradation of MO can fit pseudo-first-order kinetics in the range of 0–60 min (Fig. 8B):
lnðC 0 =CÞ ¼ kt
Fig. 4. Raman spectra of (a) neat PANi powder, (b) P/TS membrane, and (c) TS800 °C membrane.
where C0 is the initial concentration, C is the concentration of MO at time t, and the degradation rate constant k are 0.018 min1, 0.021 min1, 0.014 min1 and 0.010 min1 for P/TS-0.5, P/TS-1, P/ TS-2 and P/TS-4, respectively. When the polymerization time exceeds 1 h, the degradation rate of MO on P/TS samples decreases gradually under visible light though it is still superior to those on P/S membrane and TiO2. The above results reveal that the polymerization time of PANi has a significant influence on the photocatalytic activity of the as-prepared photocatalysts. We consider that the 3D open structure of the fibrous photocatalyst has a beneficial effect on the photocatalytic activity. By rough
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Fig. 5. SEM images of TS nanofiber membranes coated with PANi nanoparticles with different reaction time: (A) 0.5, (B) 1, (C) 2, and (D) 4 h. The insets in (A) and (B) are the SEM images at high magnifications.
Fig. 6. TGA curves of TS membrane, P/TS nanofiber membranes with different polymerization time and PANi powder. The inset shows low weight loss region of the curves.
Fig. 7. Time dependent UV–vis absorption spectra of the MO solution with P/TS-1. The insets are the corresponding digital photos of MO solutions at the same time when the UV–vis spectra are taken.
estimation, the fibrous structure is over 90% in porosity, which makes the diffusion of reactants easier. In addition, the surface of TS nanofibers contains a large number of hydroxyl groups, thus forming hydrogen bonding interactions with aniline monomers. Such a strong interaction ensures that the aniline monomers are uniformly adsorbed on the surface of TS nanofibers during the formation of the composites [38]. For P/TS photocatalysts, the positively charged backbone of PANi emeraldine salt on the surface undergoes chemical
interactions with the anionic dye (MO). Such an adsorption is favorable in promoting the photodegradation of the dyes [35]. Although TS nanofibers are nearly inactive under visible light, the combination of TS nanofibers with PANi leads to high photocatalytic activity for the degradation of MO. PANi has a band gap of 2.81 eV, narrower than that of TiO2, which shows absorption in the visible-light region. Thus PANi may function as a photosensitizer to TiO2. The excited-state electrons in PANi can readily migrate to the conduction band (CB) of TiO2 and
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Fig. 8. (A) Visible-light photocatalytic degradation curves and (B) kinetic linear fitting curves of MO on TiO2 powder, TS, P/S and P/TS nanofiber membranes with different polymerization time.
Fig. 9. (A) Recycling photocatalytic tests on P/TS-1 nanofiber membranes under visible-light irradiation, and (B) SEM image of the P/TS-1 membrane after five cycles. The inset in (B) is the SEM image at high magnification.
subsequently transfer to the surface to react with water and oxygen, yielding hydroxyl and superoxide radicals with strong oxidation capability [28]. Herein, fast photogenerated charge separation and relatively slow charge recombination occur, which significantly enhances the photocatalytic activity of the as-prepared P/ TS photocatalysts. The polymerization time of PANi (namely PANi loading amount) significantly influences the morphology of P/TS membrane and the photodegradation of MO (as shown in Fig. 8). The photocatalytic activity of P/TS membrane initially increases and then decreases with the increase in polymerization time. The optimum polymerization time for PANi on the TS nanofiber template is 1 h, which can be ascribed to the balance between the increase in PANi loading and the decrease in light absorption due to coating or coverage of PANi on the TS nanofibers. High amount of PANi loading prevents TiO2 from absorbing visible light, thus causing a rapid decrease in irradiation passing through the reaction system [21]. When the amount of PANi surpasses the threshold value, the excessive PANi nanoparticles tend to form a relatively thick layer and even aggregate on the surface of TS nanofibers, thus hindering the migration of excited electrons from the outer PANi layer to the inner TS nanofibers. Consequently, the number of radicals decreases and the photodegradation of the dye pollutant is thus affected [28]. Therefore, the amount of PANi loading should be adequately controlled.
The P/TS nanofiber membranes could be easily taken out of solutions after use owing to the excellent flexibility of the selfstanding TS nanofiber membranes. Furthermore, we studied the stability of the P/TS-1 nanofiber membrane with the best photocatalytic activity. Fig. 9A shows the recycling test results for the P/TS1 sample under visible-light irradiation. The degradation of MO slightly declines from 87% to 70% after five cycles, still providing a good performance. The most likely explanation for such a decrease is that the presence of residual organic dye in the nanofibers blocks part of active sites on the photocatalyst [39]. Fig. 9B is the SEM image of the membrane after five cycles of photocatalytic tests. It can be seen that the PANi nanoparticles are still densely immobilized on the surface of TS nanofibers after immersion in water for about 10 h (shown at high magnification in the inset image), and there is no apparent change compared with Fig. 5B. Obviously, the P/TS nanofiber membranes exhibit good photocatalytic stability for the photodegradation of dye pollutant. Conclusions In summary, PANi coated TiO2/SiO2 nanofiber membranes have been successfully prepared through an effective route. The new photocatalysts exhibit highly improved visible-light photocatalytic activity for the degradation of dye pollutant. The large specific surface area and porous 3D nanostructure of the composite
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membranes make their surface active sites fully accessible to reactants. The outstanding photoactivity of the P/TS may result from the synergistic effect of TiO2 and PANi. The optimum polymerization time for PANi proves to be 1 h, which can be attributed to the balance between the maximum increase in PANi loading and the minimum decrease in light absorption. The recycling tests reveal that the P/TS nanofiber membrane is stable and effective for the degradation of dye pollutant. The as-prepared nanofiber membranes prove to be flexible and easy to separate from the treated water, thus having potential applications in photocatalysis and water remediation fields. Acknowledgment The authors are grateful for financial support from the National Natural Science Foundation of China (51125011, 51373037). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis. 2014.03.009. References [1] X.Y. Zhang, Y.J. Sun, X.L. Cui, Z.Y. Jiang, Int. J. Hydrogen Energy 37 (2012) 811– 815. [2] H.J. Kim, Y.G. Shul, H.S. Han, Top. Catal. 35 (2005) 287–293. [3] W.Y. Dong, C.W. Lee, X.C. Lu, Y.J. Sun, W.M. Hua, G.S. Zhuang, S.C. Zhang, J.M. Chen, H.Q. Hou, D.Y. Zhao, Appl. Catal. B: Environ. 95 (2010) 197–207. [4] W.Y. Dong, Y.J. Sun, Q.W. Ma, L. Zhu, W.M. Hua, X.C. Lu, G.S. Zhuang, S.C. Zhang, Z.G. Guo, D.Y. Zhao, J. Hazard. Mater. 229 (2012) 307–320. [5] A. Greiner, J.H. Wendorff, Angew. Chem. Int. Ed. 46 (2007) 5670–5703. [6] X. Xia, S.L. Li, X. Wang, J.X. Liu, Q.F. Wei, X.W. Zhang, J. Mater. Sci. 48 (2013) 3378–3385. [7] P.S. Kumar, S. Nizar, J. Sundaramurthy, P. Ragupathy, V. Thavasi, S.G. Mhaisalkar, S. Ramakrishna, J. Mater. Chem. 21 (2011) 9784–9790. [8] M. Guo, B. Ding, X.H. Li, X.L. Wang, J.Y. Yu, M.R. Wang, J. Phys. Chem. C 114 (2010) 916–921. [9] D. Vu, Z.Y. Li, H.N. Zhang, W. Wang, Z.J. Wang, X.R. Xu, B. Dong, C. Wang, J. Colloid Interface Sci. 367 (2012) 429–435. [10] K. Yoon, Y. Yang, P. Lu, D.H. Wan, H.C. Peng, K.S. Masias, P.T. Fanson, C.T. Campbell, Y.N. Xia, Angew. Chem. Int. Ed. 51 (2012) 9543–9546.
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