Fibers and Polymers 2015, Vol.16, No.6, 1336-1342 DOI 10.1007/s12221-015-1336-7
ISSN 1229-9197 (print version) ISSN 1875-0052 (electronic version)
Synthesis and Characterization of Photocatalytic and Antibacterial PAN/Ag2CO3 Composite Nanofibers by Ion Exchange Method Prem Singh Saud1,2, Bishweshwar Pant1, Zafar Khan Ghouri1, Gopal Panthi1, Soo-Jin Park4, Weidong Han1, Mira Park3*, and Hak-Yong Kim1,3* 1
Department of BIN Fusion Technology, Chonbuk National University, Jeonju 561-756, Korea 2 Department of Chemistry, Tribhuvan University, Dhangadhi 977, Nepal 3 Department of Organic Materials and Fiber Egineering,Chonbuk National University, Jeonju 561-756, Korea 4 Department of Chemistry, Inha University, Incheon 402-751, Korea (Received January 27, 2015; Revised April 15, 2015; Accepted May 13, 2015) Abstract: Highly photocatalytic and antibacterial Ag2CO3 nanoparticles were incorporated into PAN nanofibers by electrospinning technique followed by ion exchange reaction between silver nitrate and sodium bicarbonate at room temperature. The samples were characterized by field-emission scanning electron microscopy (FE-SEM), Transmission electron microscopy (TEM), X-ray diffraction (XRD), photoluminescence (PL), and Fourier transformed infrared (FT-IR) spectroscopy. The as prepared sample was found to exhibit an excellent photocatalytic activity toward degradation of methylene blue (MB) in aqueous solution and antibacterial properties against Escherichia coli (E. coli) under visible light. The perfect recovery of catalyst after reaction and its unchanged efficiency for cyclic use showed that it will be an economically and environmentally friendly photocatalyst for the water purification. Keywords: Photocatalyst, Antibacterial, Ion exchange reaction, Nanoparticles, PAN/Ag2CO3 Composites
of the conduction band (CB) that consists of relatively delocalized s and/or p orbitals which are largely dispersed and can accommodate high photogenetated electrons and holes mobility resulting to the enhancement of photocatalytic activity [8]. Incorporation of p-block element into a narrow band gap oxide develops a new highly efficient semiconductor photocatalysts [7,8,11]. Ag2CO3 semiconductor photocatalyst has been frequently investigated due to its highly efficient photocatalytic activity for the dye degradation and antibacterial properties. However, very poor stability due to the photocorrosion resulting from metallic Ag formation limits its application [12]. Recently, to improve the photocatalytic activity or stability of Ag2CO3, heterojunction composites semiconductors have been investigated [13-15]. As-synthesized heterojunction composites semiconductors generally possessed randomness, uncontrollable combination, irregular structure and are usually obtained in powder or granular form, which in many cases constrains their use for water purification and also for reusability. Therefore, fabrication organic polymer/Ag2CO3 composite nanofibers might extensively improve the catalytic performance or stability of Ag2CO3 and may provide a sufficient area for interaction without agglomeration of semiconductor NPs during water treatment. Regarding this, electrospinning technique has been proven to be a simple, effective, low cost, and versatile technique to fabricate micro-to nanoscale fibers having prominent features like high specific surface area and large aspect ratio. These properties of nanofibers make it a suitable candidate in some predominant fields like sensors, tissue engineering, water purification, and etc., [16,17]. From the past decade, PAN has been used as stabilizers
Introduction Organic pollutants and toxic water pollutants produced by some industries result in a serious environmental problem. Recent years, semiconductor photocatalysts have great potential attention in environmental remediation in the field of waste water treatment due to their extremely fast degradation rate and low toxicity [1]. In the past decade, various visible light sensitive semiconductor photocatalysts like CdS/TiO2 [2], TiO2/ZnO [3], TiO2/RGO/nylon-6 [4], SrCO3 [5], C3N4-TiO2 [6] etc, have been developed and reported to be good photocatalysts either in decomposition of organic compounds or antibacterial activities utilizing visible light. It is well known that photodegradation of organic pollutants and destruction of bacteria using semiconductor involves the generation of electrons and holes pair, migrating to the surface of semiconductor, and react with adsorbed organic molecules and cell of bacteria by a series of redox process, in consequence, giving rise to decomposing organic pollutants and destruction of bacteria. In recent year, Ag-containing nanocomposites such as Ag2CO3 [7], AgSbO3 [8], Ag2CO3/TiO2 [9], Ag3PO4/TiO2 [10] have been investigated due to their promising highefficient photocatalytic and antibacterial effects. In the case of Ag-containing transition metal complex oxide semiconductor, the top of the valence band (VB) of them consists of hybridized Ag 4d and O 2p orbitals. This hybridization lifts energy at the top of the valance band to a higher energy and makes the band-gap narrower. On the other hand, the bottom *Corresponding author:
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Photocatalytic and Antibacterial PAN/Ag2CO3 Hybrid Mats
and produce variety of synthetic fibers [18]. Furthermore, PAN-based electrospun nanofiber has been used as reusable catalyst due to its hydrophobicity, low density, and high environmental stability properties [19]. In the PAN/Ag2CO3 composite nanofiber, PAN act as a polymer template and Ag2CO3 nanoparticles have been exposed completely and densely on the fiber surface without agglomeration, thus they could share the photo-excited electrons and achieve novel collective photocatalytic and antibacterial properties. In recent trend, numerous organic polymer/NPs composite nanofiber like cellulose/TiO2 [20], nylon6/TiO2 [21], nylon6/ Ag [22], PAN/ZnS [23], PU/TiO2 [24], PCL/TiO2 [25] and etc., have been investigated for the degradation of organic dye as well as antibacterial activities under visible or UV light. Generally, the loading of semiconductor NPs into the nanofiber is achieved by adding the NPs in the polymer solution prior to the electrospinning or post treatment of electrospun nanofibers. Mixing of the NPs in the polymer solution prior to electrospinning may cause aggregation of NPs where as loading of NPs after electrospinning process possesses poor attachment of NPs and lacks reusability. Moreover, it is difficult to decorate the nano sized fibers by micro sized semiconductor NPs. Therefore, it is desirable to develop a facile and feasible approach to achieve a good dispersion of NPs throughout polymer nanofiber surfaces. In this contribution, we demonstrated the fabrication of Ag2CO3 NPs decorated PAN nanofibers by simple and versatile electrospinning process followed by ion exchange reaction between silver nitrate and sodium bicarbonate. The polymer not only provides the reactive sites for in situ ion exchange with ions, but also performs as clapboards to spatially separate NPs, avoiding their aggregation [26]. The improved properties of composite mat compared to the pristine PAN nanofiber were evaluated for photocatalytic dye degradation and antibacterial performances.
Experimental Materials N,N-dimethylformamide (DMF, 99.5 assay, Showa Chemical Ltd., Japan), polyacrylonitrile (PAN, MW 150,000 g/mol, Sigma-Aldrich), methylene blue (Showa Chemical Ltd., Japan), silver nitrate (Showa Chemical Ltd., Japan) were used in this study without further treatment. Preparation of PAN/Ag2CO3 Nanofiber First of all, 10 % PAN solution was prepared by dissolving the polymer granules in N,N-dimethylformamide (DMF) with vigorous stirring at room temperature to form homogenous solution. After stirring at room temperature for 12 h, solution having 1 and 5 wt% of silver nitrate (based on polymer solution) were prepared. Thus prepared sol-gel solution was subjected to electrospining at 15 kV maintaining a tip- tocollector distance of 15 cm. The obtained PAN/ AgNO3 fiber
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mats were dried for 2 h in the air in order to remove the residual solvent. For the fabrication of PAN/Ag2CO3 nanofiber, as-synthesized electrospun PAN/AgNO3 mats were immersed into a NaHCO3 aqueous solution (0.2 M) containing 0.1 M PVP at room temperature for ion exchange reaction. Within a few min, the color of the composite nanofibers was changed from white to yellow indicating the formation of Ag2CO3 nanoparticles on the surface of polymer nanofibers via the reaction of Ag+ with HCO3−. At last, as-prepared nanofibers were washed several times with distilled water to remove the PVP residue and immediately dried at 60 oC for 1 h. The mats obtained from 1 %, and 5 % AgNO3 precursor solution were represented as S1, and S2, respectively. Characterization The morphology was investigated using FE-SEM (S4700, Hitachi, Japan). The EDX spectrum of PAN/Ag2CO3 nanofibers were also recorded with the same FE-SEM instrument. High resolution images of different NPs were obtained via transmission electron microscopy (TEM, JEM2010, JEOL, Japan). Information about the phase and crystallinity was obtained with a Rigaku X-ray diffractometer (XRD, Rigaku, Japan) with Cu K (=1.540 A)˚ radiation over Bragg angles ranging from 100 to 600. The bonding configuration of the polymer with Ag2CO3 NPs was characterized by means of Fourier-transform infrared (FTIR, FT/IR4200, Jasco international Co., Ltd.). The UV-visible spectra were obtained with a UV-visible spectrometer (Lambda 600, PerkinElmer, USA) over the range of 200-800 nm. Photoluminescence (PL) spectrum was recorded by Perkin Elmer Instruments (LS-55). Photocatalytic Activity Investigation Photocatalytic activities of PAN/Ag2CO3 nanofiber photocatalysts were evaluated by monitoring the photodegradation of methylene blue aqueous solution under solar light irradiation according to our previous work [27]. The experiment was conducted in natural environment on sunny day (September, 2014) between 11:0 a.m. to 3:0 p.m. For the photodegradation experiments, 5×5 cm2 (about 150 mg) PAN/Ag2CO3 nanofiber was put in 50 ml of a 10 ppm MB aqueous solution. Under magnetic stirring, the mixed solution was irradiated under sunlight. In addition, a control experiment with 150 mg of pristine PAN mat and catalyst free were also carried out to monitor photocatalytic activity of PAN mat and self-degradation of dye, respectively. At regular intervals of time, 2 ml of aliquots were taken out and the concentration of the dye was measured by recording the UV absorbance in the range of 200-900 nm, using a UV-vis spectrophotometer. In this experiment, the ability test of reused PAN/Ag2CO3 mat was also performed after full treatment. For this purpose, the used mat was washed several times with distilled water and then photodegradation of MB dye was carried out under the same aforementioned condition.
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Antibacterial Property Antibacterial activity of pristine PAN, S1, and S2 mats were investigated by the zone inhibition method using Escherichia coli (E. coli) as the model organism at room condition. Using a spread plate method, one colony of E. coli was taken out from the original stock in an agar plate, centrifuged at 200 rpm/min and was cultured lysogeny broth (LB) medium and were grown overnight in LB medium at 37 oC for 24 h. Different nanofibers mats with same dimension were transferred on the inoculated plates, and were then incubated at 37 oC for 24 h.
Results and Discussion Figure 1 shows the morphology of different mats with distribution of Ag2CO3 NPs on the surface of nanofiber. As in Figure 1(a), pristine nanofibers exhibited smooth, bead free, and continuous morphology with average diameter of ~310 nm whereas treating the PAN/AgNO3 mat with NaHCO3 led to the formation of Ag2CO3 NPs which are attached to the surface of nanofibers (Figure 1(b) and (c)). Both structure and distribution of Ag2CO3 NPs had been rationally tailored by adjusting the content of AgNO3 in precursor solution. In S1 mat, only few NPs can be seen on the surface of the nanofiber which is due to the lesser content of AgNO3 (1 %)
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in the nanofiber. The average diameter of the S1 sample was found to be slightly increased (~400 nm). After increasing the content of AgNO3 to 5 %, we achieved a uniform layer of Ag2CO3 NPs distributed throughout the PAN nanofibers (Figure 1(c)) and the average diameter of the nanofiber was further increased to ~700 nm. To clearly assess the assembling structure of the Ag2CO3 NPs on the surface of nanofibers, TEM image was taken. Figure 1(d) shows the TEM image of S2 sample. As in the figure, it can be seen clearly that the NPs are well attached on the surface of nanofibers. The typical XRD pattern of the pristine PAN, S1 and, S2 mats are shown in Figure 2(a). In pristine PAN nanofiber, a crystalline peak centers at about 17 o can be assigned to the PAN polymer phase. PAN/Ag2CO3 composite nanofiber mat showed extra peaks at around 18.61 o, 20.60 o, 32.76 o, 39.75 o, 43.45 o, 49.01 o, 53.01 o, and 57.73 o corresponding to the crystalline plane of (021), (110), (-101), (-130), (031), (131), (121), (150), and (060) respectively, which confirms the formation of PAN/Ag2CO3 composite [28]. However, only few XRD peaks can be seen on S1 mat nanofiber as compared to that of S2 mat nanofiber due to lesser content of Ag2CO3 NPs on the PAN nanofiber. Furthermore, EDX results obtained from FE-SEM images (Figure 2(b)) also showed the presence of silver, carbon, and oxygen element in composite nanofibers which confirms the incorporation of
Figure 1. FE-SEM images of pristine PAN nanofibers mat (a), S1 mat (b), and S2 mat (c), and TEM image of S2 mat (d).
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Figure 2. XRD pattern of different mats (a) and FE-SEM EDX of S2 mat (b).
Figure 3. PL measurements for the pristine PAN nanofibers compared to S2 nanofiber mat.
Ag2CO3 NPs in PAN nanofibers and simultaneously supports the XRD analysis. PL emission pairs in semiconductor particles and to investigate the efficiency spectra have been widely used to understand the recombination rate of electron/hole of charge carrier trapping, migration and transfer. The photoluminescence (PL) spectra of PAN nanofiber and PAN/Ag2CO3 nanofiber composites can be seen in Figure 3. The intensity of the PL spectra is an indicator for the electrons/hole recombination rate, low intensity indicates lower combination and vice versa [29]. From the figure, it can be observed that PAN/ Ag2CO3 nanocomposites exhibit lower emission intensity than that of Ag2CO3 nanofiber. The lower PL intensity indicates low recombination rate, which is preferable in case of utilizing the materials as catalysts in the photoreactions [27]. Fourier transformation infrared spectroscopy (FTIR) analysis was performed in our study in order to investigate the interaction between the polymer and the inorganic phase. Figure 4 shows the typical FT-IR spectra of PAN and PAN/ Ag2CO3 composite nanofibers. PAN nanofiber shows the
Figure 4. FT-IR spectra of pristine PAN nanofiber and S2 nanofiber mats.
characteristic absorption peaks of the nitrile group at around 2241 cm-1, besides, a series of characteristic bands in the regions 2930 cm-1, 1451 cm-1, 1380-1360 cm-1, and 12901260 cm-1 are ascribed to the vibrations of different modes in methylene group of PAN. The characteristic absorption band of CO32− could be observed at 705 cm-1, indicating the formation of the Ag2CO3 on the PAN/Ag2CO3 nanofiber [19,30]. Electrospun membranes are emerging materials in the water purification field as filter membrane. However, filtration efficiency of as synthesized nanofiber has been hindered by poor mechanical strength of membrane [31]. Therefore, we observed the effect of Ag2CO3 nanoparticle on mechanical properties of composites nanofiber. The tensile strength of different as-synthesized nanofiber is shown in Figure 5. It revealed that S2 mat has the highest tensile strength than that of S1 and pristine nanofiber. Here, number of Ag2CO3 nanoparticles incorporated in nanofiber might play crucible
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Figure 5. Stress-strain curves of pristine PAN nanofiber mat, S1 mat, and S2 mat.
role to enhance mechanical properties of as-synthesized nanofiber. Increase in mechanical strength with inclusion of nanoparticle has been also reported in the literature [32]. Numerous uniform nanoparticles dispersed in the polymer increase the interaction between the nanoparticles and polymer matrix and consequently improve the mechanical properties of composite nanofiber [33]. The photocatalytic behavior of the products was evaluated for the degradation of methylene blue (MB) dye under solar light irradiation. As shown in Figure 6(a), the rate of degradation of methylene blue by PAN/Ag2CO3 nanofiber is significantly higher than that of PAN nanofiber. This significantly higher rate of MB degradation could be explained by combined degradation properties of Ag2CO3 nanofiber and absorption properties of PAN [7]. The reuse of catalysts is very important parameter in assessing the practical application of photocatalysts in waste water treatment. In order to evaluate the reusability, PAN/Ag2CO3 composite photocatalysts was recycled and reused for three cycles, and the photocatalytic performances are shown in Figure 6(b). The efficacy of the nanocomposite was found to be slightly decreased on the successive cycles which may be due to the blockage of active absorption sites of PAN nanofiber. Mechanism of the photocatalytic activity of the Ag2CO3 NPs has been already discussed in literature [7,34]. Briefly, the solar light radiation leads to excite the electrons from the valence band of to the conduction level of Ag2CO3 leaving holes behind. These photogenerated electrons and holes migrate to the surface of Ag2CO3. Then photogenerated electrons are captured by dissolved O2 in the dyes solutions to produce •O2− radicals. These •O2− radicals can directly oxidize dyes or/and immediately react with H+ ions to generate H2O2, following on converting Into •OH radicals to oxidize dyes. Simultaneously, photogenetated holes can directly oxidize dyes, as well as react with H2O and/or OH−
Figure 6. Comparison of the MB photodegradation by different specimens under solar radiation (a) and the catalytic reusability of S2 nanofiber mat up to three cycles (b).
ions to produce •OH, then oxidizing dyes [35]. The goal of this work is not only to make photocatalytic materials but also perform antibacterial activities with the same material. Therefore, the antibacterial performance of as-prepared nanocomposites was carried out by the zone inhibition test method using E. coli as the model organism, which is shown in Figure 6. Pristine PAN nanofibers mat showed no zones of inhibition, suggesting no antibacterial activity towards the E. coli. On the other hand, PAN/Ag2CO3 composite nanofibers showed zones of inhibition which confirms that the composite nanofibers mat have antibacterial efficiency. The antibacterial effect of the S2 mat was pronounced higher than that of S1 mat as larger inhibition zone can be seen in the case of S2 mat. The higher antibacterial activity of S2 mat is due to the higher content of Ag2CO3 NPs on the surface of nanofibers as observed by FESEM and TEM images (Figure 1(c) and 1(d)). The mechanism of antibacterial activity of Ag2CO3 towards E.
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References
Figure 7. Bactericidal activity of E. coli, exposed to pristine mat (a), S1 mat (b), and S2 mat (c).
coli is similar to the dye degradation. Ag2CO3 is a semiconductor and it can generate electron-hole pairs under ambient light [36]. The electron-hole pairs react with O2 or OH− to give rise to active oxygen species, which are able to react with DNA, cell membranes, and cellular proteins, leading to bacterial cell death [37-39].
Conclusion PAN/Ag2CO3 nanofibers were fabricated by simple electrospinning technique followed by ion exchange reaction between silver nitrate and sodium bicarbonate at room temperature. The as-synthesized PAN/Ag2CO3 nanofibers displayed much enhanced photocatalytic activity in the degradation of MB as well as destruction of E. coli in comparing with pristine PAN nanofiber under visible light irradiation. The above demonstration revealed that asprepared materials have potential as economically friendly photocatalyst and water filter membrane due to its reusability.
Acknowledgments This research was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MISP) (Grant Number 2014R1A4A1008140). This research was also supported by Ministry of Education, Science Technology (MEST) through the Human Resources and Training Project for Regional Innovation (Grant Number 2012-10-A-04-046-13-010100). We thank Ms. Hyelan Kim, Mr. Jong-Gyun Kang, and Mrs. Yun-Young Choi, Center for University Research Facility, for taking high-quality FESEM, TEM, and uv-vis spectrometer, respectively.
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