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Photocatalytic zinc oxide on flexible polyacrylonitrile nanofibers via sol–gel coaxial electrospinning
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Photocatalytic zinc oxide on flexible polyacrylonitrile nanofibers via sol–gel coaxial electrospinning Rungthiwa Methaapanon, Kanchat Chutchakul, Varong Pavarajarn∗ Centre of Excellence in Particle and Material Processing Technology, Department of Chemical Engineering, Chulalongkorn University, Bangkok, Thailand
A R T I C LE I N FO
A B S T R A C T
Keywords: Zinc oxide Photocatalyst Flexible Nanofiber
Photocatalysts on nanofibers provide a large surface area for high photocatalytic activity, prevent nanoparticle aggregation, and facilitate easy separation from the process fluids. In this work, zinc oxide (ZnO) photocatalysts on flexible polyacrylonitrile (PAN) were prepared via single-step deposition by sol–gel coaxial electrospinning. A core/sheath structure consisting of the PAN polymer covered with a ZnO–polyvinylpyrrolidone (PVP) mixture was fabricated and then calcined at high temperatures to remove the PVP and crystallize the ZnO. The concentrations and viscosities of PAN and PVP were varied to find the appropriate conditions for nanofiber production. The thermal behavior and optimal calcination temperature were investigated via thermogravimetric analysis and Fourier transform infrared spectrometry. ZnO/PAN coaxial nanofibers fabricated using 18% PVP and 12% PAN and calcined at 450 °C in air for 2 h had a ZnO-to-carbon ratio of 0.51. The nanofibers retained their flexibility and exhibited photocatalytic activity toward methylene blue degradation.
1. Introduction The diverse potential applications of heterogeneous photocatalytic technology, including environmental remediation, production of renewable fuel, and organic syntheses, have been attracting attention in many industries. Among various studied materials, zinc oxide (ZnO) is one of the most significant and promising semiconductors for photocatalysis because it has a suitable band gap for utilizing UV light, as well as good photocatalytic activity, chemical stability, and nontoxicity [1]. ZnO nanoparticles show higher photocatalytic efficiency than bulk ZnO owing to their high surface area and high crystallinity. However, the use of nanoparticles suspended in solution as a photocatalyst is limited by the difficulty of separating the nanoparticles from the solution and their strong tendency to aggregate into larger particles, decreasing their photocatalytic activity [2]. ZnO can be formed into films composed of nanoparticles to avoid these problems [3,4], but the 2D thin film structure results in significant losses in surface area and photocatalyst activity. Granules or pellets of nanoparticles can be easily recovered from the solution [5]; however, the low surface-to-volume ratio of pellets limits light penetration to the inner part of the photocatalyst. Nanofibers consisting of nanosized ZnO grains offer the same high surface area advantages as nanoparticles, and the length of the fibers
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facilitates easy separation [6,7]. Ceramic nanofibers can be simply fabricated using a sol–gel process combined with an electrospinning technique [8,9]. Although experimental results have shown that the photocatalytic activity of nanofibers is comparable to that of nanoparticles [6], unfortunately, the natural brittleness of ceramic nanofibers causes them to disintegrate into powder after repeated use. In contrast to fragile pure ceramic nanofibers, flexible ZnO nanofibers can prevent catalyst deterioration while maintaining their advantages for photocatalysis. Previous works have attempted to achieve this flexibility by mixing ZnO nanoparticles or zinc acetate into a polymer solution from which nanofibers were fabricated [10–12]. Subsequent calcination is typically necessary to obtain photocatalytically active crystalline ZnO [13,14]. The required calcination temperature also results in decomposition of the polymer in the composite [15,16], so the nanofibers are brittle, and their structure may be destroyed under stress. In addition, poor dispersion of nanoparticles in the polymer solution can lead to nonuniform integration and agglomeration of nanoparticles in the nanofibers. Some of the nanoparticles are embedded in the fibers, which limits their exposure and activity at the surface. Alternatively, flexible nanofibers can be decorated with ZnO nanoparticles or nanowires by gas-phase or solution deposition [17–19]. The process requires two steps: flexible nanofiber fabrication and post-deposition of ZnO. This research introduces the fabrication of ZnO nanoparticles on
Corresponding author. E-mail address: [email protected] (V. Pavarajarn).
https://doi.org/10.1016/j.ceramint.2019.12.058 Received 29 June 2019; Received in revised form 30 September 2019; Accepted 5 December 2019 0272-8842/ © 2019 Published by Elsevier Ltd.
Please cite this article as: Rungthiwa Methaapanon, Kanchat Chutchakul and Varong Pavarajarn, Ceramics International, https://doi.org/10.1016/j.ceramint.2019.12.058
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flexible nanofibers via a single-step coaxial electrospinning process. Polyacrylonitrile (PAN) is supplied as the core, and ZnO sols dispersed in polyvinylpyrrolidone (PVP) are formed into a covering sheath. A subsequent high-temperature step removes the PVP and calcines the ZnO. The temperature is controlled to maintain the flexibility of the core polymer. The impact of the coaxial electrospinning process parameters on the morphology, crystalline phase, and structure of the polymer before and after calcination are investigated, along with the nanofiber photocatalytic activity.
10 ppm MB solution. The solution was magnetically stirred in the dark for 30 min to reach adsorption equilibrium of MB on the surface of the catalyst before the reaction. Next, the solution was irradiated with six UV-A lamps (Phillips TLC 15w/05, Netherlands) under continuous stirring to maintain uniform dispersion of the catalyst in the solution. The degradation rate of MB was measured by a UV–Vis spectrophotometer (Shimadzu, UV- 1700, Kyoto, Japan) at the maximum absorption wavelength of MB (664 nm) at 10, 20, 30, 40, and 60 min and every hour until the MB was completely degraded.
2. Experimental procedure
3. Results and discussion
2.1. Nanofiber fabrication
3.1. Fabrication of ZnO/PAN coaxial nanofibers
The electrospinning solution for the PAN core was prepared by dissolving PAN (Mw = 150,000) in N,N′-dimethylformamide (DMF) at 600 °C and stirring for 1 h. The PAN concentration of core solution was varied to investigate its effect on the nanofiber properties. The ZnO sol for the sheath solution was prepared by dissolving 3.29 g of zinc acetate and various amounts of PVP (Mw ~ 1,300,000) in 20 mL of DMF. A mixture of 0.26 mL of distilled water, 0.18 mL of HCl, and 1.58 mL of diethanolamine in 5 mL of DMF was mixed into the ZnO precursor solution and stirred continuously for 2 h to obtain a transparent solution. The electrospinning solutions were delivered to the coaxial nozzles by two separate plastic syringes. The coaxial electrospinning process was initiated at an electric field strength of 1 kV/cm (with a charge of 25 kV on the collector drum, which was 25 cm from the nozzles). The flow rate was maintained at 0.8 mL/h for both the core and sheath solutions. The nanofibers in each batch were collected for 6 h and held for 24 h for complete evaporation of the solvent. The electrospun products were calcined in a box furnace. The temperature of the furnace was raised from room temperature to target temperatures between 400 and 500 °C at a heating rate of 10 °C/min, and then maintained for 2 h for calcination.
3.1.1. Effect of PVP concentration in sheath solution The PVP concentration in the sheath solution was varied from 8 to 18 wt%, which corresponded to solution viscosities of 0.52–3.43 Pa s, on the basis of previous studies suggesting that the PVP concentration must be higher than 7.5 wt% to produce PVP fibers [20,21]. It is important that the viscosity of the sheath solution is higher than that of the core solution to successfully produce a core/sheath structure. The viscous stress applied by the sheath solution on the core solution must overcome the interfacial tension between the two solutions in order to form the compound Taylor cone and the subsequent jet for coaxial electrospinning [22]. Therefore, the PAN concentration of the core solution was maintained at 8 wt% (a viscosity of 0.11 Pa s), which is lower than that of the PVP solutions. The core/sheath structure was confirmed by TEM, as shown in Fig. 1a. When the viscosity of the sheath solution was reduced to less than that of the core solution, only a simple rod structure was observed (Fig. 1b). As shown in Fig. 1c, the average diameter of the as-spun coaxial fibers ranged from 403 ± 73 to 516 ± 93 nm and gradually increased with increasing viscosity of the PVP solution. After calcination at high temperature, the PVP sheath decomposed, leaving ZnO nanoparticles on the PAN fibers, as shown in Fig. 2. The
2.2. Characterizations 2.2.1. Physical properties The morphology, core/sheath structure and size, and composition of the nanofibers were determined by scanning electron microscopy (SEM, JEOL JSM-6400, Tokyo, Japan), energy-dispersive X-ray spectroscopy (EDX), and transmission electron microscopy (TEM, JEOL JSM-2100, Tokyo, Japan). The viscosity of the solutions was measured using a cone-and-plate-type viscometer (BROOKFIELD, DV-II+Pro, Middleboro, MA). Thermogravimetric analysis (TGA) was used to study the decomposition temperature and thermal behavior of the nanofibers (Mettler-Toledo TGA/DSC1 STARe System, Columbus, OH). The samples were heated under an oxygen flow of 40 mL/h heated from 25 to 1,000 °C at a ramp rate of 10 °C/min. Fourier transform infrared (FT-IR) spectrometry (Nicolet 6700, Waltham, MA) was used to investigate the functional groups in the products. The crystalline phase of the synthesized photocatalyst (ZnO) was analyzed by X-ray diffraction (XRD, Bruker AXS D8 Advance, Karlsruhe, Germany) using CuKα radiation with a wavelength of 1.5406 Å at 40 kV in a 2θ range of 20°–70°. The tensile strength of the fiber products was measured by a universal testing machine (Hounsfield H 10 KM, Surrey, England). The fiber films were cut to dimensions of 1 × 3 cm2. The thickness of the films was controlled by the electrospinning time (6 h). The specimens were pulled at a rate of 10 N/min during measurement. 2.2.2. Photocatalytic activity test The photocatalytic activity of the ZnO/PAN nanofiber structures was evaluated via degradation of methylene blue (MB, C16H18N3SCl.3H2O) solution under UV irradiation. The coaxial fiber content was equivalent to 1 mg of the catalyst per 10 mL of the
Fig. 1. (a) Coaxial structure obtained using a sheath solution with a higher viscosity than the core solution. (b) Simple rod structure obtained from a sheath solution with lower viscosity. (c) Diameter of coaxial nanofibers fabricated using PVP solutions with various viscosities (PVP concentrations) and a fixed PAN concentration of 8 wt% in the core solution. 2
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Fig. 2. (a) Coaxial fibers before calcination and (b) ZnO particles on PAN fiber after calcination at 450 °C. The insets show high-magnification TEM images of the corresponding structures. (c) Diameter of ZnO-decorated nanofibers after calcination at 450 °C as a function of PVP concentration.
Fig. 3. SEM images of ZnO/PAN coaxial nanofibers prepared using PAN concentration of (a) 4 wt%, (b) 6 wt%, (c) 8 wt%, (d) 10 wt%, and (e) 12 wt% after calcination at 450 °C. The PVP concentration in the sheath solution was fixed at 12 wt%.
noticeable reduction in the nanofiber diameter after calcination also confirmed PVP sheath removal. Note that the diameter of the PAN core increased slightly with increasing PVP concentration, despite the constant 8 wt% PAN concentration. The change in the core diameter indicated the interaction between the concentrations of PVP in the sheath and PAN in the core that resulted in the Taylor cone. 3.1.2. Effect of PAN concentration in core solution The PAN concentration was varied from 4 to 12 wt%, and the PVP concentration of the sheath solution was fixed at 12 wt%, which gave it a viscosity slightly higher than that of the 12 wt% PAN solution to obtain the core/sheath structure. Solutions with PAN concentrations higher than 12 wt% were so viscous that they clogged the electrospinning needle. PAN concentrations of 4 and 6 wt% produced a bead morphology (Fig. 3a and b) similar to that observed at low polymer concentrations in previous studies [23]. The fibers fabricated from 8 to 12 wt% PAN appeared smooth, as shown in Fig. 3c–e. The effect of PAN concentration on the size of the as-spun fibers is shown in Fig. 4. As the PAN concentration increased from 8 to 10 wt%, the diameter of the as-spun fibers gradually increased. The size increased sharply when the PAN concentration was increased from 10 to 12 wt%. The PAN concentration in the core solution had a larger effect on the size of the fibers than the PVP concentration in the sheath solution. The diameter of the fibers increased by as much as 6 times when the viscosity of the PAN solution increased from 0.11 to 0.68 Pa s. In other words, the PAN concentration was the key factor determining the diameter of the coaxial fibers.
Fig. 4. Diameter of ZnO-decorated nanofibers before (●) and after calcination at 450 °C (■), and viscosity of PAN solution as a function of PAN concentration. The PVP concentration was fixed at 12 wt%.
3.1.3. Effect of high-temperature calcination TGA and differential scanning calorimetry (DSC) were applied to determine the appropriate temperature for ZnO calcination and PVP removal. The TG and DSC diagrams of as-spun coaxial nanofibers are shown in Fig. 5. The TG spectra exhibit four gravimetric steps. The broad range between 0 and 200 °C corresponds to desorption of physisorbed water. The second step, indicated by a strong thermopositive peak at 280 °C in the DSC curve, is well correlated with nitrile group polymerization (cyclization) of PAN to stabilized PAN below 300 °C [24]. The reaction consumed some of the nitrile groups, resulting in mass loss. The third step, which began at approximately 400 °C, is
consistent with exothermic PVP decomposition [25]. Roughly 20% of the mass was dissipated during this step. The shoulder of the thermopositive peak from this event partially overlaps the heat release due to PAN cyclization; therefore, the exact onset temperature could not be identified. TG-DSC analysis of plain PVP nanofibers suggested that PVP decomposition began at approximately 200 °C and was complete at slightly above 400 °C. As calcination was conducted under an oxidizing condition, no carbon residue from PVP was expected to appear on the remaining nanofibers. The last step, at approximately 510 °C, which 3
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Fig. 7. FT-IR spectra of nanofibers: (a) as-spun PVP fibers, (b) as-spun PAN fibers, (c) as-spun ZnO coaxial fibers, (d) ZnO coaxial fibers after calcination at 450 °C, and (e) PAN fibers after calcination at 450 °C. The dashed, dotted, and dash-dotted lines show the vibrational characteristics associated with PAN, PVP, and stabilized PAN, respectively.
Fig. 5. TGA and DSC results of the as-spun ZnO/PAN coaxial fibers.
[30]. As a result of the long calcination time, the PAN formed a conjugated cyclic structure, as indicated by the C=C peak of the aromatic ring at 810 cm−1. This thermo-oxidative stabilization of PAN prevented melting of the core PAN nanofibers during the high-temperature process [31]. PVP removal was confirmed by the disappearance of the C=O-bond-related peak at 1660 cm−1. The structure consisted of ZnO nanoparticles on the core fibers owing to PVP removal in the sheath, as shown earlier in Fig. 2b. The ZnO/PAN coaxial fibers fabricated using 18 wt% PVP and 12 wt% PAN exhibited the highest ZnO-to-carbon ratio of 0.51. 3.2. Properties of ZnO nanofibers 3.2.1. Crystallinity of ZnO The crystalline phase is an important factor influencing the photocatalytic activity of ZnO. Fig. 8a shows TEM images of ZnO/PAN nanofibers after calcination at 450 °C for 2 h in air; the ZnO nanocrystals have an average grain size of approximately 65 ± 10 nm. The selected area electron diffraction pattern (SAED) suggests that the nanoparticles are single crystals. The lattice fringes could also be observed using high-
Fig. 6. TGA and DSC results of ZnO/PAN coaxial nanofibers after calcination at 450 °C for 2 h in air.
was indicated by a sharp thermopositive peak in the DSC curve, is associated with decomposition of the PAN polymer. There was no further significant weight loss as the temperature was increased above 600 °C, indicating complete loss of the organic constituent; only ZnO remained. The TG-DSC analysis established that the range of calcination temperature at which PVP in the sheath is removed and the flexible PAN core structure of the final ZnO nanofibers is maintained lies between 400 and 500 °C. The TG plot of the coaxial ZnO/PAN nanofibers after calcination at 450 °C for 2 h (Fig. 6) shows that the compound fibers were stable up to approximately 500 °C. The absence of a peak associated with PVP suggests complete removal of PVP during the 2 h calcination. At higher temperature, a sudden significant mass loss appears, accompanied by a strong thermopositive peak at 523 °C in the DSC diagram. This mass loss corresponds to decomposition of residual PAN in the fibers. No further mass loss is observed beyond 600 °C, suggesting complete depletion of the organic materials. Fig. 7 shows the FT-IR spectra of PVP, PAN, and the ZnO coaxial nanofibers before and after calcination. Before calcination, the ZnO coaxial nanofibers exhibited a combination of the vibrational characteristics of PAN and PVP, including the C≡N nitrile group at approximately 2,242 cm−1 from PAN [26], as well as C=O stretching at 1660 cm−1 and C–N stretching at 1285 cm−1 from PVP [27]. Various vibrational modes of the aliphatic CH groups at approximately 2,925 and 1,425–1,450 cm−1 [28,29] are associated with both PAN and PVP. The FT-IR spectrum of coaxial ZnO fibers (Fig. 7d) after calcination resembled that of calcined pristine PAN fibers (Fig. 7e), indicating stabilized PAN structure in the core of the fibers [29]. The intensities of the C≡N band and the related CH peaks clearly decreased. A new peak at 1,590 cm−1 corresponds to C=N, confirming cyclization of PAN
Fig. 8. ZnO/PAN nanofiber structure after calcination at 450 °C for 2 h in air: (a) TEM image showing the ZnO nanoparticle distribution on the PAN fiber (inset shows SAED pattern), (b) HRTEM image showing lattice fringes of c-axisoriented ZnO, and (c) XRD pattern. 4
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the original as-spun fibers. The yield strength of the calcined fibers also decreased by 18%, possibly owing to the transformation of PAN by heat. When the calcination temperature was increased to 500 °C, however, the stress–strain curves of the obtained fibers showed no plastic region, indicating brittle behavior as PAN began to decompose. 3.2.3. Photocatalytic activity The photocatalytic behavior of the ZnO/PAN coaxial nanofibers was compared with that of ZnO powder synthesized by the same sol–gel process. All the products were calcined at 450 °C. Both the ZnO/PAN fibers and ZnO powders showed activity toward the degradation of MB in solution under UV irradiation (Fig. 11). Using the pseudo-first-order kinetic model, the rate constants of the reaction using the ZnO/PAN nanofibers and ZnO powder were 0.218 and 0.283 h−1, respectively. This activity is comparable to the photocatalytic activity of the pure metal oxide nanofibers reported in a previous work [34]. However, the photocatalytic activity of the ZnO/PAN coaxial nanofibers was slightly lower than that of the ZnO powder, possibly because the surface area of ZnO particles that adhered to the polymer core was reduced.
Fig. 9. Crystallite sizes of ZnO/PAN coaxial fibers after calcination at 450 °C for different calcination times, as calculated by the Debye–Scherrer equation.
4. Conclusions ZnO/PAN coaxial nanofibers were fabricated by a combination of a sol–gel process and electrospinning, yielding nanofibers with a PAN core and a ZnO/PVP sheath. The PAN concentration and viscosity of the core determined the diameter of the coaxial fibers. The viscosity of the PAN solution must also be lower than that of the PVP solution to maintain a stable core/sheath structure. After calcination at 450 °C, the PVP in the sheath decomposed, and the ZnO was transformed into crystallites and remained on the PAN nanofibers. The calcined ZnO/ PAN nanofibers were flexible and exhibited photocatalytic activity for MB degradation.
Fig. 10. Stress–strain curves of coaxial fibers before calcination and after calcination at 450 and 500 °C.
Declaration of competing interest None. Acknowledgment This research was funded by the Thailand Research Fund (DBG5580006). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ceramint.2019.12.058.
Fig. 11. Photocatalytic MB degradation using (a) ZnO/PAN nanofibers, and (b) ZnO powder (450 °C, 10 ppm dye concentration, 1 mg/10 mL of solution).
References resolution TEM (HRTEM) and exhibited a d-spacing of 2.59 Å, which corresponds to the (002) plane of ZnO [32]. The XRD pattern shown in Fig. 8c resembles the powder diffraction pattern of ZnO with wurtzite (hexagonal) structure, indicating that the nanoparticles did not have a preferential orientation on the fibers [33]. The size of these ZnO crystallites, as calculated by the Debye–Scherrer equation, was found to increase with calcination time, as shown in Fig. 9.
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Photocatalytic zinc oxide on flexible polyacrylonitrile nanofibers via sol–gel coaxial electrospinning Rungthiwa Methaapanon, Kanchat Chutchakul, Varong Pavarajarn∗ Centre of Excellence in Particle and Material Processing Technology, Department of Chemical Engineering, Chulalongkorn University, Bangkok, Thailand
A R T I C LE I N FO
A B S T R A C T
Keywords: Zinc oxide Photocatalyst Flexible Nanofiber
Photocatalysts on nanofibers provide a large surface area for high photocatalytic activity, prevent nanoparticle aggregation, and facilitate easy separation from the process fluids. In this work, zinc oxide (ZnO) photocatalysts on flexible polyacrylonitrile (PAN) were prepared via single-step deposition by sol–gel coaxial electrospinning. A core/sheath structure consisting of the PAN polymer covered with a ZnO–polyvinylpyrrolidone (PVP) mixture was fabricated and then calcined at high temperatures to remove the PVP and crystallize the ZnO. The concentrations and viscosities of PAN and PVP were varied to find the appropriate conditions for nanofiber production. The thermal behavior and optimal calcination temperature were investigated via thermogravimetric analysis and Fourier transform infrared spectrometry. ZnO/PAN coaxial nanofibers fabricated using 18% PVP and 12% PAN and calcined at 450 °C in air for 2 h had a ZnO-to-carbon ratio of 0.51. The nanofibers retained their flexibility and exhibited photocatalytic activity toward methylene blue degradation.
1. Introduction The diverse potential applications of heterogeneous photocatalytic technology, including environmental remediation, production of renewable fuel, and organic syntheses, have been attracting attention in many industries. Among various studied materials, zinc oxide (ZnO) is one of the most significant and promising semiconductors for photocatalysis because it has a suitable band gap for utilizing UV light, as well as good photocatalytic activity, chemical stability, and nontoxicity [1]. ZnO nanoparticles show higher photocatalytic efficiency than bulk ZnO owing to their high surface area and high crystallinity. However, the use of nanoparticles suspended in solution as a photocatalyst is limited by the difficulty of separating the nanoparticles from the solution and their strong tendency to aggregate into larger particles, decreasing their photocatalytic activity [2]. ZnO can be formed into films composed of nanoparticles to avoid these problems [3,4], but the 2D thin film structure results in significant losses in surface area and photocatalyst activity. Granules or pellets of nanoparticles can be easily recovered from the solution [5]; however, the low surface-to-volume ratio of pellets limits light penetration to the inner part of the photocatalyst. Nanofibers consisting of nanosized ZnO grains offer the same high surface area advantages as nanoparticles, and the length of the fibers
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facilitates easy separation [6,7]. Ceramic nanofibers can be simply fabricated using a sol–gel process combined with an electrospinning technique [8,9]. Although experimental results have shown that the photocatalytic activity of nanofibers is comparable to that of nanoparticles [6], unfortunately, the natural brittleness of ceramic nanofibers causes them to disintegrate into powder after repeated use. In contrast to fragile pure ceramic nanofibers, flexible ZnO nanofibers can prevent catalyst deterioration while maintaining their advantages for photocatalysis. Previous works have attempted to achieve this flexibility by mixing ZnO nanoparticles or zinc acetate into a polymer solution from which nanofibers were fabricated [10–12]. Subsequent calcination is typically necessary to obtain photocatalytically active crystalline ZnO [13,14]. The required calcination temperature also results in decomposition of the polymer in the composite [15,16], so the nanofibers are brittle, and their structure may be destroyed under stress. In addition, poor dispersion of nanoparticles in the polymer solution can lead to nonuniform integration and agglomeration of nanoparticles in the nanofibers. Some of the nanoparticles are embedded in the fibers, which limits their exposure and activity at the surface. Alternatively, flexible nanofibers can be decorated with ZnO nanoparticles or nanowires by gas-phase or solution deposition [17–19]. The process requires two steps: flexible nanofiber fabrication and post-deposition of ZnO. This research introduces the fabrication of ZnO nanoparticles on
Corresponding author. E-mail address: [email protected] (V. Pavarajarn).
https://doi.org/10.1016/j.ceramint.2019.12.058 Received 29 June 2019; Received in revised form 30 September 2019; Accepted 5 December 2019 0272-8842/ © 2019 Published by Elsevier Ltd.
Please cite this article as: Rungthiwa Methaapanon, Kanchat Chutchakul and Varong Pavarajarn, Ceramics International, https://doi.org/10.1016/j.ceramint.2019.12.058
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flexible nanofibers via a single-step coaxial electrospinning process. Polyacrylonitrile (PAN) is supplied as the core, and ZnO sols dispersed in polyvinylpyrrolidone (PVP) are formed into a covering sheath. A subsequent high-temperature step removes the PVP and calcines the ZnO. The temperature is controlled to maintain the flexibility of the core polymer. The impact of the coaxial electrospinning process parameters on the morphology, crystalline phase, and structure of the polymer before and after calcination are investigated, along with the nanofiber photocatalytic activity.
10 ppm MB solution. The solution was magnetically stirred in the dark for 30 min to reach adsorption equilibrium of MB on the surface of the catalyst before the reaction. Next, the solution was irradiated with six UV-A lamps (Phillips TLC 15w/05, Netherlands) under continuous stirring to maintain uniform dispersion of the catalyst in the solution. The degradation rate of MB was measured by a UV–Vis spectrophotometer (Shimadzu, UV- 1700, Kyoto, Japan) at the maximum absorption wavelength of MB (664 nm) at 10, 20, 30, 40, and 60 min and every hour until the MB was completely degraded.
2. Experimental procedure
3. Results and discussion
2.1. Nanofiber fabrication
3.1. Fabrication of ZnO/PAN coaxial nanofibers
The electrospinning solution for the PAN core was prepared by dissolving PAN (Mw = 150,000) in N,N′-dimethylformamide (DMF) at 600 °C and stirring for 1 h. The PAN concentration of core solution was varied to investigate its effect on the nanofiber properties. The ZnO sol for the sheath solution was prepared by dissolving 3.29 g of zinc acetate and various amounts of PVP (Mw ~ 1,300,000) in 20 mL of DMF. A mixture of 0.26 mL of distilled water, 0.18 mL of HCl, and 1.58 mL of diethanolamine in 5 mL of DMF was mixed into the ZnO precursor solution and stirred continuously for 2 h to obtain a transparent solution. The electrospinning solutions were delivered to the coaxial nozzles by two separate plastic syringes. The coaxial electrospinning process was initiated at an electric field strength of 1 kV/cm (with a charge of 25 kV on the collector drum, which was 25 cm from the nozzles). The flow rate was maintained at 0.8 mL/h for both the core and sheath solutions. The nanofibers in each batch were collected for 6 h and held for 24 h for complete evaporation of the solvent. The electrospun products were calcined in a box furnace. The temperature of the furnace was raised from room temperature to target temperatures between 400 and 500 °C at a heating rate of 10 °C/min, and then maintained for 2 h for calcination.
3.1.1. Effect of PVP concentration in sheath solution The PVP concentration in the sheath solution was varied from 8 to 18 wt%, which corresponded to solution viscosities of 0.52–3.43 Pa s, on the basis of previous studies suggesting that the PVP concentration must be higher than 7.5 wt% to produce PVP fibers [20,21]. It is important that the viscosity of the sheath solution is higher than that of the core solution to successfully produce a core/sheath structure. The viscous stress applied by the sheath solution on the core solution must overcome the interfacial tension between the two solutions in order to form the compound Taylor cone and the subsequent jet for coaxial electrospinning [22]. Therefore, the PAN concentration of the core solution was maintained at 8 wt% (a viscosity of 0.11 Pa s), which is lower than that of the PVP solutions. The core/sheath structure was confirmed by TEM, as shown in Fig. 1a. When the viscosity of the sheath solution was reduced to less than that of the core solution, only a simple rod structure was observed (Fig. 1b). As shown in Fig. 1c, the average diameter of the as-spun coaxial fibers ranged from 403 ± 73 to 516 ± 93 nm and gradually increased with increasing viscosity of the PVP solution. After calcination at high temperature, the PVP sheath decomposed, leaving ZnO nanoparticles on the PAN fibers, as shown in Fig. 2. The
2.2. Characterizations 2.2.1. Physical properties The morphology, core/sheath structure and size, and composition of the nanofibers were determined by scanning electron microscopy (SEM, JEOL JSM-6400, Tokyo, Japan), energy-dispersive X-ray spectroscopy (EDX), and transmission electron microscopy (TEM, JEOL JSM-2100, Tokyo, Japan). The viscosity of the solutions was measured using a cone-and-plate-type viscometer (BROOKFIELD, DV-II+Pro, Middleboro, MA). Thermogravimetric analysis (TGA) was used to study the decomposition temperature and thermal behavior of the nanofibers (Mettler-Toledo TGA/DSC1 STARe System, Columbus, OH). The samples were heated under an oxygen flow of 40 mL/h heated from 25 to 1,000 °C at a ramp rate of 10 °C/min. Fourier transform infrared (FT-IR) spectrometry (Nicolet 6700, Waltham, MA) was used to investigate the functional groups in the products. The crystalline phase of the synthesized photocatalyst (ZnO) was analyzed by X-ray diffraction (XRD, Bruker AXS D8 Advance, Karlsruhe, Germany) using CuKα radiation with a wavelength of 1.5406 Å at 40 kV in a 2θ range of 20°–70°. The tensile strength of the fiber products was measured by a universal testing machine (Hounsfield H 10 KM, Surrey, England). The fiber films were cut to dimensions of 1 × 3 cm2. The thickness of the films was controlled by the electrospinning time (6 h). The specimens were pulled at a rate of 10 N/min during measurement. 2.2.2. Photocatalytic activity test The photocatalytic activity of the ZnO/PAN nanofiber structures was evaluated via degradation of methylene blue (MB, C16H18N3SCl.3H2O) solution under UV irradiation. The coaxial fiber content was equivalent to 1 mg of the catalyst per 10 mL of the
Fig. 1. (a) Coaxial structure obtained using a sheath solution with a higher viscosity than the core solution. (b) Simple rod structure obtained from a sheath solution with lower viscosity. (c) Diameter of coaxial nanofibers fabricated using PVP solutions with various viscosities (PVP concentrations) and a fixed PAN concentration of 8 wt% in the core solution. 2
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Fig. 2. (a) Coaxial fibers before calcination and (b) ZnO particles on PAN fiber after calcination at 450 °C. The insets show high-magnification TEM images of the corresponding structures. (c) Diameter of ZnO-decorated nanofibers after calcination at 450 °C as a function of PVP concentration.
Fig. 3. SEM images of ZnO/PAN coaxial nanofibers prepared using PAN concentration of (a) 4 wt%, (b) 6 wt%, (c) 8 wt%, (d) 10 wt%, and (e) 12 wt% after calcination at 450 °C. The PVP concentration in the sheath solution was fixed at 12 wt%.
noticeable reduction in the nanofiber diameter after calcination also confirmed PVP sheath removal. Note that the diameter of the PAN core increased slightly with increasing PVP concentration, despite the constant 8 wt% PAN concentration. The change in the core diameter indicated the interaction between the concentrations of PVP in the sheath and PAN in the core that resulted in the Taylor cone. 3.1.2. Effect of PAN concentration in core solution The PAN concentration was varied from 4 to 12 wt%, and the PVP concentration of the sheath solution was fixed at 12 wt%, which gave it a viscosity slightly higher than that of the 12 wt% PAN solution to obtain the core/sheath structure. Solutions with PAN concentrations higher than 12 wt% were so viscous that they clogged the electrospinning needle. PAN concentrations of 4 and 6 wt% produced a bead morphology (Fig. 3a and b) similar to that observed at low polymer concentrations in previous studies [23]. The fibers fabricated from 8 to 12 wt% PAN appeared smooth, as shown in Fig. 3c–e. The effect of PAN concentration on the size of the as-spun fibers is shown in Fig. 4. As the PAN concentration increased from 8 to 10 wt%, the diameter of the as-spun fibers gradually increased. The size increased sharply when the PAN concentration was increased from 10 to 12 wt%. The PAN concentration in the core solution had a larger effect on the size of the fibers than the PVP concentration in the sheath solution. The diameter of the fibers increased by as much as 6 times when the viscosity of the PAN solution increased from 0.11 to 0.68 Pa s. In other words, the PAN concentration was the key factor determining the diameter of the coaxial fibers.
Fig. 4. Diameter of ZnO-decorated nanofibers before (●) and after calcination at 450 °C (■), and viscosity of PAN solution as a function of PAN concentration. The PVP concentration was fixed at 12 wt%.
3.1.3. Effect of high-temperature calcination TGA and differential scanning calorimetry (DSC) were applied to determine the appropriate temperature for ZnO calcination and PVP removal. The TG and DSC diagrams of as-spun coaxial nanofibers are shown in Fig. 5. The TG spectra exhibit four gravimetric steps. The broad range between 0 and 200 °C corresponds to desorption of physisorbed water. The second step, indicated by a strong thermopositive peak at 280 °C in the DSC curve, is well correlated with nitrile group polymerization (cyclization) of PAN to stabilized PAN below 300 °C [24]. The reaction consumed some of the nitrile groups, resulting in mass loss. The third step, which began at approximately 400 °C, is
consistent with exothermic PVP decomposition [25]. Roughly 20% of the mass was dissipated during this step. The shoulder of the thermopositive peak from this event partially overlaps the heat release due to PAN cyclization; therefore, the exact onset temperature could not be identified. TG-DSC analysis of plain PVP nanofibers suggested that PVP decomposition began at approximately 200 °C and was complete at slightly above 400 °C. As calcination was conducted under an oxidizing condition, no carbon residue from PVP was expected to appear on the remaining nanofibers. The last step, at approximately 510 °C, which 3
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Fig. 7. FT-IR spectra of nanofibers: (a) as-spun PVP fibers, (b) as-spun PAN fibers, (c) as-spun ZnO coaxial fibers, (d) ZnO coaxial fibers after calcination at 450 °C, and (e) PAN fibers after calcination at 450 °C. The dashed, dotted, and dash-dotted lines show the vibrational characteristics associated with PAN, PVP, and stabilized PAN, respectively.
Fig. 5. TGA and DSC results of the as-spun ZnO/PAN coaxial fibers.
[30]. As a result of the long calcination time, the PAN formed a conjugated cyclic structure, as indicated by the C=C peak of the aromatic ring at 810 cm−1. This thermo-oxidative stabilization of PAN prevented melting of the core PAN nanofibers during the high-temperature process [31]. PVP removal was confirmed by the disappearance of the C=O-bond-related peak at 1660 cm−1. The structure consisted of ZnO nanoparticles on the core fibers owing to PVP removal in the sheath, as shown earlier in Fig. 2b. The ZnO/PAN coaxial fibers fabricated using 18 wt% PVP and 12 wt% PAN exhibited the highest ZnO-to-carbon ratio of 0.51. 3.2. Properties of ZnO nanofibers 3.2.1. Crystallinity of ZnO The crystalline phase is an important factor influencing the photocatalytic activity of ZnO. Fig. 8a shows TEM images of ZnO/PAN nanofibers after calcination at 450 °C for 2 h in air; the ZnO nanocrystals have an average grain size of approximately 65 ± 10 nm. The selected area electron diffraction pattern (SAED) suggests that the nanoparticles are single crystals. The lattice fringes could also be observed using high-
Fig. 6. TGA and DSC results of ZnO/PAN coaxial nanofibers after calcination at 450 °C for 2 h in air.
was indicated by a sharp thermopositive peak in the DSC curve, is associated with decomposition of the PAN polymer. There was no further significant weight loss as the temperature was increased above 600 °C, indicating complete loss of the organic constituent; only ZnO remained. The TG-DSC analysis established that the range of calcination temperature at which PVP in the sheath is removed and the flexible PAN core structure of the final ZnO nanofibers is maintained lies between 400 and 500 °C. The TG plot of the coaxial ZnO/PAN nanofibers after calcination at 450 °C for 2 h (Fig. 6) shows that the compound fibers were stable up to approximately 500 °C. The absence of a peak associated with PVP suggests complete removal of PVP during the 2 h calcination. At higher temperature, a sudden significant mass loss appears, accompanied by a strong thermopositive peak at 523 °C in the DSC diagram. This mass loss corresponds to decomposition of residual PAN in the fibers. No further mass loss is observed beyond 600 °C, suggesting complete depletion of the organic materials. Fig. 7 shows the FT-IR spectra of PVP, PAN, and the ZnO coaxial nanofibers before and after calcination. Before calcination, the ZnO coaxial nanofibers exhibited a combination of the vibrational characteristics of PAN and PVP, including the C≡N nitrile group at approximately 2,242 cm−1 from PAN [26], as well as C=O stretching at 1660 cm−1 and C–N stretching at 1285 cm−1 from PVP [27]. Various vibrational modes of the aliphatic CH groups at approximately 2,925 and 1,425–1,450 cm−1 [28,29] are associated with both PAN and PVP. The FT-IR spectrum of coaxial ZnO fibers (Fig. 7d) after calcination resembled that of calcined pristine PAN fibers (Fig. 7e), indicating stabilized PAN structure in the core of the fibers [29]. The intensities of the C≡N band and the related CH peaks clearly decreased. A new peak at 1,590 cm−1 corresponds to C=N, confirming cyclization of PAN
Fig. 8. ZnO/PAN nanofiber structure after calcination at 450 °C for 2 h in air: (a) TEM image showing the ZnO nanoparticle distribution on the PAN fiber (inset shows SAED pattern), (b) HRTEM image showing lattice fringes of c-axisoriented ZnO, and (c) XRD pattern. 4
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the original as-spun fibers. The yield strength of the calcined fibers also decreased by 18%, possibly owing to the transformation of PAN by heat. When the calcination temperature was increased to 500 °C, however, the stress–strain curves of the obtained fibers showed no plastic region, indicating brittle behavior as PAN began to decompose. 3.2.3. Photocatalytic activity The photocatalytic behavior of the ZnO/PAN coaxial nanofibers was compared with that of ZnO powder synthesized by the same sol–gel process. All the products were calcined at 450 °C. Both the ZnO/PAN fibers and ZnO powders showed activity toward the degradation of MB in solution under UV irradiation (Fig. 11). Using the pseudo-first-order kinetic model, the rate constants of the reaction using the ZnO/PAN nanofibers and ZnO powder were 0.218 and 0.283 h−1, respectively. This activity is comparable to the photocatalytic activity of the pure metal oxide nanofibers reported in a previous work [34]. However, the photocatalytic activity of the ZnO/PAN coaxial nanofibers was slightly lower than that of the ZnO powder, possibly because the surface area of ZnO particles that adhered to the polymer core was reduced.
Fig. 9. Crystallite sizes of ZnO/PAN coaxial fibers after calcination at 450 °C for different calcination times, as calculated by the Debye–Scherrer equation.
4. Conclusions ZnO/PAN coaxial nanofibers were fabricated by a combination of a sol–gel process and electrospinning, yielding nanofibers with a PAN core and a ZnO/PVP sheath. The PAN concentration and viscosity of the core determined the diameter of the coaxial fibers. The viscosity of the PAN solution must also be lower than that of the PVP solution to maintain a stable core/sheath structure. After calcination at 450 °C, the PVP in the sheath decomposed, and the ZnO was transformed into crystallites and remained on the PAN nanofibers. The calcined ZnO/ PAN nanofibers were flexible and exhibited photocatalytic activity for MB degradation.
Fig. 10. Stress–strain curves of coaxial fibers before calcination and after calcination at 450 and 500 °C.
Declaration of competing interest None. Acknowledgment This research was funded by the Thailand Research Fund (DBG5580006). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ceramint.2019.12.058.
Fig. 11. Photocatalytic MB degradation using (a) ZnO/PAN nanofibers, and (b) ZnO powder (450 °C, 10 ppm dye concentration, 1 mg/10 mL of solution).
References resolution TEM (HRTEM) and exhibited a d-spacing of 2.59 Å, which corresponds to the (002) plane of ZnO [32]. The XRD pattern shown in Fig. 8c resembles the powder diffraction pattern of ZnO with wurtzite (hexagonal) structure, indicating that the nanoparticles did not have a preferential orientation on the fibers [33]. The size of these ZnO crystallites, as calculated by the Debye–Scherrer equation, was found to increase with calcination time, as shown in Fig. 9.
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