Electrospun PolyacrylonitrileâIonic Liquid Nanofibers for Superior

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Electrospun Polyacrylonitrile-Ionic Liquid Nanofibers for Superior PM Capture Capacity 2.5

Lin Jing, Kyubin Shim, Cui Ying Toe, Tim Fang, Chuan Zhao, Rose Amal, Kening Sun, Jung Ho Kim, and Yun Hau Ng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b12313 • Publication Date (Web): 26 Feb 2016 Downloaded from http://pubs.acs.org on February 27, 2016

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ACS Applied Materials & Interfaces

Electrospun Polyacrylonitrile-Ionic Liquid Nanofibers for Superior PM2.5 Capture Capacity Lin Jing,ab Kyubin Shim,c Cui Ying Toe,a Tim Fang,d Chuan Zhao,d Rose Amal,a Ke-Ning Sun,*b Jung Ho Kim*c and Yun Hau Ng*a a. Particles and Catalysis Research Group, School of Chemical Engineering, The University of New South Wales, Sydney, NSW, 2052, Australia. b. Beijing Key Laboratory for Power Source and Green Catalysis, School of Chemical Engineering and Environment, Beijing Institute of Technology, Beijing, 100081, P. R. China. c. Institute for Superconducting and Electronic Materials (ISEM), Australian Institute for Innovative Materials (AIIM), University of Wollongong, North Wollongong, NSW2500, Australia. d. School of Chemistry, The University of New South Wales, Sydney 2052, Australia.

KEYWORDS: polyacrylonitrile, electrospinning, nanofiber, PM2.5, ionic liquid

ACS Paragon Plus Environment

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ABSTRACT:

Ambient fine particulate matter (PM) affects both human health and climate. To reduce the PM2.5 (mass of particles below 2.5 μm in diameter) concentration of individual’s living environment, ionic liquid modified polyacrylonitrile (PAN) nanofibers with superior PM2.5 capture capacity was prepared by electrospinning. The ionic liquid diethylammonium dihydrogen phosphate (DEAP) with high viscosity and hydrophilicity was involved during the electrospinning process. Observations by

scanning electron microscopy (SEM), transmission electron microscopy (TEM) and water contact angle measurement suggested the modification of DEAP on PAN effectively altered the morphology (roughness) and surface properties (hydrophilicity) of the PAN nanofibers. The PM2.5 capture measurement was performed in a closed and static system, which mimicked the static hazy weather without wind flow. As a result, DEAP modified PAN nanofibers exhibited significantly enhanced PM2.5 capture capacity compared to that of the bare PAN nanofibers. This can be attributed to the improved surface roughness (i.e. improved adsorption sites), hydrophilicity and dipole moment of PAN upon DEAP modification.


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Introduction Recent years, suspended particulate matter (PM) in the air has been considered as the most serious pollutant, especially in developing countries such as China and India. PM pollution has strong effects on human health, atmospheric visibility and global climate.1, 2 It is well known that the PM is a complex mixture of extremely small particles and moisture, which is mainly generated by incomplete combustion of fossil fuel, biomass burning, and vehicle emissions.3, 4 According to the reference of

particle sizes, PM can be classified by PM2.5 and PM10,

respectively (mass of particles with aerodynamic diameters below 2.5 and 10 μm).5,

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In

particular, PM2.5, with the relatively larger surface areas, can adsorb more moisture and toxic air pollutant such as organic compounds, oxidant gases, and transition metals.7-9 Moreover, PM2.5 poses greater health risks because it can penetrate human bronchi, lungs, even blood, owing to the small particle size.10, 11 Many effective methods have been developed and applied to alleviate the PM2.5 pollution, such as improving the quality of fuels, using better combustion methods to improve the combustion efficiency, and reducing the PM2.5 emission by after-combustor treatment. However, these strategies are usually costly and technologically challenging for implementation. In addition, it is really time-consuming to change the long-term hazy weather that is already being a very serious problem in many metropolitan regions like Beijing and Shanghai.12, 13 Hence, reducing the PM2.5 concentration of the individual’s living environment seems to be a much more imminent solution to protect them from the PM2.5 exposure. At the moment, facial mask is used to prevent the inhalation of PM2.5 during the outdoor activities, whereas electrostatic air cleaner is employed to improve the air-conditioned indoor by removing dust, pollen, and other fine particles. The


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interconnected fibers network structure in the masks and air cleaners played a crucial role in blocking and adsorbing the fine particles from the air. Recently, Cui and co-workers developed novel nanofiber air filters made of different polymers (i.e. polyacrylonitrile, polyvinylpyrrolidone, polystyrene, polyvinyl alcohol and polypropylene) for indoor air-quality protection under a continuous airflow system.14 According to their study, polyacrylonitrile (PAN) nanofibers exhibited the highest PM2.5 removal efficiency benefited from the small fiber diameter and surface chemistry. Although electrospun nanofibers made of the above polymer nanofibers have been adopted for fine particles filtration under continuous airflow system (please refer to Table S1 in the supporting information and its relevant references), to the best of our knowledge, studies on PM2.5 capture by using PAN nanofibers under static condition are extremely rare. We consider the adsorption of PM2.5 on nanofibers under static situation is mimicking the typical operational condition of a mask while high flux systems represent the working condition of a device (e.g. vacuum). Both systems can tackle the PM2.5 issue from different perspectives and have their own merits. There are many strategies that can be used to further improve the performance on PM2.5 capture. For this purpose, application of the electrospinning process

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can be suggested as an

easy and economical way to obtain one-dimensional (1D) fiber structure. PM2.5 capture on PAN nanofibers utilizes adsorption (both physical and chemical adsorption). PM2.5 presents in the polluted cities contains a high level of atmospheric moisture, in which the moisture acts as a binder to all fine organic carbon, forming a complex system.7, 18, 19 Combining the principle of adsorption and the significant presence of moisture, it is expected that the modification of hydrophilicity and the adsorption sites on the surface of nanofibers can affect the affinity of the nanofibers towards PM2.5 complex.


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In this work, therefore, we prepared the PAN nanofibers by introducing small volumes of highly viscos ionic liquid, i.e. diethylammonium dihydrogen phosphate (DEAP), during the electrospinning process. Compared to the bare smooth PAN nanofibers, the surface properties (viscosity and surface tension) and dipole moment of the as-prepared PAN nanofibers can be easily controlled upon the modification of different amount of DEAP. As a result, the obtained DEAP modified PAN samples showed improved surface area and hydrophilicity. By studying in the typical static PM2.5 removal system, PM2.5 capture capacity of PAN/DEAP nanofibers was found to be significantly enhanced.

Results and Discussion The electrospinning process of bare PAN nanofibers and modified PAN nanofibers with different volumes of DEAP (0.1, 0.5 and 1 wt%) is illustrated in Figure S1. The nanofibers were directly collected on aluminium foil substrate to form the membrane. The membrane could be conveniently detached or transferred from the substrate for analysis. Figure S2 shows the digital photographs of PAN/DEAP membranes obtained with different electrospinning times (15, 30 and 60 min). The thickness and robustness of the membranes increased with the increasing electrospinning time, while the light transparency decreased. As the thickness of the membrane increases, optical light transparency obviously deceases. Depending on the application environment in which light transparency may or may not be required, the transparency of this electrospun membrane is controllable. On the other hand, the excessive thickness may block the flow of PM2.5, while the low thickness may be easily damaged. Therefore, a balance in between the robustness and transparency of the membrane is important. To fulfill above requirements, herein, all samples were prepared with 30 min of electrospinning time.


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Figure 1. SEM images and the corresponding TEM images of (A) PAN, (B) PAN/DEAP(0.1), (C) PAN/DEAP(0.5), and (D) PAN/DEAP(1). The morphological features of bare PAN nanofibers, PAN/DEAP(0.1), PAN/DEAP(0.5), and PAN/DEAP(1) were firstly carried out by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The respective SEM and TEM images are shown in Figure 1. All samples exhibited interconnected nanofiber network with window sizes below 3 μm (Figure 1A1-D1), indicating its potential use in blocking the penetration of particles with size larger than 3 μm. For bare PAN and PAN/DEAP(0.1) samples, uniform distribution of nanofibers with average diameter of 200 nm can be observed. With the increasing DEAP content, beadednanofibers were observed on both of the PAN/DEAP(0.5) and PAN/DEAP(1) samples. The formation of the beads has been reported in other studies20, 21 where it was attributed to the


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change of the viscosity and surface tension of the PAN nanofibers through the addition of the viscous ionic liquid. The improved viscous forces can pose a disturbance at the jet surface (refer to Figure SI for the experimental configuration), while the electrical forces could alter the capillary breaking of the electrospun jets by surface tension, which can both lead to the formation of the beaded-nanofibers. As can be seen in Figure 1C1 and 1D1, however, majority of the nanofibers observed in these samples were non-beaded. For bare PAN sample, the surface of the nanofibers was smooth (Figure 1A2-1A4). Interestingly, new features were observed on the surface of PAN/DEAP samples, as indicated in Figure 1B, 1C, and 1D. The nanofibers and beaded-nanofibers in all PAN/DEAP samples exhibited considerably rough surface after the addition of DEAP, which could contribute to the improved surface area and adsorption sites for the PM2.5 capture. This can be further confirmed by Brunauer-Emmett-Teller (BET) results (Table S2). Compared to the bare PAN membrane (9.4 m2g-1), the surface area of PAN/DEAP(0.1), PAN/DEAP(0.5) and PAN/DEAP(1) membranes was measured to be 12.3, 13.2 and 11.9 m2g-1, respectively. It is to be noted that PAN/DEAP(1) exhibited a smaller surface area which can be due to the increased beads on the nanofibers. We believed that the improved roughness can be attributed to the change of surface tension of the precursor solution, which offered the nanofiber a backward pulse when it was stretched by the electrical force during the nanofiber formation process. We also examined the PAN and PAN/DEAP nanofibers using N2 adsorption-desorption isotherms (Figure S3). The presence of hysteresis loops indicates the mesoporous structure of these samples. Accompanied with the obvious rougher surface in all PAN/DEAP samples as shown in SEM and TEM images, their N2 sorption volumes are higher than that of the bare PAN nanofiber thus resulting in a generally larger BET surface area in PAN/DEAP samples. All nanofibers demonstrate wide pore size distribution without a distinct


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central pore size value, which are dominantly derived from the spaces between the interlinked nanofibers. As all PAN and PAN/DEAP samples show comparable shape of isotherm with the main difference in surface area, we believe that the PM2.5 is adsorbed on the fiber through similar adsorption mechanism as suggested by the FTIR results.

Figure 2. (A) FTIR, (B) N1s XPS and (C) water contact angle of PAN, PAN/DEAP(0.1), PAN/DEAP(0.5) and PAN/DEAP(1).


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Fourier transform infrared spectroscopy (FTIR) spectra of the bare PAN and PAN/DEAP(0.1, 0.5 and 1) samples are depicted in Figure 2A. The spectrum of pure PAN contains prominent peaks at 2937, 2241, and 1470 cm−1 due to the stretching vibration of methylene (-CH2-), stretching vibration of nitrile groups (-C≡N), and bending vibration of methylene, respectively.22, 23

PAN deposited from N,N-dimethylmethanamide (DMF) solution often has a peak at about

1670 cm−1, which is assigned to the vibration of the C=O bonds formed in the hydrolyzed PAN nanofibers and the stretching vibration of the C=O bonds in residual solvent DMF.24 PAN/DEAP(0.1) sample presented almost the identical peaks with bare PAN. With the increasing DEAP content, however, FTIR spectra of both PAN/DEAP(0.5) and PAN/DEAP(1) presented a new peak at around 953 cm-1, which can be assigned to N-H wagging vibration from DEAP (inset of Figure 2A).25 To further confirm this, X-ray photoelectron spectroscopy (XPS) was employed to investigate the chemical states of N in the PAN/DEAP samples. Figure 2B shows the XPS N1s spectra of the four samples. For bare PAN sample, the peak with binding energy at 399.0 eV can be attributed to the C≡N bond from PAN. The peak at 400.3 eV observed for DEAP is ascribed to the -NH- group.26, 27 After the addition of different amount of DEAP, positive shift of 0.02 eV, 0.12 eV and 0.25 eV can be observed on the PAN/DEAP(0.1), PAN/DEAP(0.5), and PAN/DEAP(1) samples, respectively. Moreover, the oxygen content of the PAN/DEAP samples increased with the increasing DEAP content (Table S3). These results strongly demonstrated the combination of PAN with DEAP. To determine the wettability of the samples, their water contact angle was measured and shown in Figure 2C and Table S2. The water angle of the droplet on the as-prepared PAN was below 90° (Figure 2C1), which indicated hydrophilicity. After the addition of different amount of DEAP, the droplets on the PAN/DEAP


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samples spread out faster than that of the bare PAN, indicating the improvement of the surface hydrophilicity (Figure 2C2-4). The PM2.5 capture measurement was performed in a static system as illustrated in Figure S4. All of the four samples were firstly attached on the top of the lid. The PM2.5 was then generated by burning cigarette and collected in the container in the presence of moisture. We noted that the smoke generated by cigarette shared the similar ingredient with that of the PM 2.5 pollution in the air.28,

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The capture of PM2.5 on the samples in this work was mainly dominated by the

Brownian motion of PM2.5 particles, which was a random motion and resulted from the quick collision of the particles suspended in the reactor.30 This capture system mimicked the static hazy weather without wind flow. The morphology of the samples after the PM2.5 capture measurement was performed by SEM (Figure S5). After 60 min of PM2.5 capture, the morphology of the PAN/DEAP(0.1), PAN/DEAP(0.5), and PAN/DEAP(1) samples changed significantly compared to the bare PAN with agglomerated PM2.5 wrapped on the nanofibers and deposited in the nanofiber network. Moreover, after the PM2.5 adsorption tests, PAN/DEAP(0.5) and PAN/DEAP(1) turned darker than that of the bare PAN and the PAN/DEAP(0.1), which implied that more PM deposits were found on these samples. To estimate the PM2.5 capture capacity of the four samples, thermogravimetric analysis (TGA) of each sample before and after the PM2.5 capture measurement was performed and shown in Figure S6. Clearly, the difference of the weight loss for each sample before and after PM 2.5 capture was mainly caused by the adsorbed PM2.5. Take the TGA of PAN/DEAP(0.1) for example (Figure S7), the weight loss of 8% (before 300℃) and 5% (after 650℃) can be mainly attributed to the desorption and decomposition of captured PM2.5, respectively. The significant weight loss (87%) between 300 ℃ and 650 ℃ is mainly due to the decomposition of PAN


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nanofibers. The capacity of the sample can be calculated by using the weight of the captured PM2.5 divided by the weight of the bare sample. According to the TGA results (Figure S5), the capacity of PAN/DEAP(0.1, 0.5 and 1) samples towards PM2.5 adsorption improved significantly (14.9 wt%, 23.4 wt% and 21.9 wt%) after the addition of DEAP (Table S2), compared to that of the bare PAN (5.2 wt%). Based on the above results, PAN/DEAP(0.5) sample showed the best PM2.5 capture performance.

Figure 3. (A) SEM images and the corresponding C1s XPS of PAN/DEAP(0.5) with different time of PM2.5 capture, (B) The photos of PAN/DEAP(0.5) sample before and after 1h of PM2.5 capture, (C) FTIR of PAN/DEAP(0.5) with different time of PM2.5 capture.


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To further understand the PM2.5 capture process, the morphological features of PAN/DEAP(0.5) with different times of PM2.5 capture were performed by SEM (Figure 3A). With 10 min of PM2.5 capture, some deposits were observed on the nanofibers. With time increased to 30 min, more and more PM2.5 were attached and bound tightly on the nanoparticles. For extended length of 1h, the incoming PM2.5 directly attached on the deposit that had already wrapped the nanofibers and merged together to form the fusion of the attached substance with the nanofibers. In addition, the colour of the membrane changed from white to brown after 1h capture, which also implied the existence of PM2.5 on the sample (Figure 3B). The corresponding C1s XPS spectra exhibited the increased peaks at around 284.7 eV and 288.1 eV with the increase of the capture time, which can be assigned to the C=C and C=O bonds from the PM 2.5, respectively. From the FTIR spectra (Figure 3C), the peaks for C=O, C=C, C-C, C-N and C-O increased with the increase of the capture time, which indicated that the PM2.5 had been gradually adsorbed on the sample. The ash of the burnt cigarette was also characterized by FTIR as shown in Figure S8. The ash showed the similar peaks with that of the PM2.5, which confirmed the successful capture of PM2.5 on the nanofibers. In addition, the peak for -OH observed in Figure 3C was also gradually enhanced, which can be attributed to the adsorption of moisture due to the improved hydrophilicity.


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Figure 4. Illustration of the PM2.5 capture process. We believed that the enhanced PM2.5 capture capacity of the PAN can be attributed to the improved hydrophilicity and the rough surface of the nanofibers after addition of DEAP. In the PM2.5 system (Figure 4), the moisture can be adsorbed on the surface of the PM2.5 particles. When the particles with adsorbed moisture attached to the hydrophilic surface of the nanofibers, they can be easily captured. Apart from the hydrophilicity of the nanofiber surface, the improved roughness can provide more adsorption sites, contributing to the enhanced PM2.5 capture capacity. On the other hand, dipole moment of the material can provide useful insight towards the understanding of the PM2.5 capture. Generally, a high dipole moment has a strong dipoledipole interaction and intermolecular force, which can contribute to the enhanced electrical binding of PM2.5 on the polymer surface.14 PM2.5 particles can be electrically charged, which resulted from the quick collision of the particles in the reactor. When the electrically charged PM2.5 particles attached on the PAN nanofibers with a high dipole moment (3.5D), they can be electrostatically adsorbed. The addition of DEAP with a higher dipole moment of 3.8D can also


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trigger a stronger electrostatic interaction between the PAN/DEAP nanofibers and PM2.5 particles, resulting in an improved PM2.5 capture efficiency. Conclusion In conclusion, we prepared the PAN nanofibers by electrospinning in the presence of highly viscos and hydrophilic ionic liquid DEAP. The surface properties of the obtained PAN nanofibers have been modified by DEAP, which induced rough surface (i.e. adsorption sites) and improved hydrophilicity. The interlinked nanofibers membrane with desired “window” size and surface chemistry can effectively facilitate high affinity towards moisture-bound PM2.5 particles. By studying the PM2.5 capture performance of the samples in a typical static system, the PM2.5 capture capacity on the PAN/DEAP samples has been significantly improved compared to the bare PAN sample. We believed that these cost-effective and facile materials find potential applications in the field of PM2.5 removal devices and environmental protection systems.

ASSOCIATED CONTENT Supporting Information. Experimental section, additional table and figures. This material is available free of charge via the Internet at http://pubs.acs.org.” AUTHOR INFORMATION Corresponding Author E-mail: [email protected]; [email protected]; [email protected] Notes


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The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was financially supported by the Australia Research Council Discovery Project (DP 110101638) and partially supported from the China Scholarship Council (CSC, no. 201406030034). The authors appreciate the facility and technical assistances supported by UNSW Mark Wainwright Analytical Centre. We also thank Dr. Bill Gong from Solid State & Elemental Analysis Unit of UNSW for his help in XPS measurements and analysis, Mr. Tim Fang for his help in FTIR measurements, and Dr. Zhi-Yu Yang for his help in design graphitic image. REFERENCES (1) Huang, R.-J.; Zhang, Y.; Bozzetti, C.; Ho, K.-F.; Cao, J.-J.; Han, Y.; Daellenbach, K. R.; Slowik, J. G.; Platt, S. M.; Canonaco, F.; Zotter, P.; Wolf, R.; Pieber, S. M.; Bruns, E. A.; Crippa, M.; Ciarelli, G.; Piazzalunga, A.; Schwikowski, M.; Abbaszade, G.; Schnelle-Kreis, J.; Zimmermann, R.; An, Z.; Szidat, S.; Baltensperger, U.; El Haddad, I.; Prevot, A. S. H., High Secondary Aerosol Contribution to Particulate Pollution during Haze Events in China. Nature 2014, 514, 218-222. (2) Apte, J. S.; Marshall, J. D.; Cohen, A. J.; Brauer, M., Addressing Global Mortality from Ambient PM2.5. Environ. Sci. Technol. 2015, 49, 8057-8066. (3) Chow, J. C.; Watson, J. G., Review of PM2.5 and PM10 Apportionment for Fossil Fuel Combustion and Other Sources by the Chemical Mass Balance Receptor Model. Energy Fuels 2002, 16, 222-260.


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