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Low-resistance Dual-purpose Air Filter Releasing Negative Ions and Effectively Capturing PM 2.5

Xinglei Zhao, Yuyao Li, Ting Hua, Pan Jiang, Xia Yin, Jianyong Yu, and Bin Ding ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b00351 • Publication Date (Web): 15 Mar 2017 Downloaded from http://pubs.acs.org on March 20, 2017

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ACS Applied Materials & Interfaces

Low-resistance Dual-purpose Air Filter Releasing Negative Ions and Effectively Capturing PM2.5 Xinglei Zhao,† Yuyao Li,† Ting Hua,† Pan Jiang,† Xia Yin,† Jianyong Yu,§ and Bin Ding*,†,§

†

Key Laboratory of Textile Science & Technology, Ministry of Education, College of

Textiles, Donghua University, Shanghai 201620, China. §

Nanofibers Research Center, Modern Textile Institute, Donghua University, Shanghai

200051, China.

*

Corresponding author: Prof. Bin Ding (E-mail: [email protected])

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ABSTRACT: The fatal danger of pollution due to particulate matter (PM) calls for both high-efficiency and low-resistance air purification materials, which also provide healthcare. This is however still a challenge. Herein, low-resistance air filter capable of releasing negative ions (NIs) and efficiently capturing PM2.5 was prepared by electrospinning polyvinylidene fluoride (PVDF) fibers doped with negative ions powder (NIPs). The air-resistance of fibrous membranes decreased from 9.5 to 6 Pa (decrease of 36%) on decreasing the average fiber diameter from 1.16 to 0.41 µm. Moreover, the lower rising rate of air-resistance with reduction in pore size, for fibrous membranes with thinner fiber diameter was verified. In addition, a single PVDF/NIPs fiber was provided with strong surface potentials, due to high fluorine electronegativity, and tested using atomic force microscopy. This strong surface potential resulted in higher releasing amounts of NIs (RANIs). Interestingly, reduction of fiber diameter favored the alleviation of the shielding effects on electric field around fibers and promoted the RANIs from 798 to 1711 ions cc-1. Moreover, by regulating the doping contents of NIPs, the RANIs increased from 1711 to 2818 ions cc-1. The resultant fibrous membranes showed low air resistance of 40.5 Pa. Field-tests conducted in Shanghai showed stable PM2.5 purification efficiency of 99.99% at high RANIs, in the event of haze.

KEYWORDS: electrospinning, negative ion, low-resistance, nanofiber, air purification


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1. INTRODUCTION An average person breathes more than twenty thousand times and in the process exchanges ten thousand liters of air every day.1 Hence, air is the fundamental necessity of life. However, pollution of air as result of a large number of pollutants emitted from industries and automobiles is a severe threat to human health.2 Particulate matter (PM), one of the major air pollutants, particularly in many developing countries, is very lethal to humans.3 According to a recent report by the United States Environmental Protection Agency annually 2.1 million deaths occur worldwide due to the increasing levels of PM2.5.4,5 Purification of air, which involves removal of particulate matter from air, can effectively counter the harmful effects of high concentrations of PM2.5. High-performance air purification materials are capable of capturing PM2.5 with high efficiency and simultaneously provide a low resistance to clean air.6 However, the currently used filtration materials for air purification, such as glass fiber,7 spun-bond fiber,8,9 and needle-punched fiber10 have the disadvantage of high air-resistance, due to direct contact of air molecules with the fibers and low filtration efficiency due to large pore sizes. Meanwhile, electret melt-blown fibrous materials have the advantage of low initial air-resistance. However, their use is limited due to charge dissipation within the material and related safety hazards.11,12 Another important development trend for air filtration was to prepare new materials with the function of providing healthcare. Negative ions (NIs) provide a very effective cure against chronic bronchitis, asthma, hypertension, hyperlipidemia, and neurasthenia.13-16 More importantly, the immune system of the body can be boosted if the concentration of NIs in the air exceeds 2000 ions cc-1.17,18 Therefore, artificial generation of negative ions in air is a very useful method for protecting human health. Unfortunately, however, the application of this method is very limited currently. For this reason, the concept of an air filter that is capable of releasing NIs for healthcare and efficiently capturing PM2.5 has been introduced here for the first time. 3 ACS Paragon Plus Environment


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Electrospun nanofiber, which is the cutting edge technology for advanced fibrous materials, has the advantage of possessing ultrathin diameters (10-1000 nm), extensively interconnected pores, and an adjustable porosity as also the ease and scalability of fabrication methods using different source materials.19,20 Hence, these fibers have great potential as novel nanoscale building blocks and are used in the fabrication of high-performance materials for air filtration.21 Therefore, the development of electrospun filter materials including polyimide,22 polyvinyl

chloride-polyurethane,23

poly(lactic

acid)/titania,24

polyacrylonitrile,25

and

polymide-6626 has attracted much attention. Moreover, composite nanofibers hybridized by nanoparticles, which enhance the electret effect of nanofibers, have been reported.27,28 However, the efficiencies of the aforementioned fibrous materials for capturing fine particles and their air permeabilities are still not satisfactory, due to unsuitable pore sizes and dense structures of the fiber assemblies. In addition to this, nearly no effort has been made for developing nanofiber-based air purification materials along with the ability to release NIs. Therefore, the challenge was to produce homogeneous, bicomponent, and low-resistance air purification materials capable of releasing NIs and effectively capturing PM2.5. In this study, a robust methodology for producing low-resistance air filter, which also releases NIs has been presented. This has been achieved by regulating the diameter of single fiber, polymeric category, and doping content of negative ions powder (NIPs). The effects of decreased fiber diameter on the reduction of air resistance and the relationship between the air resistance and pore size have been thoroughly investigated. Studies on the effect of high electronegativity of PVDF fibers on the releasing amounts of NIs (RANIs) showed that higher electronegativity can result in stronger surface potential and consequently higher RANIs. More importantly, the alleviative shielding effect, due to the reduction of fiber diameter on the electric potential difference inside the negative ions powder, is instrumental in promotion of RANIs. Moreover, optimization of the doping contents of NIPs increases the RANIs. Taking advantage of the enhanced slip-flow of airstream, the resultant fibrous membranes 4 ACS Paragon Plus Environment

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present a low air-resistance and stable PM2.5 purification efficiency at high RANIs, in the event of haze. 2. EXPERIMENTAL SECTION 2.1. Materials. PVDF (DY-1, Mw = 680000, density = 1.77 g cm-3) was obtained from Shandong DEYI New materials Co., Ltd., China. Polyvinyl butyral (PVB, B-72, Mw = 190000, density = 1.07 g cm-3) was supplied by J&K Scientific Co. Ltd., China. Polysulfone (PSU, Udel P-1700LCD, Mw = 370000, density = 1.37 g cm-3) was supplied by SOLVAY Shanghai Technology Park, China. NIPs with the average particles size of 60 nm (the composition of NIPs: Al2O3 35.10 wt%, SiO2 34.81 wt%, B2O3 11.02 wt%, MgO 4.70 wt%, Fe2O3 10.18 wt%, Na2O 0.91 wt%, K2O 0.04 wt%, P2O5 0.22 wt%, TiO2 0.26 wt%, FeO 1.35 wt%, other composition 1.41 wt%) were supplied by Dongguan Yingde plastic cement materials Co. Ltd., China. N,N-Dimethylfomamide (DMF) was provided by Macklin Biochemical Co. Ltd., China. The non-woven substrate (polyethylene terephthalate) for the collection of nanofibers, kindly supplied by Hainan Xinlong Nonwoven Co. Ltd., China, possessed negligible filtration efficiency (2%) and pressure drop (0 Pa), when the airflow velocity was 5.3 cm s-1. 2.2. Preparation of Solutions. Polyvinylidene fluoride (PVDF) solution was prepared by dissolving PVDF powder (30 g) in DMF (70 g) at 80 °C while stirring for 24 h. PVB solution was prepared by dissolving PVB powder (16.8 g) in DMF (43.2 g) at 25 °C while stirring for 14 h. PSU solution was prepared by dissolving PSU powder (13.2 g) in DMF (46.8 g) at 25 °C while stirring for 14 h. The PVDF polymer solutions (18 wt%) containing 8 wt% NIPs were prepared as follows: NIPs (4.8 g) were taken in four undefiled glass bottles and then DMF was added to the each bottle with vigorous stirring using magnetic stirrers. Moreover, to ensure homogeneous dispersion of NIPs in DMF, the mixture was ultrasonically dispersed for 1 h. Finally, PVDF powder (9.6 g) was added to the corresponding NIPs/DMF dispersions and then these multicomponent solutions were subjected to vigorous stirring to ensure complete 5 ACS Paragon Plus Environment


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dissolution of the polymer. The procedures for PVB and PSU polymer solutions containing 8 wt% NIPs were with the same as that of the PVDF polymer solution (18 wt%) containing 8 wt% NIPs. The procedures for 14, 16, and 20 wt% PVDF polymer solutions containing 8wt% NIPs were also the same as that of the 18 wt% PVDF polymer solution containing 8 wt% NIPs. 2.3. Fabrication of nanofibrous membranes. The nanofibers were fabricated using the electrospinning equipment DXES-4 (Shanghai Oriental Flying Nanotechnology Co. Ltd., China). In a typical procedure, the polymer solution was sucked into plastic syringes and clamped to the supporting frame, which continuously moved right and left. Later, the homogeneous solution was extruded through 5 G metallic needles with a governable infusion velocity of 0.5 mL h-1 and a high DC voltage of 30 kV was simultaneously applied at the tips of the needles, which led to a stable jet flow. The fibrous membranes obtained were assembled on the earthed metallic tumbling barrel covered by a nonwoven substrate, which rotated at a velocity of 50 rpm, with 15 cm distance between the tip and collector. The temperature and relative humidity during the process were 23 ± 2 °C and 48 ± 4%, respectively. The resultant PVDF fibrous membranes prepared from various solution concentrations and contained 8 wt% NIPs were denoted as PVDF-14/NIPs-8, PVDF-16/NIPs8, PVDF-18/NIPs-8, and PVDF-20/NIPs-8. The resultant 16 wt% PVDF fibrous membranes with various concentrations of NIPs were denoted as PVDF/NIPs-8, PVDF/NIPs-10, PVDF/NIPs-12, and PVDF/NIPs-14. 2.4. Characterization of the Membranes. Scanning electron microscope (SEM, TM 3000, Hitachi Ltd., Japan HQ) was used to study the morphology of PVDF/PTFE fibrous membranes. The fiber diameter was measured using image analysis software (Adobe Photoshop CS6). An automatic filtration performance tester, purchased from Huada Instrument and Equipment Co. Ltd., China was employed to assess the filtration performances. The charge neutralized sodium chloride (NaCl) aerosol particles were generated by atomizing air pump and then passed through the test samples with valid test area of 100 cm2. The NaCl 6 ACS Paragon Plus Environment

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aerosols were detected using laser airborne particle counter, which has ten particle size distribution channels and the correspondent particle size were 0.3, 0.5, 1.0, 3.0, 5.0 and 10.0 µm. For each distribution channels, the geometric standard deviation of particle size is less than 1.86. The ambient temperature and humidity in the test room were 25 ± 2 °C and 45 ± 5%, respectively. The filtration efficiency was calculated from the equation η=1- ε1 ⁄ε2 , where ɛ1 and ɛ2 represented the quantities of NaCl aerosol in the downstream and upstream of the filter, respectively. The test equipment for the evaluation of filtration efficiency could accurately determine the values up to three decimal places. The pressure drops of the samples were measured by a flow gauge and two electronic pressure transmitters (as shown in Figure S1). The filtration properties of as-prepared membranes were measured with the nonwoven substrate together. The RANIs were evaluated by a negative ion detector (EP009, Beijing YiPai Textile Instruments Co. Ltd., China), at a temperature of 25 ± 2 °C and relative humidity of 45 ± 2%. The most penetrating particle size of the fibrous membranes was tested using the TSI 3880 equipment (TSI Instruments Co. Ltd., America). The condensation particle counter was used to probe the aerosol. This instrument allows measuring the particles in the size-range from 10 to 615 nm in up to 96 channels. From the aerosol stream sucked into the condensation particle counter, the particles larger than 615 nm are removed by a buile-in inertial impactor. The experimental data of the number of particles’ counts in each channel were saved on the disc of a PC. The surface potentials of single fiber was tested using atomic force microscopy (Bruker’s Dimension Icon, USA), at a temperature of 25 ± 3 °C and relative humidity of 45 ± 4%. In addition, the PVDF and PSU fibrous membranes were tested after eliminating the charge, which was caused by the electrospinning process, by using the method of isopropyl alcohol soaking. Meanwhile, The PVB fibrous membranes were tested after eliminating the charge by using the method of n-hexane soaking. The aperture sizes of fabricated membranes were measured using a capillary flow porometer (CFP-1100AI, Porous Materials Inc., USA), based on Laplace's equation. Porosity of the fabricated membrane was 7 ACS Paragon Plus Environment


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obtained using the following formula:

porosity =

D0 D1 D0

× 100%

(1)

where D0 represents the density of raw PVDF and D1 specifies the density of fibrous membranes. 3. RESULTS AND DISCUSSION 3.1. Morphologies, structures, and air-resistances of fibrous membranes. The main focus of this work was to fabricate nanofibrous materials with decreased air-resistance, which could also carry out the functions of releasing NIs and capturing PM2.5 with high efficiency. With this intention functional nanofibrous materials were designed based on the following three conditions: (1) the diameters of nanofibers should be close to the mean free path of air molecules (65.3 nm), which favors the traverse of air molecules across the nanofibers with lower drag force and reduces air-resistance; (2) the polarity of molecular groups in polymer fibers should be high enough to synergize with the NIPs so as to promote the RANIs; (3) more NIPs should be naked on the surfaces of fibers to alleviate the shielding effects of polymeric components from the electric potential differences between the crystals of NIPs. The first objective was achieved by employing a versatile, readily accessible, and efficient fiber-fabrication technology of electrospinning. This approach involved the choice of polymeric species and the optimization of processing parameters. To investigate the effect of fiber diameter on air-resistance, the PSU, PVB, and PVDF fibrous materials with fiber diameters ranging from micrometer scale to sub-micrometer scale were fabricated. Since electrospun nanofibers were formed by evaporation or solidification of polymer fluid jets[29], the fiber diameters depended primarily on the polymer composition as well as on the humidity in the interval of jets. The SEM images of representative nanofiber media, electrospun from the three polymeric materials, are shown in Figures 1a-c. All the pristine PSU, PVB, and PVDF fibrous membranes showed randomly oriented three8 ACS Paragon Plus Environment

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dimensional structures with gradually reducing fiber diameter. The micro-sized fiber diameters of the PSU fibrous membranes ranged from 0.8 to 1.8 µm and the average fiber diameter was 1.16 µm (Figure S2).This was the result of rapid evaporation of solvent and phase separation of the polymer-solvent-water triphasic system.30,31 However, the average diameters of PVB fibers and PVDF fibers were reduced and were 0.68 µm and 0.41 µm, respectively (Figure S2). The reduction in diameters of these two polymeric fibers was a result of combined effects of low solution viscosity, high solution conductivity, and insensitivity of polymer-solvent systems towards moisture in air (Figure S3). Both, porosity and thickness have an enormous effect on the flow behavior of airstream inside a fibrous membrane. Moreover, to effectively compare the air-resistances of fibrous membranes with reduced fiber diameters, porosities and thicknesses (the fiber density of each polymer collected on the substrate can be seen in Table S1) of the three fibrous membranes were adjusted to almost the same values (Figure 1d) by regulating the electrospinning time and humidity. Interestingly, as illustrated in Figure 1e, the pressure drop decreased as the mean fiber diameter decreased, the filtration efficiency remaining almost the same (Figure S4). This was consistent with our previous studies and could be ascribed to the reduction of viscous forces induced by random motion and impact of air molecules, the fiber diameter being close to the mean free path of air molecules.32-34 In addition, the effect of porosity and thickness on the slip-effect can be seen in Supplementary Discussion. In addition to this, airflow velocity also has enormous consequences for the transformation of air-resistance.35 It is clear from Figure 1f that the pressure drop of PSU fibrous membranes showed a faster escalating trend with an increase in airflow velocity than that of the PVB and PVDF fibrous membranes. This could be a consequence of strong direct impact of air molecules on micro-sized fibers, revealing the vital role of fiber diameter on the reduction of pressure drop (The effect of air velocity on the slip-effect can be seen in Supplementary Discussion). However, a sharp increase of pressure drop could be seen for all the three fibrous 9 ACS Paragon Plus Environment


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membranes when the airflow velocity was more than 13.3 cm s-1. This was due to the reduction in diffusion of air molecules with increase in airflow velocity.36

Figure 1. Morphologies, structures, and pressure drops of PSU, PVB, and PVDF fibrous membranes. SEM images of (a) PSU fibrous membrane, (b) PVB fibrous membrane, and (c) PVDF fibrous membrane. (d) Porosities and thicknesses of PSU, PVB, and PVDF fibrous membranes. The pressure drops of PSU, PVB, and PVDF fibrous membranes (e) with different fiber diameters, and (f) at different airflow velocities. 3.2. The relationship between air-resistance and pore size. The pore size, developed as a result of the overlap of fibers, had great influence on the flow behavior of airstream inside the fibrous membranes that further influenced the pressure drop. As illustrated in Figure 2a, a slight effect of pore size on the pressure drop was seen when the pore size was more than 15 10 ACS Paragon Plus Environment

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µm and the pressure drops of three fibrous membranes were also the same. This result could be ascribed to the fact that the ratio of fiber diameter to the aperture size of the fabricated material was very small, due to which most of the air would have passed through the fibrous membrane from the pore area. However, the sharp increase in pressure drop of PSU fibrous membranes could be clearly associated with pore sizes smaller than 15 µm. This could explain the influence of fiber diameter (1.16 µm) on the flow behavior of airstream. However, for fibrous membranes with average fiber diameters of 0.68 and 0.41 µm, the effect of fiber diameter on flow behavior of airstream could be neglected, till the pore size reached 10.5 µm. As expected a steep slope of pressure drop was clearly seen for PSU, PVB, and PVDF fibrous membranes when the pore sizes were reduced to less than 9, 7.5, and 7.3 µm. This further confirmed the greater influence of fibers with larger diameters on the pressure drop. But it is worth noting that the porous structure of fibrous membranes, constructed from fibers with different diameters, also influenced the flow behavior of airstream to a large extent.32 This could be judged from the higher pressure drop of PSU fibrous membrane when the pore sizes were same for the three fibrous membranes. This could be attributed to PSU fibrous membrane having larger thickness (Figure S5). In addition, as shown in Figure 2b, the pressure drops of PVDF fibrous membranes tested under increased airflow velocities showed an escalating trend, while the filtration efficiencies of PVDF fibrous membranes exhibited a decrease trend (Figure S6) with the increase of airflow velocities. In order to study the distribution of airflow field inside the fiber assembly, computer simulation (the detailed simulation process can be seen in Supplementary Discussion) was carried out based on the weight, packing density, porosity, and relevant structural features of the as-spun PSU, PVB, and PVDF fibrous membranes, as shown in Figures 2c-e. Herein, Stokes flow was the prevailing air fluid, in which the Reynolds number was small (Re ≤ 1) and force due to inertia was far lower than the viscous force. Periodic boundary condition was applied in the airflow direction along the Z axis to keep the simulation system constant and 11 ACS Paragon Plus Environment


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eliminate the influence of boundary effect. In addition to this, the flow rate and temperature of the simulated airstream were maintained at 5.3 cm s-1 and 20 °C, respectively, to generate the most authentic test conditions. As shown in Figures 2f-h, the airflow velocity inside the fibrous membranes with average fiber diameters exhibited a gradually increasing trend, which could be ascertained from the change of red color within the streamline. This increase in velocity could be due to the reduction of viscous forces induced by random motion and impact of air molecules, with the fiber diameter being close to the mean free path of air molecules.37

Figure 2. The relationship between pressure drop and pore size. (a) The pressure drops of PSU, PVB, and PVDF fibrous membranes with varying pore sizes. (b) The pressure drops tested at different airflow velocities for PVDF fibrous membrane having average pore size of 4.5 µm. The simulative images of fibrous membranes: (c) PSU fibrous membrane, (d) PVB fibrous membrane, and (e) PVDF fibrous membrane. The corresponding simulative


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distribution of airflow inside fibrous membranes: (f) PSU fibrous membrane, (g) PVB fibrous membrane, and (h) PVDF fibrous membrane. 3.3. The effect of polymeric structure on RANIs. As illustrated in Figure 3a, the mechanism of formation of NIs involves the ionization of air induced by the electric potential difference between the crystals of NIPs. When the electrons formed by high electric potential get attached to the water molecules, the water molecules turn into NIs (the relationship between RANIs and ambient humidity can be seen in Supplementary Discussion and Figure S7). Moreover, in the process of electrospinning, the system comprising the polymer solution was subjected to a high positive voltage, during which the large number of charges generated insitu were incorporated into the entire multicomponent solution system. Intensification of the electric potential difference between the crystals of NIPs favored the enhancement of the RANIs. The process for the formation of NIs is as follows:38

H2O OH⁻ + H⁺ OH⁻ + nH2O → OH⁻ (H2O)n

(2) (3)

Based on this, it could be concluded that the molecular structure of fibers could enhance the electric potential difference between the crystals of NIPs. To probe the synergistic effects between the polymeric structure and NIPs on the RANIs, PSU/NIPs-8, PVB/NIPs-8, and PVDF-18/NIPs-8 fibrous membranes with the almost similar fiber diameters (average fiber diameter was 0.5, 0.53, and 0.55 µm, respectively) (Figure S8) were fabricated by regulating the electrospinning time (30 min). As demonstrated in Figure 3b, higher RANIs and lower basis weight could be clearly obtained for PVDF fibrous membranes. This was attributed to the strong electronegativity of PVDF molecular segment, which enhanced the electric potential difference between the crystals of NIPs and promoted the formation of NIs. In addition, the presence of a large number of hydroxyls in case of PVB fibrous membranes also favored the promotion of RANIs.39 However, nonpolar bonds such as 13 ACS Paragon Plus Environment


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those in the benzene ring and oxygen-sulfur bonds in the PSU fibers did not affect the electric potential difference between the NIPs particles, resulting in the relatively lower RANIs. To testify the above analysis, the surface potentials of single fibers were analyzed using AFM. As shown in Figure 3c, the average surface potential of PVDF fibrous membrane was -288 mV, which was higher than that of PVB fibrous membrane (211 mV) and PSU fibrous membrane (92 mV), confirming the effect of strong electronegativity of PVDF molecular segment on reinforcing the electric potential difference. The introduction of NIPs changed the structural properties (pore size and porosity) of fibrous membranes and also their corresponding filtration characteristics. As illustrated in Figure 3d, the filtration efficiencies of PSU/NIPs-8, PVB/NIPs-8, and PVDF-18/NIPs-8 tested after the charge elimination by using the method of isopropyl alcohol (IPA) soaking were 99.24%, 98.15%, and 98.72%, respectively, while their pressure drops were 42, 37, and 30 Pa, respectively. The lower pressure drop of PVDF fibrous membrane could be ascribed to its fluffy structure as a result of stronger charge repulsions between the fibers in their formation process (Figure S9). In addition, owing to its remarkably higher porosity, the PVDF/NIPs-8 membrane showed a quality factor of 0.132 Pa-1, which was higher than that of the other samples, further confirming the contribution of a stacking structure on the reduction of pressure drop of nanofibrous membranes.


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Figure 3. The RANIs and filtration performances of PSU/NIPs-8, PVB/NIPs-8, and PVDF18/NIPs-8 fibrous membranes. (a) Schematic representation of the fabrication process of the air filter, capable of releasing NIs. (b) The RANIs of PSU/NIPs-8, PVB/NIPs-8, and PVDF18/NIPs-8 fibrous membranes with different basis weights. (c) The surface potentials of single PSU/NIPs-8, PVB/NIPs-8, and PVDF-18/NIPs-8 fibers. (d) The filtration efficiencies, pressure drops, and quality factors of PSU/NIPs-8, PVB/NIPs-8, and PVDF-18/NIPs-8 fibrous membranes with average basis weight of 3.6, 4.3, and 5.5 g m-2, respectively. 3.4. The effect of fiber diameter on the RANIs. The covering of polymeric components on NIPs shields the electric potential difference generated between the particles of NIPs, which leads to the reduction of RANIs. Hence, the PVDF/NIPs-8 fibrous membranes with varying fiber diameters were fabricated in order to probe the relationship between fiber diameter and RANIs. As shown in Figure 4a, the RANIs showed an increasing trend with the reduction of fiber diameter from 0.71 to 0.39 µm (Figure S10). This confirmed that reduction in the amount of polymeric components led to an increase in RANIs. However, further decrease of the fiber diameter to 0.21 µm, led to a significant reduction of the RANIs. This was inconsistent with the hypothesis mentioned above and could be caused by the formation of the beads-on-string structure (Figure S10), resulting in a lot of NIPs stored in the beads. However, as shown in Figure 4b, the PVDF-14/NIPs-8 could reach higher RANIs when the basis weight reached only up to 2.4 g m-2, compared to other fibrous membranes with larger fiber diameters. This could be the consequence of increased NIPs in thinner fibers with the increase of basis weight of fibrous membrane, which was in favor of increasing the RANIs. The filtration properties of PVDF-x/NIPs-8 fibrous membranes with varying fiber diameters (the porosity and thickness can be seen in Figure S11) were studied using 300 nm NaCl aerosol particles. As illustrated in Figure 4c, the filtration efficiency of PVDF-16/NIPs8 fibrous membrane was 99.13%, which was higher than that of the other three fibrous membranes (PVDF-14/NIPs-8: 79.89%, PVDF-18/NIPs-8: 97.12%, and PVDF-20/NIPs-8: 15 ACS Paragon Plus Environment


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82.14%). This could be ascribed to the relatively thinner fiber of PVDF-16/NIPs-8 compared to PVDF-18/NIPs-8 and PVDF-20/NIPs-8 fibrous membranes and without beads compared to PVDF-14/NIPs-8 fibrous membrane. All the filtration efficiencies was tested after the charge elimination by using the method of IPA soaking. In this case, the main filtration mechanisms of those fibrous membranes are interception, diffusion, and impaction. In addition to this, PVDF-16/NIPs-8 fibrous membrane showed higher pressure drop (38 Pa) than that of other three fibrous membranes, due to smaller pore size. However, the PVDF-16/NIPs-8 membrane also showed a higher quality factor of 0.127 Pa-1 (Figure 4d), which further confirmed the role of fiber diameter on the improvement of filtration efficiency and reduction of pressure drop for nanofibrous membranes.

Figure 4. The RANIs and filtration performance of PVDF-x/NIPs-8 fibrous membranes with various fiber diameter. (a) RANIs of PVDF-x/NIPs-8 fibrous membranes with varying fiber diameters. (b) RANIs of PVDF-x/NIPs-8 fibrous membranes with varying basis weights. (c) 16 ACS Paragon Plus Environment

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Filtration efficiencies and pressure drops, and (d) quality factors of PVDF-x/NIPs-8 fibrous membranes with varying fiber diameters. 3.5. Regulating NIs of the nanomaterial. The crucial factor that determined the RANIs was the amount of NIPs. Therefore, the combination of an optimal fiber diameter, the best polymer composition, and an appropriate amount of NIPs could further increase the RANIs. However, increase in the amount of NIPs changed the morphology of fibrous membranes, as shown in Figures 5a-d. PVDF fibrous membranes fabricated from the polymer solution with increased concentration of NIPs from 8 to 10 wt%, led to a slight increase of the mean fiber diameter from 0.39 to 0.42 µm (Figure S12), due to enhanced viscosity of the polymeric solution. However, increase in the NIPs concentration from 12 to 16 wt% resulted in a slight change of the average fiber diameter, which could be the consequence of the synergistic effects of enhanced solution viscosity and reinforced solution conductivity. Moreover, more rough nano-structures (Figures 5a-d and the inset TEM pictures) appeared on the surfaces of all the PVDF fibers, due to increasing amounts of the NIPs and the solution instability at the stretching stage of the jet flow.40 The increase in NIPs could effectively promote the RANIs for fibrous membranes. As illustrated in Figure 5e, the RANIs showed an increased trend with the increasing amount of NIPs from 8 to 12 wt% inside the PVDF fibrous membranes fabricated with the same electrospinning time. In addition, the RANIs reached a high value of 2800 ions cc-1 when the basis weight of PVDF/NIPs-12 fibrous membranes reached 9.6 g m-2 and such a high RANIs was beneficial for human health. This could be due to the gradual increase of electric field around the single fiber, with increase of NIPs both, inside and on the surface of fibers, as shown in Figure 5f. However, the reduction of the initial RANIs could be seen for the PVDF/NIPs-14 fibrous membranes (990 ions cc-1 with basis weight of 0.79 g m-2), which could be the consequence of less fibers in the same electrospinning time. However, the RANIs of the PVDF/NIPs-14 fibrous membrane also could reach high value of 2770 ions cc-1 17 ACS Paragon Plus Environment


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when it reached a lower basis weight of 6.4 g m-2 due to higher content of NIPs in thinner fibers, leading to higher electric field. However, the lower basis weight in the same electrospinning time was caused by slight blocking of needle. In addition, to probe the effects of the induced charges developed in the process of electrospinning on the RANIs, the surface potentials were evaluated. As demonstrated in Figure 5g, the initial surface potential and RANIs were 2.3 kV and 2818 ions cc-1, respectively. However, the surface potential after charge elimination using the IPA was 20 V but the RANIs after charge elimination still maintained a high value of 2800 ions cc-1, which indicated that the surface potential had no effect on the RANIs. The ample number of RANIs of PVDF/NIPs-x fibrous membrane prompted further elaborate investigation of air filtration performances. Figure 5h demonstrated that the filtration efficiencies of PVDF/NIPs-x fibrous membranes were 98.13%, 97.64%, 99.79%, and 96.42%, whereas their corresponding pressure drops were 37.5, 37.5, 40.5, and 39.5 Pa, respectively. These results depicted the significant contribution of larger basis weight on the promotion of filtration efficienc.41 Furthermore, under higher airflow velocities of 10, 14.1, and 16.6 cm s-1, the filter could still retain robust efficiencies of 99.21%, 98.93%, and 98.33%, respectively, as demonstrated in Figure 5i. Due to nearly unchanged filtration efficiencies and slow increase of air resistance under extremely high air speed, the PVDF/NIPs-12 fibrous membrane could achieve quality factor (QF) value of 0.042 Pa-1 at 16.6 cm s-1 velocity (Figure 5j). In addition, the most penetration particle size and the corresponding minimum efficiency of PVDF/NIPs-12 fibrous membranes were tested. As shown in Figure S13, the most penetrating particle size was 0.106 µm and the corresponding minimum efficiency was 99.91%. Bases on the RNAIs and filtration performance, therefore the PVDF/NIPs-12 would be carried out in the following study.


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Figure 5. Morphologies, RANIs, filtration performances of PVDF/NIPs-x fibrous membranes. The SEM and TEM images of (a) PVDF/NIPs-8 fibrous membrane, (b) PVDF/NIPs-10 fibrous membrane, (c) PVDF/NIPs-12 fibrous membrane, and (d) PVDF/NIPs-14 fibrous membrane. (e) The RANIs. (f) Schematic showing the mechanism of the decreased shielding effects on electric field around fibers with the increase of NIPs. (g) The relationship between RANIs and surface potential of PVDF/NIPs-12 fibrous membranes. (h) Filtration efficiencies, pressure drops and quality factors of PVDF/NIPs-x fibrous membranes. (i) Filtration efficiencies and pressure drop, and j) quality factors of PVDF/NIPs-12 fibrous membranes under various airflow velocities. 3.6. The evaluation of ambient PM2.5 purification performance. The long-term performance of the low-resistance air filter was evaluated using the PVDF/NIPs-12 membrane characterized by RANIs of 2800 N cc-1 under the hazardous conditions of PM2.5 index > 500 (as shown in Figure S14). The long-term RANIs is shown in Figure 6a, after continuously working for 300 min, the PVDF/NIPs-12 membrane still maintained a high RANIs, 19 ACS Paragon Plus Environment


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demonstrating the good stability of the releasing of NIs. In addition, after 600 min, the filter also still showed a high PM2.5 purification efficiency of 99.99%, as illustrated in Figure 6b. This behavior could be attributed to the efficient physical interception of the fibrous membrane without loss of filtration efficiency as observed for electric materials. In contrast, the filtration efficiency of the commercial melt-blown fibrous membranes decreased drastically from 99.99 to 87.45% due to gradual deterioration, which could result in potential safety hazards as reported previously.[42] To demonstrate the combined benefits of low air-resistance and high PM2.5 purification efficiency, the clean air delivery rate (CADR) of PVDF/NIPs-12 fibrous membranes was determined (Figure S15). This parameter is defined as the time taken for the PM2.5 concentration to decrease from 500 to 35 µg m-3. As illustrated in Figure 6c, the PVDF/NIPs12 filter membrane required only 13 min to achieve this, which was higher than our previous work (PAN fibrous membranes: 15 min).32 The main reason was the PVDF/NIPs-12 fibrous membrane can effectively remove all kinds of particles with sizes from 0.3 µm to 2 µm with very high efficiency, as shown in Figure 6d. Furthermore, the two commercial materials (C1 and C2) were also tested for comparison purposes. C1 and C2 required much longer durations of 37 min and 19 min, respectively. Their relatively poor performances were likely due to the lower purification efficiency of C1 (86.6%) making it difficult to remove the PM2.5 and the high pressure drop of 136 Pa for C2 leading to decrease in air capacity handled per unit time. Moreover, as demonstrated in Figure 6e, the PVDF/NIPs-12 fibrous membrane maintained a CADR of 13 min without any change, even after 20 test cycles. This result could be attributed to the physical interception of the fibrous membrane and the strong adsorption of nanofibers towards particles due to the robust dipole–dipole and induced-dipole intermolecular forces. This result is consistent with other studies.25


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Figure 6. The PM2.5 purification efficiency, time for removal of PM2.5, and filtration efficiency. (a) The long-term RANIs of PVDF/NIPs-12 fibrous membranes. (b) Long-term PM2.5 purification efficiencies of PVDF/NIPs-12 fibrous membranes and commercial samples. (c) Time for removal of PM2.5 from 500 to 35 µg cm-3 for PVDF/NIPs-12 fibrous membranes, PAN, and commercial samples. (d) Filtration efficiency of PM particles with various size. (e) The long-term recycling performance of PVDF/NIPs-12 fibrous membranes for removing PM2.5 from 500 to 35 µg cm-3. (f) Comparison of the filtration performances of current filtration media with PVDF/NIPs-12 fibrous membranes at airflow velocity of 5.3 cm s-1. To compare the commercially available products, such as glass fibers and electret meltblown fibers, the filtration efficiencies versus their pressure drops at airflow velocity of 5.3 cm s-1 were plotted, as shown in Figure 6f. The commercial glass fibers could achieve relatively high filtration efficiencies due to the ultrafine diameters of their fibers (400-800 nm) and compact stacking structure. However, they were inevitably subjected to exceedingly large pressure drops.43 Meanwhile, low air resistance is a typical feature of electret meltblown microfibrous materials with moderate filtration efficiency. However, comparatively low mechanical filtration efficiency (~30%) and gradual ineffectiveness of charge in the course of utilization immensely deteriorates the filtration efficiency and leads to potential 21 ACS Paragon Plus Environment


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safety hazard. Taking advantage of the high mechanical filtration efficiency with extraordinary charge stability, the novel electret PVDF/NIPs-12 fibrous membranes were equipped with both, high filtration efficiency of 99.99% and low pressure drop of 80 Pa, which was higher than the previously reported electrospun fibrous materials, such as polyacrylonitrile/fluorinated polyurethane[44], polyamide-46[45], polylactic acid[46], etc. In addition, we also explored the feasibility of scaling-up of the electrospinning process for the as-prepared air filters. The requirements for large-scale production of the air filters have been discussed in the Supplementary Discussion section.

4. CONCLUSION In summary, novel low-resistance nanofibrous membranes capable of releasing negative ions and effectively capturing PM2.5 were successfully fabricated, which could potentially be applicable as air purification materials. The decrease of air-resistance for PSU, PVB, and PVDF fibrous membranes, with reduction of fiber diameter from 1.16 to 0.41 µm, was investigated. Moreover, the fibrous membranes with thinner fiber diameter are provided with lower rising rate of pressure drop along with the increase of pore size. In addition to this, due to the high electronegativity of PVDF fibers, synergistic effects of polymer structure and negative ion release were seen, which resulted in higher (RANIs). More importantly, the reduction of fiber diameter and the doping content of NIPs favored the RANIs from 798 to 2818 ions cc-1. Ultimately, the resultant fibrous membranes exhibited high PM2.5 purification efficiency of 99.99%, a low pressure drop of 40.5 Pa, and high RANIs of 2818 ions cc-1. Furthermore, the structural design of these new electrospun nanofibrous membranes, characterized by multilayered and cave-like structures, would pave the way for new types of nanofibrous materials, with applications in various fields.


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ASSOCIATIED CONTENT Supporting information. Experimental setup for property evaluation of filter media (Figure S1), the fiber diameter distribution of PSU, PVB, and PVDF fibrous membranes (Figure S2), the viscosity of PSU, PVB, and PVDF polymer solution (Figure S3), the fiber density of each polymer collected on the substrate (Table S1), the filtration efficiencies of PSU, PVB, and PVDF fibrous membranes (Figure S4), the thickness and filtration efficiencies of PSU, PVB, and PVDF fibrous membranes with various pore size (Figure S5), the thickness and filtration efficiencies of PSU, PVB, and PVDF fibrous membranes with various pore size (Figure S6), the RANIs of PVDF/NIPs-12 fibrous membranes tested under various humidity (Figure S7). the SEM images and the distribution of fiber diameter of PSU/NIPs-8, PVB/NIPs-8, and PVDF/NIPs-8 fibrous membranes (Figure S8), the porosity of PSU/NIPs-8, PVB/NIPs-8, and PVDF/NIPs-8 fibrous membranes (Figure S9). the SEM images and the distribution of fiber diameter of PVDF-14/NIPs-8, PVDF-16/NIPs-8, PVDF-18/NIPs-8 and PVDF-20/NIPs-8 fibrous membranes (Figure S10), the porosities and thicknesses of PVDF-14/NIPs-8, PVDF16/NIPs-8, PVDF-18/NIPs-8, PVDF-20/NIPs-8 fibrous membranes (Figure S11), the distribution of fiber diameter of PVDF/NIPs-8, PVDF/NIPs-10, PVDF/NIPs-12 and PVDF/NIPs-16 fibrous membranes (Figure S12), The MPPS of PVDF/NIPs-12 fibrous membranes (Figure S13), experimental setup for filter media property evaluation using real PM (Figure S14), PM2.5 purification efficiency measurement (Supplementary Methods), equipment for purification efficiency measurement of simulated PM2.5 (Figure S15), and Supplementary Discussion. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author 23 ACS Paragon Plus Environment


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*

Prof. Bin Ding, Email: [email protected].

Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS This work is supported by the Key Technologies R&D Program of China (Nos. 2015BAE01B01 and 2015BAE01B02), the National Natural Science Foundation of China (Nos. 51673037 and 51503030), the ‘DHU Distinguished Young Professor Program’, the Fundamental Research Funds for the Central Universities (No. 16D110115), and the Shanghai Sailing Program (No. 15YF1400600).


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(44) Wang, N.; Zhu, Z.; Sheng, J.; Al-Deyab, S. S.; Yu, J.; Ding, B. Superamphiphobic Nanofibrous Membranes for Effective Filtration of Fine Particles. J. Colloid Interface Sci. 2014, 428, 41-48. (45) Mikheev, A. Y.; Shlyapnikov, Y. M.; Kanev, I. L.; Avseenko, A. V.; Morozov, V. N. Filtering and Optical Properties of Free Standing Electrospun Nanomats From Nylon-4,6. Eur. Polym. J. 2016, 75, 317-328. (46) Wang, Z.; Zhao, C.; Pan, Z. Porous Bead-on-string Poly(Lactic Acid) Fibrous Membranes for Air Filtration. J. Colloid Interface Sci. 2015, 441, 121-129.


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