Anionic Surfactant-Triggered Steiner Geometrical Poly(vinylidene

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Functional Nanostructured Materials (including low-D carbon)

Anionic Surfactant Triggered Steiner Geometrical Poly(vinylidene fluoride) Nano-Fiber/Nets Air Filter for Efficient Particulate Matter Removal Xiong Li, Ce Wang, Xiaohua Huang, Tonghui Zhang, Xuefen Wang, Minghua Min, Lumin Wang, Hongliang Huang, and Benjamin S. Hsiao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b16564 • Publication Date (Web): 14 Nov 2018 Downloaded from http://pubs.acs.org on November 15, 2018

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ACS Applied Materials & Interfaces

Anionic Surfactant Triggered Steiner Geometrical Poly(vinylidene fluoride) Nano-Fiber/Nets Air Filter for Efficient Particulate Matter Removal Xiong Li,†,‡ Ce Wang,‡ Xiaohua Huang, Tonghui Zhang,‡ Xuefen Wang,‡,* Minghua Min,†,* Lumin Wang,† Hongliang Huang,† and Benjamin S. Hsiao§ †Key

Laboratory of Oceanic and Polar Fisheries, Ministry of Agriculture and Rural Affairs, East

China Sea Fisheries Research Institute, Chinese Academy of Fishery Sciences, Shanghai, 200090, PR China ‡State

Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of

Materials Science and Engineering, Donghua University, Shanghai, 201620, PR China §Department

of Chemistry, Stony Brook University, Stony Brook, NY 11794-3400, United

States Key

Laboratory of Open-Sea Fishery Development, Ministry of Agriculture and Rural Affairs,

PR China

KEYWORDS: Steiner geometry, poly(vinylidene fluoride), nano-fiber/nets, particulate matter, air filtration

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ABSTRACT: The emergence of Steiner minimal tree is of fundamental importance, and designing such geometric structure and developing its application has practical effect in material engineering and biomedicine. We used a cutting-edge nano-technology, electro-spinning/netting, to generate a Steiner geometrical poly(vinylidene fluoride) (PVDF) nano-fiber/nets filter for removing airborne particulate matter (PM). Manipulation of surface morphologies by precise control of charged situation enabled the creation of two dimensional (2D) nano-nets with Steiner geometry. A significant crystalline phase transition of PVDF from α-phase to β-phase was triggered by the dipole orientation and the intermolecular interactions derived from the electrostatic potential analysis. Particularly, the synergy of electrical interaction (ion-dipole and dipoledipole) and hydrophobic interaction facilitated the formation of Steiner geometric structure during the evolution process of nano-nets. The resultant PVDF nano-fiber/nets air filter exhibited high filtration efficiency of 99.985% and low pressure drop of 66.7 Pa under the airflow velocity of 32 L/min for PM0.26 removal by the safest physical sieving mechanism. Furthermore, such filter possessed robust structure integrity for reusability, comparable optical transmittance, superior thermal stability, and prominent purification capacity for smoke PM2.5. The successful construction of such fascinating Steiner geometrical PVDF nano-nets will provide new insights into the design and exploitation of novel filter media for air cleaning and haze treatment.


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INTRODUCTION

Steiner minimal tree, as a well-known geometrical architecture, is constituted by interconnecting given points A1, …, An in the plane using lines of shortest possible total length.1 Such geometry has been of considerable interest in network structure design and operations research,2 especially serving as the geometric foundation for the mechanics.3-4 Notably, emerging evidences from nature, such as spider webs, honeycombs, sisal fibers, even the soap bubbles, and so forth,4-7 indicate the fascinating Steiner geometry particularly inspiring the development of novel biomimetic structural materials for various applications.2,

8-9

Specifically, recent advances in

electro-spinning/netting nano-fiber/nets, as the forefront of advanced nano-technology, have endowed the newly constructional 2D nano-nets with ideal and weighted Steiner geometry.10-13 Such identified nano-nets, comprised of interlinked ultrafine nano-fibrils, exhibit numerous prominent characteristics, like smaller pore size and fibril diameter (1 μm).31 Therefore, Wang et al.32-33 developed novel electrical-type nano-fiber air filters to enhance the removal efficiency towards the fine PM, i.e. triboelectric nanogenerator (TENG) enhanced PI and PVDF nano-fiber filters. While the highest removal efficiency for the PM with particle size of 1 μm),31 as shown in Figure 6a. Interestingly, benefiting from the Steiner geometrical PVDF nano-nets in nano-fiber framework


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with small pore size, the interception and adhesion of the target particles will be particularly enhanced and obviously improve the filtration efficiency (Figure 6b). Meanwhile, the interconnected tortuous airflow channels and controllable packing density (i.e. basis weight, determining the transparency of the filter) of the PVDF nano-fiber/nets air filter will become the pivotal contributors to the low resistance of airflow.12 We thus investigated the contribution of Steiner geometrical nano-nets promoting the air filtration performance for the fine airborne particles. In this study, monodisperse sodium chloride (NaCl) aerosol particles with a mass mean diameter of 260 nm were employed to carry out the filtration performance evaluation. Figure 6c showed the filtration efficiency and pressure drop of the PVDF nano-fibers and nano-fiber/nets air filters (with basis weight of 3.04 g/m2) as a function of the airflow velocity, considering the various practical purposes. It could be clearly observed that, with the increase of velocity from 20 to 80 L/min, the filtration efficiency of both air filters remained virtually unchanged, and the pressure drop exhibited increased tendency, which was directly proportional to the velocity and complied with the Darcy’s theory.53 As it was expected, the filtration efficiency of the nanofiber/nets filter was considerably higher than that of the nano-fibers filter, such as the values of 99.987% and 61.5% were achieved respectively for the filter with and without nano-nets, when the airflow velocity was 30 L/min. Moreover, the nano-fiber/nets filter could even maintain the filtration efficiency of >99.94% for the fine PM capture at the highest velocity of 80 L/min, fully demonstrating its enhanced structural robustness derived from the three-way junction of nanonets. The relatively stable filtration efficiency of the nano-fibers filter might be due to the large basis weight (3.04 g/m2) used in this study, and therefore the airflow velocity (20~80 L/min) was not high enough that could decrease its filtration efficiency. Here, it should be mentioned that, besides the small pore size provided the sieving capacity for the fine PM, β-PVDF with high


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dipole moment of 2.1 D permitted the strong adhesion between the fine PM and the nano-fibrils by virtue of the high polarity effect of β-phase.54 While, as shown in Figure 6c, the pressure drop of the nano-fiber/nets filter was higher than that of the nano-fibers filter under the certain airflow velocity, which was mainly due to the smaller pore size of the layer-by-layer stacked nano-nets.55 But the pressure drop of such nano-fiber/nets filter was still lower and competitive when the velocity was less than 60 L/min, especially the value of 64.89 Pa obtained at the velocity of 30 L/min. This could be attributed to the extremely small fibril diameter of the nano-nets (half of the mean free path of air molecules of 65.3 nm), which could exhibit a particularly enhanced “slip effect”, i.e. slip flow of air molecule on the periphery of the fibrils, resulting in lower drag force.56 Additionally, the trade-off parameter, quality factor (QF = –ln(1–η)/Δp, η is the filtration efficiency and Δp is the pressure drop), is commonly used to assess the comprehensive filtration performance, i.e. the higher QF value the better filtration performance of the air filter.29 As shown in Figure 6d, the nano-fiber/nets filter could perform a robust QF value of 0.182 Pa-1 under the airflow velocity of 20 L/min, and all the QF values were accordingly larger than those of the nano-fibers filter, confirming the crucial role of the completely covered 2D nano-nets with Steiner geometry on promoting the air filtration performance.

Figure 7a-f showed the surface morphology of the PVDF nano-fibers and nano-fiber/nets filters after the filtration of airborne fine PM. Obviously, only a minority of fine particles were attached by the nano-fibers (Figure 7a-c), which was due to the large pore size of PVDF nanofibers filter and the lower adhesion behavior derived from the lower dipole moment of α-PVDF (1.2D).49 While almost all the fine PM were screened and captured on the nano-nets surfaces without damaging its structural integrity (Figure 7d-f), further confirming the fabulous filtration efficiency of PVDF nano-fiber/nets filter. To achieve an effective reusability of such filter (with


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Figure 7. Representative surface FE-SEM images of (a-c) PVDF nano-fibers and (d-f) PVDF nano-fiber/nets filters after the filtration of airborne fine PM. (g) Cyclic filtration performances of the air filter after cleaning post-treatment (with basis weight of 3.04 g/m2 and airflow velocity of 30 L/min). (h-i) Representative surface FE-SEM images of the nano-fiber/nets filter after the sixth cleaning process.


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basis weight of 3.04 g/m2) considering its potential practical applications, a facile cleaning posttreatment with appropriate agent was adopted to regenerate the polluted nano-fiber/nets filter. Here, we directly immersed it into the water, and the fine NaCl aerosol particles could be rapidly dissolved into the water and completely cleaned the filter. Then, after drying, the regenerated filter was repeatedly used for the filtration measurement (under airflow velocity of 30 L/min) and cleaning post-treatment to identify its reusability. Notably, after six cycles of the filtrationcleaning process, the nano-fiber/nets filter still maintained high filtration efficiency of 99.96% and low pressure drop of 55.45 Pa (Figure 7g). Furthermore, the fascinating Steiner geometry of the nano-nets was also integrally reserved after the sixth cleaning process as shown in Figure 7hi, demonstrating the robust three-way junction of the nano-nets and the excellent reusability of this filter.

The optical transmittance of the air filter, determined by the basis weight (i.e. packing density), is another significant character that has been evaluated. Figure 8a presented the photographs of the free-standing PVDF nano-fiber/nets (peeled off from the mesh, see Supporting Information Figure S6) with transmittances of approximately 83, 67, 55, 44, 41, and 29%. The corresponding basis weights were 0.54, 0.99, 1.64, 2.21, 2.47, and 3.04 g/m2, respectively. It could be observed that the logo of “Donghua University” placed under the nano-fiber/nets with transmittance of 83% and 67% were clearly visible, while it began to become blurry from the transmittance of 55%, and finally turned into completely invisible at 29%. The air filtration performance of the nano-fiber/nets filters with different transmittance levels were assessed under the airflow velocity of 32 and 85 L/min. This two velocity are commonly regarded as the industrial testing standard and the latest research advance for the air filters.15 The results were correspondingly depicted in Figure 8b-c. As can be seen, with the increase of the optical transmittance


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(a)

T-83

T-67

T-55

T-44

T-41

T-29

(b)

(c)

Figure 8. (a) Photographs of the free-standing PVDF nano-fiber/nets at different transmittances. (b) The filtration performances of the nano-fiber/nets air filters with different transmittance levels at airflow velocity of (b) 32 and (c) 85 L/min. (decreasing the basis weight), the filtration efficiency and pressure drop all exhibited reduced tendency, i.e. from 99.985% and 66.7 Pa to 77.7% and 3.92 Pa at 32 L/min, and from 99.93% and 376.5 Pa to 32.5% and 13.73 Pa at 85 L/min. Wherein, >94% removal at 55% transmittance


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and >90% removal at 44% transmittance for the fine PM capture were obtained at the corresponding airflow velocity. These findings will provide a reference for the nano-fiber/nets coating on the real window screen with an optimal transmittance level, thereby favoring the penetration of sufficient sunlight.

The thermal stability of the Steiner geometrical nano-nets was further investigated to evaluate its potential in the practical high temperature environment. Figure 9a showed the photographs and surface FE-SEM images of the PVDF nano-fiber/nets filters after thermal treatment at different temperature for 2h. As can be seen, with the temperature increased from 25 to 150 °C, the overall appearance of the filters and the surface morphology of nano-fiber/nets were almost unchanged, including the coverage rate, pore geometry and pore size of the nano-nets. When the temperature increased to 180 °C, the neighboring nano-fibers seemed to slightly fuse together at the intersection, and this can be explained by that the melting temperature of β-PVDF nanofiber/nets was about 180 °C (Figure S5). But the nano-net virtually maintained its original morphology. Further increasing the temperature to 200 °C, the nano-fiber/nets on the mesh surface totally melted into a film with big holes. To demonstrate the influence of such thermal treatment on the removal of fine PM, we thus studied the air filtration performance of the nanofiber/nets filters (with basis weight of 3.04 g/m2) after thermal treatment with different temperatures. As shown in Figure 9b-c, under the airflow velocity of 32 and 85 L/min, both filtration efficiency and pressure drop remained nearly constant at temperature below 180 °C, especially 99.767% removal with 63.52 Pa resistance and 99.547% removal with 367.2 Pa resistance were respectively gained at 180 °C. While the seriously damaged filter at 200 °C exhibited terrible filtration performance for the fine PM. These results were consistent with the above surface morphology analysis, indicating the excellent thermal stability of the as-prepared


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Steiner geometrical nano-nets at temperature of ≤ 180 °C. Here, it should be mentioned that, most of the PM sources, such as the coal furnace exhaust, vehicle exhaust, and biomass burning, contain various particles with temperature of 50-200 °C.26 Therefore, the PVDF nano-fiber/nets air filters will be expected to be stable when used for removing PM from these exhausts.

To comprehensively elaborate the air filtration performance of our Steiner geometrical nanofiber/nets filter, its filtration efficiency as a function of pressure drop in conjunction with the QF curve was compared to those of H&V commercial filters,57 electro-spun nano-fibers filters, and the state-of-the-art nano-fiber/nets filters, as shown in Figure 10a. Wherein, similar basis weight, airflow velocity and PM size were taken into account for this comparison purpose. As can be seen, the nano-fiber/nets filters possessed much better fine PM capture performance and higher QF values than those of electro-spun nano-fibers filters and glass fiber high efficiency particulate air (HEPA) filters. Since the tremendous basis weight (>100 g/m2) used and the potential safety hazard from the electret degradation, the electret melt-blown fiber HEPA filters with high QF level were not considered for the plot.34 Compared with the filtration performance of PMIA and PA-56 nano-fiber/nets filters,13, 58 as the blue stars in the red region showed in Figure 10a, our PVDF filter exhibited relatively lower pressure drop and higher QF value, suggesting it as promising platform for efficiently removing the fine PM with low air resistance. The detailed parametric configurations of all air filters were summarized in Table S2. Additionally, to evaluate the long-term purification capacity of PVDF nano-fiber/nets filter in the practical hazy environment, considering the difference between real PM and NaCl aerosol particles, a recycling test was performed to purify the polluted air via removing smoke PM2.5 from >600 μg/m3 to 600 μg/m3 to

Anionic Surfactant-Triggered Steiner Geometrical Poly(vinylidene

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