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Hierarchical Metal-Organic Framework Assembled Membrane Filter for Efficient Removal of Particulate Matter Won-Tae Koo, Ji-Soo Jang, Shaopeng Qiao, Wontae Hwang, Gaurav Jha, Reginald M. Penner, and Il-Doo Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b02986 • Publication Date (Web): 21 May 2018 Downloaded from http://pubs.acs.org on May 21, 2018

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ACS Applied Materials & Interfaces

Hierarchical Metal-Organic Framework Assembled Membrane Filter for Efficient Removal of Particulate Matter Won-Tae Koo,† Ji-Soo Jang,† Shaopeng Qiao,§ Wontae Hwang,† Gaurav Jha,‡ Reginald M. Penner,‡ and Il-Doo Kim†,* †

Department of Materials Science and Engineering, Korea Advanced Institute of Science and

Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea §

Department of Physics and ‡Department of Chemistry, University of California, Irvine,

California 92697-2025, United States

*E-mail: [email protected]

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ABSTRACT

Here, we propose heterogeneous nucleation assisted hierarchical growth of metal-organic framework (MOF) for efficient particulate matter (PM) removal. The assembly of 2dimensional (2D) Zn-based zeolite imidazole framework (2D-ZIF-L) in deionized water over a period of time produced hierarchical ZIF-L (H-ZIF-L) on hydrophilic substrates. During the assembly, the 2nd nucleation and growth of ZIF-L occurred on the surface of the 1st ZIF-L, leading to the formation of flower-like H-ZIF-L on the substrate. The flower-like H-ZIF-L was easily synthesized on various substrates, namely, glass, polyurethane 3D foam, nylon microfibers, and non-woven fabrics. We demonstrated H-ZIF-L assembled polypropylene microfibers as a washable membrane filter with highly efficient PM removal property (92.5 ± 0.8% for PM2.5 and 99.5 ± 0.2% for PM10), low pressure drop (10.5 Pa at 25 L min–1), longterm stability, and superior recyclability. These outstanding particle filtering properties are mainly attributed to the unique structure of the 2D-shaped H-ZIF-L, which is tightly anchored on individual fibers comprising the membrane.

Keywords: metal-organic framework, ZIF, hierarchical, particulate matter, filter


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1. INTRODUCTION Recently, air pollution issue has become a serious global problem. With the rapid development of technology, various pollutants, such as COx, NOx, NO3–, SiO2, SO2, SO42–, and so on, are generated by the industry, and they form particulate matters (PMs) in air.1 These PMs seriously affect human circulatory and/or respiratory systems.2 In particular, PM2.5 and PM10, i.e., PM with sizes under 2.5 µm and 10 µm, respectively, easily infiltrate the human body, causing lung cancer and pneumonia.3,4 According to the World Health Organization (WHO), for safety, the criteria for PM concentration in air is less than 10 µg m– 3

for PM2.5, and 20 µg m–3 for PM10. However, in East Asia and developing countries, the

concentration of PMs frequently exceeds the standard,5,6 thus this issue is treated as an important environmental agenda in each country. Therefore, it is imperative to find the practical solution for PM. Conventionally, removal of PMs is achieved by electrostatic precipitators, centrifugal collectors, fabric filters, and wet scrubbers.7 However, they have inherent problems such as large energy consumption and large volume space. To address this issue, a number of researchers have attempted to develop next-generation air filter systems.8-11 For instance, the Cui group reported electrospun nanofiber based air filters for the removal of PM2.5.12 The small particles were easily captured at the surface of the nanofibers with high surface area and high aspect ratio, and whose efficiency exceeded that of conventional filters. In addition, Ko et al.13 synthesized hierarchical Ag nanowires on a nylon mesh backbone to remove PMs by using electrostatic force. The electrical potential that was applied to the hierarchical Ag nanowires effectively attracted negatively charged PMs. However, for such air filter systems, specific nanofabrication tools and use of high cost nanomaterials are needed. Thus, air filters with high efficiency for removal of PMs should be rationally designed via relatively simple and low cost fabrication technique. 3 ACS Paragon Plus Environment


Meanwhile, metal-organic frameworks (MOFs) have been exploited in diverse fields, including catalysis,14 gas storage,15,16 gas sensors,17-19 and energy devices,20,21 due to their large surface area, high porosity, and diverse structures.22 Metal ions and organic ligands are linked by coordinative bond, leading to the formation of crystalline structures. Importantly, MOFs have a number of micropores (< 2 nm) in their structure, which make them useful for gas separation or filtration.23 In addition, MOFs can be applied to various functional devices because they have a number of active sites in their structure; (i) open metal sites and (ii) functional groups of organic ligands.22 Moreover, the dangling bonds of metal ions or organic ligands on MOF surface form partial surface charge.24-26 Considering their gas filtration property, large specific surface area (> 1500 m2 g–1), and high activity, MOFs can be regarded as very promising materials in the field of air filters for PM removal. Very recently, Wang group demonstrated the high feasibility of MOFs for removal of PMs by assembling particletype MOFs on plastic mesh or electrospun polymeric nanofibers.11, 27 Although the MOFbased composite materials showed efficient filtering properties toward PM, these materials still need further improvement because particular shaped polyhedron MOFs with nanoscale sizes are not sufficient to entirely remove PM2.5 or PM10 in air. In this work, we first observed the facile growth of hierarchical MOFs (H-MOFs) comprised of 2D MOF films on various substrates and we applied H-MOFs as air filters for efficient removal of particulate matter. Previously, H-MOF structures have been synthesized by a two-step process requiring the assistance of surfactants.28-30 In other words, to synthesize H-MOF, the assembly of MOF was conducted twice by using different kinds of metal ions and organic linkers, or specific surfactants were introduced during the synthesis of MOFs. Thus, precise shape control of MOFs on the supporting receiver layers was very limited. To overcome this limitation, we manipulated the growth dynamics of 2D MOFs for synthesis of flower-like H-MOFs. The hierarchically assembled MOFs were obtained by promoting 4 ACS Paragon Plus Environment

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heterogeneous nucleation of 2D MOFs on oxygen plasma treated substrate. The fast growth of the 2D MOFs caused secondary nucleation and vertical growth across their central planes, leading to the flower-like H-MOF structures. The flower-like H-MOFs were easily assembled on various substrates, including hydrophobic polyurethane 3D foam and hydrophilic nylon microfibers. The H-MOFs assembled on polypropylene microfibers showed superior properties in terms of PM2.5 removal efficiency, pressure drop, long-term stability, and outstanding recyclability even after washing. The physical PM screening of H-MOFs comprised of 2D sheets and electrostatic PM screening imparted by the partially charged surface of H-MOFs played key roles in the improvement of air filter properties. To the best of our knowledge, this is the first report on the one-pot synthesis of flower-like H-MOFs on various substrates and their effective PM filtration properties.

2. EXPERIMENTAL SECTION 2.1. Materials. 2-methylimidazole (99.0%) was purchased from Aldrich. Zinc nitrate hexahydrate ([Zn(NO3)2·6H2O], 98%), ethanol (EtOH, 98%), methanol (MeOH, 99.9%), glass slides (25 mm X 75 mm), and nylon filter membrane (nylon microfibers, pore size 0.2 µm) were purchased from Sigma-Aldrich. Polyurethane (PU) 3D foam and non-woven fabrics (polypropylene [PP] microfibers) were purchased from commercial sources. All chemicals were used without further purification. 2.2 Synthesis of H-ZIF-L on various substrates. Firstly, various substrates, namely, glass, PU 3D foam, nylon microfibers, and PP microfibers were washed with EtOH and dried at 50 ºC for 12 h. Then, all substrates were treated by O2 plasma system (CUTE, Femtoscience) at a pressure of 5.00 x 10–2 torr. The O2 plasma was generated at 100 W and 50 kHz for 4 min. Subsequently, the precursors for ZIF growth were added into deionized water (DI-water, 100 mL) in which 0.586 g of Zn(NO)3·6H2O and 1.298 g of 25 ACS Paragon Plus Environment


methylimidazole were dissolved. The ZIF-L layers were assembled for 6 h without stirring at room temperature. After the assembly, the samples were washed with DI-water and EtOH, and dried at 50 ºC overnight. 2.3. Synthesis of ZIF-8 on PP microfibers. Firstly, PP microfibers were treated with the O2 plasma under the same condition. Then, the PP microfibers were placed in MeOH solution (100 mL) in which Zn(NO3)2·6H2O (0.293 g) and 2-methylmidazole (0.649 g) were dissolved. After assembly for 6 h, the samples were washed with EtOH and dried at 50 °C overnight. 2.4. Synthesis of ZIF-L. ZIF-L was synthesized by room temperature precipitation of Zn(NO3)2·6H2O (0.293 g) and 2-methylmidazole (0.649 g) dissolved in DI-water (100 mL). The precipitated ZIF-L was purified using EtOH and dried at 50 °C overnight. 2.5. PM removal measurement. PM removal measurement was conducted by a homemade measurement system. Two chambers were linked by the channel, and a filter (5 cm x 5 cm) was positioned at one side of the channel. The burning of incense generated PMs in one chamber, and air was supplied to carry the generated PMs to the other clean chamber by using conventional fan with a flow rate of 0.2 m s–1. After stabilization of the PM concentrations, the concentration of PMs was monitored by a commercialized PM meter (CEM). The efficiency of PM removal was calculated by comparing PM concentrations with and without the filters. The pressure drop was investigated by a differential pressure transmitter (Sensys). 2.6. Characterization. To analyze the microstructure and morphology of samples, we carried out scanning electron microscope (SEM, Magellan 400 XHR system, FEI) analysis. In addition, energy-dispersive X-ray spectroscopy (EDS) elemental mapping images were obtained by using SEM with an EDS detector (80 mm2, Aztec software, Oxford Instruments). To investigate the crystal structure of the samples, X-ray diffraction (XRD, SmartLab, 6 ACS Paragon Plus Environment

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Rigaku) analysis was carried out using Cu Kα radiation (λ = 1.5418 Å). The Zeta potentials of the samples were investigated by Zeta potential analyzer (ELSZ-2000, Otsuka electronics). The optical transmittance of the samples was measured by using a UV-visible spectrometer (UV-3100, Shimadzu) with bare glass as a reference sample. The optical microscope (BX51M, Olympus) analysis was conducted to obtain the low-magnification image. The attenuated total reflectance (ATR) Fourier transform-infrared spectroscopy (FT-IR) spectrum was obtained by using Nicolet iS50 FT-IR spectrometer (Thermo Fisher Scientific Instrument).

3. RESULTS AND DISCUSSION In this work, we used 2D Zn based zeolite imidazole framework (ZIF-L) for the growth of a H-MOF layer as a PM absorbing layer, considering its high thermal and chemical stability.31 The synthetic process of H-MOFs is described in Figure 1a. To synthesize HMOFs and investigate the associated growth dynamics, we used O2 plasma treated glass (amorphous SiO2) substrate as a receiver layer. The O2 plasma treatment increases the hydrophilicity of the substrate, which can promote the heterogeneous growth of MOFs. Then, the substrate was placed in deionized water (DI-water) containing dissolved Zn(NO)3·6H2O and 2-methylimidazole. The precipitation reaction at room temperature over a period of time produced hierarchical ZIF-L (H-ZIF-L) structures by secondary nucleation and growth behavior (See details in the MATERIALS AND METHODS section). Figure 1b–i shows ex situ SEM images of samples depending on assembly-time evolution. As a reference, the bare glass substrate showed smooth surface without any particles (Figure 1b,f). During the assembly of ZIF-L, Zn ions were easily bound to glass because the surface of glass became super-hydrophilic after O2 plasma treatment. The ATR FT-IR spectrum and EDS elemental mapping images clearly exhibited the adsorption of Zn 7 ACS Paragon Plus Environment


ions on the surface of glass substrate. Then, deprotonated 2-methylimidazole dissolved in DIwater coordinated with Zn ions, leading to the nucleation of ZIF-L. After 1 h, a number of ZIF-L nuclei were randomly seeded on the glass substrate (Figure S2a,b). The sizes of ZIF-L crystallites ranged from 2 to 3 µm. After an increase of the precipitation time to 2 h, the growth of ZIF-L occurred (Figure S2c). Most of the ZIF-L grew parallel to the substrate while some of crystallites also grew vertically. Since ZIF-L has a 2D shape with microscale dimension, we also observed 2D sheet structures with increased diameter on the glass substrate (Figure S2d). After 3 h, the density of ZIF-L on the substrate increased, and the substrate was partially covered with ZIF-L (Figure S2e). In addition, the shape of ZIF-L crystallites retained the 2D structure (Figure S2f), which is consistent with previous reports.31-33 In the case of the sample assembled for 4 h, the substrate was densely covered with ZIF-L (Figure 1c). Very interestingly, the secondary nucleation and growth of ZIF-L on ZIF-L were observed in the SEM image (Figure 1g). The 2nd ZIF-L was vertically grown from the middle of the 1st ZIF-L surface. Then, a further increase of the assembly time to 5 h produced the hierarchical structure of ZIF-L and the surface of the glass was covered with ZIF-L (Figure 1d). Figure 1h clearly shows the morphology of H-ZIF-L. The 2nd ZIF-L with a 2D shape was perpendicularly grown on the 1st layer of ZIF-L, leading to the formation of the hierarchical ZIF-L structure. After an assembly time of 6 h, many H-ZIF-L covered the glass substrate (Figure 1e). As shown in Figure 1i, the magnified SEM image clearly shows a flower-like H-ZIF-L structure. Interestingly, newly grown 2nd ZIF-L sheets were oriented at about 60º with respect to each other, while they were perpendicular to the 1st ZIF-L. Further growth of ZIF-L with longer assembly time (8 h) resulted in H-ZIF-L with morphology similar that of the samples assembled for 6 h (Figure S3), and some of homogenously synthesized 2D-ZIF-L were also stacked on the surface. Furthermore, energy-dispersive X-


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ray spectroscopy (EDS) analysis of H-ZIF-L confirmed the elements of Zn, C, and N (Figure S4), which came from organic linkers (2-methylimidazole [C4H6N2]) and Zn nodes. Figure 2a shows a schematic illustration of the hierarchical growth behavior of ZIF-L. The growth dynamics of H-ZIF-L can be explained by secondary nucleation and growth of ZIF-L on ZIF-L. Firstly, ZIF-L rapidly grows on a substrate by heterogeneous nucleation. The O2 plasma treated super-hydrophilic substrate efficiently attracts Zn ions. Then, deprotonated methylimidazoles predominantly bind with Zn ions immobilized on the substrate, instead of forming homogeneous ZIF-L in solution. Under these conditions, the surface of the substrate is quickly covered by a 2D sheet of ZIF-L (Figure 2b). Then, secondary nucleation and growth of ZIF-L occurred on the plane of 1st ZIF-L layer. Since the speed of heterogeneous nucleation and growth are much faster than that of homogeneous nucleation and growth, the secondary nucleation and growth of precipitates can mainly occur on the surface of the precipitates.34 In our case, the solution was not stirred during the assembly process, which can lead to the relative supersaturation state. Relative supersaturation state generally promotes a rapid nucleation rate,35,36 that also contributes to the secondary nucleation and growth of ZIF-L. The 2nd ZIF-L was vertically grown on the 1st ZIF-L due to the structural features of ZIF-L. ZIF-L is consisted of [ZnN4] tetragonal units connected by imidazole linkers. The [ZnN4] on the outer side of the ZIF-L z-axis is connected to three imidazole linkers and one free imidazole (pink rectangle in Figure 2b), while [ZnN4] on the inside of ZIF-L is connected to four organic linkers.31 Thus, the growth of ZIF-L in the z-direction is suppressed and the 2D sheet structure is formed in the case of conventional ZIF-L synthetic method. However, with the help of heterogeneous nucleation on the substrate, ZIF-L is rapidly grown along the xy-plane. Then, the outer free imidazoles of ZIF-L in z-axis can coordinate to Zn ions, leading to the secondary growth of ZIF-L on the plane of 1st ZIF-L (blue rectangle in Figure 2c). In addition, 3rd growth of ZIF-L can also 9 ACS Paragon Plus Environment


occur on 1st ZIF-L vertically (yellow rectangle in Figure 2d). At this time, the nucleation of both the 2nd and 3rd ZIF-L is generated by linking Zn ions and free imidazoles along the z– axis, and they are formed perpendicular to 1st ZIF-L and located on the same xy plane. Interestingly, the 2nd ZIF-L and 3rd ZIF-L are inclined at an angle of 60º to the direction of the 1st ZIF-L due to the tetragonal structure of [ZnN4]. As a result, flower-like H-ZIF-L assembled from 2D ZIF-L sheets can self-assemble even at room temperature without any other inputs (Figure 1i). In contrast, without using a substrate, the assembly of Zn precursors and 2-methylimidazole in DI-water solution for 8 h only produced 2D sheet shaped ZIF-L (Figure S5), similar to previous report.31 To confirm the formation mechanism of H-ZIF-L on the glass substrate, we conducted X-ray diffraction (XRD) analysis (Figure 2e). The samples assembled for 1 h and 2 h did not show any peaks related to ZIF-L due to the low amounts of ZIF-L. The sample assembled for 3 h exhibited very weak intensity of ZIF-L peaks. On the other hand, the sample assembled for 4 h clearly revealed the peaks that were in good agreement with the simulated data in a previous report.31 Importantly, the intensity of the (004) plane was stronger than that of (020) plane, demonstrating the preferential growth of ZIF-L on the xy plane of the substrate. The weak intensity of (200), (400), and (800) was also observed in the XRD data. After 5 h, the intensity of (020) significantly increased due to the 2nd growth of ZIF-L on the 1st ZIF-L. In addition, the XRD data of the sample assembled for 6 h clearly exhibited increased intensity of reflections for the (020), (200), (400), and (800) planes, which indicated the growth of the 2nd and 3rd ZIF-L on the 1st ZIF-L. To verify the degree of ZIF-L orientation, we calculated the crystallographic preferred orientation (CPO) by using the peak intensity of (004), (020), and (400) plane. The CPO(020)/(004) was 0.14 for 4 h, 0.31 for 5 h, and 0.37 for 6 h. In addition, the CPO(400)/(004) was 0.28 for 4 h, 0.39 for 5 h, and 0.43 for 6 h. These results revealed that the growth of H-ZIF-L depended on the assembly time; (i) ZIF-L was preferentially grown to 10 ACS Paragon Plus Environment

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xy-plane on the substrate, thus the (004) peak was stronger than other peaks observed in the initial assembly period. (ii) the value of CPO(020)/(004) and CPO(400)/(004) increased by increasing the assembly time, indicating the formation of the 2nd and 3rd ZIF-L on the 1st ZIF-L. To demonstrate robust and reproducible growth of H-ZIF-L, we extended the hierarchical growth of MOF on various scaffolds with diverse structures. Hydrophobic polyurethane (PU) 3D foam and hydrophilic nylon microfibers were used as scaffolds, and the same synthetic processing steps were followed for growth of H-ZIF-L on each scaffold. Here, PU 3D foam and nylon microfibers were treated by O2 plasma to increase hydrophilicity. Figure S6a,b show the O2 plasma treated PU 3D foam with a number of macropores. After self-assembly for 6 h in DI-water solution containing Zn precursors and 2methylimidazole, PU 3D foam was covered with flower-like H-ZIF-L (H-ZIF-L_PU 3D foam) (Figure S6c). The magnified SEM image clearly exhibited hierarchical structure of ZIF-L (Figure S6d). In addition, flower-like H-ZIF-L could also be grown on nylon microfibers with an average diameter of 25 µm (Figure S7). The hierarchical structure of ZIF-L on the nylon microfibers (H-ZIF-L_nylon microfibers) was clearly observed in the magnified SEM image (Figure S7d). The XRD results for H-ZIF-L assembled PU 3D foam indicated the formation of crystalline ZIF-L in comparison with that of pristine PU 3D foam (Figure S8a). Likewise, the XRD results also confirmed the crystal structure of ZIF-L on the nylon microfibers (Figure S8b). These results emphasize the versatile growth behaviors of heterogeneous nucleation-assisted hierarchical MOFs on a diversity of receiving substrate types. To demonstrate the potential suitability of H-ZIF-L as air filtering layers for PM removal, we grew high density H-ZIF-L on the commercialized nonwoven fabric, i.e. polypropylene (PP) microfibers (H-ZIF-L_PP microfibers). As a control sample, we prepared PP microfibers decorated with Zn based polyhedron ZIF (ZIF-8), i.e. particle-shaped ZIF-8 11 ACS Paragon Plus Environment


covered PP microfibers (ZIF-8_PP microfibers), by replacing the DI-water solution with methanol solution. Figure 3a-c show the SEM images of PP microfibers, ZIF-8_PP microfibers, and H-ZIFL_PP microfibers. Pristine PP microfibers with an average diameter of 20 µm have a number of inter-pores between microfibers (Figure 3a). The morphology of ZIF-8_PP microfibers was similar to that of pure PP microfibers, however the high resolution SEM image clearly showed that tiny particles of ZIF-8 covered the surface of the PP microfibers (Figure 3b). In the case of H-ZIF-L_PP microfibers, H-ZIF-L were densely assembled on the surface of PP microfibers (Figure 3c). The magnified SEM image shows the characteristic hierarchical structure of ZIF-L on PP microfibers. XRD analysis was carried out to investigate the crystal structure of ZIF-8 and ZIF-L grown on PP microfibers (Figure 3d). The distinct peaks related to ZIF-L were observed, while the peaks of ZIF-8 were lower in intensity due to the small size of ZIF-8 compared with that of PP microfibers. As shown in Figure S9, PP microfibers, ZIF-8_PP microfibers, and H-ZIF-L_PP microfibers exhibited very similar apparent optical transparency. The average transmittance of pristine PP microfibers in the wavelength range of 400–800 nm was 32.25% (Figure S10). After the deposition of ZIF-8 and H-ZIF-L on the PP microfibers, the relative transmittance retained was 95% for ZIF-8_PP microfibers and 90% for H-ZIF-L_PP microfibers compared with that of PP microfibers (Figure 3e). These results revealed that H-ZIF-L can be easily deposited onto the nonwoven fabric based conventional air filters without any significant loss of optical transmittance, which is essential for the development of a transparent air filter. In addition, the SEM images of H-ZIF-L_PP microfibers after the folding test exhibited tightly immobilized H-ZIF-L on PP microfibers (Figure S11), demonstrating the high stability and robustness of the H-ZIF-L_PP microfibers. The PM removal properties (PM2.5 and PM10) of PP microfibers, ZIF-8_PP microfibers, and H-ZIF-L_PP microfibers were examined using a homemade air filtering system at room 12 ACS Paragon Plus Environment

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temperature (Figure 4a). Two chambers were connected to the channel, and the samples were placed in the channel (inset photo image in Figure 4a). Then, incense was burned in one of the chambers for more than 5 min, to generate PMs with the size distribution of 0.5–10 µm (Figure S12). After stabilization, we measured the concentration of PMs in the two chambers using commercialized PM concentration meter. The efficiency of PM removal was calculated by the following equation:

Efficiency of PM removal = (C − C) / C

(1)

where C0 is the concentration of PM without the filter and C is the concentration of PM with the filter. Figure 4b shows the removal efficiency of the samples to PM2.5 and PM10. The error bars represent the PM filtering properties of three independent samples of PP microfibers, ZIF-8_PP microfibers, and H-ZIF-L_PP microfibers. The H-ZIF-L_PP microfiber based filter exhibited the highest removal properties (92.5 ± 0.8% for PM2.5 and 99.5 ± 0.2% for PM10) compared with PP microfibers (73.8 ± 1.1% for PM2.5 and 97.5 ± 0.5% for PM10) and ZIF-8_PP microfibers (80.7 ± 1.1% for PM2.5 and 99.4 ± 0.2% for PM10). In the case of PM10 removal, both ZIF-8 and H-ZIF-L showed an enhanced removal efficiency compared with PP microfibers. On the other hand, for PM2.5, the removal property of H-ZIFL_PP microfibers was about 20% higher than PP microfibers and about 10% higher than ZIF8_PP microfibers. The long-term stability of H-ZIF-L_PP microfiber was investigated by conducting the long-time exposure test. The H-ZIF-L_PP microfibers exhibited high PM removal efficiency (90.2 ± 1.1% for PM2.5 and 99.1 ± 0.8% for PM10) even after 48 h of filtering test (Figure 4c). Moreover, to demonstrate the recyclability of H-ZIF-L_PP microfibers, we cleaned the filter with DI-water after PM filtering measurements. The filter was rinsed with DI-water 5 times, and dried at 50 ºC for 1 h. The photo images in Figure 4d 13 ACS Paragon Plus Environment


clearly exhibited color change of the H-ZIF-L-PP microfibers after cleaning. Then, PM removal tests and cleaning procedure were repeatedly conducted by using H-ZIF-L_PP microfibers, and pristine PP microfibers as a reference. Since the nonwoven fabric of pristine PP microfibers purchased from a commercial source was tightly fixed with binders (Figure S13a), the membrane itself was not damaged upon repeated cleaning (rinsing with DI-water). Therefore, the pristine PP microfibers exhibited stable PM removal properties after cyclic PM tests and cleaning (Figure S13b), showing that the pristine PP microfibers based fabric is an effective backbone materials for washable air filters. As a result, the H-ZIF-L_PP microfibers showed high PM removal properties even after cyclic measurement and cleaning process (Figure 4d). After 12 cycles, the PM10 removal efficiency was still high (99.2 ± 0.4%) and the PM2.5 removal property slightly decreased (78.8 ± 1.6%) compared to its initial value (90.2 ± 1.1% for PM2.5). The slight decrease of removal efficiency can be described by the partial deformation of H-ZIF-L to ZIF-L (yellow arrows in Figure 4f). During repeated washing, some parts of H-ZIF-L were collapsed, thus causing the decrease in impaction yield of PM2.5 on H-ZIF-L. However, the efficiencies of H-ZIF-L_PP microfibers for 12 cycles were still higher than the initial efficiency of pristine PP microfibers (73.8 ± 1.1% for PM2.5). The ex situ SEM images revealed the robustness of H-ZIF-L_PP microfibers during cleaning (Figure 4e,f). The PMs (red circles in Figure 4e) attached on the surface of H-ZIF-L_PP microfibers were easily washed out with DI-water without the damage (peeling off) of H-ZIF-L (Figure 4f), demonstrating high stability and efficient PM removal even after multiple cyclic measurements. The improvement of PM efficiency of H-ZIF-L_PP microfibers is attributed to two reasons: (i) The increase of interception and impaction of PM by the structure of H-ZIF-L, and, (ii) The increase of electrostatic removal of PM by the positively charged surfaces of HZIF-L. In principle, fibrous filters block PMs in air by the interception and impaction of PM 14 ACS Paragon Plus Environment

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by randomly entangled fiber network. The collision of PMs into the surface of fibers results in the trapping of the PMs on fabric filters. Thus, the increase of fiber density leads to enhanced PM removal efficiency, but it also induces high pressure drop. The conventional nonwoven fabrics that consisted of PP microfibers with an average diameter of 20 µm (Figure S13a) showed high air permeability through numerous pores (a size of few to hundreds µm) between PP microfibers, but low PM removal efficiency. However, in the case of H-ZIF-L on PP microfibers, active filtering sites sufficiently increased while maintaining low pressure drop (10.5 Pa at 25 L min–1) compared with pristine PP microfibers (9.0 Pa) and ZIF-8_PP microfibers (9.1 Pa), due to its hierarchically assembled structure and microporous nature of MOFs. The H-ZIF-L consisted of vertically stacked 2D ZIF-L with an average diameter of 10 µm and thickness of few hundreds nm, which means that the side of each 2D ZIF-L (~ 5 µm) was utilized for the removal of PMs. Therefore, the PM2.5 was effectively captured by H-ZIFL. In addition, the surface of H-ZIF-L is positively charged due to the dangling bond of Zn ions exposed on the surface of ZIF, promoting the adsorption of the negatively charged PMs, such as NO3– and SO42–, on the surface of H-ZIF-L. To investigate the surface charge of ZIF8 and ZIF-L, we carried out Zeta potential analysis (Figure 4g). The Zeta potential of the samples at pH 7.0 was 71.0 ± 0.8 mV for ZIF-L and 65.1 ± 0.2 mV for ZIF-8. Therefore, ZIF-L can more effectively remove PMs than ZIF-8. Although the PM removals by the impaction, interception, and electrostatic interactions on H-ZIF-L_PP microfibers were saturated and slightly decreased due to the blocking of surface of H-ZIF-L by the captured PMs, the PM removal properties can be easily regenerated by eliminating the captured PMs on the H-ZIF-L via washing. The air filtering properties reported in recent literatures are summarized in Table S1. The H-ZIF-L_PP microfibers exhibited high PM removal efficiency and extremely low pressure drop, demonstrating the superiority of the H-ZIF-L_PP microfibers compared with other materials (Table S1). 15 ACS Paragon Plus Environment


CONCLUSIONS In summary, we have demonstrated, for the first time, the robust growth of hierarchical 2D assembled ZIF-L on several diverse substrates (glass, PU 3D foam, nylon microfibers, and PP microfibers) and we have quantitatively demonstrated the efficient PM filtering effect of these nanomaterials. The hydrophilic substrate treated with O2 plasma induced fast heterogeneous nucleation and growth of ZIF-L in DI-water solution, and the secondary nucleation and growth of ZIF-L occurred on the basal plane of the 1st ZIF-L, generating hierarchical structures of ZIF-L. In particular, the H-ZIF-L assembled PP microfibers exhibited superior air filtering properties in terms of PM removal efficiency (92.5 ± 0.8% for PM2.5 and 99.5 ± 0.2% for PM10), pressure drop (10.5 Pa at 25 L min–1), long-term stability. Even after 12 cycles of filtering and cleaning, the air filtration performance was well preserved, demonstrating that it is a washable air filter.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publication website at http://pubs.acs.org. Figures showing additional SEM, EDS mapping, and photo images; XRD analysis; Optical transmittance analysis; supplementary table.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ORICD 16 ACS Paragon Plus Environment

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Reginald M. Penner: 0000-0003-2831-3028 Il-Doo Kim: 0000-0002-9970-2218 Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by Wearable Platform Materials Technology Center (WMC) funded by National Research Foundation of Korea (NRF) Grant of the Korean Government (MSIP) (No. 2016R1A5A1009926). This work was also supported by the Ministry of Trade, Industry & Energy (MOTIE, Korea) under Industrial Technology Innovation Program (No.10070075).

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Figure 1. (a) Schematic illustration of the synthetic process of H-ZIF-L on the substrate. SEM image of (b,f) bare glass, and H-ZIF-L assembled for (c,g) 4 h, (d,h) 5 h, and (e,i) 6 h. Low magnification images of the substrate (b,c,d,e), and high magnification images of single H-ZIF-L (f,g,h,i).


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Figure 2. Schematic illustration of H-ZIF-L growth on glass substrate: (a) 3D view and (b,c,d) xz-plane view. (b) 1st growth of ZIF-L, (c) 2nd growth of ZIF-L, and (d) 3rd growth of ZIF-L. (e) XRD patterns of H-ZIF-L depending on the assembly time.



Figure 3. SEM image of (a) PP microfibers, (b) ZIF-8_PP microfibers, and (c) H-ZIF-L_PP microfibers. Inset SEM images show the surface of the samples. (d) XRD analysis of PP microfibers, ZIF-8_PP microfibers, and H-ZIF-L_PP microfibers. (e) Optical transmittance change of PP microfibers after ZIF-8 and H-ZIF-L loading. The transmittance of pristine PP microfibers was 32.25% compared with bare glass.


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Figure 4. (a) Photo image of homemade PM removal measurement setup. Inset photo image shows cross-section of the channel. (b) PM removal efficiencies of PP microfibers, ZIF-8_PP microfibers, and H-ZIF-L_PP microfibers. (c) Long-term PM removal properties of H-ZIFL_PP microfibers. Inset photo image shows H-ZIF-L_PP microfibers (10 cm x 10 cm). (d) Cyclic PM removal properties of H-ZIF-L_PP microfibers after filtering and cleaning. Inset photo image shows the H-ZIF-L_PP microfibers after filtering test and cleaning. Ex situ SEM images of H-ZIF-L_PP microfibers (e) after PM removal and (f) after cleaning. (g) Zeta potentials of ZIF-8 and ZIF-L in aqueous solutions at pH 7. 25 ACS Paragon Plus Environment


Table of contents


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