Supporting Information

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1 Supporting Information In situ Active Poling of Nanofibers Network for Gigantically Enhanced Particulate Filtration Chun Xiao Li,, Shuang Yang Kuang,, Yang Hui Chen,, Zhong Lin Wang,,,#, Congju Li, and Guang Zhu*,,,,ǁ CAS Center for Excellence in Nanoscience, Beijing Key Laboratory of Micro-Nano Energy and Sensor, Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing , China School of Nanoscience and Technology, University of Chinese Academy of Sciences, Beijing , China Department of Mechanical, Materials and Manufacturing Engineering, The University of Nottingham Ningbo China, Ningbo , China ǁ New Materials Institute, The University of Nottingham Ningbo China, Ningbo , China Center on Nanoenergy Research, School of Physical Science and Technology, Guangxi University, Nanning , China # School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332, United States Corresponding author.* zhuguang@binn.cas.cn S-1

2 1. EXPERIMENTAL SECTION 1.1. Preparation of the electrospinning solution. 0.5 g of PAN (MW = g. mol -1, Sigma-Aldrich) was added into 5.75 g of N, N-dimethylformamide (DMF), and the solution was then stirred at 90 for 4 h to form a homogeneous solution with a concentration of 8 wt%. 0.5 g of polyvinyl alcohol (PVA) (MW = g. mol -1, Sigma-Aldrich) was added into 7.83 g of distilled water, and the solution was then stirred at 50 for 6 h to form a homogeneous solution with a concentration of 8 wt%. 0.5 g of polystyrene (PS) (MW = g. mol -1, Sigma-Aldrich) was added into 3.67 g of DMF, and the solution was then stirred at 60 for 6 h to form a homogeneous solution with a concentration of 12 wt% Electrospinning of nanofibers. The electrospinning process was performed by using Ucalery ET-2535H eletrospinning machine (Beijing Ucalery Technology & Development Co., Ltd., China), which consisted of a high voltage power supply, a syringe pump, and a roller covered with a layer of the metal network in connection to the negative pole of a voltage source for collecting the fibers. A 5 ml syringe with a 23-gauge needle tip was used to load the polymer solution, which was connected to the positive pole of the voltage source. The operating voltage, the pump rate and the needle-collector distance were set to be 8.0 ± 1.0 kv, mm/min, and 18 cm for PAN electrospinning, respectively. The three parameters were 16.0 ± 1.0 kv, 0.02 mm/min, and 20 cm for PVA electrospinning, respectively. They were set to be 12.0 ± 1.0 kv, 0.01 mm/min, 20 cm for PS electrospinning, respectively. The temperature and humidity during the electrospinning were kept at 25 ± 2 and 25 ± 2%, respectively. All of the electrospun nanofibers were dried at 60 for 6 h before being used. S-2

3 1.3. Assembly of the APNF. After electrospinning, another layer of the metal network of the same mesh grade was fixed on top of the electrospun nanofibers to form a sandwich-structured air filter with an area of 6 6 cm 2 (Figure 1d). A dc voltage source is loaded across the metal layers. The polarity of the dc voltage did not affect the measured removal efficiency Removal efficiency measurement. As air quality with PM 2.5 concentration greater than 300 µg/m 3 is categorized as serious pollution" 15, the PM 2.5 concentration prior to the filtration was kept higher than 300 µg/m 3. The APNF was fixed between two glass bottles with a connection pipe having a diameter of 2.1 cm, and an electric fan was fixed at the downstream side of the APFN to generate airflow passing through the APFN. The airflow velocity is adjusted by changing the voltage applied onto the electric fan. The PM particulates concentration was measured by two particle counters (CEM DT-9881M) at the upstream and the downstream sides of the APFN COMSOL Simulation. A two-dimensional model was used to reveal the electric field distribution between the two layers of the metal network. For the case of 500 mesh grade, as depicted in Figure S4a, 50 metal circles (20 µm in diameter) were set in a line to represent the cross section of one layer of the metal network. The interval between adjacent circles was set to be 50 µm. Correspondingly, 50 metal circle (20 µm in diameter) were set in the same order as the other metal layer. The distance between two electrodes was set to be 200 µm. The electric potential of one metal layer was set to be zero, and the other was set to be a positive value. For the case of 200 mesh grade, as depicted in Figure S4c, 20 metal circle (50 µm in diameter) were set in a line as one electrode, and the metal circle interval was set to be 130 µm. Symmetrically, 20 metal S-3

4 circle (50 µm in diameter) were set in the same order as the other electrode. The distance between two electrodes was set to be 200 µm. The electric potential of one electrode was set to be zero, and the other was set to be 2.0 kv Other characterizations. The microscopic morphology was characterized by a field emission SEM (Hitachi SU8020 SEM). The thickness of the electrospun nanofibers was obtained by a Stylus Profilometer (KLA-Tencor D-100, USA). The fiber diameter was gauged by using an image analysis software (Adobe Photoshop CS6). The aperture sizes of the electrospun PAN nanofiber membranes were measured using bubble pressure filter aperture analyzer (3H-2000PB, Beijing BeiShiDe Instrument Technology Co. Ltd. China). The porosity of membranes were measured using a true density and porosity tester (3H-2000TD, Beijing BeiShiDe Instrument Technology Co. Ltd. China). The pressure drop was measured by a differential pressure gauge (CEM, DT-8890A). The airflow volume is detected by a gas mass flow meter (MF-5700-N-200) and the airflow velocity was calculated through the airflow volume divided by the cross-sectional area of the connection pipe between the glass bottles. S-4

5 Figure S1. Fiber diameter distribution (a) and pore size distribution (b) of the electrospun PAN nanofibers. S-5

6 Figure S2. Photograph of the experimental setup for characterizing the filtering materials. S-6

7 Figure S3. SEM images of the metal layers. (a) 500 mesh grade. (b) 200 mesh grade. S-7

8 Figure S4. Electric field distribution obtained by COMSOL numerical simulation. (a) Cross-sectional view of the electric field distribution between two metal layers of 500 mesh grade. The enlarged inset shows the metal wire diameter of 20 µm and the pore size of 30 µm. (b)the line distribution of the electric field defined by the red dotted line in (a). (c) Cross-sectional view of the electric field distribution between two metal layers of 200 mesh grade. And the wire diameter is 50 µm and the pore size is 80 µm. (d) The line distribution of the electric field defined by the red dotted line in (c). S-8

9 Figure S5. Molecular models and SEM images of different polar polymers. (a-c) Molecular models of different polymers including PAN, PVA, and PS with the calculated dipole moments. Gray, C; yellow, H; red, N; blue, O. (d-f) SEM images of PAN, PVA, and PS nanofibers with the same average diameter of 200 nm. S-9

10 Figure S6. The thickness of the electrospun PAN nanofibers. (a-c) The lateral profiles reveal the thickness of 10.4 µm, 13.0 µm and 14.5 µm, respectively. The insets show the screenshots during the test. S-10

11 Table S1. The filtration performance of the APNF compared with the normal nanofibers at the airflow velocity of 0.21 m/s. Sample T (µm) E (%) P (Pa) QF (Pa -1 ) m (g/m 2 ) Normal NF APNF T, thickness. E, PM 2.5 removal efficiency. P, pressure drop. QF, quality factor. m, dust holding capacity. QF = - ln (1 - E%) / P. S-11

12 Table S2. The filtration performance of the APNF and the normal PAN nanofibers as affected by the thickness at the airflow velocity of 0.21 m/s. Sample T (µm) E (%) P (Pa) QF (Pa -1 ) Normal NF APNF (2 kv) Normal NF APNF (2 kv) Normal NF APNF (2 kv) T, thickness. E, PM 2.5 removal efficiency. P, pressure drop. PAN, polyacrylonitrile. QF, quality factor. QF = - ln (1 E %) / P. S-12