Triboelectric Nanogenerator Enhanced Multilayered .

Nano riboelectric nanogenerator enhanced multilayeredantibacterial nanofiber air filters for efficient removal ofultrafine particulate matterGuang Qin Gu1,2,§, Chang Bao Han1,2,§, Jing Jing Tian1,2,§, Tao Jiang1,2, Chuan He1,2, Cun Xin Lu1,2, Yu Bai1,2,Jin Hui Nie1,2, Zhou Li1,2 ( ), and Zhong Lin Wang1,2,3 ( )1Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing 100083, ChinaSchool of Nanoscience and Technology, University of Chinese Academy of Sciences, Beijing 100049, China3School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0245, USA§Guang Qin Gu, Chang Bao Han, and Jing Jing Tian contribute equally to this work.2Received: 1 December 2017ABSTRACTRevised: 4 January 2018We developed a high-efficiency rotating triboelectric nanogenerator (R-TENG)enhanced multilayered antibacterial polyimide (PI) nanofiber air filters for removingultrafine particulate matter (PM) from ambient atmosphere. Compared to singlelayered PI nanofiber filters, the multilayered nanofiber filter can completelyremove all of the particles with diameters larger than 0.54 µm and showsenhanced removal efficiency for smaller PM particles. After connecting with aR-TENG, the removal efficiency of the filer for ultrafine particles is further enhanced.The highest removal efficiency for ultrafine particulate matter is 94.1% at thediameter of 53.3 nm and the average removal efficiency reached 89.9%. Despitean increase in the layer number, the thickness of each individual layer of thefilm decreased, and hence, the total pressure drop of the filter decreased insteadof increasing. Moreover, the nanofiber film exhibited high antibacterialactivity because of the addition of a small amount of silver nanoparticles. Thistechnology with zero ozone release and low pressure drop is appropriatefor cleaning air, haze treatment, and bacterial control.Accepted: 7 January 2018 Tsinghua University Pressand Springer-Verlag GmbHGermany, part of SpringerNature al property,electrospinning,Ag-polyimide nanofiber,air filter,ultrafine particle1IntroductionAir pollution is a major public, environmental, and healthissue contributing to 7 million premature deathsworldwide, annually [1, 2]. Particulate matter (PM),as one of the major airborne pollutants, has raisedserious concerns in recent years [3–5]. PM is categorizedby the size of the particle as coarse, fine, and ultrafineparticles (UFPs) with aerodynamic diameters within2.5–10 µm (PM10), 2.5 µm (PM2.5), and 0.1 µm (PM0.1),respectively. Numerous studies have demonstratedthe association of long-time PM2.5 polluted air exposureAddress correspondence to Zhong Lin Wang, zlwang@gatech.edu; Zhou Li, zli@binn.cas.cn

2Nano Res.with morbidity and mortality from respiratory andcardiovascular diseases [6–9]. Hence, the air qualityindex related to the concentrations of PM2.5 and PM10is used to reflect the daily air pollution level.However, most of the particles (73%) are in the ultrafinefraction, whereas most of the mass (82%) could beattributed to particles in the size range of 0.1 to 0.5 µm[10]. Moreover, UFPs are much more harmful to publichealth. Compared to PM2.5 with larger particles, UFPshave a much higher number concentration and surfacearea [11], enhanced oxidation capacity [12, 13], greaterinflammatory potential [11], and higher pulmonarydeposition efficiency [14, 15]. The UFPs cannot befiltered out by the nose and bronchioles and their smallsize allows them to be breathed deeply into the lungs,where they are able to penetrate alveolar epitheliumand enter the pulmonary interstitium and vascular spaceto be absorbed directly into the blood stream [16].The body also does not have efficient mechanisms forclearing the deeper part of the lung as only a very smallfraction of natural particles would be as small as these[17]. The UFPs are highly chemically reactive owingto their small size and large surface area [18]. Further,they also carry a large amount of toxic compounds ontheir surfaces [19]. For example, in air emissions fromincinerators, heavy metals, dioxins, hydrocarbons,and other organic chemicals can adhere to the surfaceof UFPs and increase their toxicity [20]. Among thevarious hazards of PM pollutants, microorganisms inPM2.5 and PM10 are speculated to be responsible forvarious allergies and for the spreading of respiratorydiseases [21]. Therefore, it is necessary to develop airfilters to remove these UFPs and microorganisms forthe sake of people’s health.Nowadays, electrostatic precipitation and fibrousfilters are widely utilized to remove PMs. One majordrawback of the electrostatic precipitators is that theyinevitably ionize air, and hence produce ozone [22],which causes negative effects on human health withthe possibility of causing cancer [23, 24]. As for fibrousfilters, they present the advantage of high efficiencyto remove particles larger than their holes. However,the removal efficiency for UFPs, whose diameters aremuch smaller than the holes of the fibrous filter,decreases significantly. Moreover, the pressure dropincreases with dust loading [25, 26].Nanogenerators, in particular, piezoelectric nanogenerators[27–29] and triboelectric nanogenerators (TENGs) [30–34],were invented by Wang’s group in 2006 [35] and 2012[36], respectively. TENGs, based on triboelectrificationand electrostatic induction effects, were invented to harvestenergies from all kinds of mechanical movements suchas human motion, wind, water wave [37–42]. One ofthe greatest advantages of TENG is the generation ofa high open-circuit voltage (VOC), which is generallyhundreds of times higher than that generated byelectromagnetic generators [43]. For example, a TENGbased on collision between polytetrafluoroethylene (PTFE)pellets and electrodes can yield a space electric field ashigh as 12 MV/m and a self-powered triboelectricfilter based on this TENG shows a removal efficiencyof 95.5% for PM2.5 [44]. Furthermore, the highvoltage of TENGs does not ionize the surrounding air,and therefore, no ozone is produced.In our previous study, a rotating triboelectric nanogenerator(R-TENG)-enhanced electrospinning nanofiber filmfilter was fabricated for efficient PM removal [45].However, only a single layer of charged nanofiber filmcannot provide adequately high removal efficiencyfor ultrafine particulate matters. Moreover, the threatof microorganisms such as bacteria to human health isnot considered. In this study, we developed a high-efficiencyR-TENG-enhanced multilayered antibacterial polyimide(PI) nanofiber air filters for the removal of ultrafine PMand microorganisms in ambient atmosphere. Comparedwith the single-layered nanofiber filter, the multilayerednanofiber filter can remove all of the particles withdiameters larger than 0.54 µm and show enhancedremoval efficiency for smaller PM particles. Afterconnecting with the R-TENG, the removal efficienciesfor ultrafine particles are further enhanced. The highestremoval efficiency for ultrafine particulate matter is94.1% at the diameter of 53.3 nm and the averageremoval efficiency reached 89.9%. Although the layernumber was increased, the thickness of each layer ofthe films decreased, and hence, the total pressure dropof the filter did not increase, but decreased. Moreover,the nanofiber film exhibited high antibacterial activitybecause of a small amount of silver nanoparticlesincorporated in it. This technology with zero ozonerelease and low pressure drop is effective for cleaningair, haze treatment, and bacterial control. www.editorialmanager.com/nare/default.asp

3Nano Res.22.1Experimental sectionElectrospinningThe solution used for electrospinning was a polyamideacid (PAA) solution in dimethylformamide (DMF) witha solid content of 15.5 wt.%. Silver nitrate was firstdissolved in DMF and then added to the polymersolution, keeping the Ag ratio at 3 wt.% in the finalAg-polyimide (PI) nanofiber. A 5 mL syringe with a22-gauge needle tip was used to load the polymersolution. A syringe pump (model LSP01-1A, LongerPump) was used to pump the solution out of the needletip. The electrospinning nanofibers were collected bya 100 mesh stainless-steel wire mesh. The voltage appliedbetween the needle tip and the mesh was 18 kV. Duringelectrospinning, the nanofibers lay across the meshholes to form the air filter.2.2Antibacterial testA fresh bacterial solution was first acquired by culturingE. coli and S. aureus in an autoclaved lysogenic broth(LB) medium overnight at 37 C (bacteria were in thestationary phase); the rotation speed was 120 rpm.Subsequently, 1 mL of the E. coli and S. aureus solutionwas pipetted from the phase incubated overnight intoanother flask containing 100 mL of fresh LB. The mixturewas then cultured at 37 C for another 5 h to obtain abacterial suspension at the exponential growth phase.Thereafter, the bacterial suspension was centrifuged at10,000 rpm to separate the bacteria from the culture medium,and then the bacteria were washed and resuspendedin a sterilizing saline to achieve a concentration of 106colony-forming units (CFU)/mL for the sterilization test.2.3 PM generation and evaluation of the filteringefficiencyThe PM generation and efficiency measurements wereperformed in a 30 m3 lab. The PM particles used inthis work were generated by burning cigarette. The PMparticles from cigarette smoke had a wide particlesize distribution ranging from 10 nm to 10 µm. Bydiluting the smoke PM by air and waiting for 30 minto allow an even dispersion of the smoke in the air, theinflow concentration was controlled to a hazardouspollution level equivalent to the PM2.5 index 300. Ahandheld particle counter (3016-IAQ, Lighthouse), ascanning mobility particle sizer (SMPS 3938L75, TSI),and an aerodynamic particle sizer (3321, TSI) wereused to detect the PM particle number concentrationbefore and after filtration. The removal efficiency wascalculated by comparing the number concentrationbefore and after filtration.2.4Pressure drop and estimation of ozoneThe pressure drop was measured using a differentialpressure gauge (Testo 510) and the gas velocity wasmeasured by an anemometer (Testo 450-V1). Ozonewas analyzed by an ozone monitor (aeroQUAL, series200).2.5CharacterizationField emission scanning electron microscopy (FE-SEM)images were obtained using FEI Nova NANOSEM450 SEM with an acceleration voltage of 5 kV for imaging.The open-circuit voltage was determined using anoscilloscope (DSO-X 2014A; Agilent). The absorbancespectrum of Ag nanoparticles dissolved in DMF wasrecorded on an ultraviolet visible–near infrared (UV vis NIR) spectrophotometer (UV 3600).3Results and discussionThe fabrication of the antibacterial PI air filter byelectrospinning is schematically illustrated in Fig. 1(a).A given quantity of silver nitrate was first dissolvedin DMF. Then, the resulting solution was added to asolution of PAA, to obtain an electrospinning solution.Further details could be found in the Experimentalsection. As reported previously, DMF reacted with silvernitrate, forming Ag nanoparticles according to Eq. (1)[46, 47]HCONMe2 2Ag H2O 2Ag Me2NCOOH 2H (1)UV vis absorbance spectrum of the Ag nanoparticlesin DMF solution is shown in Fig. S1 in the ElectronicSupplementary Material (ESM). The plasmon peak ofAg nanoparticles is observed at 430 nm. Figure 1(b)shows the energy dispersive spectrum (EDS) patternsof PI nanofibers and silver nanoparticle-dopedwww.theNanoResearch.com www.Springer.com/journal/12274 NanoResearch