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Air Filter Commercial Fibrous Air Filter Transparent Air Filter ARTICLE Received 1 Sep 2014 | Accepted 6 Jan 2015 | Published 16 Feb 2015 DOI: 10.1038/ncomms7205 Transparent air filter for high-efficiency PM2.5 capture Chong Liu1,*, Po-Chun Hsu1,*, Hyun-Wook Lee1, Meng Ye2, Guangyuan Zheng3, Nian Liu4, Weiyang Li1 & Yi Cui1,5 Particulate matter (PM) pollution has raised serious concerns for public health. Although outdoor individual protection could be achieved by facial masks, indoor air usually relies on expensive and energy-intensive air-filtering devices. Here, we introduce a transparent air filter for indoor air protection through windows that uses natural passive ventilation to effectively protect the indoor air quality. By controlling the surface chemistry to enable strong PM adhesion and also the microstructure of the air filters to increase the capture possibilities, we achieve transparent, high air flow and highly effective air filters of B90% transparency with 495.00% removal of PM2.5 under extreme hazardous air-quality conditions (PM2.5 mass concentration 4250 mgmÀ 3). A field test in Beijing shows that the polyacrylonitrile trans- parent air filter has the best PM2.5 removal efficiency of 98.69% at high transmittance of B77% during haze occurrence. 1 Department of Materials Science and Engineering, Stanford University, Stanford, California 94305, USA. 2 Department of Civil and Environmental Engineering, Stanford University, Stanford, California 94305, USA. 3 Department of Chemical Engineering, Stanford University, Stanford, California 94305, USA. 4 Department of Chemistry, Stanford University, Stanford, California 94305, USA. 5 Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, USA. * These authors contributed equally to this work. Correspondence and requests for materials should be addressed to Y.C. (email: [email protected]). NATURE COMMUNICATIONS | 6:6205 | DOI: 10.1038/ncomms7205 | www.nature.com/naturecommunications 1 & 2015 Macmillan Publishers Limited. All rights reserved. ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms7205 articulate matter (PM) pollution in air affects people’s living particles are different because of their chemical compositions, quality tremendously, and it poses a serious health threat to morphologies and mechanical properties15. Some rigid inorganic Pthe public as well as influencing visibility, direct and PM particles are mainly captured by interception and impaction indirect radiative forcing, climate, and ecosystems1–7.PMisa on a filter surface. Some soft PM containing a lot of carbon complex mixture of extremely small particles and liquid compounds or water such as those from combustion exhaust droplets1,8. On the basis of the particle size, PM is categorized would deform on filter surfaces and require stronger binding by PM2.5 and PM10, which refer to particle sizes below 2.5 and during the process of attaching to the filter. It is important to 10 mm, respectively. PM2.5 pollution is particularly harmful since study the surface properties of the air filters to enhance PM it can penetrate human bronchi and lungs owing to the small particle capture. However, in existing air filter technology, not 9 particle size . Hence, long-term exposure to PM2.5 increases much work has been performed to study the properties of filter morbidity and mortality10–14. Recently, there have been serious materials. There are two types of air filters in common use15. One PM pollution problems in developing countries with a large is a porous membrane filter, which is similar to a water filtration manufacturing industry such as China. Figure 1a,b shows images filter (shown in Fig. 1c). This type of air filter is made by creating of a location in Beijing during clear and hazy days, respectively. pores on solid substrate, and it usually has very small pore size to During hazy days, the visibility decreased greatly and the air filter out PM with larger sizes and the porosity of this type of filter quality was poor because of extremely high levels of PM2.5. is low (o30%). Hence, the filtration efficiency is high, although Measures taken by the public during hazy days are mostly the pressure drop is large. Another type of air filter is a fibrous air focused on outdoor individual protection, such as using mask filter, which captures PM particles by the combination of thick filters, which are often bulky and resistant to air flow15. In indoor physical barriers and adhesion (shown in Fig. 1d). This type of spaces, protection is only available in modern commercial filter usually has porosities 470% and is made of many layers of buildings through filtering in ventilation systems or central air thick fibres of diverse diameters from several microns to tens of conditioning; residential housing seldom has filtration protection microns. To obtain a high efficiency, this type of filter is usually from PM. Moreover, all these active air exchanges by mechanical thick. The deficiency of the second type of filter is the bulkiness, ventilation consume enormous amounts of energy because of the nontransparency and the compromise between air flow and filter massive use of pumping systems16. When people stay indoors efficiency. without sufficient air exchange, indoor air quality is also of great Existing technology would not meet the requirements of a 17 concern . It would be ideal if passive air exchange—that is, transparent high-efficiency PM2.5 filter. Here we demonstrate a natural ventilation—through windows could be used for indoor novel polymer nanofibre filter technology, which has attractive air filtration. Owing to the large surface area of most windows, air attributes of high filtering efficiency, good optical transparency, exchange is very efficient. Protection at windows requires the air low resistance to air flow and light weight as shown in Fig. 1e. We filters to not only possess a high PM-capture ability but also a found that when the surface chemistry of the air filter is high optical transparency for natural lighting from the sun and optimized to match that of PM particles, that the single fibre sight-viewing at the same time. Here we introduce the concept of capture ability becomes much more than the existing fibrous a transparent air filter for indoor air-quality protection at the filters. Therefore, the material use in the air filter can be reduced window for the first time. significantly to a transparent level to enable both transparency to The PM2.5 pollution particles in air have complicated sunlight and sufficient air flow. In addition, when the fibre 2 À compositions including inorganic matter (such as SiO2,SO4 diameter is decreased to the nanometre scale, with the same À and NO3 ) and organic matter (such as, organic carbon and packing density, the particle-capture possibility is increased a lot elemental carbon) from diverse sources including soil dust, because of large surface area, which also ensures effective PM vehicular emission, coal combustion, secondary aerosols, indus- capture with a much thinner air filter. This transparent filter can trial emission and biomass burning1,18–21. The behaviour of PM be applied to all types of situations including personal masking Air flow Air flow Strong Air flow h surface adhesion Small opening 200 nm size exclusion Thick Micron size Porous membrane air filter Commercial fibrous air filter Transparent air filter Figure 1 | Particulate matter pollution and comparison among different air filters. (a) Photograph of a random place in Beijing during a sunny day. (b) Photograph of the same place in Beijing during a hazy day with hazardous PM2.5 level. (c) Schematics of porous air filter capturing PM particles by size exclusion. (d) Schematics of bulky fibrous air filter capturing PM particles by thick physical barrier and adhesion. (e) Schematics of transparent air filters that capture PM particles by strong surface adhesion and allowing a high light and air penetration. 2 NATURE COMMUNICATIONS | 6:6205 | DOI: 10.1038/ncomms7205 | www.nature.com/naturecommunications & 2015 Macmillan Publishers Limited. All rights reserved. NATURE COMMUNICATIONS | DOI: 10.1038/ncomms7205 ARTICLE and building windows. By controlling the surface chemistry and the window screen as a supporting and adhering substrate, the air microstructure of the air filters, we report transparent ultrathin filter is mechanically robust. Nanofibres with different surface filters of B90% transparency with 495.00% removal, of B60% properties are made by changing the functional groups on the transparency with 499.00% removal and B30% transparency polymer side chains and also by coating different materials using with 499.97% removal of PM2.5 particles under extreme a sputtering method. The chosen polymers are available in large hazardous air-quality condition. quantity and at low cost, including polyacrylonitrile (PAN), polyvinylpyrrolidone (PVP), polystyrene (PS), polyvinyl alcohol (PVA) and polypropylene (PP). The coating materials are copper Result and carbon. PP, copper and carbon are all commonly used Transparent air filter surface screening. To find the most materials in commercial fibrous or porous membrane air filters15. effective material for transparent air filters, different polymers The molecular models and formulae of the different polymers are and polymers with other coatings were investigated for PM shown in Fig. 2b. The polarity and hydrophobicity are different capture. Electrospinning was used to make polymer nanofibrous between each polymer and the dipole moments are 3.6, 2.3, 0.7, air filters (shown in Fig. 2a). Electrospinning has
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