High Efficiency, Transparent, Reusable, and Active PM2.5 Filters by

Jun 13, 2017 - Air quality has become a major public health issue in Asia including China, Korea, and India. Particulate matters are the major concern...
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Letter pubs.acs.org/NanoLett

High Efficiency, Transparent, Reusable, and Active PM2.5 Filters by Hierarchical Ag Nanowire Percolation Network Seongmin Jeong,† Hyunmin Cho,† Seonggeun Han,† Phillip Won,† Habeom Lee,† Sukjoon Hong,‡ Junyeob Yeo,§ Jinhyeong Kwon,*,† and Seung Hwan Ko*,†,∥ †

Applied Nano and Thermal Science Lab, Department of Mechanical Engineering, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Korea ‡ Department of Mechanical Engineering, Hanyang University, 55 Hanyangdaehak-ro, Sangnok-gu, Ansan, Gyeonggi-do 15588, Korea § Novel Applied Nano Optics (NANO) Lab, Department of Physics, Kyungpook National University, 80 Daehak-ro, Bukgu, Daegu 41566, Korea ∥ Department of Mechanical Engineering/Institute of Advanced Machinery and Design (SNU-IAMD), Seoul National University, Gwanak-ro, Gwanak-gu, Seoul 08826, Korea S Supporting Information *

ABSTRACT: Air quality has become a major public health issue in Asia including China, Korea, and India. Particulate matters are the major concern in air quality. We present the first environmental application demonstration of Ag nanowire percolation network for a novel, electrical type transparent, reusable, and active PM2.5 air filter although the Ag nanowire percolation network has been studied as a very promising transparent conductor in optoelectronics. Compared with previous particulate matter air filter study using relatively weaker short-range intermolecular force in polar polymeric nanofiber, Ag nanowire percolation network filters use stronger long-range electrostatic force to capture PM2.5, and they are highly efficient (>99.99%), transparent, working on an active mode, low power consumption, antibacterial, and reusable after simple washing. The proposed new particulate matter filter can be applied for a highly efficient, reusable, active and energy efficient filter for wearable electronics application. KEYWORDS: PM2.5 filter, metal nanowire percolation network, transparent filter, active filter, reusable filter, low cost

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circulatory system or respiratory system.10−13 For example, the PM sized under 10 μm (PM10) can pass through the body and settle on lung. Moreover, when the PM is sized under 2.5 μm (PM2.5), it penetrates into the alveolus and blood vessel. The identity of the PM is harmful heavy metals such as cadmium, arsenic, lead, and zinc. Therefore, the development of the filtration technology for the PM is strongly needed. However, the conventional PM filter systems have many inherent problems because they consume large energy and require large volume space. Additionally, multilayer structure

n environment issue for the sustainable growth becomes an important agenda worldwide. Increasing concerns on the Earth’s atmosphere including air pollution and global warming by greenhouse gas and fast industrialization with huge urbanization attracted many researchers from various fields.1−3 While the global warming is being controlled by the international agreements such as the Kyoto protocol, the air pollution is considered as a low priority task.4−6 However, the air pollution leads to very serious long-term effects on the atmospheric environment as well as public health. The major factor of the air pollution consists of COx, NOx, SO2, ozone, and particle matters.7−9 Particularly, the seriousness of the particulate matter (PM) is growing since the particulate matter tends to be absorbed and eventually settled on the human’s © XXXX American Chemical Society

Received: April 4, 2017 Revised: June 2, 2017

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DOI: 10.1021/acs.nanolett.7b01404 Nano Lett. XXXX, XXX, XXX−XXX

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Figure 1. (a) Schematic of fabrication procedure of the Ag nanowire network based TRAP filter for particulate matter collector. Ag nanowires were transferred to a nylon mesh (hole size: approximately 53 μm) with uniform distribution by vacuum filtration method. An optical microscope image of Ag nanowire with high aspect ratio is shown in inset (aspect ratio: 2000). (b) Digital camera image and (c) magnified microscope image of TRAP filter. The fabricated TRAP filter shows little color difference between Ag nanowires and nylon backbone structure due to the high transparent of Ag nanowire network. The surface of nylon mesh is completely covered with Ag nanowire, which results in spider-web-like structure. (d) COMSOL simulation of the air flow velocity (left figure) and pressure drop (right table) for the TRAP filter.

working in an active working principal where filter function can be adjusted on demand. Metal nanowire networks have been widely studied for flexible transparent conductors in optoelectronics applications due to its superior electrical, mechanical, and optical properties and chemical stability against oxidation.20−23 Besides the optoelectronic applications, metal nanowire networks have various ideal properties for active mode high efficiency transparent PM filter development. However, there has been no research on their application in PM filter field. In this research, we present a first demonstration of Ag nanowire percolation network application in developing a Transparent, Reusable and Active PM (TRAP) filter with a very high efficiency and small volume by using hierarchical Ag nanowire percolation network. The proposed TRAP filter has various advantages and superior to previous filter studies. TRAP air filter system is designed to collect PM by electrostatic force in a more active manner upon applying voltage to the transparent Ag nanowire percolation network. Compared with short-range intermolecular force used in previous polar

and high power pump system are required to maintain the large pressure difference between inlet and outlet of the filter14,15 and makes filters very bulky and opaque. In order to remove those limitations, various new PM filter systems based on new nanomaterials and production technology were developed. Cui group applied various polar polymeric nanofiber network like polyacrylonitrile (PAN), nylon, and polyimide to develop high efficiency transparent PM filters.16−19 While the polar polymeric nanofiber network filter system presented high filtration efficiency and mass productivity, its working principle was dependent only on conventional surface chemistry by changing the functional groups on the polymer side chains to optimize intermolecular force between polar polymeric nanofiber and particulate matters. However, polar polymeric nanofiber network air filter using surface chemistry is only for single time use and inherently passive device in that the PM filter function cannot be turned on/off or adjusted according to the user’s demand. There is a strong need for a new type of high efficiency PM filter that can be reusable multiple times and B

DOI: 10.1021/acs.nanolett.7b01404 Nano Lett. XXXX, XXX, XXX−XXX

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Figure 2. (a) Schematic (top) and digital camera image (bottom) of TRAP filter experimental setup. The PM counter sensor is placed in the left side of the chamber, while the PM is generated from incense at the right side of the chamber. TRAP filter is sandwiched between the two chambers. (b) Transient evolution of PM2.5 removal performance for (i) only ionizer on (5 V), (ii) only TRAP filter on (10 V), (iii) both trap filter and ionizer on, and (iv) both off. The PM density showed saturation at 1000 μg/m3 due to detector limit. (c) Transient evolution of PM2.5 density for various TRAP filter voltage condition. (d) Magnified view of graph (c) after 120 s from the PM detection for various TRAP filter voltage conditions.

fabricated TRAP filter is shown in Figure 1b. Since Ag nanowires are initially dispersed in ethanol solution, the electrical conductivity and average pore size of the TRAP filter could be easily modulated by adjusting the density of Ag nanowire in solution. In here, small quantity of Ag nanowire was used to fabricate TRAP filter, where the sheet resistance is fixed at approximately 5−7 Ω/sq for the consistency of study. Figure 1c presents the magnified optical image of the surface of the nylon fiber covered with Ag nanowire percolation network. The transferred Ag nanowire meshes were hanging between nylon mesh as well as covered on the nylon mesh, which resulted in spider-web-like structure (Figure S1). Afterward, the fabricated TRAP filter was annealed to enhance electrical and mechanical property by welding junctions between Ag nanowires. Pressure drop is a big issue for filter efficiency enhancement. A numerical simulation along with measurement was performed to check an effect of air flow velocity and pressure drops after passing through the TRAP filter in Figure 1d. The numerical simulation was executed by increasing specific area ratio (net wires area/hole area) from 0 to 10%. The pressure drop was proportional to the increased specific area ratio. The numerical simulation result indicated that the pressure drop was 3.51 Pa at 2.5% of specific area ratio much smaller than that of a commercial filter (300−800 Pa) and previous study (130−200 Pa).16 Along with numerical simulation, pressure drop across the TRAP filter and air flow velocity in the TRAP filter system was measured with a differential pressure gauge and a digital anemometer (Figure S2). Particularly, the measured pressure drop (10.34 ± 3.44 Pa) across the TRAP filter in the chamber was close to the

polymeric nanofiber PM filter studies, TRAP filter uses both long-range electrostatic force and short-range van der Waals force when low electrical energy is applied on the metal nanowire network, which is much stronger than intermolecular force to more efficiently collect particulate matters. TRAP filter showed a very high efficiency (>99.99%) compared with polar polymeric nanofiber network PM filter (>95−99%).17 Additionally, unlike typical electrical filter system, the fabricated Ag nanowire percolation network based TRAP filter system can be operated at extremely low air flow resistance and low energy consumption without an additional pump system due to large pore size and thin single layer structure. Additionally, Ag nanowire has an antibacterial effect to kill any harmful microbials. Finally, the fabricated TRAP filter system can be reusable for multiple times after simple cleaning process. In this study, a transparent percolation network of Ag nanowire was prepared through a simple all-solution process24 and used as a transparent electrode for high efficiency electric type PM2.5 filter. The Ag nanowire was synthesized by a modified polyol method and has high aspect ratio with length and diameter of 200 μm and 100 nm, respectively. Detailed synthesis can be found in an experimental section. Figure 1a shows an overall fabrication process for the TRAP filter using Ag nanowire percolation network. A nylon mesh was employed as a backbone supporting structure for Ag nanowire networks. The hole size of the nylon mesh was approximately 53 μm, which meets high air permeability and provides a enough secure base structure for the Ag nanowire network. To fabricate a TRAP filter, vacuum filtration method was adopted to transfer Ag nanowire on the nylon mesh. A digital image of the C

DOI: 10.1021/acs.nanolett.7b01404 Nano Lett. XXXX, XXX, XXX−XXX

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Figure 3. SEM images of Ag nanowire percolation network based TRAP filter for (a) as-fabricated, (b) after PM filtration, and (c) after cleaning process with EG. After PM filtration, the collected dust particles on Ag nanowire percolation network (yellow dotted box area in (b)) were verified by EDX mapping analysis for carbon (left), oxygen (middle), and silver (right). The dust particles consist of the carbon and oxygen, which are the major components of the incense. (d) A schematic explanation for the mechanism of PM removal by TRAP filter.

fixed at 10 V. The PM2.5 removal performance of the ionizer and the applied voltage on the TRAP filter was verified by simple tests of (i) only ionizer on (5 V), (ii) only TRAP filter on (10 V), (iii) both ionizer (5 V) and TRAP filter (10 V) on or (iv) both off, respectively. Case (iii) showed superior PM2.5 removal performance over other cases in Figure 2b. These results signify that 99.99% PM2.5 was captured by TRAP filter when the applied voltage was on, while the PM2.5 easily passed the TRAP filter when the applied voltage was off. This implies that cooperative work of TRAP filter and ionizer is important for successful PM removal and that ionizer alone does not have a dominant influence. Case (iii) showed the best PM2.5 removal performance when the applied voltage on the TRAP filter was 10 V. Figure 2c shows the transient evolution of the PM2.5 density at various applied voltage conditions (0−10 V) on the TRAP filter while ionizer was on. Applying voltages to the TRAP filter caused quick and dramatic reduction of PM2.5 density compared with no bias condition (black line in Figure 2c). Inset picture displays a digital image of the chamber with a working TRAP filter after PM introduction with burning incense. The right side of the chamber was entirely filled with high density of PM smoke (over 1000 μg/m3), while the left

simulation value. The specific area ratio of the TRAP filter was found to be very small (around 2.5%) measured from SEM image, and it hardly disturbed air passage and induced smaller pressure drop. The proposed TRAP filter can remove PM2.5 in a more active manner compared with previous polar polymer nanofiber based PM filter. By controlling the applied voltage to the filter, the PM removal can be controlled actively. The fabricated percolation network of Ag nanowire on nylon mesh was applied as a transparent electrode for the electric type PM2.5 filter system. A schematic of experimental setup for TRAP filter system is presented in Figure 2a. The experiment setup consisted of two divided glass chambers. A particulate matter counter sensor, which measures PM2.5 density of the air in real time, was installed at the left side of the chamber. At the right side of the chamber, a negative ionizer was located to charge PMs. The TRAP filter was inserted in the middle of the two chambers. Before staring an experiment, PMs filled the right side of the chamber by burning incense. Then, incense continuously introduced PMs to the right side of the chamber nearby the ionizer. While a turn-on voltage of the ionizer was set at 5 V, the maximum applied voltage for the TRAP filter was D

DOI: 10.1021/acs.nanolett.7b01404 Nano Lett. XXXX, XXX, XXX−XXX

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Figure 4. (a) UV−vis measurement and (b) FT-IR measurement of TRAP filter for as-fabricated (black line), after PM filtration (green line) and after cleaning (red line). FT-IR measurement indicated that the cleaning process by using polar solvent was successful. The insets are digital images of TRAP filter after PM filtration (left) and after cleaning process (right). (c) XPS characterization for the carbon content was verified on (i) nylon filter only, (ii) as-fabricated TRAP filter, (iii) TRAP filter after PM filtration, and (iv) TRAP filter after cleaning process.

The morphology and chemical content on the TRAP filter surface before/after PM filtration were examined by SEM characterization and EDX (energy dispersive X-ray) mapping. The Ag nanowire percolation networks on the backbone nylon fiber in as-fabricated TRAP filter were clearly observed in Figure 3a. In contrast, dust-covered Ag nanowire networks and condensed dust particles were observed after PM filtration in Figure 3b. The chemical components of the dust particles were verified by EDX analysis. The EDX element mapping images showed that the dust particles mainly consisted of carbon and oxygen contents, matched for the major components of the

side of the chamber exhibited a clear view to read the background paper after activating TRAP filter. The TRAP filter efficiency was measured to be over 99.99% (experimental section in Supporting Information). In addition, TRAP filter started to filter PM instantly as soon as it is on. When the voltage condition was 2.5 V, the PM2.5 density steadily saturated to around 20 μg/m3. At higher voltage (5, 7.5, and 10 V) applied to TRAP filter, the PM2.5 density was much more reduced to around 5 μg/m3 after only 300 s as shown in Figure 2d. E

DOI: 10.1021/acs.nanolett.7b01404 Nano Lett. XXXX, XXX, XXX−XXX

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Nano Letters incense. To check the reusability of TRAP filter, cleaning process was carried out for the TRAP filter after PM filtration by polar solvent washing. A cleaning effect for various polar solvents was checked in Figure S3. The TRAP filter after PM filtration was soaked for several seconds in ethanol, DI water, and ethylene glycol (EG), respectively. Interestingly, cleaning efficiency was increased with the dipole moment of the solvents, known as ethanol, 1.69 D; DI water, 1.85 D; and ethylene glycol, 2.31 D.25 This simple result can be explained in that the charged PMs dissolved well in the solvent with higher dipole moment caused by strong intermolecular force, which resulted in higher cleaning efficiency. Therefore, ethylene glycol was adopted as cleaning solvent in this study. Figure 3c indicates that the dust particles disappeared after the EG cleaning process, and the Ag nanowire percolation networks on the nylon fiber clearly appeared again. PMs were completely removed without causing any mechanical damages on the Ag nanowire percolation network after the cleaning process (Figures S4 and S5). Therefore, the TRAP filter is expected to be reusable after a simple cleaning process. The mechanism of PM removal by TRAP filter is explained in Figure 3d. First, the particulate matters were generated from the incense and became negatively charged by the ionizer. At the same time, positive voltage was applied to the Ag nanowire percolation network of TRAP filter. Since the Ag nanowire networks have a very large surface to volume ratio as well as good electrical conductivity, it provides high efficiency to collect PMs on the filter. Eventually, when a voltage was applied to TRAP filter, strong electrostatic attraction force (Coulomb force) occurred between the PMs and TRAP filter as described in eq 1. Kε

q1q2 r2

= F (Coulomb force)

−m12m2 2 24π 2ε0 2εr 2k bTr 6

with the inset images. The transparency was slightly decreased after cleaning process; however, a similar peak trend was observed. Figure 4b suggests that as-fabricated TRAP filter and cleaned TRAP filter had analogous FT-IR spectra, while dustcovered TRAP filter showed different signal trends in 900− 1800 cm−1. Inset digital pictures in Figure 4a show dustcovered TRAP filter and cleaned TRAP filter, respectively. TRAP filter became transparent again after cleaning step. Therefore, the cleaning process by using polar solvent was successfully demonstrated in an optical property viewpoint. Meanwhile, the chemical property differences before and after cleaning of the TRAP filter was inspected by XPS analysis. Figure 4c represents carbon peaks of (i) nylon filter, (ii) asfabricated TRAP filter, (iii) TRAP filter after PM filtration, and (iv) TRAP filter after cleaning process. The nylon filter has strong C−C bond at around 284.5 eV and weak C−O or C−N bond at around 286 eV as major components for the nylon.26,27 Similar signal peak to the nylon was detected at the TRAP filter because there was no carbon composition on the Ag nanowire itself. A strong CO bond at around 287 eV and relatively small C−C peak was observed on the TRAP filter after filtration, from dust-covered Ag nanowire network. This result was quite consistent with the EDX mapping analysis. Various chemical bonding peaks were found at the TRAP filter after cleaning process. The peak trends of as-fabricated and cleaned TRAP filter indicated that the PMs captured on the TRAP filter were removed well after cleaning process without damaging the TRAP filter. When the TRAP filter is reused after cleaning, the durability and reliability of the TRAP filter need to be verified by repeated cleaning test. Figure 5a shows a repeated PM2.5 removal

(1)

= V (Keesom force) (2)

In contrast, in the previous study, the major mechanism for the PM capture was originated from the relatively weaker intermolecular forces such as van der Waals force (Keesom force), caused from the permanent dipoles of polar polymer filters and PMs.16−19 It is generally known that intermolecular force (van der Waals force) is much smaller than electrostatic force (Coulomb force). Therefore, PM removal efficiency of polar polymer filters was lower than the electric force based filter. To efficiently capture the PMs, TRAP filter uses stronger electrostatic force rather than using weaker intermolecular force. During the TRAP filter cleaning, electrostatic force was not present and intermolecular interaction was used instead. The intermolecular interaction between dipole solvent and PMs was high enough to separate the PMs from the TRAP filter. The Ag nanowire percolation network has been usually used for a flexible transparent conductor as an ITO replacement. This implies that the PM filter also can be made in a transparent form. The optical characteristics of TRAP filter were examined for as-fabricated, after filtration, and after cleaning processed TRAP filter with UV−vis and FT-IR measurement. Figure 4a represents optical transmittance differences of the TRAP filter. An obvious peak difference observed in wavelength range of 250−400 nm for the TRAP filter after filtration and cleaning process, which corresponds

Figure 5. (a) Repeated PM2.5 removal performance test and (b) electrical resistance change for the TRAP filter reusability and reliability demonstration. TRAP filter maintained good PM removal performance and sheet resistance after each polar solvent cleaning process. F

DOI: 10.1021/acs.nanolett.7b01404 Nano Lett. XXXX, XXX, XXX−XXX

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Nano Letters performance test after cleaning process. Each filtration test was performed for 550 s while the real-time PM2.5 counter sensor was operated. The filter system including glass chambers was washed by acetone, ethanol, and DI water after a single experiment in order to prevent any side effect from residual PMs. The PM2.5 density of every single experiments initially dropped to 15 μg/m3 within 120 s and maintained under 10 μg/m3 until the end of experiment. Additionally, the electrical resistance of the TRAP filter was measured after each washing process. The normalized electrical resistance showed little change after each cleaning process, and most of Ag nanowire networks are stable due to the backbone nylon filter (Figure 5b). The fabricated TRAP filter in this study demonstrates good PM removal performance with excellent durability and reliability for multiple-times reuse. In conclusion, we presented the first environmental application demonstration of Ag nanowire percolation network, which has been frequently used as a flexible transparent conductor as an alternative to ITO. We demonstrated an electrical type of PM2.5 filter that is transparent, reusable, and active using a highly conductive and transparent Ag nanowire percolation network electrode. The TRAP filter was fabricated by a simple vacuum filtration method on the nylon mesh filter, and it was optimized under low working voltage condition (99.99%) compared with polar polymeric nanofiber network PM filter because the longrange electrostatic force is much stronger than intermolecular force to more efficiently collect particulate matters. In addition to high efficiency PM removal, TRAP filter can be operated at a very low air flow resistance thus low power consumption, and it is transparent and possesses antibacterial effect to kill any harmful microbials. Finally, the fabricated TRAP filter system can have excellent durability and reliability for the environment applications and thus can be reusable for multiple times after simple cleaning process. Methods. Synthesis of Long Ag Nanowires. Long Ag nanowires (>100 μm in length,