Color-changing Microfibers-based Multifunctional Window Screen for

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Functional Nanostructured Materials (including low-D carbon)

Color-changing Microfibers-based Multifunctional Window Screen for Capture and Visualized Monitoring of NH3 Zhen Wang, Xinxin Yuan, Shan Cong, Zhi-Gang Chen, Qingwen Li, Fengxia Geng, and Zhigang Zhao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b02516 • Publication Date (Web): 12 Apr 2018 Downloaded from http://pubs.acs.org on April 12, 2018

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ACS Applied Materials & Interfaces

Color-changing Microfibers-based Multifunctional Window Screen for Capture and Visualized Monitoring of NH3 Zhen Wang,†,‡ Xinxin Yuan,† Shan Cong,† Zhigang Chen,†,‡ Qingwen Li,†,‡ Fengxia Geng,§ Zhigang Zhao,†,‡,* †

Key Lab of Nanodevices and Applications,

Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou 215123, P. R. China ‡

School of Nano-Technology and Nano-Bionics, University of Science and Technology of China, Hefei 230000, China §

College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, China *

E-mail: [email protected]

Abstract: Air pollution is one of the most serious issues affecting the world today. Instead of expensive and energy-intensive air-filtering devices, fiber-based transparent air filter coated on window screen is seen as one of the state-of-the-art filtration technology to combat the seriously growing problem, delivering the advantages of simplicity, convenience and high filtering efficiency. However, such window screen is currently limited to particulate matter (PM) filtration, and ineffective with other air pollutants. Here, we report the use of a newfangled type of color-changing fibers, porous Prussian blue analogues (CuHCF)/polymer composite microfibers, for transparent window screens towards air pollutant filtration. To increase pollution filtration, pores and dimples are purposely introduced to the fibers using binary solvent systems through a non-solvent induced phase separation mechanism. Such composite microfibers overcome some of the limitations of those previously used fibers, and could simultaneously capture PM2.5, PM10 and NH3 with high efficiency. More interestingly, a distinct color change is observed upon exposure to air pollutants in such window screens, which provides multifunctional capability of simultaneous pollutant capture and naked-eye screening of the pollutant amount. Specifically, in case of long-term exposure to low-concentration NH3, the symbol displayed in such window screen changes from yellow color to brown, and the coloration rate is directly controlled by NH3 concentration, which may serve as a careful reminder for those people who are repeatedly exposed to low-concentration ammonia gas (referred to as chronic poisoning). In contrast, after short-term exposure to a high concentration of 1

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ammonia gas, the yellow symbol immediately becomes blackened, which provides timely information about the risk of acute ammonia poisoning or even ammonia explosion. Further spectroscopic results show that the chromatic behaviors in response to different concentrations of NH3 are fundamentally different, which is related to the different locations of ammonia in the lattice of CuHCF, either in its interstitial sites or at the Fe(CN)6 vacancy sites, largely distinguished by the absence or presence of atmospheric moisture.

KEYWORDS: color-changing, porous microfibers, window screen, visualization, NH3 capture

2


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Introduction Currently, air pollution has become a pressing issue in a myriad of cities around the world due to the massive increase in air pollutants emitted from human activities, which is strongly linked to cause respiratory diseases, cardiovascular diseases and heart diseases in urban communities.1-4 Among the various sources of air pollution, ammonia gas (NH3) and particulate matter (PM) are two of key air pollutants.5-6 NH3 is a hazardous gas that can cause irritation of the eyes, skin, and respiratory system, and also can increase the risks of soil erosion combined with deterioration in water quality in rivers and lakes.7-9 More seriously, NH3 is explosively flammable when mixed with air, although it is much less well-known. For example, an ammonia leak caused a deadly explosion at a poultry abattoir in north-east China in 2013, which killed at least 119 people.10 As for PM, it is a complex mixture of small solid particles and liquid droplets, which can be mainly classified into two types on the basis of size as PM2.5 and PM10. It is well known that PM2.5 can cause asthma, respiratory inflammation, jeopardizes lung functions and even promotes cancers.2-3, 11 No matter what the type of air pollutant, it is necessary to deploy effective protection from the dangers of air pollution for the public health or safety. For protection against PM2.5, some efforts have been made toward personal and building protection. For example, in case of indoor buildings, people tackle this problem by using a ventilation system or central air conditioning network. Such PM filtering system is normally sophisticated, expensive and high energy-consuming. To overcome this issue, Cui’s group developed a simple and low-cost technique for PM2.5 capture in indoor building based on transparent polymeric fiber air filter coated on window screen, which has attractive attributes of high filtering efficiency, good optical transparency, low resistance to air flow and light weight.12-16 More recently, the biomass-based nanofabrics (e.g. protein and cellulose) have been also shown to be promising for air-filtration applications, benefiting from their rich surface functional groups for strong interactions with air pollutants.17-19 However, previous efforts on these polymeric fiber-based window screens mainly aim at the protection against PM2.5, and such window screens for simultaneous protection against other air pollutants such as NH3 have been less explored. On the other hand, employing device visualization as part of an overall device design strategy is an interesting recent research direction in science and technology, 3



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which enables devices to function in intelligent and naked-eye screening modes.20-27 For example, recent works have enabled successful fabrication of smart energy storage devices by incorporating electrochromic materials such as W18O49, polyaniline into electrodes inside energy storage devices, in which the level of energy stored can be visually conveyed through recognizing variations in pattern color schemes.20 Using a WO3-film electrochromic device array, a pressure sensor that can both directly visualize and record applied pressure/stress is also developed.21 Accordingly, in the window screen, it would be rather beneficial if the window screen itself could visually monitor the pollutant capture situation through color variations of the screen. However, to the best of our knowledge, no attempt has yet been made to generate such multifunctional window screens. Presumably, chemo-chromic materials (e.g. Prussian

blue,

tungsten

oxide,28

polypyrrole,29

bromocresol

green30)

are likely to play a key role in realizing such visualized monitoring in the window screens, since they have the ability to change color upon exposure to the toxic gases including ammonia, acetone, and etc, and can be potentially used as air filtering materials for air pollutant monitoring. Here, we report the first use of a newfangled type of color-changing microfibers, porous Prussian blue analogues (CuHCF)/polymer composite microfibers, for transparent window screens towards air pollutant filtration. Such composite microfibers overcome some of the limitations of those previously used fibers, and could simultaneously capture PM2.5, PM10 and NH3 with high efficiency. By controlling the fiber microstructure and surface chemistry, the fibers could effectively remove 99.9% PM particles in highly polluted air, and have a high ammonia removal capacity of 1.8 mmol/g. More interestingly, the air pollutant capture can be directly visualized by color changes in such window screen, which simultaneously provides pollutant capture and naked-eye screening of the pollutant amount.

Results and Discussion At our first effort, we come to polar polymeric fibers which have large amounts of surface functional groups. As reported before, the polar polymeric fibers could capture PM particles by surface adhesion between air pollutants and surface functional groups, while allowing a high light and air penetration.12-13 One example is polyvinyl butyral (PVB) fibers prepared by electrospinning method. From its 4


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chemical structure, it can be clearly seen that PVB basically has several polar functional groups such as hydroxyl group, ester group, which is also confirmed by our Fourier transform infrared spectroscopy (FTIR) analysis (Figure 1a). Due to the existence of various polar functional groups present at the surface of PVB, the PVB fibers, denoted as fiber A, have strong dipole-dipole and induced-dipole intermolecular forces with PM particles so that such PVB fibers can shut off PM particles from entering the indoor environment when installed on window screens (Figure 1b).12-13 Figure 1c provides a vivid demonstration to block the PM particles from the sources (right bottle) entering the indoor environment (left bottle) using the type of air filter. The right bottle contains very high levels of PM2.5 (the initial PM2.5 mass concentration >500 µg/m3) generated by burning incense, while a PVB fibers-based air filter with optical transmittance of 80% is placed in the connection of two bottles. Even after approximately 1 h, the left bottle is still very clear, suggesting that the PM particles can be effectively captured by the PVB fiber-based filter. Scanning electron microscope (SEM) images of a PVB fibers-based filter before and after filtration provides further support for the effective PM2.5 capture. As can be seen from Figure 1d, e, the pristine PVB fibers are ~800 nm in diameter. After filtration, the PVB fibers are coated with many PM particles with different sizes. Using the experimental setup shown in Figure 1b, the removal efficiency for PM particles can be estimated via the formula (1- CL/CR), where CL and CR are the PM concentration in the left and right bottle, respectively. Quantitatively, a high PM2.5 removal efficiency level of >95% can be achieved by the PVB fibers-based filters at all optical transmittance levels of 10-90%, which is attributed to the intermolecular or interatomic attraction forces between polar PVB fibers and PM particles.31 (Figure 1f). On the other side, although less noticeable, the as-prepared PVB fibers-based filters are also capable of capturing NH3 from the atmosphere to a certain extent, since the lone pairs on the nitrogen in NH3 can form a through-bond interaction with the ester group by the empty orbitals in PVB.32 The NH3 capture can be assessed from a detectable decrease of NH3 concentration when the PVB fibers-based filters are placed in a sealed bottle (5 L) containing NH3 at an initial concentration of 80 ppm. However, the removal capacity of NH3 for the PVB fiber-based filters is rather limited, only 0.31 mmol/g at the maximum even with a thick filter with a thickness of about 10 µm (corresponding to 10% optical transmittance)), which is probably in relation 5



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with the relatively weaker short-range intermolecular force in PVB fibers (Figure 1g).

Figure 1. (a) FTIR spectra of electrospun PVB fibers. The inset shows a general molecular structure of PVB with the polar functional groups circled in red. (b) Photograph of a transparent PVB fibers-based window screen. (c) Demonstration of using a transparent PVB fibers-based air filter to shut off PM from the outdoor (right bottle) entering the indoor (left bottle) environment. (d, e) SEM images of the PVB fibers-based air filters before and after filtering the PM. (f) PM2.5 and PM10 removal efficiencies of PVB transparent filters at different transmittances. The area densities of the pure PVB fibers-based filters with different transmittances range from 0.02 to 0.49 mg/cm2, which demonstrates the lightness of the filters. The area densities corresponding to the transmittances are listed in Table S2 in Supporting Information. Error bars are the standard deviation for five replicates. (g) The NH3 adsorption ability of pure PVB fibers-based filters in ambient air. To improve the NH3 removal efficiency of our air filters, special attention is given to CuxFey[Fe(CN)6]2 (copper hexacyanoferrate, CuHCF), a member of the Prussian blue family with electronically active metal sublattices and an open framework structure built by a cyanide-bridged network of octahedral units (Figure 2a). In its open framework structure, two kinds of adsorption sites are considered to be able to accommodate ammonia due to their large tunnel sizes:33 one is vacancy site surrounded by six NH3-capturing metals around [Fe(CN)6] vacancy (Figure 2a), since 6


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the lone pairs on the nitrogen in NH3 can form a through-bond interaction with the empty orbitals of Fe atom. The other is interstitial site surrounded by a cubic framework, where both hydrated and dehydrated forms of ammonia could be captured at the site. The tunnel-driven NH3-trapping process in CuHCF can be described as follows: In the dehydrated situation, CuHCF trap NH3 according to equation (1): CuxFey[Fe(CN)6]3 + 6NH3

(NH3)6CuxFey[Fe(CN)6]3

(1)

In contrast, in the hydrated case such as atmospheric environment, ammonia is adsorbed in CuHCF in the form of NH4+ according to equation (2): CuxFey[Fe(CN)6]3+4NH3+4H2O

(NH4)4CuxFey[Fe(CN)6]3[OH]4

(2)

Accordingly, our experimental findings suggest that the CuHCF in powder form shows a high ammonia removal capacity of 19.5 mmol/g in an atmospheric environment (Figure S1). Therefore, the enhancement of ammonia removal capacity is strongly expected when CuHCF is added into PVB fibers. By the addition of CuHCF into the PVB precursor solution before electrospinning, smooth and solid CuHCF/PVB composite fibers with diameter of about 1000 nm can be produced, denoted as fiber B (Figure 2b). However, the ammonia removal capacity of the smooth composite fibers is still as low as 0.52 mmol/g (Figure 2g, h), since many of the tunnels that have free access to NH3 may be largely blocked by PVB polymer. To further increase the ammonia removal capacity, pores and dimples are skillfully introduced to the surface of the electrospun CuHCF/PVB composite fibers through a non-solvent induced phase separation mechanism, using binary solvent systems systems with different properties (Figure 2c). Briefly, two types of porous fibers, denoted as fiber C and D, are fabricated via an electrospinning technique from the feed solutions which are prepared by dissolving PVB at 8% (w/v) in a binary solvent mixture of tetrahydrofuran (THF) and dimethyl sulphoxide (DMSO) at 90:10 or 95:5 volume ratios, respectively. THF has a low boiling point and a high solubility for PVB, while DMSO has a high boiling point and a low solubility for PVB. The opposite nature of the two solvents causes a distinct difference in the evaporation rate between THF and DMSO. During electrospinning, the more volatile THF would evaporate first accompanied by phase separation, making the PVB polymer precipitate from the DMSO, which eventually yields polymer fibers with porous structures. Figure 2d shows the two morphologies of the resultant porous fibers prepared using two different solvents. It is observed that fiber C is populated with many nano-scaled 7



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pores of ellipsoidal shape, with the major axis along the fiber direction (Figure 2d). For fiber D, the fiber surface is decorated with even more number of and larger elliptic nanopores, appearing to be rather rough and uneven (Figure 2e). There are also some internal porous structures seen at fiber D (Figure S2). Nitrogen adsorption isotherms provide quantitative data on the surface area increment on porous fibers compared to smooth fibers (Figure 2f). The BET surface area of smooth fibers A, smooth fibers B, porous fibers C, and porous fibers D are found to be about 1.13, 2.41, 12.87 and 30.91 m2/g, respectively. Benefiting from the desirable porous nanostructure, the fibers with roughened or porous surface have been shown to capture more NH3 from the atmosphere than smooth surface fibers. As shown in Figure 2g, the ammonia removal capacities of porous fibers C and D are significantly raised to 0.98 and 1.81 mmol/g, respectively.

Figure 2. (a) The crystal structure of CuHCF. Its major structural feature is that it has an open framework structure, and the open framework has a number of interstitial sites and vacancies where both hydrated and dehydrated forms of ammonia can be accommodated. (b) Schematics showing the electrospun fabrication of porous 8


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CuHCF/PVB composite fibers for transparent air filter through a non-solvent induced phase separation mechanism. (c-e) SEM images of different CuHCF/PVB composite fibers. (f) Nitrogen adsorption isotherms of three different CuHCF/PVB composite fibers and pure PVB fiber. (g) Time dependence of NH3 adsorption to different fibers-based filters. (h) Comparison of NH3 adsorption ability of four fibers-based filters.

Further study interestingly reveals that the resultant porous CuHCF/PVB fibers can display changes in color when exposed to NH3, based on the fascinating NH3-related chromic behavior of CuHCF powders. For instance, the initial bright yellow color of as-prepared CuHCF, can be changed immediately into black in exposure to NH3 with a high concentration of 5000 ppm (a concentration which is close to the explosive limit of NH3) for less than 20 min, while instead be gradually deepened into chestnut brown over 2 consecutive days if the concentration of NH3 is as low as 25 ppm (a concentration which cannot be easily perceived as odour by human olfaction but is still considered to be hazardous by Safety Data Sheets34) (the inset in Figure 3a). The deepened color of CuHCF powders in exposure to NH3 can be further quantified by the elevated absorption monitored at given wavelength, e.g. 600 nm, in the absorption spectra, as shown in Figure 3a. The chromic behavior of the CuHCF inspires us to develop a “smart” window screen using the color-changing fibers for visualized capture of the air pollutants. In our design, the “smart” window screen consists of a yellow icon or symbol, “Suzhou”, our city's name, on a transparent background framed within a brown border (Figure 3b, 3d, S3). The symbol is constituted by a 20 µm-thick porous CuHCF/PVB composite fiber film. When exposure to a low concentration of ammonia gas such as 25 ppm, the yellow symbol appears to keep unchanged within hours, however, gradually changes from yellow color to brown during long-term exposure (1-7 days) (Figure 3b, S4). Thus, our window screen may serve as a careful reminder for those people who are repeatedly exposed to low-concentration ammonia gas (referred to as chronic poisoning). The absorbance at 600 nm has been measured for a CuHCF/PVB composite fiber symbol at increased exposure times in diluted NH3 with different concentrations, such as 25, 50, 200 and 500 ppm, respectively (Figure S5). Gradually deepened colors are evidenced by the fact that the absorbance progressively increases from an initial value of 0.58 to a 9



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saturated value over 0.77 after exposing the symbol to four concentrations of NH3 for a sufficient time (7 days). Further, the coloration rate can be roughly characterized by an absorbance-concentration plot (Figure 3c). Noticeably, in exposure to 200 ppm NH3, the absorbance increase is achieved faster than in exposure to 25 ppm NH3, but it finally amounts to a similar value, indicating that the coloration rate is directly controlled by NH3 concentration under low-concentration conditions. Therefore, the environmental NH3 concentration could be roughly but directly readout through naked-eye observation, as simply judged by the rate of color change of the symbol, which make the ammonia determination rapid, convenient and easy. In contrast, after 20-minute exposure to a high concentration of ammonia gas such as 5000 ppm, the yellow symbol becomes blackened (Figure 3d), which provides timely information about the risk of acute ammonia poisoning or even ammonia explosion. It should be mentioned that the warning of long-term exposure to low concentration of NH3 is an irreversible process, while the warning of short-term exposure to high concentration of NH3 given by our window screen is, by and large, reversible and repeatable. For example, after exposure to air for 2 hours, the colored symbol upon exposure to high concentration of NH3 develops a yellowish brown color (Figure 3d). When the symbol is alternately exposed to high-concentration ammonia and air, the color changes between yellowish brown and black, and such cycle can be repeated at least 5 times (Figure S6). In contrast, long-term exposure to low concentration of NH3 can lead to irreversible coloration (Figure S7). Figure S8 shows the color change of symbols in the exposure of variable NH3 concentration.

10


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Figure 3. (a) The images and corresponding UV-Vis spectra of CuHCF powders without and with absorbing NH3. (b) Photographs of a multifunctional transparent window screen based on the color-changing CuHCF/PVB fibers, which consist of a yellow icon or symbol, “Suzhou”, our city's name, on a transparent background framed within a brown border. The window screen could serve as a careful reminder for those people who are repeatedly exposed to low-concentration ammonia gas such as 25 ppm, since it changes from yellow color to black brown during long-term NH3 exposure. (c) Plot the absorbance at 600 nm versus the NH3 concentration after exposure to low-concentration NH3 for 1 day. The inset in c shows the corresponding UV-Vis spectra of “Suzhou” symbols at different NH3 concentrations. (d) After exposure to a high concentration of ammonia gas such as 5000 ppm, the yellow symbol becomes blackened in a very short time, which provides timely information about the risk of acute ammonia poisoning or even ammonia explosion.

The difference in ammonia capturing mode is further illustrated based on the results of multiple spectroscopic studies. As depicted in Figure 4a, all XRD peaks observed for the as-prepared sample can be indexed as the expected cubic Cu[Fe(CN)6]2 with space group Fm 3 m (JCPDS 53-0084), in which octahedrally coordinated transition 11



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metals such as Cu and Fe are linked by CN ligands, forming a face-centered cubic structure.35-36 After long-term exposure to 25 ppm NH3, although the change in color appearance is observed, the sample shows nearly identical XRD pattern with the pristine CuHCF, indicating that the crystal structure of CuHCF is well maintained after low-concentration NH3 adsorption, probably due to the interstitial occupation of ammonia ions assisted by water molecules. In sharp contrast, the black sample obtained by exposure to high-concentration NH3 shows a XRD pattern totally different to that of the pristine CuHCF, suggesting a possible phase transition. Besides the appearance of a large number of new peaks, the featured (200) peak of CuHCF shifts towards higher angles while the (220) peak shifts to lower angles, indicating the distortion of the unit cell probably arising from the occupation of Fe(CN)6 vacancy with NH3 coordinating to surrounded metals in the face-centered-cubic (fcc) unit cell.36 The direct coordination binding of NH3 molecule at the vacancy site can probably account for the excellent NH3 capture ability of the CuHCF, especially exposed at very high concentration of NH3. FTIR spectrum gives further clues to the different locations of ammonia in the lattice of CuHCF during exposure to low/high concentration NH3 (Figure 4b). Ammonia-related absorption peaks at 1220 and 1410 cm-1 appear after NH3 absorption, corresponding to the symmetric deformation of Fe−NH3 and degenerate deformation of NH4+/NH3,33 with the former mainly observed at a exposure concentration of 5000 ppm and the latter at a exposure concentration of 25 ppm. From the discrepancies in FTIR spectra, it can be inferred that the adsorbed ammonia can bind to Fe sites at a high-concentration exposure while exist mainly as interstitial ions at a low-concentration exposure, which is in consistence with that deduced from XRD patterns. Notably, besides the peak at 2178 cm-1 related to the v(CN) band of FeIII-CN-CuII in the pristine CuHCF,37-38 a shoulder peak at 2093 cm-1 can be also observed in the sample after exposure to either high or low concentration of NH3. The newly-appeared peak at 2093 cm-1 can be ascribed to the FeII-CN-CuII links,35 so its appearance indicates that the ammonia occupation, either occupying in the interstitial or vacancy sites, would cause the reduction of neighboring Fe(III) centers to Fe(II) in the lattice of CuHCF. XPS can give further chemical state information for Fe atoms before and after ammonia occupation. As shown in Figure 4c, for the pristine CuHCF, the Fe 2p spectrum can be fitted with two sets of doublets at the binding energy of 711.1, 724.8, and 709.7, 722.6, respectively, indicating that 12


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iron is in the mixed-valence Fe(III)-Fe(II) state in the sample. The percentages of Fe3+ and Fe2+ can then be determined to be 75% and 25% by evaluating the peak areas attributed

to

the

two

different

oxidation

states.

However,

after

low-or

high-concentration NH3 exposure, one notices a total disappearance of the Fe 2p3/2 doublet attributed to Fe3+ as well as an increase in the Fe 2p1/2 doublet attributed to Fe2+ in the XPS spectra, both indicative of the reduction of Fe atoms from Fe(III) to Fe(II) upon NH3 adsorption.

Figure 4. (a) XRD, (b) FTIR and (c) XPS spectra of pristine CuHCF, CuHCF after long-term exposure to low-concentration of NH3, and CuHCF after exposure to high-concentration of NH3. The recovery characteristics of the symbol after exposure to a high concentration of ammonia gas can also be examined by multiple spectroscopic studies. As we mentioned before, the colored symbol upon exposure to high concentration of NH3 could recover its original light color after exposure to air for 2 hours. Such a process is accompanied by the generation/degeneration of some peaks in the XRD patterns (Figure S9). Further evidence can also be found with FTIR spectra (Figure S10), as the color changes from black to yellowish brown, the peak at 1232 cm-1 ascribing to Fe-NH3 decreases while the peak at 1406 cm-1 featuring the existence of NH4+ increases.33 Accordingly, it can be inferred that storing in air could easily re-oxidize Fe atoms, thus significantly weakening the binding towards NH3. Such multifunctional window screen can be used not only to function as visual colorimetric absorbents for ammonia, but also still acquire excellent PM2.5 removal efficiency under extremely hazy air conditions. A transparent window screen constructed with porous CuHCF/PVB fibers instead of pure VB fibers are shown in Figure 5a. After PM2.5 capture, the captured PM particles tightly attach to the surface 13



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of each fiber to form a thin layer coating, which subsequently aggregate to form large particles especially at the junctions of the fibers with the continuous feeding of PM2.5 particles (Figure 5b), which is similar to the case of pure PVB fibers. To optimize the removal of both PM2.5 and NH3, a set of porous CuHCF/PVB fibers-based window screens with varied coating thickness are prepared, showing the transmittance of ~85, ~72, ~45, ~30%, respectively (Figure S11). For all window screens with different transmittances, the PM2.5 and PM10 removal efficiencies are kept as high as 97-100% and 99-100%, respectively. Furthermore, in spite of its porous structure, penetration testing shows that transparent window screens also have high air penetration and stability. As can be seen from Figure 5d, a transparent CuHCF/PVB fibers-based window screen is placed between a fan with a velocity of 3 m·s-1 and a bundle of paper tassels. When a strong, continuous wind blows from the fan, the paper tassels flutter quickly in front of the CuHCF/PVB fibers-based window screen, which clearly demonstrated great penetration of air through the transparent screen. After continuously working for 100 h, that transparent window screen are still stable, and their PM2.5 and PM10 removal efficiency almost keeps unchanged (Figure 5e).

Figure 5. (a, b) SEM images of the porous CuHCF/PVB fibers (here the fiber D is used) before and after filtration, showing the PM deposition. (c) PM removal efficiency for the porous CuHCF/PVB fibers-based window screen with different transmittance. Compared with pure PVB fibers, the area densities of the CuHCF/PVB fibers-based filters are increased to be 0.038-0.58 mg/cm2 due to the higher molecular weight of CuHCF (Table S2). (d) Photograph demonstrating great penetration of air 14


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through the transparent window screen. (e) Durability tests of the CuHCF/PVB fibers-based window screen.

Conclusion We

have

manufactured

a

novel

multifunctional

window

screen

using

color-changing fibers, electrospun porous CuHCF/PVB microfibers with high surface areas, in which pores and dimples are purposely introduced to the fibers through a non-solvent induced phase separation mechanism. It is shown that the window screen has high air filtration efficiency, high PM2.5 removal efficiency (>99.5% removal of PM2.5 at ~73% of transmittance), and excellent ammonia capture capacity (>1.8 mmol/g). More interestingly, the as-designed window screen can display changes in color as a response to the captured air pollutants, which provides multifunctional capability of simultaneous pollutant capture and naked-eye screening of the pollutant amount. We believe that the present study may serve as a starting point for the future rational design of transparent window screens with new features and functionalities for achieving a healthier indoor environment.

Experimental Methods Synthesis of CuHCF: All reagents were of analytical reagent grade without further purification. In a typical example, 50 mL aqueous solution of 0.5 mol/L copper sulfate was mixed with 50 mL 0.1 mol/L potassium ferrocyanide solution, and stirred with 250 rpm at room temperature for 1 hour. After the reaction, the mixture was precipitated by centrifugation at 3000 rpm for 10 min. The precipitate was then washed with ethanol two more times followed by centrifugation to ensure thorough cleaning of the reaction product. Finally, a bright yellow powder was obtained after drying at 60 ℃ in vacuum.

Electrospinning: All fibers were fabricated by electrospinning using different precursor solutions that have different compositions (see Table S1 in Supporting Information). All precursor solutions were firstly vigorously stirred at room temperature before use. The precursor solution was then loaded into a 5 mL syringe with a plastic capillary tip with an inner diameter of 0.6 mm, and ejected with a flow rate of 1 mL/h. The distance between the syringe needle tip and the collector was adjusted to 10 cm. For uniformly collecting the as-prepared fibers, a rotating 15



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cylindrical drum wrapped with stainless-steel cloth mesh made by stainless-steel wires of 0.13 mm diameter interwoven at a pitch of 0.5 mm was used as the collector. The rotate speed of the cylindrical counter collector was 500 rpm, the movement speed of the needle was 50 mm/s, and a high voltage of 25 kV was applied by a voltage-regulated DC power supply.

PM filtration efficiency test: For all PM-based performance tests, the PM particles used in this work are generated by burning incense smoke. The PM particle number concentration is detected by a particle counter (CEM), and the removal efficiency is calculated by comparing the number concentration before and after filtration. In the typical PM capture test, the initial PM2.5 mass concentration is controlled to a hazardous pollution level (>500 µg/m3), and the wind velocity is set to be 0.21 m s-1.

NH3 removal capacity test: The NH3 capture is evaluated by the decrease of NH3 concentration when the fibers-based filters are placed in a sealed bottle (5 L) containing NH3 at an initial concentration of 80 ppm. For the sake of accuracy, a thick filter with thickness of 10 µm is used to determine the removal capacity. The NH3 concentrations in the sealed bottle reactor are measured by an ammonia gas detector with its probe penetrating through the bottle stopper. Subsequently, the (C0 − CT ) ⋅ V m removal capacity is calculated using the following equation , where C0 is

the initial NH3 concentration, CT is the residual NH3 concentration, V is the volume occupied by NH3, and m is the mass of adsorbents. For visualized monitoring, the NH3 concentrations are kept at 25, 50, 200, 500 and 5000 ppm, depending on the experimental requirements.

Acknowledgement This work was supported by the National Natural Science Foundation of China (51572286, 21503266, 51772319 and 51772320), the Outstanding Youth Fund of Jiangsu Province (BK20160011), F.X.G. acknowledges the support from the National Natural Science Foundation of China (51772201), the Thousand Young Talents Program, and the Jiangsu Specially-Appointed Professor Program. S.C. acknowledges the support from the Youth Innovation Promotion Association, CAS (2018356). 16


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Supporting Information Characterization methods; Compositions of precursor solutions; Magnified SEM images of fiber D; UV-Vis spectra of CuHCF/PVB composite fiber film under different condition; Photographs of our multifunctional transparent window screens; XRD patterns and FTIR spectra of CuHCF samples under different conditions.

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Table of Contents Graphic

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Color-changing Microfibers-based Multifunctional Window Screen for

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