Large Area Co-Assembly of Nanowires for Flexible Transparent Smart

Jun 30, 2017 - Electrochromic devices with controllable color switching, low cost, and energy-saving advantages have been widely used as smart windows...
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Large Area Co-Assembly of Nanowires for Flexible Transparent Smart Windows Jin-Long Wang, Yi-Ruo Lu, Hui-Hui Li, Jian-Wei Liu,* and Shu-Hong Yu* Division of Nanomaterials & Chemistry, Hefei National Laboratory for Physical Sciences at the Microscale, Collaborative Innovation Center of Suzhou Nano Science and Technology, Department of Chemistry, CAS Centre for Excellence in Nanoscience, Hefei Science Centre of CAS, University of Science and Technology of China, Hefei 230026, China S Supporting Information *

ABSTRACT: Electrochromic devices with controllable color switching, low cost, and energy-saving advantages have been widely used as smart windows, rear-view car mirrors, displays, and so on. However, the devices are seriously limited for flexible electronics as they are traditionally fabricated on indium tin oxide (ITO) substrates which will lose their conductivity after bending cycles (the resistance significantly changed from 200 Ω to 6.56 MΩ when the bending radius was 1.2 cm). Herein, we report a new route for large area coassembly of nanowires (NWs), resulting in the formation of multilayer ordered nanowire (NW) networks with tunable conductivity (7−40 Ω/sq) and transmittance (58−86% at 550 nm) for fabrication of flexible transparent electrochromic devices, showing good stability of electrochromic switching behaviors. The electrochromic performance of the devices can be tuned and is strongly dependent on the structures of the Ag and W18O49 NW assemblies. Unlike the ITO-based electronics, the electrochromic films can be bent to a radius of 1.2 cm for more than 1000 bending cycles without obvious failure of both conductivity (ΔR/R ≈ 8.3%) and electrochromic performance (90% retention), indicating the excellent mechanical flexibility. The present method for large area coassembly of NWs can be extended to fabricate various NW-based flexible devices in the future.

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INTRODUCTION Development of portable, flexible, and stretchable devices is of great interest for applications of various electronics such as flexible electronics,1−3 pressure sensors,4−6 and piezoelectric nanogenerators.7 Electrochromic devices fabricated on conductive electrodes show different optical properties when applied with different bias voltages and have shown wide applications in smart windows, antiglare rear-view mirrors, and electronic displays.8−14 Traditionally, flexible electrochromic devices are fabricated on ITO related flexible electrodes, which is far from completely flexible (the resistance significantly changed from 200 Ω to 6.56 MΩ when the bending radius is 1.2 cm; see Figure S1).15−17 Recently, metal nanowire (NW), graphene, and carbon nanotube networks have been selected to fabricate flexible electrodes to overcome the brittleness, low infrared transmittance and low abundance limit suitability of ITO.18,19 However, flexible films made of carbon nanotube (CNT) and graphene exhibit sheet resistances and optical properties of 100−1000 Ω/sq at 80−90% optical transmittance which are still not good enough for most applications.20−24 Thin films of metal NWs, in comparison, hold great promise because of their high conductivity and optical transmittance. 25 Compared with disordered NW networks, manipulating NW assemblies for macroscopic-scale NW architectures can precisely © 2017 American Chemical Society

tailor and balance the optical transmittance and the conductivity of the flexible transparent electrodes.26,27 In this work, Ag and W18O49 NWs have been manipulated to fabricate 2D NW assemblies with tunable conductivity (7−40 Ω/sq) and transmittance (58−86% at 550 nm) for the flexible transparent electrochromic devices. Unlike the ITO-based electronics, the electrochromic films can be bent to a radius of 1.2 cm for more than 1000 bending cycles without obvious failure for both conductivity (ΔR/R ≈ 8.3%) and electrochromic performance (90% retention), indicating the excellent mechanical flexibility.

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EXPERIMENTAL SECTION Synthesis of Ag NWs. All chemicals and solvents were purchased from Shanghai Chemical Reagent Co. Ltd. and used without further purification. Ag NWs with an average diameter of 60 nm and length of several micrometres were prepared by the polyol method.28 Typically, 5.86 g of poly(vinylpyrrolidone) (PVP, MW ≈ 40 000) was added to 190 mL of glycerol in a 500 mL round bottle flask and was kept at 100 °C for about 1 h to form a homogeneous solution. After cooling down to room Received: April 13, 2017 Published: June 30, 2017 9921

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decorated with PVP were first prepared as previously reported28,29 (Figure S2) and then mixed in DMF and CHCl3 solution (volume ratio is 1.5:1) for the followup coassembly process on the LB trough. The typical process for the fabrication of flexible electrochromic conductive electrodes with 2D NW architectures was shown in Figure 1a. In brief, the devices were

temperature, 1.58 g of AgNO3 was added to the solution. Then the solution was heated from room temperature to 210 °C in 20 min with slow stirring. When the temperature reached 60 °C, the NaCl solution (59 mg of NaCl dissolved in 0.5 mL of deionized water and 10 mL of glycerol) was added to the flask. When the temperature reached 210 °C, the heating was stopped immediately and 200 mL deionized water was added to the solution. The solution was kept undisturbed for 24 h to remove the Ag nanoparticles from the NWs. The obtained Ag NWs were washed once with deionized water and dispersed into 60 mL aqueous solution for further use (the concentration of Ag NWs was 0.014 g/mL). Synthesis of W18O49 NWs. Uniform W18O49 NWs were prepared by a modified solvothermal method.29 Typically, 0.0001 g of PVP and 0.03 g of WCl6 (Alfa Aesar) were added into 40 mL of ethanol. The mixture was stirred by magnetic stirring to form a homogeneous solution. The solution was then added into a 50 mL Teflon-lined stainless steel autoclave and maintained at 180 °C for 24 h. After that, the autoclave was cooled to room temperature naturally and W18O49 NWs were obtained for further use (the concentration was 0.0004 g/mL). Fabrication of Transparent Electrochromic Films. The well-defined NW networks were assembled by a modified Langmuir−Blodgett (LB) technique. The assembly process was operated in a LB trough (Nima Technology, 312D) at room temperature using Millipore Milli-Q water (resistivity 18.2 MΩ cm) as subphase. The prepared W18O49 NW solution (2 mL; 0.0004 g/mL) and Ag NW solution (0.1 mL, 0.15 mL, 0.2 mL, 0.25 mL, 0.3 mL; 0.014 g/mL) were centrifuged and washed in absolute ethanol for several times. Then W18O49 and Ag NWs were separately dispersed into a solution of 0.75 mL N,Ndimethylformamide (DMF) and then 0.5 mL of trichloromethane (CHCl3) was separately added to the two kinds of NW suspension. After that, Ag NW suspension was added into the W18O49 NW suspension. The mixed NW suspension was added onto the water subphase drop by drop using a syringe. Then, the NWs were compressed with a compression rate of 20 cm2·min−1 until a fold formation that parallels the barrier direction. By depositing two layers of the aligned NWs with crossing angles, the transparent electrochromic films composed of NW networks were obtained. To further control the electrochromic property of the films, different layers of W18O49 NWs assembled by LB technique were transferred onto the Ag/W18O49 NW networks with the same process. Device Characterization. Scanning electron microscopy (SEM) was carried out with a field emission scanning electron microanalyzer (Zeiss Supra 40 scanning electron microscope at an acceleration voltage of 5 kV). The X-ray diffraction patterns (XRD) were measured on a Philips X’Pert Pro Super X-ray diffractometer equipped with graphite-monochromatized Cu KR radiation. UV−vis spectra were recorded on UV-2501PC/ 2550 at room temperature (Shimadzu Corporation, Japan). The electrical resistance and mechanical stability of the devices were tested by a Keithley 4200 SCS and mechanical system (Instron 5565A). Electrochemical measurements were carried out using a three electrode system on an IM6ex electrochemical workstation (Zahner, Germany).

Figure 1. (a) Schematic illustration for the fabrication of flexible transparent smart windows. (b) SEM image of the coassembled Ag/ W18O49 NWs networks on the polyethylene terephthalate (PET) substrate. The mass ratio (mAg:mW18O49) is 14:4. (c) SEM image of eight layers W18O49 NWs assembled on the Ag/W18O49 NW films. (d) Photograph of assembled W18O49 NW monolayer on the water−air interface on a large-scale LB trough used for the fabrication of NW assemblies. (e) Photograph of flexible transparent electrochromic film composed of two layers of Ag/W18O49 NW networks with 8 layers of assembled W18O49 NW films. (f) Electrochromic switching behaviors of the electrochromic film for 6 different places as shown in (e). The insets are typical photographs of the film in bleached (o V) and colored (−1 V) state.

fabricated by coassembly of Ag and W18O49 NWs using the LB technique, of which the Ag NWs were used as conductive components and the W18O49 NWs played a role as nanoscale spacers to separate the Ag NWs with tunable distances. Transferring the films from the LB trough to the PET substrate twice, the Ag/W18O49 NW networks with interlocking model can be fabricated as transparent electrodes. Figure 1b shows uniform distributions of two kinds of NWs in the Ag/W18O49 NW networks. By increasing the mass ratio (4:14, 21, 28, 35, 42) between W18O49 and Ag NWs (mW18O49:mAg), the density of the well-aligned Ag NWs will increase (Figure S3). At the same time, the transmittance at the wavelength of 550 nm which is the average wavelength of visible light and the most sensitive wavelength for human beings can be tuned from about 86% to 80%, 75%, 69%, 60% with the corresponding sheet resistance changing from about 40 Ω/sq to 25 Ω/sq, 17 Ω/sq, 13 Ω/sq, 7 Ω/sq, respectively (Figure S4). W18O49 as the most commonly used inorganic electrochromic material has been widely studied for the applications of smart windows.33−37 Herein, the highly transparent form of W18O49 NW networks in the assembly not

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RESULTS AND DISCUSSION Manipulating the Assembly of Ag and W18O49 NWs for Flexible Transparent Electrochromic Film. PVP as surfactant has been reported to play an important role in selfassembly processes.30−32 Herein, Ag and W18O49 NWs 9922

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Figure 2. (a) Schematic illustration of the curved Ag and W18O49 NW film with electrochromism property. (b,c) Photographs of the as-prepared film attached on the curved surface of the beaker prior and after applying an external voltage, respectively. (d) In situ electrical resistance change of flexible electrochromic film after 0, 100, 200, 300, 500, and 1000 bending cycles. (e) Electrochromic switching behaviors of the flexible electrochromic film after 0, 100, 200, 300, 400, 500, and 1000 bending cycles. The film is composed of eight layers of W18O49 NW on top of two layers of Ag/W18O49 NW networks and mAg:mW18O49is 14:4.

conductive and electrochromic devices, we fabricated samples into long strip shapes for the bending test. As shown in Figure 2d, the long strip shape film device (L = 6 cm) was bent to a radius of 1.2 cm and the in situ electrical resistance change during the bending test was recorded by interconnecting the mechanical system (Instron 5565A) with a Keithley 4200 SCS. As a contrast, the resistance change of the first 50 cycles for both the as-prepared sample and ITO-PET film (purchased from Zhuhai Kaivo Optoelectronic Technology Co., Ltd.) were recorded. The resistance increased by about 0.45 Ω from 36.15 Ω to 36.6 Ω after the film device was bent to a radius of 1.2 cm and there was almost no change after it was released. Even after 1000 bending cycles, resistance variation of the film device was just about 3 Ω (ΔR/R ≈ 8.3%) (Figure 2d) indicating the excellent mechanical stability under bending stress. While for the ITO-PET film, the resistance reached about 6.56 MΩ (bent state) from 200 Ω (initial state) when it was bent and eventually was stable at about 30 KΩ (released state) after 50 bending cycles (Figure S1). Besides, the in situ switching behaviors of the film device after 0, 100, 200, 300, 400, 500, and 1000 bending cycles were also tested as shown in Figure 2e. The results show that the ΔT (transmittance contrast between the coloration (−1 V) and bleached (0 V) state) remains more than 90% (from ∼35% to ∼32%) even after 1000 bending cycles. Adjustment of Electrochromic Performance. To further control the electrochromic property of the films, different layers

only contributes to the fabrication of transparent conductive electrodes but also enables the films to own electrochromic property. Transferring more layers of the assembled W18O49 NWs (Figure 1c) onto the Ag/W18O49 NW networks will enhance the electrochromic property of the films. With a largescale LB trough (80 × 40 cm2), we have overcome the size limit of the traditional LB technique and fabricated a electrochromic film with dimension of 20 cm × 16 cm as shown in Figure 1d,e. As is well-known, the LB technique has been widely used for the assembly of nanomaterials with uniform monolayer structure. Figure 1f shows the electrochromic switching behaviors of 6 different places of the large-scale film, indicating much higher uniformity of the film when compared with the commonly used spray-coating method (Figure S5). Mechanical Stability of the Electrochromic Film. Compared with the rigid ITO materials, the flexible nature of Ag and W18O49 NWs has enabled the film to be flexible devices with both conductivity and electrochromism (Figure 2a). Figure 2b,c shows that the assembled NW film with up to 15 cm × 15 cm can be attached on the curved surface of the beaker, which shows obvious and uniform blue color when applied with a negative voltage. As shown in the insets of Figure 2b,c, the transparent beaker changed to semitransparent with uniform blue color when the color of the NW film changed, indicating the flexible NW films can be potentially used as flexible smart windows. To further evaluate the mechanical flexibility of the 9923

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study the switching stability of the film devices, 100 cycles of electrochromic switching for the electrochromic films with different layers of W18O49 NWs were carried out (Figure S8), and Figure 3e shows the average values of ΔT after 100 cycles of electrochromic switching, indicating that the ΔT became more stable with the increasing of W18O49 NW layers, especially when increasing the number of W18O49 NW layers to more than ten. Figure 3f,g shows the typical electrochromic switching of the film device with 15 layers of W18O49 NW for 100 switching cycles, indicating that there is almost no change of ΔT (more than 95% contrast retention). Moreover, the electrochromic film is very stable and ΔT of the film shows a nearly constant even after 1000 switching cycles for 7 h (Figure S9). Besides, when increased the W18O49 NWs to 20 layers, the transmittance of the film was lower than 5% at colored state, which can be applied as smart windows to fully isolate the line of sight from the outside world. According to the in situ switching behaviors of the films, the coloration and bleaching time (time reaching 90% of the full response) was recorded around 1.3 and 1.4 s for the pure Ag/W18O49 NW networks and with the W18O49 NW layers increased, the response time changed accordingly from 3.5 and 3.3 s to 9 and 12.3 s (Figure S10a−g), which were consistent with our previous report.43 The fast switching time should be attributed to the good contact between Ag and W18O49 NWs. When the thickness of W18O49 NW films was increased from 20 to 30 layers, the transmittance of the film becomes less than 60% at the wavelength of 632.5 nm, and the film with 50 layers of W18O49 NWs even cannot return back to the bleached state at one bias cycle, of which the coloration and bleaching time increase to about 12 and 18 s (Figure S10h,i). With sufficient time, the film with 50 layers of W18O49 NWs will return back to the bleached state, and when applied with different positive bias of 0.1, 0.2, 0.3, and 0.4 V, the response time of the film decreased from 480 to 320, 250, and 160 s, respectively (Figure S11). Besides, Ag/W18O49 NW networks with different densities of Ag NWs and 10 layers of W18O49 NWs on the top were also fabricated. It is found that the NW films show better response time with the decrease of sheet resistance. When the sheet resistance of the film decreased from 104 to 44, 22, 13, and 8 Ω/ sq, respectively, the coloration/bleaching time decreased from 12.6 s/16.9 s, to 6.1 s/12.9 s, 4.8 s/11.4 s, 4.5 s/9.5, and 4.1 s/8.5 s (Figure S12). When only composed of 2, 5, 8, and 10 layers of Ag/W18O49 NW networks (mAg:mW18O49 is 14:4) (Figure S13a), the sheet resistances of the films significantly reduced from 46.7 to 7.8, 3.2, and 1.9 Ω/sq. At the same time, the transmittance at 632.5 nm of these devices significantly reduced from 85% to 69%, 55%, and 30% at the bleached state. The coloration/bleaching time of the 2, 5, 8, and 10 layers of NW networks increased from 1.3 s/1.4 s to 1.46 s/2.29 s, 3.03 s/5.68 s, and 3.92 s/ 7.78 s, respectively (Figure S13b). It is well-known that the Ag NWs are not chemically stable without protection and will transform to the more stable nanoparticles as they are easily oxidized.13,44 Herein, the same phenomenon (Figure S14) was also found after 100 cycles of electrochromic switching for the Ag/W18O49 NW networks as Ag nanowire would be oxidized by LiClO4 which is a strong oxidizing agent, which was the main reason for the loss of transmittance at the bleached state.13 While increasing the W18O49 NW layers on the Ag/W18O49 NW networks, the loss of transmittance became less (Figure 3d,e and Figure S8),

of W18O49 NWs can be well assembled onto the Ag/W18O49 NW networks with the 2D stacking architectures. Compared with the previous Ag-based electrochromic devices which usually require several steps for conductive electrode fabrication, the strategy introduced here has simplified the operating steps and brought the best of both NW materials.38−40 Besides, the solution-based process under ambient conditions makes the fabrication of the electrochromic devices much easier and flexible.14 In the previous reports,36,41,42 different transmittances of the smart windows or electronic displays needed in various environments were mostly realized by applying various voltages, meaning that a potential transformer was needed during applications. The flexible NW assembly device with controlled layers of W18O49 NWs can be fabricated as shown in Figure 3a,

Figure 3. (a) Schematic illustration of the flexible film devices with controlled layers of W18O49 NWs. (b,c) Optical graphs of the asprepared film device prior and after applied with external voltage. (d,e) Electrochromic switching behaviors of the as-prepared films with different layers of W18O49 NWs monitored at 632.5 nm at a voltage of 0 V for 20 s and −1 V for 20 s for cycles. (f,g) Electrochromic switching for 4000 s of the as-prepared NW film device with 15 layers of W18O49 NWs and the enlarged plots of the first 6 cycles and last 6 cycles. mAg:mW18O49 is 14:4.

which can show different transparence at one film when applied with a constant negative voltage. When external voltage is switched off (0 V), the flexible devices with different W18O49 NW layers (from 0 to 10) showed almost the same transparency (Figure 3b), indicating that the W18O49 NWs have almost no influence on the film devices. However, when a negative voltage of −1 V is applied, the influence of different layers of W18O49 NWs would be displayed as shown in Figure 3c, indicating that more layers of W18O49 NWs will induce the deeper color and lower optical transmittance of the film. Figure 3d shows the electrochromic switching behaviors of the as-prepared films with different W18O49 NW layers monitored at the wavelength of 632.5 nm where the highest contrast can be obtained. Each cycle contains an applied voltage of 0 V for 20 s and −1 V for another 20 s. The ΔT can be tuned from 5.5% to 18%, 22%, 28%, 35%, 56%, 68% at the wavelength of 632.5 nm corresponding to the as-prepared films with 0, 3, 5, 8, 10, 15, 20 layers of W18O49 NWs, while the transmittance spectra were shown in Figure S6. The coloration efficiency of films with different layers W18O49 NWs were calculated to be about from 32.67 to 35.7 cm2/C as shown in Figure S7. To 9924

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Journal of the American Chemical Society indicating the coverage of W18O49 NWs on the Ag NWs played a role in protecting the Ag NWs. Fabrication of Electrochromic Pixels and Solid Devices for Smart Windows. The flexible and implantable electrochromic devices have wide applications for future displays. The display panels with different pixel displays can be used for static or dynamic images. For example, flexible electrochromic film devices with display pixels and another one with characters of “NANO WIRE” were fabricated with the assistance of masks as shown in Figure 4a. The example images of the flexible

Figure 5. (a) Structural schematic diagram of solid electrochromic device. (b,c) Photographs of the bleached and colored state of solid electrochromic devices at bending state. The insets are photographs of the devices without bending. (d) Photographs of the electrochromic glasses model. (e) Photographs of the electrochromic window model.

window shows obvious color and transmittance change (Figure 5e) when compared with the original state shown in the inset of Figure 5e. Furthermore, large area electrochromic device with dimension of 18 cm × 15 cm can also be fabricated as shown in Figure 6. Although only prototypes are fabricated at present, it has demonstrated the possibility for scale-up fabrication of flexible displays, smart windows, or electronic displays.

Figure 4. (a) Schematic illustration for the fabrication of electrochromic films with display pixels and characters of “NANO WIRE”. (b−d) Photographs of the film with display pixels in the bleached state, colored state, and bent state. (e−g) Photographs of the film with characters of “NANO WIRE” that in bleached state, colored state, and while being bent manually. The film is composed of eight layers of W18O49 NWs on top of two layers of Ag/W18O49 NW networks and mAg:mW18O49 is 14:4.

electrochromic devices with bleached and colored state are shown from Figure 4b−g. These patterned film devices had quick response to the coloration state when an external voltage was applied even during the bending process (SI Video 1) indicating the excellent electromechanical stability of NW film devices. The solid electrochromic device can be potentially applied in various fields with special demands especially in flexibility and portability. For instance, we fabricated a flexible solid electrochromic device using the electrochromic films for the applications in portable electrochromic glasses and smart windows. Figure 5a shows the structural schematic diagram of the solid electrochromic device of which the Ag/W18O49 NW film and ITO-PET film were used as the work and counter electrode, 0.5 M LiClO4/PC (propylene carbonate) with 10 wt % PMMA (poly(methyl methacrylate)) as stiffer was used as solid electrolyte.45,46 The flexibility of the representative electrochromic windows at bleached and colored states is shown in Figure 5b,c and it still maintains the functionality of quick response to electric signal even at bending state. A model of electrochromic eyeglass was assembled as shown in Figure 5d and the colored state can be obviously observed when a negative voltage is applied. We also installed the solid electrochromic device on a glass window model. The colored state of the smart

Figure 6. Photograph of the as-prepared solid electrochromic device with about 18 cm × 15 cm.

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CONCLUSION In summary, we present a new route to fabricate flexible Ag/ W18O49 NW films as both transparent electrodes and electrochromic devices with tunable resistances (7−40 Ω/sq) and transmittance (58−86% at 550 nm). Compared with previous flexible electrochromic devices on ITO-PET substrate, our flexible NW architectures maintain their functionalities, both conductivity and electrochromism after 1000 bending cycles. The flexible solid electrochromic windows constructed based on the well-defined assembled Ag/W18O49 NWs may find wide applications in solar cell, smart windows, displays, and other next-generation flexible devices. Furthermore, the present route for large area coassembly of NWs is expected to be extended to design various NW-based flexible devices in the future. 9925

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b03227. Electrical resistance change of the ITO-PET film; SEM images; XRD patterns; optical transmittance spectra; electrochromic switching behavior; coloration and bleaching time; sheet resistance and transmittance (PDF) Response to the coloration state when an external voltage is applied (AVI)

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AUTHOR INFORMATION

Corresponding Authors

*[email protected] *[email protected] ORCID

Shu-Hong Yu: 0000-0003-3732-1011 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the funding support from the National Natural Science Foundation of China (Grants 21431006, 21761132008, 51471157, 21401183), the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (Grant 21521001), Key Research Program of Frontier Sciences, CAS (Grant QYZDJ-SSW-SLH036), the National Basic Research Program of China (Grant 2014CB931800), and the Users with Excellence and Scientific Research Grant of Hefei Science Center of CAS (2015HSC-UE007), the Youth Innovation Promotion Association of CAS (2014298), and the Anhui Provincial Natural Science Foundation (1508085QB28).

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DOI: 10.1021/jacs.7b03227 J. Am. Chem. Soc. 2017, 139, 9921−9926