Rapid Fabrication of Noniridescent Structural Color Coatings with

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Rapid Fabrication of Non-Iridescent Structural Color Coatings with High Color Visibility, Good Structural Stability and Self-healing Properties Fantao Meng, Malik Muhammad Umair, Kashif Iqbal, Xin Jin, Shufen Zhang, and Bingtao Tang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b01522 • Publication Date (Web): 18 Mar 2019 Downloaded from http://pubs.acs.org on March 18, 2019

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ACS Applied Materials & Interfaces

Rapid Fabrication of Non-Iridescent Structural Color Coatings with High Color Visibility, Good Structural Stability and Self-healing Properties Fantao Meng, † Malik Muhammad Umair, † Kashif Iqbal, § Xin Jin, ‡ Shufen Zhang, † and Bingtao Tang*, †, ‡

†State

Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian

116023, P. R. China ‡Eco-chemical

Engineering Cooperative Innovation Center of Shandong, Qingdao

University of Science and Technology, Qingdao 266042, P.R. China §Textile

Processing Department, National Textile University, Faisalabad, Pakistan

E-mail: [email protected]; Tel: +86-411-84986267

KEYWORDS: amorphous photonic structures, spray coating, color visibility, structural stability, textile coloring

ABSTRACT: Artificial construction of amorphous photonic structures (APSs) is an important approach for obtaining non-iridescent structural colors and shows a great potential for practical application in paints, textile coloring, display or other color-related 1 ACS Paragon Plus Environment

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fields. However, the structural colors are usually dim due to the influence of incoherent scattering, and the point contact among the microspheres leads to poor structural stability. This paper presents an innovative strategy for the constructing non-iridescent structural color coatings with high color visibility, good structural stability and self-healing properties by combining APSs with polymers. Color visibility is significantly improved without the addition of black light-absorbing substances due to the inherent properties of polysulfide microspheres (PSFMs). At the same time, the introduction of waterborne polyurea (WPU) in the system enhanced the structural stability and imparted the self-healing properties. The prepared coatings can be applied to various substrates and even to the coloration of soft fabrics, which not only achieves excellent performance but can also be easily patterned on bulk scale.

INTRODUCTION

In nature, structural color belongs to the class of physical colors which are generated from the interaction of visible light with micro-/nanostructure, such as scattering, interference or diffraction.1-6 In particular, the non-iridescent structural colors with unique properties are exhibited by amorphous photonic structures (APSs) with characteristic sizes on the order of the wavelength of visible light.7-11 Among the currently reported methods for artificially constructing the APSs, spraying colloidal nanoparticles is simple, rapid and well controlled process to achieve large-area assembly on flat and curved 2 ACS Paragon Plus Environment

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surfaces.12-18 However, color visibility and structural stability are still two core issues that need to be solved for the practical application of the APSs.

The coherent scatter of the characteristic wavelength is not prominent because the scattering effect of the amorphous structure on light is relatively strong, which may result in a large drop in color saturation and produce whitish color or no structural color.19 Therefore, the strategy of doping of APSs with the black light-absorbing substances14-22 is usually espoused to improve the color saturation because of low color visibility, which undoubtedly increases the difficulty and complexity of the self-assembly process. In addition, the red color generated by following above strategy is not real due to the incoherent scattering.16, 23-24 These limitations can be tackled by using the microspheres with optical absorption properties25 for improving the color visibility whilst obtaining an amorphous structure.

Another challenge in the construction of APSs is the point contact among the microspheres

resulting

in

the

poor

structural

stability.

Polyacrylate

(PA),17

Poly-dopamine (PDA),13 poly (vinyl alcohol) (PVA)20 and polyurethane (PU) precursor26 have been added as binders to improve mechanical stability. Compared with the above materials, polyurea adhesives provide a strong adhesion with different application substrates due to the high internal cohesive energy, strong polar groups, and the ability to form intramolecular and intermolecular hydrogen bonds.27 Interestingly, polyurea also 3 ACS Paragon Plus Environment


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offers large number of urea groups in different molecular chains, through which quadruple hydrogen bonds shaped like double-teeth can be formed to construct a physical cross-linking network for inducing the self-healing properties. 28-30

Different from simple construction of ordered photonic crystals (PCs) 25, this paper aims to solve two core problems of color visibility and structural stability of APSs in practical applications. We synthesized waterborne polyurea (WPU) and introduced it into the amorphous structure of polysulfide microspheres (PSFMs) to fabricate a new non-iridescent structural color coating with high color visibility, good structural stability and self-healing properties. Significant improvement in the color visibility of APS coatings can be achieved by one step assembly, since the intrinsic characteristics of PSFMs avoid the doping of black light-absorbing materials or any modification. As a high-efficiency adhesive, WPU can be filled in the gaps among the microspheres to achieve structural locking after the assembly. Therefore, the APS coatings could be endowed with good mechanical stability on rigid and soft substrates, such as glass and textiles. Furthermore, a protective film formed by respraying the WPU, imparts self-healing properties to the coating while enhancing the structural stability. This system is efficient, eco-friendly and feasible to achieve excellent performance for bulk-scale application of APSs in the paints, textile coloring, display, sensing, separation and other color-related fields.31-32 4 ACS Paragon Plus Environment

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RESULTS AND DISCUSSION

The emulsion of PSFM and WPU was injected into an airbrush and then air-sprayed onto a preheated substrate under a certain pressure to facilely form the uniform APS coatings, as schematically shown in Figure 1.

Figure 1. Schematic of the facile formation of APS coatings on various substrates by spray coating.

According to an appropriate weight ratio, PSFMs and WPU were systematically mixed to form a homogenous emulsion. An airbrush with a nozzle size of 0.2 mm pushed the mixed emulsion to generate foggy vapor under a pressure of 50 kPa onto preheated substrates (such as rigid glass slides placed on a hot plate at 50 °C). After the rapid volatilization of solvent under the mild conditions, PSFMs were disorderly assembled and evenly coated onto the substrate to fabricate an amorphous structure. The compact and homogenous APS coatings were continuously covered via line-by-line movement of the airbrush at a relatively low speed. Moreover, the thickness can easily be controlled by 5 ACS Paragon Plus Environment


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adjusting the spraying amount, and the color of the coatings can be modulated by tailoring the diameters of PSFMs. Multiple colors output on various substrates can be achieved through a mask process.

APS coatings with eight non-iridescent structural colors were formed on the glass substrates by spraying the mixed emulsion containing WPU (8 wt %) and PSFMs with various diameters of 188, 198, 213, 225, 230, 252, 261 and 273 nm (Figure 2a). The corresponding reflective spectra (Figure 2b) of the eight APS coatings manifested that the peak positions are consistent with the observed color. The reflection peaks of the red-colored sample exhibited almost no change upon varying the detection angle from 10° to 50° (Figure 2c), which indicated the non-iridescent character of APSs color. The scanning electron microscope (SEM) image and its two-dimensional fast Fourier transform (2D-FFT) showed only the short-range ordered amorphous structure of the APS coating (Figure 2d and inset of Figure 2d). The previously reported strategies involved fabrication of APSs

14-21

by colloidal

microspheres with no optical absorption properties, and required addition of black light-absorbing substances to enhance the color saturation. However, this strategy can critically influence some optical properties, such as brightness, and increase the difficulty as well as the complexity of the self-assembly process. Moreover, the red color is difficult to achieve by using above microspheres because the observed colors of APS coatings 6 ACS Paragon Plus Environment

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originate from the constructive interference and incoherent scattering.16,33 In our work, the obtained PSF coatings displayed high color visibility without addition of black light-absorbing substances, and the red color was relatively obvious because the optical absorption properties and high refractive index (as high as 1.858) of PSFMs

25

can

significantly reduce the interferences of incoherent scattered light. Noticeably, the APS coating of PSF exhibited high color visibility compared with the SiO2 coating, demonstrated by the image of the red-colored samples and the corresponding reflection and absorption spectra (Figure 2e and Figure S1, Supporting Information). In addition, the color of the SiO2 coating became whiter and brighter with increasing thickness of coating.

According to the corresponding energy dispersive spectroscopy (EDS) mapping image of nitrogen and the other three elements (Figure 2f and Figure S2, Supporting Information), WPU was uniformly distributed in the gaps among the PSFMs because of its small average diameter of approximately 47 nm (Table S1, Supporting Information). The increase in addition ratio of WPU from 2–10 wt % almost had no effect on the color of the red-colored samples due to the intrinsic characteristics of PSFMs, although the effective refractive index of the APS coating slightly increased after WPU spread in the gaps (Figure S3, Supporting Information). APS coatings with a uniform thickness of approximately 7–8 μm were obtained (Figure S4, Supporting Information). The change in 7 ACS Paragon Plus Environment


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thickness did not influence the visibility of red-colored samples in the presence of PSFMs as demonstrated in Figure S5 in the Supporting Information.

Figure 2. (a) Images of eight APS coatings containing PSFMs of different particle sizes and WPU on glass slides under 45° and 90° observation. The weight ratio of WPU and PSFMs was 8 wt %. (b) Reflection spectra of eight samples. (c) Scattering spectra of the sample (d-261) in (a) by simulating diffuse reflection with different detection angle (10°– 50°). Inset is the schematic diagram of diffuse reflection mode. (d) SEM image of the red-colored sample (d-261) in (a). Inset is the corresponding 2D-FFT spectra. (e) Comparison of red-colored PSF and SiO2 samples. (f) Comparison of N elemental

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mapping for cross-section SEM images of red-colored samples (d-261) with and without the addition of WPU showing the distribution of WPU among the microspheres.

It is worth mentioning that WPU is an important type of high-viscosity adhesive, containing water as a dispersing agent making it suitable for eco-friendly applications. The advantages of WPU film are numerous such as excellent wear resistance, physical and mechanical properties, water resistance, solvent resistance and good flexibility. These favorable properties make the WPU an ideal functional material for textiles, shoes, automobiles, and packaging and construction applications.

In present study, WPU with a small diameter and low glass transition temperature was prepared via the reaction of PEA2000, IPDI, PEA230 and AAS (Figure 3a). Synthesized WPU is a typical multiblock copolymer with microphase-separated hard and soft segments, and with highly polar urea groups in the molecular structure. Fourier transform infrared (FTIR) spectra (Figure 3b) showed the absence of characteristic peak of isocyanate around 2270 cm-1, and the appearance of the characteristic peak corresponding to the urea group around 1630 cm-1 in the synthesized WPU. The molecular chain of WPU has a strong ability for diffusion movement, and the glass transition temperature of the soft segment is approximately −58 C, as defined by the temperature corresponding to the peak of the loss modulus in the dynamic thermodynamic analysis (DMA) diagram (Figure 3c and Figure S6, Supporting Information). In our work, the lower glass 9 ACS Paragon Plus Environment


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transition temperature of WPU supported the film formation at relatively lower temperature and improved the adherence of adjacent microspheres after the completion of assembly process.

To evaluate mechanical stability, the red-colored samples were rubbed on the sandpaper by applying an external force. After the friction test, the surface of the sample (containing 8 wt % WPU) was able to maintain relatively good integrity, and the color was also retained due to the locking effect of WPU (Figure 3d), so we choose 8 wt % as the optimized weight ratio of WPU and PSFMs.

Figure 3. (a) The schematic route to synthesize WPU. (b) FTIR spectra of IPDI and WPU. (c) DMA diagram of loss modules for WPU sample. (d) Images of red-colored 10 ACS Paragon Plus Environment

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APS coatings with different weight ratio of WPU and PSFMs on glass slides after friction test.

As shown in Figure 4a, WPU emulsion can be sprayed on the prepared APS coating to form a transparent protective film of WPU, which not only enhanced the structural stability but also imparted the self-healing properties. Figure 4b shows five different APS coatings on glass after respraying the WPU emulsion, which is demonstrated by the corresponding reflective spectra in Figure S8 in the Supporting Information. In addition, the surface and the cross sectional SEM images of the red-colored sample showed the APS coating covered with a thin layer of WPU film (Figure S9, Supporting Information). The protective WPU film is almost transparent as presented by the comparison of transmission spectra of the glass with or without WPU film (Figure 4c).

One of the main ways to self-heal based on reversible bonds is by external triggering, or internally induced by making the edges of the damaged interfaces close. The self-healing effect of the WPU film was validated by following procedure. Firstly, the WPU film was rubbed with sandpaper. The sample was dabbed with several drops of water on its damaged surface34 as wetting enhances the creep tendency of WPU molecular chains. Next, the sample was heated to evaporate the water for quick and intuitive visualization of healing effects. Consequently, the multiple hydrogen bonds between the different macromolecular chains of WPU rebuilt along the damaged interface 11 ACS Paragon Plus Environment


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and formed a physically crosslinked network. We reasoned that the self-healing behavior may be attributed to the formation of hydrogen bonds by intentionally wetting the damaged site to accelerate the movement of the molecular chains along the damage interface as schematically shown in Figure 4f. Stimulation of water is necessary during this process, and no healing was observed in a dry environment when the network was scratched. The marks at the interfaces almost disappeared when the water completely evaporated after 30 min at 70 C (Figure 4d). Similarly, slight scratches on the APS coating covered with the WPU film healed under the conditions described above (Figure 4e).

Figure 4. (a) Schematic diagram of preparation of APS coatings covered by WPU film. (b) Images of five APS coatings on glass slides after respraying the WPU. (c) Transmission spectra of glass with and without covered WPU film. (d) Photos of the


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WPU film before and after self-healing. (e) Photos of the APS coating covered by WPU film before and after self-healing. (f) The schematic self-healing mechanism based on hydrogen bonding of urea groups.

The APS coating of high color visibility and good mechanical stability can be applied to the coloration of fabrics by the spraying method. As the dyeing may cause environmental pollution, and the color may fade due to the external factors. Therefore, the spraying PSFMs on the fabrics using water as a solvent, makes this strategy far more economical and eco-friendly. It is also simple, well-controlled and feasible process for large scale application. Figure 5a shows the complex pattern sprayed with four different particle sizes of PSFMs and WPU over a wide area of silk, in which the four colors are distinct and contrasting. To demonstrate the application scope of this strategy on other types of substrates, a pattern of five-color school logo is assembled on a steel plate (Figure 5b), and the Chinese calligraphy patterns of six colors is created on the PVC film (Figure 5c).

Three types of Chinese calligraphy patterns (without WPU, with WPU, with WPU/WPU film) (Figure 5d) were fabricated to test the fastness of APSs on silk fabric, the microstructure of which were shown in the SEM images (Figure 5e). The surface morphology of the fabrics after spraying was not destroyed, and the two-directional structure of warp and weft on the silk surface was still clearly observed. At high 13 ACS Paragon Plus Environment


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magnification, the APS coating was found to be uniformly covered on the single fiber, in which PSFMs were arranged in an amorphous state, and WPU was distributed in the gaps. After respraying the WPU emulsion on the pattern, the fiber surface became relatively smooth due to the coverage of the WPU film, as demonstrated by the comparison of SEM images at different magnifications (Figure 5e). The coating with WPU still maintained relatively good integrity after the friction test, and the color also remained unimpaired, as confirmed by the images of the patterns (Figure 5d) and the corresponding standard cotton cloth (Figure S10, Supporting Information). Simulating the washing process by stirring at 1000 r/min for 15 min, the APSs with WPU on the fabric were almost intact after washing, and no obvious color fading was observed. However, after the friction and simulated washing tests, the structure of APSs without WPU underwent a noticeable deterioration, and the color changed greatly. The presence of WPU film further enhanced the structural stability of the Chinese calligraphy patterned on the silk as presented by the corresponding images (Figure 5d and Figure S10, Supporting Information). The friction and simulated washing tests results showed the impressive performance of the APS coatings on silk, and further affirmed the potential of the employed spraying strategy for practical applications.


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Figure 5. (a) Complex pattern sprayed with four different particle sizes of PSFMs and WPU on silk. (b) School logo sprayed with five different particle sizes of PSFMs and WPU on a steel plate. (c) Chinese calligraphy pattern of school name sprayed with six different particle sizes of PSFMs and WPU on PVC film. (d) SEM images of three types of red APS coatings (without WPU, with WPU, with WPU/WPU film) on silk under different magnifications. Comparison of insets confirms the distribution of WPU among the PSFMs. (e) Images of three types of red-colored Chinese calligraphy patterns 15 ACS Paragon Plus Environment


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(without WPU, with WPU, with WPU/WPU film) on silk before (untreated) and after friction and simulated washing tests. The weight ratio of WPU and PSFMs was all 8 wt %.

CONCLUSION

In summary, we fabricated the non-iridescent structural color coatings with high color visibility and good structural stability by spraying the PSFMs in combination with the WPU. According to the results, PSFMs possessed inherent optical absorption properties and high refractive index, which significantly improved the color visibility of APS coatings without addition of black light-absorbing substances. WPU as a binder improved the structural stability of APS coating on the various substrates, such as glass or even flexible fabrics. In addition, a protective film of WPU formed on a prepared APS coating, not only enhanced the structural stability but also imparted the self-healing properties. A rapid patterning on various substrates can be achieved by a simple masking process. This system is facile, economic and eco-friendly, and has potential applications in paints, textile coloring and other color-related fields.

EXPERIMENTAL SECTION

Materials. Absolute ethanol was purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). 1,2,3-trichloropropane （TCP ）(AR, 98 %) was bought from TCI 16 ACS Paragon Plus Environment

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(Shanghai) Chemical Industry Development Co., Ltd. Pluronic F127 was provided by Sigma Aldrich. Sublimed sulfur (AR, ≥ 99.5 %) was purchased from Tianjin Tianli Chemical Reagent Co., Ltd. Poly(propylene glycol) bis (2-aminopropyl ether) (PEA) (Mn 2000 and 230) and isophorone diisocyanate (IPDI) were purchased from Aladdin Chemistry Co., Ltd. (Shanghai, China). Sodium hydroxide was bought from Tianjin Dongli District Tianda Chemical Reagent Factory. Acetone (AR) was provided by Tianjin Komiou Chemical Reagent Co., Ltd. Sodium 2-((2-aminoethyl) amino) ethanesulfonate was bought from Shanghai Haohong Biomedical Technology Co., Ltd. All reagents were used without further purification.

Preparation of Monodispersed PSFM emulsion. As shown in the synthetic schematic (Figure S11, Supporting Information), sodium hydroxide and sublimed sulfur were orderly added to distilled water (500 mL) according to the stoichiometric ratio, and then the mixture was heated to boiling point and stirred in a water bath for 1 h to obtain a Na2S2 solution. The Na2S2 solution (50 mL), mixture of deionized water and ethanol (250 mL) were proportionally poured in a 500 mL three-necked flask under magnetic stirring at a particular temperature. The reaction solution was continuously stirred for 6 h to obtain the PSFM emulsion after Pluronic F127 (0.5 g) and TCP (1.25 g) were orderly added. The emulsion of PSF colloidal spheres with eight different diameters were



obtained by adjusting the temperature and the amount of ethanol (Table S1 and Figure S12, Supporting Information).

Preparation of WPU emulsion. PEA 2000 (4 g) and acetone (4.5 g) were mixed in a 250 mL four-necked flask under mechanical stirring at room temperature with a N2 atmosphere. IPDI (2.22 g), PEA 230 (1.265 g) and AAS (0.95 g) were orderly added to the reactive system at intervals of 5 min. Deionized water was added to the system to disperse the emulsion after 10 min. Finally, the acetone was removed via distillation under reduced pressure to obtain the emulsion of WPU with small diameters.

Spraying process for preparing APS coatings. The concentration of PSFM and WPU emulsion was adjusted to 10 wt % in deionized water. The mixture prepared according to a certain mass ratio was ultrasonically dispersed for 30 min. The concentration of resprayed WPU emulsion was adjusted to 3 wt %. The pressure, distance, temperature of the surface of the substrate, emulsion concentration and solvent characteristics all affect the optical properties of the assembled APSs. When the pressure is too high or the airbrush is closer to the substrate, the thin liquid layer may first form on the substrate’s surface due to incomplete drying, and the microspheres are assembled orderly to exhibit iridescence.

Characterization and property measurement. SEM images were observed by Nova Nano SEM450 scanning electron microscope, which is equipped with an energy 18 ACS Paragon Plus Environment

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dispersive X-ray spectroscopy spectra (EDX, X-Max, Oxford, England). The samples were covered with gold before SEM observation. The average particle size was measured by using a Zetasizer NanoZS-90 measurement system (DLS, Malvern instruments, Malvern, UK). Optical photographs of all samples were taken by a Nikon D7000 digital camera. The reflection spectra irradiated vertically by incident light were measured by EQ2000 spectrometer. The scattering spectra in the simulated diffuse reflection mode were measured by Hitachi U-4100 spectrophotometer. The infrared reflection spectrum was investigated by Nicolet Avatar 320 FTIR spectrometer. Glass transition temperature is obtained by dynamic thermodynamic analysis on a DMA1 (METTLER TOLEDO) Dynamic mechanical analyzer. The sample was cut into a 10.0 mm × 4.8 mm × 0.14 mm rectangular strip. The heating rate was 5°C/min at a frequency of 1Hz.

The friction test to verify the mechanical stability of the APSs on the glass is shown in Figure S7 in the Supporting Information. The APSs coated on the slide were placed facing a 3000-grit sandpaper. Then a 200 g weight was applied to the slide. Afterwards, the slide was dragged straight forward by 10 cm at a speed of 5 cm/s. Simulated washing test of the APSs on silk: The silk coated with the APSs was placed in a 150 mL beaker filled with water and then magnetically stirred at a rate of 1000 r/min for 15 min. Friction test of the APSs on silk：Using a finger wrapped in standard cotton cloth, the silk surface coated with APSs was rubbed back and forth for 15 times in one direction with a vertical 19 ACS Paragon Plus Environment


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downward force of 9 N, according to the ISO 105-X12:2001 standard for textile color fastness test. The area pressed by the finger was approximately a circle of 1.1 cm in diameter, and the force exerted on silk was approximately 94704 Pa.

ASSOCIATED CONTENT

Supporting Information. Supporting Information consists of the reflection and absorption spectra of the red-colored PSF and SiO2 samples, EDS mapping images of the red-colored samples with and without the addition of WPU, images of red-colored APS coatings with different weight ratio of WPU and PSFMs, SEM image of the cross section for the red-colored PSF sample, images of red-colored APS coatings with different thickness, DMA diagram of storage modulus for WPU sample, schematic diagram of friction test, the reflection spectra of five samples covered by WPU film, SEM images of the red-colored PSF sample covered by WPU film, images of standard cotton cloth after friction test, schematic diagram of synthesis of PSFMs, the Zeta-Sizer of the PSFMs and WPU, and SEM images of PSFMs with different diameters.

AUTHOR INFORMATION Corresponding Author Bingtao Tang,* E-mail: [email protected]. Tel: +86-411-84986267

Present Addresses 20 ACS Paragon Plus Environment

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†

State Key Laboratory of Fine Chemicals, Dalian University of Technology, P.O. Box

89, West Campus, 2# Linggong Rd, Dalian 116024, China.

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS

This work was financially supported by the National Natural Science Foundation of China (21878043, 21576039, 21421005, 21536002 and U1608223), Program for Innovative Research Team in University (IRT_13R06), Fundamental Research Funds for the Central Universities (DUT18ZD218).

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