Straightforward Approach to Antifogging, Antireflective, Dual-Function

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Straightforward Approach to Antifogging, Antireflective, DualFunction, Nanostructured Coatings Ying Wang,†,‡,§ Xin Ye,†,∥ Bolin Li,∥,⊥ Junhui He,*,‡ and Wanguo Zheng*,∥

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‡

Functional Nanomaterials Laboratory, Center for Micro/Nanomaterials and Technology and Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Zhongguancundonglu 29, Haidianqu, Beijing 100190, China § University of Chinese Academy of Sciences, Beijing 100049, China ∥ Research Center of Laser Fusion, China Academy of Engineering Physics, Mianyang, Sichuan 621900, China ⊥ Joint Laboratory for Extreme Conditions Matter Properties, Southwest University of Science and Technology, Mianyang, Sichuan 621900, China S Supporting Information *

ABSTRACT: Here, we report a straightforward approach to fabricate antifogging antireflective dual-function nanostructured coatings, where antireflective nanograsses were etched into antifogging polymer coatings by self-masking reactive ion etching (RIE). The transmittance of coatings increases with the etching time, and the maximum transmittance reaches up to 98.9% in 180 s. The effective refractive index of grass-like nanostructure was calculated to be 1.15 and its optical property was simulated via the finite difference time domain (FDTD) model. The antifogging property of polymer coatings remains unchanged after RIE, which results from the hygroscopicity of polymer matrix. This strategy surpasses traditional design concepts of antifogging polymer coatings by combining excellent antireflective and antifogging properties on the same outermost layer, which demonstrates that it is probable to achieve multifunction on a single layer of a single composition.

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INTRODUCTION Antifogging coatings have attracted great interest in the field of scientific research, which could be applied to solve fogging problem on the surface of materials, such as glazed windows, eyeglasses, optical lenses, and so on.1,2 Both superhydrophilic and hygroscopic antifogging coatings are widely considered as two approaches to achieving antifogging property.3−5 According to the Wenzel and Cassie−Baxter models,6,7 when the surface of material is superhydrophilic, water droplets tend to spread and form a transparent water sheet on the surface of material, and the contact angle of water droplet is close to 0°, which eliminates the scattering caused by light shed on water droplets. About hygroscopic antifogging coatings, when the surface of material contains a large number of hydrophilic groups, such as hydroxyl or carboxyl groups, water molecules on the surface can be rapidly adsorbed, and form hydrogen bonds with the material, which exist in the form of bound water molecules in the material, preventing the formation of fog.5,8 Antireflective coatings have enormous prospects in practical applications, which can improve the photoelectric conversion efficiency of solar cells and enhance the clarity and contrast of optical imaging.9,10 According to the Fresnel equation,11,12 the antireflective property can be obtained via tailoring the reflective index and thickness of coatings.13 Varied © XXXX American Chemical Society

methods have been used to fabricate antireflective coatings, including etching,14−16 chemical vapor deposition (CVD),17,18 sol−gel process,19,20 and self-assembly.21,22 The integration of antireflective and antifogging properties on transparent material surfaces has an even broader range of application. Efforts have been dedicated to the preparation of dual-functional coatings with both antifogging and antireflective properties.23−26 Researchers designed different surface nanostructures combing nanotechnology and functional materials,27−30 and some surface engineering strategies, including etching,15 CVD,17 layer-by-layer assembly,31 and nanoimprinting,32 were also used for antifogging and antireflective properties. There have been intensive studies on superhydrophilic antifogging and antireflective coatings. For example, highly porous silica coatings were fabricated via layerby-layer assembly of poly(diallyldimethylammonium chloride) (PDDA)-silicate complexes with poly(acrylic acid) (PAA) followed by calcination.33 Polymer films with controllable surface structure were developed by tailoring the period and height of artificial moth-eye nanostructures followed by depositing a layer of hydrophilic SiO2 nanoparticles.34 Received: July 23, 2019

A

DOI: 10.1021/acs.langmuir.9b02304 Langmuir XXXX, XXX, XXX−XXX

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Figure 1. Schematic illustration of the preparation procedure of antifogging antireflective coatings. The red and yellow arrows in the SEM image point to a nanohole and a nanocrack, respectively. sonication in water for 10 min, the treating time and oxygen flow being set to 5 min and 800 mL·min−1, respectively. Next, an antifogging precursor solution was prepared according to our previous reports.36,37 Then, the cleaned slide glasses were coated by the antifogging precursor solution using a dipping time of 60 s and a withdrawing speed of 50−60 mm/min. Finally, the coatings were subjected to a thermal treatment of 5−15 min. RIE of Antifogging Polymer Coatings. The coatings were etched by oxygen (O2) and argon (Ar) plasma in an RIE system (Plasmalab 80 Plus RIE, Oxford Instrument (China), Shanghai, China). First, the RIE chamber was cleaned by a standard plasma cleaning process. Then, the samples were placed on the sample holder in the RIE chamber. The RIE process was performed using a mixture of O2 (5 SCCM) and Ar (50 SCCM) as the etching gas, with a chamber pressure of 10 Pa and a RF power of 60 W. During the etching process, varied periods of etching time (60, 120, 180, and 240 s) were tried to optimize the antireflective property of coated slide glasses. Characterization of Dual-Functional Coatings. The surface morphology and structure of antifogging polymer coatings before and after the RIE process were characterized by scanning electron microscopy (SEM) on a Hitachi S-4800 field-emission scanning electron microscope at 5 kV. The optical property characterization of samples were carried out on a PerkinElmer Lambda 950 spectrophotometer. The transmittance spectra and reflectance spectra were recorded from 400 to 1100 nm. The refractive index and extinction coefficient were measured by a J.A. Woollam V-VASE spectroscopic ellipsometer. Water contact angle measurements were carried out on a Kino SL200B3 automatic contact angle meter, the angle precision of which is ±0.2°. ATR-FTIR spectra were recorded via a Varian Excalibur 3100 spectrometer.

However, few studies have been done toward hygroscopic antifogging and antireflective coatings. Very recently, He’s group35 reported antifogging and antireflective dual-functional composite thin films. The antireflective function originated from hollow silica nanoparticles with low refractive index at the outmost layer. The antifogging property derived from the formation of hydrogen bonding between water molecules and hydroxyl groups of hygroscopic polymer, which acted as the inner layer. That design represented a breakthrough as compared to traditional polymer coatings, because the realization of antifogging performance requires the antifogging layer as the outmost layer, and so does the realization of antireflective property. However, the integration of hygroscopic antifogging and antireflective properties on transparent material surfaces faces a tougher challenge of compromising these two functions on a single surface by a single composition. To our best knowledge, no single layer has so far been reported to simultaneously acquire both hygroscopic antifogging and antireflective performances. Herein, for the first time, we designed and fabricated a hygroscopic antifogging and antireflective dual-functional polymer coating as a single layer of a single composition via a two-step approach. In the first step, a glass substrate was coated with a polymer solution by dip-coating, and the selected polymer contained sufficient hydroxyl groups or amino groups, which could form hydrogen bonding with water molecules.5,36,37 Then we endowed the polymer coating with a nanograss surface structure via a simple self-masking reactive ion etching (RIE) procedure used previously for quartz substrates.16 The polymer nanograss coating simultaneously demonstrated both excellent antireflective and antifogging properties. This approach represents a new concept and a significant progress toward multifunctional coatings as compared to previous approaches.

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RESULTS AND DISCUSSION Morphology of Coatings. The preparation procedure of dual-functional nanostructured coatings is schematically illustrated in Figure 1. After fabricating a polymer coating by dip-coating, we etched the coating under oxygen (O2) and argon (Ar) in an RIE system. By changing the time of RIE (60, 120, 180, and 240 s), we attained coatings with varied morphologies. The surface morphologies of coatings with and without applying RIE were observed by scanning electron microscopy (SEM). The polymer coating without RIE treatment has a very smooth surface (Figure 1), only containing some nanocracks (as pointed by the yellow arrow) and nanoholes (as pointed by the red arrow) caused by evaporation of water molecules during heat treatment. The coating has a uniform thickness, which was estimated to be about 3.2 μm. The surface

EXPERIMENTAL SECTION

Chemicals. Poly(acrylamide), poly(vinyl alcohol) (PVA, fully hydrolyzed), and poly(acrylic acid) (PAA, 10−30 wt %) were purchased from Aladdin and Beijing Enoch Technology Company, respectively. A Millipore Mill-Q Plus 185 purification system provided deionized (DI) water with a resistivity higher than 18.2 MΩ cm. Slide glasses were attained from Beihua Fine Chemicals, and their size was 7.5 cm × 2.5 cm × 0.1 cm. Fabrication of Antifogging Polymer Coatings. Antifogging polymer coatings were prepared on slide glasses by dip-coating and thermal treatment. First, slide glasses were cleaned by O2 plasma after B

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Figure 2. SEM images of coatings etched for varied periods of time: (a−d) top views of coatings etched for 60 s, 120 s, 180 and 240 s, respectively, (e−h) side views of coatings etched for 60, 120, 180, and 240 s, respectively, (i−l) cross-section views of coatings etched for 60, 120, 180, and 240 s, respectively. Scale bars: 500 nm.

Figure 3. (a) Transmission spectra of coatings: line 0, pristine coating without etching; lines 1−4, coatings etched for 60, 120, 180, and 240 s, respectively. (b) Photographs of coatings etched for 0, 60, 120, 180, and 240 s, respectively.

micromask covering on the substrate by Ar. The chemical etching is attributed to the dissociation of O2 into oxygen radicals followed the formation of volatile etching products of CO, CO2, and H2O, as shown in eq 1.40,41 The micromask by physical sputtering deposition of reactor wall and electrode builds a passivating layer on the sidewall of nanostructures, thus, leading to the etching anisotropy during the RIE process.40

morphology significantly changed after RIE (Figure 2). When the etching time was 60 s, the coating presented dense and small nanodots (Figure 2a,e), and these nanodots were measured to be about 40 nm in average diameter and about 65 nm in average height (Figure 2i). When the etching time increased to 120 s, grass-like nanostructures (Figure 2b,f,g) were obtained, and the diameter and height of nanostructures were estimated as about 72 and 130 nm, respectively, from their cross-section view (Figure 2g). After RIE for 180 and 240 s, the heights of nanograsses became about 241 nm (Figure 2c,g,k) and 334 nm (Figure 2d,h,l), respectively, and their average diameters were nearly unchanged (ca. 87 nm). The height of nanostructures was found to increase linearly with increase of the etching time (Figure S1). Thus, the etching time effects significantly on both the morphology and height of obtained nanostructures. It is also noted that the tips of near nanograsses were inclined to agglomerate as commonly observed for nanowires.38,39 In the O2 and Ar plasma process, the self-organized grass-like nanostructures observed on the polymer coatings arise from chemical etching through O2 and

− O(ion) + Cx HyOz(solid) → CO(gas) + CO2(gas) + H 2O(gas)

(1)

Antireflective Properties of Coatings. The optical properties of polymer coatings before and after RIE were investigated by transmission spectroscopy. In Figure 3 and Table 1, the pristine polymer coating without etching had a maximum transmittance of 90.5% and an average transmittance of 87.6% (400−1100 nm). After etching, the transmittance increased significantly. Figure 3c lists the maximum transmittance and average transmittance in 400− 1100 nm of coatings etched for varied periods of time. The coating by 60 s RIE exhibited good transparency with the C

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schematically shown in Figure 4. The coating surface demonstrates a pillar-like morphology with a pillar diameter

Table 1. Maximum Transmittance and Average Transmittance of Coatings coatings

0s

60 s

120 s

180 s

240 s

Tmax (400−1100 nm) Tave (400−1100 nm)

90.5% 87.6%

92.7% 88.4%

96.6% 91.7%

98.9% 94.6%

97.5% 93.5%

maximum transmittance of 92.7% (505 nm) and average transmittance of 88.4%. By etching for 120 s, the maximum and average transmittances of coatings became 96.6% (510 nm) and 91.7%, improving significantly by 3.9% and 3.3%, respectively, as compared to the coating etched for 60 s, which apparently resulted from the change of structure from smaller nanodots to nanograsses. When the coating was etched for 180 s, the maximum transmittance and average transmittance reached as high as 98.9% (535 nm) and 94.6%, respectively. However, the coating by 240 s RIE had a slightly lower transmittance of 97.5% (560 nm) than the coating etched for 180 s. The larger height of nanograsses by 240 s RIE can apparently bring more light scattering and thus result in the lower transmittance.42 In addition, the peak of transmission spectrum redshifts with the etching time (short arrows in Figure 3a), which is in accord to the changing height of nanostructures. The reflection spectra further demonstrate that the etched coatings had a significant antireflective property (Figure S2). The average reflectance of coating etched for 60 s is reduced by 2−3% compared to that of pristine coating without etching in the wavelength range of 380−760 nm. After etching for 120, 180, and 240 s, respectively, the coatings displayed even much lower reflectance, which is about 6−8% lower than that of pristine coating. The reflectance of etched coatings decreases with increasing the etching time, however the even larger nanostructures cause the reflectance of the coating etched for 240 s to be higher than that of the coating etched for 180 s, which agrees well with the transmittance results (Figure 3a). Photographs of the coating surfaces etched for varied periods of time were captured under a fluorescent lamp (Figure 3b). The pristine coating strongly reflected the fluorescent light and the underlying words became blurred. In contrast, the words under the coatings by RIE gradually become more and more visible with the extension of RIE time from 60 to 240 s (Figure 3b, the photographs from left to right). Meanwhile, different reflection colors appeared on the coating surfaces with the increment of nanograss height.15,19 The transmittance of all nanostructured coatings are higher than that of pristine coating without RIE, which indicates that the polymer coatings by RIE exhibit excellent antireflection property. The antireflective property should arise from the reduction of refractive index of antifogging coating between air and glass substrate, which is caused by the RIE treatment.14,16 Thus, we calculated the refractive index of nanograss coatings to understand the relationship between the nanostructure and the antireflective property. The coating etched for 180 s was taken as an example for the finite difference time domain (FDTD) simulation. First, the coating was divided into two layers, the etched layer and the layer without etching. According to the SEM images in Figure 2, the etched nanostructured layer was divided into a number of small units with a four-square close-packed structure. The pillars in each unit cell represent the polymer nanograsses obtained by etching, and the area between the pillars was designated as air. Then the corresponding 3D simulation model was built and is

Figure 4. Finite difference time domain model of coating etched for 180 s. (a) XY view (top view) and (b) 3D simulation model (brown lines represent the direction of incident wave propagation).

of 87 nm and a spacing of 70 nm between pillars. The effective refractive index could be calculated according to eq 2: neff = [n0 2(1 − f ) + n p2f ]1/2

(2)

where f is the nanostructure filling factor and n0 and np are the refractive indexes of air and polymer, respectively. As shown in Figure 5a, the refractive index (np, red line) and extinction coefficient (k, blue line) of polymer were measured by ellipsometry, and the average refractive index is about 1.55 for the UV−vis region (380−760 nm). Thus, the effective refractive index neff was calculated to be 1.15 according to eq 2. The transmission spectrum of the coating for the UV−vis region (380−760 nm) was obtained by the FDTD simulation, which is flatter than the measured spectrum. The simulated maximum and average transmittance of coating are 98.8% and 97.9%, respectively, and the simulated transmission spectrum is nearly consistent with the measured spectrum of coating (Figure 5b). It is noted that the simulated transmittance of nanostructured coating is somehow higher than the measured transmittance in the wavelength range of 600−760 nm. Such a deviation may have resulted from the rough nanograss surface, which could bring light scattering but was not considered in the FDTD simulation. Antifogging Properties of Coatings. Tests on the antifogging property were carried out by placing the coatings at −6 °C for 24 h in a refrigerator and then exposing them to humid laboratory air. As demonstrated in Figure 6, when the coatings were exposed to the air of 20−40% relative humidity, the blank portion of slide glasses fogged immediately, and the words underneath became blurred because of water vapor condensation that caused significant light scattering. In contrast, the coated parts remained crystal clear and the words underneath were clearly visible. The polymer coating without RIE has sufficient free hydroxyl groups or amino groups (Figure 6a), which could form hydrogen bonding with water molecules and absorb surface water vapor, demonstrating excellent antifogging property.5 After etching for different periods of time (Figure 6b−e), although a partial polymer had been converted to volatile etching products of CO, CO2, and H2O, the remaining nanostructured polymer coatings could still show excellent antifogging performance. Water contact angles of coating surfaces were also measured using water droplets of 3 μL (Figure 7) toward better understanding their antifogging mechanism. The contact angle of pristine coating without etching was 87°, and those of coatings etched for 60−180 s were nearly 70°. When the D

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Figure 5. (a) Refractive index and extinction coefficient of coating without etching. (b) Measured and simulated transmission spectra of coating etched for 180 s.

Figure 6. Digital images (red dot-dashed squares) exhibiting the antifogging property of (a) pristine coating without etching and coatings etched for 60 (b), 120 (c), 180 (d), and 240 s (e), respectively, where the underneath letters were clearly visible. However, the uncoated parts fogged immediately and the underneath letters were blurred severely.

Figure 7. Images of water contact angles on pristine coating without etching (a) and coatings etched for 60 (b), 120 (c), 180 (d), and 240 s (e), respectively.

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etching time increased to 240 s, however, the contact angle of coating decreased to 23°. Apparently, the coating etched for 240 s was more hydrophilic than the other coatings, which might have resulted from the larger height of nanograsses and rougher surface. According to the Wenzel model, if pristine coatings are hydrophilic, the etched coatings would have a smaller contact angle than the original coating due to the structural change. Therefore, coatings in the current work do not have to be superhydrophilic to be effectively antifogging, and the antifogging property originates from the hygroscopic property of polymer.5,35,43 ATR-FTIR spectra further support the above results (Figure S3). A wider absorption peak at 3290 cm−1, which is attributed to hydroxyl groups and amino groups, was found in both the pristine coating and the coating etched for 180 s. These observations indicate that the grass-like nanostructured coatings generally maintained the same chemical composition as the pristine coating even though the grass-like morphology was fabricated by RIE, and thus, the antifogging property should result from the hygroscopic property of the polymer.5,35

CONCLUSIONS

In conclusion, we developed a straightforward approach to the fabrication of antifogging antireflective dual-function nanostructured coatings. Antireflective nanograsses were fabricated by etching into an antifogging polymer coating by a simple selfmasking RIE. Along with increase of etching time, the height of nanograsses increases and the transmittance of coating is gradually enhanced. The maximum transmittance of nanograss coatings reaches up to 98.9%. The nanograss coatings also show excellent antifogging property, which is similarly attributed to the hygroscopicity of polymer matix.5 The remarkable antireflective and antifogging properties of polymer coatings demonstrate that it is promising to achieve multifunction on a single layer of a single composition. In addition, to our best knowledge, it is the first example that etches hygroscopic antifogging polymer coatings with self-masking to achieve antireflective property and is a meaningful breakthrough as compared with the traditional design concepts of antifogging polymer coatings. However, it should be pointed that the mechanical strength of coatings must be improved in E

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future before any possible practical applications, probably by improving the composition and structure of coatings.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.9b02304. Figures S1−S3 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Fax: +86 10 82543535. E-mail: [email protected]. *E-mail: [email protected]. ORCID

Xin Ye: 0000-0001-7092-109X Junhui He: 0000-0002-3309-9049 Wanguo Zheng: 0000-0002-1909-7549 Author Contributions †

These two authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21571182), the National Key Research and Development Program of China (2017YFA0207102), the Science and Technology Commission of Beijing Municipality (Z151100003315018), and a Chinese Academy of Sciences Grant (CXJJ-14-M38).

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REFERENCES

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DOI: 10.1021/acs.langmuir.9b02304 Langmuir XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.langmuir.9b02304 Langmuir XXXX, XXX, XXX−XXX