Hollow Rodlike MgF2 with an Ultralow Refractive Index for the

Double-layer broadband antireflective coatings with an average ... and wide-spectrum high-transmittance properties and good mechanical strength...
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Hollow Rodlike MgF2 with an Ultralow Refractive Index for the Preparation of Multifunctional Antireflective Coatings Lei Bao, Zihan Ji, Hongning Wang, and Ruoyu Chen* Changzhou University, No. 1 Gehu Road, Changzhou, Jiangsu 213164, PR China S Supporting Information *

ABSTRACT: Antireflective coatings with superhydrophobic, self-cleaning, and wide-spectrum high-transmittance properties and good mechanical strength have important practical value. In this research, hollow nanorodlike MgF2 sols with different void volumes were prepared by a template-free solvothermal method to further obtain hollow nanorod-like MgF2 crystals with an ultralow refractive index of 1.14. Besides, a MgF2 coating with an adjustable refractive index of 1.10−1.35 was also prepared by the template-free solvothermal method. Then through the combination of base/acid two-stepcatalyzed TEOS and hydroxyl modification on the surface of nanosilica spheres, the SiO2 coating with good mechanical strength, a flat surface, and a refractive index of 1.30−1.45 was obtained. Double-layer broadband antireflective coatings with an average transmittance of 99.6% at 400−1400 nm were designed using the relevant optical theory. After the coating thickness was optimized by the dip-coating method, the double-layer antireflective coatings, whose parameters were consistent with those designed by the theory, were obtained. The bottom layer was a SiO2 coating with a refractive index of 1.34 and a thickness of 155 nm, and the top layer was a hollow rodlike MgF2 coating with a refractive index of 1.10 and a thickness of 165 nm. The average transmittance of the obtained MgF2−SiO2 antireflective coatings was 99.1% at 400−1400 nm, which was close to the theoretical value. The hydrophobic angle of the coating surface reached 119° at first, and the angle further reached 152° after conducting surface modification by PFOTES. In addition, because the porosity of the coating surface was only 10.7%, the pencil hardness of the coating surface was 5 H and the critical load Lc was 27.05 N. In summary, the obtained antireflective coatings possessed superhydrophobic, self-cleaning, and wide-spectrum high-transmittance properties and good mechanical strength.

1. INTRODUCTION According to Fresnel reflection theory, single-layer antireflective coatings can achieve the best effect of zero reflection at only a single wavelength, whereas double-layer or multilayer antireflective coatings can achieve the effect of zero reflection at several wavelengths to further achieve an antireflective effect in a wide spectrum range. Because of the use of a more solar energy spectrum, double-layer or multilayer antireflective coatings can significantly improve the photoelectric or photothermal conversion efficiency of photovoltaic and solar thermal modules.1,2 In addition, considering that antireflective coatings are often used outdoors, the self-cleaning property and mechanical strength of the coating surface are also very important. Therefore, it is of great value to prepare antireflective coatings with comprehensive excellent performance, e.g., broadband antireflective and self-cleaning properties and good mechanical strength. The design of double-layer broadband antireflective coatings mainly focuses on the two parameters of the top and bottom layers, i.e., the refractive index and coating thickness, where the refractive index of the top layer is lower than that of the bottom layer.3 Among the several parameters of the double-layer antireflective coatings, the refractive index of the top layer is a © XXXX American Chemical Society

very sensitive parameter in which the lower the refractive index, the easier it is to design an antireflective coating with a broader band. The preparation of a low-refractive-index coating is the key to the preparation of double-layer broadband antireflective coatings. If the coating is wrapping the air with a refractive index of 1, then the refractive index can be small. Therefore, during the preparation of double-layer antireflective coatings by the sol−gel method, some templates are often used to make pores in the coating to form a low-refractive-index coating as the top layer. The templates used include PEG-200,4 P123,5 F127, CTAB,6 and PPG.7 The refractive index of the coating varies from 1.13 to 1.20, depending on the porosity of the coating. The nanosilica spheres are bridged by polymer compounds such as poly(propylene oxide) (PPO),8 PPG,9 and HDMS10 to form voids on the coating surface, and thus the refractive index of the coating can be reduced to about 1.13. However, the porosity of the coating prepared by the template method is too high, which will result in a poor mechanical property. And it is very easy for the coating to adsorb tiny dust Received: March 4, 2017 Revised: May 25, 2017 Published: June 11, 2017 A

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licane (TEOS), ethanol (EtOH, 99.9%), anhydrous methanol (CH3OH, 99.9%), ammonia−water (25%−28%), hydrochloric acid (HCl, 37%), hydrogen peroxide (H2O2, 30%), magnesium acetate (Mg(CH3COO)2·4H2O, 99%), hydrofluoric acid (HF, 40%), and toluene were obtained from Sinopharm (Shanghai, China). The water was deionized. 2.2. Preparation of the MgF2 Sol. Mg(CH3COO)2·4H2O (1.73 g) was dissolved in 47.60 g of CH3OH. After being stirred for a while, 0.645 g of HF was added dropwise. After the mixture was stirred for 20 min, it was then transferred to a 100 mL solvothermal reactor. (The safe temperature of PPL material in use is below 280 °C.) The reaction took place at 180, 200, 220, 240, and 260 °C for 24 h. (A dual temperature-control system was adopted to prevent accidental increases in temperature.) The mixture was then cooled to room temperature and coated after being aged for 7 days. The mass concentration of the MgF2 sol was 1.5%. 2.3. Preparation of the SiO2 Sol. Absolute EtOH, NH4OH, and TEOS were added to the beaker in sequence at a molar ratio of EtOH/NH4OH/TEOS = 115:4:1. After being stirred at 50 °C for 6 h and then aged at 25 °C for 4 days, a 20 nm SiO2 sol was obtained. The sol was refluxed in an oil bath for 24 h to eliminate ammonia. When the ammonia was eliminated, H2O2 was added to the above-mentioned SiO2 nanoparticle (SiO2−NP) sol (molar ratio of H2O2/SiO2-NPs = 1:5.3) according to our previous research. After being refluxed at 108 °C for 5 h, a SiO2 nanoparticle sol with hydroxyl modification could be obtained. HCl, H2O, and TEOS were added to the hydroxyl-modified SiO2 sol in sequence (molar ratio of HCl/H2O/TEOS = 0.03:4:1), followed by being stirred at room temperature for 6 h. Then, the base/ acid two-step-catalyzed SiO2 sol was obtained in a molar ratio of TEOS/SiO2-NPs = 5:5, 4:6, 3:7, 2:8, and 1:9. The concentration of the SiO2 sol was 1.5%. 2.4. Preparation of Double-Layer Antireflective Coatings and Hydrophobic Modification. The high-borosilicate glass (100 mm × 25 mm × 3 mm) was used as the substrate, and the cleaning treatment was carried out according to our laboratory’s previous research.18 A SYDC-100 dip-coating machine was used to place a onelayer SiO2 coating on the glass substrate at a speed of 80−120 mm/ min. After being dried at 80 °C for 30 min and heat treated at 400 °C for 2 h, a one-layer MgF2 coating was prepared at a speed of 160−200 mm/min. After being dried in an oven at 150 °C for 2 h and cooled to room temperature, MgF2-SiO2 double-layer broadband antireflective coatings could be obtained. PFOTES was used to conduct hydrophobic modification on the coated substrate. The specific steps were as follows. The coated slide glass was placed in a Teflon container and sealed with a stainless steel autoclave containing a few droplets (10−20 μL) of PFOTES in the bottom. There was no direct contact between the substrate and the PFOTES droplets. The autoclave was put in an oven at 150 °C for 2 h to enable the vapor of PFOTES to react with the hydroxyl groups on the coating surface. Finally, the autoclave was opened and placed in an oven at 160 °C for 1.5 h to volatilize unreacted PFOTES molecules on the coating.19 2.5. Characterization. The structures of the SiO2 nanoparticle sol and the MgF2 nanoparticle sol were observed with a high-resolution transmission electron microscope (TEM, JEM-2100, JEOL). The surface micromorphology of the double-layer coatings was observed with a field emission scanning electron microscope (FESEM, SUPRA55, Zeiss). The microstructure of the coatings was observed with an atomic force microscope (AFM, NanoMan VS, Veeco). The phase and crystal form of the MgF2 coating were analyzed by an X-ray diffractometer (PC-based D/max 2500, Cu Kα, λ = 0.154056 nm, tube voltage 40 kV, tube current 100 mA). The transmittance spectrum of the coatings in the range of 400−1400 nm was recorded with a UV− vis spectrophotometer (UV-1700, Shimadzu). Fourier transform infrared (FT-IR) spectra were collected on a Nicolet Fourier spectrophotometer using KBr pellets of the solid samples. The refractive index of the coating was measured by a spectroscopic ellipsometer (E03, ELLITOP Scientific). The hydrophobic angle of the coating was measured with a contact angle measuring instrument with a HARKE-SPCA standard. The abrasion resistance of the coating

particles in the air during use, reducing the optical properties of the coating. Besides, the use of polymer compounds in antireflective coatings will result in the degradation of the antireflective coating’s structure under ultraviolet irradiation,11 leading to a decrease in optical efficiency and mechanical properties. The hollow-structured material can reduce the refractive index of the body material by wrapping the air with a refractive index of 1, so it is one of the most effective means for preparing the low-refractive-index coatings. Commonly, hollow-structure SiO2 is adopted to prepare the antireflective coating with a low refractive index.12,13 Because MgF2 has the lowest refractive index among the existing numerous inorganic materials (with a refractive index of 1.38), it is of great advantage to prepare the coating with an ultralow refractive index by MgF2.14,15 However, the synthesis of MgF2 nanoparticles with a hollow structure has not been reported so far. By using the hollowstructured MgF2, it is possible to prepare a coating with an ultralow refractive index. Because most of its voids are encapsulated in the interior of the material and the voids between the nanoparticles are small, the adsorption of dust by the small holes on the coating surface can be greatly reduced. In addition, MgF2 with an ultralow refractive index can easily be combined with other materials to prepare different kinds of antireflective coatings, which is an attractive scheme. Superhydrophobic coatings with a contact angle greater than 150° are self-cleaning. Usually, the preparation of a superhydrophobic surface requires a micronano composite structure to form a certain degree of roughness. However, if the surface roughness is too high, diffuse reflection will occur on the surface, reducing the antireflective effect. Therefore, the roughness of the coating surface is required to achieve a balance between the antireflection and the surface hydrophobicity.16 At present, most of the antireflective coatings are prepared by spherical SiO2 or MgF2 materials. When the surface roughness of the nanosphere coating is too low (Rq < 10 nm),17 the hydrophobic angle is usually in the range of 90− 130° after the modification by the low-surface-energy material. If a nanorod material longer than 100 nm can be introduced into the antireflective coatings, both the antireflective effect and the surface hydrophobicity (which requires ideal roughness) can be realized, resulting from the irregular embossment on the surface caused by the rodlike substance. In this research, a template-free solvothermal method was adopted to synthesize the hollow rodlike MgF2 sols with different void volumes to prepare the MgF2 antireflective coating with an ultralow refractive index. Besides, the SiO2 coating with an adjustable refractive index ranging from 1.30 to 1.45 was prepared by base/acid two-step catalysis and hydroxyl modification on the surface of SiO2. On the basis of the parameters of double-layer broadband antireflective coatings at 400−1400 nm designed by multilayer theory, the broadband antireflective coatings that had properties that were similar to the theoretical optical properties were prepared by optimizing the refractive index and coating thickness of the top and bottom layers. Finally, after the hydrophobic treatment was conducted on the coating surface, the antireflective coatings were prepared with comprehensive features of superhydrophobic, broadband antireflective, and good mechanical properties.

2. MATERIAL AND METHODS 2.1. Materials. 1H,1H,2H,2H-Perfluorooctyltriethoxysilane (PFOTES, 95%) was purchased from Aladdin. TetraethylorthoxylsiB

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Langmuir was evaluated with a pencil hardness tester (291-type, Erichsen), and the adhesion strength between the coating and the substrate was measured with a scratch tester (WS-92).

3. RESULTS AND DISCUSSION 3.1. Template-Free Solvothermal Synthesis of Hollow Nanorod-like MgF2 Material and the Properties of the Coatings. A coating with an ultralow refractive index is the key to preparing double-layer broadband antireflective coatings. Because hollow-structured material has a lower refractive index than body material, the synthesis of hollow-structured MgF2 has been researched. The XRD results of the dried powder MgF2 sol prepared by the solvothermal method at 180−260 °C are shown in Figure 1. The diffraction peaks of MgF2 have

Figure 2. TEM images of the MgF2 sol obtained by the solvothermal method at different temperatures: (a) 180, (b) 200, (c) 220, (d) 240, (e) 240, and (f) 260 °C.

the heat treatment, the intermediate product was further cured, and finally the hollow rodlike MgF2 formed by the selfassembly process. The relationship between the refractive index of the coating and the porosity of the coating is given by

Figure 1. XRD image of MgF2 sol particles with temperatures of 180, 200, 220, 240, and 260 °C respectively from (a) to (e).

nP 2 = (n2 − 1)(1 − P) + 1

appeared at all five solvothermal temperatures, corresponding to the crystal planes of 110, 101, 111, 210, 220, 002, and 301 (PDF: 41-1443).20 Among all of these peaks, the characteristic diffraction peak of MgF2 synthesized at 240 °C is the sharpest peak, showing the best crystallinity. It can obviously be seen that when the reaction temperature is 240 °C the peak of the 110 crystal plane is sharper than that at other temperatures as a result of the superior growth of the 110 plane at 240 °C, which is consistent with the result in Figure 2e. The TEM results of MgF2 sols synthesized at different temperatures are shown in Figure 2. The results show that at the solvothermal temperature of 180 °C, MgF2 presents short, thick rods with solid crystal structure; that at 200 °C, MgF2 begins to show hollow structure, with long, thin hollow rods; that at 220 °C, MgF2 has a short, thick hollow structure; and that at 240 °C, MgF2 has good dispersion, showing long, thin hollow rod structure with a length of about 200 nm, a diameter of about 15 nm, and a wall thickness of about 1.5 nm. When the temperature further rose to 260 °C, the length of the hollow rodlike structure was shortened to about 100 nm, with a diameter of about 25 nm and a wall thickness of about 3 nm. Through preliminary calculation, the void volumes of MgF2 synthesized at 240 and 260 °C were almost the same. In Figure 2e, a clear diffraction pattern with a stripe spacing of 0.223 nm can be seen in the solid part of the hollow MgF2 structure at 240 °C by an electron diffraction test, which corresponded to the 110 crystal plane of MgF2. During the formation of hollow rodlike MgF2, intermediate product Mg(OH)2−xFx formed because of the excess magnesium ions.21 In the later stage of

where nP represents the refractive index of the porous coating, n represents the refractive index of the dense coating, and P represents the porosity of the coating.22 In this research, several hollow rodlike MgF2 crystals with complete morphology in the sol prepared by solvothermal synthesis at 240 °C were chosen as the targets. The crystal was considered to be an approximate hollow cylinder. The diameter, length, and wall thickness were measured to estimate the volume of the hollow part. The result showed that the volume of the hollow part occupied about 63% of the whole crystal (P = Vvoid/(Vvoid + Vsolid)). The relationship between the material cavity and the material body refractive index is given by nhollowMgF2 = nMgF2(1 − P) + nairP

where nhollowMgF2 represents the refractive index of the hollow MgF2, nMgF2 represents the refractive index of the body material MgF2, and P represents the porosity of the cavity volume of hollow MgF2.23 The refractive index of the hollow crystal was calculated to be 1.14. As far as we know, this kind of material has the lowest refractive index in the published literature. The MgF2 sols obtained at different solvent temperatures were coated on the glass substrate, with the coating thickness being about 140 nm. The results of the ellipsometer test are shown in Figure 3. The results show that the refractive index of the coating is closely related to the void volume of MgF2. The MgF2 prepared by solvothermal synthesis at 180 °C had a solid structure with the refractive index being 1.35, which was close C

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nanoparticles obtained by base-catalyzed TEOS decreases from 50 to 90%. In the base/acid two-step-catalyzed SiO2 sol, there were two kinds of structures, i.e., chain-like SiO2 polymers obtained by acid-catalyzed TEOS and particle-like SiO2 obtained by basecatalyzed TEOS.25,26 Depending on the proportion of the two materials, a series of coatings with different refractive indexes could be obtained. The refractive index of the mixed coating was between that of the coating by the acid-catalyzed sol and that of the coating by the base-catalyzed sol.27 The change in porosity between the SiO2 nanoparticles in the coating was the main reason for the change in the coating’s refractive index. When the nanosilica solid spheres had a higher proportion in the coating, the refractive index of the coating was low because of the high porosity between the particles. When the solid spheres had a lower proportion, the porous part was filled by the chainlike SiO2 polymers with a high refractive index, resulting in a decrease in the porosity and an increase in the refractive index. Although the sol, with a high proportion of SiO2 nanoparticles, prepared by the base/acid two-step-catalyzed method had an adjustable refractive index, the chemical bonds formed between the nanoparticles and chainlike SiO2 polymers were very few. This could result in worse surface hardness of the coating with the pencil hardness usually being only 4−5 B.28 In our previous work,29 the number and intensity of chemical bonds between the nanoparticles and the chain-like SiO2 polymers were enhanced by the hydroxyl modification on the surface of the SiO2 nanospheres, and the obvious structure of chain−solid sphere−chain was obtained (Figure 5a). After the coating was calcined, the hydroxyl groups were further

Figure 3. Refractive index and porosity of MgF2 coatings at different solvent temperatures.

to the refractive index of the MgF2 body material. When the temperature of the solvothermal synthesis increased to 200 °C, the hollow structure began to appear. And the refractive index of the coating decreased to 1.24 as the void wrapped the air with a refractive index of 1.0. As the temperature kept rising, the void volume in MgF2 also increased and the void volume reached the top in the case of 240 and 260 °C, with the refractive index of the coating being 1.10. The refractive index of the coating was lower than that of the SiO2−MgF2 coating prepared by Cui et al. with the copolycondensation method where the refractive index of the latter was 1.12.24 The refractive index and the porosity results of MgF2 coatings prepared at different solvothermal temperatures are shown in Figure 3. The refractive index of the MgF2 coating is adjustable, ranging from 1.10 to 1.35. The porosity of the MgF2 coatings prepared at different temperatures was calculated to be 9.1−76.8%, which was caused partially by the hollow structure of MgF2 and partially by the voids between the hollow rods in the coating. The void volume of the MgF2 body synthesized at 240 °C occupied 63%; therefore, the porosity of the nanorods in the coatings prepared by this sol was only 13.8% (the porosity of SiO2 nanoparticles in the SiO2 antireflective coating is usually 53%), indicating that the MgF2 crystals in the coatings were densely bonded. 3.2. Structure and Properties of SiO2 Coatings with Different Refractive Indexes. According to the theoretical design results of double-layer broadband antireflective coatings, the bottom layer has a slightly higher refractive index than the top layer and the bottom layer also has better adhesion performance, better mechanical strength, and a flatter surface. The measurement results of the refractive index of the base/ acid two-step-catalyzed SiO2 coatings with different weight ratios are shown in Figure 4. Figure 4 shows that the refractive index of the coating decreases from 1.30 to 1.45 when the weight ratio of the SiO2

Figure 4. Refractive index and porosity of base/acid two-stepcatalyzed SiO2 coatings with different weight ratios.

Figure 5. (a) TEM image of the SiO2 sol and (b) surface morphology of the SiO2 coating. D

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Langmuir condensed, and the surface hardness reached 6−7 H. The FESEM results show that the coating surface is smooth and compact (Figure 5b). The base/acid two-step-catalyzed method could be used to prepare antireflective coatings with an adjustable refractive index ranging from 1.30 to 1.45. Because of the combination of hydroxyl modification on the nanoparticle surface, the coating had good mechanical strength and a flat surface, which was good for superimposing other coatings on the coating surface. 3.3. Theoretical Design and Optical Properties of MgF2−SiO2 Double-Layer Coatings. According to the optical coating theory,30 double-layer coatings were designed in this article. Considering the vertical incidence of light, the characteristic matrix of the double-layer coatings is given by

Figure 6. (a) Theoretical and experimental transmittance curves of MgF2−SiO2 double-layer broadband antireflective coatings and the single SiO2 and MgF2 coating transmittance curves and SEM crosssection images of (b) the SiO2 coating, (c) the MgF2 coating, and (d) the MgF2−SiO2 coating.

⎡ B ⎤ ⎡ cos δ1 i/η1 sin δ1⎤ ⎡ cos δ2 i/η2 sin δ2 ⎤⎡ 1 ⎤ ⎥··⎢ ⎥⎢ ⎥ ⎢⎣ ⎥⎦ = ⎢ ⎢⎣iη1 sin δ1 cos δ1 ⎥⎦ ⎢⎣iη2 sin δ2 C cos δ2 ⎥⎦⎣ ηg ⎦

where η1 is the refractive index of the bottom layer, δ1 is the thickness of the bottom layer, η2 is the refractive index of the top layer, δ2 is the thickness of the top layer, and ng is the refractive index of the substrate. Because the incident medium is air, the η0 of the incident medium is equal to n0. The reflectance of the double-layer coating is given by

antireflective property in the range of ultraviolet to nearinfrared wavelengths. The deviation of the transmittance between the experimental and theoretical values mainly occurs in the range of 400−500 nm, which may be due to the scattering of incident light caused by the rough surface of the hollow MgF2 coating.32 3.4. Hydrophobicity and Mechanical Properties of MgF2−SiO2 Double-Layer Coatings. Hydrophobic antireflective coatings have the advantage of being used outdoors as a result of their self-cleaning property. After the surface of the double-layer antireflective coatings was modified with PFOTES, the test results of contact angles were shown in Figure 7. The results show that the contact angle of the unmodified MgF2−SiO2 double-layer coatings is 119° (Figure 7a). After the surface modification by PFOTES, the contact angle increases to 152° (Figure 7b), achieving the superhydrophobic effect. It is unexpected that the coating surface that was not treated at low surface energy could reach such large hydrophobic angles because the coatings prepared by various inorganic substances were usually hydrophilic and few of them had a contact angle greater than 90°. Through further analysis of the AFM image of the MgF2− SiO2 double-layer coating surface, it can be seen that the hollow nanorod-like MgF2 with a diameter of about 15 nm and a length of about 200 nm formed a rough structure on the coating surface. The AFM results (Figure 7b) indicate that the surface roughness of the coating, Rq, is 29.4 nm, and the Ra value is 23 nm. From the FESEM image (Figure 6), it can be clearly seen that rodlike MgF2 forms air grooves on the surface of the coating, and the air is easily trapped by the water droplets in the valley of the rough structure to form air vesicles. The water droplets would consequently float above the air and were supported by the protruding solid, achieving the apparent hydrophobic effect, which was a typical Cassie heterogeneous interface.33 And this was also the main reason that the coatings could achieve the hydrophobic effect without low-surfaceenergy treatment or fluorosilane modification. Figure 8 shows the FTIR spectra of the MgF2 coating and the PFOTES- and PFOTES-modified MgF2 coatings. The results show that the characteristic peak of the Si−O bond (1134 cm−1) appears on the spectrum of PFOTES-modified MgF2 and that this Si−O is shifted when comparing it to the characteristic peak of the Si−O bond (1074 cm−1) on the spectrum of PTOETS. The characteristic peak of the C−Si

⎛ n B − C ⎞ ⎛ n 0B − C ⎞ R (λ ) = ⎜ 0 ⎟·⎜ ⎟* , T = 1 − R ⎝ n 0B + C ⎠ ⎝ n 0B + C ⎠

A series of double-layer, double-wave broadband antireflective coatings can be designed by the above equation and the corresponding parameters. The results show that the average transmittance of the double-layer coatings reached the highest values, 99.1% in the range of 400−780 nm and 99.6% in the range of 400−1400 nm. When the refractive index of the bottom layer was 1.34, the corresponding thickness was 155 nm, and when the refractive index of the top layer was 1.10, the corresponding thickness was 165 nm. On the basis of the above results, the SiO2 coating, with a mass ratio of acid sol/base sol = 2:8 by base/acid two-step catalysis and with a refractive index of 1.34, was chosen as the bottom layer. The top coating was the MgF2 coating with a refractive index of 1.10 prepared with the hollow rodlike MgF2 sol at the solvothermal temperature of 240 °C. The thickness of the coating could be precisely controlled by the withdrawal speed used in the dip-coating method. By optimizing the withdrawal speed, we could obtain double-layer antireflective coatings whose thickness was close to the theoretical value. As shown in Figure 6, the cross sections of the single SiO2 coating, the single MgF2 coating, and the double-layer coatings are clearly defined. The thickness of the SiO2 coating is 155 nm, and the refractive index is 1.34. The thickness of the MgF2 coating is 165 nm, and the refractive index is 1.10. The transmittance curves of single SiO2, MgF2, and the double-layer broadband antireflective coatings are shown in Figure 6. Accordingly, τT = (Σλλmax TλSλΔλ)/(Σλλmax SλΔλ), where min min Tλ is the transmittance of the corresponding wavelength, Sλ is the solar irradiance intensity for air mass 1.5 (ASTM G173-03), and Δλ is the wavelength interval.31 The weighted-average light transmittance of the double-layer antireflective coatings in the range of 400−1400 nm is 99.1%, very close to the theoretical value of 99.6%. The double-layer coatings exhibit an excellent E

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Figure 7. MgF2−SiO2 double-layer broadband antireflective coatings. (a) FESEM image of surface topography. The chart is the result of the unmodified hydrophobic angle. (b) AFM image of the modified hydrophobic angle test. (c) Surface hydrophobicity of the coating.

hydroxyl groups on the MgF2 surface, and PFOTES was anchored to the coating surface in the form of a Si−O bond, which further reduced the coating’s surface energy and rendered the coating surface with a superhydrophobic effect. Mahadik et al. also modified the organosilane on the silica surface by a similar method, and similar results were achieved.34,35 After conducting hydroxyl modification on the surface of SiO2 nanoparticles, the bonding strength between the nanoparticles and the linear silicate polymers was obviously enhanced. Because the porosity between the MgF2 nanorods was only 13.8%, the nanorods in the top layer were densely bonded and the hardness of the coating was 5 H according to the ISO 15184 standard (Supporting Information). The binding force between MgF2 crystals might be due to the fact that MgF2 is an ionic bond compound and a large number of hydroxyl groups existed on the surface of the coating. Hydrogen bonding between the fluorine element and hydroxyl group also further enhanced the bonding force of the coating surface. The adhesion force between the coating and the substrate was measured by the scratch test.36 And the critical load of the coating, Lc, was 27.05 N (Supporting Information), which showed good scratch resistance. In the experiment in question, no obvious damage was found on the MgF2−SiO2 coating by visual inspection after being rubbed 100 times, and the transmittance values before and after rubbing were very similar to each other, as shown in Figure. 9. The average transmittance after scratching decreased by merely 0.10% (400−1400 nm). The hydrophobic angle of the coating decreased to 140° after scratching.

Figure 8. (a) FTIR spectra of the MgF2 coating and PFOTES- and PFOTES-modified MgF2 coatings and (b) mechanism diagram of modifying MgF2 by PFOTES.

bond (820 cm−1) and the stretching vibration characteristic peaks of the saturated C−H bond also appear on the spectrum of the modified MgF2 (2830 and 2940 cm−1), which does not appear on the infrared spectrum of MgF2. Besides, the peaks of hydroxyl groups on the PFOTES-modified MgF2 coating are significantly reduced (3445 cm−1), which resulted from the fact that the surface hydroxyls were largely covered after their condensation reactions with PFOTES. After PFOTES was modified on the surface of the MgF2 coating by a similar CVD method, the ethoxy groups of PFOTES reacted with the

4. CONCLUSIONS A series of double-layer antireflective coatings ranging from the ultraviolet to the near-infrared band, i.e., 400−1400 nm, were designed in this research by optical coating theory. A series of hollow nanorod-like MgF2 sols with different void volumes were prepared by the solvothermal method to obtain MgF2 crystals with an ultralow refractive index of 1.14. The aboveobtained MgF2 sols were used to prepare the antireflective F

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ACKNOWLEDGMENTS The authors acknowledge the financial support of the National Natural Science Foundation of China (grant no. 21571024) and Industry and Research Perspective in Jiangsu Province (grant no. BY2015027-01).

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Figure 9. Transmittance curves and surface hydrophobicity of MgF2− SiO2 double-layer broadband antireflective coatings before and after rubbing.

coating with an adjustable refractive index ranging from 1.10 to 1.35. The SiO2 sols with chain−solid sphere−chain structure and with different proportions of chains and spheres were synthesized by base/acid two-step-catalyzed TEOS. The aboveobtained SiO2 sols were used to prepare the antireflective coating with an adjustable refractive index ranging from 1.30 to 1.45. And through the hydroxyl modification on the surface of SiO2 nanoparticles, the mechanical strength of the SiO2 coating prepared by base/acid two-step catalysis was greatly enhanced. On the basis of the designed parameters of the double-layer antireflective coatings and by the many optimization methods, the double-layer antireflective coatings were prepared where the bottom layer was a SiO2 coating with the refractive index being 1.34 and the thickness being 155 nm, and the top layer was a MgF2 coating with the refractive index being 1.10 and the thickness being 165 nm. The average transmittance of the prepared double-layer coatings in the range of 400−1400 nm reached 99.1%, which was close to the theoretical value. The pencil hardness of the surface was 5 H, and the critical load was 27.05 N. The hydrophobic angle of the coating surface was 119° without any modification, and the hydrophobic angle increased to 152° after the surface was modified with PFOTES. In summary, the double-layer antireflective coatings prepared in this article had comprehensive excellent properties of broadband antireflection, wear resistance, and superhydrophobicity.

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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/acs.langmuir.7b00737. SiO2−MgF2 double coatings’ mechanical properties. Acoustic emission spectrum of scratches between the coatings and the substrate. Scratches made by a 6 H pencil on the surface of the coatings. (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Ruoyu Chen: 0000-0001-8023-4293 Notes

The authors declare no competing financial interest. G

DOI: 10.1021/acs.langmuir.7b00737 Langmuir XXXX, XXX, XXX−XXX

Article

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DOI: 10.1021/acs.langmuir.7b00737 Langmuir XXXX, XXX, XXX−XXX