Multifunctional Surfaces with Outstanding Mechanical Stability on

Feb 14, 2013 - Glass Substrates by Simple H2SiF6‑Based Vapor Etching ... month outdoor exposure, the as-etched glass showed remarkable stability in ...
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Multifunctional Surfaces with Outstanding Mechanical Stability on Glass Substrates by Simple H2SiF6‑Based Vapor Etching Lin Yao†,‡ and Junhui He*,† †

Functional Nanomaterials Laboratory 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 ‡ Graduate University of Chinese Academy of Sciences, Beijing 100864, China S Supporting Information *

ABSTRACT: Mechanically robust antireflective glass surfaces play an important role in the performance of many optical and optoelectronic devices. In this paper, we have demonstrated a simple method to create a high performance wide-range antireflective layer on glass surface by H2SiF6-based vapor etching at low temperature (5−20 °C). The maximum transmittance of 99.0% was achieved under optimal etching conditions. Scratch tests showed that the surface had excellent mechanical strength, and its pencil hardness is above 6H. After 2 month outdoor exposure, the as-etched glass showed remarkable stability in their antireflection property. The as-etched glass was endowed superhydrophilic and antifogging property after annealing and O2-plasma treatment, which provide an additional advantage for operating outdoors or in high-humidity environments. The composition, morphology, and formation mechanism of the hierarchically nanostructured surface were discussed in detail on the basis of experimental results. A new mechanism was proposed to account for the etching−morphology relationship.

1. INTRODUCTION

wafers, surface micromachining, and creation of frosted glass for decorative applications.23,24 Wet chemical etching of silicate glasses in aqueous HF solutions is a subject which has been studied over many years, especially in integrated circuit technology. Most contemporary studies of fluoride etching on glass surfaces have been performed using HF or HF-based solution (such as adding HCl, HNO3, NH4F, or H2SiF6).25−27 Significantly less work has been reported on the use of H2SiF6 as the sole etchant to enhance the optical and wetting performance of glass with desired mechanical robustness. Fluorosilicic acid is a strong acid in aqueous solution, which cannot exist in the vapor state and will decompose into HF and SiF4 under ordinary conditions.28 Recently, large quantities of fluorosilicic acid have been readily produced as a byproduct in the fertilizer industry or recovered from the absorption process of pyrolyzed SiF4 gas.29 It is worth mentioning that Chinyama et al. fabricated antireflective coatings with excellent durability by etching in silica saturated fluorosilicic acid (H2SiF6·SiF4).20 After immersion in the solution at 60 °C for 30 min, the glass has a transmittance improvement of about 8%. To the best of our knowledge, however, very little work has been reported of using H2SiF6based vapor etching to fabricate porous glass surface structure with high transmittance.

Antireflective coatings (ARCs) are extremely useful and badly in need in daily life because of enhancement of light transmission.1,2 ARCs could be employed for a variety of applications: optical and electro-optical systems in telecommunications, glass lenses, solar cells, architectural windows, and displays.3−6 Glass is one of the most common substrates for these devices due to its low cost, high transmittance in visible and near-IR region, excellent barrier properties against water and oxygen, and low thermal expansion coefficient.7,8 Many efforts have been dedicated to the design and fabrication of ARCs on glass. Currently, porous silica coatings as a promising candidate have already attracted a great deal of scientific and technological interest. Methods previously used for constructing porous silica layer on glass substrate include etching,7,9,10 spin-coating,11 spray-coating,12 direct dip-coating,13 layer-by-layer dip-coating,14,15 layer-by-layer spray-coating,16 and chemical vapor deposition.17 Among these methods, etching is a simple, inexpensive, and straightforward method to directly create porous layer on the surface of glass. Etching methods can be mainly classified into two types: liquid-phase etching and vapor-phase etching. For a long time, different etchants such as NaOH, KOH, NH4OH, H2SO4, HNO3, and HF were used in varied etching reactions of glasses.7,18−22 In the past several decades, fluoride etchants have been widely used for the etching of silica in the glass industry and semiconductor industry, including etching of glass fibers, removing oxide impurities from stainless steel and silicon © 2013 American Chemical Society

Received: January 9, 2013 Revised: February 6, 2013 Published: February 14, 2013 3089

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Figure 1. SEM images of slide glasses etched for varied periods of time: 0 (a), 12 (b), 18 (c), 24 (d), 30 (e), and 48 h (f). (g) Enlarged view of slide glass etched for 30 h. (h) Cross-sectional image of slide glass etched for 30 h. Etching condition: 30 mL of H2SiF6 solution (1.0 mol/L) at 15 °C. rinsed with ultrapure water, and dried by compressed air for following characterization. 2.3. Characterization. Transmission and reflection spectra in the wavelength range of 300−1500 nm were recorded using a Varian Cary 5000 UV−vis−NIR spectrophotometer. Water contact angles (WCAs) of coating surfaces were measured at ambient temperature on a JC2000C contact angle/interface system (Shanghai Zhongchen Digital Technique Apparatus Co.), the angle precision of which is ±0.5°. Water droplets of 3 μL were dropped carefully onto the coating surfaces. Once a water droplet contacted the sample surface, the machine began to take photos at a speed of 30 photos/s; i.e., the interval between the contact moment and the first image was 33 ms. The measurement was carried out on three different areas of the sample surface. Scanning electron microscopy (SEM) observations and energy dispersive spectrum (EDS) analysis were carried out on a Hitachi S-4300 field emission scanning electron microscope. The specimens were coated with a layer of gold by ion sputtering before SEM observations. The roughness and morphology of the surfaces were characterized by atomic force microscopy (AFM) on an MM8SYS scanning probe microscope (Bruker AXR). The sample for AFM, antifogging test, and scratch test was the slide glass etched under optimal conditions. In the durability test, the slide glass was etched under optimal conditions, except the etching time was 24 h. For the sake of simplicity, it will not be described repeatedly below. 2.4. Scratch and Durability Tests. Pencil hardness was measured by a pencil hardness tester (Elcometer 3086 Motorised Pencil Hardness Tester) according to the State Standard Testing Method (GB/T6739-1996, equivalent of American Society for Testing and Materials (ASTM) D3363). The pencil was held firmly against the film at a 45° angle and pushed away by the tester in a 6.5 mm stroke at a speed of 0.5 mm/s. The degree of hardness of a pencil was systematically increased until the pencil visually scratched the film surface. The hardest pencil that would not scratch the film surface was determined from the scale of 6B (the softest) to 6H (the hardest), the hardness of which was used to represent the pencil hardness of the film surface. The unetched and etched slide glasses were situated on the roof of the main building of Technical Institute of Physics and Chemistry, CAS, Beijing, as shown in Figure S2. The durability test was carried out under the conditions of 12−38 °C and 18−90% RH for 2 months.

In this article, we report the fabrication of porous glass surfaces with controllable wettability and AR property by H2SiF6-based vapor phase etching. We designed and fabricated an ingenious reaction vessel which allowed the reaction to be carried out smoothly at low temperatures (10−20 °C). Enhanced transmittance (up to 99.0%) was achieved in a wide wavelength range. Scratch and durability tests showed the etched glass has excellent mechanical properties and transmittance stability after 2 month outdoor exposure. In addition, antifogging effect was demonstrated, which may offer an additional advantage for outdoor uses as a solar cell substrate in a high-humidity environment or underwater.30,31 The morphology, composition, surface, and optical properties could be tailored by varying the reaction conditions.

2. EXPERIMENTAL SECTION 2.1. Materials. Commercial slide glass (Sail brand; size: 7.5 cm × 2.5 cm × 0.1 cm; composition: 72.5% SiO2, 13.7% Na2O, 9.80% CaO, 3.5% MgO, 0.4% Al2O3, and 0.1% K2O, which is approximately agreeable with that obtained by EDS analysis (Figure S1)), ethanol, and acetone were purchased from Beijing Chemical Reagents. Fluorosilicic acid (H2SiF6, 99.0%) was purchased from Xilong Chemical Reagent Company. All chemicals were analytic grade and used without further purification. Ultrapure water with a resistivity higher than 18.2 MΩ·cm was used in all experiments and was obtained from a three-stage Millipore Mill-Q Plus 185 purification system (Academic). 2.2. Etching Procedure. Slide glasses were etched by a one-step acid (H2SiF6) vapor phase etching process at low temperatures (5−20 °C). In a typical procedure, a slide glass was first cleaned under sonication (100 W) for 20 min in a mixed solvent of ultrapure water, ethanol, and acetone (volume ratio 1:1:1) and then rinsed under sonication (100 W) for 10 min in ultrapure water. The cleaned slide glass was then placed horizontally on a bracket, which was placed in Teflon container of a homemade sealed reactor. There was some H2SiF6 solution of given concentration (0.75−1.50 mol L−1) and volume (20−40 mL) at the bottom of the Teflon container. The reactor was placed in a thermostat at given temperature (5−20 °C) for a given period of time (12−48 h). Finally, the slide glass was taken out, 3090

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3. RESULTS AND DISCUSSION 3.1. Morphologies and Structures. As shown in Figure 1a, unetched glass surface is smooth with flaws or cracks, which are small (ca. 6 nm in width) and almost closed. Figures 1b−e show SEM images of glass substrates after etching for varied periods of time (12, 18, 24, 30, and 48 h). After a reaction time of 12 h, those surface detects (flaws or cracks) began to be etched open and gradually transformed into much longer and wider cracks (ca. 10 nm in width) as the etching proceeded. When the reaction time was extended to 30 h, the etching deepened internally and a hierarchical structure formed (Figure 1g). Further extension of reaction time (above 48 h) resulted in a porous coating of network-connected nanocracks with enlarged pores (ca. 17 nm in width). Figure S3 shows the dependence of average crack width on the etching time. Clearly, the morphology and layer thickness could be controlled by changing the etching conditions. In Figure 1h, it is noted that the depth of etching was about 60 nm after 30 h. It is believed that the formation of nanoscaled cracks was caused mainly by acid attack. The detailed mechanism will be discussed in the Etching Mechanism section. Figure 2 shows AFM images of unetched slide glass and asetched slide glass. Though wider cracks (ca. 17 nm in width)

glasses at varied temperatures and using varied H2SiF6 concentrations and volumes for varied periods of time. Nearly all the as-etched slide glasses show much enhanced transmittance in the wavelength range of 400−1500 nm as compared with unetched slide glass. Figure 3 shows the

Figure 3. Transmission and reflection spectra of unetched slide glass and slide glass as etched at 15 °C for 30 h using 30 mL of H2SiF6 solution (1.00 mol L−1).

transmission and reflection spectra of slide glass etched under the optimal conditions. Before etching, the slide glass showed an average reflectance of ca. 9% and an average transmittance of ca. 90% in the wavelength range of 400−1500 nm. After etching for 30 h, the reflectance decreased over the whole spectral range. The lowest reflectance (0.52%) and highest transmittance (98.95%) were obtained at ca. 400 and 475 nm, respectively. As we know, to achieve the maximum transmission and minimum reflection, the refractive indices of materials involved should satisfy the formula nc = (na × ns)0.5, where nc, na, and ns are the refractive indices of coating, air, and substrate, respectively. na can be approximated as 1 and the refractive index of glass ns is 1.5, so that nc must be 1.22 to make the coating achieve the maximum transmission and minimum reflection. Besides, based on the effective medium theory,7 neff of a certain composite layer can be calculated by neff = [nc2f + na2(1 − f)]1/2, where f is the filling factor. The porous layer has a lower neff than the solid layer due to the existence of air in the interstitial space. This is consistent with the trends of transmittance change with the experimental conditions in Figure S4. As shown in Figure S4a, with increase of the H2SiF6 concentration, the maximum transmittance of as-etched slide glass first increases, then reaches a maximum value, and finally decreases, and the maximum transmittance wavelength has a red-shift from 402 nm (0.75 mol/L, 96.40%) to 472 nm (1.00 mol/L, 97.64%) to 584 nm (1.25 mol/L, 96.33%) to 677 nm (1.50 mol/L, 95.56). As shown in Figure S4b, with increase of the reaction temperature, the maximum transmittance of asetched slide glass also first increases, then reaches a maximum value, and finally decreases, and the maximum transmittance wavelength is from 466 nm (5 °C, 93.58%) to 516 nm (10 °C, 95.26%) to 472 nm (15 °C, 97.64%) to 640 nm (20 °C, 93.56%). As shown in Figure S4c, with increase of the H2SiF6 volume, the maximum transmittance of as-etched slide glass has a similar trend, and the maximum transmittance wavelength is from 658 nm (20 mL, 95.49%) to 472 nm (30 mL, 97.64%) to 630 nm (40 mL, 94.52%). As shown in Figure S4d, with increase of the etching time, the maximum transmittance of asetched slide glass again has a similar trend, and the maximum transmittance wavelength is from 398 nm (18 h, 96.01%) to 402 nm (24 h, 98.52%) to 473 nm (30 h, 98.98%) to 405 nm

Figure 2. AFM images of (a) unetched slide glass and (b) slide glass as etched at 15 °C for 30 h using 30 mL of H2SiF6 solution (1.00 mol L−1).

and deeper pores formed on the as-etched slide glass, its surface became smoother compared with untched slide glass. The rootmean-square (rms) roughness of slide glass before and after etching was estimated and is 0.496 and 0.268 nm, respectively, further supporting the above observations. Compared to traditional antireflective coatings formed by layer-by-layer assembly or dip-coating of SiO2 nanoparticles, the as-etched glass surface is much smoother and denser.15 This feature may endow the antireflective structure better mechanical strength and durability, which will be proved in the following pencil hardness tests. 3.2. Optical Performance. Effects of etching conditions on transmittance were investigated. Figure S4 shows the transmission spectra of unetched slide glass and as-etched slide 3091

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the decomposing of H2SiF6 (Figure 4b, left). After a certain period of time, a condensed film of HF, H2SiF6, and H2O would form and continue growing on the surface. Considering the existence of SiF4 and the development of etching, more H2SiF6 and H2O would form in the condensed film (Figure 4b, right). When the etching was complete, the liquid H2SiF6/HF/ H2O film could be evaporated or rinsed off. The dissolution of glass is a heterogeneous reaction, which makes it difficult to control the dissolution process and the etching rate. Therefore, it is extremely important to understand the relationship between the HF concentration, the ionic composition of the solution, and the etching mechanism. The specific property of HF-containing solutions to attack the glass is related to the presence in solution of the fluorine-containing species: F−, HF, HF2−, SiF6−, and H2F2, which results from HF dimerization reaction. Since it is nearly impossible for the SiF6− ion bonding with Si, the reaction process can be concluded as shown in Scheme 1.25 A and B are in a dynamic equilibrium in

(36 h, 98.47%) to 472 nm (48 h, 97.64%). However, the transmittance increases monotonically with increase of the H2SiF6 concentration and reaction temperature in the wavelength range of 900−1500 nm (near-infrared region). As shown in Figure S4a, when the concentration of was 1.5 mol/L, the largest enhancement in transmittance reached 7.23% (790 nm) in the near-infrared range. After the start of etching, the etching depth increased and thus the air fraction of composite film increased, resulting in the decease of neff. When the neff of composite film was close to the ideal value of 1.22, light reflection could be effectively suppressed. As discussed above, the AR property of porous structure originated from the tunability in neff. Too high or too low H2SiF6 concentration may result in a relative quick or slow etching rate for the same etching time to obtain the most ideal neff. As the etching progressed, nanoscaled particles, which might act as scattering centers and reduce light transmission, were formed simultaneously on the surface of glass, as shown in Figure S5. 3.3. Etching Mechanism. In 1923, Jacobson proved that fluorosilicic acid is a nonvolatile acid, like H2CO3 and H2SO3, and cannot exist in the vapor state under ordinary conditions with four different types of experiment.28 Therefore, there would be only HF, SiF4, and H2O existing in the gas phase of the homemade enclosed reactor. We took the lead from the Helms and Deal’s work32 to develop a model of vapor phase etching of SiO2 in light of the above results. The basis of the model is the initial assumption that etching would proceed only if a condensed HF containing liquid layer is presented on the surface of glass. It was proved by a simple experiment: after etching, the slide glass was taken out and immediately placed in a beaker filled with 200 mL of pure water; the pH value of pure water changed from 7 to 3.5−4.5. Nevertheless, examination of the change in pH value at varied temperatures and for varied periods of etching time revealed that the change was not very regular. The above speculation was also confirmed by Figure 4a.

Scheme 1. Reaction Mechanism of the Dissolution of SiOH Unit in HF Solution

acidic solution. The first reaction step is the elimination of OH− (for A) or H2O (for B) from the surface to form the reactive intermediate D because OH− groups are very poor leaving groups and need some other groups, which are HF2− and H2F2, as helpers to eliminate them. Because of the FH part parallel to the HO side of the silanol group and the second F atom close to the silicon atom, the elimination and addition can take place consecutively. Then a more stable Si−F bond is formed. The current data show that the dissolution of silanol in HF solutions is the pivotal reaction step. It provides a valid explanation for the formation of hierarchical structures. There are more silanol groups on the glass defects (flaws or cracks); thus, etchant could follow the glass interior tens of nanometers, not just on the surface of glass. Besides, the incorporation of other oxides in glass modifies its interconnected three-dimensional siloxane structure and also has important implications for the etching process. In glasses, network modifiers such as Na2O, K2O, CaO, and Al2O3 are bonded ionically to the silicate network, and therefore they are relatively mobile.21 In acidic etching solutions, these mobile metal ions could be leached out of the glass and replaced by ion-exchanged H3O+ ions. Take Na2O as an example, when H3O+ ions diffuse into the vitreous network, ion exchange takes place as the following reaction:

Figure 4. (a) Digital image comparison of unetched slide glass (upper) and slide glass etched by 30 mL of H2SiF6 (1.0 mol L−1) at 15 °C for 72 h (lower). (b) Schematic illustration of the processes occurring during the etching of glass.

After a long reaction time (72 h), the liquid layer on the glass surface was clearly visible. Based on the assumption, the etching should have proceeded in the same way as the HF liquid solution etching. The etching process is schematically summarized in Figure 4b. Initially, the surface of glass was exposed to a vapor phase mixture of HF, SiF4, and H2O due to

≡Si−O−Na + H3O+ → ≡Si−OH + Na + + H 2O 3092

(1)

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The change in the amount of network-modifying oxides that occurs in the etching process was estimated by the EDS spectra on the top surfaces of the glass before and after etching. And the change in atomic ratio of M (M refers to cations in the glass) is revealed in Table 1 to compare the corrosion behavior Table 1. Atomic Percentages of Elements Revealed by EDS on the Top Surface of Unetched Slide Glass and Slide Glass Etched at 15 °C for 30 h Using 30 mL of 1.00 mol−1 L H2SiF6 element

Si

O

Na

Ca

Mg

Al

K

at. % in unetched slide glass at. % in asetched slide glass

23.62

58.79

7.59

2.34

1.80

0.36

0.17

22.85

59.98

7.43

2.14

1.72

0.35

0.11

Figure 5. 3-D finite model describing the evolution of surface cracks with etching time (t0 = 0 h, t1 = 12 h, t2 = 30 h). Etching condition: 30 mL of H2SiF6 solution (1.0 mol/L) at 15 °C. The white scale bars on the micrographs represent 200 nm.

of different cations. All the metal cation contents were lowered to varied extents after etching. The bonding energy between metal cation and nonbridge oxygen has a major impact on the ion exchange rate. For instance, the bonding energy of Na−O and Al−O are 94 and 330−422 kJ mol−1, respectively. Both of the bonding energies are lower than the Si−O−Si network bonding energy (443 kJ mol−1), which makes them easily attacked. As the Al−O bonding energy is high, it is relatively resistant to ion exchange. Thus, the atomic ratio of Al in slide glass shows little change after etching. Different methods have been developed to determine the etching or dissolution rate of glasses, such as (a) for some early studies, it was based on the weight loss of dispersed powders with known surface areas, especially in a slurry reactor.33,34 (b) Some methods are based on partially masking the surface of a glass body, e.g., by photoresist or wax, and the dissolution rate can be determined by measuring the depth of the recessed etched region.35 (c) Other methods are based on monitoring the decrease in film thickness during etching by ellipsometry or other optical interference methods.36,37 However, due to the imperfections of the theory and the lack of a variety of parameters at 15 °C, the etching rate calculation needs further study and discussion. 3.4. Morphology−Etching Relationship. The etching process not only removes the surface material of glass but also changes its surface morphology. Since the silicate etching definitely has a directionality effect, the type of glass surface structure would play an important role to initiate the HF etching reaction. For instance, glasses etch more rapid than mica and coesite in HF solution. Combined with previous works,21,26 we developed a three-dimensional finite model to describe the etching behavior of a surface which contains a distribution of cracks. Figure 5 shows how a closed flaw or microcrack on an unetched glass is etched open, and this surface defect is gradually transformed into a cusp as etching proceeds. At time t0, etching has not yet started, and each crack remains closed. At time t1, the surface detects gradually transform into much longer and wider cracks. At time t2, the surface detects are etched open and hierarchically porous structure forms. Some cusps intersect and coalesce with one another. After etching, the surface of glass is still smooth and relatively dense, which indicates that the diffusion of fluorine species over the glass surface into the closed crack is faster as compared to the dissolution rate. This enables the etching method with two advantages. On one hand, the smoothness

and compactness of surface are conducive to maintain the strength and durability of glass surface. On the other hand, the trend of increasingly deep crack and the formation of hierarchically porous structure are beneficial to enhancement of transmittance in a wide spectrum from visible to nearinfrared. 3.5. Wettability and Antifogging Function. Wetting is one of the most important functions of solid surfaces. It is an interface phenomenon between a solid surface and a liquid fluid. In particular, hydrophilic surfaces which have antifogging function have attracted wide attention these days. In the current etching process, after etching under varied conditions, the maximum WCA is 44.1° and the minimum WCA is 31.8°, as shown in Figure S6. At least three different locations were chosen for WCA measurements on the surfaces, and there were very little differences in the WCAs measured at these locations, indicating the uniformity of the surfaces. Compared to unetched slide glass (WCA: 13.9°), the WCAs of as-etched slide glasses increased to a certain extent (20°−30°), though they were still hydrophilic. The observed increases of WCAs might arise from the formation of Si−F bonds. The Si−F group has lower surface energy than the silanol group, which renders the surface more hydrophobic. Since the fluorine content of surface is relatively small, the contact angle increases only by 20°−30° under the conditions of this experiment. As shown in Figures 6a and 6b, the WCA of unetched slide glass is 13.9° and the WCA changed to 35.4° after etching at 15 °C for 30 h. In order to improve the mechanical stability and hydrophilicity of the surface, the as-etched slide glass was annealed at 720 °C for 135 s followed by oxygen plasma (120 W) treatment for 25 min.38,39 The oxygen plasma treatment rendered the as-etched and annealed slide glass a significant amount of the Si−OH group instead of the Si−F group. Therefore, the WCA of as-treated slide glass decreased to 3.2° (within 0.5 s), as shown in Figure 6c. The antifogging behavior of as-treated slide glass was also studied. The unetched and astreated slide glasses were simultaneously placed over a glass of 80 °C hot water for 5 s and then immediately placed on a white paper with dark letters (room temperature (ca. 60% RH)). As shown in the lower part of Figure 6d, the control slide glass 3093

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be good enough to ensure an economical and acceptable lifetime. Thus, a simple 2 month outdoor exposure test was designed and carried out on the etched slide glass. A suitable durability test should take into account existing environmental conditions. The temperature for the current test ranged from 12 to 37 °C, and the humidity ranged from 18% RH to 96% RH. Figure 8 shows the transmission spectra of an etched slide

Figure 6. Digital images of water droplets after 0.5 s of spreading on the surface of (a) unetched slide glass, (b) as-etched slide glass, and (c) slide glass after etching, annealing, and O2-plasma treatment. The volume of water droplets used was 3 μL. (d) Comparison of the antifogging behavior of unetched (lower part) and post-treated etched slide glass (upper part). The as-etched glass was fabricated by 30 mL of H2SiF6 solution (1.0 mol/L) at 15 °C for 30 h.

Figure 8. Transmission spectra of as-etched slide glass before and after aging outdoors for 2 months.

fogged immediately, and many water droplets were observed on the glass surface. In sharp contrast, the as-treated glass remained clear (the upper part of Figure 6d). Thus, the etched slide glass exhibited a superior antifogging property after oxygen plasma treatment. 3.6. Scratch Tests. The pencil hardness test measures the ability of a surface to resist scratches posed by pencils of varied hardness from 6B to 6H, which are the softest and hardest pencils, respectively, in the tester. The SEM image of scratch by 6H pencil is shown in Figure 7a. After magnification, the flaws

glass before and after outdoors exposure, respectively. After 2 months, the maximum transmittance changed from 98.52% to 96.24% and only decreased by 2.28%. Interestingly, the maximum transmittance wavelength had a red-shift after outdoors exposure. This might be caused by a subtle change of the surface structure due to some environmental conditions (such as storms, strong wind, and dust). It is known that AR coatings made by bottom-up methods (e.g., layer-by-layer, dip-coating, etc.) usually confront some problems. For example, the coatings usually do not strongly adhere to substrates (especially in water or some organic solvents), or the building blocks may degrade in the course of their use through weathering. In contrast, the as-etched slide glass is much more resistant to chemical, physical, and thermal actions. Normal water washing, elevated temperature (below the glass-transition point), and light almost would not impact on the antireflective property of as-etched slide glasses. Thus, employing the current method to prepare antireflective surfaces would greatly reduce the cost of cleaning and is more suitable for outdoor long-term applications.

Figure 7. (a) SEM image of as-etched slide glass scratched by 6H pencil and (b) enlarged view of the rectangled area in (a). The asetched glass was fabricated by 30 mL of H2SiF6 solution (1.0 mol/L) at 15 °C for 30 h.

4. CONCLUSION In the current work, we designed an ingenious reaction vessel and prepared AR surfaces with excellent mechanical properties on glass substrates via the H2SiF6-based vapor etching process. The reflectance of as-etched slide glass decreased by 7.78% at 475 nm, and the maximum transmittance reached as high as 98.95% under the optimal conditions. After annealing followed by O2-plasma treatment, the WCA of etched glass decreased to 3.2° in 0.5 s, and the as-treated surface exhibited superhydrophilicity and superior antifogging property. In addition, the as-etched glass showed outstanding hardness and durability. The pencil hardness is above 6H, which is usually not accessible by conventional coating approaches. The transmittance of asetched glass has only decreased by 2.28% after 2 month outdoor exposure. Among different AR coating fabrication methods, the H2SiF6based vapor etching has four main advantages. (1) The etching process is easy and does not require either special etching agent or expensive equipment, which makes it safer and economically more feasible and attractive. (2) As in the closed reactor, there

or cracks of etched slide glass were still clearly visible as shown in Figure 7b. (The bright particles in Figure 7b were pencil residues.) Thus, the 6H pencil could not mar the surface structure of etched slide glass; i.e., the glass surface still had excellent hardness after etching. He’s group40 reported hydrothermal treatment of SiO2−nanoparticle or polymer− nanoparticle thin films for enhanced mechanical property. In a 4H pencil test, however, the coatings were almost stripped off, indicating that the coatings could not pass the 4H pencil hardness test. In contrast, the as-etched slide glass exhibited better hardness property (>6H) to adapt to the needs of practical applications. 3.7. Durability Tests. For long-term applications, such as solar collector coverings, it is important that their surface coatings not only have enhanced sunlight transmission but good durability as well. In another word, their durability should 3094

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is little toxic emission from the process and thus little impact on the environment. (3) The glass so treated not only has excellent antireflection property but also exhibits excellent mechanical strength and durability. (4) This method is expected to be extendable to the etching of various SiO2based and Si-based materials. All these advantages point to its significant prospects for the preparation of large-area AR surfaces.

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ASSOCIATED CONTENT

S Supporting Information *

Figures S1−S6. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel 86-10-82543535; Fax 86-10-82543535; e-mail jhhe@mail. ipc.ac.cn. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National High Technology Research and Development Program (“863” Program) of China (Grant 2011AA050525), the Knowledge Innovation Program of the Chinese Academy of Sciences (CAS) (Grants KGCX2YW-370 and KGCX2-EW-304-2), and the “Hundred Talents Program” of CAS.

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REFERENCES

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