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Superhydrophobic antireflective coating with high transmittance.

Abstract The aim of this study is to fabricate a superhydrophobic antireflective (AR) coating that can be deposited on the covering of a solar cell system. First, AR coatings were synthesized on glass substrates with an average transmittance over 96% by layer-by-layer deposition of polyelectrolyte. Superhydrophobic sol gel was prepared by hydrolyzing tetraethoxysilane and then reacting it with hexamethyldisilazane. The sol gel, aged at 20[degrees]C for 96 h, was used to spin-coat a superhydrophobic film with a water contact angle of 163[degrees] and a transmittance of ~91%. The superhydrophobic sol gel was spin-coated on the top of an AR coating to form a superhydrophobic AR coating on a glass substrate. The average transmittance, advancing contact angle, and contact angle hysteresis of the superhydrophobic AR coating, which was spin-coated from sol gel aged for 96 h or 168 h, were 94.5 [+ or -] 0.7%, 154.0[degrees] [+ or -] 1.5[degrees], and 15.4[degrees] [+ or -] 0.3[degrees] or 96.4 [+ or -] 0.2%, 158.4[degrees] [+ or -] 4.4[degrees], and 1.8[degrees] [+ or -] 0.3[degrees], respectively. Strategies for obtaining a superhydrophobic AR coating are discussed herein.

Keywords Superhydrophobic surface, Au tire Water contact angle, Sol gel

Introduction

The covering of a solar cell system is made of glass or plastic. Glass and common plastics have indices of refraction in the range of 1.45-1.7, (1), (2) so they reflect from 4% to over 6.5% of normally incident light on each air/substrate interface. The transmitted energy of a solar cell system will be lost considerably from multiple reflections. Accordingly, an antireflection (AR) coating on a covering is used to increase the efficiency of a solar cell system. Many approaches (3-10) are available for fabricating AR coatings. After some time, the surfaces of the coverings of a solar cell system in a natural environment become contaminated. Therefore, a self-cleaning coating on the covering helps to increase the efficiency of the solar cell system. Self-cleaning phenomenon is usually understood as the result of a rough surface with hierarchical micro- and nanostructures, formed of a material with a low surface energy, which makes the surface superhydrophobic with both a high water contact angle (WCA) (greater than 150[degrees]) and a low sliding angle (less than 5[degrees]). (11) Wenzel (12), (13) and Cassie and Baxter (14) recognized the importance of the surface roughness in achieving highly hydrophobic surfaces. Nishino et al. (15) showed that regularly aligned and close-packed C[F.sub.3] groups are associated with the lowest possible surface free energy of a flat surface. Therefore, the highest WCA that a smooth surface can reach is 120[degrees]. Since the microstructures of a superhydrophobic coating strongly scatter visible light, high transparency in the visible region and the superhydrophobic property are mutually exclusive in principle. Consequently, the transmittance of most of the optically transparent superhydrophobic coatings reported in the literature (16-29) is, not very high. The transmittance of ZnO-based, (16-18) carbon nanotubes-based, (19), (20) or BN-based (21) transparent superhythophobic films is about 80% or even lower. The transmittance of some superhydrophobic AR films (22-26) is close to that of untreated glass (~90.6%). Chen and colleagues (27) fabricated an AR superhydrophobic silica-based surface using a tetraethoxysilane (TEOS)/polypropylene glycol hybrid. They reported transmittance values relative to that of an untreated glass substrate, rather than absolute values. Rubner and colleagues (6) made AR coatings with an average transmittance of greater than 98% by the layer-by-layer (LBL) assembly of polyelectrolytes. Shiratori and colleagues (28) synthesized ultra-water-repellent films by the LBL assembly of polyelectrolytes and Si[0.sub.2] nano-particles, but they did not represent any data concerning the light transmittance of the films. Rubner and colleagues (29) fully exploited the advantages of LBL processing to optimize the level of roughness to maximize superhydrophobicity while minimizing light scattering. The film with 20 bilayers of polyelectrolytes with Si[0.sub.2] nanoparticles exhibited superhydrophobic behavior, but its average transmittance was about the same as that of an uncoated glass. Manca et al. (30) fabricated superhydrophobic AR surfaces with a WCA of 168[degrees] and stable self-cleaning effect during 2000 h of outdoor exposure. The coatings produced an AR effect of more than 3% increase in optical transmittance. Zhang et al. (31) used roughness parameters to relate the superhydrophobic and AR properties of boehmite films. Their sample B1-300 showed the best optical transmittance, between 95% and 96%, over the entire visible spectrum. Li et al. (32) used LBL deposition process to fabricate highly transparent superhydrophobic coatings on a quartz substrate with an average transmittance higher than 97%, which was composed of the underlying AR nanoporous silica layer and the top transparent superhydrophobic Si[0.sub.2] nanoparticle layer.

In this study, AR coatings with a transmittance of higher than 96% were first fabricated on glass substrates by the LBL deposition of polyelectrolytes. Superhydrophobic sol gel was prepared by hydrolyzing TEOS and then adding the hydrolyzed TEOS with hexamethyldisilazane (HMDS). Subsequently, the superhydrophobic sol gel was spin-coated on the top of an AR coating to yield a superhydrophobic AR coating with a transmittance of ~96%. Different strategies for obtaining a superhydrophobic AR coating are also discussed.

Materials and methods

Synthesis of an AR coating on glass substrate

AR coatings on glass substrates were prepared by the LBL deposition of poly(allylamine hydrochloride)/poly(acrylic acid) (PAH/PA A) polyelectrolyte, which has been described in detail elsewhere. (6) The glass substrate was placed on a support that was suspended in the polyelectrolyte. Accordingly, PAH/PAA polyelectrolyte was deposited on both sides of the glass substrate. To synthesize an AR coating, we first assembled a PAH/PAA multilayer at pH 8.5 and 3.5 for PAH and PAA, respectively (denoted as 8.5/3.5 PAH/PAA). A 5.5 bilayer of 8.5/3.5 PAH/PAA was synthesized. Immersing the 8.5/3.5 PAH/PAA multilayer in an HCI solution at pH 2.4 for 65 s caused a pH-induced phase separation. A brief rinse (~15 s) in water at pH 5.5 and subsequent drying yielded a swollen nanoporous multilayer that exhibited a composite refraction index between that of the air and the polymer film. Several AR coatings with an average transmittance in the visible spectrum of higher than 96% were prepared for the later fabrication of superhydrophobic AR coatings on glass substrates.

Preparation of superhydrophobic sol gel

Superhydrophobic sol gel was prepared as described in detail elsewhere. (33) The optimal TEOS:[C.sub.2][H.sub.5]OH:[H.sub.2]0 molar ratio was 1:48:4 (pH 0.5), realized by hydrolyzing TEOS at 70[degrees]C for 2 h, and then adding HMDS (HMDS/TEOS = 2) by the HMDS(20) method. (33) The HMDS(20) method divided the HMDS into 20 parts, which were put into the sol successively at different positions. The sequential addition of HMDS proceeded for 3 h. Upon the fourth or fifth addition of HMDS and supersonic oscillation of the sol, part of the sol transformed into transparent jelly. This transparent jelly retransformed into transparent sol when additional [C.sub.2][H.sub.5]OH ([C.sub.2][H.sub.5]OH/TEOS = 24) was added and oscillated supersonically for 1-2 h. Besides the fixed 3 h, this period of 1-2 h was required to break up the transparent jelly. Then, HMDS continued to be added until the end of the procedure. The transparent jelly was then no longer present. To obtain an ideal superhydrophobic sol gel, while maintaining transparency as high as possible, further [C.sub.2][H.sub.5]OH ([C.sub.2][H.sub.5]OH/TEOS = 6) was added to the sol gel at the end of the addition of HMDS. Then, all of the sol gels were sealed hermetically in bottles and aged in a refrigerator at 20[degrees]C until they were used to spin-coat films on glass substrates. Sol gel A was spin-coated when aged for 48, 72, and 96 h. Sol gel B was spin-coated when aged for 72 and 96 h. Sol gels C and D were spin-coated when aged for 96 and 168 h, respectively. The samples that were spin-coated from sol gels A, B, C, and D were designated as samples A, B, C, and D, respectively.

Fabrication of superhydrophobic IR coating on glass substrate

After the syntheses of AR coatings on glass substrates, and preparation and aging of sol gel for 48, 72, 96, or 168 h to make it superhydrophobic, the superhydrophobic sol gel was spin-coated on the top of an AR coating to fabricate a superhydrophobic AR coating on a glass substrate.

Characterization of superhydrophobic AR coatings

The surface morphology and root-mean-square (RMS) roughness of the films were investigated using an atomic force microscope (AFM: Dimension D3100S-1) in tapping mode; a UV-visible spectrophotometer was adopted to obtain the transmittance spectra of the films. The WCA of the superhydrophobic films, determined using a contact angle tester, was an average of three independent measurements for each sample.

The classical way to measure advancing and receding angles is to tilt the sample until the drop just begins to roll down. An interesting alternative approach is to fix the tilt but increase the drop volume until movement begins. Both of the methods were adopted to provide the measurements presented here.

Results and discussion

AR coatings

Owing to the ionic character of [PAW.sup.+] and [PAA.sup.-], the AR coating of the PAH/PAA multilayer is hydrophilic and susceptible to be attacked by polar water molecules. The ionic bonding between [PAH.sup.+] and [PAA.sup.-] was transformed into covalent amide bonding by heat treatment at 220[degrees]C for 2 h. (34) The heat-treated sample was immersed in deionized water for 1 week. Figure 1 represents transmittance profiles of an AR coating immediately after the preparation (96.9%), after heat treatment (96.4%) and after immersion in deionized water for 1 week (97.0%). The transmittance was slightly reduced by heat treatment, but increased again to almost the value of the initial coating during the water immersion test. Figures 2a-2c display the AFM 3D morphology of the AR coating before heat treatment, after heat treatment, and after immersing in water for 1 week, respectively. There was minute change in pore size with the uniform distribution of the pores over the scanned region in Fig. 2a. There was a large pore in the central region: and the surrounding area possessed a solid structure (Fig. 2b). Such structure is not good for obtaining high transmittance. Nanoporous pores were uniformly distributed once again over the scanned region in Fig. 2c. These results demonstrated that the AR coating in the form of a PAH/PAA nanoporous multilayer with heat treatment at 220[degrees]C for 2 h was waterproof.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

Superhydrophobic coatings

[C.sub.2][H.sub.5]OH ([C.sub.2][H.sub.5]OH/TEOS = 6) was added to the sol gel after addition of HMDS (Experimental section) to disperse hydrolyzed TEOS molecules and prevent water or alcohol condensation. Accordingly, hydrolyzed TEOS reacted with HMDS to produce more hydrophobic bonds, while the transparency of the sol gel was maintained as high as possible. Table 1 presents the effects of aging on the WCA and transmittance of the films, which were spin-coated from sol gels A, B, and C onto an untreated glass substrate. Data are expressed in the form of "average values of three samples [+ or -] standard deviation" in Tables 1 and 2. All of the films were superhydrophobic and quite transparent, exhibiting transmittance levels of close to that of the uncoated glass substrate (~90.6%). Therefore, the static superhydrophobic property of prepared sol gels was fully ascertained.

From Table 1, the effect of aging on the hydrophobic property of the sol gel is evident. N[H.sub.3] produced by the reaction of hydrolyzed TEOS with HMDS plays a key role in the aging of the sol gel. TEOS was further hydrolyzed under the catalyzation of N[H.sub.4]OH and the hydrolyzed TEOS then reacted with HMDS to yield inure hydrophobic bonds. Hence, the WCA of spin-coated films generally increased with aging period.
Table 1: Effects of aging on the WCA and transmittance (%) of
the films that were spin-coated from sol gels A, B, and C onto
an untreated glass substrate

Aged period                         Sol gel A
(h):

                           48              72              96

WCA            150.2[degrees]  151.6[degrees]  155.5[degrees]
                     [+ or -]        [+ or -]        [+ or -]
                 1.7[degrees]    1.2[degrees]    2.5[degrees]

Transmittance   90.7 [+ or -]   91.2 [+ or -]   90.4 [+ or -]
(%)                       0.9             0.1             0.6

Aged period         Sol gel B                       Sol gel C
(h):

                           72              96              96

WCA            152.3[degrees]  157.9[degrees]  157.6[degrees]
                     [+ or -]        [+ or -]        [+ or -]
                 1.9[degrees]    1.5[degrees]    1.7[degrees]

Transmittance   91.0 [+ or -]   90.1 [+ or -]   90.2 [+ or -]
(%)                       0.3             1.1             1.3


Figure 3 displays a photograph of a water droplet, which was deposited on the coating by spin-coating from sol gel C. Since the surface of the film was superhydrophobic, the water droplet could not be smoothly deposited onto the film surface until its volume increased to 50.87 [micro]L. The WCA at the left and right side of the water droplet and their average values were 162.60[degrees], 164.47[degrees], and 163.53[degrees], respectively.

[FIGURE 3 OMITTED]

Superhydrophobic AR coatings

Since longer-aged sol gel produced superhydrophobic films with higher WCA and all of the films were quite transparent, sol gels C and D were used to spin-coat a superhydrophobic film on top of the AR coating.

Table 2 summarizes transmittance of the as-assembled AR coatings, the AR coatings after heat treatment and the superhydrophobic AR coatings, as well as the advancing WCA and contact angle hysteresis (CAH). Figure 4 shows the transmittance profiles of the AR coating and the superhydrophobic AR coating spin-coated from sol gel C (sample ST-C1) and sol gel D (sample ST-D1). The speed of spin-coating for sample C was 3500 rpm. The seal of the bottle that contained sol gel D was first opened after aging for 168 h. Upon opening, the gel was quite sticky. Therefore, the speed of spin-coating for sample D was, increased up to 5000 rpm to insure the complete spreading out of the sol gel over the entire AR coating. The calcination process slightly reduced the transmittance of the AR coatings. The transmittance of the superhydrophobic AR coatings of samples C was as high as 94.5 [+ or -] 0.7%, and their advancing WCA and CAH were 154.0[degrees] [+ or -] 1.5[degrees] and 15.4[degrees] [+ or -] 0.3[degrees], respectively. Nevertheless, samples D simultaneously possessed a very high transmittance of 96.4 [+ or -] 0.2%, 158.4[degrees] [+ or -] 4.4[degrees] advancing WCA and a CAH as low as 1.8[degrees] [+ or -] 0.3[degrees]. Figure 5 represents a photograph of the advancing and receding contact angles of sample D. Advancing WCA, receding WCA. CAH and the sample tilt angles were 164.0[degrees], 162.0[degrees], 2.0[degrees], and 2.0[degrees], respectively.
Table 2: Hydrophobic and optical properties of superhydrophobic
AR coatings that were spin-coated from sol gels C and D at their
developing stages

       Aged   [T.sub.1]   [T.sub.2]   [T.sub.3]   Advancing WCA
     period         (%)         (%)         (%)
        (h)

Sol      96  96.6 [+ or  96.3 [+ or  94.5 [+ or  154.0[degrees]
gel              -] 0,5      -] 0.4      -] 0.7        [+ or -]
C                                                  1.5[degrees]

Sol     168  97.0 [+ or  96.8 [+ or  96.4 [+ or  158.4[degrees]
gel              -] 0.1      -] 0.1      -] 0.2        [+ or -]
D                                                  4.4[degrees]

               CAH

Sol  15.4[degrees]
gel       [+ or -]
C     0.3[degrees]

Sol   1.8[degrees]
gel       [+ or -]
D     0.3[degrees]

[T.sub.1] (%), [T.sub.2] (%), and [T.sub.3] (%) are transmittance
of the as-assembled AR coatings, the AR coatings after heat
treatment and the superhydrophobic AR coatings, respectively


The average transmittance of multilayer films with 20 bilayers of [[PAH (7.5)/(50 + 20 nm) Si[O.sub.2] (9.0.)].sub.20] and top layers obtained according to procedure of Rubner and colleagues (29) was almost the same as that of an uncoated glass substrate (~90.6%). The WCA and CAH of the films before and after calcination were 161[degrees] and 5[degrees], and 157[degrees] and 7[degrees], respectively. Without top layers of single 20 nm silica nanoparticles, the WCA and CAH before and after calcination were 154[degrees] and 20[degrees], and 152[degrees] and 18[degrees], respectively. A comparison of the data in Table 2 with those in Table 1 of Rubner and colleagues, (29) shows that the superhythophobic behavior of sample D was superior to that of the [[PAH (7.5)/(50 + 20 nm) Si[O.sub.2] (9.0.)].sub.20] multilayer with top Livers of single 20 nm silica nanoparticles, whereas the superhydrophobic behavior of sample C was similar to that of the [[PAH (7.5)/(50 + 20 nm) Si[O.sub.2] (9.0.)].sub.20] multilayer without top layers. Importantly, the transmittance of the superhydrophobic AR coatings herein apparently exceeded that of the [[PAH (7.5)/(50 + 20 nm) Si[O.sub.2] (9.0.)].sub.20] multilayer system with or without top layers.

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

Table 3 summarizes the hydrophobic properties, roughness parameters, and average transmittance of visible light of some superhydrophobic films in the literature. Although CAH in the study of Myoung and colleagues (16) was just 2[degrees], transmittance of the film was only 80%. The sliding angle in the investigation of Rao and colleagues (25) was only 2[degrees], but the transmittance of the film was around 90%. CAH of the film in the study of Shang et al. (23) was as high as 50[degrees]. CAH in the research of Manca et al. (30) was lower than 5[degrees], and the transmittance of the film was ~93%. The transmittance of the films in the investigation of Zhang et al. (31) and De and colleagues (35) reached 94% or even higher, but no dynamic contact angle data were reported. The sliding angle in the study of Li et al. (32) was only 1[degrees], and the transmittance of the film was as high as ~97%. However, they used a quartz substrate, which is much more expensive than a glass substrate. The transmittance of the film in the research of Seeger and colleagues (36) was ~94%, but the sliding angle reached 20[degrees]. Superhydrophobic AR coatings with transmittance apparently higher than that of an untreated glass substrate are promising to be applied on the covering of a solar cell system.
Table 3: Hydrophobic and optical properties of some
superhydrophobic AR films in the literature

                                  WCA                     CAH

Myoung and               162[degrees]              2[degrees]
colleagues
(16)

Shang et                 165[degrees]             50[degrees]
al. (23)

Rao and     161[degrees]~172[degrees]
colleagues
(25)

Rubner and  152[degrees]~161[degrees]  5[degrees]~20[degrees]
colleagues
(29)

Manca et                 160[degrees]             <5[degrees]
al. (30)

Seeger and      160[degrees] [+ or -]
colleagues                 2[degrees]
(36)

De and          168[degrees] [+ or -]
colleagues                 3[degrees]
(35)

Zhang et
al. (31)

A1-100                   103[degrees]

A1-300                   152[degrees]

Al-500                   152[degrees]

B1-300                   154[degrees]

B2-300                   154[degrees]

Li et al.                157[degrees]
(32)

Kaless et               >150[degrees]
al. (37)

This work

ST-C1                    153[degrees]             15[degrees]

ST-D1                    164[degrees]              2[degrees]

                    Sliding angle  Transmittance        RMS  [S.sub.dr]
                                             (%)  Roughness         (%)
                                                       (nm)

Myoung and             5[degrees]             80
colleagues
(16)

Shang et                                   90~91     5.3 nm       <1.02
al. (23)

Rao and     2[degrees]-9[degrees]            ~90  10.0~1129
colleagues
(25)

Rubner and                                 90~91  42.4~48.2
colleagues
(29)

Manca et              <3[degrees]            ~93     90~120
al. (30)

Seeger and   20[degrees] [+ or -]            ~94
colleagues             4[degrees]
(36)

De and                                       ~94         30
colleagues
(35)

Zhang et
al. (31)

A1-100                                     92~93        2.7         2.5

A1-300                                     94~95       28.1        71.7

Al-500                                     93~94       16.0        46.1

B1-300                                     95~96       27.6        97.8

B2-300                                     94~95       13.8        24.2

Li et al.              1[degrees]            ~97      ~74.8
(32)

Kaless et                                    ~98
al. (37)

This work

ST-C1                                        ~95       47.6        50.0

ST-D1                  2[degrees]            ~96       25.5        33.4


Figures 6a and 6b display 3D AFM photographs of superhydrophobic AR coatings that were spin-coated from sol gels C and D, respectively. The RMS roughness values as shown in Figs. 6a and 6b were 47.6 and 25.5 nm, respectively. The 3D AFM morphology of the AR coating (Figs. 2a-2c) looked like many differently sized conic pillars. However, the 3D AFM photographs of superhydrophobic AR coatings demonstrated complicated morphologies that differed considerably from those of an AR coating. The surface morphologies as depicted in Figs. 6a and 6b differed from each other. A careful observation of these morphologies revealed that submicrometer-sized structures were composed of many nanometer-sized structures, which made the surface similar to the surface of a lotus leaf with its two-scale roughness characteristic. Figure 7 shows a scanning electron microscopy image of a superhydrophobic AR coating that was spin-coated from sol gel D. Numerous nanopores were uniformly distributed over the surface in Fig. 7 that was conducive for the passing of the visible light. Therefore, when the superhydrophobic sol gel was spin-coated on the top of an AR coating, the decrease of transmittance was less than 1%. Figure 8 shows an optical image of several water droplets on a superhydrophobic AR coating.

[FIGURE 6 OMITTED]

[FIGURE 7 OMITTED]

The refractive indices, calculated from transmittance profiles and electromagnetic wave theory, of untreated glass, AR coating, superhydrophobic coating, and superhydrophobic AR coating were 1.56 [+ or -] 0.01, 1.44 [+ or -] 0.01, 1.59 [+ or -] 0.01, and 1.47 [+ or -] 0.01, respectively. An AR coating must satisfy the relation [n.sub.e] = [([n.sub.0][.sub.s]).sup.1/2], where [n.sub.e], [n.sub.0], and [n.sub.s] are refraction indices of the coating, environment, and substrate, respectively. The refraction index of air is ~1, so [n.sub.e] is 1.25 for the glass substrate in the present study. The refractive index of our AR coatings (1.44 [+ or -] 0.01) was larger than 1.25. Complete AR was not obtained, but the transmittance level reached 96.8 [+ or -] 0.1%. The refractive index of superhydrophobic coatings, 1.59 [+ or -] 0.01, was obviously larger than 1.25 due to the structure roughness required for superhydrophobicity. Consequently, the transmittance level of a superhydrophobic coating was only 89-91 % (Table 1). When the superhydrophobic sol gel was spin-coated on the top of an AR coating, the transmittance level was certainly lower than the original AR coating due to the large refractive index of superhydrophobic sol gel. Because the thickness of the superhydrophobic coating was very small (40-50 nm) under a high speed (5000 rpm) spin-coating, the refractive index of a superhydrophobic AR coating (1.47 [+ or -] 0.01) was only slightly larger than that of an AR coating (1.44 [+ or -] 0.01).

[FIGURE 8 OMITTED]

Strategy for obtaining a superhydrophobic AR coating

Zhang et al. (31) analyzed the effect of roughness parameters on the superhydrophobic and AR properties of boehmite film. Table 3 shows some of their experimental data. The roughness parameter [S.sub.dr] measures the extent to which the actual surface area exceeds the projected two-dimensional area. The table demonstrates a very wide range of RMS roughness values (5.3-1129 nm) in the literature on superhydrophobicity, because superhydrophobicity is the cooperation of a rough surface with special micro--and nanostructures and a low surface energy material. The transmittance through a superhydrophobic film with an RMS roughness of greater than a few hundred nm clearly cannot reach 95% owing to the severe scattering of visible light. From Table 3, the highest RMS roughness of superhydrophobic AR films, including the present work, with a transmittance of at least 95% was 74.8 nm. Perhaps an appropriate [S.sub.dr] can be regarded as a prerequisite for superhydrophobicity. An excessively small RMS roughness and [S.sub.dr] are not conducive to superhydrophobicity. Of the five films of Shang et al. (23), all with 0.5 nm < RMS roughness < 8.7 nm and [S.sub.dr] < 1.02%, only film C (RMS roughness = 5.3 nm) was superhydrophobic. Furthermore, the 50[degrees] CAH of film C was too large. In the work of Zhang et al. (31) A1-100 (RMS roughness 2.7 nm, [S.sub.dr] 2.5%) was the only film that was not superhydrophobic. The superhydrophobic AR films of Zhang et al. (31) and those herein have a relatively high [S.sub.dr] (24.2% < [S.sub.dr] < 97.8%). Comparison of the roughness parameters of the A1-300 and B1-300 films of Zhang et al. (31) seems to imply that a larger [S.sub.dr] provides higher transmittance of a film. However, the roughness parameters of A1-500 and B2-300 of Zhang et al. (31) and of ST-C1 and ST-D1 herein contradict this finding. The CAH of sample ST-D1 (2[degrees]) was smaller than that of sample ST-C1 (15[degrees]).

Since a superhydrophobic surface and a highly transparent coating are mutually exclusive in principle, a superhydrophobic AR coating seems to be impossible to obtain in one step. One approach that is frequently used to produce a superhydrophobic AR coating is to fabricate a porous film with a particular roughness on a substrate to obtain a high transmittance. A material with a low surface energy that contains -- C[F.sub.3] or -- C[H.sub.3 ]groups (23), (25), (29), (31), (32), (37) was deposited on top of a porous film by chemical vapor deposition or dipping to generate superhythophobicity. Since the low-energy molecule layer is usually very thin, with a thickness of only few nanometers, it hardly affects the transmittance. Instead of depositing a film on the substrate, Kaless et al. (37) conducted a plasma treatment of transparent PMMA substrates to obtain an average transmittance in the visible spectral range of more than 98%. Shortly after plasma treatment, the PMMA substrates normally exhibit hydrophilic behavior, such that water droplets wet the surfaces. Kaless et al. (37) obtained superhydrophobicity by coating with fluoroalkyl silanes. In this work, another method was used to form superhydrophobic AR coatings. Highly transparent coatings were prepared on glass substrates by the LBL deposition of PAH/PAA polyelectrolyte. The roughness parameters of these coatings were 6 nm < RMS roughness < 14 nm and [S.sub.dr] < 1.07--neither of which were conducive to superhydrophobicity. A superhydrophobic sol gel was spin-coated on the top of a highly transparent coating to fabricate a superhydrophobic AR coating on a glass substrate. Sol gels that were formed by the reaction of HMDS and hydrolyzed TEOS simultaneously provided hydrophobic groups -- C[H.sub.3] and roughness required for superhydrophobicity. Accordingly, no additional surface chemical modification was needed.

The impact resistance of the superhydrophobic AR coating

The object of this research is to fabricate a superhydrophobic AR coating that can be applied on the covering of a solar cell system. This covering is susceptible to the impact of hailstones or gravel. Therefore, the impact test was performed to understand the impact resistance of superhydrophobic AR coatings. The dimension of the glass substrate was 3 mm x 20 mm x 20 mm. The hardness of the plastic bullet was 21.3 [+ or -] 0.7 HV and diameter was 0.56 cm. The speed of the bullet before impacting glass was 29.9 m/s, which was measured by a ballistic pendulum. If air resistance is ignored, it was equivalent to the speed of an object which falls from a height of 45.6 m. Every superhydrophobic AR coating was subjected to the normal impact of the bullet 9 times. The impact points were uniformly distributed over the coating. Transmittance, advancing contact angle, and CAH before and after the impact test of three superhydrophobic AR coatings are given in Table 4. After the impact test, transmittance of the three superhydrophobic AR coatings decreased 0.4 [+ or -] 0.4%, but magnitude of transmittance was still as high as 96.0 [+ or -] 0.4%. Advancing WCA of three coatings increased 7.2[degrees] [+ or -] 5.0[degrees]. Before impact test, CAH was as low as 1.8[degrees] [+ or -] 0.3[degrees]. After impact test, CAH of three coatings increased 4.6[degrees] [+ or -] 0.2[degrees], but its magnitude was still as low as 6.4[degrees] + 0.3[degrees]. The increase of CAH is attributable to the high speed impact of the plastic bullet which caused the RMS roughness of three superhydrophobic AR coatings to increase from 32.7 [+ or -] 7.1 to 46.2 [+ or -] 3.8 nm. The results of impact test prove that the adhesion force between the coating and the substrate is very strong.
Table 4: Transmittance (T, %), advancing WCA and CAH of
superhydrophobic AR coatings before and after impact test Before
impact test

                  Before                        After
                  impact                       impact
                    test                         test

        T (%)  Advancing        CAH  T (%)  Advancing        CAH
                     WCA                          WCA

Sample   96.4      153.3        1.3   95.5      167.5        6.0
1              [degrees]  [degrees]         [degrees]  [degrees]

Sample   96.6      164.0        2.0   96.5      168.0        6.8
2              [degrees]  [degrees]         [degrees]  [degrees]

Sample   96.2      158.0        2.0   96.0      161.3        6.3
3              [degrees]  [degrees]         [degrees]  [degrees]


Conclusions

In this study, highly transparent coatings were fabricated on glass substrates with a mean transmittance of over 96% by layer-by-layer deposition of polyelectrolyte. Superhydrophobic sol gel was prepared by hydrolyzing TEOS and then reacting the hydrolyzed TEOS with HMDS. Superhydrophobic sol gel was spin-coated on the top of a highly transparent coating to fabricate a superhydrophobic transparent coating with a very high transmittance of 96.4 [+ or -] 0.2%, 158.4[degrees] [+ or -] 4.4[degrees] advancing WCA and a CAH as low as l.8[degrees] [+ or -] 0.3[degrees]. The first step, to obtain a superhydrophobic transparent coating with an average transmittance in the visible range of ~96%, is to deposit a highly transparent coating with a mean transmittance of over 96%. The principles on which a superhydrophobic surface is based differ from those on which a highly transparent coating is based. If the first step is to deposit a coating with the roughness required for superhydrophobicity, the transmittance of the resultant coating will not be able to reach 96%.

Acknowledgments

The authors would like to thank the National Science Council of the Republic of China. Taiwan, for financially supporting this research under Contract No. NSC_100-2221-E-224-075-.

[c] American Coatings Association & Oil and Colour Chemists Association 2013

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S.-D. Wang ([??]), Y.-Y. Shu

Institute of Materials Science. National Yunlin University of Science & Technology, 123. Sec 3. University Rd., Douliu, Yunlin, Taiwan, ROC

e-mail: sdwang@yuntech.edu.tw

DOI 10-1007/S11998-012-9468-9
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Date:Jul 1, 2013
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