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In this work self-cleaning and transparent surfaces were produced on glass surface with simultaneous wide-angle and good optical transmittance on the visible region. These properties are pursued by combination of multi-scale surface topology based on silica nanoparticles (SNPs), index grading and interference coating, as well as polytetrafluoroethylene (PTFE) self-assembly, using two approaches. In the first, two-layer approach (glass/SNPs/PTFE), the resulting samples presented a water contact angle (WCA) of 169° ± 2° with very low hysteresis, as well as significant antireflection. The second, three-layer approach (glass/SNPs/silica aerogel/PTFE), produced surfaces with WCA of 158° ± 2° with also very low hysteresis (<5°), in addition to normal transmittance of 99% or higher, which decreased less than 2% at 20° incidence. These results show that proper structure-coated glass, with a combination of interference and graded-index effects, may provide simultaneous wide-angle antireflection and self-cleaning properties.
©2012 Optical Society of America
1. Introduction
Anti-reflective (AR) coatings on glass have been subject of great technological and scientific attention for low-loss transmission optical systems, and particularly they enhance efficiency in many applications, such as solar panels and cells, video display screens, automobile windscreens, eyeglasses, windows of buildings, and so on [1–3].
In this work, two approaches are used for AR coatings. The first consists of a graded-index layer provided by textured surfaces [4], produced by silica nanoparticles, and can be used for broadband antireflection in a wide range of angles of incidence. The second approach is also interferometric and, in its simplest case, involves a quarterwave layer of transparent material, whose refractive index is the geometric average of the substrate and incident medium refractive indices [5,6]. More elaborate interferometric antireflection AR coating designs are obtained can be produced as a function of bandwidth, thickness, number of layers and available coating materials [7], but usually these are not suitable to wide-angle AR properties, unless a gradual refractive index is used, as in rugged filters [7,8].
For single layer antireflection of BK7 or soda-lime glass, AR film materials with low refractive index, around n = 1.22 [8], can be used, such as silica aerogels, which are a class of mesoporous solids usually with 90% to 99.8% vol. of air [7–11]. Silicate compounds have been used as precursors to aerogels since 1931 [12]. However the structure of an aerogel was then extremely fragile, which made it brittle and thus with low applicability. Nowadays resistant aerogels are achieved by adding water-repellent silane during the sol-gel process [13–16], and the final hydrophobicity can be changed by adding different precursors during synthesis [16].
Such hydrophobicity can be described by two main regimes. The Wenzel regime relates a roughness factor (r) to the solid surface, and the apparent contact angle of a liquid (θc) given by [17,18]:
where θ is the Young’s angle that describes the liquid contact angle on a smooth plane surface. The second regime was proposed by Cassie and Baxter as an extension of Wenzel’s equation to predict an increase of the apparent liquid contact angle when air is trapped between the liquid and the solid surfaces [17,18]:where f2 is the liquid surface area fraction in contact with air, and fraction f1 = 1- f2 refers to the solid-air interface area.Equation (2) reduces to Eq. (1) when f2 is zero (with f1 = r). Wenzel’s theory is applicable when the liquid completely fills the interstices on the solid surface, which then becomes hydrophilic. The Cassie-Baxter theory assumes that the liquid rests on top of the roughness features, and thus on a composite air-solid surface [17,18].
Superhydrophobicity is attained when a water drop contact angle becomes larger than 150° (in the static sense). On the other hand, when a small tilt angle is required for a water drop dislocation (associated with low hysteresis between advancing and receding water drop contact angles on a surface), which can be made possible by combination of micro and nanostructures with low chemical surface energy, the dynamic self-cleaning property (Lotus effect) can be attained [18–25]. Previous reported articles on material sciences, to our best knowledge, reported high hydrophobicity but still low AR on glass, and transparency in the visible region at normal incidence only [20,21]. Besides this, in this work, we distinctively take advantage of infrared wavelengths – much larger than the rough surface structures necessary for self-cleaning, for decrease of light scattering – to investigate the possibility of simultaneous self-cleaning and significant AR, through a combination of surface topology, interference and index grading, in a wide-angle range.
2. Experimental
The first set of films was prepared using a two-layer configuration composed of multi-scale silica nanoparticles, which created a graded-index on glass through topology, and of a PTFE layer, which, due to a combination of refractive index and physical thickness, acts as an AR interference coating. The second approach is based on a three-layer configuration that consists of the graded-index topology from the silica nanoparticle clusters, a silica aerogel interference coating, and a thin, dynamically superhydrophobic PTFE overlayer [26]. The PTFE refractive index typically lies in the range between 1.35 and 1.38, but may present significant dependence on layer thickness [27]. Thicknesses are 200 and 55 nm in the first and second approaches, respectively. For the aerogel layer, approximate values for refractive index and thickness are 1.22 and 200 nm. In both approaches, the silica layer, composed of nanoparticle aggregates ranging from nanometric to micrometric sizes (see SEM and AFM pictures in Fig. 1 , presents variable surface texture with thicknesses of a few microns. More importantly, its effective refractive index varies gradually between 1.00 (air) and 1.45 (fused silica).
Fig. 1 SEM and AFM pictures, where are presented the texture, in the scale shown of 10 μm, of the nanoparticle aggregates on the glass surface and the (left) and the topology produced by clustering of nanoparticles in the nanoscale level (right).
Silica nanoparticles. All commercial solvents and reagents were used as received without further purification. Fumed silica nanoparticles (Aerosil® R972 - Degussa/Evonik), previously treated with dimethyldichlorosilane (Aldrich) were diluted in toluene to obtain a 1 wt% concentration. Afterward the solution was dip coated on a soda-lime glass substrate, at a speed of 1 mm/s. The resulting films were dried for 24h at room temperature.
Silica aerogel. Silica aerogel was prepared via sol-gel synthesis using tetraethoxysilane (TEOS) (Aldrich) as precursor, following the procedure: TEOS hydrolysation in presence of ethanol 95% (Fluka) and deionized water under acidic conditions (HCl 0.1M Aldrich) at 60°C, with a molar ratio of 1/3.8/1.1/7x10−4 to TEOS/ethanol/H2O/HCl. After 90 min of stirring, aqueous ammonium hydroxide (Merck) was added with ethanol 95% at room temperature, resulting in the final molar ratios 1/38.8/3.6/7x10−4/2x10−3 to TEOS/ethanol/H2O/HCl/NH4OH. Following gelation and ageing at 50°C, the gel was washed three times with ethanol and hexane. The aerogel in hexane was derivatized with trimethylchlorosilane (TMCS), and sonicated for 30 min to create a fluid sol, suitable for dip coating [11–16]. After dip-coating, the film was dried for 2h at 250°C.
WCA measurements. Water contact angles (WCAs) were measured using the sessile drop method by deposition of 4-6 μL droplets of deionized water on horizontal dip-coated glass surfaces [21–24]. The droplets were observed directly in cross section with an Olympus BX-41 microscope, whereas their images were digitally captured using a 1.4 mega pixel computer-controlled digital CCD camera. The contact angle values were determined as averages of more than twenty measurements, performed in different areas on each sample surface. The sliding angle measurements were adjusted using a mechanical level goniometer. To determine hysteresis, advancing and receding contact angles were measured in both sides of the droplet, and in at least three different locations on each sample surface.
Atomic Force Microscopy (AFM) and Ellipsometry. Film micro and nano-structures were characterized by Atomic Force Microscopy (AFM). Thicknesses were measured with spectral ellipsometer Sopra GES-5E.
Transmittance. Transmittances at various angles of incidence were measured with a Cary 5000 UV-VIS-IR spectrophotometer. A goniometer was attached to the equipment to perform wide-angle measurements.
PTFE layer monitoring. This layer, with thickness around 55 nm, was deposited by thermal physical vapor deposition (PVD) at a base pressure of 2.10−6 Torr. A quartz microbalance monitor was used for thickness control. Furthermore the samples were characterized by Atomic Force Microscopy (AFM), which also was used for thickness post-deposition determination.
3. Results
Figure 1 (left) presents a multi-scale distribution of silica nanoparticles on the glass surface, as shown by the Scanning Electron Microscopy (SEM) pictures. Figure 1 (right) shows the 3D AFM image of silica nanoparticles aggregates. A combination of nano and microstructures favors attainment of superhydrophobicity.
At first, the clustered nanoparticle-covered glass presented a WCA and 161° ± 2°, which indicates a statically superhydrophobic surface, and hysteresis of 11° ± 2°. These results were expected, since the SNPs (Aerosil® R972) were previously treated with dimethyldichlorosilane, a well-known silane used to prepare hydrophobic coatings. On the other hand, the two-layer approach, where the silica nanoparticles (SNPs) were directly covered by a PTFE layer, also allowed for full superhydrophobicity (static and dynamic) with a water contact angle of 169° ± 2° and low hysteresis ≤ 2° (Fig. 2 ). These results indicate that besides the multi-scale topology, the PTFE overlayer plays a fundamental role in the low angular hysteresis, associated a self-cleaning property, due to its low surface energy and nanotexture.
Figures 3(a) -3(c) depicts the measured transmittance of coated (up to two layers) and uncoated glass sample at normal incidence and at 20° incidence for p-polarized and s-polarized light, respectively. The transparency results at normal incidence, in Fig. 3(a), are similar to those previously reported by Manca et al. and by Bravo et al. in the visible spectrum [20,21]. However, here we also attain: (i) visible wide-angle transparency, as shown in Figs. 3(b)-3(c) at ± 20° incidence for p-polarized and s-polarized light; as well as (ii) significant AR in the near infrared, with the high optical surface transmittance shown in the insets. It is worth mentioning that the transmittance for p-polarized light with 20 degrees angle of incidence is higher because, for p-polarized light only, there is a decrease in reflectance as angles of incidence get closer to Brewster angles. Furthermore, this two-layer approach, besides its simplicity, already allows for simultaneous wide-angle AR and self-cleaning performance.
Fig. 3 Spectral transmittance of coated (up to two layers) and uncoated glass sample (lower curve) at: (a) normal incidence, (b) 20° incidence for p-polarized light and (c) 20° incidence for s-polarized light in the visible spectrum. For these samples, PTFE was deposited on front surface only. Insets - Spectral transmittance of coated (up to two layers) and uncoated front glass surface (lower curve) in the near infrared spectrum at: (a) normal incidence, (b) ± 20°incidence for p-polarized light and (c) ± 20° incidence for s-polarized light.
In order to further improve the AR effect, in the second approach, the silica nanoparticles were dip-coated for an additional silica aerogel layer, which was later followed by the PTFE coating. Superhydrophobicity was attained with high WCA, as shown in Fig. 4 (left), and self cleaning, shown in Fig. 4 (right).
Fig. 4 Droplet image with WCA of 158° ± 2° (left) and self-cleaning property due to the low angular hysteresis (less than 5°) is made possible by silica nanoparticle clusters covered by silica aerogel and PTFE coating (second approach) (right) (Media 1).
The PTFE layer, deposited by PVD, produces a self-assembled, nanotextured surface, shown in Fig. 5(a) , and, when combined with silica nanoparticles and silica aerogel, attained a micro and nanostructured surface, shown in Fig. 5(b), that allows superhydrophobicity with low angular hysteresis. The roughness was measured by AFM, whose results are shown in Fig. 5(c). The surface roughness results from the PTFE layer (55 nm) and aerogel layer (200 nm), the latter over the silica nanoparticle aggregates. Besides the coatings anti-reflection effect, a higher roughness allows for more air trapping in the Cassie-Baxter regime, thus favoring superhydrophobicity.
Fig. 5 (a) PTFE nanotexture produced by PVD deposition and controlled by time and rate deposition. (b) 3D AFM image of silica nanoparticles, aerogel and PTFE layers (second approach). This combination produces a multi-scale level configuration. (c) Corresponding roughness measurement results.
Figures 6(a) -6(c) show the transmittance curves at normal and at ± 20° incidence with s and p polarized light, respectively. The average transmittance of the three layer coated surface in the 650-700 nm range was around 99%. Here those properties are attained with further dynamic superhydrophobicity (low hysteresis) that allows self-cleaning and transparency in a wide-angle range. Figures 6(b)-6(c) indicate a decrease of only 1 to 2% at 20° incidence using p and s-polarizations, respectively, in relation to transmittance at normal incidence. In addition to the high WCA (158° ± 2°), self-cleaning is attained and a high visible transmittance in wide-angle range by the sol-gel process, which involves mild chemical conditions in its preparation and is much less costly than photolithography. The insets in Figs. 6(a)-6(c) show the transmittance in the near infrared spectrum, which present up to 99.5% of transmittance between 1600 at 2000 nm.
Fig. 6 Spectral transmittance (a) at normal incidence, (b) 20° incidence for p-polarized light and (c) 20° incidence for s-polarized light. Insets – Similarly, spectral transmittance of uncoated and up to three layer coated glass surface in the near infrared spectrum at: (a) normal incidence, (b) ± 20° incidence for p-polarized light and (c) ± 20° incidence for s-polarized light.
4. Conclusions
A simple combination of surface topology, through index grading by multi-scale clusters of silica nanoparticles and PTFE interference coating (two-layers) indicated the viability of simultaneous transparency and superhydrophobic properties on glass, with a contact angle of 169° ± 2° and angular hysteresis ≤ 2°, as well as a high wide-angle transmittance in the visible region. An additional silica aerogel layer, combined with the silica nanoparticle multi-scale clusters and the PTFE layer, improves light transmittance to around 99%, at normal incidence in the 650 nm to 700 nm range, and dynamic superhydrophobicity is still attained with WCA of 158° ± 2° and angular hysteresis less than 5°. Better antireflection is attained from a combination of the low silica aerogel refractive index with an optical thickness of nearly a quarter-wave in the region between 600 nm and 700 nm. This visible transmittance remains high even in a wide angular range. Similarly with much better antireflection due to lower scatter loss in the near infrared spectrum, transmittance up to 99.5% is attained between 1600 at 2000 nm. Both approaches to superhydrophobic and self-cleaning coatings can be simply and cost-effectively produced in large areas with simultaneous antireflection and self-cleaning properties.
Acknowledgments
We would like to acknowledge the Center of Electronic Microscopy at Universidade Federal do Rio Grande do Sul for the SEM characterization, the Degussa Company for providing the silica nanoparticles and the Brazilian agency Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) for financial support.
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