Letter pubs.acs.org/NanoLett
Superomniphobic, Transparent, and Antireflection Surfaces Based on Hierarchical Nanostructures Prantik Mazumder,*,† Yongdong Jiang,‡ David Baker,† Albert Carrilero,§ Domenico Tulli,§,⊥ Daniel Infante,§,# Andrew T. Hunt,‡ and Valerio Pruneri*,§,∥ †
Corning Incorporated, Sullivan Park, Corning, New York 14831, United States nGimat Co., 1824 Willow Trail Parkway, Norcross, Georgia 30093, United States § ICFO−Institut de Ciències Fotòniques, Av. Carl Friedrich Gauss, 3, 08860 Castelldefels, Barcelona, Spain ∥ ICREA−Institució Catalana de Recerca i Estudis Avançats, Passeig Lluís Companys, 23, 08010 Barcelona, Spain ‡
ABSTRACT: Optical surfaces that can repel both water and oil have much potential for applications in a diverse array of technologies including self-cleaning solar panels, anti-icing windows and windshields for automobiles and aircrafts, lowdrag surfaces, and antismudge touch screens. By exploiting a hierarchical geometry made of two-tier nanostructures, primary nanopillars of length scale ∼100−200 nm superposed with secondary branching nanostructures made of nanoparticles of length scale ∼10−30 nm, we have achieved static contact angles of more than 170° and 160° for water and oil, respectively, while the sliding angles were lower than 4°. At the same time, with respect to the initial flat bare glass, the nanotextured surface presented significantly reduced reflection (90%) is only attained when the soot thickness is smaller than one micron for which the oil contact angle is not high enough to be superoleophobic. To the best of our knowledge, a surface with optical
Figure 2. Direct optical transmission as a function of wavelength. The antireflection effect and corresponding change in transmission of the nanostructuring of one of the two surfaces are clearly visible.
Table 1. Contact Angles in Degrees before and after the Branching Nanostructures, and Low Surface Tension Monolayer Fluorosilane Is Applied to the Flat and Nanopillar Corning Glass Substrates water (72.1 mN/m) flat glass nanopillars branching nanostructures branching nanostructures on nanopillars
113 144 165 172
± ± ± ±
3 4 3 4
oleic acid (32.9 mN/m)
hexadecane (27.7 mN/m)
78 ± 2 118 ± 2 157 ± 3 163 ± 3
67 ± 2 104 ± 3 150 ± 3 153 ± 3
interface develops, where air is trapped underneath the liquid.3,10−13 Thus, there is a reduction in true area of contact between the liquid and the solid and a significant area of the liquid meniscus could be suspended in air. The apparent B
dx.doi.org/10.1021/nl501767j | Nano Lett. XXXX, XXX, XXX−XXX
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Figure 3. Wetting of branching nanostructures on nanopillar substrate after application of fluorosilane: water and hexadecane droplets with contact angle of >170° and 150°, respectively (top), and image with three liquid droplets of different surface tension: water (bottom middle), oleic acid (bottom left), and hexadecane (bottom right).
transmission greater than >90% and oil contact angle greater than >150° has never been reported. In this letter, we report a nanotextured glass surface with high transmission, low haze, and superomniphobic properties. Hierarchical structure is made of primary nanopillars and secondary branching nanostructures. The initial hydrophobic and oleophobic properties of the primary nanopillar structure were further increased by the branching nanostructures. Branching nanostructures of small thicknesses could then be used, and this, combined with the antireflection property of the nanopillars,25 produced high optical transmission and low haze. We were able to produce surfaces with contact angles of more than 170°, 160°, and 150° for water, oleic acid, and hexadecane, respectively. The surface also showed very low contact line pinning or hysteresis, very high optical transmission of about 94% (average value over the visible), and very low haze of about 1%. Methods. The primary nanopillars on the glass surface are fabricated by creating nanoscale metal masks on glass followed by reactive ion etching of the latter. Ultrathin metal films are first deposited on the flat glass surface. In a subsequent and rapid thermal annealing step the metal film is dewetted into discrete nanoparticles. These metal nanoparticles are subsequently used as masks during reactive ion etching of the surface. After removal of the metal masks, the surface is covered by monolithically integrated nanopillars whose geometry could be controlled by the process conditions. Details of the process could be found in a separate reference.1 Two types of secondary nanostructures, branching nanostructures and nanonodules, were deposited on the primary nanopillars by the NanoSpray CCVD process using the proprietary Nanomiser2 device. The structures were controlled by the process conditions such as solution concentration, composition, and deposition temperature.26,27 All the nanostructures were treated with a low surface energy fluorosilane. The glass samples were placed in a fluorosilane solution for a certain period of time followed by rinsing by a solvent and deionized water and drying in nitrogen. The CCVD
Figure 4. Durable hierarchical nanostructure. Top: solid nanonodules on nanopillars (scale bar is 200 nm). Bottom: measured direct spectral transmittance showing the improvements due to primary nanopillars, secondary nanonodules on nanopillars, and resistance against rubbing of the hierarchical structure (nanonodules on nanopillars).
C
dx.doi.org/10.1021/nl501767j | Nano Lett. XXXX, XXX, XXX−XXX
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substrates with only nanobranching structures are 165° ± 3, 157° ± 3, and 150° ± 3. Finally, the contact angles of 172° ± 4, 163° ± 3, and 153° ± 2 are obtained on the substrate with primary nanopillars superposed with secondary nanobranching structures. To the best of our knowledge, this is the first such combination of superoleophobicity (Hexadecane CA > 150°) and high transmission (>92%). The improvement when one uses a substrate with nanopillars rather than a flat one is also significant if one considers the fact that starting contact angles are already very high. The rolling angles for the different liquids on both the hierarchical branching nanostructures on nanopillar and branching nanostructures on flat surfaces are similar, increasing from water (
