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ORNL superhydrophobic glass coating offers clear benefits
April 7, 2017 May 19, 2015 by Brian Wang
A moth's eye and lotus leaf were the inspirations for an antireflective water-repelling, or superhydrophobic, glass coating that holds significant potential for solar panels, lenses, detectors, windows, weapons systems and many other products.
The discovery is based on a mechanically robust nanostructured layer of porous glass film. The coating can be customized to be superhydrophobic, fog-resistant and antireflective.
"While lotus leaves repel water and self-clean when it rains, a moth's eyes are antireflective because of naturally covered tapered nanostructures where the refractive index gradually increases as light travels to the moth's cornea," said Tolga Aytug, lead author of the paper and a member of ORNL's Materials Chemistry Group. "Combined, these features provide truly game-changing ability to design coatings for specific properties and performance."
Journal of Materials Chemistry C – Monolithic graded-refractive-index glass-based antireflective coatings: broadband/omnidirectional light harvesting and self-cleaning characteristics
To be superhydrophobic, a surface must achieve a water droplet contact angle exceeding 150 degrees. ORNL's coating has a contact angle of between 155 and 165 degrees, so water literally bounces off, carrying away dust and dirt. This property combined with the suppression of light reflection from a glass surface is critical for improved performance in numerous optical applications, Aytug said.
The base material—a special type of glass coating—is also highly durable, which sets it apart from competing technologies, according to Aytug, who described the process.
"We developed a method that starts with depositing a thin layer of glass material on a glass surface followed by thermal processing and selective material removal by etching," he said. "This produces a surface consisting of a porous three-dimensional network of high-silica content glass that resembles microscopic coral."
The fact the coating can be fabricated through industry standard techniques makes it easy and inexpensive to scale up and apply to a wide variety of glass platforms.
"The unique three-dimensionality interconnected nanoporous nature of our coatings significantly suppresses Fresnel light reflections from glass surfaces, providing enhanced transmission over a wide range of wavelengths and angles," Aytug said. The Fresnel effect describes the amount of light that is reflected versus the amount transmitted.
Where solar panels are concerned, the suppression of reflected light translates into a 3-6 percent relative increase in light-to-electricity conversion efficiency and power output of the cells. Coupled with the superhydrophobic self-cleaning ability, this could also substantially reduce maintenance and operating costs of solar panels. In addition, the coating is highly effective at blocking ultraviolet light.
Other potential applications include goggles, periscopes, optical instruments, photodetectors and sensors. In addition, the superhydrophobic property can be effective at preventing ice and snow buildup on optical elements and can impede biofouling in marine applications.
Aytug emphasized that the impact abrasion resistance of the coating completes the package, making it suitable for untold applications.
"This quality differentiates it from traditional polymeric and powder-based counterparts, which are generally mechanically fragile," Aytug said. "We have shown that our nanostructure glass coatings exhibit superior mechanical resistance to impact abrasion – like sand storms – and are thermally stable to temperatures approaching 500 degrees Celsius."
Other ORNL authors of the paper, titled "Monolithic Graded-Refractive-Index Glass-based Antireflective Coatings: Broadband/Omnidirectional Light Harvesting and Self-Cleaning Characteristics," were Andrew Lupini, Gerald Jellison, Pooran Joshi, Ilia Ivanov, Tao Liu, Peng Wang, Rajesh Menon, Rosa Trejo, Edgar Lara-Curzio, Scott Hunter, John Simpson, Parans Paranthaman and David Christen.
A revolutionary impact on the performance of many optical systems and components can come from the integrative design of multifunctional coatings. Such coatings should be mechanically robust, and combine user-defined optical and wetting functions with scalable fabrication formulations. By taking cues from the properties of some natural biological structures, we report here the formation of low-refractive index antireflective glass films that embody omni-directional optical properties over a wide range of wavelengths, while also possessing specific wetting capabilities. The coatings comprise an interconnected network of nanoscale pores surrounded by a nanostructured silica framework. These structures result from a novel fabrication method that utilizes metastable spinodal phase separation in glass-based materials. The approach not only enables design of surface microstructures with graded-index antireflection characteristics, where the surface reflection is suppressed through optical impedance matching between interfaces, but also facilitates self-cleaning ability through modification of the surface chemistry. Based on near complete elimination of Fresnel reflections (yielding over 95% transmission through a single-side coated glass) and corresponding increase in broadband transmission, the fabricated nanostructured surfaces are found to promote a general and an invaluable [similar]3–7% relative increase in current output of multiple direct/indirect bandgap photovoltaic cells. Moreover, these antireflective surfaces also demonstrate superior resistance against mechanical wear and abrasion. Unlike conventional counterparts, the present antireflective coatings are essentially monolithic, enabling simultaneous realization of graded index anti-reflectivity, self-cleaning capability, and mechanical stability within the same surface. The concept represents a fundamental basis for development of advanced coated optical quality products, especially where environmental exposure is required.
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