Transparent superhydrophobic surfaces using a spray coating process☆
Introduction
Although the availability of solar energy has variability depending on time and location it remains one of the attractive energy resources [1]. Maintaining clean surfaces and decreasing the accumulation of dust and other pollutants is important for increasing the efficiency of systems for solar energy conversion [2], [3] such as photovoltaic panels and solar mirrors. Dust is a term generally applied to solid particles with diameters less than 500 µm [4]. Accumulation of a film of dust is considered an environmental stimulus, third in importance after irradiance and air temperature [5]. Previous studies [6] have reported the soiling rate in different regions by studying declines in power plant performance and found that system efficiency due to soiling declines by an average of 0.2% per day without rainfall in dry climates. In addition, in photovoltaic systems rain and soiling by salty aerosols can lead to substantial leakage currents [7]. Using antisoiling coatings shows potential to reduce dust accumulation and maintain higher system efficiency. Requirements for low-soiling surfaces include hydrophobicity, low surface energy, and non-stickiness [8], weather and abrasion resistance and high transparency [9].
An inspiration for self-cleaning surfaces is the lotus leaf, which exhibits superhydrophobic (SH) properties with a water contact angle (WCA) of about 160° and a roll-off angle of 2–4° [10], [11]. Self-cleaning approaches that use of superhydrophilic [12] and superhydrophobic [13], [14], [15] coatings has been reported before but the lack of a coating scalable process has limited to smaller areas. SH coatings have low surface energy and low adhesion forces [16] which reduces the rate of dust accumulation and the superhydrophobicity facilitates near-complete cleaning of the surface during rain events. Durable anti-soiling coatings are still not available and it is a topic of on-going research to address this need. [9].
The transparent antisoiling coatings we describe in this work were deposited with a scalable spray coating process which produced thin layers of highly transparent SH silica nanoparticles bound to the substrate using a polymer binder. The spray deposition of the nanoscale nanostructured features on the surface due to the nanoparticles (50–200 nm) can also provide antireflective properties [17]. Producing superhydrophobic and superhydrophilic nanoparticle films with increased optical transmittance has been reported by multiple groups [15], [18], [19] and the surface feature sizes was kept below 200 nm in order to minimize light scattering at the shortest transmission wavelength (~ 300 nm).
The application of durable SH coatings on large surfaces has been hampered by the lack of an appropriate scalable deposition process. Developing a spray coating process that can produce transparent and durable layers was the goal of our study. In the past, we have developed spin coating deposition methods that were used to optimize the formulations of the transparent SH coatings [19]. Some of the best performing coatings (high WCA) were produced with the spin coating process using Cerakote [20] as the polymer binder [19].
Section snippets
Materials
Glass samples were used as substrates for the deposition of nanoparticle films by spray coating. The glass samples were cleaned by sonication in acetone, iso-propyl alcohol (IPA), and deionized water for 10 min. For the coating process, we used a chemical solution that consisted of 3 g Cerakote (MC-156 High Gloss Ceramic Clear) [20], 60.0 g of Acetone (or IPA), 0.3 g of commercial hydrophobic silica nanoparticle (Aerosil® r8200, fumed silica), and 0.3 g of fluorinated silica nanoparticles [19]. For
Results and discussion
The chemical solution that was used for the spin coating [19] was based on 3 g of Cerakote, 3 g of PCBTF, 20 g of IPA and 0.2 g of silica nanoparticles produced high quality SH surfaces (WCA > 165°). However, for the spray coating process we found that this concentration of binder was high and tended to mask or overcoat the silica nanoparticles, which resulted in a coating that was not superhydrophobic. We found that it was necessary to reduce the concentration of the polymer binder (Cerakote) and
Conclusions
Although there are many damage mechanisms that impact the coating durability outdoors, in this work, we discuss the effect of falling sand on low-cost superhydrophobic (self-cleaning) surface coatings that were deposited using a spray coating process. The antisoiling coatings are formed from thin layers of highly transparent superhydrophobic silica nanoparticles that are bound to the substrate using a polymer binder. Our results show that the WCA depends strongly on the ratio of the polymer
Acknowledgments
This work was supported by the DOE SunShot Initiative (SuNLaMP program) and the Laboratory Director’s Research and Development Program of Oak Ridge National Laboratory. Oak Ridge National Laboratory is operated for the U.S. Department of Energy by UT-Battelle under Contract No. DE-AC05-00OR22725.
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Notice of Copyright: This manuscript has been authored by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan).