Hierarchically Designed Nanoparticles for High-Transparency, Self-Cleaning Surfaces

Superhydrophobic surfaces provide an exotic opportunity for engineering diverse interfacial phenomena through their extreme liquid repellence.¹ One smart strategy for rendering superhydrophobicity is to simply roughen hydrophobic surfaces, which is often coined as the lotus effect, producing a static contact angle (<i>θ</i>_c) of &gt;150° and a contact angle hysteresis (Δ<i>θ</i> ) of &lt;10° for water droplets. The <i>θ</i>_c of flat hydrophobic surfaces is typically 100–120° because of their low surface chemical energy, whereas superhydrophobicity (<i>θ</i>_c &gt; 150° and Δ<i>θ</i> &lt; 10°) can be achieved when the surfaces are structured below or above wavelength scale.² Superhydrophobic surfaces can trap air beneath water droplets (i.e. Cassie state), which offers a means to promptly eliminate contaminants and to avoid icing and fouling in various environmentally benign applications.³<br/>Although superhydrophobic coatings have been investigated for over twenty years, there are still space for improvement in terms of scale up, anti-reflection function, and vision resolution, which restricts their usage in low-maintenance glass-based applications. Particularly, to impart an anti-reflection function to superhydrophobic coatings, their thickness and refractive index must be precisely adjusted to the wavelength of incident light and must be uniform over the entire coated area. In addition, such coated surfaces must possess an optical flatness (i.e. root-mean-square surface roughness &lt; one-tenth of a reference wavelength) to exhibit glass-like transparency.² To address these issues, in this study, we develop a scalable, high-precision-thickness superhydrophobic coating that reduces surface reflectance without degrading visual clarity. The rationale for this technology originates from hierarchically designed surfaces in which each three-dimensional array of colloidal or fumed silica nanoparticles acts independently to provide anti-reflection and self-cleaning functions, respectively. This design strategy is a key breakthrough because there is a fundamental trade-off between the optical and wetting properties; a roughened surface is desirable for rendering superhydrophobicity, which adversely affects visual clarity. The developed superhydrophobic surfaces exhibit outstanding droplet mobility such that their self-cleaning capability is fast and effective for various sizes of contaminants compared with uncoated glass and even commercial coatings. The optical properties of high transmittance and high clarity are valid over a broad range of wavelengths and incident angles, which enable high-performance solar energy and display applications. As a proof-of-concept, we conducted experiments on photovoltaic devices in a dusty environment and observed improved power conversion efficiency and a complete recovery of device performance using the hierarchically designed surfaces developed herein.<br/><b>References</b><br/>[1] Lafuma, A. & Quéré, D. Superhydrophobic states. <i>Nat. Mater.</i> <b>2</b>, 457–460 (2003).<br/>[2] Wang, D. <i>et al.</i> Design of robust superhydrophobic surfaces. <i>Nature</i> <b>582</b>, 55–59 (2020).<br/>[3] Geyer, F. <i>et al.</i> When and how self-cleaning of superhydrophobic surfaces works. <i>Sci. Adv.</i> <b>6</b>, eaaw9727 (2020).