Superhydrophobic–Oleophobic Visible–Transparent Antireflective Nanostructured Anodic HfO₂ Multifunctional Coatings for Potential Solar Panel Applications

The fabrication of superhydrophobic-oleophobic HfO2 coatings presents a challenge. Here, we synthesize via the self-organized anodizing of aluminum-on-hafnium layers sputter-deposited onto SiO2 and glass substrate arrays of highly aligned HfO2 nanostructures of dissimilar shapes, sizes, spacings, and population densities termed as nanorods, nanopillars, nanohoodoos, and nanopillars-in-domains, which were then modified with various self-assembled monolayers. The treatment in fluoroalkyl-trimethoxysilane (FAS-17) appeared to be the most effective in making the hafnium-oxide nanoarrays superhydrophobic, with water contact angles of 153° (nanopillars), 155° (nanorods), 160° (nanohoodoos), and 174° (nanopillars-in-domains), the latter revealing the smallest roll-off angle of <1°. The FAS-17-modified HfO2 nanorods and pillars-in-domains demonstrated the best oleophobic properties with a contact angle for ethylene glycol of 140° and for rapeseed oil of 121°. In addition, the HfO2 nanorod films were highly transparent and antireflective in the visible spectral range and substantially less transparent and increasingly reflective in the near infrared. An outstanding combination of robust superhydrophobic-oleophobic properties with the specific optical behavior of the HfO2 nanoarrays makes them attractive for application as self-cleaning visible-transparent heat-repelling dielectric coatings for solar cell coverglass and concentrator photovoltaics.