Zirconia nanotube coating – UV resistant superhydrophobic surfaces

Superhydrophobic surfaces have always piqued an interest for weather-proof coating solutions often exploiting the ‘Lotus effect’ ^[1], self-cleaning as effective under active-photocatalytic conditions. Sub-micron structures are developed via electro-chemical anodization resulting in oxide film formation and is a versatile and cost-effective fabrication technique with high transferability and scale-up. Nanostructured surfaces offer superior surfaces for enhanced molecular adhesion as a function of surface roughness. Recently, such surfaces were created by hydrophobization of anodized Titania, ^[2] although effective lacked a long-term stability of their hydrophobic modifications due to it being susceptible to UV-mitigated degradation. In light of this limitation, we developed Zirconia based optically transparent super hydrophobic coatings. Zirconia nanotubes were synthesized via a single-step anodization in the absence of a hydrofluoric acid pre-etching step, improving operator safety and recording shorter experimental duration. These nanotubular zirconia surfaces were rendered super-hydrophobic with Octadecyl-phosphonic acid (OPA) monolayers via a simple immersion in bulk-solution for a minimum duration of six hours at ambient conditions.^[3] The zirconia-OPA composites reported prolonged hydrophobicity under sunlight in ambient conditions and when immersed in water for a period of 20 to 50 days and when subjected to high-power UV-lamp irradiation. These specimens were also tested for their optical transparency and report on the tunability of optical response as a function of nanotubular oxide-film thickness. In addition to this, high mechanical stability of the coating is reported when subjected to a jet of water. Nanotubular zirconia, when functionalized with Octadecyl-phosphonic acid (OPA- SAM) renders the surface superhydrophobic and resistant to UV damage and can serve as optically clear coatings on photo-functional surfaces.

References

1. The Lotus Effect: Superhydrophobicity and Metastability. Langmuir 2004, https://doi.org/10.1021/la036369u

2. Self-Assembly and Bonding of Alkanephosphonic Acids on the Native Oxide Surface of Titanium. Langmuir 2001, https://doi.org/10.1021/la010649x

3. Wetting behavior of zirconia nanotubes, RSC Adv. 2021, https://doi.org/10.1039/D1RA04751E