Understanding and controlling the hydrophobicity/-philicity of surfaces is critical for a range of applications. Traditional hydrophobic surfaces consisting of a polymer coating over a roughened surface often deteriorate in harsh environments, giving rise to increased wettability. Recently, however, researchers from the Massachusetts Institute of Technology (MIT) have shown that a series of rare-earth oxides ceramics (REOs) are intrinsically hydrophobic, a property related to unique structuring of water molecules at the oxide surfaces.
Sequential snapshots of a water droplet impinging on a surface coated with a thin layer of ceria (∼200 nm thick); the droplet cannot form hydrogen bonding with the REO and surface tension causes the water droplet to bounce off the coating illustrating the superhydrophobic nature of these materials. Scale bar 2.5 mm. Reproduced with permission from Nature Mater. (DOI: 10.1038/NMAT3545). © 2013 Macmillan Publishers Ltd.
As described in the January 20 online edition of Nature Materials (DOI: 10.1038/NMAT3545), G. Azimi and co-workers sintered REO pellets of the lanthanide series including ceria and lutecia. Upon subjecting them to thermal and abrasive testing, the REOs show minimal change to their hydrophobic nature. Superhydrophobic surfaces were generated by texturing the REOs using a range of techniques, such as sputter-coating onto smooth and microstructured silica surfaces. Contact angles as large as 160° were measured and video snapshots of water droplets falling onto the microstructured surface show the droplets bouncing off the surface.
The origin of the hydrophobicity of the REOs can be attributed to their electronic structure. While most ceramics and metals are hydrophilic, due to coordinative unsaturation which allows water to form bonds with available valence orbitals, the 4f orbitals of rare-earth atoms are completely shielded by the electrons in the filled 5s and 5p orbitals. They are thus not available to hydrogen bond with water molecules. This causes the water adjacent to the REO surface to exhibit a hydrophobic hydration structure.
This work highlights the flexibility and scalability of these intrinsically hydrophobic REOs, where they can be fabricated using standard ceramic processing methods. The research also addresses problems of robustness, and may lead to new hydrophobic applications in harsh environments.
