Sol–gel route for the building up of superhydrophobic nanostructured hybrid-coatings on copper surfaces

• A nanostructured hybrid coating for copper with controlled wettability is proposed.
• Copper oxides affect coating wettability if copper is treated at high temperatures.
• A scheme of the evolution steps for copper oxide formation is depicted.
• The deposition process is optimized in order to limit copper oxide formation.
• Stable superhydrophobic coatings are obtained by thermal annealing at 200 °C.

A wet chemical route is herein presented with the aim of building up a superhydrophobic coating on copper (Cu). A thin film of flower-like alumina – obtained by sol–gel – was deposited and combined with fluoroalkylsilane moieties, resulting in a hybrid coating with excellent repellence to water (static contact angle of 179 ± 1°) and self-cleaning properties (contact angle hysteresis of 5 ± 1°). The wetting performances were strictly related to the peculiar morphology of the coating's inorganic component and to the chemistry of the outer organic layer. The combination of the nanometric alumina lamellas with the micrometric roughness of sandblasted Cu surface proved to be essential to the formation of the hierarchical scaled structure allowing superhydrophobicity. However, the surface extension of alumina layer and its functional effectiveness were threatened by Cu oxides occasionally formed during the annealing steps necessary to stabilize the coating. Field emission-scanning electron microscopy (FE-SEM) and X-ray photoelectron spectroscopy (XPS) analyses of the surfaces, in fact, revealed the simultaneous presence of different chemical species and morphologies. Grains with cube-like aspect (attributable to Cu2O) were formed on coated surfaces thermally treated at 200–300 °C, while microwires of CuO appeared at higher temperature. Once the thermal conditions are carefully tuned and the temperature kept not higher than 200 °C, it is possible to limit the presence of Cu oxides which, in turn, means preserving a high level of performances, also avoiding brittleness phenomena and keeping unchanged the surface optical properties. The coating's stability and the maintenance of superhydrophobicity were preliminarily investigated following the water contact angle evolution after immersion of samples in ethanol in an ultrasonic bath.