Sol-gel processes are very adapted and versatile to deposit layers with controlled homogeneity, thickness, porosity and associated surface structures. A specific route has been explored for the preparation of sol-gel nanostructured surfaces with high specific area, controlled porosity, and optical size dependant properties.

Our preparation approach based on a leaching process was developed for alumina based coatings [Tadanaga, J. Am. Ceram. Soc., 1997]. Alumina based spin- or dip-coated films on a large variety of substrates are annealed and dipped in boiling water. SEM observations show the formation of surface structures resulting from successive dissolution-precipitation mechanisms of the initial alumina-based film. Characteristic sizes of these structures (distance between spines, texture depth) can be easily tuned by adjusting different process parameters.

Moreover, the formulation can be changed to induce chemical composition, morphology and properties modifications of the final films: doping with TiO₂ for photocatalytic applications, hierarchical morphologies, increased film thickness and surface area.

Antireflective and wettability effects due to these nanostructures are currently studied and seem to be promising. Samples of such “hot-water-treated” alumina based coated glass have been optically characterised in the visible and near infra-red range. The reflection spectrum at normal incidence demonstrates a significant reduction of intensity for wavelengths in the range [400 – 1200 nm].

Moreover, the nanostructured surfaces are highly hydrophilic and therefore exhibit very interesting antifogging properties. If the nanostructures surfaces are silanised with fluorinated agents, the surface energy of the coating can be drastically reduced. Large superhydrophobic surfaces have been obtained exhibiting high water contact angle with highly mobile drops.

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