Superhydrophobic surfaces based on self-organized TiO2-nanotubes

In this study, the interaction between a self-organized TiO2-nanotube layer formed by an anodizing procedure and a post-treatment with different water repellent coatings is investigated. Therefore, a conceptional model of these interactions is experimentally explored. Ti6Al4V samples are anodized using a fluoride-containing electrolyte. Using an anodizing voltage of 30 V, a homogeneous layer of TiO2-nanotubes with a pore diameter of 68.1 +/- 3.8 nm and oxide layer thickness of 589.2 +/- 23.7 nm could be generated on the surface. The anodization increases the surface area in comparison to the original state. To achieve water repellent surface properties the TiO2-nanotubes are functionalized by a selection of coating systems. Two different coating systems (coating 1: fluoroalkylsilane based; coating 2: perfluoropolyether based) are investigated within this study. By means of physical and chemical analyses both morphology and composition of the TiO2-nanotube surface are characterized. The scanning electron microscope (SEM) analyses illustrate that the tube morphology is not covered by the different coating systems. The chemical composition of the two different coatings is investigated by X-ray photoelectron spectroscopy (XPS). The results indicate that only the boundaries of the TiO2-nanotubes are covered by the coatings. However, the XPS results reveal that the different coatings do not fully infiltrate the tubular structures. Only the first nanometers (15-20 nm) are covered. To characterize and evaluate the superhydrophobic properties of the modified surfaces, the dynamic contact angles are measured. Each individual coating system shows a contact angle in excess of 150 degrees. Surfaces that are simply degreased in an alkaline solution, without post-treatment with a water repellent coating show a contact angle of 21.9 +/- 5.4 degrees. Hence, it becomes evident that only the combination of a surface nanostructuring and a water repellent coating leads to superhydrophobic surface properties. (C) 2015 Elsevier B.V. All rights reserved.