Preparation of self-organized porous anodic niobium oxide microcones and their surface wettability

Porous anodic niobium oxide with a pore size of ∼10 nm was formed at 10 V in glycerol electrolyte containing 0.6 mol dm−3 K2HPO4 and 0.2 mol dm−3 K3PO4 at 433 K. After prolonged anodizing for 5.4 ks, niobium oxide microcones develop on the surface. X-ray diffraction patterns of the anodized specimens revealed that the initially formed anodic oxide is amorphous, but an amorphous-to-crystalline transition occurs during anodizing. As a consequence of the preferential chemical dissolution of the initially formed amorphous oxide, due to different solubility of the amorphous and crystalline oxides, crystalline oxide microcones appear on the film surface after prolonged anodizing. The surface is superhydrophilic. After coating with fluorinated alkylsilane, the surface becomes superhydrophobic with a contact angle of 158° for water. The surface is also oil repellent, with a contact angle as high as 140° for salad oil.