“…Porous anodic alumina (PAA) layers are nowadays used for various purposes, for instance as photonic crystals [16] or as a template material for other material nanostructures such as nanowires [7,17]. Within the last decade self-organized nanostructured oxide layers (ordered nanoporous or nanotubular) were successfully grown by an optimized anodization process in fluoride containing electrolytes on a range of other valve metals, such as Ti [8][9][10][18][19][20], Zr [21][22][23], Hf [24,25], Nb [26,27], Ta [28][29][30], and W [31][32][33]. In particular, anodic TiO 2 nanostructures are extensively explored, as TiO 2 offers unique properties and a high functionality for various applications such as photocatalysis, self-cleaning surfaces, solar energy conversion, catalysis, biomedicine, sensors, and optical coatings.…”