Mechanistic aspects of the self-organization process for oxide nanotube formation on valve metals

In this paper we address several key aspects to the formation mechanism of self-organized oxide nanotube layers grown by anodization of valve metals and their alloys in fluoride ion containing electrolytes. We suggest that (i) the self-organized structure is produced as a result of an autocatalytic reaction, in which electrochemical oxidation and chemical dissolution of oxide accelerate each other; (ii) in the initial growth stage the competition for oxidizable area between neighboring initial growth spots is a key element in self-organization; and (iii) the diameter of the nanotubes on different materials and as a function of anodization voltage is strongly related with the anodic growth factor (nm/V) of the valve metal oxides. Additionally, for multilayer pore structure growth the present work provides insight into the sites of highest reactivity in repeated anodization experiments (bottom of the pores, in between pores). (c) 2007 The Electrochemical Society.