The role of the Ti surface roughness in the self-ordering of TiO2 nanotubes: a detailed study of the growth mechanism

Abstract

Highly ordered TiO₂ nanotubes (NTs) were synthesized by electrochemical anodization of Ti foils. We investigated the effect of the Ti surface roughness (applying different pre-treatments prior to the anodization) on the length, growth rate and degree of self-organization of the obtained NT arrays. The mechanisms related to the TiO₂ NT formation and growth were correlated not only with the corresponding anodization curves but also with their appropriate derivatives (1^st order) and suitable integrated and/or obtained parameters, to reveal the onset and end of different electrochemical regimes. This enables an in-depth interpretation (and physical–chemical insight), for different levels of surface roughness and topographic features. We found that pre-treatments lead to an extremely small Ti surface roughness, offer an enhanced NT length and also provide a significant improvement in the template organization quality (highly ordered hexagonal NT arrays over larger areas), due to the optimized surface topography. We present a new statistical approach for evaluating highly ordered hexagonal NT array areas. Large domains with ideally arranged nanotube structures represented by a hexagonal closely packed array were obtained (6.61 μm²), close to the smallest grain diameter of the Ti foil and three times larger than those so far reported in the literature. The use of optimized pre-treatments then allowed avoiding a second anodization step, ultimately leading to highly hexagonal self-ordered samples with large organized domains at reduced time and cost.