Highly ordered TiO2 nanotubes (NTs) were synthesized by electrochemical anodization of Ti foils. We investigated the effect of the Ti surface roughness (applying different pretreatments prior to the anodization) on the length, growth rate and degree of selforganization 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 pretreatments 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 m 2 ), 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.

A. acknowledges the Faculty of Sciences of Oporto University for the financial support under the FCT project NANA/NMed-SD/ 0156/2007. J.V. and J.P.A. acknowledge financial support through FSE/POPH and Fundac -ão Gulbenkian ("Programa Gulbenkian de Estímulo a Investigac -ão Científica"), respectively.

The fundamental understanding of the barrier layer (δ b ) growth in TiO 2 nanotubes (NTs) is here established and compared with the classical metal oxidation theory from Mott and Cabrera. The role of δ b in the anodization of TiO 2 NTs under different applied potentials and times was analyzed using scanning transmission electron microscopy (STEM). Contrary to the well-known case of anodic aluminum oxide, we found that δ b of TiO 2 NTs progressively grows over time due to the nonsteady anodization regime. We then establish a relation between the phenomenological growth of the barrier layer with time and applied voltage, δ b (V,t) using the high-field Mott and Cabrera conduction theory.The developed model was found to be in excellent agreement with the experimental data from both STEM and anodization curves. On the basis of these results, the relationship between δ b and the anodization time and potential can now be quantitatively understood.Theory of the oxidation of metals goes back to the late 1940s, when Mott and Cabrera discussed the growth of oxide thin films formed by anodic oxidation under an applied electric field. 1 In the Mott−Cabrera picture, the oxide growth of Ti and other valve metals (Al, Hf, Ta, W, etc.) is governed by the high-field conduction mechanism. 1−3 Under higher fields, the entry of a cation across the metal/oxide interface into the oxide is the oxide growth ratedetermining step. Thus, during oxidation, both the rate of oxidation and the rate-limiting process depend on the thickness of the oxide. 1 According to the underlying theory, the growth kinetics of the passive film is described by the relation between current density (j) and the electric field strength (E = V/δ b , where V is the applied potential and δ b the oxide thickness)Under this approach, the electrochemical oxidation of metals can lead to (i) stable continuous oxide 2 films, if the oxide is insoluble to the electrolyte, or (ii) nanoporous oxide films if the oxide is fairly soluble in the presence of an acidic electrolyte. 4 Indeed, in past decades, Al and Ti electrochemical anodization together with other valve metals (Hf, Ta, W, etc.) has been widely studied because highly regular hexagonal arrangements of pores or nanotubes can be obtained. Both anodic aluminum oxide (AAO) nanoporous and anodic TiO 2 nanotubes (NTs) have stimulated considerable scientific and technological interest with extensive use in practical nanostructures. 5−9 In particular, the distinct properties of anodic TiO 2 NTs make it highly attractable for a wide range of applications, mainly in renewal energy sources such as H 2 generation by water photoelectrolysis and dye-sensitized solar cells (DSCs). 6,7Because Zwilling et al. first introduced the anodic oxidation of Ti using fluoride-based electrolytes, 10 anodization parameters such as electrolyte composition, applied potential, time, temperature, and Ti surface roughness were found to significantly influence the growth and morphology of TiO 2 NTs. This influence is seen in the crucial geo...

The introduction of voids in a magnetic thin-film alters the stray field distribution and enables the tailoring of the corresponding physical properties. Here we present a detailed study on thin magnetic nanohole arrays (NhAs) grown on top of hexagonally-ordered anodic aluminum oxide (AAO) substrates. We address the effect of AAO topography on the corresponding electrical and magneto-transport properties. Optimization of the AAO topography led to NhAs with improved resistance and magnetoresistance responses, while retaining their most important feature of enhanced coercivity. This opens new pathways for the growth of more complex structures on AAO substrates, a crucial aspect for their technological viability.

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