The role of the alumina barrier layer thickness (δ(b)) on the growth of Ni nanowires (NWs) in porous anodic alumina (PAA) has been revealed. By varying the final anodization voltage to form dendrites at the bottom of the nanoporous structure, we are able to optimize δ(b) (in the 2-16 nm range), allowing us to obtain a Ni pore filling percentage (f(p)) of almost 100% for δ(b) = 10 nm. However, deviations from this optimal δ(b)-value led to a strong decrease of f(p). Moreover, an increase of the electrodeposition efficiency (EE) and NW homogeneity was also verified for δ(b) up to 10 nm. Such increase in nominal δ(b) leads to a consistent growth rate in all pores and consequently a complete and uniform nanopore filling. On the other hand, the decrease in electrodeposition efficiency visible for δ(b) > 10 nm is related with hydrogen evolution and dielectric breakdown of the insulator layer due to the required high deposition voltages. Non-uniform NW growth is then visible, with the consequent decrease in f(p). The control of the pore filling and length homogeneity of the fabricated 1D metallic nanostructures, combined with the ability to adjust the pore dimensions of PAA, can bring novel approaches for the fabrication of nano-objects and thus exciting new applications.

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.

With the increasing demand for high quality methods for the fast fabrication of extremely high aspect ratio nanoparticles, the research for efficient, low-cost and simple techniques has become fundamental. A promising approach on the synthesis of these materials is here addressed. Pulsed electrodeposition in porous anodic alumina templates was improved enabling, for the first time, a simple and cost effective fabrication method for vertically aligned nanomaterials with aspect ratios never reported with this method. Iron nanowires were electrodeposited and the effect of electrolyte molar concentration, temperature and stirring, pulse shape and barrier layer thickness on the deposition quality was investigated to potentially increase the template filling and the nanowires length. The electro-deposition temperature and current density were also found to be determinant parameters affecting NWs crystallography. A methodology of surface response design of experiment was conducted to retrieve the optimum values for the deposition parameters. With the determined optimized process, we were able to obtain filling ratios up to 93% and aspect ratios over 10 times higher than previous reports for an alternating current method. The high deposition homogeneity combined with the simplicity of the pulsed deposition method, can open new opportunities for the nanofabrication of nanowires.

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...

We investigate equilibrium properties of an exchange-spring magnetic system constituted of a soft layer (e.g. Fe) of a given thickness on top of a hard magnetic layer (e.g. FePt). The magnetization profile M (z) as a function of the atomic position ranging from the bottom of the hard layer to the top of the soft layer is obtained in two cases with regard to the hard layer: i) in the case of a rigid interface (the FePt layer is a single layer), the profile is obtained analytically as the exact solution of a sine-Gordon equationwith Cauchy's boundary conditions. Additional numerical simulations also confirm this result. Asymptotic expressions of M (z) show a linear behavior near the bottom and the top of the soft layer. In addition, a critical value of the number of atomic planes in the soft layer, that is necessary for the onset of spin deviations, is obtained in terms of the anisotropy and exchange coupling between the adjacent plane in the soft layer. ii) in the case of a relaxed interface (the FePt layer is a multilayer), the magnetization profile is obtained numerically for various Fe and FePt films thicknesses and applied field.

Multilayered WO 3 nanosquare platelet films were successfully grown on transparent TCO substrates by spray-coating WO 3 nanoparticles aqueous suspension prepared by the sol−gel method. This work assesses the influence of two annealing schemes in the photoresponse of WO 3 photoelectrodes with different film thicknesses. The photoelectrochemical characterization reveals that the slow-heating ramp produces a photoelectrode with an improved photocurrent density of 1.6 mA cm −2 at 1.23 V vs RHE. Comparing photoelectrodes with the same film thickness, the slow-heating ramp yields higher photocurrent densities, 80% more than the conventional fast-heating ramp. The effect of the annealing ramp on the morphology and crystalline-phase structure of WO 3 photoelectrodes is correlated with the photocurrent density. The slow-heating ramp annealing unveils film morphology with both higher porosity degree and higher nanosquare platelets dimensions. X-ray diffraction (XRD) structural analyses disclose that the films grow in monoclinic crystalline phase with a textural preferential direction [002], often related to improved photocurrent performances. The crystallite sizes and lattice microstrain are estimated using a simple X-ray diffraction broadening method, the Williamson-Hall analysis. A quantified correlation between the WO 3 lattice defects, intergrain strains, and performance is done. The proposed deposition method paves the way for producing efficient and scalable photoelectrodes of WO 3 for photoelectrochemical water splitting by using low-cost and simple manufacturing processes.

No abstract