Fabrication of porous anodic alumina (PAA) templates with straight pores and with hierarchical structures through exponential voltage decrease technique

Abstract

The oxide barrier layer at the bottom of the pores has been successfully thinned by applying an exponential voltage decrease process followed by a wet chemical etching. The impact of the potential drop on the porous anodic alumina (PAA) structure has been deeply investigated, as well as the electrolyte temperature, the number of potential steps and the exponential decay rate. The results presented herein evidence that straight pores can be obtained and simultaneously remove the dielectric layer in spite of applying the exponential voltage decay during the PAA synthesis, through a smart adjustment between the anodization conditions and exponential voltage decay parameters. Additionally, the PAA structure can be tuned to fabricate hierarchically nanoporous templates with secondary pores ranging from 2 up to 10 branches. The presented simple procedure aims to become a standard step for the fabrication of the next generation PAA templates based devices.

Recent Advances in Nanoporous Anodic Alumina: Principles, Engineering, and Applications

“…With a different approach to alter the pore diameter, an improvement of the process control by mitigating the tendency to form branched pores that are commonly formed upon exponential decrease of anodization potential was demonstrated [ 177 ]. The development of the process was a consequence of attempts to achieve a thinner barrier layer and allowed to tailor a step-less architecture with a gradually decreasing pore diameter.…”

A Bioinspired Hierarchical Underwater Superoleophobic Surface with Reversible pH Response

“…Among the numerous methods, templating approach is particularly interesting for the growth of nanostructures and is an ideal procedure for mimicking biological surfaces. [ 32–35 ] Compared to other typical nanoporous templates, anodic aluminum oxide (AAO) membranes have received much attention because of their excellent thermal and chemical stability as well as structural parameters controllability. [ 36,37 ] AAOs with a high areal density (up to 10 11 cm −2 pores) and narrow pore‐size distribution over a large area have been applied in various fields, including energy storage and conversion, [ 38 ] sensors, [ 39 ] optical devices, [ 40 ] molecular separation, [ 41–43 ] catalysis, [ 44 ] tissue engineering, [ 45 ] drug delivery, [ 46 ] as well as other applications and aspects.…”

A Review on Anodizing of Aerospace Aluminum Alloys for Corrosion Protection

“…Finally, a new steady porous growth state is achieved. The observed current reduction is attributed to the delay between the applied potential and the thickness of the barrier layer underneath the pores [67,68]. From a morphological perspective van Put et al [69] and Curioni et al [68] showed that an oxide with varying morphology across its thickness is achieved when the potential is dropped during the process, with finer pores generated below the initially present larger pores.…”

“…On the contrary, if the voltage is slowly reduced the current will adjust to the new condition much faster [53]. Besides the rate at which the potential is dropped, other factors influence the speed of current recovery, such as the values of the original and final voltage, the difference between voltages, and the temperature of the electrolyte [53,67]. The recovery effect is often related to the initial chemical thinning of the barrier layer, followed by a stage of field assisted dissolution in which the film is dissolved faster but also a certain degree of growth takes place at the metal/film interface.…”

“…The impact of the electrolyte temperature on the oxide barrier layer thickness is controversial. In the literature claims can be found reporting barrier layer thinning with an increase of the electrolyte temperature [43,57], while others report no significant changes [54,67,80].…”