nd templates for the growth of separated metal nanowires [13, 25, 26], or supported oxide or polymer nanotubes [27, 28]. However, for several applications, it is desirable that the APA thickness h is comparatively high, h ≥ 100 μm. This can give the film the required robustness for use as either a standalone membrane in case of, e.g., battery separator  or lithographic etching mask [18, 29], or the required high aspect ratio (a.r. = h/d, where d is the pore diameter) when using the layer as a template for, e.g., nanowire electrodeposition (where a.r. ≥ 1,000 can be required [21, 30]). However, the film growth rate vg reported so far for conventional mild anodization (MA) of Al has been in the order of 0.03–0.1 μm/min , which makes the growth of thick APA quite time consuming. This slow growth hinders the advantage of the relatively cheap setup for the fabrication of APA, discouraging its use for both the development of prototype structures in the research academy, and for possible industrial fabrication processes. The current interest in speeding up the APA growth has recently found a viable way in the application of a single-step process combining MA with industrial hard anodization (HA) conditions , made possible by the protecting oxide layer grown during the preliminary MA phase.
Another possible route for fast APA fabrication could be the identification of proper additives, which can conveniently change the environmental conditions for anodization. Ionic liquids (ILs) are a class of solvents that have recently attracted a renewed interest as chemical additives in a number of reactions [32, 33]. To our knowledge, the only use of APA and IL system has been so far as an additive in the fabrication of cobalt nanowires inside an APA template . In this work, we report on the use of ILs in the fabrication and characterization of APA, starting from a well-known APA fabrication EL such as oxalic acid. The idea behind using IL additives in this system is that the convective flow of the IL component species can facilitate the displacement of the EL ions useful for anodization, and avoid formation of strong temperature gradients between the anode and the beaker walls, which are in direct contact with the refrigerating bath.
Materials and Methods
We used 0.25-mm thick foils of polycrystalline Al (Goodfellows, 99.999% purity). The foils were cut with scissors into rectangular pieces of single face areaS ~ 15 × 3 mm2, and flattened back to roughly planar surface, after scissors curling, by pressing each of them between two new glass slides.
Degreasing was performed by hand brushing (>10 s) with acetone-wet lens paper, 3 min sonicating in warm (60 °C) acetone, rinsing in de-ionized (DI) water, sonicating another 3 min in warm (60 °C) ethanol, and thoroughly washing (>30 s) in running DI water.
After degreasing, an electropolishing (EP) step was performed on the Al foil, which was partly dipped in a 250-mL beaker filled up to 200 mL with a 1:5 v/v HClO4:C2H5OH mixture, and kept inside a refrigerating bath set atTbath =+7 °C. The cathode was a Pt plate, also partly dipped in the EL, kept at a gap distance ofg ~ 11 mm from the Al anode. The process was run for 7 min without stirring, at constant currentiEP.
In our setup, both sides of the dipped Al foil come into contact with the EL, the anodic contact being provided from the top, outside the EL. The dipped single face surface area for EP wasSEP ~ 12 × 3 mm2, and as a result the constant current density wasJEP = iEP/2SEP ~ 170 mA/cm2. During the process, hydrogen gas evolution could be observed at the cathode. The final Al surface looked mirror-like.
Since in our setup immersion of the beaker in the refrigerating bath was not compatible with stirring onto a magnetic plate, the temperature close by the Al anode was probably higher thanTbath(~+10 °C difference has been measured in several cases). We tried to minimize this effect during anodization as compared to EP, by keeping the anode as close as possible to the external temperature controlled bath, using in this case a much smaller be