te 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 beaker (50 mL, filled up to 30 mL).
As the starting anodization EL, we chose an aqueous (DI water) solution of oxalic acid ((COOH)2, Sigma-Aldrich, Italy), and decided to run this process as well as the EP atTbath =+7 °C without stirring. We have only run single anodization processes, and considered the inner APA surface (in contact with the Al substrate) as the test surface for the layer quality, that is the regular pore arrangement. The outer APA surface (in contact with the EL) has been checked as well, soon after anodization, for an estimation of the outer pore mouth diameterdout.
The anodizations were all run in galvanostatic mode, changing as a parameter the anodization currenti and so the current densityJ = i/2Sanod, whereSanodis the dipped anode surface. AnSanod ~ 10 × 3 mm2was used, significantly smaller thanSEP, to be sure to anodize only electropolished Al surface, and to avoid that possible side effects occurring at the ambient air–EL meniscus insist on the same region during the two consecutive processing steps, (namely EP and anodization).
As ILs to test we chose two different commercially available room temperature (RT) water-soluble ILs, namely 1-butyl-3-methylimidazolium 2-(2-methoxyethoxy) ethyl sulfate (C13H26N2O6S, “IL1”) and 1-butyl-3-methylimidazolium tetrafluoborate (C8H15BF4N2, “IL2”), both from Sigma-Aldrich. The choice was driven by the former being particularly rich in oxygen, possibly taking place in the anodization reaction in spite of the oxalic acid and/or water oxygen, and the latter being very easily soluble in water.
The APA outer and inner surfaces were imaged by means of atomic force microscopy (AFM) with a MFP-3D instrument (Asylum Research, USA), operating in Tapping mode with gold-coated silicon cantilever probes NSG10 (NT-MDT, Russia). The probes had a nominal resonance frequency ~250 kHz and standard tip (apex diameter ~10 nm, aspect ratio ~2.6). Apart from surface quality inspection, the AFM images of the outer APA surface have also been used for quantitative determination 