aker (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 of the sample roughness by means of the root mean square (RMS) of the distribution of sample features height. The RMS values of at least three AFM images acquired in different regions with 10 μm scan size have been averaged for each APA sample.
The APA top surface wettability was measured with different solvents by means of sessile drop method, using a DataPhysics OCAH 200 at laboratory conditions (temperature 17–20 °C, relative humidity 40–60%). Droplets of ~1 μL volume (drop diameter ~1 mm) have been used in all cases. Similarly to the RMS measurements, for the liquid contact angles (CAs), the values on at least three different regions on each sample have been averaged as well. For both used liquids and especially for the IL2 solution, the CAs were soon decreasing in time after touching the APA surface. The reported values have been measured immediately after contact (t < 5 s).
For removal of the Al substrate, which was necessary to determine the APA thickness h, we adopted a non-standard technique. The reason was that in our samples the Al was sandwiched between two adjacent APA films, and in these conditions, we found the standard dissolution in either saturated HgCl2 [1, 4, 6, 34–38] or CuCl2 [7, 8] not to be so effective as for one-side APA films on Al. Therefore, we decided to run a second EP-like process in much harder conditions than during the Al smoothing step, namely at RT and current density ~10 JEP. The hard Al etching was accompanied by a strong hydrogen bubbling at the cathode and by a typical noise, while quasi-periodic (1–2 Hz) Al spitting off between the two APA films was visible through the beaker walls. We stopped the process when we could see that a significant loss of Al had already occurred at the bottom of the dipped sample, sufficient for optical inspection (typically after a time t ~ 30 s). The APA thickness h was then measured by optical micrographs acquired in reflection perpendicular to the film sections, with a ± 1 μm resolution uncertainty.
Results and Discussion
Anodization in Bar