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 Bare Oxalic Acid
Independent of the anodization mode, if either potentiostatic or galvanostatic, in the steady state both i and V should be constant over the process time t, as for ionic conduction it is i ~ eαV, with α an appropriate constant [5, 14]. However, V(t) is linearly increasing in Fig. 1a after the initial transients, with different rates increasing in turn with J. This behavior shows that we were not in a condition of equilibrium for the different anodization reactions. The reason can be that in our setup immersion of the EL beaker in the cooling bath was not compatible with stirring onto a magnetic plate. Therefore, a local depletion of ions in the EL close to the anode, along with formation of a stable ion concentration gradient, can have occurred over time, which leads to a progressive increase in V in order for the power supply to keep J constant.
The h values obtained at tend = 30 min as a result of the corresponding anodizations have been plotted in Fig. 1b. If the same current efficiency of the process was maintained in all the anodizations and for all the process time period, a linear relationship between J and h was expected [5, 22]. However, after the initial increase in h with increasing J a tendency to saturation is observed in Fig. 1b. Obviously, a progressive reduction of the current efficiency has occurred, probably due to the appearance of side reactions different from the anodization ones .
Actually, side reactions can also occasionally lead to catastrophic events, such as shown in Fig. 1a for the curve at J = 200 mA/cm2. In that case after reaching a critical value Vcrit ~ 95 V in tcrit ~ 12 min, V started to decrease with some fluctuations, and finally increased up to the maximum power supply voltage. The reason for the latter increase was that the Al foil was cut at the air–EL meniscus, and the piece of anode dipped in the EL fell on the bottom of the beaker, opening the circuit. The APA thickness measured for this sample was hcrit = 11 μm, such that the respective critical electric f