rate the smaller pores and thus senses the respective APA as an almost continuous, smooth surface.
Anodizations in Oxalic Acid–IL Solutions
The general effect of addition of an IL in the EL, as compared to all the possible different anodization conditions for our setup, can also be seen in Fig. 3a. With respect to the dotted line curve, obtained for anodization run with external cooling and without stirring, the dashed line curve beneath it was obtained in the same EL also without stirring yet at RT. This curve presents a quite constantV level (after the initial transients). Obviously the higher EL temperature allowed for maintaining a higher ionic mobility as well, and no local ion depletion at the anode occurred, different from the cooled EL condition, (Figs. 1a,2a dotted line). A similar effect of approximately constantV was observed when the anodization was run at RT and stirring was also activated, as shown by the continuous line curve beneath the dashed one. In this case, theV level was even lower, as probably ion exchange and transport was further eased by the mechanical agitation, which was added to the thermal one. As a drawback obviously some instabilities were generated in the system, which resulted in strong fluctuations ofV. The situation of both cooled and stirred 0.03 M oxalic acid EL is not shown, as it was not experimentally accessible in our setup, but we would also expect a similar situation of approximately constantV(t), with intermediateV level lying between the high (RT, stirring) and the low (cooling, no stirring) ion mobility conditions. TheV(t) curves for anodization with cooled EL and no stirring (i.e., low ion mobility condition) but with IL contentc = 0.5% are also reported in Fig. 3a (thick black line for IL2 and thick gray line for IL1, respectively). One can see that theV level at regime was constant also in the latter cases, but significantly lower than for all the no-IL containing EL conditions. This was obviously due to the increased electrical conductivity of the EL after injection of the IL ions.
From Fig. 3a, it is clear that by keeping V low via the IL additive one can run anodization at comparatively high i as compared to standard values reported in the literature for bare oxalic acid EL, and still operate in MA condition. This should allow for avoiding detrimental effects such as the barrier breakdown observed in Fig. 1a. Furthermore, in case of two-step anodization it would help to keep the conditions as close as possible to the V desired for optimal ordered APA growth, which for potentiostatic process in oxalic acid is in the 40–60 V range [5, 10, 17].
The resulting conductivity of the IL1-added EL was four times as much for the IL2-added EL. This could make one think that the performance of IL1 solutions in APA growth would be better. However, APA films were observed after anodizations run with IL1 solutions only for the lowest relative concentration values explored, namelyc = 0.01%, and with comparatively lowJ = 20 mA/cm2. In those conditions,h was quite low, as expected (hIL1 = 5–10 μm). For higherJ and/or higherc, no APA film was obtained at all, and on the contrary black pits were always observed on the Al substrate. Atc = 0.5% andJ = 600 mA/cm2, in particular, anodization resulted in complete dissolution of the anode during successive rinsing in DI-water. Obviously the result of the high oxygen content, associated with the quite high conductivity (i.e., ion mobility), makes the dominating effect of IL1 to be a heavy ion bombardment of Al, rather than a support to the flow of the EL ions.
For the IL that on the contrary demonstrated to provide APA after most preliminary test anodizations, namely IL2, we decided to systematically investigate the results of the processes run for different combinations ofc andJ parameters, in the rangec = 0.01–2% andJ = 20–600 mA/cm2. In particular, in Fig. 3b, a part of the (c,J) “phase” space for all the combinations of the valuesc = 0.1%, 0.5%, and 1.0% andJ = 100, 200, and 400 mA/cm2is shown, with respect to the resulting APA growth rate. I