ion 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. It turns out that a local maximum ofvgwas found for the point (c,J) = (0.5%, 200 mA/cm2), with valuevgmax(IL2) = 1.1 μm/min. This growth rate is higher than for the same acidic EL with 10-fold higher concentration (see Fig. 1b) and more than three times higher than for the EL solution with the same acid concentration (V(t) dotted line curve in Fig. 3a,vg ~ 0.3 /min).
Obviously, some improvement due to IL2 ions occurs only at intermediate c and J values. For J, side effects can be imagined to negatively affect APA formation on excessive increase of this parameter, such as an extraordinary local EL heating, with a consequent loss of current efficiency. On the other hand, too many IL ions in solution can overwhelm the other EL ions, and decrease the current efficiency in turn. We can work out the molar ratio of the IL–oxalic acid species in the experimentally observed best condition of 0.5% v/v for IL2. The numbers of moles for each species can be calculated as nox = MoxVox, where Mox is the molarity and Vox the volume of oxalic acid, and nIL2 = mIL2/MWIL2, where mIL2 is the mass and MWIL2 the molecular weight of IL2, respectively. Therefore, the molar ratio is nox/nIL2 = MoxMWIL2/ρIL2c, with ρIL2 the mass density of IL2. Since it is MWIL2 = 226.03 g/mole and ρIL2 = 1.21 g/mL, for c = 0.5% it turns out nox/nIL2 ~ 1.1. Therefore, the best improvement in vg on addition of IL2 is obtained for a ~ 1:1 ratio of the IL2 moles with respect to the moles of the oxalic acid. When this ratio was increased of a factor two, it was not possible to grow APA any more even with IL2. On the contrary, anodization run with the same ratio, obtained for example by doubling both concentrations (Mox = 0.06 M and cIL2 = 1%), produced APA with consistent h values. The color of the respective outer APA surfaces was also quite similar, pale yellow in all cases, as usually observed due to inclusion of oxalate ions [5, 14].
The molar ratio for the same relative concentrationc = 0.5% in the case of the other IL can also be calculated. For IL1, bei