t 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, being MWIL1 = 338.42 g/mole and ρIL1 = 1.19 g/mL, it turns outnox/nIL1 ~ 1.7. This value is of the same order of magnitude as for IL2. Anyway even forc = 0.1%, no APA was obtained in the case of IL1, such that this negative result can only be assigned to an inherent chemical difference between the interaction of the two ILs with the oxalic acid.
IL Aided Characterization and Processing
After using the IL2 as an additive in anodization, we have also tried to take advantage of its properties in the characterization of the system. In Fig. 2b, the liquid CAs θ measured on the APA surfaces described in Figs. 12a have been reported. The filled squares represent the CAs obtained with DI-water as the wetting phase. One can see that all the respective APA films look rather hydrophilic (θwater < 90°). Most samples showed quite similar values (θwater ~ 29°), whereas only the sample with smaller pores showed a significantly higher θwater. The reason is probably that for that sample the pores were too small to be filled by the water, and the drop was actually sitting on a mixed APA–air interface . In practical terms, the water “probe” did not allow for high enough resolution to sense the smallest APA pores. A similar resolution limit affected also the topographic measurements by AFM, as observed in the RMS roughness plot of Fig. 2a (see the previous subsection for discussion). In Fig. 2b, the open squares report instead the CAs obtained with a c = 10% v/v solution of IL2 in the DI-water wetting phase, θIL2. In this case, the CA values were all quite similar to each other, and lower than for the bare water, θIL2 ~ 16°. This means that the IL2 solution had higher wetting power than water, and could wet even the smallest APA pores. Actually a similar behavior is expected from any IL, which should work such as a highly polar solvent that can easily penetrate voids of a few nanometer diameter only . Therefore, IL2 can also be used successfully for this kind of characterization of the porous APA surface morphology.
The effect of addit