## 3 Results and Discussion
### Fabrication and Characterization of AAM
The comparison of the two images clearly discloses that the electric charge passed did not affect cell shape and population that is fixed at 6.5 \(\times\) 10\({}^{12}\) m\({}^{-2}\), i.e., the same value found for PAA formed in mixed 0.4 M  
Figure 9: Top views of PAA formed in 0.4 M H\({}_{3}\)PO\({}_{4}\) at 160 V and 0 \({}^{\circ}\)C: (**a,b**) inner, and outer surface at 20 C cm\({}^{-2}\), respectively; (**c,d**) inner, and outer surface at 160 C cm\({}^{-2}\), respectively. H\({}_{3}\)PO\({}_{4}\) and Al(OH)\({}_{3}\). It is noteworthy that, also in pure H\({}_{3}\)PO\({}_{4}\), the cell population is less than the pore population independently of the passed electric charge. This means that, also in pure 0.4 M H\({}_{3}\)PO\({}_{4}\), the channels do not have the same height, but only some of them extend through the total thickness of PAA, while other ones stop their growth, such as those shown in Figures 7 and 8. The pore diameter distribution on the outer PAA surface is shown in Figure 10 for different anodizing conditions. It can be observed that at the lowest concentration of Al(OH)\({}_{3}\), pore diameters change from about 40 nm at 20 C cm\({}^{-2}\) to 160 nm at 160 C cm\({}^{-2}\). In addition, a bimodal distribution is distinguishable at the lowest charge with a second peak at about 70 nm. Additionally, at higher Al(OH)\({}_{3}\) concentration (Figure 10b), a bimodal distribution appears at 20 C cm\({}^{-2}\), but, in this case, the most intense peak is the second one at about 50 nm, while the weaker one is at about 40 nm. In any case, Figure 10 confirms that PAA morphology formed at higher Al(OH)\({}_{3}\) concentration changed slightly with the electric charge. In fact, the distribution curves are partially overlapped, with the exception of the curve relative to 160 C cm\({}^{-2}\) that is centered around 100 nm and is distinct from the other ones. It is also interesting to emphasize the decrease in the pore population as the electric charge was increased. In pure 0.4 M H\({}_{3}\)PO\({}_{4}\) solutions, a bimodal distribution has been found at 20 C cm\({}^{-2}\), with two peaks of the same intensity centered at 65 and 100 nm, respectively. At 160 C cm\({}^{-2}\), the distribution curve is centered around 200 nm, which is the highest pore diameter obtainable in all the investigated solutions [46]. Finally, it has to be observed that all the distribution curves are rather narrow, independently of both solution composition and charge passed. For better detailing the influence of passed charge on PAA porosity, the pore mean size has been plotted as a function of charge density for different Al(OH)\({}_{3}\) concentrations, Figure 11. The useful information by such a plot is the slope of the curves because it is clear evidence of how strong the effect is of the passed charge on the pore size. For instance, it can be observed that the mean pore diameter changes very little with electrical charge density at 0.13 M, while it is very significant at 0.04 M. Since the passed electrical charge is linked to the anodizing time, the slopes of Figure 11 curves provide the rate of variation of pore size during anodizing, in practice. Therefore, such a plot is another valuable tool for tailoring PAA layers according to the desired application. Figure 10: Diameter distribution of PAA formed at 160 V and 272.15 \({}^{\circ}\)K in (**a**) 0.4 H\({}_{3}\)PO\({}_{4}\) and 0.04 Al(OH)\({}_{3}\); (**b**) 0.4 M H\({}_{3}\)PO\({}_{4}\) and 0.09 5 M Al(OH)\({}_{3}\). Tables 1 and 2 summarize the morphological features of PAA formed at 160 V and 0 \({}^{\circ}\)C in mixed Al(OH)\({}_{3}\) and 0.4 M H\({}_{3}\)PO\({}_{4}\) electrolytes, therefore can be considered as useful guidelines for PAA designing, while plots and SEM pictures provide more in-depth details. At this aim, it must be taken into account that the increase in Al(OH)\({}_{3}\) concentration leads to the growth of compact PAA featured by small, and not uniformly distributed pores, each other separated by a more or less thick oxide.