## 3 Results and Discussion  
### Fabrication and Characterization of AAM  
The effect of the addition of Al(OH)\({}_{3}\) is clearly evidenced in Figure 2 where aluminum mass oxidized and PAA moles formed are plotted vs. bath composition. The two curves shown in Figure 2a clearly evidence that the oxidized aluminum is partially converted into PAA, therefore a dissolution process must occur for accounting of the aluminum mass unconverted into PAA. Consequently, the aluminum anodizing occurs with an efficiency less than 1, whose values depend on Al(OH)\({}_{3}\) concentration in the electrolyte, as shown in Figure 2b evidencing that the maximum efficiency of 80 \(\pm\) 5% is attained for 0.13 M Al(OH)\({}_{3}\). The findings of Figure 2 can be attributed to a variation in the dissolution extent of the anodic oxide as the concentration of Al(OH)\({}_{3}\) increases. The most likely cause of such a variation is the common ion effect that controls the solubility of any sparing substance in a solution that contains an ion in common [80]. The influence of Al(OH)\({}_{3}\) concentration on PAA morphology is evidenced in Figure 3 where FESEM images are reported for the lowest and highest anodizing electrical charges. In detail, Figure 3b,d show the top (outer surface) views of PAA grown at 160 V in 0.4 M H\({}_{3}\)PO\({}_{4}\) and 0.04 M Al(OH)\({}_{3}\) for 20 (b) and 160 C cm\({}^{-2}\) (d), while Figure 3a,c show the typical morphology of the pore base formed at the interface metal/oxide (inner surface). Each geometrical unit discloses the shape and size of the cell featuring the structure of PAA [3]. Ideally, such units should be hexagonal with size depending on the anodizing voltage at a value of about 433.2 nm [6], whereas, in the present case, they are not uniform in either shape or size. The cells present a pentagonal shape, prevalently, while the few hexagonal cells are not regular in the shape with variable inter-pore distances. The most relevant issue is the cell population independence of the passed charge since values of 6.5 \(\times\) 10\({}^{12}\) cells m\({}^{-2}\) were always found. On the contrary, the surfaces exposed to the solution show more relevant differences (Figure 3b,d). After 20 C cm\({}^{-2}\), different pores for shape and size, and randomly distributed over the surface are clearly distinguishable. Typically, a mean diameter of about 60 \(\pm\) 5% nm was measured. Differently, after 160 C cm\({}^{-2}\), the surface shows a more regular structure with pores of a rather uniform size, prevalently circular in shape with a mean diameter of about 167 \(\pm\) 5% nm. Some ellipsoidal pores are also evident whose formation can be attributed to the coalescence of neighboring circular pores due to the chemical dissolution of the inter-pore oxide owing to the long exposure time to the solution. Hence, pores with a higher diameter and more regular shape were formed at 160 than 20 C cm\({}^{-2}\). As the Al(OH)\({}_{3}\) concentration was increased from 0.04 M to 0.095 M in otherwise identical conditions, the PAA morphology for 20 C cm\({}^{-2}\) is shown in the images of Figure 4. The inner surface, in contact with the metal (Figure 4a), is identical to that of Figure 3a with a cell population value of 6.5 \(\times\) 10\({}^{12}\) m\({}^{-2}\). The outer surface, in contact with the electrolyte (Figure 4b), discloses, in comparison with Figure 3b, a reduction of about 10 nm in the pore mean size, from 62.5 \(\pm\) 5% to 51.68 \(\pm\) 5%, together with a thickening of the inter-pore oxide.