## 3 Results and Discussion
### Fabrication and Characterization of AAM
As the anodizing time increases, pore mean size increases because oxide dissolution can occur at a higher extent with consequent enlargement of pore mouth (Figure 5a), while pore population decreases because the narrowest pores generated at the initial stages of anodizing are subject to closure by the formation of the more compact film. Therefore, anodizing time (i.e, passed charge) has two opposite effects because  
Figure 5: Pore mean size (**a**), and surface pore population (**b**) of PAA layers obtained by Al anodization at 160 V and 0 \({}^{\circ}\)C in 0.4 M H\({}_{3}\)PO\({}_{4}\) added with different amounts of Al(OH)\({}_{3}\). determines on the one hand enlargement of the pore with consequent size enhancement, but on the other hand it causes the closure of the narrowest pores. About the generation of new pores over anodizing, Figure 5b shows an increase in pore population with Al(OH)\({}_{3}\) up to 0.095 M, where a sharp decrease to 0.13 M starts towards values that are independent of the passed charge. Likely, such a behavior can be explained by inferring that the pore onset occurs prevalently in the first stages of anodizing. The pore population variation with Al(OH)\({}_{3}\) concentration is rather limited, with the exception of 160 C cm\({}^{-2}\) where the greatest variation was found (from 0.69 to 1.67 m\({}^{-2}\)). The increase in population up to 0.095 M could be likely attributed to the opening of some new but shallow pores. Such a new pore generation will occur at a higher extent as the longer is the electrode immersion time in solution (just 160 C cm\({}^{-2}\)). This interpretation is supported by the difference between the pore population at the outer PAA surface and the cell population at the bottom one. One pore should be located inside one cell, therefore the two values of the population should be identical, but this is not, as shown in Table 1. In practice, only the pore generated in the first instants of anodizing leaves its imprint on the aluminum, whose surface consequently becomes scalloped. Over anodizing, additional pores are generated  
Figure 6: Top-views of PAA layers formed in 0.4 M H\({}_{3}\)PO\({}_{4}\) and 0.13 M Al(OH)\({}_{3}\) at 160 V and 0 \({}^{\circ}\)C: (a) 20 C cm\({}^{-2}\); (b) 40 C cm\({}^{-2}\); (c) 80 C cm\({}^{-2}\); (d) 160 C cm\({}^{-2}\). at the oxide/electrolyte interface, but their height is lower than the initially formed ones, which are separated from the oxidizing aluminum by the barrier layer. In practice, two types of channels are formed, which can be distinguished on the basis of their length. Initially formed pores that extend throughout the PAA thickness, and successively formed pores that stop their growth at different depths of penetration into the oxide layer. Such a difference can be attributed to the electric field strength that, according to the most recent literature [82; 83], contributes to the growth of the initially formed pores, because the oxide barrier layer is thin, and, consequently, the electric field reaches high strength values. On the contrary, as the PAA layer is thickening, the electric field strength is diminishing, and with it, the contribution to the channel's growth. Figure 7 shows a schematic diagram of PAA with pores that stop its growth at some depth inside the oxide layer. Such a scheme justifies the difference between the pore population evaluated at the PAA surface in contact with the electrolyte and cell density evaluated at the PAA surface in contact with aluminum. As a consequence, a few ordered nanostructure is formed with channels of different lengths. The scheme of Figure 7 is confirmed by the FESEM images of Figure 8 where sectional views of PAA formed at 160 V and 0 \({}^{\circ}\)C in different electrolytes are shown. Channels of different heights are evident.