## 3 Results and Discussion
### Fabrication and Characterization of AAM
It can be also observed that the thickness of the pore wall increases with the concentration of Al(OH)\({}_{3}\) in solution, so that, for instance, the channel-free cross-section is heavily reduced as PAA is formed in 0.4 M H\({}_{3}\)PO\({}_{4}\) and 0.13 M Al(OH)\({}_{3}\). Moreover, Figure 8 shows that also the morphological disorder of the PAA layer increases with Al(OH)\({}_{3}\) concentration likely owing to a non-uniform thickening of the pore wall, determining the loss of parallelism between the channels. A more in-depth analysis of the images of Figure 8 indicates that the most ordered PAA layers can be obtained by anodization in 0.4 M H\({}_{3}\)PO\({}_{4}\) and 0.04 M Al(OH)\({}_{3}\) (Figure 8b), even if the order degree of Figure 8 appears satisfying. Its drawback is the very low thickness of the PAA layer which is about 6.4 \(\pm\) 5% \(\upmu\)m as shown in Table 2 where the PAA layer thickness is shown for the different anodizing conditions. Figure 2.0 Schematic view of PAA with channel of different heights some of which stop after onset at the outer oxide surface in contact with the electrolyte. Additionally, the scalloped surface imprinting oxidizing aluminum is shown. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Electrolyte**} & \multicolumn{4}{c}{**PAA Layer Thickness, \(\mu\)m \(\pm\) 5\%**} \\ \cline{2-5}  & **20 C cm\({}^{-2}\)** & **40 C cm\({}^{-2}\)** & **80 C cm\({}^{-2}\)** & **160 C cm\({}^{-2}\)** \\ \hline
0.4 M H\({}_{3}\)PO\({}_{4}\) & & & & \\
0.04 M & 11.63 & 21.40 & 40.97 & 82.60 \\ Al(OH)\({}_{3}\) & & & & \\ \hline
0.4 M H\({}_{3}\)PO\({}_{4}\) & & & & \\
0.095 M Al(OH)\({}_{3}\) & & & & \\ \hline
0.4 M H\({}_{3}\)PO\({}_{4}\) & & & & \\
0.13 M & 6.4 & 15.7 & 29.1 & 59.8 \\ Al(OH)\({}_{3}\) & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: PAA layer thickness dependent on the electrolyte composition and passed electrical charge for aluminum anodizing at 160 V and 0 \({}^{\circ}\)C.  
Figure 8: Section views of PAA formed at 160 V and 0 \({}^{\circ}\)C after 160 C cm\({}^{-2}\) in **(a)** 0.4 M H\({}_{3}\)PO\({}_{4}\), **(b)** 0.4 M H\({}_{3}\)PO\({}_{4}\) and 0.04 M Al(OH)\({}_{3}\); **(c)** 0.4 M H\({}_{3}\)PO\({}_{4}\) and 0.095 M Al(OH)\({}_{3}\); **(d)** 0.4 M H\({}_{3}\)PO\({}_{4}\) and 0.13 M Al(OH)\({}_{3}\). In order to better evidence the effect of Al(OH)\({}_{3}\) addition to 0.4 M H\({}_{3}\)PO\({}_{4}\) aqueous solutions, the morphology of PAA formed in pure 0.4 M H\({}_{3}\)PO\({}_{4}\) was also investigated, in otherwise identical conditions. Really, a similar study was already conducted, but the attention was focused on tailoring the PAA as a template for fabrication of Ni nanostructured array [46], while in the present case, the investigation has been aimed to compare the morphology of PAA formed at different anodizing times in bath containing different Al(OH)\({}_{3}\) amounts. Figure 9, to be compared with Figure 3, shows the morphological features of the inner and outer surface after passing 20 and 160 C cm\({}^{-2}\). It can be observed that narrow pores are distributed on the outer surface formed at 20 C cm\({}^{-2}\). They are not uniformly distributed on the surface, and their shape and diameter are rather variable. After 160 C cm\({}^{-2}\), a relevant modification occurred due to the large dissolution of the inter-pore oxide leading to a strong increase in the pore size also enhanced by the probable coalescence of neighboring pores. As a consequence, a highly porous surface is formed. The micrographs of the inner surface (Figure 9a,c) show the formation of well-defined cells, even if they are rather irregular in shape and size.