## 3 Results and Discussion  
### Chemical Surface and Morphological Characterizations  
Chemical characterization of NPASs surfaces was performed by XPS. The analysis of the core level spectrum obtained for each element found on the sample surface permits us to estimate its atomic concentration percentage (A.C. (%)). Table 1 presents the average values obtained for the main chemical elements found as well as the corresponding O/Al ratio. The presence of non-characteristic sample elements, such as nitrogen and carbon (this latter in a high percentage), is associated to both environmental [44] and fabrication process contaminations, with the latter being the reason for sulphur and phosphorus presence (phosphoric acid solutions are employed as etching agents for the removal of the oxide barrier layer); small percentages (\(\leq\)0.6%) of other non-characteristic elements (N, Cr,... ) were also found. Moreover, slight oxidation of the samples surfaces was also observed when comparing the experimental O/Al ratio and the theoretical one ((O/AL)\({}^{\text{T}}\) = 1.5). Surface morphology of the NPASs was determined by analyzing SEM micrographs such as those shown in Figure 2. Pore radii distributions, obtained by computer-assisted image analysis [37], are displayed in the Supplementary Information (Figure S3). It is worth noting that experimental NPAS-Sf and NPAS-Ph samples displayed a highly ordered hexagonal pore arrangement and a monodisperse pore radii distribution, whereas the pore arrangement in the Anodisc commercial sample was randomly distributed, and it displayed two or more peaks in the pore size distribution at both sides, which might be associated to these pores that are occluded or stop growing during the membrane fabrication process. Furthermore, experimental NPAS samples only showed small differences in pore radii for top and bottom surfaces according to their symmetric structure, as was \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Sample** & **C (\%)** & **O (\%)** & **Al (\%)** & **N (\%)** & **S (\%)** & **P (\%)** & **O/Al** \\ \hline NPAS-Sf & 16.9 & 53.0 & 25.7 & 0.7 & 2.1 & 1.9 & 2.07 \\ NPAS-Ph & 19.2 & 51.7 & 27.2 & 0.6 & & 0.9 & 1.90 \\ NPAS-And & 16.6 & 50.8 & 27.8 & 0.5 & & 3.1 & 1.83 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Average atomic concentration percentages of the main chemical elements found on NPASs surfaces. already reported in previous works [19; 45; 46; 47], but they were significantly different in the case of the asymmetric Anodisc sample. Values of morphological parameters (pore radii (r\({}_{\text{p}}\)), interpore distance (D\({}_{\text{int}}\)), and porosity (\(\Theta=(2\pi\sqrt{\lambda})(\text{r}_{\text{p}}/\text{D}_{\text{int}})^{2}\)) [48] obtained for each NPAS are summarized in Table 2, where average values of r\({}_{\text{p}}\) and \(\Theta\) are indicated in the case of NPAS-Sf and NPAS-Ph samples. For these two experimental samples, the obtained values were in good agreement with those previously reported for nanoporous alumina layers synthesized under similar conditions, and they showed similar porosities of around 10% [2; 48]. However, due to the random pore distribution and lower geometrical reproducibility of the NPAS-And sample, precision in the estimated values of the morphological parameters for each opposite surface was remarkably lower than those measured for the highly ordered NPAS samples. In this particular sample, alternative techniques based on gas penetration seem to confirm our results, indicating an average porosity of \(29.3\pm 0.5\%\) for similar NPAS-And samples [49], thus being consistent with our estimations based on SEM image analysis. Finally, it should be pointed out that thickness of the different NPASs under study were approximately constant at around 60 \(\upmu\)m.  
Figure 2: SEM micrographs of NPASs: NPAS-Sf top view (**a**) and bottom view (**b**); NPAS-Ph top view (**c**) and bottom view (**d**); Anopore: bottom view (**e**) and top view (**f**).