Figure 6

FESEM images of PAAs’ evolvements, a two-layer PAA sample with upper larger pores and bottom smaller pores, which is first anodized under 160 V at 0°C for 20 min in 5 wt% H₃PO₄ and second anodized 40 V at 30°C for 2 h in 0.3 M oxalic acid, b two-layer PAA structure fabricated under 180 V at 20°C in the mixture of 5 wt% phosphoric acid, 1 wt% citric acid and 5 wt% ethanol, c three-layer configuration by three-step anodization at 150 V for 1 min, 40 V for 25 min and 150 V for 1 h, d a typical cross-section of PAA fabricated under 40 V at 0°C in 0.3 M oxalic acid for 10 h, e combined nanopore-microstep hierarchical structure, which is obtained by first anodization under 5 V and 0°C in a mixture of 1 wt% H₃PO₄, 0.42 wt% NaCl and 20 wt% ethanol, and second anodization under 40 V and 40°C in 0.3 M oxalic acid, f upper aluminum micro-scale step structure with lower general PAA layer, which is obtained by first anodization in oxalic acid and second anodization on the other side in Cl⁻ containing electrolyte until the current dropped dramatically. The anodizing condition in (f) is as same as that in (e)