A single hard anodization step was performed to obtain straight pores (SP) with a uniform pore diameter from top to bottom (Figure 1a). The length of the pores of all SP is 30 µm. To widen the pores, wet chemical etching with aqueous solution of 5% H3PO4 was performed for 0 (SP1), 45 (SP2), 90 (SP3), and 120 min (SP4).