## 3 Results and Discussion  
In order to quickly fabricate and reduce the cost of the tubular AAO process, a batch of aluminum tubes was set in the anodization mold. Silicone plugs were used to seal the bottoms of the tubes so that AAO formed on the outer surfaces of the aluminum tubes rather than on the inner surfaces. Based on our previous anodization process that is illustrated in Figure 1[1], the solutions of each tank individually were (a) electro-polishing electrolyte (HClO\({}_{4}\) solution); (b) water; (c) anodization electrolyte (oxalic acid solution); (d) AAO removal solution (H\({}_{3}\)PO\({}_{4}\) solution); (e) aluminum removal solution (CuCl\({}_{2}\) + HCl solution); and, (f) AAO pore widening solution (H\({}_{3}\)PO\({}_{4}\) solution). The experimental parameters of solution temperature and applied voltage were controlled using a cooling/heating machine and programmable power supply, respectively. The aluminum tube, functioning as the working electrode, was in contact with a conducting copper sheet as the anode, and an aluminum sheet was in contact with the cathode as the counter. Figure 1: Schematic diagram of the producing flow and instrument setup with an overhead conductive rod for fabricating the tubular anodic aluminum oxide (AAO) template. The operating process is conducted in the order from (**a–f**), but after each step, the sample is rinsed in (**b**) the water bath. The solutions in the tanks are (**a**) electro-polishing electrolyte; (**b**) water; (**c**) anodization electrolyte; (**d**) AAO removal solution; (**e**) aluminum removal solution; and, (**f**) AAO pore widening solution. Figure 2 shows the set up to remove Al substrate, including Al removal solution (CuCl\({}_{2}\) + HCl solution), pump, and pipes. After the removal solution was pumped into Al tube to etch Al for ten of minutes, translucent tubular AAO was obtained. The chemical displacement reaction between Al and Cu can be expressed as: 2 Al(s) + 3 CuCl\({}_{2(s)}\) \(\rightarrow\) 3 Cu(s) + 2 AlCl\({}_{3(s)}\). By the way, HCl promotes the AlCl\({}_{3}\) formation, and the reaction is denoted as: 2 Al(s) + 6 H\({}^{+}\)(aq) + 6 Cl\({}^{-}\)(aq) \(\rightarrow\) 2 AlCl\({}_{3(s)}\) + 3 H\({}_{2(g)}\). Because this is an exothermic reaction during Al-Cu displacement reaction the cooling for etching bath is critical. Figure 3 shows the detailed Al removing processing flow. Most of Al substrate can be removed after a 15 min removing process. Figure 3: Optical images of tubular Al/AAO film after etching Al substrate for (**a**) 2 min; (**b**) 4 min; (**c**) 6 min; (**d**) 8 min; (**e**) 10 min; (**f**) 12 min; (**g**) 15 min; and, (**h**) the finish product cleaned with water. Figure 2: (**a**) Schematic diagram and (**b**) working situation of experimental setup for removing aluminum substrate from Al/AAO sample. Figure 4 shows the optical images of the initial 6061 tube and anodic tubular aluminum oxide made by the assistance of electrochemical and wet-etching process. It includes the following steps: (a) the aluminum tube that contains many scratches; (b) most of rough scratches on aluminum tube were removed after mechanical grinded; (c) a mirror-like aluminum surface was achieved after electrolytic polished; (d) a uniformity AAO film was formed on the Al surface through the first anodization process; (e) ordering patterns were formed on the Al tube after removing the first anodization film; (f) AAO film was formed on the ordering pattern Al surface through the second anodization; and, (g) a translucent AAO film was presented after removing Al substrate. Figure 5 shows a schematic diagram of the tubular AAO micro-morphology. In the beginning, AAO film forms hexagonal channels because of the internal stress balance. Subsequently, the hexagonal channels are changed to a circular tube by anodization electrolyte etching. While the high purity Al tube is anodized, circular holes form on the hexagonal patterns of the anodic film. In Figure 5a, anodic aluminum oxide with regular pores on the Al tube surface was shown.