Structure characterization of the TNT arrays (a) Schematic diagram of an anodized Ti wire with a TNT array outer layer wrapping the inner Ti core. (b) FESEM image of an anodized Ti wire which outer TNT array was partially peeled off. (c) FESEM image of a free-standing cylindrical TiO2 tubule with its Ti core completely removed by chemical etching. Top (d) and bottom (e) view of the TNT array by FESEM. (f) TEM image of annealed TNTs. Inset: SAED of the TNTs. (g) XRD spectrum of the annealed TNT array.