**2.5 Anodization-annealing**

Dong et al. reported the fabrication of black titania NTs using an anodization approach followed by heat treatment at higher temperature as indicated schematically in **Figure 6** [16]. Precursor titania NTs were fabricated using 10-hour anodization on a Ti foil for two times at 60 V potential in an ethylene glycol solution having 0.25 wt.% NH4F and 2 vol% deionized water. The titania NTs were removed from the reaction system after first anodization and then subjected to second anodization. The repeated washing and cleaning of anodized Ti foil was achieved using ethanol and distilled water. The pure cleaned analytes were then dried at 120°C, and then annealed at 450°C for an hour under ambient environment. The removal of top oxide layer from the substrate leads to the development of layer of black titania.
