context: (2) A Ti metal precursor is anodized in an alkali electrolyte (e.g. NaOH and Ba(OH)2 solutions). The alkaline anodization (Fig. 6B) could be considered an electro-assisted hydrothermal-like process with 2 stages where TiO2·2H2O anodized from the titanium anode reacts with the hot alkaline surroundings generated by a continuous electric field. In the first stage, a fast anodic reaction on the titanium surface creates a thin TiO2·2H2O nanosheet layer (eqn (1) and (2)) which would react with alkali to form titanate nanowire bundles on the titanium anode (eqn (3)). In particular, it should be mentioned that the OH− concentration should be above 4 M to ensure the formation of titanate rather than titania. In the second stage, the anodized TiO2·2H2O would also break down and disperse into the electrolyte to form a 3D hierarchical porous structure with a large specific surface area.
answer:  titanium 