Anodically Formed TiO2 Thin Films: Evidence for a Multiparameter Dependent Photocurrent-Structure Relationship

A series of nanostructured TiO2 thin films, characterized from the presence of an ordered array of vertically-aligned titania nanotubes, were prepared by anodic oxidation of titanium foils, and compared with a reference sample prepared by sol–gel dip-coating. The size of the nanotubes and their packing density were modified by changing anodization parameters, but using the same type of electrochemical bath in order to prepare a homogeneous set of titania thin films. These thin films were investigated in terms of photoelectrochemical responses and nanostructure characteristics. The nanostructured thin films show a photocurrent around twenty times higher than the reference film per unit of illuminated area, and over hundred times higher per gram of TiO2 present in the film well demonstrating that the nanostructured film possesses significantly higher performances in the generation of photocurrent. In terms of relationship between characteristics and packing density of the titania nanotubes, as well as photocurrent generation, a complex trend was observed, with the photocurrent density passing through a minimum as a function of the anodization time (in the 1–6 h range) and through a maximum as a function of the anodization voltage (in the 20–60 V range), using as electrolyte an ethylene glycol solution containing 0.3 wt% NH4F and 2 wt% water. This trend was interpreted in terms of effect of the anodization parameters on various characteristics of the nanotube array (packing density, geometrical surface area, nanotube wall-thickness and length).