Effects of geometric and crystal structures on the photoelectrical properties of highly ordered TiO2 nanotube arrays

The photoelectrical properties of highly ordered TiO2 nanotube (TNT) arrays have been systematically and quantitatively studied and found to be closely related to their geometric and crystal structures. The geometric characteristics, including the nanotube diameter and length, were modified by adjusting the anodization potentials and durations, while the crystal structure was modified by thermal annealing at different temperatures. The nanotube array samples with the mixed crystalline phases possess higher photoconversion efficiency than those with the single anatase or rutile phase. The optimal content of rutile phase is about twice of that of anatase phase. In terms of the influence of the geometric structure, the TNT arrays with larger inner diameters and longer tube lengths have better photoelectrical properties. A geometric roughness factor has been applied to describe the combinative effect of the geometric characteristics. The TNT sample with the geometric roughness factor of 125.32 shows the superior photoconversion efficiency of 13.2%. The underlying mechanism has also been discussed in detail.