Enhancing photoelectrochemical water splitting performance of TiO2 nanotube arrays by controlling morphological properties

Anodic TiO2 nanotube (TONT) with various morphological features was developed by simple electrochemical anodization in the variation of applied voltage where the morphological properties such as pore diameter, wall thickness, and inter-tube spacing can be adjusted to survey the influence on the photoelectrochemical (PEC) activity. Thus, in the applied voltage from 20 V to 60 V at different reaction times, the identical thickness of 3.0 ± 0.2 µm was maintained. From the field-emission scanning electron microscopy, the geometric factors were compared and the porosity of TONT films was calculated. The results demonstrated that the porosity of TONT film grown at 20 V (denoted as 20 V) is the highest and that the porosity steadily reduced with increasing applied voltage. On the basis of these geometric factors, the PEC performance of TONT film (20 V) shows the highest photocurrent of 1.64 mA/cm2 at 0 V vs. Ag/AgCl. This was attributed to the lowest series resistance due to the high porosity for the hole or ions pathway and to the high crystallite size for the beneficial charge transport. On the other hand, TONT films (above 40 V) exhibit a similar photocurrent with high series resistance for opposite reasons.