Improving Photoelectrochemical Water Splitting Activity of TiO₂ Nanotube Arrays by Tuning Geometrical Parameters

Here we report a study to improve the solar water splitting activity of TiO2 photoanodes by tuning the porosity of the nanotube arrays. Through modifying the electrochemical anodization conditions, the average wall thickness and inner diameter of the synthesized TiO2 nanotube arrays were controlled in the range of 8–20 nm and 40–145 nm, respectively, corresponding to a variation of the porosity from 47.1% to 75.7%. The photoelectrochemical (PEC) measurements demonstrate that the sample with maximum porosity (75.7%) shows the peak ultraviolet conversion efficiency of 7.02%. Further analysis reveals that the photoconversion efficiency increases monotonously with porosity rather than with wall thickness and/or inner diameter. We suggest that the large porosity can ensure a much shorter hole diffusion path toward wall surface and accelerate ion migration in the tube to overcome the kinetic bottleneck, thus enhancing the PEC water splitting efficiency of the TiO2 nanotube arrays.