Effect of Ti foil size on the micro sizes of anodic TiO2 nanotube array and photoelectrochemical water splitting performance

Anodic TiO2 nanotube (NT) array is promising for the flexible and efficient photoanode in photoelectrochemical water splitting (PECWS) cell. However, the photocurrent response of pristine anodic TiO2 NT photoanode in literature has a ca 50 times difference, viz. from 0.05 to 1 mA/cm2. Improvement of the pristine TiO2 NT is the base for achieving a high efficient anode. Here, we examine the size effect in different scale on the PECWS performance with manipulating the macro size of the Ti foil. With decrease of the Ti foil size from 6 to 1 cm2, corresponding to the anodic TiO2 NT growth active area of 3.45 to 0.65 cm2, the photocurrent response increased by 50.6%, achieving 1.13 mA/cm2 at 1.23 VRHE (V versus reversible hydrogen electrode). The Ti foil size also significantly influences the micro sizes of the nanotubes including crystallite size, double wall thickness, inner diameter and tube length, which have profound effects on WS efficiency. The relationships between involved length scales, a span of six orders of magnitude from ten nanometers (10−8 m) to centimeter (10−2 m), and the PECWS efficiency is analyzed and discussed. Transient i-t curves are used to represent the chemical kinetics during the growth of anodic TiO2 NT array. Finally, photon capture scheme is proposed to explain the physics behind the multi length scale effect of the TiO2 NT photoanode. The need of quantitative models during the scale-up of the PECWS process is stressed.