Photoelectrochemical water splitting on highly smooth and ordered TiO2 nanotube arrays for hydrogen generation

To improve the photoelectrochemical (PEC) water splitting efficiency for hydrogen production, we reported the fabrication of lotus-root-shaped, highly smooth and ordered TiO2 nanotube arrays (TiO2 NTs) by a simple and effective two-step anodization method. The TiO2 NTs prepared in the two-step anodization process (2-step TiO2 NTs) showed better surface smoothness and tube orderliness than those of TiO2 NTs prepared in one-step anodization process (1-step TiO2 NTs). Under illumination of 100 mW/cm2 (AM 1.5, simulated solar light) in 1 M KOH solution, water was oxidized on the 2-step TiO2 NTs electrode with higher efficiency (incident-photon-to-current efficiency of 43.4% at 360 nm and photocurrent density of 0.90 mA/cm2 at 1.23 VRHE) than that on the 1-step TiO2 NTs electrode. The effective photon-to-hydrogen conversion efficiency was found to be 0.18% and 0.49% for 1-step TiO2 NTs and 2-step TiO2 NTs, respectively. These results suggested that the structural smoothness and orderliness of TiO2 NTs played an important role in improving the PEC water splitting application for hydrogen generation.