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 V_RHE) 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.