Electrochemical reduction induced self-doping of Ti³⁺ for efficient water splitting performance on TiO₂ based photoelectrodes

Hetero-element doping (e.g., N, F, C) of TiO₂ is inevitably accompanied by significantly increased structural defects due to the dopants' nature being foreign impurities. Very recently, in situ self-doping with homo-species (e.g., Ti³⁺) has been emerging as a rational solution to enhance TiO₂ photoactivity within both UV and visible light regions. Herein we demonstrate that conventional electrochemical reduction is indeed a facile and effective strategy to induce in situ self-doping of Ti³⁺ into TiO₂ and the self-doped TiO₂ photoelectrodes showed remarkably improved and very stable water splitting performance. In this study, hierarchical TiO₂ nanotube arrays (TiO₂ NTs) were chosen as TiO₂ substrates and then electrochemically reduced under varying conditions to produce Ti³⁺ self-doped TiO₂ NTs (ECR-TiO₂ NTs). The optimized saturation photocurrent density and photoconversion efficiency on the ECR-TiO₂ NTs under simulated AM 1.5G illumination were identified to be 2.8 mA cm^-2 at 1.23 V vs. RHE and 1.27% respectively, which are the highest values ever reported for TiO ₂ based photoelectrodes. The electrochemical impedance spectra measurement confirms that the electrochemical induced Ti³⁺ self-doping improved the electrical conductivity of the ECR-TiO₂ NTs. The versatility and effectiveness of the electrochemical reduction method for Ti³⁺ self-doping in P25 based TiO₂ was also examined and confirmed. This journal is