Electrochemical Reduction of TiO ₂ Nanotubes: Real-Time Structural Insights and Photoelectrochemical Enhancement via Oxygen Vacancy Engineering

TiO 2 nanotube (NT) electrodes of controlled lengths were electrochemically reduced to introduce oxygen vacancies (Ti 3+ sites), thereby strongly boosting their photoelectrochemical (PEC) activity. Precise control over NT lengths of 1.85 μm, 4.50 μm, and 8.00 μm was achieved via electrochemical anodization times of 15, 60, and 120 min, respectively. Real-time in situ Raman spectroscopy revealed that the most significant structural transformations during electrochemical reduction occur within the first three min, as reflected by pronounced shifts in Raman band intensities and positions. Among the tested conditions, reduction at −1.2 V and −1.6 V versus a saturated calomel electrode (SCE) was the most effective, with the 8.00 μm NTs anodized for 120 min exhibiting the most favorable defect formation. Electrochemical (EC) and PEC measurements substantiated the Raman findings, unequivocally linking oxygen vacancy engineering to enhanced performance. Strikingly, the 120 min anodized, 8.00 μm, R-TiO 2 NTs reduced at −1.6 V exhibited the fastest charge-transfer kinetics and the highest photocurrent densities under 365 nm illumination, outperforming the electrodes with anodization periods shorter or longer than 120 min. These results establish that PEC enhancement by electrochemical reduction is not a generic effect but is critically governed by NT length and anodization duration. In situ Raman analysis thus provided direct mechanistic insights into defect formation, firmly corroborated by EC and PEC analyses.