Electrochemically Reduced TiO2 Nanotube Arrays for Photoelectrochemical Hydrogen Production and Pollutant Degradation: Balancing Charge Separation and Reactive Oxygen Species Generation

Photoelectrochemical (PEC) systems offer a sustainable route for hydrogen (H2) production and the degradation of organic pollutants. This study investigates the PEC performance of electrochemically reduced titanium dioxide nanotube arrays (rTNTA) on Ti felt as photoanodes for the oxidative degradation of indigo carmine (IC) dye and simultaneous H2 evolution. rTNTA exhibits a 5-fold enhancement in photocurrent density (3.75 mA cm-2 at 1.23 V vs RHE) compared to pristine TNTA (0.73 mA cm-2), attributed to Ti3+ self-doping that enhances charge separation and carrier transport. The H2 Faradaic efficiency (FE) was 100%, and total organic carbon (TOC) removal reached 96% after the prolonged PEC reaction. The O2 FE was higher than 85% even in the presence of dye. In-situ UV-vis spectroscopy reveals that rTNTA achieves a 2-fold increase in dye degradation rate (0.021 vs 0.011 min-1 for TNTA), implying that the improvement in photocurrent does not directly translate to a proportionate increase in the IC dye degradation efficiency. Continuous-flow electron spin resonance spectroscopy reveals that rTNTA follows a four-electron OER pathway, with hydroxyl radical (<middle dot>OH) generation showing lower FE (1.48%) than that of unreduced TNTA (3.11%). These findings suggest that, despite the enhanced photocurrent, the lower selectivity of rTNTA for generating reactive oxygen species (ROS) such as <middle dot>OH and reactive surface holes resulted in a moderate enhancement in the IC dye degradation. Thus, optimizing the balance between charge separation and ROS generation is key for enhancing PEC systems for pollutant degradation.