• • BiOI-TC with twin crystal defects achieves a CH3CH2OH evolution rate of 6.2 μmol g−1 h−1 with 100% selectivity, a 12-fold increase over pristine BiOI, directly enabling high-purity ethanol production from CO2.
• • The introduction of twin crystals enhances light absorption, CO2 adsorption, charge transfer efficiency, and carrier lifetime, as confirmed by UV-Vis, chemisorption, and PL spectroscopy, addressing the bottleneck of poor charge dynamics in BiOI.
• • In situ FTIR identifies *COOH, *CO, *COCO, *CHO, and *CH2 as key intermediates for CH3CH2OH generation, while *CH3 (1456 cm−1) is uniquely detected on BiOI-LD, steering selectivity toward C2H6 and demonstrating defect-mediated pathway control.
• • EPR spectroscopy shows increased superoxide radical generation in BiOI-TC, correlating with higher reactivity; this radical-mediated mechanism contributes to the 12-fold activity boost and 100% ethanol selectivity.