Unveiling the critical role of crystallinity in photogenerated hole transfer behavior of TiO2 with an aminoxyl radical mediator

Integrating aminoxyl radical mediators (ARMs) with titanium dioxide (TiO₂) based photoelectrochemical (PEC) processes presents a promising approach for achieving mild and sustainable chemical transformations. However, a comprehensive understanding of the interaction between ARMs and PEC processes remains elusive. This study investigates the PEC behavior of photoelectrodes fabricated from single- and polycrystalline TiO₂ in the presence of a representative ARM, 4-acetamido-2,2,6,6-tetramethylpiperidine 1-oxyl radical, in acetonitrile. Interestingly, we observe contrasting PEC behavior: photocurrent enhancement for single-crystalline TiO₂ and photocurrent suppression for polycrystalline TiO₂. We attribute this difference to subtle variations in surface properties arising from the material’s crystallinity. Photoelectrochemical impedance spectroscopy (PEIS) is employed to gain deeper insights into the influence of surface defects on charge transfer and recombination mechanisms. The PEIS results suggest that an anomalous hole transfer mechanism mediated by surface trap states governs the PEC performance of polycrystalline TiO₂.