Electrochemical Imaging and Redox Interrogation of Surface Defects on Operating SrTiO₃ Photoelectrodes

We introduce electro­chemical imaging and nano-resolved measurements of catalytic intermediates on operating SrTiO₃ photo­electrodes. Spatially resolved redox titrations of photo­generated reactive oxygen species (ROS) were used to profile changes in ROS coverage and reactivity at pristine and ion-milled defective areas on n-doped (100) SrTiO₃. Adsorbed ROS reached a potential-dependent limiting coverage of ∼0.1 monolayer and did not differ significantly between milled and pristine areas. However, the reaction kinetics between a solution-phase mediator and adsorbed ROS were found to be significantly decreased over ion-milled areas. Using a nano­electrode, we resolved k_si values of 5 and 300 m³/s·mol for these bimolecular reactions at defective and pristine sites, respectively. Ion-milled areas also showed significantly decreased activity toward photo-oxidations, providing evidence that photo­generated ROS mediate fast charge-transfer reactions with solution-phase species at the semiconductor–electrolyte interface. Our results provide spatially resolved direct evidence of the impact of surface defects on the performance of photo­electro­chemical systems. Scanning electro­chemical microscopy offers a powerful method for evaluating the reactivity of an operating electro­chemical interface by using redox titrations that detected as few as 30 attomoles of adsorbed ROS.