Distinguishing Direct and Indirect Photoelectrocatalytic Oxidation Mechanisms Using Quantitative Single-Molecule Reaction Imaging and Photocurrent Measurements

Light-driven semiconductor-catalyzed oxidation reactions are of fundamental importance in photocatalysis and photoelectrocatalysis for removing organic contaminants in wastewater, solar energy conversion, and fine chemical synthesis. The underlying reaction mechanism is often unclear because it is difficult to measure directly and specifically the semiconductor-catalyzed reaction rates. For example, an organic molecule could be oxidized "directly" by photogenerated holes that are transported from the semiconductor interior to the semiconductor electrolyte interface or "indirectly" by photogenerated intermediates (e.g., hydroxyl radical, superoxide anion, or hydrogen peroxide) that are produced at the semiconductor surface in aqueous solution. New experimental approaches that can distinguish these pathways are thus desirable. Here we introduce quantitative single-molecule, single-particle fluorescence imaging to measure the photoelectrocatalytic oxidation rate of a model organic substrate, amplex red, on the surface of individual rutile TiO2 nanorods. Our approach probes the oxidation product selectively before it becomes further degraded (which complicates bulk reaction kinetics measurements) while also avoiding interparticle charge transfer kinetics. By examining the reaction rate scaling relations versus light intensity at fixed potential and versus potential at fixed light intensity, together with the corresponding photocurrent scaling reactions, we demonstrate that amplex red oxidation on a TiO2-nanorod photoanode proceeds via an indirect mechanism.