Flux-matching conditions at TiO₂ photoelectrodes: Is interfacial electron transfer to O₂ rate-limiting in the TiO₂-catalyzed photochemical degradation of organics?

A flux-matching condition has been applied to determine whether O₂ reduction is rate-limiting under photocatalytic conditions for the degradation of CHCl₃ at rutile TiO₂ single-crystal electrodes. In this approach, the potential dependence of the photooxidation current density is compared to the potential dependence of the current density for O₂ reduction. The potential at which the oxidation and reduction fluxes are equal determines the operating potential and the steady-state flux that will flow through the crystal under no applied bias. If this flux-matching condition occurs when the cathodic flux equals the flux of photogenerated carriers, then the predicted quantum yield should approach unity; otherwise, recombination should be significant in the TiO₂. Our measurements indicate that significant recombination will occur for the oxidation of typical organic molecules in H₂O over a range of pH values. The data also indicate that Pt catalysis of O₂ reduction should be beneficial for the oxidation of organic molecules, as would the use of alternate electron acceptors such as Fe(CN)₆ ^3-. The O₂ reduction data and rotating disk electrode data collected in this work allow a quantitative comparison to theoretical estimates of the electron transfer rate constant for O₂ reduction at TiO₂. We also present an elucidation of the previously published theoretical treatments of TiO₂ charge transfer rate constants in view of the new data collected herein.