Flux-Matching Conditions at TiO_2 Photoelectrodes: Is Interfacial Electron Transfer to O_2 Rate-Limiting in the TiO_2-Catalyzed Photochemical Degradation of Organics?

A flux-matching condition has been applied to determine whether O_2 reduction is rate-limiting under photocatalytic conditions for the degradation of CHCl_3 at rutile TiO_2 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 O_2 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 TiO_2. Our measurements indicate that significant recombination will occur for the oxidation of typical organic molecules in H_2O over a range of pH values. The data also indicate that Pt catalysis of O_2 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 O_2 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 O_2 reduction at TiO_2. We also present an elucidation of the previously published theoretical treatments of TiO_2 charge transfer rate constants in view of the new data collected herein.