FLUX-MATCHING CONDITIONS AT TIO2 PHOTOELECTRODES - IS INTERFACIAL ELECTRON-TRANSFER TO O-2 RATE-LIMITING IN THE TIO2-CATALYZED PHOTOCHEMICAL DEGRADATION OF ORGANICS

A flux-matching condition has been applied to determine whether O-2 re duction is rate-limiting under photocatalytic conditions for the degra dation of CHCl3 at rutile TiO2 single-crystal electrodes. In this appr oach, the potential dependence of the photooxidation current density i s compared to the potential dependence of the current density for O-2 reduction. The potential at which the oxidation and reduction fluxes a re 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 p hotogenerated carriers, then the predicted quantum yield should approa ch unity; otherwise, recombination should be significant in the TiO2. Our measurements indicate that significant recombination will occur fo r the oxidation of typical organic molecules in H2O over a range of pH values. The data also indicate that Pt catalysis of O-2 reduction sho uld be beneficial for the oxidation of organic molecules, as would the use of alternate electron accepters such as Fe(CN)(6)(3-). The O-2 re duction data and rotating disk electrode data collected in this work a llow a quantitative comparison to theoretical estimates of the electro n transfer rate constant for O-2 reduction at TiO2. 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 her ein.