KINETICS OF STOCHASTIC CHARGE-TRANSFER AND RECOMBINATION EVENTS IN SEMICONDUCTOR COLLOIDS - RELEVANCE TO PHOTOCATALYSIS EFFICIENCY

We report stochastic calculations in compact 2-D lattices modeling the competition between electron-hole recombination and reactive processe s on the surface of illuminated TiO2 colloidal particles. Computer sim ulations-in which holes perform unbiased random walks, react with oxid izable donors, or neutralize stationary electrons, while the latter re duce O-2-confirm that conventional kinetic concepts and rate laws are invalid in such domains. Bimolecular carrier recombination never follo ws second-order kinetics: single excitons decay exponentially and mult iple pairs annihilate with second-order rate coefficients asymptotical ly approaching a t(-1/2) dependence. The occurrence of discrete events at the microscopic level further implies that (1) anodic and cathodic processes do not necessarily occur synchronously and (2) the fastest, rather than the slowest, charge transfer reaction is yield-determinin g. We carry out a rigorous data reduction analysis of experimental rec ombination rates and explore photon flux, [O-2], and particle size eff ects on the quantum yield of photocatalytic oxidations in these system s. Under typical steady-state illumination conditions, the model predi cts that primary photooxidation yields should be nearly independent of photon flux and [O-2] over wide ranges and increase with particle rad ius.