Ma. Grela et Aj. Colussi, KINETICS OF STOCHASTIC CHARGE-TRANSFER AND RECOMBINATION EVENTS IN SEMICONDUCTOR COLLOIDS - RELEVANCE TO PHOTOCATALYSIS EFFICIENCY, Journal of physical chemistry, 100(46), 1996, pp. 18214-18221
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.