Photocatalysis of Semiconductor Nano-Particles: Explicit Kinetics and Uniqueness of the Reactions

In this review, some special characteristics of the reactions in semiconductor photocatalysis are presented. At first, since a pair of the redox reactions take place at the same particle, a particle-based kinetic method was presented and applied for the Langmuir–Hinshelwood kinetics to describe the photocatalytic oxidation as a function of both the reactant concentration and the light intensity. Since the surface electron transfer (ET) reactions are the subject of electrochemistry, the difference in the characteristics from particulate semiconductor photocatalysis was pointed out by showing each electric potential near the solid surface. Different from ET in electrochemistry, the ET frequency is limited by the photon absorption in photocatalysis. In the estimation of the reaction rate, the validity of Marcus theory in photocatalysis was argued. Almost all photocatalytic reactions are irreversible, because, before the charge recombination, the oxidation and/or reduction must take place at the same particle. Then, the kinetics for irreversible reaction was discussed. As an exception, the reversible reduction reaction of methylviologen with a hole scavenger was presented. By changing pH, the energy levels of thermalized electrons in TiO2 particles were estimated, and the difference of the flat band potentials between anatase and rutile was clearly explained. Thus, various uniqueness of photocatalytic reactions in aqueous suspension of semiconductor particles were demonstrated.