The classical macroscopic photoelectrochemical system consists of a semiconductor in electric contact with a counter-electrode. For N-type semiconductor particles of sufficiently small dimension, the particles polarize into anodic and cathodic regions, effectively forming microscopic photoelectrochemical cells. The illuminated surface of a particle catalyzes a photooxidation reaction, while the “dark” side of the particle facilitates a concomitant reduction.
Photoelectrochemical oxidation may be thought of as a special case of photochemical oxidation (PCO). Photochemical oxidation entails the generation of radical species that enable oxidation reactions, with or without the electrochemical interactions involved in semiconductor-catalyzed systems, which occur in photoelectrochemical oxidation.