has applications in the detoxification of air and water, hydrogen production, and other applications. Reaction mechanism The process by which a photon initiates a chemical reaction directly is known as photolysis; if this process is aided by a catalyst, it is called photocatalysis. If a photon has more energy than a material's characteristic band gap, it can free an electron upon absorption by the material. The remaining, positively charged hole and the free electron may recombine, generating heat, or they can take part in photoreactions with nearby species. If the photoreactions with these species result in regeneration of the electron-donating material—i.e., if the material acts as a catalyst for the reactions—then the reactions are deemed photocatalytic. PECO represents a type of photocatalysis whereby semiconductor-based electrochemistry catalyzes an oxidation reaction—for example, the oxidative degradation of an airborne contaminant in air purification systems. The principal objective of photoelectrocatalysis is to provide low-energy activation pathways for the passage of electronic charge carriers through the electrode electrolyte interface and, in particular, for the photoelectrochemical generation of chemical products. With regard to photoelectrochemical oxidation, we may consider, for example, the following system of reactions, which constitute TiO2-catalyzed oxidation. TiO2 (hv) → TiO2 (e− + h+) TiO2(h+) +RX → TiO2 + RX.+ TiO2(h+) + H2O → TiO2 + HO. + H+ TiO2(h+) + OH− → TiO2 + HO. TiO2(e−) + O2 → TiO2 + O2.− This system shows a number of pathways for the production of oxidative species that facilitate the oxidation of the species, RX, in addition to its direct oxidation by the excited TiO2 itself. PECO concerns such a process where the electronic charge carriers are able to readily move through the reaction medium, thereby to some extent mitigating recombination reactions that would limit the oxidative process. The “photoelectrochemical cell” in this case could be as simple as a very small particle of the semiconductor catalyst. Here, on the “light” side a species is oxidized, while on the “dark” side a separate species is reduced. Photochemical oxidation (PCO) versus PECO 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