Free radicals play dominant roles in almost all photocatalytic processes. Among these free radicals, the most prominent are the hydroxyl radicals, formed primarily from oxidation of water or OH⁻ by photogenerated holes. Superoxide anion radicals, formed by reduction of di-oxygen by photogenerated electrons, are no less important. Their formation reduces the recombination rate.¹  In addition, superoxides participate in the process as active species (usually in the secondary steps of degradation) and, upon reacting with protons, as a secondary source for OH radicals via homolytic cleavage of H₂O₂. Other radicals, of lesser importance in photocatalysis, are oxygen atoms. The large differences between the redox potential of the hydroxyl radical and that of many organic compounds, together with very low activation energy and weak steric hindrance (typical for small size radicals), are manifested by strong activity that does not differentiate between different organic compounds.