Generation and Detection of Reactive Oxygen Species in Photocatalysis

The detection methods and generation mechanisms of the intrinsic reactive oxygen species (ROS), i.e., superoxide anion radical (^•O₂^-), hydrogen peroxide (H₂O₂), singlet oxygen (¹O₂), and hydroxyl radical (^•OH) in photocatalysis, were surveyed comprehensively. Consequently, the major photocatalyst used in heterogeneous photocatalytic systems was found to be TiO₂. However, besides TiO₂ some representative photocatalysts were also involved in the discussion. Among the various issues we focused on the detection methods and generation reactions of ROS in the aqueous suspensions of photocatalysts. On the careful account of the experimental results presented so far, we proposed the following apprehension: adsorbed ^•OH could be regarded as trapped holes, which are involved in a rapid adsorption-desorption equilibrium at the TiO₂-solution interface. Because the equilibrium shifts to the adsorption side, trapped holes must be actually the dominant oxidation species whereas ^•OH in solution would exert the reactivity mainly for nonadsorbed reactants. The most probable routes of generating intrinsic ROS at the surfaces of two polymorphs of TiO₂, anatase and rutile, were discussed along with some plausible rational reaction processes. In addition to the four major ROS, three ROS, that is organic peroxides, ozone, and nitric oxide, which are less common in photocatalysis are also briefly reviewed.