Superoxide radical enhanced photocatalytic performance of styrene alters its degradation mechanism and intermediate health risk on TiO2/graphene surface

Enhancement of reactive oxygen species (ROS) on semiconductor coupled by carbon material promotes photocatalytic performance toward aromatic hydrocarbons, while the contribution to their degradation mechanism and health risk is not well understood. Herein, photocatalytic degradation of styrene on TiO2 and TiO2/reduced graphene oxide (TiO2/rGO) surface is compared under dry air condition to investigate the role of O-center dot(2) in styrene degradation. TiO2/rGO shows 4.8 times higher degradation efficiency than that of TiO2, resulting in 16% reduced production of intermediates with identical composition. The improved formation of O-center dot(2) on TiO2/rGO is confirmed responsible for these variations. Theoretical calculation further reveals the enhancement of O-center dot(2) thermodynamically favoring conversion of styrene to acetophenone, turning the most dominant intermediate from benzoic acid on TiO2 to acetophenone on TiO2/rGO. The accumulated formation of acetophenone on TiO2/rGO poses increased acute threat to human beings. Our findings proclaim that ROS promoted photocatalytic performance of semiconductor after carbon material composition ultimately changes the priority order of degradation pathways to form by-product with higher threat toward human beings. And more attentions are advised focusing on the relevance with degradation efficiency, intermediate and toxicity of aromatic hydrocarbons on carbon material based photocatalyst.