Particularities of photocatalysis and formation of reactive oxygen species on insulators and semiconductors: cases of SiO2, TiO2 and their composite SiO2-TiO2

Highly defected tubular SiO2 is found to outperform the activity of TiO2-P25 and the SiO2-TiO2 composite in photocatalytic H-2 generation from methanol-water solution under simulated solar light AM 1.5. The enhanced performances of SiO2 come from the particularities of the reaction mechanism and ROS (reactive oxygen species) generation. The SiO2 exposed to light generates solely O-1(2) (singlet oxygen). The TiO2-P25 produces uniquely large quantities of OH radicals whereas the formation of O-2(-) is evidenced only over SiO2-TiO2, along with small amounts of OH. The TiO2 works as a photocatalyst by intermediation of OH radicals. In contrast, the organic substrate is activated on the surface of SiO2 by the intra-band gap, isolated, surface quantum defects. Distinct reaction mechanisms, involving the participation of photogenerated charges and ROS, are proposed. The material-related ROS production can be of great practical importance in fields such as biology (germ inactivation), medicine (photodynamic therapy by O-1(2)), and synthesis of oxygenated organic compounds of great added value.