Singlet Oxygen 1O2 in Photocatalysis on TiO2. Where Does It Come from?

Features of the near-infrared phosphorescence of singlet oxygen 1O2 photosensitized by TiO2 in solutions have been investigated. The short-lived phosphorescence of 1O2 with spectral maximum at approximately 1300 nm is observed to follow excitation by pulsed laser radiation at 355 nm of TiO2 suspended in CCl4 and water H2O. The shorter lifetime and spectral shift of this phosphorescence, as compared with phosphorescence of 1O2 sensitized by phenalenone in these solvents, are attributed to the adsorbed state of emitting singlet oxygen. The observed laser pulse energy dependence indicates the two-photon and three-photon nature of this phosphorescence initiation in CCl4 and H2O, respectively. The formation of 1O2 in CCl4 is assumed to be provided by the sequence of one-photon excitation of TiO2, giving rise to superoxide anion O2– and one-photon photodetachment of electron from O2–. The photodetachment of electron from O2– in water H2O is a two-photon process because of higher solvation energy of superoxide anion in water. The processes that govern the rise and decay of singlet oxygen phosphorescence sensitized by TiO2 are discussed. The factors affecting the yield of singlet oxygen provided by electron photodetachment are also considered.