Detection of Superoxide Ions from Photoexcited Semiconductors in Non-Aqueous Solvents Using the ESR Spin-Trapping Technique

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Abstract
The ESR spin-trapping technique using 5,5-dimethyl-1-pyrroline N-oxide (DMPO) and N-t-butylbenzylideneamine N-oxide (PBN) as spin-trap reagents has been applied to detect active oxygen radicals generated by the photoexcitation of powdery semiconductors (TiO2, WO3, CdS, and Fe2O3) in non-aqueous solvents such as dimethylsulfoxide (DMSO), benzene, ethanol, and acetonitrile. Appreciable amounts of superoxide ion (O2−) were detected from TiO2, WO3, Fe2O3, and CdS suspensions under photoexcitation. The hyperfine splitting constants (hfsc) of the superoxide ion and carbon(C)-centered radical spin adducts for DMPO and PBN could be determined in various non-aqueous solvents. Some characteristic features for the production of the superoxide ion and C-centered radicals are discussed in connection with the energy-level diagrams of the semiconductors and the redox potentials of the superoxide ion and non-aqueous solvents.