EPR studies of electron and hole trapping in titania photocatalysts

In situ EPR spectroscopy at cryogenic temperatures is used to observe paramagnetic products formed when titania photocatalysts are irradiated with UV–visible light in the presence of reactant molecules. Irradiation in vacuo, in the absence of reactants, produces weak EPR signals of trapped holes (O−) and trapped electrons (Ti3+). When high photon fluxes are used, the intensities of the trapped electron signals are enhanced dramatically when irradiation is stopped. This process is completely reversible on restoring the irradiation, and is attributed to a trapping of EPR invisible conduction band electrons once irradiation is stopped. The trapped electrons are excited back into the conduction band when irradiation is resumed. In the presence of adsorbed organic compounds, products of valence band hole trapping by the organic molecules are detected. Methyl radicals are formed by attack of valence band holes on adsorbed acetic acid. The valence band holes are also able to cleave carbon–silicon bonds, forming methyl radicals from tetramethylsilane. Benzyltrimethylsilane derivatives form both methyl radicals and benzyl radicals through cleavage of all four carbon–silicon bonds. The relevance of these observations to photocatalysed organic reactions in which carbon–carbon bond formation occurs via radical intermediates is discussed.