Electron Traps in Rutile TiO2 Crystals: Intrinsic Small Polarons, Impurities, and Oxygen Vacancies

ABSTRACTRutile TiO2 is well known for its ability to “trap” photoinduced electrons at Ti4+ ions and form Ti3+ ions with an unpaired d1 electron. This has been shown experimentally to result in a large family of similar, yet slightly different, Ti3+-related centers that include both intrinsic small polarons and donor-bound small polarons. In these latter centers, the Ti3+ ion is located next to an oxygen vacancy or an impurity such as fluorine, lithium, or hydrogen. These small polarons are easily formed in commercially available bulk single crystals of rutile TiO2 by illuminating oxidized (and nominally undoped) samples at temperatures between 5 and 30 K with sub-band-gap laser light (e.g., 442 nm) or by slight reducing treatments (in the case of hydrogen). Once formed, the ground states of the defects are readily studied at low temperature with magnetic resonance (EPR and ENDOR). Single crystals of rutile TiO2 provide complete sets of angular dependence data, and thus allow detailed information about the ground-state models of the electron traps to be extracted in the form of g matrices and hyperfine matrices. In this review, the differences and similarities of the various Ti3+-related trapped electron centers are described.