Electron Traps in Rutile TiO 2 Crystals: Intrinsic Small Polarons, Impurities, and Oxygen Vacancies
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Electron Traps in Rutile TiO2 Crystals: Intrinsic Small Polarons, Impurities, and Oxygen Vacancies Larry E. Halliburton Department of Physics and Astronomy West Virginia University, Morgantown, WV 26506, U.S.A. ABSTRACT Rutile 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-bandgap 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. INTRODUCTION Titanium dioxide (TiO2) is a wide-band-gap semiconductor with an established record as a unique and versatile photocatalyst [1]. A strategy recently adopted to optimize TiO2 for specific applications involves modifying the optical and electrical properties of the material by adding or removing selected donors and acceptors (impurities as well as native defects). Success in these efforts, however, requires a complete understanding of the fundamental characteristics of these donors and acceptors in TiO2, including their electronic structure and optical response. Although much of the recent experimental work in TiO2 has focused on powders, thin films, nanoparticles, nanotubes, nanowires, etc., it is much more informative to use bulk crystals to initially and fully investigate these point defects. Electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) are experimental techniques well suited to study paramagnetic point defects in bulk crystals. Information obtained from a bulk crystal can then be used to interpret the EPR spectra observed in TiO2 powders and nanostructures. Electron traps in rutile TiO2 have recently been studied in detail because large single crystals of rutile are commercially available and EPR spectra from different centers have sharp lines and are easily resolved, thus allowing complete sets of spinHamiltonian parameters to be determined. Specifically, EPR and ENDOR have been used to study the intrinsic small polarons, the fluorine donors, and the S = 1/2 and S = 1 charge states of oxygen vacancies in fully oxidized TiO2 (rutile)
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